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Abstract
Thin films of the highly bismuth-substituted neodymium iron garnet Nd0.5Bi2.5Fe5-yGayO12 (Bi2.5Ga:NIG) were prepared on Nd2BiFe4GaO12 (Bi1Ga1:NIG) buffer layers on glass substrates. These thin films exhibited a large magneto-optical (MO) effect, demonstrating that the magnetic anisotropy of this material can be varied by substituting gallium for iron. As gallium content was increased from 0 to 1, the Bi2.5Ga:NIG thin films showed Faraday rotation angles varying from 21.9 to 14.9°/μm, and effective magnetic anisotropy energy values varying from −3.3 × 104 to 1.3 × 104 erg/cm3. The variation of the effective magnetic anisotropy is discussed in terms of the shape anisotropy depending on the saturation magnetization and the induced magnetic anisotropy caused by the different thermal expansion coefficients for the thin films and the glass substrate. The significant decrease in saturation magnetization upon gallium substitution is believed to result from 85% of the gallium being preferentially substituted at tetrahedral sites in the garnet, which is consistent with changes observed in the Faraday rotation spectra of the thin films.
© 2017 Optical Society of America
1. Introduction
Magneto-optical (MO) devices such as indicators [1–3], spatial light modulators for three dimensional holographic displays [4, 5] and waveguide-type isolators [6, 7] have attracted much attention. Bismuth-substituted rare-earth iron garnets having the general formula R3-xBixFe5O12 show particular promise with regard to these applications, since they exhibit the largest MO figure of merit in the visible light region [8–14]. This MO effect is enhanced by increasing the extent to which bismuth is substituted for the rare-earth elements at dodecahedral sites, as a result of the significant spin-orbit interaction between the 6p orbitals of bismuth and the 2p orbitals of oxygen [15–19]. Various thin film preparation techniques such as a pulsed laser deposition method [12, 13], a thermal decomposition method [20–23], an ion beam sputtering [10, 11], a radio frequency (RF) magnetron sputtering [24], etc., were studied to obtain highly bismuth-substituted iron garnet thin films, while liquid phase epitaxy (LPE) method [8] was already established for single crystalline bismuth-substituted iron garnet with bismuth content of 1. As a result, the fully bismuth-substituted iron garnet Bi3Fe5O12 (BIG) exhibiting an extremely large Faraday rotation of 24 – 30°/μm at approximately 520 nm were obtained [11, 25–31]. However, considering the significant light absorption of BIG thin films and the difficulty in controlling the magnetic anisotropy of these materials [26, 27], it is preferable to employ bismuth content of less than 3. Our own group previously reported that Y0.5Bi2.5Fe5O12 (Bi2.5:YIG) thin films prepared on Gd3Ga5O12 (GGG) single crystal substrates by MOD method [31], has better crystallinity and a larger MO figure of merit than BIG thin films. Moreover, we found that bismuth and gallium-substituted neodymium iron garnets, Nd0.5Bi2.5Fe5-yGayO12 (Bi2.5Ga:NIG), prepared on GGG substrates had a larger figure of merit than that for Bi2.5:YIG, and that the magnetic anisotropy of these new materials could be readily controlled [32].
Moreover, we have developed a technique to grow polycrystalline highly bismuth-substituted iron garnet thin films on glass substrate, in which they can be grown on a buffer layer, bismuth-substituted iron garnet films with bismuth content of 1 [33–35]. We consider that this technique could be applied not only for glass substrates, but also other materials such as silicon, if bismuth-substituted iron garnet films with bismuth content 1 could be grown as reported in Ref. 31. When considering MO applications that require low production costs and large dimensions, the use of glass substrates becomes important. However, precise control of the magnetic anisotropy of Bi2.5Ga:NIG on glass substrates has not yet been studied.
In the present work, we investigated the magnetic anisotropy of Bi2.5Ga:NIG thin films with increasing gallium content from 0 to 1 prepared on Bi1Ga1:NIG buffer layers on glass substrates by MOD method. To control the magnetic anisotropy of these thin films, the saturation magnetization was changed by varying the amount of gallium substituted for iron ions. The magnetic anisotropy and MO properties of these Bi2.5Ga:NIG thin films are discussed herein.
2. Experimental
Bi2.5Ga:NIG thin films and Bi1Ga1:NIG buffer layers were prepared using the MOD method. The MOD solutions employed in this study were made with Nd, Bi, Fe and Ga carboxylates, at a total carboxylate concentration of 4% by mass. MOD solutions having Nd:Bi:Fe:Ga ratios of 2:1:4:1, 0.5:2.5:5:0 and 0.5:2.5:4:1 (Kojundo Chemical Laboratory Co., Ltd.) were used to produce the Bi1:NIGG buffer layer and the Bi2.5Ga:NIG layers. The solutions were mixed to obtain the appropriate compositions for gallium contents of 0.25, 0.5 and 0.75.
The above MOD solutions were spin-coated onto glass substrates (Eagle XG, Corning Inc.) at 3000 rpm for 60 s, followed by drying on a hot-plate at 100 °C for 10 min and pre-annealing to decompose the organic materials at 450 °C for 10 min. These procedures were repeated three times for each substrate to obtain a Bi1Ga1:NIG buffer layer thickness of approximately 90 nm, after which each sample was annealed in a furnace at 700 °C for 3 h to promote crystallization. The spin-coating, drying and pre-annealing processes were then repeated five times for each buffer layer to obtain a Bi2.5Ga:NIG thin film with a thickness of approximately 150 nm, after which the samples were annealed at 700 °C for 3 h. The layer thicknesses of these specimens were measured by ellipsometry (MM-16, Horiba, Ltd.).
X-ray diffraction (XRD) measurements, comprising out-of-plane 2θ/ω scans and in-plane 2 θχ/φ scans, were carried out using a high-resolution XRD instrument (SmartLab, Rigaku Co., Ltd.) with Cu-Kα radiation. Faraday spectra and Faraday hysteresis curves were acquired with an MO spectrometer (BH-M800, Neoark Co., Ltd.). Magnetization curves in the out-of-plane and in-plane directions were acquired using a vibrating sample magnetometer (VSM-5-19, Toei Industry Co., Ltd.) and magnetic anisotropy energies were measured at 2 kOe with a torque magnetometer (TRT-2, Toei Kogyo Co., Ltd.). Transmission spectra were measured with a spectrometer (V-570, JASCO Co., Ltd.). The surface of the thin films was observed with a field emission scanning electron microscopy (FE-SEM) (SU8230, Hitachi High-Tech. Co., Ltd.) and the compositions of the thin films were analyzed using an energy dispersive X-ray spectrometer (EDX) (X-Max SDD, Oxford instruments Co., Ltd.).
3. Results and discussion
Figure 1 and 2 show the XRD patterns obtained from Bi2.5Ga:NIG thin films with increasing gallium content from 0 to 1 in the out-of-plane and in-plane directions, respectively. For each sample, the diffraction pattern indicates a polycrystalline garnet structure. The in-plane data also demonstrate the presence of an unidentified secondary phase for samples with gallium content of 0.25 and 0.5, as indicated by the asterisks in Fig. 2, while no additional phases are evident in the out-of-plane XRD patterns. This result indicates that small amounts of a secondary phase segregated on the surface can be detected via the in-plane measurements, which are more sensitive to the surface structure.
The in-plane and out-of-plane lattice parameters of Bi2.5Ga:NIG thin films with increasing gallium content from 0 to 1 were determined using the Nelson-Riley function [36] and are plotted against the gallium content in Fig. 3. The calculated equivalent lattice parameter a, assuming a cubic unit cell is also plotted. It can be seen that all the lattice parameters decreased slightly with increasing gallium content, and that the in-plane parameter was always larger than the out-of-plane parameter. This dependence on the gallium content suggests that gallium ions, having a smaller ionic radius (62 pm) [37], substituted for iron ions (67 pm) in the lattice [38]. In addition, for gallium content of 0, the a value of 1.261 nm is consistent with the value of 1.262 nm calculated from Vegard’s law using literature data for BIG [12] and Nd3Fe5O12 (NIG) [39].
The lattice distortion, (aip - a)/a, was determined to be 0.071, 0.069, 0.058, 0.058 and 0.048% for gallium contents of 0, 0.25, 0.5, 0.75 and 1.0, respectively, as shown in Fig. 4. Here, aip is the in-plane lattice parameter. We attribute this distortion to the different thermal expansion coefficients for Bi2.5Ga:NIG and the glass substrate. Although the thermal expansion coefficient for Bi2.5Ga:NIG is not known, it is likely similar to that reported for BIG thin films (12.9 × 10−6 K−1) [40]. Comparing this value to that for the glass substrate, 3.17 × 10−6 K−1, we would expect that the thin films would experience tensile stress as the temperature decreased from the annealing temperature of 700 °C to ambient.
The Faraday rotation spectra of Bi2.5Ga:NIG thin films with increasing gallium content from 0 to 1 are shown in Fig. 5. All spectra exhibit characteristics of a garnet structure, having a peak around 520 nm [30–35, 41, 42]. As gallium content increases from 0 to 1, the Faraday rotation angle decreases and the peak becomes less sharp. This change is caused by the imbalance in contributions from the tetrahedral and octahedral sites in the garnet, as discussed in detail below.
Faraday hysteresis loops for Bi2.5Ga:NIG thin films with increasing gallium content from 0 to 1 obtained at the wavelengths corresponding to the maximum Faraday rotation around 520 nm are shown in Fig. 6. As the gallium content increases from 0 to 1, the hysteresis loops are seen to become more square while the rotation angle slightly decreases.
The MO figures of merit of Bi2.5Ga:NIG thin films with gallium contents of 0, 0.5 and 1 are shown in Fig. 7. We used MO figure of merit defined with an equation Q = 2|θF|/α = 2|θF|t/ln(1/T) [28, 43, 44], where θF (deg/μm) is the measured Faraday rotation angle, α = t/ln(1/T) is the absorption coefficient, t is the thickness of thin films, and T is the transmittance. In Fig. 7, the maximum value of MO figure of merit was larger than 8.5 for the gallium content of 0. This value was higher than those reported for Y0.5Bi2.5Fe5O12 (Bi2.5:YIG) thin films [30], Bi2.5Ga:NIG thin films on GGG [32] and BIG films [28], and was the highest values for a single layer, as far as we know.
Figure 8 shows SEM images for Bi2.5Ga:NIG thin films with gallium contents of 0, 0.5 and 1. In the images, no cracks were observed, although precipitates of impurity phases with diameters of approximately 0.1 μm found, while severe cracks were observed on the surface of BIG films prepared reported in Ref. 29.
The composition ratios for Bi2.5Ga:NIG thin films measured by EDX are summarized in Table 1. We should note that these values included signals from both Bi2.5Ga:NIG layers and Bi1Ga1:NIG buffer layers, since the EDX measurement detected X-ray signals from the depth of larger than 1 μm. Figure 9 shows measured gallium content for iron plotted as a function of the nominal compositions in the MOD solutions, where averaged nominal compositions of Bi2.5Ga:NIG layers and Bi1Ga1:NIG buffer layers were shown as a green solid line. We consider that measured gallium contents agree with those in MOD solutions, and the difference could be within the error.
Table 1. The composition ratios for Bi2.5Ga:NIG thin films
The magnetization curves obtained from Bi2.5Ga:NIG thin films with increasing gallium content from 0 to 1 are presented in Fig. 10. The red and blue lines represent hysteresis loops acquired in the out-of-plane and in-plane directions, respectively. These data show that the saturation magnetization, Ms, for the samples decrease from 137.2 to 46.3 emu/cm3 as the gallium content is increased. In addition, the shapes of the hysteresis loops acquired in the out-of-plane direction match those of the out-of-plane Faraday hysteresis loops shown in Fig. 6.
The saturation magnetization and maximum Faraday rotation angle in the vicinity of 520 nm are plotted as functions of the gallium content in Fig. 11. The saturation magnetization decreases by 67% as gallium content increases from 0 to 1, while the Faraday rotation angle decreases by 35%. We believe that this can be attributed to an imbalance in the site occupation of gallium ions. The saturation magnetization for Bi2.5Ga:NIG results from the magnetic moments of neodymium ions as well as iron ions in tetrahedral and octahedral sites. If the gallium ions preferentially substitute for iron ions at tetrahedral sites, the saturation magnetization for the Bi2.5Ga:NIG would rapidly decrease, since the direction of the magnetic moments at the tetrahedral sites is opposite that at the octahedral sites.
The magnetic moment of Bi2.5Ga:NIG with gallium content of 0 (Bi2.5:NIG) at room temperature, nB (Bi2.5:NIG), was estimated using the equation
Based on the magnetic moments of NIG and Y3Fe5O12 (YIG) (i.e., nB (NIG) and nB (YIG)) [45–47], we used [nB (NIG) − nB (YIG)] and nB (YIG) to find nB (Nd3+) and nB (Fe3+), respectively. Consequently, we determined a magnetic moment for Bi2.5:NIG at room temperature of 3.79 μB, which is consistent with the experimental value of 3.71 μB ( = 137.2 emu/cm3) in Fig. 10. To express the degree of imbalance in gallium substitution, we can write the chemical formula for Bi2.5Ga:NIG as {Nd3-xBix}[Fe2-zaGaza] × (Fe3-zdGazd)O12. Here { }, [ ] and () indicate dodecahedral, octahedral and tetrahedral sites, respectively, and za and zd are the occupancy ratios of gallium ions substituting for iron ions at octahedral and tetrahedral sites, respectively.
In the case of Bi2.5Ga:NIG, the magnetic moment of the iron ions should be nB (Fe3+) × [(3 − zd) − (2 − za)]. Considering that for Bi2.5Ga:NIG at gallium content of 1 (or za + zd = 1), the decrease in the saturation magnetization was 67%, zd was determined to be 85%. This result agrees with the value of approximately 90% reported for liquid phase epitaxial-grown garnet thin films [48–50].
The variations in the Faraday rotation spectral shapes that are evident in Fig. 5 also agree with an imbalance in gallium substitution. The spectral shape in the vicinity of 520 nm originates from optical transitions from O 2p to Fe 3d orbitals in the tetrahedral and octahedral sites, with the deconvoluted tetrahedral spectra being narrower than the octahedral spectra. The results shown in Fig. 5 demonstrate a broadening of the spectra as gallium content is increased, which is consistent with a diminishing contribution of tetrahedral sites to the Faraday rotation.
Finally, we consider the magnetic anisotropy of the Bi2.5Ga:NIG thin films. The torque curves obtained from thin films of Bi2.5Ga:NIG with increasing gallium content from 0 to 1 at an applied magnetic field of 2 kOe are presented in Fig. 12. All samples exhibit two-fold symmetry in these plots, indicating uniaxial magnetic anisotropy, although it should be noted that small background signals indicating four-fold symmetry could not be subtracted completely. The effective magnetic anisotropy constants, Keff, and the uniaxial magnetic anisotropy constants, Ku, are both plotted as functions of gallium content in Fig. 13. Here Ku was calculated as Ku = Keff + 2πMs2. It is apparent that Keff changed from negative to positive as the gallium content increased, in good agreement with the change in the hysteresis curve shapes in Fig. 10. In contrast, the Ku value for each specimen is positive and decreases from 8.5 × 104 to 2.6 × 104 erg/cm3 as gallium content increases from 0 to 1. The positive Ku values are believed to result from the tensile stress discussed above, and the decrease is attributed to a reduced magneto-strictive effect as the gallium content is increased [51].
4. Conclusions
Bi2.5Ga:NIG thin films with increasing gallium content from 0 to 1 prepared on Bi1Ga1:NIG buffer layers on glass substrates by the MOD method exhibited large Faraday rotation angles of 21.9 – 14.9°/μm. The uniaxial magnetization anisotropy constant for all samples was positive, which was attributed to the induced magnetic anisotropy resulting from lattice distortion. In addition, the effective magnetic anisotropy constant changed from −3.3 × 104 to 1.3 × 104 erg/cm3 as gallium content increased from 0 to 1. The observed variations in the saturation magnetization and the Faraday rotation angle were explained by assuming that 85% of the gallium was preferentially substituted at tetrahedral sites in the garnet. This theory is consistent with changes in the Faraday rotation spectra shape with gallium content.
Funding
Nanotechnology Platform Program (Molecule and Material Synthesis) of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan; Nagoya University.
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