Figure represents the magnetisation curves for dry milled zinc ferrites at room temperature. All samples show superparamagnetic properties at room temperature with low coercivity. Table summarizes the saturation magnetisation (Ms) and coercivity (Hc) values of dry milled zinc ferrite samples. A high enhancement of Ms value can be observed in 5 hours milled sample. However, 10 hours milled sample shows no much increase in saturation magnetisation comparing with 5 hours milled sample.
The grain size of 10 hours milled sample is slightly greater than 5 hours milled sample. But the Ms value is not decreasing in 10 hours milled sample. Hence a direct relation between grain size and saturation magnetisation is not possible from our results. However, we can say that milling time strongly influence the saturation magnetisation.
The high saturation magnetisation value for milled zinc ferrites cannot be explained by superparamagnetism alone, but there exists some other magnetic ordering together with superparamagnetism that contribute to magnetisation. A ferrimagnetic ordering that is short range ordered is developing in the core of the particle as a result of cation redistribution. This forms magnetic clusters in the core. Inside the clusters, a Fe3+ ion in A site and its twelve nearest neighbours in B sites are coupled through AB spin interaction. This leads to magnetic behaviour, which is responsible for the residual magnetisation in milled sample. The formation of magnetic clusters as a result of cation redistribution is highly probable in coprecipitated samples, which are produced at low temperature within a short period of time. This result is in accordance with the results obtained by various researchers. X-ray photoelectron spectroscopy studies by V.Sepelak and co-workers indicated the migration Fe3+ ions to A sites with milling.
A marginal increase in a-Fe2O3 is observed with milling from XRD results. a-Fe2O3, which is antiferromagnetic, should decrease the magnetisation in milled samples. But the increase of Ms suggests that the cation redistribution contributes much more to saturation magnetisation comparing with the increase of a-Fe2O3. But the a-Fe2O3 may act as a facilitator in producing cation redistribution.
The surface of the particle may behave differently and modify the magnetic properties in nanoparticles. In these materials the superexchange interaction occurs through O2- ions. The absence of the oxygen ion from the surface results in broken exchange bonds between Fe3+ ions, which induces surface spin disorder. This significantly modifies the magnetic properties in nanoparticles where surface to volume ratio is high. Thus surface effects dominate the magnetic properties of fine particles since decreasing the particle size increases the ratio of surface spins to the total number of spins. Some researchers reported that the high energy ball milling may produce spin-glass like surface disorder. Low temperature studies will give some idea about this and is carried out and discussed in next section.
Hence from the discussion, a mixed ferrimagnetic and superparamagnetic behaviour is predominating at room temperature magnetic properties and the role of surface spin disorder and spin-glass like properties are expected at low temperatures. Figure 1 clearly shows nonsaturation of magnetisation curves for all samples even by applying a 10 KOe field. This suggests the coexistence of superparamagnetism and ferrimagnetism in our samples. The ferrimagnetic part tends to saturate, whereas the superparamagnetic part increases linearly, resulting in lack of saturation. The saturation magnetisation and coercivity values suggest our samples have only a small ferrimagneic contribution and are dominated by superparamagnetic type clusters.
The grain size of 10 hours milled sample is slightly greater than 5 hours milled sample. But the Ms value is not decreasing in 10 hours milled sample. Hence a direct relation between grain size and saturation magnetisation is not possible from our results. However, we can say that milling time strongly influence the saturation magnetisation.
The high saturation magnetisation value for milled zinc ferrites cannot be explained by superparamagnetism alone, but there exists some other magnetic ordering together with superparamagnetism that contribute to magnetisation. A ferrimagnetic ordering that is short range ordered is developing in the core of the particle as a result of cation redistribution. This forms magnetic clusters in the core. Inside the clusters, a Fe3+ ion in A site and its twelve nearest neighbours in B sites are coupled through AB spin interaction. This leads to magnetic behaviour, which is responsible for the residual magnetisation in milled sample. The formation of magnetic clusters as a result of cation redistribution is highly probable in coprecipitated samples, which are produced at low temperature within a short period of time. This result is in accordance with the results obtained by various researchers. X-ray photoelectron spectroscopy studies by V.Sepelak and co-workers indicated the migration Fe3+ ions to A sites with milling.
A marginal increase in a-Fe2O3 is observed with milling from XRD results. a-Fe2O3, which is antiferromagnetic, should decrease the magnetisation in milled samples. But the increase of Ms suggests that the cation redistribution contributes much more to saturation magnetisation comparing with the increase of a-Fe2O3. But the a-Fe2O3 may act as a facilitator in producing cation redistribution.
The surface of the particle may behave differently and modify the magnetic properties in nanoparticles. In these materials the superexchange interaction occurs through O2- ions. The absence of the oxygen ion from the surface results in broken exchange bonds between Fe3+ ions, which induces surface spin disorder. This significantly modifies the magnetic properties in nanoparticles where surface to volume ratio is high. Thus surface effects dominate the magnetic properties of fine particles since decreasing the particle size increases the ratio of surface spins to the total number of spins. Some researchers reported that the high energy ball milling may produce spin-glass like surface disorder. Low temperature studies will give some idea about this and is carried out and discussed in next section.
Hence from the discussion, a mixed ferrimagnetic and superparamagnetic behaviour is predominating at room temperature magnetic properties and the role of surface spin disorder and spin-glass like properties are expected at low temperatures. Figure 1 clearly shows nonsaturation of magnetisation curves for all samples even by applying a 10 KOe field. This suggests the coexistence of superparamagnetism and ferrimagnetism in our samples. The ferrimagnetic part tends to saturate, whereas the superparamagnetic part increases linearly, resulting in lack of saturation. The saturation magnetisation and coercivity values suggest our samples have only a small ferrimagneic contribution and are dominated by superparamagnetic type clusters.
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