Structure, Magnetism, and the Magnetocaloric Effect of MnFe₄Si₃ Single Crystals and Powder Samples

Abstract

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.

Elasticity and magnetocaloric effect inMnFe4Si3

“…The strong change of the isomer shift of the 4d and 6g site with opposite sign at the transition is surprising, as such strong change in isomer shift is often explained by a volume change affecting the electron density at the nucleus. Though, the isomer shift should change with equal sign [28] and it was already mentioned that the volume changes only slightly about the transition [8,9]. The hyperfine field of the 4d site in MnFe 4 Si 3 is only ≈ 75% of the value for the averaged 6g site, see figure 5.…”

“…The importance of next nearest-neighbor interactions is not surprising, especially for the 6g site, in view of the inter-atomic distances given in the introduction. The fit performed with the Bean and Rodbell model leads to a value of η = 0.28 (9) [26]. In order to assess the influence of the magnetic transition on the lattice dynamics,…”

“…The deviation from the linear trend in the ZFC (initial ) measurement of c 33 could be related to additional stress due to the magnetic interaction. The spins, which lie in the ab plane [9], need to overcome the magnetocrystalline anisotropy in order to point along the c direction. This stress was probably not present in the paramagnetic and the FC phase, as indicated by the linear behavior of c 33 .…”

“…For this purpose, the Mn 5-x Fe x Si 3 series is particularly interesting because the volume change due to the magnetic transition is quite small (see Ref. [8,9] or the supplemental material). This small change in volume allows for synthesizing single crystals without breaking and cracking upon crossing the transition temperature, as usually observed for other MCE materials with a first order transition [10].…”

“…Recent results [8,9] indicate that this crystal structure is only approximate, because the metal sites are split with different occupations. However, for the following data analysis, the approximation of the P 6 3 /mcm structure is a useful simplification.…”