Synthesis And Magnetic Properties Of Y₃Fe₅O₁₂ (YIG) Nano-particles Doped With Rare Earth Ions

In this article, Re³⁺—substituted garnet nanoparticles Y3-xRexFe5O12 (R=Gd,Dy, Nd, Sm and La) were fabricated by a sol-gel method. Compared to PEG, citricacid is helpful to reduce the synthesis temperature, at the same time, most of Fe3+,Y³⁺ ions form chelate complexes by adjusting PH=2, which not only decreases thesynthesis temperature, but also evade the formation of intermediate phase, that isY_3-xRe_xFe₅O₁₂ could be formed by a single-step process, where an amorphousprecursor powder is formed first and then transformed to YIG during calcinations.Results of XRD,IR and DTA-TG show that all samples have only garnet structureand the sizes of particles rang from 34 to 69nm, the crystallization of samplesoccurs around 750℃,furthermore, with increasing the treating temperature, thecrystallization of samples becomes more and more complete, and the sizes ofparticles are increased. Because of different ionic radius of every rare earth ion,we conclude the maximum level of La³⁺, Gd³⁺, Dy³⁺, Nd³⁺ Sm³⁺ are x=0.8,3.0,2.2,2.0,3.0 respectively. Results of VSM show that the saturation magnetization ofY_3-xGd_xFe₅O₁₂, Y_3-xDy_xFe₅O₁₂ particles is not only obviously less than pure YIG,but also decreases with increasing the Re³⁺ concentration (x), but the values of theirchanges are different. Because the extent of the magnetic order of Dy³⁺ is smallerthan the extent of magnetic order of Gd³⁺, and the effective magnetic moments inc-sites formed by Dy³⁺ are smaller than the magnetic moments formed by Gd3+,therefore, the saturation magnetization of Y_3-xDy_xFe₅O₁₂ is larger than thesaturation magnetization of Y_3-xGd_xFe₅O₁₂ when they have the same Reconcentration x at room temperature, especially when the Re concentration x>1.0.Because of the large ionic radius of non-magnetic La³⁺ ions, the substitution ofLa³⁺ ions gives rise to a microscopic structure distortion of the a-and d-sites todifferent degrees, which produces two different kinds of chemical environmentsaround the a-and d-iron sites, thus, with increasing the La³⁺ concentration, Thesaturation magnetization increases firstly and then decreases as the La³⁺concentration (x) is increased, and gets it's maximal saturation magnetization whenthe La concentration x=0.2 . Because Nd3+ ions are magnetic light rare earth ions,the c sub-lattice formed by the Nd3+ ions is coupled ferromagnetically to the totaliron moment so the saturation magnetization increases as the Nd concentration (x)is increased, and gets its maximum at x=1.0, however, when the Nd concentrationx>1.0, the microstructures of Y3-xNdxFe5O12 begin to distort and produces a similarsituation as the Y3-xLaxFe5O12, therefore the saturation magnetization decreasesapparently as the Nd concentration (x) is increased from 1.0 to 2.0. At roomtemperature, we think the magnetic moment of Sm3+ is too small to change themagnetic moment of samples when Sm3+ ions are doped in YIG, while the Sm3+radius is far larger than the Y3+ radius, so when the amount of Sm3+ is large, themicrostructures of Y3-xSmxFe5O12 begin to distort, and the effective magneticmoments formed by Fe3+ decrease, so with increasing the Sm concentration x from0.0 to 1.0, the saturation magnetization changes smoothly, while x≥1.0, thedecrease of the saturation magnetization becomes more apparent. Meanwhile, it isobserved that may be due to the enhancement of the surface spin effects, thesaturation magnetization rises as the particle size is increased.