Year

2012

Degree Name

Doctor of Philosophy

Department

Institute for Superconducting and Electronic Materials

Abstract

In this thesis, the magnetocaloric materials namely, the β-Co(OH)2 nanosheets, La0.8Gd0.2Fe11.4Si1.6Bx compounds, La0.8Gd0.2Fe11.4Si1.6Bxcompounds and the Mn0.94Ti0.06CoGe alloy have been systematically investigated with their structural, magnetic and magnetocaloric properties being studied in detail.

We report the synthesis of β-Co(OH)2 nanosheets using microwave assisted hydrothermal and conventional chemical reaction methods. A magnetic transition at an onset temperature, T1 = 96 K, and a sign of antiferromagnetic state transition at the Néel temperature, TN = 9 ~ 10 K, can be determined from M-T curves and M-H curves magnetic properties investigation. It is found that a sign of field-induced first order transition below 9 K, acts the role of reversing the magnetocaloric effect from negative to positive value. The large reversible magnetic-entropy change ΔSM of 17 J/kg K around 11 K for a field change of 5 T indicates that this material is useful for refrigeration applications at low temperatures.

The effects of boron doping on the itinerant-electron metamagnetic (IEM) transition and the magnetocaloric effects (MCEs) in the cubic NaZn13-type La0.8Gd0.2Fe11.4Si1.6 compound have been investigated. The Curie temperature, TC, of La0.8Gd0.2Fe11.4Si1.6Bx compounds with x = 0, 0.03, 0.06, 0.2 and 0.3 was found to increase from 200 K to 222 K with increase in boron doping, x. The maximum values of the isothermal magnetic entropy change, ΔSM, (derived using the Maxwell relation for a field change ΔB = 0 – 5 T) in La0.8Gd0.2Fe11.4Si1.6Bx with x = 0, 0.03, 0.06, 0.2 and 0.3 are 14.8, 16, 15, 7.5 and 6.6 J kg-1 K-1 respectively, with corresponding values of the refrigerant capacity, RCP of 285, 361, 346, 222 and 245 J kg-1.

The large ΔSM value observed for the low level B doped La0.8Gd0.2Fe11.4Si1.6B0.03 and La0.8Gd0.2Fe11.4Si1.6B0.06 compounds is attributed to the first order nature of the IEM transition while the decrease of ΔSM at x = 0.2 and 0.3 is due to a change to a second order phase transition with increase in B doping. The nature of the magnetic phase transitions is also reflected by the magnetic hysteresis of ~ 3.7, 9, 5.7, 0.4 and 0.3 J kg-1 for x = 0.0, x = 0.03, 0.06, 0.2 and 0.3 respectively.

In an effort to improve the magnetocaloric effects (MCEs) of the NaZn13-type La0.8Gd0.2Fe11.4Si1.6 compound, the effect of boron doping on the magnetic properties and magnetocaloric properties has been investigated. The magnetic entropy change (ΔSM) for the La0.8Gd0.2Fe11.4Si1.6 compound, obtained for a field change of 0 – 5 T using the Maxwell relation exhibits a spike and appears to be overestimated and is thus corrected by using the Clausius-Clapeyron equation (CC). The ΔSM determined from the CC equation is estimated to be 19.6 J kg-1K-1. However, large hysteretic losses which are detrimental to the magnetic refrigeration efficiency, occur in the same temperature range. In this work, we report a significant reduction in hysteretic losses by doping the La0.8Gd0.2Fe11.4Si1.6 compound with a small amount of boron to get La0.8Gd0.2Fe11.4Si1.6Bx compounds. The hysteresis loss decreases from 131.5 to 8.1 J kg -1 when x increases from 0 to 0.3, while ΔSM, obtained for a field change of 0 – 5 T, varies from 19.6 to 15.9 J kg-1K-1. This also simultaneously shifts the TC from 174 K to 184 K and significantly improves the effective refrigerant capacity (RCeff) of the material from 164 to 305 J kg-1.

Structural, magnetic and magnetocaloric properties of the Mn0.94Ti0.06CoGe alloy have been investigated using x-ray diffraction, DC magnetization and neutron diffraction measurements. Two phase transitions have been detected at Tstr = 235 K and TC = 270 K, respectively. A giant magnetocaloric effect has been obtained around Tstr associated with a structural phase transition from the low temperature orthorhombic TiNiSi-type structure to the high temperature hexagonal Ni2In-type structure, which is confirmed by neutron study. In the vicinity of the structural transition, Tstr, the magnetic entropy change, -ΔSM reached a maximum value of 14.8 Jkg-1K-1 under a magnetic field of 5T which is much higher than that previously reported on the parent compound MnCoGe. To investigate the nature of the magnetic phase transition around TC = 270 K from ferromagnetic to paramagnetic state, we performed a detailed critical exponent study. The critical components, γ, β and δ determined using the Kouvel-Fisher method, the modified Arrott plot as well as the critical isotherm analysis agree well and are close to the theoretical prediction of the mean-field model.

Structural and magnetic properties of Mn0.94Ti0.06CoGe have been studied by a combination of bulk magnetisation and neutron diffraction measurements over the temperature range 5 K - 350 K. The crystal structural transition occurs at Tstr (~ 235 K) with a change in symmetry from the low temperature orthorhombic TiNiSi-type structure (space group Pnma) to the high temperature hexagonal Ni2In-type structure (space group P63/mmc) and the magnetic phase transition takes place around TC = 270 K. It is found that the structural transition around Tstr is incomplete and there is a co-existence of the orthorhombic and hexagonal structures between Tstr and TC (~ 270 K).

High pressure x-ray diffraction studies up to 10.4 GPa were performed on the Mn0.94Ti0.06CoGe alloy using synchrotron radiation and a diamond anvil cell. No structural phase transitions occurred in the entire range of our measurements. Unit cell parameters were determined up to 10.4 GPa and the calculated unit cell volumes were found to be well represented by a third order Birch-Murnaghan equation of state. The bulk modulus determined from the pressure – volume data was found to be, B0 = 231.72 ± 7.79 GPa. This study, employing high resolution synchrotron x-rays has helped clarify the behaviour of the Mn0.94Ti0.06CoGe alloy under high pressure.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.