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.

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.

We study the spin Hall magnetoresistance effect in ferrimagnet/normal metal bilayers, comparing the response in collinear and canted magnetic phases. In the collinear magnetic phase, in which the sublattice magnetic moments are all aligned along the same axis, we observe the conventional spin Hall magnetoresistance. In contrast, in the canted phase, the magnetoresistance changes sign. Using atomistic spin simulations and x-ray absorption experiments, we can understand these observations in terms of the magnetic field and temperature dependent orientation of magnetic moments on different magnetic sublattices. This enables a magnetotransport based investigation of noncollinear magnetic textures.

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.

The field dependence of the magnetocaloric effect (MCE) in Mn_1.22Fe_0.73P_0.47Si_0.53 is studied in terms of the entropy change (Δ S) and the temperature change (Δ T) for applied magnetic fields up to 5 and 14 T, respectively. The magnetic fields required to saturate the MCE in this system are ∼ 1.7 and 4-5 T for Δ S and Δ T, respectively. The MCE field dependence is compared with the two approaches of the literature: 1) latent heat model and 2) the power law evolution predicted from the universal analysis of the MCE. It turns out that both of these methods are unsuitable to describe the MCE field evolution in MnFe(P,Si) materials.