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.     

Phase diagram of a geometrically frustrated triangular-lattice antiferromagnet in a magnetic field

The magnetic phase diagram of a geometrically frustrated triangular-lattice antiferromagnet is evaluated as a function of magnetic field and anisotropy using a trial spin state built from harmonics of a fundamental ordering wave vector. A noncollinear incommensurate state, observed to be chiral and ferroelectric in CuFeO2, appears above a collinear state with 4 sublattices (SLs). The apparent absence of multiferroic behavior for predicted chiral, noncollinear 5-SL states poses a challenge to theories of the ferroelectric coupling in CuFeO2.     

Local spin resonance and spin-peierls-like phase transition in a geometrically frustrated antiferromagnet

Inelastic magnetic neutron scattering reveals a localized spin resonance at 4.5 meV in the ordered phase of the geometrically frustrated cubic antiferromagnet ZnCr2O4. The resonance develops abruptly from quantum critical fluctuations upon cooling through a first order transition to a co-planar antiferromagnet at T(c) = 12. 5(5) K. We argue that this transition is a three dimensional analog of the spin-Peierls transition.     

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.     

Absence of long-range order with long-range potentials

A particular form of Mermin's inequality is analyzed for repulsive inverse power potentials [V(r)=e ² r ^–m/m] in ad-dimensional space. For long-range potentials (m d) the system is put into a stabilizing background. Long-range order is shown to be excluded ford (m + 2)/2 whenm d, while for short-range potentials (m > d) we recover Mermin's result (d 2). For Coulomb systems (m=d – 2) and the experimentally studied electron surface layer (d = 2,m=1), long-range order cannot be excluded by the present argument.     

Coexistence of long-range order and spin fluctuation in geometrically frustrated clinoatacamite Cu2Cl(OH)3

Muon spin rotation experiments are carried out on clinoatacamite, Cu2Cl(OH)3, which is a new geometrically frustrated system featuring a three-dimensional network of corner-sharing tetrahedral 3d Cu2+ spins. A long-range antiferromagnetic order occurs below 18.1 K with a surprisingly small entropy release of about 0.05Rln2/Cu. Below 6.5 K, the static long-range order transforms abruptly into a metastable state with nearly complete depolarization of muon spins which suggests strong fluctuation. The system then enters a state in which partial long-range order and spin fluctuation coexist down to the lowest experimentally attainable temperature of 20 mK. This work presents a novel system for studying geometric frustration.     

Absence of long-range order in a dipole system

A new field representation is found to consider the statistical mechanics of a system of classical dipoles. It appears that the functional of this field is locally degenerate so that a phase transition is impossible in the dipole 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     

First order phase transition in the frustrated triangular antiferromagnet CsNiCl3

By means of high-resolution ultrasonic velocity measurements, as a function of temperature and magnetic field, the nature of the different low temperatures magnetic phase transitions observed for the quasi-one-dimensional compound CsNiCl3 is established. Special attention has been devoted to the field-induced 120 degrees phase transition above the multicritical point in the H-T phase diagram where the elastic constant C44 reveals a steplike variation and hysteresis effects. These results represent the first experimental evidence that the 120 degrees phase transition is weakly first order and contradict the popular notion of new universality classes for chiral systems.     

Absence of long-range order in certain random systems

It is proved that a classical X-Y model where the field conjugate to the order parameter is static and random shows no long-range order at any temperature for spatial dimensions d ? 4. The proof can be extended to systems with higher order parameter dimension n.     

Absence of long-range order in one-dimensional spin systems

For a one-dimensional Ising model with interaction energy $$E\left\{ \mu \right\} = - \sum\limits_{1 \leqslant i< j \leqslant N} {J(j - i)} \mu _\iota \mu _j \left[ {J(k) \geqslant 0,\mu _\iota = \pm 1} \right]$$ it is proved that there is no long-range order at any temperature when $$S_N = \sum\limits_{k = 1}^N {kJ\left( k \right) = o} \left( {[\log N]^{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}} } \right)$$ The same result is shown to hold for the corresponding plane rotator model when $$S_N = o\left( {\left[ {{{\log N} \mathord{\left/ {\vphantom {{\log N} {\log \log N}}} \right. \kern-\nulldelimiterspace} {\log \log N}}} \right]} \right)$$      

A frustrated three-dimensional antiferromagnet: stacked J1-J2 layers

We study a frustrated 3D antiferromagnet of stacked J(1)-J(2) layers. The intermediate 'quantum spin liquid' phase, present in the 2D case, narrows with increasing interlayer coupling and vanishes at a triple point. Beyond this, there is a direct first-order transition from Néel to columnar order. Possible applications to real materials are discussed.     

Absence of magnetic order in Yb3Ga5O12: relation between phase transition and entropy in geometrically frustrated materials

From muon spin relaxation spectroscopy experiments, we show that the sharp peak (lambda-type anomaly) detected by specific heat measurements at 54 mK for the ytterbium gallium garnet compound, Yb3Ga5O12, does not correspond to the onset of a magnetic phase transition, but to a pronounced building up of dynamical magnetic pair correlations. Beside the lambda anomaly, a broad hump is observed at higher temperature in the specific heat of this garnet and other geometrically frustrated compounds. Comparing with other frustrated magnetic systems we infer that a ground state with long-range order is reached only when at least 1/4-1/3 of the magnetic entropy is released at the lambda transition.     

Persistence of magnons in a site-diluted dimerized frustrated antiferromagnet

We present inelastic neutron scattering and thermodynamic measurements characterizing the magnetic excitations in a disordered spin-liquid antiferromagnet with non-magnetic substitution. The parent compound Ba(3)Mn(2)O(8) is a dimerized, quasi-two-dimensional geometrically frustrated quantum disordered antiferromagnet. We substitute this compound with non-magnetic V(5+) for the S=1 Mn(5+) ions, Ba(3)(Mn(1-x)V (x))(2)O(8), and find that the singlet-triplet excitations which dominate the spectrum of the parent compound persist for the full range of substitution examined, up to x=0.3. We also observe additional low-energy magnetic fluctuations which are enhanced at the greatest substitution values.