Structure and frustrated magnetism of the two-dimensional triangular lattice antiferromagnet Na2BaNi(PO4)2

A new frustrated triangular lattice antiferromagnet Na₂BaNi(PO₄)₂ was synthesized by high temperature flux method. The two-dimensional triangular lattice is formed by the Ni²⁺ ions with S =1. Its magnetism is highly anisotropic with the Weiss constants θ_CW =-6.615 K (H⊥c) and -43.979 K (H||c). However, no magnetic ordering is present down to 0.3 K, reflecting strong geometric spin frustration. Our heat capacity measurements show substantial residual magnetic entropy existing below 0.3 K at zero field, implying the presence of low energy spin excitations. These results indicate that Na₂BaNi(PO₄)₂ is a potential spin liquid candidate with spin-1.     

Valence bond ground states in a frustrated two-dimensional spin-1/2 Heisenberg antiferromagnet

We study a class of two-dimensional spin-1/2 Heisenberg antiferromagnets, introduced by Klein [1], in which the nearest-neighbor term is supplemented by next-nearest-neighbor pair and four-body interactions, producing additional frustration. For certain lattices, including e.g. the hexagonal lattice, we prove that any finite subset which admits a dimer covering has a ground state space spanned by valence bond states, each of which consists only of nearest-neighbor (dimer) singlet pairs. We also establish linear independence of these valence bond states. The possible relevance to resonating-valence-bond theories of high-temperature superconductors is briefly discussed. In particular, our results apply both to regular subsets of the lattice and to subsets with static holes.Work supported in part by N.S.F. Postdoctoral Research FellowshipsWork supported by N.S.F. Grant No. DMR-83-18051     

NMR study of the partially disordered state in the frustrated fcc antiferromagnet GdInCu4

Hyperfine Interactions - The magnetic structure of the frustrated fcc antiferromagnet GdInCu4 was investigated by NMR. Local spin configurations around In and Cu sites were found to be practically...     

Li2VO(Si,Ge)O4, a prototype of a two-dimensional frustrated quantum heisenberg antiferromagnet

NMR and magnetization measurements in Li2VOSiO4 and Li2VOGeO4 are reported. The analysis of the susceptibility shows that both compounds are two-dimensional S = 1/2 Heisenberg antiferromagnets on a square lattice with a sizable frustration induced by the competition between the superexchange couplings J1 along the sides of the square and J2 along the diagonal. Li2VOSiO4 undergoes a low-temperature phase transition to a collinear order, as theoretically predicted for J2/J1>0.5. Just above the magnetic transition the degeneracy between the two collinear ground states is lifted by the onset of a structural distortion.     

Large magnetostriction and negative thermal expansion in the frustrated antiferromagnet ZnCr2Se4

A detailed investigation of ZnCr2Se4 is presented which is dominated by strong ferromagnetic exchange but orders antiferromagnetically at TN=21 K. Specific heat and thermal expansion exhibit sharp first-order anomalies at the antiferromagnetic transition. TN is shifted to lower temperatures by external magnetic fields and finally is fully suppressed by a field of 65 kOe. The relative length change DeltaL/L(T) is unusually large and exhibits negative thermal expansion alpha below 75 K down to TN indicating strong frustration of the lattice. Magnetostriction DeltaL/L(H) reveals large values comparable to giant magnetostrictive materials. These results point to a spin-driven origin of the structural instability at TN explained in terms of competing ferromagnetic and antiferromagnetic exchange interactions.     

Magnetoelastic coupling and symmetry breaking in the frustrated antiferromagnet alpha-NaMnO2

The magnetic and crystal structures of the alpha-NaMnO2 have been determined by high-resolution neutron powder diffraction. The system maps out a frustrated triangular spin lattice with anisotropic interactions that displays two-dimensional spin correlations below 200 K. Magnetic frustration is lifted through magneto-elastic coupling, evidenced by strong anisotropic broadening of the diffraction profiles at high temperature and ultimately by a structural phase transition at 45 K. In this low-temperature regime a three-dimensional antiferromagnetic state is observed with a propagation vector k=(1/2,1/2,0).     

Mössbauer study on a two-dimensional triangular antiferromagnet YFeMnO4

Magnetic ordering of YFeMnO₄ has been studied with Mössbauer spectroscopy in the temperature range between 4.2 K and 300 K on the conditions of gamma rays parallel to and perpendicular to thec-axis. It is revealed that the spin system orders like a Heisenberg spin system in a triangular antiferromagnet with an anisotropic axis parallel to thec-axis.     

Magnetic interactions in the geometrically frustrated triangular lattice antiferromagnet CuFeO2

The spin-wave excitations of the geometrically frustrated triangular lattice antiferromagnet CuFeO2 have been measured using high resolution inelastic neutron scattering. Antiferromagnetic interactions up to third nearest neighbors in the ab plane (J1, J2, J3, with J{2}/J{1} approximately 0.44 and J{3}/J{1} approximately 0.57), as well as out-of-plane coupling (J{z}, with J{z}/J{1} approximately 0.29) are required to describe the spin-wave dispersion relations, indicating a three-dimensional character of the magnetic interactions. Two energy dips in the spin-wave dispersion occur at the incommensurate wave vectors associated with multiferroic phase and can be interpreted as dynamic precursors to the magnetoelectric behavior in this system.     

阻挫三角反铁磁AgCrO2螺旋自旋序的第一性原理研究

基于密度泛函理论的广义梯度近似(GGA)和投影缀加波(PAW)方法,分别从共线和非共线磁性结构出发,研究了自旋阻挫三角反铁磁AgCrO₂的基态、磁性以及电子结构,从理论计算的角度给出了基态磁性结构.计算结果表明:AgCrO₂具有120°螺旋自旋序反铁磁基态,其自旋螺旋面平行于(110)面或(11^-0)面;由于Cr离子间的自旋几何阻挫,导致沿晶体的a,b和a+b方向上均形成了螺旋自旋转动角为120°的 关键词： 第一性原理 交换相互作用 阻挫 反铁磁     

Statics and dynamics of incommensurate spin order in a geometrically frustrated antiferromagnet CdCr2O4

Using elastic and inelastic neutron scattering we show that a cubic spinel, CdCr2O4, undergoes an elongation along the c axis (c > a = b) at its spin-Peierls-like phase transition at T(N) = 7.8 K. The Néel phase (T < T(N)) has an incommensurate spin structure with a characteristic wave vector Q(M) = (0, delta,1) with delta approximately 0.09 and with spins lying on the ac plane. This is in stark contrast to another well-known Cr-based spinel, ZnCr2O4, that undergoes a c-axis contraction and a commensurate spin order. The magnetic excitation of the incommensurate Néel state has a weak anisotropy gap of 0.6 meV and it consists of at least three bands extending up to 5 meV.     

阻挫三角反铁磁YMnO3电子结构和磁性研究

基于广义梯度近似(GGA)的密度泛函理论,从非共轴的磁性结构出发,运用第一性计算得到了自旋阻挫反铁磁YMnO3的电子结构和磁性.考虑在位库仑势作用(U),研究了d电子关联作用对电子结构、能隙、和磁性的作用.计算得到,由于电子强关联和自旋阻挫的作用,Mn3d和O2p的电子态密度分布发生很大变化,费米面附近的基态能隙变大,Mn3d和O2p态电子杂化减弱,Mn离子的磁矩增加到3.92μB.我们的结果比局域化自旋密度近似LSDA计算结果更好地符合于实验测量值.     

Magnetic behavior of the two-dimensional antiferromagnet RbFeF4

RbFeF₄ shows anomalous magnetic susceptibility behavior rather similar to that of KFeF₄, in which the existence of two-dimensional antiferromagnetic ordering has recently been clearly demonstrated. A neutron diffraction study of powder samples reveals that RbFeF₄ orders antiferromagnetically in three dimensions at T_N = 134 K. In addition around T_N, the diffraction patterns indicate a magnetic ridge, thus giving, in accordance with KFeF₄, the evidence of the two-dimensional nature of the magnetism in RbFeF₄.     

A Mössbauer study of the quasi two-dimensional antiferromagnet TlFe2−x Se2

TlFe_2?x Se₂ compounds withx from 0.36 to 0.40 have been examined with Mössbauer spectroscopy. The compounds order antiferromagnetically with broad magnetic transition regions whose width increases with decreasing Fe content. The Néel temperature decreases with x and the temperature variation of the magnetic hyperfine field indicates that the transition is of first order. In samples without vacancy ordering the Fe nuclei experience a different temperature variation of the hyperfine field than in compounds with ordered vacancies.