92 research outputs found
Magnetic Properties of Holmium{Erbium Alloys
This thesis is dedicated to the memory of my late professor A.R. Mackintosh — meant as a student’s tribute to his work of great insight. Through our unfortunately short acquaintance I owe Allan my interest in the field of solid state physics in general and rare earth magnetism in particular. I am grateful to my supervisors Jens Jensen from the Ørsted Laboratory, Niels Bohr Institute and Des McMorrow from the Solid state department, Risø National Laboratory. For their help and suggestions on various problems, I also wish to thank the people in the solid state department, my fellow students and my family. The work falls into three main parts, namely experiments, theory and calculations. On the other hand the examined system shows several different interesting features. I have chosen to start by supplying the background knowledge, occasionally mentioning the implications on the features. Afterwards I discuss each of the features individually. This procedure has led to some overlapping in the report, but on the other hand I found it the most instructive way
Magnetic skyrmions and skyrmion clusters in the helical phase of CuOSeO
Skyrmions are nanometric spin whirls that can be stabilized in magnets
lacking inversion symmetry. The properties of isolated skyrmions embedded in a
ferromagnetic background have been intensively studied. We show that single
skyrmions and clusters of skyrmions can also form in the helical phase and
investigate theoretically their energetics and dynamics. The helical background
provides natural one-dimensional channels along which a skyrmion can move
rapidly. In contrast to skyrmions in ferromagnets, the skymion-skyrmion
interaction has a strong attractive component and thus skyrmions tend to form
clusters with characteristic shapes. These clusters are directly observed in
transmission electron microscopy measurements in thin films of CuOSeO.
Topological quantization, high mobility and the confinement of skyrmions in
channels provided by the helical background may be useful for future
spintronics devices.Comment: 5 pages, 3 figures, 4 pages supplemen
Quantum critical scaling for a Heisenberg spin-1/2 chain around saturation
We demonstrate quantum critical scaling for an S = 1/2 Heisenberg antiferromagnetic chain compound Cu(C4H4N2)(NO3)(2) in a magnetic field around saturation, by analyzing previously reported magnetization [Y. Kono et al., Phys. Rev. Lett. 114, 037202 (2015)], thermal expansion [J. Rohrkamp et al., J. Phys.: Conf. Ser. 200, 012169 (2010)], and NMR relaxation data [H. Kuhne et al., Phys. Rev. B 80, 045110 (2009)]. The scaling of magnetization is demonstrated through collapsing the data for a range of both temperature and field onto a single curve without making any assumption for a theoretical form. The data collapse is subsequently shown to closely follow the theoretically predicted scaling function without any adjustable parameters. Experimental boundaries for the quantum critical region could be drawn from the variable range beyond which the scaled data deviate from the theoretical function. Similarly to the magnetization, quantum critical scaling of the thermal expansion is also demonstrated. Further, the spin dynamics probed via NMR relaxation rate 1/T-1 close to the saturation is shown to follow the theoretically predicted quantum critical behavior as 1/T-1 proportional to T-0.5 persisting up to temperatures as high as k(B)T similar or equal to J, where J is the exchange coupling constant
A Prismatic Analyser concept for Neutron Spectrometers
A development in modern neutron spectroscopy is to avoid the need of large
samples. We demonstrate how small samples together with the right choice of
analyser and detector components makes distance collimation an important
concept in crystal analyser spectrometers. We further show that this opens new
possibilities where neutrons with different energies are reflected by the same
analyser but counted in different detectors, thus improving both energy
resolution and total count rate compared to conventional spectrometers. The
technique can be combined with advanced focusing geometries and with
multiplexing instrument designs. We present a combination of simulations and
data with 3 energies from one analyser. The data was taken on a prototype
installed at PSI, Switzerland, and shows excellent agreement with the
predictions. Typical improvements will be 2 times finer resolution and a factor
1.9 in flux gain compared to a Rowland geometry or 3 times finer resolution and
a factor 3.2 in flux gain compared to a single flat analyser slab
Experimental and theoretical thermodynamic studies in BaMgReO -- the ground state in the context of Jahn-Teller effect
We address the degeneracy of the ground state multiplet on the 5
Re ion in double perovskite BaMgReO using a combination of
specific heat measurements and density functional calculations. For
BaMgReO, two different ground state multiplets have previously been
proposed - a quartet (with degeneracy =4) [1] and a doublet (=2) [2].
Here we employ two independent methods for the estimation of phonon
contribution in heat capacity data to obtain the magnetic entropy ,
which reflects the degeneracy of the ground state multiplet through
ln. In both cases, we obtain a better fit to ln2
indicating evidence of =2 degeneracy in the range from 2 to 120~K. The
detailed nature of the ground state multiplet in BaMgReO remains an
open question.Comment: 5 pages, 4 figure
Structure, heat capacity and Raman spectra of mm-sized BaMgWO single crystals synthesized by BaCl-MgCl flux method
We present a new method of BaMgWO single crystal synthesis that
allows to grow larger crystals using BaCl and MgCl flux.
Difficulties to grow single crystal of a size suitable for macroscopic material
property measurements caused the majority of characterisation being published
on polycrystalline samples. Single crystal diffraction and energy dispersive
X-ray analysis confirmed high quality of synthesised samples. Heat capacity
measurements from 300~K to 2~K do not show any transitions. However, Raman
spectra measured down to 77~K contain additional peaks at all temperatures
probed, which is in a contrast with only 4 Raman active modes expected from the
reducible representation. This calls for a more detailed study of potential
symmetry breaking that could also influence the electronic properties of the
material
Note: Versatile sample stick for neutron scattering experiments in high electric fields
We present a versatile high voltage sample stick that fits into all cryomagnets and standard cryostats at the Swiss Spallation Neutron Source, Paul Scherrer Institut, and which provides a low effort route to neutron scattering experiments that combine electric field with low temperature and magnetic field. The stick allows for voltages up to 5 kV and can be easily adapted for different scattering geometries. We discuss the design consideration and thermal behavior of the stick, and give one example to showcase the abilities of the device. (C) 2014 AIP Publishing LLC
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