Monte Carlo simulations of the fcc kagome lattice: Competition between triangular frustration and cubic anisotropy

Abstract

The impact of local cubic anisotropy on the magnetic states of the Heisenberg model on the fcc kagome lattice are examined through classical Monte Carlo simulations. Previous simulations revealed that the macroscopic degeneracy of the two-dimensional (2D) kagome exchange-coupled co-planar spin system partially persists in the 3D case of ABC stacked layers giving rise to a discontinuous phase transition. Local cubic anisotropy is shown to remove this degeneracy by re-orienting the spins out of the co-planar configuration. In addition, the re-oriented states are shown to carry a uniform magnetic moment. Simulation results indicate that the effect of anisotropy is to transform the first-order phase transition to a continuous one. These results are relevant to Ir-Mn alloys which have been widely used by the magnetic storage industry in thin-film form as the antiferromagnetic pinning layer in spin valves.

Monte Carlo simulations of a kagome lattice with magnetic dipolar interactions

“…The most popular compound for use as the antiferromagnet in spin valves for magnetic recording is IrMn 3 , which in its ordered crystalline fcc phase is composed of ABC stacked [111] planes of magnetic Mn ions on kagome sites. 4,5,[11][12][13][14][15] The large number of studies over the past few decades on the near-neighbor antiferromagnetic exchange Heisenberg kagome model have shown that the large degeneracy associated with the 120…”

Impact of further-range exchange and cubic anisotropy on magnetic excitations in the fcc kagome antiferromagnet IrMn3

“…Anisotropy serves to lift the spin vectors out of the plane and induce a finite magnetization in a 111 direction. 13 This effect is characterized by α, the cosine of the angle between each sublattice spin and its anisotropy axis (α = cos(S i • n i ), i = A, B, C), and β, the cosine of the angle with respect to the other two anisotropy axes (β = cos(S i • n j ), i = j) where α 2 + 2β 2 = 1. The modified ground state energy per spin that includes further-neighbor exchange is given by (compare Eqs.…”

“…Note that there are eight degenerate ground states corresponding to the eight 111 axes. 13 The analysis of spin excitations in this section correspond to a single domain [111] crystal. Powder averaged results are discussed in the following section.…”

“…This release of frustration drives the transition to be continuous. 13 This scenario was supported by spin wave calculations of the NN Heisenberg model with and without anisotropy which demonstrated that in the absence of anisotropy, the zero energy flat mode exists only in certain high symmetry directions in reciprocal space and that the addition of anisotropy induces a gap (Ref.14, hereafter referred to as I). Monte Carlo studies of [111] thin films confirmed that the q=0 magnetic order remains in these geometries and the impact of surface axial anisotropy was also considered.…”

Solitonlike magnetization textures in noncollinear antiferromagnets

“…The presence of the flat band is a direct consequence of the absence of interlayer couplings within our model. If we consider interlayer exchange interaction including the terms outside the plane (111) in the gradient expansion, the flat band is modified as is shown in 14,21 . …”