Phase Transitions, 2014 Vol. 87, Nos. 10 11, 938 952, http://dx.doi.org/10.1080/01411594.2014.953509
Effect of thermal treatment on magnetic and dielectric response of SrM hexaferrites obtained by hydrothermal synthesis Andrzej Hilczera*, Bart»omiej Andrzejewskia, Ewa Markiewicza, Katarzyna Kowalskab and Adam Pietraszkob a
Institute of Molecular Physics, Polish Academy of Sciences, Smoluchowskiego, Pozna n, Poland; b Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Ok olna, Wroc»aw, Poland (Received 7 May 2014; accepted 29 June 2014) Electric, dielectric and magnetic properties of SrFe12O19 hexaferrite ceramics obtained from hydrothermally synthesized single-phase nanopowders were studied in wide temperature range. The effect of space charge polarization, related to highly conducting grains with poor conducting grain boundaries, was found to be apparent at high temperatures and at low frequencies. The activation energy of relaxation of the (Fe3C Fe2C) dipoles in low conducting grain boundary regions was found to amount to 0.20 eV for non-annealed ceramics and to increase to 0.32 eV after thermal treatment. The temperature and frequency dependences of the dielectric permittivity for non-annealed and annealed SrFe12O19 ceramics were found to be correlated with respective dependences of the electric conductivity. We relate the observed increase in the saturation magnetization after annealing to an increase in coherent spin rotation in greater grains, which are however still below the critical single-domain size. Keywords: strontium M-type hexaferrite ceramics; hydrothermal synthesis; electric heterogeneity; dielectric and electric response; magnetic properties
1. Introduction Among the hexagonal ferrite family containing six types of compound of different chemical formula, the hexaferrites of M-type (AFe12O19; A D Ba, Sr, Pb) have been used for a long time due to their applications in permanent magnets, microwave devices, data storage and recording and magnetooptical devices.[1] As the nanoparticles were found to exhibit novel material properties, different from those in the bulk, the hexaferrites have gained increasing attention due to their potential applications in hyperthermia cancer therapy [2,3] and magnetically rewritable ferroelectic memories and electrically rewritable magnetic memories.[1,4,5] The hexaferrites consisting of 3d transition metal cations (Fe3C and M2C D Co, Cu, Fe, Mn, Ni, Zn), heavy alkaline cations (Ba2C, Sr2C, Pb2C) and oxygen anions (O2¡) have complex crystal structure.[1] The simplest description of the structure is based on the concept of three blocks termed S, R and T.[1,6,7] The S-block, of chemical formula 2 (MFe2O4) and cubic spinel structure, is built of two layers of four oxygen atoms and three transition metal atoms adjacent to each layer. Four of the transition metal cations are located at octahedral sites coordinating six oxygen anions and two metal cations are at tetrahedral sites with four oxygen ligands. The R-block, of chemical formula (Ba/Sr/Pb) *Corresponding author. Email:
[email protected] Ó 2014 Taylor & Francis
Phase Transitions
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Fe6O11 and hexagonal packing, consists of three layers: the top and the bottom layers have four oxygen atoms, whereas the central layer contains a heavy alkaline atom and three oxygen atoms. Five transition metal cations are located at octahedral sites and one Fe3C cation takes a trigonal bipyramidal site with five oxygen ligands. The T-block, of chemical formula (Ba/Sr/Pb)2Fe8O14, contains four layers: the top and bottom layers consist of four oxygen atoms and two adjacent layers with heavy alkaline atom and three oxygen atoms are sandwiched between the top and bottom layer. Six of the Fe3C cations are located at the octahedral sites and two Fe3C cations take the tetrahedral sites. The structure of the hexaferrites can be considered as consisting of various combinations of the S, R and T blocks. The M-type hexaferrites contain two chemical units in the unit cell, which is built of an RSRS combination, where the asterisk denotes that the block is turned for 180 around the c-axis. The Y-type hexaferrite unit cell has three chemical units with (TS)3 blocks, whereas the Z-type hexaferrites contain two chemical units in the unit cell with the following combination of the blocks: RSTRSTS. The long wavelength magnetic structures apparent in hexaferrites make them interesting for magnetoelectric applications. Ferroelectric polarization induced by non-collinear magnetic structures has been discussed theoretically and explained by spin-current and inverse Dzyaloshinskii Moriya models, which stated that spontaneous polarization can be induced only in spiral magnetic structures with cycloidal component.[8 10] The origin of the magnetoelectric coupling has been studied intensively in Y- and Z-type hexaferrites [4,5,11 18] to search for materials applicable in electric field controlled magnetic memories or magnetic field controlled electric memories. An intrinsic multiferroic properties have been reported in Ba2Mg2Fe12O22, where both the spontaneous polarization P and magnetization M were found to exist in zero magnetic field and P can be switched by very low magnetic field (
