Ferromagnetic TM2BC (TM = Cr, Mn) monolayers for spintronic devices with high Curie temperature.

Transition metal boro-carbide (TM2BC) structures crystallize in the layered orthorhombic structure in their bulk phases. In this study, however, we find that TM2BC (TM = Cr, Mn) prefer a tetragonal (t) crystal structure in their monolayer phases due to the occurrence of strong sp2 bonds between the metal and B/C atoms instead of sp3 + sp2 bonds which exist in the orthorhombic phase. The calculated energy difference between the orthorhombic and the tetragonal structures based on density functional theory (DFT) is more than 1 eV per unit cell. In addition, t-Cr2BC and t-Mn2BC monolayers are dynamically and thermally stable with their magnetic metal electronic structures. For further investigations, we combine our DFT calculations with the Monte Carlo simulations and find that both t-TM2BC monolayers show ferromagnetic properties. The calculated Curie temperatures are 846 K and 128 K for t-Cr2BC and t-MnBC sheets, respectively. In addition, we examine the magnetic anisotropy energies (MAE) of t-TM2BC monolayers and find that both structures prefer out-of-plane as the easy axis magnetization direction and the applied electric field can easily modulate the MAE of the monolayers. Our theoretical calculations reveal that t-TM2BC (TM = Cr, Mn) sheets have great potential for the future design of controllable spintronic devices with their tunable MAE properties.

A tetragonal phase Mn2B2 sheet: a stable room temperature ferromagnet with sizable magnetic anisotropy.

Exploring the magnetic properties of two-dimensional (2D) metal boride (MBene) sheets for spin-based electronics is gaining importance for developing electronic devices. Through combined first-principles calculations and Monte Carlo simulations, we present a new tetragonal Mn2B2 (tetra-Mn2B2) sheet. The tetra-Mn2B2 sheet shows metallic ferromagnetism (2.65 µB per Mn atom) with excellent stability. Moreover, it is demonstrated that the tetra-Mn2B2 sheet holds promise for experimental synthesis within an acceptable range from the results of stability tests of tetra-Mn2B2. We also find that the magnetic anisotropy (MAE) of the 2D tetra-Mn2B2 sheet is significantly increased under an electric field. The Curie temperature (TC) of the tetra-Mn2B2 sheet is calculated as 406 K. This 2D tetra-Mn2B2 with a high Curie temperature can serve as a promising candidate for future magnetoelectronics applications.

Tuning the electronic structure of RhX3 (X = Cl, Br, I) nonmagnetic monolayers: effects of charge-injection and external strain.

Atomistic exploration and electronic modification of 2D materials have been a central focus of research since the breakthrough of graphene. In the present study, we introduce and reveal the structure, stability and electronic features of novel RhX3 (X: Cl, Br, I) monolayer systems within the framework of density functional theory. Phonon dispersion spectra and equilibrium molecular dynamics calculations confirm the stability of the phases studied at room and elevated temperatures. The structures are slightly modified because of thermal excitations and maintain their stabilities up to 1000 K. We show that the electronic and magnetic nature of pristine monolayers can be tuned by external effects, i.e. both mechanically and electrically. RhCl3, RhBr3 and RhI3 monolayers are nonmagnetic and indirect-gap semiconductors intrinsically, but display indirect-to-direct band gap transitions at particular strain values. The systems gain a net magnetic moment and are transformed into metals by negative charging. The optical properties, such as the absorption coefficient, optical conductivity, energy loss spectrum, refractive index and extinction coefficient, are also presented. This interesting class of nanomaterials are promising candidates for several applications in nanotechnology and optoelectronics with good thermal stability, mechanical flexibility, and tunable electronic properties.

Influence of time dependent longitudinal magnetic fields on the cooling process, exchange bias and magnetization reversal mechanism in FM core/AFM shell nanoparticles: a Monte Carlo study.

Using Monte Carlo simulations, we have investigated the dynamic phase transition properties of magnetic nanoparticles with ferromagnetic core coated by an antiferromagnetic shell structure. Effects of field amplitude and frequency on the thermal dependence of magnetizations, magnetization reversal mechanisms during hysteresis cycles, as well as on the exchange bias and coercive fields have been examined, and the feasibility of applying dynamic magnetic fields on the particle have been discussed for technological and biomedical purposes.

Dynamic hysteretic features of Ising-type thin films.

In order to elucidate the nature of hysteresis characteristics in a magnetic Ising-type thin film with a certain thickness, such as types of frequency dispersion curves, decay of hysteresis loop area, corresponding coercive field and remanent magnetization values, etc., we investigate the hysteretic response of each layer within effective-field theory. Throughout the analysis, the best appropriate parameter values are chosen since they would allow us to observe the reversed magnetic hysteresis after a certain value of external field frequency. This eccentric phenomenon has prompted us to associate it to the domain nucleation and growth mechanism in the dynamic process. Exotic shapes of the response for different layer indices in two different regimes of modified surface exchange are particularly emphasized.

Nonequilibrium phase transitions and stationary-state solutions of a three-dimensional random-field Ising model under a time-dependent periodic external field.

Nonequilibrium behavior and dynamic phase-transition properties of a kinetic Ising model under the influence of periodically oscillating random fields have been analyzed within the framework of effective-field theory based on a decoupling approximation. A dynamic equation of motion has been solved for a simple-cubic lattice (q=6) by utilizing a Glauber-type stochastic process. Amplitude of the sinusoidally oscillating magnetic field is randomly distributed on the lattice sites according to bimodal and trimodal distribution functions. For a bimodal type of amplitude distribution, it is found in the high-frequency regime that the dynamic phase diagrams of the system in the temperature versus field amplitude plane resemble the corresponding phase diagrams of the pure kinetic Ising model. Our numerical results indicate that for a bimodal distribution, both in the low- and high-frequency regimes, the dynamic phase diagrams always exhibit a coexistence region in which the stationary state (ferro or para) of the system is completely dependent on the initial conditions, whereas for a trimodal distribution, the coexistence region disappears depending on the values of the system parameters.

Effective-field-theory analysis of the three-dimensional random-field Ising model on isometric lattices.

An Ising model with quenched random magnetic fields is examined for single-Gaussian, bimodal, and double-Gaussian random-field distributions by introducing an effective-field approximation that takes into account the correlations between different spins that emerge when expanding the identities. Random-field distribution shape dependencies of the phase diagrams and magnetization curves are investigated for simple cubic, body-centered-cubic, and face-centered-cubic lattices. The conditions for the occurrence of reentrant behavior and tricritical points on the system are also discussed in detail.