Ferrimagnetism | EPFL Graph Search

A ferrimagnetic material is a material that has populations of atoms with opposing magnetic moments, as in antiferromagnetism, but these moments are unequal in magnitude so a spontaneous magnetization remains. This can for example occur when the populations consist of different atoms or ions (such as Fe2+ and Fe3+). Like ferromagnetic substances, ferrimagnetic substances are attracted by magnets and can be magnetized to make permanent magnets. The oldest known magnetic substance, magnetite (Fe3O4), was classified as a ferromagnet before Louis Néel discovered ferrimagnetism in 1948. Since the discovery, numerous uses have been found for ferrimagnetic materials, such as hard drive platters and biomedical applications. History Until the twentieth century, all naturally occurring magnetic substances were called ferromagnets. In 1936, Louis Néel published a paper proposing the existence of a new form of cooperative magnetism he called antiferromagnetism. While working with

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

Magnetic and structural properties of Ni-substituted magnetoelectric Co4Nb2O9

Virgile Yves Favre, Hadi Papi, Henrik Moodysson Rønnow, Ivica Zivkovic

The magnetic and structural properties of polycrystalline Co4-xNixNb2O9 (x = 1, 2) have been investigated by neutron powder diffraction, magnetization and heat capacity measurements, and density functional theory (DFT) calculations. For x = 1, the compound crystallizes in the trigonal P (3) over bar c1 space group. Below T-N = 31 K it develops a weakly noncollinear antiferromagnetic structure with magnetic moments in the ab plane. The compound with x = 2 has crystal structure of the orthorhombic Pbcn space group and shows a hard ferrimagnetic behavior below T-C = 47 K. For this compound a weakly noncollinear ferrimagnetic structure with two possible configurations in the ab plane was derived from neutron diffraction study. By calculating magnetic anisotropy energy via DFT, the ground-state magnetic configuration was determined for this compound. The heat capacity study in magnetic fields up to 140 kOe provides further information on the magnetic structure of the compounds.

2019

Ferrimagnetic 120 degrees magnetic structure in Cu2OSO4

Helmuth Berger, Virgile Yves Favre, Arnaud Magrez, Henrik Moodysson Rønnow, Romain Sibille, Gregory Scott Tucker, Lin Yang, Ivica Zivkovic

We report magnetic properties of a 3d(9) (Cu2+) magnetic insulator Cu2OSO4 measured on both powder and single crystal. The magnetic atoms of this compound form layers whose geometry can be described either as a system of chains coupled through dimers or as a kagome lattice where every third spin is replaced by a dimer. Specific heat and DC susceptibility show a magnetic transition at 20 K, which is also confirmed by neutron scattering. Magnetic entropy extracted from the specific heat data is consistent with an S = 1/2 degree of freedom per Cu2+, and so is the effective moment extracted from DC susceptibility. The ground state has been identified by means of neutron diffraction on both powder and single crystal and corresponds to an similar to 120 degrees spin structure in which ferromagnetic intradimer alignment results in a net ferrimagnetic moment. No evidence is found for a change in lattice symmetry down to 2 K. Our results suggest that Cu2OSO4 represents a type of model lattice with frustrated interactions where interplay between magnetic order, thermal and quantum fluctuations can be explored.

AMER PHYSICAL SOC2020

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

Virgile Yves Favre

EPFL2021