The impact of surface morphology on the magnetovolume transition in magnetocaloric LaFe_11.8Si_1.2

Coupling Phenomena in Magnetocaloric Materials

“…For convex regions at the sample surface, a scenario similar to the one described for the corners of a cube applies (see Section 3.1): nucleation is facilitated at convex regions if the volume increases during the transition. On the other hand, for concave regions nucleation is suppressed due to the strain fields that build up more easily in such confined geometries . Local strain fields produced by strongly concave and convex surface morphologies can therefore lead to local T C shifts comparable with the effect of hydrostatic pressure.…”

“…On the other hand, for concave regions nucleation is suppressedd ue to the strain fields that build up more easily in such confined geometries. [82] Locals train fields produced by strongly concavea nd convexs urface morphologies can therefore lead to local T C shifts comparable with the effect of hydrostatic pressure.O nt he other hand, the absence of strain fields in flat regionso ft he samplel eads to a uniform growth of the ferromagnetic phase.T hough it has only been shown partly in experiments to this point, it is feasible to assumet hat nucleation and growth and hence also the thermal hysteresis can be tuned by the surface shape of a sample,o peningu papromising way to control the extrinsic properties of magnetocaloric materials.O ne should note, however, that these are results of temperature-driven transitions,b ut transitions in magnetocaloric materials can be drivenbyboth changing external magneticfields and temperature. [83] Fort he case of am agnetic-field-induced transition, competingi nfluenceso fs train and the demagnetizing field are expected.…”

Contributions to Hysteresis in Magnetocaloric Materials

“…For MCE materials with first order transitions, measured hysteresis can be altered, in a number of ways. An appreciation of the fact that the first nucleation site where the new phase seeds within the host phase, for example from a paramagnetic to ferromagnetic phase, will occur in a small nucleation volume, where it is either (i) the locally highest magnetic field region controlled by demagnetization, (ii) the lowest strain field region, enabling the nucleation volume to change size, (iii) along the easy axis of magnetisation, if the material has magnetocrystalline anisotropy. In principle, it is challenging to imagine a scenario where the material is engineered such that the onset field of both the magnetizing and demagnetizing transition is modified usefully, in the same material, for example, an engineered composite sample which is both strain relieving and strain enhancing.…”

“…In principle, it is challenging to imagine a scenario where the material is engineered such that the onset field of both the magnetizing and demagnetizing transition is modified usefully, in the same material, for example, an engineered composite sample which is both strain relieving and strain enhancing. A recent study examining the role of the sample topography may offer some clues as to how this might be achieved . Samples with irreversibility are in a mixed state as the transition evolves.…”

“…Samples with irreversibility are in a mixed state as the transition evolves. Scanning Hall probe images can be used to build up an understanding of the distribution of local M–H loops showing the regions of the sample where the transition has already occured (as illustrated in Figure ). This type of information can be correlated with sample topography as well as providing input into the engineering models, such as the Preisach model, allowing estimation of the impact of hysteresis on the cooling cycle …”