Frustration effects in the disordered system CsMnFeF6, studied by neutron scattering, ac susceptibility, and magnetization measurements

In CsMnFe${\mathrm{F}}_6$, which has a cubic structure ($a=10.5\mathrm{}$ Å), space group $\mathrm{Fd}3m$, the magnetic ions ${\mathrm{Mn}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ form regular tetrahedra. The system should exhibit frustration effects, since the nearest-neighbor interactions are antiferromagnetic in character. We performed neutron scattering, ac susceptibility, and magnetization measurements both on a powder and a single crystal, for temperatures $1.2\le T\le 300$ K and applied magnetic fields $0\le H\le 50$ kOe. It was found that already at $T=300\mathrm{}$ K there are well-developed clusters of antiferromagnetically coupled spins. These clusters have a weak resulting moment. Lowering the temperature to $T\simeq 80$ K leads to an increase of the effective moment due to the reduction of the thermal motions, whereas this moment remains constant for $T\lesssim 80$ K. At $T=26.3\mathrm{}$ K the cluster moments freeze in—a process which is accompanied by a ferromagnetic ordering of the resulting moments of the clusters. It is suggested that an applied magnetic field breaks the large domains, containing many clusters, into microdomains containing only a few clusters, or even one.