Magnetic resonance study of Ni nanoparticles in single-walled carbon nanotube bundles

We present a detailed study of the electron magnetic resonance (EMR) properties of Ni nanoparticles (NPs) placed in the bundles of single-walled carbon nanotubes produced by arc discharge with Ni catalyst. The behavior of EMR signals has been investigated in the 10-300 K temperature range for the initial powderlike materials and those diluted in a nonmagnetic matrix. The magnetic response evolves between two modes, ferromagnetic and superparamagnetic, depending on both the temperature and distribution of Ni nanoparticles in the sample. The behavior of EMR spectra shows that the initial materials retain the ferromagnetic character of the NP ensemble even at room temperature. This is most likely due to dipole-dipole interactions and macroscopic demagnetizing fields stemming from powderlike composition of the samples. For the diluted materials, the actual superparamagnetic signal is observed at room temperature. As temperature is reduced, the behavior of the EMR parameters reflects a gradual transition from free rotated magnetic moments of NPs to those ordered along the "easy" magnetic axes (blocked state). In the 300-130 K temperature range, anomalous temperature dependence of the resonance magnetic field H-res was observed. It is examined in terms of competition between the single-particle anisotropy energy and dipole interactions between the Ni nanoparticles. Finally, a transition to a blocked state occurs at blocking temperature estimated as T-b approximate to 40 K. At lower temperatures, both the dense and diluted samples behave identically. (c) 2006 American Institute of Physics.