We synthesized NiO nanoparticles in the pores of mesoporous silica, with particle sizes ranging from 2.6 to 22 nm, and investigated their crystal structure and magnetic properties. The size dependence of the crystal structure exhibited a change in behavior across the critical size of 3 nm. The lattice constants attained maximal values at approximately 3 nm and asymptotically decreased with increasing particle size, reaching the values for the bulk crystal. The rhombohedral distortion of crystallographic structure increased with decreasing particle size for particle sizes below ≈10 nm and was maximum at ≈3 nm. The size dependence of lattice strain also exhibited a maximum at 3 nm. The magnetic properties such as the coercive field, anisotropy energy, blocking temperature, and shell moment exhibited maxima for particle sizes between 3 and 10 nm. These results suggest that the magnetic behavior is generated by the change in the magnetic anisotropy constant originating from the changes in the crystal structure.

We applied anisotropic stress to LaMnO3+δ nanoparticles synthesized in one-dimensional pores of mesoporous silica and investigated the effects of the anisotropic strain on the nanoparticles, which consisted of ferromagnetic and antiferromagnetic particles. The nanoparticles in the one-dimensional pores were crushed without stress in the direction parallel to the silica walls that separated the pores when the mesoporous silica containing the nanoparticles is pressurized. X-ray diffraction patterns of the nanoparticles indicated that the lattice strain increased with increasing applied pressure. The lattice constants were found to reduce anisotropically, and the values saturated above a critical strain. Both the ferromagnetic and antiferromagnetic transition temperatures for the LaMnO3+δ nanoparticles initially increased with increasing pressure and then remained constant at around the critical strain. These results indicate that anisotropic stress causes distortion of the shape of nanoparticles. This induces an increase in the lattice strain and the anisotropic compression of the crystal structure, which, in turn, results in an increase in the transition temperatures.

We report a study on the pressure response of the anisotropy energy of hollow and solid maghemite nanoparticles. The differences between the maghemite samples are understood in terms of size, magnetic anisotropy and shape of the particles. In particular, the differences between hollow and solid samples are due to the different shape of the nanoparticles and by comparing both pressure responses it is possible to conclude that the shell has a larger pressure response when compared to the core.

No abstract