Reducing the nucleation barrier in magnetocaloric Heusler alloys by nanoindentation

Abstract

Magnetocaloric materials are promising as solid state refrigerants for more efficient and environmentally friendly cooling devices. The highest effects have been observed in materials that exhibit a first-order phase transition. These transformations proceed by nucleation and growth which lead to a hysteresis. Such irreversible processes are undesired since they heat up the material and reduce the efficiency of any cooling application. In this article, we demonstrate an approach to decrease the hysteresis by locally changing the nucleation barrier. We created artificial nucleation sites and analyzed the nucleation and growth processes in their proximity. We use Ni-Mn-Ga, a shape memory alloy that exhibits a martensitic transformation. Epitaxial films serve as a model system, but their high surface-to-volume ratio also allows for a fast heat transfer which is beneficial for a magnetocaloric regenerator geometry. Nanoindentation is used to create a well-defined defect. We quantify the austenite phase fraction in its proximity as a function of temperature which allows us to determine the influence of the defect on the transformation.

Heterogeneous nucleation and heat flux avalanches in La(Fe, Si)13 magnetocaloric compounds near the critical point

“…It is known that the hysteresis width of La(Fe,Si) 13 is influenced by the strength of the magnetic field 4 , by hydrostatic pressure 5 and by substitution element at Fe sites. Moreover, as pointed out on several works 6,7 , and particularly on those regarding La(Fe,Si) 13 based materials 8 , the transformation process of first order magnetocaloric materials is due to the motion of phase boundaries between FM and PM phases. This motion takes place on a complex energy landscape influenced by several factors.…”

Coupling Phenomena in Magnetocaloric Materials

“…Strain fields at the surface of a sample can be caused by morphological features and defects. For instance, the shift of the transition temperature in the vicinity of an artificial mechanical defect (induced by plastic deformation with a nano‐indenter tip) in a thin film of Ni 48.4 Mn 32.8 Ga 18.8 was studied by Niemann et al . In these magnetocaloric Heusler compounds, the martensitic transition can be induced by stress, either globally, or locally .…”

“…For instance, the shift of the transition temperature in the vicinity of an artificial mechanical defect (induced by plastic deformation with a nano‐indenter tip) in a thin film of Ni 48.4 Mn 32.8 Ga 18.8 was studied by Niemann et al . In these magnetocaloric Heusler compounds, the martensitic transition can be induced by stress, either globally, or locally . The latter stress‐induced martensite formation can be tracked by atomic force microscopy (AFM) imaging, revealing an elastic stray field around the defect that locally influences the martensite start temperature and globally has an impact on the hysteresis of the transformation.…”

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

“…To influence and decrease thermal hysteresis in Heusler alloys, different approaches were proposed: 1) the use of artificial phase nucleation sites, 2) training by thermal cycling, and 3) minor loops …”

“…For epitaxial Ni–Mn–Ga films, which exhibit an identical microstructure like Ni–Mn–Ga–Co films, it was shown that well‐defined defects act as artificial phase nucleation sites by locally changing and reducing the nucleation barrier. The remanent indents (1 μm) created by nanoindentation promote the martensitic transformation, just on a local scale of about 10 μm because the influence radius is limited by the rigid substrate . For epitaxial Ni–Mn–Ga–Co films grown on MgO substrates it was shown that thermal cycling of the functional film reduces thermal hysteresis drastically by increasing the martensitic start‐temperature, which has been attributed to an increased nucleation density and a reduced constrain of the rigid substrate by delamination …”