Nanoscale Phase Separation and Large Refrigerant Capacity in Magnetocaloric Material LaFe_11.5Si_1.5

Abstract

Ferromagnetic transitions, the magnetocaloric effect, and the atomic-scale microstructure in an annealed bulk material of LaFe 11.5 Si 1.5 were investigated using magnetic measurements, Mossbauer spectroscopy, high-resolution X-ray diffraction analysis, and high-resolution transmission electron microscopy. The results provide evidence of the coexistence of two ferromagnetic phases with different Curie temperatures. The phase with a Curie temperature of 216 K corresponds to a Sirich phase of a NaZn 13 -type structure with a small lattice volume, whereas the phase with a Curie temperature of 185 K corresponds to a Si-poor phase of a NaZn 13 -type structure with a large lattice volume. This phase coexistence is observed on a nanometer scale and it can account for inverse thermal hysteresis of the ferromagnetic transition in the Si-rich phase during warming. Furthermore, the phase coexistence helps the bulk material achieve a refrigeration capacity of 170 J kg −1 due to a combination of the magnetocaloric effect in each phase. It is determined that the magnetocaloric effect in the Si-rich and the Si-poor phase brings about a maximum of the total entropy change of −3.55 and −6.80 and J kg −1 K −1 at respective Curie temperatures, respectively. Fundamentally, the phase coexistence indicates coloring of Si atoms in the lattice of a NaZn 13 -type structure and can be attributed to nanoscale phase separation of a disordered precursor during annealing of the bulk material. The nanoscale phase separation may occur in other magnetocaloric materials, thus allowing for improvement of their refrigeration capacity.

Zero Thermal Expansion Effect and Enhanced Magnetocaloric Effect Induced by Fe Vacancies in Fe2Hf0.80Nb0.20 Laves Phase Alloys

“…The XRD result, linear scanning of SEM image, and Mössbauer spectrum confirm the single-phase behavior of the Fe1.95 alloy; thus, this magnetic phase separation can be attributed to Fe vacancies introduced into the Fe1.95 alloy, as reported in the vacancy-induced phase separation in La 0.70 Ca 0.30 Mn 0.92 Cr 0.08 O z and Fe–Cr-based alloys . The inhomogeneous distribution of Fe vacancies disrupts the movement of phase boundaries during the FM–AFM transition, thus inducing a two-step magnetization change, as shown in the M–T curve of the Fe1.95 alloy . The Fe2.00 alloy exhibits a relatively weak magnetic phase separation, leading to a singular broad M–T curve and a correspondingly broad magnetic entropy change.…”

“…The isomer shifts for both the AFM and FM phases remain similar, implying an identical Fe valence state. However, the quadrupole splitting of Fe at the 2a site for the AFM phase significantly exceeds that of Fe at the 2a site for the FM phase This discrepancy indicates pronounced chemical environment distortions around Fe at the 2a site upon the transformation of the FM phase into the AFM phase with increasing temperature . For the FM phase, the hyperfine field of Fe at the 6h site (18.0 T) and 2a site (14.1 T) is akin to the values observed in Fe 2 Hf 0.83 Ta 0.17 (19.0 and 13.6 T, respectively) .…”

Frustration-Induced Inversion of the Magnetocaloric Effect and Metamagnetic Transition in Substituted Pyrochlore Iridates

“…In recent years, solid-state magnetic cooling based on the magnetocaloric effect (MCE) has gained considerable attention as an alternative to the traditional gas compression refrigeration technology. − The MCE is defined and measured using primarily two broad categories: (1) the change in the temperature of a magnetic material due to the application of a magnetic field in an adiabatic process (△ T ad ) and (2) the change in the magnetic entropy (△ S M ) with the application of a magnetic field in an isothermal condition. In the conventional MCE, spins align upon the application of a magnetic field leading to a reduction of magnetic entropy: the materials heat up when the magnetic field is applied and cool down when the field is removed.…”

Ultra-low hysteresis in giant magnetocaloric Mn1-V Fe0.95(P,Si,B) compounds

“…The reason for the different value of hysteresis for M-T and DSC measurement is not clear yet. Recently, the origins for zero-hysteresis in the materials with IEM are investigated from the aspect of nanoscale phase separation [6,44,45] and the secondary phases [46]. Thus, we propose an explanation based on the in-field DSC data.…”