Khozima Hamasha, Qassem I Mohaidat, Mahdi Lataifeh,Ibrahim Bsoul, Sami H Mahmood

(1. Department of Basic Sciences, Al-Huson University College, Al-Balqa Applied University, Al-Salt, Jordan; 2. Physics Department, Yarmouk University, Irbid 21163, Jordan; 3. Physics Department, Al al-Bayt University, Mafraq 13040, Jordan; 4. Physics Department, The University of Jordan, Amman 11942, Jordan)

Abstract: Ga-doped Yttrium Iron Garnet (Y3GaxFe5-xO12, Ga:YIG) was prepared by solid state reaction method and sintering at 1 300 ℃. Rietveld analysis of X-ray diffraction patterns indicated that all samples crystallized in a single cubic structure (space group Ia-3d) with decreasing lattice constant as Ga concentration increased. SEM surface micrograph images of YIG samples showed highly compacted grains with small reduction in the grain size with increasing Ga concentration. Raman spectroscopy measurements confirmed the replacement of Fe3+ ions by Ga3+ ions in the garnet structure was revealed by the observed blue shifts in Raman spectra. The saturation magnetization decreased from 28.2 to 4.98 emu g-1 with increasing x from 0.0 to 1.0 due to the preferential substitution of Ga3+ ions for Fe3+ ions at tetrahedral sites. Room temperature Mössbauer spectra for the samples revealed a reduction of the hyperfine field values for octahedral and tetrahedral sites, and the development of additional components with increasing Ga concentration. Analysis of the magnetic data and Mössbauer spectra confirmed that spin canting in the substituted garnets plays an important role in explaining the observed reduction of the saturation magnetization as x increased.

Key words: rare earth iron garnet; structural characteristics; scanning electron microscope; raman spectroscopy; magnetic properties; mössbauer spectroscopy

1 Introduction

Yttrium iron garnet (Y3Fe5O12, YIG) and substituted YIG have been studied extensively due to their interesting electromagnetic properties. Specifically,they have been widely used in microwave devices,memory devices, magnetic-optical devices, and electromagnetic wave absorbers[1,2].

YIG has cubic structure with eight chemical formula units (twenty-four Y3+ions, forty Fe3+ions, and ninety-six O-2ions) per unit cell. The Y3+ions occupy(24c) dodecahedral sites, and of the forty Fe3+ions,sixteen occupy octahedral (16a) sites and twenty-four occupy (24d) tetrahedral sites[3-6]. Y3+ions have zero magnetic moment and therefore the magnetic properties of YIG arise from the super-exchange interactions between thea-site Fe3+ions and thed-site Fe3+ions in which their magnetic moments aligned antiparallel.Each Fe3+ion has a 3d5electronic configuration which leads to magnetic moment of 5mB(atT=0 K).

The question concerning the site selectivity of nonmagnetic ion in doped YIG have received special attention by researchers due to the tunability of the magnetic properties of these materials, and their importance for constructing YIG-based microwave devices and magnetic bubble digital memories[1].Specifically, the magnetization (M) behavior of Y3Ga0.25Fe4.75O12and Y3Ga0.75Fe4.25O12as a function of temperature was investigated, and good agreement between the experimental observations and theoretical predictions based on the molecular field theory was reported[1]. Based on the early work of Gilleo and Geller, it became clear that Al3+and Ga3+ions preferred tetrahedral sites in yttrium iron garnets[7,8]. Other studies showed that Al3+ions prefered tetrahedral site for small concentrations while with higher concentration the tendency of occupying octahedral site was increased[9-10]. Bsoulet alshowed that Ga3+ions in the system Er3Fe5-xGaxO12substituted Fe3+ions at tetrahedral sites in the composition range up tox= 0.6,while a small fraction of Ga3+ions replaced Fe3+ions at the octahedral sites in the sample withx= 0.8[11].

The site selectivity of nonmagnetic substituents in YIG has a critical influence on the magnetic properties of the garnets, and may depend on the synthesis route,as well as the heat treatment of the samples. The magnetic dilution of garnets by partial substitution of Fe3+ions by nonmagnetic ions is essential for the stability of the magnetic bubbles, and improvement of device performance[11]. To the best of our knowledge,research articles concerned with a comprehensive characterization of Ga3+substituted yttrium iron garnet by using both Raman and Mössbauer spectroscopies,in addition to the conventional structural and magnetic studies, are still scarce in the literature. The present work is concerned with the synthesis of single-phase Ga-doped YIG using solid state reaction technique.X-ray diffraction (XRD), Raman spectroscopy,scanning electron microscopy (SEM), vibrating sample magnetometer (VSM), and Mössbauer spectroscopy(MS) will be used in order to study the effects of the substitution of Ga3+ions on structural and magnetic properties of the Y3GaxFe5-xO12(x= 0.0, 0.2, 0.4, 0.8,1.0) garnets.

2 Experimental

Y3GaxFe5-xO12samples withx= 0.0, 0.2, 0.4,0.6, 0.8 and 1.0 were prepared by conventional ball milling of stoichiometric mixtures of high purity Y2O3,Fe2O3and Ga2O3starting powders, and sintering at temperatures sufficient to induce solid state reaction.The milling was carried out at rotational speed of 250 rpm for 16 h, using hardened stainless-steel bowls and milling balls, with a ball-to-powder mass ratio of 10:1.The resulting precursor mixture was pelletized then sintered at 1 300 ℃ for 2 h. For phase identification,X-ray diffraction (XRD) patterns were collected using Philips PW 1720 X-ray diffractometer with Cu-Ka radiation (λ = 1.540 5 Å). The samples were scanned over the angular range 15° < 2θ< 75° with 0.02°scanning step and speed of 1 °/min. The XRD patterns were analyzed using X’pert HighScore 2.0.1 software for phase identification, and Rietveld refinement of the crystal structure was performed using FullProf suite 2 000 software.

Raman spectra of the samples were obtained at room temperature using Thermo Fisher Scientific(DXRTM2 Smart) Raman microscope with an excitation line of 532 nm. The Raman signals were collected at 900 lines/mm grating by the charge coupled device(CCD) detector.

The grain morphology and the grain size were examined with a scanning electron microscope (SEM,Nova Nano 450) system. The magnetic measurements were performed using vibrating sample magnetometer(VSM MicroMag 3900, Princeton Measurements Cooperation), operating at a maximum applied magnetic field of 10 kOe, and Quantum Design 9TPPMS EverCool-II magnetometer.

Room temperature Mössbauer spectra of the samples were collected over 1024 channels using a standard constant acceleration spectrometer with57Co source dispersed in rhodium matrix. Fitting software based on least-squares analysis was used to fit the spectra, and the centroid of a-iron spectrum at room temperature was used as the origin of the velocity scale.

3 Results and discussion

3.1 XRD analysis

Fig.1 shows the refined XRD patterns for Y3GaxFe5-xO12samples (withx=0.0, 0.2, 0.4, 0.6,0.8, and 1.0) prepared by ball milling method and sintering at 1 300 ℃. All patterns indicated the presence of a single garnet phase with space group Ia-3d. The absence of other impurity phases indicated incorporation of Ga3+ions at Fe3+sites in the garnet structure.

Fig.1 Refined XRD patterns for garnet samples (The residual difference between the experimental and calculated data is given in the lower part of the plots)

The goodness of fit (c2value) and the lattice parameters are listed in Table 1. The lattice constant(a=12.373 4 Å) of the sample withx= 0.0 is in good agreement with previously reported values[9,12,13].Moreover, the values of lattice constantaand the unit cell volumeVare found to decrease monotonically with the increase of Ga concentration. Such decrease can be attributed to the smaller ionic radius of Ga3+ion at tetrahedral and octahedral sites (0.47 Å, 0.620 Å, respectively) compared with that of Fe3+ion(0.49 Å, 0.645 Å, respectively)[14]. This decrease is demonstrated by the upward shift of the angular position of the (420) peak with the increase of Ga substitution as shown in Fig.2. This behavior is consistent with the recently reported lattice contraction in Ga-substituted erbium iron garnets[11].

Table 1 χ2 value, lattice constant a, unit cell volume V, and density ρx for all garnet samples

Fig.2 Enlarged view of (420) peak for Y3GaxFe5-xO12 garnet samples

The bond length of Fe(a)–O and Fe(d)–O, where Fe(a)and Fe(d)represent Fe3+ions at octahedral (a) and tetrahedral (d) sites, respectively, are given in Table 2.With Ga doping, the bond length of Fe(d)– O fluctuated slightly (about 0.5% over the entire substitution range),whereas a small decrease (about 1.2%) was observed in the bond length of Fe(a)–O, in agreement with previously reported bond length contraction[11,15].

The X-ray density exhibited a monotonic increase with increasing the concentration of Ga as shown in Fig.3, which culminated to 2.34% increase atx= 1.0.This increase is attributed partially to the decrease of the cell volume, and to the increase in molecular mass. In fact, the observed 2.34% increase in the X-ray density of the sample withx= 1.0 is equal to 1.88%due to the increase in molecular mass plus 0.46% due to the decrease of the cell volume.

Table 2 Bond length Fe(a) - O and Fe(d) - O obtained from rietveld refinement for Y3GaxFe5-xO12 garnet samples

Fig.3 Density of Y3GaxFe5-xO12 garnets as a function of x

3.2 SEM analysis

Fig.4 shows representative SEM images of all Y3GaxFe5-xO12samples prepared using ball milling technique along with the grain size distribution histograms of each sample. These images were characterized using ImageJ software[16]. Generally, the figure shows that all samples have compacted grains and composed of grains with different shapes and nearly with good grain to grain connectivity.

The sample withx=0.0 consists of grain sizes of the range from about 1 to about 3.3 μm and with an average grain size about 1.75 μm, which is in good agreement with previous studies[9]. Moreover, the small irregular splinters in this sample, and generally in all samples, could have resulted from the fractured surfaces.

With increasing the concentration of Ga+3ions,not only the range of the grains became smaller but also the average grain size decreased to about 1.23 μm for the sample withx=1.0. In addition, the grain size distribution for the sample withx=1.0 is narrower and more uniform compared to other samples. These results indicated that the Ga substitution for Fe in the YIG lattice resulted in a small reduction in the grain size, whereas the average grain sized remained in the microscale range for all samples.

3.3 Raman spectroscopy

To further study the effect of Ga3+substitution on the structure of YIG, Raman spectra of the system Y3GaxFe5-xO12with (x=0.0, 0.2, 0.4, 0.6, 0.8, and 1.0)were recorded at RT in the range (100 – 700) cm-1.Raman spectroscopy is another tool that can be used to study the lattice distortion for the corroboration of the XRD results. From group theory, there are 25 (3A1g+8Eg+14T2g) Raman active modes that are expected for the cubic YIG structure. Fig.5 shows the Raman spectra for all samples which are in good agreement with previous studies[17-20].

Fig.6 shows the Raman spectrum for the sample withx=0.0, which is in good agreement with previous studies[20-23]. The peaks at 123.56, 164.86, 186.44,232.14, 436.10, and 570.64 cm-1are attributed toT2gmodes, whereas the peak at 409.60 cm-1is attributed toEgmode, and the peak at 498.85 cm-1toA1gmode.On the other hand, the peaks at 263.76, 660.39, and 671.98 cm-1, and the peak 331.67 cm-1are attributed to the combinations ofEg+T2gandEg+T1gvibrational modes, respectively. In addition, the peaks observed in the range 100 - 400 cm-1are primarily associated with the translational motion of Y3+ions in the dodecahedral lattice, while the peaks that are observed in the range(400 - 700) cm-1can be attributed to the Fe-O bonds in both the tetrahedral and octahedral lattices.

Fig.5 Raman spectra for the Y3GaxFe5-xO12 garnet samples recorded at room temperature

Fig.6 Room temperature Raman spectrum for the sample with x =0.0 (Lorentzian fit was used in order to determine the peak locations)

Raman spectra indicated that as the concentration of Ga3+ions increased, the peaks are shifted towards higher wavenumber as shown Fig.5. For comparison,the major Raman active modes of the samples withx=0.0 andx=1.0 are shown in Table 3, which are consistent with previous studies[17,23]. The behavior of Raman shifts is consistent with the observed decrease of the lattice constant, which tends to increase the force constants, leading to higher frequencies of the vibrational modes. Moreover, these shifts confirm the substitution of Fe3+by Ga3+in the YIG structure.

Table 3 Major Raman shift corresponding to Raman-active modes A1g, Eg, and T2g at room temperature for the samples with x = 0.0 and x = 1.0

3.4 Magnetic measurement

Fig.7 Magnetic hysteresis loops for Y3GaxFe5-xO12 garnet samples

Table 4 The saturation magnetization Ms for Y3GaxFe5-xO12 garnet samples

Room temperature (RT) hysteresis loops of all synthesized garnets are shown in Fig.7. The measurements revealed magnetic saturation at the highest applied field strength of 10 kOe, and negligible coercivity in all samples which are consistent with previous studies[9,13,24]. The saturation magnetization(Ms) for all samples was determined directly from the corresponding hysteresis loops, and the results are listed in Table 4.

TheMsvalue was found to decrease monotonically with the increase of Ga3+concentration (from 28.2 emu·g-1forx=0 to 4.98 emu·g-1atx=1.0) as shown in Fig.8. The saturation magnetization for the pure sample is consistent with previously reported results[9,13].

Fig.8 The variation of Ms with Ga concentration for Y3GaxFe5-xO12 garnet samples

Y3+ion has no permanent magnetic moment,and thus the net magnetization in the YIG structure originates from the superexchange interactions between Fe3+ions occupying tetrahedral (d) and octahedral(a) sites, resulting in 37.8 emu·g-1for a perfectly collinear magnetic structure (at 0 K). The strength of the superexchange interaction depends on the angle of the Fe3+(d)-O-2-Fe3+(a) linkage in which the Fe3+(d)and Fe3+(a) are coupled antiferromagnetically. The substitution of Fe3+ions by non-magnetic Ga3+ions at the (d) sites should decrease the net magnetic moment and reduce the saturation magnetization of the garnet.Complete substitution of Ga3+ions at tetrahedral (d)sites should result in zero net magnetic moment of the garnet withx= 1.0. Thus, the finite saturation magnetization of 4.98 emu·g-1for the sample withx= 1.0 is an indication that a small fraction of the Ga3+ions replaced Fe3+ions at octahedral sites. Now we analyze the magnetic data using a simple model based on collinear magnetic structure, in which the saturation magnetization is proportional to the net magnetic moment per molecule. According to the model, if the room temperature magnetization of 28.20 emu·g-1is assumed to arise from a net magnetic moment of 5μB(for one Fe3+ion) per molecule, the saturation magnetization for the sample withx= 1.0 should arise from 0.67μBper molecule (taking into consideration the different cell volume and X-ray density for this sample). This net magnetic moment suggests that the fraction of Ga3+ions occupying the octahedral sites in this sample is 0.067, which indicates that the tetrahedral site selectivity of Ga3+ions in this sample is 93.3%. This value is slightly higher than that of 90% reported by Gelleret al[25]. The site selectivity determined from analysis of our magnetic data,however, is based on other assumptions, including the stability of the collinear magnetic structure across the entire range of Ga3+substitution. However, the substitution of nonmagnetic ions for Fe3+ions leads to deterioration of the net magnetic moments of the magnetic sublattices, and a consequent reduction of the strength of the superexchange interactions, resulting in spin canting and serious deviation from collinear magnetic structure, which may be an alternative explanation of the reduction of the magnetization.Therefore, Mössbauer spectroscopy was employed to provide an insight into the site selectivity of the Ga3+ions in the synthesized garnets and shed light on the true mechanism responsible for the reduction of the saturation magnetization with the increase of Ga content.

3.5 Mössbauer spectroscopy

Room temperature Mössbauer spectra of Y3GaxFe5-xO12garnets are shown in Fig.9. The fitted parameters obtained from the best fit of the Mössbauer spectra are shown in Table 5. Mössbauer spectrum for the un-substituted sample (x= 0.0) was best fitted with three sextets, corresponding to the Fe3+cations in three different environments. The components with higher hyperfine fields and center shift (CS) in the range about 0.3 – 0.4 mm·sec-1are usually assigned to six-coordinated Fe3+ions at octahedral sites, whereas the lower hyperfine field components with CS about 0.2 mm·sec-1are assigned to four-coordinated Fe3+at tetrahedral sites[26-28]. Consequently, the first two components with hyperfine fieldsBhf= 497 and 485 kOe, and center shifts CS = 0.37 and 0.36 mm·sec-1,respectively, are assigned to Fe3+ions at octahedral sites, whereas the component with lower hyperfine field of 401 kOe and CS = 0.15 mm·sec-1is assigned to Fe3+ions at tetrahedral sites. The relative sub-spectral intensities of octahedral and tetrahedral components indicated that the ratio of Fe3+ions at these sites is 37:63, which is consistent with the theoretical ratio of 40:60 for stoichiometric iron garnets.

Fig.9 Mössbauer spectra for the Y3GaxFe5-xO12 garnet samples (filled circles) together with the theoretical fit (continuous line)

Table 5 The hyperfine parameters and the percentage relative intensities (I) obtained at room temperature for Y3GaxFe5-xO12 garnet samples

The spectra of the samples withx= 0.2 - 0.6 were also fitted with three components assigned to Fe3+ions at tetrahedral and octahedral sites as before. Moreover, a weak magnetic component,with intensity less than 1%, attributed to anα-Fe2O3phase was observed for the sample withx= 0.6. This phase was not observed in the XRD spectra due to the fact that its fraction in the sample was below the detectability limit of the diffraction technique. The spectral intensities of the octahedral and tetrahedral components indicated random distribution of the Ga3+ions at tetrahedral and octahedral sites, with preference for tetrahedral sites as indicated by the fraction of Ga3+ions (ft) at tetrahedral sites (Fig.10).The monotonic decrease of the hyperfine fields at all sites is attributed to the decrease of the average net magnetic moment of the magnetic sublattices induced by the substitution of nonmagnetic Ga3+ions for Fe3+ions[29], thus confirming the random substitution of Ga3+ions at both tetrahedral and octahedral sites. The increase of the width of the sub-spectral components with the increase of Ga3+concentration is due to the random statistical distribution of the Ga3+ions at all sites, which introduces new slightly different chemical environments around the Fe3+ions, leading to slightly different hyperfine parameters, and a consequent line broadening[30-32].

Fig.10 The fraction ft of Ga3+ ions in the tetrahedral site of Y3Fe5-x GaxO12 garnets as a function of x

The spectra of the samples withx= 0.8 and 1.0 were each fitted with five components, three of which correspond to Fe3+ions at octahedral site, and two correspond to Fe3+ions at tetrahedral sites, in addition to the same weak magnetic component that was assigned to the a-Fe2O3phase. The evolution of additional components arises from the increase of Ga3+content at tetrahedral and octahedral sites, which leads to the development of new Fe3+sites with significantly different statistical averages of the number of Ga3+ions,resulting in the development of new components with significantly different hyperfine parameters. The width of the spectral components continues to increase as a consequence of the statistical distribution of Ga3+ions at all sites, and the hyperfine field continues to decrease as a consequence of the reduction of the net magnetic moments of the magnetic sublattices. Additionally,the relative spectral intensities of the octahedral and tetrahedral components indicated that the fraction of Ga3+ions at tetrahedral sites increased with the increase of Ga3+concentration (x) as demonstrated by Fig.10.However, the highest fraction of Ga3+of 0.68 atx=0.1 is significantly lower than the fraction determined from analysis of the magnetic data assuming collinear magnetic structure in the garnets. This result clearly indicates that the reduction of the net magnetic moment of the garnets as a consequence of Ga3+ions preference for tetrahedral sites cannot explain the observed large reduction of the saturation magnetization, and spin canting plays an important role in explaining this behavior.

4 Conclusions

Polycrystalline samples of Y3Fe5-xGaxO12garnets with space group Ia-3d were successfully synthesized by solid state reaction and sintering at 1 300 ℃.Partial substitution of Fe3+ions by Ga3+ions resulted in lattice contraction, and increase of molecular mass,leading to a monotonic increase of the X-ray density with increasingx. These results indicated successful incorporation of the Ga3+ions in the garnet lattice.The observed shift in some Raman active modes also confirmed the successful substitution of magnetic Fe3+by non-magnetic Ga3+ions in the YIG structure.Room temperature magnetization hysteresis loops revealed soft ferromagnetic behavior for all samples with decreasing behavior ofMsasxincreased.This behavior indicated preference of Ga3+ions for tetrahedral sites, whereas the non-vanishing of the saturation magnetization for the sample withx= 1.0 is an indication of partial substitution of Ga3+ions at octahedral sites. Analysis of the magnetic data with the assumption of collinear magnetic structure, and of Mössbauer spectra of the samples, clearly indicated that spin canted non-collinear magnetic structure plays an important role in the observed reduction of the saturation magnetization of the garnets.