Electron Paramagnetic Resonance Of Ferrite Nanoparticles. Koksharov Yu.A., Pankratov D.A., Gubin S.P., Kosobudsky I.D., Beltran M., Khodorkovsky Y., Tishin A.M. //Journal of Applied Physics. 2001. V. 89. № 4. P. 2293-2298
Three types of iron-based oxide nanoparticles (weight compositions Fe2O3, BaFe2O4, and BaFe12O19) embedded in a polyethylene matrix are studied using the electron paramagnetic resonance technique. All nanoparticles are found to be multiphase. Thermal variations of electron paramagnetic resonance spectra reveal the presence of two phases in the Fe2O3 nanoparticles. One such phase undergoes an antiferromagnetic-like transition near 6 K. Nanoparticles of BaFe2O4 demonstrate a resonance anomaly near 125 K that could indicate the presence of a magnetic phase. Reduced magnetic anisotropy in BaFe12O19 nanoparticles may be related to either structural imperfection or particle smallness (effective diameter of less than 10 nm). Our data clearly show that low temperature experiments are desirable for the correct identification of nanoparticles by means of the elecstron paramagnetic resonance technique.
Iron-based oxide materials are technologically important materials in which there is an unceasing interest. Whereas γ-Fe2O3 is the most popular general-purpose magnetic tape material, BaFe12O19 and its substituted derivatives are considered to be the most promising candidates for use in highdensity recording media because of their chemical stability and suitable magnetic characteristics. Advanced barium ferrite tapes, which utilize particles with a diameter of 40–50 nm, an aspect ratio near 4 and coercivity of about 2000 Oe, offer superior high-density recording performance. Even the pure-phase BaFe12O19 is doped with other cations in order to reduce its magnetocrystalline anisotropy. Several methods are often used to prepare ferrite nanoparticles: glass crystallization, ball milling, chemical coprecipitation, and the citrate precursor technique. It is quite unpleasant that very unlike magnetic properties have been observed in materials that have a similar nanoparticle size but that were produced by different methods. This may be due to structural order–disorder differences. It was also found, with respect to microcrystalline γ-Fe2O3 particles (about 100 nm in diameter), that the degree of order in the distribution of vacancies affects their magnetic properties, suggesting that atoms inside a particle can be significantly influenced by canting effects.
It is interesting to compare the properties of iron-based oxide nanoparticles that have various iron/oxygen ratios, all prepared by the same method. Here we report on an electron paramagnetic resonance study of three types of iron-based oxide nanoparticles with weight compositions of Fe2O3, BaFe2O4, and BaFe12O19. It is well known that bulk Fe2O3 exists either as ferromagnetic γ-Fe2O3 (maghemite) or antiferromagnetic α-Fe2O) (hematite). Maghemite nanoparticles seem to be a model for the experimental study of nanoparticles and for testing the validity of theoretical concepts. Bulk hematite is antiferromagnetic below the Morin temperature (TM ≈ 260 K) and weakly ferromagnetic above TM. In hematite nanoparticles, the TM shifts to helium temperatures and no marked anomalies are revealed in the magnetization near 260 K. Nonferromagnetic BaFe2O4 has a rhombic structure and often appears as an impurity phase during the preparation of BaFe12O19. To our knowledge, the present work is the first communication about ferrite BaFe2O4 in nanoscale form. To date, the preparation of nanoparticles of the barium hexaferrite BaFe12O19 has been a difficult task. Only recently BaFe12-2xTixCoxO19 particles with a mean diameter of approximately 8 nm have been prepared as ferrofluid.
Electron paramagnetic resonance (EPR) is accepted as a very useful technique to study the properties of bulk paramagnetic compounds, including their transitions to the magnetic ordering state. Below the critical temperature, ferromagnetic (FMR) or antiferromagnetic (AFMR) resonances are usually detected. EPR spectra in paramagnetic samples can give information about the resonance-active ion valence and the symmetry of the ligand environment. Angular variation analyses of FMR signals in ferromagnetic single-crystal and textured samples offer the opportunity to deduce the sign and the value of the magnetic anisotropy. Selected nanoparticle systems (including γ-Fe2O3, Fe3O4, Mn0.6Fe0.4Fe2O4, and Mn–Zn ferrite) were investigated by the EPR method. Unfortunately, in the case of nanoparticles the theory of EPR leaves much to be desired since it either remains semiqualitative or requires laborious numerical calculations. However, a study of the thermal variation of EPR spectra in nanoparticles can be very informative because of the high sensitivity of the EPR technique to various phase transitions.
Three types of iron-based oxide (ferrite) nanoparticles in a polyethylene matrix were studied by the EPR technique. The EPR spectra suggest that Fe2O3 nanoparticles contain ferromagnetic (presumably, γ-Fe2O3) and antiferromagnetic (α-Fe2O3) phases. The strong superparamagnetic resonance (SPR) signal (g ≈ 2.1) that dominated at high temperatures in typical of γ-Fe2O3 nanoparticles. The weak EPR signal of the rhombic symmetry (g ≈ 4.3) that appears at low temperatures may be attributable to an α-Fe2O3 phase that undergoes an antiferromagneticlike transition near 6 K. The BaFe2O4 nanoparticles, the bulk counterpart of which is non-ferrimagnetic, demonstrate an EPR anomaly near 125 K that could indicate the presence of a BaFe2O4 phase. The BaFe12O19 nanoparticles reveal an EPR signal that is significantly narrowed at high temperatures by superparamagnetic fluctuations. This is evidence of the reduced magnetic anisotropy energy that may be due to structural imperfections or to the particle’s smallness (effective diameter <10 nm). A comparison of high- and low temperature EPR spectra in Fe2O3 and BaFe2O4 nanoparticles clearly shows that low temperatures experiments are desirable for the correct identification of nanoparticles by means of the EPR technique.