WO2008020817A1

WO2008020817A1 - Read head and magnetic device comprising the same

Info

Publication number: WO2008020817A1
Application number: PCT/SG2007/000259
Authority: WO
Inventors: Yuankai Zheng; Guchang Han; Kebin Li; Jinjun Qiu; Ping Luo
Original assignee: Agency For Science, Technology And Research
Priority date: 2006-08-17
Filing date: 2007-08-16
Publication date: 2008-02-21

Abstract

A read head is provided, which includes a first layer structure having a pinned ferromagnetic layer, a first spacer layer and a second layer structure including a synthetic anti-ferromagnetic (SAF) multi-layer structure. The SAF multi-layer structure includes a first ferromagnetic free layer, a second ferromagnetic free layer, and a second spacer layer arranged in between the two ferromagnetic layers. A first magnetization of the first ferromagnetic free layer and a second magnetization of the second ferromagnetic layer are perpendicular to a fixed magnetization of the pinned ferromagnetic layer.

Description

READ HEAD AND MAGNETIC DEVICE COMPRISING THE SAME

Field of the Invention

Embodiments of the present invention relate to a magnetic device, in particular a read head, for reading magnetically-recorded data.

Background

A magnetic disk drive generally includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating magnetic disk and an actuator arm that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating magnetic disk.

A high performance read head employs a giant magneto-resistance (GMR) sensor for sensing the magnetic signal fields from the rotating disk. The GMR effect was first described by Baibich et al in Phys. Rev. Lett. 61, 2472(1988). A spin valve structure as described by B. Dieny et al in Phys. Rev. B43, 1297(1991) has been widely used in the hard disk. A standard spin valve employing GMR effect comprises two ferromagnetic layers separated by a non-ferromagnetic spacer, such as Cu. The magnetization of one ferromagnetic layer is fixed by adjacent anti-ferromagnetic layer or permanent magnetic layer, thereby preventing rotation in the presence of an external magnetic field. The magnetization of the other ferromagnetic layer is not fixed and can freely rotate in the presence of an external field.

The read head in this context is also referred to as magnetic transducer, read sensor, magnetic sensor or MR (magnetoresistive) sensor. Currently there are two modes in the MR sensor. One is the current-in-plane (CIP) mode, the other is the current-perpendicular-to-plane (CPP).

The CIP mode is used in the hard disk drive due to its low noise and relatively high signal. Generally insulator layers are required to separate the shield layers from the sensor, which would limit the gap length in the CIP head. In addition, as signal is proportional to the track width of the sensor in CIP mode, the CIP mode is not suitable to be used in ultra-high density hard disk.

On the other hand, signal is inversely proportional to the track width in CPP mode. Therefore, the CPP sensor is promising for ultra-high density recording. US patent 5668688 describes a CPP spin valve type magneto-resistance transducer. US Patents 5729410, 5898547 and 5901018 also disclose conventional magnetic head structure using the magnetic-tunneling-junction (MTJ) elements. It is reported by Dexin Wang et al. that the MTJ element has a large resistance change ratio ΔR/R of 70% and above in IEEE Trans. MAG. 40, 2269 (2004).

As the area density increases further, reduction in both of the track width and the gap length is required. The gap can be reduced in the CPP sensor due to the removal of the insulator layers between the shield layers. In order to suppress the magnetic noise, a bias field is required to stable the free ferromagnetic layer of the read head. The bias field is inversely proportional to the track width. As the track width decreases, the bias field increases. Subsequently, the sensitivity of the CPP sensor is reduced. Furthermore, the additional biasing layer may cause the increment of the thickness of the CPP sensor, thereby increasing the gap.

There is a need to provide high sensitivity read head with smaller track width and narrower gap length. Summarv of the Invention

An embodiment of the invention provides a read head, which includes a first layer structure having a pinned ferromagnetic layer, a first spacer layer and a second layer structure including a synthetic anti-ferromagnetic (SAF) multi-layer structure. The SAF multi-layer structure includes a first ferromagnetic free layer, a second ferromagnetic free layer, and a second spacer layer arranged in between the two ferromagnetic layers. A first magnetization of the first ferromagnetic free layer and a second magnetization of the second ferromagnetic layer are perpendicular to a fixed magnetization of the pinned ferromagnetic layer.

Another embodiment of the invention provides a magnetic device includes a read head according to the embodiment of the invention.

Brief Description of the Drawings

Fig. 1 A shows a 3-D view of a CIP read head with top and bottom shields.

Fig. 1 B shows an ABS view of a CIP read head with top and bottom shields. Fig. 2A shows an ABS view of a CPP read head with abutted hard-biased scheme.

Fig. 2B shows a view of a read head with in-stack exchange-biased scheme.

Fig. 3A shows a view of a read head with long distance in-stack biased scheme and side-shielding layer. Fig. 3B shows the magnetization states of the free layer and reference layer under different external field. Fig. 3C shows a transfer curve of the read head of Fig. 3A.

Fig. 4 shows the layer structure of a read head according to one embodiment of the present invention. Fig. 5A shows magnetization states of a read head under different external field according to one embodiment of the present invention. Fig. 5B shows a - A -

transfer curve of a read head according to one embodiment of the present invention.

Figs. 6A, 6B and 6C show read heads according to the embodiments of the present invention. Fig. 7A shows a transfer curve according to one embodiment of the present invention. Fig. 7B shows a transfer curve according to one embodiment of the present invention.

Detailed Description

Fig. 1A shows a 3D view of a conventional CIP read head 100 with top shield 101 and bottom shield 102. The read head 100 comprises the shields 101 ,102, sensor 104 and electrodes 103. There is a gap 105 between the sensor 104 and the respective shields 101 , 102 to insulate the sensor 104 from the respective shields 101 , 102. An air-bearing surface (ABS) of the CIP read head 100 is indicated by 106.

The detailed ABS view of the CIP read head 100 is shown in Fig.1B. The sensor 104 comprises a reference layer 112, a spacer layer 113 and a free layer 114. The magnetization of the free layer 114 is biased by the bias layers 115,116. The electrodes 117,118 are in contact with the sensor 104. A current 124 passes through the sensor 104 to detect the resistance of the sensor 104. The magnetization 123 of the free layer 114 has the same direction as the magnetization 122 of the bias layers 115,116. The magnetization 121 of the reference layer 112 is perpendicular to the magnetization 123 of the free layer 114. The bottom shield 110 is isolated from the sensor 104 by an insulator layer 111. The top shield 120 is also isolated from the sensor 104 by another insulator layer 119. The total gap length includes the thickness of the insulator layers 111 and 119. The thick insulator layers 111 ,119 make the gap length of the CIP read head 100 unsuitable for high density recording. Fig. 2A shows the ABS view of a conventional CPP read head 200 with abutted hard bias scheme. The CPP read head 200 comprises a bottom electrode 210, a top electrode 214, a reference layer 211 , a spacer layer 212 and a free layer 213. The top electrode 214 and the bottom electrode 210 also serve as the top and bottom shield layers. Bias layers 215 and 216 are isolated from the sensor 204 by the insulator layers 217 and 218, respectively. The magnetization

219 of the reference layer 211 is fixed to be perpendicular to the ABS of the read head 200 in the absence of external field. As seen, the magnetization 219 of the reference layer 211 is also perpendicular to the magnetization 221 of the free layer 213. The stray field generated from the hard bias layers 215, 216 bias the magnetization 221 of the free layer 213 to be parallel to the fixed magnetization

220 of the bias layers 215, 216. The current 222 passes through the sensor 204 in a direction perpendicular to the layer plane. There is no insulator layer between the top electrode 214 and the bottom electrode 210 (also top and bottom shield layers) and the sensor 204, so a much smaller gap length can be achieved when compared to the CIP read head of Fig. 1B. However, as the track density increases, the side-reading effect plays a significant role during the read process, which would affect the read performance of the CPP read head. Since the bias layers 215, 216 are located at the side of the sensor 204, no side-shield can be applied to suppress the side-reading effect in this scheme.

Fig. 2B shows a conventional CPP read head 250 with an in-stack bias scheme. The read head 250 comprises a reference layer 261 with a fixed magnetization 266, a spacer layer 262, a free layer 263 with a magnetization 267, another spacer layer 264 and a bias layer 265 with a fixed magnetization 268. The ferromagnetic coupling field between the free layer 263 and the bias layer 265 is much larger than the stray field of the bias layer 265, so the magnetization 267 of the free layer 263 is parallel to the magnetization 268 of the bias layer 265. The interlayer coupling field can be adjusted by changing the thickness of the spacer layer 264. In the extreme case, the spacer layer 264 can be removed. The magnetization 267 of the free layer 263 is perpendicular to the magnetization 266 of the reference layer 261. The side-shield can be used in this scheme, but the thick bias layer 265 is required to maintain the stability of the bias layer 265. The thick bias layer 265 makes it difficult to reduce the gap length of the read head. Another issue in the stray field bias scheme is low sensitivity for a small track width sensor. The bias field increases as the track width reduces, so the sensitivity reduces as the track width decreases.

A conventional CPP read head with long-distance bias scheme as shown in Fig. 3A uses side-shield to suppress the side-reading effect. US pat. 6680829, US pat applications 20020030947, 20030189802, 20030174446, 20040047087 also disclosed side shield structures to improve the side reading effect.

In Fig. 3A, the read head 300 comprises a bottom electrode 310, a reference layer 311 , a spacer layer 312, a free layer 313, another spacer layer

314, a bias layer 319 and a top electrode 320. The multi-layer structure including the reference layer 311 , the spacer layer 312, the free layer 313, another spacer layer 314 and the bias layer 319 is also referred to as sensor 304. The side-shield

315, 316 are located at both sides of the sensor 304. And the side-shield 315, 316 are isolated from the sensor 304 and the bottom electrode 310 by the respective insulator layers 317, 318. In this case, the bias layer 319 is stacked above the free layer 313, with a weak interlayer coupling inbetween. The stray field from the bias layer 319 can stabilize the magnetization 322 of the free layer 313 to be anti-parallel (i.e. in a parallel but opposite direction) to the magnetization 321 of the bias layer 319. The magnetization 323 of the reference layer 311 is fixed and perpendicular to the magnetization 322 of the free layer 313. The current 324 passes through the sensor 304 in a direction perpendicular to the plane of its multi-layer structure. Though the side-shield layer can be used in this structure, thick hard bias layer 319 is required to maintain the stability of the bias layer 319, which would increase gap length of the CPP read head 300. Fig. 3B shows the magnetization state of the long-distance in-stack CPP read head of Fig. 3A, and Fig. 3C shows the transfer curve of the long-distance in- stack CPP read head of Fig. 3A.

In Fig. 3B, when there is no external field, the total field 333 is perpendicular to the magnetization 331 of the reference layer 311. The magnetization 332 of the free layer 313 is aligned in parallel to the total field 333, and perpendicular to the magnetization 331 of the reference layer 311. The resistance 335 of the sensor in this state corresponds to the middle point B in Fig. 3C.

When an upward external field 344 is applied as shown in Fig. 3B, the total field 343 will rotate the magnetization 342 of the free layer 313 upward to the right. The magnetization 341 of the reference layer 311 remains unchanged. The resistance 345 of the sensor in this state increases which corresponds to point C in Fig. 3C.

When a downward external field 354 is applied, the total field 353 will rotate the magnetization 352 of the free layer 313 downward to the right. The magnetization 351 of the reference layer 311 remains unchanged. The resistance 355 of the sensor in this state decreases, which corresponds to point A in Fig. 3C.

The external magnetic field from the recorded magnetic medium thus causes a change in the resistance of the sensor, and the data as recorded can be determined by determining the change of the resistance of the sensor.

In all the layer structures of Figs. 1-3, the magnetization of the bias layer is perpendicular to the magnetization of the reference layer. Therefore, the magnetization processes need to be performed twice to set these two magnetizations. Z. Lu et al. described a method to set up reference layer and free layer's magnetization in "Doubly exchange-biased FeMn/NiFe/Cu/NiFe/CrMnPt spin valves," IEEE Trans. Magn. 36, 2899(2000). Paper "Spin valves with spin- engineered domain-biasing scheme," Appl. Phys. Lett., 82, 4107(2003) to Z.Q.Lu et al and US. Patent application No: 2004/0095690A1 described a one step method to initialize the free layer and the reference layer by means of the spin-flop switching.

Embodiments of the present invention provide a read head with high sensitivity when track width and gap length scale down, which is suitable for ultrahigh density recording.

Fig. 4 shows the 3-D view of a read head according to an embodiment of the invention. The read head 400 includes a first layer structure 401 having a pinned ferromagnetic layer, a first spacer layer 402, and a second layer structure 410. The second layer structure 410 may include a synthetic anti-ferromagnetic (SAF) multi-layer structure, which comprises a first ferromagnetic (FM) free layer 403, a second spacer layer 404 and a second ferromagnetic (FM) free layer 405. The pinned ferromagnetic layer 401 has a fixed magnetization 406. The first ferromagnetic free layer 403 of the second layer structure 410 has a first magnetization 407, and the second ferromagnetic free layer 405 has a second magnetization 408, wherein the first magnetization 407 and the second magnetization 408 are perpendicular to the fixed magnetization 406 of the pinned ferromagnetic layer 401.

In one embodiment, the pinned ferromagnetic layer 401 serves as a reference layer of the read head 400, as it has a fixed magnetization 406. In another embodiment, said reference layer 401 may be a bias layer which, for example, generates a stray field 409 to align the first magnetization 407 and the second magnetization 408 of the second layer structure 410 to be perpendicular to the stray field 409 and the fixed magnetization 406 of the pinned ferromagnetic layer 401. In one example, if the first FM layer 403 and the second FM layer 405 are of the same ferromagnetic material, the spin-flop field is represented as Hsf =(H_s*H_k)^1/2, wherein Hs and Hk is the saturation field and the anisotropic field of the SAF layer, respectively. When the stray field 409 is larger than the spin flop field Hs_f, the stray field 409 can bias the first magnetization 407 and the second magnetization 408 of the SAF structure 410 to be perpendicular to the stray field 409 and the fixed magnetization 406. In one example, the stray field 409 is in a range from 20 Oe to 500 Oe. In other examples, the stray field 409 may be in other ranges in consideration of the spin flop field of the SAF multi-layer structure.

In accordance with this embodiment, as the reference layer 401 acts as the bias layer, gap length of the read head 400 can be reduced. In addition, magnetization process is not required to set the bias field of the bias layer. The bias field is not dependent on the track width of the read head, but on the interlayer coupling stiffness and the anisotropy energy of the ferromagnetic layer. Therefore, the track width of the read head can be scaled down to achieve a high sensitivity.

According to one embodiment, the fixed magnetization 406 of the pinned ferromagnetic layer 401 is fixed in a direction parallel to an air bearing surface 420 of the read head 400, as shown in Fig. 4. As the pinned ferromagnetic layer 401 serves as both reference layer and bias layer in an embodiment, a bias field 406 as generated by the bias layer 401 is parallel to the air bearing surface 420 of the read head 400.

In another embodiment, the first ferromagnetic free layer 403 is anti- ferromagnetically coupled with the second ferromagnetic free layer 405 through the second spacer layer 404. Thus, the first magnetization 407 is anti-parallel to the second magnetization 408. In an embodiment, the total moment of the SAF structure 410 is zero or substantially close to zero, such that the SAF structure 410 is balanced. According to another embodiment, the synthetic-antiferromagnetic multilayer structure 410 may include more than two FM layers, which is anti- ferromagnetically coupled to the adjacent FM layer through the spacer layer. In a preferred embodiment, the total moment of this SAF multi-layer structure is zero or substantially close to zero.

In a further embodiment, the read head 400 may include a top electrode arranged on the second layer structure 410 and a bottom electrode arranged under the first layer structure 401, which are not shown in Fig. 4. Current may flow from the top electrode to the bottom electrode through the layer structures of the read head 400 in the read operation. In another embodiment, the top electrode and the bottom electrode may act as a top shield layer and a bottom shield layer, respectively.

Fig. 5A shows the magnetization state of the read head 400 in Fig. 4, and Fig. 5B shows the transfer curve of the read head 400.

In Fig. 5A, in the absence of external field, the stray field 409 aligns the first magnetization 523 of the first FM layer 403 and the second magnetization

524 of the second FM layer 405 to be perpendicular to the fixed magnetization

522 of the reference layer 401. The second magnetization 524 of the second FM layer 405 is anti-parallel to the magnetization 523 of the first FM layer 403. The total field 525 is also anti-parallel to the fixed magnetization 522 of the reference layer 401. In this state, the resistance 526 corresponds to the middle point B in

Fig. 5B.

When a downward external field 511 is applied, the total field 515 turns downwards to the left. Due to the spin-flop effect of the SAF layer structure 410, the first magnetization 513 of the first FM layer 403 and the second magnetization 514 of the second FM layer 405 are aligned in an angle of near 90 degree relative to the total field 515. The first magnetization 513 of the first FM layer 403 rotates downwards to the right and the second magnetization 514 of the second FM layer 405 rotates in an anti-parallel direction to the first magnetization 513 of the first FM layer 403. The fixed magnetization 512 of the reference layer 401 remains unchanged. As the magnetization of the SAF layer structure 410 rotates to a direction parallel to the fixed magnetization 512, the resistance of the read head 400 decreases, which corresponds to the resistance 516 of point A in Fig. 5B.

When an upward external field 531 is applied, the total field 535 turns upwards to the left. Now the magnetization 533 of the first FM layer 403 rotates downwards to the left and the magnetization 534 of the second FM layer 405 rotates upwards to the right, in an anti-parallel direction to the magnetization 533 of the first FM layer 403. As the magnetization of the SAF layer structure 410 rotates to a direction anti-parallel to the fixed magnetization 532, the resistance of the read head 400 increases, which corresponds to the resistance 536 of point C in Fig. 5B.

Figs. 6A to 6C shows a read head according to other embodiments of the invention.

The read head 600 according to one embodiment of the invention as shown in Fig. 6A is similar to the read head 400 of Fig. 4. As seen, the read head 600 includes a first layer structure 601 having a pinned ferromagnetic layer 602, a first spacer layer 610, and a second layer structure 611. The second layer structure 611 may include a synthetic anti-ferromagnetic multi-layer structure, which includes a first ferromagnetic free layer 612, a second ferromagnetic free layer 616 and a second spacer layer 614 inbetween. The first magnetization 613 of the first ferromagnetic free layer 612 and the second magnetization 617 of the second ferromagnetic free layer 616 are perpendicular to a fixed magnetization 603 of the pinned ferromagnetic layer 602.

In this embodiment, the first layer structure 601 further comprises an anti- ferromagnetic layer 604 pinning the ferromagnetic layer 602. The anti- ferromagnetic layer 604 may comprise at least one conductive material selected from the group consisting of IrMn, FeMn, NiMn, PtMn or alloys thereof. The anti- ferromagnetic layer 604 may comprise other anti-ferromagnetic materials in other embodiments. By the anti-ferromagnetic layer 604, the magnetization 603 of the pinned ferromagnetic layer 602 is fixed. In an embodiment, the magnetization 603 is fixed in a direction parallel to an air bearing surface 620 of the read head. In another embodiment, the pinned ferromagnetic layer 602 acts as a bias layer generating a stray field 605, which is in the plane of the air bearing surface 620 of the read head. The stray field 605 may align the first magnetization 613 and the second magnetization 617 of the second layer structure 611 to be perpendicular to the air bearing surface of the read head, as shown in Fig. 6A.

Fig. 6B shows a read head 630 according to another embodiment of the invention. The read head 630 includes a first layer structure 631 having a pinned ferromagnetic layer, a first spacer layer 640, and a second layer structure 641. The second layer structure 641 may include a synthetic anti-ferromagnetic multilayer structure, which includes a first ferromagnetic free layer 642, a second ferromagnetic free layer 646 and a second spacer layer 644 inbetween. The first magnetization 643 of the first ferromagnetic free layer 642 and the second magnetization 647 of the second ferromagnetic free layer 646 are perpendicular to a magnetization 637 of the first layer structure 631.

In this embodiment, the pinned ferromagnetic layer of the first layer structure 631 includes an anti-ferromagnetic multi-layer structure. For example, the pinned ferromagnetic layer may comprise a third ferromagnetic layer 632 having a third magnetization 633, a fourth ferromagnetic layer 636 having a fourth magnetization 637 and a third spacer layer 634 arranged inbetween. In one embodiment, the third ferromagnetic layer 632 and the fourth ferromagnetic layer 636 are anti-ferromagnetically coupled through the third spacer layer 634.

According to another embodiment, the first layer structure 631 may further include an anti-ferromagnetic layer 638 pinning the magnetization of the pinned ferromagnetic layer. The anti-ferromagnetic layer 638 may comprise at least one conductive material selected from the group consisting of IrMn, FeMn, NiMn, PtMn or alloys thereof.

In one embodiment, the magnetization 633 and 637 is fixed in a direction parallel to an air bearing surface 650 of the read head. In another embodiment, the pinned ferromagnetic multi-layer (632, 634, 636) acts as a bias layer generating a stray field 639, which is in the plane of the air bearing surface 650 of the read head. The stray field 639 may align the first magnetization 643 and the second magnetization 647 of the second layer structure 641 to be perpendicular to the bias field and the air bearing surface of the read head, as shown in Fig. 6B.

Fig. 6C shows a read head 660 according to another embodiment of the invention. The read head 660 is similar to the read head 630 of Fig. 6B, which includes a first layer structure 661 having a pinned ferromagnetic layer, a first spacer layer 670, and a second layer structure 671. The second layer structure 671 may include a first ferromagnetic free layer 672, a second ferromagnetic free layer 676 and a second spacer layer 674 inbetween. The pinned ferromagnetic layer of the first layer structure 661 includes an anti-ferromagnetic multi-layer structure, comprising a third ferromagnetic layer 662 having a third magnetization 663, a fourth ferromagnetic layer 666 having a fourth magnetization 667 and a third spacer layer 664 arranged inbetween. In one embodiment, the third ferromagnetic layer 662 and the fourth ferromagnetic layer 666 are anti- ferromagnetically coupled through the third spacer layer 634, such that the third magnetization 663 and the fourth magnetization 667 are anti-parallel to each other.

According to one embodiment, the first layer structure 661 may further include an anti-ferromagnetic layer 668 pinning the magnetization of the pinned ferromagnetic multi-layer. In another embodiment, the first layer structure further comprises a bias layer 680 having a bias field 681. The bias layer 680 and the ferromagnetic layers 662, 666 may comprise a ferromagnetic material, such as Co, Fe, Ni or alloys thereof. The third magnetization 663, the fourth magnetization 667 and the bias field 681 are fixed in a direction parallel to an air bearing surface 690 of the read head, for example, through the anti-ferromagnetic layer 668. The bias layer 680 may also comprise a synthetic anti-ferromagnetic multi-layer structure in an embodiment. In another embodiment, the bias layer 680 and the pinned layer (662,664, 666) may be a SAM multi-layer with non-zero moment.

The bias layer 680 generates a stray field 683. If the stray field 683 is larger than the spin-flop field of the SAF multi-layer 671, it may align the first magnetization 673 and the second magnetization 677 of the second layer structure 671 to be perpendicular to the bias field 681 and the air bearing surface

690 of the read head, as shown in Fig. 6C.

In the above embodiments of the invention, the pinned ferromagnetic layer 401, 602, 632, 636, 662, 666 of the first layer structure may comprise a ferromagnetic materials, such as Co, Fe, Ni or alloys thereof. The first spacer layer 402, 610, 640, 670 may be a conductive layer such as Cu, Au, Ag, Al, Ru, Ta in one example when the read head is a spin-valve read head, or may be an insulator layer such as AIOx, MgO, TaO, AINx, TiO in another example when the read head is a tunneling-magnetoresistance (TMR) read head. The first ferromagnetic free layers 403, 612, 642, 672 and the second ferromagnetic free layers 405, 616, 646, 676 may comprise soft-ferromagnetic materials, such as Co, Fe, Ni, CoFe, NiFe, CoFeNi, CoFeB and alloys thereof. The first ferromagnetic free layer and the second ferromagnetic free layer comprise the same soft- ferromagnetic materials in one embodiment, and comprise different soft- ferromagnetic materials in another embodiment. The second spacer layers 404, 614, 644, 674 and the third spacer layers 634, 664 of the SAF multi-layer structure may comprise at least one conductive materials selected from the group consisting of Ru, Ta, Cu, Al, Ag, Au, etc.

Another embodiment of the invention relates to a magnetic device comprising the read head 400, 600, 630, 660 as illustrated in the above embodiments. The magnetic device may further comprise a write head such that the magnetic device may be used for both read and write operations. In one embodiment, the magnetic device is a hard disk drive. The magnetic device may be other kind of drives in other embodiments.

Fig. 7A shows a transfer curve of a read head according to one embodiment of the invention. From the simulation result of Fig. 7A, linear signal (magnetoresistive ratio) can be achieved for the read head.

Fig. 7B shows a transfer curve of a read head according to the embodiment of the invention, wherein linear signal (resistance) is achieved.

From the simulation results of Figs. 7A and 7B, the read head according to the embodiments of the invention achieves a higher sensitivity.

Whilst the present invention has been described with reference to preferred embodiments it should be appreciated that modifications and improvements may be made to the invention without departing from the spirit and scope of the invention as defined in the following claims.

Claims

1. A read head comprising: a first layer structure comprising a pinned ferromagnetic layer, said pinned ferromagnetic layer having a fixed magnetization; a first spacer layer arranged on the first layer structure; and a second layer structure arranged on the first spacer layer, said second layer structure comprising a synthetic anti-ferromagnetic multi-layer structure; wherein the synthetic anti-ferromagnetic multi-layer structure comprises a first ferromagnetic free layer having a first magnetization, a second ferromagnetic free layer having a second magnetization, and a second spacer layer arranged in between the first ferromagnetic free layer and the second ferromagnetic free layer; wherein the first magnetization and the second magnetization are perpendicular to the fixed magnetization of the pinned ferromagnetic layer.

2. The read head of claim 1 , further comprising a top electrode arranged on the second layer structure and a bottom electrode arranged under the first layer structure.

3. The read head of claim 2, wherein the top electrode acts as a top shield layer and the bottom electrode acts as a bottom shield layer.

4. The read head of claim 1 , wherein the first layer structure further comprises an antiferromagnetic layer pinning the pinned ferromagnetic layer.

5. The read head of claim 4, wherein the anti-ferromagnetic layer comprises at least one conductive material selected from the group consisting of IrMn, FeMn, NiMn, PtMn or alloys thereof.

6. The read head of claim 1 , wherein the pinned ferromagnetic layer is an anti-ferromagnetic multi-layer structure.

7. The read head of claim 6, wherein the pinned ferromagnetic layer comprises a third ferromagnetic layer; a fourth ferromagnetic layer; and a third spacer layer arranged in between the third ferromagnetic layer and the fourth ferromagnetic layer.

8. The read head of claim 7, wherein the third ferromagnetic layer is anti- ferromagnetically coupled with the fourth ferromagnetic layer through the third spacer layer.

9. The read head of any one of claims 1 to 8, wherein the pinned ferromagnetic layer serves as a reference layer.

10. The read head of claim 9, wherein the reference layer is a bias layer.

11.The read head of any one of claims 1 to 9, wherein the first layer structure further comprises a bias layer.

12. The read head of any one of claims 1 to 11 , wherein magnetization of the pinned ferromagnetic layer is fixed in a direction parallel to an air bearing surface of the read head.

13. The read head of any one of claims 10 to 12, wherein the bias layer generates a stray field and a bias field.

14. The read head of claim 13, wherein the bias field is parallel to an air bearing surface of the read head.

15. The read head of claim 13 or 14, wherein the stray field of the bias layer aligns the first magnetization of the first ferromagnetic free layer and the second magnetization of the second ferromagnetic free layer in a direction perpendicular to the bias field.

16. The read head of claim 1 , wherein the first ferromagnetic free layer is anti- ferromagnetically coupled with the second ferromagnetic free layer through the second spacer layer.

17. The read head of claim 1 , wherein total moment of the second layer structure is zero or substantially close to zero.

18. The read head of claim 1 , wherein the first spacer layer comprises at least one conductive material selected from the group consisting of Cu, Ru, Ag, Al, Au and Ta.

19. The read head of claim 1 , wherein the first spacer layer comprises at least one insulating material selected from the group consisting of AIOx, MgO, AINx, TiO and TaO.

20. The read head of claim 1 , wherein the first ferromagnetic free layer comprises at least one material selected from the group consisting of Co, Fe, Ni, CoFe, NiFe, CoFeNi, CoFeB or alloys thereof.

21. The read head of claim 1 , wherein the second ferromagnetic free layer comprises at least one material selected from the group consisting of Co,

Fe, Ni, CoFe, NiFe, CoFeNi, CoFeB or alloys thereof.

22. The read head of claim 1 , wherein the second spacer layer comprises at least one conductive material selected from the group consisting of Cu, Ru, Ag, Al, Au and Ta.

23. A magnetic device comprising the read head according to any one of claims 1 to 22.

24. The magnetic device of claim 23, further comprising a write head.

25. The magnetic device of claim 23, wherein said magnetic device is a hard disk drive.