English abstract
Single-phased nanocrystalline Y3Fe5-2.,Al,Cr,012 materials with x = 0.0, 0.2, 0.4 and 0.6 have been prepared by the conventional ceramic technique at 1200°C using a stoichiometric mixture of the starting materials Y203, Fe2O3, Al2O3 and Cr203, followed by ball milling for 1, 3 and 5 hours to produce nanosized samples. Energy dispersive X-ray spectroscopy (EDXS) using scanning electron microscope shows that the weight percentages of the Y and O atoms are consistent within 2% accuracy. TEM shows that we have distributions in the particle sizes between 20 and 300 nm, small particles agglomerate as Alt/Cr* co-doping increases. On increasing the milling duration, the large particles were pulverized; hence numbers of small particles increased as well as aggregation. PowderCell analyses of X-ray diffraction patterns of the samples show them to crystallize in the (la3d) space group and the corresponding lattice constant decreases with increasing Al" and Cr* contents (x). The average crystallite size ranges between 24 and 36 nm for all samples during milling. XRD spectra reveal that for the milling time of 3 hours, the rare-earth garnet YIG starts to decompose into the perovskite Y FeO3 and hematite a-Fe2O3. The Fe Mössbauer measurements at 295 and 78 K show a superposition of two magnetic sextets and a paramagnetic doublet; this doublet shows some reduction in intensity with decrease in the temperature. The existence of the doublet at 78 K suggests that the blocking temperature is less than LN temperature. From the intensity of the Fert sites and assumed site occupancies of the Cr* and Al" ions, it may be deduced that the paramagnetic doublet is associated with both octahedral and tetrahedral sites. The hyperfine fields for the octahedral and tetrahedral sites are reduced by milling. The 300 and 113 K magnetization measurements for the samples ball-milled for 1 and 5 hours show similar trend decrease for the magnetization with co-doping and milling; saturation magnetization monotonically decrease with increasing x. The reduction in particle sizes for 5-hour milled samples may cause the sharp decrease in magnetization. Néel temperature (TN) does not change with milling, but it decreases with increasing concentration (x). This implies that the magnetic super-exchange interactions between the Fe + ions at different sites (octahedral and tetrahedral) were not affected by milling.
