Influence of the transition width on the magnetocaloric effect across the magnetostructural transition of Heusler alloys

Structural, magnetic, and magnetocaloric properties of the multiferroic host double perovskite compound Pr2FeCrO6

We have investigated the crystal structure and the nature of the magnetic ground state of the polycrystalline compound Pr2FeCrO6 (PFCO) through X-ray diffraction (XRD), magnetization, and magnetocaloric effect studies. Analysis of the XRD pattern reveals that the PFCO compound exhibits a B-site disordered orthorhombic crystal structure. The random distribution of Fe3+ and Cr3+ magnetic sublattices at the B-sites of the crystallographic unit cell helps to generate several fascinating magnetic properties. The compound exhibits three distinct anomalies in both the temperature dependence of the magnetization and the magnetic entropy change (-ΔS) curves, namely, (i) a G-type canted antiferromagnetic (AFM) ordering of the transition metal ions (TN1), (ii) a progressive spin reorientation (SR) transition (TSR), and (iii) an AFM ordering of Pr3+ sublattices at very low temperature (TN2). Surprisingly, a novel "diamagnetism-like" behavior appears in the low-temperature region for low applied field values. Moreover, we have also constructed the thermal evolution of the magnetic crystal structures in different transition regions with the help of irreducible representations of the crystal symmetry. Overall, our study of B-site disordered PFCO may help to encourage basic fundamental and applied research on disordered rare-earth and transition metal-based perovskite systems due to their interesting magnetic properties over a broad temperature range.

Influencing Martensitic Transition in Epitaxial Ni-Mn-Ga-Co Films with Large Angle Grain Boundaries

Magnetocaloric materials based on field-induced first order transformations such as Ni-Mn-Ga-Co are promising for more environmentally friendly cooling. Due to the underlying martensitic transformation, a large hysteresis can occur, which in turn reduces the efficiency of a cooling cycle. Here, we analyse the influence of the film microstructure on the thermal hysteresis and focus especially on large angle grain boundaries. We control the microstructure and grain boundary density by depositing films with local epitaxy on different substrates: Single crystalline MgO(0 0 1), MgO(1 1 0) and Al2O3(0 0 0 1). By combining local electron backscatter diffraction (EBSD) and global texture measurements with thermomagnetic measurements, we correlate a smaller hysteresis with the presence of grain boundaries. In films with grain boundaries, the hysteresis is decreased by about 30% compared to single crystalline films. Nevertheless, a large grain boundary density leads to a broadened transition. To explain this behaviour, we discuss the influence of grain boundaries on the martensitic transformation. While grain boundaries act as nucleation sites, they also lead to different strains in the material, which gives rise to various transition temperatures inside one film. We can show that a thoughtful design of the grain boundary microstructure is an important step to optimize the hysteresis.

Following the Martensitic Configuration Footprints in the Transition Route of Ni-Mn-Ga Magnetic Shape Memory Films: Insight into the Role of Twin Boundaries and Interfaces

Magnetic shape memory Heuslers have a great potential for their exploitation in next-generation cooling devices and actuating systems, due to their “giant” caloric and thermo/magnetomechanical effects arising from the combination of magnetic order and a martensitic transition. Thermal hysteresis, broad transition range, and twinning stress are among the major obstacles preventing the full exploitation of these materials in applications. Using Ni-Mn-Ga seven-modulated epitaxial thin films as a model system, we investigated the possible links between the phase transition and the details of the twin variants configuration in the martensitic phase. We explored the crystallographic relations between the martensitic variants from the atomic-scale to the micro-scale through high-resolution techniques and combined this information with the direct observation of the evolution of martensitic twin variants vs. temperature. Based on our multiscale investigation, we propose a route for the martensitic phase transition, in which the interfaces between different colonies of twins play the major role of initiators for both the forward and reverse phase transition. Linking the martensitic transition to the martensitic configuration sheds light onto the possible mechanisms influencing the transition and paves the way towards microstructure engineering for the full exploitation of shape memory Heuslers in different applications.

Magnetostructural transformation and magnetocaloric effect of Sn-bonded Mn0.66Fe0.34Ni0.66Fe0.34Si0.66Ge0.34 composite

Magnetostructural coupling in MnMX (M = Co or Ni, X = Si or Ge) system attracts considerable attention for the accompanied multi-magnetoresponsive effects. However, due to the large stress generated from the structural transformation, the alloys become shattered or powder-like, hindering the further investigation and their applications. The possible solution is to embed the MnMX powders into metal matrix. In this paper, we choose Mn_0.66Fe_0.34Ni_0.66Fe_0.34Si_0.66Ge_0.34 as a representative of MnMX alloy and produce Mn_0.66Fe_0.34Ni_0.66Fe_0.34Si_0.66Ge_0.34/Sn composite bulk by hot pressing. The magnetostructural-coupled composites exhibit an improved rate of the transformation temperature shift by magnetic field and broadened operating temperature range. Additionally, we also propose a simple formula based on the entropy-temperature diagram to calculate the isothermal entropy change, which is consistent with the results obtained by the Maxwell relation.