WO2015126326A1

WO2015126326A1 - Magnetic head with spin torque oscillator for microwave assisted magnetic recording

Info

Publication number: WO2015126326A1
Application number: PCT/SG2015/000047
Authority: WO
Inventors: Tiejun Zhou; Mingsheng Zhang; Hong Jing CHUNG; Hon Seng WONG; Zhejie Liu
Original assignee: Agency For Science, Technology And Research
Priority date: 2014-02-18
Filing date: 2015-02-16
Publication date: 2015-08-27

Abstract

There is provided a magnetic head for microwave assisted magnetic recording including: a recording main pole configured to generate a recording magnetic field during operation for writing information to a recording medium, a shield arranged to form a gap to the recording main pole, wherein the recording magnetic field from the recording main pole causes a gap magnetic field in the gap between the recording main pole and the shield, and a spin torque oscillator disposed in the gap between the recording main pole and the shield, the spin torque oscillator comprising a field generation layer configured to generate a high frequency magnetic field for reducing the coercivity of a target recording area of the recording medium. In particular, the spin torque oscillator is configured such that a predetermined crystallographic axis or crystal plane orientation of the field generation layer is substantially aligned with a direction of the gap magnetic field in the field generation layer during operation. There is also provided a corresponding method of fabricating such a magnetic head.

Description

MAGNETIC HEAD WITH SPIN TORQUE OSCILLATOR FOR MICROWAVE ASSISTED MAGNETIC RECORDING

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of priority of Singapore Patent Application No. 10201400112S, filed 18 February 2014, the contents of which being hereby incorporated by reference in its entirety for all purposes.

TECHNICAL FIELD

[0002] The present invention generally relates to microwave assisted magnetic recording (MAMR), and more particularly, to a magnetic head with spin torque oscillator for MAMR and a method of fabrication thereof.

BACKGROUND

[0003] Microwave assisted magnetic recording (MAMR) is one of the most promising technologies that has the potential to support recording density up to 3-4 Tbit/in². Currently, spin-torque oscillator (STO) is the only device that can produce very localized AC magnetic field of about 1 kOe or higher for significantly reducing the switching field (coercivity) of recording medium. However, reliability has been a major concern for STO because of the high critical current (Ic) required to make stable precession of magnetization in the field generation layer (FGL) of the STO, through which stable high frequency AC magnetic field can be generated. For example, high critical cuixent may damage the STO device due to the excessive heat generated, thereby affecting the long-term reliability of the STO device. On the other hand, lower critical current is advantageous as it allows lower energy consumption.

[0004] A need therefore exists to provide a magnetic head with spin torque oscillator for MAMR that seeks to overcome, or at least ameliorate, one or more of the deficiencies of conventional magnetic heads, such as to reduce the conventionally high critical current required for stable precession in the FGL. It is against this background that the present invention has been developed. SUMMARY

[0005] According to a first aspect of the present invention, there is provided a magnetic head for microwave assisted magnetic recording comprising:

a recording main pole configured to generate a recording magnetic field during operation for writing information to a recording medium;

a shield arranged to form a gap to the recording main pole, wherein the recording magnetic field from the recording main pole causes a gap magnetic field in the gap between the recording main pole and the shield; and

a spin torque oscillator disposed in the gap between the recording main pole and the shield, the spin torque oscillator comprising a field generation layer configured to generate a high frequency magnetic field for reducing the coercivity of a target recording area of the recording medium;

wherein the spin torque oscillator is configured such that a predetermined crystallographic axis or crystal plane orientation of the field generation layer is substantially aligned with a direction of the gap magnetic field in the field generation layer during operation.

[0006] Preferably, the predetermined crystallographic axis of the field generation layer is a c-axis of the field generation layer.

[0007] In an embodiment, the spin torque oscillator is disposed on a surface of the magnetic head, said surface being configured to be substantially normal to said direction of the gap magnetic field such that said predetermined principle axis of the field generation layer is substantially aligned with said direction of the gap magnetic field.

[0008] Preferably, said surface of the magnetic head is a surface of the recording main pole or a surface of the shield.

[0009] Preferably, said surface is inclined at an angle in the range of about 15° to 40° with respect to an axis across the gap.

[0010] In another embodiment, the field generation layer comprises an array of oblique structures, and the spin torque oscillator is disposed in the gap between the recording main pole and the shield such that the oblique structures extend in a direction substantially aligned with said direction of the gap magnetic field. [0011] Preferably, the oblique structures extend at an angle in the range of about 15° to 40° with respect to a vertical axis of the array.

[0012] In a further embodiment, the field generation layer comprises structures generally having a predetermined crystal plane orientation, and the spin torque oscillator is disposed in the gap between the recording main pole and the shield such that said predetermined crystal plane orientation is substantially aligned with said direction of the gap magnetic field.

[0013] Preferably, said predetermined crystal plane orientation is the (101) crystal plane orientation.

- [0014] Preferably, the spin torque oscillator further comprises a reference layer and an intermediate layer disposed between the field generation layer and the reference layer, and wherein the reference layer comprises a soft magnetic material or a negative anisotropy magnetic material configured to generate a demagnetization field for reducing a predetermined component of the gap magnetic field in the field generation layer.

[0015] Preferably, said predetermined component of the gap magnetic field is parallel to a longitudinal axis of the recording main pole.

[0016] Preferably, the reference layer comprises an in-plane layer and a perpendicular layer, the in-plane layer comprises the soft magnetic material or the negative anisotropy magnetic material, and the perpendicular layer comprises a perpendicular magnetic anisotropy material.

[0017] Preferably, the soft magnetic material or the negative anisotropy magnetic material is Co, Coir, CoFe, Fe, NiFe, NiFeMo, NiFeCo, NiFeCr, CoCrPt, Colr-X, or a combination thereof, where X is selected from the group consisting of Cr, Ru, Fe, Ni, Pt, Mn, Ti, and Pd.

[0018] Preferably, the spin torque oscillator further comprises at least one polarization layer disposed between the field generation layer and the reference layer for enhancing spin polarization ratio and reducing damping constant of the spin torque oscillator.

[0019] Preferably, the at least one polarization layer is made of Co, Fe, CoFe, Co/Cu/Co trilayers, NiFe, NiCoFe, CoFeB, CoFeCr, Ni, NiFeMo or a combination thereof. [0020] In an embodiment, the reference layer is a switchable perpendicular reference layer.

[0021] In another embodiment, the reference layer is a pinned reference layer.

[0022] Preferably, the field generation layer comprises a negative anisotropy magnetic material.

[0023] Preferably, the negative anisotropy magnetic material is Co, Coir, CoFe, Fe, NiFe, CoCrPt, CoFeMo, CoFeCr, Colr-X, or a combination thereof, where X is selected from the group consisting of Cr, Ru, Fe, Ni, Pt, Mn, Ti and Pd.

[0024] According to a second aspect of the present invention, there is provided a method of fabricating a magnetic head for microwave assisted magnetic recording, the method comprising:

forming a recording main pole configured to a recording magnetic field during operation for writing information to a recording medium;

arranging a shield to form a gap to the recording main pole, wherein the recording magnetic field from the recording main pole causes a gap magnetic field in the gap between the recording main pole and the shield; and

disposing a spin torque oscillator in the gap between the recording main pole and the shield, the spin torque oscillator comprising a field generation layer configured to generate a high frequency magnetic field for reducing the coercivity of a target recording area of the recording medium;

wherein the method further comprises configuring the spin torque oscillator such that a predetermined crystallographic axis or crystal plane orientation of the field generation layer is substantially aligned with a direction of the gap magnetic field in the field generation layer during operation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Embodiments of the present invention will be better understood and readily apparent to one of ordinary skill in the art from the following written description, by way of example only, and in conjunction with the drawings, in which: FIG. 1 depicts a schematic diagram of a magnetic head for microwave assisted magnetic recording (MAM ) according to an example embodiment of the present invention and a recording medium in which information may be written;

FIGs. 2A to 2C depict schematic diagrams of various spin torque oscillators (STO) according to various embodiments of the present invention;

FIG. 3 depicts a top view of the spatial field distribution of the AC magnetic field generated by the field generation layer (FGL) of the STO in an exemplary illustration;

FIGs. 4A and 4B depict an example of the precession of the magnetization in the FGL and the switching of the reference layer, respectively, under a head gap magnetic field (H_az) of 8000 Oe with an applied direct current of 1 mA to the reference layer;

FIG. 5 depicts a graph of the switching time of the reference layer against the available head gap magnetic field of 8000 Oe under different damping constants of 0.15, 0.2 and 0.25 in an exemplary illustration;

FIG. 6 depicts a graph showing the required critical current (Ic) against the y- component of the head gap field (Hy) in the FGL in an exemplary illustration;

FIG. 7A depicts a magnetic field distribution in the gap between the recording main pole and the shield with the FGL disposed therein in an exemplary illustration;

FIG. 7B illustrates an arrangement of the STO such that the c-axis of the FGL is substantially aligned with the direction of the gap magnetic field in the FGL during operation according to an embodiment of the present invention;

FIG. 7C depicts a plot of the x- and y-components of magnetic field distribution in the gap along the x-axis;

FIG. 7D depicts an arrangement of the STO in the gap of the magnetic head such that the c-axis of the FGL is substantially aligned with the direction of the gap magnetic field in the FGL during operation according to an embodiment of the present invention;

FIG. 7E depicts a schematic diagram illustrating a surface of the magnetic head configured to be substantially normal to the direction of the gap magnetic field such that the c-axis of the FGL disposed on such a surface is substantially aligned with the direction of the gap magnetic field in the FGL during operation according to an embodiment of the present invention; FIG. 8 A depicts a schematic diagram of an array of oblique structures forming the FGL whereby the oblique structures are formed so as to extend in a direction (and thus the c-axis of the FGL) substantially aligned or matching with the direction of the gap magnetic field during operation according to an embodiment of the present invention;

FIG. 8B depicts a transmission electron microscopy (TEM) cross-sectional image of the array of nanostructures as an exemplary illustration;

FIG. 9 depicts a schematic diagram illustrating an exemplary material (Coir alloy) forming the FGL having a predetermined crystal plane (101) substantially aligned with a direction of the gap magnetic field in the FGL according to an embodiment of the present invention;

FIG. 10 illustrates the self-compensation of the y-component of the head gap field in the FGL based on self-demagnetization effects from the reference layer according to an embodiment of the present invention;

FIGs. 11A and MB illustrate the magnetization of the composite reference layer according to an embodiment of the present invention, with and without the head gap field;

FIGs. 12A and 12B illustrate the magnetization of the reference layer according to an embodiment of the present invention, with and without the head gap field;

FIG. 13 depicts a schematic diagram of an STO with additional spin polarization layers incorporated according to an embodiment of the present invention; and

FIG. 14 depicts a flow diagram of a method of fabricating the magnetic head for microwave assisted magnetic recording according to an embodiment of the present invention. DETAILED DESCRIPTION

[0026] Embodiments of the present invention provide a magnetic head with spin torque oscillator (STO) for microwave assisted magnetic recording (MAMR) that seeks to overcome, or at least ameliorate, one or more of the deficiencies of conventional magnetic heads. As known in the art, in the MAMR technique, writing of information in a recording medium by the recording main pole is assisted by the STO positioned in the vicinity of the recording main pole. In particular, the recording/writing magnetic field generated by the recording main pole for writing information in the recording medium is typically not strong enough to switch the magnetization of a target recording area of the recording medium. In this regard, the STO is configured to generate a high frequency field (microwave) from the precession of magnetization in the field generation layer (FGL) of the STO. This is intended to reduce the coercivity of the target recording area when the high frequency field is applied thereo, thus assisting the recording/writing magnetic field to write information in the target recording area. However, as discussed in the background, reliability has been a major concern for conventional STO because of the high critical driving current (Ic) required to be applied to the STO in order to make stable precession of magnetization in the FGL of the STO, which for example may damage the STO device due to the excessive heat generated.

[0027] Therefore, embodiments of the present invention seek to provide a magnetic head with spin torque oscillator for MAMR which advantageously reduces the conventionally high critical current required for stable precession of magnetization in the FGL. This advantageously minimizes/avoids damages to the STO as well as enabling the magnetic head including the STO to be more power efficient. Further preferred embodiments of the present invention also enable the STO to operate in an alternative magnetic field in the magnitude of about 8,000 to about 10,000 Oe, enable the STO to generate large AC magnetic field (microwave) with high and tunable frequency, and/or enable the magnetic head to have a wide fabrication/operation window (i.e., more susceptible to fabrication errors such as misalignment of the STO in the gap (e.g., up to about 8% to 10% misalignment error)).

[0028] FIG. 1 depicts a schematic diagram of a magnetic head 100 for MAMR according to an example embodiment of the present invention and a recording medium (perpendicular magnetic recording medium) 102 in which information- may be written. As shown in FIG. 1, the magnetic head 100 comprises a recording main pole 110 configured to generate a recording magnetic field during operation for writing information to the recording medium 102, a shield 114 arranged to form a gap to the recording main pole 110 (whereby the recording magnetic field from the recording main pole 110 causes a gap magnetic field in the gap between the recording main pole 110 and the shield 114), and a STO 120 disposed in the gap between the recording main pole 110 and the shield 114.

[0029] FIG. 2A depicts a schematic diagram of the STO 120 according to an embodiment of the present invention. The STO 120 comprises a FGL (oscillation layer) 122 configured to generate a high frequency magnetic field (microwave) for reducing the coercivity of a target recording area of the recording medium 102. In particular, according to embodiments of the present invention, the STO 120 is configured such that a predetermined crystallographic axis or crystal plane orientation of the FGL 122 is substantially aligned with a direction of the gap magnetic field in the FGL 122 during operation. Aligning the predetermined crystallographic axis (preferably, c-axis) or crystal plane orientation (preferably, (101) orientation) of the FGL 122 with the direction of the gap magnetic field in the FGL 122 has been surprisingly found to make precession of magnetization in the FGL 122 much easier, thus advantageously reducing the critical current required to be applied to the STO 120 in order to make stable precession of magnetization in the FGL 122. This aspect of the present invention will be described in further details later below, along with specific examples of how such an alignment can be implemented according to embodiments of the present invention.

[0030] In order that the invention may be readily understood and put into practical effect, the magnetic head 100 will now be described in further details by way of a non- ' limiting example. As shown in FIG. 1, the magnetic head 100 may comprise a reader/reproducing module/unit 140 and a writer/recording module/unit 142. For example, the reader module 140 may comprise a magnetic reproducing element/sensor 144 interposed between two opposing magnetic shields 146 for detecting/sensing the magnetization direction recorded in the recording medium 102 so as to obtain the information stored therein. The writer module 142 may comprise a magnetic core 148 formed by the recording main pole 110 and the magnetic shield 114, an electromagnetic coil 150 for exciting the magnetic core 148, and the STO 120. For example, as shown in FIG. 2A, the STO 120 may comprise a FGL 122, an intermediate or spacer layer 124 of which is made of a non-magnetic material such as Cu, Cr, Ag, or Au, and a reference or polarization layer 126. For example, the in-plane reference or polarization layer 126 can be made of Co, Fe, CoFe, NiFe, NiFeCo, NiFeMo, CoFeCr, Coir, Colr-X (e.g., X=Cr, Ru, Fe, Ni, Pt, Mn, Ti, or Pd), any other soft magnetic materials with antiferromagnetic pinning layer, or a combination thereof. The in-plane reference or polarization layer 126 can also be made of any CoCrPt alloys with c-axis in the film plane. The perpendicular FGL 122 can be Co/Pt, Co/Pd, Co/Ni multilayer films, CoPt, CoPt-X (e.g., X=Cr, Ni, Rh, Ru, Cu, Pd, Fe, etc), any Co-Pt alloys, or any magnetic materials with perpendicular anisotropy.

[0031] During operation, the electromagnetic coil 150 is powered to excite the magnetic core 148 for the recording main pole to generate a recording magnetic field for writing information to the recording medium 102. In addition, a direct/driving current is applied to the STO 120 for the STO 120 to generate a high frequency magnetic field (microwave). In this regard, the electron spins passing through the reference layer 126 is polarized based on the direction of magnetization in the reference layer 126, which produces a spin polarized current. This spin polarized current is transmitted through the spacer layer 124 and applies a spin transfer torque on the FGL 122, which causes the magnetization of the field generating layer 122 to oscillate into a precessional state 123, thereby generating the high frequency magnetic field (microwave).

[0032] For ease of reference and consistency, a three-dimensional coordinate system 180 having an x-axis, a y-axis and a z-axis as shown in FIG. 1 is used as a reference when appropriate to describe various parameters (such as a direction, an orientation or a dimension) of the magnetic head 100, including various components thereof, and the magnetic field, unless stated otherwise. For example, the x-axis is parallel to a direction extending between the recording main pole 110 and the shield 114, the y-axis is parallel to a longitudinal axis of the recording main pole 110, and the z-axis is perpendicular to both the x-axis and y-axis. It will be appreciated that the present invention is not limited to the coordinate system 180 shown in FIG. 1.

[0033] By way of an example only and without limitation, the gap between the recording main pole 110 and the shield 114 may be about 50 nm, and the magnetic head 100 may be configured to operate at about 5 nm above the recording medium 102 (i.e., head-to-medium separation (HMS)). It will be appreciated to a person skilled in the art that various dimensions for the gap between the recording main pole 110 and the shield 114 may be implemented as appropriate such as 25-40 nm, while the HMS can change from 3 nm to about 10 nm.

[0034] FIG. 2B depicts a schematic diagram of the STO 120 according to another example embodiment of the present invention. In this embodiment, the STO 120 comprises a FGL 122 made of a negative anisotropy material, a spacer layer 124, and a switchable perpendicular reference layer 246. As a non-limiting example, the FGL 122 may be made of a negative anisotropy material having an anisotropy energy constant (K_u) of about -6 xlO⁶ to -lOxlO⁶ erg/cc, a saturation magnetization (M_s) of about 10000 emu/cc, and a polarization (η) of about 0.4 to 0.5. The reference layer 246 may be made of a material having an anisotropy energy constant (K_u) of 2 to 5xl0⁶ erg/cc, saturation magnetization (M_s) of about 600 to 800 emu/cc, and a polarization (η) of about 0.4 to 0.5. In this embodiment, the switchable reference layer 246 advantageously address the alternative head gap field, and the use of negative anisotropy material for the FGL 122 has been found to result in a large angle and stable precession as well as a large in-plane AC magnetic field. For example, the FGL 122 can be made of Coir, Co/Fe multilayer films or any magnetic materials with negative anisotropy energy such as Co, Fe, NiFeCo, NiFeMo, CoFeCr, and Colr-X (e.g., X= Cr, Ru, Fe, Ni, Pt, M , Ti, or Pd). The switchable reference layer 246 can be made of Co/Pt, Co/Pd, Co/Ni multilayers, CoPt, FePt, CoPt-X, FePt-X (e.g., X=Cr, Ni, Rh, Ru, Cu, Pd, Fe, Co, etc), any other Co-Pt alloys, or any magnetic materials with perpendicular anisotropy.

[0035] FIG. 2C depicts a schematic diagram of the STO 120 according to a further example embodiment of the present invention. In this embodiment, the STO 120 comprises a FGL 122 made of a negative anisotropy material (which may be the same as the FGL 122 shown in FIG. 2B), a spacer layer 124, and a reference layer 260 including a first reference layer 264 and a second reference layer 266, both of which are made of a negative anisotropy material and are pinned (non-switchable) in-plane reference layers. The seedlayer 261 may be configured for inducing a face-centered cubic (FCC) (111) or hexagonally close-packed (HCP) (0002) orientation and is preferably made of materials such as Pt, Pd, Ru, NiW, or NiF. The antiferromagnetic pinning layer 262 is configured to pin the first and second reference layers 264, 266 and is preferably made of materials such as IrMn or PtMn. The reference layers 264 and 266 can be any magnetic materials with negative anisotropy energy, such as Co, Fe, NiFeCo, NiFeMo, CoFeCr, Coir or Co/Fe multilayers, and Colr-X (e.g., X= Cr, Ru, Fe, Ni, Pt, Mn, Ti, or Pd), which are antiferromagnetically coupled through the Ru layer 265 (e.g., thickness of about 0.8-1.0 nm). With the above configuration, the two antiferromagnetically coupled reference layers. 264, 266 was found to cancel the stray field from reference layer 260, which makes the FGL precession easier and therefore advantageously reducing the STO driving current.

[0036] By way of an example only and without limitation, for a gap between the recording main pole 110 and the shield 114 of about 50 nm, exemplary dimensions (length x width x thickness) of each layer of the STO 120 may be about 40 nm x 40nm x lOnm for the FGL 122, 40nm x 40nm x 2 to 5nm for the spacer layer 124, and 40nm x 40nm x 10 to 20nm for the reference layer 126, 246, 264 (the thickness being along the x- axis when the FGL 122 is oriented as shown in FIGs. 1 and 2).

[0037] FIG. 3 depicts a top view of the spatial field distribution of the AC magnetic field generated by the FGL 122 in an exemplary illustration. It can be observed from FIG. 3 that the AC magnetic field changes from being linear at the center to being circular on both sides of the FGL 122. The opposite chirality of the circular AC magnetic field on both sides of the FGL 122 can be used for improving the writing efficiency. In FIG. 3, the outer circles represent the AC magnetic field strength produced by the STO 120 in the recording media 102 at a HMS of 5 nm, the middle circles represent the AC magnetic field strength produced by the STO 120 in the recording media 102 at a HMS of 10 nm, while the inner circles represent the AC magnetic field strength produced by the STO 120 in the recording media 102 at a HMS of 15 nm. As can be clearly seen, the AC magnetic field strength decreases fast with increase of the HMS. Compared to linear AC magnetic field, circular AC magnetic field is more efficient during the writing process. In particular, with circular AC magnetic field having frequency close to the nature resonance frequency of the media magnetization, the media magnetization can be easily activated to precess with a large precession angle, resulting in easier switching and higher efficiency.

[0038] FIG. 4A illustrates an example of the precession of the magnetization in the FGL 122 and FIG. 4B illustrates the switching of the reference layer 246 under a head gap magnetic field (H_az) of 8000 Oe with an applied direct current of 1 mA to the reference layer 246. In this example, the FGL 122 is made of a negative anisotropy material having an anisotropy energy constant (K_u) of -8xl0⁶ erg/cc, saturation magnetization (M_s) of 1000 emu/cc, and a damping constant (a) of 0.02. The reference layer 246 is made of a material having an anisotropy energy constant (K_u) of 0.25xl0⁷ erg/cc, saturation magnetization (M_s) of 1100 emu/cc, and a damping constant (a) of 0.02. FIG. 4A(a) shows the switching of M_z, while FIGs. 4A(b) and (c) show the precession processes of M_x and M_y of the FGL magnetization under an alternative head gap field of 8000 Oe. It can be observed that from FIGs. 4A(b) and (c) that both before switching and after complete switching of the reference layer 246, the FGL 122 precesses steadily with the same frequency. However, it can also be observed that the precession of the FGL magnetization is not steady during the switching process of the reference layer 246, which may affect the recording process, resulting in unwanted writings or write-in errors. FIGs. 4B(a), (b) and (c) demonstrate the response of the M_x, M_y and M_z of the reference layer magnetization under alternative head gap field of 8000 Oe. A clear precession switching of reference layer 246 can be seen from FIG. 4B(a) and the switching the reference layer 246 was found to take some time. In this case, it was close to 1 ns, which may be considered too long for practical purposes.

[0039] In the example, it is noted that the reference layer 246 is switchable under the alternative gap magnetic field without changing the high frequency field (microwave) generated by the FGL 122 because the symmetry of the microwave is not changed before and after switching. If the reference layer 246 is not switchable, when the head gap field changes directions, the effective magnetic field acting on FGL 122 changes as well, resulting in the frequency change of the FGL magnetization precession. In the example shown in FIGs. 4A and 4B, it can be observed that the switching of the reference layer 246 may take too long. Therefore, according to embodiments of the present invention, the switching time of reference layer 246 was. advantageously reduced to be less than 0.2 ns, which is about the head field rising time, in order to follow the clock of head field. In particular, it was surprisingly found that the switching time depends on the anisotropy energy constant (K_u) and the damping constant (a) and this is described in further detail below with reference to FIG 5. [0040] FIG. 5 depicts a graph 500 of the switching time of the reference layer 246 against the available head gap magnetic field of 8000 Oe under different damping constants of 0.15, 0.2 and 0.25 for illustration purposes only. It can be observed that the switching time of reference layer 246 decreases with the anisotropy energy of the reference layer 246, while it increases with a decrease in the damping constant. Therefore, this demonstrates that the damping constant plays an important role in reducing the switching time of the reference layer 246, and demonstrates that a switching time of 0.2 ns or below can be achieved by configuring the anisotropy energy constant and the damping constant accordingly, which is required for effective writing of information in the recording medium 102. In an embodiment, it was found that a damping constant of about 0.15 or more is needed to obtain a switching time of less than 0.2 ns (e.g., see FIG. 5). Preferably, the damping constant is about 0.3 to 0.5 or less depending on the materials available. In an embodiment, a Co/Pt multilayer with high perpendicular anisotropy and Ll₀ FePt can have a damping constant more than 0.15. On the other hand, the switching time can also be reduced by decreasing the anisotropy energy of the reference layer 246. In an embodiment, the anisotropy energy of Co/Pt multilayers is tuned by changing the relative thickness of Co (e.g., vary from about 0.2 nm to about 1 nm) and Pt (e.g., vary from about 0.2 nm to about 0.6 nm), while keeping the damping constant similar. For example, the anisotropy energy constant can be changed from zero to about 1 x 10⁶ erg/cc, while the damping constant needs to be about 0.15 or more.

[0041] In another embodiment, FIG. 6 depicts a graph 600 illustrating the required critical current (Ic) against the y-component of the head gap field (H_y) in the FGL 122. It was surprisingly found that the critical current increases linearly with the y-component of the head gap field in the FGL 122 as can be observed from FIG. 6. Without wishing to be bound by theory, a possible explanation for the critical current increasing linearly with the y-component of the head gap field is because the spin tor ue field (¾ττ) is found to be proportional to the STO driving current, i.e., H_srr∞ if there is

any y-component in the head gap field, the spin torque field needs to be high enough to balance the y-component of the head gap field in order to have a stable magnetization precession. Accordingly, based on this finding, embodiments of the present invention advantageously configures the STO 120 such that the y-component of the head gap field in the FGL 122 during operation is minimized/eliminated in order to reduce the critical current required to achieve stable precession of magnetization in the FGL 122. Preferred configurations of the STO 120 in the magnetic head 100 for minimizing/eliminating the y-component of the head gap field in the FGL 122 will now be. described with reference to FIGs. 7 to 9.

[0042] For illustration purposes, FIG. 7 A illustrates a magnetic field distribution in the gap between the recording main pole 110 and the shield 114 with the FGL 122 disposed in the gap, which indicates a field angle of about 28 to 30° with respect to the x- direction. FIG. 7C depicts a plot of the x- and y-components of magnetic field distribution in the gap across the x-axis. It can be observed that the head gap field is not uniform. In particular, the total head gap field field (H_totai) is about 8000 Oe near the main pole 110, while it is about 6000 Oe close to the front shield 114. The y-component (Hy) is about 4000 Oe with the highest value of about 5000 Oe and lowest value of about 2000 Oe close to the front shield 114. Therefore, it can be seen that the y-component value is very high and not favorable for large-angle out-of-plane precession, which is required for the generation of large in-plane AC magnetic field in the recording media 102 for effective assisted writing.

[0043] It will be appreciated to a person skilled in the art that the head gap field (and thus the direction of the magnetic field in the FGL 122 of the magnetic head 100) depends on various parameters such as the head geometry and the flying height. For example, the head gap field distribution depends on both the head geometry and the distance between the head surface and the soft underlayer, which is tailorable (both magnitude and direction) to meet the STO requirements. For STO design, it is necessary to consider both the head gap field strength and its spatial distribution. Therefore, it will be appreciated to a person skilled in the art that the field angle is not limited to being about 28° to 30° with respect to the x-direction as shown in FIG. 7A and may be other angles depending on various parameters such as those as mentioned above. For example, the field angle may be in the range of about 15° to about 40° with respect to the x-axis, such as 15° to 35°, 20° to 30°, 25° to 40°, 28° to 40°, 28° to 35°, and 28° to 30° according to preferred embodiments of the- present invention. The magnitude of the head gap field may be in the range of about 3000 Oe to about 8000 Oe. The STO 120 may then be arranged in the gap accordingly based on the actual direction of the gap magnetic field in the FGL 122.

[0044] For simplicity, embodiments the present invention will be described based on the example as described above whereby the direction of the gap magnetic field in the FGL 122 is about 28° to 30° with respect to the x-direction. In the example, it can be observed from FIG. 7A that the y-component of the head gap field in the FGL 122 is significant if the FGL 122 is positioned/oriented as shown in FIG. 7 A (vertically along the y-axis and parallel to the y-z plane). In a first example embodiment, to minimize/eliminate the y-component of the head gap field in the FGL 122, a predetermined crystallographic axis of the FGL 122 is substantially aligned with a direction 710 of the gap magnetic field in the FGL 122 during operation. In a preferred embodiment, the predetermined crystallographic axis is a c-axis 720 of the FGL 122 as illustrated in FIG. 7B. In particular, the c-axis 720 of the FGL 122 is substantially aligned or matched with the direction 710 of the head gap field in order to minimize/eliminate the y-component of the head gap field in the FGL 122 so as to reduce the critical current required to have stable precession in the FGL 122. As illustrated, in FIGs 7B and 7D, the c-axis 720 of STO/FGL is oriented away from the x-axis by about 28° to 30°.

[0045] FIG. 7E depicts a schematic diagram of an exemplary implementation according to the first example embodiment to align/match the predetermined crystallographic axis (c-axis 720) of the FGL 122 with the direction of the gap magnetic field in the FGL 122 during operation. In particular, a surface of the magnetic head 100 is configured to be substantially normal to the direction of the gap magnetic field such that the c-axis 720 of the FGL 122 is substantially aligned with the direction of the gap magnetic field in the FGL 122. Preferably, the surface of the magnetic head 100 is a surface 750 of the recording main pole 110 or a surface 752 of the shield 114 facing/defining the gap and near or adjacent the air bearing surface (ABS) 760 of the magnetic head 100. For example, the recording main pole 110 and/or the shield 114 may be trimmed to form such an inclined surface 750, 752. In a preferred embodiment, the surface is inclined at an angle in the range of about 15° to 40° with respect to the air bearing surface 760 (i.e., the x-axis) of the magnetic head 100, such as 15° to 35°, 20° to 30°, 25° to 40°, 28° to 40°, 28° to 35°, and 28° to 30° according to preferred embodiments of the present invention. With such an inclined surface 750, 752, the STO 120 may then simply be disposed on the inclined surface and the c-axis of the FGL 122 would naturally be substantially aligned with the head gap field.

[0046] FIG. 8 A depicts a schematic diagram of an exemplary implementation according to a second example embodiment for configuring the FGL 122 such that a predetermined crystallographic axis (in particular, c-axis) of the FGL 122 is substantially aligned with a direction of the gap magnetic field in the FGL 122 during operation. In this example embodiment, the FGL 122 comprises an array of oblique structures (nanostructures such as nanorods or nano wires) 810, and the STO 120 is disposed in the gap between the recording main pole 110 and the shield 114 such that the oblique structures 810 extend in a direction (and thus the c-axis of the FGL 122) substantially aligned or matching with the direction of the gap magnetic field.

[0047] For example, the FGL 122 is deposited using an oblique sputtering technique to form an array of oblique structures 810 extending in a direction away from a vertical axis 820 of the array such that the c-axis of the FGL 122 is also tilted away from the vertical axis 820. Accordingly, the STO 120 having the FGL 122 with obliquely deposited structures 810 can be disposed on a surface of magnetic head 100 (e.g., a surface of the recording main pole 110 or a surface of the shield 114 facing/defining the gap and near or adjacent the air bearing surface (ABS) 760 of the magnetic head 100) with the tilted c-axis of the FGL 122 aligned with the gap magnetic field in the FGL 122 without requiring forming an inclined surface 750, 752 as shown in FIG. 7D. In a preferred embodiment, the oblique structures 810 are inclined or extend in a direction at an angle in the range of about 15° to 40° to the vertical axis 820 of the array 810, such as 15° to 35°, 20° to 30°, 25° to 40°, 28° to 40°, 28° to 35°, and 28° to 30° according to preferred embodiments of the present invention. Oblique sputtering techniques are known in the art and a skilled person would know how to deposit an array of oblique structures having the desired angle of inclination, thus details thereof will not be described herein.

[0048] As an illustration only, FIG. 8B depicts a transmission electron microscopy (TEM) cross-sectional image of an array of nanostructures 810 formed according to this embodiment of the present invention. The films were deposited with a ultrahigh vacuum system at a sputtering pressure of about 3-5 mT. The sputtering angle (Θ) as shown in FIG. 8A may be configured accordingly for forming the nanostructures 810 at a desired angle, such as about 30° to 60°. For example, suitable materials for forming the nanostructures 810 may be Cr and Cr alloys, such as CrRu, CrW, CoCrRu, and CrMo.

[0049] FIG. 9 depicts a schematic diagram of an exemplary implementation according to a third example embodiment for configuring the FGL 122 such that a predete mined crystal plane of the FGL 122 is substantially aligned with a direction of the gap magnetic field in the FGL 122 during operation. In particular, it is noted that the angle between the c-axis and the (101) plane of the nanostructures in the FGL 122 is about 28°, which is very close to the head gap field direction. Therefore, in this embodiment, a seed layer with a preferred orientation (i.e., (101) plane) is introduced to control the orientation of the nanostructures formed in the FGL 122 such that the crystal plane of the FGL would generally align with the head gap field during operation. Accordingly, the STO 120 having the FGL 122 with controlled/predetermined crystal plane orientation can be disposed on a surface of magnetic head 100 (e.g., a surface of the recording main pole 110 or a surface of the shield 114 facing/defining the gap and near or adjacent the air bearing surface (ABS) 760 of the magnetic head 100) with the crystal plane orientation of the FGL 122 generally aligned/matched with the gap magnetic field in the FGL 122. For example, Cr and Cr alloys, such as CrRu, CrMo, CrCoRu, or CrPt, with (110) orientation is a suitable seed layer for the above-described controlled orientation of FGL 122 (101) orientation. For example, the (110) plane of Cr (or Cr alloys) has similar atomic arrangement and lattice parameters as the (101) plane of FGL 122. Therefore, (101) plane of FGL 122 can be epitaxially grown on top of (110) plane of Cr (or Cr alloys). Cr (110) can be easily grown on top of amorphous substrate, such as glass substrates or Si02/Si wafers.

[0050] It will be appreciated that the above exemplary implementations described are non-limiting and various implementations may be realized without deviating from the scope of the present invention as long as the STO 120 is configured such that a predetermined parameter (preferably, a predetermined crystallographic axis or crystal plane orientation) is substantially aligned with a direction of the gap magnetic field in the field generation layer during operation so as to minimize/eliminate the y-component of the head gap field in the FGL 122.

[0051] In a further embodiment, it was found through experiments that misalignment of the predetermined parameter (e.g., c-axis) with the head gap field may typically occur during fabrication. Furthermore, the c-axis of the FGL 122 may have a degree of dispersion. That is, an extent of the y-component of the head gap field may still be present in the FGL 122 during operation despite attempting to configure the predetermined parameter of the FGL 122 so as to be aligned with a direction of the gap magnetic field in the FGL 122. This may not allow the critical current to be reduced as much as intended or desired. As an example, a 5° misalignment between the c-axis of the FGL 122 and the head gap field may result in a y-component of the head gap field in the FGL 122 measuring 700 Oe. This may in turn increase the critical current required from about 1.6mA to about 6.5 mA, which is greatly undesirable.

[0052] Therefore, according to an embodiment of the present invention, the STO 120 is further configured to compensate such a y-component of the head gap field (H_y) in the FGL 122 due to fabrication errors such as misalignment resulting in the presence of the y-component of the head gap field in the FGL 122 during operation. Preferably, up to 8% to 10% alignment error may be compensated. As described hereinbefore, the y- component of the head gap field (H_y) can be compensated by the spin torque field (H_STT) which is proportional to the STG driving current. For example, a y-component of 500 Oe needs more than 6 mA driving current to compensate the y-component, which is considered too high and adversely affects long term reliability of the STO 120. Without wishing to be bound by theory, it is believed that the y-component of the head gap field in the FGL 122 may be self-compensated using self-demagnetization effects based on the principles as shown in FIG. 10. In particular, from FIG. 10, it can be seen that the effective field {H_ef_j) in FGL 122 can be zero if the y-component (H_y) of the head gap field can be balanced by the demagnetization field (%.,·) from the reference layer, that is, H_eff = H —H_d__r => 0 . The demagnetization field at the FGL 122 generated by the reference layer 126 is always opposite to the y-component of the head gap field. If the reference layer 126 is made of negative anisotropy materials (or soft materials), the moment will be tilted away from the film normal, that will generate a demagnetization field that is opposite to the y-component of the head gap field in the FGL 122. Based on this, it is possible to at least partially cancel the y-component of the head gap field in the FGL 122 due to fabrication errors.

[0053] FIGs. 11 A and 1 IB depict an exemplary implementation for compensating the y-component of the head gap field according to an exemplary embodiment of the present invention. In this embodiment, the reference layer 126 is a composite reference layer and comprises ah in-plane layer 1110 and a perpendicular layer 1112 coupled together. FIG. 11 A illustrates that without the head field, the magnetization of composite reference layer is tilted away from the film normal. However, as shown in FIG. 1 IB, with the presence of the head gap field, the magnetization of the composite reference layer will be pointed to the film normal, with which the system symmetry can be maintained. This is important because if the system symmetry is not maintained, the FGL precession mode and frequency will be changed, which may resulting in unwanted writing/erasing.

[0054] For example, by using composite reference layer 126, it was found through experiments that the magnetization switching of the composite reference layer 126 is faster and the magnetization of the composite reference layer 126 is responsive to the y- component of the head gap field. Therefore, with this self-compensation of the y- component in the FGL 122, the STO operation/fabrication window can be significantly increased (e.g., able to tolerate up to about 8 to 10% error in alignment, and preferably at least 5%, while the STO critical driving current can be significantly reduced. For example, the in-plane layer of the composite reference layer 126 can be Co, Coir, CoFe, Fe, NiFe, CoCrPt, Colr-X (e.g., X= Cr, Ru, Fe, Ni, Pt, Mn, Ti, or Pd), or any other soft magnetic materials and magnetic materials with negative anisotropy energy. For example, the perpendicular layer 112 of the composite reference layer 126 can be Co/Pt, Co/Pd, Co/Ni multilayer films, CoPt, CoPt-X (e.g., X=Cr, Ni, Rh, Ru, Cu, Pd, Fe, etc), any Co- Pt alloys, or any magnetic materials with perpendicular anisotropy. In particular, the magnetic material with negative anisotropy energy is part of the reference layer 126. The composite reference layer 126 has been found to possess a number of benefits. One is tunable Κ„/¾ of the reference layer, which enable faster switching of the reference layer. Second, the composite reference layer 126 is responsive to the y-component of the head gap field, H_y, and therefore reducing the STO current. Furthermore, the in-plane layer 1110 has higher spin polarization rate, and therefore help to enhance the spin torque effect and reduce STO driving current.

[0055] FIGs. 12A and 12B depict another exemplary implementation for compensating the y-component of the head gap field according to another exemplary embodiment of the present invention. In this embodiment, the reference layer 126 is made of a negative anisotropy material with variable K_u and M_Si such as Co, Fe, NiFe, CoCrPt, Coir, Colr-X (e.g., X=Cr, Fe, Ni, Cu, Ru, Pt, Pd, Mn, Ti, etc), or Co/Fe multilayer films with changeable K_u of 0 to -10⁷ erg/cc) and M_s of 500 to 1200 eum/cc. FIG. 12A illustrates that without the head gap field, the magnetization of the reference layer 126 lies in the film plane. However, as shown in FIG. 12B, with the presence of the head gap field, the magnetization of the reference layer 126 will be turned to the film normal, with which the system symmetry can be maintained.

[0056] For example, by using a reference layer 126 having negative anisotropy energy, it was found through experiments that the magnetization switching of the reference layer 126 is much shorter and the magnetization of the reference layer 126 is responsive to the y-component of the head gap field. Therefore, with this self- compensation of the y-component in the FGL 122, the STO operation/fabrication window can be significantly increased (e.g., able to tolerate up to about 8 to 10% in alignment, and preferably at least 5%) while the STO critical driving current can be significantly reduced. For example, the reference layer 126 can be Co, Coir, CoFe, Fe, NiFe, CoCrPt, Colr-X (e.g., X= Cr, Ru, Fe, Ni, Pt, Mn, Ti, or Pd), or any other soft magnetic materials and magnetic materials with negative anisotropy energy. Coir, Colr-X (e.g., X=Cr, Ru, Fe, Ni, Pt, Mn, Ti, Pd, etc) or any other magnetic materials with negative anisotropy energy can also be used as the reference layer 126. The anisotropy energy (K_u) and the saturation magnetization (M_s) of the reference layer 126 can be tuned by element doping to meet the system requirements.

[0057] According to a further embodiment of the present invention, to further reduce the critical current required for stable precession of magnetization in the FGL 122, one or more spin polarization layers 1310 are incorporated into the STO 120 as shown in FIG. 13. This is a physical approach to increase the spin polarization ratio and reduce the damping constant so as to reduce the critical current. In the embodiment of FIG. 13, two spin polarization layers 1310 are disposed on either side of the spacer layer 124 and between the FGL 122 and the reference layer 126. Inserting the spin polarization layers 1310 into the film stack advantageously further enhances the efficiency of the STO 120 and reduces the critical current required. For example, the FGL materials Coir has high damping constant and low spin polarization rate, while the spin polarization layer, such as Co, Co/Cu/Co trilayers, Fe, CoFe, NiFe, CoFeB, CoFeCr, Ni, NiFeMo, etc, or a combination thereof, has low damping constant and high spin polarization rate. By combining high spin polarization and low damping constant materials for the FGL 122, the effective damping constant can be reduced and the spin polarization rate can be increased. With this spin polarization layer 1310, the spin polarization rate of the injected current from the reference layer can be increased, leading to high spin torque efficiency and therefore, lower STO current.

[0058] FIG. 14 depicts a flow diagram of a method 1400 of fabricating the magnetic head 100 for microwave assisted magnetic recording according to an embodiment of the present invention. The method 1400 comprises a step 1402 of forming a recording main pole 110 configured to a recording magnetic field during operation for writing information to a recording medium 102, a step 1404 of arranging a shield 114 to form a gap to the recording main pole 110, wherein the recording magnetic field from the recording main pole 110 causes a gap magnetic field in the gap between the recording main pole 110 and the shield 114, a step 1406 of disposing a STO 120 in the gap between the recording main pole 110 and the shield 114, the STO 120 comprising a FGL 122 configured to generate a high frequency magnetic field for reducing the coercivity of a target recording area of the recording medium 102, and a step 1408 of configuring the STO 120 such that a predetermined crystallographic axis or crystal plane orientation of the FGL 122 is substantially aligned with a direction of the gap magnetic field in the FGL 122 during operation.

[0059] It will be appreciated that the magnetic head 100 described herein can be implemented in a magnetic data storage system such as a hard disk drive (HDD) or other magnetic storage device which utilizes MAMR. In addition, the recording medium 102 used in MAMR is known in the art and thus need not be described in detail herein. For example, it is known that the recording medium may comprise a substrate, a soft magnetic layer disposed on the substrate, and a perpendicular magnetic recording layer for recording information (based on perpendicular magnetization direction of a target recording area of the recording layer) disposed on the soft magnetic layer.

[0060] In summary, embodiments of the present invention provide a magnetic head for microwave assisted magnetic recording which advantageously reduces the driving/critical current required to achieve stable precession of magnetization in the FGL 122, while being able to generate large in-plane AC magnetic field with high frequency. Embodiments of the present invention also allow errors in configuring the FGL 122 for minimizing/eliminating the y-component of the gap field in the FGL 122 to be self- compensated thus allowing the critical current to be effectively reduced. Further embodiments of the present invention enable the efficiency of the STO 120 to be further increased and critical current to be further reduced such as by incorporating additional spin polarization layers into the STO 120.

[0061] While embodiments of the invention have been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The scope of the invention is thus indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced.

Claims

CLAIMS is claimed is:

A magnetic head for microwave assisted magnetic recording comprising:

The magnetic head according to claim 1, wherein the predetermined

crystallographic axis of the field generation layer is a c-axis of the field generation layer.

The magnetic head according to claim 1 or 2, wherein the spin torque oscillator is disposed on a surface of the magnetic head, said surface being configured to be substantially normal to said direction of the gap magnetic field such that said predetermined principle axis of the field generation layer is substantially aligned with said direction of the gap magnetic field. 4. The magnetic head according to claim 3, wherein said surface of the magnetic head is a surface of the recording main pole or a surface of the shield. The magnetic head according to claim 4, wherein said surface is inclined at an angle in the range of about 15° to 40° with respect to an axis across the gap.

The magnetic head according to claim 1 or 2, wherein the field generation layer comprises an array of oblique structures, and the spin torque oscillator is disposed in the gap between the recording main pole and the shield such that the oblique structures extend in a direction substantially aligned with said direction of the gap magnetic field.

The magnetic head according to claim 6, wherein the oblique structures extend at an angle in the range of about 15° to 40° with respect to a vertical axis of the array.

The magnetic head according to claim 1 or 2, wherein the field generation layer comprises structures generally having a predetermined crystal plane orientation, and the spin torque oscillator is disposed in the gap between the recording main pole and the shield such that said predetermined crystal plane orientation is substantially aligned with said direction of the gap magnetic field.

9. The magnetic head according to claim 8, wherein said predetermined crystal plane orientation is the (101) crystal plane orientation.

The magnetic head according to any one of claims 1 to 9, wherein the spin torque oscillator further comprises a reference layer and an intermediate layer disposed between the field generation layer and the reference layer, and wherein the reference layer comprises a soft magnetic material or a negative anisotropy magnetic material configured to generate a demagnetization field for reducing a predetermined component of the gap magnetic field in the field generation layer.

11. The magnetic head according to claim 10, wherein said predetermined component of the gap magnetic field is parallel to a longitudinal axis of the recording main pole.

12. The magnetic head according to claim 10, wherein the reference layer comprises an in-plane layer and a perpendicular layer, the in-plane layer comprises the soft magnetic material or the negative anisotropy magnetic material, and the perpendicular layer comprises a perpendicular magnetic anisotropy material.

13. The magnetic head according to any one of claims 10 to 12, wherein the soft magnetic material or the negative anisotropy magnetic material is Co, Coir, CoFe, Fe, NiFe, NiFeCo, NiFeMo, CoFeCr, CoCrPt, Colr-X, or a combination thereof, where X is selected from the group consisting of Cr, Ru, Fe, Ni, Pt, Mn, Ti and Pd.

14. The magnetic head according to any one of claims 10 to 13, wherein the spin torque oscillator further comprises at least one polarization layer disposed between the field generation layer and the reference layer for enhancing spin polarization ratio and reducing damping constant of the spin torque oscillator.

15. The magnetic head according to claim 14, wherein the at least one polarization layer is made of Co, Fe, CoFe, Co/Cu/Co trilayers, Fe, NiCoFe, CoFeB, CoFeCr, Ni, NiFeMo, or a combination thereof.

16. The magnetic head according to claim 10, wherein the reference layer is a switchable perpendicular reference layer.

17. The magnetic head according to claim 10, wherein the reference layer is a pinned reference layer.

18. The magnetic head according to any one of claims 1 to 17, wherein the field generation layer comprises a negative anisotropy magnetic material.

19. The magnetic head according to claim 18, wherein the negative anisotropy magnetic material is selected from the group consisting of Co, Coir, CoFe, Fe, NiFe, CoCrPt, NiFeCo, NiFeMo, CoFeCr, and Colr-X, where X is selected from the group consisting of Cr, Ru, Fe, Ni, Pt, Mn, Ti, and Pd.

20. A method of fabricating a magnetic head for microwave assisted magnetic

recording, the method comprising: