Synthesis And Magnetic Properties Of BiY₂Fe₅O₁₂ Nano-particles Doped With Rare Earth Ions

Rare-earth garnet particles are very attractive objects of magnetic research,This is because they are single magnetic domain and accordingly theirproperties and their mutual interaction can be studied without magnetic domaineffects. These materials have a uniquely defined cation distribution and do notpresent any site inversion problems which can arise in other ferrites. From anindustrial viewpoint, they are applicable to media for high-density magnetic ormagneto-optical information storage. Among the magnetic material,polycrystalline yttrium iron garnets and substituted ones have received a greatdeal of attraction in laser, microwave devices and magneto-optics.Recently, the bismuth-yttrium iron garnet(Bi-YIG) was found that it hasenhanced Faraday rotation effect of one order of magnitude larger than that ofyttrium iron garnet(YIG). Furthermore, replacement of Y³⁺ ions by Bi³⁺ ionscan decrease the crystallization temperature for 100℃to 300℃in contrast toYIG. These properties make the bismuth-yttrium iron garnet much morepromising for applications in magneto-optical discs and magneto-opticaldisplay devices.1,single-phase polycrystalline BiY₂Fe₅O₁₂ was synthesized by sol-gelmethod. Purity and mainsize are taken into account, and best conditions of themethod were found . The best method is that: polycrystalline samples with theformula BiY₂Fe₅O₁₂ were initially prepared from a solution of analytically puregrade Bi(NO₃)₂ 5H₂O, Fe(NO₃)₃ 9H₂O, Y(NO₃)₃ and polyethylene glycol(PEG,molecular weight 20000) in appropriate stoichiometric ratios at 80℃untilgelation. Gels were dried at 100℃for 36h to form the precursors which werefurther preheated at 450℃for 1h and calcined at 850℃,900℃,950℃,1000℃for 3h. The mainsize of the nanoparticles increase with the increase of theanneling temperature. But the they are single magnetic domain and the mainsizehas no influence on magnetic properties of garnet nano-particles.2,Substituted YIG exhibits excellent magnetic properties. It is well-knownthat garnet has the composition A₃B₂B'₃O₁₂(cubic space group, Ia3d),with eightformula units per cell. In the case of BiY₂Fe₅O₁₂, the A= Y³⁺,Bi³⁺ site iseight-fold dodecahedrally coordinated(c site), The B=Fe³⁺ site six-foldoctahedral(a site) and the B'=Fe3+ site four-fold tetrahedral(d site). Y3+,Bi3+ arediamagnetic and 5 μ(T=0K) per formula unit results from negative Bsuperexchange (antiferromagnetic) interactions are dominant to a-a and d-dinteractions. We choose the ions to substitute into garnet in view of magneticmoment and mainsize.Rare-earth ions have large ion ridius and can substitute Y3+ and enter into c site.Gd3+,Sm3+ and La3+ have different magnetic moments and their magneticmements are 7.98μB ,1.74μB and0μB individually. So they can influence themagnetic properties of garnet differently. Synthesis method of RexBiY2-xFe5O12are equal to BiY2Fe5O12'. The process of crystallization include the perovskitephase which is the intermediate compound and can be expressed by thechemical reaction: 1/3Bi2O3+(2-x)/3Y2O3+ x/3Re2O3+Fe2O3→2 Rex/3Bi1/3Y(2-x)/3FeO3 (1) 3 Rex/3Bi1/3Y(2-x)/3FeO3+Fe2O3 →RexBiY2-xFe5 O12 (2) XRD patterns and DTA/TG curves indicate that the post-annealing above850℃is necessary to obtain a single garnet phase of samples. Mainsize ofgarnet increase with the increase of post-annealing temperature which is from850℃to 1000℃. The saturation magnetization (Ms) at room temperature was obtained fromthe hysteresis curve for each sample, and we found that a linear decrease ofsaturation magnetization as x increases to the three system. But they havedifferent slopes of decrease. As to GdxBiY2-xFe5O12 and SmxBiY2-xFe5O12, Themagnetization is given by: Mtotal(T, Happ)= Md(T, Happ)?Ma (T, Happ) ?Mc (T, Happ) where Mc is the sub-lattice magnetization of 'c'sub-lattice. Mc (T, Happ)will increase with the increasing content of magnetic ions, which accordinglylead to the decrease of magnetization of samples. The magnetic moment ofGd3+ and Sm3+are 1.74μB and 7.98μB individually. So we can see that whyGdxBiY2-xFe5O12 has large slope of decrease of saturation magnetization.LaxBiY2-xFe5O12 also has a slope of decrease of saturation magnetizationthough La3+'magnetic mement are 0μB. It maybe relate to the reason that theionic radius of La3+(0.1061nm) is larger than that of Y3+(0.0892nm) and itinfluence the cation distribution of Fe3+ ions of d site and a site. 3,Cr3+ and Al3+ can substitute Fe3+ and enter into garnet. BiY2Fe5-XCrxO12and BiY2Fe5-XAlxO12 are synthesized by sol-gel method. The process of...