Ferromagnet CsNiF3 Research Articles

Solitons in quasi-one-dimensional magnetic materials and their study by neutron scattering

A review is made of theoretical concepts from nonlinear dynamics of quasi-one-dimensional magnetic materials and of experimental investigations of solitons by inelastic neutron scattering and of studies of anomalies of thermodynamic quantities such as the specific heat, magnetization, susceptibility, etc. The main investigated substances are the quasi-one-dimensional ferromagnet CsNiF3 and the antiferromagnet tetramethylammonium manganese chloride (TMMC) which share the same crystal structure. They can be regarded as quasi-one-dimensional magnetic materials with easy-plane anisotropy. The spin dynamics of such a system subjected to an external magnetic field applied in the easy plane can be reduced to the sine-Gordon equation. A detailed analysis is given of the recent experiments on CsNiF3 and TMMC carried out using unpolarized and polarized neutrons and demonstrating that in a certain range of temperatures and fields this dynamics includes nonlinear excitations which can be described qualitatively as solitons of the sine-Gordon equation. Another group of quasi-one-dimensional crystals with easy-axis anisotropy is considered: it belongs to Ising-like magnetic materials. In the absence of an external field these materials exhibit soliton-type nonlinear excitations in the form of antiphase domain walls. The available experimental data confirm the concept of a soliton gas of excitations in quasi-one-dimensional magnetic materials.

The dynamics of longitudinal spin fluctuations

The dynamical processes underlying the longitudinal spin fluctuation spectra in Heisenberg systems are investigated in two distinct physical regimes. Very recent polarized-neutron data in the one-dimensional ferromagnet CsNiF3 have supported our previous prediction of a regime in which longitudinal spectra are dominated by a coupling between spin and exchange energy densities. A quantum-mechanical analysis is found to give results in good quantitative agreement with the experimental line shapes. The second regime investigated deals with much smaller (and less easily accessible) wave vectors. Here, a new hydrodynamic mode (‘‘second magnon’’) may in principle be supported at not too high temperatures. The observability of this feature in real systems (in particular, EuO) is examined.

Magnons and solitons in CsNiF3 investigated by nuclear spin-lattice relaxation

By nuclear spin-lattice relaxation of133Cs we studied linear and nonlinear magnetic excitations in the one-dimensional easy-plane ferromagnet CsNiF3. The measurements were performed in the temperature range from 3 to 20 K and with external magnetic fields from 3 to 55 kOe applied perpendicular to the chain direction. Universal dependence on\(\sqrt H /T\) was obtained forT/T1. A quantitative interpretation of theT1 data in terms of two-magnon and soliton contributions could be achieved by considering renormalization effects resulting from magnon-magnon and soliton-magnon interaction as well as from quantum corrections. The instability of solitons due to out-of-plane fluctuations expected in high magnetic fields is discussed.

Refractive index and birefringence measurements of the short-range order in the one-dimensional Heisenberg ferromagnet CsNiF3

Magnetic short-range order in the one-dimensional Heisenberg ferromagnet CsNiF3 has been investigated by linear magnetic birefringence and optical phase retardation measurements The short-range order was found to extend beyond 120K. The contributions of out-of-plane fluctuations and of the interchain interaction have been recognised. The values of the photoelastic coefficients and of the lattice dilatations are also reported.

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.

Quasielastic neutron scattering at the 1-D ferromagnet CsNiF3 with an applied transverse field

By means of quasielastic neutron scattering we studied the magnetic field induced field induced crossover in CsNiF3 in the transverse magnetic field. The susceptibility at q = 0 shows a broad maximum in its temperature dependance, which is best described by the static properties of the classical sine-Gordon chain.

High energy electronic Raman scattering in the one-dimensional ferromagnet CsNiF3

High energy (6000 to 14000 cm−1) electronic Raman scattering in the one-dimensional ferromagnet CsNiF3 has been studied in the short-range order region above the three dimensional magnetic ordering temperature (2.6 K). Our measurements involve transitions from the 3A2g ground term to the 3T2g and 3T1ga excited terms of the Ni2+ ion 3d8 configuration. Besides broad phonon assisted bands, the spectrum consists of purely electronic sharp lines. The successive moments of the sharp lines depend upon the spin correlation functions, which were experimentally monitored through a temperature or a magnetic field variation. Our experimental results qualitatively agree with the predicted ones.

Raman scattering in the one-dimensional ferromagnet CsNiF3

Reports phonon, magnon and high-energy (6800 to 14000 cm-1) electronic Raman scattering in CsNiF3. One-magnon scattering is observed with an applied magnetic field which ferromagnetically aligns the spins: both experimental frequency and selection rules for polarised spectra are interpreted. The high-energy electronic spectra involve transitions, within the Ni2+ ion (3d)8 configuration, from the 3A2g ground level to the 3T2g and 3T1ga excited states; each transition shows sharp zero-phonon electronic lines in addition to a broad phonon-assisted band. The widths and the peak positions of the sharp lines are related to spin correlation functions which depend upon the temperature and the applied magnetic field, as experimentally observed. The intensities of the polarised spectra are also discussed in terms of spin correlation functions for all the irreducible representations which can be assigned to the excited states.

Optical absorption and static spin correlation functions in the one-dimensional ferromagnet CsNiF3

The authors report the temperature dependence (1.6<T<25K) of the zeroth and first moments of the absorption line due to the 3A2g to 3T2g( Gamma 3) excitonic transition in the one-dimensional easy-plane ferromagnet CsNiF3. They have found a linear dependence of its position (or of the first moment) on the temperature, a quite unusual behaviour which is claimed to be due to its 1D character. The authors have calculated these moments in terms of one- and two-ion static spin correlation functions. In particular, the linear shift of the first moment with temperature is qualitatively well explained. They discuss quantitatively their results in the light of 1D-Heisenberg, X Y or easy-plane models.

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.

Nuclear magnetic resonance and relaxation studies of the linear ferromagnet CsNiF3

The nuclear spin lattice relaxation rate in the one-dimensional easy-plane ferromagnet CsNiF 3 is found to decrease exponentially at low temperature and is shown to be dominated by the spin wave excitations. Saturation of the 133 Cs line shift at low temperature is observed and correlated with the magnetization saturation.

Spin waves in CsNiF3with an applied magnetic field

The spin wave dispersion in the planar 1D ferromagnet CsNiF3 has been measured by inelastic neutron scattering in an external field. The spin wave linewidths are found to decrease with increasing field and become resolution-limited for H>10 kG at 4.2K. At high fields, H>10 kG, both energies and intensities are found to follow linear spin wave theory. The analysis resolves a discrepancy between the anisotropy constants, A, derived earlier from spin-wave and susceptibility measurements, respectively. It turns out that a quantum correction is necessary in order to get consistent parameters. Two parameters namely the nearest-neighbour exchange, J/k=11.5+or-0.05K and A/k=8.9+or-0.2K, are needed to fully describe the system. The temperature dependence of the spin waves in an external field was also studied.

FIR-investigations on the one-dimensional ferromagnet CsNiF3

Abstract The magnetic field and temperature dependence of the ferromagnetic resonance of the 1d easy plane type magnet CsNiF3, was determined by far infrared techniques. The single ion anisotropy of the Ni2+ ions turned out to be D = 3.3 cm−1. The g-factor for the magnetic field within the easy plane was found to be 2.28. The temperature dependence of the position of the resonance can be satisfactory explained by spin wave renormalization.