Structure and magnetic properties of magnetocaloric Mn$_{3}$Fe$_{2}$Si$_{3}$

The magnetocaloric effect (MCE) is thermodynamically described by the temperature change of a magnetic material upon adiabatic change of magnetic field. This effect affords a sustainable technique for refrigeration, if cheap and environment-friendly materials are used. Therefore, it attracts the attention of many scientific studies.In this thesis we are interested in the compound Mn$_{3}$Fe$_{2}$Si$_{3}$, which belongs to the Mn$_{5-x}$Fe$_{x}$Si$_{3}$ (0$\leqslant$ x$\leqslant$5). These compounds fit to the new paradigm that the existence of different magnetic sites in a compound is beneficial for a large MCE. First part of this thesis is devoted to macroscopic investigation on the single crystalline specimen of the Mn$_{3}$Fe$_{2}$Si$_{3}$. The magnetic field vs temperature of Mn$_{3}$Fe$_{2}$Si$_{3}$ is similar yet not identical to the parent compound Mn$_{5}$Si$_{3}$. The Mn$_{3}$Fe$_{2}$Si$_{3}$ compound exhibits two antiferromagnetic phase transitions AF1 and AF2. The magnetic response is anisotropic, e.g. the transition AF2 to PM is visible only, if the field is applied perpendicular [001]. Compared to Mn$_{5}$Si$_{3}$, which features a field driven transition from AF1 to AF2, no remarkable increase is observed in the magnetization of Mn$_{3}$Fe$_{2}$Si$_{3}$ and accordingly only a small isothermal entropy change is derived. The second and third part of this thesis is about Crystal and magnetic structure investigation by means of synchrotron radiation and neutron diffraction. The results show that Mn$_{3}$Fe$_{2}$Si$_{3}$ crystallizes in hexagonal space group $P$6$_{3}$/$mcm$. No clear sign of a structural phase transition down to lowest temperature, nor any indication of a clear response of the lattice parameter to the magnetic transitions. Mn$_{3}$Fe$_{2}$Si$_{3}$ has a collinear magnetic structure in the AF2 phase and a non-collinear magnetic structure in AF1 phase. The both phases have ordered moments already on the M1 site and 2/3 of the M2 sites. By comparing the magnetic structure of Mn$_{3}$Fe$_{2}$Si$_{3}$ and the parent compound Mn$_{5}$Si$_{3}$, we find strong similarities in the magnetic structure with the distinct difference of the ordering on the M1 site. We associate these differences with the strength of the magneto-caloric properties