N.C.S.R. "Demokritos"


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Magnetoelectric oxides

Magnetoelectric compounds are very rare since special requirements are necessary. In order the magnetoelectric tensors to have non-zero values; an inversion center should be combined with the operator which reverses all currents. In these materials magnetic and electric properties are coupled so that (i) for H=0 the magnetic moment responds to an applied electric field (M=aikEi) and (ii) for E=0 the electric polarization can change with a magnetic field (P=aikHk) as well. Although several studies exist from the early sixties, the exact microscopic explanation of the magnetoelectric effect still has not been achieved also corroborated from the rare existence of magnetoelectric compounds. One of them, known from the sixties, with high anisotropic magnetic properties is the orthorhombic form of the mixed Al2-xFexO3 compound which is piezoelectric and ferrimagnetic. This compound was discovered by Richardson when they heated a mixture consisting of Fe2O3 and Al2O3 in oxygen at 1400oC. By using x-ray diffraction, magnetization and Moessbauer spectroscopy techniques we have studied the magnetoelectric Al2-xFexO3 (x=0.8, 0.9 and 1.0) compound. The magnetic measurements revealed magnetic transitions at TN=180, 210 and 260 K for x=0.8, 0.9 and 1.0 respectively, which can be attributed to the Neel temperatures of the ferrimagnetic to paramagnetic phase transitions for all samples. Moessbauer spectra for the three samples were recorded between 4.2 K and 295 K. Above Neel temperature the paramagnetic spectra can be analyzed by three quadrupole doublets associated with the octahedral Fe1, Fe2 and Fe4 sites. The values of the hyperfine parameters show that iron ions are in Fe+3 in high spin state. The spectrum area of the doublet with larger quadrupole splitting increases with x, and in combination with x-ray diffraction results, it can be attributed to the iron which occupies the Fe4 site. Below TN(x) the Moessbauer spectra are magnetically split and at T=4.2 K consist of six broad lines, indicating either a hyperfine magnetic field distribution or the three octahedral sites give three unresolved sextets. The most probable value of the hyperfine magnetic field follows a power law indicative for a second order transition, in agreement with magnetic moment and ac-susceptibility measurements.

Relevant journal articles

  • Pissas, M., Stamopoulos, D., Sanakis, Y., Simopoulos, A. (2008) Magnetic properties of the magnetoelectric Al2-xFexO3 (x = 0.8, 0.9 and 1), Journal of Physics Condensed Matter 20, -.
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