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Register a new userAnalysis of magnetic neutron-scattering data of two-phase ferromagnets
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We have analyzed magnetic-field-dependent small-angle neutron scattering (SANS) data of soft magnetic two-phase nanocomposite ferromagnets in terms of a recent micromagnetic theory for the magnetic SANS cross section [D. Honecker and A. Michels, Phys. Rev. B $mathbf{87}$, 224426 (2013)]. The approach yields a value for the average exchange-stiffness constant and provides the Fourier coefficients of the magnetic anisotropy field and magnetostatic field, which is related to jumps of the magnetization at internal interfaces.
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The study of atomically thin ferromagnetic crystals has led to the discovery of unusual magnetic behaviour and provided insight into the magnetic properties of bulk materials. However, the experimental techniques that have been used to explore ferromagnetism in such materials cannot probe the magnetic field directly. Here, we show that ballistic Hall micromagnetometry can be used to measure the magnetization of individual two-dimensional ferromagnets. Our devices are made by van der Waals assembly in such a way that the investigated ferromagnetic crystal is placed on top of a multi-terminal Hall bar made from encapsulated graphene. We use the micromagnetometry technique to study atomically thin chromium tribromide (CrBr3). We find that the material remains ferromagnetic down to monolayer thickness and exhibits strong out-of-plane anisotropy. We also find that the magnetic response of CrBr3 varies little with the number of layers and its temperature dependence cannot be described by the simple Ising model of two-dimensional ferromagnetism.
We discuss various methods to obtain the resolution volume for neutron scattering experiments, in order to perform absolute normalization on inelastic magnetic neutron scattering data. Examples from previous experiments are given. We also try to provide clear definitions of a number of physical quantities which are commonly used to describe neutron magnetic scattering results, including the dynamic spin correlation function and the imaginary part of the dynamic susceptibility. Formulas that can be used for general purposes are provided and the advantages of the different normalization processes are discussed.
The study of magnonic thermal Hall effect has recently attracted attention because this effect can be associated with topological phases activated by Dzyaloshinskii-Moriya interaction, which acts similar to a spin-orbital coupling in an electronic system. A topological phase transition may arise when there exist two or more distinct topological phases, and this transition is often revealed by a gap closing. In this work, we consider a ferromagnetic honeycomb lattice described by a Hamiltonian that contains Heisenberg exchange interaction, Dzyaloshinskii-Moriya interaction, and an applied Zeeman field. When expanding the spin operators in the Hamiltonian using Holstein-Primakoff (HP) transformation to the order of $S^{1/2}$, where $S$ is the magnitude of spin, the thermal Hall conductivity stays negative for all values of parameters such as the strength of Zeeman interaction and temperature. However, we demonstrate in this work that by including the next order, $S^{-1/2}$, in HP transformation to take into account magnon-magnon interaction, the Hartree type of interaction gives rise to topological phase transitions driven by temperature. When the temperature increases, we find that the gap of the magnonic energy spectrum closes at Dirac points at a critical temperature, $T_c$, and the gap-closing is indeed the signature for a topological phase transition as confirmed by showing that the Chern numbers are distinct above and below $T_c$. Finally, our analysis points out that thermal Hall conductivity exhibits sign reversal at the same temperature. This phenomenon can be used in experiments to verify the topological nature of magnons in honeycomb magnets.
The physical characterisation and understanding of molecular magnetic materials is one of the most important steps towards the integration of such systems in hybrid spintronic devices. Amongst the many characterisation techniques employed in such a task, Inelastic Neutron Scattering (INS) stands as one of the most powerful and sensitive tools to investigate their spin dynamics. Herein, the magnetic properties and spin dynamics of two dinuclear complexes, namely [(M(hfacac)$_2$)$_2$(bpym)] (where M = Ni$^{2+}$, Co$^{2+}$, abbreviated in the following as Ni$_2$, Co$_2$) are reported. These are model systems that could constitute fundamental units of future spintronic devices. By exploiting the highly sensitive IN5 Cold INS spectrometer, we are able to gain a deep insight into the spin dynamics of Ni$_2$ and to fully obtain the microscopic spin Hamiltonian parameters; while for Co$_2$, a multitude of INS transitions are observed demonstrating the complexity of the magnetic properties of octahedral cobalt-based systems.
Motivated by recent progress on synthesizing two-dimensional magnetic van der Waals systems, we propose a setup for detecting the topological Berezinskii-Kosterlitz-Thouless (BKT) phase transition in spin-transport experiments on such structures. We demonstrate that the spatial correlations of injected spin-currents into a pair of metallic leads can be used to measure the predicted universal jump of $2/pi$ in the ferromagnet spin-stiffness as well as its predicted universal square root dependence on temperature as the transition is approached from below. Our setup provides a simple route to measuring this topological phase transition in two-dimensional magnetic systems, something which up to now has proven elusive. It is hoped that this will encourage experimental efforts to investigate critical phenomena beyond the standard Ginzburg-Landau paradigm in low-dimensional magnetic systems with no local order parameter.
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