International Journal of Materials and Chemistry

p-ISSN: 2166-5346    e-ISSN: 2166-5354

2012;  2(2): 72-74

doi: 10.5923/j.ijmc.20120202.05

Magnetic Properties of Magnesium Doped Li-Cr Ferrites

A. M. Rais1, A. Addou2, M. Ameri1

1Département de physique, Faculté des Sciences, Université Djilali Liabes , Sidi Bel Abbes, Algeria

2Laboratoire STEVA, Département de chimie, Université de Mostaganem, Mostaganem, Algeria

Correspondence to: A.  M. Rais, Département de physique, Faculté des Sciences, Université Djilali Liabes , Sidi Bel Abbes, Algeria.

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Copyright © 2012 Scientific & Academic Publishing. All Rights Reserved.

Abstract

Mixed ferrites Cr1.05 Li0.5 Mg x Fe1.45-(2/3)x O4 (0 ≤ x ≤ 0.4) doped with Magnesium have been studied using x-ray diffraction, Mössbauer spectroscopy and magnetic measurements. X-ray diffraction patterns show that all samples have single phase cubic spinel structure. The temperature-dependent magnetic measurements revealed that magnetic compensation disappears when Fe3+ in A-site is partially replaced by Mg2+. Moreover, below the compensation temperature the observed magnetic moment of these ferrites increases with magnesium content. The magnetization data at all concentrations are discussed in the light of Néel’s molecular field model taking into account the cations distribution obtained using the analysis of Mössbauer spectra.

Keywords: Magnetic Compensation, Crystal Structure, X-ray Diffraction, Mössbauer Spectroscopy, Cations Distribution

1. Introduction

Li0.5Fe2.5O4 has been the subject to extensive technical and fundamental studies both in its pure form as well as its substituted form [1-3]. Various researchers have reported the effect of additions of divalent, trivalent and tetravalent ions in lithium ferrites and the different parameters have been measured depending on the desired application [4–6]. The reason behind these studies has been that the crystal structure and presence solely of Fe3+ ions in the host material has allowed detailed modelling of the exchange interactions giving rise to its ferrimagnetic order and secondly that subsequent substitution either with magnetic or with nonmagnetic cations has allowed studies of a variety of different magnetic states arising from the perturbed magnetic exchange interactions between ions.
The chromium-doped lithium ferrites Li0.5Fe2.5-xCrxO4 are among the few systems exhibiting the effect of magnetic compensation and Gorter et al [7, 8] were the first to observe this phenomenon in Cr1.25Li0.5Fe1.25O4. These are ferrimagnetic materials in which the various temperature-dependent magnetizations of the spin-up and spin-down sublattices cross over at temperatures below the Curie temperature, resulting in a change of sign of the net spontaneous magnetization at that temperature. In a previous paper [9], we agreed with Kuznetsov et al [10] that Li0.5 Fe2.5-xCrxO4 ferrites have a completely inverted spinel structure for concentrations up to x = 1.25.
In this work, we report the effect of Mg2+ substitution for Fe3+ on the magnetic compensation of Li-Cr ferrite. Moreover, using a Mossbauer study of this system, we propose a cations distribution and investigate its relationship with the magnetic compensation effect.

2. Experimental

Five samples of the ferrite system Cr1.05 Li0.5 Mg x Fe1.45-(2/3)x O4 (0 ≤ x ≤ 0.4) were prepared by the conventional double-sintering ceramic technique.
The X-ray data were collected using a Philips PW1820 vertical goniometer with monochromator attached to a CuKα PW1700 generator operating at a voltage of 40 kV.
The magnetization measurements were performed using a vibrating sample magnetometer (VSM) of 10-5 emu sensitivity in the magnetic field range of 0 kOe to 13.5 kOe and in the temperature range of 77 K to 600 K.

3. Results and Discussion

X-ray diffraction analysis showed the formation of single phase cubic spinel structure for all five samples. A representative diffraction of Cr1.05Li0.5Fe1.45O4 is shown in Fig.1. The major peaks are indexed while the minor peaks indicate the presence of superlattice structures which suggest an ordered arrangement of Li2+ and Fe3+ cations on the octahedral sublattice.
The lattice parameter (a) was obtained by extrapolation to θ = 900 for different indexed planes against the Nelson-Riley function. The lattice parameter value of Cr1.05Li0.5Fe1.45O4 agrees well with the literature taking into account our preparation technique. As can be seen in the inset of Fig.1, within error bars the lattice parameter increases slowly with magnesium content.
Figure 1. X-ray representative spectrum of Cr1.05Li0.5Fe1.45O4. The inset shows the lattice parameter a (Angstroms) versus magnesium content
This variation reflects the larger radius of Mg2+ as compared to Fe3+ .
Figure 2 shows the Mossbauer spectra of Cr1.05 Li0.5 Mg x Fe1.45-(2/3)x O4 (0 ≤ x ≤ 0.4) at 77 K. The spectra were fitted with two magnetic sextet assigned to the octahedral [B] and tetrahedral (A) sites but constrained to have relative areas proportional to the occupancies stated in Table 1.
The line width of the outer absorption line of each spectrum shows a broadening and increases with magnesium content. This could give an estimate of the distribution of the hyperfine fields at each site. The 77 K spectra shows patterns similar to room temperature ones, but with less broadened outerlines. All the hyperfine interaction parameters except the relative areas, given in Table 1, were varying freely in the fitting process. Note that all the magnetic hyperfine fields are decreasing slowly with increasing magnesium content.
Figure 2. Mössbauer spectra of Cr1.05 Li0.5 Mg x Fe1.45-(2/3)x O4 at 77K
Taking the fitted absorption areas as proportional to iron site occupancies, Table 2 shows the proposed cation distrbutions. It is well known from previous studies that Mg2+ occupies A-sites [11] and Cr3+ occupies B-sites [12]. On the other hand from earlier reports, Lithium-Chromium ferrites [10] have been interpreted with Li+ occupying preferentially the octahedral sites. The rest of the tetrahedral and octahedral sites have been filled with Fe3+ cations in accordance with our Mössbauer absorption areas results.
Table 1. Mössbauer parameters of Mg doped Li-Cr ferrites at 77 K
     
On the basis of Neel’s molecular field model [14] and the cation distributions given in Table 2, the magnetic moment in ferrites is mainly from the parallel-uncompensated electron spin of the individual ions, and the spin alignments in the two sub-lattices are arranged antiparallel. Also, the A–B exchange interaction is predominant over the A–A and B–B interactions. Hence the net magnetic moment of the lattice is given by the algebraic sum of the magnetic moments of A and B sub-lattices, i.e. M0 = MB - MA . Given the electronic configurations of the B- and A-site cations Fe3+ (3d5 ) and Cr3+ (3d3 ), we can estimate the sublattice magnetizations (Fe3+ has no orbital momentum, whereas in Cr3+ it is crystal-field frozen). The calculated spin magnetic moments M0 at 0 K are shown in Table 2 and appear to decrease with magnesium content.
Measured saturation magnetization MS of all compositions against temperature are shown in Figure 3. At x=0, MS values are in good agreement with those of Gorter et al [7] as well as the compensation temperature TK (310K) and the Curie temperature TC (500 K). The Curie temperatures were obtained using the method of the “intersecting tangents” to MS against T curves. The inset shows TC and TK versus magnesium content.
It is interesting to point the gradual increase of MS with the addition of magnesium at temperatures below TK . This trend is consistent with the calculated M0 values (Table 2) obtained using the cation distribution deduced from our Mössbauer analysis. We could not extract the experimental values of MS at 0 K from our data because the long extrapolation from 77 K to 0 K would make the comparison with the calculated M0 unreliable.
As expected, the inset shows a decreasing TC as more diamagnetic Mg2+ replaces Fe3+. Moreover, Figure 3 shows that MS decreases with magnesium content at temperatures between TK and TC, this is also expected since TK gradually approaches TC as the inset shows. Indeed, on extrapolating the compensation line, the intersection with the Curie line represents the point where compensation disappears, i.e. x≈0.4. This value is confirmed by our direct observation at this concentration.
Table 2. Cations distribution of Cr1.05 Li0.5 Mg x Fe1.45-(2/3)x O4 and calculated magnetic moments MA , MB and M0 in Bohr magnetons(μB). Round brackets stand for A-sites and square brackets stand for B-sites
     
This effect of the disappearance of magnetic compensation can be interpreted on the basis of Neel’s molecular field model [14] and the concept of ‘weak’ magnetic sublattice as introduced by Belov[15]. In the case of Cr1.05 Li0.5 Mg x Fe1.45-(2/3)x O4, we may consider A-sites as the magnetically strong sub-lattice because of the relatively higher content of Fe3+ cations and the B-sites as the weak sub-lattice. As diamagnetic Mg2+ substitutes Fe3+ in A-sites only, exchange interactions A-A are modified in such a way that the magnetization MA of A sub-lattice becomes weaker. This means that the decrease rate of MA with temperature becomes higher and subsequently the compensation occurs at a point TK approaching gradually TC. The compensation disappears at a magnesium content as such that A sub-lattice has weakened enough so to disable the compensation of B sub-lattice over the whole temperature range up to TC . Note that as Mg2+ substitutes Fe3+ in A-sites, the net moment M= MB - MA increases because MA decreases while MB stays almost constant. This is due to the relatively unchanged content of the magnetic cations Fe3+ and Cr3+ in B-sites.

4. Conclusions

We may conclude from the study of the effect of Mg2+ on the magnetic compensation of Chromium-Lithium ferrites that:
1. The magnetic compensation in Cr1.05Li0.5Fe1.45O4 disappears when Fe3+ in A-site is partially replaced by Mg2+.
2. Below the compensation temperature, the observed magnetic moment of these ferrites increases with magnesium content.
3. The magnetic moments calculated using a cations distribution consistent with the Mössbauer study of these ferrites show an increasing trend with magnesium content. This trend agrees with the magnetic measurements.
Figure 3. Saturation magnetizations versus temperature of Cr1.05 Li0.5 Mg x Fe1.45-(2/3)x O4. The inset shows the Curie and compensation temperatures versus magnesium content

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