Best practices in evaluation of the magnetocaloric effect from bulk magnetization measurements

Conventional magnetometry is irreplaceable in evaluating bulk magnetization of materials over broad temperature and field ranges. The technique is also effective in quantifying hysteresis that may be associated with magnetic and structural phase transitions that occur during the magnetizing/demagnetizing cycling, and the derived magnetic field-induced isothermal entropy change – one of the most important properties in the field of magnetocalorics. Both systematic and random errors present during the measurements of magnetization, however, may lead to erroneous conclusions. Using two well-known materials – elemental Gd and intermetallic Gd5Si2Ge2 as examples, we consider best practices in performing reliable and rapid magnetization measurements for proper characterization of magnetocaloric properties

Manipulating the stability of crystallographic and magnetic sub-lattices: A first-order magnetoelastic transformation in transition metal based Laves phase

A first-order magnetoelastic transition (FOMT) is found near the triple point between ferromagnetic, antiferromagnetic and paramagnetic phases in the magneto-chemical phase diagram of (Hf1-xNbx)Fe2 Laves phase system. We show that bringing different magnetic states to the edge of stability, both as a function of the chemical composition and under the influence of external stimuli, such as temperature, pressure and magnetic field, is essential to obtain and control FOMTs. Temperature dependent X-ray diffraction experiments reveal a discontinuity in the lattice parameter a and the unit cell volume without the change in the crystal symmetry at the FOMT. Under applied pressure, the transition temperature drastically shifts downward at a remarkable rate of −122 K/GPa. It is this first-order magnetic transition that leads to a negative thermal expansion (NTE) with average ΔV/(VΔT) ≈ −15 × 10−6 K−1 observed over a 90 K broad temperature range, which is uncommon for magnetoelastic NTE materials. Density functional theory calculations and microstructural analyses demonstrate that the unusual broadness of the FOMT originates from phase separation between ferro- and antiferromagnetic phases, which in turn is rooted in partial segregation of Hf and Nb and a peculiar microstructure. This new understanding of the composition-structure-property relationships in transition metal based Laves phases is an essential step toward a better control and more precise tailoring of rich functionalities in this group of material

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 Fe2Pbased 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 element specific in the vicinity of the first-order ferromagnetic transition. The experimental spectra are compared with first-principle calculations and charge-transfer multiplet simulations in order to derive the magnetic moments. Even though signatures of a metamagnetic behaviour 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

Nature of the first-order magnetic phase transition in giant-magnetocaloric materials

This thesis reports on advanced characterizations of giant magnetocaloric materials that show a first order magnetic phase transition (FOMT). The results are of great interest not only for the design of new magnetic refrigerants, but also for a better understanding of the FOMT. This thesis paves the way for further developments and understanding of giant magnetoclaoric materials that exhibit a FOMT. One can already observe a surge of interest for magnetocaloric characterization techniques probing cyclic quantities in the literature. A comparison of the various (Mn,Fe)2(P,X) compounds with X= As, Si, Ge has shown that the key ingredient for control of the giant magnetocaloric properties is the latent heat of the FOMT. More specifically, to the Fe2P system, it has been shown that the metalloid elements have an important influence on the FOMT. The X-ray magnetic circular dichroism study at the transition metals L-edges contributes to the work on fundamental aspects of the FOMT in Fe2Pmaterials by characterizing the evolution of the magnetic and electronic properties across the FOMT. The first growth of (Mn,Fe)2(P,Si) single crystal that show a ferro-paramagnetic FOMT opens new perspectives and is an important pre-requisite for characterization techniques requiring single crystals.RST/Radiation, Science and TechnologyApplied Science

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'.RST/Fundamental Aspects of Materials and Energ

Correlation between deformation and total entropy change at the first-order magnetic transition of Fe2P-based magnetocaloric materials

While there is a growing consensus that lattice is an important source of entropy change at structural or elastic first-order magnetic transitions (FOMT) with finite volume change, the case of volume preserving transitions (ΔV = 0) remains more controversial. Here, the evolution of the total transition entropy change within the Fe2P family is presented as a function of the discontinuity on the ratio of the cell parameters. A striking correlation between ΔStr and Δ(c/a) is observed. This relationship is a first step towards a systematization of the FOMT properties in the Fe2P materials system. It also demonstrates that deformation can play a leading role in volume preserving FOMT

Magnetic properties, anisotropy parameters and magnetocaloric effect of flux grown MnFe4Si3 single crystal

International audienceThe transition-metal based alloy MnFe 4 Si 3 not only is a potential candidate for room temperature magnetocaloric applications, but also shows a large magnetic anisotropy forming an interesting case study in the search for rare-earth free permanent magnets. However, former polycrystalline and single crystal studies led to major disagreements about the order of the magnetic transition and the magnetocrystalline anisotropy scheme, which are two essential points for the understanding of this alloy. Here, magnetic, magnetocaloric properties and the magnetic anisotropy of MnFe 4 Si 3 (Mn~0 .86 Fe~4 .24 Si~2 .90) are investigated on a high quality single crystal grown by flux method, and compared to polycrystalline materials. Using the recently proposed criterion of field dependence of the magnetocaloric effect, we show that the ferromagnetic transition is more likely to be of second order, which is fully compatible with the absence of thermal hysteresis at the ferromagnetic transition in the present MnFe 4 Si 3 crystal. The c axis is confirmed to be the hard magnetic axis, both in single crystal and polycrystalline MnFe 4 Si 3, and a large, dominant, K 1 anisotropy constant (~−2.5 MJ m −3) is found at low temperatures

Best practices in evaluation of the magnetocaloric effect from bulk magnetization measurements

Conventional magnetometry is irreplaceable in evaluating bulk magnetization of materials over broad temperature and field ranges. The technique is also effective in quantifying hysteresis that may be associated with magnetic and structural phase transitions that occur during the magnetizing/demagnetizing cycling, and the derived magnetic field-induced isothermal entropy change – one of the most important properties in the field of magnetocalorics. Both systematic and random errors present during the measurements of magnetization, however, may lead to erroneous conclusions. Using two well-known materials – elemental Gd and intermetallic Gd5Si2Ge2 as examples, we consider best practices in performing reliable and rapid magnetization measurements for proper characterization of magnetocaloric properties.</p