NMR spectral densities and two dimensional diffusion in TiS2(NH3)1.0 and TaS2(NH3)x

NMR measurements of proton spin-lattice relaxation times T₁ and T_1? in the layered intercalation compounds TiS₂(NH₃)_1.0 and TaS₂(NH₃)_x (x = 0.8, 0.9, 1.0) are reported as functions of frequency and temperature (100 K – 300 K). These observations probe the spectral density of magnetic fluctuations due to motions of the intercalated molecules at frequencies accessible to the T₁ (4–90 MHz) and T_1? (1–100 kHz) measurements. Since the average molecular hopping time (τ) can be changed by varying temperature, different regions of the spectral density can be examined. For T > 200 K, both T^?1₁ and T^?1_1? vary logarithmically with frequency, reflecting the two dimensional character of the molecular diffusion. The temperature dependence of T₁ suggests that a more accurate picture of the short time dynamics is required. No dependence of relaxation rate on vacancy concentration is found.     

Magnetic phase diagram of Ni(NO3)2 6NH3

The magnetic phase diagram of Ni(NO₃)₂6NH₃ was determined from the field and temperature dependence of the magnetic susceptibility. The zero temperature exchange and anisotropy fields were determined to be H_E(0) ≈ 26 kOe and H_A(0) ≈ 0.7 kOe respectively.     

Dielectric dispersion in the incommensurate phases of [N(CH3)4]2CoCl4 and [N(CH3)4]2ZnCl4

The complex dielectric permittivity ?(ω) of [N(CH₃)₄]₂CoCl₄ and [N(CH₃)₄]₂ZnCl₄ along the a-axis was measured between 0.35 MHz and 100 MHz. It has been found that for both substances the relaxation frequencies are about 5 MHz at T_c. The dielectric relaxation of both substances could be described by a polydispersive process β = 0.74 in the vicinity of T_c. However, for the temperature region of (T?T_c) > 0.6 for [N(CH₃)₄]₂CoCl₄ the dielectric absorption seems to be rather monodispersive.     

Polarisation remanente dans les varietes de basse temperature de (NH4) 3AlF6 ET (NH4) 3FeF6

Dielectric and thermocurrent measurements have been carried out on (NH₄) ₃AlF₆ and (NH₄) ₃FeF₆ ceramic samples. A maximum of permittivity is observed close to the transition temperature (T_{T(NH4) 3AlF6} = 217K; T_T(NH43FeF6 = 264K. In the low-temperature phase a polarization current of about 10^-9A is obtained and can be reversed when the sign of the polarization field is changed, a property which could correspond to a ferroelectric behavior. However, no pyroelectric current is detected when the temperature decreases from T_T. Another hypothesis, based on a field-induced polarization, has been considered : the depolarization current could be due to charge displacements from potential minima favored by rising temperature. In any way, the low-temperature phase is characterized by a remanent polarization.     

High magnetic field behavior of MEM(TCNQ)2

The charge-transfer complex MEM(TCNQ)₂ is a spin-Peierls system with a non-magnetic, singlet ground state at T=0. We report high-field magnetization data which provide some evidence for a new magnetic spin-Peierls phase at fields above 190 kOe. The experimental results are compared to those for TTFCuBDT.     

EPR studies of linewidth anomalies at phase transitions in [N(C2H5)4]2MnCl4

We have studied [N(C₂H₅)₄]₂MnCl₄ crystal by X-band CW EPR spectra in the temperature range 170-300 K. The angular dependences of linewidth ΔH were measured and described in the light of a double-layer system (2D) with exchange interactions. Two temperature anomalies of linewidth ΔH were found at T₁=225 K and T₂=192 K on cooling. Different behaviors of ΔH anomalies recorded for an external magnetic field parallel and perpendicular to the ab crystallographic plane indicate ordering/disordering of MnCl₄ groups in this plane and their displacement along the c-axis which occurs in the temperature of about 225 K.     

Magnetic susceptibility studies of (CH2)3(NH3)2FeCl2Br2 and (CH2)6(NH3)2FeCl2Br2

The magnetic susceptibility of the layered compounds (CH₂)₃(NH₃)₂FeCl₂Br₂ and (CH₂)₆(NH₃)₂FeCl₂Br₂ has been measured in the range 80 < T < 300 K. The results follow a Curie-Weiss behavior in the range 120 < T < 300 K but are field dependent for T < 120 K. The results are interpreted in terms of a two-dimensional antiferromagnetic interaction which is canted. A comparison with the corresponding pure chloride compounds is given.     

Magnetic phase diagrams of Mn(Nb0.5Ta0.5)2O6

The magnetic phase diagrams of Mn(Nb_0.5Ta_0.5)₂O₆ have been determined by means of neutron diffraction. Two spin-flop-transitions G_xA_z → A_xG_z → G_y have been detected in a magnetic field parallel to the x-direction. The first phase boundary increases as the temperature rises up to a triple point at 4.74 K and 20.5 kOe, whereas the second phase boundary decreases until another triple point at 4.55 K and 35.5 kOe is reached. In fields parallel to the z-direction a transition G_xA_z → G_xG_z has been revealed, the phase boundary meets the paramagnetic one in a triple point at 4.65 K and 55 kOe.     

The heat capacity of the layer compounds (CH3CH2CH2NH3)2MnCl4 and (CH3CH2CH2NH3)2CdCl4 from 10 K TO 300 K

The heat capacity of the layer compounds tetrachlorobis (n-propylammonium) manganese II and tetrachlorobis (n-propylammonium) cadmium II, (CH₃CH₂CH₂NH₃)₂MnCl₄ and (CH₃CH₂CH₂NH₃)₂CdCl₄ respectively, has been measured over the temperature range 10 K ?T ? 300 K.Two known structural phase transitions were observed for the Mn compound in this temperature region: at T = 112.8 ± 0.1 K (ΔH_t= 586 ± 2 J mol^?1; ΔS_t = 5.47 ± 0.02 J K^?1mol^?1) and at T =164.3 ± (ΔH_t = 496 ± 7 J mol^?1; ΔS_t =3.29 ± 0.05 J K^?1mol^?1). The lower transition is known to be from a monoclinic structure to a tetragonal structure, while the upper is from the tetragonal phase to an orthorhombic one. From comparison with the results for the corresponding methyl Mn compound it is deduced that the lower transition primarily involves changes in H-bonding while the upper transition involves motion in the propyl chain.A new structural phase transition was observed in the Cd compound at T= 105.5 ± 0.1 K (ΔH_t= 1472.3 ± 0.1 J mol^?1; ΔS_t = 13.956 ± 0.001 J K^?1mol^?1), in addition to two transitions that have been observed previously by other techniques. The higher of these transitions(T = 178.7 ± 0.3 K; ΔH_t = 982 ± 4 J mol^?1 ΔS_t = 6.16 ± 0.02 J K^? mol^?1) is known to be between two orthorhombic structures, while the structural changes at the lower transition (T= 156.8 ± 0.2 K; ΔH_t = 598 ± 5 J mol^?1, ΔS_t = 3.85 ± 0.03 J K^?1 mol^?1) and at the new transition are not known. It is proposed that these two transitions correspond respectively to the tetragonal to orthorhombic and monoclinic to tetragonal transitions in the propyl Mn compounds.In addition to the structural phase transitions (CH₃CH₂CH₂NH₃)₂MnCl₄ magnetically orders at t? 130 K. The magnetic contribution to the heat capacity is deduced from the heat capacity of the corresponding diamagnetic Cd compound and is of the form expected for a quasi 2-dimensional Heisenberg antiferromagnet.     

metamagnetic behavior of linear chain pyridine compounds: Co(Pyridine)2Cl2, Fe(Pyridine)2Cl2, Fe(Pyridine)2(NCS)2 and Ni(Pyridine)2Cl2

Magnetic phase transitions in the pyridine (pyr) compounds Co(pyr)₂Cl₂, Fe(pyr)₂Cl₂, Fe(pyr)₂(NCS)₂ and Ni(pyr)₂Cl₂ have been observed at applied magnetic fields of ～0.7, 0.7, 1.1 and 2.7 kG respectively. These low field phase transitions are observed in the Fe and Ni compounds at T = 4.2 K, and in the Co compound at T < 3K, and are consistent with metamagnetic behavior. Magnetic saturation is not achieved in any of these compounds for fields of 60 kG, reflecting high anisotropy.     

The NMR study of the mixed compound (CH3NH3)2Cu(Cl0.1Br0.9)4

The mixed compound (CH₃NH₃)₂Cu(Cl_0.1Br_0.9)₄ which has random bonds of ferromagnetic and antiferromagnetic interactions has been investigated by the pulsed NMR of ¹H, ^63,65Cu and ^79,81Br. The temperature dependence of the line width of ¹H showed the existence of the magnetic phase transition at 15 K in the applied field of 4 kOe. The moments of Cu²⁺ have been shown to lie within the c-plane and have some randomness from the field dependence of the NMR spectrum and the spin-echo decay time of the copper and the bromine nuclei.     

Field dependent susceptibility of the 2D ferromagnet (CH3NH3)2CuCl4 in the paramagnetic phase

The temperature dependence of the ac susceptibility (χ) at constant applied magnetic field was investigated in the paramagnetic region of the quasi-2D ferromagnet (CH₃NH₃)₂CuCl₄. Above the Curie temperature (T_C=8.85 K) a maximum in the χ(T,H) curves was observed at T_m(H). The temperature at the maximum increases with increasing applied field. This anomaly is related to short range fluctuations close the order transition. The behavior of T_m(H) is governed by the gap exponent of the scaling function (Δ=γ+β). We found Δ=2.2±0.1 in very good agreement with the previously known values of γ and β.     

Zero-point spin reduction in a two-dimensional Heisenberg antiferromagnet Cu(HCOO)2 · 4H2O

The copper NMR in a two-dimensional Heisenberg antiferromagnet Cu(HCOO)₂ ·4H₂O (T_N = 17 K) was observed using a standard spin- echo method at liquid He temperatures. The field-dependence of the ⁶³Cu NMR frequency was measured for H₀ (? 56 kOe) | b-axis. The result was interpreted by taking into account a zero-point spin reduction of about 35% in addition to the rotation of the antiferromagnetic spin axis. The amount of the observed spin reduction is in good agreement with the prediction of spin-wave theory.     

Two-dimensional critical behaviour in antiferrodistortive Al ur6(CIO4)3 and Ga ur6(ClO4)3

Critical behaviour with dimensionality d = 2 has been observed for the 300 K antiferrodistortive phase transition in Al ur₆(ClO₄)₃ and Ga ur₆(ClO₄)₃ by means of the temperature dependence of the ESR parameter D. The systems exhibited d = 2 behaviour in the static critical behaviour for T<T_c?40 K for T>T_c + 40 K. From the ESR data including line width measurements the local order parameter relaxation rate ω₁ has been obtained for various temperatures above T_c, with a lowest value of ω₁ = 150 MHz at T_c + 15 K     

The heat capacity of the layer compound tetrachlorobis (methylammonium) manganese—II (CH3NH3)2MnCl4, from 10 to 300k

The heat capacity of the layer compound, tetrachlorobis (methylammonium) manganese II, (CH₃NH₃)₂MnCl₄, has been measured over the range 10K <T<300K. In this region, two structural phase transitions have been observed previously by other techniques: one transition is from a monoclinic low temperature (MLT) phase to a tetragonal low temperature (TLT) phase, and the other is from TLT to an orthorhombic room temperature (ORT) phase. The present experiments have shown that the lower transition (MLT→TLT) occurs at T = 94.37±0.05K with ΔH_t = 727±5 J mol^?1 and ΔS_t = 7.76±0.05 J K^?1 mol^?1, and the upper transition (TLT→ORT) takes place at T = 257.02±0.07K with ΔH_t = 116±1J mol^?1 and ΔS_t = 0.451±0.004 J K^?1mol^?1. These results are discussed in the light of recent measurements on (CH₃NH₃)₂CdCl₄, and also with regard to a recent theoretical model of the structural phase transitions in compounds of this type.In addition to the structural phase transitions, (CH₃NH₃)₂MnCl₄ also undergoes magnetic ordering at T < 150K. The magnetic component to the heat capacity, as deduced from a corresponding states comparison of the heat capacity of the present compound with that of the Cd compound, is shown to be consistent with the behaviour expected for a quasi 2-dimensional Heisenberg antiferromagnet.     

准二维电荷密度波导体磷酸钨青铜(PO2)4(WO3)2 m(m=6)的磁电阻研究

对准二维电荷密度波导体磷酸钨青铜(PO₂)₄(WO₃)_2m(m=6)在2—300K温区内的磁电阻及在2K时的Δρ/ρ_０-B关系进行了 实验研究.利用电子磁击穿模型对其低温端的磁阻增强行为进行了分析解释,理论和实验相 符合,并估算出高温端第一个Peierls能隙的大小为3.0meV,电子在低温下的迁移率为0.0 42m²V^-1s^-1 关键词： 低维导体磁电阻 电荷密度波     

Absorption spectra of Ni2+ in (NH4)2Mg(SO4)2·6H2O and Co2+ in Na2Zn(SO4)2·4H2O

Optical absorption spectra of Ni²⁺ in (NH₄)₂Mg(SO₄)₂·6H₂O and Co²⁺ in Na₂Zn(SO₄)₂·4H₂O single crystals have been studied at room and liquid nitrogen temperatures. From the nature and position of the observed bands, a successful interpretation could be made assuming octahedral symmetry for both the ions in the crystals. The splitting observed for ${}^3\text{T}{}_{\mathrm{1g}}\text{(F)}$ band in Ni²⁺ and ${}^4\text{T}{}_{\mathrm{2g}}\text{(F)}$ band in Co²⁺ at liquid nitrogen temperature have been explained as due to spin-orbit interaction. The extra band observed at 16,325 cm^-1 in the case of Ni²⁺ at low temperature has been interpreted to be the superposition of vibrational mode of SO^2-₄ radical on ${}^3\text{T}{}_{\mathrm{1g}}\text{(F)}$ band. The observed band positions in both the crystals have been fitted with four parameters B, C, Dq and ζ.     

Neutron diffraction study of Ho(Rh0.3Ir0.7)4B4

Using powder neutron diffraction techniques, we have examined the magnetic order of the pseudoternary compound Ho(Rh_0.3Ir_0.7)₄B₄ below the Néel temperature T_N=2.7K. The magnetic structure consists of stacked antiferromagnetic basal plane sheets forming a body centered tetragonal unit cell, with a sublattice magnetization corresponding to 9.6±0.6μ_B per Ho³⁺ion at 1.5 K. Magnetic intensity versus temperature measurements indicate that the transition is second order and reveal no anomalous effects when the compound becomes superconducting at T_c=1.34K.     

Antiferromagnetic resonance in ferroborate NdFe3(BO3)4

The AFMR spectra of the NdFe₃(BO₃)₄ crystal are measured in a wide range of frequencies and temperatures. It is found that by the type of its magnetic anisotropy the compound is an “easy-plane” antiferromagnet with a weak anisotropy in the basal plane. The effective magnetic parameters are determined: anisotropy fields H_a1=1.14 kOe and H_a2=60 kOe and magnetic excitation gaps Δν₁=101.9 GHz and Δν₂=23.8 GHz. It is shown that commensurate-incommensurate phase transition causes a shift in resonance field and a considerable change in absorption line width.At temperatures below 4.2 K nonlinear regimes of AFMR excitation at low microwave power levels are observed.     

Phase transition in the Heisenberg ferromagnet Cu(NH4)2Br4·2H2O in a magnetic field

The effect of a magnetic field on phase transitions in the Heisenberg ferromagnet Cu(NH₄)₂Br₄·2H₂O is investigated. It is found that the singularity shift of the susceptibility χ(P, T) in a magnetic field is approximated by power functions with the indexes ω = 2.5 and ? = 0.58.