Electron paramagnetic resonance spectra near the spin-glass transition in iron oxide nanoparticles
Electron paramagnetic resonance spectra near the spin-glass transition in iron oxide nanoparticles. Koksharov Yu.A., Gubin S.P., Kosobudsky I.D., Yurkov G.Yu., Pankratov D.A., Ponomarenko L.A., Mikheev M.G., Beltran M., Khodorkovsky Y., Tishin A.M. //Physical Review B: Condensed Matter and Materials Physics. 2001. V. 63. № 1. P. 124071-124074
Electron paramagnetic resonance (EPR) in iron-oxide nanoparticles (∼ 2.5 nm) embedded in a polyethylene matrix reveals the sharp line broadening and the resonance field shift on sample cooling below TF ≈ 40 K. At the same temperature a distinct anomaly in the field-cooled magnetization is detected. The temperature dependences of EPR parameters below TF 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.
Oxide nanostructured media are considered as very perspective materials for high density magnetic recording. This is the reason for the great interest in the maghemite (γ-Fe2O3) nanoparticles (NP). The strong decreasing of the saturation magnetization in maghemite NP, in comparison with the bulk counterpart, gave rise to a conception of spin canting. The principle question is whether only the surface spins of a particle resist being aligned with even a large external magnetic field, or if such a property inheres in the core spins as well. Recent studies of low field zero field cooled (ZFC) and field cooled (FC) magnetization curves of γ-Fe2O3 NP evidence the existence of a spin-glass-like surface layer that undergoes a magnetic transition to a frozen state below ≈ 42 K. Analogous spin-glass-like behavior below about 50 K was also found in oxygen passivated iron NP, as well as in NiFe2O4 NP. It seems natural to study these spin-glass-like phenomena in NP by an EPR technique, which has been proven to be a very useful tool for exploring spin dynamics in various ferromagnets and antiferromagnets and, especially, in spin glasses, including reentrant alloys. Whereas measurements of the magnetic moment provide integral sample characteristics, EPR data give information about local magnetic properties and, in principle, about the nature of spin-spin interactions, the distribution of internal fields, and spin-spin correlations. As a rule, in canonical bulk spin glasses (SG’s) the EPR resonance field Hres and the EPR linewidth ΔH are roughly temperature independent at high temperatures, but change rapidly [ΔH~exp(-T/Tg)] if T<Tg ,where Tg is the spin-freezing temperature. Contrary to usual magnetic phase transitions, for which the linewidth diverges at the critical temperature, bulk SG reveal a finite value of ΔH at Tg . Due to the very complicated magnetism of spin glasses, there is no completely adequate theory of the linewidth temperature dependence. The increase in the linewidth is usually attributed either to a broadening from a distribution of internal local fields, or to a slowing down of the spin-relaxation rate on approaching Tg.
Some microscopic features of SG and NP systems are similar, e.g., the maximum of ac and low-field ZFC susceptibility at a certain temperature Tm, as well as the irreversibility (splitting between ZFC and FC curves). Spin-glass-like behavior in the NP systems is usually considered as a result of the random dipole-dipole interaction between NP at low enough temperatures, when all the particle moments are blocked along the anisotropy axes. Correlations between the particle moments develop in a similar way to the correlations between spins in spin glasses. A lot of magnetic NP systems, similarly to SG, show a broadening and a low-field shift of EPR lines with a temperature decreasing. Nagata and Ishihara proposed a phenomenological description for these anomalies in superparamagnetic systems. They derived a simple power relation between the shift (relative to a high temperature value) of the resonance field δHres and the EPR linewidth ΔH. For randomly oriented particles it was found that δHres~(ΔH)3. This theory does not take into consideration effects of magnetic transitions in nanoparticle systems and, probably, it should not hold below Tg. Some spin-glass concepts have been used for analysis of low-temperature anomalies of EPR spectra in NP. However, qualitative differences in behavior of EPR spectra in NP and in bulk spin glasses are still lacking. It should be stressed, that the present work is devoted to the study of the intrinsic spinglass state which takes place inside an individual particle, resulting from interactions between spins, which form its internal magnetic structure.
We report the results of static magnetization and EPR measurements on iron-oxide NP embedded in a polyethylene matrix. The samples were prepared by the high-speed thermal decomposition of an iron-containing compound in a solution/melt of polyethylene in vaseline oil in an inert atmosphere at 220°C. This method allows for the fabrication of particles with bimodal lognormal diameter distribution F(D) and effective sizes below 10 nm. For the samples studied, our small-angle X-ray diffraction measurements show that the maximum of F(D) is near 2.5 nm. A room-temperature Mossbauer spectrum of the samples can be considered as a superposition of two doublets of nearly equal intensity, with the isomer shift 0.35 ± 0.01 mm/s (with reference to α-Fe) and values of the quadrupole splitting 0.75 ± 0.01 mm/s and 1.27 ± 0.02 mm/s. These parameters are close to those for γ-Fe2O3 superparamagnetic NP. The iron content in powderlike samples is approximately 30 wt. %.
In summary, the thermal behavior of the EPR linewidth, the effective resonance field, as well as the FC magnetization curve demonstrate anomalies near 40 K, which can be related to the SG freezing in the NP surface layer. We found a linear increasing of the EPR linewidth excess below TF, in good agreement with the thermal behavior of the exchange anisotropy field predicted by the random-field model. The violation of the Nagata-Ishihara relation suggests essentially different mechanisms of EPR line broadening in the superparamagnetic regime and below spin-glass freezing. Our data indicate that in iron-oxide NP the predominant cause of the EPR linewidth changing below the spin-freezing temperature is the influence of the exchange anisotropic field on the resonance conditions.