D. Visser

Neutron scattering investigation of the static critical properties of Rb2CrCl4

We have investigated the static zero-field critical properties of the ionic ferromagnet Rb2CrCl4, using neutron scattering, under conditions such that the quasi-static approximation is valid. A general consideration of the spin Hamiltonian of Rb2CrCl4, with S = 2, shows that it approximates the two-dimensional (2D) XY model, with additional weak perturbations. Rb2CrCl4 orders three dimensionally at Tc = 52.3 K, and very close to Tc there is a regime of three-dimensional critical behaviour. However, both below and above Tc there is a crossover to regimes of two-dimensional critical fluctuations. In these regimes, there is excellent agreement between experiment and theoretical predictions for a finite-sized 2D XY model, suggesting that Rb2CrCl4 is an ideal realization of this system.

INFLUENCE OF CHIRAL SYMMETRY ON THE CRITICAL BEHAVIOR OF STACKED TRIANGULAR ANTIFERROMAGNETS

We review and extend our recent specific-heat results of three antiferromagnetic quasi-onedimensional magnets, CsMnBr3, CsNiCl3, and CsMnI3, with chiral symmetry. In zero field, CsMnBr3 exhibits an unusually large critical specific-heat exponent α in good agreement with predictions for chiral XY symmetry. The anisotropy-crossover exponent ɸ has been determined in low fields applied perpendicular to the c axis. ɸ is clearly smaller than unity and differs from theoretical predictions. For CsNiCl3 we find a crossover in fields B||c from conventional universality behavior for B=0 to chiral Heisenberg behavior at the multicritical point BM=2.3 T and further to chiral XY behavior at still higher fields. The critical exponents and also the amplitude ratios of the specific heat are in excellent agreement with theory. A magnetic field applied 13° off the c axis modifies the phase diagram, but the critical parameters remain essentially unchanged. Finally, specific-heat measurements of CsMnI3 confirm the crossover for easy-axis antiferromagnets with chiral symmetry and yield an exponent α=0.28±0.06 at the multicritical point. At 7 T, we find α=0.33±0.06. These values are again in very good agreement with the predictions for the chiral Heisenberg and chiral XY universality classes.

Magnetic excitations and phase transition of CsFeBr3 in an external magnetic field

In CSFeBr3 the Fe2+ ion with effective spin one has locally a singlet ground state (m=0). The antiferromagnetic interactions between neighbouring Fe-ions are too weak as compared with the anisotropy constant to introduce long range order in the absence of an external field. By inelastic neutron scattering we studied the magnetic excitations in an external magnetic field up to 5 Tesla applied along thec-axis. A linear Zeeman splitting was observed with a Landé factorg=2.4. The field renormalizes the dispersion curves in such a way that the exchange interaction has decreasing influence with increasing field. Theoretical calculations according to the excitonic model of Lindgård describe the experimental results very well. At 4.1 Tesla a phase transition appears to a commensurate long range order with a 120° arrangement of the spins in the hexagonal plane. Within the limits of experimental observation this phase transition has no influence on the dynamical behaviour. No critical phenomena could be observed. The dynamical structure factor |Gj(ℚ)|2 of the lower Zeeman split modes decreases with increasing field.

Magnetic excitations of the singlet ground state antiferromagnet CsFeBr3 in a magnetic field

Abstract The magnetic excitations in the quasi-one-dimensional singlet ground state antiferromagnet CsFeBr3 have been studied by inelastic neutron scattering in the presence of an applied magnetic field, H‖c and H⊥c. With H‖c, the dispersion curves renormalise so that the exchange interaction has a decreasing influence with increasing field. H⊥c has little or no effect on the dispersion of the magnetic excitations.

Magnetic ordering in the quasi-one-dimensional induced moment ferro- and antiferromagnets AFeX3

Abstract The ternary halide hexagonal perovskites AFeX 3 shows a local singlet ground state behaviour. Depending on the strength of the exchange constants and the single-ion anisotropy, these systems will show an induced magnetic moment. In this paper we discuss the magnetic ordering behaviour in this series of compounds and the effect on the magnetic ordering due to an applied magnetic field.

Crystallographic structure and magnetic ground state of CeSix

Abstract The intermetallic compound CeSi x ( x =1.80, 1.85) has been investigated by neutron powder diffraction using the diffractometer D2B at the ILL, Grenoble. For both samples the coexistence of two structural phases was observed. In the case of CeSi 1.85 , these two phases are of the same spacegroup but with a different temperature variation of their lattice parameters. A dependence of the magnetic ground state of CeSi 1.80 on the heat treatment of the sample was also observed. The importance of the ordering of holes in the Si lattice of CeSi 2 in determining the ground state properties of CeSi x is discussed.

Temperature and magnetic field dependent correlations in the singlet ground state system CsFeBr3

The magnetic excitations in CsFeBr3 have been measured with inelastic scattering of cold neutrons to high precision at 80 mK. The fact that the lowest frequency mode softens with decreasing temperature but stabilizes at 0.11 THz below 2.5 K is the indication that CsFeBr3 remains a SGS system forT→0.From dispersion curves measured earlier in a magnetic field along the chain axis experimental intensities were derived and in turn correlation lengths. Correlation lengths were also calculated using the new parameters for exchange and anisotropy. The experimental results and the calculations both show that the correlation lengths increase for increasing magnetic field, flatten off around the phase transformation at 4.1 Tesla with a maximum of about 35 Å and then decrease again for higher fields. No critical phenomena connected with this phase transformation could be detected.The “Mirror Mode” published earlier was found to be of spurious origin due to double scattering. It does not exist.

Critical behavior of antiferromagnets with chiral symmetry

Abstract The quasi-one-dimensional and two-dimensional triangular lattice antiferromagnets give rise to new critical behavior which is due to an additional symmetry of chirality. Experiments performed recently on easy-axis and easy-plane compounds, notably specific-heat experiments of high accuracy, clearly demonstrate that critical exponents and amplitude ratios of the specific heat are in good agreement with predictions for chiral behavior. It is also possible to observe a crossover induced, e.g. by magnetic fields, from non-chiral to chiral universality classes.

An investigation of the magnetism of the rare earth Heusler alloy Cu2GdIn using SQUID magnetometry

Abstract The magnetic properties of Cu 2 GdIn have been investigated over the temperature range 2–300 K and in magnetic fields up to 5 T. The results indicate that the alloy, which is highly ordered in the L2 1 Heusler structure, becomes antiferromagnetic below 10 K. Above this ordering temperature the static susceptibility follows a Curie-Weiss law with an effective moment of 7.89μ B close to the free ion value of Gd. Magnetisation measurements below 10 K as a function of magnetic field suggest the existence of a metamagnetic phase transition.

Magneto-clastic distortions in Heusler alloys

Magnetic and neutron scattering experiments have been carried out on a new series of Heusler alloys based on the composition (Au1Pd2−x)MnIn. A magnetic moment of ≈4 μB is confined to the manganese atoms and for x < 1.4 the compounds order antiferromagnetically below ≈ 150 K. Dependent upon composition, three distint structures are observed: antiferromagnetism of type I. II and IIIA. This represents the first observation of type I ordering in a Heusler system. The configurational symmetry associated with these structures has a well-defined anisotropy, producing a distinct lattice distortion in the case of types I and III order. There has been no observation of any lattice distortion associated with the onset of type II antiferromagnetism. The structural distortion, which depends upon the size of the Mn moment, has been investigated as a function of temperature.