Structural and Magnetic Properties of Some Delafossite Magnetic and Multiferroic Materials.

Abstract

In recent years, there has been renewed interest in geometrically frustrated magnetic systems. Delafossites (CuFeO2 and CuCrO2), and Credenerite (CuMnO2) are ABO2-type materials, having two dimensional (2D) layered structures consisting of triangular lattices made up of magnetic B-site trivalent cations, have been studied as typical frustrated triangular-lattice spin systems. In case of Delafossites (CuFeO2 and CuCrO2) have a perfect triangular lattice system because of B-sites cations have and in d-orbitals while Credenerite (CuMnO2), the crystal structure of CuMnO2 does not consist of perfect triangular lattices, rather it consist of isosceles-triangular lattices due to Jahn-Teller distortion of with an orbital degrees of freedom. Geometrical spin frustration has been recognized to be one of the candidates for the magneto electric multiferroic materials. In such systems, spin ordering is suppressed by competing exchange interactions well below the conventional ordering scale set by the Weiss temperature. Due to this, they have expected to possess unconventional magnetic states, such as spin glass, Griffith phase, exchange bias, spin liquid, spin ice etc. The frustrated system, owing to the vast degeneracy arising from competing magnetic interactions, often displays complex magnetic orders at low temperatures like noncentrosymmetric, noncollinear. These exotic magnetic structures sometimes break the crystal symmetry and can be an origin of the ferroelectricity. Multiferroics are the materials having two or more long range ferroic ordering simultaneously. Multiferroic materials with strong magnetoelectric coupling (appearance of magnetization M in an electric field E, or appearance of electric polarization P by the application of magnetic field H) can enable 4- state logic devices. The present thesis is focused on the structural, magnetic, transport and magneto electric behavior of CuMnO2 and CuCrO2 at low temperature. In order to give systematic discussion, I have organized my thesis into six chapters. Summary of each chapter is given below: In Chapter 1, we will be introducing the geometrically frustrated magnetic system, in which unconventional magnetic ordering, multiferroic behavior and their importance in physical and practical application has been discussed. Unconventional magnetic ordering, means spin glass, exchange bias, Griffith like phase and the definition of multiferroics, its origin and types of multiferroics have been discussed in details. Furthermore, the electrical transport models used to explain the electrical properties of these materials such as the Arrhenius model and Variable Range Hopping (VRH) model will be discussed. The main motivation behind the thesis work has also been given in this chapter. Chapter 2 deals with synthesis and characterization techniques such as neutron diffraction, synchrotron X-ray diffraction for structural and phase identification, UV- visible to study the optical absorbance properties (determination of optical band gap), X-ray photoemission spectroscopy, superconducting interference device (SQUID) for magnetic measurement and electrical transport measurement at low temperature. Chapter 3 is devoted to the structural magnetic optical and electrical properties of Crednerite system. Neutron diffraction, synchrotron X-ray diffraction, magnetic, X-ray photoemission spectroscopy (XPS) and UV-visible spectroscopy measurements have been carried out on CuMnO2 and 5% Fe doped CuMnO2 samples. It has been observed that with Fe doping, the apical Mn–O distances decrease while the equatorial distances slightly increase, reducing the distortion in MnO6 octahedra. Moreover, when Fe is doped along with k1 = (1/2,1/2,1/2) the magnetic peaks can also be indexed with the propagation vector k2 = (1/2,1/2,0) indicating the appearance of ferromagnetic coupling between ab-planes. Value of magnetization is increased with Fe doping but coercivity is decreased. These might be due to direct Mn–Mn exchange and Mn–O–Cu–O–Mn super–super exchange interactions. The UV-vis data showed the increase in one of the two energy gaps, on Fe doping, indicating the usefulness of these materials as wide band gap magnetic semiconductors. In Chapter 4, DC and AC magnetic measurement have been discussed to establish the Griffith phase behavior in CuMnO2 system. It has been observed that in DC magnetization data, a sharp downturn is observed well above Curie-Weiss temperature in which χ-1 dc (T) curve obeys the Curie Weiss law and downturn decreases with application of magnetic field. The existence of GP can also be confirmed by analyzing magnetic susceptibility which for a Griffith Phase, should be characterized by an exponent less than unity, that is χ(T) ∝(T/TC-1)-(1-λ) where 0≤λ ≤1. The anomaly observed in χ' and χʺ is attributed to the onset of GP. A steady decay in χ’(T) is observed in the frequency range 5Hz to 500Hz except the onset temperature, 54K where an increase in slope is observed which is consistent with the onset of GP. As the frequency increases, the intensity in both χ'(T) and χʺ (T) decreases. Also with increase of frequency, the anomaly shifts towards higher temperature. The GP anomaly in χ' gradually diminishes with the application of small dc field, in accordance with the extreme sensitivity shown by the GP in other compounds. This behavior of χac with the applied magnetic field also supports the existence of Griffith like phase above the AFM order in the present system. In Chapter 5, we have studied structural, magnetic and magneto-dielectric and pyroelectric with magnetic field measurement to explain multiferroic and magnetoelictric coupling in CuCrO2 system. In pyroelectric current measurement no electric polarization was observed in CuCrO2 without fields. However, in the presence of poling electric field of 400kV/m, the sign reversal of polarization observed by reversing the direction of poling electric field. The polarization starts developing just above TN which increases with lowering temperature. The observed magnitude of polarization in presence of magnetic field 3T and 5T becomes ~32µC/m2 and ~ 37µC/m2 respectively. Spin ice behavior has been confirmed by AC and DC magnetic measurements. We have observed a new spin freezing transition below 32K. It is observed that this transition is thermally driven. Further, the analysis yielded the nature of the spin freezing to be a single ion process. Chapter 6 deals with the conclusions of the whole work presented in the thesis. Some plans for the future research are also given in this chapter.