SPIN

Electric field-induced magnetization switching in multiferroics is intriguing for both fundamental studies and potential technological applications. Here, we review the recent developments on electric field-induced magnetization switching in multiferroic heterostructures. Particularly, we study the dynamics of magnetization switching between the two stable states in a shape-anisotropic single-domain nanomagnet using stochastic Landau–Lifshitz–Gilbert (LLG) equation in the presence of thermal fluctuations. For magnetostrictive nanomagnets in strain-coupled multiferroic composites, such study of magnetization dynamics, contrary to steady-state scenario, revealed intriguing new phenomena on binary switching mechanism. While the traditional method of binary switching requires to tilt the potential profile to the desired state of switching, we show that no such tilting is necessary to switch successfully since the magnetization’s excursion out of magnet’s plane can generate a built-in asymmetry during switching. We also study the switching dynamics in multiferroic heterostructures having magnetoelectric coupling at the interface and magnetic exchange coupling that can facilitate to maintain the direction of switching with the polarity of the applied electric field. We calculate the performance metrics like switching delay and energy dissipation during switching while simulating LLG dynamics. The performance metrics turn out to be very encouraging for potential technological applications.

A model calculation is presented for the coherent magnon transmission and thermal transport at ferromagnetic nanojunction boundaries. The system consists of a Gd ultrathin film sandwiched between two Fe semi-infinite ferromagnetically ordered crystals. The dynamic of the system is analyzed using the equations of motion for the spin precession amplitudes on the lattice sites, valid for the range of temperatures of interest. The coherent transmission and reflection cross sections at the nanojunction boundary are calculated using the matching method. These calculations are presented for arbitrary directions on the boundary, for all accessible frequencies in the propagating bands, at variable temperatures and for a given thicknesses of the ultrathin nanojunction, with no externally applied magnetic field. The model is applied in particular to the Fe/Gd(5)/Fe system with a ferromagnetic Gd nanojunction. Our model yields the total integrated coherent thermal conductivity due to coherent magnons transmission via the sandwiched five Gd spin layers of the nanojunction. It elucidates, in particular, the dependence of the coherent magnons transmission and thermal transport in relation to the spatially inhomogeneous magnetic order of the atomic planes of the nanojunction for a given thickness.

Circular magnetic spin valve nanopillars in a dual vortex configuration have dynamic characteristics strongly dependent on the interlayer dipole coupling. We report here on frequency domain properties of such nanopillars obtained by micromagnetic simulations. After the free layer is chirality switched with spin transfer torque, a radial spin wave eigenmode spectrum forms in the free layer with unusually large edge amplitude. The structure of these modes indicate a departure from the magnetostatic processes typically observed experimentally and treated analytically in low aspect ratio isolated disks. Our findings give new details of dynamic chirality control and relxation in nanopillars and raise potential signatures for experiments.

We performed a detailed study on the perpendicular magnetic anisotropy (PMA) of TbCo film by using two kinds of nonmagnetic underlayers, Ta and Ta/Cu with different thicknesses. We found that for both the Tb-rich and Co-rich TbCo alloy films, the PMA strength decreases considerably with the increase of Ta underlayer thickness, while their net saturation magnetization $M_{\mathrm{\text{s}}}$ exhibit opposite varying trends. The $M_{\mathrm{\text{s}}}$ value continues to increase for the Co-rich TbCo samples while decrease for the Tb-rich films. Interestingly, an additional Cu layer inserted between the Ta and TbCo layers can efficiently recover the PMA and $M_{\mathrm{\text{s}}}$. We attribute such observed variation behaviors of magnetic properties to the increased disordering of Tb magnetic moments at the Ta/TbCo interface, which has been verified by both experimental measurements and micromagnetic simulations.

The alternating Fe–Mn layered structures of the compounds FeMnAs_xP${}_{1-x}$ display properties which have been demonstrated experimentally as very promising as far as commercial applications of the magnetocaloric effect are concerned. However, the theoretical literature on this and other families of magnetocaloric compounds still adopts simple molecular-field models in the description of important statistical mechanical properties like the entropy variation that accompanies applied isothermal magnetic field cycling, as well as the temperature variation following adiabatic magnetic field cycles. In the present paper, a random phase approximation Green function theoretical treatment is applied to such structures. The advantages of such approach are well known since the details of the crystal structure are easily incorporated in the model, as well as a precise description of correlations between neighbor spins can be obtained. We focus on a simple one-exchange parameter Heisenberg model, and the observed first-order phase transitions are reproduced by the introduction of a biquadratic term in the Hamiltonian whose origin is related both to the magnetoelastic coupling with the phonon spectrum in these compounds as well as with the values of spins in the Fe and Mn ions. The calculations are compared with experimental magnetocaloric data for the FeMnAs_xP${}_{1-x}$ compounds. In particular, the magnetic field dependence for the entropy variation at the transition temperature predicted from the Landau theory of continuous phase transitions is reproduced even in the case of discontinuous transitions.

First principles calculations of spin-dependent electronic transport properties of magnetic tunnel junction (MTJ) consisting of MgO adsorbed graphene nanosheet sandwiched between two CrO₂ half-metallic ferromagnetic (HMF) electrodes is reported. MgO adsorption on graphene opens bandgap in graphene nanosheet which makes it more suitable for use as a tunnel barrier in MTJs. It was found that MgO adsorption suppresses transmission probabilities for spin-down channel in case of parallel configuration (PC) and also suppresses transmission in antiparallel configuration (APC) for both spin-up and spin-down channel. Tunnel magneto-resistance (TMR) of 100% is obtained at all bias voltages in MgO adsorbed graphene-based MTJ which is higher than that reported in pristine graphene-based MTJ. HMF electrodes were found suitable to achieve perfect spin filtration effect and high TMR. I–V characteristics for both parallel and antiparallel magnetization states of junction are calculated. High TMR suggests its usefulness in spin valves and other spintronics-based applications.