Magnetic Materials
Pressure Effects on Magnetic and Transport Properties in CoFe-Based Spin Valve
2020 Volume 61 Issue 8 Pages 1483-1486
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2020 Volume 61 Issue 8 Pages 1483-1486
We have studied the magnetoresistance of an enhanced-biased spin valve device under high pressure. The magnetoresistance decreases by 0.0014 up to 2 GPa with increasing pressure, which is inferred to be due to slight deviation from an antiparallel-spin configuration of the free and pinned layers. In the pressure range between 2 and 2.75 GPa, the exchange bias field generated in the pinned layer decreases and the coercivity of the free layers clearly increases by ∼5 Oe, which is likely to be related to less hydrostatic pressure.
Fig. 1 Magnetoresistance of the exchange-biased spin valve device at room temperature under high pressure. The resistance, R is normalized by that at +1200 Oe, RP. The R/RP of the antiparallel configuration of the pinned (P) and free (F) layers in the magnetic field range between −500 Oe and 0 Oe is suppressed gradually with increasing pressure.
Application of pressure to a material is a powerful experimental method in condensed matter physics because it unveils various exotic phenomena in some materials, like high-Tc supercondutivity of H2S.1) Furthermore, pressure also can systematically modify physical parameters associated with transport, thermal, optical and magnetic properties in the identical sample, which greatly helps us to interpret physics underlying the exotic phenomena.2) So far, tons of high-pressure experiments on bulk samples have been conducted. However, there exist very few experimental studies on pressure effects on spintronic devices with nano structures. One of the rare examples is pressure-induced enhancement of giant magnetoresistance (GMR) in [Fe(20 Å)/Cr(30 Å)]20 magnetic multilayers, which is due to a crossover from biquadratic to bilinear interlayer exchange coupling.3–6) Recent rapid evolution of spintronics has revealed a variety of novel phenomena related to the spin current in the ferromagnetic (FM)/nonmagnetic (NM) hybrid structures.7) In addition, band calculations have suggested that the spin polarization of Co-based Heusler alloys could be enhanced by high pressure.8) Since the enhancement of the spin polarization makes it more efficient to produce and detect a pure spin current, novel pressure effects on spintronic phenomena are expected. We think that a lot of such novel pressure effects on spintronic devices remain undiscovered. Accordingly, more and more high-pressure studies should be conducted. Thus, we have attempted to apply high pressure to spintronic devices to find out innovative pressure-induced effects.
In the present study, to examine pressure-dependence of GMR effect and mechanisms of exchange bias phenomena and to promote high-pressure research on spintronic devices, we have measured magnetoresistance (MR) of an exchange-biased spin valve (EBSV) device under high pressure. The EBSV device, which consists of antiferromagnetic (AFM), FM, NM, and FM layers, exhibits a striking MR in a rather small magnetic field.9,10) The spins of the FM layer (pinned layer), which is adjacent to the AFM layer, are reversed by a rather large external magnetic field (HP) due to the exchange bias field from the AFM layer.11,12) On the other hand, the spins of another FM layer (free layer) are easily reversed by a small magnetic field (HF). Thus, the antiparallel-spin configuration of the pinned and free layers appears between HP and HF, which gives rise to a high-MR state. The MR can be easily measured in a current in-plane (CIP) configuration by DC two-probe method. We report that application of pressure modifies the MR of an EBSV device, coercivity in the free layer and exchange bias field generated in the pinned layer.
We used an EBSV device consisting of substrate/Ta (5 nm)/Ru (2 nm)/IrMn (7 nm, AFM layer)/CoFe (2 nm, pinned FM layer)/Cu (3 nm, NM layer)/CoFe (2.5 nm, free FM layer)/Cu (1 nm) layers. The surface of the device was coated by SiO2 to prevent the device from being deteriorated by a pressure transmitting medium during experiments. The device and a manganin wire were put into a Teflon cell filled with Daphne oil 7373 (produced by Idemitsu Kosan) as a pressure transmitting medium. It has been reported that the Daphne oil 7373 is solidified at around 2 GPa and room temperature.13) Since the liquid pressure transmitting medium transmits an applied load to the sample isotropically in accordance with Pascal’s law, the solidification gives rise to loss of hydrostaticity and uniaxial characteristics of the applied pressure, which could affect the properties of the EBSV device. The manganin wire was used as a manometer. The Teflon cell was placed in a piston-cylinder-type pressure cell, consisting of inner (NiCrAl alloy) and outer (CuBe alloy) cylinders, an upper piston (WC alloy) and a lower plug (CuBe alloy).14) The hydrostatic pressure was generated up to 2.75 GPa by applying load to the Teflon cell through the piston. The pressure applied to the device was calibrated from a linear relationship between pressure and resistance of the manganin wire, P = α(ΔR/R).15) In advance, the coefficient α of the manganin wire used was determined to be 45.27 GPa by measuring resistance of the manganin wire and a bar-shaped specimen of Bi, which exhibits a marked decrease of resistance associated with a I–II phase transition at 2.55 GPa.16) The resistances of the manganin and Bi samples were recorded by a DC four-probe method. The MR of the EBSV device was measured in a current in-plane (CIP) configuration by a DC two-probe method in the magnetic field range between −1200 and +1200 Oe at room temperature.
Figure 1 shows pressure dependence of the MR of the EBSV device at room temperature. The data are normalized by the resistance at +1200 Oe, where the spins of the pinned and free layers are parallel to each other. The resistance at +1200 Oe is denoted as RP. With sweeping magnetic field from +1200 Oe to −1200 Oe, the R/RP jumps at around −40 Oe due to reversal of the spins of the free layer and gradually decreases to R/Rp = 1 at around −600 Oe due to reversal of the spins of the pinned layer. Then, with sweeping magnetic field from −1200 Oe to +1200 Oe, the R/RP increases gradually at around −250 Oe, drops at around +16 Oe, and approaches R/RP = 1. The high- and low-resistance states are ascribed to antiparallel- and parallel-spin configurations of the free and pinned layers, respectively. The hysteretic behavior between the positive and negative field sweeps is ascribed to coercivity of CoFe of the pinned and free layers. These are typical phenomena of an EBSV device. With applying pressure, the R/RP of the antiparallel-spin configuration is gradually suppressed, while that of the parallel-spin configuration is almost preserved. To evaluate the suppression of the R/RP of the antiparallel-spin configuration, the maximum of the R/RP, which is denoted as (R/RP)MAX, are plotted as a function of pressure in Fig. 2. The (R/RP)MAX decreases by Δ(R/RP)MAX = 0.0014 up to 2 GPa almost linearly to pressure and seems to be constant in the pressure range between 2 and 2.75 GPa. With decreasing pressure from 2.75 GPa, the (R/RP)MAX increases linearly and is likely to return to the original value before applying pressure, which indicates that the decrease in (R/RP)MAX with increasing pressure is intrinsic. In Co/Cu multilayers, it was reported that the MR ratio oscillates from large to small values as a function of the thickness of the Cu layers tCu, which is ascribed to oscillations of the interlayer exchange coupling of the Co layers from AFM to FM.17) Similarly, in the EBSV device studied, we infer that the decrease in tCu by the application of pressure gives rise to slight variation of the interlayer exchange coupling of the pinned and free CoFe layers from AFM toward FM. The variation of the interlayer exchange coupling slightly cants the spins of the pinned and free layers away from the antiparallel-spin configuration, which could resultantly decrease (R/RP)MAX. Here, we compare tCu dependence of the MR between the EBSV device and the Co/Cu multilayers. Assuming that the EBSV device is compressed isotropically, the decrease in tCu is estimated to be ΔtCu/tCu = −0.48% (ΔtCu = −0.015 nm for tCu = 3 nm) at 2 GPa from the bulk modulus of Cu (137.8 GPa18)). Thus, Δ(R/RP)MAX/ΔtCu amounts to 0.090 nm−1 for the present EBSV device. On the other hand, for the Co/Cu multilayers, Δ(MR ratio)/ΔtCu is evaluated to be 0.10 nm−1 at around tCu = 3.5 nm at T = 300 K from the MR ratio versus tCu curve, as shown in Fig. 2 of Ref. 17). These values are comparable to each other, which suggests that the observed decrease in (R/RP)MAX is due to variation of the interlayer exchange coupling of the pinned and free layers.
Magnetoresistance of the exchange-biased spin valve device at room temperature under high pressure. The resistance, R is normalized by that at +1200 Oe, RP. The R/RP of the antiparallel configuration of the pinned (P) and free (F) layers in the magnetic field range between −500 Oe and 0 Oe is suppressed gradually with increasing pressure.
Maximum of R/RP as a function of pressure. The open square and cross denote the data taken in the compressing and decompressing processes, respectively.
Next, to estimate magnetic fields where the spins of the pinned and free layers are reversed, we define magnetic fields at half maximum as HP1, HP2, HF1 and HF2, as shown in Fig. 3. The HP1, HP2, and the absolute value of average of the HP1 and HP2, associated with the pinned layer, are plotted as a function of pressure in Fig. 4. Though we cannot find a definite trend up to 2 GPa due to scattering of the HP1 and HP2, the magnitude of the HP2 decreases above 2 GPa. As a result, the absolute value of the average of HP1 and HP2, which corresponds to the exchange bias field generated in the pinned layer, has a tendency to decrease slightly from 440 Oe to 423 Oe above 2 GPa. This result might imply the exchange bias field from the IrMn layer is weakened with pressure. It was reported that the exchange bias field decreases as the fcc crystallinity of the IrMn layer becomes worse.12) As described in Experimental Procedures, the applied pressure could be less hydrostatic above 2 GPa due to the solidification of the Daphne oil 7373. The less hydrostatic pressure could distort the fcc structure of the IrMn layer into a lower-symmetry one, which possibly results in the decrease in the exchange bias field. On the other hand, the HF1 and HF2, which correspond to coercivity of the free layer, are plotted as a function of pressure in Fig. 5. The magnitudes of HF1 and HF2 are constant up to 2 GPa, but clearly increase by 4∼5 Oe above 2 GPa with increasing pressure, which indicates increase in the coercivity of the free layer. It has been reported that the Daphne oil 7373, which was used as a pressure-transmitting medium, is solidified at around 2 GPa and room temperature.19) The solidification gives rise to loss of hydrostaticity and uniaxial characteristics of the applied pressure. The rise of the coercivity seems to be related to the solidification. The less hydrostatic pressure condition above 2 GPa possibly introduces strains to the CoFe layers. The strains prevent magnetic domain walls from moving when spins of the free layers are reversed, resulting in the slight enhancement of the coercivity of the free layer. Similar effects should occur also in the pinned layers, but are invisible. This would be because the magnitudes of the HP1 and HP2 are too large and scattered to detect the variation of the HP1 and HP2 by 4∼5 Oe.
The HP1, HP2, HF1, and HF2 are defined as magnetic fields at half maximum of R/RP. The HP1 and HP2 are associated with the coercivity of the pinned layer. The HF1 and HF2 are associated with that of the free layer.
Pressure dependence of the HP1, HP2, and average of the magnitudes of HP1 and HP2. The average of the magnitudes decreases above 2 GPa.
Pressure dependence of HF1 and HF2. The magnitudes of the HF1 and HF2 increase clearly above 2 GPa.
We have reported the MR measurements of the EBSV device under high pressure to aim to promote high-pressure research on spintronic devices with nano structures. A decrease of the MR by 0.0014 with pressure is observed up to 2 GPa. This behavior is inferred to be due to slight deviation from an antiparallel-spin configuration of the pinned and free layers, which is caused by pressure-induced variation of the interlayer exchange coupling of the pinned and free layers. The Δ(R/RP)MAX/ΔtCu value evaluated from the present high-pressure data and the bulk modulus of Cu is comparable to the Δ(MR ratio)/ΔtCu estimated from the data reported previously for the Co/Cu multilayers. With increasing pressure from 2 GPa, the exchange bias field generated in the pinned layer decreases and the coercivity of the free layer clearly increases, which is likely to related to less hydrostatic (uniaxial) pressure due to the solidification of a pressure-transmitting medium (Daphne oil 7373). The former and latter phenomena are possibly caused by lowering a symmetry of the fcc structure of the IrMn layer and by strains introduced into the free layers, respectively.
We thank K. Yamanoi and T. Ogawa for their kind technical support. This work was partly supported by Qdai-jump Research Program from Kyushu University, by Grants-in-Aid for Scientific Research (C) (Grant Number 18K04875) from the Japan Society for the Promotion of Science, and by JSPS Program for Fostering Globally Talented Researchers, Grant Number JPMXS05R2900005.
