The cooling sector contributes significantly to the global emission of greenhouse gases due to the used refrigerants as well as the indirect CO$_2$ emission as a result of the energy consumption to run refrigerators and air conditioners. Magnetocaloric (MC) refrigeration attracts a lot of attention since it can be more energy efficient and environmentally friendly than current vapor compression technology. The concept uses a solid-state magnetic material that heats up and cools down cyclically when exposed to a changing magnetic field. The development of multicaloric materials using more than one transformation-inducing stimulus opens up further possibilities to enhance the efficiency of the caloric cooling cycle. We suggested an approach that uses magnetic field and uniaxial pressure alternatingly to trigger the cyclic phase transition in a multicaloric material [1]. This approach requires a certain thermal hysteresis to prevent the back transformation and to lock the material in its high-magnetization phase. In order to transform the material back, an external stress can be applied leading to an elastocaloric effect (ECE)

In this work, we present the concept of the new multi-stimuli cooling concept including potential material systems and methods to tailor their functional and mechanical properties. The main objective is adopting the properties for a classical MC cycle towards the extended needs of the multi-stimuli cycle and tailoring the desired properties by intrinsic and extrinsic means. Therefore, we focus on Ni-(Co)-Mn-based Heusler alloys due to their adjustable transition temperature and large MCE for the first magnetic field application. We investigated for Ni-Co-Mn-In and Ni-Co-Mn-Ti Heusler alloys the influence of different microstructures, which have been designed by chemical variation as well as different processing routes, on the width of the phase transition, the magnetocaloric performance as well as the elastocaloric performance [2, 3]. We found that introducing secondary phases highly enhances the long-term stability under cyclic stress application while maintaining the caloric performance [4].

The system of Ni-Co-Mn-Ti exhibits a large MCE and good mechanical properties for a single-phase microstructure. Therefore, it is an ideal candidate to tailor the thermal hysteresis and the sensitivity of the phase transition towards external magnetic field and stress by chemical means. We show that both can be achieved by varying the Ni/Co ratio as well as by adjusting the transition temperature and austenite Curie temperature of the compound. By further optimizing the heat treatment, we could also enhance the sharpness of the phase transition and reach large isothermal entropy changes of up to 40 Jkg$^{-1}$K$^{-1}$ in 2 T [3] making this compound a suitable candidate for the caloric multi-stimuli cooling cycle.

[1] T. Gottschall et al., Nature Mat. 17, 929–934 (2018)
[2] L. Pfeuffer et al., Acta Materialia 221, 117390 (2021)
[3] L. Pfeuffer et al., Acta Materialia 217 1175157 (2021)
[4] A. Taubel et al., Acta Mater. 201, 425-434 (2020)