Solitons in one-dimensional antiferromagnet with uniaxial symmetry

\u3cp\u3eDynamical solitons in a one-dimensional antiferromagnet with both a uniaxial anisotropy and an applied magnetic field along the uniaxial axis are investigated theoretically. It is shown that two types of solitons, sine-Gordon kink solitons and pulse solitons are possible. The crossover between both types of solitons is discussed.\u3c/p\u3

Field dependence of the thermal conductivity of CoBr2.6H2O

\u3cp\u3eThe thermal conductivity of CoBr26H2O has been measured in the temperature range 1.5-30 K and in magnetic fields up to 90 kOe. The specific field dependence of the data, especially with the field along the easy axis, indicates that the magnons do not contribute to the heat transport, but mainly act as a scattering source for the phonons. The data can be understood qualitatively by confronting them with calculated field-dependent magnon spectra. The actual scattering mechanism seems to be governed by the two-magnonone-phonon process.\u3c/p\u3

Stabilization of metastable expanded face-centered-tetragonal manganese

The structural and magnetic properties of Mn prepd. on single cryst. face-centered-tetragonal (fct) Co(001) were investigated. Mn grows coherently up to at least 50 monolayers (ML) and adopts a metastable expanded fct(001) phase [c/a=1.055(5)]. This new fct-Mn phase was recently predicted theor. by Hafner and Spisak [Phys. Rev. B 72, 144420 (2005)]. Studies of magnetic Mn/Co interface exchange interactions prove the room temp. antiferromagnetic state for thicknesses above 2.5 ML. The magnetic anisotropy of the thin Mn is high enough to induce a significant exchange anisotropy for Mn thicknesses as low as 6 ML. The potential of fct-Mn to become a novel model system for systematic studies on the exchange interactions at antiferromagnet/ferromagnet interfaces is discussed. [on SciFinder (R)

Granular Co/Ag Multilayers : relation between nanostructure, and magnetic and transport properties

The relation between structural properties, magnetization, and magnetoresistance of sputtered Co/Ag multilayers is studied in as-deposited and annealed multilayers with thin Co layers. Nuclear-magnetic-resonance experiments and magnetization measurements showed that decreasing the nominal Co thickness below 10 Å causes a gradual transition from an anisotropic ferromagnetic layered system to a granular system consisting of a layered array of Co pancakes. Annealing extends the granular regime to larger Co thicknesses, reduces the roughness, and transforms the Co clusters to a more isotropic shape. The effect of these structural changes on magnetization and magnetoresistance is discussed

Magnetic interactions in CoBr2·6H2O

\u3cp\u3eHigh-precision measurements of the heat capacity of CoBr2·6H2O were performed between 1.2 and 11 K. A fit of the high-temperature magnetic specific heat to the high-temperature-series expansion for a two-dimensional XY antiferromagnet yielded Jk=-2.5 K. A combination with antiferromagnetic-resonance results gave J1xx=-2.4 K, J1yy=-2.3 K, J1zz=-0.3 K for the intersublattice interaction and J2xx=-0.5 K, J2yy=-0.5 K, and J2zz=-0.07 K for the intrasublattice interaction. A calculation of the Curie constant, the perpendicular susceptibility, and the paramagnetic phase transition based upon these values gave satisfactory agreement with the experimental data. From the critical entropy (57%), as well as the critical behavior of the sublattice magnetization (β=0.31), a somewhat bidimensional character could be concluded. The effect of deuteration will be briefly considered.\u3c/p\u3

Substitution effects of Bi2Sr2Ca(Cu1-xMx)2O8+z (M=Co,Ni) single crystals

Single crystals of Bi2Sr2Ca(Cul_,M,)20a+ z superconductors doped with Co and Ni have been prepared by the travelling-solvent-floating-zone (TSFZ) method. The measured concentration of M/Cu is 2.3%, 4.010, 5.5% for M=Co and 2%, 5% for M=Ni. It is found that the c-axis shrinks with the increasing concentration of the doping element. The critical temperature T, is sharply depressed by the substitution of both elements with an initial rate of about 5 K per percentage doping. The influence of the doping elements as impurities on the different pinning regimes of Bi-2212 single crystals is discussed. Refereed version of the contributed paper presented at the 1995 T aiwan International Conference on Superconductivity, August 8-11, 1995, Hualien, Taiwan, R.O.C

Single-ion anisotropy of localized-electron compounds

We present a computational analysis of the influence of taking the full multiplet structure into account in calculations of the single-ion contribution to the magnetocrystalline anisotropy (MCA) of 3d transition-metal (TM) ions in localized-electron compounds at T=0 K. For atoms with dn (n=1–4,6–9) configurations, at sites with a local cubic, tetragonal, or trigonal symmetry the single ion MCA on the basis of the Hund’s rule ground state term only [crystal field (CF) theory] is compared with the single-ion MCA on the basis of a fully relativistic first principles atomic theory including the intra-atomic d-d interaction. Solid state effects are taken into account by effective crystal fields and the exchange field. Under certain, realistic, conditions the use of the full multiplet theory is shown to have a significant effect on the calculated single-ion MCA. We also discuss the effect on the overall cubic anisotropy constants for cubic crystals containing transition metal atom sublattices for which the d-metal atoms are located on sites with a local tetragonal and trigonal symmetry. Possible refinements of the theory are discussed

Classical aspects of quantum spin chains (abstract)

\u3cp\u3eA variational estimate of the free energy of a quantum spin chain based on the stochastic behavior of the quantization axes is proposed. It allows a description of the nonlinear excitations and quantum spin waves within the same framework. For an easy-plane ferromagnet with an external field in the plane it reveals that the nonlinear excitations (solitons) are predominantly described by the classical degrees of freedom. Considerable effort has been devoted to the theoretical description\u3csup\u3e1\u3c/sup\u3e and experimental verification of the existence of nonlinear excitations. In this field magnetic chain systems have played an important part.\u3csup\u3e2\u3c/sup\u3e Chain systems with an easy-plane anisotropy, such as (C\u3csub\u3e6\u3c/sub\u3e H\u3csub\u3e11\u3c/sub\u3e NH\u3csub\u3e3\u3c/sub\u3e) CuBr \u3csub\u3e3\u3c/sub\u3e (CHAB) and CsNiF\u3csub\u3e3\u3c/sub\u3e, which are subjected to a symmetry breaking field, were found to be soliton bearing.\u3csup\u3e3\u3c/sup\u3e Theoretically the description of the nonlinear effect is based on the mapping of the spin dynamics on a sine-Gordon model. Further research\u3csup\u3e4\u3c/sup\u3e indicated that the interpretation of the thermodynamical properties of real systems in terms of the sine-Gordon model required however quantum and anisotropy corrections. Since the dipolar interactions between the spins in the chain are considered to be negligible, the direction of the spin chain in configuration space with respect to any preferred direction in spin space does not enter explicitly in the Hamiltonian. This makes it interesting to define a site-dependent z direction in spin space which makes an angle (θ\u3csub\u3en\u3c/sub\u3e, φ\u3csub\u3en\u3c/sub\u3e) with the z direction in configuration space. The model Hamiltonian can then be written in terms of the spin operators defined with respect to the site-dependent quantization axis. The representation with equal quantization axis for all the sites is related to the representation with different quantization axes at each site by a rotation in spin space. The unitary transformation that describes the rotation contains the angles (θ\u3csub\u3en\u3c/sub\u3e, φ\u3csub\u3en\u3c/sub\u3e), which we have treated as random variables. Based on this observation we have proposed a new variational approach for the quantum spin chain. We will outline the main results here. A full account is published elsewhere.\u3csup\u3e5\u3c/sup\u3e In our approach the free energy of the system can be separated in a quantum part and a classical part. The free energy F of the chain is then approximated by an upper bound F<F\u3csub\u3ecl\u3c/sub\u3e +F\u3csub\u3eqm\u3c/sub\u3e, where βF\u3csub\u3ecl\u3c/sub\u3e =-ln[E(exp-βV\u3csub\u3ecl\u3c/sub\u3e)] and βF\u3csub\u3eqm\u3c/sub\u3e =-ln Tr(exp-β〈H\u3csub\u3eqm\u3c/sub\u3e 〉). The first part is a configurational classical free energy, depending on the choice of the probability distribution for the quantization axes via the expectation E. The second contribution is the free energy of a modified quantum chain, whose parameters are classical averages and can be obtained from the configurational classical free energy. For easy-plane ferromagnetic chains, the nonlinear excitations are comprised in the classical part, whereas the quantum part contains predominantly linear excitations. For those thermodynamical quantities that are insensitive to the contribution of the quantum excitations, the chain is thermodynamically equivalent with a classical sine-Gordon system. The out-of-plane corrections are incorporated in the quantum part of the free energy. This behavior is corroborated experimentally by measurements of the excess heat capacity of CHAB for different magnetic fields with the same in-plane component.\u3csup\u3e6\u3c/sup\u3e The validity of our approach is, however, not restricted to easy-plane ferromagnetic chains. Also for more complex systems with, for example, an orthorhombic or helical symmetry, the introduction of stochastic quantization axes can shed new light on some of the thermodynamical properties of these quantum systems in terms of their classical equivalent.\u3c/p\u3

Theoretical and experimental magnetic specific heat of CsMnCl3 • 2H2O

\u3cp\u3eDetailed specific-heat measurements between 1.1 and 52 K on the S=52 Heisenberg linear-chain system CsMnCl3 • 2H2O are reported. The results could be interpreted with the aid of newly developed theoretical estimates for the magnetic contribution and a lattice contribution characteristic for a chemically layered compound. A value of Jk=-3.3±0.3 K is obtained, which will be compared with the results from other studies.\u3c/p\u3

Anisotropic classical chain:Numerical calculations of thermodynamic properties

\u3cp\u3eThermodynamic properties are computed for the classical linear chain with orthorhombic anisotropy in an external magnetic field. Special attention has been given to crossover effects between different model systems as a function of temperature and field. The ordering temperature of quasi-one-dimensional systems is computed as a function of the interchain interactions and the anisotropy. Results are compared with other theories.\u3c/p\u3