High-frequency ferromagnetic resonance on ultrafine cobalt particles

Abstract

We report on high-frequency ͑300-700 GHz͒ ferromagnetic resonance ͑HF-FMR͒ measurements on cobalt superparamagnetic particles with strong uniaxial effective anisotropy. We derive the dynamical susceptibility of the system on the basis of an independent-grain model by using a rectangular approach. Numerical simulations give typical line shapes depending on the anisotropy, the gyromagnetic ratio, and the damping constant. HF-FMR experiments have been performed on two systems of ultrafine cobalt particles of different sizes with a mean number of atoms per particles of 150Ϯ20 and 310Ϯ20. In both systems, the magnetic anisotropy is found to be enhanced compared to the bulk value, and increases as the particle size decreases, in accordance with previous determinations from magnetization measurements. Although no size effect has been observed on the gyromagnetic ratio, the transverse relaxation time is two orders of magnitude smaller than the bulk value indicating strong damping effects, possibly originating from surface spin disorders. ͓S0163-1829͑99͒50706-6͔ RAPID COMMUNICATIONS R3936PRB 59 M. RESPAUD et al.

Solvent Dependent Shape and Magnetic Properties of Doped ZnO Nanostructures

“…As a consequence, the nanoparticles reproduced some of the unusual properties of gas phase clusters such as a high magnetic moment and high anisotropy constant. [59][60][61][62] The high-frequency ferromagnetic resonance (HF-FMR) spectra of two systems of Co nanoparticles with sizes of 1.5 and 1.9 nm (called Coll.1 and Coll.2, respectively in ref. [60]) have been measured in the superparamagnetic (SP) regime as a function of the temperature.…”

“…[59][60][61][62] The high-frequency ferromagnetic resonance (HF-FMR) spectra of two systems of Co nanoparticles with sizes of 1.5 and 1.9 nm (called Coll.1 and Coll.2, respectively in ref. [60]) have been measured in the superparamagnetic (SP) regime as a function of the temperature. [60] They reveal broad absorption spectra (∆B = 1250 Gauss at T = 298 K for Coll.1).…”

Magnetic resonance in nanoparticles: between ferro- and paramagnetism

“…[6][7][8][9][10][11][12][13][14][15][16][17][18][19]. Various explanation of these "anomalies" have been proposed; their basic idea is to account for specific features of the spins disposed on the particle surface.…”

“…A number of magnetic resonance experiments were performed by various authors on assemblies of randomly oriented nanoparticles (as a rule, nanoparticles were embedded in a diamagnetic matrix to weaken or even exclude the interparticle interaction) [6][7][8][9][10][11][12][13][14][15][16][17][18][19]. For the most part the agreement between experimental data and theoretical predictions is rather poor and does not provide an opportunity of accurate quantitative analysis of the experimental results.…”

Spin-Polarized Electronic Structure

“…Studies of small free-standing FeN, CoN and NiN clusters have revealed that their magnetic moments are signifi cantly larger than the corresponding bulk magnetizations [59][60][61][62][63][64][65]. Non-vanishing magnetic moments have even been observed in small clusters of some 4d transition metals, for example Pd, Ru and Rh, which are non-magnetic in the solid state [66][67][68][69][70].…”