Magnetic Excitations in K2CoxFe1−xF4: A Mixed Magnetic System with Competing Spin Anisotropies
K2CoxFe1−xF4 is a good physical realisation of a two dimensional mixed magnetic system with competing anisotropies. The end members K2CoF4 and K2FeF4 have the K2NiF4 structure (Birgeneau et al. 1970) in which the magnetic ions are at the corners and body centre positions of a tetragonal unit cell. The a-b planes thus contain quadratic layers of magnetic ions within which the nearest neighbour antiferromagnetic exchange interaction is much greater than that between spins in adjacent layers, leading to quasi two dimensional magnetic properties. In K2CoF4 (Breed et al., 1969) the pseudo spin S = 1/2 aligns along the c-axis due to an anisotropy in the exchange interaction, giving a uniaxial ordered phase. In K2FeF4 (Macco et al., 1978) a single ion anisotropy forces the pseudo spin S = 2 in the a-b plane perpendicular to the c direction and a small in-plane anisotropy aligns the spins along the a or b axis of the magnetic unit cell. (Rotated 45° about c, relative to the chemical unit cell.) K2FeF4 therefore orders in a planar phase.
KeywordsSpin Wave Magnetic Excitation Tetragonal Unit Cell Spin Wave Dispersion Magnetic Unit Cell
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