Magnetic Modulation in Mechanical Alloyed Cr1.4fe0.6o3 Oxide

We have synthesized Cr1.4Fe0.6O3 compound through mechanical alloying of Cr2O3 and Fe2O3 powders and subsequent thermal annealing. The XRD spectrum, SEM picture and microanalysis of EDAX spectrum have been used to understand the structural evolution in the alloyed compound. The alloyed samples are matching to rhombohedral structure with R3C space group. The observation of a modulated magnetic order confirmed a systematic diffusion of Fe atoms into the Cr sites of lattice structure. A field induced magnetic behaviour is seen in the field dependence of magnetization data of the annealed samples. The behaviour is significantly different from the mechanical alloyed samples. The experimental results provided the indications of considering the present material as a potential candidate for opto-electronic applications.

the compound. The correlation between its crystal structure and magnetism would also be relevant to gain the properties of Cr-Fe interface [2].
The antiferromagnetic ordering of spins in both α-Fe 2 O 3 and Cr 2 O 3 has been explained in terms of super exchange interactions (Cr-O-Cr, Fe-O-Fe) [4,8]. Neutron diffraction study attributed the drastic variation of T N (~310 K for Cr 2 O 3 and ~950 K for Fe 2 O 3 ) to a different kind of magnetic structure, although α-Fe 2 O 3 and Cr 2 O 3 have shown identical crystal structure. Earlier reports [4,7,8] suggested that the spin moments of Cr 3+ (3d 3 [4]. 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 Fe 2-x Cr x O 3 compound is not clear till date. Attempts were made to understand the structural and magnetic properties of Fe 2-x Cr x O 3 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 such important issues related to the effect of modulated spin structure on the properties of Fe 2-x Cr x O 3 compound.
In this work, we have synthesized Cr 1.4 Fe 0.6 O 3 compound using the novel technique of Mechanical alloying [18]. We have investigated the associated structural and magnetic evolution in different states of mechanical alloying and annealing of the samples. We also attempted to identify the nature of magnetic order in Cr 1.4 Fe 0.6 O 3 , whether antiferromagnet or ferromagnet or the mixed properties of ferromagnet and antiferromagnet.

A. Sample preparation
The stoichiometric amounts of high purity α-Fe 2 O 3 and Cr 2 O 3 were mixed for the preparation of Cr 1.4 Fe 0.6 O 3 compound. The initial colours of α-Fe 2 O 3 and Cr 2 O 3 are red and green, respectively. The mixture of Fe 2 O 3 and Cr 2 O 3 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 10mm Silicon Nitride and 5mm 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 (Si 3 N 4 ) 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, if it happened at al. 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 o C in atmospheric conditions. After annealing for 1 hour, 3 hours and 17 hours, individual pellet was directly air quenched to room temperature . We denoted the mechanical alloyed samples 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 0 C are denoted as SNt, where t = 1, 3 and 17 for annealing time 1 hour, 3 hours and 17 hours, respectively.

B. Sample characterization and measurements
The crystalline phase of alloyed and annealed samples was characterized by recording the Xray Diffraction spectra in the 2θ range 10-90 0 with step size 0.01 0 . The Cu-K α 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 fluorescence (EDAXF) 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 T C to the temperature lower than T C 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. , where 2θ C is the position of peak center, λ is the wavelength of X-ray radiation (1.54056 Å), ω is the full width at the half maximum of peak height (in degrees). The average grain size of the alloyed compound is found to be in nanometer range (shown in Table I Fig. 3a for MA48 sample. The particle size (~ 500 nm) of MA48 sample estimated from the SEM picture (Fig. 3b) is much larger than that obtained from XRD data (~ 22 nm). This represents the fact that SEM is not the suitable tool to estimate the particle size, as the scale (micrometer range) of SEM data is larger in comparison with the particle size in nanometer range. This means the SEM picture indicates the size of multigrained particles. The mapping (Fig. 3c-e) suggests a uniform distribution of Cr, Fe, O atoms over the zone. Achieving some basic knowledge of the structural properties (i.e., size, shape, composition, and crystal structure) of the samples, we have attempted below to understand the magnetic properties of the samples. when the sample changes from bulk α-Fe 2 O 3 to SN17. We noted that the position of MZFC maximum at T m is highly sensitive to the magnitude of measurement field. This is confirmed from the decrease of T m ~ 810 K (at 1 kOe) to 750 K (at 2 kOe) in MA48 sample (Fig. 4b).

(b) Magnetic Properties
The paramagnetic to canted ferromagnetic ordering temperature T N ~ 950 K of bulk α- nanoparticles [25], originating from the competitive spin ordering at the core-shell interface [26]. Viewing the absence of such field induced magnetic ordering in mechanical alloyed (with out heat treatment) samples, although grain size is in nanometer range, we believe that the field induced magnetic behaviour in our annealed samples is not solely due to particle size effect, but can be attributed to a modulated magnetic ordering due to diffusion of two different magnetic elements [27]. The induced magnetic behaviour in annealed samples is also reflected in the field dependence of differential magnetization (dM/dH) curves (Fig. 6).
The differential susceptibility (dM/dH) increases to exhibit a maximum for mechanical alloyed samples when the applied field approaches to zero value, whereas dM/dH exhibits a minimum near to the zero field value for the annealed samples. The observation of induced magnetism confirms an experimental evidence of modulated spin structure in this compound, which was suggested from Neutron diffraction study [8].

IV. CONCLUSIONS
The magnetic properties of mechanical alloyed Cr 1.4 Fe 0.6 O 3 compound strongly depends on the structural change associated with the variation of milling time and annealing temperature.