Reducing Hysteresis Losses by Heating Minor Loops in Magnetocaloric Ni–Mn–Ga–Co Films

Abstract

Magnetocaloric materials relying on first‐order phase transitions give the highest entropy changes compared to second‐order transitions. However, they have the drawback of a transformation hysteresis, which reduces the efficiency of a cooling cycle. Here an approach to reduce the thermal hysteresis losses by performing incomplete transformation cycles, so called minor loops, is presented. A freestanding, epitaxially grown Ni–Mn–Ga–Co film is used as a model system to analyze the direct martensitic and reverse transformation paths in detail. The additional residual phase fraction only facilitates the forward martensitic transformation, but not the reverse transformation. This pronounced asymmetry between cooling and heating demonstrates that the forward transformation to martensite is controlled by nucleation and the associated excess energy contributes to hysteresis losses. For the reverse transformation, however, nuclei of austenite are always present and accordingly this transformation is controlled by growth of the austenitic phase. The experiments reveal that only the use of heating minor loops is useful to improve the efficiency of magnetocaloric epitaxial films and by this, the ratio of usable magnetization difference divided by hysteresis loss can be increased by 64 %.

Influences of Si Substitution on Existence, Structural and Magnetic Properties of the CoMnGe Phase Investigated in a Co–Mn–Ge–Si Thin-Film Materials Library

“…The presence of residual austenite upon a structural transition to martensite is a known phenomenon in Heusler alloys and the same principles as described by Niemann et al and Diestel et al will be applicable here for different geometries. 33,34 Localized strain at the interface between substrate and thin film can also restrain the structural transition and have a significant impact on the hysteresis width of the transition. 35,36 This also explains the observed large hysteresis widths of >100 K. Thermal cycling could reduce the hysteresis width and increase 15,37 The latter showed an increase of the structural transition temperature for a substitution of Ge with Si.…”

Probing the Martensitic Microstructure of Magnetocaloric Heusler Films by Synchrotron Diffraction

“…The simultaneous occurrence of different sets has recently been suggested to have an impact on hysteresis . Not all boundaries between different sets are compatible and thus some residual austenite can remain there and act as nucleus for the reverse transformation to the austenite.…”

“…As described in detail in ref. , the major origin of hysteresis is the excess energy necessary for nucleation and growth of both phases coexisting during a first order transition.…”

Following the Martensitic Configuration Footprints in the Transition Route of Ni-Mn-Ga Magnetic Shape Memory Films: Insight into the Role of Twin Boundaries and Interfaces

“…defects, verse transition (martensite to austenite) the itic phase [35]. Residuals of untransformed suggested to serve as the starting points for scribed martensitic interfaces are generally the untransformed austenitic phase [36].…”

“…Residuals of untransformed austenitic phase inside the martensitic phase have been suggested to serve as the starting points for the reverse transition. In the literature, the above-described martensitic interfaces are generally suggested to be the regions maintaining the residuals of the untransformed austenitic phase [36].…”