title: Local structure and phase transition in compositionally complex magnetocalorics
Creators: name: Koch, David
Publisher: name: UNSPECIFIED
Year: 2024

description: Magneto-functional materials play a pivotal role in modern society’s pursuit of sustainability
through renewable energy sources. Among these materials, solid-state cooling systems based on
the magnetocaloric or multicaloric effect offer a promising way for reducing energy consumption
in cooling applications. The global demand for cooling solutions is steadily rising, emphasizing
the need for energy-efficient alternatives to conventional vapor compression techniques. Ni-Mn-
Z-based Heusler alloys, where Z is a main group element, represent a promising candidate for
solid-state cooling due to their martensitic phase transitions and unique functional properties.
These transitions, from a high-temperature cubic phase (austenite) to a low-symmetry lowtemperature
phase (martensite), have been the subject of extensive experimental and theoretical
research since the 1990s. Additionally, these alloys exhibit the characteristic L2₁ atomic structure,
which is typical for Heusler alloys.
In 2015, the discovery of the "all-d-Heusler" system Ni(Co)MnTi, which lacks main group
elements and features a tunable magneto-structural phase transition, opened new ways for
research. These alloys, free of p-electrons, presented intriguing questions regarding the origins
of structural instability and ordering mechanisms. Compared to traditional Heusler alloys,
all-d-Heusler compounds are less brittle and more ductile, offering improved mechanical stability
and machinability, making them promising candidates for solid-state refrigeration.However,
the inclusion of Co in substantial amounts adds complexity to the composition, introducing
challenges in unraveling the chemical ordering in these compositionally complex magnetocaloric
alloys.
This thesis aims to investigate chemical ordering in the Ni-Co-Mn-Ti system at both long
and short length scales and explore its interaction with structural instability. Understanding
this material system has the potential to lead to the discovery of more systems with martensitic
phase transitions composed of abundant 3-d metals. To investigate the structure and phase
transition of this complex system, advanced scattering techniques, including laboratory x-ray
diffraction, neutron diffraction, and high-energy x-ray diffraction at a synchrotron source, are
employed. While long range order can be studied in polycrystalline materials, single crystalline samples are essential for probing short-range order and anisotropic elastic precursors related to
the martensitic phase transition.
Key findings include the absence of long range L2₁order in Ni(Co)MnTi, in contrast to
Ni-Mn-Z Heusler compounds. The inclusion of Co introduces sensitivity to the degree of chemical
long range order, offering a means to tune phase transitions. Additionally, Co allows to modify
the martensite structures. This modification enhances phase compatibility and improves thermal
hysteresis.
Furthermore, the investigation of diffuse x-ray scattering in single crystals unveils a complex
behavior of the elastic moduli for ferromagnetic Ni₃₇Co₁₃Mn₃₃Ti₁₇ with a diverging Zener
anisotropy constant approaching the phase transition. A soft mode in the TA₂ phonon branch
is found, even though no p-d hybridization is present. Short-range L2₁ order can be induced by
low-temperature annealing and shows an influence on the phase transition temperatures and
soft mode behaviors, emphasizing the significance of local order on physical properties.
In conclusion, this thesis contributes to a comprehensive understanding of the Ni-Co-Mn-Ti
system, shedding light on its complex phase transitions, chemical ordering, and the interplay
between magnetism and lattice interactions. The insights gained pave the way for the design of
improved magnetocaloric materials composed of abundant 3-d metals, with potential applications
in energy efficient solid-state refrigeration.