Spin fluctuations in an octahedral antiferromagnet Mn3Pt alloy

A new spin frustration system with an octahedral configuration of magnetic atoms was found. In neutron scattering measurements for an ordered Mn3Pt alloy, inelastic diffuse scattering intensity distributes along the first Brillouin zone-boundary of the fee structure in the paramagnetic and the F-phases. Within the calculations based on a RPA, the experimental data in the paramagnetic phase are reproduced using antiferromagnetic couplings between the spins up to the third-neighbor exchange interactions. In the F-phase below the Neel temperature, a part of the Mn spins still fluctuates at the special site. The existence of the F-phase is a characteristic of the octahedral spin frustration system. The effects of the chemical disorder on the spin frustration are also studied using a specimen with a small atomic LRO parameter.