Search Results (7)

In this paper, vacuum annealing has been adopted to introduce atomic cluster micro-regions inside Gd-based metallic microfibers to further explore the effect of the structural changes on the magnetocaloric properties and the mechanism which is systematically expressed. The experimental results indicate that the as-prepared Gd-based metallic microfibers have favorable amorphous formation ability and thermal stability. After annealing @ 380 °C, the maximum magnetic entropy change −ΔS_m^max, refrigerating capacity (RC), and relative cooling power (RCP) values of the Gd-based metallic microfibers are 7.20 J/kg·K, 459.4 J/kg, and 588.7 J/kg, respectively. Combined with the transmission electron microscopy analysis results, the internal organizational order of the annealed microfibers is significantly altered, and the atomic clusters formed in localized regions, which reduce the magnetocrystalline anisotropy of the microfibers. While under the uni-action of an external magnetic field, the magnetic moment rotation state and magnetic domain structure distribution of the micro-region atoms will be changed obviously, thereby changing the general magnetic properties and magnetocaloric properties of the metallic microfibers. The above research results can promote the engineering application of Gd-based metallic microfibers in the field of magnetic refrigeration. Full article

Magnetic anisotropy strongly influences the performance of the magnetocaloric effect. We investigated the magnetocaloric properties of the NdAlGe single crystal with I4₁md structure. The temperature-dependent magnetization revealed significant anisotropic properties; stable antiferromagnetic transition at T_N = 6 K for H//a and meta-magnetic spin reorientation at low temperature (T ≤ 5 K) within an intermediate field (H = 2 T) for H//c. During the metamagnetic spin reorientation, the abrupt change of the magnetic entropy leads to a significant magnetocaloric effect with negative magnetic entropy change (∆S_M) by −13.80 J kg⁻¹ K⁻¹ at T_C = 5.5 K for H = 5 T along the H//c axis. In addition, the antiferromagnetic state for H//a shows the inverse magnetocaloric effect(I-MCE) by positive entropy change ∆S_M = 2.64 J kg⁻¹ K⁻¹ at T_N = 6 K for H = 5 T. This giant MCE accompanied by the metamagnetic transition resulted in a significantly large relative cooling power (158 J/kg at H = 5 T) for H//c. The giant MCE and I-MCE can be applied to the rotational magnetocaloric effect (R-MCE) depending on the crystal orientations. NdAlGe exhibits rotational entropy change ∆S_c−a = −12.85 J kg⁻¹ K at T_peak = 7.5 K, H = 5 T. With comparison to conventional MCE materials, NdAlGe is suggested as promising candidate of R-MCE, which is a novel type of magnetic refrigeration system. Full article

An experimental study of the rotational magnetocaloric effect in Ni(en)(H₂O)₄SO₄∙2H₂O (en = ethylenediamine) single crystal is presented. The study was carried out at temperatures above 2 K and was associated with adiabatic crystal rotation between the easy plane and hard axis in magnetic fields up to 7 T. The magnetocaloric properties of the studied system were investigated by isothermal magnetization measurement. The experimental observations were completed with ab initio calculations of the anisotropy parameters. A large rotational magnetic entropy change ≈12 Jkg⁻¹K⁻¹ and ≈16.9 Jkg⁻¹K⁻¹ was achieved in 5 T and 7 T, respectively. The present study suggests a possible application of this material in low-temperature refrigeration since the adiabatic rotation of the single crystal in 7 T led to a cooldown of the sample from the initial temperature of 4.2 K down to 0.34 K. Finally, theoretical calculations show that S = 1 Ni(II)-based systems with easy-plane anisotropy can have better rotational magnetocaloric properties than costly materials containing rare-earth elements in their chemical structures. Full article

GdFeTeO₆ and GdGaTeO₆ have been prepared and their structures refined by the Rietveld method. Both are superstructures of the rosiaite type (space group $P\overline{3}1c$). Their thermodynamic properties have been investigated by means of magnetization M and specific heat C_p measurements, evidencing the formation of the long-range antiferromagnetic order at T_N = 2.4 K in the former compound and paramagnetic behavior down to 2 K in the latter compound. Large magnetocaloric effect allows considering GdFeTeO₆ for the magnetic refrigeration at liquid hydrogen stage. Density functional theory calculations produce estimations of leading Gd–Gd, Gd–Fe and Fe–Fe interactions suggesting unique chiral 120° magnetic structure of Fe³⁺ (S = 5/2) moments and Gd³⁺ (J = 7/2) moments rotating in opposite directions (clockwise/anticlockwise) within weakly coupled layers of the rosiaite type crystal structure. Full article

Magnetic refrigeration is a fascinating superior choice technology as compared with traditional refrigeration that relies on a unique property of particular materials, known as the magnetocaloric effect (MCE). This paper provides a thorough understanding of different magnetic refrigeration technologies using a variety of models to evaluate the coefficient of performance (COP) and specific cooling capacity outputs. Accordingly, magnetic refrigeration models are divided into four categories: rotating, reciprocating, C-shaped magnetic refrigeration, and active magnetic regenerator. The working principles of these models were described, and their outputs were extracted and compared. Furthermore, the influence of the magnetocaloric effect, the magnetization area, and the thermodynamic processes and cycles on the efficiency of magnetic refrigeration was investigated and discussed to achieve a maximum cooling capacity. The classes of magnetocaloric magnetic materials were summarized from previous studies and their potential magnetic characteristics are emphasized. The essential characteristics of magnetic refrigeration systems are highlighted to determine the significant advantages, difficulties, drawbacks, and feasibility analyses of these systems. Moreover, a cost analysis was provided in order to judge the feasibility of these systems for commercial use. Full article

Octacyanometallate-based compounds displaying a rich pallet of interesting physical and chemical properties, are key materials in the field of molecular magnetism. The [M(CN)₈]ⁿ⁻ complexes, (M = W^V, Mo^V, Nb^IV), are universal building blocks as they lead to various spatial structures, depending on the surrounding ligands and the choice of the metal ion. One of the functionalities of the octacyanometallate-based coordination polymers or clusters is the magnetocaloric effect (MCE), consisting in a change of the material temperature upon the application of a magnetic field. In this review, we focus on different approaches to MCE investigation. We present examples of magnetic entropy change ΔS_m and adiabatic temperature change ΔT_ad, determined using calorimetric measurements supplemented with the algebraic extrapolation of the data down to 0 K. At the field change of 5T, the compound built of high spin clusters Ni₉[W(CN)₈]₆ showed a maximum value of −ΔS_m equal to 18.38 J·K⁻¹ mol⁻¹ at 4.3 K, while the corresponding maximum ΔT_ad = 4.6 K was attained at 2.2 K. These values revealed that this molecular material may be treated as a possible candidate for cryogenic magnetic cooling. Values obtained for ferrimagnetic polymers at temperatures close to their magnetic ordering temperatures, T_c, were lower, i.e., −ΔS_m = 6.83 J·K⁻¹ mol⁻¹ (ΔT_ad = 1.42 K) and −ΔS_m = 4.9 J·K⁻¹ mol⁻¹ (ΔT_ad = 2 K) for {[Mn^II(pyrazole)₄]₂[Nb^IV(CN)₈]·4H₂O}_n and{[Fe^II(pyrazole)₄]₂[Nb^IV(CN)₈]·4H₂O}_n, respectively. MCE results have been obtained also for other -[Nb(CN)₈]-based manganese polymers, showing significant T_c dependence on pressure or the remarkable magnetic sponge behaviour. Using the data obtained for compounds with different T_c, due to dissimilar ligands or other phase of the material, the ΔS_m ~ T_c^−2/3 relation stemming from the molecular field theory was confirmed. The characteristic index n in the ΔS_m ~ ΔHⁿ dependence, and the critical exponents, related to n, were determined, pointing to the 3D Heisenberg model as the most adequate for the description of these particular compounds. At last, results of the rotating magnetocaloric effect (RMCE), which is a new technique efficient in the case of layered magnetic systems, are presented. Data have been obtained and discussed for single crystals of two 2D molecular magnets: ferrimagnetic {Mn^II(R-mpm)₂]₂[Nb^IV(CN)₈]}∙4H₂O (mpm = α-methyl-2-pyridinemethanol) and a strongly anisotropic (tetren)Cu₄[W(CN)₈]₄ bilayered magnet showing the topological Berezinskii-Kosterlitz-Thouless transition. Full article

Thanks to the strong magnetic anisotropy shown by the multiferroic RMn₂O₅ (R = magnetic rare earth) compounds, a large adiabatic temperature change can be induced (around 10 K) by rotating them in constant magnetic fields instead of the standard magnetization-demagnetization method. Particularly, the TbMn₂O₅ single crystal reveals a giant rotating magnetocaloric effect (RMCE) under relatively low constant magnetic fields reachable by permanent magnets. On the other hand, the nature of R³⁺ ions strongly affects their RMCEs. For example, the maximum rotating adiabatic temperature change exhibited by TbMn₂O₅ is more than five times larger than that presented by HoMn₂O₅ in a constant magnetic field of 2 T. In this paper, we mainly focus on the physics behind the RMCE shown by RMn₂O₅ multiferroics. We particularly demonstrate that the rare earth size could play a crucial role in determining the magnetic order, and accordingly, the rotating magnetocaloric properties of RMn₂O₅ compounds through the modulation of exchange interactions via lattice distortions. This is a scenario that seems to be supported by Raman scattering measurements. Full article