Macroscopic Quantum Effects in Nanometer-Scale Magnets

Theory of quantum tunneling of the magnetization in magnetic particles de Raedt, Hans; Miyashita, S.; Saito, K.; García-Pablos, D.; García, N. Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. We study the response of the magnetization to a time-dependent applied magnetic field H(t) in a model for a uniaxial magnet. It is shown that a staircase structure in the magnetization curve results from Landau-Zener tunneling between different pairs of nearly-degenerate energy levels. This mechanism might be relevant to the analysis of the hysteresis of nanoscale magnets at low temperatures, allowing one to extract the energy splittings from the hysteresis curve. We investigate the dependence of the staircase structure on the sweep rate dH(t)/dt, and point out some universal features of the staircase in uniaxial magnets. We also study the effect of a step-wise ͑instead of continuous͒ increase in the field, and show that the size of the steps depends sensitively on the procedure used to change the applied field. ͓S0163-1829͑97͒06042-6͔

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“…To illustrate the application of the above concepts let us consider the simplest case, a single spin- 1 2 system described by the Hamiltonian…”

Section: Introductionmentioning

confidence: 99%

Theory of quantum tunneling of the magnetization in magnetic particles

et al. 1997

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“…I refer here to the giant spin Hamiltonian for a nanomagnet as a "high-energy" Hamiltonian, but of course it is obvious that Hamiltonians like (8) or (9) are themselves the result of the truncation of an even higher energy (more "microscopic") electronic spin Hamiltonian like, eg.,…”

Section: Figmentioning

confidence: 99%

“…Even if we knew all the couplings J αβ ij , K αβ j (which is hardly likely given the internal complexity of most nanomagnets), truncation of H o (S) at low energies is practically impossible if L > 5 − 10, even with supercomputers. Instead one attempts to measure the parameters in (8) or (9), thereafter treating them as "fundamental" (similar situations are encountered in most problems involving strong interactions, ranging from QCD to Fermi liquid theory). Moreover, even this is not easy -experiments such as ESR (Electron Spin Resonance) can only parametrize terms with small ℓ or r in (8).…”

Section: Figmentioning

confidence: 99%

“…Great scientific interest has been generated (as well as some rather misleading articles in the more popular press) by recent experiments in crystals of magnetic macromolecules [1,2,3,4,5,6,7], which show resonant tunneling of their magnetisation. Other experiments which have also generated considerable press include coherence experiments on the ferritin biological macromolecule [8,9], and tunneling experiments on domain walls in magnetic wires [10,11]. I thus spent some time explaining how the theoretical methods may be applied to these real systems.…”

Section: Introductionmentioning

confidence: 99%

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QUANTUM ENVIRONMENTS: SPINS BATHS, OSCILLATOR BATHS, and applications to QUANTUM MAGNETISM

Stamp

1998

Proceedings From the Institute for Nuclear Theory

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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 nanomagnets.

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“…In most practical applications of this "central spin model", the environmental nuclear spins will be either inside the object carrying the giant spin, or near it, in some substrate, or solvent, or surrounding matrix. The central spin may be a magnetic grain [5] or a magnetic macromolecule such as ferritin [7] or, on a smaller scale [8], M n 12 -ac. Similar models were also introduced some time ago to describe the large superparamagnetic "spin clusters" which are believed to exist in many disordered magnets at low temperature, such as Si : P near the metal-insulator transition [9], or "giant magnetic polarons" [10].…”

Section: Introductionmentioning

confidence: 99%

Effective Hamiltonian in the Problem of a "Central Spin" Coupled to a Spin Environment

Tupitsyn

Prokof’ev

Stamp

1997

Int. J. Mod. Phys. B

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We consider here the problem of a "giant spin", with spin quantum number S ≫ 1, interacting with a set of microscopic spins. Interactions between the microscopic spins are ignored. This model describes the low-energy properties of magnetic grains or magnetic macromolecules (ferromagnetically or antiferromagnetically ordered) interacting with a surrounding spin environment, such as nuclear spins.Our aim is to give a general method for truncating the model to another one, valid at low energies, in which a two-level system interacts with the environmental spins, and higher energy terms are absorbed into a new set of couplings. This is done using an instanton technique.We then study the accuracy of this technique, by comparing the results for the low energy effective Hamiltonian, with results derived for the original giant spin , coupled to a macroscopic spin, using exact diagonalisation techniques. We find that the low energy central spin effective Hamiltonian gives very accurate results (with increasing accuracy for large S), provided the typical coupling energies between the giant spin and the microscopic spins are not too large, and provided temperature and external field are sufficiently low. The essential limitation to the applicability of the low-energy effective Hamiltonian is just the semiclassical WKB approximation itself, which inevitably fails for very small S.Our results thus justify previous use of this effective Hamiltonian in calculations of the effects of nuclear spins on the dynamics of nanomagnetic systems.

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Macroscopic Quantum Effects in Nanometer-Scale Magnets

Cited by 248 publications

References 29 publications

Theory of quantum tunneling of the magnetization in magnetic particles

Theory of quantum tunneling of the magnetization in magnetic particles

QUANTUM ENVIRONMENTS: SPINS BATHS, OSCILLATOR BATHS, and applications to QUANTUM MAGNETISM

Effective Hamiltonian in the Problem of a "Central Spin" Coupled to a Spin Environment

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