Tuning structural instability toward enhanced magnetocaloric effect around room temperature in MnCo(1-x)Zn(x)Ge

The Effects of La Doping on the Crystal Structure and Magnetic Properties of Ni2In-Type MnCoGe1-xLax (x = 0, 0.01, 0.03) Alloys

In this experiment, a series of MnCoGe_1−xLa_x (x = 0, 0.01, 0.03) alloy samples were prepared using a vacuum arc melting method. The crystal structure and magnetic properties of alloys were investigated using X-ray diffraction (XRD), Rietveld method, physical property measurement system (PPMS), and vibrating sample magnetometer (VSM) analyses. The results show that all samples were of high-temperature Ni₂In-type phases, belonging to space group P6₃/mmc (194) after 1373 K annealing. The results of Rietveld refinement revealed that the lattice constant and the volume of MnCoGe_1−xLa_x increased along with the values of La constants. The magnetic measurement results show that the Curie temperatures (T_C) of the MnCoGe_1−xLa_x series alloys were 294, 281, and 278 K, respectively. The maximum magnetic entropy changes at 1.5T were 1.64, 1.53, and 1.56 J·kg⁻¹·K⁻¹, respectively. The respective refrigeration capacities (RC) were 60.68, 59.28, and 57.72J·kg⁻¹, with a slight decrease along the series. The experimental results show that the doping of La results in decreased T_C, basically unchanged magnetic entropy, and slightly decreased RC.

Mixed-spin system with supersolid phases: magnetocaloric effect and thermal properties

Recently, it has been shown that two dimensional frustrated mixed-spin systems with anisotropic exchange interactions display supersolid phases in their ground state phase diagrams even in the absence of long-range interactions. In this paper, using cluster mean field theory, we investigate the effects of thermal fluctuations on the ground state phases of this kind of system and show that various thermal solids and thermal insulators emerge around the ground state solid and Mott insulating phases. We also study the thermodynamic properties and magnetocaloric effect of these systems and demonstrate that at low temperatures, a large cooling rate is seen in the vicinity of the solid-supersolid, solid-superfluid and Mott insulator-superfluid critical points, with the large accumulation of the entropy and the minimums of the isentropes. Our results show the sign change of the magnetocaloric parameter inside the solids and the Mott insulator, which is a characteristic of ordered phases.

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.

Magnetocaloric Materials with Multiple Instabilities

The magnetocaloric effect is a well-known phenomenon where the temperature of a magnetic material varies upon application or removal of a magnetic field. This effect is anticipated to be applied to magnetic refrigeration technology, which is environmentally benign. For practical applications, it is essential to explore and expand the materials horizon of novel magnets that exhibit giant magnetocaloric effects to achieve sufficient cooling efficiency. In this article, several attempts to enhance the magnetocaloric effect are reviewed from the viewpoint of the competition or cooperation between the ferromagnetic interaction and other magnetic, electronic, and structural instabilities in strongly correlated materials. The results indicate that both the competition and cooperation between them promote the first-order nature of the magnetic transition, leading to giant magnetocaloric effects. Therefore, exploiting multiple instabilities is a promising strategy for exploring new magnetocaloric materials.

Large magnetocaloric effect and near-zero thermal hysteresis in the rare earth intermetallic Tb1-x Dy x Co2 compounds

We report the magnetocaloric effect in a Tb_1-x Dy _x Co₂ compound which exhibits a wide working temperature window around the Curie temperature (T _C) and delivers a large refrigerant capacity (RC) with near-zero thermal hysteresis. Specifically, the wide full width at half maxima ([Formula: see text]) can reach up to 62 K and the RC value changes from 216.5 to 274.3 J Kg^-1 when the external magnetic field increases to 5 T. Such magnetocaloric effects are attributed to a magnetic and structural transition from a paramagnetic and cubic phase to a ferromagnetic (M _S along [1 1 1] direction) and rhombohedral phase or ferromagnetic (M _S along [0 0 1] direction) and tetragonal phase.

Magnetocaloric effect and negative thermal expansion in hexagonal Fe doped MnNiGe compounds with a magnetoelastic AFM-FM-like transition

We report a detailed study of two successive first-order transitions, including a martensitic transition (MT) and an antiferromagnetic (AFM)-ferromagnetic (FM)-like transition, in Mn_1-xFe_xNiGe (x = 0, 0.06, 0.11) alloys by X-ray diffraction, differential scanning calorimetry, magnetization and linear thermal expansion measurements. Such an AFM-FM-like transition occurring in the martensitic state has seldom been observed in the M(T) curves. The results of Arrott plot and linear relationship of the critical temperature with M² provide explicit evidence of its first-order magnetoelastic nature. On the other hand, their performances as magnetocaloric and negative thermal expansion materials were characterized. The isothermal entropy change for a field change of 30 kOe reaches an impressive value of -25.8 J/kg K at 203 K for x = 0.11 compared to the other two samples. It demonstrates that the magneto-responsive ability has been significantly promoted since an appropriate amount of Fe doping can break the local Ni-6Mn AFM configuration. Moreover, the Fe-doped samples reveal both the giant negative thermal expansion and near-zero thermal expansion for different temperature ranges. For instance, the average thermal expansion coefficient ā of x = 0.06 reaches -60.7 × 10^-6/K over T = 231-338 K and 0.6 × 10^-6/K over T = 175-231 K during cooling.

Realization of magnetostructural coupling by modifying structural transitions in MnNiSi-CoNiGe system with a wide Curie-temperature window

The magnetostructural coupling between structural and magnetic transitions leads to magneto-multifunctionalities of phase-transition alloys. Due to the increasing demands of multifunctional applications, to search for the new materials with tunable magnetostructural transformations in a large operating temperature range is important. In this work, we demonstrate that by chemically alloying MnNiSi with CoNiGe, the structural transformation temperature of MnNiSi (1200 K) is remarkably decreased by almost 1000 K. A tunable magnetostructural transformation between the paramagnetic hexagonal and ferromagnetic orthorhombic phase over a wide temperature window from 425 to 125 K is realized in (MnNiSi)1-x(CoNiGe)x system. The magnetic-field-induced magnetostructural transformation is accompanied by the high-performance magnetocaloric effect, proving that MnNiSi-CoNiGe system is a promising candidate for magnetic cooling refrigerant.