Low-temperature Spin Dynamics Research Articles

Spin dynamics in the square-lattice antiferromagnet.

We apply the Schwinger boson mean-field theory to the square-lattice Heisenberg antiferromagnet at low temperatures. For spin \textonehalf{} we confirm the renormalized classical behavior of the correlation length ${\ensuremath{\kappa}}^{\ensuremath{-}1}(T)$ without appealing to spin-wave theory. We also present a detailed calculation of the dynamical structure factor. A quasielastic peak is featured near $\mathbf{q}=(\ensuremath{\pi},\ensuremath{\pi})$, while for $|\mathbf{q}\ensuremath{-}(\ensuremath{\pi},\ensuremath{\pi})|\ensuremath{\gg}\ensuremath{\kappa}$, spin-wave ridges appear. The uniform susceptibility interpolates between spin-wave results at $T=0$ and the high-temperature series of Rushbrooke and Wood. Our theory is distinct from theories in which fermionic spinon excitations determine the low-temperature spin dynamics.

Low temperature spin dynamics in TMMC from nuclear spin-lattice relaxation rate

Measurements of proton spin-lattice relaxation rate in (CH) 4NMnCl 4 (TMMC) in the temperature range (1.2–4.2 K) and for different magnetic fields (0.085-2 T) for both H ∥ c and H ⊥ c are presented and compared with theoretical calculations based on Villain theory for damped paramagnons in 1D magnets. The implication of the disagreement found between theory and experiment is briefly discussed.

A polarised neutron study of linear and non-linear spin fluctuations in CsNiF3

Inelastic neutron scattering and neutron polarisation analysis have been used to study the low-temperature spin dynamics of the one-dimensional (1d) ferromagnet CsNiF3. Measurements were made in the temperature range 2<or=T<or=12K with a magnetic field of 10 kG applied perpendicular to the chain direction. At the lowest temperatures the neutron polarisation induced by single-magnon scattering differs significantly from that expected for an isotropic Heisenberg system, but is in excellent agreement with a recent quantum calculation for a 1d, easy-plane Heisenberg magnet. The experiment demonstrates that the cross-over to isotropic spin dynamics is not complete at the temperature and magnetic fields for which solitons have been reported in CsNiF3. The authors suggest that this fact may be responsible for the agreement between experiment and Mikeska's (1978) sine-Gordon theory of in-plane spin dynamics.

Optical absorption and spin dynamics in the 1d ferromagnet CsNiF3

Low temperature spin dynamics in the anisotropic 1d ferromagnet CsNiF3 results in a linear thermal broadening of one zero phonon line, and in a dramatic decrease of its linewidth when applying a magnetic field parallel to the easy plane. Comparison is made with both experimental (neutron scattering) and theoretical results.

Abstract: Spin dynamics in the linear antiferromagnet TMMC: Effect of a magnetic field at low temperature

We present an investigation of the effect of an applied external magnetic field on the low temperature spin dynamics of the linear antiferromagnet (CH3)4NMnC13 (TMMC). The theoretical results without adjustable parameters are compared with experimental values obtained for H, D, and 14N in the field range 1–77kOe and in the temperature interval 1.5–4K.; in high fields the comparison is satisfactory while at low fields there appears to be an extra contribution to the spin-lattice relaxation not included in the theory. (AIP)

Numerical studies of the low temperature spin dynamics in PdMn (10 at%)

Employing both matrix diagonalization and equation-of-motion techniques we have calculated the density of spin wave modes and the dynamic structure factor for the spin glass PdMn (10 at%). We used a functional form for the exchange interaction suggested by high field magnetization measurements with an overall multiplicative factor adjusted to give agreement with the low temperature specific heat. Our results for the dynamic structure factor for momentum transfer Q=0.8 Å−1 are compared with recent inelastic neutron scattering data.

SPECTRAL EXCITATIONS OF LINEAR MAGNETIC CHAINS

SPECTRAL EXCITATIONS OF LINEAR MAGNETIC CHAINS

Spin waves in random ferromagnets

We consider the low-temperature spin dynamics of substitutionally disordered ferromagnetic alloys. A nearest-neighbor Heisenberg Hamiltonian is assumed. The linear spin-wave approximation is made. However, the disorder is treated exactly by use of a computer-simulation technique which works for very large finite models. Densities of states, partial densities of states, and the scattering law are presented for spins diluted with nonmagnetic atoms and for mixtures of atoms having different exchange couplings. We give detailed results for the simple cubic lattice as well as representative cases of body-centered- and face-centered-cubic lattices. The over-all behavior is discussed in terms of the development of bands of excitations associated with each constituent as the concentrations are varied from the impurity limits. We also compare our calculations with analytical approximations based on effective-medium theory and on exact moments.