Theory of Phase Transitions

Romano

2014

Phys. Rev. E

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At low temperatures, some lattice spin models with simple ferromagnetic or antiferromagnetic interactions (for example, nearest-neighbor interaction being isotropic in spin space on a bipartite three-dimensional lattice) produce orientationally ordered phases exhibiting nematic (second-rank) order, in addition to the primary first-rank one; on the other hand, in the literature, they have been rather seldom investigated in this respect. Here we study the thermodynamic properties of a three-dimensional model with dipolar-like interaction. Its ground state is found to exhibit full orientational order with respect to a suitably defined staggered magnetization (polarization), but no nematic second-rank order. Extensive Monte Carlo simulations, in conjunction with finite-size scaling analysis, have been used for characterizing its critical behavior; on the other hand, it has been found that nematic order does indeed set in at low temperatures, via a mechanism of order by disorder.

Section: Computational Aspects and Finite-size Scaling Theorymentioning

confidence: 99%

Nematic order by thermal disorder in a three-dimensional lattice spin model with dipolarlike interactions

Romano

2014

Phys. Rev. E

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“…In an attempt to elucidate the mechanisms underpinning the magnetic properties of this molecule we revisited 5 19 . The curves represent the calculated intensities I n n , with n and n denoting the number of initial and final states in the transition processes, where n = 0 is the ground state.…”

Section: Introductionmentioning

confidence: 99%

Magnetization steps in the molecular magnet Ni4Mo12 revealed by complex exchange bridges

Georgiev

2020

Phys. Rev. B

We study the behavior of the magnetization and the magnetic susceptibility of molecular magnets with complex bridging structure. Our computations are based on a post-Hartree-Fock method accounting for the intricate network of interatomic bonds and an effective spin-like Hamiltonian that captures the essential magnetic features of magnetic molecules. The devised method and the constructed Hamiltonian are further employed to characterize the magnetic properties of the molecular magnet Ni 4 Mo 12 . The obtained results reproduce both quantitatively and qualitatively the main features of the magnetic spectrum. Furthermore, the computations for the magnetization and the low-field susceptibility are in very good agreement with their experimental counterparts. In this respect, they improve upon the results obtained with conventional Heisenberg models.

“…The physical properties, such as energy spectra, susceptibility, etc., of magnetic clusters at the nanoscale depend on their size, shape and the presence of different bondings among the constituent chemical elements and thus the distribution of ligands between the magnetic ions (for more details see [1][2][3][4] and references therein). Some prominent examples are Mn based magnetic compounds [5][6][7][8][9][10][11] and spin clusters with Ni magnetic ions [12][13][14][15] .…”

Section: Introductionmentioning

confidence: 99%

Magnetic excitations in molecular magnets with complex bridges: the tetrahedral molecule Ni4Mo12

Georgiev

2019

Eur. Phys. J. B

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We investigate the spectroscopic magnetic excitations in molecular magnets with complex intermediate structure among the magnetic ions. Our approach consists in introducing a modified spin Hamiltonian that allows for discrete coupling parameters accounting for all energetically favorable spatial distributions of the valence electrons along the exchange bridges connecting the constituent magnetic ions. We discuss the physical relevance of the constructed Hamiltonian and derive its eigenvalues. The model is applied to explore the magnetic excitations of the tetrameric molecular magnet Ni 4 Mo 12 . Our results are in a very good agreement with the available experimental data. We show that the experimental magnetic excitations in the named tetramer can be traced back to the specific geometry and complex chemical structure of the exchange bridges leading to the splitting and broadness of the peaks centered about 0.5 meV and 1.7 meV.