Research On The Structure And Magnetic Properties Of Rare-earth Ions Doped Yttrium Iron Garnet

Yttrium iron garnet (YIG, Y₃Fe₅O₁₂) materials have been widely used inmagneto-optical and microwave communication applications, such as for opticalisolators,oscillators, circulators and phase shifters, due to their large Faraday rotation,low propagation loss, high saturation magnetization and smaller linewidth in termsof magnetic resonance. For large modi? cations of the magnetic properties of YIGmaterials, the doping method is usually easier to use, that is the Y/Fe elements couldbe replaced by other elements. Therefore, much research on the substitution of YIGmaterials with respect to the magneto-optical and magnetic properties has been done.The magnetic properties of rare earth iron garnets (RIG) have attracted the interestof many researchers in the last four decades. That is due to their extensiveapplications in materials science and technology, as they are good electricalinsulators,as they also can change Ms,Tc,|¤H independently . The general formulaunit {R3}c[Fe2]a(Fe3)dO12 where the ?°R³⁺?±cations occupy dodecahedral ?°c?±sites,the?°Fe³⁺?±cations occupy the octahedral ?°a?±sites, and the tetrahedral ?°d?±sites in thecrystal structure. The unit cell has cubic symmetry and contains eight formula unitsfor a total of 160 atoms. The condition for site occupancy is ionic size. R³⁺ ioncannot occupy the octahedral and tetrahedral sites because of its large ion radius, soR³⁺ ion can only occupy dodecahedral sites which have larger space. In YIG, all thetetrahedral and octahedral cavities are occupied by Fe³⁺ and dodecahedral cavitiesare occupied by Y³⁺. Nowadays, despite the remarkable utility of small magneticparticles in magnetic recording media, permanentmagnets,microwave devices and soon have been proved, their present applications and a full understanding of theirmagnetic properties remain a challenge. In this paper, structure and magnetic properties of Yttrium iron garnet and rare-earth ion doped Yttrium iron garnet materials were studied. YIG,Y3-xRxFe5O12(R=Gd,Tb,Dy x=0.0,0.2,0.4,0.6,0.8,1.0) were fabricated by a sol–gel method ,using citric acid as a surfactant. The molar ratio is 2:1 between Citric acid added to and the expected metal ion. Then NH3.H2O was added to adjust pH=2.0.The resulting solution was heated at 80℃in order to obtain the gel; to obtain the YIG and Y3-xRxFe5O12 (R=Gd,Tb,Dy x=0.0,0.2,0.4,0.6,0.8,1.0) samples, the gels were dried initially at 110℃to form the precursors which were further preheated at 450℃for 1h and calcined at 600℃,700℃,800℃,900℃,1000℃for 3h. The structure of the samples are investigated by X-ray diffraction (XRD) and Scanning Electron Microscope(SEM), the magnetic properties are studied by Vibrating Sample Magnetometry (VSM)and Ferromagnetic Material Curie Temperature Experiment-meter.Results of XRD show that all samples have only garnet structureand , with increasing the treating temperature,thecrystallization of samples becomes more and more complete,and the sizes of particles are increased. According to the Scherrer's relationship, we calculated crystalline sizes for samples with the same R concen- tration (x) but treated at different temperature. The obtained sizes increase as the heating temperature is increased from700℃to1000℃. Furthermore, we can also observe that for similar annealing, the crystallite sizes of Y3-xRxFe5O12 (03+(0.97),Tb³⁺(1.0),Dy³⁺(0.99A) all bigger than Y³⁺(0.88A) which leads to tiny variation about the crystalline lattice.Results of VSM show that the saturation magnetization of Y3-xGdxFe5O12, Y_3-xTb_xFe₅O₁₂, Y_3-xDy_xFe₅O₁₂ (x=0.2,0.4,0.6,0.8,1.0)particles are not only obviously less than pure Yttrium iron garnet( YIG),but also decrease with increasing the Re³⁺ concentration(x) in a linear manner,but the values of their changes are different. The reason :we substituted someR³⁺(R= Gd,Tb,Dy)for Y³⁺, so the ion distribution structure can be represented by writing the garnet formula as {Y3-xRx}[Fe2](Fe3)O12({ }-c sub-lattice, [ ]- a sub-lattice, ( )- d sub-lattice,R=Gd,Tb,Dy). BecauseR³⁺ ions in Y3-xRxFe5O12 are magnetic, whose magnetic moment are 7.94 B (Gd³⁺),9.70 B (Tb³⁺),10.60 B (Dy³⁺)(Y³⁺ ion is non-magnetic and its magnetic moment is 0 B), so there are three magnetic sub-lattices: one (c) formed by the R³⁺ ions occupying the dodecahedral sites, another (a) formed by Fe³⁺ ions occupying the octahedral sites and the third (d) formed by the Fe³⁺ ions occupying the tetrahedral sites. The two iron sub-lattices are coupled anti-ferromagnetically by the super-exchange interaction. The c sub-lattice is coupled anti- ferromagnetically with the tetrahedral sub-lattice. At room temperature, the three sub-lattice moments align along the[1 1 1] direction. Therefore, the magnetic structure for such a mixed garnet can be represented by writing the garnet formula as and the net magnetic moments is M = Mc-|Md-Ma|.In the case of Y3-xRxFe5O12, the saturation magnetization (Ms) decreases with increasing concentration (x) in a linear manner, which could be related to the fact that at room temperature the Fe³⁺ ions net magnetic moments play a leading role and the magnetic moments of R³⁺ ions align opposite to the effective moments formed by Fe³⁺ ions.Results of VSM show also show that the saturation magnetization for samples with the same R concentration (x) are increased as the heating temperature is increased. Clearly,the saturation magnetization increases linearly with the increasing of the particle diameter.When meet some concentration x, the magnetic moment of R³⁺ play a leading role at 0K, (?)°c(?)±sites magnetic moments antiparallel Fe³⁺ ions net magnetic moments. As a result of the coupling between(?)°c(?)±sites and (?)°a(?)±,(?)°d(?)±sites are weak, the rate of (?)°c(?)±sites magnetic moments decline is greater than。When a certain temperature, the overall magnetizationMs is zero,Mc =Md-Ma, the direction of Mc is antiparallel to the net magnetic moment of Fe³⁺ ion, at a higher temperature ,the magnetic moment of Fe³⁺ play a leading role, there is Ms。Because the extent of the magnetic order of Dy³⁺is smaller than the extent of magnetic order of Tb³⁺, the extent of the magnetic order of Tb³⁺is smaller than the extent of magnetic order of Gd³⁺,and the effective magnetic moments in c-sites formed by Dy³⁺are smaller than the magnetic moments formed by Tb³⁺, the effective magnetic moments in c-sites formed by Tb³⁺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-xTb_xFe₅O₁₂the saturation magnetization of Y_3-xTb_xFe₅O₁₂ is larger than the saturation magnetization of Y3-xGdxFe5O12 when they have the same concentration x at room temperature,The interaction between Fe³⁺ ions in (?)°a(?)±and (?)°d(?)±sites is strongly antiferromagnetic due to the strong superexchange interaction via the oxygen ions O2(?)...