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Ultra-low-density BiFeO3 nanoparticles have been prepared by pulsed laser deposition and their structure and magnetic properties have been studied. Annealing increases crystallinity and the size of the particles leading to an alteration of magnetic properties, observed from magnetic studies and evaluated using high-resolution transmission electron microscopy , selected area electron diffraction and x-ray diffraction patterns analysis. Transmission electron microscopy results show that the BiFeO3 as-deposited nanoparticles annealed up to 400 °C exhibit a orthorhombic distorted perovskite structure without secondary phase and with diameters varying from 9 nm (as-deposited) to 17 nm (annealed at 400 °C). Magnetic data exhibit exchange bias and magnetic blocking effects at low temperatures and typical superparamagnetic behavior at high temperatures. Meanwhile, the BiFeO3 nanoparticles annealed at 500 °C exhibit a rhombohedrally distorted perovskite structure with typical antiferromagnetic properties and diameter of about 56 nm. The analysis of magnetic relaxation time using the Arrhenius equation suggests a superparamagnetic blocking process of ferromagnetic clusters on the surface of the nanoparticles at low temperature. The magnetic properties are discussed considering the interactions between nanoparticles and the co-existence of different magnetic phases within the nanoparticles: an ordered antiferromagnetic core and ferromagnetic clusters on the surface. [ABSTRACT FROM AUTHOR]

The magnetic properties of sub-micrometric magnetic structures of Ni and permalloy (Ni80Fe20) patterned stripes are studied as functions of temperature. The samples are produced by combining an interferometric lithographic technique with sputtering. At room temperature, ferromagnetic resonance and hysteresis data indicate the existence of an in-plane uniaxial anisotropy in both samples. The NiFe sample has an easy axis along the stripes direction in the entire studied temperature range (2 < T < 300 K). On the other hand, an interesting change in the easy magnetization direction is observed in Ni samples, from parallel to perpendicular to the axis of the stripes, when the temperature is decreased. The results are interpreted in terms of a competition between shape and magnetoelastic anisotropies. [ABSTRACT FROM AUTHOR]