Cite this Article
Hao Runrun, Zang Ruxue, Zhou Tie, Kang Shishou, Yan Shishen, Liu Guolei, Han Guangbing, Yu Shuyun, Mei Liangmo. Magnetization-direction-dependent inverse spin Hall effect observed in IrMn/NiFe/Cu/YIG multilayer structure. Chinese Physics B, 2019, 28(3): 037202  
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Magnetization-direction-dependent inverse spin Hall effect observed in IrMn/NiFe/Cu/YIG multilayer structure
Hao Runrun, Zang Ruxue, Zhou Tie, Kang Shishou †, Yan Shishen, Liu Guolei, Han Guangbing, Yu Shuyun, Mei Liangmo
School of Physics and State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China

 

† Corresponding author. E-mail: skang@sdu.edu.cn

Project supported by the National Basic Research Program of China (Grant No. 2015CB921502), the National Natural Science Foundation of China (Grant Nos. 11474184 and 11627805), the 111 Project, China (Grant No. B13029), and the Fundamental Research Funds of Shandong University, China.

Abstract

The magnetization-direction-dependent inverse spin Hall effect (ISHE) has been observed in NiFe film during spin Seebeck measurement in IrMn/NiFe/Cu/yttrium iron garnet (YIG) multilayer structure, where the YIG and NiFe layers act as the spin injector and spin current detector, respectively. By using the NiFe/IrMn exchange bias structure, the magnetization direction of YIG (MYIG) can be rotated with respect to that of NiFe (MNiFe) with a small magnetic field, thus allowing us to observe the magnetization-direction-dependent inverse spin Hall effect voltage in NiFe layer. Compared with the situation that polarization direction of spin current (σs) is perpendicular to MNiFe, the spin Seebeck voltage is about 30% larger than that when σs and MNiFe are parallel to each other. This phenomenon may originate from either or both of stronger interface or bulk scattering to spin current when σs and MNiFe are perpendicular to each other. Our work provides a way to control the voltage induced by ISHE in ferromagnets.

PACS: 72.25.Mk;72.25.Rb;72.25.-b;75.70.Cn
Keyword:spin current;magnetic insulator;inverse Hall effect
1. Introduction

Generation and manipulation of pure spin current, which have attracted a great deal of attention, play important roles in spintronic devices.[1,2] Spin pumping,[3–6] spin Hall effect (SHE),[7,8] and spin Seebeck effect (SSE) provide several means to generate pure spin current.[8–10] Particularly, SSE in ferromagnetic insulators is an attractive one, which generates the spin current by a thermal gradient applied across a ferromagnetic insulator without any charge flow. When a spin current is injected into an adjacent material with large spin–orbit coupling, a transverse charge current will be induced in the direction perpendicular to both its polarization direction and flow direction via inverse spin Hall effect (ISHE).[4,11] The non-magnetic 5d metals, such as Pt, Ta, and W, are the common materials for spin current detectors.[12] In addition, comparatively large ISHE signals in ferromagnetic metals (FMs) have also been observed.[13–18] However, the situation in FM is more complicated since the conducting electrons are polarized, which means that the numbers of electrons with opposite spin are not equal. The scattering strength on spin electrons in FM depends on the relative orientation of the spin-polarized charge current and FM magnetization (MFM), which has been observed in the FM/normal mental/FM (or FM/insulator/FM) spin valve structure.[19–22] Very recently, in a ferroic collinear multilayer spin valve, the detection of the spin current emitted by ferromagnetic resonance spin pumping has been reported.[23] In that case, the amplitude of the spin current depends on the relative alignment of yttrium iron garnet (YIG) and Co magnetization. The dependence of spin dissipation on magnetization direction at the normal mental/FM interface has been observed,[24,25] and the magnetization direction dependence of pure spin current in FMs when the polarization direction of the spin current (σs) is non-colinear with MFM has attracted much attention.[26,27]

In this paper, we provide a scheme to observe the magnetization-direction-dependent ISHE in FM. In the IrMn/NiFe/Cu/YIG multilayer structure shown in Fig. 1, we used the ferromagnetic insulating YIG thin film as the spin injector, and the ferromagnetic metal NiFe layer as the spin detector. Due to the interfacial exchange anisotropy between NiFe and IrMn, the NiFe shows up unidirectional anisotropy. Since the YIG film is magnetically soft in plane, the magnetization direction of the YIG (MYIG) layer can be rotated by a small magnetic field while the magnetization direction of the NiFe (MNiFe) layer is fixed. As shown in Figs. 1(a) and 1(b), the MNiFe is parallel and perpendicular to MYIG, respectively. Thus the σs of the spin current emitted by the SSE due to the thermal gradient across the YIG layer can be parallel or perpendicular to MNiFe. Therefore, the ISHE voltages in the NiFe layer in both cases can be observed. The Cu layer was inserted between the NiFe and YIG layers to clearly decouple the two layers.[15] Since the thickness of the Cu layer is much smaller than its spin diffusion length and it has very weak spin–orbit coupling, the spin current can pass through the Cu layer in the IrMn/NiFe/Cu/YIG sample with the ISHE voltage induced in the Cu layer negligible.

Fig. 1. Schematic setup for measuring the inverse spin Hall effect (ISHE) in NiFe layer induced by longitudinal spin Seebeck. Here, ΔT and M denote the temperature gradient and magnetization directions of ferromagnets, respectively. An exchange bias field is induced in NiFe. The orientations of spin current which is emitted by the longitudinal spin Seebeck are parallel and perpendicular to the magnetization direction of the NiFe layer in panels (a) and (b), respectively.
2. Experimental details

The YIG films of 100 nm were deposited on (111) single crystal substrates of gadolinium gallium garnet (GGG). The YIG/GGG films were annealed at 85 °C in a tube furnace flowing oxygen gas for 2 h. After annealing the YIG film, the IrMn(8)/NiFe(8)/Cu(4) were deposited on YIG, and then 100 nm Au electrodes were also deposited. A a control sample of IrMn(8)/NiFe(8)/Cu(4) on naturally oxidized Si substrate (SiO2-Si) was also prepared. A 10 nm SiO2 capping layer was deposited to protect from oxidation for all samples. All films were prepared with a magnetron sputtering system at room temperature, and the base pressure was better than 3×10−6 Pa. The IrMn/NiFe/Cu and the Au films were both patterned with shadow masks, whose sizes are shown in Fig 2(b). During deposition of IrMn(8)/NiFe(8)/Cu(4), an in-plane static field of about 400 Oe was applied to induce an easy magnetization axis in the NiFe/IrMn layers. As shown in Fig. 2(a), the morphology of the YIG film, with a root mean square (RMS) roughness of 0.21 nm, tracks the atomically flat terraces. The magnetic hysteresis loop of the YIG film measured by an alternative gradient magnetometer (AGM) with magnetic field swept in the film plane at room temperature is shown in Fig. 2(c). The YIG film is magnetically soft with an in-plane saturation field of about 30 Oe, and its saturation magnetization is determined to be about 1.59 kG (4πMs). As shown in Fig. 2(d), the out-of-plane saturation field is roughly equal to the value of 4πMs.

Fig. 2. (a) Atomic force microscopy (AFM) image of 100 nm YIG film on GGG (111). (b) The size of the sample and the measurement method. Here, thermal gradient ΔT is along the z direction, the easy magnetization axis of the NiFe layer is along the +y direction, and the rotatable magnetic field is applied in the x–y plane. (c) The in-plane hysteresis loop of 100 nm YIG. (d) The out-of-plane hysteresis loop of 100 nm YIG.

The longitudinal SSE measurement method is shown in Fig. 2(b). A temperature gradient was applied along the z direction. The DC voltages were measured with Keithley 2182 nanovoltmeters between electrodes a and b (y direction, Vab), and c and d (x direction, Vcd) with a rotatable magnetic field applied in the x–y plane. The easy magnetization axis of NiFe was along the +y direction.

3. Results and discussion

To compare the amplitudes of SSE voltages for MNiFe⊥σs (Vab) and σs ∥ MNiFe (Vcd), MNiFe and MYIG should be fixed and rotatable with a rotatable magnetic field, respectively, during the SSE measurement in the sample of IrMn(8)/NiFe(8)/Cu(4)/YIG(100)/GGG. A small rotatable magnetic field of 35 Oe, which is larger than the saturation field of YIG, was applied to rotate MYIG. Before the SSE measurement with a small magnetic field, a large magnetic field of 1000 Oe was first applied along the +y direction (the easy axis of NiFe) to saturate MNiFe. After removing the magnetic field of 1000 Oe, the SSE measurement with a rotatable magnetic field of 35 Oe was performed.

The IrMn(8)/NiFe(8)/Cu(4)/YIG(100)/GGG magnetic hysteresis loop with a magnetic field swept along the easy magnetization axis of NiFe is shown in Fig. 4(a). The bias field of about 230 Oe is much larger than the rotatable magnetic field of 35 Oe. To examine whether MNiFe was changed with the small rotatable magnetic field of 35 Oe, the comparison measurement was performed on IrMn(8)/NiFe(8)/Cu(4)/SiO2. When MNiFe rotated with the applied magnetic field, both anomalous Nernst and spin Seebeck voltages would be induced due to the thermal gradient across the NiFe layer in the IrMn(8)/NiFe(8)/Cu(4)/SiO2. As shown in Fig. 3(b), compared to that for H = 1000 Oe, the voltage for H = 35 Oe was negligible, indicating a nearly unchanged MNiFe in IrMn(8)/NiFe(8)/Cu(4)/SiO2 when a magnetic field of 35 Oe was applied. As shown in Fig. 3(a), the bias field of NiFe in the sample of IrMn(8)/NiFe(8)/Cu(4)/SiO2 is about 200 Oe, which is a bit smaller than that of NiFe in the sample of IrMn(8)/NiFe(8)/Cu(4)/YIG(100)/GGG. Therefore, MNiFe in the sample of IrMn(8)/NiFe(8)/Cu(4)/YIG(100)/GGG should be nearly unchanged when a small rotatable magnetic field of 35 Oe was applied. The magnetization direction dependence of SSE voltages measured in the sample of IrMn(8)/NiFe(8)/Cu(4)/YIG(100)/GGG with a small rotatable magnetic field of 35 Oe is shown in Fig. 4(b). We can find that the amplitude of Vcd is about 30% larger than that of Vab, indicating a larger SSE voltage induced for σs ∥ MNiFe (Vcd) than that for σs⊥ MNiFe (Vab). When a rotatable magnetic field of 1000 Oe was applied in the sample of IrMn(8)/NiFe(8)/Cu(4)/YIG(100)/GGG, MYIG and MNiFe were both saturated and σs was always parallel to MNiFe. As shown in Fig. 4(c), the nearly identical amplitudes of Vab and Vcd are observed during the SSE measurement with a rotatable magnetic field of 1000 Oe. We should point out that the value of (Rab−Rcd)/(Rab+Rcd) is less than 1.5%, where Rab and Rcd are the resistances between electrodes a and b, and c and d, respectively. Therefore the amplitude difference of Vab and Vcd (as shown in Fig. 4(b)) cannot result from the non-uniformity of the sample. In addition, the spin current emitted by the SSE in the YIG layer may flow into the IrMn layer after passing through the Cu and NiFe layers. If we assume that the spin diffusion length of NiFe λNiFe = 2.5 nm,[28] the spin current flowing into the IrMn layer after passing through the 8 nm NiFe layer will be very weak, thus allowing us to ignore the ISHE voltage induced in the IrMn layer.

Fig. 3. (a) The in-plane hysteresis loop of IrMn(8)/NiFe(8)/Cu(4)/SiO2 by a magnetic field sweeping along the easy magnetization axis of NiFe. (b) Magnetization direction dependence of SSE voltages for the sample of IrMn(8)/NiFe(8)/Cu(4)/SiO2 with rotatable magnetic fields of 35 Oe and 1000 Oe.
Fig. 4. (a) The IrMn(8)/NiFe(8)/Cu(4)/YIG(100)/GGG in-plane hysteresis loop by sweeping a magnetic field along the easy magnetization axis of NiFe. Panels (b) and (c) show the magnetization direction dependence of the SSE voltages for the sample of IrMn(8)/NiFe(8)/Cu(4)/YIG(100)/GGG with rotatable magnetic fields of 35 Oe and 1000 Oe, respectively.

In general, the VSSE induced in the sample of IrMn(8)/NiFe(8)/Cu(4)/YIG(100)/GGG can be expressed as[14]

a product of three terms. The first term includes the spin injection coefficient A, the thermal gradient ΔT across the YIG layer, and the sample dimension L. The second term includes the electrical resistivity ρNiFe and the spin Hall angle of NiFe. The third term describes the decay of pure spin current due to spin diffusion length λNiFe and layer thickness tNiFe of NiFe. We assume that the spin Hall angles of NiFe for σs⊥MNiFe and σs ∥ MNiFe are the same. The scattering strength to spin current at NiFe/Cu interface, which is related to the interfacial spin injection coefficient A, may be different. The value of A for σs ∥ MNiFe may be larger than that for σs⊥MNiFe, thus allowing more spin current injection into the NiFe layer for σs ∥ MNiFe. This may induce a larger ISHE voltage for σs ∥ MNiFe than that for σs⊥MNiFe. Besides, the bulk scattering strength may be different in the two cases, where different spin diffusion lengths in NiFe may be induced. In comparison with that for σs ∥ MNiFe, the bulk scattering strength to spin current for σs⊥MNiFe may be stronger, which may induce a shorter spin diffusion length in the NiFe layer. If we assume that λNiFe = 2.5 nm[15] for σs ∥ MNiFe and λNiFe < 2.5 nm for σs⊥MNiFe, the value of the third term of Eq. (1) for σs⊥MNiFe is less than that for σs ∥ MNiFe, indicating a samller voltage induced for σs⊥MNiFe. In the SMR measurement, the larger spin dissipation for σs⊥MNiFe at the interface of normal mental/FM has been observed.[24,25,29] Therefore, the smaller SSE voltage for σs⊥MNiFe than that for σs ∥ MNiFe may originate from either or both of stronger interface and bulk scatterings for σs⊥MNiFe.

4. Conclusion

The magnetization-direction-dependent ISHE has been observed in IrMn/NiFe/Cu/YIG multilayer structure. When a small rotatable magnetic field is applied, the magnetization direction of the YIG layer can be rotated, while the magnetization direction of the NiFe layer is fixed by the exchange bias field. By this means, we can regulate the relative orientation of σs and MNiFe. The spin Seebeck voltage for σs ∥ MNiFe is about 30% larger than that for σs⊥MNiFe. This amplitude difference of SSE signals may originate from either or both of stronger interface scattering at Cu/NiFe and bulk scattering in NiFe when σs⊥MNiFe. Our result provides a way to control the ISHE voltage induced in ferromagnets.

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