Date of Award
Doctor of Philosophy
Dr. Pnina Ari-Gur
Dr. Clement Burns
Dr. Yang Ren
Dr. Arthur McGurn
Ni-Mn-X Heusler alloys, demonstrating strong coupling between crystalline structure and magnetic state, were studied. They undergo field-induced, first-order transformations from a low symmetry martensite to a high-symmetry austenitic phase around room temperature. The substantial difference between the entropies of the two phases results in a large adiabatic temperature change, called “Giant Magnetocaloric Effect (GMCE)”. Consequently, these alloys are promising refrigerants for near-room temperature cooling systems. This magnetic cooling is an energy-efficient and eco-friendly technology.
Crystalline structures and magnetic states of these alloys, which determine their magnetocaloric performances, highly depend on their composition. To examine new paths to optimize their magnetocaloric performances, this research is focused on the crystalline and magnetic behavior of a series of alloys under various experimental conditions (one Ni-Mn-In, three Ni-Mn-In-Co and two Ni-Mn-Ga). Additionally, phase transformation temperatures, co-existing phases, site occupancies, the effects of a magnetic field on the phase transformation temperature and hysteresis were also studied.
Their chemical compositions were determined by the RBS and EDS techniques. Rietveld refinements of diffraction data, reveals austenitic structure of all these alloys is cubic L21 (𝐹𝑚3̅ 𝑚) and upon cooling, they transform into monoclinic martensitic phases (P 1 2/m 1 space group). Martensitic phase, except for Ni-Mn-Ga, is a mixture of two modulated monoclinic phases: either 5M & 7M or 6M & 8M. Ni-Mn-Ga alloys undergo inter-martensitic phase transformations from 7M modulated monoclinic phase to a non-modulated L10 tetragonal phase, upon cooling.
Magnetic nature was determined by thermomagnetic, AC-susceptibility, and neutron diffraction. The austenitic phase of the Ni-Mn-In and Ni-Mn-In-Co alloys is ferromagnetic due to strong ferromagnetic interactions between Mn(4a-sites) and Mn(4b-sites). In the Ni-Mn-In-Co alloys, the interactions between Co atoms enhance the ferromagnetism of the austenite. The Ni- Mn-Ga alloys in the current study are paramagnetic in the austenitic phase and they order ferromagnetically in their martensitic phase. Magnetic interactions in the martensitic phase become complex with the variation of interatomic distances between magnetic atoms due to the modulations of the martensitic phase. Consequently, different magnetic natures, ferromagnetic, antiferromagnetic, and spin-glass-like are present in the martensitic phase.
Both magnetic field and temperature drive the martensitic transformation. The hystereses associated with magnetic transformations are significantly higher than those of the crystalline transformations, and are approximately proportional to the square of the magnetic field. The hystereses associated with crystalline phase transformation have a minimum at a certain field. Because of the difference between the two transformations they merge only upon heating and under a certain magnetic field.
In all studied martensitic transformations (i.e. upon cooling) the lattice entropy decreases. However, the effect is larger when the austenite’s magnetic nature has higher entropy than the martensite does. Therefore, a magneto-structural transformation from antiferromagnetic martensite to cubic ferromagnetic austenite produces a large GMCE. However, it is vital to consider the thermal hysteresis losses associated with both phase transformations when calculating the GMCE.
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Madiligama, Amila S. B., "Crystallography and Magnetism of Magnetocalorically Important Ni-Mn-X Based Heusler Alloys" (2016). Dissertations. 2476.