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Publication# Magnetic force microscopy contrast formation and field sensitivity

Résumé

The magnetic force microscope (MFM) is an established experimental tool for imaging stray fields with high spatial resolution and sensitivity. The MFM contrast can however contain contributions from the sample topography, variations in the surface Kelvin potential and magnetic contributions arising from grain-to-grain variations of the areal density of the magnetic moment, apart from the contrast generated by the micromagnetic pattern of the sample. Differential imaging techniques can be used to disentangle these contrast contributions. The calibration of the response of the MFM tip on different spatial wavelengths of the field allows a quantitative determination of the magnetic vector field in the plane parallel to the sample surface scanned by the tip. Generally, the tip becomes less sensitive for smaller spatial wavelengths. Obtaining a high spatial resolution thus requires a high measurement sensitivity that can be obtained by MFM operation in vacuum and by using & nbsp;high-quality factor cantilevers. As a result, field sensitivities better than 80 mu T/root Hz can be obtained, even with low magnetic moment tips.

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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.

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.

Artem Korshunov, Irina Safiulina

Long-range magnetic ordering and short-range spin correlations in layered noncentrosymmetric orthogermanate Li2MnGeO4 were studied by means of polarized and unpolarized neutron scattering. The combined Rietveld refinement of synchrotron and neutron powder diffraction data at room temperature within the Pmn2(1) space group allowed us to specify the details of the crystal structure. According to the additional Bragg peaks in low-temperature neutron diffraction patterns a long-range antiferromagnetic ordering with the propagation vector k = (1/2 1/2 1/2) has been found below T-N approximate to 8 K. Symmetry analysis revealed the model of the ground state spin structure within the C(a)c (no. 9.41) magnetic space group. It is represented by the noncollinear ordering of manganese atoms with a refined magnetic moment of 4.9 mu(B)/Mn2+ at 1.7 K, which corresponds to the saturated value for the high-spin configuration S = 5/2. Diffuse magnetic scattering was detected on the neutron diffraction patterns at temperatures just above T-N. Its temperature evolution was investigated in detail by polarized neutron scattering with the following XYZ-polarization analysis. Reverse Monte Carlo simulation of diffuse scattering data showed the development of short-range ordering in Li2MnGeO4, which is symmetry consistent on a small scale with the long-range magnetic state below T-N. The reconstructed radial spin-pair correlation function S(0)S(r) displayed the predominant role of antiferromagnetic correlations. It was found that spin correlations are significant only for the nearest magnetic neighbors and almost disappear at r approximate to 12 angstrom at 10 K. Temperature dependence of the diffuse scattering implies short-range ordering long before the magnetic phase transition. Besides, the spin arrangement was found to be similar in both cases above and below T-N. As a result, an exhaustive picture of the gradual formation of magnetic ordering in Li2MnGeO4 is presented.

2020