eprintid: 34165
rev_number: 14
eprint_status: archive
userid: 7806
dir: disk0/00/03/41/65
datestamp: 2023-12-14 10:14:06
lastmod: 2024-01-08 15:53:53
status_changed: 2023-12-14 10:14:06
type: doctoralThesis
metadata_visibility: show
creators_name: Shetiwy, Ahmed Elghandour
title: Thermodynamic and Magnetic Studies
on Correlated Electron Systems
with Competing Interactions
subjects: ddc-530
divisions: i-130700
adv_faculty: af-13
abstract: 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.
date: 2023
id_scheme: DOI
id_number: 10.11588/heidok.00034165
ppn_swb: 1877228737
own_urn: urn:nbn:de:bsz:16-heidok-341654
date_accepted: 2023-12-06
advisor: HASH(0x55a9a64bf348)
language: eng
bibsort: SHETIWYAHMTHERMODYNA
full_text_status: public
place_of_pub: Heidelberg
citation:   Shetiwy, Ahmed Elghandour  (2023) Thermodynamic and Magnetic Studies on Correlated Electron Systems with Competing Interactions.  [Dissertation]     
document_url: https://archiv.ub.uni-heidelberg.de/volltextserver/34165/1/Diss_2023_Ahmed%20Eltamimy%20Abdelrahman%20Elghandour%20Shetiwy.pdf