Low-Temperature Properties of Quasi-One-Dimensional Molecule-Based Ferromagnets

2004

Phys. Rev. B

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We present a comparative study of bosonic languages to describe one-dimensional Heisenberg ferrimagnets. The ferrimagnetic Schwinger-boson mean-field theory demonstrated by Wu et al., the antiferromagnetic modified spin-wave theory designed by Takahashi, and its ferrimagnetic variant proposed by Yamamoto et al. are employed to calculate the energy structure and the thermodynamics of various ferrimagnets. A modified spin-wave scheme, which introduces a Lagrange multiplier keeping the native energy structure free from temperature and thus differs from the original Takahashi scheme, is particularly stressed as a useful tool to investigate one-dimensional quantum ferrimagnetism. The antiferromagnetic limit of these descriptions is also considered

Section: Resultsmentioning

confidence: 57%

Bosonic representation of one-dimensional Heisenberg ferrimagnets

2004

Phys. Rev. B

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“…This new spin-wave scheme, which is now referred to as the modified spin-wave theory, was further applied to quantum antiferromagnets and ferrimagnets. The modified spin-wave scheme is highly successful for extensive ferrimagnets [24][25][26][27][28][29] and still applies well for two-dimensional antiferromagnets [30][31][32][33][34]. As for one-dimensional antiferromagnets, there exists a pioneering argument [35], but it looks unsatisfactory for interpreting experimental and numerical observations.…”

mentioning

confidence: 99%

Spin–Wave Description of Haldane-Gap Antiferromagnets

Hori

2003

J. Phys. Soc. Jpn.

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Modifying the conventional antiferromagnetic spin-wave theory which is plagued by the difficulty of the zero-field sublattice magnetizations diverging in one dimension, we describe magnetic properties of Haldane-gap antiferromagnets. The modified spin waves, constituting a grand canonical bosonic ensemble so as to recover the sublattice symmetry, not only depict well the ground-state correlations but also give useful information on the finite-temperature properties.PACS numbers: 75.10. Jm, 05.30.Jp, 75.40.Mg Haldane's conjecture [1,2] that one-dimensional Heisenberg antiferromagnets should exhibit qualitatively different low-energy structures based on whether the constituent spins are integral or fractional sparked renewed interest in the field of quantum magnetism. An energy gap immediately above the ground state was indeed observed in a quasi-one-dimensional Heisenberg antiferromagnet Ni(C 2 H 8 N 2 ) 2 NO 2 (ClO 4 ) [3] and a rigorous example of such a massive phase was also found out [4,5]. The energy gaps in magnetic excitation spectra, that is, spin gaps, are now one of the most attractive and important topics. In the context of theoretical progress, we may be reminded of quantized plateaux in the groundstate magnetization curves [6], a dramatic crossover from one-to two-dimensional quantum antiferromagnets [7], and an antiferromagnetic excitation gap accompanied by ferromagnetic background [8][9][10][11]. From the experimental point of view, metal oxides such as spin-Peierls compounds Cu 1−x M x GeO 3 (M = Zn, Mg) [12,13], Haldanegap antiferromagnets R 2 BaNiO 5 (R = rare earth) [14] and ladder materials Sr n−1 Cu n+1 O 2n (n = 3, 5, 7, · · ·) [15] have significantly contributed to systematic investigations of the mechanism of gap formation.

“…Their simple magnetic structures, describable within isotropic exchange Hamiltonians [17,18], are suitable enough to compare oligonuclear ferrimagnets with those of one dimension in their intrinsic features. The theoretical tool we employ here is a recently developed modified spin-wave theory, which is quite useful in understanding thermal [19][20][21] as well as groundstate [22,23] properties of various one-dimensional ferrimagnets. We inquire further into zero dimension and dynamic properties.…”

Section: Introductionmentioning

confidence: 99%

“…The theoretical tool we employ here is a recently developed modified spin-wave theory, which is quite useful in understanding thermal [19][20][21] as well as groundstate [22,23] properties of various one-dimensional ferrimagnets. We inquire further into zero dimension and dynamic properties.…”

Section: Introductionmentioning

confidence: 99%

Nuclear spin-lattice relaxation in ferrimagnetic clusters and chains: A contrast between zero and one dimensions

Hori

2003

Phys. Rev. B

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Motivated by ferrimagnetic oligonuclear and chain compounds synthesized by Caneschi et al., both of which consist of alternating manganese(II) ions and nitronyl-nitroxide radicals, we calculate the nuclear spin-lattice relaxation rate 1/T1 employing a recently developed modified spin-wave theory. 1/T1 as a function of temperature drastically varies with the location of probe nuclei in both clusters and chains, though the relaxation time scale is much larger in zero dimension than in one dimension. 1/T1 as a function of an applied field in long chains forms a striking contrast to that in finite clusters, diverging with decreasing field like inverse square root at low temperatures and logarithmically at high temperatures.