Magnetic modulation in mechanical alloyed Cr1.4Fe0.6O3oxide
© Bhowmik RN et al 2008
Received: 22 October 2008
Accepted: 29 December 2008
Published: 29 December 2008
The compound Cr1.4Fe0.6O3 has been synthesized through mechanical alloying of Cr2O3 and α-Fe2O3 powders and subsequent thermal annealing. The XRD spectrum, SEM picture and microanalysis of EDAX spectrum have been used to understand the structural evolution during alloy formation. The alloyed samples have been matched to rhombohedral structure with R3C space group. The observation of a modulated magnetic order confirmed the systematic diffusion of Fe atoms into the Cr sites of lattice structure. A field induced magnetic behaviour has been noted in the field dependence of magnetization data of the annealed samples. This feature is significantly different from that of the mechanical alloyed samples. The experimental results also provided the indications of considering the present material as a potential candidate for opto-electronic applications.
PACS Codes: 75.30.-m, 75.20.En, 81.40.Rs
In recent years extensive research efforts have been given to the formation of new magnetic oxides, considering their potential applications in the field of micro-electronics and multifunctional devices [1, 2]. In order to search for the new class of materials, it has been found that solid solution of mixed metal oxides might be a potential candidate, having advantage of easy alloying due to ionic radius of same order [3, 4]. Fe2-xCrxO3 is one such compound series, which can be formed through the alloying of α-Fe2O3 and Cr2O3 oxides [5, 6]. The interesting points are that both α-Fe2O3 and Cr2O3 stabilized into rhombohedral crystal structure with space group R3C [4, 7] and both are antiferromagnetic insulator . Cr2O3 oxide is a well known compound for the prediction and experimental observation of large magneto-electric (ME) effect [9, 10]. The ME effect is generally small in symmetric non-magnetic insulators . The low magnetic moment and low antiferromagnetic ordering temperature (TN) of single phase Cr2O3 (~310 K) is also not suitable for obtaining large magneto-electric effect, as well as for applications [12, 13]. Recently, the exhibition of large magnet-electric effect in Cr2O3 was attributed  to a possible change of magnetic space group symmetry. This triggered the alloying of Fe2-xCrxO3series by mixing a suitable amount of Fe2O3 (TN ~ 950 K) [4, 8] in Cr2O3 oxide. The study of this series also remained attractive due to its promising applications in the field of opto-electronic materials. The immediate effect is that Fe2-xCrxO3 series has shown the enhancement in photo-conductivity with the increase of Cr content [2, 15]. In order to understand the modified photo-conductivity, as well as magneto-electric effect, one needs to realize the correlation between crystal structure and magnetic ordering in the compound. The correlation between its crystal structure and magnetism would also be relevant to gain the properties of Cr-Fe interface .
In literature, the antiferromagnetic ordering of spins in both α-Fe2O3 and Cr2O3 has been explained in terms of super exchange interactions (Cr-O-Cr, Fe-O-Fe) [4, 8]. Neutron diffraction study explained the drastic variation of TN (~310 K for Cr2O3 and ~950 K for Fe2O3) due to a different kind of magnetic structure, although α-Fe2O3 and Cr2O3 have shown identical crystal structure. Earlier reports [4, 7, 8] suggested that the spin moments of Cr3+ (3d3) ions in Cr2O3 are arranged in + - + - (+ ≡ up spin, - ≡ down spin) sequence along the  axis, whereas the spins of Fe3+ (3d5) ions in α-Fe2O3 are ordered in the + - - + sequence. Hence, it is expected that the substitution of Fe3+ (3d5) by Cr3+ (3d3) in Fe2-xCrxO3 solid solution would modify the sequence of spins ordering, as well as the nature of superexchange interactions. Form the phase diagram of Fe2-xCrxO3  one could estimate an equal probability of Fe-O-Fe, Fe-O-Cr, Cr-O-Cr superexchange interactions near to x = 1 and no significant change in the ordering of spins. This resulted in a slow variation of TN with Cr content in the region 0.90 < x ≤ 1.52 . However, the nature of spin ordering may not follow the conventional structure due to the change of magnetic space group symmetry. The modulated (perturbed) local magnetic order could be expected during the diffusion of Fe atoms into Cr sites. To our knowledge, the magnetic ordering of Fe2-xCrxO3 compound is not clear till date. Attempts were made to understand the structural and magnetic properties of Fe2-xCrxO3 compound by reducing the particle size in nanometer range using various chemical routes [16, 17]. However, enough attention was not given in the reported works to realize the effect of modulated spin structure on the properties of Fe2-xCrxO3compound.
In this work, Cr1.4Fe0.6O3 compound has been synthesized using the novel technique of Mechanical alloying . Attempts have been made to investigate the associated structural and magnetic evolution in different states of mechanical alloying and annealing of the samples. The nature of magnetic order, i.e., antiferromagnet or ferromagnet or the mixed properties of ferromagnet and antiferromagnet, was understood for Cr1.4Fe0.6O3 compound.
2.1. Sample preparation
The stoichiometric amounts of high purity α-Fe2O3 and Cr2O3 were mixed for the preparation of Cr1.4Fe0.6O3compound. The initial colours of α-Fe2O3 and Cr2O3 were red and green, respectively. The mixture of Fe2O3 and Cr2O3 was ground using agate mortar and pestle for nearly two hours in atmospheric conditions. The ground powder was mechanical alloyed using Fritsch planetary mono mill (pulverisette 7). The material and balls (combination of 10 mm Silicon Nitride and 5 mm Tungsten Carbide) mass ratio was maintained to 1:7. The mechanical alloying was carried out in atmospheric condition up to 84 hours in a silicon nitride (Si3N4) bowl with rotational speed 300 rpm. The non-magnetic balls and bowl (Silicon Nitride and Tungsten Carbide) were selected to avoid the magnetic contamination during the milling process. The milling process was intermediately stopped to monitor the uniform alloying of the mixture and to minimize the local heat generation that might occur during continuous milling. A small quantity of alloyed sample after 24 hours and 48 hours was taken out to check the structural phase evolution. The alloyed samples with different milling hours were made into pellets. The pellets of 84 hours milled sample were placed in alumina crucibles and annealed at 700°C in atmospheric conditions. After annealing for 1 hour, 3 hours and 17 hours, individual pellet was directly air quenched to room temperature. The mechanical alloyed samples was denoted as MAh, where h = 0, 24, 48 and 84 for alloying time 0, 24 hours, 48 hours and 84 hours, respectively. The samples annealed at 700°C were denoted as SNt, where t = 1, 3 and 17 for annealing time 1 hour, 3 hours and 17 hours, respectively.
2.2. Sample characterization and measurements
The crystalline phase of alloyed and annealed samples was characterized by recording the X-ray Diffraction spectra in the 2θ range 10–90° with step size 0.01°. The Cu-Kα radiation from the X-ray Diffractometer (model: X pert Panalytical) were employed to record the room temperature spectrum of each sample. The scanning electron microscopic (SEM) picture of the samples was taken using HITACHI S-3400N model. Elemental composition of the samples was obtained from the energy dispersive analysis of x-ray (EDX) spectrum. The magnetic properties of the samples were studied by the measurement of magnetization as a function of temperature and magnetic field using vibrating sample magnetometer (Model: Lakeshore 7400). The temperature (300 K–900 K) dependence of magnetization was measured by attaching a high temperature oven to the vibrating sample magnetometer. The temperature dependence of magnetization was carried out at 1 kOe magnetic field by increasing the temperature from 300 K to 900 K (ZFC mode) and reversing back the temperature to 300 K in the presence of same applied field 1 kOe (FC mode). It should be noted that the ZFC mode followed here is slightly different from the conventional zero field cooling (ZFC) measurement, where the sample is first cooled without applying magnetic field from the temperature greater than TC to the temperature lower than TC and magnetization measurement in the presence of magnetic field starts with the increase of temperature. The field dependence of magnetization at 300 K was measured within ± 15 kOe.
3. Results and discussion
3.1. Structural properties
Grain size (<d>), lattice strain (ε) based on Willimson-Hall method and cell parameters of the samples are calculated using XRD data.
Grain size (nm)
Lattice parameter a(Ǻ)
Lattice parameter c(Ǻ)
5.0386 ± 0.0014
13.7498 ± 0.0002
302.30 ± 0.0840
0.4817 ± 0.0756
4.9521 ± 0.0021
13.6063 ± 0.0004
288.96 ± 0.1221
0.5121 ± 0.0365
4.9535 ± 0.0016
13.6041 ± 0.0002
289.08 ± 0.0923
0.5771 ± 0.0604
4.9546 ± 0.0013
13.6082 ± 0.0003
289.29 ± 0.0781
0.4495 ± 0.0416
4.9526 ± 0.0019
13.6142 ± 0.0002
289.19 ± 0.1090
0.3709 ± 0.0118
4.9520 ± 0.0018
13.6084 ± 0.0002
288.99 ± 0.1099
0.3202 ± 0.0121
4.9512 ± 0.0012
13.6070 ± 0.0002
288.87 ± 0.0997
4.9510 ± 0.0011
13.5996 ± 0.0001
288.69 ± 0.0667
3.2. Magnetic properties
M(H) = χ1H + χ2H2 + χ3H3 + (rest of the terms)
The magnetic properties of mechanical alloyed Cr1.4Fe0.6O3 compound strongly depends on the structural change associated with the variation of milling time and annealing temperature. The variation of cell parameters is attributed to the effect of diffusion of Fe3+ ions into the boundary of Cr3+ ions. The structural analysis, using XRD and SEM with EDX spectrum, may suggest the completion of alloy formation during milling process, but the magnetic behaviour confirmed that the alloying process is, still, continued at 700°C to approach the structure dominated by Cr2O3 phase. Experimental results suggested that magnetic ordering of mechanical alloyed Cr1.4Fe0.6O3 compound, annealed at 700°C, belongs to neither a canted ferromagnetic ordering of α-Fe2O3 nor a typical antiferromagnetic ordering of Cr2O3. Rather, a field induced magnetic order appeared in the annealed samples, which we attribute to the appearance of a modulated spin structure in the compound. This experimental work may instigate subsequent investigations on the non-linear magneto-opto-electronic properties of similar materials.
We thank to CIF, Pondicherry University for providing experimental facilities. We also thank to FIST Program in the department of Physics for providing XRD measurement facilities. RNB also thanks UGC for providing financial support (F.No. 33-5/2007(SR)).
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