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Thermodynamic and Magnetic Studies on Correlated Electron Systems with Competing Interactions

Shetiwy, Ahmed Elghandour

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In this work the ground states and thermodynamic properties of selected materials with competing magnetic interactions are investigated mainly by heat capacity and magnetization studies down to low temperatures and in high magnetic fields. Due to replacement of Mn2+ by Ni2+, competing magnetic anisotropies and mixed spins in Ni0.25Mn0.75TiO3 yield a suppression of long-range magnetic order as compared to its undoped parent compound and a spin-reoriented low-temperatures phase appears. While short-ranged magnetic order persists up to about 4 × TN, glass-like behavior, a quasi-linear temperature dependence of the specific heat and the presence of weakly-coupled moments highlight the unusual ground state and its thermal excitations. Geometric frustration is studied in the example of the disordered pyrochlore structure Ho2Zr2O7 and is evidenced by the absence of long-range magnetic order at least down to T = 280 mK. The analysis of residual entropy excludes a spin ice state in Ho2Zr2O7. Instead, it exhibits a disordered ground state with shortranged antiferromagnetic correlations and a spin-frozen state below T = 0.6 K is found. Magnetic frustration is also relevant in the hexagonal systems Nd3BWO9 and Pr3BWO9 in which the rare earth ions form a distorted Kagom´e lattice. Again, no long-range magnetic order appears in Pr3BWO9 at least down to T = 90 mK while Nd3BWO9 orders antiferromagnetically at TN = 0.3 K. In the former, specific heat shows a Schottky anomaly at T = 5.7(4) K. Magnetic entropy changes in both systems suggest an effective Jeff = 1/2 orbital ground state. While Nd3BWO9 exhibits field-induced spin freezing at T∗ = 4.3 K as shown by AC-susceptibility data, Pr3BWO9 does not show such field-induced anomalies. A narrow 1 3-magnetization plateau is observed in Nd3BWO9 at ∼ 0.8 T whereas in Pr3BWO9, the magnetization increases monotonically with increasing the field with no signs of plateaus. In both systems, the theoretical saturation magnetization is not reached even in high-magnetic fields of B = 58 T and no further magnetization plateaus appear in the high field regime.

Document type: Dissertation
Supervisor: Klingeler, Prof. Dr Rüdiger
Place of Publication: Heidelberg
Date of thesis defense: 6 December 2023
Date Deposited: 14 Dec 2023 10:14
Date: 2023
Faculties / Institutes: The Faculty of Physics and Astronomy > Kirchhoff Institute for Physics
DDC-classification: 530 Physics
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