4f-electric quadrupolar effects on the lattice constants and magnetism of RCu2Ge2 (R=rare-earths)

The temperature dependence of the lattice constants of the alloys, RCu₂Ge₂ (R=Nd, Tb, Dy, Ho and Tm) shows the existence of a lattice strain due to the alignment of the 4f-electric quadrupole at low temperatures. This strain could be related to the deviation of the Néel temperatures (T _N ) from that predicted by de Gennes scaling, thereby indicating the role of the quadrupole on the magnetism. On the other hand, in PrCu₂Ge₂, for which the enhancement ofT _N is the largest in thisR series, the lattice constants interestingly behave as if the 4f-quadrupole moment is almost quenched and the anomalous magnetism in this case is attributed to some degree of 4f-delocalisation.     

Effect of interchain coupling on spin correlation functions of spin-1 Heisenberg antiferromagnet CsNiCl3

The inelastic neutron-scattering experiment on CsNiCl₃ gives much bigger total intensity of multi-particle continuum than the theoretical calculation by the (1+1$1+1$)-dimensional O(3) non-linear σ-model. Three-dimensional effect has to be considered. A scenario was proposed where the interchain interaction is treated as an effective staggered magnetic field. We propose another model to approximately include the effect of the interchain coupling, and use the large-N expansion to do the calculations. In this circumstance we find that the Néel temperature is about 5 K, and also that for a range of wave-vectors the single magnon is unstable and will decay into three magnons.     

Three-body interaction and phonon dispersion relations in aluminium

Based on an ab initio cohesive energy calculation and a model of three-body interaction, the pair potential can be calculated using the Möbius inversion theorem in the theory of numbers. Then the atomic force constants and the phonon dispersion for A1 are evaluated both with and without three-body interaction. Compared with experiments, the results show that taking the three-body interaction into account considerably improves the dispersions. Contrary to previous work, the method for calculating the atomic force constants and phonon dispersions presented here is simple, with only two adjustable parameters.     

Measurement of the coupling strength distribution in ferromagnetic (F)/antiferromagnetic (AF) exchange bias systems

We propose a method for determination of the distribution function P(j) of the coupling energy density j in polycrystalline textured ferromagnetic (F)/antiferromagnetic (AF) film systems. P(j) governs the entire film coupling J and the exchange bias field H_e and was not measurable until now. The method is verified by torquemetry in a high magnetic field and by reversing its rotation sense. The transition to a new magnetic steady state after rotation reversal is analyzed within a Stoner–Wohlfarth model including thermal relaxation. This transition is completed earlier for strongly coupled grains than for grains with smaller j, which is reflected in the torque curves. We determined P(j) for a sputtered NiFe(16 nm)/IrMn(0.8 nm) film at T=50 K in the hysteretic range of coupling energies and found that P strongly decreases for increasing j.     

Dispersive phonon imaging of InAs

The phonon-focusing patterns of ballistic phonons in InAs are measured in the frequency range 0.1 to 1 THz, in an effort to test the global validity of lattice dynamics models for this semiconductor. Phonon caustic patterns depend sensitively on the shapes of constant frequency surfaces. Several tunnel-junction detectors with sensitivity onsets in this frequency range are used to measure dispersive shifts in the phonon caustics. The measured caustic positions are compared to those predicted by rigid-ion and bond-charge models. Similar to the case of InSb studied by Hebboul and Wolfe, a 6-parameter bond-charge model (BCM)-with force constants determined by neutron, X-ray, and Raman scattering-reproduces the phonon-imaging data both qualitatively and quantitatively. Comparisons of the focusing patterns with an 11-parameter rigid-ion model (RIM) do not show good agreement. New structures are predicted in the phononfocusing patterns at frequencies above about 1.2 THz — presently outside our experimental range-which are highly sensitive to the theoretical modeling.     

Hydrogen bonds mediated magnetism in Cu(bmen)2Pd(CN)4

Results of synthesis, X-ray structure analysis, electron spin resonance, susceptibility, magnetization and specific heat measurements of powdered Cu(bmen)₂Pd(CN)₄ (,N^′-dimethyl-1,2-diaminoethane) are reported. Its structure is formed of quasi-linear chains of the [-Cu(bmen)₂-NC-Ni(CN)₂-CN-]_n composition; these are interlinked by hydrogen bonds (HBs) leading to two-dimensional patterns. Upon magnetic, spectral and thermodynamic measurements the compound was identified as an S=1/2 Heisenberg antiferromagnet on a square lattice with due to the dominant role of HBs in creating a square network. The long-range ordering observed at 0.24 K is proposed to be of a Néel type. The possibility of tuning the exchange interactions in various directions is considered.     

Two-dimensional antiferromagnet in an external staggered field

We calculate the temperature-dependent energy gap in the magnetic excitations spectrum of the two-dimensional antiferromagnet in the presence of a staggered magnetic field, using the nonlinear sigma model.     

Magnetic transitions in layered triangular antiferromagnets

Magnetic states and phase transitions of the layered triangular antiferromagnets in an applied field are studied. It is shown that in compounds like VBr₂ and VCl₂ quantum effects change the ground-state structure and cause successive phase transitions as the magnetic field increases. Coplanar structures of different spin configuration are realized far from the saturation field and a noncoplanar structure of umbrella-type configuration is realized near this field. The ground-state phase diagram is constructed, and a finite region of fields where the collinear phase is also possible is indicated.     

Spin-assisted photoluminescence of polycrystalline α-MnSe

We have studied photoluminescence of α-MnSe in the antiferromagnetic and paramagnetic phases. Two broad, possibly multi-component emission bands centred at about 1.53 and 1.66 eV are observed at 20 K. The frequencies and intensities of these bands show step-like shift in the vicinity of T_N. On the basis of difference in peak energies at T=1.8 K and T_N the d-p-d superexchange interaction in excited state is estimated to be J_2g(exc)=0.918 meV.     

Electrical and magnetic properties of TmCoIn5 and YbCoIn5 single crystals

Electrical and magnetic properties of TmCoIn₅ and YbCoIn₅ single crystals were investigated by means of electrical resistivity and magnetization measurements in the temperature range from 300 to 0.5 K under the magnetic field up to 5 T. TmCoIn₅ is an antiferromagnetic metal with a Néel temperature T_N=2.6 K. YbCoIn₅ shows non-magnetic behavior, reflecting of divalent Yb ion.     

Symmetry constraints on phonon dispersion in graphene

We calculate the phonon dispersion for graphene with interactions between the first, second, and third nearest neighbors in the framework of the Born-von Karman model taking into account the constraints imposed by the lattice symmetry. Analytical expressions give the nonzero sound velocity for the out-of-plane (bending) mode. The dispersion of four in-plane modes is determined by coupled equations. Values of the force constants are found in fitting with frequencies at critical points and elastic constants measured on graphite.     

Properties of exchange interaction in Yb3Fe5O12 under extreme conditions

In Yb₃Fe₅O₁₂, the exchange effective field can be expressed as H_eff=−λ·M_Fe=−λχ_eff·H_e=−γ·H_e where γ is named as the exchange field parameter and H_e is the external magnetic field. Then, in this paper, by the discussions on the characteristics of the exchange field parameter γ, the properties of exchange interaction in ytterbium iron garnet (Yb₃Fe₅O₁₂) are analyzed under extreme conditions (high magnetic fields and low temperatures). Our theory suggests that the exchange field parameter γ is the function of the temperatures under different external magnetic fields, and γ=a+b·T+c·T², where the coefficients a, b, c are associated with the external magnetic fields and the magnetized directions. Thus, the temperature-dependence, field-dependence and anisotropic characteristics of the exchange interaction in Yb₃Fe₅O₁₂ are revealed. Also, excellent fits to the available experiments are obtained.     

Optical phonon modes in a free-standing quantum wire with ring geometry

The confined longitudinal-optical (LO) phonon and surface-optical (SO) phonon modes of a free-standing quantum wire with ring geometry are discussed within the dielectric continuum (DC) approximation. Two branches of SO phonon modes have been investigated. The frequencies of the SO phonons are found to be dispersed and radius dependent for small size systems. When the wave vector qz→∞, the frequencies of each SO modes converge to the frequency values of the single planar heterostructure.     

Magnetic linear dichroism studies of in situ grown NiO thin films

We report thickness dependence of magnetic linear dichroism (MLD) of in situ grown NiO(0 0 1) films on Ag(0 0 1) substrate at the Ni L₂ absorption edge. Antiferromagnetic domains at the surface of NiO(0 0 1) films are found to be preferentially aligned in-plane. For films thinner than a critical thickness _tc$t_{\mathrm{c}}$ (20–40ML), we observe a softening of the in-plane magnetic domain alignments with increasing film thickness, arising from the strain-relaxation effects. Films thicker than _tc$t_{\mathrm{c}}$ exhibits a residual in-plane anisotropy, possibly related to the finite-thickness effects.     

Magnetic-field-induced transition from metastable spin glass to possible antiferromagnetic-ferromagnetic phase separation in Cd0.5Cu0.5Cr2O4

Using ac susceptibility, dc magnetization and heat-capacity measurements, we have investigated the magnetic properties of Cd_0.5Cu_0.5Cr₂O₄. Cd_0.5Cu_0.5Cr₂O₄ has an extraordinary magnetic phase including a metastable spin-glass (SG) phase at zero field, a possible phase separation scenario of AFM/FM above ∼0.5 T field, and at intermediate fields, an apparent pseudo reentrant spin-glass (RSG) plateau is observed. These phenomena are closely correlated with the pinning effect of the Cu²⁺ sublattice on the frustrated lattice.     

Influence of hydrogenation on the structural and magnetic properties of compounds based on cerium and crystallizing in the tetragonal CeFeSi-type structure

The hydrogen absorption properties of CeMnGe, CeFeSi and CeCoX (X=Si and Ge) have been investigated. Neutron powder diffraction performed on deuteride CeCoGeD indicates that D-atoms are inserted in the pseudo-tetrahedral interstices [Ce₄] of the tetragonal CeFeSi-type structure of this compound. Magnetization and electrical resistivity measurements reveal that the hydrogenation of: (i) CeCoSi and CeCoGe leads to the transition antiferromagnet→spin fluctuation behaviour; (ii) CeMnGe suppress the magnetic ordering of the Ce-moments. These results which suggest a lost of ordered magnetic moment on the Ce site after hydrogenation could result from the chemical effect of hydrogen which prevails over the unit cell expansion effect.     

Influence of temperature on critical fields in ZnCr2Se4

The complex ac dynamic magnetic susceptibility was used to study the influence of temperature on critical fields in polycrystalline ZnCr₂Se₄ spinel. An antiferromagnetic order with a Néel temperature T_N=20.7 K and a strong ferromagnetic exchange evidenced by a positive Curie-Weiss temperature θ_CW=55.1 K were established. An increasing static magnetic field shifts T_N to lower temperatures while a susceptibility peak at T_m in the paramagnetic region—to higher temperatures. The non-zero and negative values both of the second and third harmonics of susceptibility suggest only a parallel spin coupling in ferromagnetic clusters in the range between the Néel and Curie-Weiss temperatures. Below T_N the magnetic field dependence of susceptibility, χ_ac(H), shows two peaks at critical fields H_c1 and H_c2. The values of H_c1 decrease slightly with temperature while the values of H_c2 drop rapidly with temperature. The strong changes of H_c2 temperature induced are mainly responsible for a spin frustration of the re-entrant type in the spinel under study.