Superparamagnetic Ferrites Realization and Physical Obstacles
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ABSTRACT In superparamagnetic materials, the change of the direction of the magnetization is not associated with the movement of Bloch walls, but with thermal fluctuation of the magnetization vector. Therefore, the resonance frequency of the Bloch walls is no longer limiting the maximum frequency for applications. The limit found in superparamagnetic materials is given by the frequency of electron spin resonance. This behavior was verified for spinelle type ferrites made of ceramic or polymer coated oxide nanoparticles produced by the microwave plasma process. By selecting the composition of the spinelle type ferrites the energy of magnetic anisotropy controlling the susceptibility and the maximum frequency for applications can be adjusted. Superparamagnetic materials have their frequency limit beyond 2 GHz. Coating of the particles reduces dipole - dipole interaction destroying superparamagnetism. Even when the susceptibility is in the order of magnitude of today's commercial ferrites, the saturation magnetization is found to be smaller than the theoretically expected value. This phenomenon is partly clarified by soft X-ray magnetic circular dichroism (SXMCD) measurements, showing a significant orbital magnetic moment anti arallel to the direction of the spin moment. Additionally, it was found that the amount of Fe ' ions is possibly larger than expected by thermodynamic data of bulk materials. INTRODUCTION Maghemite (y-Fe 2Oa) with grain sizes in the range of micrometers is ferrimagnetic. Pro-
vided that the particle size is sufficiently small, nanocrystalline iron oxides are known to be superparamagnetic. A thermally fluctuating vector of magnetization characterizes superparamagnetic materials. Provided the particles fulfil the condition Kv < kT (1) (K ... constant of magnetic anisotropy, v ... volume of the particle, Kv ... energy of magnetic anisotropy, kT ... thermal energy) the material is superparamagnetic [1,21. This leads to zero coercivity. This condition may be fulfilled in two different ways [3, 4]: Either the particles are embedded in a liquid and free to rotate or the particles are bond together in a solid. In the first case. Brownian superparamagnetism with relatively large relaxation times is observed. The second case leads to the N6el superparamagnetism. In this case the orientation of the electron spins is fluctuating relative to the particle. In this case the relaxation time may be significantly less than one nanosecond. The proof for this type of superparamagnetism is the Mol3bauer effect. The MOssbauer spectra of superparamagnetic materials show a pure quadrupole splitting above and a magnetic sextet structure below the blocking temperature. The transition temperature between the ferrimagnetic and the superparamagnetic state as given in Eq. 1 is called the "blocking temperature". Superparamagnetic materials exhibit a magnetisation curve free of hysteresis in the range between the blocking temperature and the Curie temperature. Below the blocking temperature a non-zero remanence is observed.
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