Magnetization orientation dependence of the quasiparticle spectrum and hysteresis in ferromagnetic metal nanoparticles

We argue that ferromagnetic transition metal nanoparticles with fewer than approximately 100 atoms can be described by an effective Hamiltonian with a single giant spin degree of freedom. The total spin $S$ of the effective Hamiltonian is specified by a Berry curvature Chern number that characterizes the topologically non-trivial dependence of a nanoparticle's many-electron wavefunction on magnetization orientation. The Berry curvatures and associated Chern numbers have a com...

This chapter provides an introduction to the fundamental physical ideas and models relevant to the phenomenon of magnetic hysteresis in nanoparticle assemblies. The concepts of single-domain particles and superparamagnetism are discussed. The mechanisms of magnetization by coherent rotation and the role of temperature in the gradual decay of magnetization are analyzed in the framework of simple analytical models. Modern numerical techniques (Monte Carlo simulations, Magnetiza...

Magnetic nanoparticles are important building blocks for future technologies ranging from nano-medicine to spintronics. Many related applications require nanoparticles with tailored magnetic properties. However, despite significant efforts undertaken towards this goal, a broad and poorly-understood dispersion of magnetic properties is reported, even within mono-disperse samples of the canonical ferromagnetic 3d transition metals. We address this issue by investigating the mag...

Ensembles of gold nanoparticles present a magnetic behavior which is at odds with the weakly diamagnetic response of bulk gold. In particular, an unusual ferromagnetic order has been unveiled by several experiments. Here we investigate if the combined effect of orbital magnetism of conduction electrons and interparticle dipolar interaction can lead to magnetic ordering. Using different model systems of interacting mesoscopic magnetic dipoles, together with a microscopic descr...

High energy resolution spectroscopic studies of quantum magnets have proven to be extremely valuable in directly accessing magnetodynamics quantities, such as energy barriers, magnetic interactions, lifetime of excited states and fluctuations at the most fundamental level. Here, we explore the existence of a new flavor of low-energy spin-excitations for quantum spins coupled to an electron bath. In sharp contrast to the usual tunneling signature of two steps symmetrically cen...

We present the results of Monte Carlo simulations of the magnetic properties of individual spherical nanoparticles with the aim to explain the role played by surface anisotropy on their low temperature magnetization processes. Phase diagrams for the equilibrium configurations have been obtained, showing a change from quasi-uniform magnetization state to a state with hedgehog-like structures at the surface as $k_S$ increases. Through the simulated hysteresis loops and the anal...

We review recent theoretical developments about the role of spins, electron-electron interactions, and spin-orbit coupling in metal nanoparticles and semiconductor quantum dots. For a closed system, in the absence of spin-orbit coupling or of an external magnetic field, electron-electron interactions make it possible to have ground states with spin $S > 1/2$. We review here a theoretical analysis which makes predictions for the probability of finding various values of spin $S...

The combined effect of electron-electron interactions and spin-orbit scattering in metal nanoparticles can be studied by measuring splitting of electron levels in magnetic field ($g$ factors) in tunneling spectroscopy experiments. Using random matrix theory to describe the single-electron states in the metal particle, we find that even a relatively small electron-electron interaction strength (ratio of exchange constant $J$ and mean level spacing $\spacing$ $\simeq 0.3$) sign...

We develop and validate a computational approach to nanomagnets. It is built on the spin wave approximation to a Heisenberg ferromagnet whose parameters can be calculated from a first principles theory (e.g. density functional theory). The method can be used for high throughput analysis of a variety of nanomagnetic materials. We compute the dependence of the magnetization of an iron nanomagnet on temperature, size and shape. The approach is applied to nanomagnets in the range...

We discuss numerical and theoretical results for models of magnetization switching in nanoparticles and ultrathin films. The models and computational methods include kinetic Ising and classical Heisenberg models of highly anisotropic magnets which are simulated by dynamic Monte Carlo methods, and micromagnetics models of continuum-spin systems that are studied by finite-temperature Langevin simulations. The theoretical analysis builds on the fact that a magnetic particle or f...