Transition Shifts in Heat-Assisted Magnetic Recording and Magnetoresistance Enhancement by Wave-Vector Filtering

Abstract

In the first part of this thesis, micromagnetic simulations are employed to study transition shifts induced by nonequilibrium spin dynamics in HAMR. These dynamical effects, including superparamagnetic (SP) switching, spin temperature lag, and switching time, cannot be captured by static analytical models. It is found that changes in head velocity and damping both introduce transition shifts due to their effects on the spin cooling rate. The transition positions obtained from simulations are compared to the predictions of an analytical model. It is shown that the analytical model overestimates the write temperature when the thermal profile is wide because SP switching is neglected; whereas for a narrow thermal profile, the analytical model underestimates the write temperature because the switching time is assumed to be zero. By investigating the transition shifts due to nonzero head field risetime (~0.1 ns), the effect of SP switching is found to be the same order of magnitude as the risetime and is dominant. Approaches based on spontaneous magnetization, damping constant of a block of spins, and spin energy are proposed to determine spin temperature and its lag relative to lattice temperature. The lag is determined to be of the order of 0.01 ns with typical HAMR parameters. Finally, the switching speed of a HAMR process is extracted from the head velocity effect, and is found to be ~0.05 ns. This high switching speed is confirmed by atomistic simulations under decreasing temperature. In the second part, the magnetoresistance (MR) of Fe/Ag/Fe/InAs/Ag(100) in the current-perpendicular-to-plane geometry is studied based on a tight-banding model with full spd bands, the Landauer-Büttiker formalism, and the recursive Green’s function technique. Results show that the system's MR can reach values above 1000%. This MR enhancement mainly is a result of the wave-vector filtering effect imposed by the InAs layer, restricting conductance within a small region around the Gamma point in the 2D Brillouin zone. Calculations also reveal that when the Fermi level sits in the InAs band gap, the MR gradually saturates as a function of the InAs thickness; whereas when the Fermi level is in the InAs conduction band and close to the band bottom, the MR exhibits an oscillatory behavior with a large period. MR oscillations are also observed with respect to the Ag thickness, with amplitudes determined by the Fermi level position relative to the InAs conduction band edge. The oscillation periods in both cases can be well explained by the concept of quantum-well states, and are determined by the spanning vector of the Fermi surface belly of the corresponding material. These MR oscillations are due to the quantum interference of conduction electrons near the Gamma point. The MR and area-resistance product (RA) profiles at a wide range of Fermi energies are compared. Near the MR peak (with MR above 1000%) in the InAs conduction band, the RA can be as low as 8.8 Ωμm^2. This feature of large MR but small RA results from the wave-vector filtering effect of doped InAs, and it makes the structure under study distinct from conventional giant-magnetoresistance systems (small MR, small RA) and magnetic tunnel junctions (large MR, large RA).