Abstract
SrTiO 3 (100) sub/BiFeO 3/CoFe/Ru/CoFe/Cu/CoFe/Ta structure was prepared by a combination of chemical solution deposition and sputtering method, and followed by a systematical investigation for the structural, magnetic and magnetoresistance properties at room temperature (RT) as a function of CoFe and Ru thicknesses. It was revealed that introduction of synthetic CoFe/Ru/CoFe as a pinning layer increased the giant magentoresistance (MR) ratio to 8.3 at RT. This enhancement of MR ratio might be attributed to (i) the increase of pinning field, and (ii) suppression of the influence of the surface roughness of BiFeO 3 by inserting the synthetic CoFe/Ru/CoFe layer.
| Original language | English |
|---|---|
| Article number | 072901 |
| Journal | Applied Physics Letters |
| Volume | 101 |
| Issue number | 7 |
| DOIs | |
| Publication status | Published - 2012 Aug 13 |
ASJC Scopus subject areas
- Physics and Astronomy (miscellaneous)
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Enhancement of magnetoresistance using CoFe/Ru/CoFe synthetic ferrimagnetic pinned layer in BiFeO 3 based spin-valves. / Naganuma, Hiroshi; Bae, In Tae; Miyazaki, Takamichi; Kubota, Miho; Inami, Nobuhito; Kawada, Yuki; Oogane, Mikihiko; Mizukami, Shigemi; Han, X. F.; Ando, Yasuo.
In: Applied Physics Letters, Vol. 101, No. 7, 072901, 13.08.2012.Research output: Contribution to journal › Article › peer-review
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TY - JOUR
T1 - Enhancement of magnetoresistance using CoFe/Ru/CoFe synthetic ferrimagnetic pinned layer in BiFeO 3 based spin-valves
AU - Naganuma, Hiroshi
AU - Bae, In Tae
AU - Miyazaki, Takamichi
AU - Kubota, Miho
AU - Inami, Nobuhito
AU - Kawada, Yuki
AU - Oogane, Mikihiko
AU - Mizukami, Shigemi
AU - Han, X. F.
AU - Ando, Yasuo
N1 - Funding Information: 3 based spin-valves Naganuma Hiroshi 1 a) Bae In-Tae 2 Miyazaki Takamichi 3 Kubota Miho 1 Inami Nobuhito 1 Kawada Yuki 1 Oogane Mikihiko 1 Mizukami Shigemi 4 Han X. F. 5 Ando Yasuo 1 1 Department of Applied Physics, Graduate School of Engineering, Tohoku University , 6–6–05 Aoba Aramaki Aoba-ku, Sendai 980–8579, Japan 2 Small Scale Systems Integration and Packaging Center, State University of New York at Binghamton , Binghamton, New York 13902, USA 3 Tohoku University , Department of Instrumental Analysis, 6-6-11 Aoba, Aramaki, Aoba, Sendai 980-8579, Japan 4 WPI Advanced Institute for Materials Research, Tohoku University , Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan 5 Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences , Beijing 100190, China a) E-mail addresses: naganuma@mlab.apph.tohoku.ac.jp and hrhr2004jp@hotmail.com . 13 08 2012 101 7 072901 24 03 2012 12 07 2012 13 08 2012 2012-08-13T09:37:11 2012 American Institute of Physics 0003-6951/2012/101(7)/072901/3/ $30.00 SrTiO 3 (100) sub/BiFeO 3 /CoFe/Ru/CoFe/Cu/CoFe/Ta structure was prepared by a combination of chemical solution deposition and sputtering method, and followed by a systematical investigation for the structural, magnetic and magnetoresistance properties at room temperature (RT) as a function of CoFe and Ru thicknesses. It was revealed that introduction of synthetic CoFe/Ru/CoFe as a pinning layer increased the giant magentoresistance (MR) ratio to 8.3% at RT. This enhancement of MR ratio might be attributed to (i) the increase of pinning field, and (ii) suppression of the influence of the surface roughness of BiFeO 3 by inserting the synthetic CoFe/Ru/CoFe layer. 23656025 BiFeO 3 (BFO) material has been attracting much attention because antiferromagnetic plane switch can occur by switching ferroelectric domain of 71° and 109° at room temperature (RT). 1–3 However, due to antiferromagnetic spin configuration of BiFeO 3 , magnetic switching can not easily be detected macroscopically. In the case of BiFeO 3 -based spin-valve structure, if the ferroelectric domains of 71° and 109° are antiferromagnetically coupled with pinning magnetic layer, magnetization switching can be induced by polarization switching of the pinning layer. This means that magnetoresistance can be controlled by electric field through ferroelectric domain switching, which is useful for the voltage controlled magnetic random access memory (V-MRAM) application. There have been several reports on the magnetoresistance 4–6 and the electric field control of magnetoresistance change 7 for BiFeO 3 -based spin-valve. However, pinning effect of BiFeO 3 is weak and the magnetoresistance ratio is below 2% at RT due to imperfection of parallel (P) and antiparallel (AP) spin configuration. In this study, we prepared BiFeO 3 /CoFe bilayer and BiFeO 3 -based spin-valve structure using CoFe/Ru/CoFe synthetic ferromagnetic, and investigated the magnetic and magnetoresistance properties at RT with various thicknesses of CoFe and Ru in order to enhance the magentoresistance (MR) ratio in the BiFeO 3 -based spin-valves. BiFeO 3 films were prepared by chemical solution deposition (CSD) method on SrTiO 3 (100) substrates, and followed by postannealing at 600 °C for 10 min in air. The lattice mismatch between SrTiO 3 (100) ( a = 0.3905 nm) and bulk BiFeO 3 (100) ( a = 0.3952 nm) is 1.20%. The film thickness ( t BFO ) of BiFeO 3 was approximately 100 nm. The detail of preparation process of CSD was mentioned elsewhere. 8 The metal magnetic layers of Co 50 Fe 50 ( t CF )/Ta cap and Co 50 Fe 50 (4)/Ru( t Ru )/Co 75 Fe 25 (4)/Cu(3)/Co 75 Fe 25 (4)/Ta cap were prepared by an ultra-high-vacuumed d.c. magnetron sputtering with applying magnetic field of 140 Oe along in-plane direction (the direction of applying magnetic field is called as easy axis). We made a selection of Co 50 Fe 50 material as pinning layer based on to our previous study. 9 In order to create synthetic structure, the thickness of Ru ( t Ru ) was systematically changed from 0.4 to 0.8 nm in accordance with our previous study. 10 The surface morphology of the films was examined by atomic force microscopy (AFM; SPI3800N, SII). The crystal structure and orientation were determined by x-ray diffraction analysis (XRD; D8 Discover, Bruker Co., Ltd.). The magnetic properties were measured by superconducting quantum interference device (SQUID) magnetometer (MPMS, Quantum design). The current-in-plane magnetoresistance at RT was measured by conventional four probe method. Figure 1 shows the XRD patterns and an AFM image exhibiting surface morphology of BiFeO 3 films ( t BFO = 100 nm). The XRD peaks of BiFeO 3 appeared left side of peaks of SrTiO 3 substrate. Phi-scan measurement using ( 2 ¯ 02) BiFeO 3 showed that the epitaxial growth of BiFeO 3 on SrTiO 3 substrate. Therefore, the peak shift can be considered that the BiFeO 3 was compressed for in-plane direction due to large unit cell compared with SrTiO 3 , and then the BiFeO 3 was expanded to out-of-plane direction. The secondary phases and metal layer peaks were not confirmed in the XRD profiles. The crystal symmetry was confirmed by x-ray reciprocal space mapping using (204) peak. The crystal symmetry of BiFeO 3 was rhombohedral structure with space group of R 3c, which is consistent with our previous study. 11 The surface morphology of BiFeO 3 showed roughness ( R a ) of 1.0 nm and a few spike projections were confirmed in the AFM image. Figure 2 shows the magnetic properties of the SrTiO 3 (100)/BiFeO 3 (100 nm)/Co 50 Fe 50 (1 ≤ t CF ≤ 4 nm)/Ta cap bilayers with various thicknesses of CoFe layer. H ex , the exchange bias field, was determined from a shift of the centers of the magnetization ( M-H ) curves along the field axis. Here, the easy axis is parallel to the applied magnetic field during deposition of metal layers and the hard axis is perpendicular to easy axis in the in-plane dimension. While the H ex was observed for the CoFe thickness between 2 and 4 nm, it disappeared at with the CoFe thickness ( t CF ) of 1 nm. The M-H curves for t CF = 1 nm was hard to saturate. [Fig. 2(d) ] It is worth noting that the shape of M-H curves for the hard axis also showed almost the same shape as M-H curve for easy axis (not shown), which indicates the H ex was distributed for the film plane. When focused on squareness of the M-H curves, the roundish shape was still existed for all samples, which indicate the effect from the surface roughness was also reflected even for the thicker samples. The saturation magnetization in dimension ( M s in emu/cm 2 ) linearly decreased with decreasing the t CF and fitting line almost came across the point of origin. This indicates that the dead layer between BiFeO 3 and CoFe might be small. Thus, it can be considered that the distribution of H ex at t CF = 1 nm might be attributed to the surface roughness of BiFeO 3 . For the spin-valve structure, the thick CoFe ( t CF = 4 nm) was used for pinning layer in order to suppress the influence of roughness. Figure 3 shows the M-H curves and magnetoresistance curves for SrTiO 3 (100)/BiFeO 3 (100 nm)/Co 50 Fe 50 (4)/Ru( t Ru )/Co 75 Fe 25 (4)/Cu(3)/Co 75 Fe 25 (4)/Ta cap. The thickness of Ru ( t Ru ) layer varied between 0.4 and 0.8 nm. The MR ratio increased with increasing the thickness of Ru with the largest MR ratio of 8.3% that was observed at t Ru = 0.8 nm. To the best of our knowledge, this is the highest MR ratio at RT reported to date using the BiFeO 3 based spin-valves. 4–6 7 For the t Ru = 0.4 nm, pinning field and H ex estimated from the M-H curve was small compared with t Ru = 0.6 and 0.8 nm, which indicates the AP and P states were clearly achieved in the case of t Ru = 0.6 and 0.8 nm. Two possible reasons for the large MR ratio could be considered as follows. The pinning field oscillates as a function of Ru thickness and the maximum was at second peak of 0.8 nm in accordance with our previous study using the same sputtering machine. 10 Thus, the large pinning field created at 0.8 nm leads to the obvious AP and P state, which is the first reason for enhancement of MR ratio. As described in the Fig. 1(d) , the BiFeO 3 layer had surface roughness of R a = 1.0 nm and the spike projections were periodically observed in the AFM image. This relative large surface roughness might be the reason for the interfacial spin scattering and/or magnetic coupling between pinned layer and free layer, these degrade the MR ratio. The insertion of the synthetic pinning layer with the total thicknesses above 8.0 nm, reduced the influence of surface roughness of BFO, which is the second reason for the enhancement of MR ratio. According to previous study, 12 extrinsic H ex due to the distribution of spin arrangement still existed at least above R a = 0.5 nm, which implies that the synthetic layer might effectively enhance MR ratio even though the surface was relatively flat. However, the influence of roughness on MR ratio needs further systematic investigation by using samples with various surface roughness values in near future. The SrTiO 3 sub./BFO(100)/Co 50 Fe 50 ( t CoFe )/Ta cap and SrTiO 3 sub./BFO(100)/Co 50 Fe 50 (4)/Ru( t Ru )/Co 75 Fe 25 (4)/Cu(3)/Co 75 Fe 25 (4)/Ta films were prepared by combination of CSD and sputtering methods. The MR ratio increased with increasing the t Ru of synthetic pinning layer with the maximum MR ratio of 8.3% obtained at t Ru = 0.8 nm. When the BiFeO 3 directly face to CoFe layer, the spin direction of CoFe layer was distributed approximately to the thickness of 1.0 nm due to the roughness from BiFeO 3 , which can be reduced by inserting the synthetic layer. The enhancement of MR ratio by inserting the synthetic structure is attributed to the strong synthetic coupling and reduction of influence of surface roughness from BiFeO 3 . We thank Professor S. Okamura in Tokyo University of Science for preparing the samples of the BiFeO 3 films. This work was partly supported by Grant-in-Aid for challenging Exploratory Research (Grant No. 23656025), by JSPS through FIRST program, by NSFC-JSPS international Joint Research Fund, by Grant for Advanced Industrial Technology Development by NEDO, and by research grant from TANAKA HOLDINGS Co. Ltd., and ITB is also grateful to New York Center of Excellence, New York State Office of Science, Technology, and Innovation, and Empire State Development Corporation for partial financial support. FIG. 1. (a) θ/2θ XRD profile, (b) expanded θ/2θ XRD profile at around BFO (300), (c) phi-scan profile using ( 2 ¯ 02), and (d) surface morphology of BiFeO 3 . FIG. 2. (a)–(d) M-H curves of SrTiO 3 (100) sub./BiFeO 3 /CoFe/Ta with various CoFe thicknesses ( t CoFe ), (e) exchange bias field ( H ex ), and (f) saturation magnetization in dimension ( M s ). FIG. 3. M-H curves and magnetoresistance curves for (a) SrTiO 3 (100)/BiFeO 3 (100 nm)/Co 50 Fe 50 (4)/Cu(3)/Co 75 Fe 25 (4)/Ta cap, (b)–(d) SrTiO 3 (100)/BiFeO 3 (100 nm)/Co 50 Fe 50 (4)/Ru( t Ru )/Co 75 Fe 25 (4)/Cu(3)/Co 75 Fe 25 (4)/Ta cap.
PY - 2012/8/13
Y1 - 2012/8/13
N2 - SrTiO 3 (100) sub/BiFeO 3/CoFe/Ru/CoFe/Cu/CoFe/Ta structure was prepared by a combination of chemical solution deposition and sputtering method, and followed by a systematical investigation for the structural, magnetic and magnetoresistance properties at room temperature (RT) as a function of CoFe and Ru thicknesses. It was revealed that introduction of synthetic CoFe/Ru/CoFe as a pinning layer increased the giant magentoresistance (MR) ratio to 8.3 at RT. This enhancement of MR ratio might be attributed to (i) the increase of pinning field, and (ii) suppression of the influence of the surface roughness of BiFeO 3 by inserting the synthetic CoFe/Ru/CoFe layer.
AB - SrTiO 3 (100) sub/BiFeO 3/CoFe/Ru/CoFe/Cu/CoFe/Ta structure was prepared by a combination of chemical solution deposition and sputtering method, and followed by a systematical investigation for the structural, magnetic and magnetoresistance properties at room temperature (RT) as a function of CoFe and Ru thicknesses. It was revealed that introduction of synthetic CoFe/Ru/CoFe as a pinning layer increased the giant magentoresistance (MR) ratio to 8.3 at RT. This enhancement of MR ratio might be attributed to (i) the increase of pinning field, and (ii) suppression of the influence of the surface roughness of BiFeO 3 by inserting the synthetic CoFe/Ru/CoFe layer.
UR - http://www.scopus.com/inward/record.url?scp=84865430311&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=84865430311&partnerID=8YFLogxK
U2 - 10.1063/1.4745504
DO - 10.1063/1.4745504
M3 - Article
AN - SCOPUS:84865430311
VL - 101
JO - Applied Physics Letters
JF - Applied Physics Letters
SN - 0003-6951
IS - 7
M1 - 072901
ER -