H. Yibole

Taming the First‐Order Transition in Giant Magnetocaloric Materials

Large magnetically driven temperature changes are observed in MnFe(P,Si,B) materials simultaneously with large entropy changes, limited (thermal or magnetic) hysteresis, and good mechanical stability. The partial substitution of B for P in MnFe(P,Si) compounds is found to be an ideal parameter to control the latent heat observed at the Curie point without deteriorating the magnetic properties, which results in promising magnetocaloric properties suitable for magnetic refrigeration.

Magnetocaloric effect, cyclability and coefficient of refrigerant performance in the MnFe(P, Si, B) system

MnFeP0.595Si0.33B0.075 has recently been presented as a top class magnetocaloric material combining a large magnetocaloric entropy change, a large temperature change, limited thermal hysteresis, and an enhanced mechanical stability. By providing practical rules to control the transition temperature in the MnFe(P,Si,B) system, we demonstrate that this new material was not a single composition and that a giant magnetocaloric effect (MCE) can be observed over a broad temperature range, a point of great interest for applications. As important prerequisite is the cyclability of the MCE. The thermal hysteresis and the recovery of the MCE during field oscillations have been addressed for MnFe(P,Si,B) materials. It is found that when the thermal hysteresis becomes about as large as the field induced shift of the transition, the MCE becomes partially irreversible and a strong decrease in the cyclic temperature change occurs. For an intermediate field change, typically 1 T, the limit for thermal hysteresis is about...

Moment evolution across the ferromagnetic phase transition of giant magnetocaloric (Mn, Fe)(2)(P, Si, B) compounds

A strong electronic reconstruction resulting in a quenching of the Fe magnetic moments has recently been predicted to be at the origin of the giant magnetocaloric effect displayed by Fe2P-based materials. To verify this scenario, x-ray magnetic circular dichroism experiments have been carried out at the L edges of Mn and Fe for two typical compositions of the (Mn,Fe)2(P,Si,B) system. The dichroic absorption spectra of Mn and Fe have been measured in the vicinity of the first-order ferromagnetic transition. The experimental spectra are compared with first-principles calculations and charge-transfer multiplet simulations in order to derive the magnetic moments. Even though signatures of a metamagnetic behavior are observed either as a function of the temperature or the magnetic field, the similarity of the Mn and Fe moment evolution suggests that the quenching of the Fe moment is weaker than previously predicted.

Field Dependence of the Magnetocaloric Effect in MnFe(P,Si) Materials

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 (ΔS) and the temperature change (ΔT) 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 ΔS and ΔT, 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.

Transition Metal Based Magneto Caloric Materials for Energy Efficient Heat Pumps

Magnetic refrigeration near room temperature is considered as an environmentally benign alternative for the current compressor-based cooling technology. Two materials that are based on ferromagnetic transition metal compounds are considered as the most promising candidates to be used in real world applications. Here we discuss the main features of these materials.

A universal metric for ferroic energy materials

After almost 20 years of intensive research on magnetocaloric effects near room temperature, magnetic refrigeration with first-order magnetocaloric materials has come close to real-life applications. Many materials have been discussed as potential candidates to be used in multicaloric devices. However, phase transitions in ferroic materials are often hysteretic and a metric is needed to estimate the detrimental effects of this hysteresis. We propose the coefficient of refrigerant performance, which compares the net work in a reversible cycle with the positive work on the refrigerant, as a universal metric for ferroic materials. Here, we concentrate on examples from magnetocaloric materials and only consider one barocaloric experiment. This is mainly due to lack of data on electrocaloric materials. It appears that adjusting the field-induced transitions and the hysteresis effects can minimize the losses in first-order materials. This article is part of the themed issue ‘Taking the temperature of phase transitions in cool materials’.

First-order ferromagnetic transition in single-crystalline (Mn,Fe)2(P,Si)

(Mn,Fe)2(P,Si) single crystals have been grown by flux method. Single crystal X-ray diffraction demonstrates that Mn0.83Fe1.17P0.72Si0.28 crystallizes in a hexagonal Fe2P crystal structure (space group P6¯2m) at both 100 and 280 K, in the ferromagnetic and paramagnetic states, respectively. Magnetization measurements show that the crystals display a first-order ferromagnetic phase transition at their Curie temperature (TC). The preferred magnetization direction is along the c axis. A weak magnetic anisotropy of K1 = 0.28 × 106 J/m3 and K2 = 0.22 × 106 J/m3 is found at 5 K. A series of discontinuous magnetization jumps is observed far below TC by increasing the field at constant temperature. These magnetization jumps are irreversible, occur spontaneously at a constant temperature and magnetic field, but can be restored by cycling across the first-order phase transition.

High field measurement of the magnetocaloric effect in MnFe(P, Si) materials

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. 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 ΔTad, a tunable Curie temperature (TC) 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. 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 ΔTad 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 ΔTad 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, ΔTd and quantifying the B* in MnFe(P, X) is required for further optimizations.