arXiv subjects

X. F. Han

Publications and source records attributed to X. F. Han.

27 records · Page 2Linked to original sources

arXiv2015

Determining Spin Polarization of Seebeck Coefficients via Anomalous Nernst Effect

Recently, Seebeck coefficients of ferromagnetic conductors are found to be spin-dependent. However straightforward method of accurately determining its spin polarization is still to be developed. Here, we have derived a linear dependence of anomalous Nernst coefficient on anomalous Hall angle with scaling factor related to spin polarization of Seebeck coefficient, which has been experimentally verified in [Co/Pt]n superlattices. Based on the dependence, we have also evaluated spin polarization of Seebeck coefficient of some ferromagnetic conductors. Besides, we have also found a new mechanism to generate pure spin current from temperature gradient in ferromagnetic/nonmagnetic hybrid system, which could improve efficiency from thermal energy to spin current.

cond-mat.mtrl-sci↗

arXiv2014

Low frequency noise peak near magnon emission energy in magnetic tunnel junctions

We report on the low frequency (LF) noise measurements in magnetic tunnel junctions (MTJs) below 4 K and at low bias, where the transport is strongly affected by scattering with magnons emitted by hot tunnelling electrons, as thermal activation of magnons from the environment is suppressed. For both CoFeB/MgO/CoFeB and CoFeB/AlO$_{x}$/CoFeB MTJs, enhanced LF noise is observed at bias voltage around magnon emission energy, forming a peak in the bias dependence of noise power spectra density, independent of magnetic configurations. The noise peak is much higher and broader for unannealed AlO$_{x}$-based MTJ, and besides Lorentzian shape noise spectra in the frequency domain, random telegraph noise (RTN) is visible in the time traces. During repeated measurements the noise peak reduces and the RTN becomes difficult to resolve, suggesting defects being annealed. The Lorentzian shape noise spectra can be fitted with bias-dependent activation of RTN, with the attempt frequency in the MHz range, consistent with magnon dynamics. These findings suggest magnon-assisted activation of defects as the origin of the enhanced LF noise.

cond-mat.mes-hall↗

arXiv2013

Thermal transport due to quantum interference in magnetic tunnel junctions

We study the thermal transport in magnetic tunnel junctions. Thermal gradients across the tunneling barrier appear around the Fowler-Nordheim tunneling regime, due to the current-induced heat caused by quantum interference. Both thermovoltage and thermal temperature follow a linear response with the applied current, which is an evidence for a thermoelectric effect. By increasing the barrier transparency, the dynamics of thermoelectric properties is observed with the current. Accordingly, a large range of the Seebeck coefficient, 10 - 1000 μV/K, has been obtained in magnetic tunnel junctions.

cond-mat.mtrl-sci↗

arXiv2012

Transport Measurements on Nano-engineered Two Dimensional Superconducting Wire Networks

Superconducting triangular Nb wire networks with high normal-state resistance are fabricated by using a negative tone hydrogen silsesquioxane (HSQ) resist. Robust magnetoresistance oscillations are observed up to high magnetic fields and maintained at low temperatures, due to the eective reduction of wire dimensions. Well-defined dips appear at integral and rational values (1/2, 1/3, 1/4) of the reduced flux f = Phi/Phi_0, which is the first observation in the triangular wire networks. These results are well consistent with theoretical calculations for the reduced critical temperature as a function of f.

cond-mat.mes-hall↗

arXiv2012

Edge superconducting state in Nb thin film with rectangular arrays of antidots

Superconducting Nb thin films with rectangular arrays of submicron antidots have been systemically investigated by transport measurements. In low fields, the magnetoresistance curves demonstrate well-defined dips at integral and rational numbers of flux quanta per unit cell, which corresponds to a superconducting wire network-like regime. When the magnetic field is higher than a saturation field, interstitial vortices interrupt the collective oscillation in low fields and form vortex sublattice, where a larger magnetic field interval is observed. In higher fields, a crossover behavior from the interstitial sublattice state to a single-loop-like state is observed, characterized by oscillations with a period of $Φ_0/πr_{eff}^2$, originating from the existence of edge superconducting states with a size $r_{eff}$ around the antidots.

cond-mat.mes-hall↗

arXiv2012

Wire network behavior in superconducting Nb films with diluted triangular arrays of holes

We present transport measurement results on superconducting Nb films with diluted triangular arrays (honeycomb and kagomé) of holes. The patterned films have large disk-shaped interstitial regions even when the edge-to-edge separations between nearest neighboring holes are comparable to the coherence length. Changes in the field interval of two consecutive minima in the field dependent resistance $R(H)$ curves are observed. In the low field region, fine structures in the $R(H)$ and $T_c(H)$ curves are identified in both arrays. Comparison of experimental data with calculation results shows that these structures observed in honeycomb and kagomé hole arrays resemble those in wire networks with triangular and $T_3$ symmetries, respectively. Our findings suggest that even in these specified periodic hole arrays with very large interstitial regions, the low field fine structures are determined by the connectivity of the arrays

cond-mat.supr-con↗

arXiv2011

Abnormal magnetoresistance behavior in Nb thin film with rectangular antidot lattice

Abnormal magnetoresistance behavior is found in superconducting Nb films perforated with rectangular arrays of antidots (holes). Generally magnetoresistance were always found to increase with increasing magnetic field. Here we observed a reversal of this behavior for particular in low temperature or current density. This phenomenon is due to a strong 'caging effect' which interstitial vortices are strongly trapped among pinned multivortices.

cond-mat.mes-hall↗

arXiv2005

Magnetic Vortex Core Dynamics in a Ferromagnetic Dot

We report direct imaging by means of x-ray photoemission electron microscopy of the dynamics of magnetic vortices confined in micron-size circular Permalloy dots that are 30 nm thick. The vortex core positions oscillate on a 10-ns timescale in a self-induced magnetostatic potential well after the in-plane magnetic field is turned off. The measured oscillation frequencies as a function of the aspect ratio (thickness/radius) of the dots are in agreement with theoretical calculations presented for the same geometry.

cond-mat.mes-hall↗

arXiv2003

Magnon and phonon assisted tunneling in a high-magnetoresistance tunnel junction using Co$_{75}$Fe$_{25}$ ferromagnetic electrodes

Magnetoelectric properties of the spin-valve-type tunnel junction of Ta(5\ nm)/Ni$_{79}$Fe$_{21}$ (25 nm)/Ir$_{22}$Mn$_{78}$ (10 nm)/Co$_{75}$Fe$_{25}$ (4 nm)/Al (0.8 nm)-oxide/Co$_{75}$Fe$_{25}$ (4 nm)/Ni$_{79}$Fe$_{21}$ (20 nm)/Ta (5 nm) are investigated both experimentally and theoretically. It is shown that both magnon and phonon excitations contribute to the tunneling process. Moreover, we show that there are two branches of magnon with spin $S=1/2$ and 3/2 respectively. The theoretical results are in good agreement with the experimental data.

cond-mat↗