Magnetization and ferromagnetic resonance (FMR) experiments have been performed on $3\mathrm{nm}$ NiO noninteracting nanoparticles. The results indicate that the high temperature behavior is determined by the noncompensated moments of particle core, whose blocking is centered at $T\approx 60\mathrm{K}$ [field dependent maximum of the zero-field-cooled (ZFC) magnetization]. On the other hand, the low temperature behavior is determined by surface cluster spins whose thermal fluctuations freeze in a cluster-glass-like state at $T\approx 15\mathrm{K}$ (weakly field dependent maximum of ZFC magnetization), giving the major contribution to the effective anisotropy, as shown by the rapid increase of coercivity, remanent magnetization, and confirmed by the temperature dependence of signal intensity, resonant field, and linewidth of FMR spectra.

The spin-flop transition of acicular hematite nanoparticles of different size synthesized by chemical route and annealed at different temperatures has been investigated as a function of temperature. Transmission electron microscopy (TEM) measurements show that particles have an ellipsoidal shape, with a major axis of $330\pm 50\mathrm{nm}$ and a minor axis of $70\pm 10\mathrm{nm}.$ TEM and x-ray diffraction experiments show that nanoparticles are made of hematite crystallites, which grow on increasing the annealing temperature. Both the Morin transition temperature ${(T}_M)$ and the spin-flop transition field ${(H}_{\mathrm{sf}})$ have been found to increase for increasing crystallite size $\mathrm{}\left(d\right):$ for instance, for $d=36\mathrm{}\mathrm{nm}$ particles $T_M=164\mathrm{}\mathrm{K}$ and the value of $H_{\mathrm{sf}}$ extrapolated at $T=0\mathrm{}$ ${(H}_{\mathrm{sf}0\mathrm{}})$ is 1.7 T, whereas for bulk hematite $T_M=263\mathrm{}\mathrm{K}$ and $H_{\mathrm{sf}0\mathrm{}}=6.5\mathrm{}\mathrm{T}.$ Both $H_{\mathrm{sf}0\mathrm{}}$ and $T_M$ follow a $1/d$ dependence (at a faster rate for $H_{\mathrm{sf}0\mathrm{}}),$ indicating that their variation is mainly driven by surface effects.

We present magnetization measurements on ∼3-nm $({\mathrm{Fe}}_{0.26}{\mathrm{Ni}}_{0.74}{)}_{50}{\mathrm{B}}_{50}$ ferromagnetic amorphous nanoparticles. Our results show evidence of the surface contribution to the magnetic properties in the $M\left(H\right),$ $M\left(T\right)\mathrm{}$ and relaxation measurements. We observe a large increase in the magnetization with decreasing temperature, a nonsaturated component in the $M\left(H\right)\mathrm{}$ curves, the presence of two maxima in the zero-field-cooling $M\left(T\right)\mathrm{}$ data, and two viscosity regimes in magnetization relaxation data. The results have been interpreted by a simple model where we consider each single-domain particle as a core plus shell system with a uniaxial anisotropy acting on the core and a surface anisotropy acting on the shell. Monte Carlo simulations based in this core-shell model qualitatively reproduce all the observed phenomena.

The magnetic properties of $15\mathrm{nm}$ ${\mathrm{Mn}}_3{\mathrm{O}}_4$ particles, in powder form and dispersed in a polymer at different concentrations (17.6%, 1.5%), have been investigated by magnetization and ESR measurements. Below the ferrimagnetic transition temperature $(T_c=42\mathrm{K})$, marked differences are observed among the three samples in the temperature dependence of the magnetization, hysteresis loop shape and type and intensity of ESR spectra. They are due to different strengths of interparticle interactions and different contributions from free spins (and free cluster spins) at the particle surface. With decreasing temperature their thermal fluctuations slow down and spins freeze in random directions, according to their local atomic environment. Their fraction is found to decrease with increasing inter-particle interactions, i.e. going from the more diluted particle dispersion to the powder sample.

We report on Monte Carlo simulations of the magnetization behavior of $\sim 3\mathrm{nm}$ of ${\left({\mathrm{Fe}}_{0.26}{\mathrm{Ni}}_{0.74}\right)}_{50}{\mathrm{B}}_{50}$ and ${\left({\mathrm{Co}}_{0.25}{\mathrm{Ni}}_{0.75}\right)}_{65}{\mathrm{B}}_{35}$ ferromagnetic amorphous nanoparticles dispersed in a polymer matrix. Simulations based on a core-shell model, describing the nanoparticles as consisting of a ferromagnetically ordered core and a disordered surface shell, reproduce satisfactorily the significant features observed in hysteresis cycles, i.e., $H_c\left(T\right)$ and $M_r\left(T\right)$ curves. The results provide clear evidence of the major role played at low temperature by surface anisotropy and surface-core exchange interaction in determining some anomalies of the hysteresis cycles and the large increase in effective particle anisotropy at low temperature.