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Magnetic Structure and Multiferroic Properties of Ga-Substituted M-type Hexaferrites (PbFe12-xGaxO19)

Magnetic Structure and Multiferroic Properties of Ga-Substituted M-type Hexaferrites (PbFe12-xGaxO19)
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Mutual control of magnetization (M) and electric polarization (P) by the electric (E) and magnetic (H) fields is a major challenge in condensed-matter science and applications that may be realized by multiferroic materials. Recently, multiferroics have attracted considerable attention owing to their potential applications in various devices such as multibit memories or sensitive magnetic field sensors. One interesting class of materials to look at is the hexaferrites composed of magnetoplumbite-related members, such as M type (i.e., AFe12O19: A = Pb, Ca, Sr, Ba, etc.), Y type (i.e., A2Me2Fe12O22: Me = transition metal), Z type (i.e., A3Me2Fe24O41), etc., where types of elemental blocks and their stacking order are different from each other. A ferrimagnetic M-type hexaferrite with the simplest crystalline structure among the hexagonal ferrites is an industrially mass-produced uniaxial hard magnet. There have been extensive research efforts to partially replace Fe ions by some other cations to improve magnetic and dielectric properties of M-type hexaferrites. The influence of the cationic distribution and the consequent sublattice magnetization of M-type hexaferrites have been studied by neutron diffraction and Mössbauer spectroscopy measurements. Among numerous studies done on the partial replacement of Fe ions by diamagnetic or paramagnetic ions, the Ga-substitution is expected to play a unique role. This is because Ga ion is trivalent, with its ionic radius comparable to that of Fe3+. Thus, the trivalent Ga ions are expected to minimize the formation of oxygen vacancy defects and consequently to improve the electrical resistivity and the dielectric loss. Furthermore, since Ga ions are nonmagnetic, the Ga-ion substitution for Fe ions would modulate the degree of the superexchange interaction between neighboring Fe ions and thus alter magnetic properties. We have studied magnetic structure and properties of Ga-substituted Pb-hexaferrites having the stoichiometry of PbFe12-xGaxO19 with x = 6 (i.e., Fe:Ga = 1:1). According to the neutron diffraction results, this compound is characterized by a collinear spin structure below its Curie temperature (~325 K). Analysis of the neutron diffraction patterns further indicates that the magnetic-moment direction of Fe3+ ions located at the octahedral 2a sublattice is downward while that of the unsubstituted PbFe12O19 is upward at room temperature. With decreasing temperature, the Fe3+ magnetic moment at the octahedral 2a sublattice undergoes a reorientation to the upward direction while that of the unsubstituted PbFe12O19 remains upward down to 5 K. This selective local spin reversal at the 2a sublattice of PbFe6Ga6O19 was attributed to the weakening of the superexchange interaction between the octahedral 2a site and the tetrahedral 4fIV site upon the preferential substitution of Ga ions for Fe ions at these two neighboring sites. Comparison of the neutron diffraction results with dc magnetization responses and ac susceptibilities further indicates that the paramagnetic-ferrimagnetic transition at ~325 K (Tc) is followed by the local spin reversal at lower temperatures. We also have measured the directional dependence of their dielectric properties and polarization. The dielectric constant as a function of temperature along the a-axis, ε_α (T) exhibits a broad peak near 70K. In contrast, the anomaly of ε along the c- axis is hardly discernible. In the high temperature region (T>150K), ε shows no apparent anomaly. Loss tangent also exhibits a little sharp peak near 50K. Polarization reached up to 20nC/cm2 at 5K along a-axis, where as the P along c-axis is very small (~2nC/cm2). With dielectric constant, these results indicate that ferroelectric polarization lies with ab-plane (perpendicular to c-axis) in the hexagonal phase. The spontaneous polarization value itself is relatively small compared with that of conventional ferroelectric, but is comparable with that of the typical magnetically induced multiferroic materials. Polarization is induced below 70K, which is coincident with magnetic transition (spin reversal) temperature. These results indicate the electric polarization is attributed to a magnetic transition. From magnetic structure analysis, we note that Fe3+(4fIV) - Fe3+(4fIV) pair across neighboring zigzag chains has parallel spins, whereas Fe3+(2a)-Fe3+(4fIV) has antiparallel spins. The exchange striction shifts ions in a way that optimizes the spin-exchange energy: ions with antiparallel spins are pulled to each other, whereas ion having parallel spins move away from each other. This leads to the distortion, which breaks inversion symmetry and induces net polarization in ab-plane.
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