Layered manganese-halide compounds exhibit quasi-two-dimensional magnetic behavior in the temperature region immediately above the ordering temperature where soliton excitation can be experimentally detected as an Arrhenius, exp(E/T), temperature-dependent electron paramagnetic resonance linewidth, where E is the soliton energy. When a nonmagnetic impurity such as magnesium is introduced into the Mn lattice, experimental linewidth data indicate that the excitation energy is dramatically reduced and the temperature range over which Arrhenius behavior is observed is widened to include higher temperatures. These effects occur for very small (less than 1%) impurity concentrations.

It is shown that the introduction of a very small amount of nonmagnetic impurities into the magnetic sites of a classical two-dimensional antiferromagnet creates a new type of static (impurity-pinned) soliton that affects the Arrhenius, exp(-E/T), temperature-dependent electron paramagnetic resonance linewidth by drastically changing the parameter E. Data just above the transition temperature for (C3H7NH3)(2)MxMn1-xCl4 confirm the existence of these impurity-pinned solitons.

Recent theoretical and experimental work confirm the existence of solitons in the nearly classical two-dimensional (2D) Heisenberg antiferromagnet. Previous ac susceptibility measurements on (n-propyl ammonium)2MnCl4 or (PAMC), a 2D spin 5/2 antiferromagnet AFM shows that the susceptibility is strongly dependent on a weak magnetic field (≈0.2 Oe) at temperatures below TC. These data provided the motivation for a calculation of the field dependence of the static soliton structure factor. This is done by assuming the small field dependent term as a perturbation around the static soliton solution. The dynamic part of the correlation function is assumed to result from soliton–magnon scattering in the Born approximation. These field-dependent and dynamic results are combined to obtain the dynamic field-dependent correlation function.

Modulated microwave absorption (MMA) spectra from Josephson junction formations on a scratched Nb wire have been studied at 9.3 GHz and 4 K. The peak-to-peak separation, delta H of the Josephson lines was found to vary linearly with P-1/2, where P is the applied microwave power, in contrast to a recent interpretation of junction formation in pressed lead pieces by Rubins, Drumheller, and Trybula. The interpretation of the MMA data on Nb are given in terms of the theory of Vichery, Beuneu, and Lejay for superconducting loops containing weak links.

It has previously been assumed that spin waves were the dominant excitations in lower-dimensional magnets. Recently, however, it has been shown that nonlinear excitations or solitons rather than spin waves influence the dynamic thermal quantities such as the spin correlation function which can be investigated experimentally through the electron paramagnetic resonance linewidth. In this review the influence of both spin waves and solitons on the temperature-dependent linewidth in the fluctuation region immediately above the ordering temperature is discussed. It is seen that both excitations result in a theoretical Arrhenius temperature-dependence, (∆H~ exp (E/T) where E=6πJs 2 for spin waves and E=4πJs 2 for solitons, J is the nearest neighbor exchange constant, and s is the value of the spin. In experiments, quantum (s=1/2) layered copper compounds exhibit the temperature dependence expected from spin waves even though nonlinear excitations have been shown to exist in these systems. On the other hand nearly classical (s=5/2) manganese compounds have the temperature dependence expected from solitons. The calculation of the linewidth from both spin waves and solitons is reviewed and compared with experimental data to show that solitons dominate the dynamics of the layered, nearly classical magnet.

We have studied several quasi-one-dimensional spin 1/2 ferromagnets ranging from Heisenberg to Ising anisotropy to test the Johnson and Bonner prediction that the low temperature thermodynamic behavior of these systems in external fields depends on the exchange anisotropy and that there are certain ranges of the anisotropy and fields in which bound magnons or spin waves dominate thermodynamic properties. The experimental results confirm that for the samples tested bound magnons dominate susceptibility across available range of the anisotropy (γ<1.05) and for fields up to 9 T near the Heisenberg limit.

The title compound, (C4NH10)CuCl3, is shown to exist in two phases, the previously reported alpha phase and a new beta phase. DSC studies indicate that the beta phase transforms to the a phase at 93 degrees C. The beta phase is stable at the room temperature, with the a phase metastable with respect to the beta phase at room temperature. Crystals of the beta phase are monoclinic, C2/c, with a 17.327(3) Angstrom, b = 8.360(2) Angstrom, c = 12.005(2) Angstrom, and beta 100.92 (3)degrees with Z = 8 for p = 1.883 g/cm(3). The structure contains chains of bibridged Cu2Cl62- dimers running parallel to the c direction. This is in contrast to the previously reported a form, which contains uniform chains of face-shared octahedra. The chains in the alpha phase lie parallel to the unique (monoclinic) axis. Thus, no crystallographic relationships exist between the two compounds and the phase transition must be first order in nature. A unique feature of the phase transition is a decrease in volume as the crystal is heated through the transition. This is a result of the formation of the more compact chains of face-shared octahedra in the high-temperature phase, Magnetic susceptibility studies of the beta phase are indicative of competing ferromagnetic and antiferromagnetic coupling, with the onset of long-range order at 7.5 K. The data are interpreted in terms of a ladder chain consisting of ferromagnetic dimers coupled into antiferromagnetic chains through short Cl-... Cl contacts.

Experimental evidence is presented for the existence of nonlinear localized spin excitations (solitons) in the classical two-dimensional Heisenberg magnet doped with nonmagnetic impurities. These solitons are shown to provide the dominant mechanism for spin relaxation in electron paramagnetic resonance linewidths immediately above the ordering temperature for layered Mn compounds with a small impurity concentration. The new gapless excitations are pinned by the nonmagnetic impurities thereby becoming topologically unstable and lower in energy than the nonlinear excitations (skyrmions) that may exist in the pure material. Concentration-dependent results preclude a spin-wave mechanism for this effect.

The temperature dependence of the high-field spin cluster resonance observed in the one-dimensional ferromagnet FeTAC at low temperatures is shown to depend on the numbers of ferrous ions in the chains. The longer the chain, the lower is the temperature at which deviations from the previously obtained linear relationship between In(intensity) and 1/T occur. Calculations for the distribution of chain lengths obtained by the random substitution of a nonmagnetic impurity are made for several impurity concentrations and compared with the experimental data points for a pure FeTAC sample. The results indicate that, in this case, other types of crystal defect must limit the chain lengths.

It is shown that collective glassy behavior occurs not only in strongly substitutionally disordered systems as assumed so far, but also in weakly disordered systems, namely, Rb1-x(NH4)xH2AsO4 with x = 0.01 and x = 0.02. The observed As-75 NQR and NMR line shapes can only be described quantitatively by the compressible random bond-random field pseudospin Ising model, showing a much greater random bond than random field effect even for x --> 0. A discontinuous change in the Edwards-Anderson order parameter has been observed at the transition temperature where ferroelectric long range ordering occurs as predicted by theory.

Electron paramagnetic resonance measurements at 36 GHz and in the range 12–18 GHz were made on a single crystal of the 1D Ising ferromagnet [(CH3)3 NH] FeCl3⋅2H2O, known as FeTAC, containing a nominal 10% of the isomorphous compound CoTAC. While the 4.2 K spectra obtained with the external field parallel to the chain axis were similar to the spin cluster resonance spectra previously identified in pure FeTAC, differences in the low-field satellite structure were observed, which are tentatively attributed to differences in the dipolar interaction and to shorter ferrous chains. An appreciable decrease in the strength of the demagnetizing field is associated with the tendency of the Co2+ moments to align perpendicularly to the chain axis.

The magnetic properties of the powdered layered structures of 3-ammoniumpyridinium tetrabromocuprate(II) and 3-ammoniumpyridinium tetrachlorocuprate(II) have been studied from 4.2 to 150 K. The data were interpreted using both 1D and 2D series expansions for the Heisenberg model, combined with the appropriate mean-field corrections, yielding interlayer exchanges with J2h/k values of (−11±2) K for the chloride salt and (−52±7) K for the bromide salt. The intralayer exchange (J1h/k) values were found to be (14±2) K and (20±2.5) K for the chloride and bromide salts, respectively. The new results are compared with previous results for the eclipsed layered structure series NH3(CH2)nNH3CuX4, where X=Br or Cl and n=2, 3, 4, or 5. As the halide-halide separation distance decreases the transition from magnetic isolation of the layers to a strong interlayer exchange is observed. The new study allows confirmation of the power dependence on the halide-halide separation.

The electron-paramagnetic-resonance technique has been used to obtain an unambiguous identification of resonances between spin-cluster states from the distinctive satellite spectra observed in the ranges 10-36 GHz and 2.6-4.2 K from a large single crystal of the one-dimensional Ising ferromagnet [(CH3)3NH]FeCl3.2H2O, known as FeTAC. The frequency and temperature dependencies of the spectra are explained by the addition of the term DELTAS(x) to the Zeeman term in the effective S = 1/2 spin Hamiltonian describing the first set of excited cluster states. The spin-cluster levels are the eigenvalues of the resulting tridiagonal matrix, and lead to a satellite structure in FeTAC which is independent of the chain length for chains of more than about 30 Fe2+ ions.

Magnetic resonance spectra at 12–36 GHz are reported for Fe2+(3d6, S=2) in the 1D Ising ferromagnet [(CH3)3NH]FeCl3⋅2H2O, known as FeTAC, between 2.5 and 13 K. The main signal was identified as a spin-cluster resonance (SCR) because of its characteristic temperature dependence below 6 K and the linear dependence of the resonance field on the microwave frequency for H parallel to the easy axis at 4.2 K. The temperature dependence of the line intensity was fitted to the function exp[−(2J+hν)/kBT], with 2J/kB=(41±1) K, and the linear field dependence was explained by the value gz=8.26±0.05 in the S=1/2 representation and a demagnetization field of several hundred gauss, which depended on the sample shape. A frequency-dependent satellite structure observed on the low field side of the main line, which appeared as a broad line below 15 GHz, is tentatively associated with energy shifts of the lower-lying spin-cluster states produced by the magnetic dipole interaction and other terms in the Hamiltonian.

The EPR linewidths of (C3H7NH3)2Mn1−xCdxCl4 have been measured in the paramagnetic region as a function of temperature. An Arrhenius behavior exp(b/T) is observed with b strongly dependent on Cd concentration. This behavior in the pure compound is characteristic of soliton-like (skyrmion) excitations with b related to the excitation energy. In the doped compound, gapless nonlinear excitations are possible, resulting in a much different temperature-dependent linewidth. It is concluded that skyrmions dominate the low-temperature thermodynamics of two-dimensional magnets.

The electron paramagnetic resonance (EPR) spectra of NH+ radicals in NH4H2AsO4 (ADA) and mixed Rb1-x(NH4)xH2AsO4 (RADA) (x = 0.35) in the temperature region 4.2 to 300 K are measured. It is shown that in ADA at room temperature (RT) NH3+ molecules undergo fast rotation around the principal axes of the crystal field tensor, and make rapid jumps between them. These motions provide nearly isotropic averaging of the spectra. Below T(N) = 216 K rapid rotation only around the a- and b-axes is present. In contrast to ADA, in RADA the NH3+ groups rotate rapidly only around the a- and b-axes even at RT and inversion tunneling of nitrogen is suggested. The hyperfine structure of the ammonia proton is isotropically averaged down to 115 K in ADA and down to 60 K in RADA.

Zero-field ac susceptibility and isothermal magnetization of the quasi-2D alkanediammonium copper tetrahalide series, [NH3(CH2)nNH3]CuX4, where n=4, 5, 6, and 10 with X=Cl and Br, are reported. The 3D antiferromagnetic ordering temperature Tc for the Cl compounds is determined. It is shown that the critical susceptibilities decay exponentially as the temperature increases (T≳Tc). A power-law divergence at Tc is seen in the Br compounds with n=7 and 10. This behavior is characteristic of 3D ferromagnetic ordering at Tc. The critical exponent γ for the initial susceptibility (T≳Tc) has been obtained for the Br compounds. It is found that there is a second (minor) peak below Tc in the Br compounds with n=5 and 7. The transition associated with this peak may be interpreted as a long range (spontaneous) ordering due to very small spin anisotropies, such as a spin canting between the layers. In the Br compounds the isothermal magnetization is suppressed in low fields, and the value of the critical exponent δ estimated from the isothermal data is considerably smaller than that given by standard models. Spatial- and spin-dimensionality crossovers are apparent in the both initial susceptibility and isothermal magnetization data.

The syntheses, crystal structures, and powder magnetic studies of several new quasi-planar bibridged Cu(n)X(2n+2)(2-) oligomers (n = 3, 4, 6, and 7; X = Cl or Br) are reported, based on the 1-methylpyridinium (C6H8N)(+) and 1,2-dimethylpyridinium (C7H10N)(+) cations. These include (C7H10N)(2)Cu3Br8, (C6H8N)(2)Cu4Cl10, (C7H10N)(2)Cu6Cl14, and (C7H10N)(2)Cu7Br16. Crystallographic data: (C7H10N)(2)Cu3Br8, triclinic, space group P (1) over bar, a = 7.947(2) Angstrom, b = 8.799(2) Angstrom, c = 9.840(2) Angstrom, alpha = 86.95(2)degrees, beta = 76.23(2)degrees, gamma = 71.54(2)degrees, V = 633.6(3) Angstrom(3), Z = 2, d(x) = 2.78 g/cm(3), and R = 0.0483; (C6H8N)(2)Cu4Cl10, monoclinic, space group P2(1)/n, a = 11.759(2) Angstrom, b = 9.056(2) Angstrom, c = 12.048(3) Angstrom, beta = 106.21(2)degrees, V = 1232.1(5) Angstrom(3), Z = 2, d(x) = 2.15 g/cm(3), and R = 0.0321; (C7H10N)(2)Cu6Cl14, triclinic, P (1) over bar, a = 8.997(3) Angstrom, b = 9.288(3) Angstrom, c = 11.540(4) Angstrom, alpha = 80.53(2)degrees, beta = 67.82(2)degrees, gamma = 60.22(2)degrees, V = 714.7(4) Angstrom(3), Z = 1, d(x) = 2.34 g/cm(3), and R = 0.0363; (C6H8N)(2)Cu7Br16, triclinic, P (1) over bar, a = 7.237(2) Angstrom, b = 10.880(2) Angstrom, c = 12.880(2) Angstrom, alpha = 89.47(2) Angstrom, beta = 75.08(2)degrees, gamma = 79.48(2)degrees, V = 962.7(3) Angstrom(3), Z = 1, d(x) = 3.35 g/cm(3), and R = 0.0520. A common feature of the structures is the existence of oligomers containing quasi-planar symmetric bibridged finite chains of edge-sharing CuX(4), (X = halide) monomeric units. The n = 3 oligomers aggregate into chains through the formation of asymmetric bibridged linkages between terminal copper ions on adjacent trimers. In the n = 4 salt, the oligomers aggregate into stacks in which pairs of the copper ions extend their coordination sphere by forming a long, semicoordinate bond to a halide ion from a neighboring oligomer. For both the n = 3 and n = 4 salts, the pyridinium cations lie parallel to and directly above and below the anionic oligomers, separating the chains. In the n = 6 rind n = 7 salts, the stacks formed in this manner interdigitate, forming two-dimensional slabs. The slabs are separated by the organic cations. The magnetic properties of compounds are dominated by antiferromagnetic intraoligomer interactions. Thus, the n = 4 and n = 6 salts depopulate into singlet ground states at low temperature. In contrast, the n = 3 and n = 7 oligomers have S = 1/2 ground states. Expressions for the magnetic susceptibility of the n = 6 and n = 7 oligomers were obtained by diagonalization of a nearest neighbor Heisenberg Hamiltonian. The data were fit to these expressions, with inclusion of a mean field correction for interoligomer exchange. The intra-oligomer exchange coupling constants are J(1)/k = -153 K for n = 3; J(1)/k = -60 K, J(2)/k = -40 K for n = 3; J(1)/k = -23 K, J(2)/k = -30 K, and J(3)/k = -52 K for n = 5 and J(1)/k = -90 K, J(2)/k = -90 K and J(3)/k = -120 K.At low temperature, the n = 3 oligomer exhibits ferromagnetic behavior. Since the oligomer has depopulated to a spin 1/2 ground state, the system can be modeled as a spin 1/2 ferromagnetic chain with J'/k = 22.8 K.

The structure and magnetic properties and EPR spectra are reported for two copper chloride complexes of CuCl64-stoichiometry. (1-Methylpiperazinium)2CuCl6 belongs to the triclinic space group P1BAR with a = 12.448 (8) angstrom b = 11.891 (8) angstrom, c = 6.577 (4) angstrom, a = 84.16 (1)-degrees, beta = 86.74 (1)-degrees, gamma = 93.56 (1)-degrees, and Z = 2. The structure contains isolated cations, CuCl42- anions, and lattice Cl- ions. The CuCl42- anions have nearly D2d symmetry with an average trans Cl-Cu-Cl angle of 145.8-degrees. The crystals of (piperazinium)2CuCl6.MeOH are tetragonal, of space group P4(2)/n, with a = 13.597 (8) angstrom, c = 11.376 (8) angstrom, and Z = 4. The structure is composed of isolated piperazinium cations, Cl- ions, and methanol molecules and chains of nearly square pyramidal CuCl53- anions. The primary distortion of the CuCl53- species away from idealized C4v symmetry involves a small C2v distortion im basal plane (trans Cl-Cu-Cl angles = 166.8 (1) and 178.2 (1)-degrees, respectively). The basal Cu-Cl distances average 2.313 angstrom, while the apical Cu-Cl distance is 2.641 (3) angstrom. These anions are linked together in chains via extremely long interactions of 3.936 (3) angstrom between the apical Cl of one anion and the Cu atom of its neighbor. Magnetic susceptibility data are indicative of weak antiferromagnetic coupling in this latter compound. Analysis of the EPR line widths is consistent with a two-dimensional magnetic system. These results are interpreted in terms of a two-halide exchange pathway of the type Cu-Cl...Cl-Cu between chains.

Powder magnetic susceptibility measurements on two new, low-dimensional copper-halide systems are reported: (piperazinium)2CuCl6·CH3OH, and (1,2-dimethylpyridinium)2Cu3Br8. The data are analyzed using the high- and low-temperature series expansions for a 1D,S = ½, Heisenberg Hamiltonian, with a mean-field correction to model the interaction between chains. The piperazinium system shows very weak antiferromagnetism with a possibility of two-dimensional behavior. The pyridinium trimer system shows more strongly ferromagnetically coupled doublet ground states between trimers, which comprise the chain.