Single phase Mn_0.66Fe_1.29P_1-xSi_x (0 ≤ x ≤ 0.42) compounds were synthesized using the melt-spinning (rapid solidification) technique. All the compounds form in the Fe₂P-type hexagonal structure, except a Co₂P-type orthorhombic structure of the Si-free Mn_0.66Fe_1.29P compound. The compounds with 0.24 ≤ x ≤ 0.42 present a FM-PM phase transition, while the compounds with lower Si content show an AFM-PM phase transition. In the Mn_0.66Fe_1.29P_1-xSi_x compounds, T_C and ΔT_hys are not only Si content dependent, but also magnetic field dependent. By increasing the Si content from x = 0.24 to 0.42, T_C increases from 195 to 451 K and ΔT_hys is strongly reduced from ∼61 to ∼1 K. T_C increases and ΔT_hys decreases with increasing magnetic field, ΔT_C/ΔB is about 4.4 K/T. Mn_0.66Fe_1.29P_1-xSi_x compounds show large saturation magnetic moments with values up to 4.57 μ_B/f.u. A large MCE with a small thermal hysteresis is obtained simultaneously in Fe-rich Mn_0.66Fe_1.29P_1-xSi_x melt-spun ribbons.

Neutron diffraction, Mössbauer spectroscopy, magnetometry, and in-field x-ray diffraction are employed to investigate the magnetoelastic phase transition in hexagonal (Mn,Fe)2(P,Si) compounds. (Mn,Fe)2(P,Si) compounds undergo for certain compositions a second-order paramagnetic (PM) to a spin-density-wave (SDW) phase transition before further transforming into a ferromagnetic (FM) phase via a first-order phase transition. The SDW-FM transition can be kinetically arrested, causing the coexistence of FM and untransformed SDW phases at low temperatures. Our in-field x-ray diffraction and magnetic relaxation measurements clearly reveal the metastability of the untransformed SDW phase. This unusual magnetic configuration originates from the strong magnetoelastic coupling and the mixed magnetism in hexagonal (Mn,Fe)2(P,Si) compounds.

Magnetic cooling is a highly efficient refrigeration technique with the potential to replace the traditional vapor compression cycle. It is based on the magnetocaloric effect, which is associated with the temperature change of a material when placed in a magnetic field. We present experimental evidence for the origin of the giant entropy change found in the most promising materials, in the form of an electronic reconstruction caused by the competition between magnetism and bonding. The effect manifests itself as a redistribution of the electron density, which was measured by X-ray absorption and diffraction on MnFe(P,Si,B). The electronic redistribution is consistent with the formation of a covalent bond, resulting in a large drop in the Fe magnetic moments. The simultaneous change in bond length and strength, magnetism, and electron density provides the basis of the giant magnetocaloric effect. This new understanding of the mechanism of first order magneto-elastic phase transitions provides an essential step for new and improved magnetic refrigerants.

The spatial and temporal correlations of magnetic moments in the paramagnetic regime of (Mn,Fe)2(P,Si) have been investigated by means of polarized neutron diffraction and muon-spin relaxation techniques. Short-range magnetic correlations are present at temperatures far above the ferromagnetic transition temperature (TC). This leads to deviations of paramagnetic susceptibility from Curie-Weiss behavior. These short-range magnetic correlations extend in space, slow down with decreasing temperature, and finally develop into long-range magnetic order at TC.

We present direct measurements of the magnetocaloric effect on a Fe₂P-based compound induced by a milliseconds pulsed magnetic field of 1 T to test their possible use in high frequency (up to 100 Hz) thermomagnetic cycles. The reported measurements were performed with an innovative and versatile non-contact set up based on the mirage effect. The adiabatic temperature change of a MnFeP_0.45As_0.55 sample is presented and compared with measurements performed varying the same magnetic field in a time interval of 1 s and 100 ms. These results demonstrate the absence of kinetic constraints in the first-order phase transition of this sample induced on the milliseconds time scale. The study of the materials' response to millisecond magnetic field pulses represents a fundamental test for the development of more powerful and efficient magnetic refrigerators.

(Mn, Fe)₂(P, Si)-type compounds are, to date, the most promising materials for refrigeration and energy conversion applications due to the combination of highly tunable giant magnetocaloric effect (GMCE) and low material cost.[1, 2] The GMCE of these compounds originates from the first-order magnetoelastic transition around the magnetic phase-transition temperature T_C. However, the phase-transition temperature shows a peculiar thermal-history dependence in these compounds. As-prepared (Mn, Fe)₂(P, Si) displays a significantly lower T_C upon first cooling than on second and subsequent cooling processes. Since this behavior is only observed in as-prepared samples it is called the 'virgin effect'. The difference in T_C between the first and second cooling processes of the as-prepared sample, hereafter referred to as ΔT_C0, is taken as a measure of how strong the virgin effect is. The virgin effect is not exclusive to (Mn, Fe)₂(P, Si) compounds being observed in other GMCE materials[3, 4], however its origin was for a long time unknown. In this study, we report our high-resolution neutron diffraction experiments that finally shed light on the origin of the virgin effect. Additionally, recovery of the virgin effect induced by thermal activation was observed experimentally.