Magnetic correlations in the paramagnetic phases of MnAs

The magnetic scattering in the paramagnetic phases of MnAs has been investigated in a spin polarised neutron scattering experiment. The magnetic correlations are found to indicate ferromagnetic short-range order which persists in both the orthorhombic as well as the high temperature hexagonal modification of MnAs. In particular, the unusual magnetic properties of the orthorhombic phase, as revealed by magnetisation measurements, are not mirrored in the magnetic scattering results which have been obtained in an almost zero external field experiment. The magnitude and the scattering vector dependence of the magnetic moment do not change as a function of temperature. It is argued that the unusual temperature dependence of the magnetic susceptibility is a phenomenon induced by the application of an external magnetic field and arises due to a coupling of magnetic and lattice degrees of freedom.