Effect of Substituted Concentration on Structure and Magnetic Properties of Y3Fe5-XInxO12 - Vu Thi Hoai Huong

VNU Journal of Science: Mathematics – Physics, Vol. 34, No. 4 (2018) 28-34 28 Effect of Substituted Concentration on Structure and Magnetic Properties of Y3Fe5-xInxO12 Vu Thi Hoai Huong, Dao Thi Thuy Nguyet, Nguyen Phuc Duong* ITIMS, Hanoi University of Science and Technology, No. 1, Dai Co Viet, Hai Ba Trung, Hanoi, Vietnam Received 09 November 2018 Revised 18 December 2018; Accepted 18 December 2018 Abstract: This study examines the effect of substituted concentration on the structure and magnetic properties of Y3Fe5-xInxO12 (x = 0.1 ÷ 0.7) powder samples prepared using the sol-gel method. The morphological properties of the samples were analysed using XRD, SEM. The single phase of garnet was obtained in x = 0.1 ÷ 0.6 samples. The lattice parameters of the samples exhibit a linear increase with the increasing In3+ content, which can be explained by a substitution of In3+ ions for Fe3+ ions, considering the larger ionic radius of In3+ compared with that of Fe3+. Crystallite sizes were determined via the XRD data which are of 38 – 49 nm while the particle sizes were estimated from SEM images to be in range of 50 - 100 nm. Magnetization and Curie temperature of the single phase samples were studied by magnetization curves in fields up to 10 kOe and in the temperature range from 80 K to 560K. With the increase of In3+, the magnetization gradually increases while the Curie temperature decreases due to the occupation of In atoms at the a sites and the reduction of intersublattice interaction, respectively. Keywords: Yttrium iron garnet, Indium substitution, cation distribution, magnetization, Curie temperature. 1. Introduction Yttrium ferrite garnet Y3Fe5O12 (YIG) is known as one of the most important garnets for microwave applications. It is material that widely used in microwave devices, transformers, electric generators, bio- processing, and storage devices [1-7] because of its combine good high-frequency dielectric properties with a ferrimagnetic order. YIG has three different crystallographic sites with 16 Fe3+ ions in the octahedral [a] site, 24 Fe3+ ions in the tetrahedral site (d) and 24 Y3+ ions in the dodecahedral {c} site. The Fe atoms on the octahedral sites couple anti-ferromagnetically to the Fe atoms on the tetrahedral ________ Corresponding author. Tel.: 84-915527063. Email: duong@itims.edu.vn https//doi.org/ 10.25073/2588-1124/vnumap.4298 V.T.H. Huong et al. / VNU Journal of Science: Mathematics – Physics, Vol. 34, No. 4 (2018) 28-34 29 sites (Jad < 0) and there is no coupling to the dodecahedral sites because yttrium is a nonmagnetic atom. The magnetization of YIG per formula unit calculated by 3 tetrahedral and 2 octahedral sites, corresponding to a net moment of 5(3 - 2)µB = 5µB. The interest in the structural, microstructural and magnetic properties of YIG has been due to the fact that all these properties can be widely varied by dopant substitutions [8-10]. In previous studies, nonmagnetic ion as V5+ was substituted on Fe ions at d site and that is reason leading to reduce magnetization and Curie temperature of the system [11, 12]. In most applications of ferrite garnets, enhancement of the magnetization of the materials is required which can be achieved by substituting non-magnetic ions on the a sublattice. In this work, In3+ ions were substituted for Fe3+ and the effect of the presence of non-magnetic ions on the structure and magnetic properties were investigated. 2. Experimental 2.1. Sample preparation The Y3Fe5-xInxO12 nanoparticle samples (x = 0 ÷ 0.7) were prepared by using sol-gel method. Starting materials for preparation of the samples were high purity Fe(NO3)3, Y2O3, In2O3 (99,99%, Sigma Aldrich). The oxides were dissolved in HNO3 1M to form nitrate solutions. The metal nitrate solutions were mixed the required amount of the metal ions in a stoichiometric ratio of Y: Fe: In = 3: (5 – x): x. An aqueous solution of citric acid was added into the solution with the total cation/citric acid molar ratio is 1/3. The mixtures were stirred at 400 rpm and slowly evaporated at 80C to form gels. The gels were dried at 95C for more than 12 hours in order to form xero-gels. The nanoparticle samples were obtained after burning the xero-gels at 400C in 2 hours and annealing at 900C in 5 hours. 2.2. Analytical methods The phase structure and crystallite sizes of the powder samples at room temperature were investigated using X–ray diffraction (XRD) (Cu-Kα, Siemens D-5000) operating with Cu-Kα radiation ( =1.54060 Å) in the range of 20-70 by a 2 scan mode and 0.03 scan step at room temperature. The grain size and morphology of samples were measured by using Scanning Electron Microscope (SU3500). Magnetic measurements were carried out by using a vibrating sample magnetometer (VSM) in maximum applied magnetic field of 10 kOe and at temperatures from 88 to 550 K. For the magnetic measurements, the nanoparticles were pressed in the forms of platelets. 3. Results and discussion 3.1. Structure, cation distribution and morphology of particle samples Fig.1 shows the XRD patterns of the Y3Fe5-xInxO12 (x = 0 – 0.7) nanoparticle samples sintered at 900C in 8 hours in which, the planes of a cubic unit cell of garnet structure were indexed as (321), (400), (420), (422), (521), (611), (444), (640), (642), (800), (752), (840). On the others XRD studies of samples sintered at 800C and 850C in 8 hours, the amount of YFeO3 phase is present. Therefore, the temperature of 900C in 8 hours is the lowest temperature of the sol-gel method for preparing In substituted YIG nanoparticle samples. This temperature is also lower than the 1000C of the mechanical alloying method [13] and conventional mix oxide route [14]. V.T.H. Huong et al. / VNU Journal of Science: Mathematics – Physics, Vol. 34, No. 4 (2018) 28-34 30 Figure 1. XRD patterns of Y3Fe5-xInxO12 (x = 0 – 0.7) samples. Small figure is enlarged image of diffraction peak (420). The samples with the In concentrations, x ≤ 0.6 are of single phase while the sample x = 0.7 exhibited the second phase orthoferrite YFeO3. Similar result is also observed in other study on indium substituted YIG polycrystalline samples [15]. The lattice constant, a, was computed using the interplanar spacing d and the corresponding Miller indices (h,k,l). It is also observed that a increases from12.38 Å to 12.442 Å as x increasing from 0.1 to 0.7. Fig. 2 shows monotonous increasing of lattice constants with increasing of In3+ contents. The observed variation in lattice constant can be explained by Vegard’s law, according to that, if the radius of displacing ion is larger than the displaced ion, the lattice is expanded and the lattice constant increases [16]. The ionic radius of In3+ (0.79 Å – in octahedral site) is larger than that of Fe3+ (0.645 Å – in octahedral site; 0.49 Å – in tetrahedral site), resulting in the gradual increase of the lattice constant with increasing In3+ content. The diffraction peaks (420) of the substituted samples are found to shift to lower Bragg angles in comparison with the pure sample (x = 0) as shown in the small Fig. 1. The value of a obtained at x = 0.1÷ 0.4 samples in this study are similar to the polycrystalline samples with the same concentration prepared by conventional mixed oxide [14] and mechanical alloying [13]. Table 1. Lattice parameter a (Å) and average crystallize size DXRD (nm) of Y3Fe5-xInxO12 (x = 0 ÷ 0.7) x = 0 x = 0.1 x = 0.2 x = 0.3 x = 0.4 x = 0.5 x = 0.6 x = 0.7 a (Å) 12.371 12.38 12.387 12.398 12.407 12.423 12.433 12.442 DXRD (nm) 38 49 47 46 43 41 43 42 The average crystal size DXRD determined based on the width of the (420) peak using the Debye – Scherrer varies from 41 to 49 nm as shown in Table 1. On the other hand, the morphological shape particle of samples were characterized by SEM images shown that average sizes are in the range of 50 – 100 nm. SEM image of Y3Fe5-xInxO12 (x = 0.2; 0.3; 0.4; 0.6) samples were indicated in Fig. 3, in which, due to the influence of temperature during annealing process, the grains are melted and clustered V.T.H. Huong et al. / VNU Journal of Science: Mathematics – Physics, Vol. 34, No. 4 (2018) 28-34 31 together, forming a porous structure. The difference in average crystal size and particle size is explained by the fact that each particle observed on the SEM image contains several crystallites which have dimensions in the orders of DXRD values. The structural heterogeneity region between these grains is also known as crystal dislocation region where the crystal growth temporarily halted [17]. x = 0.2 x = 0.3 Figure 2. Lattice constant of Y3Fe5-xInxO12 (x = 0 ÷ 0.7) samples x = 0.4 x = 0.6 Figure 3. SEM micrographs of Y3Fe5-xInxO12 (x = 0.2; 0.3; 0.4; 0.6) 3.2. Magnetic properties Magnetic properties of samples were studied by means of origin magnetization curves measured in magnetic fields up to 10 kOe at temperature from 88 K to 570 K. Fig. 4 demonstrates the M(H) curves of Y3Fe5-xInxO12 (x = 0.1 ÷ 0.6) samples at 88 K. At all of the survey temperatures, the M(H) curves reach saturation state in a magnetic field greater than 2 kOe. The value of the saturation magnetization Ms were determined based on the flat part of the curves in the higher field region. Ms(T) curves in Fig. 5 show the dependence of Ms value of samples on the temperature. The saturation magnetization Ms(0) at 0 K were determined by extrapolation of the graphical plot of Ms against T to T = 0 according to a modified Bloch law [18]: Ms(T) = Ms(0) (1 – BTα) where B is the Bloch’s constant and α is the Bloch exponent The experimental curves were the best reproduced with α 1.6 as shown in Fig. 5. The extrapolated magnetic moments in ground state express in Bohr magneton per formula unit (µB/f.u) were calculated according to the formula: m(0) = Ms(0)×W/5585 where W is the molar mass. In order to evaluate the effect of substituted concentrations on the magnetic moment of samples, the experimental magnetization values are identified at 0 K and compared with theory values. Applying the Néel model for the assumption that In ions substituted in a sublattice, the theoretical values of m(0)theo of substituted samples are identified based on formula (4): m(0)theo = m(tetra) – m(octa) = 3×5 µB – (2 – x)×5 V.T.H. Huong et al. / VNU Journal of Science: Mathematics – Physics, Vol. 34, No. 4 (2018) 28-34 32 µB where m(tetra) and m(octa) are magnetization of tetrahedral and octahedral sites at 0 K. The theory and experimental values m(0)exp of magnetizations are listed in Table 2. Figure 4. The M(H) curves of Y3Fe5-xInxO12 (x = 0.1 ÷ 0.7) samples at 88 K Figure 5. Temperature dependence of magnetization values Ms of Y3Fe5-xInxO12 (x = 0 ÷ 0.7) samples The increase of magnetic moment with increasing indium contents indicates that In atoms prefer to enter the a site. The experiment values are smaller than the theoretical values for all the samples which can be explained due to both dislocations and canting of the magnetic moments in the main phase. The canting effect was reported previously for Sc substituted YIG samples in which non-magnetic Sc atoms occupy the a site [18]. The Curie temperature TC values of Y3Fe5-xInxO12 samples are listed in Table 2. The TC of YIG sample is similar to that of the bulk counterpart [19] while the TC of substituted samples decreases gradually with x. The reduction of TC is due to the magnetic dilution effects. Comparison is made among the TC values of magnetically diluted YIG systems which is shown in Table 2. It is seen that with the same substitution level, TC decreases faster in the systems with substitution takes place at the d sites compared to that at the a sites. This phenomenon can be explained by the fact that the number of pair interactions between Fe ions in the a and d sublattice for the case (3-x)×2 is larger than that for the case 3×(2-x) where x represents the mole fraction of non-magnetic ions. Table 2. Saturation magnetization Ms (0) and Curie temperature TC of Y3Fe5-xInxO12 (x = 0 ÷ 0.7) samples x 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Mexp (0 K) (emu/g) 35.5 41.05 43.58 44.5 46.56 47.58 49 50 m(µB/f.u.)exp (0 K) 4.72 5.47 5.84 6.04 6.34 6.54 6.78 6.98 m(µB/f.u.)theo (0 K) 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 TC (K) 560 546 530 512 493 478 460 405 TC (K){Y}3[Fe2](Fe3-xAlx) O12 [20] - - - - - 480 - - TC (K) {Y3-xCax}[Fe2-xSnx](Fe3) O12 [21] - 530 - 502 - 463 - - V.T.H. Huong et al. / VNU Journal of Science: Mathematics – Physics, Vol. 34, No. 4 (2018) 28-34 33 4. Conclusion In summary, the present study has shown that Y3Fe5-xInxO12 (x = 0 ÷ 0.7) powders with size between 50 – 100 nm were fabricated using the sol-gel method. The single phase structural was observed for x as large as 0.6. At x = 0.7, a small amount of orthoferrite YFeO3 appears (~ 2%). The value of lattice constant increases with increasing x. Our results show that In3+ ions can replace for Fe with the concentration up to x = 0.7. 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