Magnetic properties of CeAlSi and CeAlGe

This TPIV experiment deals with first measurements of CeAlSi and CeAlGe magnetic properties and allows to a comparison between these last two. The first step is a description of MPMS measurement, then the paramagnetism and ferromagnetism theory leads to an essential knowledge for the future analysis of principal magnetic phenomenon. Turning now to practical aspects, this experiment investigates the CeAlSi and CeAlGe magnetization depending on the temperature and on the mag- netic field, which allows to the determination of the Curie-Weiss temperature, the transition temperature and the effective magnetic moment of this two alloys. The study of magnetization as function of the magnetic field shows hysteresis and paramagnetic regimes for both compounds; Brillouin fit has been taken in account. Analytical methods, used to calculate previous above parameters, are given in this report.

Construction d'un microscope à effet tunnel à basse température et études d'impuretés magnétiques en surfaces

Laurent Claude

This thesis reports on the design and construction of a novel experimental setup for the study of surface magnetic systems. This setup consists of a scanning tunneling microscope (STM) designed for ultra high vacuum (UHV) operating at T ≲ 1 K and strong magnetic fields (0 - 8.5 T). Such extreme experimental conditions have influenced the choice of the construction materials and components as well as the general design principles. This work comprises the design, realization and first tests of the STM, cryostat, damping system, sample preparation UHV chambers and transfer system. In parallel to this new setup realization, we have investigated the magnetic properties of dilute transition metal impurities deposited on non-magnetic metal surfaces. Measurements of those systems were performed with x-ray magnetic circular dichroism (XCMD). We have started by adressing the host electronic density effect on magnetic properties of the impurities. In order to make contact with the Friedel [1] and Anderson [2] models, 3d impurities (Fe and Ni) were deposited on alkali metal substrates (K, Na et Li) whose electronic structure is close to the Fermi electron gas. The gradual increase of the host electronic density with decreasing atomic weight going from K to Li allow us to study the effects of s - d electron hybridization on the magnitude magnetic moment of the impurity . In particular, we show that the magnetic moment of Fe is strongly reduced in going from K to Li, while the Ni moment disappears on Na and Li. The XMCD data also show that the orbital magnetic moment is atomic-like on K, but compared to the spin moment, significantly reduces as the substrate electron density increases. Finally, we report on a variable temperature STM study of the nucleation and growth processes of Pd/Pt(111). The experimental results are analyzed on the basis of mean field theory applied to surface diffusion processes and solved by numerical integration. This study allows us to highlight the different atomistic processes that contribute to the nucleation and growth of a prototype epitaxial system.

EPFL2005

Microfluidic magnetic particles manipulation

Amar Rida

In this thesis work we propose new concepts of transporting and manipulating magnetic particles on microfluidic scale. The objective is to propose solutions to the problems related to the integration of the magnetic inductive components within a microfluidic network for magnetic particle manipulation. A second part of this work is the study of the physics of magnetic particle pattern formation when a suspension of the particles is subjected to an external magnetic field. The first concept that we propose is based on the dynamic motion of a selfassembled structure of ferromagnetic beads that are retained within a microfluidic flow using a local alternating magnetic field. We show that an alternating magnetic field induces a rotational motion of the magnetic particles, thereby strongly enhancing the fluid perfusion through the magnetic structure that behaves as a dynamic random porous medium. The result is a very strong particle-liquid interaction that can be controlled by adjusting the magnetic field frequency and amplitude, as well as the liquid flow rate. This fact is at the basis of very efficient liquid mixing. We anticipate that the intense interaction between the fluid and magnetic particles with functionalized surfaces holds large potential for the development of future bead-based assays. The second concept is the transport of magnetic particles on microfluidic scales over long-range distances using an array of simple planar coils placed in large static magnetic field. This magnetic field imposes a permanent magnetic moment to the microparticles, so that a very small magnetic field gradient of a simple planar coil is sufficient to displace the microparticles. We demonstrate that the repulsive and attractive magnetic forces generated by adjacent strongly overlapped coils, allow an efficient and well controlled magnetic particle displacement along one-dimensional or two dimensional paths. Finally, we study the dynamics of supraparticles structure (SPS) patterns of a ferromagnetic particle suspension in an alternating magnetic field. We show that the application of an alternating magnetic field to a ferromagnetic particle suspension results in a phase separation of the supraparticles structures (SPS) patterns into periodic structures of columns. Moreover, we investigate the dynamic motion, relative to a microfluidic flow, of such magnetic SPS. The latter are locally manipulated with an alternating magnetic field that is transverse to a microfluidic channel. We show that the SPS in the microchannel are composed of weakly aggregated and open columnar-like structures, allowing large particles surfaces to be in contact with the fluid flow.

EPFL2004

Order and dynamics in Kagome like compound Cu2OSO4 and other quantum magnets

Virgile Yves Favre

This thesis presents results of studies of novel compounds modeling complex fundamental physics phenomena. Cu2OSO4 is a copper based magnetic Mott Insulator system, where spin half magnetic moments form a new type of lattice. These intrinsically quantum pins are exhibiting atypical magnetic order and spin dynamics. The recent success in the growth of large single crystals of Cu2OSO4 enabled to perform measurements probing its static and fluctuating properties. The peculiarity of this sample is that its atoms are forming layers, with a geometry close to the intensively studied Kagomé lattice, but with a third of its spins replaced by dimers. This quantum magnetism system has been probed in its bulk, by the means of heat capacity and DC-susceptibility measurements, revealing a transition to a magnetically long range ordered state upon cooling, the details of which are revealed by neutron scattering. Single crystal inelastic neutron scattering shed light on the spin-dynamics in the system, with clear spin waves appearing as fluctuations around the peculiar ground state of the system: a 120 degrees spin configuration where the magnetic moment of the spin-dimer causes the sample to be globally ferrimagnetic. The presented results indicate that Cu2OSO4 represents a new type of model lattice with frustrated interactions where interplay between magnetic order, thermal and quantum fluctuations can be explored. The magnetic excitations of the compound can be modeled by a yet-to-be-understood internal effective mean-field that no simple magnetic coupling seems to reproduce. K2Ni2(SO4)3 is another compound that allows for the existence of non-trivial topological phases. This thesis presents results of the study of the unusual magnetic behavior of K2Ni2(SO4)3. No clear sign of well-established magnetic long range order has been observed down to dilution temperatures. Neutron scattering reveals the details of the competition between frustration and fluctuations that prevent order from settling in. Low temperature spin excitations take the form of a continuum at 500 mK, but also of broad, energy independent continua at higher temperatures. Bulk and neutron scattering measurements are put in perspective and linked together with a view to building up a better understanding of how quantum spin liquids can be stabilized in general, and in particular in this model compound. Finally, the last contribution of this thesis to the field of condensed matter physics regards the establishment of a state-of-the-art technique to fit heat capacity and unit cell volume of samples to try and make the extraction of magnetic information from specific heat measurements more robust. This newly-developed technique consists in modeling lattice contributions with better accuracy by using data from multiple experimentally accessible quantities to consolidate the fitting scheme. This method has been cautiously applied to several compounds at the forefront of research in experimental physics.

EPFL2021