Small Antiferromagnetic Clusters

Buczko, R.; Wilamowski, Ż.; Jantsch, W.

doi:10.12693/aphyspola.90.747

Cited by 3 publications

(3 citation statements)

References 8 publications

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“…Εe, 75.50.Ρp, 76.50.+g Usually it is believed that in a macroscopic, 3-dimensional antiferromagnet (AF) each magnetic sublattice is statically magnetised, but the magnetic moments compensate each other. Α more sophisticated discussion of the nature of AF, however, shows that the AF coupling leads rather to an AF correlαtion of the sublattices, but not necessarily to their static magnetization [1][2][3]. Someadditional coupling, e.g., to a nuclear spin system, can lead to a symmetry breakdown and thus to the formation of static staggered magnetization.…”

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confidence: 99%

Oscillating Antiferromagnetism of Ultrathin EuTe Layers

et al. 1997

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We study magnetic resonance on EuTe/PbTe superlattices. Analysis of the magnetic dipole anisotropy in the superlattices and of the EPR amplitude of isolated Eu ions in the PbTe wells shows that the diffusion at interfaces is very small. The real thickness of EuTe can be evaluated with an accuracy better than one monolayer. Since for EuTe layers thicker than 2 monolayers there is no difference in the character of the antiferromagnetic resonance observed for even and odd numbers of monolayers, we conclude that there is no static magnetization of the antiferromagnetic sublattices. Nevertheless, long range antiferromagnetic order is clearly evident.PACS numbers: 75.50. Εe, 75.50.Ρp, 76.50.+g Usually it is believed that in a macroscopic, 3-dimensional antiferromagnet (AF) each magnetic sublattice is statically magnetised, but the magnetic moments compensate each other. Α more sophisticated discussion of the nature of AF, however, shows that the AF coupling leads rather to an AF correlαtion of the sublattices, but not necessarily to their static magnetization [1][2][3]. Someadditional coupling, e.g., to a nuclear spin system, can lead to a symmetry breakdown and thus to the formation of static staggered magnetization. So far, there is no solid experimental evidence whether an AF is of static or of oscillatory character. In macroscopic systems the oscillation frequency tends to zero. In finite systems, the socalled macroscopic quantum coherence (MQC) rate is discussed. Some resonances, with their frequency depending on the size of the AF, were found in AF nanoclusters [4], but their interpretation induced some controversy [1].Twodimensional AF's are intensively investigated since some classes of high temperature superconductors are 2D antiferromagnets [5]. AF correlations are well evidenced but no static staggered magnetization is proven in these materials. Moreover, there is a concept according to which coupling of conduction carriers to "oscillating" AF planes leads to the formation of superconducting pairs. Thin AF MnTe and MnSe layers were investigated by neutron diffraction. It was shown

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Oscillating Antiferromagnetism of Ultrathin EuTe Layers

et al. 1997

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“…Such a conclusion is strongly supported by the fact that quasi-2D layers are characterised by a much higher Néel temperature. It is also confirmed by the fact that the mean binding energy of a quantum string, in contrast to the classical model, increases for short strings [2].…”

Section: Dere H2p Andh ? D E S C R I B E T H E S E C O N D M O M E N mentioning

confidence: 66%

Magnetic Properties of Semiconductor-Antiferromagnet Superlattices

et al. 1996

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Magnetic resonance investigations of ultra-thin antiferromagnetic EuTe layers show a specific behaviour in the quasi-2D antiferromagnetic ordering: (i) an anisotropy of the critical broadening, (ii) a substantial increase in the Néel temperature and (iii) an anisotropic shift of the resonance frequency which diverges at the Néel point. The results show that exchange coupling is stronger in quasi-2D than in 3D antiferromagnetic samples and that correlation of spin chains aligned in the perpendicular direction occurs already well above the Néel point.PACS numbers: 75.50. Ee, 75.50.Pp, 76.50.+g There is a substantial difference between the macroscopic and the microscopic nature of antiferromagnets (AFs). Macroscopic AFs are described by stable Néel sublattices with opposite spin orientation and the magnetizations of the sublattices compensate each other completely. For microscopic objects, in contrast, quantum mechanical calculations show that there is no stable magnetization of any sublattice. Neighbouring spins are anti-correlated but the mean value of each individual spin vanishes. There is no obvious answer which type of AF is more appropriate for ultra-thin AF 1ayers where one dimension is a microscopic one. In this paper we show that magnetic phase transitions of such layers can be regarded as an ordering of short quantum AF strings which are oriented from one interface to another.The Eu chalcogenides are known as magnetic model systems where coupling among the spins is described by pure Heisenberg coupling. We investigate the magnetic resonance ("EPR") of a series of MBE grown EuTe/PbTe superlattices (SLs) and we compare the experimental results obtained with those from a feẁ (973)

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“…The quantum mechanical calculation of finite AF clusters [6] shows that for an even number of spins, N, for axial symmetry and positive anisotropy parameter D > 0, a non-magnetic singlet (M = 0) is a ground state of the cluster. The first excited state, separated by an energetic distance, Δ, is another non-magnetic singlet.…”

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Ultra-Fast Spin Dynamics in Diluted Magnetic Semiconductors

et al. 1996

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In nano-size antiferromagnetic systems a spatially inhomogeneous field leads to the formation of a staggered magnetization. Thereby the total magnetic moment does not change but the formation of a net magnetic moment at the border of the cluster leads to an energy gain. This type of magnetism is characterised by an ultra-fast dynamics. We suggest it is also responsible for the formation of the exciton magnetic polaron.PACS numbers: 75.10.Jm, 75.50.-y A few recently discovered effects in diluted magnetic semiconductors are still not fully understood as yet. In particular, the characteristic time for the creation of an exciton magnetic polaron (EMP) is extremely short. It is much faster than any longitudinal relaxation time [1][2][3]. In addition, there is a "mysterious" coupling between antiferromagnetic (AF) layers in antiferromagnet-semiconductor superlattices [4] and also between antiferromagnetic micrograins which are characterised by a new type of magnetic resonance [5]. In this paper we suggest that for the explanation of this type of phenomena one has to consider the response of the magnetic system on a local, spatially inhomogeneous perturbation. The discussed effect has a pure quantum character therefore we analyse a rigorous solution of the HamiltonianThe first term in Eq. (1) describes the exchange energy, the second some magnetic anisotropy. We examine the solutions for linear chains rigorously. This is the simplest possible case where the above-mentioned phenomena can be demonstrated. Moreover, we limit our discussion to chains with an even spin number, N. For odd spin number the ground state is characterised by a 2S + 1 degeneracy, i.e., by a nonvanishing magnetic moment. In that case, the discussion of the magnetic properties is more complicated since the Curie magnetism caused by the magnetic (969)

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Small Antiferromagnetic Clusters

Cited by 3 publications

References 8 publications

Oscillating Antiferromagnetism of Ultrathin EuTe Layers

Oscillating Antiferromagnetism of Ultrathin EuTe Layers

Magnetic Properties of Semiconductor-Antiferromagnet Superlattices

Ultra-Fast Spin Dynamics in Diluted Magnetic Semiconductors

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