Processing and Application of Ceramics 8 [4] (2014) 211–218 DOI: 10.2298/PAC1404211O Synthesis, structure and magnetic properties of Y3Fe5-xAlxO12 garnets prepared by the soft chemical method Pedro Paulo Silva Ortega1, Miguel Angel Ramirez1, César Renato Foschini2, Filiberto González Garcia3, Mario Cilense4, Alexandre Zirpoli Simões1,∗ 1Universidade Estadual Paulista - UNESP, Faculdade de Engenharia de Guaratinguetá, Av. Dr. Ariberto Pereira da Cunha, 333, Bairro Portal das Colinas, CEP 12516-410, Guaratinguetá, SP, Brazil 2Universidade Estadual Paulista - UNESP, Faculdade de Engenharia de Bauru, Dept. de Eng. Mecânica, Av. Eng. Luiz Edmundo C. Coube 14-01, Zip-Code: 17033-360, Bauru, SP, Brazil 3Universidade Federal de Itajubá, Instituto de Física e Química, Ave. BPS 1303, Itajubá, 37500-903, Minas Gerais, MG, Brazil 4Universidade Estadual Paulista - UNESP- Instituto de Química – Laboratório Interdisciplinar em Cerâmica (LIEC), Rua Professor Francisco Degni s/n, Zip-Code: 14800-90-, Araraquara, SP, Brazil Received 13 October 2014; Received in revised form 19 December 2014; Accepted 22 December 2014 Abstract A study was undertaken about the structural, morphological and magnetic properties at room temperature of crystalline aluminium substituted yttrium iron garnet, YIG (Y3Fe5-xAlxO12 with 1.5< x< 1.7) nanoparticles pre- pared by polymeric precursor method at the temperature of 700 °C for 2 hours. The single-phase character and the well-defined structure of YIG nanoparticles were confirmed by X-ray diffraction, excluding the presence of any other phases. The Raman spectra showed that the changes of lattice vibration would influence interaction between the Fe ion and the host. Mean crystallite size of the single-phase powder was about 46–65 nm. Parti- cles’ morphology was investigated by high-resolution transmission electron microscopy, which shows that the particles were agglomerated. From hysteresis loops, particles’ efficiency range from 91.4% to 95.9% as Fe/Al ratio decreases. Saturation magnetization was affected by the particle size and Fe/Al stoichiometric ratio. We observe that the saturation magnetization increases as the Fe/Al ratio is raised due to enhancement of the surface spin effects. Keywords: yttrium iron garnet, powder, chemical synthesis, characterization, magnetic properties I. Introduction In recent years, hyperthermia a therapeutic procedure used in cancer treatment has been the subject of ex- tensively studies. It is based in the principle that tem- peratures around 42 °C destroy cancerous cells preserv- ing the surrounding healthy tissues, once the malignant cells are less resistant to this increase in temperature [1]. Hyperthermia has a notable lack of side effects, which makes it an attractive substitute or combined treatment for chemotherapy and radiotherapy. Though many dis- tinct modes of hyperthermia are available, such as elec- tromagnetic radiation, lasers, microwaves and radiofre- ∗Corresponding author: tel: +55 12 3123 2228 e-mail: alezipo@yahoo.com quency, they have serious limitations in tumor target- ing and precise localization of the thermal energy [2,3]. Therefore, the ideal mechanism for a proper use in hy- perthermia must be capable of heating the cancer site affecting the less possible the surrounding healthy tis- sues. Gilchrist et al. [4] suggested that the selectiv- ity required can be achieved through a powdered mag- netic material with controlled particle size which must be one micron or less in diameter. Hence, the mag- netic particles can be guided or localized in a specific target through an external magnetic field [1]. Kumar’s recent review [5] pointed out many types of magnetic materials that have been studied for their hyperther- mia potential, most of them iron oxide-based materi- als. As it is known, the iron-containing oxide phases 211 P.P.S. Ortega et al. / Processing and Application of Ceramics 8 [4] (2014) 211–218 with A3B5O12 cubic garnet structure possess unique magnetic, magneto-optical, thermal, electrical and me- chanical properties, such as ferrimagnetism, excellent creep, high thermal conductivity, high electrical resis- tivity, moderate thermal expansion coefficients, radia- tion damage resistance, energy-transfer efficiency and controllable magnetization saturation. [6] Yttrium iron garnets (YIG), Y3Fe5O12, belong to this class of ma- terials. Since their discovery in 1956 by Bertaut and Forrat [7], YIG have attracted attention due to their in- teresting properties, as low dielectric loss, narrow res- onance linewidth in microwave region and good satu- rated magnetization value [8], and as an important ma- terial in optical communication and magneto-optical de- vices [6–10]. YIG crystallizes in cubic structure, space group Ia3d, with eight molecules in a unit cell of lat- tice constant a = 12.376± 0.004 Å [10]. There are three sublattices: tetrahedral (d), octahedral (a) and dodeca- hedral (c). They are surrounded by four, six and eight oxygen ions, respectively. The formula unit can be writ- ten as Y3Fe2(FeO4)3 and yttrium ions occupy 24c (do- decahedral) sites, two iron ions occupy 16a (octahedral) sites and the other three iron ions occupy 24d (tetra- hedral) sites. The only magnetic ion in YIG is Fe3+. A magnetic moment of 5 µB per formula unit results from antiferromagnetic superexchange interaction be- tween these Fe3+ ions in octahedral and tetrahedral sites through the intervening O2– ions [6,10,11]. Akhtar et al. [12] showed that magnetic properties (such as rema- nence and saturation) depend on particle size, magnetic dilution and superexchange interaction of the YIG fer- rites. Although YIG high saturated magnetization value is significant for hyperthermia, its controllable Curie temperature, Tc (or Curie point) is an equally important property. Above Curie temperature ferromagnetic mate- rial becomes paramagnetic. When exposed to an exter- nal magnetic field, YIG generates heat due to both Neel and Brown relaxation [13]. Once these particles reach Curie temperature they no longer generate heat. This phenomenon prevents the particles from overheating and damaging human tissues. The substitution of metal ions in the ferrite structure can affect the magnetic, elec- trical and dielectric properties of ferrites [14]. As Gras- set et al. demonstrated [15], aluminium substituted YIG (Y3Fe5-xAlxO12, 0 ≤ x ≤ 2) decreases Curie point near body temperature by adjusting Fe/Al ratio, mak- ing these particles self-regulated and safe-guarded for hyperthermia applications. A variety of synthesis meth- ods for obtaining YIG has been described in literature, such as the mechanochemical method [11], solid state reaction [16], citrate sol-gel [17], microwave-induced combustion [18], hydroxide coprecipitation [19], copre- cipitation in microemulsion [20], metal alkoxides hy- drolysis [21], glycothermal synthesis [22] and glass- crystallization [23]. Although, in literature was found that only the hydroxide coprecipitation method per- form aluminium substituted YIG. In this paper, poly- meric precursor method, or Pechini method, was used to synthesize aluminium substituted YIG, Y3Fe5-xAlxO12 (1.5 < x < 1.7), which has not yet been reported in literature. This technique was originally developed by Maggio Pechini in 1967 [24]. It is based on the chelation of a metallic cation by a carboxylic acid, such as citric acid, and further polymerization promoted by the addi- tion of ethylene glycol and consequent polyesterifica- tion [25]. This method possesses important advantages: possibility of working with aqueous solutions, synthesis at low temperatures free of contamination, low cost, no necessity of high atmosphere control, good stoichiomet- ric control at a molecular level and produces powders in a nanometric scale [26–28]. II. Experimental procedure The reactants used were all reagent grade: Y(NO3)3×6 H2O (99.9%) and AlC5H5O7 (99.9%) were provided by Aldrich, FeC6H5O7×NH4OH (99.5%) by Merck, and C6H8O7 (99.5%) and C2H6O2 (99.5%) by Synth. The polycrystalline nanoparticles of the composition Y3Fe5-xAlxO12 with 1.5 < x < 1.7 were prepared by the polymeric precursor method. All reagents were weighed according to the previous defined stoichiometries. The resins were obtained dissolving ammonium iron (III) citrate, yttrium ni- trate hexahydrate and aluminium citrate in water by constant mixing at 80 °C. Afterwards, citric acid and ethylene glycol were added to the solution. Citric acid complex the metal ions and ethylene glycol promotes the polymerization. The result is a yellow coloured, clear, homogeneous polyester resin. Alumina crucibles were used for calcination of each resin. The first part consisted of preparing a metal-citrate complex for three different stoichiometries, with x values equal to 1.5, 1.6 and 1.7. The second part was to decompose the organic material at 700 °C for 2 hours. Calcination temperature and time were carefully programmed in a mufla oven. After the thermal treatment, the nanometric powders were obtained. Phase formation and crystallographic structure were characterized by X-ray diffraction (XRD) on a Rigaku Dmax/2500PC diffractometer using Cu Kα radiation (λ = 1.5406 Å). The cubic cell parameter and the crystallite size were determined from the FULL- PROF software (Version 0.2 Mar 98, LLB Juan Rodriguez-Carvajal). The peak shape was described by a Thompson-Cox-Hastings pseudo-Voigt function. The refinable parameters are the isotropic Lorentzian and Gaussian contribution, respectively, attributed to the size and microstrains. For each diffraction pattern, the Gaussian contribution associated to microstrains was found negative and inferior to 0.4%. Raman spectra were collected (Bruker RFS-100/S Raman spectrome- ter with Fourier transform). A 1064 nm YAG laser was used as the excitation source, and its power was kept at 150 mW. Fourier transform infrared spectroscopy (FTIR) was recorded using a Bruker Equinox 55 spec- 212 P.P.S. Ortega et al. / Processing and Application of Ceramics 8 [4] (2014) 211–218 trometer in diffuse reflection mode. The spectra were measured between 385 and 4000 cm-1 region with a res- olution of 4 cm-1. Size and morphology of the particles were studied by field emission gun scanning electron microscopy (FEG-SEM). A Zeiss Supra 35-VP micro- scope was used, operating under a 6 kV incident elec- tron beam. Specimens for transmission electron micro- scope (TEM) were obtained by drying droplets of as- prepared samples from an ethanol dispersion which had been sonicated for 5 min onto 300 mesh Cu grids. High- resolution transmission electron microscope (HRTEM) images and selected area diffraction (SAD) patterns were then taken to an accelerating voltage of 200 kV on a Philips model CM 200 instrument. To measure the DC magnetic field, a Hall probe was employed. Magnetization measurements were done by using a vibrating-sample magnetometer (VSM) from Quantum Design™. All measurements were taken at room temperature. Dielectric properties were measured using an impedance analyser, model 4192 of HP. From the capacitance dependence temperature curves, the Curie temperature was determined. III. Results and discussion In this presented paper, aluminium substituted YIG, with formula Y3Fe5-xAlxO12 (1.5 < x < 1.7), was pre- pared by polymeric precursor method. The influence of stoichiometry on phase formation was monitored. The diffraction patterns reveals that yttrium aluminium iron garnet appears as a single phased for all stoichiometries at 700 °C. The Bragg peaks are in accordance with the literature [29], proving that the particles are pure and homogenous. X-ray diffraction patterns reveal that the structure of powders is bcc and the garnet phase has been obtained after calcining. Small shifts of diffraction lines of Bragg angles with increasing the Al concentra- tion were observed. This is due to the fact that the ra- dius of Al3+ is 0.5 Å and that of Fe3+ is 0.61 Å [29], and by substitution of Al3+ instead of Fe3+ slight changes of lattice parameter would be expected. From the Fig. 1 it was clear that the 2θ position of each crystal plane showed almost no differences with respect to the doping concentration of Fe3+, implying that the crystal planes were not damaged by doping. Y, Al and O were still in their own original plane. Doping just replaced a Fe3+ ion with an Al3+ ion without changing the whole crystal structure. The average crystallite size of different sam- ples was in the range 46–65 nm. Lattice parameters were 1.2280, 1.2265 and 1.2260 nm for x value equal to 1.5, 1.6 and 1.7, respectively. FT-IR spectrums for the powders are shown in Fig. 2. The presence of multiple bonds in the Y3Fe5-xAlxO12 powders can be noticed. All powders analysed presented O–H bond stretching from 3200 to 3600 cm-1, probably due to water absorption during test. Wavenumbers rang- ing from 825 to 930 cm-1 represent axial strain of C– O bonds in carbonates and carboxylates, respectively, proving the presence of these compounds in the sam- Figure 1. X-ray diffraction pattern of Y3Fe5-xAlxO12 powders synthesized at 700 °C for 2 h by the polymeric precursor method Figure 2. Infrared spectroscopy of Y3Fe5-xAlxO12 powders synthesized at 700 °C for 2 h by the polymeric precursor method (inset is FT-IR in the range from 500 to 1000 cm-1) ples. Wavenumbers ranging from 545 to 680 cm-1 are assigned to stretching mode of YIG tetrahedra [15], characteristic of metal-oxygen bonds in ceramics. There was, however, a vibration band noted that is associ- ated to the deformation of O–H bonds near 1680 cm-1. This is attributed to water adsorbed at the powder sur- face when the sample was in contact with the environ- ment. The spectrum exhibits three bands at 759, 680 and 602 cm-1 assigned to the stretching mode of YIG tetra- hedral [30]. In the case of aluminium substituted YIG, these bands are slightly broadened and shifted towards higher wavenumbers. Unlike previous studies [20], IR spectra show very weak bands characteristic of carbon- ates (1400 and 1520 cm-1). The band at 2339 cm-1 was previously assigned by Vaqueiro et al. to atmospheric CO2 [20]. Nevertheless, this band did not disappear af- ter long degassing time and could be assigned to another absorber which nature remains unknown. Figure 3 shows the Raman spectra of YIG polycrys- talline powders with different aluminium doping con- centrations. It is already known from X-ray diffrac- tion analyses that there is no crystal structural change 213 P.P.S. Ortega et al. / Processing and Application of Ceramics 8 [4] (2014) 211–218 (a) (b) (c) Figure 3. Raman spectra of Y3Fe5-xAlxO12 powders synthesized at 700 °C for 2 h by the polymeric precursor method with: a) x = 1.5, b) x = 1.6 and c) x = 1.7 in theses powders. However, Raman spectra do show some quite obvious variations in the molecular vibra- tional modes as the doping concentration is changed. This clearly indicates that Al-doping does replace ions in the crystal, thus changing the original YIG vibrational modes toward the YAG Raman spectrum. We believe that the replaced ions are the central aluminium ions in the tetrahedrons and octahedrons. First, let us examine the peak at 720 cm-1. As the doping concentration is in- creased, the relative intensities of these peaks are grad- ually decreased and eventually disappear from the spec- trum as aluminium doping is 1.6. This may be explained as follows. As Al3+ ions are doped into the YIG crys- tals, they gradually fill into the tetrahedrons and octahe- drons and replace Fe3+ as their central ion. The greater mass of Fe3+ ion makes the tetrahedrons and octahe- drons heavier than before, thus, weakens these vibra- tions and causes the decrease in their relative intensi- ties. We speculate that there might be a phase change point somewhere in between 1.5 and 1.6 which causes the complete disappearance of these vibrational modes of the aluminium-centred tetrahedrons and octahedrons. As pointed out in the reference [7], these peaks are the internal stretching or external vibrations between alu- minium ion and oxygen, or between aluminium-centred tetrahedrons or octahedrons and oxygen. These peaks disappear at high doping concentrations. This may be attributed to the various displacement situations of alu- minium by iron. In an YAG crystal, there are 16 octahe- dral a lattices, each with six coordinating oxygen ions and 24 tetrahedral d lattices, each with four coordinat- ing oxygen ions. Fe3+ ion displacement may take place in either one of these lattices. It may be reasonably as- sumed that ion displacement in these lattices may be sequential. That is, one group may start after the other finishes. Since the Al3+ induced vibrational modes give rise to higher peak intensities in this region than those in the previous region, the remaining spectral signals may, therefore, be attributed to those lattices in which their Al3+ ions are not yet displaced. The peaks at 300, and 520 cm-1 are from vibrations of Y3+ ions. No frequency shift is observed as indicated in Fig. 3. The determination of hydrodynamic particle size was made by mercury porosimetry and is important because most biotechnological processes use liquid media. The observed hydrodynamic particle size (Fig. 4) is larger than the crystallite size and confirms the aggregation phenomenon. The theoretical number of crystallites per particle is determined as the ratio of the hydrodynamic particle size to the crystallite size and it is nearly a con- stant function of x. The average values are 2.5, 1.8 and 2.0 for x values equal to 1.5, 1.6 and 1.7, respectively. This may be explained by the fact that the samples were calcined at the same temperature and during the same (a) (b) (c) Figure 4. Hydrodynamic particle size distribution of Y3Fe5-xAlxO12 with: a) x = 1.5, b) x = 1.6 and c) x = 1.7 214 P.P.S. Ortega et al. / Processing and Application of Ceramics 8 [4] (2014) 211–218 time and may indicate a constant sintering process. The aluminium substituted YIG powder is composed of par- ticles with a hydrodynamic average diameter particle size of approximately 0.3, 0.6 and 0.8 µm for x value equal to 1.5, 1.6 and 1.7, respectively. Field emission gun scanning electron microscope (FEG-SEM) was used for better evaluation of the Y3Fe5-xAlxO12 nanoparticles prepared by polymeric precursor method (Fig. 5). The particles are present in agglomerates, independently of the stoichiometry. Ob- viously, the obtained solids are composed of grains with no regular shape. For example, the volumetric plate- like grains coexist with spherically shaped particles, and they are partially fused to form hard agglomerates. Par- ticles’ morphology is similar for all compositions, but the agglomerates seem to enlarge as the Fe/Al ratio de- creases. Afterwards, it is possible to separate the par- ticles by ball milling in order to reduce the size of the agglomerates and increase the specific area. It is inter- esting to note that almost identical microstructure was observed for all samples. As the Fe/Al ratio decreases individual particles seem to be plate-like crystals with an average particle size of around 0.3 µm. Figure 6 shows a bright field image of the sample (on the left), and corresponding selected area electron diffraction pattern (on the right). In the electron mi- crograph, grain-like particles with dimensions ranging from several nanometers to several tens of nanometers are observed. Compared to hydrodynamic average di- ameter, particles with lower size were observed spe- cially due to the agglomerates. It should be noted that it was possible to separate particle with size between 10 and 100 nm after 30 min of centrifugation at a constant rotation speed. As the specimen was tilted with respect to the in- cident electron beam, image contrast for several parti- cles changed significantly, suggesting that these parti- cles were crystalline in nature. SAD shows predomi- nantly a halo pattern in a small diffraction angle region. There are diffraction rings together with several weak spots present. This indicates that the majority of the particles were still in the crystalline form, but the sym- metry of unit cells slightly changed. Although the ex- act mechanism of the phase formation of Y3Fe5-xAlxO12 with 1.5 < x < 1.7 garnets has not yet been identified, the common feature of both their synthesis and mag- netic properties of the crystalline YIG has play special importance. In almost all regions of the specimen, ag- glomerates were observed, whereas the small particles were dispersed on exceptionally small fraction of the total area. It is therefore concluded that the particles are crystalline and consistent with the results from X-ray diffraction, leading to the conclusion that no interme- diate state exists between the amorphous and the crys- talline YIG. Otherwise, the diffraction pattern of YIG structure becomes disordered as Fe/Al ratio decreases. It should, however, be noted that the diffraction spots do not exactly fall on the circumference of the diffraction rings but fluctuate with respect to the radial direction. This fluctuation was not observed in the X-ray diffrac- tion patterns. In order to understand the role of aluminium in the substituted garnet a study of magnetic properties was undertaken. Figure 7 shows hysteresis loops of Y3Fe5-xAlxO12 samples thermal treated at 700 °C for 2 h at room temperature. In the YIG, the A = Y3+ site is eightfold dodecahedrally coordinated (c site), the B = Fe3+ site six-fold octahedral (a site) and the B0 = Fe3+ site four-fold tetrahedral (d site). Y3+ is diamag- netic and the magnetic moment results from negative superexchange (antiferromagnetic) interaction between Fe3+ ions in these two different a and d sites. In this case, the a–d interactions are dominant compared to a–a and d–d interactions [23]. As it can be seen, the particles are superparamagnetic. Fe3+ ions occupy tetrahedral (d) and octahedral (a) sites antiferromagnetically, therefore, the magnetic moment is a response to the negative su- perexchange interaction between Fe3+ in a and d sites [15]. At first, Al3+ ions preferably substitutes Fe3+ ions in d sites for small quantities, and tend to octahedral substitution as Al3+ concentration is increased [6]. As Fe3+ ions are substituted by the non-magnetic Al3+ ions in d and a sites the superexchange interactions of iron ions in the lattice is reduced [6], consequently reducing the magnetization, as it can be seen for lower Fe/Al ra- tios. As it is well known, tetrahedral site is smaller than octahedral site. In substituted YIG a non-magnetic sub- stituent M3+ ion occupies predominantly an octahedral or tetrahedral site for small x as it is larger than Fe3+. In other words, the smaller ion persistently seeks a smaller site [31]. Reduction of magnetization for higher Al3+ concentration is due to the substitution of Fe3+ for Al3+ in octahedral and tetrahedral sites in substituted YIG and the reduction of super exchange interactions in the lattice. The percentage efficiency calculated from hys- teresis loops are 91.4%, 93.7% and 95.9% for x equal to 1.5, 1.6 and 1.7, respectively. Therefore, the efficiency seems to increase as Fe/Al ratio decreases. However, less energy is converted to heat as Fe/Al ratio decreases, once the magnetization saturation is proportional to the presence of Fe3+ ions in the compound. The temperature dependence of dielectric permittiv- ity is shown in Fig. 8. The dielectric permittivity mea- sured at 10 kHz are 14300, 11600 and 4660 for x equal to 1.5, 1.6 and 1.7, respectively, and the phase transi- tions are around 53, 47 and 37 °C. A structural phase transition corresponds to the transition of a non polar state to a polar state at Tc. The dielectric permittivity increases gradually with an increase in temperature up to the transition temperature (Tc) or Curie point and then decreases. Also, the maximum dielectric permit- tivity (εm) and the corresponding maximum temperature (Tm), depend on the composition. The magnitude of the dielectric permittivity decreases for Y3Fe3.5Al1.5O12 to Y3Fe3.4Al1.6O12 and Y3Fe3.3Al1.7O12, respectively, and the Curie temperature shifts toward a lower temperature 215 P.P.S. Ortega et al. / Processing and Application of Ceramics 8 [4] (2014) 211–218 (a) (b) (c) Figure 5. FEG-SEM of Y3Fe5-xAlxO12 powders synthesized at 700 °C for 2 h by the polymeric precursor method with: a) x = 1.5, b) x = 1.6 and c) x = 1.7 (a) (b) (c) Figure 6. TEM and SAD images of Y3Fe5-xAlxO12 powders synthesized at 700 °C for 2 h by the polymeric precursor method with: a) x = 1.5, b) x = 1.6 and c) x = 1.7 216 P.P.S. Ortega et al. / Processing and Application of Ceramics 8 [4] (2014) 211–218 Figure 7. M-H loops of Y3Fe5-xAlxO12 powders synthesized at 700 °C for 2 h by the polymeric precursor method Figure 8. Temperature dependence of dielectric permittivity at 10 KHz of Y3Fe5-xAlxO12 (1.5 < x < 1.7) powders synthesized at 700 °C for 2 h by the polymeric precursor method which indicates that dielectric polarization is a relax- ation type in nature. The region around the dielectric peak is broadened due to a disorder in the cations ar- rangement in one or more crystallographic sites of the structure. The reduction of dielectric permittivity indi- cates that Al3+ ions are doped into the YIG crystals, they gradually fill into the tetrahedrons and octahedrons and replace Fe3+ as their central ion. The higher the alu- minium content is, the lower the Curie temperature is because the number of the main magnetic interaction Jad (where Jad is the constant exchange between cations in a and d sites) per magnetic ion per formula was re- duced. So in the composition range investigated, Tc val- ues are near room temperature. To improve confiability, temperature-dependent magnetization curves and differ- ential scanning calorimetry (DSC) should be performed. IV. Conclusions Y3Fe5-xAlxO12 (YIG, with 1.5 < x < 1.7) nanopar- ticles with no impurities were synthesized by the poly- meric precursor method at 700 °C for 2 h. XRD patterns suggest that the powder crystallizes in bcc garnet struc- ture and doping just replaced a Fe3+ ion with an Al3+ ion. Raman analysis revealed that as Al3+ ions are doped into the YIG crystals, they gradually fill into the tetrahe- drons and octahedrons and replace Fe3+ as their central ion. Field emission electron scanning microscopy shows particles’ morphology similar for all compositions, but the agglomerates seem to enlarge as the Fe/Al ratio de- creases. Transmission electron microscopy and selec- tion area diffraction evidenced disordered structure as Fe/Al ratio decreases and that almost all YIG particles are crystalline. 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