Journal of Magnetism and Magnetic Materials 324 (2012) 2019–2023 Contents lists available at SciVerse ScienceDirect Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm Electric field-induced magnetoresistance in spin-valve/piezoelectric multiferroic laminates for low-power spintronics D.T Huong Giang n, 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 abstract Article history: Received November 2011 Received in revised form 10 January 2012 Available online 17 February 2012 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 magnetization is accomplished by strain mediated magnetoelectric coupling Practically, the magnetization 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 Keywords: Magnetoresistance Magnetization switching Spin-valve multilayer Piezoelectric Multiferroic Introduction as follows: 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, function on the basis of the magnetic field-induced magnetization 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 information 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 [2–4] GMR in the spin-valve structure is related to the spin depending scattering 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 [1,57] Ryị ẳ RP ỵ ẵRAP 2RP ị1cosyị n Corresponding author Tel.: ỵ84 37549332; fax: ỵ84 37547460 E-mail address: giangdth@vnu.edu.vn (D.T Huong Giang) 0304-8853/$ - see front matter & 2012 Elsevier B.V All rights reserved doi:10.1016/j.jmmm.2012.01.038 ð1Þ 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 influenced by an external strain thanks to the inverse magnetostriction (Villary effect) [7] For the case of the positive magnetostriction, 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 [5–7] and Refs therein Developing the principle of the above mentioned strain-driven magnetization rotation, E-field induced large magnetic anisotropy has been achieved in several multiferroic heterostructures via strain mediated magnetoelectric (ME) coupling [8–12] MingLiu et al [11] have investigated 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 piezoelectric coefficients allows to clarify the role of corresponding inplane strains There, the coercivity and magnetoresistance enhancement of 100% and 3% were reported, respectively This paper deals with a power efficient E-field tunable magnetization realized in the spin-valve-based {IrMn/Fe50Co50/Cu/ Ni80Fe20/Si}/PZT laminates In this structure, only the pinned 2020 D.T Huong Giang et al / Journal of Magnetism and Magnetic Materials 324 (2012) 2019–2023 (magnetostrictive) Fe50Co50 layer 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 distributed 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 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 150 mm thick glass substrate under working pressure of mTorr and the base pressure of  10 À 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 ferromagnetic layers then aligns the pinning direction of the antiferromagnetic IrMn layer Spin-valve/PZT composites were manufactured by bonding the  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, respectively For the later measurement, the electric current IR ¼1 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 VPZT ¼800 V) applied across the normal direction of the PZT slab Results and discussion Shown in Fig 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 Sweeping the magnetic fields from positive to negative ones, the magnetization remains almost saturation in high positive magnetic fields Fig 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) Fig Full range in-plan magnetic hysteresis loops under zero and applied E-field of 12 kV/cm for the longitudinal (a) and transversal (b) spin-valve/PZT configurations D.T Huong Giang et al / Journal of Magnetism and Magnetic Materials 324 (2012) 2019–2023 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 d31 ¼ d32 o0 Then the squared shape PZT slab suffered an equal in-plane bi-axial compressive stress As usual, one can assume that, the spinvalve 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 (d33 40) In consequences, the in-plane magnetization, in particular the inplane remanence, will decrease under applied E-fields in both longitudinal and transversal configurations This argument, however, can describe the magnetic behavior of the longitudinal configuration 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 spinvalve 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 in Fig It can be seen from this model that the E-fields (and the effective compressive stress) have no effect in high magnetic 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 (see Fig 2a,b) For an analysis of details, magnetization and magnetoresistance data are presented together in Figs and For the longitudinal Fig Establishment of electric field-induced magnetic anisotropy under a compressive stress s o0 parallel (a) and perpendicular (b) to the easy axis 2021 Fig Magnetic (a) and magnetoresistance (b) hysteresis loops for longitudinal configuration 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 magnetoresistance value GMRmax(¼ DR/R) shown in Fig 4b decreases from 2.09% at E¼ to 1.79% at E¼ 12 kV/cm Illustrated in Fig 5a,b are the E-field dependence of the maximal magnetoresistance GMRmax and the magnetic field at which GMR reached the maximum H(GMR)max for the longitudinal spin-valve/PZT configuration under investigation 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 magnetizations and then the enhancement of the high resistance state 2022 D.T Huong Giang et al / Journal of Magnetism and Magnetic Materials 324 (2012) 2019–2023 Fig Electric field dependence of the GMRmax (a) and H(GMR)max (b) for the longitudinal configuration Fig Electric field dependence of the GMRmax (a) and H(GMR)max (b) for the transversal configuration Indeed, the maximal magnetoresistance GMRmax increases 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 GMRmax and H(GMR)max show a clear tendency to increase with increasing E-fields up to 14 kV/cm and approach to the saturation 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 Conclusion remarks A novel magnetization switching type, names as electric fieldinduced 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 Fig Magnetic (a) and magnetoresistance (b) hysteresis loops for transversal configuration 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 D.T Huong Giang et al / Journal of Magnetism and Magnetic Materials 324 (2012) 2019–2023 References [1] [2] [3] [4] [5] [6] S Maekawa, Concepts in Spin Electronics, Oxford Science Publications, 2006 W 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H(GMR)max for the longitudinal spin-valve/ PZT configuration under investigation It is interesting that, both GMRmax and H(GMR)max shows a clear tendency to decrease with increasing E-fields The saturation,... magnetic fields from positive to negative ones, the magnetization remains almost saturation in high positive magnetic fields Fig Schematic spin-valve/ PZT laminate configurations: the magnetic easy axis... later and maximal GMR is higher Conclusion remarks A novel magnetization switching type, names as electric fieldinduced magnetic anisotropy has been realized in spin-valve/ PZT multiferroic laminates