Zhuhong Liu

Metamagnetic phase transformation in Mn50Ni37In10Co3 polycrystalline alloy

This letter reports on an alloy design of Mn50Ni37In10Co3 based on the principle of Mn-Mn ferromagnetic coupling via Co doping. The alloy is shown to exhibit a metamagnetic martensitic transformation and a high saturation magnetization of 118 emu/g in its austenitic state. The transformation generates a large magnetization difference of 89 emu/g, more than 200% of what is reported in the literature for similar alloys. A complete magnetic field induced martensitic transformation was achieved at 170 K. Such a high magnetization difference provides a strong driving force for magnetic-field-induced transformation, making this material a promising candidate for magnetic actuation applications.

Micromagnetic simulation of spin-transfer switching in a full-Heusler Co2FeAl0.5Si0.5 alloy spin-valve nanopillar

We investigated the spin-transfer switching in a full-Heusler Co2FeAl0.5Si0.5 alloy spin-valve nanopillar through micromagnetic simulation. A two-step switching hysteresis loop due to the fourfold in-plane magnetocrystalline anisotropy of Co2FeAl0.5Si0.5 layers was obtained. The simulation explains the experimental result of the resistance versus current hysteresis loop and yields good agreement with the measured critical current. Furthermore, the magnetization trajectory and magnetization distribution were shown and analyzed to elucidate the different characters of two-step switching.

Large magnetization change and magnetoresistance associated with martensitic transformation in Mn2Ni1.36Sn0.32Co0.32 alloy

In this paper we report on the realization of magnetic field-induced martensitic phase transformation in Mn-rich Heusler alloy Mn2Ni1.36Sn0.32Co0.32. The saturation magnetization of the austenite reached 111 emu/g at 70 kOe, which decreased rapidly to 8 emu/g upon transforming to martensite. This is attributed to the crystallographic distortion from cubic structure to tetragonal structure with c/a > 1, turning the Mn moments at B sites and D sites from parallel alignment to antiparallel alignment. A large magnetoresistance of 40% was observed through the field-induced transformation. The increase of conduction electrons accompanying this field-induced martensitic transformation is estimated to be 67%. These intriguing properties render the alloy a good candidate for applications in smart devices.

Martensitic and magnetic transformation behaviours in Mn50Ni42-xSn8Cox polycrystalline alloys

This study investigated the effect of Co substitution for Ni in Mn50Ni42Sn8 alloy with the aim of increasing the magnetic driving force for inducing its martensitic transformation. The martensitic transformation temperatures, enthalpy and entropy changes were found to decrease progressively with increasing Co content, while the transformation hysteresis increased. Co substitution for Ni also significantly increased the magnetization of the austenite, but with negligible effect on that of the martensite. A large magnetization difference 109 emu g(-1) was achieved across the transformation in a Mn50Ni34Sn8Co8 alloy. The large magnetization difference between the two phases provides enhanced thermodynamic driving force for the transformation. Consequently, the martensitic transformation was induced by the application of a magnetic field in Mn50Ni36Sn8Co6 and Mn50Ni34Sn8Co8 alloys. The effect of Co substitution for Ni on the magnetic interaction among the constituents for the austenite and martensite was clarified in this study, which provides a guide for alloy design for magnetoactuation applications.

Transition from Anomalous Hall Effect to Topological Hall Effect in Hexagonal Non-Collinear Magnet Mn 3 Ga

We report experimental observation of large anomalous Hall effect exhibited in non-collinear triangular antiferromagnet D019-type Mn3Ga with coplanar spin structure at temperatures higher than 100 K. The value of anomalous Hall resistivity increases with increasing temperature, which reaches 1.25 μΩ · cm at a low field of ~300 Oe at room temperature. The corresponding room-temperature anomalous Hall conductivity is about 17 (Ω · cm)−1. Most interestingly, as temperature falls below 100 K, a temperature-independent topological-like Hall effect was observed. The maximum peak value of topological Hall resistivity is about 0.255 μΩ · cm. The appearance of the topological Hall effect is attributed to the change of spin texture as a result of weak structural distortion from hexagonal to orthorhombic symmetry in Mn3Ga. Present study suggests that Mn3Ga shows promising possibility to be antiferromagnetic spintronics or topological Hall effect-based data storage devices.

Numerical simulation of vortex dynamics in type-II superconductors in oscillating magnetic field using time-dependent Ginzburg–Landau equations

Time-dependent Ginzburg-Landau equations were solved by the finite difference scheme for a superconducting sample in steady and oscillating magnetic fields for 3D geometry. The dynamic behaviour of penetrating and leaving magnetic vortices in superconductor with the oscillating magnetic field was simulated. Carrier concentration density and the average magnetization of the sample were studied as a function of the external oscillating magnetic field. Anomalies in carrier concentrations at certain magnetic field values were observed and discussed. It was also observed that the area swept by magnetization versus external magnetic field is magnetic oscillation frequency dependent, which increases with increasing frequencies. It was suggested that this effect may cause instability in the superconducting characteristics of the sample over a number of cycles. Calculated energy patterns showed consistency with vortex patterns in the steady magnetic field. Magnetic oscillations initiated oscillations in energy components, ripples in superconducting energy are subjected to the entrance and leaving of vortices, while instability observed in interaction energy is referred to vortex relaxation time.

Phase-Field Simulation of Strain-Assisted Current-Induced Magnetization Dynamics in a Magnetic Tunnel Junction

We investigated the effect of substrate misfit strain theoretically by using a phase-field microelasticity model in a simple sandwich structure of a magnetic tunnel junction with perpendicular anisotropy. The critical current for magnetization flipping decreases with increasing tensile strain except in the region above a threshold strain, which results from magneto-elastic coupling interaction between perpendicular magnetic anisotropy and substrate strain. Magnetization reversal speeds with different substrate strains and switching currents are also presented, which suggest ways to improve the magnetization switching efficiency and decrease the power consumption of magnetic devices.

Micromagnetic simulation of electric field-modulation on precession dynamics of spin torque nano-oscillator

Understanding electric field effects on precession dynamics is crucial to the design of spin transfer torque devices for improving the performance in nano-oscillator. In this letter, the precession dynamics of a CoFeB/MgO multi-layer structured nano-oscillator under externally applied electric field is predicted using a micromagnetic simulation. It is revealed that the electric field can modify the range of oscillation spectra in single frequency mode. With the increase in electric field, there is a red-shift of the resonant frequency. When a positive electric field pulse is applied, a phase lag of the spin precession is induced, which is proportional to the pulse amplitude and duration. The present work is expected to stimulate future experimental efforts on designing devices with electric-field modulated spin transfer torque nano-oscillators.Understanding electric field effects on precession dynamics is crucial to the design of spin transfer torque devices for improving the performance in nano-oscillator. In this letter, the precession dynamics of a CoFeB/MgO multi-layer structured nano-oscillator under externally applied electric field is predicted using a micromagnetic simulation. It is revealed that the electric field can modify the range of oscillation spectra in single frequency mode. With the increase in electric field, there is a red-shift of the resonant frequency. When a positive electric field pulse is applied, a phase lag of the spin precession is induced, which is proportional to the pulse amplitude and duration. The present work is expected to stimulate future experimental efforts on designing devices with electric-field modulated spin transfer torque nano-oscillators.

Boundary Pinning Effects on the Frequency Spectra of Point-Contact Spin-Torque Oscillators

Micromagnetic simulation is used to investigate the frequency spectra of a perpendicularly magnetized nanocontact spin-torque oscillator subject to different boundary pinning conditions. Boundary confinement is critical to the output spectra: spectra under two limiting conditions are attributed to the different pinning strengths at the boundaries. The correlation between boundary pinning strength and output spectra may facilitate the design of future nanocontact microwave devices.

Magnetic anisotropy energy of ferromagnetic shape memory alloys Ni2X(X=Fe, Co)Ga by first-principles calculations

First-principles calculations were employed to explore magnetocrystalline anisotropy energy (MAE) of Ni2X(X=Fe, Co)Ga alloys. The MAE of Ni2FeGa is found to show a concave behavior as a function of tetragonal distortion and easy-axis of magnetization in martensitic phase is along long axis, which have been interpreted by the shift of Fe dxy+dyz peak in minority spin channel near Fermi level. The substitution of Ni by Co in Ni2FeGa alloys rotates magnetic easy axis from long axis to short axis in non-modulated phase while substitution of Fe by Co did not, which is in agreement with experiment. Magnetic anisotropy constant and magnetic stress have been estimated with calculated MAE of martensite phases. By comparing first-principles estimated values of magnetic and twinning stresses, we confirmed the condition, whether large magnetic field-induced strains in FSMAs could be obtained or not. This information can provide theoretical guidance in searching new types of FSMAs with large magnetic field induced strain.