Magnetic and structural properties of high quality magnetocaloric MnFe 4 Si 3 single crystals are investigated macroscopically and on an atomic scale. Refinements of combined neutron and x-ray single crystal diffraction data introduce a new structural model in space group P 6 characterized by partial ordering of Mn and Fe into layers perpendicular to c on one of the transition metal sites. A second transition metal site is exclusively occupied by iron. MnFe 4 Si 3 has a phase transition to a ferromagnetically ordered phase at approximately 300 K and displays a strong anisotropy of the 1 magnetization and the magnetocaloric effect with the easy axis of magnetization in the a, b-plane. This is confirmed by a refinement of the magnetic structure in the magnetic spacegroup P m ′ which shows that the spins on the sites with mixed occupancy of Mn and Fe are aligned in the a, b-plane. A significant magnetic moment for site exclusively occupied by iron could not be refined. The thermal evolution of the lattice parameters exhibit an anisotropic behavior and clearly reflects the onset of magnetic ordering. Comparison of the ordered moment and the effective paramagnetic moment hints towards itinerant magnetism in the system.

The room temperature magnetocaloric material MnFe 4 Si 3 was investigated with nuclear inelastic scattering (NIS) and resonant ultrasound spectroscopy (RUS) at different temperatures and applied magnetic fields in order to assess the influence of the magnetic transition and the magnetocaloric effect on lattice dynamics. The NIS data give access to phonons with energies above 3 meV, whereas RUS probes the elasticity of the material in the MHz frequency range and thus low-energy, ∼ neV, phonon modes. A significant influence of the magnetic transition on the lattice dynamics is observed only in the low-energy, long-wavelength limit. MnFe 4 Si 3 and other compounds in the Mn 5-x Fe x Si 3 series were also investigated with vibrating sample magnetometry, resistivity measurements and Mössbauer spectroscopy in order to study the magnetic transitions and to complement the results obtained on the lattice dynamics.

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The magnetic excitation spectrum of the magnetocaloric compound MnFe 4 Si 3 has been investigated by means of polarized and unpolarized inelastic neutron scattering on single crystals. Spectra were collected in the ferromagnetic phase (T C ≈ 305 K), as well as in the paramagnetic state, in order to understand the nature of the magnetism in MnFe 4 Si 3. Spin-wave measurements at 1.5 K reveal a strong anisotropy of the magnetic exchange interactions along the (h00) and (00l) reciprocal directions of the hexagonal system, which also manifests itself in the q-dependent linewidths in the paramagnetic state. The correlation lengths indicate a short-range order, while the average linewidth is of the order of k B T C pointing to a behavior typical of many ferromagnets. In addition, the in-and out-of-plane spin fluctuations are found to be isotropic around T C and can be suppressed by a magnetic field of 2 T.