High-throughput design of all-d-metal Heusler alloys for magnetocaloric applications

Due to their versatile composition and customizable properties, A$_2$BC Heusler alloys have found applications in magnetic refrigeration, magnetic shape memory effects, permanent magnets, and spintronic devices. The discovery of all-$d$-metal Heusler alloys with improved mechanical properties compared to those containing main group elements, presents an opportunity to engineer Heuslers alloys for energy-related applications. Using high-throughput density functional theory calculations, we screened magnetic all-$d$-metal Heusler compounds and identified 686 (meta)stable compounds. Our detailed analysis revealed that the inverse Heusler structure is preferred when the electronegativity difference between the A and B/C atoms is small, contrary to conventional Heusler alloys. Additionally, our calculations of Pugh ratios and Cauchy pressures demonstrated that ductile and metallic bonding are widespread in all-$d$-metal Heuslers, supporting their enhanced mechanical behaviour. We identified 49 compounds with a double-well energy surface based on Bain path calculations and magnetic ground states, indicating their potential as candidates for magnetocaloric and shape memory applications. Furthermore, by calculating the free energies, we propose that 11 compounds exhibit structural phase transitions, and propose isostructural substitution to enhance the magnetocaloric effect

Tailoring magnetocaloric effect in all-d-metal Ni-Co-Mn-Ti Heusler alloys: a combined experimental and theoretical study

Novel Ni-Co-Mn-Ti all-d-metal Heusler alloys are exciting due to large multicaloric effects combined with enhanced mechanical properties. An optimized heat treatment for a series of these compounds leads to very sharp phase transitions in bulk alloys with isothermal entropy changes of up to 38 J kg$^{-1}$ K$^{-1}$ for a magnetic field change of 2 T. The differences of as-cast and annealed samples are analyzed by investigating microstructure and phase transitions in detail by optical microscopy. We identify different grain structures as well as stoichiometric (in)homogenieties as reasons for differently sharp martensitic transitions after ideal and non-ideal annealing. We develop alloy design rules for tuning the magnetostructural phase transition and evaluate specifically the sensitivity of the transition temperature towards the externally applied magnetic fields ($\frac{dT_t}{\mu_0dH}$) by analyzing the different stoichiometries. We then set up a phase diagram including martensitic transition temperatures and austenite Curie temperatures depending on the e/a ratio for varying Co and Ti content. The evolution of the Curie temperature with changing stoichiometry is compared to other Heusler systems. Density Functional Theory calculations reveal a correlation of T$_C$ with the stoichiometry as well as with the order state of the austenite. This combined approach of experiment and theory allows for an efficient development of new systems towards promising magnetocaloric properties. Direct adiabatic temperature change measurements show here the largest change of -4 K in a magnetic field change of 1.93 T for Ni$_{35}$Co$_{15}$Mn$_{37}$Ti$_{13}$

High‐Throughput Design of Magnetocaloric Materials for Energy Applications: MM´X alloys

Magnetic refrigeration offers an energy efficient and environmental friendly alternative to conventional vapor‐cooling. However, its adoption depends on materials with tailored magnetic and structural properties. Here a high‐throughput computational workflow for the design of magnetocaloric materials is introduced. Density functional theory calculations are used to screen potential candidates in the family of MM'X (M/M’ = metal, X = main group element) compounds. Out of 274 stable compositions, 46 magnetic compounds are found to stabilize in both an austenite and martensite phase. Following the concept of Curie temperature window, nine compounds are identified as potential candidates with structural transitions, by evaluating and comparing the structural phase transition and magnetic ordering temperatures. Additionally, the use of doping to tailor magnetostructural coupling for both known and newly predicted MM'X compounds is predicted and isostructural substitution as a general approach to engineer magnetocaloric materials is suggested

Designing Multicaloric Materials with Martensitic Phase Transitions for Future Cooling Applications

The demand for cooling devices and the corresponding energy costs are constantly expanding, driven by the growth of global population and the economies of fast-developing countries in warm climates. Novel caloric cooling solutions are an alternative that do not rely on environmentally harmful refrigerants and can provide a better energy-efficiency compared to the conventional vapor-compression technology. Especially magnetocaloric cooling is in focus of research activities including material research and well-performing device development. In order to optimize the material, many requirements need to be taken into account and each material system behaves differently under the application of an external magnetic field. This work focuses on the development of the MM'X material family with a conventional magnetocaloric effect and the various systems of Heusler alloys showing the inverse magnetocaloric effect. Both systems have in common that they experience a martensitic transition between a high-magnetization and a low-magnetization state. The MM'X base system of MnNiGe can be tuned by the isostructural alloying method, which is investigated in detail in this work. Therefore, Mn is substituted partially by Fe as well as Ge by Si. This enhances the magnetization change of the magnetostructural phase transition, reduces the amount of expensive Ge and allows for tailoring the transition temperature. The resulting alloys show very large isothermal entropy changes for small pieces of material. A difficulty that arises for this system is the mechanical integrity together with the low magnetic-field dependence of the transition temperature. The very good sensitivity of the transition towards hydrostatic pressure reveals barocaloric purposes as a very attractive field of application for these materials. A direct comparison with the versatile family of Ni-Mn-based Heusler alloys underlines their high potential for magneto- and multicaloric applications. With stoichiometric changes, the phase transition can be adjusted and also the magnetic-field dependence of the transition temperature is found to scale directly with the difference of the transition temperature to the austenite Curie temperature. The most promising system for low magnetic field changes is Ni(-Co)-Mn-In. Even though Ni(-Co)-Mn-Sn shows similar isothermal entropy changes, adiabatic temperature changes cannot compete. The drawback of a significant thermal hysteresis of around 10 - 15 K, which hinders a good cyclic performance of Heusler alloys, can be turned into an advantage by considering the novel approach of a multi-stimuli cycle, which exploits the thermal hysteresis to lock the material completely in its transformed state after a magnetic-field application. The necessary reverse transformation can be carried out by the application of pressure/stress as a second stimulus requiring a good pressure/stress-sensitivity of the transition temperature. Among the Heusler alloys, the novel all-d Heusler alloys of Ni-Co-Mn-Ti represent a promising material system for this approach. Their magnetocaloric performance is compared to the other Heusler alloys in small magnetic field changes of 2 T as well as for higher and faster field changes since the multi-stimuli approach allows for concentrated magnetic fields. Detailed investigations on the microstructure give insights that are crucial in order to understand the transition behavior. Analyzing the temperature-, magnetic field-, and pressure-induced phase transitions allows for assessing the potential of using the different Heusler systems for magnetocaloric and/or multicaloric cooling applications. This thesis puts the general properties of different material systems in a broad context and aims at providing principal design rules for the studied systems in order to develop and tailor well-performing caloric materials

Influence of magnetic field, chemical pressure and hydrostatic pressure on the structural and magnetocaloric properties of the Mn–Ni–Ge system

The magnetic, structural and thermomagnetic properties of the MM'X material system of MnNiGe are evaluated with respect to their utilization in magnetocaloric refrigeration. The effects of separate and simultaneous substitution of Fe for Mn and Si on the Ge site are analysed in detail to highlight the benefits of the isostructural alloying method. A large range of compounds with precisely tunable structural and magnetic properties and the tuning of the phase transition by chemical pressure are compared to the effect of hydrostatic pressure on the martensitic transition. We obtained very large isothermal entropy changes $\Delta S_{\rm iso}$ of up to $-37.8$ J ${\rm kg}^{-1}$ ${\rm K}^{-1}$ based on magnetic measurements for (Mn,Fe)NiGe in moderate fields of 2 T. The enhanced magnetocaloric properties for transitions around room temperature are demonstrated for samples with reduced Ge, a resource critical element. An adiabatic temperature change of 1.3 K in a magnetic field change of 1.93 T is observed upon direct measurement for a sample with Fe and Si substitution. However, the high volume change of 2.8% results in an embrittlement of large particles into several smaller fragments and leads to a sensitivity of the magnetocaloric properties towards sample shape and size. On the other hand, this large volume change enables to induce the phase transition with a large shift of the transition temperature by application of hydrostatic pressure (72 K ${\rm GPa}^{-1}$ ). Thus, the effect of 1.88 GPa is equivalent to a substitution of 10% Fe for Mn and can act as an additional stimulus to induce the phase transition and support the low magnetic field dependence of the phase transition temperature for multicaloric applications

High‐Throughput Design of Magnetocaloric Materials for Energy Applications: MM´X alloys

Magnetic refrigeration offers an energy efficient and environmental friendly alternative to conventional vapor-cooling. However, its adoption depends on materials with tailored magnetic and structural properties. Here a high-throughput computational workflow for the design of magnetocaloric materials is introduced. Density functional theory calculations are used to screen potential candidates in the family of MM'X (M/M’ = metal, X = main group element) compounds. Out of 274 stable compositions, 46 magnetic compounds are found to stabilize in both an austenite and martensite phase. Following the concept of Curie temperature window, nine compounds are identified as potential candidates with structural transitions, by evaluating and comparing the structural phase transition and magnetic ordering temperatures. Additionally, the use of doping to tailor magnetostructural coupling for both known and newly predicted MM'X compounds is predicted and isostructural substitution as a general approach to engineer magnetocaloric materials is suggested

Ce and La as substitutes for Nd in Nd2Fe14B-based melt-spun alloys and hot-deformed magnets: A comparison of structural and magnetic properties

Ce and La as very cheap rare-earth elements were used to substitute Nd in nanocrystalline melt-spun ribbons of nominal compositions (Nd1−xREx)13.6FebalCo6.6Ga0.6B5.6 (x = 0, 0.1, 0.2, … 1 for RE = Ce) and (x = 0, 0.1, 0.2, … 0.5 for RE = La). Ce substitution gradually decreased the Nd2Fe14B lattice constants and produced CeFe2 segregation from x = 0.7. La substitution led to lattice expansion along the c-axis and induced segregation of α-Fe and Nd2Fe17 at x = 0.5. Grain coarsening was observed in the Ce-substituted samples while La was found to suppress grain growth. Cerium worsened the magnetic properties of as-spun powders after an initial improvement in (Nd0.9Ce0.1)13.6FebalCo6.6Ga0.6B5.6 alloy which showed a coercivity (µ0Hc) of 1.54 T and a remanence (Br) of 0.81 T. Coercivity dropped with increasing La concentration but remanence increased from 0.73 T in the base composition to 0.88 T at x = 0.3. The Curie temperatures (TC) showed a slight decrease in both cases until x = 0.4. It then dropped abruptly for increasing Ce fractions and increased at x = 0.5 La. For x = 0.2 and 0.3 Ce and x = 0.2 La fractions, the melt-spun samples were further processed by hot-pressing and hot-deformation. The hot-pressed (Nd0.8La0.2)13.6FebalCo6.6Ga0.6B5.6 alloy measured lower coercivity but increased remanence comparing to the Ce-substituted alloys. However, this composition responded poorly to hot-deformation, severe cracking being induced in the process. Due to enhanced hot-workability, best magnetic properties were obtained after deformation for the (Nd0.7Ce0.3)13.6FebalCo6.6Ga0.6B5.6 alloy (µ0Hc = 1.09 T, Br = 0.97 T and energy product (BH)max = 170 kJ/m3)

Making a Cool Choice: The Materials Library of Magnetic Refrigeration

The phase‐down scenario of conventional refrigerants used in gas–vapor compressors and the demand for environmentally friendly and efficient cooling make the search for alternative technologies more important than ever. Magnetic refrigeration utilizing the magnetocaloric effect of magnetic materials could be that alternative. However, there are still several challenges to be overcome before having devices that are competitive with those based on the conventional gas–vapor technology. In this paper a rigorous assessment of the most relevant examples of 14 different magnetocaloric material families is presented and those are compared in terms of their adiabatic temperature and isothermal entropy change under cycling in magnetic‐field changes of 1 and 2 T, criticality aspects, and the amount of heat that they can transfer per cycle. The work is based on magnetic, direct thermometric, and calorimetric measurements made under similar conditions and in the same devices. Such a wide‐ranging study has not been carried out before. This data sets the basis for more advanced modeling and machine learning approaches in the near future

Influence of Gd-rich precipitates on the martensitic transformation, magnetocaloric effect, and mechanical properties of Ni–Mn–In Heusler alloys - A comparative study

A multi-stimuli cooling cycle can be used to increase the cyclic caloric performance of multicaloric materials like Ni–Mn–In Heusler alloys. However, the use of uniaxial compressive stress as an additional external stimulus to a magnetic field requires good mechanical stability. Improvement in mechanical stability and strength by doping has been shown in several studies. However, doping is always accompanied by grain refinement and a change in transition temperature. This raises the question of the extent to which mechanical strength is related to grain refinement, transition temperature, or precipitates. This study shows a direct comparison between a single-phase Ni–Mn–In and a two-phase Gd-doped Ni–Mn–In alloy with the same transition temperature and grain size. It is shown that the excellent magnetocaloric properties of the Ni–Mn–In matrix are maintained with doping. The isothermal entropy change and adiabatic temperature change are reduced by only 15% in the two-phase Ni–Mn–In Heusler alloy compared to the single-phase alloy, which results from a slight increase in thermal hysteresis and the width of the transition. Due to the same grain size and transition temperature, this effect can be directly related to the precipitates. The introduction of Gd precipitates leads to a 100% improvement in mechanical strength, which is significantly lower than the improvement observed for Ni–Mn–In alloys with grain refinement and Gd precipitates. This reveals that a significant contribution to the improved mechanical stability in Gd-doped Heusler alloys is related to grain refinement