Macroscopic Quantum Tunneling Of Magnetization Research Articles

On Random Switching of Magnetization

Random switching of magnetization is studied using a continuous master equation derived for magnetization dynamics driven by a jump-noise process. This paper is concerned with numerical analysis of the Kramers-Brown approximation as well as with low temperature random magnetization switching where the so-called phenomena of macroscopic quantum tunneling of magnetization has been experimentally observed and theoretically predicted.

Macroscopic quantum tunneling of magnetization explored by quantum-first-order reversal curves

A method to study the fundamental problem of quantum double well potential systems that display magnetic hysteresis is proposed. The method, coined quantum-first-order reversal curve (QFORC), is inspired by the first-order reversal curve, based on the Preisach model for hysteresis. We successfully tested the QFORC method in the hysteresis of the Mn12Ac molecular magnet, which is governed by macroscopic quantum tunneling of magnetization. The QFORC reproduces well the experimental magnetization behavior. It is possible to separate the thermal activation and tunneling contributions from the magnetization variation, as well as associate the magnetization jumps with specific quantum transitions.

Switching of molecular magnets

AbstractIn this paper we consider magnetic properties of single‐molecule magnets (SMMs). The main focus, however, is on possible mechanisms of magnetic switching of SMMs. One mechanism considered is the phenomenon of macroscopic quantum tunneling of magnetization (QTM) in an external time‐dependent magnetic field. When the molecule is additionally attached to magnetic leads, the phenomenon of QTM is associated with transfer of charge and spin between the leads, so the SMM can work as a charge and/or spin pump. Another possibility follows from spin momentum transfer from conduction electrons to the molecule, which effectively may reverse spin moment of the molecule attached to magnetic leads, provided the bias voltage exceeds some threshold value determined by the anisotropy barrier. To calculate the basic transport characteristics we apply the rate equation to a model Hamiltonian that includes exchange interaction of tunneling electrons with the molecule's magnetic core and electron correlations in the LUMO level. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Quantum tunneling of magnetization in dipolar spin-1 condensates under external fields

We study the macroscopic quantum tunneling of magnetization of the $F=1$ spinor condensate interacting through dipole-dipole interaction with an external magnetic field applied along the longitudinal or transverse direction. We show that the ground state energy and the effective magnetic moment of the system exhibit an interesting macroscopic quantum oscillation phenomenon originating from the oscillating dependence of thermodynamic properties of the system on the vacuum angle. Tunneling between two degenerate minima are analyzed by means of an effective potential method and the periodic instanton method.

Macroscopic quantum tunnelling of magnetization in spinor Bose-Einstein condensates

We investigate macroscopic quantum tunneling of magnetization in a a spinor Bose- Einstein condensate trapped in a double-well potential. Using the mean field theory applicable for large condensates, and employing the single spatial mode approximation we derive dynamical equations for magnetization, independent from density mode, for certain initial conditions. We show that they can be reduced to standard Josephson junction dynamics equations under certain conditions.

분자 자성체의 자기 특성과 양자역학적 효과

Magnetism of molecular nanomagnet, which attracted a lot of academic attention after the discovery of the macroscopic quantum tunneling of magnetism, is reviewed. Molecular nanomagnet is metal-organic material in which magnetic ions are regularly located in the organic skeleton. Also, the interaction between the molecules is very small and those molecules form macroscopic molecular crystal in which molecules are residing at the element points in the crystal. Molecular nanomagnets show a lot of interesting features, especially, equivalence of macroscopic magnetic properties and molecular magnetic properties. In this paper, research results on molecular nanomagnet with microscopic tool like NMR are reviewed mainly. The new method to observe the quantum tunneling of magnetization discovered in Mnl2-ac with NMR is shown and the research results on the microscopic aspects of the macroscopic quantum tunneling of magnetization using the new method are shown. Also, the physical aspect of the level crossing effect which has been reported originally with NMR in molecular nanomagnet is reviewed with experiment results. The research results on the molecular nanomagnets will reveal the important information about the limit of the miniaturization of magnetic memory units and give us the basic scientific knowledge which is needed for the application for the quantum computation. Moreover, academically, many quantum mechanical theories which have not been checked the validity can be checked with experiments.

On the non-standard rhombic spin Hamiltonian parameters derived from Mössbauer spectroscopy and magnetism-related measurements

The orthorhombic standardization of spin Hamiltonian parameters is increasingly adopted in the electron magnetic resonance area. The aim of this paper is to elucidate the ramifications of orthorhombic standardization for other spectroscopic and magnetic techniques, which also employ the spin Hamiltonian formalism. This is illustrated by examples derived from the Mössbauer spectroscopy, magnetic moments and magnetic susceptibility, photoinduced changes of magnetization, and other magnetism-related measurements. Implications of standardization in the studies of magnetic ordering, Haldane gap for integer spin systems, the macroscopic quantum tunnelling of magnetization, specific heat measurements, the spin wave theory, and inelastic neutron scattering are also discussed. Several sets of the non-standard zero-field splitting (ZFS) parameters for transition ions at orthorhombic symmetry sites, expressed in various notations and units, are standardized. Calculations are performed using the computer package CST, which yields the standardized ZFS parameter sets. The results are presented in a unified way in the extended Stevens notation b k q and units of cm −1 together with the conventional D and E parameters, which prevail in the studies dealt with in this paper. This enables a direct comparison with the available data for similar ion/host systems. The standardization reveals several inconsistencies in interpretation of the experimental data obtained by various techniques.

Magnetization reversal in individual nanoparticles: macroscopic quantum tunneling of magnetization

The combination of highly sensitive Superconducting Quantum Interference Device (SQUID) with high quality nanoparticles allowed us to check for the first time simple classical models proposed nearly 50 years ago by Neel, Stoner and Wohlfarth. For nanoparticles containing about 10/sup 5/ to 10/sup 6/ /spl mu/B the quantitative agreement with the Neel-Brown theory of thermal activated magnetization reversal allowed us to identify unambiguously the magnetization reversal mechanism as uniform rotation. In the case of insulating barium ferrite nanoparticles containing about 10/sup 5/ /spl mu//sub B/ and below 0.4 K, strong deviations from this model are found which are quantitatively in agreement with the predictions of the theory of macroscopic quantum tunneling of magnetization in the low dissipation regime.

Macroscopic Quantum Tunneling of Magnetization of Single Ferrimagnetic Nanoparticles of Barium Ferrite

Presented are switching field measurements of individual ferrimagnetic and insulating BaFeCoTiO nanoparticles containing about ${10}^{5}{\ensuremath{\mu}}_{B}$ at very low temperatures (0.1--6 K). For temperatures higher than 0.4 K, the quantitative agreement with the N\'eel-Brown theory of thermal activated magnetization reversal allowed us to identify unambiguously the magnetization reversal of uniform rotation. Below 0.4 K, strong deviations from this model are evidenced which are quantitatively in agreement with the predictions of the theory of macroscopic quantum tunneling of magnetization in the low dissipation regime.

Quantum tunneling effect in magnetic particles

During the past five yeas, researchers have achieved advances in both sample elaboration and measurement techniques in an effort to witness macroscopic quantum tunneling of magnetization. Important advances include experimental results of relaxation measurements, single particle measurements, and domain wall junction, quantum coherence and quantum resonance measurements. Quantum resonance measurements have shown unambiguously quantum tunneling of magnetization at a scale of one nanometer.

Nanoscale magnetic particles: Synthesis, structure and dynamics

In recent years, new synthetic routes, which include wet techniques and synthesis in confined geometries, have been developed for the preparation of nanoscale magnetic particles. The shape of the particles obtained is usually smooth and rounded all over, due to the influence of the surface energy. The final shape, however, depends on the preparation method and is much influenced by the substrate used, as this may stress and deform the particles in order that they adapt to it. Changes in lattice parameters, in comparison to the bulk, and surface modifications give way in many cases to changes in the magnetic and transport properties. For very small ferromagnetic metal particles (clusters), both enhanced magnetism, due to localization of the valence d-electrons, and giant magnetic moments for some nonmagnetic clusters are observed. Magnetic phenomena in real single-domain particles reveal that the statistics of the magnetization reversal cannot be described by a thermal activation over a single-energy barrier. Macroscopic quantum tunnelling of magnetization is observed at very low temperatures in several systems. Other interesting electrical and magnetic phenomena, such as finite size effects, coercivity enhancement due to surface effects, and giant magnetoresistance, are observed in fine particle systems.

Macroscopic quantum tunnelling of magnetization in a single crystal of nanomagnets

THE precise manner in which quantum-mechanical behaviour at the microscopic level underlies classical behaviour at the macroscopic level remains unclear, despite seventy years of theoretical investigation. Experimentally, the crossover between these regimes can be explored by looking for signatures of quantum-mechanical behaviour—such as tunneling—in macroscopic systems1. Magnetic systems (such as small grains, spin glasses and thin films) are often investigated in this way2–12 because transitions between different magnetic states can be closely monitored. But transitions between states can be induced by thermal fluctuations, as well as by tunnelling, and definitive identification of macroscopic tunnelling events in these complex systems is therefore difficult13. Here we report the results of low-temperature experiments on a single crystal composed of super-paramagnetic manganese clusters (Mn12-ac), which clearly demonstrate the existence of quantum-mechanical tunnelling of the bulk magnetization. In an applied magnetic field, the magnetization shows hysteresis loops with a distinct 'staircase' structure: the steps occur at values of the applied field where the energies of different collective spin states of the manganese clusters coincide. At these special values of the field, relaxation from one spin state to another is enhanced above the thermally activated rate by the action of resonant quantum-mechanical tunnelling. These observations corroborate the results of similar experiments performed recently on a system of oriented crystallites made from a powdered sample4.

Macroscopic quantum tunneling in small magnetic particles

Macroscopic quantum tunneling of magnetization in small antiferromagnetic clusters is studied for the spin-1/2 anisotropic (Jz ≡ J, Jx=Jy≡Jxy) Heisenberg model with the use of both perturbation theory and an exact numerical diagonalization technique. Splitting of the two lowest levels due to the lifting of the degeneracy between them (in the case Jxy=0) by the in-plane exchange (Jxy ≠ 0) is found to occur in the N/2th order of perturbation theory. Tunneling is absent in systems with an odd number of sites, in which the levels are doubly degenerate in zero magnetic field.

Low temperature magnetic relaxation and quantum tunneling in nanocrystalline particles (abstract)

We report measurements of the magnetic relaxation rate versus temperature for ferrofluid and magnetic-glass samples, which are formed by a modification of nanocomposite material consisting of nanocrystalline CoFe2O4 and polymer. The magnetic properties of the samples have also been studied by using SHE-SQUID at different temperatures (1.8–300 K) with low and high applied magnetic field (−5 T to 5 T). The magnetic relaxation in two samples show a perfect logarithmic dependence on the time, i.e., M(t)=M(t0)[1−S ln(t/t0)], in accordance with the ZFC-FC results which indicate that there is wide energy distribution. The temperature independence of magnetic viscosity S≡[1/M(t0)dM/d]ln t below several Kelvin for the two samples gives clear evidence of macroscopic quantum tunneling of magnetization, in accordance with current theories of quantum tunneling of magnetization.

Quantum tunneling by magnetostatic interaction of fine ferroparticles (clusters)

This paper considers macroscopic quantum tunneling of magnetization in the system of fine ferroparticles (clusters) with account of their magnetostatic interaction. This effect is found to be fully determined only by the magnetostatic interaction of particles. It is shown that the quantum tunneling effect results in the resonance absorption peak of a nonuniform ac magnetic field.

Measurement of the dynamics of the magnetization reversal in individual single-domain Co particles

The first magnetization measurements of individual single-domain particles (50–150 nm) patterned by electron-beam lithography are presented. The angular dependence of the switching field is measured and the temperature dependence (0.1–6 K) of the reversal process is investigated by two independent methods: (i) by measuring the switching field and the width of the switching field distribution as a function of temperature and field sweeping rate, (ii) by measuring the probability for switching at constant applied magnetic field as a function of time and temperature. We found that mean values like the mean switching field and the mean switching time verify a simple model based on thermal activation between 1 and 6 K. Below 1 K thermal activation is not sufficient anymore to explain their variation. However the width of the switching field distribution and the probability of switching are in disagreement with such a model. The results are discussed in view of Macroscopic Quantum Tunnelling of magnetization.

Stochastic resonance in a bistable magnetic system

Theoretical model of magnetostochastic resonance is developed. It is shown that in the presence of noise the signal-to-noise ratio for a bistable uniaxial ferromagnetic can be a million times greater than that for a one-stable ferromagnetic. The phenomenon can be used for magnetic field measurements, detection of macroscopic quantum tunneling of magnetization and investigation of electron tunneling that depends upon magnetization.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Time-dependent phenomena at low temperature in magnetic digital compact cassette tape

In this paper we report on the low-temperature magnetic relaxation of CrO2 particles of digital compact cassette tapes. The observed temperature dependence of the magnetic viscosity, S≡∂M/∂ ln t, shows three different regimes below room temperature. The most remarkable is that for T≤5 K, S(T) remains constant, suggesting the occurrence of macroscopic quantum tunneling of magnetization.

Quantum tunneling of magnetization in single domain particles

We present a study of the magnetic relaxation of several ferrofluids composed of particles of about 40 Å in diameter (Fe3O4FeC, CoFe2O4). Our key observation is a nonthermal character of the relaxation below 3 K for the CoFe2O4 ferrofluid and below 1 K for the FeC ferrofluid. The crossover temperature from thermal to nonthermal (quantum) regime is in accordance with theoretical suggestions of macroscopic quantum tunneling of magnetization in single domain particles.

Small metal particles

This review article is devoted to results of theoretical and experimental investigations of properties of nanometer metal particles. First, the modern theory of equilibrium structures and shapes of small particles is presented. The next Section deals with the thermodynamics of small particles. It reflects the present state of the theory of surface forces for small particles, including crystalline ones. Validity conditions are pointed out for the standard approach, and an alternative approach is described which is adequate for particles surrounded by their saturated vapor. In it the Laplace pressure is not a real physical force, but a formal quantity that describes the size dependence of the chemical potential. Specific features are described of melting and interphase fluctuations in small particles. In describing the electron properties of the particles emphasis is laid on the smoothed level density and the size dependence of the Fermi energy. Some effects caused by this dependence are presented, in particular the mutual charging of particles of different sizes which manifests itself in an anomalously strong attraction between particles and so on. Magnetic properties of small particles are also described including macroscopic quantum tunneling of magnetization.