Spin-chirality duality in a spin ladder with four-spin cyclic exchange

Effect of four-spin cyclic exchange on magnetism is studied in the two-leg S=1/2 ladder. We develop an exact spin-chirality duality transformation, under which the system is self-dual when the four-spin exchange J4 is half of the two-spin exchange. Using the density-matrix renormalization-group method and the duality relation, we find that the four-spin exchange makes the vector-chirality correlation dominant. A "chirality short-range resonating-valence-bond" phase is identified for the first time at large J4.     

Electron spin resonance of Gd in the intermetallic compounds YCu,YAg, and LaAg: Wave vector dependence of the exchange interaction

The discrepancy between the Gd EPR g shift and linewidth in YCu, YAg, and LaAg is resolved using a wave vector dependent exchange parameter J(k,k'). Various experimentally observable quantities are related to J(k,k') using a partial wave expansion.     

Mixed spin Ising model with four-spin interaction and random crystal field

The effects of fluctuations of the crystal field on the phase diagram of the mixed spin-1/2 and spin-1 Ising model with four-spin interactions are investigated within the finite cluster approximation based on a single-site cluster theory. The state equations are derived for the two-dimensional square lattice. It has been found that the system exhibits a variety of interesting features resulting from the fluctuation of the crystal field interactions. In particular, for low mean value D of the crystal field, the critical temperature is not very sensitive to fluctuations and all transitions are of second order for any value of the four-spin interactions. But for relatively high D, the transition temperature depends on the fluctuation of the crystal field, and the system undergoes tricritical behaviour for any strength of the four-spin interactions. We have also found that the model may exhibit reentrance for appropriate values of the system parameters.     

On the theory of spin exchange structures

All possible types of spin ordering manifested in spin-spin correlation functions are determined. Some general characteristics of arbitrary spin structures predicted by macroscopic theory are examined, including energy associated with inhomogeneity, anisotropy, and energy in external fields.     

Anomalous hall effect in some uranium intermetallic compounds

We report Hall effect measurements on the uranium intermetallic compounds UAl₂, U_0.97La_0.03Al₂, U_0.93La_0.07Al₂, U_0.85La_0.15Al₂, UPt₃ and UPd₃. We find in all of these substances, a large positive Hall effect at high temperatures and maxima or minima at low temperatures. We have compared our data with the results of magnetic skew scattering theories and found qualitative agreement at high temperatures in the cases of U_1-xLa_xAl₂ and UPt₃, but not in the case of UPd₃.     

Ground state of a spin system with two- and four-spin exchange interactions on the triangular lattice

We study a spin system with both two- and four-spin exchange interactions on the triangular lattice as a possible model for the nuclear magnetism of solid ³He layers adsorbed on grafoil. The ground state is analyzed by the use of the mean-field approximation. It is shown that the four-sublattice state is favored by introduction of the fourspin exchange interaction. A possible phase transition at a finite temperature into a phase with the scalar chirality is predicted. Application of a magnetic field is shown to cause a variety of phase transitions.     

X-Ray isochromats of some chromium intermetallic compounds

A systematic investigation of the fine structure near the short wavelength limit of the continuous X-ray spectrum for the chromium compounds CrB, CrSb, and Cr₂Te₃ has been made by means of a vacuum spectrometer with a curved (radius one meter) topaz crystal. Information about the density of states in the conduction band and the characteristic energy losses in these compounds is given.     

Magnetic properties of some Cr-Ga intermetallic compounds

Magnetic susceptibility measurements in the range 10 – 300 K on the compounds Cr₃Ga, CrGa, Cr₅Ga₆ and CrGa₄ are reported here. All except CrGa₄ appeared to be Pauli paramagnetic, but CrGa₄ appeared to be basically diamagnetic with a parasitic weak ferromagnetic component.     

Continuum of compensation points in the mixed spin Ising ferrimagnet with four-spin interaction and next-nearest neighbor coupling

We investigate the magnetic properties of the mixed spin-1/2 and spin-1 Ising ferrimagnetic system with four-spin interaction J₄ and next-nearest neighbor (NNN) coupling J′. We perform exact ground-state calculations and use the finite cluster approximation, based on a single cluster theory, to derive the state equations for the two-dimensional square lattice. The main attention has been paid to the study of the phase diagram for both the transition and compensation temperatures. We find a number of characteristic behaviors. The model with only NNNs induces one compensation point while the four-spin interaction does not. The investigation of the model with both interactions shows a number of characteristic behaviors. In particular, the presence of the four-spin interaction, according to J₄ and J′, may lead to one, two or possibly a continuum of compensation points. This phenomenon may have important applications in technology such as thermomagnetic writing and erasing at the compensation point.     

Spin structures with frustrated moments in RNiAl intermetallic compounds

Magnetic order in the ternary equiatomic intermetallic compounds PrNiAl, DyNiAl and HoNiAl is investigated by means of neutron powder diffraction and dc- and ac-susceptibility measurements. DyNiAl shows two magnetic phases: It is ferromagnetic between T₁ =15 K and T_c = 31 K and a tilted antiferromagnet below T₁. HoNiAl is a ferromagnet in a narrow temperature region just below T_c = 14.5 K, but furthermore exhibits two tilted antiferromagnetic phases (T₁ =5.5 K, T₂ = 12.5 K). The corresponding magnetic structures differ in the ordering of some of the magnetic moments which are coupled by frustrated bonds. The existence of those frustrated moments, which is induced by the topology is a common phenomenon in the hexagonal RNiAl series. They are manifested with reduced mean values of their moments in the elastic neutron spectra. Another interesting observation is the scaling of the second temperature T₁ at which the frustrated spins change their order: For all heavy rare earths investigated so far (R = Tb, Dy, Ho) we find T₁: T_N,C ≈ 1: 2. PrNiAl displays an incommensurate sinusoidal (TSW) structure with XY spin behaviour below T_N = 6.9 K, and only one ordered phase is found. The observed ordering temperatures roughly follow the de Gennes scaling factor.     

Hyperfine interaction of111Cd in In-Pd intermetallic compounds

Perturbed - angular correlation (PAC) spectroscopy was applied to study the hyperfine parameters for¹¹¹In probes in In-Pd intermetallic compounds. The PAC spectra measured in In₃Pd, In₃Pd₂ and InPd₂ compounds reflect the number, population and symmetry of nonequivalent indium probe sites predicted by the crystallographic data. The temperature dependence of the observed electric field gradients was measured in the temperature range 80–873 K.     

Electron spin resonance of Gd in intermetallic compounds with CsCl structure

The g-shift and the thermal broadening of the ESR linewidth of Gd doped YRh and ScM (M = Ru, Rh, Pd, Ag, Cu) are reported. The results seem to be influenced by non s-band conduction electrons.     

Quantum spin liquid in an antiferromagnet with four-spin interactions

A quantum Monte Carlo study of the anisotropic antiferromagnet with four-spin interaction (K) and with spin S=1/2 on a square lattice is reported. Temperature dependences of the capacity, susceptibility, spin correlation functions, and correlation length, and field dependences of magnetization are calculated. Phase diagrams of the ground-state antiferromagnet and gap (K>0) and gapless (K<0) quantum spin liquids are determined in the exchange-anisotropy-four-spin-coupling-constant (K), magnetic-field (H)-temperature (T), and temperature-(T)-K planes for an exchange anisotropy of 0.1 J. Fiz. Tverd. Tela (St. Petersburg) 39, 1404–1409 (August 1997)     

Magnetic structures and related properties of some rare-earth intermetallic compounds, RCu2

The magnetic structures and some relevant bulk magnetic properties of R(Cu, Ni)₂ (R = Tb, Tb_zY_1−z, Dy, Ho, Er and Tm) are summarized. Basically, the magnetic structures are antiferromagnetically modulated with propagation vector a^*. For R = Tb, Dy, Ho the a-axis anisotropy dominates and the structures are longitudinally modulated. For R = Tm, Er (probably) the b-axis anisotropy dominates and this results in transversely modulated structures. For R = Tb, Dy the structures are collinear, For R = Ho, Er, Tm (probably) an incommensurate modulation coexists with the commensurate a^*-axis modulation at the lowest temperature.     

Long-lived nuclear spin states in the solution NMR of four-spin systems

The existence of long-lived nuclear spin states in four-spin systems is explored by solution-state NMR experiments. Long-lived states are proved to exist in three different natural product molecules, each containing either a AA'BB' or a AA'XX' proton spin system. The measured state lifetimes are between four and eight times the spin-lattice relaxation time constants.     

Electron spin decoherence in quantum dots due to interaction with nuclei

We study the decoherence of a single electron spin in an isolated quantum dot induced by hyperfine interaction with nuclei. The decay is caused by the spatial variation of the electron wave function within the dot, leading to a nonuniform hyperfine coupling A. We evaluate the spin correlation function and find that the decay is not exponential but rather power (inverse logarithm) lawlike. For polarized nuclei we find an exact solution and show that the precession amplitude and the decay behavior can be tuned by the magnetic field. The decay time is given by (planck)N/A, where N is the number of nuclei inside the dot, and the amplitude of precession decays to a finite value. We show that there is a striking difference between the decoherence time for a single dot and the dephasing time for an ensemble of dots.     

Magnetic properties of some gadolinium—Cobalt intermetallic compounds

Crystallographic and magnetic properties of the single crystals having the compositions of GdCo₃, Gd₂Co₇, GdCo₅, Gd_2.2Co_16.6 and Gd₂Co₁₇ were studied in order to contribute to the investigation of the magnetic anisotropy in the Gd–Co system. The single crystal used were prepared by means of the Bridgman method using BN-coated alumina crucibles.The room-temperature magnetocrystalline anisotropy constants K₁ of GdCo₃, Gd₂Co₇, GdCo₅, Gd_2.2Co_16.6 and Gd₂Co₁₇ are determined as 8.1 × 10⁶, 1.3 × 10⁷, 4.1 × 10⁷, 3.0 × 10⁶ and -2.7 × 10⁶ erg/cm³, respectively. it is also found that the magnetocrystalline anisotropy of Gd–Co phases in the stable region of the Th₂Zn₁₇-type structure changes very sensitively from negative to positive as the ratio Gd/Co increases above the stoichiometric ratio of 2/17.     

Cycloidal vortex motion in easy-plane ferromagnets due to interaction with spin waves

The dynamics of a non-planar vortex in a two-dimensional easy-plane ferromagnet of finite size is studied. Spin dynamics simulations show small cycloidal oscillations of the vortex around its mean path. In contrast to an earlier phenomenological theory we give a physical explanation: The oscillations occur due to the interaction of the vortex with coherent spin waves which are excited by this vortex at the moment when it starts to move, in order to conserve the total energy and angular momentum. The calculation of these quantities yields the frequencies and amplitudes of the cycloidal oscillations in good agreement with the simulation data. Received 9 December 2002 Published online 4 June 2003 RID="a" ID="a"e-mail: franz.mertens@uni-bayreuth.de