New materials for future refrigeration

Until now, researchers have been studying the so-called magnetocaloric effect, which can be observed when certain metals and alloys are exposed to a magnetic field: materials spontaneously change their magnetic order and temperature, and that makes them promising candidates for magnetic cooling devices. Recently, it has been found that it is possible to increase this effect in certain materials by simultaneously adding other stimuli, such as a stress field or, more specifically, a mechanical load. These are the so-called multi-caloric materials, of which a small range is already known about.

The research team that conducted this study selected a special nickel-manganese-indium alloy as one of the most promising materials. It is one of the magnetic shape memory alloys. If there is an external stimulus, such as a magnetic field, these structures morph into each other, resulting in noticeable alterations in the material. “The special feature of the selected compound is, however, that at the temperature at which the crystal structures change, the magnetic properties of the compound also change abruptly: structure and magnetism are strongly coupled”, notes Lluís Mañosa, professor at the Department of Condensed Matter Physics and member of the Institute of Nanoscience and Nanotechnology of the UB (IN2UB).

A custom-made measuring device
In order to determine the material properties that are necessary for an efficient cooling process, the team of the University of Barcelona had to develop a unique, specially designed calorimeter that enables the simultaneous application of a magnetic field and pressure to the sample.

To do this, the researchers harnessed a familiar method from materials testing and adapted it for their purposes, subjecting the sample to uniaxial mechanical stress. While the magnetic flux densities ranged up to 6 Tesla (T), which is 120,000 times stronger than the Earthʼs magnetic field, the peak compressive stress applied was a moderate 50 megapascals. For the given sample size, that force roughly corresponds to a mass of 20 kilograms.

“This means that one can apply this kind of pressure with just one hand. And that is the decisive aspect for future applications, because such manageable mechanical loads are relatively easy to implement”, says Mañosa. “Ou challenge was to integrate accurate measurements of both compressive stress and strain into our calorimeter without distorting the measurement conditions”, he adds.

The researchers recorded various parameters simultaneously -such as temperature change, magnetic flux density, compressive stress, and the alloyʼs entropy- during programmed cooling and heating phases near a specific temperature at which the given material experiences transformations in the crystal lattice that lead to a change in magnetization. In the alloy they used, this process occurs at room temperature, which is also advantageous for later practical application.

The measurements represent the sampleʼs behavior in a four-dimensional space. Mapping this space in a meaningful way requires a raft of experiments, resulting in large-scale measurement campaigns. The interaction of the different stimuli in multi-caloric materials has hardly been investigated so far. The nickel-manganese-indium alloy is the best-researched prototype compound in this class of materials to date. UB professor and IN2UB member Antoni Planes notes that “results have shown that cycles that use mechanical stress and magnetic field simultaneously have a better cooling performance than those that only use one of these stimuli”.

Reference article:

A. Gràcia-Condal, T. Gottschall, L. Pfeuffer, O. Gutfleisch, A. Planes, L. Mañosa, "Multicaloric effects in Metamagnetic Heusler Ni-Mn-In under uniaxial stress and magnetic field", in Applied Physics Reviews, 2020.DOI: doi: 10.1063/5.0020755