Magnetic Excitations in the Disordered System Mg 1-x Co x Cl 2

We report in this paper our experiments about the magnetic excitations in the disordered system Mg1-xCoxCl2. The compound CoCl2 has the hexagonal CdC12 structure and is antiferromagnetic below 24.9K. The or- dered state spin structure is such that spins within a metal layer form a ferromagnetic sheet and the spins in adjacent layers are antiparallel/1/. The spins are confined into the c-plane and the anisotropy within this plane is very small. When the external magnetic field is applied in the c-plane, a transition from the antiferromagnetic to ferromagnetic(saturated paramagnetic) phase takes place at the critical field denoted by Hs. S When CoCl2 is randomly diluted with diamagnetic MgCl2, an imbalance between the ferromagnetic and antiferromagnetic exchange interactions in the c-plane occurs. Then, Mg1-xCoxCl2 is a candidate for a planar(XY) spin-glass.