T. Chattopadhyay

Long-range antiferromagnetic order in CeCu5.5Au0.5

The magnetic ordering previously discovered in CeCu6−xAux heavy-fermion alloys is shown to be of long-range antiferromagnetic type by elastic neutron scattering performed on a polycrystalline sample. The data are compatible with an incommensurate wave vector (0.17, 0, 0.514). Although a definite assignment needs a single-crystal study, the magnetic structure is clearly different from the type of magnetic correlations in pure CeCu6 found in inelastic scattering.

Antiferromagnetic ordering in Gd2CuO4

Abstract We have carried out neutron diffraction and antiferromagnetic resonance investigations of Gd 2 CuO 4 single crystals. The former shows that the magnetic moments of the gadolinium ions order antiferromagnetically at T N =6.4 K with the propagation vector k =(0, 0, 0). The two Gd atoms of the primitive unit cell related by the centre of symmetry have oppositely directed moments. The antiferromagnetic resonance spectra indicate that Gd 2 CuO 4 belongs to the class of “easy plane” antiferromagnets and show the presence of a spin reorientation phase transition at H c =0.88 T ( H ‖[100], T =1.8 K). From these data we conclude that Gd 2 CuO 4 has a collinear antiferromagnetic structure.

Magnetic excitations of Nd in Nd2−xCexCuO4 (x = 0 and 0.13)

Abstract We have studied the wave vector dependence of the magnetic excitation spectrum of Nd in Nd 2− x Ce x CuO 4 ( x = 0,0.13) by inelastic neutron scattering experiments on single crystals. The results are analyzed with the help of model calculations which are performed in the context of the mean field random-phase approximation (RPA). This enables us to obtain direct information on the coupling constants between the rare earth ions. In Nd 2 CuO 4 we observe a dependence of the coupling constants on the initial and final state that is involved in a crystal field excitation. Furthermore, our results indicate that the interaction of the Nd spins is opposite to the ordering enforced by the Ndue5f8Cu exchange. We argue that this will lead to instability of the Nd-ordering in the Ce-doped systems which might be important for the onset of high γ-values.

Magnetic diffuse scattering from the frustrated antiferromagnet MnS2

Abstract Magnetic diffuse scattering has been investigated in the frustrated antiferromagnet MnS 2 by neutron diffraction with a linear multidetector in the flat-cone geometry in the temperature range 55–300 K. We have performed mean-field calculations of the magnetic diffuse scattering under the quasistatic approximation, which agree with the experimental results in general.

μSR investigation of the magnetic ordering in CeCu5.5Au0.5

Abstract A zero-field μSR investigation of the alloy CeCu 5.5 Au 0.5 at the ISIS pulsed muon source has shown that the substitution of Cu by Au drives the heavy-fermion intermetallic compound CeCu 6 to a magnetically ordered state at about T N = 0.95 K. This result is in agreement with those of magnetization, specific heat and neutron diffraction investigations.

Influence of Ce-doping on the magnetic excitations of Nd in Nd2−xCexCuO4

Abstract We present a study of the spin-wave excitations of Nd in Nd2−xCexCuO4 (x = 0, 0.09, 0.15, 0.18) by inelastic neutron scattering. Upon Ce-doping one observes a softening of the spin-wave branches. This is mainly explained by the decrease of the Cuue5f8Nd exchange field at the Nd site, while the Ndue5f8Nd exchange interactions remain almost unchanged. Consequently a high density of states of spin-wave excitations at low energies is created. The resulting calculated specific-heat shows a large γ = C/T-value at low temperatures, which indicates that the novel heavy-fermion behavior in these compounds is mainly due to magnetic correlations of the Nd ions.

Spin correlations in MnS2

Abstract To understand the origin of the first order antiferromagnetic phase transition in MnS2 we have extended our previous diffuse neutron scattering investigations. Diffuse scattering above TN is centered at an incommensurate position. The temperature dependence of the corresponding wave vector is similar to that of a lock-in transition found in a three-dimensionally ordered system. Mean field calculations reproduce the experimental diffuse scattering only approximately and cannot explain the centering of the diffuse scattering at an incommensurate position.