Session P15: Focus Session: Spins in Metals - Spin Current Generation and Spin Motive Force

Slonczewski-like torque due to Rashba spin-orbit coupling in thin magnetic layers

Recent experiments report that large Rashba spin-orbit coupling (RSOC) can exist for ferromagnetic nanostructure with structural asymmetry. Previous theories of spin-transfer torque (STT) induced by RSOC addressed this problem by introducing an effective field due to RSOC. However, the Rashba field is not sufficient, and another STT perpendicular to the Rashba STT is required to explain experimental results. In this work, we propose the mechanism of such STT based on nonadiabaticity and examine the effect of the torque. By studying the domain wall (DW) motion, we demonstrate that the DW motion is significantly affected by our result. We show that the effect of the RSOC-induced torque on the DW velocity can be so large that the DW can move along the current direction without assuming negative nonadiabaticity or spin polarization. Our result is discussed in comparison with experimental results.   

Spin-orbit coupling of Pt studied by circular dichroism in soft x-ray ARPES

Pt has a large spin-orbit coupling (SOC) and is reported to exhibit the largest spin Hall conductivity among all materials studied to date [1,2]. To establish the role of SOC in the electronic structure, we investigate the bulk electronic structure of Pt(111) using circular dichroism (spin-orbit dichroism) in soft x-ray (SX)-ARPES. We have measured band dispersions along $\Gamma$-K-X, L-W and $\Gamma$-L and the complete set of Fermi surfaces of Pt. Calculated band dispersions including SOC gives a very good match with the experimental results [2,3], thus demonstrating the role of SOC. Our results also show a $k$-dependent suppression of spin-orbit dichroism, implying a $k$-dependent quenching of the spin polarization [3]. [1] T. Kimura {\it et al.}, Phys. Rev. Lett. {\bf 98}, 156601 (2007). [2] G. Y. Guo {\it et al.}, Phys. Rev. Lett. {\bf 100}, 096401 (2008). [3] M. Gradhand {\it et al.}, Phys. Rev. B {\bf 80}, 224413 (2009).   

Spin pumping with coherent elastic waves

The generation and detection of pure spin currents is an important topic for spintronic applications. Spin currents may be generated, e.g., via spin pumping. In this approach, a precessing magnetization relaxes via the emission of a spin current. Conventionally, electromagnetic waves, i.e. microwave photons, are used to drive the magnetization precession. We here show that a spin current can also be pumped by means of an acoustic wave, i.e. microwave phonons. In the experiments, coherent surface acoustic wave (SAW) phonons with a frequency of 1.55 GHz traverse a ferromagnetic thin film/normal metal (Co/Pt) bilayer. The SAW phonons drive the resonant magnetization precession via magnetoelastic coupling [1]. We use the inverse spin Hall voltage in the Pt film as a measure for the generated spin current and record its evolution as a function of time and external magnetic field magnitude and orientation. Our experiments show that a spin current is generated in the exclusive presence of a resonant elastic excitation. This establishes acoustic spin pumping as a resonant analogue to the spin Seebeck effect and opens intriguing perspectives for applications in, e.g., micromechanical resonators. \\[4pt] [1] M. Weiler \textit{et al.}, Phys. Rev. Lett. \textbf{106}, 117601 (2011)   

Giant spin accumulation and long-distance spin precession in metallic lateral spin valves

The non-local spin injection technique in lateral spin valves (LSVs) has provided not only scientific interests to study spin transport and relaxation in a nanowire but also potential spintronic device applications. The non-local method involves no charge-current flow but spin accumulation in the nonmagnetic wire. In order to increase the output signal, related to the spin accumulation splitting for electrochemical potential, efficient spin injection into nonmagnet from ferromagnet as well as high applied current are indispensable. The spin resistance mismatch between the ferromagnet and the nonmagnet needs to be overcome by using high interface resistance such as tunnel barrier for the efficient spin injection, while a low junction resistance is preferred for applying high current. In this talk, I will discuss a guideline to design metallic LSVs with high output signal. The interface resistance of around 0.1 $\Omega$$\mu$m2, several orders of magnitude smaller than that of a typical tunnel junction, could effectively overcome the spin resistance mismatch in the metallic system [1] and leads to the giant spin accumulation signal over 200 $\mu$V in LSVs with NiFe/MgO/Ag junctions [2,3]. The Hanle effect measurements demonstrate a long-distance collective 2$\pi$ spin precession along the 10 $\mu$m long Ag wire. This result will accelerate the development of novel spintronic devices utilizing the pure spin current and the spin accumulation.\\[4pt] [1] Y. Fukuma, L. Wang, H. Idzuchi and Y. Otani, Appl. Phys. Lett. 97, 012507(2010).\par [2] Y. Fukuma, L. Wang, H. Idzuchi, S. Takahashi, S. Maekawa and Y. Otani, Nature Mater. 10, 527(2011).\par [3] L. Wang, Y. Fukuma, H. Idzuchi, G. Yu, Y. Jiang and Y. Otani, Appl. Phys. Exp. 4, 093004(2011).   

Temperature and temporal evolution of nonlocal spin signals

An unusual non-monotonic temperature dependence of spin signals was previously observed for nanoscale metallic nonlocal spin valves (NLSV): The spin signal increases as the temperature decreases from room temperature, reaches a maximum value around 50 K, and then decreases as the temperature approaches 4 K. This has been interpreted as due to a high rate of surface spin-flip scattering in the nonmagnetic channel, but the origins of the high surface spin-flip rate are yet to be understood. In this work, we show that for an as fabricated Py-Cu NLSV device this temperature dependence is clearly observed. The device was then stored in the ambient environment for a period of 5 months. Afterwards, an increase of the spin signals was found, and more interestingly the temperature dependence became monotonic. From room temperature to 50 K the spin signal increases, but from 50 K to 4 K the spin signal levels off instead of decreasing further. We conclude that the surface spin-flip scattering originates from the magnetic impurities embedded in the Cu channel near the side surfaces. The impurities are introduced into the Cu during the fabrication procedure. Upon oxidizing the Cu in the ambient environment, the surface impurities are buried in copper oxide and become less accessible to the conduction electrons. Therefore the surface spin-flip rate is reduced over time, resulting in a larger spin signal and monotonic temperature dependence.   

Continuous dc Spinmotive Force in a Patterned Ferromagnetic Film

Recently, a motive force of spin origin, i.e., a ``spinmotive force", has been theoretically predicted[1] and experimentally observed[2,3]. A spinmotive force reflects the spin of electrons in an essential manner and is a new concept relevant to electronic devices. However, problems remain, one of which is the continuous generation of such a spinmotive force. In this study, we present experiment and theory that demonstrate the continuous generation of a dc spinmotive force by exciting a ferromagnetic resonance of a single comb-shaped ferromagnetic thin film[4]. Experimental results are well reproduced by theoretical calculations, offering a quantitative and microscopic understanding of this spinmotive force. \newline [1] S. Barnes and S. Maekawa, PRL 98, 246601 (2007). [2] S. A. Yang et al. PRL 102, 067201 (2009). [3] P. N. Hai et al. Nature 458, 489 (2009). [4] Y. Yamane, K. Sasage et al., to appear in PRL.   

Theory of anomalous Hall and spin Hall effects in ferromagnetic metals

We give a unified theory of the anomalous Hall effect (AHE) and the spin Hall effect (SHE) in ferromagnetic metals (FM). We do this by extending Kondo's theory of the AHE in FM and including the short range spin-spin correlations. We find a novel relation between the SHE and the second order nonlinear spin fluctuation in FM near Curie temperature Tc, which has been hidden for about 50 years, since Kondo gave a relation between the AHE and the first order nonlinear spin fluctuation in pure FM near Tc in 1962. Our results show an essential difference between the AHE and SHE in terms of the symmetry. Our theory can be applied to the recent SHE experiment in FM near Tc by Y. Otani et al.   

Spin motive force in a Rashba system

Spin motive force (SMF) is the nonconservative spin force induced by magnetization dynamics. In addition to spin-transfer torque (STT), SMF is considered as one of the representative interactions between magnetization and conduction electrons. It has been reported that large Rashba-type spin-orbit coupling (RSOC) can show up in magnetic nanostructures containing thin ferromagnetic layers with structural asymmetry. In this work, we investigate the effect of RSOC on SMF. The additional SMF induced by RSOC is proportional only to time variation of magnetization while the conventional SMF is proportional to both time and space variation of magnetization. This result indicates that SMF can be induced from even homogeneous magnetic structure. The resulting RSOC SMF has so large magnitude that the feedback effect on the magnetization dynamics can be significant. Combining RSOC SMF and the previously known Rashba STT effect, we derived that Gilbert damping is one or two orders of magnitude enhanced by RSOC. Lastly, we show that the symmetry breaking nature of RSOC and the induced SMF greatly help the electrical distinction of magnetization structure. It is expected that RSOC will play a key role in extending applicable area of SMF.   

Light-induced pure spin current

We propose theoretically that a pure spin current without an accompanying charge current can be generated by light in magnetic tunneling junctions. The principle is based on a photovoltaic effect combined with the spin selectivity of the magentic electrodes of the junction. We demostrate this effect in graphene nanostructures by atomic first principles calculation. The results show that appreciable pure spin-currents and open circuit spin bias are generated in pure graphene nanostructures, and it can reach significant values if half metal with high spin polarization is used as the electrodes.   

Perpendicular switching of a single ferromagnetic layer induced by in-plane current injection

We demonstrate switching of a ferromagnetic Pt/Co/AlOx layer with perpendicular anisotropy through lateral current-injection. Magnetization reversal occurs as an unpolarized electric current is injected parallel to an in-plane magnetic field of moderate magnitude. The switching direction depends on the sign of the current with respect to that of the in-plane field. The critical switching current scales with the lateral dimensions of the layer and duration of the current pulse. Our measurements also indicate that the switching efficiency increases with the magnetic anisotropy of the Co layer and oxidation of the top Al layer. The symmetry of this effect corresponds to an in-plane torque perpendicular to the current. We will discuss possible contributions to this torque, including Rashba-induced spin accumulation and the spin Hall effect [1]. \\[0pt] [1] I. M. Miron, K. Garello, G. Gaudin, P.-J. Zermatten, M. V. Costache, S. Auffret, S. Bandera, B. Rodmacq, A. Schuhl, and P. Gambardella, Nature 476, 189 (2011).   

Current induced magnetization dynamics in spin-orbit coupled thin film ferromagnets

We consider the effect of an in-plane current on the magnetization dynamics of a two-dimensional spin-orbit coupled nanoscale itinerant ferromagnet. By solving the appropriate kinetic equation for an itinerant electron ferromagnet, we show that Rashba spin-orbit interaction provides transport currents with a switching action, as observed in a recent experiment (I. M. Miron \textit{et al}., Nature 476, 189 (2011)). The dependence of the effective switching field on the magnitude and direction of an external magnetic field in our theory agrees well with experiment. We comment on the possibility of finding materials in which this spin-orbit switching effect can be achieved at moderate current densities   

Spin-flipping at Sputtered Co/Ag Interfaces

We measured at 4.2K the Current-Perpendicular-to-Plane Magnetoresistances (CPP-MRs) of sputtered ferromagnetically coupled [Co(3 nm)/Ag(1.8 or 2.0 nm)]$_{n}$Co(3 nm) multilayers with $n$ = 0 to 8 that were imbedded in the middle of symmetric, Py-based, double exchange-biased spin-valves. The measurements yielded the parameter for spin-flipping at the Co/Ag interface, $\delta _{Co/Ag}=\mathop {0.30}\nolimits_{-0.1}^{+0.05} $. $\delta $ is related to the probability $P$ of spin-flipping at the Co/Ag interface by $P$ = [(1-exp(-$\delta )$]. Despite the expected larger lattice-mismatch-induced disorder at the Co/Ag interface, this value is similar to that reported earlier for the Co/Cu interface [1], suggesting that such disorder does not play a major role in $\delta _{F/N}$ at ferromagnetic/non-magnetic (F/N) interfaces. \\[4pt] [1] B. Dassonneville et al., Appl. Phys. Lett. 96, 022509 (2010).   

Large non-inverted and inverted spin signals in break-junction-based nonlocal spin valves

A nonlocal spin valve (NLSV) is a lateral structure with a ferromagnetic (F) spin injector, an F spin detector and a nonmagnetic (N) channel. A pure spin current can be generated in the N channel by electrical injection through the injector, and can be detected as a spin signal between the spin detector and the N channel. For a typical metallic NLSV, one expects a regular spin signal meaning that the nonlocal resistance is high for the parallel (P) states of the injector and the detector and low for the antiparallel (AP) states. We investigate spin signals of NLSV's with a break junction formed between the detector and the copper N channel by electrostatic discharge. We observed both non-inverted and inverted spin signals with large magnitudes. An inverted spin signal has low nonlocal resistance values for the P states and high values for the AP states. The magnitude is up to 90 milliohms at 4 K and up to 30 milliohms at 300 K, larger than the typical metallic NLSV with similar dimensions. The large magnitude can be explained by the spin-charge coupling across the highly resistive break-junction. The signs of the spin signals (non-inverted vs. inverted) are determined by the local spin-dependent density of states near the break-junction.