Book/Dissertation / PhD Thesis

FZJ-2021-00727

http://join2-wiki.gsi.de/foswiki/pub/Main/Artwork/join2_logo100x88.png Structure and Dynamics of Magnetocaloric Materials

Maraytta, N. (Corresponding author)FZJ*

2021
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag Jülich
ISBN: 978-3-95806-557-4

Please use a persistent id in citations: http://hdl.handle.net/2128/27805

Abstract: The search for more efficient use of energy has been leading to a growing interest in the research field of magnetocaloric materials. The magnetocaloric effect (MCE) describes the change of temperature or entropy of a material when exposed to a change of the magnetic field and forms the basis of magnetocaloric refrigeration technologies. This utilization of the effect can offer a novel method for cooling that is economically feasible and ecologically friendly, and hence the effect attracts the attention of many researches. MCE is identified by the temperature change ($\Delta$T$_{ad}$) in an adiabatic process, and by the entropy change ($\Delta$S$_{iso}$) in an isothermal process.Part of this thesis is devoted to the investigation of the magnetocaloric effect (MCE) by direct measurements in pulsed magnetic fields as well as by analyzing the magnetization and specific heat data collected in static magnetic fields. The emphasis is on the direct measurement of the adiabatic temperature change $\Delta$T$_{ad}$ in pulsed magnetic fields as it provides the opportunity to examine the sample-temperature response to the magnetic field on a time scale of about 10 to 100 ms, which is on the order of typical operation frequencies (10 - 100 Hz) of magnetocaloric cooling devices. Furthermore, the accessible magnetic field range is extended to beyond 70 T and the short pulse duration provides nearly adiabatic conditions during the measurement. In the last years there has been an upsurge in the knowledge of the MCE and many materials have been investigated for their MCE characteristics. In the context of this thesis, the magnetocaloric properties of the single crystalline compounds MnFe$_{4}$Si$_{3}$ and Mn$_{5}$Ge$_{3}$ are investigated. Moreover, the nuclear and magnetic structure of the AF1' phase of the single crystalline compound Mn$_{5}$Si$_{3}$ are determined. For the MnFe$_{4}$Si$_{3}$, we have studied the magnetic and magnetocaloric response to pulsed and static magnetic fields up to 50 T. We determine the adiabatic temperature change $\Delta$T$_{ad}$ directly in pulsed fields and compare to the results of magnetization and specific heat measurements in static magnetic fields. The high ability of cycling even in high fields confirms the high structural stability of MnFe$_{4}$Si$_{3}$ against field changes, an important property for applications. The magnetic response to magnetic fields up to $\mu_{0}$H = 35 T shows that the anisotropy can be overcome by fields of approx. 7 T. For the Mn$_{5}$Ge$_{3}$, we have investigated the field direction dependence of the thermo-magnetic behavior in single crystalline Mn$_{5}$Ge$_{3}$. The adiabatic temperature change $\Delta$T$_{ad}$ in pulsed fields, the isothermal entropy change $\Delta$S$_{iso}$ calculated from static magnetization measurements and the heat capacity have been determined for field parallel and perpendicular to the easy magnetic direction [001]. The isothermal magnetization measurements yield furthermore the uniaxial anisotropy constants in second and fourth order, K$_{1}$ and K$_{2}$. We discuss how the anisotropy affects the magneto-caloric effect (MCE) and compare the results to the related [...]

Note: Dissertation, RWTH Aachen University, 2021

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Contributing Institute(s):

1. Streumethoden (JCNS-2)
2. Streumethoden (PGI-4)
3. JARA-FIT (JARA-FIT)

Research Program(s):

1. 899 - ohne Topic (POF4-899) (POF4-899)

Experiment(s):

1. HEiDi: Single crystal diffractometer on hot source (SR9b)
2. POLI: Polarized hot neutron diffractometer (SR9a)

Appears in the scientific report 2021

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