Paramagnetic fluctuations of the magnetocaloric compound MnFe$_4$Si$_3$

Abstract

Inelastic neutron scattering technique is employed to investigate the paramagnetic spin dynamics in a single crystalline sample of the magnetocaloric compound MnFe$_4$Si$_3$. In the investigated temperature range, 1.033$\times T_{C}$ to 1.5$\times T_{C}$, where $T_C$ is the Curie temperature, the spin fluctuations are well described by the ferromagnetic Heisenberg model predictions. Apart from the Heisenberg exchange, additional pseudo-dipolar interactions manifest through a finite long-wavelength relaxation rate that vanishes at the transition temperature ($T_C = 305$\,K). Based on the characteristic extend of spin fluctuations in wave-vector and energy space we determine that the nature of magnetism in MnFe$_4$Si$_3$ is localized above room temperature. This contrasts with the most celebrated Mn and Fe based magnetocaloric materials that are considered as itinerant magnets.