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The structural and magnetic properties of the intermetallic compounds NdMn2−xTixSi2(x = 0, 0.1, 0.2, and 0.3) have been studied by x-ray and high resolution neutron powder diffraction, specific heat, dc magnetization, and differential scanning calorimetry measurements over the temperature range 3–450 K. The Curie temperature and Néel temperature of NdMn2Si2 decrease from TC = 36 K and TN = 380 K to TC = 14 K and TN = 360 K, respectively, on substitution of Ti (x = 0.3) for Mn. The magnetocaloric effect at the first order ferromagnetic phase transition at TC, has been investigated in detail. Under a change of magnetic field of 0–5 T, the maximum value of the magnetic entropy change is 27 J kg−1 K−1 at x = 0, reducing to 15.3 J kg−1 K−1 at x = 0.1 and 10 J kg−1 K−1 at x = 0.3; importantly, no thermal or field hysteresis losses occur (eliminated from 0.3 K and 28.5 J kg−1 at x = 0 around TC) with increase in Ti concentration. Combined with the lack of any hysteresis effects, these findings indicate that NdMn1.9Ti0.1Si2 compound offers potential as a candidate for magnetic refrigerator applications in the temperature region below 35 K.

Structural, magnetic and magnetocaloric properties of the Mn0.94Ti0.06CoGe alloy have been investigated using x-ray diffraction, DC magnetization and neutron diffraction measurements. Two phase transitions have been detected, at T-str = 235 K and T-C = 270 K. A giant magnetocaloric effect has been obtained at around Tstr associated with a structural phase transition from the low temperature orthorhombic TiNiSi-type structure to the high temperature hexagonal Ni2In-type structure, which is confirmed by neutron study. In the vicinity of the structural transition, at T-str, the magnetic entropy change, -Delta S-M reached a maximum value of 14.8 J kg(-1) K-1 under a magnetic field of 5 T, which is much higher than that previously reported for the parent compound MnCoGe. To investigate the nature of the magnetic phase transition around T-C = 270 K from the ferromagnetic to the paramagnetic state, we performed a detailed critical exponent study. The critical components gamma, beta and delta determined using the Kouvel-Fisher method, the modified Arrott plot and the critical isotherm analysis agree well. The values deduced for the critical exponents are close to the theoretical prediction from the mean-field model, indicating that the magnetic interactions are long range. On the basis of these critical exponents, the magnetization, field and temperature data around T-C collapse onto two curves obeying the single scaling equation M(H, epsilon) = epsilon(beta)f +/- (H/epsilon(beta+gamma)).

An epitaxial La0.8Ca0.2MnO3/LaAlO 3 (LCMO/LAO) thin film was fabricated using the pulsed laser deposition technique to evaluate the correlation between the crystal structure and the magnetocaloric effect. In our study, the LCMO film was 200 nm in thickness and appeared to have a strong out-of plane texture. We found that each column in the LCMO thin film layer is a domain which has a different ordering direction. These microscopic feature results in anisotropic properties of magnetization, entropy, and relative cooling power. The film exhibited a paramagnetic-to-ferromagnetic second order phase transition at 249 K. The lack of any hysteresis loss also confirmed that the material is intrinsically reversible. In addition, the large magnetization of the thin film results in an entropy change larger than those of all other perovskite type materials. Consequently, the relative cooling power is significantly enhanced.

The temperature dependent magnetization of nanocomposite alloys has been fit with a modified Handrich-Kobe equation with an asymmetric exchange fluctuation parameter combined with the Arrott-Noakes equation. The two equations of state are combined to calculate the entropy change in the magnetocaloric effect associated with the ferromagnetic to paramagnetic phase transformation. The complete fit for the M(T) of (Fe70Ni30)88Zr7B4Cu nanocomposite powder is accomplished by combining the two theories. We investigate the broadening of the second-order transition arising from asymmetric exchange parameters and resulting from the fluctuations of interatomic spacing found in an amorphous matrix and the asymmetric dependence of exchange energy on interatomic spacing. The magnetic entropy curve revealed extra broadening with a refrigeration capacity (RC) value of 135 J/kg at 5 T, which is comparable to (Fe76Cr8-xMoxCu1B15) ribbons, which have a RC value of 180 J/kg for the same applied field. Broadening of the magnetic entropy can lead to larger RC values and a wider working temperature range, making nanocomposite alloys promising for magnetocaloric applications.

Magnetic properties and the magnetocaloric effect (MCE) have been investigated in La0.7Ca0.3MnO3 single crystal with applied field along both the ab-plane and the c-direction. Due to the magnetocrystalline anisotropy, the crystal exhibits anisotropy in the MCE. Upon application of a 5 T field, the magnetic entropy changes (SM), reaching values of 7.668 J/(kg K) and 6.412 J/(kg K) for both the ab-plane and the c-direction, respectively. A magnetic entropy change of 3.3 J/(kg K) was achieved for a magnetic field change of 1.5 T at the Curie temperature, TC = 245 K. Due to the absence of grains in the single crystal, the SM distribution here is much more uniform than for gadolinium (Gd) and other polycrystalline manganites, which is desirable for an Ericsson-cycle magnetic refrigerator. For a field change of 5 T, the relative cooling power, RCP, reached 358.17 J/kg, while the maximum adiabatic temperature change of 5.33 K and a magnetoresistance (MR) ratio of 507.88% at TC were observed. We analysed the magnetization of La0.7Ca0.3MnO3 single crystal at TC and estimated several parameters of spin fluctuation on the basis of a self-consistent renormalization theory of spin fluctuations, with reciprocal susceptibility above TC. We found that the magnetic property of La0.7Ca0.3MnO3 is weakly itinerant ferromagnetic. A large reversible MCE and no hysteresis loss with a considerable value of refrigerant capacity indicate that La0.7Ca0.3MnO3 single crystal is a potential candidate as a magnetic refrigerant.

The crystal and magnetic structure of the magnetocaloric compound FeMnP0.5Si0.5 has been studied by means of neutron and X-ray powder diffraction. Single phase samples of nominal composition FeMnP0.5Si0.5 have been prepared by the drop synthesis method. The compound crystallizes in the Fe2P-type structure (P-62m) with the magnetic moments aligned along the a-axis. It is found that the Fe atoms are mainly situated in the tetrahedral 3g site while the Mn atoms prefer the pyramidal 3f position. The material is ferromagnetic (TC=382 K) and at 296 K the total magnetic moment is 4.4 µB/f.u. It is shown that the magnetic moment in the 3f site is larger (2.5 µB) than in the 3g site (1.9 µB).

In this paper, the influence of the Mn content on the magnetocaloric response of ribbon-shaped amorphous samples of Fe80−xMnxB20 x=10, 15, 18, 20, and 24 , has been studied. For this purpose, the temperature and field dependence of the magnetic entropy change SM have been obtained from magnetization curves. The partial substitution of Fe by Mn leads to a monotonous change in the Curie temperature TC of the alloys from 438 K for x=10 to 162 K for x=24, in agreement with the coherent-potential approximation. These Curie temperatures could make them good candidates to be used for magnetic refrigeration at room temperature. For an applied field of 1.5 T, the maximum entropy change SM pk passes from 1 J K−1 kg−1 x=10 to 0.5 J K−1 kg−1 x=24 , and the refrigerant capacity varies between 117 J kg−1 x=10 and 68 J kg−1 x=24 . A linear relationship between SM pk and the average magnetic moment per transition metal atom Fe,Mn has been presented. © 2010 American Institute of Physics.

The magnetocaloric response of mechanically alloyed Fe75Nb10B15 powders was studied for samples with different amorphous and nanocrystal volume fractions. Thermomagnetic properties scale following a 3 law for different milling processes, where is the milling frequency. Curie temperature of the amorphous phase increases as the amorphous fraction increases due to its progressive enrichment in B. The peak magnetic entropy change, SM pk , as well as the refrigerant capacity increase with increasing amorphous fraction. The field dependence of SM pk can be explained by the multiphase character of the studied samples.

Nanostructured materials for refrigeration applications are experimentally realized by molecular beam epitaxial growth of Co/Cr superlattices using mean-field theoretical concepts as guiding principles. Magnetocaloric properties are deduced from measurements of the temperature and field dependence of the magnetization of our samples. More generally, the potential of artificial antiferromagnets for near room-temperature refrigeration is explored. The effects of intraplane and interplane exchange interactions on the magnetic phase diagram in Ising-type model systems are revisited in mean-field considerations with special emphasis on tailoring magnetocaloric properties. The experimental results are discussed in light of our theoretical findings, and extrapolations for future improved nanostructures are provided.

The structural and magnetic properties of the re-entrant ferromagnet PrMn2Ge0.8Si1.2 have been investigated by various experimental methods. Similar to the canonical re-entrant ferromagnet SmMn2Ge2, multiple magnetic phase transitions have been detected in PrMn2Ge0.8Si1.2 over the temperature range from 10 to 550 K with re-entrant ferromagnetism occurring around ∼ 54 K. The magnetocaloric effect has been measured in terms of the isothermal magnetocaloric entropy change and found to be positive at the re-entrant ferromagnetic transition with a maximum value of around 1.9 J/kg K at 58 K for a magnetic field change of 0–3 T. On the other hand, the entropy change becomes negative ( ∼ −0.5 J/kg K) at the antiferromagnetic to ferromagnetic transition for the same field change.

Khan, R. A. A., Ghomashchi, R., Xie, Z., & Chen, L. (2018). Ferromagnetic shape memory heusler materials: Synthesis, microstructure characterization and magnetostructural properties. Materials, 11(6). Available here