The Substitution Effect Of Ga On Magnetoresistance And Magnetic In Sr₂FeMoO₆ System

Most recently, interest has been emerging in the magnetotransport properties of the ordered double perovskite Sr₂FeMoO₆ (SFMO) and related half-metallic ferromagnets. It has been known that SFMO has a double perovskite structure with ionic ordering on the transition metal sites, and is metallic with ferromagnetic Curie temperature of 410 K～450 K. Polycrystalline bulk samples of this ordered double perovskite exhibit large negative MR effect due to grain boundary tunneling, which persists well above the room temperature. This suggests that the double perovskite holds promise for utilization as electrodes for spin valve and magnetic tunnel junction devices. X-ray diffraction (XRD) shows that the compounds in the range of 0≤x≤0.6 all form a good-quality solid solution. No impurity phase is observed. But the relative intensity of diffraction peak at 19.5°which characterizes the degree of cationic order in SFMO decreased gradually with the increased ironic of Ga. This reveals that the cationic order had been restrained as the impurity replacement increased. At the lower Ga doping (x<0.3) the system displays the long distance order of ferromagnets, however the Curie temperature TC decreases gradually with the increased content of Ga. With the contentment of Ga in the samples of x=0.3 and 0.4 in the range of low temperature the phase of spin-glass was discovered and the cooling temperature Tf was also been discovered. The system enters a mixed phase that displayed the characteristics of both a spin glass and a ferromagnet. A cusp in the zero-field-cooled dc magnetization, a frequency-dependent peak in ac susceptibility in both dc and ac magnetic properties all point towards glass behaive. It should be pointed out that the exchange bias which was attended because of the value of theoretic research and practice application was discovered in the sample of x=0.4, it was the fist time that the exchange bias was found in the compound of double perovskite SFMO.The transport properties and magnetotransport were researched in the system of Sr₂Fe_1-xGa_xMoO₆(0≤x≤0.6), All the samples exhibit semiconductor behavior over the entire measured temperature region. The semiconductor to metal transition temperature was not found up to x = 0.6. We find that at the low doping the LFMR of SFMO (not shown) can be greatly enhanced by the replacing of Fe with the nonmagnetic Ga ion up to a temperature of 300 K. In spite of the observed increase in LFMR, low TC makes the highly doped compounds useless for practical applications. Usually the LFMR was due to electron spin-dependent transfer across grain boundaries. Since all the samples were processed simultaneously and the preparated condition for the samples is exactly alike, Hence, the appreciable increase in MR for Ga-doping systems is thought to weaken the double exchange barrier after Ga doping in SFMO system. In such a case, Ga ions may act as a barrier for electron transport along the chain in the ferromagnetic segregation and weaken the ferromagnetic exchange. However, the magnitude of the intergrain MR in those samples does not decrease close to room temperature. However at the high doping of x≥0.4 the LFMR was restrained because the smaller ferromagnetic grain and the spin-glass state of disorder which made the direct tunneling impossible.