Neutron scattering study of anisotropic spin correlations in quasi two-dimensional antiferromagnet MnTiO3

An anisotropic three-dimensional neutron critical scattering has been observed from a “quasi” two-dimensional antiferromagnet MnTiO₃ in the vicinity of the Néel temperature. When temperature increases, the scattering profile transforms gradually to a ridge-like form, indicating that “cross-over” from three-dimensional to two-dimensional character appears at about 16 degrees above T_N.     

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.     

Magnon Raman scattering in the quasi 1-D antiferromagnet CsCoBr3

Raman scattering from magnons has been observed in the three magnetic phases of CsCoBr₃. In the 1-D Ising phase T > 28K a broad band at 96 cm^?1 is observed. This band grows in intensity but shows little renormalisation (100.5 cm^?1 at 14K) in the partially disordered antiferromagnetic phase 14K < T < 28K. For T < 14K additional structure at 111.5, 123.5, 133, and 141 cm^?1 is attributed to magnon-magnon combination bands. Two extra magnon branches are expected for this ferrimagnetic phase. One of these has an energy of ≈ 11 cm^?1.     

Critical line broadening of the EPR in the two-dimensional antiferromagnet crystals Rb2MnCl4 and Rb2Mn0,8Cd0,2Cl4

The EPR spectrum of quasi-two-dimensional antiferromagnets Rb₂Mn_xCd_1?xCl₄ (x=1,0; 0,8) has been studied in a critical temperature range. Two theoretical approaches - the scaling theory and the soliton's theory - are used to explain experimental data on temperature of the EPR linewidth. In the first interpretation critical exponents are determined. For both crystal two temperature regions with different critical exponents are found. It is shown that the soliton's theory with an anomaleous great excitement energy describes the experimental data satisfactory.     

Irradiation- and substitution- induced disorder in the quasi one-dimensional antiferromagnet Ag0.50Cr0.50PS3

The deviation from the 1D antiferromagnetic behaviour in Ag_0.50Cr_0.50PS₃ is greatly enhanced by substitutional or electron - irradiation - induced disorder. Accordingly, we interpret this deviation as a contribution of unpaired spins related to finite length of antiferromagnetic segments and to isolated magnetic defects. In the case of short segments, a strongly field dependent susceptibility is observed in the temperature range of 1D interactions, suggesting a possibility of 1D spin glass behaviour.     

Non-Fermi liquid behavior in the magnetotransport of quasi two-dimensional heavy Fermion compounds CeMIn5

We observe several non-Fermi liquid behaviors in the normal-state transport properties of CeMIn₅ (M: Rh and Co) under pressure at low temperatures: (1) The dc-resistivity shows T-linear dependence, ρ_xx∝T. (2) The magnitude of Hall coefficient |R_H| increases rapidly with decreasing temperature, and reaches a value much larger than |1/ne| at low temperatures. (3) The magnetoresistance displays T- and H-dependence that strongly violate Kohler's rule, and is well scaled by the tangent of the Hall angle, . These non-Fermi liquid properties in the electron transport are remarkably pronounced when the AF fluctuations are enhanced in the vicinity of the quantum critical point. Since all of these salient features have been also reported for high-T_c cuprates, we infer that the non-Fermi liquid transport properties capture universal features of strongly correlated electron systems in the presence of strong antiferromagnetic fluctuations.     

Surface-layer effect in MnTiO3 ceramics at low temperatures

MnTiO₃ ceramics were prepared via the traditional solid-state reaction route. The low-temperature (100–330 K) dielectric properties of MnTiO₃ have been systematically investigated in the frequency range from 100 Hz to 5 MHz. Our results showed that MnTiO₃ exhibits intrinsic dilectric response in a wide temperature range up to 200 K. A relaxation appears near room temperature. This relaxation can be enhanced by annealing treatment in oxygen and weakened in nitrogen. Reducing the sample thickness gradually leads to the disappearance of the relaxation. Based on these experimental facts, the relaxation was ascribed to be a Maxwell–Wagner relaxation due to surface-layer effect.     

Two-magnon Raman scattering in the two-dimensional antiferromagnet K2FeF4

Two-magnon Raman scattering in the planar quadratic antiferromagnet K₂FeF₄ is investigated. The temperature dependence of the energy shift is in good agreement with second-order Green-function theory, as is the linewidth at low temperature. Numerical results, including renormalization, are the Heisenberg exchange $\text{J}\text{k}{}_{\mathrm{B}}\text{= ?14.5 ± 0.7}\text{K}$ and the anisotropy $\text{Δ(T = 0) = gμ}{}_{\mathrm{B}}\text{H}{}_{\mathrm{A}}\text{4|J|S = 0.18 ± 0.05}$, but with $\text{J[1 + Δ(T = 0)]}\text{k}{}_{\mathrm{B}}\text{= ?17.06 ± 0.10}\text{K}$.     

D.C. Protonic conductivity in the quasi two-dimensional SnCl2.2H2O

An experimental study on the temperature dependence of d.c. protonic conductivity in stannous chloride dihydrate single crystal is reported and briefly discussed. Two contributions to conductivity are put in evidence. The first is a critical contribution arising near the order-disorder phase transition temperature, the second is a background contribution which dominates at high temperatures.     

Spin gap in quasi-one-dimensional S=3/2 antiferromagnet CoTi2O5

Quasi-one-dimensional(1D) antiferromagnets are known to display intriguing phenomena especially when there is a spin gap in their spin-excitation spectra. Here we demonstrate that a spin gap exists in the quasi-1D Heisenberg antiferromagnet CoTi₂O₅ with highly ordered Co²⁺/Ti⁴⁺ occupation, in which the Co²⁺ ions with S = 3/2 form a 1D spin chain along the a-axis. CoTi₂O₅ undergoes an antiferromagnetic transition at T     

Charge density wave instability in the quasi two-dimensional metal γ-Mo4011

Transport and specific heat properties have been studied on the orthorhombic molybdenum oxide γ-Mo₄0₁₁. The anisotropy of the electrical resistivity establishes that this compound is a quasi two-dimensional metal, as expected from crystal structure data. Both the resistivity and the thermopower show that an electronic transition, probably due to a charge density wave instability takes place at T_c = 100 K. Low temperature specific heat data provide an estimation of the Debye temperature and of the electronic density of states in the low temperature metallic phase.     

Spin flop in one-dimensional Heisenberg antiferromagnet KCuF3

Following Yamada and Kato [J. Phys. Soc. Japan 63 (1994) 289], we have calculated the frequency－field diagram of antiferromagnetic resonance of KCuF_3 using an eight-sublattice model and mean field approximation. A spin flop is found to take place from the [110]_p to a direction perpendicular to [110]_p in the c-plane at H\simeq 42172A/m, which is in good agreement with the data obtained by measuring magnetic susceptibility.     

NMR study in a single crystal of the one-dimensional antiferromagnet CsCuCl3

The Cu²⁺ hyperfine field directions were determined by means of the ⁶³Cu magnetic resonance on a single crystal of CsCuCl₃ in a dc magnetic field. They make an angle 57° with the c axis. It was suggested a possible spiral spin configuration with the Cu²⁺ spins lying in the c plane. From the temperature dependence of the nuclear resonance frequency it was deduced an energy gap of about 4°K for this antiferromagnetic compound.     

Mössbauer characterization of tin dopant ions in the antiferromagnetic ilmenite MnTiO3

Mössbauer parameters of ¹¹⁹Sn diamagnetic dopant cations in an antiferromagnetic compound having the ilmenite structure are for the first time reported. The spectra reveal a well resolved hyperfine splitting pattern of combined magnetic and quadrupole interactions (at 5 K, δ=0.19 mm/s, H₁=52.5 kOe, eV_ZZQ_3/2=−0.80 mm/s, θ≈0°). This spectral component whose contribution (A₁=82%) represents more than four fifths of the total amount of the dopant (Sn/(Mn+Ti)=1/200) is assigned to Sn(IV) ions located in the bulk of MnTiO₃, on the Mn(II) site, and with a Mn(II) vacancy in their nearest surrounding. Two spectral components with minor contributions are also observed: one of them (H₂≈25 kOe, A₂=8%) can be assigned to Sn(IV) ions, in the MnTiO₃ lattice as well, on a site where they exhibit a weaker spin polarization (this site could be the Ti(IV) one) and the other (H₃=0 kOe, A₃=10%) to SnO₂ or/and Ti_1−xSn_xO₂ clusters. The Néel temperature of MnTiO₃ probed by the ¹¹⁹Sn dopant (T_N=69±2 K) agrees well with the values previously provided by ESR and antiferromagnetic resonance measurements. Variation of H₁ with temperature follows close the Brillouin function for S=5/2. No perturbation appears in the Mössbauer spectra around T=90 K where a broad peak, characteristic of 2D magnetic interactions, is observed on the static magnetic susceptibility curve.     

Magnetic properties of incommensurate antiferromagnet β-KMnCl3

Magnetic properties of orthorhombic incommensurate antiferromagnet β-KMnCl₃ have been studied as a function of temperature down to 0,5K and in magnetic fields up to 70 k0e. The exchange parameters were estimated with molecular-field theory. Magnetic field behaviour of β-KMnCl₃ is in accordance with change of magnetic structure from helical into fan structure and then into induced ferromagnetic state. The Neel temperature decreases with increase magnetic field overage 0,05K on 1 k0e.     

Nuclear spin lattice relaxation in the linear antiferromagnet CsNiCl3

Anisotropic effects have been found in the low temperature (< 10 K) nuclear spin lattice relaxation rates in the linear paramagnet CsNiCl₃ in a strong magnetic field (～ 60 kOe). The existence of a small single ion anisotropy term may be the cause of these effects.     

Spin-wave response in the one dimensional anisotropic antiferromagnet CsCoCl3

The magnetic excitation spectrum of CsCoCl₃ has been studied by neutron scattering. Evidence is presented that the long wavelength spectrum consists of a sharp spin-wave-like peak superposed on a broader band of excitations that extends up to the Ising-limit zone-boundary frequency. In contrast, the zone boundary response is sharp, suggesting that the continuum has a greater breadth at the zone centre. Further, it is found that the spectral weight of longitudinal scattering within the spin-wave band is small.     

Critical opalescence in SrTiO3

We have measured the temperature and angular dependence of the total scattered light intensity near the structural phase transition of SrTiO₃ and interpreted it in terms of the theory of Ornstein and Zernike. It is concluded that in light scattering the direct coupling to the phonon density fluctuations or central mode in SrTiO₃ prevails over the indirect coupling via the soft mode.     

Half-field EPR line in the two-dimensional ferromagnet K2CuF4

Room temperature measurements of the half-field EPR line in the two- dimensional ferromagnet K₂CuF₄ are reported. Changes of the intensity and line-width vs the angle between the applied field and the c-axis indicate the necessity of an additional term in the perturbing Hamiltonian besides the dipolar interaction. Its appearance at half the main Zeeman field at each angle means that there is no observable dynamic effect on the shift of the resonance position in this salt.