Xin-Yi Chen

Spin polarization induced by an external electric field in a hybrid magnetic-electric barrier

Recent studies indicated that under zero bias there is no spin polarization in an antiparallel magnetic-electric barrier structure, where double δ-function magnetic fields point in the opposite direction. Our research demonstrates that an external electric field can make such a hybrid structure spin polarized, however, the degree of spin polarization is smaller than 5% for the GaAs system. It is also shown that the electric barrier can greatly suppress the current density and affect the degree of spin polarization. The results can be explained by the variation of the effective potential induced by the electric field.

Rashba spin-orbit effect on traversal time in ferromagnetic/semiconductor/ferromagnetic heterojunction

Based on the group velocity concept, we investigated the traversal time of a quasi-one-dimensional waveguide that contains the ferromagnetic/semiconductor/ferromagnetic heterojunction in the presence of the Rashba spin-orbit interaction. The significant quantum size, quantum coherence, and Rashba spin-orbit interaction are considered simultaneously. The results indicate that as the strength of the spin-orbit coupling increases, the traversal time considerably decreases. The results also indicate that the traversal time is not prolonged linearly as the length of the semiconductor increases but shows step-like behavior.

Spin-filter diode based on ZnSe/Zn1−xMnxSe/Zn1−yMnySe/ZnSe heterostructures

We propose and demonstrate a spin-filter diode based on semimagnetic semiconductor ZnSe/Zn1−xMnxSe/Zn1−yMnySe/ZnSe heterostructures. It is found that the degree of spin polarization greatly depends on the electric field direction and its magnitude. There is significant difference of the spin polarization between under forward bias and under reverse bias. It is also found that the spin polarization will reverse under relatively small magnetic field, which is originated from resonant enhancement effect for spin-up electrons tunneling through effective steplike potential of the corresponding structure.

Spin filtering in parallel magnetic-barrier structures

Recent studies have indicated that the final wave vector kf of the electron is exactly the same as the initial wave vector ki for electrons tunnelling through a parallel magnetic-barrier structure, where double δ-function magnetic fields point in the same direction. In contrast to the anti-parallel magnetic-barrier configuration, we have found that the parallel magnetic-barrier configuration as well as identical initial and final wave vectors cannot concur. We have derived the correct formula of the transmission and have shown that the spectra of both the transmission and the conductance are quite different from published results. Numerical results further confirm that spin filtering in the magnetic-electric barrier is relatively weak because the intrinsic Zeeman term between the electronic spin and the inhomogeneous magnetic field is local and it strength is small for the GaAs system.

A tunable spin filter in periodic diluted magnetic semiconductor/semiconductor superlattices

We propose a tunable spin filter based on periodic diluted magnetic semiconductor/semiconductor (DMS/S) superlattices. It is found that periodic DMS/S superlattices can filter high efficiently not only spin-up electrons but also spin-down ones over a broad range of incident energies. The positions and widths of spin-filtering bands can be manipulated by adjusting the parameters of the superlattices or the external magnetic field. It is also found that the defect layer within the system generally leads to a strong suppression of spin-dependent transmission and makes the superlattices filter single-energy electrons. The results obtained may lead to potential applications in the field of spintronics.

Electrochemical control of the phase transition of ultrathin FeRh films

We investigate the electrical manipulation of the phase transition in ultrathin FeRh films through a combination of ionic liquid and oxide gating. The 5 nm-thick FeRh films show an antiferromagnetic-ferromagnetic transition at around 275 K with in-plane magnetic field of 70 kOe. A negative gate voltage seriously suppresses the transition temperature to ∼248 K, while a positive gate voltage does the opposite but with a smaller tuning amplitude. The formation of electric double layer associated with a large electric field induces the migration of oxygen ions between the oxide gate and the FeRh layer, producing the variation of Fe moments in antiferromagnetic FeRh accompanied by the modulation of the transition temperature. Such a modulation only occurs within several nanometers thick scale in the vicinity of FeRh surface. The reversible control of FeRh phase transition by electric field might pave the way for non-volatile memories with low power consumption.

Tunneling anisotropic magnetoresistance driven by magnetic phase transition

The independent control of two magnetic electrodes and spin-coherent transport in magnetic tunnel junctions are strictly required for tunneling magnetoresistance, while junctions with only one ferromagnetic electrode exhibit tunneling anisotropic magnetoresistance dependent on the anisotropic density of states with no room temperature performance so far. Here, we report an alternative approach to obtaining tunneling anisotropic magnetoresistance in α′-FeRh-based junctions driven by the magnetic phase transition of α′-FeRh and resultantly large variation of the density of states in the vicinity of MgO tunneling barrier, referred to as phase transition tunneling anisotropic magnetoresistance. The junctions with only one α′-FeRh magnetic electrode show a magnetoresistance ratio up to 20% at room temperature. Both the polarity and magnitude of the phase transition tunneling anisotropic magnetoresistance can be modulated by interfacial engineering at the α′-FeRh/MgO interface. Besides the fundamental significance, our finding might add a different dimension to magnetic random access memory and antiferromagnet spintronics.Tunneling anisotropic magnetoresistance is promising for next generation memory devices but limited by the low efficiency and functioning temperature. Here the authors achieved 20% tunneling anisotropic magnetoresistance at room temperature in magnetic tunnel junctions with one α′-FeRh magnetic electrode.

Highly ordered arrays of macroscopically long Pb nanobelts with atomic-level controlled thickness and width on Si

We report growth of ordered arrays of superlong Pb nanobelts using Al decorated Si(111) substrates as a template. By depositing Al at substrate temperature of 650–700°C, each original Si(111) terrace is divided into two distinct strips, a γ-phase strip and a mixed √7×√7 and √3×√3 structure strip. In situ scanning tunneling microscopy observation reveals that Pb atoms preferentially nucleate on the γ-phase strips and form uniform array of nanobelts with a width from 10to100nm and a thickness from 2.3to20nm, which can delicately be controlled by Al coverage and Pb coverage.

Asymmetric effect on spin polarization in a spin-filter device using nonmagnetic triple-barrier structure

We investigate the asymmetric effect in a spin-filter device, which is based on the Rashba spin-orbit coupling effect and uses a nonmagnetic tunneling diode. The structural asymmetry is introduced by unequivalence of the two quantum wells in the spin-filter device. It is found that the structural asymmetry can greatly change spin-filtering efficiency. For some asymmetric spin-filter structures, one can see spin-dependent enhancement in the transmission. Moreover, the current density can increase or decrease greatly depending on the degree of the structural asymmetry.

Temperature- dependent transport properties of FeRh

The finite-temperature transport properties of FeRh compounds are investigated by first-principles Density Functional Theory-based calculations. The focus is on the behavior of the longitudinal resistivity with rising temperature, which exhibits an abrupt decrease at the metamagnetic transition point, T = Tm between ferroand antiferromagnetic phases. A detailed electronic structure investigation for T ≥ 0 K explains this feature and demonstrates the important role of (i) the difference of the electronic structure at the Fermi level between the two magnetically ordered states and (ii) the different degree of thermally induced magnetic disorder in the vicinity of Tm, giving different contributions to the resistivity. To support these conclusions, we also describe the temperature dependence of the spin-orbit induced anomalous Hall resistivity and Gilbert damping parameter. For the various response quantities considered the impact of thermal lattice vibrations and spin fluctuations on their temperature dependence is investigated in detail. Comparison with corresponding experimental data finds in general a very good agreement.