D. Petitgrand

Neutron diffraction study of (Ce, La)Ru2Si2 alloys in an external field

Abstract The magnetic phase diagram of Ce1−xLaxRu2Si2 alloys (x = 0.20 and 0.30) has been determined by neutron diffraction. Besides the zero-field incommensurate modulated structure (k1 = [0.31, 0, 0]) and the polarized paramagnetic state, two additional phases are observed: another incommensurate structure and a commensurate one ( k 3 = [ 1 3 , 1 3 , 0] ). The magnetic modulation at H = 0 remains practically unaffected by La dilution up to x = 0.80.

Antiferromagnetism in YBa2Cu3O6

Abstract We report on the first single-crystal determination of the magnetic order in YBa 2 Cu 3 O 6 by neutron scattering. The magnetic structure is found to consist of antiferromagnetic square-lattice layers of Cu(2) ions which are antiferromagnetically stacked along the c-axis. Magnetic moments of Cu(2) lie in the (ab) plane and have an amplitude of 0.6 μ B whereas Cu(1) show no ordered moment.

Study of the structural phase transition in Gd2−xCexCuO4

Abstract The structural phase transition in Gd 2− x Ce x CuO 4 ( x = 0, 0.12) characterized by the rotation of the CuO 4 -squares around the c axis has been studied by different diffraction techniques. The order parameter of this transition is found to increase continuously below 658(1) K for Gd 2 CuO 4 and below 707.2(7) K for Gd 1.88 Ce .12 CuO 4 with an unusual temperature dependence. The rotation of the CuO 4 -squares is coupled to a spontaneous strain along the c -axis. Structure analysis of the undistorted high temperature T′-phase in Gd 2 CuO 4 indicates persisting local distortions.

Spin dynamics in Nd2CuO4 and Pr2CuO4

Abstract The low-energy part of the copper spin-wave excitation spectrum in the undoped antiferromagnets Nd2CuO4 and Pr2CuO4 has been studied by inelastic neutron scattering. In Nd2CuO4, magnetic scattering observed around the (½½Q) rod exhibits pronounced energy gaps for both the “in-plane” and “out-of-plane” spin fluctuations over the whole Brillouin zone. The two gaps appear to be dispersionless along the tetragonal axis direction and remarkably increase when temperature decreases. In Pr2CuO4, the spin-wave gaps are also observed with smaller energies at low temperatures.

Two-dimensional antiferromagnetic excitations in Nd2CuO4 crystal

Abstract Inelastic neutron scattering experiments have been used to study magnetic excitations of Cu2+ spins in a Nd2CuO4 single crystal. These magnetic excitations are characteristic of the strong 2D antiferromagnetic coupling within CuO2 layers. As the temperature increases, the magnetic cross section shifts to lower frequency suggesting the existence of a temperature-dependent anisotropy gap in the spin-wave spectrum.

Interplay of copper and neodymium magnetic moments in the magnetic structure of Nd2CuO4 investigated by neutron scattering

Abstract Magnetic neutron scattering experiments have been performed on a single crystal of Nd 2 CuO 4 in the temperature range 1.5–300 K in order to investigate the magnetic moments of Cu 2+ and Nd 3+ and their respective roles in the magnetic structures observed below T N =260 K. Our results show that a magnetic moment is present on Nd 3+ in all three phases. The magnitude of this moment increases with decreasing temperature, whereas some Nd-Cu spin correlations change abruptly at the I–II and II–III phase transitions.

Antiferromagnetic phase stability by Nd3+ -Cu2+ interplanar coupling in Nd2CuO4

Abstract Nd 2 CuO 4 undergoes three successive antiferromagnetic phase transitions associated with the Cu 2+ spins. As its tetragonal structure leads to a complete frustration of the interplane interactions, it raises the question of how the correlated antiferromagnetic planes couple to give 3D long range order. In order to explain the three transitions, we have evolved a phenomenological approach taking into account a non-zero moment on Nd 3+ atoms.

Magnetisation density in YBa2Cu3O6.5 by polarised neutron diffraction

Abstract A polarised neutron diffraction experiment has been carried out on a single crystal of YBa2Cu3O6.5. The projection of the induced magnetisation density along the (a + b) direction reveals an induced moment only on Cu(1) showing that some Cu2+, not too strongly coupled, are present in the basal plane.

Inelastic neutron scattering study of frustrated Heisenberg triangular magnet CuFeO2

We have studied the magnetic excitation spectrum of the geometrically frustrated triangular lattice antiferromagnets of CuFeO2 with collinear ground state and CuFe1−xAlxO2 (0.014≤x≤0.030) with noncollinear ground state, using inelastic neutron scattering measurements on the single crystals. We have reported almost complete spin-wave dispersion relations and the temperature dependence of the magnetic excitation spectrum in both pure and slightly diluted samples. While the higher-energy spin-wave branches remain almost unchanged between the pure and diluted samples, the energy gap of the lowest-energy branch in CuFeO2 becomes zero as a result of the dilution, suggesting the retrieval of the original Heisenberg spin character of the orbital singlet Fe3+. Furthermore, by comparing with recent elastic neutron diffraction and x-ray measurements, we discuss the interrelation among the magnetic ordering, crystal lattice distortion and magnetic excitation qualitatively.

Pseudodipolar Interaction and Antiferromagnetism of R2CUO4 Compounds (R=PR,ND, SM and EU)

It is shown that the pseudodipolar interaction allows to explain static and dynamic properties of non-collinear antiferromagnets R2CuO4 in the temperature range above the rare-earth (RE) ordering temperature. Experimental data for inelastic neutron scattering in Pr2CuO4 are presented. Parameters of the pseudodipolar interactions are determined. Comparison of the spin-wave neutron scattering in Pr2CuO4 and Nd2CuO4 reveals a strong dependence of the in-plane anisotropy on the type of the RE ion. The ground-state energy in magnetic field applied parallel to the CuO2 planes is derived and analyzed. In particular it is shown that the spin-flop transition is of second order if the field is applied along the [1,1,0] direction.