Elsevier

Scripta Materialia

Volume 48, Issue 7, 1 April 2003, Pages 961-966
Scripta Materialia

Enhanced magnetisation in nanocrystalline high-energy milled MgFe2O4

https://doi.org/10.1016/S1359-6462(02)00600-0Get rights and content

Abstract

The changes in magnesium ferrite (MgFe2O4) caused by high-energy milling are investigated by means of Mössbauer spectroscopy, magnetisation measurements, and electron microscopy. The observed enhancement of the magnetisation in nanoscale milled MgFe2O4 is discussed with respect to the mechanically induced cation redistribution and spin canting.

Introduction

The high-energy milling as a solid-state processing method has been the subject of great interest in recent years [1]. The highly nonequilibrium nature of the milling process creates the opportunity to prepare solids with improved and/or novel physical and chemical properties [2]. Owing to the flexibility of the structure of spinel ferrites, (Me1−xFex)[MexFe2−x]O4, providing a wide range of physical behaviour, they have been considered as very convenient model systems for such processing studies [3], [4], [5], [6]. Here, round and square brackets denote cation sites of tetrahedral (A) and octahedral [B] coordination, respectively. Me is divalent metal cation; x represents the so-called degree of inversion (defined as the fraction of (A) sites occupied by Fe3+ cations).

Nanosized milled spinel ferrites exhibit interesting physical and chemical properties markedly different from those of their bulk counterparts. The high-field magnetisation irreversibility [7], the variation of the Néel temperature with grain size [8], [9], a high coercivity [10], and an altered (reduced [7], [11] or enhanced [12], [13], [14]) magnetic moments in comparison with the corresponding bulk materials have been observed in nanoscale milled ferrimagnetic spinels. Mechanical treatment was found to be an useful activation method leading to an enhanced chemical reactivity of nanostructured spinel ferrites [15], [16].

A better understanding of the nanostructure-magnetism relationship in the high-energy milled spinel ferrites is crucial not only for basic science (the development of an atomistic and microscopic theory of the mechanochemical processes, as an example [17]) but also because of the technological applications in catalysis, ferrofluids and information storage [18]. This experimental work focuses on the study of magnetic properties, spin structure and ion configuration in nanocrystalline milled MgFe2O4. Although extensive studies have recently been performed on the nanosized milled MgFe2O4 [3], [9], [19], [20], no measurements, to the best of our knowledge, of the magnetic properties of this metastable solid have been reported.

Section snippets

Experimental

The nanocrystalline MgFe2O4 sample was produced by high-energy milling coarse powders of high purity MgFe2O4 [3] in a planetary ball mill EI 2×150 (product of the Institute of Solid State Chemistry and Mechanochemistry, Novosibirsk) at room temperature. A ceramic-covered grinding chamber and balls made of α-Al2O3 were used. The ball-to-powder weight ratio was 50:1. Grinding experiments were performed in air at 750 rpm.

Mössbauer measurements were carried out at 293 and 6.4 K under a magnetic

Results and discussion

The room-temperature Mössbauer measurements revealed that the milling of the bulk MgFe2O4 leads to the partly collapse of the magnetic hyperfine splitting and to the appearance of a central doublet (Fig. 1). The doublet can be understood to arise from 57Fe in ultrafine ferrite particles exhibiting superparamagnetic behaviour [3], [22]. The influence of superparamagnetic relaxation can be counteracted by reducing the sample temperature. Mössbauer spectra of both bulk and milled MgFe2O4 taken at

Conclusions

The nanocrystalline milled MgFe2O4 is structurally and magnetically disordered due to mechanically induced changes in the cation distribution and spin canting. Although the spin canting effect tends to reduce magnetic moment, enhancement of magnetisation in milled MgFe2O4 is attributed to the mechanically induced cation redistribution. The behaviour of the ZFC sample is associated with the co-existence of both ferrimagnetic and superparamagnetic phases in the milled material. The contribution

Acknowledgments

This work was supported by the Alexander von Humboldt Foundation (AvH) and Slovak Grant Agency for Science. One of the authors (V.Š.) would like to thank the AvH for the award of Humboldt Research Fellowship for Long-Term Cooperation and the United Engineering Foundation (UEF) for the award of the UEF’s Conference Fellowship.

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