DSpace at VNU: Electric field-induced magnetoresistance in spin-valve piezoelectric multiferroic laminates for low-power spintronics

Practically, the magnetizamagnetiza-tion in the magnetostrictive FeCo layer of the spin-valve structure rotates under an effective compressive stress caused by the inverse piezoelectric

Electric field-induced magnetoresistance in spin-valve/piezoelectric

multiferroic laminates for low-power spintronics

, V.N Thuc, N.H Duc Nano Magnetic Materials and Devices Department, Faculty of Engineering Physics and Nanotechnology, VNU University of Engineering and Technology, Vietnam National University, Hanoi E3 Building, 144 Xuan Thuy Road, Cau Giay, Hanoi, Viet Nam

a r t i c l e i n f o

Article history:

Received 9 November 2011

Received in revised form

10 January 2012

Available online 17 February 2012

Keywords:

Magnetoresistance

Magnetization switching

Spin-valve multilayer

Piezoelectric

Multiferroic

a b s t r a c t

Electric field-induced magnetic anisotropy has been realized in the spin-valve-based {Ni80Fe20/Cu/

Fe50Co50/IrMn}/piezoelectric multiferroic laminates In this system, electric-field control of magnetiza-tion is accomplished by strain mediated magnetoelectric coupling Practically, the magnetizamagnetiza-tion in the magnetostrictive FeCo layer of the spin-valve structure rotates under an effective compressive stress caused by the inverse piezoelectric effect in external electrical fields This phenomenon is evidenced by the magnetization and magnetoresistance changes under the electrical field applied across the piezoelectric layer The result shows great potential for advanced low-power spintronic devices

&2012 Elsevier B.V All rights reserved

1 Introduction

In the modern electronic and spintronic devices, the giant

magnetoresistance (GMR) effect has been widely used in memory

technologies and magnetic sensors These devices however,

func-tion on the basis of the magnetic field-induced magnetizafunc-tion

switching In nanostructures, however, this physical mechanism

is not efficient to control magnetic bits due to the large current

In particular, when approaching the downscaling limits (e.g in

densely packed arrays) the unavoidable distribution of writing

parameters coupled to the large stray fields will lead to spreading

program errors and may influence to neighborhood architectures

In this context, the current-induced (or spin-transfer driven)

switching mode is considered to be more efficient However,

two main facts still remain challenging its applications in

infor-mation storage technologies: firstly, all metal spintronic devices

have low resistances and secondly, further reductions in the

magnitude of the switching currents are still the subject of active

researches [1] In order to tackle these difficulties, electric (E)

field-induced magnetization switching is a perspective solution

and multiferroics consisting of ferromagnetic and ferroelectric

orders have become an active research frontier[2– ]

GMR in the spin-valve structure is related to the spin

depend-ing scatterdepend-ing In this structure, the resistance can be described by

the relationship with the angle (y) between the magnetization

directions in the pinned and free ferromagnetic layers [1,5 7]

as follows:

here RP and RAP are low and high resistances of the spin-valve structure in parallel and antiparallel configurations, respectively The magnetization orientation, however, can also be influ-enced by an external strain thanks to the inverse magnetostric-tion (Villary effect) [7] For the case of the positive magneto-striction, the magnetization is favored to align parallel to the tensile stress direction and perpendicular to the compressive stress direction In this case, the stress sensing layer is preferred with a highly magnetostrictive material and the maximal change

in the magnetization direction can reach up to 901 Practically, the pressure sensors based on GMR and spin-valve structures were already proposed[5– ] and Refs therein Developing the princi-ple of the above mentioned strain-driven magnetization rotation, E-field induced large magnetic anisotropy has been achieved in several multiferroic heterostructures via strain mediated magne-toelectric (ME) coupling[8–12] MingLiu et al.[11]have investi-gated the E-field induced magnetization and magnetoresistance

of the free (magnetostrictive) Co layer in the spin-valve based FeMn/Ni80Fe20/Cu/Co/PZN–PT multiferroic heterostructure, where the single crystal ferroelectric PZN–PT with different in-plane piezo-electric coefficients allows to clarify the role of corresponding in-plane strains There, the coercivity and magnetoresistance enhance-ment of 100% and 3% were reported, respectively

This paper deals with a power efficient E-field tunable mag-netization realized in the spin-valve-based {IrMn/Fe50Co50/Cu/

Ni80Fe20/Si}/PZT laminates In this structure, only the pinned

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journal homepage:www.elsevier.com/locate/jmmm Journal of Magnetism and Magnetic Materials

0304-8853/$ - see front matter & 2012 Elsevier B.V All rights reserved.

n

Corresponding author Tel.: þ84 4 37549332; fax: þ84 4 37547460.

E-mail address: giangdth@vnu.edu.vn (D.T Huong Giang).

(magnetostrictive) Fe50Co50layer serves as strain sensing layer.

Thanks to the ME coupling between (ferro)magnetic (spin-valve)

structure and piezoelectric (PZT) layers, the magnetization in the

pinned FeCo layer can be turned under the mechanical strains

caused by the inverse piezoelectric effect in an external E-field

(or an external voltage) The Ni80Fe20 with a close-to-zero

magnetostriction is chosen as the reference layer, which is almost

not being influenced of internal stress The PZT slabs under

investigation, however, are polycrystal with equally

distri-buted bi-axial strains The objective of this study is to investigate

the magnetization and magnetoresistance changes due to an

effective stress induced by a voltage (VPZT) applied across the

PZT slab in proper longitudinal and transversal spin-valve

configurations

2 Experimental

The spin-valve {Ta(5 nm)/Ni80Fe20(10 nm)/Cu(1.2 nm)/Fe50Co50

(8 nm)/IrMn(15 nm)/Ta(5 nm)} structures were prepared using

magnetron sputtering technique on 150mm thick glass substrate

under working pressure of 1 mTorr and the base pressure of

7  10 7

Torr A uniform magnetic field of 400 Oe was applied

parallel to the film plane during the sputtering process This

magnetic field induces a magnetic anisotropy in the

ferromag-netic layers then aligns the pinning direction of the

antiferro-magnetic IrMn layer

Spin-valve/PZT composites were manufactured by bonding the

2  12 mm2 rectangular spin-valve films on the surface of a

12  12 mm2 square piezoelectric slab (0.5 mm thick) The PZT (APCC-855) slab is out-of-plane polarized and supplied by American Piezoceramics Inc., PA, USA

In this paper, two different (longitudinal and transversal) configurations corresponding to two different alignment of the pinning (easy) direction have been prepared: (i) along the length (Fig 1a) and (ii) along the width (Fig 1b) of the spin-valve rectangular structure, respectively The magnetization and the magnetoresistance was measured in the magnetic fields applied parallel to the easy axis of spin-valve structure using VSM (Lake Shore 7400) and a collinear four-point probe methods, respec-tively For the later measurement, the electric current IR¼1 mA is passed along the length of the spin-valve structure for sensing its resistance The measurements were carried out in different external electric fields E(¼VPZT/tPZT) up to 16 kV/cm (i.e an electric voltage

VPZT¼800 V) applied across the normal direction of the PZT slab

3 Results and discussion Shown inFig 2(a,b) are the full range in-plan magnetic hysteresis loops under zero and applied E-field of 12 kV/cm for the longitudinal and transversal spin-valve/PZT configurations, respectively Sweep-ing the magnetic fields from positive to negative ones, the magne-tization remains almost saturation in high positive magnetic fields

Fig 1 Schematic spin-valve/PZT laminate configurations: the magnetic easy axis is parallel (a) and perpendicular (b) to the length of samples (named as longitudinal and transversal configurations, respectively).

The magnetic hysteresis loops occur and exhibit a two magnetization

reversal processes at negative magnetic fields, which reflect that

both FeNi and FeCo layers are pinned (in different levels) In these

cases, the reversal occurred at the low negative field is commonly

referred to the free FeNi layer and the reversal at the high negative

field is corresponding to the pinned FeCo layer In addition, the two

magnetization reversal processes are not well separated This may

relate to a weak coupling between pinned ferromagnetic FeCo and

antiferromagnetic IrMn layers, resulting already a magnetization

distribution of the FeCo layer to the first loop

The applied E-field led to strain in the PZT slab as well as in the

spin-valve structure The E-field induced magnetization, however,

occurs in low magnetic fields and changes in opposite trend: the

magnetization slightly decreases in the longitudinal configuration

but increases in the transversal one The behavior observed in the

longitudinal configuration is rather similar with that reported for

the Fe3O4/PZT and Fe3O4/PZN–PT multiferroic heterostructure,

where applied magnetic fields are along compressive stress

direction[8] For these findings, several arguments can be proposed

For a polycrystal PZT, the in-plane piezoelastic coefficients d31¼

d32o0 Then the squared shape PZT slab suffered an equal in-plane

bi-axial compressive stress As usual, one can assume that, the

spin-valve film suffers the same stress as in PZT slab In this case, there

will be no any rotation of the magnetization in the plane, but the

magnetization of the FeCo (with a given positive magnetostriction)

will move out film plane corresponding to a tensile stress (d3340)

In consequences, the plane magnetization, in particular the

in-plane remanence, will decrease under applied E-fields in both

longitudinal and transversal configurations This argument,

how-ever, can describe the magnetic behavior of the longitudinal

config-uration only It can not apply for that observed for the transversal

one Note that, the experimental setup under investigation is a

complex system The magnetic behaviors must be considered taking

into account all factors including the 3D distribution of the magnetic

moments and shape effects In this context, one may assume that an

‘‘effective’’ compressive stress along the long direction of the

spin-valve rectangular structure was established in this proper design

This stress weakens and enhances the axial magnetic anisotropy in

the longitudinal and transversal configurations, respectively, as

illustrated inFig 3 It can be seen from this model that the E-fields

(and the effective compressive stress) have no effect in high

mag-netic fields, i.e no effect to the saturation state However, it could

strongly modify the magnetic state of the magnetostrictive FeCo

layer in the magnetization processes at low magnetic fields, e.g in

the field range of the free and pined layer reversals (seeFig 2a,b)

For an analysis of details, magnetization and magnetoresistance

data are presented together inFigs 4 and 6 For the longitudinal

configuration, when the sample is subjected in an electric field E¼12 kV/cm applied through the thickness of the PZT slab, the free layers’ loop stayed almost unmodified whereas that of the pinned layer becomes less sloping and the corresponding squareness ratio is reduced (Fig 4a) It is consistent with above argument about the E-field induced weakness of the axial magnetic anisotropy and/or the E-field induced enhancement of the distribution of magnetic moments out of easy axis (Fig 3a) In this context, the spin-valve antiparallel coupling is weakened As a result, the maximal magne-toresistance value GMRmax(¼DR/R) shown inFig 4b decreases from 2.09% at E¼ 0 to 1.79% at E¼ 12 kV/cm Illustrated inFig 5a,b are the E-field dependence of the maximal magnetoresistance GMRmaxand the magnetic field at which GMR reached the maximum H(GMR)max for the longitudinal spin-valve/PZT configuration under investiga-tion It is interesting that, both GMRmax and H(GMR)max shows a clear tendency to decrease with increasing E-fields The saturation, however, does not reach even at E¼16 kV/cm Systematically, one can summary that for the longitudinal configuration the larger E-field is applied, the distribution of the FeCo magnetic moments

is stronger, the reversal occurs earlier and maximal GMR is lower

In contrast to above phenomena, the magnetization and magnetoresistance data of the transversal configuration show an opposite behavior (Fig 6) Sweeping the field from positive to negative, the magnetic hysteresis loop (Fig 6a) exhibits also a two magnetization reversal step character, but rather flat with respect

to that of the longitudinal configuration (Fig 4a) Among the above mentioned reasons, here the influence of the (shape) demagnetization effect must be important When the PZT slab is subjected in an electric field E ¼12 kV/cm, the free layer’s loop stayed almost unmodified whereas that of pinned layer exhibits a somewhat sharper squareness This supports a reinforcement of the antiparallel configuration of the free and pinned layer mag-netizations and then the enhancement of the high resistance state Fig 3 Establishment of electric field-induced magnetic anisotropy under a

compressive stressso0 parallel (a) and perpendicular (b) to the easy axis.

Fig 4 Magnetic (a) and magnetoresistance (b) hysteresis loops for longitudinal configuration.

Indeed, the maximal magnetoresistance GMRmaxincreases from a value of 2.14% at E¼0 kV/cm up to 2.34% at E¼12 kV/cm (Fig 6b) It

is consistent with the fact that the dominant effective compressive stress always forces to orient the pinned FeCo magnetization perpendicular to the length of the spin-valve element (i.e better aligned along to the magnetic easy axis) (Fig 3b) Similarly, the E-field dependence of GMRmax and H(GMR)max is illustrated in

Fig 7(a,b) for the transversal spin-valve/PZT configuration In this case, both GMRmaxand H(GMR)maxshow a clear tendency to increase with increasing E-fields up to 14 kV/cm and approach to the satura-tion higher E-fields, where a perfect alignment of FeCo magnetic moments were reinforced by the stress Systematically, here one can also summary that the larger E-field is applied, the alignment of the FeCo magnetic moments is better, the reversal occurs later and maximal GMR is higher

4 Conclusion remarks

A novel magnetization switching type, names as electric field-induced magnetic anisotropy has been realized in spin-valve/PZT multiferroic laminates through strain mediated ME coupling The results show that using proper design and external electric field

we can control the magnetization and magnetoresistance change This advance shows great implications for low-power electronics and spintronics

Acknowledgments This work was supported by Vietnam National University, Hanoi under the Grant QG 09.29 and the NAFOSTED of Vietnam under the Project number 103.02.86.09

Fig 5 Electric field dependence of the GMR max (a) and H(GMR) max (b) for the

longitudinal configuration.

Fig 6 Magnetic (a) and magnetoresistance (b) hysteresis loops for transversal

Fig 7 Electric field dependence of the GMR max (a) and H(GMR) max (b) for the transversal configuration.

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