arXiv subjects

Hiroshi Naganuma

Publications and source records attributed to Hiroshi Naganuma.

6 recordsLinked to original sources

arXiv2026

Oxygen-induced Fe surface segregation at the $L1_0$-FePd(001)/graphene heterointerface for spintronics devices: a first-principles study

We theoretically investigate the atomic-scale structure of the heterointerface formed between the (001) surface of the $L1_0$-ordered iron palladium (FePd) intermetallic alloy and graphene (Gr), namely, $L1_0$-FePd(001)/Gr, which serves as an essential component in spintronic devices. Using density functional theory (DFT) calculations, we demonstrate that the topmost surface layer consisting of Pd (Pd-terminated surface) is energetically more stable than that consisting of Fe in vacuum, and that Pd-terminated surfaces are unfavorable for graphene adsorption. In contrast, under an oxygen atmosphere, the strong Fe--O bonding stabilizes Fe-terminated surfaces. The predicted Fe--O bonds on the oxidized surface are consistent with our X-ray photoelectron spectroscopy (XPS) measurements. These results reproduce the mechanism responsible for the graphene coverage observed in recent experiments. Similar oxygen-induced Fe surface segregation has been studied in heterogeneous catalysis on FePt and FePd alloys. In this work, we exploit this mechanism as a termination-engineering strategy to fabricate high-quality 2D-material/alloy heterointerfaces for nanoscale device applications.

cond-mat.mtrl-sci↗

arXiv2023

First-principle study of spin transport property in $L1_0$-FePd(001)/graphene heterojunction

In our previous work, we synthesized a metal/2D material heterointerface consisting of $L1_0$-ordered iron-palladium (FePd) and graphene (Gr) called FePd(001)/Gr. This system has been explored by both experimental measurements and theoretical calculations. In this study, we focus on a heterojunction composed of FePd and multilayer graphene referred to as FePd(001)/$m$-Gr/FePd(001), where $m$ represents the number of graphene layers. We perform first-principles calculations to predict their spin-dependent transport properties. The quantitative calculations of spin-resolved conductance and magnetoresistance (MR) ratio (150-200%) suggest that the proposed structure can function as a magnetic tunnel junction in spintronics applications. We also find that an increase in $m$ not only reduces conductance but also changes transport properties from the tunneling behavior to the graphite $π$-band-like behavior. Additionally, we investigate the spin-transfer torque-induced magnetization switching behavior of our \color{blue} junction structures \color{black} using micromagnetic simulations. Furthermore, we examine the impact of lateral displacements (``sliding'') at the interface and find that the spin transport properties remain robust despite these changes; this is the advantage of two-dimensional material hetero-interfaces over traditional insulating barrier layers such as MgO.

cond-mat.mtrl-sci↗

arXiv2022

Density functional study of twisted graphene $L1_0$-FePd heterogeneous interface

Graphene on $L1_0$-FePd(001), which has been experimentally studied in recent years, is a heterogeneous interface with a significant lattice symmetry mismatch between the honeycomb structure of graphene and tetragonal alloy surface. In this work, we report on the density functional study of its atomic-scale configurations, electronic and magnetic properties, and adsorption mechanism, which have not been well understood in previous experimental studies. We propose various atomic-scale models, including simple nontwisted and low-strain twisted interfaces, and analyze their energetical stability by performing structural optimizations using the van der Waals interactions of both DFT-D2 and optB86b-vdW functionals. The binding energy of the most stable structure reached $E_\mathrm{B}=-0.22$~eV/atom for DFT-D2 ($E_\mathrm{B}=-0.19$~eV/atom for optB86b-vdW). The calculated FePd-graphene spacing distance was approximately 2~Å, which successfully reproduced the experimental value. We also find out characteristic behaviors: the modulation of $π$-bands, the suppression of the site-dependence of adsorption energy, and the rise of \color{blue} moiré-like \color{black} corrugated buckling. In addition, our atomic structure is expected to help build low-cost computational models for investigating the physical properties of $L1_0$ alloys/two-dimensional interfaces.

cond-mat.mtrl-sci↗

arXiv2018

Realization of a spin wave switch based on the Spin-Transfer-Torque effect

We investigate the amplification of externally excited spin waves via the Spin-Transfer-Torque (STT) effect in combination with the Spin-Hall-Effect (SHE) employing short current pulses. The results reveal that, in the case of an overcompensation of the spin wave damping, a strong nonlinear shift of the spin wave frequency spectrum occurs. In particular, this shift affects the spin wave amplification using the SHE-STT effect. In contrast, this effect allows for the realization of a spin wave switch. By determining the corresponding working point, an efficient spin wave excitation is only possible in the presence of the SHE-STT effect yielding an increased spin wave intensity of a factor of 20 compared to the absence of the SHE-STT effect.

cond-mat.mes-hall↗

arXiv2017

Experimental investigation of the temperature-dependent magnon density and its influence on studies of spin-transfer-torque-driven systems

We present the temperature dependence of the thermal magnon density in a thin ferromagnetic layer. By employing Brillouin light scattering and varying the temperature, an increase of the magnon density accompanied by a lowering of the spin-wave frequency is observed with increasing temperature. The magnon density follows the temperature according to the Bose-Einstein distribution function which leads to an approximately linear dependency. In addition, the influence of this effect in spin-transfer-torque-driven systems is presented. In particular, the increase in the magnon density with temperature sets the limit for a suppression of magnons in charge current-driven systems. Hence, the maximum possible suppression of thermal magnons occurs at a finite current.

cond-mat.mes-hall↗

arXiv2012

Revealing the spin and symmetry properties of the buried Co2MnSi/MgO interface by low energy spin-resolved photoemission

We present a novel approach to study the spin and symmetry electronic properties of buried interfaces using low-energy spin-resolved photoemission spectroscopy. We show that this method is sensitive to interfaces buried below more than 20ML (~4nm) MgO, providing a powerful tool for the non-destructive characterization of spintronics interfaces. As a demonstration, we apply this technique to characterize the Co2MnSi/MgO interface, a fundamental building block of state-of-the-art magnetic tunnel junctions based on Heusler compounds. We find that a surface state with Δ1 symmetry and minority spin character dominating the electronic structure of the bare Co2MnSi(100) surface is quenched at the Co2MnSi(100)/MgO interface. As a result, the interface spin-dependent electronic structure resembles the theoretically expected Co2MnSi bulk band structure, with majority spin electronic states of both Δ1 and Δ5 symmetry. Furthermore we find an additional thermally-induced contribution in the minority channel, mirroring the Δ1/Δ5 asymmetry of the majority channel.

cond-mat.mtrl-sci↗