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.

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.