IEEE TRANSACTIONS ON MAGNETICS, VOL 50, NO 6, JUNE 2014 2005204 Electrical Field-Induced Magnetization Switching in CoFe/NiFe/PZT Multiferroics Nguyen Thi Minh Hong1 , Pham Thai Ha1 , Le Viet Cuong1 , P T Long2, and Pham Duc Thang1,3,4 Faculty of Engineering Physics and Nanotechnology and Laboratory for Micro and Nanotechnology, University of Engineering and Technology, Vietnam National University, Hanoi 10000, Vietnam Department of Physics, Chungbuk National University, Cheongju 361-763, Korea UNESCO UNISA Africa Chair in Nanosciences and Nanotechnology, College of Graduate Studies, University of South Africa, Pretoria 0002, South Africa Nanosciences African Network, iThemba LABS-National Research Foundation of South Africa, Pretoria 0001, South Africa In this paper, we have investigated the change in magnetization of multiferroic material, based on magnetic nanostructured CoFe/NiFe film grown on the piezoelectric lead zirconate titanate (PZT), under the effect of the strain originated from PZT layer In this material, a converse magnetoelectric effect and especially, an electric field-induced magnetic anisotropy and magnetization switching process have been observed at the changing stages of applied electric voltage In addition, a significant relative change in magnetization, above 100%, is obtained, which facilitates practical applications of the materials This opens possibilities in achieving new types of memory devices, the low energy consumption devices, as well as other functionalities, such as voltage-tunable field sensing A simple theory based on strain-mediated magnetic-electric coupling is also presented to understand the origin of the change in magnetic properties of the materials Index Terms— Ferroelectrics, ferromagnetics, magnetization switching, multiferroics I I NTRODUCTION M ULTIFERROIC materials in which magnetic and ferroelectric orders coexist have emerged as one of the promising materials for multifunctional applications in data storage, field sensors, and next generation spintronic devices, owing to the advantageous possibility of controlling the magnetic state via the electric fields and vice versa [1]–[3] Recently, the research interests have been focused on the converse magnetoelectric (CME) effect and magnetization (M) switching by electric field (E) in multiferroics thanks to its potential application for new types of memory devices and low energy consumption devices [4], [5] The physical nature of electric-magnetic couplings is also of great interest but has not been thoroughly explored There are various ways to change magnetic anisotropy of magnetic films One of the conventional methods is by changing deposition conditions, e.g., substrate temperature, gas pressure, and so on A new approach recently applied for multilayered films is to use a multiferroic structure, in which the magnetic properties can be tuned by an applied electric voltage In the previous works, we reported the finding of the CME effect and the electric-voltage-controlled magnetic anisotropy in new CoFe/NiFe/piezoelectric lead zirconate titanate (PZT) heterostructured nanocomposites [6]–[8] Interestingly, we observed the switching of the magnetization at a suitably applied electric voltage Manuscript received November 22, 2013; revised January 20, 2014; accepted January 28, 2014 Date of current version June 6, 2014 Corresponding author: N T M Hong (e-mail: hongntm@vnu.edu.vn) Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org Digital Object Identifier 10.1109/TMAG.2014.2304518 This paper aims to investigate the change in magnetization of nanostructured CoFe/NiFe/PZT multiferroics, induced by the presence of a stress originated from transverse polarized PZT piezoelectric layer In addition, we present a theoretical study on the CME effect and electric field-induced magnetization switching, the role of ferromagnetic layer thickness, and bias magnetic field on the magnetization switching process in this material II E XPERIMENT NiFe and CoFe magnetic layers were magnetron sputtered in sequence on commercial 500-μm-thick PZT piezoelectric substrate having transverse polarization (APC-855, American Piezoceramics Inc.) A base pressure of × 10−7 torr was achieved prior to the deposition The deposition process was performed at room temperature in an argon gas environment with a pressure ( pAr ) of 2.2 mtorr and at a power of 50 W In this CoFe/NiFe/PZT multiferroic structure, the thickness of CoFe layer is fixed at 190 nm The total thickness of NiFe/CoFe layers was changed from 200 to 280 nm These samples are noted as S1 , S2 , S3 , and S4 , respectively Finally, a Ta thin layer was sputtered on the top of the samples to prevent oxidization for ferromagnetic layers Magnetic hysteresis loops, M(H ), were measured using a vibrating sample magnetometer (VSM) 7400 (Lakeshore) with the magnetic field (H ) applied in the film plane and normal to the film plane For the CME effect measurement, the electrodes, having the size of × 0.5 mm2 , are placed on two sides of transverse polarized PZT substrates by using silver adhesive glue (Fig 1) The CME measurement was performed using the VSM on × mm2 sample, placed in a varied 0018-9464 © 2014 IEEE Personal use is permitted, but republication/redistribution requires IEEE permission See http://www.ieee.org/publications_standards/publications/rights/index.html for more information 2005204 IEEE TRANSACTIONS ON MAGNETICS, VOL 50, NO 6, JUNE 2014 Fig Experimental setup of CoFe/NiFe/PZT multiferroics for CME measurement external bias magnetic field (Hbias) and an applied electric voltage (V ) ranged from −400 to 400 V III R ESULTS AND D ISCUSSION For the samples S1 , S2 , S3 , and S4 , magnetic hysteresis loops have been measured at room temperature and at various angles α = 0° and 90° in which α is the angle between the film plane direction and the external bias magnetic field direction The results, not shown here, indicate that in-plane magnetic anisotropy dominates for all multiferroic samples due to the magnetic anisotropic contribution of NiFe and CoFe ferromagnetic layers The magnetization change as a function of voltage applied along the PZT substrates, M(V ), measured at various bias magnetic field Hbias = 0, ±50, ±100, ±1000 Oe aligned parallel to sample plane (α = 0°), are shown in Fig The results point out an almost linear dependence of magnetization M on voltage V for all samples Under applied electric voltage, PZT substrate is elongated in the film plane and the strain in PZT substrate leads to a strain in NiFe/CoFe layers Consequently, this alters the magnetic anisotropy of the magnetic layers via magnetoelastic coupling It indicates that the elastic stress transfers from PZT piezoelectric phase to NiFe/CoFe magnetic phase In this effect, the magnetoelastic energy associated with coupling between the stress and the magnetization and stress anisotropy field Hσ can be formed in the NiFe/CoFe In this case, the elastic stress associates with magnetization of magnetostrictive materials and thus, the stress anisotropy can be induced [9] The change in magnetization, M/M, measured at α = 0° and Hbias = −50 Oe under a voltage ranged from to 400 V is larger than 100% We observe a maximum value M/M of 138% for the sample S2 (corresponding to M = 550 μemu) The strength of the stress anisotropy is dependent on the product of the saturation magnetostriction of ferromagnetic materials λ S and the applied stress σ , namely λ S σ [10] In the theoretical model, the orientations of magnetization are defined by the competition between the stress anisotropy field Hσ and magnetocrystalline anisotropy field H K We consider first the magnetoelastic energy which is given by E me = K σ sin2 θ where the stress anisotropy constant K me is expressed as K σ = (3/2)λ S σ Apparently, K σ > favors θ = 0° where the magnetization is parallel to the stress axis Contrary, K σ < favors perpendicular alignment of the magnetization (θ = 90°) This means that when the absolute values of the stress anisotropy constant K σ are larger than the magnetocrystalline anisotropy constant K , the magnetization will be aligned along the easy direction determined by the stress anisotropy For the studied samples, PZT substrate has transverse polarization and NiFe/CoFe layers have in-plane magnetic anisotropy, which means the parallel alignment of magnetization is relative to the stress axis, thus K σ > Meanwhile NiFe/CoFe layers have the magnetostriction coefficient λ S > 0, therefore σ > indicates that the stress in those layers is tensile This is in accordance with the above results of increasing M with changing V On further investigation on the dependence of M on varied V , one can observe the electric field-induced switching of magnetization which occurs at certain applied voltage, called the magnetization reversed voltage Vrev (as shown in Fig 3) At this voltage, large strains produced in PZT can transfer to NiFe/CoFe and induce a change in the orientations of the magnetization, i.e., in the magnetic anisotropy This allows us to control magnetization switching by using an electric field instead of a traditional magnetic field At the same magnetic field Hbias, Vrev decreases with increasing the thickness of NiFe In this paper, the thickness of ferromagnetic layer CoFe has been reduced to 190 nm in comparison to that in the previous works [7] Consequently, the results show a decrease of Vrev from −500 to −75 V at the same bias magnetic field of Oe This finding is interesting since it reduces the consumption energy in the applications Hence, these multiferroic structures CoFe/NiFe/PZT have a promising application for future spintronic devices For sample S3 especially, the electric field-induced magnetization switching can be achieved at Vrev = −70 V without Hbias The magnetization state can be controlled by an external voltage, even without the use of a bias magnetic field This opens a possibility for application of CME devices with size reduction and exclusion of interference effect from electromagnet or permanent magnet [11] In order to provide more explanations to the field-induced magnetization switching phenomenon, the voltage-induced magnetic susceptibility χ was derived from M(V ) data, taken at various magnetic field Hbias = −50, −100, −200, −500, and −2000 Oe and shown in Fig for one typical sample S1 As can be seen from Fig 4, first, χ has a negative value at high applied voltage When decreasing applied voltage, χ decreases to a minimum, increases then cancels at fixed voltage (Vrev ) and finally changes in sign One observes that the higher Hbias is, the higher Vrev requires At high magnetic field, the magnetization rotates progressively from easy axis to the field direction In this state, a much higher energy is necessary to switch magnetization and therefore, Vrev increases with increasing Hbias Once magnetization is aligned along the direction of the magnetic field, magnetization switching process no longer occurs and χ approaches to zero In general, magnetization switching can be decided by the competition between the magnetic field energy and applied electric field energy In addition to considering the magnetization changes of samples as a function of applied voltages at various magnetic fields, we also observed the difference of M value measured HONG et al.: ELECTRICAL FIELD-INDUCED MAGNETIZATION SWITCHING Fig 2005204 Magnetization M as a function of applied voltage V measured at various magnetic fields at angle α = 0° Fig Magnetization reversed voltage value (Vrev ) for samples measured at various magnetic fields Fig at various angles α (Table I) The results show that the dependence M(V ) still has a virtually linear increase as we increase the applied voltage on PZT substrate However, the change in magnetization decreases gradually when magnetic field direction deviates from the film plane direction Notably, this change is very small at α = 90° and even Vrev can achieve a voltage as small as 0.5 V for sample S3 at magnetic field Hbias = −100 Oe These obtained results can be explained based on the magnetization process which is described by the model for different orientations of applied magnetic fields At α = 0°, magnetization and magnetic field direction are parallel together (having in-plane magnetic anisotropy) Thus, magnetization switching process requires much more electric field energy to rotate magnetization in the reverse direction On the other hand, when α = 90°, smaller voltage is needed to Voltage dependence of magnetic susceptibility χ for sample S1 2005204 IEEE TRANSACTIONS ON MAGNETICS, VOL 50, NO 6, JUNE 2014 TABLE I VALUE OF Vrev FOR S AMPLES M EASURED AT VARIOUS A NGLE α AND AT Hbias = −100 Oe reverse magnetization from the initial state to the orientation of applied magnetic field Thus, the rotation of magnetization toward applied magnetic field direction leads to a vantage of energy, causing the decrease of Vrev value IV C ONCLUSION The magnetic properties, including CME effect and electric field-induced magnetization, of nanostructured multiferroic CoFe/NiFe/PZT films have been studied Under the strain originated from PZT piezoelectric layer in the presence of the external electric voltage, induced switching of magnetization and magnetic anisotropy have been observed This opens possibilities in achieving new types of memory devices, low energy consumption devices, as well as other functionalities, such as voltage-tunable field sensing R EFERENCES [1] S.-W Cheong and M Mostovoy, “Multiferroics: A magnetic twist for ferroelectricity,” Nature Mater., vol 6, no 1, pp 13–20, Jan 2007 [2] M Bibes and A Barthélémy, “Multiferroics: Towards a magnetoelectric memory,” Nature Mater., vol 7, no 6, pp 425–426, Jun 2008 [3] S Y Chen, D H Wang, Z D Han, C L Zhang, Y W Du, and Z G Huang, “Converse magnetoelectric effect in ferromagnetic shape memory alloy/piezoelectric laminate,” Appl Phys Lett., vol 95, no 2, pp 022501-1–022501-3, Jul 2009 [4] J Higuchi, M Ohtake, Y Sato, F Kirino, and M Futamoto, “NiFe epitaxial films with hcp and fcc structures prepared on bcc-Cr underlayers,” Thin Solid Films, vol 519, no 23, pp 8347–8350, Sep 2011 [5] J M Hu, C.-W Nan, and L.-Q Chen, “Size-dependent electric voltage controlled magnetic anisotropy in multiferroic heterostructures: Interface-charge and strain comediated magnetoelectric coupling,” Phys Rev B, vol 83, no 13, pp 134408-1–134408-6, Apr 2011 [6] N T M Hong, P D Thang, N H Tiep, L V Cuong, and N H Duc, “Voltage-controllable magnetic behavior in PZT/NiFe/CoFe nanocomposites,” Adv Nat Sci., Nanosci Nanotechnol., vol 2, pp 015015-1–015015-4, Mar 2011 [7] N T M Hong, N H Duc, and P D Thang, “Converse magnetoelectric effect in PZT/NiFe/CoFe nanocomposites,” Int J Nanotechnol., vol 10, nos 3–4, pp 206–213, Jan 2013 [8] N T M Hong, N B Doan, N H Tiep, L V Cuong, B N Q Trinh, P D Thang, et al., “Switchable voltage control of the magnetic anisotropy in heterostructured nanocomposites of CoFe/NiFe/PZT,” J Korean Phys Soc., vol 63, no 3, pp 812–816, Aug 2013 [9] H Zhang and D Zeng, “Magnetostriction and its inverse effect in Tb0.3 Dy0.7 Fe2 alloy,” J Appl Phys., vol 107, no 12, pp 123918-1–123918-5, Jun 2010 [10] R C O’Handley, Modern Magnetic Materials: Principles and Applications New York, NY, USA: Wiley, 2000 [11] J T Heron, M Trassin, K Ashraf, M Gajek, Q He, S Y Yang, et al., “Electric-field-induced magnetization reversal in a ferromagnetmultiferroic heterostructure,” Phys Rev Lett., vol 107, no 21, pp 217202-1–217202-5, Nov 2011 ... dependence of magnetization M on voltage V for all samples Under applied electric voltage, PZT substrate is elongated in the film plane and the strain in PZT substrate leads to a strain in NiFe /CoFe layers... magnetization which occurs at certain applied voltage, called the magnetization reversed voltage Vrev (as shown in Fig 3) At this voltage, large strains produced in PZT can transfer to NiFe /CoFe. .. NiFe /CoFe and induce a change in the orientations of the magnetization, i.e., in the magnetic anisotropy This allows us to control magnetization switching by using an electric field instead of a