M. H. Qin

Dielectric and magnetic properties of BiFe1-4x/3TixO3 ceramics with iron vacancies: Experimental and first-principles studies

BiFe1-4x/3TixO3 (x = 0-0.2) ceramics with Fe vacancies created by nonstioichiometric compositions were synthesized to study their crystal structures, dielectric, and ferromagnetic behaviors. X-ray diffraction and Raman spectroscopy analysis reveal a structure transition from rhombohedral to orthorhombic phases at x = 0.2. Dielectric measurement shows that the dielectric loss is significantly reduced in the Ti-doped BiFeO3 samples. With increasing x concentration, the remanent magnetization (Mr) first increases and then decreases. The maximal Mr of 0.13 emμ/g is obtained at x = 0.05. Furthermore, theoretical calculations based on the density-functional theory prove that the Ti-doping does enhance the lattice constants, band gap, and magnetization. These results show that the Ti-doped BiFeO3 with Fe vacancies could enhance resistivity and magnetism, implying a possible improvement in multiferroic behavior.

Seeking large thermoelectric effects in MgO-based tunnel junctions

There is much controversy concerning thermoelectric effects in the MgO-based magnetic tunnel junctions (MTJs) as reported in some recent publications. To clarify the problem, we give calculations from atomic first-principles systemically. Large Seebeck coefficient (S) and up-limit of figure of merit (ZT) are predicted in double- and multi-barrier MTJs, with those in single-barrier MTJs being relatively small. By restraining the phonon thermal conductance through the introduction of one vacuum barrier or numbers of MgO barriers, the up-limit of ZT can be obtained. Room temperature and ZT are predicted in an asymmetric Fe MgO Fe Vaccum Fe MTJs. The resonant quantum-well states are suggested to be responsible for the enhanced thermoelectric effects in the MTJs with double- and multi-barrier.

Stripe-vortex transitions in ultrathin magnetic nanostructures

In this work, the magnetic states in ultrathin nanostructures are investigated using Monte Carlo simulation, based on a Heisenberg model involving the short-range exchange coupling, long-range dipole-dipole interaction, and perpendicular anisotropy. An intriguing thermally driven magnetic structural transition from perpendicular stripe domain to flux closure (planar vortex) state, accompanied by an apparent thermal hysteresis effect and typical characteristics of the first-order phase transition, is revealed. Furthermore, it is found that the transition can be remarkably modulated by perpendicular anisotropy. The present work suggests a promising approach to manipulate the spin configurations in nanomagnets by adjusting temperature and perpendicular anisotropy.

Multiferroic phase competitions in perovskite manganite thin films

Based on the Mochizuki-Furukawa model, the cycloidal spin structures of orthorhombic RMnO3 manganite thin films on various magnetic substrates are simulated using Monte Carlo method. It is revealed that the long range cycloidal spin order can be modulated by the film thickness and substrate spin structure. In particular, the ferromagnetic and antiferromagnetic spin orders of the substrate in different orientations have different pinning effects on the cycloidal spin order of the thin film. The simulated results are discussed in terms of the competition between the single-ion anisotropy and spin-orbit coupling.

Nonmagnetic B-site substitution induced two-phase coexistence in multiferroic manganites: Monte Carlo simulation

The influence of B-site nonmagnetic substitution on the spiral spin ordering in multiferroic manganites is investigated by Monte Carlo simulation within the framework of classical Heisenberg model. It is revealed that the nonmagnetic substitution significantly suppressed the multiferroic phase transitions, consistent with experimental results. The boundary between ab-cycloidal phase and bc-cycloidal phase in the temperature-substitution phase diagram becomes faint gradually with increasing substitution level. A coexistence of two cycloidal spin phases is identified when the substitution level surpassed a threshold. The physical origin for the two-phase coexistence induced by the nonmagnetic B-site substitution is discussed.

Role of further-neighbor interactions in modulating the critical behavior of the Ising model with frustration.

In this work, we investigate the phase transitions and critical behaviors of the frustrated J(1)-J(2)-J(3) Ising model on the square lattice using Monte Carlo simulations, and particular attention goes to the effect of the second-next-nearest-neighbor interaction J(3) on the phase transition from a disordered state to the single stripe antiferromagnetic state. A continuous Ashkin-Teller-like transition behavior in a certain range of J(3) is identified, while the four-state Potts-critical end point [J(3)/J(1)](C) is estimated based on the analytic method reported in earlier work [Jin, Sen, and Sandvik, Phys. Rev. Lett. 108, 045702 (2012)]. It is suggested that the interaction J(3) can tune the transition temperature and in turn modulate the critical behaviors of the frustrated model. Furthermore, it is revealed that an antiferromagnetic J(3) can stabilize the staggered dimer state via a phase transition of strong first-order character.

Microwave fields driven domain wall motions in antiferromagnetic nanowires

In this work, we study the microwave field driven antiferromagnetic domain wall motion in an antiferromagnetic nanowire, using the numerical calculations based on a classical Heisenberg spin model. We show that a proper combination of a static magnetic field plus an oscillating field perpendicular to the nanowire axis is sufficient to drive the domain wall propagation along the nanowire with the axial magnetic anisotropy. More importantly, the drift velocity at the resonance frequency is comparable to that induced by temperature gradients, suggesting that microwave field can be a very promising tool to control domain wall motions in antiferromagnetic nanostructures. Furthermore, the dependences of resonance frequency and drift velocity on the static and oscillating fields, the axial anisotropy, and the damping constant are discussed in details. This work provides useful information for the spin dynamics in antiferromagnetic nanostructures for spintronics applications.

Magnetic behaviors of classical spin model on the Shastry–Sutherland lattice: Monte Carlo simulation

We study the magnetic phase diagram of a classical Heisenberg spin model on the Shastry–Sutherland lattice using the Monte Carlo method. The simulated results indicate that the particular collinear phase region can be enlarged due to the implementation of easy-axis anisotropy, leading to the broadening of the magnetization plateau in the classical Heisenberg model. Also, the random exchange term is taken into account to study the effect of inhomogeneity on the steplike magnetization feature. It is indicated that the multi-step magnetic behaviors in the low temperature range remain observable when the inhomogeneity is in the appropriate range.

Random exchange interaction effects on the phase transitions in frustrated classical Heisenberg model

In this work, the effects of the random exchange interaction on the phase transitions and phase diagrams of classical frustrated Heisenberg model are investigated by Monte Carlo simulation in order to simulate the chemical doping effect in real materials. It is observed that the antiferromagnetic transitions shift toward low temperature with the increasing magnitude of the random exchange interaction, which can be qualitatively understood from the competitions among local spin states. This study is related to the magnetic properties in the doped iron-based superconductors.

Multiferroic properties in orthorhombic perovskite manganites: Monte Carlo simulation

In this work, a random field method (based on Mochizuki-Furukawa model) is used to study the effect of the R-site ionic disorder and inhomogeneity on the multiferroic behavior in orthorhombic manganites. It is shown that both the R-site ionic disorder and inhomogeneity can drive the reorientation of the plane in which cycloidal spin order takes place and actually lead to the flop of the spins from the ab-plane to the bc-plane. The simulated results can be understood as the consequence of the competition between exchange interactions and spin-orbit coupling.