Ultra-Fast Spin Dynamics in Diluted Magnetic Semiconductors

Abstract

In nano-size antiferromagnetic systems a spatially inhomogeneous field leads to the formation of a staggered magnetization. Thereby the total magnetic moment does not change but the formation of a net magnetic moment at the border of the cluster leads to an energy gain. This type of magnetism is characterised by an ultra-fast dynamics. We suggest it is also responsible for the formation of the exciton magnetic polaron.PACS numbers: 75.10.Jm, 75.50.-y A few recently discovered effects in diluted magnetic semiconductors are still not fully understood as yet. In particular, the characteristic time for the creation of an exciton magnetic polaron (EMP) is extremely short. It is much faster than any longitudinal relaxation time [1][2][3]. In addition, there is a "mysterious" coupling between antiferromagnetic (AF) layers in antiferromagnet-semiconductor superlattices [4] and also between antiferromagnetic micrograins which are characterised by a new type of magnetic resonance [5]. In this paper we suggest that for the explanation of this type of phenomena one has to consider the response of the magnetic system on a local, spatially inhomogeneous perturbation. The discussed effect has a pure quantum character therefore we analyse a rigorous solution of the HamiltonianThe first term in Eq. (1) describes the exchange energy, the second some magnetic anisotropy. We examine the solutions for linear chains rigorously. This is the simplest possible case where the above-mentioned phenomena can be demonstrated. Moreover, we limit our discussion to chains with an even spin number, N. For odd spin number the ground state is characterised by a 2S + 1 degeneracy, i.e., by a nonvanishing magnetic moment. In that case, the discussion of the magnetic properties is more complicated since the Curie magnetism caused by the magnetic (969)

Small Antiferromagnetic Clusters

“…When the cluster is big enough and the coupling energy of AF spins to their environment becomes com-parable to the excitation energy Δ then, because of a mixing of symmetric and antisymmetric states, the oscillating character is suppressed and a stable staggered magnetization appears [7]. Anyway, to reach the classical limit of AF, where local spins are characterized by a nonvanishing magnetization, an influence of additional (staggered or, generally, spatially nonhomogeneous) local flelds is necessary.…”