Electron paramagnetic resonance spectra near the spin-glass transition in iron oxide nanoparticles

Abstract

Electron paramagnetic resonance ͑EPR͒ in iron-oxide nanoparticles ͑ϳ2.5 nm͒ embedded in a polyethylene matrix reveals the sharp line broadening and the resonance field shift on sample cooling below T F Ϸ40 K. At the same temperature a distinct anomaly in the field-cooled magnetization is detected. The temperature dependences of EPR parameters below T F are definitely different than those found for various nanoparticles in the superparamagnetic regime. In contrast to canonical bulk spin glasses, a linear fall-off of the EPR linewidth is observed. Such behavior can be explained in terms of the random-field model of exchange anisotropy.

Magnetic resonance in nanoparticles: between ferro- and paramagnetism

“…Evidently, this can be explained by the influence of the external field B ~ 3 kG which strongly exceeds the effective anisotropy field B a of the particle. Note that the blocking of maghemite nanoparticles in such magnetic fields was observed previously only at T<40 K [12,23]. Opposite to the broad signal behavior, the narrow signal remains at the same field; its amplitude decreases steeply with decrease in temperature, see Fig.…”

“…[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.…”

“…[2,8,12,15,24]). As a rule, this shift is assigned to surface phenomena, particularly, to the "exchange anisotropy" arising at the interface between ferro-and antiferromagnetic (or spin-glass) layers [26,27].…”

“…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.…”

Magnetic resonance in iron oxide nanoparticles: Quantum features and effect of size

“…The approach also fails in describing the double-feature shape of the EMR line (see, for example, Refs. [4][5][6][7][8][9][10]). …”