Structural features, magnetic and ferroelectric properties of SrFe10.8In1.2O19 compound

https://doi.org/10.1016/j.materresbull.2021.111236Get rights and content

Highlights

  • The spontaneous polarization has been found out in polycrystalline strontium ferrite SrFe10.8In1.2O19 at room temperature.

  • The crystal structure of strontium hexaferrite was considered in the framework of non- centrosymmetric space group.

  • The arising of spontaneous polarization in hexaferrite has been explained as a result of non-centrosymmetric distortion of the unit cell.

Abstract

The effect of temperature on the features of the crystal and magnetic structures, as well as the magnetic properties of the solid solution of SrFe10.8In1.2O19 hexaferrite was investigated. The appearance of ferroelectric properties was detected at room temperature, which contradicts the generally accepted opinion on the description of the crystal structure of hexaferrites within the framework of the centrosymmetric P63/mmc (No. 194) space group. The reasons of spontaneous polarization in solid solutions of hexaferrites remain controversial. No deviation from the collinear magnetic structure was found in the investigated temperature range from 1.5 to 350 K, while the thermal expansion of the unit cell was practically absent in the range from 1.5 to 50 K. The crystal structure of hexaferrites was considered in the framework of both centrosymmetric P63/mmc and non-centrosymmetric P63mc space groups, which allowed us to associate the emerging spontaneous polarization with unequal distortions of neighboring oxygen polyhedra.

Introduction

Hexaferrites with the molar formula MFe12O19 (where M = Ba, Sr or Pb) and their solid solutions are widely used in high-density magnetic recording media [1], wireless communication systems [2] and miniature electronic components such as circulators [[3], [4], [5]], phase shifters [6], filters [7] and inductors [8]. Strontium hexaferrites, which have large dielectric and magnetic losses in the microwave frequency band [9,10], can be used as microwave absorption devices [11,12] to reduce the impact of electronic pollution from gigahertz electronic telecommunication systems. Furthermore, hexaferrites are prospective multiferroics due to the relatively recent discovery of magnetoelectric effects in these materials [[13], [14], [15], [16], [17]], along with their high Curie temperature (∼732 K) [[18], [19], [20]].

The original hexaferrite SrFe12O19 is considered isostructural to magnetoplumbite PbO∗6Fe2O3, the crystal structure of which was first determined by Adelskold [21] in 1938. The unit cell of hexagonal ferrite contains two formula units (Z = 2) and includes 2 strontium ions, 24 iron and 38 oxygen ions, which create a close packing and form several types of voids: octahedra (2a, 12k and 4fVI), tetrahedra (4fIV) and trigonal bipyramids (2b) or (4e) with localized Fe ions in them.

As a result, the crystal structure of strontium hexaferrite is well described within the framework of the centrosymmetric space group (SG) P63/mmc (No. 194) [22,23]. However, this means that the unit cell contains an inversion center, which contradicts the appearance of spontaneous polarization observed in such materials [[24], [25], [26], [27]]. Therefore, the real reasons for the direct interaction between electronic and magnetic subsystems observed in [28], as well as their microscopic mechanism, are still under discussion. The ferroelectric eff ;ect was earlier observed in the initial compound BaFe12O19 [24] and Sc-substituted solid solutions BaFe12-xScxMgδO19 (x = 1.3–1.7; δ = 0 and 0.05) [28,29]. Despite the similar chemical composition and the same type of crystal structure of substituted and initial compounds, the appearance of spontaneous polarization in them was interpreted ambiguously. The polarization of hexaferrite solid solutions is associated with a spontaneous transition from a collinear ferrimagnetic uniaxial phase to a conical structure with decreasing temperature [28,29]. However, such an explanation is impossible for the initial samples with a collinear magnetic structure, the polarization of which was explained by the distortion of a single oxygen octahedron [24,26,30]. Furthermore, detailed structural information was not provided in these papers. Another proposal concerns the revision of the space group used to describe the crystal structure of M-type hexaferrites [31].

Substitution of iron by diamagnetic ions can lead to an increase in spontaneous magnetization, which depends on their preference to occupy crystallographic positions [32], and also to an increase in spontaneous polarization, when unoccupied d-shells form a strong non-symmetrical covalent bond with surrounding anions [33]. The paper is devoted to the establishment of the microscopic mechanism of the appearance of double ferroic properties in strontium hexaferrite SrFe10.8In1.2O19. The selected composition has a significant level of diamagnetic substitution, but it is insufficient for the appearance of a magnetic cycloid structure, as in [28,34]. This should allow us to understand the mechanism of the formation of the ferroelectric properties of hexaferrites and to analyze their relationship between spin, charge and structural subsystems. In the present work, we investigate the effect of ambient temperature on the features of the crystal and magnetic structures and their correlation with physical properties. The main feature of the paper is the use of the neutron diff ;raction method in a wide temperature range, which makes it possible to collect information on the evolution of both crystal and magnetic structures during one experiment.

Section snippets

Experimental procedure

The ceramic sample SrFe10.8In1.2O19 was obtained by the solid-state method. First, oxides Fe2O3, In2O3 and carbonate SrCO3 were mixed in appropriate proportions, and then the synthesis was carried out at 1200 °C (6 h) in air:SrCO3+5.4FO2O3+0.6In2O3SrFe10.8In1.2O19+CO2

The synthesized composition was pressed into tablets and annealed at 1300 °C (6 h), after which the samples were slowly cooled (∼100° C/h). The Curie temperature was determined by the ponderomotive method when measuring

Magnetic properties

The field dependences of the magnetization of the ceramic sample SrFe10.8In1.2O19 obtained at 5 and 300 K are shown in Fig. 1a. The values of the residual magnetization (σr) measured after switching off ; the external magnetic field, and the values of the coercive field (Hc) are given in Table 1. The lower values of the residual magnetization σr = 20.2 emu/g at room temperature in comparison with 27.94 emu/g at 5 K are explained by a decrease in the degree of spin collinearity of Fe3+ ions in

Conclusion

Spontaneous polarization at room temperature was observed in the solid solution of SrFe10.8In1.2O19 hexaferrite. This fact contradicts the generally accepted opinion on the description of the crystal structure of similar materials within the framework of centrosymmetric SG P63/mmc and indicates their ferroelectric properties. The discrepancy between the FC and ZFC curves characterizes the synthesized material as a magnetically inhomogeneous system. It was found that the coercive field decreases

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

Investigations were performed under financial support from the Russian Science Foundation (Agreement No. 19-19-00694 of 06 May 2019).

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