An unconventional phenomena is observed at the first-order magnetic transitions in (Mn,Fe)₂(P,Si) materials. Here, we show that the first crossing of the transition upon cooling is associated with an abnormal temperature increase. While differential scanning calorimetry can detect this recalescence-like event, purposely-designed probes were employed to quantify it. Recalescence at a magnetic transition is extremely rare. But in (Mn,Fe)₂(P,Si), it is even more remarkable by its amplitude, with the temperature rising up to +4.0 K. In (Mn,Fe)₂(P,Si), this phenomenon is associated with irreversible burst-like evolution of the microstructure (increase in defect concentration and micro-cracking) and of the crystal structure.

The lattice dynamics in MnFe0.95Si0.50P0.50 were investigated experimentally using Fe57 nuclear inelastic scattering and inelastic x-ray scattering across the first-order magnetic transition which occurs close to room temperature. The lattice dynamics characterization was supported by a macroscopic magnetic characterization, an x-ray diffraction study, and a hyperfine interactions characterization using Mössbauer spectroscopy. The Fe specific and the x-ray generalized density of phonon states were obtained both in the ferromagnetic and in the paramagnetic state. A prominent shift, 2meV at 20meV, in the x-ray generalized density of phonon states across the first-order magnetic transition, that involves vibrations with essentially Fe character, is revealed corroborated by a change in the local environment quantified in the isomer shift and the quadrupole splitting. Above 35meV the vibrational modes are practically insensitive to the magnetic transition. The entropy change induced by a 1T magnetic field across the magnetic transition, ∼10J/K/kg, is only a fraction of the Fe vibrational entropy change, 62(21)J/K/kg.

Given the potential applications of (Mn,Fe₂(P,Si))-based materials for room-temperature magnetic refrigeration, several research groups have carried out fundamental studies aimed at understanding the role of the magneto-elastic coupling in the first-order magnetic transition and further optimizing this system. Inspired by the beneficial effect of the addition of boron on the magnetocaloric effect of (Mn,Fe₂(P,Si))-based materials, we have investigated the effect of carbon (C) addition on the structural properties and the magnetic phase transition of Mn _1.25Fe _0.70P _0.50Si _0.50C _z and Mn _1.25Fe _0.70P _0.55Si _0.45C _z compounds by x-ray diffraction, neutron diffraction and magnetic measurements in order to find an additional control parameter to further optimize the performance of these materials. All samples crystallize in the hexagonal Fe ₂P -type structure (space group P-62m), suggesting that C doping does not affect the phase formation. It is found that the Curie temperature increases, while the thermal hysteresis and the isothermal magnetic entropy change decrease by adding carbon. Room-temperature neutron diffraction experiments on Mn _1.25Fe _0.70P _0.55Si _0.45C _z compounds reveal that the added C substitutes P/Si on the 2c site and/or occupies the 6k interstitial site of the hexagonal Fe ₂P -type structure.

Mn_1.000Fe_0.950P_0.595Si_0.330B_0.075compounds have been synthesized by high-energy ball milling and a subsequent solid-state reaction. The influence of the sintering conditions on the magnetic phase transition of Mn_1.00Fe_0.95P_0.595Si_0.33B_0.075samples has been systematically investigated using X-ray diffraction and magnetic measurements. The experimental results show that all the samples obtained after different heat treatment conditions crystallize in the Fe₂P-type hexagonal structure. The annealing temperature has a strong influence on the Curie temperature, which can be tuned between 265 and 298 K by sintering the samples at different temperatures between 1273 and 1373 K. The annealing time, however, does not significantly affect the Curie temperature. Both the annealing temperature and the annealing time have a significant effect on the magnetic entropy change. The magnetic entropy change under a magnetic field change of 1 T increases from 2.7 to 6.5 J kg⁻¹K⁻¹by increasing the annealing temperature from 1273 to 1373 K. By increasing the annealing time, the magnetic entropy change at first increases and then saturates after 20 h of heat treatment at 1373 K.

Recently, materials undergoing a first-order magnetic transition (FOMT) near room temperature have attracted much attentions due to the possibility to use their large magnetocaloric effect (MCE) for magnetic refrigeration [1]. Among them, the MnFe(P, X) (X = As, Ge, Si, B) family turns out to be one of the most promising due to the large isothermal entropy change ΔS, adiabatic temperature change ΔT_ad, a tunable Curie temperature (T_C) and the practical advantages. Till now, most of the MCE studies on MnFe(P, X) focused on the intermediate magnetic field range (B ≤ 2T) as it is the most relevant field for applications [2]. However, extending the field range of the MCE derivation is important from both fundamental and practical points of view. On one hand, it allows one to address the field dependence of the MCE quantities, the possible influence of the critical point, etc; On the other hand, high field ΔS or ΔT_ad data are useful for the optimization of the MCE at intermediate field. Indeed, at first glance, one can consider for FOMT that the ΔS or ΔT_ad will saturate above a given field value (B∗(ΔS) or B∗(ΔT)). The point is that in Giant-MCE materials, it might be advantageous to bring these B∗ (often at high field) as close as possible to the field used in application. Understanding the field dependence of ΔS, ΔT_d and quantifying the B∗ in MnFe(P, X) is required for further optimizations.