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We studied Mn 12 -acetate by inelastic neutron scattering and diffraction. We separated the energy levels corresponding to the splitting of the lowest S multiplet (S 10 ground state). The irregular spacing of the transition energies unambiguously shows the presence of high-order terms in the spin Hamiltonian [D 20.457͑2͒ cm 21 , B 0 4 22.33͑4͒ 3 10 25 cm 21 ]. The relative intensity of the lowest energy peaks is very sensitive to the small transverse term that is responsible for quantum tunneling, providing the first determination of this term in zero magnetic field ͓B 4 4 63.0͑5͒ 3 10 25 cm 21 ͔. PACS numbers: 75.25. + z, 75.45. + j, 78.70.Nx 0031-9007͞99͞83(3)͞628(4)$15.00

Elastic neutron scattering experiments performed in semi-conducting La1−xCaxMnO3 single crystals (x=0.05, 0.08), reveal new features in the problem of electronic phase separation and metal insulator transition. Below TN , the observation of a broad magnetic modulation in the q-dependent scattering intensity, centered at nearly identical qm whatever the q direction, can be explained by a liquid-like spatial distribution of similar magnetic droplets. A semi-quantitative description of their magnetic state, diameter, and average distance , can be done using a two-phase model. Such a picture can explain the anomalous characteristics of the spin wave branches and may result from unmixing forces between charge carriers predicted from the s-d model.

Neutron diffraction and inelastic neutron scattering in zero field have been performed on mesoscopic magnetic Mn 12 O 12 clusters, well known for their macroscopic quantum effects observed at low temperature. In addition to the static spin correlations of the cluster in its ground state, we have observed some energy levels related both to the anisotropy and to the exchange energies of the cluster, with their respective dynamical form factor. The temperature and Q dependences of the anisotropy energy levels can be qualitatively explained using a quantum model. Besides these expected modes, the most striking result is the observation of extended energy modes at energy values below those related to anisotropy. Their temperature and Q dependences differ from those expected for the energy levels of usual magnetic clusters. They indicate an additional spin coupling which must play a role in tunneling properties.

Diffuse-neutron-scattering and resistivity measurerrients have been performed on FelCr, alloys (c & 0.15). A change of sign of the short-range-order parameters near c = 0.1 indicates that the alloys exhibit short-range clustering at c & 0.1 and a tendency to form an ordered compound at c & 0.1. Concurrently, the slope of the resistivity versus temperature changes its sign. This inversion of short-range ordering has been predicted by a recent calculation. These experiments are considered to be an important confirmation of the theory.Attractive or repulsive short-range order (SRO) in A &,8, alloys is characterized by the CowleyWarren SRO parameters, ' readily determined in a diffuse-neutron-scattering or x-ray experiment. The physical parameters underlying this observation are the interaction pair potentials which may be deduced from the SRO parameters in the mean-field approximation of Clapp and Moss, allowing a comparison with theoretical calculations. Several theoretical modelshave been developed in order to relate the interaction pair potentials to the fundamental characteristics of the alloy: They involve finding a suitable expansion of the internal energy so that it may be written as a sum of pairwise interactions. In this context, the transition-metal alloys offer a serious challenge. In these alloys, it was shown that the electronic configurational energy could be written in an expansion of effective pair potentials starting from the completely disordered state (generalized perturbation theory ) although the cohesive energy cannot. The model derived for systems in a paramagnetic state reveals the strong variation of the first nearest-neighbor pair potential with the position of the Fermi level in the electronic band and the existence of two critical positions for which the sign of the pair potential changes. This yields a strong variation of the pair potential with concentration, and the possibility of an inversion of short-range ordering in some wellchosen alloys. This possibility has not been experimentally checked, probably because of the difficulties related to the nature of the alloys concerned. In consideration of the very high values of the Curie temperature for metals in the first transition series, the model was thus extended by one of the authors to the ferromagnetic state and applied to Fe alloys.This modified model predicts that the interaction pair potential between first nearest neighbors in Fel, Cr, alloys changes its sign at rather low Cr concentration (c =0.25). This result has motivated the measurements of SRO parameters by diffuse neutron scattering in Fe~,Cr, alloys that we report here. As previous experiments' had shown that this change of sign did not occur for c & 0.15 we have limited our study to three samples of concentration c =0.05, 0.1, 0.15. We show that the alloy exhibits SR clustering for c & 0.1 and a tendency towards the formation of ordered compounds for c (0.1. This experimental result givesevidence for a spectacular band effect; moreover, it allows a verification of the ...