DSpace at VNU: Converse magnetoelectric effect in PZT NiFe CoFe nanocomposites

206 Int J Nanotechnology, Vol 10, Nos 3/4, 2013 Converse magnetoelectric effect in PZT/NiFe/CoFe nanocomposites Nguyen Thi Minh Hong, Nguyen Huu Duc and Pham Duc Thang* Laboratory for Micro and Nanotechnology, Faculty of Engineering Physics and Nanotechnology, University of Engineering and Technology, Vietnam National University, Hanoi, Building E3, 144 Xuan Thuy, Cau Giay, Hanoi, Vietnam Email: hongntm@vnu.edu.vn Email: ducnh@vnu.edu.vn Email: pdthang@vnu.edu.vn *Corresponding author Abstract: Magnetoelectric effect has attracted considerable interest due to the promising approach towards novel spintronics device In this work, we study the converse magnetoelectric properties of PZT/NiFe/CoFe nanocomposites in which the piezoelectric substrate is transversely polarised and the thickness of the NiFe layer is varied In these structures, the magnetic behaviour significantly changes under an applied voltage thanks to magnetoelectric coupling between the layers By changing the NiFe thickness, this change in magnetisation reaches up to 245% at low bias magnetic field Besides, the investigation of voltage controlled magnetisation switching is expounded These results are discussed in term of a stress-induced anisotropy field model and magnetoelectric coupling between two phases Keywords: nanocomposites; magnetisation magnetoelectric effect; voltage induced Reference to this paper should be made as follows: Minh Hong, N.T., Duc, N.H and Thang, P.D (2013) ‘Converse magnetoelectric effect in PZT/NiFe/ CoFe nanocomposites’, Int J Nanotechnology, Vol 10, Nos 3/4, pp.206–213 Biographical notes: Nguyen Thi Minh Hong is working as the researcher at the Laboratory for Micro and Nanotechnology and the Faculty of Engineering Physics and Nanotechnology, University of Engineering and Technology, Vietnam National University (VNU) in Hanoi, Vietnam She obtained her MSc degree in Electromagnetic Physics from the University of Natural Science, VNU At present, she is pursuing PhD programme and her current research interests are in the field of nanomagnetics, multifferroics and micronano ferroelectrics Nguyen Huu Duc obtained his PhD degree in Physics from the University of Hanoi in 1988 He has received the French Habilitation (DHR) in Physics at the Joseph Fourier University of Grenoble in 1997, became a full professor of the VNU in 2004 and professor of merit in 2008 He serves as the head of the Laboratory for Micro and Nanotechnology at University of Engineering and Technology, VNU He extended his research on various aspects of magnetism He is author of 100 scientific papers in various international journals and of Copyright © 2013 Inderscience Enterprises Ltd Converse magnetoelectric effect in PZT/NiFe/CoFe nanocomposites 207 five monographs published in recent volumes of the Handbook of Magnetic Materials and the Handbook on the Physics and Chemistry of Rare Earths (Elsevier Science Publisher) Pham Duc Thang obtained the PhD degree in Experimental Physics from the University of Amsterdam in 2003 From 2003 to 2006 he worked as a postdoctoral researcher at the University of Twente In 2006 he joined the University of Engineering and Technology, VNU as a research staff at the Faculty of Engineering Physics and Nanotechnology He became an associate professor of the university in 2011 His current research interests are focused on nanostructured magnetic materials, functional ferroelectrics, piezoelectrics and multiferroics, micro-nano fabrication and devices This paper is a revised and expanded version of a paper entitled ‘Converse magnetoelectric effect in PZT/NiFe/CoFe nanocomposites’ presented at the ‘3rd International Workshop on Nanotechnology and Application (IWNA’2011)’, Vung Tau, Vietnam, 10–12 November 2011 Introduction The magnetoelectric effect can be classified as direct magnetolectric effect (DME) and converse magnetoelectric effect (CME) that are characterised as magnetic field induced polarisation and electric field induced magnetisation, respectively [1] To date, most of the published papers are devoted to investigations of the DME effect However, there are only several works, reporting on the CME effect in the recent years For data storage applications, it is indeed both physically interesting and technologically important to quantitatively characterise the CME effect Thus, the multiferroic materials, which include single phase magnetoelectric system and two phase magnetoelectric system, are studied widely to obtain large CME effect [2–5] In this paper, we investigate the CME effect of PZT/NiFe/CoFe nanocomposites with various NiFe ferromagnetic thicknesses Besides, the voltage controlled magnetisation switching process will be discussed Experimental procedures In this work, the ferromagnetic layers of NiFe/CoFe were directly grown in sequence by an rf magnetron sputtering (2000F, AJA International Inc.) on polycrystalline PZT substrate with transverse polarisation (APC-855, American Piezoceramics Inc.) Before deposition, the sputtering chamber was vacuumed to a base pressure of 2×10–7 Torr A power of 50 W and Ar gas pressure of 2.2×10–3 Torr have been used for deposition of NiFe and CoFe In this hybrid structure, sputtering time for CoFe layer is fixed of 30 minutes Meanwhile, thickness of NiFe layer was changed by varying sputtering time from 10, 20, 40 and 60 minutes These samples are denoted as MN13, MN23, MN43 and MN63, respectively For the structure of the present study, the thickness of NiFe/CoFe is up to 100 nm and a 500 μm thick PZT substrate is used Finally, a Ta thin layer was sputtered on CoFe layer to prevent oxidisation for ferromagnetic layers For electrical measurement, we use silver adhensive glue, electrodes according to the polarisation direction of PZT substrate as the schematic shown in Figure The dimensions of this structure were 5×5 mm2 The magnetic and CME measurements were 208 N.T Minh Hong, N.H Duc and P.D Thang carried out by using a vibrating sample magnetometer (VSM 7404, Lakeshore) The sample was subjected to an external bias magnetic field and the applied voltage was also varied from –700 V to 700 V using a voltage amplifier Studies on the morphology and crystallographic structure have been analysed in [6] Figure Geometry and working principle for converse magnetoelectric effect (see online version for colours) Hbias Electrode Results and discussion The magnetisation (M) measured as a function of angle α between the applied magnetic field and film normal direction is shown in Figure for α = 0°, 45° and 90° One observes that in-plane magnetic anisotropy dominates for all samples due to the contribution of NiFe/CoFe ferromagnetic layers Magnetisation measured along the film plane, MS//, increases when increasing NiFe thickness It reaches a maximum for sample MN43 (as presented in Figure 3) Meanwhile, coercivity µoHC// has opposite tendency, showing a minimum value for MN43 sample This rule alters coincidental when measuring at various α angles In addition, the anomalous changing of MS// and µoHC// for MN43 sample indicates that magnetic properties can be optimised by choosing suitable buffer layer thickness It is noteworthy that an optimised thickness ratio of the magnetic and ferroelectric components is a prerequisite for obtaining large CME [7] Magnetic hysteresis loop of PZT/NiFe/CoFe composites with different NiFe thickness measured at various angles α (see online version for colours) 0.0025 90o 45 o 0o M(emu) 90 o 45 o 0o 0.0000 MN23 MN13 -0.0025 0.0025 90o 45 o 0o M(emu) Figure 90 o 45 o 0o 0.0000 MN63 MN43 -0.0025 -10000 -5000 μoH(Oe) 5000 10000 -10000 -5000 μoH(Oe) 5000 10000 Converse magnetoelectric effect in PZT/NiFe/CoFe nanocomposites Saturation magnetization MS// and coercivity μo HC// of samples (see online version for colours) 2500 50 2000 MS//(μemu) 100 μoHc// (Oe) Figure 209 μ oH C//(tNiFe) M S//(tNiFe) 0 20 40 1500 60 tNiFe (minutes) For study CME, an external voltage (V), varied from –700 V to 700 V, is applied to these samples The experiment is carried out at different magnetic field aligned parallel to sample plane The results, presented in Figure 4, display linear increasing tendency of M(V) curves are observed for all samples That corresponds closely to the ferroelectric hysteresis loop P(V) curve (see in Figure 5) This indicates that elastic stress transferred from piezoelectrics to ferromagnetic layers of NiFe/CoFe, resulted in an increase of magnetisation The change in magnetisation, ΔM, measured at 700 V and 50 Oe are listed in Table for all samples In the range of applied voltage, the relative change in magnetisation is actually large, up to 245, 220, 170 and 225%, for sample MN13, MN23, MN43 and MN63, respectively Dependence of magnetisation on the applied voltage M(V) at different magnetic field (see online version for colours) M(μemu) 3000 2400 μoH=50 Oe 1800 μoH=1000 Oe 1200 μoH=5 Oe 2400 μoH=50 Oe 1800 μoH=1000 Oe μoH=200 Oe μoH=2000 Oe μoH=200 Oe μoH=2000 Oe 1200 600 600 0 MN13 -600 -600 3000 M(μemu) 3000 μoH=5 Oe M(μemu) Figure MN23 -600 -400 -200 μoH=5 Oe μoH=5 Oe 2400 μoH=50 Oe μoH=50 Oe μoH=200 Oe μoH=200 Oe 1800 μoH=1000 Oe μoH=1000 Oe μoH=2000 Oe μoH=2000 Oe 200 400 200 400 600 V(V) 1200 600 -600 MN43 -600 -400 -200 V(V) 200 400 600 MN63 -600 -400 -200 V(V) 600 210 Figure N.T Minh Hong, N.H Duc and P.D Thang Dependence of polarization on the applied voltage of PZT substrate (see online version for colours) 0.10 P(μC/cm ) 0.05 0.00 -0.05 -0.10 -300 -200 -100 100 200 300 V(V) Table The magnetisation change ΔM, the relative change in magnetisation ΔM/M and the magnetisation reversed voltage Vrev (taken at Oe) Sample ΔM (µemu) ΔM/M (%) Vrev (V) MN13 760 245 –300 MN23 700 220 –500 MN43 540 170 –600 MN63 570 225 –250 Furthermore, we observed the magnetisation switching under the applied voltage for each sample At a magnetic field, magnetisation reversal appears at a certain voltage which is called as the magnetisation reversed voltage Vrev (listed in Table for the magnetic field of Oe) In principle, the application of an electric field can induce a stress and therefore a change in magnetic moment orientation, thanks to the magnetostriction of the magnetic materials The NiFe layer plays a role in affecting voltage induced magnetisation switching process For MN43 sample, the value of Vrev is larger than that of others and this result also reflects the above mentioned magnetic hysteresis measurements Figure shows the voltage induced magnetisation susceptibility χVIM measured on sample MN13 at various magnetic field from Oe to 2000 Oe Firstly, χVIM has a positive value at high applied voltage When decreasing applied voltage, χVIM increases to a maximum, then cancels at fixed voltage (magnetisation reversed voltage, Vrev) and finally changes its sign For different magnetic fields, these values are different The higher bias magnetic field is, the higher Vrev requires Consequently, magnetisation switching can be decided by the competition between the bias magnetic field energy and applied electric field energy Recently, future spintronic devices based on new materials with voltage controllable magnetic properties are very attractive for practical applications because of lower power consumption compared to the present conventional magnetic-controlled counterparts Through voltage induced stress/strain by the piezoelectric effect, the voltage controllable magnetisation can be realised As shown in Figure 7, the sample MN13 expressed an Converse magnetoelectric effect in PZT/NiFe/CoFe nanocomposites 211 obvious influence on the magnetic hysteresis loops while applying the voltage along the sample plane It clearly reveals the increases in coercivity, from 63 Oe to 80 Oe (manipulated by 21%) at applied voltage of 600 V Figure Electric field induced magnetisation susceptibility (χVIM) of sample MN13 (see online version for colours) μoH=5 Oe χVIM (abs.unit) μoH=50 Oe μoH=200 Oe μoH=1000 Oe μoH=2000 Oe -3 MN13 -6 -600 -400 -200 200 400 600 V(V) Figure In-plane magnetic hysteresis loops of sample MN13 at different applied voltage (see online version for colours) 0.0010 600V 0V M(emu) 0.0005 0.0000 -0.0005 MN13 -0.0010 -200 -100 100 200 μoH(Oe) The change in coercivity reflects the dependence of magnetoelastic anisotropy in the magnetic layers on stress, originated from out-of-plane lattice distortions in PZT crystal This dependence can be caused by the following factors: the direction of the stressinduced anisotropy field Hσ regarding that of the external field H; the coercivity mechanism and the contribution to the effective magnetic anisotropy field energy In this case, the analysis uses the Stoner-Wohlfrauth relation between coercivity and stressinduced anisotropy field In order to analyse the effective magnetic anisotropy field we consider first the magnetoelastic energy which is given by [8]: E me = K me sin θ (1) 212 N.T Minh Hong, N.H Duc and P.D Thang where the stress anisotropy constant Kme is expressed as: K me = λσ (2) λ is the average magnetostriction coefficient quantifying the relative length change of the sample between the demagnetised and magnetised states, σ is the induced stress and θ is the angle between the magnetisation M and the direction of σ (as illustrated in Figure 8) Apparently, Kme>0 favours θ=0° which means the parallel alignment of the magnetisation direction relative to the stress axis, meanwhile Kme0 NiFe/CoFe layers have the magnetostriction coefficient λ>0 and therefore σ>0, indicating the stress in those layers is tensile This is in accordance with the above increase in magnetisation when changing applied voltage Figure Scheme of stress-induced magnetic anisotropy for sample with an in-plane easy axis (see online version for colours) The effective magnetic anisotropy constant, Keff, of ferromagnetic layer NiFe/CoFe in contact with PZT is the sum of several anisotropy contributions and can be expressed as: K eff = K V + 2K S t NiFe/ CoFe − μo M S + K me (3) where KV is the volume magnetocrystalline anisotropy constant, the second term is the surface anisotropy with its constant KS for a film thickness of tNiFe/CoFe, the third term is the shape anisotropy which favours an easy in-plane magnetisation, and the last term is the magnetoelastic anisotropy term Besides, we have: μo H C = 2K eff MS (4) Substituting equations (2) and (4) into equation (3), one can get the formula of the stress as: ⎛ ⎞ 2K S μ μo H C MS − ⎜ K V + + o MS2 ⎟ t NiFe/ CoFe ⎝ ⎠ σ= 3λ (5) Converse magnetoelectric effect in PZT/NiFe/CoFe nanocomposites 213 Hence, the sign of stress allows us determining if it is compressive or tensile Our recent study indicates that the sign of stress is positive This can be the main cause of an increase in magnetisation when applying an external voltage Theoretical work related to these results is under investigation and will be published elsewhere Conclusion We have presented in this paper the new results on converse magnetoelectric effect in the nanocomposite PZT/NiFe/CoFe A large change in magnetisation induced by voltage has been observed This indicates that the elastic stress transfers from ferroelectric substrate to ferromagnetic layers Based on the model of stress-induced magnetic anisotropy and the coupling between two phases, we also demonstrate that the stress, which is imposed by the PZT substrate on NiFe/CoFe layer, is tensile Furthermore, the effect of applied voltage and bias magnetic field on magnetisation switching process is also discussed These results are promising for future spintronic applications Acknowledgements This research was supported by project 103.02.87.09 of the Vietnam National Foundation for Science and Technology Development (NAFOSTED) References Eerenstein, W., Wiora, M., Prieto, J.L., Scott, J.F and Mathur, N.D (2007) ‘Giant sharp and persistent converse magnetoelectric effects in multiferroic epitaxial heterostructures’, Nature Materials, Vol 6, pp.348–351 Jia, Y.M., Or, S.W., Chan, H.L.W., Zhao, X.Y and Luo, H.S (2006) ‘Converse magnetoelectric effect in laminated composites of PMN–PT single crystal and Terfenol-D alloy’, Apply Physics Letter, Vol 88, No 24, pp.2902–2904 Wan, J.G., Liu, J.M., Wang, G.H and Nan, C.W (2006) ‘Electric-field-induced magnetization in Pb(Zr,Ti)O3/Terfenol-D composite structures’, Apply Physics Letter, Vol 88, No 18, pp.2502–2504 Zhou, J.P., Meng, L., Xia, Z.H., Liu, P and Liu, G (2008) ‘Inhomogeneous magnetoelectric coupling in Pb(Zr,Ti)O3/Terfenol-D laminate composite’, Apply Physics Letter, Vol 92, No 6, pp.2903–2905 Huang, Z (2006) ‘Theoretical modeling on the magnetization by electric field through product property’, Journal of Applied Physics, Vol 100, No 11, pp.4104–4108 Minh Hong, N.T., Thang, P.D., Tiep, N.H., Cuong, L.V and Duc, N.H (2011) ‘Voltagecontrollable magnetic behavior in PZT/NiFe/CoFe nanocomposites’, Advanced in Natural Sciences: Nanoscience and Nanotechnology, Vol 2, No 1, pp.5015–5019 Nan, C.W., Bichurin, M.I., Don, S and Viehland, D (2008) ‘Multiferroic magnetoelectric composites: historical perspective, status, and future directions’, Journal of Applied Physics, Vol 103, No 3, pp.1101–1135 Cullity, B.D (1972) Introduction to Magnetic Materials, Addison-Wesley, Reading, MA  ... include single phase magnetoelectric system and two phase magnetoelectric system, are studied widely to obtain large CME effect [2–5] In this paper, we investigate the CME effect of PZT/ NiFe/ CoFe nanocomposites. .. NiFe/ CoFe ⎝ ⎠ σ= 3λ (5) Converse magnetoelectric effect in PZT/ NiFe/ CoFe nanocomposites 213 Hence, the sign of stress allows us determining if it is compressive or tensile Our recent study indicates... -10000 -5000 μoH(Oe) 5000 10000 Converse magnetoelectric effect in PZT/ NiFe/ CoFe nanocomposites Saturation magnetization MS// and coercivity μo HC// of samples (see online version for colours) 2500