Effect of Er3+ ions on structure, surface morphology, optical and magnetic properties of Tb-YIG nanocrystalline films

Materials Chemistry and Physics 208 (2018) 1e7

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Effect of Er3þ ions on structure, surface morphology, optical and magnetic properties of Tb-YIG nanocrystalline films Suleiman M. Elhamali, N.B. Ibrahim*, S. Radiman School of Applied Physics, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 The pure phase of Y2.8-xTb0.2 Erx Fe5 O12 films was grown by a sol-gel method.  The Tb3þ ions effect could be compensated by adding Er3þ ions.  Er ions reduces the saturation magnetization and increases the coercivity.  An improvement in optical transparency with an increment of Er content was observed.

a r t i c l e i n f o

a b s t r a c t

Article history:

Nanocrystalline Y2.8-xTb0.2 Erx Fe5 O12, (x ¼ 0.0e2.6) films are successfully prepared under an annealed temperature of 900  C in ambient oxygen using a sol-gel method. Structural investigations, using an Xray diffractometer, confirmed that all films have a crystallographic cubic phase of pure garnet. An increment of lattice constant was observed at lower concentrations (x  0.8), which then linearly decreased with higher concentrations; thus indicating a complete incorporation of Erþ3 ions into the Tb: YIG structure. The surface morphology, obtained using field emission scanning electron microscopy and atomic force microscopy, showed a grain size nanostructure formation, a good adhesion of the film to the quartz substrate, and a smooth surface roughness. All samples showed high transparency in visible and near-infrared regions, with absorption edges below 500 nm. Vibrating sample magnetometer results at 29  C revealed a formation of a soft ferrimagnetic material. Adding Erþ3 ions reduced the saturation magnetization, Ms but, remarkably, increased the magnetic coercivity Hc. Based on a low absorption coefficient and saturation magnetization, the obtained films are a promising material for magnetooptical devices, such as optical isolators in visible and near-infrared regions. © 2018 Elsevier B.V. All rights reserved.

Keywords: Sol-gel preparation Surface morphology Magnetic materials

1. Introduction

* Corresponding author. E-mail address: [email protected] (N.B. Ibrahim). https://doi.org/10.1016/j.matchemphys.2018.01.014 0254-0584/© 2018 Elsevier B.V. All rights reserved.

Ferrimagnetic garnets have suitable magnetic and electrical properties that are required for microwave communications and magneto-optical devices [1e3]. A typical magnetic garnet, fY3 g½Fe2 ðFe3 ÞO12, belongs to the space group Ia3d (O10 h ), with eight

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formula units in a body-centred cubic unit cell [4]. In this formula, all metal cations are in special three crystallographic sites; 24 Y3þ in c-site (2.40 Å), 16 Fe3þ in a-site (2.01 Å) and 24 Fe3þ in d-site 2 3 (1.87 Å). This loose structure can accommodate an enormous variety of trivalent cations. The Y3þ ion is partially or completely replaceable by most rare earth elements (REs). This replacement allows the magnetic and magneto-optical properties of garnets to be enhanced by changing their composition. The magnetic moments in a YIG structure come from unpaired magnetic electrons in an incomplete d shell in the Fe3þ ions that occupy both a and dsites. The interaction between individual magnetic moments is strongly antiferromagnetic through a super-exchange interaction [5]. Heavy REs (R ¼ Eu, Gd, Tb …, Lu) have a large magnetic moment that comes from unpaired electrons in the 4f shell. Unlike transition metals (TMs), REs also has an orbital angular momentum L (except for Gadolinium) in addition to the large spin momentum S. The contributions of L and S, on the total angular momentum J, where J¼ (L þ S), are different from one atomic structure to another. Thus, the total magnetic moment, as well as several intrinsic magnetic properties, such as a magnetization, will be affected [6]. However, the lattice constant of a unit cell of garnet is found to be influenced by preparation method, stress and strain, and the film-substrate lattice expansion [7]. This lattice expansion is mainly caused by the large ionic radii of some REs that occupy a c-site in the garnet structure. In fact, it is hard to substitute some REs into the YIG structure at high concentrations, because their ionic radii are bigger than the ionic radius of yttrium (Y), such as Ce, Tb, and Pr for example. Therefore, such ions doped onto the YIG structure will increase lattice distortion in a crystal, compared to pure YIG. Ibrahim [8], reported that the lattice parameters of Ce: YIG films, prepared by a sol-gel method, increase with the increment of Ce content up to (x ¼ 0.25). They then found that, for x  0.3, the solubility limit of Ce in YIG was reached due to the large ionic radius of Ce compared to the Y ion. In fact, the replacement of none magnetic Yþ3 by heavy magnetic REs, such as Tbþ3 or Erþ3 ions, had received considerable attention due to their variety of magnetic properties. In this regard, the number of reports on the form of thin films is limited [9e12]. The terbium ion exhibits a larger Faraday effect that comes from a high value of its Verdet constant [13]. It also shows a strong paramagnetic effect, due to the transition between 4f 8  4f 7 5d[14]. This effect becomes stronger when Tb ion is doped onto the YIG structure at low concentrations. Geller [15] reported that some rare earth elements, which have large ionic radii, could be substituted for Y3þ ion if it is combined with proper quantities of the smaller ions. However, the lattice distortion in a crystal caused by many Tb3þ ions could be relaxed by adding other R3þ ions that have smaller ionic radii than Y3þ, such as Er, Tm, and Lu; without degradation of other properties, such as Faraday rotation [16]. The present study reports on the effect of low and high concentrations of Er ions on the nanostructure, surface morphology, optical and magnetic properties of sol-gel Tb doped YIG films. To the best of our knowledge, the preparation a series of Y2.8-xTb0.2 Erx Fe5 O12 (0  x  2.6) nanofilms, annealed in an oxygen ambient environment, has not been previously reported. The presence of an oxygen atmosphere could not reduce Fe3þ to Fe2þ ion, or change the oxygen content of YIG enough during the annealing process [17]. Kang [18] prepared amorphous YIG films that were crystallized by ex situ post-annealing (600e900  C) in oxygen and air atmospheres. They proved that an oxygen annealing atmosphere was an effective environment to yield high-quality films with narrower ferromagnetic resonance FMR, DH values and smoother surfaces. In addition, it was reported that the presence of oxygen during the annealing process of Bi0.85 Pr0.15Fe0.9 Co0.1 O3

(BPFCO) thin film suppressed the double hysteresis loop phenomenon, which was associated with the oxygen vacancy [19]. However, Erþ3 ions were chosen in this work due to their small ionic radii (1.00 Å) compared to Y (1.015 Å) [20], a high Verdet constant at visible wavelength [12], and a high magnetic moment (9.72 mB) [21]. The objectives of this work were to prepare sol-gel Y2.8-xTb0.2 Erx Fe5 O12 nanocrystalline films and study the effect caused by different Er ion concentrations; ranging from x ¼ 0 to 2.6 on the physical properties of these films. A Tb content of x ¼ 0.2 was chosen because it was found that a Tb content of less than 0.8 prevented Y ions from occupying the octahedral sites; which may give rise to distortion of lattice parameters [9,22]. The performance of the magneto-optical materials (MO) is evaluated by the figure of merit: F1 ¼ (qf/Ms)2 and F2 ¼ (qf/a), where qf is the Faraday rotation angle, Ms is the saturation magnetization and a is the optical absorption coefficient of material, respectively [23,24]. Decreasing the material absorption and the Ms value, therefore, is highly required for better performance of ferrite materials in the MO applications. This study reports low absorption coefficient besides a low Ms value of the Y2.8-x Tb0.2 Erx Fe5 O12 nanofilms. The findings should make an important contribution to developing a more compact optical isolator and circulator in the visible and NIR regions.

2. Experimental procedure A series of Y2.8-xTb0.2 Erx Fe5 O12 samples, with atomic weight ratios of x ¼ 0, 0.4, 0.8, 1.4, 1.8, 2.2, and 2.6, respectively, were prepared by a sol-gel method using high purity raw materials (99.9%) from Sigma Aldrich company-USA. Stoichiometric mixtures of Yttrium (III) Y (NO3)3.6H2O and Iron (III) Fe (NO3)3.9H2O were dissolved in 2 mL of 2-methaoxythanol, then stirred by a magnetic stirrer for 15 h at room temperature (~29  C). The Y-Fe solution was refluxed at 80 ± 2  C for 3hr. The terbium (III), Tb (NO3)3.5H2O and erbium (III), Er (NO3)3.5H2O were dissolved separately in 2 mL of acetic acid, then stirred for 3hr before being added gradually to the Y-Fe solution. Then, the mixture solutions were refluxed at 80 ± 2  C for 3hr. The resulting solution was stirred for two days to obtain a homogenous gel. It was then filtered using a 0.45 mm syringe filter to get rid of any unwanted particles before the deposition process began. The clean quartz substrate was heated to 75 ± 2  C for 1 min to enhance the adhesion of the films onto the substrate. In order to achieve well-coated nanofilms on quartz substrates, 20 mL of the gel was put onto the quartz glass followed by spin coating at 500 ± 3 rpm for 15 s followed by 3500 ± 5 rpm for 30 s. The resulting films were dried at 70  C for 25 min in an oven to get rid of all organic solvents. All samples were annealed at 900  C for 2hr in a pure oxygen ambient environment at 100 kPa pressure with a gas flow rate of 30 sccm. The samples were annealed in an oxygen atmosphere to avoid oxygen deficiency during the crystallization process of the films. Crystallographic properties were analysed by X-ray diffractometer (XRD) using CuKa radiation (l ¼ 0.15406 nm) in the 2q range 20e60 . Field emission scanning electron microscopy (FE-SEM; ZEISS Supra 55VP) was used for surface morphology and to measure the film's thickness. Surface roughness measurements were carried out using an Atomic Force Microscope (AFM). Energy dispersive X-ray spectroscopy (EDX), coupled with the FE-SEM, was used for the elemental analysis. A UVeVis spectrophotometer (Perkins Elmer- Lambda 950) was used to measure the optical properties in a wavelength range of 300e900 nm. A vibrating sample magnetometer (VSM), with a maximum magnetic filled with 5000 Oe, was used for in-plane magnetic measurements at room temperature.

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3

3. Result and discussion 12.44

x=2.6

Intensity (a.u.)

(444) (640) (640)

(611)

(521)

(420)

(422)

x=2.6

(321) (400)

x=2.2 x=1.8 x=1.4 x=0.8 x=0.4

Intensity (a.u.)

Fig. 1 shows the XRD spectra of all samples with Er concentration x from 0.0 to 2.6, which were annealed in an oxygen atmosphere at 900  C for 2 h. The obtained peaks were compared according to the reference spectrum of pure YIG card number: (01073-1377) using EVA software. From these diffractions peaks, one can observe that all samples had a pure cubic garnet structure without any secondary phases. Adding Er3þ ions into Y2.8 Tb0.2 Fe5 O12 led to a slight shift to higher diffraction angles of the main peak from 2q ¼ 32.37 to 2q ¼ 32.45 at (x ¼ 2.6). This shift, as illustrated in the upper-right inset of Fig. 2, indicated that Erþ3 ions were introduced into the YIG structure. The average crystallite size was calculated according to Scherrer's equation D ¼ kl/b cos q, where k is the shape factor (0.89), l ¼ 0.15406 nm, b is full-width half maxima (FWHM) of the most intense diffraction peak (420) and q is the diffraction angle. Calculations of average crystallite size were in the range 24e32 nm and changed slightly with increments of x concentration. Lattice constant (a) was calculated according to Bragg's equation combined with the d-interplanar distance expression of the cubic crystal system using the equation a ¼ (l2b/ 4sin2q)1/2, where b ¼ (h2þk2þl2), (hkl) are the Miller indices, l is the wavelength of the employed X-ray and q is the diffraction angle [25]. Average lattice constant values were corrected by Bradley & Jay's extrapolation function, which was used to minimize the effect of systematic errors [26,27]. The variations of lattice constant (a), as a function of Er concentration (x), are demonstrated in Fig. 2. Increases in lattice constant, from 12.389 to 12.430 Å, were observed at lower concentrations (x  0.8). This expansion was mainly attributed to the differences in ionic radii of Y3þ (1.019 Å), Er3þ (1.004 Å) and Tb3þ (1.040 Å) ions [20] that occupied the c-sites; as well as Er3þ being partly substituted for Y3þ ions. Hence, at low concentrations, the Er ions could be pushed by Tb and Y towards the occupied a and d-sites. Ftema and Ibrahim [9]. observed that the lattice constant of Tb0.8Y2.2AlyFe5yO12 films initially increased with the increment of Al3þ ions; in spite of the smaller ionic radii of Al3þ(0.54A) compared to the Fe3þ(0.65A) ions. However, the lattice constants of samples with higher concentrations (x > 0.8) were found to linearly decrease; thus indicating that Er3þ ions were completely substituted with Y3þ ions, where Er3þ is smaller in size than Y3þ. The lowest value (12.358 Å) was obtained at x ¼ 2.6, which is closest to the standard value of lattice constant 12.377 Å of pure YIG bulk [28].

Lattice constant (Å)

3.1. Structural analysis 12.42 12.40

x=2.2 x=1.8 x=1.4 x=0.8 x=0.4 x=0.0 30

31

32

33

34

35

2 degrees.)

12.38 12.36 12.34

0.0

0.4

0.8

1.2

1.6

2.0

2.4

2.8

Er content (x) Fig. 2. The variation of lattice constant with Er content (x). The inset shows the slight shift to higher diffraction angles of the main peak (420).

3.2. Surface morphology characterization To obtain an insight into the structure of the samples, a series of FE-SEM images at X100 000 magnification was taken. Fig. 3(aef) show the top views of the FE-SEM images of the Y2.8-xTb0.2 Erx Fe5 O12 films for different x values. The small white grains on the surface belong to the element Pt. Platinum (Pt) was coated onto the sample's surfaces to overcome electrostatic charging during characterization. It was observed from the images that all films formed a nanostructure grain size with a single contrast. It can be seen that the grains are too small to be observed using FE-SEM. In all samples, the grains are aggregated with void formation due to their high surface energies during preparation and annealing processes. Because the film was deposited onto the quartz substrate, the crack on the film's surface was expected (as shown in Fig. 3cef). This is attributed to the stress caused by the differences in thermal expansion coefficient between the YIG films and the quartz material [29]. Film thicknesses were measured from the cross-section backscattered FE-SEM microstructure (as shown in Fig. 4). It changed unsystematically with Er content (x). This fluctuation can be explained by the agglomeration and annealing temperature during film crystallization. The thickness values of the obtained films are in the range from 280 to 380 nm. However, it can be seen that the film showed a good adhesion on the quartz substrate. Three dimensional AFM images, for a scan area of 2 mm  2 mm, are shown in Fig. 5. The average surface roughness (Sa) was calculated to be 1.5, 2.9, 0.9, 1.7, 1.1, 1.8 and 1.2 nm for x ¼ 0 to 2.6, respectively. The film's surface is considered to be smooth; although some ridges and valleys were formed on it. This was due to the mismatched thermal expansion effects of quartz and film, as well as the agglomeration of small grain size. The EDX analysis of the sample with x ¼ 1.8 and 2.2, which are illustrated in Fig. 6, confirms the presence of Er, Tb, Y, Fe and O elements. This result reveals that both Er and Tb ions were incorporated into the YIG structure; as confirmed by the XRD analysis. However, the Si peak in the spectrum belongs to the quartz substrate.

x=0.0

3.3. Optical properties

20

30

40

50

60

2 degrees) Fig. 1. XRD patterns of Y2.8-x Tb0.2 Erx Fe5 O12 films, (x ¼ 0 to 2.6).

Optical characterization of nanocrystalline films was studied by UV/VIS Spectrophotometer in a wavelength range of 200e900 nm. Fig. 7 shows the optical transmittance spectra of the obtained films at different Er contents (x). As can be seen, all samples show high

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Fig. 3. (aef). FE-SEM micrographs of Y2.8x Tb0.2 Erx Fe5 O12 films: a (x ¼ 0.0), b (x ¼ 0.4), c (x ¼ 0.8), d (x ¼ 1.4), e (x ¼ 1.8) and f (x ¼ 2.2).

Fig. 4. The typical cross-sectional image of Y2.8x Tb0.2 Erx Fe5 O12 films: (a) x ¼ 1.8, (b) x ¼ 2.2.

transparency in the visible and near-infrared regions with absorption edges below 500 nm. The optical transmittance changed unsystematically when Er3þ was doped into the Y2.8-x Tb0.2 Fe5 O12 film. The highest value (above 94%) was obtained at x ¼ 1.8 in the near infrared region, which is higher than the reported value for Aldoped Tb: YIG film [9]. This improvement in optical transparency could be attributed to a reduction of light scattering from a smaller grain size. However, the film with x ¼ 2.2 showed a slight decrease in transparency; which was due to a large film thickness (380 nm) and surface reflection losses [30]. As the thickness increases, the more atoms are present in the film and, therefore, more states are available for the photons to be absorbed [31]. The thickness dependence was observed by Ibrahim and Arsad [8] for their Ce doped YIG thin films prepared by a sol-gel method. They reported that the high optical transmittance (95%) was observed by the

thinnest film (pure YIG) in the visible and NIR regions. The thickness effect was also studied for ZnO: Al films [32]. It was observed that the optical transmittance decreased with an increasing film thickness in the visible region. In addition, if the surface of the film is relatively rough or the grain sizes are not uniform and the grainboundary is irregular, the surface reflection and grain boundary scattering reduce the transmittance of the film. A systematic reduction of transmittance was reported as a consequence of the increase in the surface roughness of multilayer polymer films prepared by sol-gel method [33]. This reduction was attributed to the concentration of grains on the film surface, which causes a higher light reflection and scattering. The absorption coefficient (a) was plotted to better understand the optical property behaviour of the obtained films. The optical absorption coefficient (a) was measured from the film's optical

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5

Transmittance (%)

100 80 60

x=0.0 x=0.4 x=0.8 x=1.4 x=1.8 x=2.2 x=2.6

40 20 0

300

400

500

600

700

800

Wavelength (nm) Fig. 7. The optical transmission spectra of Y2.8x Tb0.2 Erx Fe5 O12 films, (x ¼ 0 to 2.6).

35 Fig. 5. AFM images of Y2.8x Tb0.2 Erx Fe5 O12 films, (x ¼ 0 to 2.2).

104 cm -1)

20

(

104 cm -1)

25

28 24 20 16

15

0.0

0.4

0.8

1.2

1.6

2.0

2.4

2.8

x=0.0 x=0.4 x=0.8 x=1.4 x=1.8 x=2.2 x=2.6

Er Content (x)

(

transmission spectra using the Beer e Lambert's law: a ¼ ðln T = tÞ, where T is transmittance and t is film thickness. Fig. 8 illustrates the variation of a against wavelength with different Er concentrations (x). Overall, the absorption band edge shifted towards a shorter wavelength and gave rise to an appreciable reduction in a at x ¼ 1.8 in the UV region. However, from the absorption coefficient spectra, there are two distinguishable regions; the high a value in wavelength range l < 300 nm and the low a value in range 300 < l < 800 nm. It is clear that the absorption coefficient a changed unsystematically with increments of Er3þ ions in the first region (see centre inset). The Y2.8 Tb0.2 Fe5 O12 sample showed the highest value of a compared to all other films. The observed a was assigned to electronic transitions of Fe3þ cations in a and d-sites; as well as a charge transfer process between Fe3þ (3d orbit) and O2 (2p orbit) ions [34], which were affected by the presence of Tb3þ and Er3þ ions at c-sites. However, in the second region, the absorption coefficient became lower in the visible region. This attributed to crystal field transitions [34e37]; then almost vanished in the near infrared region. A similar behaviour was reported previously with different cation substitutions [38,39]. Based on the optical property results, the obtained films are deemed to be a promising crystal to be applied in optical isolators and circulators in visible and near infrared regions.

32

30

10 5 0

300

400

500

600

700

800

Wavelength (nm) Fig. 8. The absorption coefficient of Y2.8x Tb0.2 Erx Fe5 O12 films, (x ¼ 0 to 2.6) as a function of wavelength. The inset at the middle shows the variation of a with Er content (x).

3.4. Magnetic characterizations The magnetic measurements of the deposited films were performed using a VSM at room temperature (~27  C). Fig. 9 shows the in-plane M  H hysteresis loops of Y2.8-xTb0.2 Erx Fe5 O12 films (x ¼ 0 to 2.6). The linear ramp, caused by the diamagnetic quartz substrate, was deducted from the total magnetization of the sample. The normal shape and low coercivity, as illustrated in the upper-left

Fig. 6. The typical EDX spectrum of Y2.8x Tb0.2 Erx Fe5 O12 films: (a) x ¼ 1.8, (b) x ¼ 2.2.

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80

80

x=0.0 x=2.6 40

40

M(emu/cm3)

Magnetization (emu / cm3 )

6

0

-40

-400

-200

0

200

400

H(Oe)

0

x=0.0 x=0.4 x=0.8 x=1.4 x=1.8 x=2.2 x=2.6

-40 -80 -4000

-2000

0

2000

4000

Applied field (Oe) Fig. 9. The in-plane M  H hysteresis loops of Y2.8-x Tb0.2 Ery Fe5 O12 films. The upperleft inset reveals that all films are soft magnetic materials (For example x ¼ 0 and 2.6).

inset of the graph, reveals that all films are soft magnetic materials. The variation of saturation magnetization (Ms) and coercivity (Hc) with Er concentration (x) are demonstrated in Fig. 10. As can be observed, a Y2.8 Tb0.2 Fe5 O12 (x ¼ 0.0) film exhibited a higher saturation magnetization, Ms of 58 emu/cm3, which is smaller than 70 emu/cm3 reported by Ibrahim and Aldbea [11] using a sol-gel method. This is due to the difference between the solvent and the precursor material used during sample preparation; as well as the ambient annealing atmosphere during the film's crystallization. Generally, a reduction in Ms values was observed for all films compared to the Tb- YIG film. These results are less than the standard value of bulk YIG [29,38]. However, the presence of Er3þ and Tb3þions creates a magnetic moment to the c-site; where the Y3þ ion is a non-magnetic ion. In fact, the change of magnetization of Y2.8-xTb0.2 Erx Fe5 O12 films was associated with the increment of the magnetic moment of the c-site and the variation of a superexchange interaction Fe3þa (Y) e O2 e Fe3þd ([) [4]. This interaction depends strongly on interionic distance Fe-O and bond angle Fe ae O e Fe d. The Feþ3 ions in a-site and d-site are anti-parallel coupled; leaving a net moment of Feþ3 ions in d-site; and therefore, the Ms value of YIG is given mainly from d-site. In this study, the magnetic Erþ3 (9.72 mB) ions are located at csites due to their large ionic radii. When Er3þ was doped into Y2.8-x Tb0.2 Fe5 O12, the overall magnetization simply decreased with the increasing Er3þ concentration (x). It is possible that Er3þ was coupled antiferromagnetically with the resultant magnetic

70

In this study, the pure phase of Y2.8-xTb0.2 Erx Fe5 O12, (x ¼ 0.0e2.6) nanocrystalline series was easily grown by a sol-gel method under an annealing temperature of 900  C in ambient oxygen. The lattice constant was found to be linearly decreased with higher concentrations of Er3þ ions. This reduction confirmed that the lattice expansion, caused by Tb3þ ions during crystal growth, could be compensated by adding Er3þ ions with a smaller ionic radius than Y3þ without degradation of other properties. The estimated average crystallite size is in the range 24e32 nm. The films have smooth surfaces with an average thickness of 300 nm. An improvement in optical transparency with increments of Er content was observed, where the highest value (above 94%) was obtained at x ¼ 1.8 in the near infrared region. Magnetic characterization showed a reduction in Ms, probably due to cation distribution inside the YIG structure, and an increase in Hc, which attributed to the nanosized dimensions of the grains. Acknowledgement

60

60

References 50

50

40

40

30 30 20 20

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Er Content (x) Fig. 10. The variation of Ms & Hc with Er content (x).

Hc (Oe)

Ms (emu/cm3)

4. Conclusion

This research was financially supported by the Malaysian Ministry of Science and technology under grant No. DLP-2014-006 and GP-K006916.

70

Ms Hc

sublattices of Fe3þ ions at d-site. Therefore, the magnetic structure of such a mixed garnet could be understood according to the cation ƒƒ! ƒƒ ƒ! ƒƒ! ) þ3 þ3 þ3 distribution as fY3xy Eryþ3 Tbþ3 x g ½Fe2 ðFe3 Þ . A similar behaviour was reported previously for different cation substitutions into the YIG structure. Arsad [40] reported that Ce3þ decreases the Ms value due to a weak ferromagnetic exchange interaction between the a-d sites of the Ce: Gd-YIG films. Ftema [9] also observed that Ms decreases in a linear manner as Al3þ increases; which is attributed to the cation distributions for their Tb0.8 Y2.2 Aly Fe5y O12 nanoparticle films. However, Aoki [41] reported that Gd3þ could be aligned ferromagnetically with the Fe3þ ions at the d-site; resulting in increased Ms values as the Gd3þ ion increases for their Ce: Gd-YAIG nano-crystal. An increase in magnetization was also observed for Ba and TiO3 codoped BiFeO3 thin film prepared by radio frequency magnetron sputtering method [42]. This increment was attributed to the strong spin e orbital coupling, which led to magnetic anisotropy on the iron sublattice and suppressed the spiral spin structure. The magnetic coercivity Hc increases almost linearly from 22 to 63 Oe with the increment of Er concentration (as illustrated in Fig. 10). The improvement of Hc is basically attributed to a small grain size. From the above results, of in-plane magnetic measurements, it can be stated that adding Er3þ ions into Tb0.2 Y3Fe5 O12 films reduces the Ms and increases the Hc value in a systematic manner. The film, as a ferrite magnetic material with high coercivity, has potential to be used in magnetic recording media [43].

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