Geometrical Spin Frustration Research Articles

Geometric spin frustration for isolated plaquettes of the lattices: An extended irreducible tensor operator method

A new strategy to search for the good quantum numbers for the corner-sharing spin systems, as archetypal plaquettes of the lattices, was suggested for the first time in order to study on geometric spin frustration. The calculations on energy spectra by using the irreducible tensor operator method with the new strategy can be much reduced. As representative examples the energy spectra for the spin pentamer of the tetrahedron with a centered spin site and the spin heptamer of three corner-sharing equilateral-triangle were examined in order to confirm efficiency of the new strategy. Through our code, with automatically searching for the good quantum numbers, the projection operators S iz , S ix and S iy matrices in the ground state space for the spin heptamer were reliably constructed.

Importance of the O−M−O Bridges (M = V5+, Mo6+) for the Spin-Exchange Interactions in the Magnetic Oxides of Cu2+ Ions Bridged by MO4 Tetrahedra: Spin-Lattice Models of Rb2Cu2(MoO4)3, BaCu2V2O8, and KBa3Ca4Cu3V7O28

The spin-lattice models relevant for the magnetic oxides Rb2Cu2(MoO4)3, BaCu2V2O8, and KBa3Ca4Cu3V7O28 were determined by evaluating the relative strengths of the spin-exchange interactions between their Cu2+ ions on the basis of spin dimer analysis. Our study shows that the O-M-O bridges (M = V5+, Mo6+) between the magnetic ions Cu2+, provided by the MO4 tetrahedra, are crucial for the spin-exchange interactions and hence for deducing the spin-lattice models needed to interpret the magnetic properties of these oxides. The spin-lattice model of Rb2Cu2(MoO4)3 is not a uniform chain but two interpenetrating spin ladders that interact weakly with geometric spin frustration. The spin-lattice model of BaCu2V2O8 is an alternating chain as expected, but the spin-exchange paths responsible for it differ from those expected. With respect to the strongest spin exchange of BaCu2V2O8, the spin exchange of KBa3Ca4Cu3V7O28 is only slightly weaker, but the strongest spin exchange of Rb2Cu2(MoO4)3 is much weaker. This difference in the spin-exchange strengths is caused by the difference in the bridging modes of the MO4 tetrahedra leading to these spin-exchange interactions.

Spin dimer, electronic band structure and classical spin analyses of spin exchange interactions and ordered magnetic structures of magnetic solids

We reviewed briefly how spin dimer, electronic band structure and classical spin analyses are employed to describe spin exchange interactions and ordered magnetic structures of magnetic solids. It was shown that super-superexchange interactions are crucial in forming strongly interacting spin units of magnetic solids and hence in determining their magnetic properties, and that incommensurate magnetic superstructures are readily described by a Heisenberg spin Hamiltonian using the classical spin approximation. We probed the weak ferrimagnetism of monoclinic Fe 2(SO 4) 3 in terms of spin dimer and mean-field-theory analyses to explain both the ordered magnetic structure and the sublattice magnetization. The classical spin and quantum mechanical descriptions of geometric spin frustration were compared to clarify their differences.

Geometrical Spin Frustration in the fcc Antiferromagnet NiS2

Geometrical Spin Frustration in the fcc Antiferromagnet NiS2

Magnetic Properties of ZnFe2O4as a 3-D Geometrical Spin Frustration System

Magnetic properties of ZnFe 2 O 4 were studied for various specimens prepared by different thermal and artificial treatments using neutron scattering, Mössbauer spectroscopy and DC and AC susceptibilities. We found a strong correlation between the magnetic Bragg peak intensity and the Mössbauer absorption line-width. Comparing with the susceptibility data, it is discussed what are the intrinsic magnetic properties of ZnFe 2 O 4 as a typical 3-D geometrical spin frustration system.

Classical spin and quantum-mechanical descriptions of geometric spin frustration.

Geometric spin frustration (GSF) in isolated plaquettes with local spin s, i.e., an equilateral-triangle spin trimer and a regular-tetrahedron spin tetramer, was examined on the basis of classical spin and quantum-mechanical descriptions to clarify their differences and similarities. An analytical proof was given for how the state degeneracy and the total spin S of their ground states depend on the local spin s. The quantum-mechanical conditions for the occurrence of GSF in isolated plaquettes were clarified, and their implications were explored. Corner sharing between plaquettes and how it affects GSF in the resulting spin systems was examined.

Direct evidence of spin frustration in the fcc antiferromagnet NiS 2

NiS2 is a well-known Mott insulator with anomalous antiferromagnetic long-range order of coexistent type I (Q M=(1,0,0), TN1=40 K) and type II (Q M= $\frac\Box1\Box\Box2\Box$ , $\frac\Box1\Box\Box2\Box$ , $\frac\Box1\Box\Box2\Box$ , TN2=30 K). Extensive neutron-scattering measurements reveal that magnetism in NiS2 is governed by geometrical spin frustration, resulting in magnetic diffuse scattering extending along the fcc zone boundary. Although the diffuse scattering exists at temperatures as high as 250 K (6TN1), it disappears rapidly below TN2, associated with minor crystal distortion. We observed a clear energy gap in addition to the low-energy spin-wave excitation at significantly below 30 K, and obtain evidence that degeneracy due to the coexistence of the two types of antiferromagnetism is relieved in the ground state via the reduction in symmetry due to distortion.

Dielectric and magnetic anomalies and spin frustration in hexagonalRMnO3(R=Y,Yb, and Lu)

Single crystals of hexagonal $R{\mathrm{MnO}}_{3}$ $(R=\mathrm{Y},$ Yb, and Lu), where Mn ions form the triangular lattice, were investigated, focusing on their dielectric/magnetic anomalies as well as geometrical spin frustration. It is found that the ratio of a Weiss temperature to ${T}_{\mathrm{N}}$ is $\ensuremath{\sim}10$ in $R{\mathrm{MnO}}_{3},$ indicating the dominant role of strong geometrical frustration. The effect of geometrical frustration also appears in specific heat, which shows a presence of a substantial amount of residual magnetic contribution below ${T}_{\mathrm{N}},$ indicating that a part of the spins are still fluctuating at $T\ensuremath{\ll}{T}_{\mathrm{N}}.$ It is also found that the dielectric anomaly at ${T}_{\mathrm{N}}$ is strongly anisotropic, suggesting a unique correlation between magnetism and dielectric properties in these compounds.

Multinary Intermetallics from Molten Al. Synthesis of SmNiAl4Ge2and YNiAl4Ge2. Possible Spin Frustration in Separated Triangular Sm3+Layers

SmNiAl4Ge2 crystallizes from molten Al containing Sm, Ni, and Ge. The compound crystallizes in the rhomohedral space group R3m with a = 4.1121(6) Å, c = 31.109(6) Å, and V = 455.5(1) Å3. The YNiAl4Ge2 analogue is also stable. The crystal structure consists of layers of [NiAl4Ge2]3- separated by monolayers of hexagonally close-packed Sm3+ ions. The Ni atoms are surrounded by eight Al atoms in the structure. Each Sm atom has an octahedral environment of six Ge atoms. The magnetism of SmNiAl4Ge2 is unusual, possibly reflecting the presence of geometrical spin frustration. The magnetization exhibits a well-defined hysteresis loop, without signs of ferromagnetism. It also exhibits irreversibility in the field-cooled and zero-field-cooled magnetization but without the typical characteristic spin glass behavior.

Variable magnetic interactions in an organic radical system of (m-N-methylpyridinium alpha -nitronyl nitroxide)

In this paper we study an organic system of geometrical spin frustration. m-N-methylpyridinium \ensuremath{\alpha}-nitronyl nitroxide (m-${\mathrm{MPYNN}}^{+}$) is a spin-1/2 organic radical. The simple salt, m-${\mathrm{MPYNN}}^{+}$\ensuremath{\cdot}${\mathrm{ClO}}_{4}^{\mathrm{\ensuremath{-}}}$\ensuremath{\cdot}1/3 (acetone), crystallizes in a trigonal P3c1 space group, where the m-${\mathrm{MPYNN}}^{+}$ molecules exist as a dimer and the dimer units form a two-dimensional (2D) triangular lattice. One-third of the ${\mathrm{C}1\mathrm{O}}_{4}^{\mathrm{\ensuremath{-}}}$ ions are in the organic layer, joining the m-${\mathrm{MPYNN}}^{+}$ molecules, and the remainder is between the layers, compensating the excess of positive charge in the organic layers. The single-crystal EPR measurements clearly indicate a 2D Heisenberg character of the magnetic system in it. m-${\mathrm{MPYNN}}^{+}$ makes a crystalline solid-solution system, m-${\mathrm{MPYNN}}^{+}$\ensuremath{\cdot}(${\mathrm{ClO}}_{4}^{\mathrm{\ensuremath{-}}}$${)}_{\mathit{x}}$\ensuremath{\cdot}${\mathrm{I}}_{1\mathrm{\ensuremath{-}}\mathit{x}}^{\mathrm{\ensuremath{-}}}$\ensuremath{\cdot}1/3 (acetone) (0\ensuremath{\le}x\ensuremath{\le}1), which also belongs to the trigonal system. Both the a and c axes are slightly lengthened with increasing the ratio of the ${\mathrm{C}1\mathrm{O}}_{4}^{\mathrm{\ensuremath{-}}}$ ion, x, in the solid solution: The unit-cell volume is increased by 3.2% when x runs from 0 to 1. The temperature dependence of the magnetic susceptibilities of the solid solutions can be well interpreted in terms of a strong ferromagnetic intradimer interaction ${\mathit{J}}_{1}$ forming a triplet state and a weak antiferromagnetic interdimer interaction ${\mathit{J}}_{2}$ which is expected to give rise to spin frustration among the triplet spin species on each side of the triangles. It is found that ${\mathit{J}}_{2}$ quickly weakens with an increase in x, while ${\mathit{J}}_{1}$ shows little dependence. There is a possibility that this organic system can be characterized as a spin-1 kagom\'e antiferromagnet at very low temperatures.

Spin Liquid Behavior of Highly Frustrated Y(Sc)Mn2and Effects of Nonmagnetic Impurity

The roles of geometrical spin frustration in magnetic and thermodynamic properties of YMn 2 and Y(Sc)Mn 2 are studied. It is shown that the ground state of Y(Sc)Mn 2 , which exhibits heavy fermion like behavior, may be regarded as a quantum spin liquid realized through the frustration. To confirm this idea, magnetic, thermal and electrical properties of the Y 0.95 Sc 0.05 (Mn 1- x Al x ) 2 pseudobinary system have been studied. The substitution of Al for Mn causes a spin liquid to spin glass transition as a result of partial raise of spin configurational degeneracy.

LOW-TEMPERATURE SPECIFIC HEAT OF YMn2 IN THE PARAMAGNETIC AND ANTIFERROMAGNETIC PHASES

The low-temperature specific heat of YMn2 has been measured at applied pressures of 0 to 7.7 kbar. A paramagnetic state is stabilized for moderate values of the applied pressure (of the order of 1.6 kbar). A large linear term in the specific heat, which decreases regularly with increasing pressure, is observed in this phase. It is ascribed to giant spin fluctuations associated with a magnetic-non magnetic instability and a strong geometrical spin frustration.