Year

2013

Degree Name

Doctor of Philosophy

Department

Institute for Superconducting and Electronic Materials

Abstract

The magnetic and electric properties of novel magnetic systems have aroused great interest in the physics research community. In the classic LaMnO3 to DyMnO3 compounds, orbital ordering and the Jahn-Teller effect play a critical role in determining the magnetic interactions. By introducing suitable alkaline earth metal ions (such as Ca2+, Sr2+) into ReMnO3 (Re = La3+, Sm3+, Nd3+, Pr3+), insulator-metal behavior was discovered which could be modified by changing the content of alkaline earth metal ions. Furthermore, a colossal magnetoresistance effect was subsequently found around the magnetic phase transition in these materials with the Mn3+/Mn4+ mixed state. On the other hand, a giant magnetoresistance effect can be achieved in many multilayer systems and in granular, heterogeneous, or phase separation magnetic alloys, on the basis of which, memory devices with high density data storage have been designed and put into large-scale industrial application. There are also other significant physical phenomena, such as the exchange bias effect (the theoretical prototype of some magnetic recording devices) and multiferroic/magnetoelectric coupling (which shows the possibility of achieving much higher data storage in the future).

In the beginning, a brief review of the magnetoresistance effect, exchange bias, and multiferroic/magnetoelectric coupling is presented, giving the basic physical principles and the latest progress as well. After the introduction, the work done during this PhD study is presented as follows:

a) Structural, magnetic, heat capacity, and dielectric properties in DyMn1-x FexO3 DyMnO3 possesses an incommensurate antiferromagnetic ordering at low temperature due to the magnetic interaction competition, which can also be regarded as a frustrated magnetic state. This special ordering produces a ferroelectric ordering according to the spin current model. To study the magnetic competition behavior, we partially replace the Mn3+ by Fe3+, considering that Fe3+ has 5 electrons on its 3d orbital, while Mn3+ only has 4 electrons on that orbital. This work adds to our understanding of the stability of spiral ordering and provides experimental evidence to demonstrate the possibility of modifying the magnetic ordering in the frustrated state.

b) Structure and magnetic properties in Nd1-xErxMnO3 .The rare earth manganese oxides have interesting physics, such as the orbital ordering in LaMnO3. As mentioned above, DyMnO3 shows multiferroic properties. The different physical behavior is strongly dependent on the rare earth element. In this work, we have chosen to study the Er3+ doping effect on the magnetic behavior in NdMnO3, considering that both Er3+ and Nd3+ are magnetic ions. This study will help to clarify the interaction between magnetic rare earth ions and transition metal ions, and the competition between two different magnetic rare earth ions.

c) Heat capacity in Nd1-xErxMnO3. This work is based on a thermodynamic study. The heat capacity can reveal the low temperature behavior of such a compound, such as an anomaly in the magnetic entropy and the ground state splitting of rare earth ions. Combined with the results from b), we can explain the competition between the crystal field (dependent on structure) and exchange field (dependent on the magnetic interaction).

d) Exchange bias in NdMnO3 and Pr0.5Y0.5Mn2Ge2. The exchange bias effect is usually observed in bilayer systems. For bulk materials, the exchange bias effect has not been thoroughly investigated. In this work, we study the simple perovskite NdMnO3 and the magnetocaloric Pr0.5Y0.5Mn2Ge2 alloy. The exchange bias field in NdMnO3 can reach -2400 Oe and 1800 Oe in various cooling fields. This work will help in the exploration of more single phase materials with the exchange bias effect. This foreshadows the possibility of a room temperature exchange bias effect in other similar or different alloys.

e) Interface structure and ferroelectricity in SmFeO3 thin film. Artificial stress engineering can modify the physical behavior in bulk materials. It is also possible to achieve a relatively strong ferroelectric state in some nonferroelectric or weak ferroelectric materials. In this work, I intended to confirm this assumption and study epitaxial SmFeO3 on Nd-SrTiO3 substrate based on structural characterization and electric/magnetic measurements. This work will stimulate interest in interface ferroelectric behavior.

Finally, I will summarize all the work described in this thesis.

FoR codes (2008)

020404 Electronic and Magnetic Properties of Condensed Matter; Superconductivity, 020406 Surfaces and Structural Properties of Condensed Matter, 091201 Ceramics, 091205 Functional Materials

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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.