Magnetic refrigeration is based on the magnetocaloric effect (MCE) and has attracted considerable attention due to its potentially higher energy efficiency, environmental friendliness and quietness compared to conventional vapour compression refrigeration. Boosting giant MCE materials with a magnetoelastic transition into commercial applications requires not only insights into the coupling between its magnetism and the lattice, but also the correlation between macroscopic performance and microstructure. In this thesis, the fundamental physical properties, including crystal structure, microstructure, magnetic structure, negative thermal expansion behaviour and the magnetocaloric effect, are studied in Mn2Sb-based intermetallic compounds with an antiferromagnetic-to-ferrimagnetic transition and Fe2Hf-based Laves phase compounds with a ferromagnetic-to-antiferromagnetic transition...@en

In this work, the magnetocaloric effect and negative thermal expansion in melt-spun Fe₂Hf_0.83Ta_0.17 Laves phase alloys were studied. Compared to arc-melted alloys, which undergo a first-order magnetoelastic transition from the ferromagnetic to the antiferromagnetic phase, melt-spun alloys exhibit a second-order transition. For Fe₂Hf_0.83Ta_0.17 ribbons, we observed a large volumetric coefficient of negative thermal expansion of −19 × 10⁻⁶ K⁻¹ over a wide temperature range of 197 – 297 K and a moderate adiabatic temperature change of 0.7 K at 290 K for a magnetic field change of 1.5 T. The magnetic field dependence of the transition temperature (dT_t/dµ₀H = 4.4 K/T) for the melt-spun alloy is about half that of the arc-melted alloy (8.6 K/T). The origin of second-order phase transition of the melt-spun alloy is attributed to the partially suppressed frustration effect, which is due to the atomic disorder introduced by the rapid solidification.

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The influence of doping with the 5d transition metal W has been studied in the quaternary (Mn,Fe)₂(P,Si) based giant magnetocaloric compounds, which is one of the most promising systems for magnetic refrigeration. It is found that W substitution can separately decrease the Curie temperature T_C and retain the thermal hysteresis ∆T_hys at an almost constant level (∼5 K) for Mn_0.6Fe_1.27-xW_xP_0.64Si_0.36 (x ≤ 0.02). Low-content W doping conserves the good magnetocaloric effect (MCE) without an obvious degradation. For x ≤ 0.02 the average magnetic entropy change |∆S_m| amounts to 11.4 Jkg⁻¹K⁻¹ for an applied magnetic field change of 2 T and the adiabatic temperature change ∆T_ad amounts to 3.9 K for an applied magnetic field change of 1.5 T. The occupancy of substitutional W atoms is determined by XRD experiments and DFT calculations. Our studies provide a good strategy to further optimize the MCE of this material family.

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The quarternary (Mn,Fe)₂(P,Si)-based materials with a giant magnetocaloric effect (GMCE) at the ferromagnetic transition T_C are promising bulk materials for solid-state magnetic refrigeration. In the present study we demonstrate that doping with the light elements fluorine and sulfur can be used to adjust T_C near room temperature and tune the magnetocaloric properties. For F doping the first-order magnetic transition (FOMT) of Mn_0.60Fe_1.30P_0.64Si_0.36F_x (x = 0.00, 0.01, 0.02, 0.03) is enhanced, which is explained by an enhanced magnetoelastic coupling. The magnetic entropy change |ΔS_m| at a field change (Δμ₀H) of 2 T markedly improved by 30% from 14.2 Jkg⁻¹K⁻¹ (x = 0.00) at 335 K to 20.2 Jkg⁻¹K⁻¹ (x = 0.03) at 297 K. For the F doped material the value of |ΔS_m| for Δμ₀H = 1 T reaches 11.6 Jkg⁻¹K⁻¹ at 294 K, which is consistent with the calorimetric data (12.4 Jkg⁻¹K⁻¹). Neutron diffraction experiments reveal enhanced magnetic moments by F doping in agreement with the prediction of DFT calculation. For S doping in Mn_0.60Fe_1.25P_0.66-ySi_0.34S_y (y = 0.00, 0.01, 0.02, 0.03, 0.04) three impurity phases have been found from microstructural analysis, which reduce the stability of the FOMT in the main phase and decrease T_C, e.g. the |ΔS_m| reduces from 7.9(12.6) Jkg^-1K^-1 (332 K) for the undoped sample to 3.4(6.2) Jkg^-1K^-1 (313 K) for the maximum doped sample for Δμ₀H = 1(2) T. Neutron diffraction experiments combined with first-principles theoretical calculation, distinguish the occupation of F/S dopants and the tuning mechanism for light element doping, corresponding to subtle structural changes and a strengthening of the covalent bonding between metal and metalloid atoms. It is found that the light elements F and S can effectively regulate the magnetocaloric properties and provide fundamental understanding of (Mn,Fe)₂(P,Si)-based intermetallic compounds.

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The novel all-d-metal Ni(Co)MnTi based magnetic Heusler alloys provide an adjustable giant magnetocaloric effect and good mechanical properties. We report that the second-order magnetic phase transition can be tailored in this all-d-metal NiCoMnTi based Heusler system by optimizing the Mn/Ti ratio, resulting in a reversible ferromagnetic-to-paramagnetic magnetic transition. A candidate material Ni₃₃Co₁₇Mn₃₀Ti₂₀ with a magnetic entropy change ∆S_m of 2.3 Jkg⁻¹K⁻¹ for a magnetic field change of 0–5 T, has been identified. The T_C and saturation magnetization M_S can be controlled by adjusting the Ni/Co concentration and doping non-magnetic Cu atoms. The compositional maps of T_C and M_S have been established. Density functional theory (DFT) calculations reveal a direct correlation between the magnetic moments and the Co content. By combining XRD, SQUID, SEM and DFT calculations, the (micro)structural and magnetocaloric properties have been investigated systematically. This study provides a detailed insight in the magnetic phase transition for this all-d-metal Ni(Co)MnTi-based Heusler alloy system.

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The influence of excess Mn on the magnetoelastic ferromagnetic-to-antiferromagnetic transition T_t in the magnetocaloric compound (Mn,Cr)₂Sb has been studied. With increasing excess Mn the magnetoelastic transition temperature for (Mn,Cr)₂Sb initially increases and then decreases. This trend is accompanied by a strong reduction of the (Mn,Cr)Sb secondary phase. With increasing excess Mn a higher Cr content was found in the (Mn,Cr)Sb secondary phase in comparison to the matrix phase. This competition for Cr leads to a nonlinear dependence of T_t with increasing excess Mn at a fixed nominal Cr content. However, we observed that T_t depends linear on the c/a ratio for a wide range of temperatures from 170 to 350 K. A compositional diagram of the c/a ratio was constructed to assist the selection of (Mn,Cr)₂Sb alloys with a desired transition temperature.

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In the field of nanoscale magnetocaloric materials, novel concepts like micro-refrigerators, thermal switches, microfluidic pumps, energy harvesting devices and biomedical applications have been proposed. However, reports on nanoscale (Mn,Fe)₂(P,Si)-based materials, which are one of the most promising bulk materials for solid-state magnetic refrigeration, are rare. In this study we have synthesized (Mn,Fe)₂(P,Si)-based nanoparticles, and systematically investigated the influence of crystallite size and microstructure on the giant magnetocaloric effect. The results show that the decreased saturation magnetization (M_s) is mainly attributed to the increased concentration of an atomically disordered shell, and with a decreased particle size, both the thermal hysteresis and T_c are reduced. In addition, we determined an optimal temperature window for annealing after synthesis of 300–600 °C and found that gaseous nitriding can enhance M_s from 120 to 148 Am²kg⁻¹ and the magnetic entropy change (ΔS_m) from 0.8 to 1.2 Jkg⁻¹K⁻¹ in a field change of Δμ₀H = 1 T. This improvement can be attributed to the synergetic effect of annealing and nitration, which effectively removes part of the defects inside the particles. The produced superparamagnetic particles have been probed by high-resolution transmission electron microscopy, Mössbauer spectra and magnetic measurements. Our results provide important insight into the performance of giant magnetocaloric materials at the nanoscale.

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The influence of partial substitution of Bi for Sb on the structure, magnetic properties and magnetocaloric effect of Mn₂Sb_1-xBi_x (x = 0, 0.02, 0.04, 0.05, 0.07, 0.09, 0.15, 0.20) compounds has been investigated. The transition temperature of the antiferro-to-ferrimagnetic (AFM-FIM) transition initially increases with increasing Bi and decreases above 9%. Density functional theory calculations indicate that the Bi atoms prefer to occupy only the Sb site, which accounts for the large magnetization jump in Mn₂Sb_0.93Bi_0.07. As large lattice parameters are found for Bi substituted Mn₂Sb, the origin of the AFM-FIM transition in Mn₂Sb_(1-x)Bi_x compounds is ascribed to an enhanced coefficient of thermal expansion along the c axis, resulting from the Bi substitution. The moderate entropy change of 1.17 J/kg K under 2 T originating from the inverse magnetocaloric effect and the strong magnetic field dependence of the transition temperature of dT_t/dµ₀H = −5.4 K/T in Mn₂Sb_0.95Bi_0.05 indicate that this alloy is a promising candidate material for magnetocaloric applications.

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