Doctor of Philosophy
Institute for Superconducting and Electronic Materials - Faculty of Engineering
Nigam, Rashmi, Study of magnetic behaviour in Ru-based superconducting ferromagnets, Doctor of Philosophy thesis, Institute for Superconducting and Electronic Materials - Faculty of Engineering, University of Wollongong, 2010. http://ro.uow.edu.au/theses/3124
Phenomena arising due to interaction between magnetic phases and superconductivity are one of the most intensively studied and intriguing fields of research due to its relevancy to superconducting and spin electronics, colossal magneto-resistance, and mechanisms of superconductivity. This drives the current interest in natural materials combining magnetic ordering properties with superconducting properties, such as ruthenium based high temperature copper-oxide superconductors, these called ruthenocuprates. These compounds display an interesting interplay of superconductivity and magnetism. The superconductivity in the Ru-based systems is not destroyed by the presence of ferromagnetic correlations, which contradicts the antagonistic nature of these two phenomena; therefore, these compounds are classified as superconducting ferromagnets. The two main ruthenocuprate systems are RuSr2R2−xCexCu2O10−δ (Ru1222), and RuSr2RCu2O8−δ (Ru1212), where R is Eu, Gd, Sm, and Y. In order to understand how the two mutually exclusive phenomena (superconductivity and magnetism) coexist within a unit cell volume in Ru-based superconducting ferromagnets, it is important to understand their magnetic behaviour. It has been over a decade now that efforts have been devoted to understanding the true nature of magnetic ordering in the Ru1212 and Ru1222 structures. However, up to date no common consensus has been reached. The current understanding of Ru1222 compounds is even more complex. In this thesis, a comprehensive model has been developed to understand the magnetic spin ordering in the Ru1212 and Ru1222 systems, and to establish a cohesive model explaining the temperature as well as field dependent magnetic behaviour of these systems. Different characterization techniques, such as x-ray diffraction, scanning electron microscopy, temperature dependent zero-field cooled and field cooled dc susceptibility, magnetization versus applied field behaviour, detailed field and frequency dependent nonlinear ac susceptibility measurements, resistivity measurements, and neutron powder diffraction were employed to establish a more profound understanding of the magnetism in Ru-based superconducting ferromagnets. Systematic investigations carried out on a series of Ru1222 samples revealed that these compounds always contain additional magnetic phases with their own magnetic behaviour, which is similar to, but yet distinctive from the main Ru1222 phase. The interaction between these two magnetic species affects the physical, magnetic, and transport properties of the material, which can be identifies by (i) the small peak above the Curie temperature (TC ~ 95 K) in the temperature dependent magnetic susceptibility measurements, (ii) the re-entrance of irreversibility in hysteresis loops above TC, which develops a bell-shaped curve in the temperature dependence of the coercive field, and (iii) double-step superconducting transitions. Experimental observation showed that for the pure Ru1222 sample, these features were highly suppressed. A model is proposed, which demonstrates that the superposition of magnetic signals from the two magnetic phases with slightly different magnetic ordering temperatures is responsible for the controversial observations of multiple magnetic transitions in Ru1222 and the features inherent to various magnetic states, such as ferromagnetic, antiferromagnetic, and spin-glass along with coexistence of magnetism and superconductivity. This variety of possible magnetic states has led to the different controversial models proposed in the literature, reflecting one or another feature observed. Further, a detailed investigation was carried out to understand the spin dynamics in the Ru1222 system. The investigation of the nonlinear susceptibility as a function of temperature and applied field suggested that the structural properties as well as the inhomogeneities of the Ru1222 system resulted in the formation of magnetic (ferromagnetic) clusters of different sizes, shapes and properties. The magnetic clustering of the system leads to observation of various features in the dc magnetization and ac susceptibility, consistent with superparamagnetism and cluster spin glass states, which can coexist or stand alone, depending on the temperature range and the strength of the applied field. The formation of a cluster spin glass state by the ferromagnetic clusters suggested that the Ru1222 system does not possess a long range ferromagnetic order, rather short range ferromagnetic order is present in the Ru1222 system. In order to understand the temperature and magnetic field response of the Ru1222 and Ru1212 systems at atomic level, neutron powder diffraction studies (NPD) were carried out on Ru0.9Sr2YCu2.1O7.9 (Ru1212Y) and RuSr2Y1.5Ce0.5Cu2O10−δ (Ru1222Y) samples. NPD measurement showed antiferromagnetic order below TM = 140 K in Ru1212Y sample. A very weak antiferromagnetic signal below 140 K was also observed in Ru1222Y sample, which seems to be due to the presence of Ru1212Y impurity phase. Neutron diffraction measurements under applied fields of 0-6 T showed that with increasing field the AFM order disappeared in both the samples, however, a field-induced ferromagnetic ordering of the Ru moments perpendicular to the c-axis developed for Ru1222Y sample but no field-induced ferromagnetism was observed in Ru1212Y sample.
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