Paramagnetism of Surface Ni Atoms in Ni@C Nanocomposites. “Anomalous” Magnetic Size Effects in Ultrasmall Ni Particles: Manifestations of Surface Magnetic Anisotropy in Squid Magnetometry and FMR Spectra

Abstract

The dimensional magnetic effects of single-domain Ni nanoparticles encapsulated by a carbon shell (Ni@C nanocomposites) have been studied. The studied samples were obtained by solid-phase pyrolysis of solid solutions of nickel phthalocyanine (NiPc) and metal-free phthalocyanine (H₂Pc): (NiPc)_x(H₂Pc)_1 – x, where 0 ⩽ x ⩽ 1. The Ni concentration in the carbon matrix varied in the range of 0–12 wt %, the sizes of the average diameter of nanoparticles in different samples were from 4 to 40 nm. The paramagnetism of the surface and near-surface atoms of Ni nanoparticles, which is due to the charge transfer from the carbon matrix with the formation of Ni ions has been studied in detail. A method for determining the blocking temperature of superparamagnets in temperature measurements of the paramagnetic susceptibility in the case of high magnetic fields is considered. The magnetic characteristics, which reflect size effects in SQUID magnetometry and FMR, exhibit “anomalous” deviations in the range of ultrasmall particles. These are high values of the blocking temperature, high values of the coercive force, and the ferromagnetic resonance linewidth, as well as a significant shift of the effective g-factor in the FMR spectra. Generalizing equations are presented that include the contributions of the surface magnetic anisotropy along with the bulk magnetocrystalline anisotropy, which is consistent with the experimental results over the entire investigated range from 4  to 40 nm. It is shown that the parameters of the total magnetic resonance spectra (FMR + ESR) are caused by the ferromagnetism of the core of nickel nanoparticles and the paramagnetism of surface and near-surface Ni ions, as well as π-electrons of the organic matrix. A general diagram of the dependence of the coercive force on particle sizes for temperatures lower than the blocking temperature (T < T_b) is also presented.