Zoran V. Popović
Center for Solid State Physics and New Materials, Institute of Physics, University of Belgrade, Pregrevica 118, 11080 Belgrade, Serbia
Magnetic properties of materials fundamentally change when the particle sizes are reduced [1]. Below a critical size (DC), which typically lies below 100 nm, normal microscopic multidomain ferromagnetic (FM) structure is energetically unfavorable, and the particles are in single domain state. In this state, the mechanism of magnetization reversal can only occur via the rotation of the magnetization vector from one easy magnetic direction to another over the magnetic anisotropy barrier [2]. As particle size decreases within the single domain range, another critical threshold (DSP) is reached, at which remanence and coercivity go to zero and the particles are in the superparamagnetic (SP) state. Such a system has no hysteresis and magnetization curves at different temperatures superimpose onto a universal curve of M/MS vs H/T (Langevin's curve).
We have measured the magnetization of undoped and Fe2+/3+ doped CeO2-y nanocrystals at various temperatures and magnetic fields [3]. In the case of Fe-doped samples, the superparamagnetic behavior of this system is revealed by nearly zero coercive field, an appearance of the blocking temperature below 20 K, as well as the M(H) dependence, which is well fitted by weighted Langevin function which takes into account particle magnetic moment distribution [3].
We have also measured Raman scattering spectra of these nonocrystalline samples. Raman mode exhibits softening and broadening by changing the valence state of Fe dopant, as a consequence of the electron-molecular vibration coupling. The electron-molecular vibration (phonon) coupling constants λ and density of electron states at the Fermi level per spin and molecule N(0)=22 (eV)-1 were determined. The Stoner condition N(0)I>1 is multiple fulfilled in the case of nano CeO2-y favoring the band ferromagnetism approach [4].
References
[1] G. Herzer, Ch. 3: Nanocrystalline soft magnetic alloys, Handbook of magnetic materials, Vol 10, edited by K.H.J. Buschow, Elsevier Science BV, p.418 (1997).
[2] E. C. Stoner and E. P. Wohlfarth, Phyl. Trans. R. Soc. London, A 240, 599 (1948).
[3] N. Paunović, Z. V. Popović, and Z. D. Dohčević-Mitrović, J. Phys: Condens. Matter 24, 456001 (2012), https://doi.org/10.1088/0953-8984/24/45/456001
[4] Z. V. Popović, Z. D. Dohčević-Mitrović, N. Paunović, and M. Radović, Phys. Rev. B 85, 014302 (2012), https://doi.org/10.1103/PhysRevB.85.014302
