2 research outputs found
Positional Disorder, Spin-Orbit Coupling and Frustration in GaMnAs
We study the magnetic properties of metallic GaMnAs. We calculate the
effective RKKY interaction between Mn spins using several realistic models for
the valence band structure of GaAs. We also study the effect of positional
disorder of the Mn on the magnetic properties. We find that the interaction
between two Mn spins is anisotropic due to spin-orbit coupling within both the
so-called spherical approximation and in the more realistic six band model. The
spherical approximation strongly overestimates this anistropy, especially for
short distances between Mn ions. Using the obtained effective Hamiltonian we
carry out Monte Carlo simulations of finite and zero temperature magnetization
and find that, due to orientational frustration of the spins, non-collinear
states appear in both valence band approximations for disordered, uncorrelated
Mn impurities in the small concentration regime. Introducing correlations among
the substitutional Mn positions or increasing the Mn concentration leads to an
increase in the remnant magnetization at zero temperature and an almost fully
polarized ferromagnetic state.Comment: 17 Pages, 13 Figure
Disorder, spin-orbit, and interaction effects in dilute
We derive an effective Hamiltonian for in
the dilute limit, where can be described in
terms of spin polarons hopping between the {\rm Mn} sites and coupled
to the local {\rm Mn} spins. We determine the parameters of our model from
microscopic calculations using both a variational method and an exact
diagonalization within the so-called spherical approximation. Our approach
treats the extremely large Coulomb interaction in a non-perturbative way, and
captures the effects of strong spin-orbit coupling and Mn positional disorder.
We study the effective Hamiltonian in a mean field and variational calculation,
including the effects of interactions between the holes at both zero and finite
temperature. We study the resulting magnetic properties, such as the
magnetization and spin disorder manifest in the generically non-collinear
magnetic state. We find a well formed impurity band fairly well separated from
the valence band up to for which finite size
scaling studies of the participation ratios indicate a localization transition,
even in the presence of strong on-site interactions, where is the fraction of magnetically active Mn. We study the
localization transition as a function of hole concentration, Mn positional
disorder, and interaction strength between the holes.Comment: 15 pages, 12 figure