Quantum Environments: Spin Baths, Oscillator Baths, and applications to Quantum Magnetism

The low-energy physics of systems coupled to their surroundings is understood by truncating to effective Hamiltonians; these tend to reduce to a few canonical forms, involving coupling to "baths" of oscillators or spins. The method for doing this is demonstrated using examples from magnetism, superconductivity, and measurement theory, as is the way one then solves for the low-energy dynamics. Finally, detailed application is given to the exciting recent Quantum relaxation and tunneling work in naomagnets.Comment: Chapter in "Tunneling in Complex Systems" (World Sci., edited T. Tomsovic); 97 pages. Published in June 199

Theory of the spin bath

The quantum dynamics of mesoscopic or macroscopic systems is always complicated by their coupling to many "environmental" modes.At low T these environmental effects are dominated by localised modes, such as nuclear and paramagnetic spins, and defects (which also dominate the entropy and specific heat). This environment, at low energies, maps onto a "spin bath" model. This contrasts with "oscillator bath" models (originated by Feynman and Vernon) which describe {\it delocalised} environmental modes such as electrons, phonons, photons, magnons, etc. One cannot in general map a spin bath to an oscillator bath (or vice-versa); they constitute distinct "universality classes" of quantum environment. We show how the mapping to spin bath models is made, and then discuss several examples in detail, including moving particles, magnetic solitons, nanomagnets, and SQUIDs, coupled to nuclear and paramagnetic spin environments. We show how to average over spin bath modes, using an operator instanton technique, to find the system dynamics, and give analytic results for the correlation functions, under various conditions. We then describe the application of this theory to magnetic and superconducting systems.Particular attention is given to recent work on tunneling magnetic macromolecules, where the role of the nuclear spin bath in controlling the tunneling is very clear; we also discuss other magnetic systems in the quantum regime, and the influence of nuclear and paramagnetic spins on flux dynamics in SQUIDs.Comment: Invited article for Rep. Prog. Phys. to appear in April, 2000 (41 pages, latex, 13 figures. This is a strongly revised and extended version of previous preprint cond-mat/9511011

Studio delle dinamiche di spin in sistemi molecolari magnetici

In this Ph.D. thesis the spin dynamics in several types of magnetic molecules has been investigated by developing models and computational techniques in the framework of the spin Hamiltonian approach. The studied systems are composed by a core of magnetic ions surrounded by shells of organic ligands. Inside each core the different spins strongly interact via superexchange, whereas the interactions between spins of different cores are prevented by the ligands. A deep understanding of the spin dynamics is crucial both for the investigation of fundamental quantum phenomena and for the potential applications. The spin dynamics has been studied both directly - through the analysis of neutron scattering cross sections on the Cr8Zn spin segment - and indirectly - through the analysis of macroscopic measurements, as magnetization and specific heat, in Cr8Cd, Cr7Ni, and Cr8Ni rings. In addition, it has been shown that NMR measurements can provide crucial information about the relaxation dynamics of magnetic molecules, such as the Fe8 nanomagnet, the V12 antiferromagnetic cluster, the Fe30 icosidodecahedral molecule and the Cr7Ni heterometallic ring. In all the studied systems the developed models well capture the main features of the experimental data

Magnetic and transport properties of Fe-4 single-molecule magnets: a theoretical insight

Here, methods of density functional theory (DFT) were employed to study the magnetic and transport properties of a star-shaped single-molecule magnet Fe-4 S = 5 complex deposited on a gold surface. The study devoted to the magnetic properties focused on changes in the exchange coupling constants and magnetic anisotropy (zero-field splitting parameters) of the isolated and deposited molecules. Molecule surface interactions induced significant changes in the antiferromagnetic exchange coupling constants because these depend closely on the geometry of the metal complex. Meanwhile, the magnetic anisotropy remained almost constant. Transport properties were analysed using two different approaches. First, we studied the change in magnetic anisotropy by reducing and oxidizing the Fe-4 complex as in a Coulomb blockade mechanism. Then we studied the coherent tunnelling using DFT methods combined with Green functions. Spin filter behaviour was found because of the different numbers of alpha and beta electrons, due to the S = 5 ground state

A Study of Periodic and Aperiodic Ferromagnetic Antidot Lattices

This thesis reports our study of the effect of domain wall pinning by ferromagnetic (FM) metamaterials [1] in the form of periodic antidot lattices (ADL) on spin wave spectra in the reversible regime. This study was then extended to artificial quasicrystals in the form of Penrose P2 tilings (P2T). Our DC magnetization study of these metamaterials showed reproducible and temperature dependent knee anomalies in the hysteretic regime that are due to the isolated switching of the FM segments. Our dumbbell model analysis [2] of simulated magnetization maps indicates that FM switching in P2T is nonstochastic. We have also acquired the first direct, two-dimensional images of the magnetization of Permalloy films patterned into P2T using scanning electron microscopy with polarization analysis (SEMPA). Our SEMPA images demonstrate P2T behave as geometrically frustrated networks of narrow ferromagnetic film segments having near-uniform, bipolar (Ising-like) magnetization, similar to artificial spin ices (ASI). We find the unique aperiodic translational symmetry and diverse vertex coordination of multiply-connected P2T induce a more complex spin-ice behavior driven by exchange interactions in vertex domain walls, which differs markedly from the behavior of disconnected ASI governed only by dipolar interactions

Single-Electron Tunneling Spectroscopy in Magnetic Nanoparticles and Molecular Magnets

This thesis deals with single-electron tunneling in transistor-like devices in which the central electrode is either a metal nanoparticle (possibly ferromagnetic) or a molecular magnet. The investigated systems split into two different categories, depending on the size of the central island. The smaller islands, such as ultrasmall magnetic metal nanoparticles and Mn12 molecular magnets, are studied in the first part of the thesis (Papers I-III). The larger metal islands, both ferromagnetic and nonmagnetic, are studied in the second part (Papers IV-V). Different size regimes result in different types of energy spectra (discrete for the small and continuous for the large islands), and thus in different ways of calculating the electric current through the system. All the systems are investigated within the regime of weak coupling to the external leads. In this regime, quantum transport is characterized by the physics of Coulomb blockade and can be described theoretically by sequential-tunneling rate equations. Papers I-III are purely theoretical, while Papers IV-V consist both of experimental and theoretical parts, the theoretical ones belonging explicitly to this thesis. In Paper I we present a theory of quantum transport through a small ferromagnetic nanoparticle in which particle-hole excitations are coupled to spin collective modes. For strong electron-magnon coupling, we find that the tunneling conductance as a function of bias voltage is characterized by a large and dense set of resonances. Their magnetic field dependence in the large-field regime is linear, with slopes of the same sign. Both features are in agreement with tunneling experiments on similar nanoparticles. Papers II and III deal with transport through a Mn12 molecule. The many-body energy spectrum (composed of spin multiplets) and spin-dependent inter-level transition matrix elements used in transport calculations are determined by means of spin density-functional theory (SDFT). This theory provides several other properties of the molecular magnet, such as the magnetic moment and magnetic anisotropy energy of its charged states, anion and cation. In transport calculations, we compare the results obtained by the SDFT with those based on a phenomenological giant-spin model. The tunneling conductance at finite bias is characterized by peaks representing transitions between spin multiplets, separated by an energy on the order of the magnetic anisotropy. We find that the orbital degrees of freedom, included in SDFT and absent in the spin model, play an important role in transport and can lead to negative differential conductance. In Paper IV we investigate spin accumulation in a Ni/Au/Ni single-electron transistor assembled by atomic force microscopy. Transport measurements in magnetic field at 1.7 K reveal no clear spin accumulation in the device (that is, no tunneling-magnetoresistance (TMR) signal is observed), which can be attributed to fast spin relaxation in the Au disk caused by strong spin-orbit interaction. From numerical simulations using the rate-equation approach of orthodox Coulomb-blockade theory, we can put an upper bound of a few nanoseconds on the spin-relaxation time for electrons in the Au disk. The focus of Paper V is on magnetic-field dependent transport in nanoscaled ferromagnetic Co/Ni/Co single-electron transistors. Magnetotransport measurements carried out at 1.8 K reveal TMR traces with negative coercive fields, which we interpret in terms of a switching mechanism driven by the shape anisotropy of the central wire-like Ni island. A large TMR of about 18% is observed within a finite source-drain bias regime. A numerical simulation within the Coulomb-blockade theory gives a TMR which is on the order of magnitude of the experimental signal. The TMR decreases rapidly with increasing bias. The vanishing of the TMR with bias is tentatively ascribed to excitations of magnons in the central island, which cause a fast decrease of the island spin polarization

60th Annual Rocky Mountain Conference on Magnetic Resonance

Final program, abstracts, and information about the 60th annual meeting of the Rocky Mountain Conference on Magnetic Resonance, co-endorsed by the Colorado Section of the American Chemical Society and the Society for Applied Spectroscopy. Held in Denver, Colorado, July 21-25, 2019