WO2013090937A1

WO2013090937A1 - E-field writable non-volatile magnetic random access memory based on multiferroics

Info

Publication number: WO2013090937A1
Application number: PCT/US2012/070198
Authority: WO
Inventors: Nian-Xiang Sun; Ming Liu
Original assignee: Northeastern University
Priority date: 2011-12-16
Filing date: 2012-12-17
Publication date: 2013-06-20

Abstract

A multiferroic stack consists of a ferromagnetic film elasticaily coupled to a ferroelectric layer actuated with a perpendicular electric field to define the resuliing electrical resistance of the stack. Alternatively, a multiferroic stack consists of a magnetic spin valve elasticaily coupled to a ferroelectric layer actuated with a perpendicular electric field to define the resulting electrical resistance of the stack. The scaling and incorporation of these stacks as writable magnetic bits into non-volatile, ullxa-low power, high-density magnetic memory storage devices is contemplated.

Description

E-FIELD WRITABLE NON- VOLATILE MAGNETIC RANDOM ACCESS

MEMORY BASED ON MULTIFERROICS

CROSS-REFERENCE TO RELATED APPLICATIONS

[ΘΘ01] This application claims the benefit of the earlier filing date of U.S. Patent Application No. 61/576,439, filed on December 16, 201 1 , the contents of which are incorporated by reference herein in their entireties.

INCORPORATION BY REFERENCE

[0002] Ail patents, patent applications and publications cited herein are hereby incorporated by reference in their entirety in order to more fully describe the state of the art as known to those skilled therein as of the date of the invention described herein.

TECHNIC AL FIELD

{0003^'J This technology relates to materials and devices based on multiferroics for use in voltage-modulated non-volatile magnetic memory storage.

BACKGROUND

[0(504] A memory device based on the magnetoresistive effect, known as the magnetic random access memory MRAM, has attracted ever-growing attention during the past decade. Due to its superiorities in terms of access time and endurance, MRAM has become one of the most promising candidates for the next-generation non-volatile memory devices. However, a number of obstacles need to be overcome before its foil commercialization, and among them, the high writing energy is one of the most important. OOOSj In modem electronics and spintronics, the magnetoresistance effect, such as anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR), and tunneling magnetoresistance (TMR) are widely used in information storage and sensors,

Magnetoresistance is the property of a material to change the value of its electrical resistance when an external magnetic field is applied to it, AMR is a material property characterized by a dependence of electrical resistance on the angle between the direction of electric current and direction of magnetization. GMR and TMR are phenomena characterized by as a significant change in the electrical resistance depending on whether the magnetization of

1

ActiveUS i04090996v.l adjacent ferromagnetic layers, spaced apart and de-coupled by either a conductive (GMR) or insulating (TM ) non-magnetic electron transport layer, are in a parallel or an antiparallel alignment. Typically in such devices, a bulky and energy-consuming electromagnet is utilized to control the orientation of magnetization and modulate the magnetoresistance. This limits the development of smaller and ultralow power electronic devices.

[0006] For example, in most of today's MRAM designs, the memory element is a magnetic tunnel junction MTJ that may consist of an insulating tunneling barrier layer sandwiched by two magnetic electrodes. The junction resistance strongly depends on the relative orientation of the magnetic moments (as in GMR and TMR), which is utilized to determine the memory state "0" or "1," in the two magnetic electrodes. The coded magnetic bits can then be read out nondestructively by detecting such resistance changes. However, in the writing process, the magnetic bits are usually encoded by changing the magnetization orientations by high external current-generated ampere fields (magnetic -write), which is relatively slow and power-consuming. This type of writing process would also cause severe cross-talk among neighboring cells when miniaturizing the device for higher storage capacity.

SUMMARY

[0007] Multiferroic materials for use in voltage-modulated non-volatile magnetic memory storage devices are disclosed. A device incorporating these materials as bits can utilize a perpendicular writing voltage bias with a ultralow writing energy, allowing high storage density at room temperatures.

[00()8| In one aspect, a multiferroic stack for use in a voltage-modulated non-volatile magnetic memory storage device includes: a magnetoresistance magnetic spin valve comprising a free ferromagnetic layer and a hard ferromagnetic layer, and a ferroelectric layer capable of mechanical strain actuation in a direction parallel to the layer when actuated with an electric field perpendicular to the plane of the ferroelectric layer, wherein the layer is elastically coupled to the free ferromagnetic layer.

[0009] In one or more embodiments, the magnetic spin valve includes an electron transport layer positioned between the free and hard ferromagnetic layers and in electrical contact with the free and hard ferromagnetic layers, wherein the free ferromagnetic layer

ActiveUS i040909 6v.l comprises a first magnetic orientation and first magnetic easy axis, the hard ferromagnetic layer comprises a second magnetic orientation and second magnetic eas axis, and the easy axes of the first and second ferromagnetic layers are parallel , and wherein the second magnetic orientation is pinned in a direction parallel to the ferromagnetic layer.

[0010] In one or more embodiments, the magnetic spin valve further includes an antiferroinagnetic layer, magnetically coupled to the hard ferromagnetic layer, wherein the antiferromagnetic layer pins the second magnetic orientation in a direction parallel to the ferromagnetic layer.

[0(511] In one or more embodiments, the ferromagnetic hard layer includes a synthetic tri- layer with a pinned layer and a reference layer separated by a non-magnetic spacer layer.

[0012] In one or more embodiments, the electron transport layer provides a low resistance electrical pathway between the free and hard ferromagnetic layers.

[1)013] In one or more embodiments, the electron transport layer provides an electron tunneling pathway between the free and hard ferromagnetic layers.

[0(514] In any of the preceding embodiments, the multiferroic stack, further including conductive layers providing electrical contact of the first ferromagnetic layer and/or the antiferromagnetic layer with external electronics.

[0015] In one or more embodiments, the multiferroic stack further including a reader for measuring the resistance of the ferromagnetic free layer.

[t)016j In any of the preceding embodiments, the multiferroic stack, further including electrodes in electrical contact with the ferroelectric layer positioned to apply an electric field perpendicular to the plane of the ferroelectric layer.

[0017] In one or more embodiments, the electrodes are positioned above and below the plane of the ferroelectric layer.

[0018] In one or more embodiments, the electrodes are in electrical contact with the bit line and the plate line of a transistor.

[t)019j In another aspect, a method of using a multiferroic stack in a voltage-modulated non-volatile magnetic memory storage device, includes: providing the multiferroic stack of any of the preceding claims, applying an electric field perpendicular to the ferroelectric layer to rotate the magnetic orientation of the free ferromagnetic layer in the plane of the ferromagnetic layer, and measuring changes in electrical resistance parallel to the magnetic field.

[0020] In one or more embodiments, the method of using a multiferroic stack further including applying a magnetic field parallel to the hard ferromagnetic orientation.

j

ActiveUS 1040909 6v,l [0(521] In one or more embodiments, the method of using a multiferroic stack including inducing up to 90° or up to 180° rotation of the first magnetic orientation.

[0022] In another aspect, a method of making a multiferroic stack for use in a voltage- modulated non-volatile magnetic memory storage device, including: providing a ferroelectric layer capable of mechanical strain actuation in a direction parallel to the layer when actuated with an electric field perpendicular to the layer, providing a magneto-resistance magnetic spin valve, eiastically coupling the ferroelectic layer with the free ferromagnetic layer of the spin valve; and providing electrical contact to the pinned layer and free layer of the

magnetoresistance magnetic spin valve, said electrical contacts positioned to apply a current, [0023] In another aspect, a multiferroic stack for use in a voltage-modulated magnetic non-volatile memory storage device, including a ferromagnetic layer with a tunable magnetic orientation, wherein the ferromagnetic layer comprises a magnetic orientation and a magnetic easy-axis which is parallel to the ferromagnetic layer, a ferroelectric layer capable of mechanical strain actuation in a direction parallel to the layer when actuated with an electric field perpendicular to the layer, wherein the layer is eiastically coupled to the ferromagnetic layer.

[0024] In another aspect, a voltage-modulated non-volatile magnetic memory storage device, including: a 1 -transistor/ i-magnetoresi tive memory cell including a

magnetoresistance magnetic spin valve comprising a free ferromagnetic layer and a hard ferromagnetic layer; a transistor comprising a gate; a word line connected to the gate of the transistor: a plate line in electrical communication with the free layer; a bit line controlled by the word line; a ferroelectric layer, wherein the layer is eiastically coupled to the free ferromagnetic layer and electrically coupled to the plate line and bit line; and an electrode in electrical communication with the pinned layer; wherein the ferroelectric material is capable of producing mechanical strain actuation in a direction parallel to the ferroelectric layer when actuated with an electric field perpendicular to the plane of the ferroelectric layer.

[t>825] In one or more embodiments the voltage- modulated non- volatile magnetic memory storage device further including a reader for measuring the resistance of the spin valve, when current is passed between the bit line and the top electrode.

[0026] These and other aspects and embodiments of the disclosure are illustrated and described below.

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ActiveUS 104090996v,l BRIEF DESCRIPTION OF TFIE DRAWINGS

1_.0027] The invention is described with reference to the following figures, which are presented for the purpose of illustration only and are not intended to be limiting. In the Drawings:

[0(528] FIG, 1 shows a schematic of a voltage-modulated AMR stack according to one or more embodiments,

[t>829] FIG, 2 shows a voltage-modulated GMR or TMR stack according to one or more embodiments.

[0030] FIG. 3 shows a schematic of the architecture of a memory unit cell in a voltage modulated MRAM device array on a CMOS platform according to one or more

embodiments.

[0031] FIG. 4 shows a schematic of the E-field manipulation of AMR in a NisoCo?o/PZN- PT stack with magnetic bias fields applied along the [00- 1] direction. The magnetic easy axis of NigoC ?!) is prepared along the [00-1] direction and perpendicular to the measured current according to one or more embodiments.

[0032] FIG. 5 shows a plot of an E-field induced effective magnetic field in a

NisoCo2o PZN-PT stack determined by measurement according to one or more embodiments.

[0033] FIG. 6 shows a plot of the E-field dependence of magnetic hysteresis loops in a igoCo2o/P -PT stack, measured while an external magnetic field was applied along the [00-1] direction of PZN-PT and the E-fieid was applied through the thickness direction according to one or more embodiments.

[0034] FIG, 7 shows a plot of a.n FMR spectrum of a N goCo o<' ^'PZN-PT stack measured while an external magnetic field was applied along the [00-1 ] direction of PZN-PT and the E-field was applied through the thickness direction according to one or more embodiments.

[0035] FIG. 8 shows a plot of E-field manipulation of AMR in a Ν¼ο<¾ο/ΡΖΝ-ΡΤ stack with magnetic bias fields of 0 Oe and 50 Oe. with magnetic easy axis along [00-1] direction and perpendicular to the measured current according to one or more embodiments.

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ActiveUS 104090996v,l [0(536] FIG. 9 A shows a plot of E-field dynamical modulation of AMR in a

NisoCo2o PZN-PT stack with magnetic easy axis along [00-1] direction and perpendicular to the measured current under an external magnetic bias field of 0 Oe as a response to a square wave of E-fields according to one or more embodiments.

[ΘΘ37] FIG. 98 shows a plot of E-field dynamical modulation of AMR in a

NisoCoao/PZN-PT stack with magnetic easy axis along [00- 1 ] direction and perpendicular to the measured current under an external magnetic bias field of 50 Oe as a response to a sine wave of E-fields according to one or more embodiments.

[t>838] FIG. 10 shows a plot of AMR curves under v arious E-fields while magnetic easy axis is parallel to measured current and perpendicular to applied external magnetic field according to one or more embodiments.

1_.0039] FTG. 1 1 A and B show two prepared configurations of spin-valve structure of FeMn isoFejo/Ca Co/PZN-PT according to one or more embodiments.

FIG. 12 A and 12B show^r plots of E-fieid control of magnetic hysteresis loops for two prepared configurations of spin-valve structure of FeMn/NigoFe₂o /Cu/Co/PZ -PT according to one or more embodiments.

[ΘΘ41] FIG. 13A and B show plots of E-field dependence of GMR hysteresis loops for one configuration of spin-valve structure of FeMn NigoFeao /Cu/Co/PZN-PT according to one or more embodiments.

[0042] FTG. I4A and B show plots of E-field dependence of GMR hysteresis loops for a second configuration of spin-valve structure of FeMn/NigoFe₂o /Cu/Co PZN-PT according to one or more embodiments.

DETAILED DESCRIPTION

[$$43] MRAMs having substantially reduced writing energy are provided. Switching is accomplished by manipulating the magnetization direction using an electric field only (electric-write) rather than high external current-generated ampere fields (magnetic -write), e.g., purely electric field- addressed MRAMs. Optionally, a range of switching can be accomplished using a combination of electric and magneiic fields. Specifically, multiferroic structures including ferromagnetic and ferroelectric layers are capable of voltage controlled

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ActiveUS i040909 6v.l magnetism through magnetoelectric (ME) coupling. This results in highly efficient actuation of AMR, GMR, and TMR switching and makes low-power tunable electronic or spintronic devices achievable.

[©044] In composite multiferroics, E-field control of magnetization switching can be of purely mechanical origin, accomplished by strain or stress mediated ME coupling. An E-field induced in-plane distortion in a ferroelectric layer caused by the inverse-piezoelectric effect can lead to a strain in a ferromagnetic layer elasticaliy coupled to the ferroelectric layer. Consequently, the strain gives rise to an effective magnetic anisotropy due to the inverse magnetoelastic effect, that is, a strain-induced change in the magnetization of the material. Magnetic anisotropy is the directional dependence of a material's magnetic properties. A magnetically anisotropic material aligns its magnetic moment (or magnetization direction) with one of the easy axes, which are energetically favorable directions of spontaneous magnetization in the material.

1_.0045] By operating a ferroelectric material below its electrical coercive field, e.g., below a field strength that changes the polarization of the material, bistable in-plane piezostrains can be obtained even when the applied voltages are switched off, which, when coupled to ferromagnetic layers, leads to permanent magnetization switching, making non-volatile memory possible. For example, the two magnetization states can be defined by different signs of E-field as "1" and "()."A positive E-field could lead to contraction of the ferroelectric layer through the inverse piezoelectric effect. This would in turn induce a reduced effective magnetic anisotropy along the length direction for the elasticaliy coupled ferromagnetic layer; this magnetic state can be defined as "0," which could be maintained when the E-field is decreased to zero. Further decreasing the applied E-field would lead to tensile strain along the length of the ferroelectric layer. This would in turn induce an enhanced effective magnetic anisotropy along the length direction for the elasticaliy coupled ferromagnetic layer. Thus, the magnetization will start to increase and is stabilized at a negative E-field value (with magnitude below the coercive field of the material), which forms another magnetic state defined as "1." State "1" can remain unchanged when the E-field is increased from this negative value to zero as well. Bistable magnetization has recently been reported by the inventors. The voltage impulse induced bistable magnetization switching was achieved using a non-volatile strain state at zero voltage in the ferroeiectrics (see e.g. Tianxiang Nan , Ziyao Zhou , Jing Lou , Ming Liu , Xi Yang , Yuan Gao , Scott Rand and Man X. Sun, "Voltage

ActiveUS 1040909 6v,l Impulse Induced Bistable Magnetization Switching in Multiferroic Heterostructures", Appl. Phys, Lett, 100, 132409 (2012), incorporated herein in entirety by reference).

[t>846] One factor in using voltage manipulation of mutiferroics for magnetic memory devices is the direction of the actuating electric field. In one or more embodiments, a perpendicular writing voltage bias is employed. Perpendicular writing voltage bias makes the multiferroic materials easy to integrate with existing nanoscale CMOS platforms for computers. In contrast, in-plane actuation electric fields required interdigitized transducer patterning, which is typically of micron size. Thus, devices incorporating voltage controlled magnetization switching according to one or more embodiments, permit higher density of switches in a device.

[t>847] As shown in FIG. I, in one embodiment, voltage-modulated anisotropic magneto re istance structure 100 is illustrated. A ferroelectric layer 1 10 is elasticaliy coupled to a ferromagnetic layer 120 having an easy axis along the y direction. As used herein "elasticaliy coupled" means capable of inducing mechanical strain. In one or more embodiments, strain is transferred from one layer to the next through direct contact of adjacent layers e.g. via intimate physical contact. Transfer of strain energy through intermediate layers is also contemplated. For example, conductive layers disposed between the FE and FM layers that are capable of transferring strain between layers may be included in the stack up.

{0048^'J The ferromagnetic layer includes a magnetic easy axis, for example along the

'y' axis. Alternatively, the ferromagnetic layer may be configured to have its easy axis pointing in the x direction, and then current would also be measured along the y direction. The labeling of easy axes as "x" or "y" is arbitrary; any angle may be selected for the magnetic easy axis.

{0049} An external electric voltage is perpendicularly applied to the ferroelectric layer

1 10, generating a perpendicular electric field 140. The applied electric field actuates the ferroelectric material, causing the formation of in-plane strain. Via elastic coupling, this strain is communicated to the eiasticaHy-coupled ferromagnetic layer 120, causing rotation of the magnetic orientation 150 of the film from orientation along the easy axis to a new orientation. Current 160 is applied along the ferromagnetic layer 120 in the x-direction, to measure the changing resistance of the layer, which depends on the angle between the direction of current 160 and direction of magnetization (AMR effect), with the maximum

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ActiveUS 1040909 6v,l resistance resulting from the magnetic orientation of the film 150 aligning parallel to the current flow in the x-direction (90° switching). The measured resistance is an indication of the state of the ferromagnetic material, and can be used to "read" the device.

[0050] The ferroelectric layer 1 10 is selected to provide the appropriate strain response when subjected to an electric field. In certain embodiments, selection is subject to the material's piezoelectric coefficient d-parameters such as d31 or d33. In certain embodiments, piezoelectric materials with high d31 or d33 coefficients are preferable. When the d31 of ferroelectric materials is used, an applied voltage along the out-of-plane direction will lead to a change of spontaneous polarization and a change in strain along the in-plane direction.

[00S1] In exemplary embodiments, the ferroelectric layer is PZN-PT (lead zinc niohate- lead titanate), or PMN-PT (lead magnesium niobate-lead titanate), or PZT (lead zinc titanate). Piezoelectric materials with high d31 or d32 parameters are preferred, which enables planar actuation with perpendicular voltage. While not being bound by theory, planar actuation with perpendicular voltage is possible because net volume of a material is relatively fixed, and if it is deformed along a perpendicular direction, it will need to shrink or expand along in-plane directions.

[0052] In exemplary embodiments, the ferroelectric layer 1 10 is a single crystal.

[ΘΘ53] The ferroelectric layer 1 10 is selected to provide spontaneous polarization perpendicular to the layer and/or to be capable of mechanical strain actuation in a direction parallel to the layer when actuated with an electric field 140 perpendicular to the layer. In one or more embodiments, the FE layer is prepoled to achieve the desired spontaneous polarization. Direction of spontaneous polarization is a material specific property, e.g. the spontaneous polarization of PMN-PT is along <1 1 i> directions, while for PZT it is along <001 >.

[0(554] In exemplary embodiments, for use in MRAM devices, the thickness of the ferroelectric layer 110 is between 0.1-10μηι, preferably 1 -5 μηι. In some embodiments, the ferroelectric layer can be patterned into nano- island arrays, which each nano-island stacked as described in Figure 1 to provide a single memory cell.

[0055} The ferromagnetic layer 150 is selected to provide an appropriate rotation of magnetic orientation when subjected to mechanical strain.

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ActiveUS 104090996v.l [0(556] In exemplary embodiments the ferromagnetic layer is NiFe, CoFe, FeGaB, or CoFeSiB. For more information on FeGaB ferromagnetic layers, please refer to reference Jing Lou, Ming Liu, David Reed, Yuhang Ren, and Nian X. Sun, "Giant Electric Field Tuning of Magnetism in INovel Multiferroic FeG B/Lead Zinc iobate Lead Titanate Heterostiuctures". Advanced Materials, 21, 471 1 (2009), incorporated herein by reference.

[6057] In one or more embodiments, the thickness of the ferromagnetic layer 150 is between 10-1000 niti preferably 40-100 nm. A film thickness of - 100 nm is preferable for magnetoresistance measurements.

[0058] The ferromagnetic layer is a single crystal, polycrystalline, or amorphous.

[0059] In one or more embodiments, a voltage induce magnetization switch incorporates a magnetic spin valve in place of the ferroelectric material of FIG. 1. FIG. 2 illustrates a voltage-modulated GMR or TMR thin-film structure 200 according to one or more embodiments, in which a ferroelectric layer 210 is elastically coupled to a magnetic spin valve 220. The spin valve consists of a free layer 230 (with an easy axis, for example, in the x direction), an electron transport layer 240, and a pinning layer configuration 250, which may contain a magnetic hard layer (with an easy axis in the x-direction) 260 and an

antiferromagnetic layer 270. Overall, the memory cell design can depend on what kind of ferroelectric 210 material is used and its crystal orientation (e.g., (001) vs. (01 1)). The magnetoresistive device design (e.g., in-plane or out-of-plane magnetization rotation, etc.) along with choice of ferroelectric material will determine how the voltage is applied for actuation.

[Θ06Θ] In operation, an external electric voltage is perpendicularly applied to the ferroelectric layer, generating an electric field 285 perpendicular to plane of the thin film layer 210. The applied electric field actuates the ferroelectric material, causing the formation of in-plane strain. Via elastic coupling, this strain is imparted on the elastically-coupled ferromagnetic free layer 230, causing rotation of the magnetic orientation of the free layer 290. Current 295 is applied along the ferromagnetic layer 220 in the x-direction, to measure the changing resistance of the layer, which depends on the angle between the magnetic direction of the pinned and hard ferromagnetic layers, with the minimum resistance resulting from the magnetic orientation of the free layer 290 aligning parallel to the current flow in the x-direction. As noted above for the embodiments disclosed with FIG. l , the measured

1 0

ActiveUS 104090996v,l resistance is an indication of the state of the ferromagnetic material, and can be used to "read" the device.

[0061] Alternatively, both ferromagnetic layers may be configured to have easy axes pointing in the y direction, and then current would also be measured along the y direction. As above, the labeling of easy axes as "x" or "y" is arbitrary; any angle may be selected for the magnetic easy axes.

[0(562] In some embodiments, a magnetic field may also be applied parallel to the easy- axis of the free layer in order to assist in switching the magnetic orientation of the free layer, although configurations with a magnetic bias are preferable. Tn certain embodiments, the magnetic field is applied by hard magnetic arrays. As will be provided in more detail in the examples below, the direction of the easy axes, magnetic field, and applied current may be configured in different directions with, respect to the ferroelectric layer in order to achieve different modes of operation and different magnetic switching ranges. In one configuration, the easy axis of the free layer 230 is oriented parallel to the easy axis of the hard layer 260 (e.g. along the [001] direction of a (011 ) cut PZN-PT ferroelectric layer) . In one or more embodiments, 180° magnetization switching is possible for this configuration (FIG. 1 1 A), but an applied magnetic field is required. In another configuration, the easy axis of the free layer is oriented in a direction not parallel to the easy axis of the hard layer 260 (e.g. along the [01- 1 ] direction of a (01 1) cut PZN-PT ferroelectric layer). In one or more embodiments, continuous 90° magnetization switching is possible for this configuration (FIG. 1 IB) without me application of a magnetic field.

[0063] The ferroelectric layer 210 is selected to provide the appropriate strain response when subjected to an electric field. In certain embodiments, selection is subject to the material's piezoelectric coefficient d-parameters such as d31 or d33. In certain embodiments, piezoelectric materials with high d31 or d33 coefficients are preferable, or an in-plane anisotropic strain stress state is preferable. When the d31 of ferroelectric materials is used, an applied voltage along the out-of-plane direction will lead to a change of spontaneous polarization and a change in strain along the in-plane direction.

[0064] The ferroelectric layer 210 may be capable of bi-stable in-plane piezostrain when actuated below its coercive field by a perpendicular electric field 285. In one or more embodiments, the FE layer is prepoled to achieve the desired spontaneous polarization.

1 1

ActiveUS i040909 6v.l Piezoelectric materials with high d31 or d32 parameters are preferred, which enables planar actuation with perpendicular voltage.

006SJ in exemplary embodiments, the ferroelectric layer 210 is PZN-PT (lead zinc niobate-lead titanate), or PMN-PT (lead magnesium niobate-lead titanate), or PZT (lead zinc titanate).. Piezoelectric materials with high d parameters (e.g.. d31 or d32) are preferred, which enables planar actuation with perpendicular voltage.

[0(566] In exemplary embodiments, the ferroelectric layer 210 is a single crystal.

[0067] In exemplary embodiments, the thickness of the ferroelectric layer 210 is between 0.1-lOurn, preferably I -Sum. In some embodiments, the ferroelectric layer can be patterned into nano-isiand arrays, which each nano- island stacked as described in Figure 1 to provide a single memory cell.

[0068] The ferromagnetic free layer 230 is selected to provide an appropriate rotation of magnetic orientation when subjected to mechanical strain. Suitable materials may be selected from those used to create magnetic spin valves as is known in the art.

[0069] In exemplary embodiments the ferromagnetic free layer 230 is Co, Ni, FeGaB, CoFeSiB, etc.

[0078] The magnetostriction constant of the free layer 230 is preferably high, e.g. -70 ppm for FeGaB. This material property determines the anisotropy of the material and its ability to maintain a stable magnetic orientation (non-volatile) when eiastically coupled to the ferroelectric layer 210. The magnitude of the magnetostriction constant also determines the magnetoelectric coupling strength, and how effective is the voltage control of magnetization switching.

[0071] The ferromagnetic free layer 230 is a single crystal, polycrystailine, or amorphous.

[0072] In one or more embodiments, the thickness of the ferromagnetic free layer 230 is betweenl-10 nm, preferably 4 nm. This thickness changes the magnitude of the electric field- induced strain that is required to switch its magnetization. Decreasing the thickness of the free layer to ~5 nm may increase the possible range of in-plane magnetization rotation. While not being bound to theory, this effect may be attributed to the suppression of the out-of-plane magnetization componen due to the enhanced demagnetization in thinner magnetic films.

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ActiveUS 104090996v,l which facilitates in-plane magnetization rotation, the thickness of the ferromagnetic layer 150 is preferably 40 nm.

[0073] The pinning layer configuration 250 may consist of a magnetic hard layer (with an easy axis in the x- or y-direction) 260 magnetically coupled to an antiferromagnetic layer 270. The hard layer 260 may also be a synthetic tri-layer with a pinned layer and a reference layer separated by an ultrathin non- magnetic spacer layer. In both of these cases, the antiferromagnetic layer 270 is selected to pin the hard layer's magnetic orientation in a direction parallel to the ferromagnetic layer. The pinning layer configuration 250 may also be a single magnetic layer with a pinned magnetic orientation.

[©074] In one or more embodiments, the antiferromagnetic layer 270 is FcMn, or PtMn, NiO, or CrMnPt. 007SJ In one or more embodiments, the antiferromagnetic layer 270 possesses antiferromagnetic properties at temperatures between 220K-500K, preferably at room temperature,

[0(576] In one or more embodiments, the thickness of the antiferromagnetic layer 270 is between 1■■ I GOnm, preferably 15nm.

[0077] The hard ferromagnetic layer 260 is selected to provide a pinned magnetic orientation.

{0078^'J In one or more embodiments, the hard ferromagnetic layer 260 is is<oFe₂o, or FeCoB.or FeGaB.

[0079] In one or more embodiments, the thickness of the hard ferromagnetic layer 260 is between 1■ 100 nm, preferably 8 nm.

[0(580] The electron transport layer 240 is selected to be conductive (GMR) for electron flow or insulating (TM ) for electron tunneling.

[0081] In one or more embodiments, the electron transport layer 240 layer is a conductive metal and can be made of Cu,Ag, or Al, preferably Cu or mixtures thereof.

[0(582] In one or more embodiments, the conductive electron transport layer 240 layer has a resistivity between 1 -10 uOhm-em.

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ActiveUS 1040909 6v,l [0(583] In one or more embodiments, the conductive electron transport layer 240 layer has a thickness between l -6nm, preferably 2 nm.

[0084] In one or more embodiments, the electron transport layer 240 layer is insulating and consists of MgO, or Al₂0j. Suitable insulating materials include those dielectric materials compatible with CMOS and other semiconducting fabrication processes.

[0085] In one or more embodiments, the insulting electron transport layer 240 layer has a thickness between 5-200 nm.

[0086] The spin valve 220 may be capped at the top and bottom with an electrical contact layer (not shown) (hut see, FIG. 1 1A). In one or more embodiments, the capping layer may be a conductive metal such as Cu, Ta, or Pt. In one or more embodiments, the thickness of the capping layer is between 1-lQnni, preferably 3 nm for passivation.

[0087] The FE layer provides a mechanical strain under an applied voltage or voltage impulse which leads to free layer magnetization rotation due to the inverse niagnetoelastic effect. The magnetization rotation in the free layer will lead to a magnetoresistance change in the TMR or GMR cell.

[0088] The lateral size of the spin valve 220 may be between 5-100 nm in either or both x- and y-direction. Reducing lateral size, e.g., to -400 nm²' would be highly desirable for pursuing high storage density, though, if too small, thermal stability and the complex magnetization at the edges of the magnetic films 230, 260 might hamper the control of in- plane magnetization rotation.

[0089] In exemplary embodiments, the actuation voltage of an e-writable device is less than 1-2V, and preferably less than IV. In one or more embodiments, the actuation E- field is in the range of lOkV/cm. As discussed in greater detail below, a magnetization switch was prepared using ~2μτη PM -PT thick ferroelectric layers in a spin valve. An uitralow voltage of -0.26 V was sufficient for MRAM device operation.

[00.90] For a Ni free layer 64 x 64x5 nm³, this computes to an extremely lo writing energy of -0.16 fj, which is drastically lower than the 70 pJ per bit for conventional MRAMs. The energy needed for writing will further be reduced when the size of the MRAM cell is reduced further.

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ActiveUS i04090996v.l [0091] With such actuation voltages, the electric field remain below the FE coercive field of the ferroelectic material (e.g., PMN-PT, approximately ±0.2 V um--T), and the fatigue problem that ferroelectries sometimes suffer from repetitive polarization reversal can also be greatly relieved. This should allow an effective alleviation of the energy loss and

improvement of MRAM device reliability. Furthermore, the dielectric breakdown vulnerability of the present device (for dielectrics such as the electron transport layer 240 ) can be avoided as the write operating voltage, far below the dielectric breakdown threshold of the ferroelectic layer 210 (e.g. >10 V μπν^~1 for PMN-PT), is applied only on the ferroelectric layer. This avoids the need to have the write and read current flow across the whole device and share the same tunnel as with other MRAM devices.

[0092] Another important performance attribute of the voltage-controlled MRAM device is the writing speed, which typically takes less than 10 nanoseconds (ns) or with a possibil ity of sub-ns. While not being bound by theory, reducing the size of the cell will lead to reduced time for writing, with the RC lime constant as the limit. (The RC time constant is the time constant (in seconds) of an RC circuit composed of resistors and capacitors. The time constant is equal to the product of the circuit resistance (in ohms) and the circuit capacitance (in farads)). This operation speed is significantly higher than conventional magnetic-write MRAM (around 2.0 ns). Finally, the fully gate voltage-controlled operation allows a good compatibility of the present device with current CMOS platforms (details below), opening up possibilities for ultrahigh densities. For instance, assuming a typical channel length of 45 nm for the bottom transistor and the lateral size of 64 nm for the upper ME spin valve, a storage density of around 88 Gb inch can in principle be realized, challenging traditional N AND Flash memory on mass data storage applications.

[0093] FIG. 3 shows one embodiment 300 of a memory cell according to one or more embodiments integrated into a functional MRAM device. An array design of the electric- field-controlled MRAM device on CMOS platforms is contemplated. The space between neighboring cells may be 45 nm, which is a typical channel length of its constituent MOS transistor. One possible layout of a memory cell array builds on planar complementary metal- oxide- semiconductor (CMOS) platforms. It has a 1 -T(transistor)/ 1 -magnetoresistive(MR) element cell architecture (for TMR or GMR sensors), where the readout is accomplished by sensing the resistance change of the cell, as discussed above. FIG 3 shows the architecture of a 1 -Titransistor)/! -magnetoresistive (MR) memory unit cell, where the writing voltage 310

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ActiveUS i04090996v.l (for E-field 285) is applied between Bitline (BL) 320 and Plateline (PL) 330 controlled by Wordline (WL) 350 controlling the gate 360 of the MOSFET/CMOS transistor. Current 370 is applied between the BitLine and the top electrode 390 to measure resistance changes (i.e., "read" the cell). Note that the embodiments of a TMR/GMR stack demonstrated in FIG. 2 and Examples 1 and 2 below use a current 295 in plane (OP) topology for the

magnetoresistanee measurement; however, as discussed above both in plane and transplane configurations are contemplated. For nanoscale MRAM device embodiments as shown in FIG. 3, current 370 perpendicular to plane (CPP) is contemplated. In certain embodiments, nanoscale MRAM devices are implemented with such magnetoresistive memory cells 380 as an array of nanoislands on a CMOS platform. Each unit cell will have patterned

ferroelectric/piezoelectric islands 21 on which the magnetoresistive memory 220 is deposited. They can be patterned when all deposition is completed. The memory is written by voltage, instead of by current as in conventional MRAM devices.

[0(594] The following non-limiting examples further illustrate certain embodiments. L!sing perpendicularly modulated voltage-based mutiferroic stacks provides a route making high-density, low voltage-controlled, non-volatile magnetic memory devices at room temperature, either stand alone or embedded in CMOS platforms for MRAM applications.

Example 1

[6095] A multiferroic heterostructure comprising ofNigoCoa) /PZN-PT was prepared by magnetron-sputtering NigoCo2o ferromagnetic film 120 with a thickness of 40 nni onto a (01 1 ) cut single crystal ferroelectric PZN-PT ferroelectric substrate 1 10 [10 mm(L) x 5 mm(Wd)x 0.5 mm(T)], which was prepoled along the thickness direction (z). In the presence of an external magnetic field during the deposition process, an in-plane magnetic easy axis in the [01-1 ] direction was produced in the ferromagnetic film 120, Ni_&oCo₂o was selected as the AMR layer 120 due to its large AMR ratio (~2%) and saturation magnetostriction constant of -20 ppm, which is important for achie ving strong ME coupling. The AMR ratio is (he largest change in resistance which can be caused in the ferromagnetic layer due to the rotation of its magnetic orientation 150.

[0096] In (01 1 ) cut single crystal PZN-PT based laminate multiferroic hcterostractures, the E-field induced effective magnetic field F can be simply expressed as

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ActiveUS 104090996v,l M-A I ^■*^■ F)

(1 )

1_.0097] where ¥ is the Y oung's Modulus, v is Poisson's ratio, M_B is the saturation magnetization, λ is the magnetostriction constant, and dy. (-3000 pC/N along [100]) and t 32 s(10()() pC/N along [01 - 1]) are linear anisotropic piezoelectric coefficients of PZN-PT and E is the applied external E-field. H_Sff is quantitatively determined by observing the

ferromagnetic resonance (FMR) spectrum shift under various E-fields.

[©09SJ As shown in FTG. 4 of the AMR setup 400, the magnetic field 410 was applied along the [01 -1] direction of the multiferroic stack 420, the E-field was 430 applied through the thickness direction of the PZN-PT, and current 440 was applied between two electrodes 450 in the [1 10] direction. The easy axis of the ferromagnetic film 120 is in [01 -1] direction.

[0099] FIG. 5, shows a near linear relationship between effective magnetic field and E- field, demonstrated through FMR measurement in the N180C020 /PZN-PT heterostructure, which is consistent with the strain as a func tion of E-field in PZN-PT. A giant E-field induced effective magnetic field, i¾_?, of 430 Oe was achieved at an E-field value of 6 kV/cm. Given the negative magnetostriction constant of isoCo₂₀ and negative anisotropic piezoelectric coefficients of PZN-PT along the [ 1001 direction, forces the magnetization to rotate in-plane from the [01-1 ] to the [100] direction, as implied by Eq. (1 ). This is also substantiated by the E-field induced hard magnetization process along the [01 - 1] direction in FIG. 6 and the FMR spectrum upward shift 700 in FIG. 7.

[0100] FIG. 8 shows E-field modulating AMR with the magnetic easy axis of the ferromagnetic layer 120 prepared along the [01-1 ] direction of the PZN-PT teiToelectric layer 1 10 and perpendicular to electric current 440 direction. Without any magnetic bias field , a minimum AMR was observed at zero E-field caused by the orthogonal of magnetization direction and electric current. With increasing strength of E-field applied through the thickness of the PZN-PT, the orientation of magnetization is rotated in-plane from [01 -1] to [100] and parallel to the measured current direction due to the effective magnetic field, which yields the maximum magnetoresistance. Similar results were observed as an external magnetic bias field of 50 Oe was applied along the [01- 1 ] direction, in which a large E-field is necessary to overcome the magnetic bias field and make magnetoresistance maximum. E-

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ActiveUS 104090996v,l field dynamically tuning of magneioresisiance was also demonstrated as shown in FIGs. 9A and B. As shown in FIG. 9 A, without any magnetic bias field, the magneioresisiance was well-modulated with a square shape of E-field (0-2 kV/cro) at the frequency of 0.5 Hz. As shown in FIG. 9 A, under a magnetic bias field of 50 Oe, magnetoresistance was periodically changed with a sine wave of E-field (1-4 kV/cm).

[0101] FIG. 10 shows a typical E-field dependence of AMR curves with the magnetic easy axis and measured electric current prepared along the [100] direction and external magnetic field applied perpendicularly to the easy axis. In conventional AMR field sensor, such as that using

and N¼oCo2o, the detection range is very limited and less than 20 Oe as shown in FIG. 10 (area I). However, while integrating the AMR sensors into multiferroics, the magnetic field range was dramatically enhanced up to 350 Oe as shown in area II, which is attributed to the E-field induced giant effective magnetic field. Thus, a multiferroic-based and voltage-controlled multiband magnetic field sensor can be realized. This technology could boost the measurement range by at least 15 times.

Example 2

[0102] In a GMR device, such as that shown in FIG. 2, the magnetoresistance depends on the relative orientation of the free layer 230 and pinned layer 260 magnetization in a spin- valve structure 220. In our study, poiycrystalline spin-valve structure of Tail 0 nm) / FeMn(15 nm) / NisoFe?o(8 nm) / Cu(2 nm) / Co(4 nm) / Ta(10 nm) was directly deposited onto a (011) cut PZN-PT ferroelectric substrate 210 without vacuum break, where Co is the free layer 230 and has a negative magnetostriction constant of -50 ppm, NisoFe₂o s the pinned/hard magnetic layer 260 with near zero magnetostriction constant. FeMn was the antiferromagnetic layer 270, and Cu was the conductive electron transport layer 240.

Tantalum was used as the electrical capping layer (not shown).

[0103] Two configurations were prepared for this spin valve stack 1 100, as shown in FIGs 1 1 A and B, by placing the magnetic field along various directions during deposition so that the easy magnetic direction 1110, measured current direction 1140, as well as the applied external magnetic field 1120 are parallel to the [100] direction (FIG. 1 lA) and [01-1] direction (FIG. 1 1 B). The magnetic field 1 130 and current 1 140 were applied parallel to the easy axis 1 1 10 in both configurations. In one configuration (FIG. 1 I B), continuous 90° magnetization switching is possible without the application of a magnetic field. In the other

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ActiveUS i04090996v.l configuration (FIG. 1 1A) 180° magnetization switching is possible, but an applied magnetic field is required.

[6104] E-field dependence of magnetic hysteresis loop for both configurations was demonstrated as shown in FlGs 12 A and 12B. With increasing E-field (in the direction of the black arrows in both plots), opposite magnetization processes of the fixe layer (Co) were observed, indicating that the effective magnetic field makes the magnetization process easier along the [100] but harder along the [01 - 1 ] direction. In addition, the coercive field was significantly enhanced by 100% in FIG. 12A (corresponding to configuration in FIG. 1 1A.) as an E-field of 6 kV/cm was applied. However, for the pined layer of NisoFe ), the hysteresis loops barely changed under various E-fiek!s due to the near zero magnetostriction constant. Typically, pinned layers have an offset hysteresis they are pinned and not easily changed. So this is why the ferroelectric layer is used to control the free layer, not the pinned layer

[0105] As shown in FTG. 13A, typical magnetoresistance hysteresis loops with a GMR ratio of 3% under various E-fields were achieved for the configuration in FIG. 11 A. Solid, heavy arrows point in the direction of increasing electric field strength in the plot. The parallel 1320 and antiparallel 1310 magnetic alignment of the adjacent Co free layer 230 and NisoFe?o hard layer 260 result in minimum and maximum values of magnetoresistance, indicating a consistency in the E-field dependence of magnetic hysteresis loops. Similar to the magnetic hysteresis loop, the coercive field in resistance loop was also changed by 100% with increasing E-field. This corresponds to a 180° magnetization switching of the Co layer as illustrated with the arrow lines 1330 and 1340. By this means, magnetoresistance changes of 3% with reduction in E-field under various magnetic fields of 55 Oe or -55 Oe were observed as shown in FIG. 13B, which corresponded to the dashed 1340 and 1330 arrows in FIG. 13 A, respectively. In this case, the resistance change arises from the E-field induced coercive field change which enables 180° magnetization switch and results in maximum GMR.

[0106] In of FIG, 1 IB, the opposite E-field dependence of magnetoresistance hysteresis loops were observed as shown in FIG. 14A (with increasing E-field magnitude indicated by the solid arrow), indicating that a bard magnetization process occurred with the application of the E-field 285. This is caused by the E-field induced magnetic anisotropy field, which enabled a magnetization rotation of 90° and is consistent with the E-field control of

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ActiveUS 1040909 6v,l ^■magnetization as shown in FIG. I2B. E-field dynamically modulating magnetoresistance was demonstrated without applying any magnetic field as shown in Fig. 14B. During the modulation, only parallel and orthogonal magnetic alignment of the free and pined layer were achieved due to the fact that the E-field induced a 90° magnetization rotation rather than a 180° magnetization switching, thus only half of the GMR ratio tunable range was achieved as denoted by the solid arrow in FIG. 14A. This result was significantly different from the GMR change in the configuration of FIG. 1 1A, where the E-field induced coercive field change resulted in 180° magnetization switching, thus enabling the maximum resistance change; which was equal to the GMR ratio. Noting that the change in electric field allows for a continuous switching between different magnetoresistance values (along the solid arrow in FIG. 14A), the possibility of a multi-bit memory device can be contemplated.

[0107] Examples 1 and 2 demonstrated an energy-efficiency technique for electronically modulating AMR and GMR in composite multiferroic heterostructures through strain mediated ME coupling. By carefully selecting ferromagnetic and ferroelectric phases and with proper design, an E-field induced large magnetic anisotropy, which rotated the magnetization by 90° and resulted in dynamic magnetoresistance modulation, was realized. In addition, E-field induced coercive field change enabled a 180° magnetization switching and yielded a maximum GMR tunable range. This E-field control of magnetoresistance in AMR and GMR multiferroic heterostructure is power efficient, and has great implications for low-power electronics.

[0108] It will be appreciated that while a particular sequence of steps has been shown and described for purposes of explanation, the sequence may be varied in certain respects, or the steps may be combined, while still obtaining the desired configuration. Additionally, modifications to the disclosed embodiment and the invention as claimed are possible and within the scope of this disclosed invention.

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ActiveUS i04090996v.l

Claims

1. A multiferroic stack for use in a voltage-modulated non-volatile magnetic memory

storage device, comprising: a magnetoresistance magnetic spin valve comprising a free ferromagnetic layer and a hard ferromagnetic layer, and a ferroelectric layer capable of mechanical strain actuation in a direction parallel to the layer when actuated with an electric field perpendicular to the plane of the ferroelectric layer, wherein the layer is elasticaily coupled to the free ferromagnetic layer,

2. The multiferroic stack of claim I, wherein the magnetic spin valve comprises: an electron transport layer positioned between the free and hard ferromagnetic layers and in electrical contact with the free and hard ferromagnetic layers, wherein the free ferromagnetic layer comprises a first magnetic orientation and first magnetic easy axis, the hard ferromagnetic layer comprises a second magnetic orientation and second magnetic easy axis, and the easy axes of the first and second ferromagnetic layers are parallel, and wherein the second magnetic orientation is pinned in a direction parallel to the ferromagnetic layer.

3. The multiferroic stack of claim 1 or 2, wherein the magnetic spin valve further comprises an antiferromagnetic layer, magnetically coupled to the hard ferromagnetic layer, wherein the antiferromagnetic layer pins the second magnetic orientation in a direction parallel to the ferromagnetic layer.

4. The multiferroic stack of claim 1, 2 or 3, wherein the ferromagnetic hard layer comprises a synthetic tri-layer with a pinned layer and a reference layer separated by a nonmagnetic spacer layer.

5. The multiferroic stack of claim 1, 2, or 3, wherein the electron transport layer provides a low resistance electrical pathway between the free and hard ferromagnetic layers.

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ActiveUS 104090996v,l

6. The multiferroic stack of claim 1, 2, or 3, wherein the electron transport layer provides an electron tunneling pathway between the free and hard ferromagnetic layers.

7. The multiferroic stack of any of the preceding claims, further comprising conductive layers providing electrical contact of the first ferromagnetic layer and/or the

antiferromagnetic layer with external electronics.

8. The ferroelectric device of claim 1, further comprising a reader for measuring the

resistance of the ferromagnetic free layer.

9. The multiferroic stack of any of the preceding claims, further comprising electrodes in electrical contact with the ferroelectric layer positioned to apply an electric field perpendicular to the plane of the ferroelectric layer.

10. The multiferroic stack of claim 9, wherem the electrodes are positioned above and below the plane of the ferroelectric layer.

1 1. The ferroeieciric stack of claim 9, wherein the electrodes are in electrical contact with the bit line and the plate line of a transistor.

12. A method of using a multiferroic stack in a voltage-modulated non-volatile magnetic memory storage device, comprising: providing the multiferroic slack of any of the preceding claims,

applying an electric field perpendicular to the ferroelectric layer to rotate the magnetic orientation of the free ferromagnetic layer in the plane of the ferromagnetic layer, and measuring changes in electrical resistance parallel to the magnetic field.

13. The method of using a multiferroic stack of claim 12, further comprising applying a magnetic field parallel to the hard ferromagnetic orientation.

14. The method of using a multiferroic stack of claims 12 or 13, comprising inducing up to 90° or up to 180° rotation of the first magnetic orientation.

15. A method of making a multiferroic stack for use in a voltage-modulated non-volatile magnetic memory storage device, comprising:

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ActiveUS 1040909 6v,l providing a ferroelectric layer capable of mechanical strain actuation in a direction parallel to the lay er when actuated with an electric field perpendicular to the lay er, providing a magnetoresistance magnetic spin valve, elastically coupling the ferroelectic layer with the free ferromagnetic lay er of the spin valve; and providing electrical contact to the pinned layer and free layer of the magnetoresistance magnetic spin valve, said electrical contacts positioned to apply a current.

16. A multiferroic stack for use in a voltage-modulated magnetic non-volatile memory storage device, comprising: a ferromagnetic layer with a tunable magnetic orientation, wherein the ferromagnetic layer comprises a magnetic orientation and a magnetic easy-axis which is parallel to the ferromagnetic layer, a ferroelectric layer capable of mechanical strain actuation in a direction parallel to the layer when actuated with an electric field perpendicular to the layer, wherein the layer is elastically coupled to the ferromagnetic layer,

17. A voltage-modulated non-volatile magnetic memory storage device, comprising: a 1 -transistor/1 -magnetoresistive memory cell comprising: a magnetoresistance magnetic spin valve comprising a free ferromagnetic layer and a hard ferromagnetic lay er; a transistor comprising a gate; a word line connected to the gate of the transistor; a plate line in electrical communication with the free layer; a bit line controlled by the word line; a ferroelectric layer, wherein the layer is elastically coupled to the free ferromagnetic layer and electrically coupled to the plate line and bit line; and

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ActiveUS 1040909 6v,l an electrode in electrical communication with the pinned layer; wherein the ferroelectric material is capable of producing mechanical strain actuation in a direction parallel to the ferroelectric layer when actuated with an electric field perpendicular to the plane of the ferroelectric layer.

18. The de vice of claim 17, further comprising a reader for measuring the resistance of the spin valve, when current is passed between the bit line and the top electrode.

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ActiveUS 104090996v,l