ELSEVIER Journal of Magnetism and Magnetic Materials 138 (1994) 222-231

1IA Journal of

A z-m A magnellc

malarlals

Optical and magneto-optical properties of Co-Ti-substituted barium hexaferrite single crystals and thin films produced by laser

ablation deposition

R. Atkinson a3*, P. Papakonstantinou a, I.W. Salter a, R. Gerber b a Department of Pure and Applied Physics, The Queen’s University of Belfast, Belfnst BT7 INN, UK

b Department of Pure and Applied Physics, University of Salford, Saljord M5 4WT, UK

Received 22 March 1994; in revised form 13 June 1994

Abstract

Ellipsometry, reflectance photometry and Kerr polarimetry have been used to determine the optical and magneto-optical properties of bulk single-crystal platelets and thin films of Co-Ti-doped barium hexaferrite. The single crystals were grown by a slow-cooling flux method and the thin films were produced by pulsed laser ablation deposition onto single-crystal Al,O, substrates at 840°C. The complex refractive index (n) and magneto-optic Voigt parameter <Q> were determined over the spectral range 350-850 nm and the reliability of the data tested by comparison with photometric measurements of reflectance. Although the optical constants of the thin films could not readily be obtained over the entire spectral range by ellipsometry, their magneto-optical performance in terms of the Kerr rotation and ellipticity compared well with predictions based on the fundamental data for the bulk material.

1. Introduction

Barium hexaferrite is a promising candidate for high-density recording for both conventional longitu- dinal [l] and perpendicular recording [2] as well as magneto-optical recording [3,4]. This is because of its high anisotropy field, chemical inertness mechani- cal hardness and its willingness to accept substitu- tional elements that can enhance magneto-optical properties at certain wavelengths [3,4].

To date there have been a number of reports of the extrinsic magneto-optical properties of this mate- rial [4,5]. However, to assess fully the performance

* Corresponding author.

potential of the material for magneto-optic readout it is necessary to obtain the intrinsic optical and mag- neto-optical material constants which appear as the diagonal and off-diagonal elements of the skew-sym- metric permittivity tensor of a gyroelectric material

161. Thus one requires information regarding the dis-

persion of the complex refractive index n (= IZ’ + in”) and the magneto-optic parameter Q ( = Q’ + iQ”>. Once such information is available it may be used to provide a direct comparison between various media through a proper figure of merit [7] which indicates the best magneto-optical performance of a medium after careful enhancement by its incorpora- tion in a quadrilayer [8] or trilayer [9] structure. These fundamental constants are also necessary for the effective design of such structures.

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R. Atkinson et al. /Journal of Magnetism and Magnetic Materials 138 (1994) 222-231 223

For the purposes of producing a useful magnetic information storage medium it will be necessary to produce high-quality thin films of this material on low-cost substrates. At present this is providing a difficult challenge because of the high temperatures ( > 650°C) necessary to produce crystallisation of the material. This may be done in-situ during deposition [lo] or through a post-deposition anneal procedure [ll]. More often than not the large difference in expansion coefficients of high-melting-point sub- strates and the barium ferrite results in unacceptable cracking of the films. Consequently, significant ef- fort is being expended to produce thin films at low temperatures [12], which is a desirable requirement for commercial exploitation.

In this paper we present a detailed account of a series of magneto-optic, ellipsometric and photomet- ric studies on a number of single-crystal platelets of Co-Ti-substituted ferrites. From these we provide comprehensive data on the optical and magneto-opti- cal constants as a function of cobalt concentration in the spectral region 350-850 nm. The results of these measurements and their reliability are assessed by comparison with some earlier photometric measure- ments [13] of intensity reflectance, where optical constants were deduced on the basis of a Kramers- Kronig analysis [ 141.

In addition, we give an account of the preparation of thin films of Co-Ti-doped ferrite by the process of pulsed laser ablation onto Al,O, substrates. The magneto-optical properties of the material produced by this process are compared with predictions based on the fundamental optical data for the bulk material.

2. Experimental

2.1. Single-crystal platelets

Five bulk single crystals of BaFe,,_,_,Co,Ti,O,, (x = y = o-0.78) w ere grown by a slow-cooling flux method described elsewhere [15]. From each crystal, small platelets about 130 p,rn thick were cut perpen- dicular to the hexagonal (0001) axis. Surfaces were then polished, etched in warm (54°C) orthophospho- ric acid and finally cleaned using lens tissue and acetone. As a result of the growth procedure, the Co-Ti content in the platelets was slightly inhomo-

geneous [15]. However, as we shall see later, com- parisons of the measured optical and magneto-optical properties of the platelets and films produced by pulsed laser ablation indicate that this was of little significance.

2.2. Films by pulsed laser ablation deposition

A schematic diagram of the pulsed laser ablation deposition (PLD) system is shown in Fig. 1. The ablated targets were sintered discs of Co-Ti-doped barium hexaferrite with composition parameter x = O-1.0. The output beam of a KrF excimer laser (wavelength A = 248 nm, 200 mJ/pulse, 23 ns pulse width and duty cycle 20 Hz) was passed through a defining aperture and focused onto the target, through an off-axis lens (focal length = 230 mm), at an angle

of incidence of 30”. The target, lens and substrate were all rotated to improve the uniformity and tex- ture of the deposited material. The fluence, or radia- tion flux density, at the target surface was 2 J/cm*, with ablated material being ejected in a direction normal to the plane of the target.

The substrate was a 10 X 10 X 1 mm Al,O, (0001) plate heated to a temperature of approxi- mately 840°C during deposition and situated approx- imately 30 mm from the target. Typically 30,000 pulses were required to deposit a film 250 nm thick. During the deposition the partial pressure of oxygen was 0.1 mbar. This was increased to 0.4 mbar after deposition, as the sample was cooled slowly to room temperature. This procedure produced c-axis-ori- ented continuous films with minimal surface particu- lates. A typical XRD pattern is shown in Fig. 2.

E

Fig. 1. Schematic of the system for producing films by laser

ablation. E: excimer laser; A: aperture; L: rotating lens; W: UV

window; P: plume; S: substrate and heater; T: target.

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224 R. Atkinson et al. /Journal of Magnetism and Magnetic Materials I38 (1994) 222-231

(OO( w

(OOOM)

A , J_ 50 70

26 (degws)

Fig. 2. X-ray diffraction pattern of thin film of Co-Ti-doped barium hexaferrite produced by laser ablation onto a single-crystal Also,

substrate.

2.3. Optical characterisation

Ellipsometric measurements were made at an an- gle of incidence of 70” at wavelength intervals of 10 nm using a rotating analyser instrument illustrated schematically in Fig. 3. The optical constants of the

‘Lm

-I- P I RKJl

f--t+ Al

d-l

I\1 A2 Fl

I[ PC I I

I

-

M

if D

Fig. 3. Schematic of the rotating analyser ellipsometer/Kerr

polarimeter systems. T: light Source; P: polariser; R: retardation plate; S: sample; E: electromagnet; Al: rotating analyser; A2:

fixed analyser; M: monochromator; D: detector; PC: computer.

material were determined for the polished single- crystal platelets only. There were two reasons for performing ellipsometric measurements on the bulk samples. First, at long wavelengths the material has a low extinction coefficient (n” = 0.01) and conse- quently, where ellipsometric measurements are made on thin films (thickness < skin depth), the evaluation of n” is very sensitive to errors in the film thickness. In the bulk samples this is unimportant. Second, the laser ablation deposition process produces films with poor uniformity in film thickness over large areas. This compounds the problems already referred to above.

2.4. Magneto-optical measurements

Using a Kerr polarimeter based on the ellipsomet- ric arrangement of Fig. 3 and incorporating an elec- tromagnet capable of producing saturating magnetic fields of 1.5 T, measurements were made of the polar Kerr rotation (19,) and ellipticity (ok) at an angle near normal incidence (- 3”). Measurements were made for the same spectral range and interval for both bulk and thin-film samples. The data col- lected for the bulk samples were used to deduce the magneto-optical constant <Q> using well known for-

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R. Atkinson et al./Journal of Magnetism and Magnetic Materials 138 (1994) 222-231 225

3.2 , I

3.0 t

2.8 1

2.6 I

2.4

I 2.2’ ’ ’ ’ ’ ’ I ’ 8 I ’ 1 0.0’ ’ ’ j I a I ’ ’ I ’

300 400 500 600 700 800 900 300 400 500 600 700 a00 9

Wavelength (nm) Wavelength (nm)

)O

Fig. 4. Dispersion of the real (n’) and imaginary (ti’) parts of the refractive index of single crystal Co-Ti-doped barium ferrite, determined

by ellipsometry.

mulae used in association with the preferred sign convention given elsewhere [16]. That is,

inQ 8, + ie, = -

n2-1’

3. Results and discussion

3.1. Optical measurements

The real and imaginary parts of the refractive index of the bulk material determined by ellipsome-

3.2

n’ x=0

600 700 a00

Wavelength (nm)

try are illustrated in Fig. 4 as a function of wave- length, h (nm) and the composition parameter X. For comparison we also provide the optical constants determined from a Kramers-Kronig analysis applied to normal incidence reflectance measurements re- ported earlier [13]. These are shown in Fig. 5.

Before beginning a discussion of these results, the size and condition of the surface of these samples must be pointed out. The bulk samples were small (N 20 mm2) and the polishing procedure was not entirely satisfactory leaving many microscopic scratches on the surfaces. An optical micrograph of these is shown in Fig. 6. This dark field picture of

1.4

L n”

0.8

0.6

600 700 a00

Wavelength (nm)

Fig. 5. Dispersion of the real (d) and imaginary (d’) parts of the refractive index of the single crystal Co-Ti-doped barium ferrite;

determined from Kramers-Kronig analysis.

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226 R. Atkinson et al. /Journal of Magnetism and Magnetic Materials 138 (1994) 222-231

the scratches illustrates the significant amount of scattering that takes place at the surfaces of these samples.

In general, despite the condition of the surfaces of the samples, the agreement between the two sets of measurements in Figs. 4 and 5 is very good indeed for both n’ and n” over the whole spectral range. However, although a detailed comparison of the variation of n” with increasing cobalt content is not particularly good, a definite systematic variation of n’ with x is clearly seen in both sets of data where the inclusion of Co2+ and Ti4+ cations into the lattice depresses the real part of the refractive index over the whole spectral range.

A point worth noting in relation to the dispersion of n’ is seen in Fig. 4a, where a point of inflection occurs at a wavelength of about 450 nm. This is common to all curves determined by ellipsometry but is not seen clearly in the Kramers-Kronig data. This is of particular significance with regard to the assessment of the reliability of the fundamental data. To understand this we must compare the measured reflectances of these samples with those predicted by the two sets of basic optical constants. The measured reflectances and theoretical predictions are given in Figs. 7 and 8. Fig. 8 corresponds to the Kramers- Kronig analysis. It should be noted that the most recent measurements of reflectance made at Queen’s University have been scaled for the purposes of comparing theoretical and experimental data. This has been done for two reasons. First, the instrumen-

Fig. 6. Dark field optical micrograph of surface damage on a

single-crystal platelet.

0.30

R . .

0.26 I-

0.24 -

0.22 -

300 400 500 600 700 800 900 Wavelength (nm)

Fig. 7. Measurements and theoretical predictions (full lines) of the

reflectance of single crystal platelets of Co-Ti-doped barium

ferrite, determined at Queen’s University, Belfast.

tation (a reflectometer) combined with very small samples makes it necessary to a apply a small geo- metric correction factor which compensates for the fact that some of the radiation, normally incident and reflected from the sample, may be lost from the system. Simply, the probing optical beam size was a little larger than the sample in some cases. Second, and of some significance, the measured specular reflectances will be modified by problems related to the undetected scattered radiation originating at de- fects on the surfaces of the samples. Consequently, the scaled reflectance R, is determined from the measured reflectance R, according to the relation

R, = R,/( a - yK4), (2) where (Y is the geometric factor and y is a scatter factor related to surface roughness. It is assumed that the scattered intensity has a he4 dependence. The constants (Y and y may be determined from Eq. (2) using a linear least-squares fitting procedure with R,

replaced by the theoretically predicted reflectances. It is worth noting that in these fitting procedures (Y

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R. Atkinson et al. /Journal of Magnetism and Magnetic Materials 138 (1994) 222-231 227

0.27

0.25 m x=0.35 - x=0.58 - x=0.78

0.23

0.21

300 400 500 600 700 800 900 Wavelength (nm)

Fig. 8. Measurements and theoretical predictions (full lines) of the

reflectance of single-crystal platelets of Co-Ti-doped barium fer-

rite, determined in relation to the Kramer+Kronig analysis.

was always very close to or slightly less than unity, which is consistent with the model and the fact that the samples were not much smaller than the optical beam size. Consequently, it appears that the main differences between the measured and predicted re-

I I

.J Ok (min>

-4

600 700 800

Wavelength (nm)

Table 1

Fitting parameters for the measured reflectivities of the single-

crystal platelets using Eq. (2) (A is expressed in nm)

Sample x (Y y X 10e9 (nrnY

1 0 0.93 2.12

2 0.1 1.01 1.20

5 0.35 1.00 1.62

7 0.58 1.03 1.50

8 0.78 1.00 2.20

flectances are largely due to scattering. The fitting parameters (Y and y are given in Table 1.

In comparing these results it will be observed that in the case of the Kramers-Kronig results the theo- retical predictions should be in perfect agreement with the reflectance measurements from which they were originally determined. In general, this is true. On the other hand, the theoretical data predicted by the independent ellipsometric data are excellent and provide the correct variation of R with increasing cobalt content, as well as the detailed undulating structure in the wavelength range 350-500 nm. It is worth noting that this structure is not seen clearly in the reflectance data of Fig. 8.

It must be concluded from the comparisons of these results that the data shown in Figs. 4 and 5 for the optical constants of Co-Ti-doped ferrites are reasonably reliable, although the ellipsometric data are to be preferred. In support of this we make the following additional points.

18,

t Ek(min)

6 3 3 j

-6 ’ ’ ’ ’ ’ ’ ’ * ’ 300 400 500 600 700 600 900

Wavelength (nm)

Fig. 9. Dispersion of the complex polar Kerr effect (0, + i ox) for single-crystal Co-Ti-doped barium ferrite.

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:!2t K. Atk’nson et al. / JNU~Z’ of J4agnetism and Magnetic Materials 138 (1994) 222-231

First, ilon-specular sc;i:tering ha:; little effect on ell ipsometric m:asuremer ts, which are essentially ratiometric, combining the Fresnel amplitude reflec- tion coefficients for the two principal states of polar- isation [17]. Second, me,asurements of intensity re- fleotance are particularly sensitive to surface scatter- ing with a. he4 clependeno: and will therefore lead to systematic errors in R and therefore in n’ and d’ (deduced from R), and this will become worse at shorter w,avelengths. It is at the shorter wavelengths that the structure in the optical properties is clearly observed. It is also worth noting that the theoretical reflectances calculated from the ellipsometrically de- termined constants are higher than those measured by SimSa [13]. This is reasonable if there are losses due to scaittering. Again, this is most obvious at short wavelengths.

3.2. Magneto-optical measurements (bulk)

Measurements of the complex polar Kerr rotation 6,( = OK + i ox) for the bulk samples are shown in Fig. 9 as functions of the cobalt concentration. From these data the real and imaginary parts of the mag- neto-optic parameter Q were determined using Eq. (1) in combination with the optical data provided by ellipsometry. The results are shown in Fig. 10. The systematic variations of Q with wavelength, which are also reflected in the Kerr rotations and elliptici- ties are seen clearly in this figure. It has already been

pointed out [3] that a peak in magneto-optical activ- ity occurs at short wavelengths (380 nm), where the next generation of magneto-optic devices are ex- pected to operate; this is due to transitions associated with the Fe3+ ions in the lattice. That this is so may be deduced from the fact that this peak is reduced as the Fe3+ ions are replaced by Co2+. At the same time there is the onset of a noticeable increase in magneto-optic activity at 720 nm that is clearly identified with increasing Co’+ ion concentration. The unambiguous systematic variation of Q with wavelength and cobalt substitution level is clearly an indicator of the reliability of the data. In addition, as we shall see, these data provide excellent theoretical predictions of the behaviour of thin films of the same material.

3.3.1 Q 1 dependence on x

It is of interest to examine the dependence of the magneto-optic parameter on the Co doping level. This is shown in Fig. 11 for two specific wave- lengths corresponding to the Fe3+ and Co2+ peaks. Clearly, there is a linear dependence of the changes in 1 Q 1 on the number of dopant atoms with no sign of a saturation of the effect. Thus, in principle, there is scope to increase 1 Q I at A = 780 nm by the further addition of cobalt. However, it must be pointed out that it is physically difficult to increase this much

beyond x = 1.

-41 ’ ’ c I I I ’ I ’ ’ 300 400 500 600 700 800 900

Wavelength (nm)

1

300 400 500 600 700 600 9

Wavelength (nm)

Fig. 10. Dispersion of the real (Q’) and imaginary (Q”) parts of the magneto-optic parameter of single-crystal Co-Ti-doped barium ferrite.

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R. Atkinson et al. /Journal of Magnetism and Magnetic Materials I38 (1994) 222-231 229

t > 090

I

0.2 0.4 0.6 0.8 1 .o

Cobalt x

Fig. 11. Variation of the modulus of the magneto-optic parameter

of Co-Ti-doped barium ferrite as a function of composition

parameter x; for wavelengths of 380 and 710 nm.

3.4. Thin-film properties

Laser-ablated thin films of Co-Ti-doped ferrite were subjected to both ellipsometric analysis and Kerr-effect measurements. In our ellipsometer, and for an incident angle of 70” the optical beam size probing the surface of the sample is approximately 5 X 10 mm. As a consequence, at long wavelengths, where absorption is low, the nonuniform thickness of the samples produced anomalous data for n’ and n” as one would expect. On the other hand, at wave- lengths below about 600 nm the agreement with bulk data is very good since in this region the films are

-lOI 300 400 500 600 700 800 900

Wavelength (nm)

optically opaque. For these reasons we do not in- clude the optical data for the films.

The beam probe size in the case of the near normal incidence Kerr polarimeter is approximately 2 X 3 mm and as a result the influence of nonuni- form thickness on the magneto-optic measurements is very much less than that for the ellipsometer. We therefore report on the the details of these measure- ments and, as an indicator of the optical and mag- neto-optical constants of the films, we compare the Kerr-effect data with predictions based on the con- stants given for bulk samples. Of course, in the case of thin films, Eq. (1) is inappropriate and a 4 X 4 matrix approach 1181 was used to take into account the multiple reflections that occur within the medium. Such calculations were carried out for the undoped ferrite (x = 0) and for maximum cobalt doping of x = 0.8. The results are shown in Figs. 12 and 13 for two films of thicknesses (d) of 450 and 390 nm, respectively.

It will be noticed that for the wavelength range 350-500 nm the Kerr spectra are similar in shape and magnitude to those of the bulk samples given in Fig. 9. This is a consequence of the fact that the films are thicker than the optical skin depth in this wavelength region. Hence, very good agreement in material parameters between bulk and thin films grown by PLD can be expected.

At longer wavelengths (around 700 nm) variations in Kerr rotations and ellipticity are observed which are not seen in the bulk spectra. This is entirely to be

-61 ’ ’ ’ ’ ’ ’ ’ / ’ ’ 300 400 500 600 700 800 900

Wavelength (nm)

Fig. 12. Measured (solid curve) and predicted (dashed curve) complex polar Kerr rotation for a ferrite (x = 0, d = 450 nm) film produced by pulsed laser deposition.

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230 R. Atkinson et al. /Journal of Magnetism and Magnetic Materials I38 (1994) 222-231

-20 300 400 500 600 700 600 900 300 400 500 600 700 600 9

Wavelength (nm) Wavelength (nm)

Fig. 13. Measured (solid curve) and predicted (dashed curve) complex Kerr rotation for a Co-Ti-doped barium ferrite (X = 0.8, d = 390 nm) film produced by laser ablation.

expected since the low extinction coefficient of the medium allows observable optical interference ef- fects within the films, which produce rapid changes in their reflectivity. This can be seen plotted theoreti- cally in Fig. 14 for the two films corresponding to Figs. 12 and 13. However, it is emphasised that these features of the magneto-optic spectra of thin films should not be confused with the intrinsic wave- length-dependent properties of the medium which are illustrated in Figs. 4, 9 and 10.

We therefore see that the magneto-optic spectra of the laser-ablated films are consistent with the intrin- sic properties of the bulk materials. Moreover, when one compares the measurements with the theoretical

0.30

0.26 - I’

0.14’ I ’ I ’ 1 ’ ’ ’ ’ ’ 300 400 500 600 700 600 900

Wavelength (nm)

Fig. 14. Theoretically predicted reflectance of Co-Ti-doped bar-

ium ferrite films (x = 0, d = 450 nm and x = 0.8, d = 390 nm) as

a function of wavelength.

predictions based on bulk data it is clear that the agreement is very good indeed with all major varia- tions being reflected in both experimental and theo- retical curves. There are some small discrepancies but, bearing in mind the possibility of residual ef- fects arising from possible film inhomogeneities, these are unimportant.

4. Conclusions

The complex refractive index and the magneto- optic Voigt parameter of Co-Ti-doped barium hexa- ferrite have been determined for bulk single crystals for the spectral range 350-850 nm. The results have been compared with similar data for the optical constants determined previously based on a Kra- mers-Kronig analysis using reflectance data. Al- though the results are similar, it appears that the ellipsometric data are more reliable with discrepan- cies being predominantly due to optical scattering resulting from defects and scratches in the surfaces of the bulk samples. It is believed that scattering has a more pronounced effect on the intensity reflectance measurements and the constants derived from them, than on the ellipsometric measurements, which are essentially ratiometric in nature.

The dispersion of the magneto-optical activity of

BaFe,,_,_,Co,Ti,O,, is marked by two distinct peaks, at wavelengths of 380 and 710 nm. These are seen to be directly related to optical transitions asso-

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R. Atkinson et al. /Journal of Magnetism and Magnetic Materials 138 (1994) 222-231 231

ciated with the Fe3+ and Co’+ ions, respectively. In addition, it is observed that the level of magneto-optic activity at these wavelengths is linearly proportional to the Co and Fe concentrations. It is perhaps worth pointing out that it is not possible, from the optical and magneto-optical data alone, to determine the distribution of the cation species among the tetrahe- dral, octahedral and bypyramidal sites of the hexago- nal lattice. However, recently, we have made consid- erable progress in correctly identifying their loca- tions using neutron diffraction, magnetisation and Mijssbauer spectroscopy data [ 19,201.

c-axis, well oriented thin films of this material have been produced by pulsed laser ablation deposi- tion onto single-crystal, (OOOl)-oriented, AI,O, sub- strates at temperatures of 850°C in an oxygen pres- sure of 0.1 mbar. The magneto-optical performance of the samples was determined by measuring the normal incidence polar Kerr effect. Comparisons of the measurements with predictions based on theoreti- cal calculations using the fundamental data taken for the bulk samples showed very good agreement. This confirms the material and structural integrity of the Co-Ti-substituted barium hexaferrite films produced by pulsed laser ablation deposition and emphasises the importance of the approach of determining the fundamental optical and magneto-optical parameters of a material as a means of assessing its potential for magneto-optic applications.

Acknowledgements

The authors are grateful to Proffessor J.M.D. Coey, Dr. J.G. Lunney and Mr. H.J. Masterson of Trinity College Dublin for their assistance with the laser ablation work, and wish to thank Dr. 2. SimSa of the Institute of Physics, Academy of Science,

Czech Republic, for supplying data for the purposes of comparison.

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