Elsevier

Materials Research Bulletin

Volume 36, Issues 7–8, May–June 2001, Pages 1531-1538
Materials Research Bulletin

The synthesis and characterization of the hexagonal Z ferrite, Sr3Co2Fe24O41, from a sol-gel precursor

https://doi.org/10.1016/S0025-5408(01)00596-7Get rights and content

Abstract

The Z ferrite Sr3Co2Fe24O41, produced from a stoichiometric aqueous inorganic sol-gel precursor, is reported and its magnetic hysteresis loop characterized for the first time. The precursor was an iron(III)hydroxide sol stabilised with NO3- counterions and doped with stoichiometric nitrate salts, which had a particle size similar to halide stabilised Ba3Co2Fe24O41 Z ferrite precursor sols investigated previously. When fired the amorphous gel formed a mixture of α-Fe2O3, BaM and CoFe2O4 from 600°C, but these products then converted directly to the pure Z phase at 1200°C, without first forming the Y ferrite (Ba2Co2Fe12O22) phase always observed prior to Z ferrite crystallization in Ba3Co2Fe24O41 systems. The resulting material was a very soft ferrite, with a very low coercivity of 5.6 kA m-1 and a magnetisation of 48.5 emu g-1 at an applied field of 5 T.

Introduction

The Z ferrites are one of the family of hexagonal ferrites discovered by researchers with Philips in the late 1950s, and have the composition Ba3Me2Fe24O41, where Me = a transition metal 2+ ion [1]. The Z ferrites all have a uniaxial anisotropy parallel to the c-axis, except for Co2Z (where Me = Co2+), which is planar at room temperature but has a complex magnetic anisotropy, with at least four different anisotropic states. At low temperatures Co2Z has an easy cone of magnetisation at an angle of 65° to the c-axis, and this remains constant up to -103°C. Between this temperature and -53°C the angle increases to 90°, and the preferred magnetisation remains in the basal plane until it switches to the c-axis at some temperature between 207 and 242°C [2], [3]. Due to this planar anisotropy at room temperature Co2Z is magnetically soft, with a very small coercivity (Hc), but it also has a large magnetic permeability which gives it a high saturation magnetisation of 50 emu g-1(=50 A m2 kg-1) [2]. Although Co2Z is of little use as a permanent magnet, it has a much higher permeability and ferromagnetic resonance than the spinel ferrites [4], up to 1.5 GHz, bringing it into the microwave region useful for inductor cores, uhf communications, EM wave absorption and other niche applications [2].

The Z phase can be thought of as an alternate stacking of two other hexagonal ferrite phases, BaM (BaFe12O19) and Co2Y (Ba2Co2Fe12O22), and although the formation process is still not fully understood it seems that the M and Y phases must coexist first before the Z phase can crystallise, probably through a topotactic reaction [5]. For this reason Z ferrites are difficult compounds to form. A temperature of at least 1200°C is required, The Z phase usually coexisting with some or all of the phases M, Y, W and spinel [6]. It has been reported that 1225°C appears to be the optimum temperature, but the Z phase was only obtained as a major component mixed with the W (BaCo2Fe16O27) phase [7]. Single phase Z is notoriously hard to produce.

Nevertheless, we have previously reported the synthesis of a range of hexagonal ferrites, including single phase Co2Z [8], manufactured from aqueous inorganic sol-gel precursors, which can also be blow spun to produce an aligned ferrite fibre for use in composite applications. The composition, phase evolution and microstructure of these Co2Z ferrites has already been investigated [9], and their microwave properties investigated [8].

In many of the hexagonal ferrites, as well as varying the Me2+ ion, barium can be substituted with another group II metal, most commonly strontium, and SrM (SrFe12O19) is a widely used hard ferrite, often with Ms and Hc values superior to those of BaM [10]. Like the M phase, the W phase can withstand total replacement of barium with strontium [11], and SrZn2W is the most commercially important W ferrite for its magnetic and microwave properties, usually when doped with another divalent metal ion, and it forms at a relatively low temperature of 1100°C [12]. Pure SrZn2W has a high saturation magnetisation of up to 77 emu g-1 and a low coercivity of around 39.8 kA m-1 (795.8 kA m-1 = 1 T) [13], despite being strongly uniaxial. However, Sr2Zn2Y demonstrates a strange, non-collinear magnetic structuring due to the Sr2+ ion distorting the lattice, and therefore the replacement of barium with strontium in Y ferrites steadily reduces the permeability [14]. The Z phase is tolerant to substitution with strontium up to 100%, but Sr3Zn2Z shows a lattice distorted by the Sr2+ ion, reducing drastically the magnetic properties of the compound compared to Ba3Zn2Z [14]. However, there appears to have been no investigation of Sr3Co2Z ferrite and its magnetic characteristics have never been published. Therefore this paper details the synthesis of Sr3Co2Z from an aqueous inorganic sol-gel precursor, and the characterization of the magnetic hysteresis loop of the resulting product.

Section snippets

Sol preparation

Previous Co2Z ferrite sols were made using a halide stabilised iron(III) sol doped with stoichiometric amounts of barium and cobalt halide salts, halides being used due to the relative insolubility of other barium salts causing instabilities in the sol. However, it was found that these halides were retained to some degree at temperatures up to 1000°C, delaying formation of the ferrite phase [9]. A halide free SrM precursor sol has been successfully made by the authors using a nitrate stabilised

Photon correlation spectroscopy (PCS)

The particle sizes of the sols were measured on a Malvern Instruments Lo-C Autosizer and series 7132 multi-8 correlator at an angle of 90°, using a 4 mw diode laser, 670 nm wavelength at 20°C. The volume and number distribution particle sizes and ranges were calculated from the cumulants results using the Malvern PCS software version 1.32 with contin algorithms. The volume distribution is a measure of the volume occupied by particles against their size, and the number distribution is a simple

Results and discussion

A nitrate-stabilised iron(III)hydroxide sol was successfully produced which was stable up to a maximum concentration of 23.5% Fe, beyond which point it started to precipitate, and at which point it had a viscosity of 16 poise. Not only was this a lower maximum concentration than the halide based sol reported previously, but the nitrate sol was much more viscous and “muddy” looking at equivalent concentrations, and was only stable for around a year when stored as 10.5% Fe. The halide based sol

Conclusions

Sr3Co2Fe24O41 was produced from a stoichiometric aqueous inorganic sol-gel precursor, and characterized magnetically. The precursor was an iron(III)hydroxide sol stabilised with NO3- counterions, which was doped with stoichiometric solutions of strontium and cobalt nitrate, and the resulting sol had a particle size similar to halide stabilised Ba3Co2Z precursor sols investigated previously. When fired the amorphous gel behaved in similar fashion to these previous gels at lower temperatures,

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  • Synthesis, characterization, and electromagnetic wave absorption properties of Sr<inf>3</inf>Co<inf>2</inf>Fe<inf>24</inf>O<inf>41</inf> hexaferrites

    2022, Journal of Magnetism and Magnetic Materials
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    It is also confirmed that the sample calcined at 1230 °C with the highest purity of the Z-type phase has the lowest HC values. The slower magnetization behavior after first sharp magnetization is typical for Sr3Co2Z hexaferrite and similar M-H behaviors also have been reported in the previous studies [7,28]. It is associated with the magnetic non-collinear spin structures which cause magnetoelectric effect in Z-type hexaferrites [29].

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