J. Campo

Long-range ferromagnetism of Mn12 acetate single-molecule magnets under a transverse magnetic field.

We use neutron diffraction to probe the magnetization components of a crystal of Mn12 single-molecule magnets. Each of these molecules behaves, at low temperatures, as a nanomagnet with spin S = 10 and strong anisotropy along the crystallographic c axis. The application of a magnetic field H(perpendicular) perpendicular to c induces quantum tunneling between opposite spin orientations, enabling the spins to attain thermal equilibrium. For T approximately < 0.9(1) K, this equilibrium state shows spontaneous magnetization, indicating the onset of ferromagnetism. These long-range magnetic correlations nearly disappear for mu0H(perpendicular) approximately > 5.5 T, possibly suggesting the existence of a quantum critical point.

Neutron diffraction and magnetism of CoO antiferromagnetic nanoparticles

We report a study on neutron diffraction and magnetic properties ol cobalt oxide CoO antiferromagnetic nanoparticles with different sizes. The nanoparticles are composed by a structurally and magnetically ordered core and a structurally ordered and magnetically disordered shell with a thickness of about 2 nm. The ordered core has cell parameters, moments direction and modulus similar to those of bulk CoO. Small differences found are attributed to an increase of the oxidation of the nanoparticles with the decrease of size. A remanent moment Mr can be induced in CoO nanoparticles by crossing the transition temperature in the presence of a magnetic field, while the magnetic structure of the antiferromagnetically ordered moments of the nanoparticles core remains unchanged after field cooling within the experimental precision, suggesting that Mr arises in the magnetically disordered shell.

Magnetic properties and structure of LaNi3/4Mn1/4O3

We report here the structure and magnetic properties of LaNi3/4Mn1/4O3. X-ray diffraction and transmission electron microscopy experiments demonstrate that this sample is a single perovskite phase and not a biphasic system as was thought in the past. The thermal evolution of the ac magnetic susceptibility shows two large magnetic anomalies at 45 and 262?K. Isothermal magnetization measurements indicate the presence of a very low ferromagnetic component below 262?K. The neutron diffraction experiments, instead, do not detect a magnetic contribution for this sample in the whole temperature range measured. The complex magnetic behaviour can be explained as a glassy ferromagnetic state, a phase with properties typical of both systems: ferromagnets and spin-glasses.