Browsing by Author "Alshalawi, Dhoha"

Now showing 1 - 1 of 1

Restricted
GEOMETRICAL FRUSTRATED MAGNETIC MATERIALS
(Universidad Complutense De Madrid, 2024-02-14) Alshalawi, Dhoha; de la Presa Muñoz de Toro, Patricia Marcela; Alonso Rodriguez, José Maria
This thesis delves into the intriguing Quantum Spin Liquid (QSL) and Spin Glass (SG) systems. It aims to unravel their unique properties and gain insights into their potential applications. Both QSL and SG systems exhibit high degrees of frustration at low temperatures and lack of long-range order. QSL maintains continuous fluctuation, while SG systems freeze at a glassy point. This study’s findings could help connect theoretical aspects and understand the unique magnetic behaviors of these systems, which differ from known magnetic interactions. Two primary materials, Ca10Cr7O28 and LaFeO3, are the central focus of this research. These materials are attractive due to the advancement of quantum and magnetism applications, including data storage, sensors, and quantum computing. Additionally, the study explores exchange bias interactions in nanoparticle materials at low temperatures, specifically the core/shell effects. The overarching goal is to contribute to understanding both systems' magnetic geometrical fluctuation in low temperatures. The investigation employs various experimental methods and equipment, encompassing structural, thermal, chemical, and magnetic measurements for both bulk and nanoparticle properties. Impurity-free materials let us to go deep into the intrinsic physical properties of these systems. The Ca10Cr7O28 emerges as a potential QSL material governed by a two-dimensional antiferromagnetic Heisenberg Hamiltonian interaction. Previous synthesis attempts suffered from incorrect precursor ratios, resulting in materials with certain degree of impurities, that can affect the analysis of the physical properties. This study successfully identifies the correct ratio (CaO: Cr2O3≈ 2.85:1) for the pure Ca10Cr7O28 sample. The research not only determines the correct synthesis ratio for pure Ca10Cr7O28 but also offers valuable insights into the impact of impurities on the crystal structure. The crystal structure, determined through X-ray diffraction (XRD) analysis, reveals a trigonal structure with Cr5+ ions arranged in two-dimensional Kagome layers and three-dimensional Zigzag chains. The Rietveld method verifies these proposed crystal structures by calculating the compound's atomic distances. The study extends to low temperatures, demonstrating structural distortions as temperature varies. These results were confirmed using neutron diffraction (ND), including calculating atomic distances between Cr-O. The study explores the impact of temperature on atomic distances and volume values. Comparing the pure with impure samples helps clarify the influence of impurities (CaO and Cr2O3) on crystal structure. This analysis highlights increased distortion in the crystal structure due to impurities, as confirmed through XRD measurements. High-resolution transmission electron microscopy (HRTEM) images were carried out in this study that resolved the role of the impurities and, at the same time, to determine a twins-free structure despite the large dimension of the crystals. The Bond Valence Sum (BVS) calculation method establishes, with ND data, the oxidation state of the compound and confirms the proposed formula of Ca10(Cr6+)1 (Cr5+)6 O28. The material Ca10Cr7O28 exhibits quantum spin liquid behavior, as evidenced by the disappearance of paramagnetic behavior at low temperatures, a lack of remanence and coercivity, and the calculation of Curie temperature. Specific heat measurements confirm short-range magnetic ordering below 3 K, with entropy showing an inverse relationship with an applied magnetic field. Besides, magnetic long-range order emerges below 35 mK. The study extends to LaFeO3, a material displaying antiferromagnetic properties with spin canting at high temperatures. The sol-gel synthesis allows for the variation of particle size, thus enabling the investigation of magnetic frustration as a function of size. HRTEM analysis reveals changes in FeO6 octahedral distortion and the disappearance of spin canting as particle size decreases. Smaller particles exhibit spin-glass behavior at the surface while maintaining an antiferromagnetic core. This creates a core/shell magnetic structure with a significant exchange bias effect. This thesis comprehensively explores Quantum Spin Liquid and Spin Glass systems, highlighting their distinctive properties and behaviors. Additionally, the study reveals the emergence of quantum spin liquid and spin-glass behavior in different materials. This is opening new insight for understanding and potential applications in the future and understanding the related theories.
37 0