F. Guillou

Direct measurement of the magnetocaloric effect in MnFe(P,X)(X = As, Ge, Si) materials

An investigation of the magnetocaloric effect (MCE) displayed by three generations of MnFe(P,X) (Xxa0=xa0As, Ge, Si) materials has been carried out by combining indirect ΔS and direct ΔTad measurements. To be able to compare the performances of the new Si-based system with the already well-known As- and Ge-based materials in optimal conditions, both the Mn/Fe and P/Si ratios of the MnxFe1.95−xP1−ySiy compounds were optimized to display the largest MCE around room temperature. Here, we show that the maximum values of ΔTad (ΔBxa0=xa01.1xa0T) and ΔS (ΔBxa0=xa01xa0T) are respectively ∼2.2xa0K and ∼8xa0Jxa0kg−1xa0K−1 in the Si-based material Mn1.25Fe0.7P0.49Si0.51. These values are very close to the MCE performances of the As-based and Ge-based compounds. A critical comparison of these three MnFe(P,X) series highlights the role played by the non-magnetic elements on the latent heat at the Curie temperature. The combination of: (i) large ΔS and ΔTad in intermediate magnetic fields, (ii) limited thermal/magnetic hysteresis, (iii) easy tunability of the Curie temperatures and (iv) practical advantages like cheap, non-critical and non-toxic starting materials; makes the MnxFe1.95−xP1−ySiy family highly promising for magnetic refrigeration applications.

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 1u2009T, the limit for thermal hysteresis is about...

Tuning the metamagnetic transition in the (Co, Fe)MnP system for magnetocaloric purposes

The inverse magnetocaloric effect taking place at the antiferro-to-ferromagnetic transition of (Co,Fe)MnP phosphides has been characterised by magnetic and direct ΔTad measurements. In Co0.53Fe0.47MnP, entropy change of 1.5u2009Jkg−1u2009K−1 and adiabatic temperature change of 0.6u2009K are found at room temperature for an intermediate field change (ΔBu2009=u20091u2009T). Several methods were used to control the metamagnetic transition properties, in each case, a peculiar splitting of the antiferro-to-ferromagnetic transition is observed.

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.

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 280u2009K, 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 K1u2009=u20090.28u2009×u2009106u2009J/m3 and K2u2009=u20090.22u2009×u2009106u2009J/m3 is found at 5u2009K. 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.