The influence of off-stoichiometry and of doping with the 5d transition metal Ta has been studied in the quaternary (Mn,Fe)₂(P,Si)-based compound, which is one of the most promising materials systems for magnetic refrigeration. It is found that Ta substitution can decrease the transition temperature T_tr, while the thermal hysteresis ∆T_hys remains about constant. A low Ta doping enhances the magnetocaloric effect (MCE). For Mn_0.6Fe_1.27-yTa_yP_0.64Si_0.36 with y = 0.01 the magnetic entropy change ∆S_m shows and enhancement of 30.7% compared to the undoped material for a low magnetic field change of 1 T. The occupancy of substitutional Ta atoms is determined by XRD and DFT calculations. The T_tr shift and enhanced MCE upon Ta doping are ascribed to the competition between a weakening of the magnetic exchange interactions and a strengthening of the hybridization. Our studies provide a good strategy to further optimize the MCE of this material family.

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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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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 all-d-metal Ni-(Co)-Mn-Ti-based Heusler alloys are found to show a giant magnetocaloric effect near room temperature and are thereby potential materials for solid-state refrigeration. However, the relative large thermal hysteresis and the moderate ferromagnetic magnetization provides limitations for real applications. In the present study, we demonstrate that introducing interstitial B atoms within Ni36.5Co13.5Mn35Ti15 alloys can effectively decrease the thermal hysteresis ΔThys (down to 4.4 K), and simultaneously improve the saturation magnetization (maximum 40% enhancement) for low concentrations of B doping (up to 0.4 at. %). In comparison to the undoped reference material, the maximum magnetic entropy change (ΔSm) for the Ni36.5Co13.5Mn35Ti15B0.4 alloy shows a remarkable improvement from 9.7 to 24.3 J kg-1K-1 for an applied magnetic field change (Δμ0H) of 5 T (30.2 J kg-1K-1 for Δμ0H = 7 T). Additionally, due to the obtained low thermal hysteresis ΔThys, the maximum reversible ΔSmrev amounts to 18.9 J kg-1K-1 at 283 K for Δμ0H = 5 T (22.0 J kg-1K-1 at 281 K for Δμ0H = 7 T), which is competitive to the traditional Ni-Mn-X-based Heusler alloys (X = Ga, In, Sn, Sb). The enhancement of the magnetic moments by B doping is also observed in first-principles calculations. These calculations clarify the atomic occupancy of B and the changes in the electronic configuration. Our current study indicates that interstitial doping with a light element (boron) is an effective method to improve the magnetocaloric effect in these all-d-metal Ni-Co-Mn-Ti-based magnetic Heusler compounds.

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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 effect of Co and Ni doping on the structure, magnetic and magnetocaloric properties of Fe-rich (Mn,Fe)₂(P,Si) compounds was studied. With increasing Co and Ni content, both the Curie temperature (T_c) and the thermal hysteresis (ΔT_hys) decreased, whereas the hexagonal P-62 m crystal structure was maintained. A pronounced reduction in hysteresis was observed upon Co doping, while a significant reduction in Curie temperature was found upon Ni doping. Mössbauer spectroscopy measurements and DFT calculations indicated the substitution of Fe at the 3f site for both Co and Ni doping. Rietveld refinement of the X-ray diffraction data showed that Co substitute atoms in the main phase and the impurity phase, while Ni exhibits an affinity to the main phase. Magnetization measurements on the Co doped samples revealed an increase in magnetization for 2 at.% of Co, followed by a decrease for higher concentrations. DFT calculations showed that the magnetic moment on the 3f site is enhanced by Co substitution, whereas an opposite trend was observed for Ni substitution.

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The magnetocaloric effect (MCE) is a thermal response of a magnetic material to a change in an external magnetic field. With the discovery of materials exhibiting a giant magnetocaloric effect in the vicinity of room temperature, several applications of this phenomenon have been proposed. First is the magnetic refrigeration, which can serve as a more eco-friendly alternative to conventional vapour-compression cooling systems. The second is the magnetic energy conversion using thermomagnetic motors and generators. It allows to transfer waste heat - currently an untapped resource – into electricity, therefore, increasing the energy efficiency of various types of industries. The development of devices for these applications facilitated the need for an optimal material to fit all the practical requirements. To this date, only a handful of materials are considered viable for commercial implementation, among which (Mn,Fe)2(P,Si) alloys, Ni-Mn-based Heusler alloys and La(Fe,Si)13 alloys are most prominent. The goal of this thesis is to identify new promising magnetocaloric materials and improve known material systems using a combination of experimental techniques, ab initio modelling and database screening. @en

The physical properties of the extensively studied Fe₂P material family, well-known for its promising magnetocaloric qualities are greatly influenced by the unit-cell parameters of this hexagonal system. This sensitivity of the various magnetocaloric properties to structural parameters is particularly important for developing a material suitable for room-temperature magnetic refrigeration. A change in the unit cell, due to added elements can induce pronounced changes in the Curie temperature and the nature of the magnetic phase transition. Li belongs to a yet unexplored group of possible dopant elements – alkali metals, and exhibits an unusual behavior upon introduction to Fe₂P. We observe a preference to replace iron atoms, as opposed to the common tendency of non-magnetic dopants to replace phosphorus, leading to a strong influence on the magnetic structure. The addition of Li introduces a deformation of the unit cell with a small change in volume and a decrease in c/a ratio, while the same crystallographic phase is maintained over a relatively wide concentration range. We show that lithium has an exceptionally strong effect on the Curie temperature of Fe₂P reaching 800 K at 20% Li compared to 240 K for the undoped material.

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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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The interest in the magnetic cooling devices has led to an intensive search for suitable well-performing magnetocaloric materials. High-throughput studies based on density functional theory (DFT) calculations can significantly simplify and increase the range of this search. In this chapter, an effective approach to the screening of magnetocaloric materials based on the information obtained from crystallographic databases is demonstrated. To identify systems of interest, several screening parameters were developed using properties of various well-known materials with magnetocaloric effect (MCE) as a reference. Along with magnetic properties, other factors important for practical applications are taken into consideration including price, availability, and toxicity of candidate materials. Combining these criteria, an algorithm for the screening process is suggested. It utilizes both information readily available in the database and additional ab-initio calculations. A step-by-step application of initial screening parameters to sort out unsuitable materials before performing more computationally heavy assessments allows fast processing of a large number of candidates. This results in a shortlist of promising compounds ranked by their potential which can serve as a guide for experimental research.

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The giant magnetocaloric effect is widely achieved in hexagonal MnMX-based (M = Co or Ni, X = Si or Ge) ferromagnets at their first-order magnetostructural transition. However, the thermal hysteresis and low sensitivity of the magnetostructural transition to the magnetic field inevitably lead to a sizeable irreversibility of the low-field magnetocaloric effect. Here, we show an alternative way to realize a reversible low-field magnetocaloric effect in MnMX-based alloys by taking advantage of the second-order phase transition. With introducing Cu into Co in stoichiometric MnCoGe alloy, the martensitic transition is stabilized at high temperature, while the Curie temperature of the orthorhombic phase is reduced to room temperature. As a result, a second-order magnetic transition with a negligible thermal hysteresis and a large magnetization change can be observed, enabling a reversible magnetocaloric effect. By both calorimetric and direct measurements, a reversible adiabatic temperature change of about 1 K is obtained under a field change of 0-1 T at 304 K, which is larger than that obtained in a first-order magnetostructural transition. To gain a better insight into the origin of these experimental results, first-principles calculations are carried out to characterize the chemical bonds and the magnetic exchange interaction. Our work provides an understanding of the MnCoGe alloy and indicates a feasible route to improve the reversibility of the low-field magnetocaloric effect in the MnMX system.

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Ni-Mn-X (X = In, Sn, and Sb) based Heusler alloys show a strong potential for magnetic refrigeration owing to their large magnetocaloric effect (MCE) associated with first-order magnetostructural transition. However, the irreversibility of the MCE under low field change of 0–1 T directly hinders its application as an efficient magnetic coolant. In this work, we systematically investigate thermal and magnetic properties, crystalline structure and magnetocaloric performance in Ni51−xMn33.4In15.6Vx alloys. With the introduction of V, a stable magnetostructural transition near room temperature is observed between martensite and austenite. An extremely small hysteresis of 2.3 K is achieved for the composition x = 0.3. Due to this optimization, the magneticfield induced structural transition is partially reversible under 0–1 T cycles, resulting in a reversible MCE.
Both magnetic and calorimetric measurements consistently show that the largest value for the reversible magnetic entropy change can reach about 5.1 J kg−1 K−1 in a field change of 0–1 T. A considerable and reversible adiabatic temperature change of −1.2 K by the direct measurement is also observed under a field change of 0–1.1 T. Furthermore, the origin of this small hysteresis is discussed. Based on the lattice parameters, the transformation stretch tensor is calculated, which indicates an improved geometric compatibility between the two phases. Our work greatly improves the MCE performance of Ni-Mn-X-based alloys and make them suitable as realistic magnetic refrigeration materials.
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