Model Of Magnetic System Research Articles

Field-Tunable Interactions and Frustration in Underlayer-Mediated Artificial Spin Ice.

Artificial spin ice systems have opened experimental windows into a range of model magnetic systems through the control of interactions among nanomagnet moments. This control has previously been enabled by altering the nanomagnet size and the geometry of their placement. Here we demonstrate that the interactions in artificial spin ice can be further controlled by including a soft ferromagnetic underlayer below the moments. Such a substrate also breaks the symmetry in the array when magnetized, introducing a directional component to the correlations. Using spatially resolved magneto-optical Kerr effect microscopy to image the demagnetized ground states, we show that the correlation of the demagnetized states depends on the direction of the underlayer magnetization. Further, the relative interaction strength of nearest and next-nearest neighbors varies significantly with the array geometry. We exploit this feature to induce frustration in an inherently unfrustrated square lattice geometry, demonstrating new possibilities for effective geometries in two-dimensional nanomagnetic systems.

Monte Carlo study of thin magnetic Ashkin Teller films at the special point

The magnetic properties and critical behaviours of an Ashkin Teller spin-1/2 model magnetic thin-film systems are studied as part of the Monte Carlo simulation method, based on the Metropolis algorithm, in a simple cubic lattice. The effect of the interaction with the exchange coupling J4 (surface and bulk) is studied. In addition, the phase diagrams present different ratios of the surface exchange interaction at the special point (Rs=J2s/J2b)sp, at which all the films thickness have the same critical temperature, these temperatures can be below, above or equal to the critical system bulk temperature. First and second order transitions appears in the system. Moreover, to the special point descriptions, we found another partially ordered phase at high temperature for certain values of the parameter’s interaction. Finally, the effective surface exponents are computed.

Magnetooptical Visualization and Mapping of Magnetic Stray Fields over the Surface of Composite Permanent Magnets Using Gadolinium–Cobalt Amorphous Films

Stray fields of magnetic systems with a known degree of structural and magnetic nonuniformities have been mapped using indicator amorphous films with a perpendicular anisotropy. The characteristic magnetooptical patterns have been determined, which correspond to the distribution of the magnetic-field component normal to the plane of the indicator films. The theoretical patterns of model magnetic systems that are similar to the studied systems in the degree of the structural and magnetic nonuniformity have been calculated. The validity of the magnetooptical visualization has been confirmed by comparison of the experimental and theoretical patterns. The possibility of the express monitoring of the parameters of the magnetic system has been shown using a laboratory model of a magnetooptical visualizer by the observation of the magnetooptical contrast of gadolinium–cobalt indicator films under the action of an ac magnetic field.

Power-Invariant Magnetic System Modeling

In all energy systems, the parameters necessary to calculate power are the same in functionality: an effort or force needed to create a movement in an object and a flow or rate at which the object moves. Therefore, the power equation can generalized as a function of these two parameters: effort and flow, P = effort * flow. Analyzing various power transfer media this is true for at least three regimes: electrical, mechanical and hydraulic but not for magnetic. This implies that the conventional magnetic system model (the reluctance model) requires modifications in order to be consistent with other energy system models. Even further, performing a comprehensive comparison among the systems, each system's model includes an effort quantity, a flow quantity and three passive elements used to establish the amount of energy that is stored or dissipated as heat. After evaluating each one of them, it was clear that the conventional magnetic model did not follow the same pattern: the reluctance, as analogous to the electric resistance, should be a dissipative element instead it is an energy storage element. Furthermore, the two other elements are not defined. This difference has initiated a reevaluation of the conventional magnetic model. In this dissertation the fundamentals on electromagnetism and magnetic materials that supports the modifications proposed to the magnetic model are presented. Conceptual tests to a case study system were performed in order to figure out the network configuration that better represents its real behavior. Furthermore, analytical and numerical techniques were developed in MATLAB and Simulink in order to validate our model. Finally, the feasibility of a novel concept denominated magnetic transmission line was developed. This concept was introduced as an alternative to transmit power. In this case, the media of transport was a magnetic material. The richness of the power-invariant magnetic model and its similarities with the electric model enlighten us to apply concepts and calculation techniques new to the magnetic regime but common to the electric one, such as, net power, power factor, and efficiency, in order to evaluate the power transmission capabilities of a magnetic system. The fundamental contribution of this research is that it presents an alternative to model magnetic systems using a simpler, more physical approach. As the model is standard to other systems' models it allows the engineer or researcher to perform analogies among systems in order to gather insights and a clearer understanding of magnetic systems which up to now has been very complex and theoretical.

Beyond heat baths II: framework for generalized thermodynamic resource theories

Thermodynamics, which describes vast systems, has been reconciled with small scales, relevant to single-molecule experiments, in resource theories. Resource theories have been used to model exchanges of energy and information. Recently, particle exchanges were modeled; and an umbrella family of thermodynamic resource theories was proposed to model diverse baths, interactions, and free energies. This paper motivates and details the family’s structure and prospective applications. How to model electrochemical, gravitational, magnetic, and other thermodynamic systems is explained. Szilárd’s engine and Landauer’s Principle are generalized, as resourcefulness is shown to be convertible not only between information and gravitational energy, but also among diverse degrees of freedom. Extensive variables are associated with quantum operators that might fail to commute, introducing extra nonclassicality into thermodynamic resource theories. An early version of this paper partially motivated the later development of noncommutative thermalization. This generalization expands the theories’ potential for modeling realistic systems with which small-scale statistical mechanics might be tested experimentally.

Relaxation of a coherent, magnetic s–p model system coupled to one and two thermal baths and a laser pulse

We study an s–p model magnetic system with a triplet ground state coupled to two temperature baths. By varying the temperatures we both generate non-thermal electronic distributions and create additional coherences in the density matrix of the system. Thus the thermally-induced magnetic response goes beyond the simple picture of majority-minority population dynamics. Furthermore, we discuss the influence of temperature induced relaxation effects on the dynamics induced by an optical perturbation for this quantum system.

Tunable random telegraph noise in individual square permalloy dots

Magnetization fluctuations caused by thermal energies are poorly understood, yet they represent a fundamental limit for many magnetic technologies. Here, thermal fluctuations of the magnetization are quantitatively characterized. The model magnetic system is thin square magnetic dots (<10 nm) with side lengths down to 200 nm. The magnetic state and the fluctuations are determined in individual dots by measuring the anisotropic magnetoresistance. The magnetization exhibits random telegraph noise with a transition rate determined by the dot size, the applied magnetic field, and the sample temperature. An energy barrier model of the system explains the dependence of the magnetic fluctuations on these parameters and is supported by additional experiments unaffected by the magnetic fluctuations. This independent identification of the mechanism producing magnetic noise, and its agreement with micromagnetic simulations, demonstrates a microscopic understanding of noise extendable to more complex noise sources such as 1/f noise.

The heat is on

Model magnetic systems known as artificial spin ices have almost always been found in frozen, athermal states. But an artificial spin ice that is designed to be thermally active has now been imaged as it explores its frustrated energy landscape.

Density functional theory study of the magnetic properties of rare earth complexes: the magnetic coupling mechanism in YIII and GdIII complexes with nitronyl nitroxide

The magnetic coupling interactions of the nitronyl nitroxide radicals bound to diamagnetic (YIII) and paramagnetic (GdIII) rare earth ions in two model magnetic systems based on novel rare earth organic radical complexes Ln(hfac)3(NITPhOCH3)2 (Ln = YIII 1, GdIII 2; hafc = hexafluoroacetylacetonate; NITPhOCH3 = 4′-methoxyo-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) have been investigated by density functional theory (DFT). The magnetic coupling mechanisms were also explored from the viewpoint of molecular orbital and spin density populations. DFT calculations show that the empty 4d-orbitals of YIII and 5d-orbitals of GdIII play an important role in the antiferromagnetic coupling between the two nitronyl nitroxide radical ligands, and that the ferromagnetic coupling between the GdIII ion and the radical magnetic centers can be attributed to the nearly complete localization of the isotropic 4f-shell and singly occupied magnetic orbital (Π*) of the nitronyl nitroxide.

Truncated forms of the second-rank orthorhombic Hamiltonians used in magnetism and electron magnetic resonance (EMR) studies are invalid—Why it went unnoticed for so long?

This paper deals with the truncated forms of the second-rank orthorhombic Hamiltonians employed in magnetism and electron magnetic resonance (EMR) studies. Consideration of the intrinsic features of orthorhombic Hamiltonians reveals that the truncations, which consist in omission of one of three interdependent orthorhombic terms, are fundamentally invalid. Implications of the invalid truncations are: loss of generality of quantized spin models, misinterpretation of physical properties of systems studied (e.g. maximum rhombicity ratio and relative parameter values), and inconsistent notations for Hamiltonian parameters that hamper direct comparison of data from various sources. Truncated Hamiltonian forms identified in our survey are categorized and systematically reviewed. Examples are taken from studies of various magnetic systems, especially those involving transition ions, as well as model magnetic systems. The pertinent studies include magnetic ordering in three- and lower dimensions, e.g. [(CH 3) 4N]MnCl 3 (TMMC), canted ferromagnets, Haldane gap antiferromagnets, single molecule magnets exhibiting macroscopic quantum tunneling, e.g. Mn 12 complexes with spin S=10. Our study provides better insight into magnetic and spectroscopic properties of pertinent magnetic systems, which calls for reconsideration of the experimental and theoretical results based on invalid truncated Hamiltonians. The physical nature of Hamiltonians used in magnetism and EMR studies and other types of inappropriate terminology occurring, especially in model magnetism studies, require separate discussion.

Signature of magnetic monopole and Dirac string dynamics in spin ice

Magnetic monopoles have eluded experimental detection since their prediction nearly a century ago by Dirac. Recently it has been shown that classical analogues of these enigmatic particles occur as excitations out of the topological ground state of a model magnetic system, dipolar spin ice. These quasi-particle excitations do not require a modification of Maxwell's equations, but they do interact via Coulombs law and are of magnetic origin. In this paper we present an experimentally measurable signature of monopole dynamics and show that magnetic relaxation measurements in the spin ice material $Dy_{2}Ti_{2}O_{7}$ can be interpreted entirely in terms of the diffusive motion of monopoles in the grand canonical ensemble, constrained by a network of "Dirac strings" filling the quasi-particle vacuum. In a magnetic field the topology of the network prevents charge flow in the steady state, but there is a monopole density gradient near the surface of an open system.

Biorthogonal method of moments of coupled‐cluster equations: Alternative derivation, further considerations, and application to a model magnetic system

AbstractThe energy expansion defining the biorthogonal method of moments of coupled‐cluster equations (MMCC) [Piecuch and Włoch, J Chem Phys, 2005, 123, 224105 and Piecuch et al., Chem Phys Lett 2006, 418, 467], which leads to the size extensive completely renormalized (CR) coupled‐cluster (CC) approach with singles, doubles, and noniterative triples employing the left eigenstates of the similarity‐transformed Hamiltonian, termed CR‐CC(2,3), is overviewed and rederived. The rederivation of the biorthogonal MMCC expansion presented in this work is based on a direct resummation and subsequent elimination of the many‐body components of the exponential wave operator of CC theory that appear at individual moment contributions in the original MMCC energy expansion [Kowalski and Piecuch, J Chem Phys, 2000, 113, 18; Kowalski and Piecuch, J Chem Phys 2001, 115, 2966], enabling one to understand why the CR‐CC(2,3) method using the biorthogonal MMCC theory is more accurate than the earlier CR‐CCSD(T) approach. The superiority of the CR‐CC(2,3) method over the CR‐CCSD(T) and other previously developed single‐reference CC methods with a noniterative treatment of triply excited clusters, including the widely used CCSD(T) approach and the triples corrections defining the CCSD(2) schemes, is illustrated by examining the singlet–triplet gap of the (HFH)− magnetic system in which two paramagnetic centers are linked via a polarizable diamagnetic bridge. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008

Quantum Phase Transition of a Magnet in a Spin Bath

The excitation spectrum of a model magnetic system, LiHoF4, was studied with the use of neutron spectroscopy as the system was tuned to its quantum critical point by an applied magnetic field. The electronic mode softening expected for a quantum phase transition was forestalled by hyperfine coupling to the nuclear spins. We found that interactions with the nuclear spin bath controlled the length scale over which the excitations could be entangled. This generic result places a limit on our ability to observe intrinsic electronic quantum criticality.

Random fields and phase transitions in model magnetic systems

Random fields occur in a wide variety of physical systems varying from type II superconductors to two-component fluids in a random medium. However, only in model magnetic systems have systematic studies as a function of both temperature and random-field strength been possible. In this article we review recent neutron and magnetic X-ray scattering studies of the magnetic ordering processes in the antiferromagnets Mn 0.75Zn 0.25F 2, Fe 0.5Zn 0.5F 2 and Fe 0.75Co 0.25TiO 3 in an applied magnetic field. These systems should all represent realizations of the three-dimensional random-field Ising model which is the simplest version of the random-field problem in models with discrete symmetry. In all cases on field cooling (FC) the systems evolve continuously from a high-temperature paramagnetic state to a low-temperature antiferromagnetic domain state. However, on cooling to low temperatures in zero field and then applying a field (ZFC) long-range order (LRO) is obtained. On subsequent heating in the three systems the LRO vanishes continuously with a rounded power-law behavior which has been labelled trompe l'oeil critical behavior. The width of the transition region scales as H 2. Reconsideration of indirect ZFC specific-heat measurements shows that the observed peaks, previously attributed to equilibrium critical fluctuations, instead arise entirely from a LRO contribution, scaling like dM s 2/dT , to the measured quantity. Here M s is the staggered magnetization. These results thus reconcile scattering and bulk property measurements of random-field Ising systems.

Nuclear magnetic order in solid 3He

Over the past 20 years our understanding of cooperative magnetic phenomena in solid 3He has expanded greatly. Starting from the concept of atomic exchange, the author explains how early experiments led to the abandonment of the naive Heisenberg nearest neighbor antiferromagnetic Hamiltonian, in favor of a multiple exchange model. He then describes the detailed experimental picture of the spin-ordered phases which has developed over the past decade, and the successes and failures of the multiple exchange model to explain the experimental results. Today solid 3He is a powerful model magnetic system for extending our general understanding of magnetic order, and should continue to play an important role for many years to come.

Riemannian geometry and the thermodynamics of model magnetic systems.

A Riemannian metrical structure of the parameter space has been introduced and investigated for magnetic systems described in the framework of quantum statistics. The introduced metric is based on the conception of the relative information. Two contrasting models have been investigated in detail: the one-dimensional Ising model, with short-range interactions, and the mean-field model of Kac, with long-range interactions. In the second case the metric tensor degenerates. The degeneration has been removed by adding the lattice energy to the original magnetic Hamiltonian. It turns out that in both cases the scalar curvature of parameter space tends toward plus infinity while approaching the critical points. The inverse of the scalar curvature, given by the second and third moments of stochastic variables, has been interpreted as a measure of the stability of the considered magnetic systems. The scalar curvature represents a joint part of fluctuations caused by the interactions of spins.

Magnons and factons in diluted antiferromagnets (invited)

Site-diluted antiferromagnets with short-range interactions can serve as a model magnetic percolation system. A length scale ξ exists such that for length scales (1/q)>ξ the structure is continuous (hydrodynamic limit), and the magnetic excitations are antiferromagnetic magnons, with a stiffness constant which depends critically upon the magnetic concentration. For length scales (1/q)<ξ, the structure is fractal, and the magnetic excitations are fractons, with a characteristic dispersion law. The fracton excitations are (strongly) spatially localized. The magnons are expected to be extended in the very long length scale limit, with the possibility of Anderson (weak) localization for length scales longer than ξ. The magnetic excitations cross over from magnon to fracton at an energy ωc∝ξ−[1+(θ/2)], where θ is the anomalous diffusion exponent. We have calculated the scattering form factor I(q,ω) for magnetic excitations of a d=3 simple cubic antiferromagnet within the effective medium approximation (EMA), as a function energy transfer ω and momentum transfer q. We find that at small fixed q (within the magnon regime), p>pc, I(q,ω) is sharply peaked at the magnon frequency. Near to pc, a second (but small) peak at ωc is visible on the asymmetric high-energy tail of I(q,ω). For larger fixed q (within the fracton regime), I(q,ω) is centered about the fracton frequency, although it is quite broad, reflecting the strong spatial scattering of fractons.