Disorder and magnetic excitations in CaC rxF e2 -xO4 (x =0 ,0.5 )

Polycrystalline CaF e₂O₄ and CaC r_0.5F e_1.5O₄ have been investigated by elastic and inelastic neutron scattering. In agreement with previous reports, CaF e₂O₄ undergoes two magnetic transitions, first to a B phase, below T_NB=200 K , then to an A phase, below T_NA=175 K , while substituted CaC r_0.5F e_1.5O₄ undergoes a magnetic transition to the B phase only, at T_N=125 K . Each phase corresponds to staggered antiferromagnetic chains coupled either ferromagnetically (A phase) or antiferromagnetically (B phase). In the A phase of CaF e₂O₄ , inelastic scattering measurements show clearly defined gapped spin waves, which can be modeled with classical calculations, based on a simple exchange Hamiltonian following the topology of the crystal structure. In contrast, in the B phase of both compounds, the interpretation of the excitation spectrum evades completely the classical approach, even at low temperature. These results are interpreted based on an interchain exchange close to the threshold between ferromagnetic and antiferromagnetic bonding geometry. This induces random interchain coupling, thus creating magnetic exchange disorder whose dominating effect is to blur out the magnetic excitation spectrum of the B phase. A magnetoelastic effect, through which the interchain coupling becomes sizably ferromagnetic, and which is not observed in CaC r_0.5F e_1.5O₄ , stabilizes the A phase at low temperature in CaF e₂O₄ .