arXiv subjects

YoshiChika Otani

Publications and source records attributed to YoshiChika Otani.

At least 37 records · Page 2Linked to original sources

arXiv2019

Relation between spin Hall effect and anomalous Hall effect in 3$d$ ferromagnetic metals

We study the mechanisms of the spin Hall effect (SHE) and anomalous Hall effect (AHE) in 3$d$ ferromagnetic metals (Fe, Co, permalloy (Ni$_{81}$Fe$_{19}$; Py), and Ni) by varying their resistivities and temperature. At low temperatures where the phonon scattering is negligible, the skew scattering coefficients of the SHE and AHE in Py are related to its spin polarization. However, this simple relation breaks down for Py at higher temperatures as well as for the other ferromagnetic metals at any temperature. We find that, in general, the relation between the SHE and AHE is more complex, with the temperature dependence of the SHE being much stronger than that of AHE.

cond-mat.mes-hall↗

arXiv2018

Active Control of Mode Crossover and Mode Hopping of Spin Waves in a Ferromagnetic Antidot Lattice

Active control of spin-wave dynamics is demonstrated using broadband ferromagnetic resonance in two-dimensional Ni80Fe20 antidot lattices arranged in hexagonal lattice with fixed lattice constant but varying antidot diameter. A strong modification in the spin-wave spectra is obtained with the variation in the antidot diameter as well as with the strength and orientation of the bias magnetic field. A broad band of modes is observed for the lattice with higher antidot diameter which decreases systematically as the antidot diameter is reduced. A crossover between the higher frequency branches is achieved in lattices with higher antidot diameter. In addition, the spin-wave modes in all lattices show a strong six-fold anisotropic behaviour due to the variation of internal field distribution as a function of the bias-field orientation. A mode hopping-like behavior is observed in the angular dispersions of spin-wave spectra for samples having intermediate hole diameters. Micromagnetic simulations qualitatively reproduce the experimentally observed spin-wave modes and the simulated mode profiles reveal the presence of extended and quantized standing spin-wave modes in these lattices. These observations are significant for large tunability and anisotropic propagation of spin waves in GHz frequency magnetic devices.

cond-mat.mes-hall↗

arXiv2018

Detection of the interfacial exchange field at a ferromagnetic insulator-nonmagnetic metal interface with pure spin currents

At the interface between a nonmagnetic metal (NM) and a ferromagnetic insulator (FI) spin current can interact with the magnetization, leading to a modulation of the spin current. The interfacial exchange field at these FI-NM interfaces can be probed by placing the interface in contact with the spin transport channel of a lateral spin valve (LSV) device and observing additional spin relaxation processes. We study interfacial exchange field in lateral spin valve devices where Cu spin transport channel is in proximity with ferromagnetic insulator EuS (EuS-LSV) and yttrium iron garnet Y$_3$Fe$_5$O$_{12}$ (YIG-LSV). The spin signals were compared with reference lateral spin valve devices fabricated on nonmagnetic Si/SiO$_2$ substrate with MgO or AlO$_x$ capping. The nonlocal spin valve signal is about 4 and 6 times lower in the EuS-LSV and YIG-LSV, respectively. The suppression in the spin signal has been attributed to enhanced surface spin-flip probability at the Cu-EuS (or Cu-YIG) interface due to interfacial spin-orbit field. Besides spin signal suppression we also found widely observed low temperature peak in the spin signal at $T \sim$30 K is shifted to higher temperature in the case of devices in contact with EuS or YIG. Temperature dependence of spin signal for different injector-detector distances reveal fluctuating exchange field at these interfaces cause additional spin decoherence which limit spin relaxation time in addition to conventional sources of spin relaxation. Our results show that temperature dependent measurement with pure spin current can be used to probe interfacial exchange field at the ferromagnetic insulator-nonmagnetic metal interface.

cond-mat.mes-hall↗

arXiv2018

Quantum Materials for Spin and Charge Conversion

Spintronics aims to utilize the spin degree of freedom for information storage and computing applications. One major issue is the generation and detection of spins via spin and charge conversion. Quantum materials have recently exhibited many unique spin-dependent properties, which can be used as promising material candidates for efficient spin and charge conversion. Here, we review recent findings concerning spin and charge conversion in quantum materials, including Rashba interfaces, topological insulators, two-dimensional materials, superconductors, and non-collinear antiferromagnets. Important progress in using quantum materials for spin and charge conversion could pave the way for developing future spintronics devices.

cond-mat.mtrl-sci↗

arXiv2018

Unveiling the mechanisms of the spin Hall effect in Ta

Spin-to-charge current interconversions are widely exploited for the generation and detection of pure spin currents and are key ingredients for future spintronic devices including spin-orbit torques and spin-orbit logic circuits. In case of the spin Hall effect, different mechanisms contribute to the phenomenon and determining the leading contribution is peremptory for achieving the largest conversion efficiencies. Here, we experimentally demonstrate the dominance of the intrinsic mechanism of the spin Hall effect in highly-resistive Ta. We obtain an intrinsic spin Hall conductivity for $β$-Ta of -820$\pm$120 ($\hbar$/e) $Ω^{-1}cm^{-1}$ from spin absorption experiments in a large set of lateral spin valve devices. The predominance of the intrinsic mechanism in Ta allows us to linearly enhance the spin Hall angle by tuning the resistivity of Ta, reaching up to -35$\pm$3%, the largest reported value for a pure metal.

cond-mat.mes-hall↗

arXiv2017

Evaluation of bulk-interface contributions to Edelstein magnetoresistance at metal/oxide interfaces

We report a systematic study on Edelstein magnetoresistance (Edelstein MR) in Co25Fe75/Cu/Bi2O3 heterostructures with a strong spin-orbit interaction at the Cu/Bi2O3 interface. We succeed in observing a significant dependence of the Edelstein MR on both Cu layer thickness and temperature, and also develop a general analytical model considering distinct bulk and interface contributions on spin relaxation. Our analysis, based on the above model, quantitatively illustrates a unique property of the spin transport near the Rashba interface, revealing a prominent role of the spin relaxation process by determining the ratios of the spin relaxation inside and outside the interface. We further find the characteristic spin transport is unaffected by temperature. Our results provide an essential tool for exploring the transport in a system with spin-momentum-locked two-dimensional states.

cond-mat.mes-hall↗

arXiv2017

Important role of magnetization precession angle measurement in inverse spin Hall effect induced by spin pumping

Here, we investigate spin Hall angle of Pt in Ni80Fe20/Pt bilayer system by using a broadband spin pumping and inverse spin Hall effect measurement. An out-of-plane excitation geometry with application of external magnetic field perpendicular to the charge current direction is utilized in order to suppress unwanted galvanomagnetic effects. Magnetization precession angle on ferromagnetic resonance for wide excitation frequency range (4-14 GHz) is estimated from the rectification voltage of anisotropic magnetoresistance (AMR) and a conventional method of using microwave power in a coplanar waveguide. A marked difference in the precession angle profiles for the different methods is observed, resulting in the large variation in estimated values of spin current density at Ni80Fe20/Pt interface. The frequency dependence of the spin current density estimated using AMR effect is found to be similar to that of the inverse spin Hall voltage. We obtain the frequency-invariant spin Hall angle of 0.067.

cond-mat.mtrl-sci↗

arXiv2016

Tuning the spin Hall effect of Pt from the moderately dirty to the superclean regime

We systematically measure and analyze the spin diffusion length and the spin Hall effect in Pt with a wide range of conductivities using the spin absorption method in lateral spin valve devices. We observe a linear relation between the spin diffusion length and the conductivity, evidencing that the spin relaxation in Pt is governed by the Elliott-Yafet mechanism. We find a single intrinsic spin Hall conductivity ($σ_{SH}^{int}=1600\pm150\: Ω^{-1}cm^{-1}$) for Pt in the full range studied which is in good agreement with theory. For the first time we have obtained the crossover between the moderately dirty and the superclean scaling regimes of the spin Hall effect by tuning the conductivity. This is equivalent to that obtained for the anomalous Hall effect. Our results explain the spread of the spin Hall angle values in the literature and find a route to maximize this important parameter.

cond-mat.mes-hall↗

arXiv2016

Experimental observation of spin to charge current conversion at non-magnetic metal/Bi2O3 interfaces

We here demonstrate the interfacial spin to charge current conversion by means of spin pumping from a ferromagnetic Permalloy (Py: Ni80Fe20) to a Cu/Bi2O3 interface. A clear signature of the spin to charge current conversion was observed in voltage spectrum of a Py/Cu/Bi2O3 trilayer film whereas no signature in a Py/Cu and Py/Bi2O3 bilayer films. We also found that the conversion coefficient strongly depended on Cu thickness, reflecting the thickness dependent momentum relaxation time in Cu layer.

cond-mat.mes-hall↗

arXiv2015

Reciprocal spin Hall effects in conductors with strong spin-orbit coupling: a review

Spin Hall effect and its inverse provide essential means to convert charge to spin currents and vice versa, which serve as a primary function for spintronic phenomena such as the spin-torque ferromagnetic resonance and the spin Seebeck effect. These effects can oscillate magnetization or detect a thermally generated spin splitting in the chemical potential. Importantly this conversion process occurs via the spin-orbit interaction, and requires neither magnetic materials nor external magnetic fields. However, the spin Hall angle, i.e., the conversion yield between the charge and spin currents, depends severely on the experimental methods. Here we discuss the spin Hall angle and the spin diffusion length for a variety of materials including pure metals such as Pt and Ta, alloys and oxides determined by the spin absorption method in a lateral spin valve structure.

cond-mat.mes-hall↗

arXiv2015

Spin relaxation characteristics in Ag nanowire covered with various oxides

We have studied spin relaxation characteristics in a Ag nanowire covered with various oxide layers of Bi2O3, Al2O3, HfO2, MgO or AgOx by using non-local spin valve structures. The spin-flip probability, a ratio of momentum relaxation time to spin relaxation time at 10 K, exhibits a gradual increase with an atomic number of the oxide constituent elements, Mg, Al, Ag and Hf. Surprisingly the Bi2O3 capping was found to increase the probability by an order of magnitude compared with other oxide layers. This finding suggests the presence of an additional spin relaxation mechanism such as Rashba effect at the Ag/Bi2O3 interface, which cannot be explained by the simple Elliott-Yafet mechanism via phonon, impurity and surface scatterings. The Ag/Bi2O3 interface may provide functionality as a spin to charge interconversion layer.

cond-mat.mtrl-sci↗

arXiv2014

Spin Relaxation Mechanism in a Highly Doped Organic Polymer Film

We report the systematic studies of spin current transport and relaxation mechanism in highly doped organic polymer film. In this study, we have determined spin diffusion length (SDL), spin lifetime, and spin diffusion constant by using different experimental techniques. The spin lifetime estimated from the electron paramagnetic resonance experiment is much shorter than the previous expectation beyond the experimental ambiguity. This suggests that significantly large spin diffusion constant, which is reasonably explained by the hopping transport mechanism in degenerate semiconductors, exists in highly doped organic semiconductors. The calculated SDL using the spin lifetime and spin diffusion constant estimated from our experiment is comparable to the experimentally obtained SDL of the order of one hundred nanometers. Moreover, the present study revealed that the spin angular momentum is almost preserved in the hopping events. In other words, the spin relaxation mainly occurs due to the spin-orbit coupling at the nanoscale crystalline grains.

cond-mat.mtrl-sci↗

arXiv2014

The 2014 Magnetism Roadmap

Magnetism is a very fascinating and dynamic field. Especially in the last 30 years it has experienced many major advances in the full range from novel fundamental phenomena to new products. Applications such as hard disk drives and magnetic sensors are part of our daily life, and new applications, such as in non-volatile computer random access memory, are expected to surface shortly. Thus it is timely for describing the current status, and current and future challenges in the form of a Roadmap article. This 2014 Magnetism Roadmap provides a view on several selected, currently very active innovative developments. It consists of 12 sections, each written by an expert in the field and addressing a specific subject, with strong emphasize on future potential. This Roadmap cannot cover the entire field. We have selected several highly relevant areas without attempting to provide a full review - a future update will have room for more topics. The scope covers mostly nano-magnetic phenomena and applications, where surfaces and interfaces provide additional functionality. New developments in fundamental topics such as interacting nano-elements, novel magnon-based spintronics concepts, spin-orbit torques and spin-caloric phenomena are addressed. New materials, such as organic magnetic materials and permanent magnets are covered. New applications are presented such as nano-magnetic logic, non-local and domain-wall based devices, heat-assisted magnetic recording, magnetic random access memory, and applications in biotechnology. May the Roadmap serve as a guideline for future emerging research directions in modern magnetism.

cond-mat.mtrl-sci↗

arXiv2013

5d transition metal oxide IrO2 as a material for spin current detection

Devices based on a pure spin current (a flow of spin angular momentum) have been attracting increasing attention as key ingredients for low-dissipation electronics. To integrate such spintronics devices into charge-based technologies, an electric detection of spin current is essential. Inverse spin Hall effect converts a spin current into an electric voltage through spin-orbit coupling. Noble metals such as Pt and Pd, and also Cu-based alloys, owing to the large direct spin Hall effect, have been regarded as potential materials for a spin-current injector. Those materials, however, are not promising as a spin-current detector based on inverse spin Hall effect. Their spin Hall resistivity rho_SH, representing the performance as a detector, is not large enough mainly due to their low charge resistivity. Here we demonstrate that heavy transition metal oxides can overcome such limitations inherent to metal-based spintronics materials. A binary 5d transition metal oxide IrO2, owing to its large resistivity as well as a large spin-orbit coupling associated with 5d character of conduction electrons, was found to show a gigantic rho_SH ~ 38 microohm cm at room temperature, one order of magnitude larger than those of noble metals and Cu-based alloys and even comparable to those of atomic layer thin film of W and Ta.

cond-mat.mes-hall↗

arXiv2013

Experimental Verification of Comparability between Spin-Orbit and Spin-Diffusion Lengths

We experimentally confirmed that the spin-orbit lengths of noble metals obtained from weak anti-localization measurements are comparable to the spin diffusion lengths determined from lateral spin valve ones. Even for metals with strong spin-orbit interactions such as Pt, we verified that the two methods gave comparable values which were much larger than those obtained from recent spin torque ferromagnetic resonance measurements. To give a further evidence for the comparability between the two length scales, we measured the disorder dependence of the spin-orbit length of copper by changing the thickness of the wire. The obtained spin-orbit length nicely follows a linear law as a function of the diffusion coefficient, clearly indicating that the Elliott-Yafet mechanism is dominant as in the case of the spin diffusion length.

cond-mat.mes-hall↗

arXiv2012

Towards coherent spin precession in pure-spin current

Non-local spin injection in lateral spin valves generates a pure spin current which is a diffusive flow of spins (i.e. spin angular momentums) with no net charge flow. The diffusive spins lose phase coherency in precession while undergoing frequent collisions and these events lead to a broad distribution of the dwell time in a transport channel between the injector and the detector. Here we show the lateral spin-valves with dual injectors enable us to detect a genuine in-plane precession signal from the Hanle effect, demonstrating the phase coherency in the in-plane precession is improved with an increase of the channel length. The coherency in the spin precession shows a universal behavior as a function of the normalized separation between the injector and the detector in material-independent fashion for metals and semiconductors including graphene.

cond-mat.mes-hall↗

arXiv2011

Detection of picosecond magnetization dynamics of 50 nm magnetic dots down to the single dot regime

We report an all-optical time-domain detection of picosecond magnetization dynamics of arrays of 50 nm Ni80Fe20 (permalloy) dots down to the single nanodot regime. In the single nanodot regime the dynamics reveals one dominant resonant mode corresponding to the edge mode of the 50 nm dot with slightly higher damping than that of the unpatterned thin film. With the increase in areal density of the array both the precession frequency and damping increases significantly due to the increase in magnetostatic interactions between the nanodots and a mode splitting and sudden jump in apparent damping are observed at an edge-to-edge separation of 50 nm.

cond-mat.mes-hall↗

arXiv2011

All-Optical Excitation and Detection of Picosecond Dynamics of Ordered Arrays of Nanomagnets with Varying Areal Density

We have demonstrated optical excitation and detection of collective precessional dynamics in arrays of coupled Ni80Fe20 (permalloy) nanoelements with systematically varying areal density by an all-optical time-resolved Kerr microscope. We have applied this technique to precisely determine three different collective regimes in these arrays. At very high areal density, a single uniform collective mode is observed where the edge modes of the constituent elements are suppressed. At intermediate areal densities, three nonuniform collective modes appear and at very low areal density, we observe noncollective dynamics and only the centre and edge modes of the constituent elements appear.

cond-mat.mes-hall↗