Surface and interparticle effects in amorphous magnetic nanoparticles

ZYSLER ROBERTO; DE BIASI EMILIO; RAMOS CARLOS; FIORANI DINO; ROMERO HECTOR

Surface Effects in Magnetic Nanoparticles

Año: 2005; p. 1 - 23

The knowledge of the magnetic properties of nanometer particles is central in basic research [1, 2] and technological applications, e.g. in high-density magnetic storage media [3, 4], hard magnets [5], and biomedicine [6]. Finite size effects induce a magnetic behaviour which may strongly differ in several aspects from those observed on conventional bulk materials. Surface effects dominate the magnetic properties of the smallest particles since decreasing the particle size ratio of the surface spins to the total number of spins increases, e.g. in a particle of diameter ~3 nm, about 70% of atoms lie on the surface, which is structurally and magnetically disordered. Consequently, the picture of a single-domain magnetic particle where all spins are pointing into the same direction, leading to coherent relaxation process, is no longer valid when one considers the effect of misaligned spins on the surface on the global magnetic properties of the particle. Defects, missing bonds and then the decrease of the average coordination number determine a weakening of the exchange interactions between surface atoms. This contributes to reduction of the magnetic transition temperature with decreasing particle size, as shown by experiments [7, 8] and Monte Carlo (MC) calculations [9]. Moreover, the symmetry breaking at the surface results in a surface anisotropy (SA), which can represent the dominant contribution to the total particle anisotropy for small enough particles. In most cases SA is strong enough to compete with the exchange energy that favours full alignment of particle spins. Actually, it is expected that the magnetization vector will point along the bulk axis (the particle anisotropy axis) in the core of the particle, and it will then gradually turn into a different direction when it approaches the surface. As a consequence of the combination of finite-size and surface effects, the profile of the magnetization is not uniform across the particle and the magnetization of the surface layer is smaller than that corresponding to the core spins. This effect has been reported in several nanoparticle systems. High-field magnetization measurements on ã-Fe2O3 [10] and Co [11] nanoparticles have shown that the magnetization is strongly influenced by surface effects, depending on the particle size. Also nickel ferrite particles show high-field open hysteresis loops, due to the large surface anisotropy, and high-field magnetic relaxation, due to the progressive overcoming of energy barriers of a spin-glass-like surface state [12, 13]. MCparticle anisotropy axis) in the core of the particle, and it will then gradually turn into a different direction when it approaches the surface. As a consequence of the combination of finite-size and surface effects, the profile of the magnetization is not uniform across the particle and the magnetization of the surface layer is smaller than that corresponding to the core spins. This effect has been reported in several nanoparticle systems. High-field magnetization measurements on ã-Fe2O3 [10] and Co [11] nanoparticles have shown that the magnetization is strongly influenced by surface effects, depending on the particle size. Also nickel ferrite particles show high-field open hysteresis loops, due to the large surface anisotropy, and high-field magnetic relaxation, due to the progressive overcoming of energy barriers of a spin-glass-like surface state [12, 13]. MCã-Fe2O3 [10] and Co [11] nanoparticles have shown that the magnetization is strongly influenced by surface effects, depending on the particle size. Also nickel ferrite particles show high-field open hysteresis loops, due to the large surface anisotropy, and high-field magnetic relaxation, due to the progressive overcoming of energy barriers of a spin-glass-like surface state [12, 13]. MC