Macroscopic Quantum Coherence and Quasidegeneracy in Antiferromagnets

Levine, Gregory; Howard, J. B.

doi:10.1103/physrevlett.75.4142

Cited by 19 publications

(36 citation statements)

References 15 publications

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“…Indeed, for systems with well-separated levels, the quasiclassical approximation usually already works reasonably for a total spin SB2-3. However, as has been demonstrated [18], to apply the same type of arguments to the splitting calculations, the (normalized) instanton action S I should exceed the value of 12. For the model employed in Ref.…”

Section: Discussionmentioning

confidence: 99%

Many-spin effects and tunneling splittings in Mn12 magnetic molecules

Raedt

Hams

Dobrovitski

et al. 2002

Journal of Magnetism and Magnetic Materials

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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. AbstractWe calculate the tunneling splittings in a Mn 12 magnetic molecule taking into account its internal many-spin structure. We discuss the precision and reliability of these calculations and show that restricting the basis (limiting the number of excitations taken into account) may lead to significant error (orders of magnitude) in the resulting tunneling splittings for the lowest energy levels, so that an intuitive picture of different decoupled energy scales does not hold in this case. ; and, at present, a substantial amount of reliable experimental data has been collected. Quantitative analysis of these experiments is a challenging theoretical problem involving fundamental issues about tunneling phenomena in mesoscopic magnetic systems. The basic prerequisite for solving this problem is our ability to evaluate accurately and reliably the energy splittings occurring as a result of tunneling between two (quasi) degenerate levels [7]. At present, carefully designed magnetic relaxation

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Section: Discussionmentioning

confidence: 99%

Many-spin effects and tunneling splittings in Mn12 magnetic molecules

Raedt

Hams

Dobrovitski

et al. 2002

Journal of Magnetism and Magnetic Materials

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“…1a. The gap decreases strongly with increasing cluster size, N, for large clusters the gap is inversely proportional to the cluster size [1,3].…”

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“…In mesoscopic system there is a hypothetical possibility of a quantum transition between two minima. Macroscopic quantum tunneling (MQT) or macroscopic quantum coherence (MQC) has been discussed [3,5,8] as a possible interpretation of the Awschalom resonance. According to our knowledge, however, there is no experimental evidence of such a transition.…”

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confidence: 99%

“…The model predicts a nonzero static Néel vector, i.e., a finite magnetization of each of the two sublattices. Within the quantum mechanical (QM) model [1][2][3], in contrast, the AF cluster of an even number of spins is characterized by a singlet ground state which is built from a superposition of the Νéel states. Thus the mean value of the net magnetic moment on each spin site vanishes.…”

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confidence: 99%

“…The rigorous QM calculations for 2D [2,3] and 3D are very difficult -only small clusters can be evaluated. The experimental data [4], however, show that in ultrathin AF layers, where spins within each monolayer are ferromagnetically correlated, an increase in the exchange energy is observed.…”

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Small Antiferromagnetic Clusters

Buczko¹,

Wilamowski²,

Jantsch³

1996

Acta Phys. Pol. A

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The analysis of the quantum mechanical model of mesoscopic size antiferromagnets shows that small antiferromagnets are characterized by a stronger exchange coupling and by an oscillating character of spins polarizations. Experimental evidence of the quantum character is discussed. We interpret the interlayer coupling in antiferromagnet superlattices and a puzzling resonance observed in nano-size antiferromagnet grains.PACS numbers: 75.10.Jm, 75.50.Rr Nano-scale antiferromagnetic (AF) grains, ultrathin layers of AF , or diluted magnetic semiconductors, which under some circumstances can be treated as a random set of AF clusters, exhibit properties which cannot be explained within classical models. Within the classical Néel model, the ground state of an AF is doubly degenerate: two different orientations of the magnetic moment in each of the AF sublattices are possible. The model predicts a nonzero static Néel vector, i.e., a finite magnetization of each of the two sublattices. Within the quantum mechanical (QM) model [1][2][3], in contrast, the AF cluster of an even number of spins is characterized by a singlet ground state which is built from a superposition of the Νéel states. Thus the mean value of the net magnetic moment on each spin site vanishes. The case of a cluster with an odd number of spins is a little bit more complicated because a single spin remains uncompensated. For simplicity reasons, in this paper we will discuss the case corresponding to an even number of spins only. Generally, the classical and the QM model predict very different properties of AF and there is no simple correspondence between small and 1arge AF. In particular, an extrapolation of the QM picture does not predict a static Néel structure of AF. A well settled Neel vector can be expected only when a coupling to an environment, e.g., to the nuclear spins by hyperfine coupling, is taken into considerations.

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