Spin injection into vanadium dioxide films from a typical ferromagnetic metal, across the metal-insulator transition of the vanadium dioxide films

A vanadium dioxide VO2 film shows metal-insulator transition (MIT) induced by changing environmental temperature. We report the temperature dependence of electromotive force properties generated in VO2/Ni80Fe20 bilayer junctions under the ferromagnetic resonance (FMR) of the Ni80Fe20 layer. An electromotive force generated in a VO2/Ni80Fe20 bilayer junction under the FMR showed a small change across the MIT temperature of the VO2 film, while the VO2 film resistance drastically changed. This behavior was not only explained with the temperature dependence of the electromotive force property generated in the Ni80Fe20 film itself under the FMR, but also with the generated electromotive forces due to the inverse spin-Hall effect (ISHE) in the VO2 film under the FMR of the Ni80Fe20 film. That is, we successfully demonstrated the spin injection from a Ni80Fe20 film into a VO2 film across the MIT temperature of the VO2 film.

2][3] Up to now, a Si-based spin transistor operated by applying a voltage has been demonstrated, 3 and to switch a spin current through light irradiation have been challenged.Meanwhile, there are no studies for switching a spin current with an environmental temperature change, not due to the so-called spin-Seebeck effect. 4In this study, to switch a spin current by a temperature change, a vanadium dioxide VO 2 film which shows metal-insulator transition (MIT) with the crystal structure change induced by changing environmental temperature is focused. 5The MIT in VO 2 films usually occurs in the temperature range between 280 K and 350 K, depending on the film thickness, 5,6 and a thinner VO 2 film shows an MIT at lower temperature.Recently, the spin injection into a VO 2 film from a ferrimagnetic insulator Y 3 Fe 5 O 12 (YIG) film by using the spin-pumping was achieved across the MIT temperature. 7However, for the practical use, to prepare high quality YIG films is needed and basically hard.Thus, in this study, the spin injection from a typical ferromagnetic metal Ni 80 Fe 20 film into aVO 2 film and the controllability of the spin current in VO 2 with environmental temperature change across the MIT are tried.We report the temperature dependence of electromotive force properties generated in VO 2 /Ni 80 Fe 20 bilayer junctions under the ferromagnetic resonance (FMR) of the Ni 80 Fe 20 layer.
Our sample structure and experimental set up are illustrated in Figures 1(a) and (b).A spin injection effect into a VO 2 film is observed as follows: in VO 2 /Ni 80 Fe 20 bilayer junctions, a pure spin-current,   , is generated in the VO 2 layer by the spin-pumping of the Ni 80 Fe 20 induced by the FMR. 8The generated spin current is converted to a charge current,   , with the inverse spin-Hall effect (ISHE) 8 in the VO 2 , which is expressed as, 10   ∝     × , where   and  are the spin-Hall angle in VO 2 films and the spin-polarization vector of the   , respectively.The converted   is detected as an electromotive force, , via the sample resistance, .Therefore, the  is expressed as, 9,11  =    ∝      × . ( That is, if the electromotive force due to the ISHE in VO 2 is detected under the FMR of the Ni 80 Fe 20 layer, it is clear evidence of the achievement of the spin injection from a Ni 80 Fe 20 layer into a VO 2 layer.We analyze the origins of the obtained electromotive forces under the FMR with some control experiments, and conclude that the spin injection from a Ni 80 Fe 20 film into a VO 2 film has been achieved and the spin current generation efficiency in VO 2 films by the spin pumping has been changed by the MIT.VO 2 thin films were formed on TiO 2 (001) single crystal substrates using a pulsed laser deposition technique (ArF excimer:  = 193 nm).The target employed was a sintered vanadium oxide pellet of V 2 O 5 (Kojundo Chemical Lab.Co., Ltd., 99.9%).The fabrication condition of VO 2 films was at a substrate temperature of 450 °C in an O 2 gas pressure of 1.0 Pa.This fabrication process is almost same as the previous result where epitaxial VO 2 films have been obtained. 6The VO 2 film thickness was set to be 10 nm.After the VO 2 depositions, the sample substrate was transferred to another vacuum chamber for the preparation of metal films.This transfer process and the next Ni 80 Fe 20 deposition process were implemented as soon as possible to keep the surface state of the VO 2 films because of the surface sensitivity to the air.An electron beam (EB) deposition technique was used to deposit Ni 80 Fe 20 (Kojundo Chemical Lab.Co., Ltd., 99.99%) to a thickness of 25 nm on the VO 2 films with a shadow mask, under a vacuum pressure of <10 -6 Pa.The deposition rate and the substrate temperature during Ni 80 Fe 20 depositions were set to be 0.1 nm/s and an ambient temperature, respectively.Finally, palladium (Pd: Furuuchi Chemical Co., Ltd., 99.99% purity) as electrodes was deposited by EB deposition through another shadow mask, under a vacuum pressure of <10 -6 Pa.The deposition rate and the substrate temperature during Pd depositions were also set to be 0.1 nm/s and an ambient temperature, respectively.
Electrical properties were evaluated using a two-probe method with a source measure unit (Keithley Instruments, 2614B).The electrical property evaluation was implemented in a small vacuum chamber equipped with a temperature variable stage using a Peltier device.To excite FMR in Ni 80 Fe 20 for investigation of the spin injection effect into VO 2 films, a coplanar waveguide connected to a vector network analyzer (VNA: KEYSIGHT Technology, N5221A) and a couple of electromagnets were used.A nano-voltmeter (Keithley Instruments, 2182A) to detect electromotive forces from the samples was used.This spin-pumping experiment system also equips with another Peltier device under the sample mount position.Leading wires for detecting the output voltage properties were directly attached at the two Pd electrodes of samples with silver paste.
Figure 1(c) shows a temperature dependence of resistance of a VO 2 film.The sample temperature was changed with a rate of 2 K/min, monitoring with a thermocouple.The VO 2 film resistance changes by three orders of magnitude and shows a hysteresis in this temperature range, reflecting the typical MIT of VO 2 films. 5,6Moreover, the VO 2 resistance is steeply changed which is a typical evidence that the VO 2 film is almost epitaxially grown. 6he inset in Figure 2(a) shows an FMR spectrum for a VO 2 /Ni 80 Fe 20 bilayer junction at 300 K, where an external magnetic field orientation angle to the sample film plane, , is 0° and the frequency of the high frequency electrical current, f, is 5 GHz.The FMR field of the Ni 80 Fe 20 film, H FMR , is 325 Oe, and the saturation magnetization of the Ni 80 Fe 20 film, M S , is estimated to be 671 G under the FMR conditions in the case of the in-plane field: 12   = √  (  + 4  ), where  and  are the angular frequency 2 and the gyromagnetic ratio of 1.86×10 7 G -1 s -1 of Ni 80 Fe 20 , respectively.The main panel of Fig. 2(a) shows output voltage properties of the same VO 2 /Ni 80 Fe 20 bilayer junction under the FMR of the Ni 80 Fe 20 layer at 300 K. Clear output voltage properties have been observed and the voltage sign has been inverted at the magnetization reversal of the Ni 80 Fe 20 layer.This voltage sign inversion associated with the magnetization reversal in Ni 80 Fe 20 is a characteristic of the typical ISHE. 9,13,14The solid lines in Fig. 2(a) are the curve fits obtained using the following equation: 9,14 () =    2 ( −   ) 2 where H is an external static magnetic field and  denotes the damping constant (18.0 Oe in this study).The first and second terms in the eq.( 4) correspond to the symmetry term to H (e.g. the ISHE) and the asymmetry term to H (e.g. the anomalous Hall effect and other effects showing the same asymmetric voltage behavior relative to the H, like parasitic capacitances), respectively.  and   correspond to the coefficients of the first and second terms in eq. ( 4), respectively.On the other hand, Fig. 2  Figure 3 shows temperature dependences of the   of a VO 2 /Ni 80 Fe 20 bilayer sample (circles) and the sample resistance (a solid line).As the VO 2 film changed from an insulator to a metal, the   became slightly small.This behavior is not only be explained with the electromotive force property generated in the Ni 80 Fe 20 film itself under the FMR 15 since the resistance of VO 2 films is drastically changed across the MIT and the ferromagnetic transition temperature is much higher than the temperature range in this study.Thus, to explain the small change of the   against the temperature, the   and the injected   into VO 2 films must be changed by controlling the environmental temperature under the relationship expressed as the eq.( 2).This is reasonable because the crystal structure of VO 2 is changed across the MIT, that is, the electronic properties in VO 2 films are drastically changed across the MIT.The electromotive force generated in the interface between the VO 2 and Ni 80 Fe 20 films in the VO 2 /Ni 80 Fe 20 junctions may be considered by using, for example, other MIT materials 5 and it is an interesting study, whereas this is also excluded by the shunting effect in the VO 2 film.That is, the ISHE in VO 2 films is dominant as origins of observed electromotive forces in the VO 2 /Ni 80 Fe 20 junctions under the FMR.Thus, we concluded that the spin injection into VO 2 films was achieved by the spin-pumping using a Ni 80 Fe 20 film.Finally, in order to investigate the spin current generation efficiency by the spin pumping, we focused on the real part of the spin mixing conductance,   ↑↓ , which represents the parameter to determine the spin pumping efficiency at the interface between the Ni 80 Fe 20 and the VO 2 layers.The   ↑↓ is expressed as, 16   ↑↓ = 4      ( −  0 ), where ,   ,   , , and  0 are the Landé g-factor, the Bohr magneton, the thickness of the Ni 80 Fe 20 layer, the Gilbert damping constant, and the intrinsic Gilbert damping constant of the Ni 80 Fe 20 , respectively.The   ↑↓ depends on the .In other words, the spin current generation efficiency depends on the . Figure 4(a) shows a frequency dependence of the half width at half maximum resonance linewidth, ∆, for a VO 2 /Ni 80 Fe 20 bilayer junction in the temperature range across the MIT.The  is obtained using the following equation: 17 where ∆ 0 is the frequency independent ∆.

FIG. 1 .
FIG. 1.(a) Bird's-eye-view and (b) top-view illustrations of our sample and orientations of external applied magnetic field H used in the experiments.JS and E correspond to the spin current generated in a VO2 film by the spin-pumping and the electromotive forces due to the ISHE in a VO2 film, respectively.(c) Temperature dependence of a VO2 film resistance.
(b) shows the output voltage properties of a Cu/Ni 80 Fe 20 bilayer junction under the FMR of the Ni 80 Fe 20 layer at 300 K.No clear output voltages were observed at the H FMR of the Ni 80 Fe 20 layer.Since Cu has small spin orbit interaction to generate the ISHE, this behavior is reasonable.This suggests the observed electromotive forces in the VO 2 /Ni 80 Fe 20 bilayer junctions under the FMR of the Ni 80 Fe 20 layer are mainly due to the ISHE in VO 2 films.Also, we can say that the electromotive force generated in the Ni 80 Fe 20 layer itself under the FMR is shunted in the Cu layer.This similar shunting effect for the samples with VO 2 can also be taken into account to explain the results.

FIG. 2 .
FIG. 2. (a) Output voltage properties of a VO2/Ni80Fe20 bilayer junction under the FMR of the Ni80Fe20 layer at 300 K.The inset shows the FMR spectrum for the bilayer junction at 300 K, and at the  of 0°.(b) Output voltage properties of a Cu/Ni80Fe20 bilayer junction under the FMR of the Ni80Fe20 layer at 300 K.The inset shows the FMR spectrum for the bilayer junction at 300 K, and at the  of 0°.

FIG. 3 .
FIG. 3. Temperature dependences of the electromotive forces generated in a VO2/Ni80Fe20 bilayer junction under the FMR of the Ni80Fe20 film (circles) and the sample resistance (a solid line).

Fig. 4 (
b) shows temperature dependence of the  of a VO 2 /Ni 80 Fe 20 bilayer junction (red circles) and a Ni 80 Fe 20 single layer sample (black circles).The  of a VO 2 /Ni 80 Fe 20 bilayer junction increased at a higher temperature, while the  of a Ni 80 Fe 20 single layer sample is almost constant in this temperature range.Since the   ↑↓ depends on the , we have concluded that the spin current generation efficiency by the spin pumping is affected by the MIT.The temperature dependence of the  of the VO 2 /Ni 80 Fe 20 bilayer junction is also an evidence of the spin injection effect from the Ni 80 Fe 20 layer into the VO 2 layer.

FIG. 4 .
FIG. 4. (a) Frequency dependence of the half width at half maximum resonance linewidth, ∆, for a VO2/Ni80Fe20 bilayer junction in the temperature range of the MIT.(b) Temperature dependences of the Gilbert damping constant, α, of a VO2/Ni80Fe20 bilayer junction and a Ni80Fe20 single layer sample.