J.P. Boucher

Experimental evidence for magnetic solitons in the antiferromagnetic chains of (CD3)4NMnCl3 (invited)

Different experimental results—neutron experiments, spin‐lattice relaxation time measurements, and an analysis of the phase diagram TN(H)—obtained on the one‐dimensional compound TMMC are reviewed and compared to the soliton model proposed for planar antiferromagnetic chains.

Magnetic‐field dependence of the phosphorus nuclear spin‐relaxation rate in the quasi‐two‐dimensional XY antiferromagnet BaNi2(PO4)2

The phosphorus nuclear spin‐lattice relaxation rate 1/T1 has been measured in the quasi‐two‐dimensional XY antiferromagnet BaNi2(PO4)2. The divergence observed just above the ordering temperature TN ≂ 23.6 K is analyzed within the context of the Kosterlitz–Thouless transition. For a magnetic field H perpendicular to the XY plane, the data presented here agree well with a recent model of freely moving vortices. The field dependence observed for H parallel suggests a drastic slowing down of the vortex velocity in that case.

NONLINEAR EXCITATIONS IN ld AND 2d MAGNETIC SYSTEMS

We present experimental results which show clearly the existence and the role played by nonlinear excitations in the quasi-1d compounds TMMC and CsCoCl 3 (solitons) and quasi-2d compounds BaM 2 (XO 4 ) 2 (M = Co, Ni; X = P, As). The role of the collisions between solitons and magnons and the applicability of the Kosterlitz-Thouless theory are discussed in the light of new inelastic neutron scattering experiments.

Soliton dynamics in the one-dimensional antiferromagnet TMMC [(CD3)4NMnCl3]

Recently the concept of solitons has been successfully applied to magnetic chains where magnetic solitons can be viewed as travelling domain walls. The one-dimensional antiferromagnetic compound (CD3)4NMnCl3 (TMMC) in a transverse field is shown to be a very good realisation of sine-Gordon chains, below T≅5 K. A review of recent experiments, both nuclear magnetic resonance and neutron diffraction, is given for this compound. New results are presented: the fluctuations associated with the solitons themselves have been observed for the first time; the magnon width gives direct evidence of the phase coupling mechanism between solitons and magnons. It is shown also that neutron spin echo measurements offer a promising way to study such narrow central peaks. The soliton dynamics in such real sine-Gordon chains is of fundamental importance. In very pure crystals the solitons move coherently while in samples containing a small amount of magnetic impurities in the chain, the solitons behave diffusively.

Magnetic fluctuation spectrum of CuGeO3: Raman scattering

Raman spectra of the magnetic fluctuation spectrum of CuGeO3 are presented for 4<T<300 K and B≤20 T. For B=0 the results show good agreement with theoretical expectations for a one‐dimensional isotropic Heisenberg antiferromagnet which exhibits a spin–Peierls transition. In the presence of a magnetic field the phase transition to the incommensurately modulated phase is observed around B=12.8 T at T=4.2 K. The spectra obtained in the high field phase clearly show evidence for a well‐defined spin fluctuation spectrum, which appears to present an intermediate case between the dimerized phase and the short‐range order regime of the uniform phase.

Investigation of the spin‐Peierls phase diagram of CuGeO3: Far‐infrared electron spin resonance in high field

Electron spin resonance on single crystals of the spin‐Peierls compound CuGeO3 has been performed in magnetic fields up to 17 T in the temperature range 1.8–20 K. Using far‐infrared laser frequencies of 245 and 525 GHz we could observe resonance conditions in the three different magnetic phases, i.e., the uniform, the dimerized, and the incommensurate phases for a magnetic field applied parallel to the a axis of the crystal. So for the first time data on electron spin dynamics in the incommensurate phase are reported for fields far from the transition to the dimerised phase. Striking effects concerning the g factor, linewidth, and hysteresis are observed.

High-field ESR in the s=1 Heisenberg antiferromagnetic chains of NENP

ESR studies of the excitation spectrum of Haldane spin systems are presented. We performed temperature-dependent high-field and high-frequency ESR measurements up to 21 T and 1042 GHz on the compound NENP [Ni(C2H8N2)2NO2ClO4]. Based on extensive experimental investigations we propose to distinguish between different classes of resonances: first, the transitions between excited states at the boundary of the Brillouin zone (q=π) below the critical field; second, the high-frequency transitions from the ground state (q=0) above the critical field and third, the low-frequency resonances at q=0 in the whole field range. New results are presented in the high-field phase, which exhibit an appreciable temperature dependence of the resonance field and at low temperatures show the existence of multiple high-frequency excitation branches. In the low-frequency regime we also observed an additional ground state excitation.

XY-Ising crossover transition in the one-dimensional antiferromagnet TMMC: Effect of a magnetic field

Quasi-elastic neutron scattering experiments have been performed in the one-dimensional antiferromagnet (CD3)4NMnCl3 (TMMC) at low temperature, 1 < T < 10 K, in the presence of a magnetic field, 0 < H < 40 kOe. Our results show that in zero field TMMC is in an XY state for T ≲ 5 K. In a field applied along a direction perpendicular to the chain axis, TMMC undergoes an XY-Ising crossover transition.

Spins and phonons in the spin-Peierls compound CuGeO3

Abstract Vibrational and spin dynamical properties of CuGeO 3 are studied using inelastic light scattering techniques as a function of temperature and magnetic field. Apart from one phonon processes, also two phonon scattering has been observed. Two spin gaps are observed in the dimerised phase, which disappear in the high field incommensurate phase. Raman data show that well-defined propagative modes still exist in the high field phase, whereas infrared absorption shows the presence of a new gap which is attributed to the Zeeman splitting of the ground state in the high field phase.

Spin excitations throughout the field-induced magnetic phase of the spin 1/2 alternating chain compound Cu(NO3)2·2.5D20

We present an inelastic neutron scattering study performed throughout the field-induced magnetic phase occurring between Hc1≃3.5T and Hc2≃4.5T in the spin 1/2 alternating antiferromagnetic chain compound Cu(NO3)2·2.5D20Cu(NO3)2·2.5D20. The exchange couplings are consistently determined from the analysis of the spin dynamics both in the gapped phase, at H=0H=0, and in the saturated phase, at H=6T, while the cancellation of the lowest energy gap in the complete field-induced magnetic phase suggests that this system is a good realization of a Bose–Einstein condensation.