Pulsed laser ablation deposition of yttrium iron garnet and cerium-substituted YIG films

Journal of Magnetism and Magnetic Materials 220 (2000) 183}194

Pulsed laser ablation deposition of yttrium iron garnet and cerium-substituted YIG "lms N.B. Ibrahim, C. Edwards*, S.B. Palmer Department of Physics, University of Warwick, Coventry CV4 7AL, UK Received 13 January 2000; received in revised form 14 March 2000

Abstract Yttrium iron garnet (YIG) thin "lms were grown on gadolinium gallium garnet substrates using pulsed laser ablation deposition (PLAD) with a XeCl excimer laser. Films were grown up to over 2 lm thick, however cracking proved to be a problem for "lms over 1 lm thick. The lattice parameter(s) of the "lms and the substrates were measured and indicated that the "lm/substrate structure was bending to accommodate strain due to the lattice mismatch. The "lms had saturation magnetisation values close to that of bulk YIG and were isotropic in the "lm plane. The magnetisation data also indicate stress-induced uniaxial isotropy. The ablation conditions were varied to produce uncracked "lms with low droplet densities. YIG melts incongruently during the laser ablation process and cone-like structures form on the ablation target lowering the ablation rates. Cerium-substituted YIG "lms were also grown in both oxygen and argon atmospheres, substituting cerium into YIG increases the lattice parameter and hence reduces the strain. The Ce-YIG "lm grown in argon was greenish indicating that cerium was in the desired oxidation state.  2000 Elsevier Science B.V. All rights reserved. PACS: 81.15.Fg Keywords: Pulsed laser ablation deposition; YIG; Ce-substituted YIG

1. Introduction YIG and substituted YIG "lms are interesting ferrimagnetic materials in part because they have potential magneto-optic applications. Cerium-substituted YIG, in particular, has been reported to have a very large Faraday rotation [1,2]. Recent reports [3}5] describe pulsed laser ablation deposition (PLAD) of epitaxial YIG "lms using KrF

* Corresponding author. E-mail address: [email protected] (C. Edwards).

(j"248 nm) excimer lasers. Within the last year, Ce-YIG has also been grown by PLAD using a tripled Nd : YAG laser [6]. However in the case of YIG, the saturation magnetisation values of the PLAD materials varied considerably from bulk YIG and some of the "lms had large numbers of micron sized droplets. This paper reports the growth of YIG and Ce-YIG "lms by PLAD using a XeCl excimer laser with a 30 ns pulse width. PLAD of YIG at this wavelength (j"308 nm) or PLAD of Ce-YIG in argon has not been previously reported. The deposition conditions were varied to produce "lms with good surface qualities.

0304-8853/00/$ - see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 3 3 1 - 0

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Laser-induced surface modi"cations to both the YIG and Ce-YIG PLAD targets were also studied.

2. Experimental details The YIG (Y Fe O ) ablation targets were pre   pared by the ceramic preparation technique described by Nicolas [7], Y O and Fe O powders     (99.9% purity) were mixed in 3 : 5 molar proportions and sintered at 1200}13003C for 6 h to form YIG. The sintered YIG was milled in a mechanical mortar and pestle and then resintered, (this procedure was repeated six times) in order to get "ne grained, uniform YIG targets. The material was pressure moulded in a 16 mm die, to obtain a low porosity pellet. Finally, the pellet was heat-treated once again in an oxygen atmosphere at 14503C for 6 h to ensure that YIG was not oxygen de"cient, PLAD of YIG is normally carried out in an oxygen atmosphere for the same reason. XRD con"rmed that the pellets were YIG and their density indicated they had less than 10% porosity. A similar procedure was used to prepare the Ce-YIG targets from 99.9% pure CeO , Y O and    Fe O oxides. The oxides were mixed in a molar   ratio of 1 : 1 : 2.5 with the aim of making a target with a composition of CeY Fe O , similar to     the targets prepared by Shintaku et al. [1] for RF sputtering. The "nal sintering temperature was 13803C as addition of Ce lowers the incongruent melting temperature. XRD measurements showed the pellet had a garnet structure with a lattice parameter of 12.436 As , however there was still a small amount of unreacted CeO present. Two  sets of YIG thin "lms were prepared by PLAD using the XeCl excimer laser. The PLAD system used has previously been described [8], the target was rotated and the laser spot moved with a wobbling lens to reduced localised target erosion. The "rst YIG "lm set was deposited using a laser #uence of &2}3 J cm\ in 0.17 mbar oxygen gas pressure, a substrate temperature of 7663C and substrate to target distance of 4 cm (the S1-series "lms). The second set was deposited with a #uence of 1}2 J cm\, in an oxygen pressure of 0.07 mbar, a substrate temperature of 8003C and substrate to target distance of 6 cm (the S2-series "lms). The

pressure and laser energy for the S1 and S2 depositions were adjusted so that the substrates were located towards the outer edge of the visible plasma but outside the more intense and directional inner core of the ablation plume. The lattice parameter of YIG is 12.376 As , gadolinium gallium garnet (GGG) which has lattice parameter of 12.383 As would be a very good substrate for growing epitaxial YIG. However it is di$cult to grow large GGG single crystals without defects. Substituted gadolinium gallium garnet (SGGG) which contains 2 mol% magnesium (lattice parameter of 12.5 As ) is commercially available and was used instead. Ce-YIG has a similar lattice parameter to SGGG, two Ce-YIG "lms were also deposited at a 4 cm target to substrate distance (the Ce-series "lms), one deposition was carried out in oxygen and one in argon at 0.17 mbar, the substrate temperature was 8003C. An X-ray di!ractometer was used to study the microstructure of S1-, S2- and Ce-series "lms which were deposited on 5;10 mm (1 1 1) orientated substrates. The "lm thicknesses, surface morphologies and droplets were studied using a scanning electron microscope (SEM). The magnetic properties were characterised using a vibrating sample magnetometer (VSM) and the optical absorption spectra were also investigated using a visible-UV spectrometer. Laser-induced target surface modi"cation was studied using SEM and energy dispersive X-ray analysis (EDS).

3. Results and discussion 3.1. XRD results The samples were cut into 2.5;5 mm quarters. One was potted and polished so the "lm thicknesses could be measured from the cross-section using backscattered electron imaging SEM. Table 1 summarises the "lm thicknesses and their surface conditions. The XRD results show that all the "lm series were epitaxial with the SGGG substrates. As the (4 4 4) "lm peaks are very close to the (4 4 4) substrate peaks, a Microcal Origin version 5 statistical curve-"tting routine was used to "t the spectra and reveal the "lm peaks. The "tting was "rstly

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Table 1 Summary of "lm thicknesses and their surface conditions Sample

Thickness ($0.05 lm)

Film condition

S1.1 S1.2 S1.3 S1.4 S1.5 S2.1 S2.2 S2.3 S2.4 S2.5 Ce(O )  Ce(Ar)

0.80 1.07 1.47 1.65 2.10 0.30 0.32 0.38 0.47 0.87 2.06 1.15

No cracks or #akes Fine cracks Partly #aked and cracked No cracks or #akes Heavily cracked and #aked No cracks or #akes No cracks or #akes No cracks or #akes No cracks or #akes No cracks or #akes Cracked No cracks or #akes

done on the substrate Ka , a doublet peaks using   a Pseudo-Voigt distribution which consists of a combination of Gaussian and Lorentz functions. The Ka centre position was calculated from the  position of Ka peak and known Ka and Ka    wavelengths. Then the "tted substrate peaks were subtracted from the spectra to reveal the "lm peaks, which were "tted iteratively with the same distribution function. Fig. 1 shows one of the XRD spectra and the curve "ts. The centre positions of the "tted peaks enabled the "lm and substrate lattice parameters to be calculated. Table 2 shows the extracted lattice

Fig. 1. Fit to XRD peaks for the S1.1 YIG "lm (correlation coe$cient"0.99781), the substrate lattice parameter is 12.476 As and the "lm lattice parameter is 12.447 As . Symbols are plotted on only 1 in 5 data points for clarity.

parameters. All of the YIG "lms, which were a pale yellow colour, grew with bigger lattice parameters than the YIG target. Generally, thicker YIG "lms ('1.0 lm) showed several overlapping X-ray peaks indicating a layered structure in the "lms with decreasing lattice parameters tending to the unstrained value for YIG. Mino et al. [9] and Tate et al. [10] have observed several layers with variable lattice parameters in Ce-YIG "lms deposited on GGG substrates by RF sputtering, they stated that the layering was caused by a combination of elastic strain and dislocation formation to relieve the lattice mismatch. XRD measures the d spacing normal

Table 2 Substrate and "lm lattice parameter(s) extracted from the XRD peaks Sample

Thickness ($0.05 lm)

Substrate lattice parameter (As )

Film lattice parameter(s) (As )

S1.1 S1.2 S1.3 S1.4 S1.5 S2.1 S2.2 S2.3 S2.4 S2.5 Ce(O )  Ce(Ar)

0.80 1.07 1.47 1.65 2.10 0.30 0.32 0.38 0.47 0.87 2.06 1.15

12.476 12.478 12.486 12.478 12.486 12.486 12.489 12.486 12.487 12.477 12.487 12.491

12.447 12.404, 12.413, 12.439, 12.396, 12.439, 12.414 12.513 12.444 12.453 12.411, 12.463

12.384 12.389 12.429, 12.417 12.379 12.419

12.393

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to the "lm thickness (d ). The in-plane d spacing, , parallel to the surface of the substrate, (d ) will be , di!erent in the case of a strained layer [9]. The XRD results indicate layers of YIG with large lattice parameters, however all the extracted lattice parameters for samples are related to d , , d must be smaller. This paradoxical result can , be explained if the substrate bends to accommodate the mismatch strain of the YIG layer as shown in Fig. 2. This implies that the YIG "lms must be under in-plane compression due to lattice mismatch between the YIG and SGGG substrate (1%). The initial "lm layer deposits epitaxially on the substrate surface, the mismatch strain increases with "lm thickness and causes the substrate to bend. Chopra [11] gives a model relating the bending radius to the "lm and substrate thicknesses and elasticities. The initial layer has a larger d (and the XRD peak occurs at lower angles) than , bulk YIG due to the compressive strain. As the "lm thickness increases the strain increases to a value, where dislocation formation will occur to reduce the strain. This results in discrete layers of "lm with di!erent lattice parameters. The upper portions of the "lm are less strained and tend to grow with the same lattice parameter as bulk YIG (12.376 As ) and the XRD peaks move towards 51.073. Fig. 2 shows how bending distorts the lattice, the upper portion ("lm) is laterally compressed but vertically ex-

Fig. 2. Distortion of lattice due to bending, the upper portion is laterally compressed but vertically expanded and visa versa for the bottom section. The dotted lines represent the undistorted lattice.

panded and vice versa for the bottom section. When a layer is stretched in one direction it contracts in the two orthogonal directions by a factor determined by Poisson's ratio (in-plane compression is equivalent to a tension applied along [1 1 1]). Matthews et al. [12] stated that if the lattice mismatch is greater than 10% the "lms will always crack and estimated a line of maximum mismatch as a function of "lm thickness for "lms which would never be expected to crack. Between these upper and lower limits there is a region where the "lms may crack if there are crack initiating defects present, calculations based upon measured lattice parameters and "lm thickness show that the S1- and S2-series "lms lie in this intermediate region. For S1-series "lms only "lms S1.1 and S1.3 did not crack while all the other "lms had cracks. The surface cracking could be due to the presence of defects (large droplets) on the "lm surfaces, see Fig. 3 where a large droplet is lying at a triple junction of cracks. The full-width half-maximum (FWHM) for all of the SGGG substrates and the YIG "lms peaks were also measured from the XRD results. Fig. 4a shows the FWHM of the substrates plotted against "lm thickness for both series. There is clear di!erence between the cracked and uncracked "lms, the FWHM of the substrates with uncracked "lms have a linear relationship with "lm thickness shown by the straight-line "t. The FWHM of the YIG "lms (see Fig. 4b) are in the range of 0.03}0.183, for multi-layered "lms the FWHM is shown for each layer. The Ce-YIG "lms were a pale orangish-yellow colour. The "lm grown in oxygen (Ce(O )) was  cracked and had a reddish brown tint whereas the "lm deposited in an argon bu!er gas (Ce(Ar)) had a greenish tint and did not crack. Argon gas inhibits formation of Ce> ion which is more stable than Ce> in an oxygen atmosphere (the target pellets are already oxygen rich since they are made from CeO not Ce O ). Gomi et al. [2] reported    that Ce-YIG grown by RF sputtering becomes a deeper green colour as the Ce> content increases and the Faraday rotation also increases. The Ce-"lms had larger lattice parameters than the CeYIG target, the 2 lm thick Ce(O ) "lm had two 

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Fig. 4. FWHM of: (a) SGGG (4 4 4) peak; and (b) YIG "lm layers versus "lm thickness (cracked "lms are labelled with C).

Fig. 3. SEM micrographs of (a) S1.5 "lm showing cracking originating from a large spherical droplet, also note the smaller angular growth features; and (b) high-quality uncraked "lm (S2.1) with a low droplet density.

layers while the thinner Ce(Ar) "lm had a single layer. 3.2. Droplet analysis Ten SEM images at di!erent spots on the "lms were taken and the droplet counting and sizing was done using Imagetools software. Following Chen [13], the term droplets will be used to refer to the spherical particles in the micron size range, which are thought to originate from liquid droplets in the ablation plume. Examination of the micrographs

showed that the vast majority of the spherical features had diameters *0.3 lm, the smaller scale structures on the "lm surfaces tend to have angular shapes and are probably growth features (see Fig. 3). Only the spherical particles with diameters *0.3 lm are counted as droplets in this analysis, Table 3 summarises the results. The total areas and volumes of the droplets were calculated from the measured radii. The percentage areas and volumes were estimated by dividing by the total surface area of the micrographs or the total volume of the "lm (micrograph area;"lm thickness). While the droplet densities of &10 cm\ seem high, the quality of the "lms can be appreciated from the percentage areas and volumes. There are only few PLAD publications which actually give numerical values, YBCO "lms with droplet densities of 3}7;10 cm\ are considered high quality [14]

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Table 3 Droplet densities, size range and estimated percentage areas and volumes for the S1-, S2- and Ce-series "lms Sample

Droplet density (;10 cm\)

Droplet diameter size range (lm)

Percentage area of droplets

Percentage volume of droplets

S1.1 S1.2 S1.3 S1.4 S1.5 S2.1 S2.2 S2.3 S2.4 S2.5 Ce(O )  Ce(Ar)

1.6 1.9 1.9 3.6 5.1 4.8 * 1.4 0.7 0.9 11.5 4.9

0.3}1.8 0.3}2.0 0.3}1.8 0.4}1.9 0.3}1.9 0.3}0.5

0.69 0.55 0.95 1.33 2.36 0.41 * 0.23 0.11 0.11 2.26 0.54

0.62 0.30 0.47 0.48 0.76 0.33 * 0.22 0.08 0.03 0.50 0.15

0.3}0.7 0.3}0.6 0.3}0.5 0.3}1.3 0.3}0.9

and droplet densities of 2;10 cm\ have been reported for metallic "lms [15]. No droplets were recorded for the S2.2 "lm as there were no features with diameters greater than 0.3 lm, although there were a few slightly smaller features with diameters of approximately 0.26 lm, this is an example of where the 0.3 lm cut o! criteria used is probably too simple. The S1-series "lms have both larger droplets and greater droplet percentage areas than the S2-series "lms. The largest droplet diameter in the S2-series is only 0.7 lm while the biggest droplet diameter for the S1-series is 2.0 lm, the percentage areas of droplets for the S2- and S1-series "lms are 0.11}0.41% and 0.55}2.36%, respectively. Hence S2-series "lms have much better quality surfaces than the S1-series "lms. The improved surface quality of the S2-series "lms may be explained as follows. Droplets are much heavier than the ionised species in the plasma, and travel at much lower velocities compared to other species evaporated from the target [13,16]. The ablation plume is distributed in a cone perpendicular to the ablation target surface. The distribution of species in the ablation plume varies with angle, there is an inner more directional component of the form cos(ch) (where c can range from 1}30) and an outer envelope of the form cos h where the plasma has cooled due to collisions with the oxygen gas [17]. In the present work the substrates were located outside the in-

tense inner core and the cone half-angle was &303. As the substrate}target distance is increased the droplet density will reduce as the solid angle subtended by the substrate is smaller, and some of the larger droplets may no longer reach or adhere to the substrate. 3.3. VSM results As the "lms are very thin compared to their lateral dimensions, the in-plane demagnetising factors (D ) tend to zero and the out-of-plane de magnetisation factor (D ) tends to 1. The in-plane  hysteresis loops obtained using a VSM show that all of the "lms have soft magnetic properties (narrow hysteresis loops). No demagnetisation corrections were necessary but a linear ramp due to the paramagnetic SGGG substrates was subtracted from the data. Fig. 5 shows the saturation magnetisation (4pM ) of the "lms versus sample thickness.  The magnetisation was calculated from the measured magnetic moments and the "lm volume, the error bars for saturation magnetisation values come mainly from the error in the "lm thickness measurement. The thickness error can be as high as 16% for the thinner "lms whereas the volume of the droplets was always less than 0.8%, see Tables 1 and 3. Within the experimental errors most of the saturation magnetisations are close to the bulk YIG (4pM "1750 G), but some "lms have values 

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three easy directions close to the "lm plane are favoured, the maximum vector sum resolved in the (1 1 1) plane is

Fig. 5. Saturation magnetisation (4pM ) for all of the "lms  versus "lm thicknesses compared with the value for bulk YIG.

over 2000 G. Dorsey et al. [4] reported values greater than 2500 G and suggested than the level was a!ected by strain-induced anisotropy. Films below 1 lm thick show angular-shaped hysteresis loops (see Fig. 6). The shapes are similar to single crystal iron magnetised along a hard [1 1 1] direction [18]. YIG is a cubic ferrimagnet, with magnetocrystalline anisotropy constants, K "  !610 J m\ (!6.1;10 erg cm\) and K "  !5.1 J m\ (!51 erg cm\) [19]. The 11 1 12 directions are the easy axes for YIG, one easy direction lies normal to the (1 1 1) "lm and will be unfavoured due to the high demagnetisation factor but there are three easy directions &19.53 from the "lm plane. It is possible to magnetize in-plane up to a certain limit (M ) by growth of easy domains   lying along these three directions giving the initial steep section. Further magnetisation would need rotation away from the easy directions giving the second stage (harder) process. Assuming only the

4pM cos (19.5)(1#2 cos(60))/3. (1)  The projections of the easy directions lie at 1203 intervals in the (1 1 1) plane, for simplicity, it is assumed that the projection of one easy direction is parallel to the applied "eld and the other two therefore lie at 603 away from the applied "eld. This simplistic analysis predicts the easy magnetisation process could extend up to 0.63M . Table 4 shows  4pM , the coercive force and the measured values  of M /M for the S2-series "lms, M /M lies       between 0.33}0.57. These angular in-plane hysteresis loop results indicate that the YIG "lms are single crystal-like "lms where the magnetic easy direction does not lie in the "lm plane. The in-plane anisotropy for the YIG thin "lms were also checked using a square sample to ensure that the principal in-plane demagnetising factors were the same. The sample was rotated through angles of 03, 453 and 903 to the applied magnetic "eld and the hysteresis loops measured using the VSM. The hysteresis loops shown in Fig. 7 show no evidence of any anisotropic behaviour. In-plane measurement on the Ce-series "lms also show that the "lms have soft magnetic properties, however the "lms have broader hysteresis loops than the YIG "lms. The coercive force for the Ce(O ) and Ce(Ar) "lms is 35 and 65 Oe, respective ly. The saturation magnetisation for the Ce-YIG "lms are lower than the standard value for bulk YIG, see Fig. 5.

Table 4 4pM , the coercive force and the measured values of M /M     for the S2-series "lms

Fig. 6. Typical angular hysteresis loop shape for a YIG "lm less than 1 lm thick.

Sample

4pM (G) 

Coercive force ($0.5 Oe)

M /M   

S2.1 S2.2 S2.3 S2.4 S2.5

1630$320 1770$290 2030$300 1660$210 1500$90

12.2 14.7 7.8 8.2 7.2

0.52 0.52 0.57 0.43 0.33

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N.B. Ibrahim et al. / Journal of Magnetism and Magnetic Materials 220 (2000) 183}194 Table 5 The experimental out-of-plane demagnetisation factors for the YIG "lms Sample

D (experimental) 

S1.1 S1.2 S1.3 S1.4 S1.5 S2.1 S2.2 S2.3 S2.4 S2.5

0.67$0.05 0.81$0.04 0.96$0.05 0.88$0.03 0.75$0.02 0.98$0.19 0.76$0.12 0.77$0.11 0.93$0.12 0.81$0.11

(a ,), E is the Young's modulus, l is the Poisson's    ratio and j is a magnetostrictive constant. For  YIG, j is negative (!2.6;10\), E is   200 GPa, l is 0.29 and a value of K of 76 J m\   (0.76;10 erg cm\) has been reported [19]. The VSM results are consistent with the XRD spectra, which also indicate strained layers in the "lms. 3.4. Optical absorption spectroscopy

Fig. 7. Sample S2.5 in-plane hysteresis loops at di!erent orientations.

The out-of-plane magnetisation loops of the samples were also measured and experimental demagnetisation factors (D ) were estimated from the  gradients. Table 5 summarises the experimental demagnetisation factors for all of the YIG samples. The values are signi"cantly lower than 1, indicating that there could be some stress-induced anisotropy present which makes it easier to magnetise the "lm in the out-of-plane direction. The stress-induced uniaxial anisotropy constant, K is given by  3 *a E  j K "!  2 a 1!l   

(2)

where *a is the lattice di!erence between the substrate lattice parameter and "lm lattice parameter

The optical transmission spectra for bare SGGG substrate, YIG (sample S2.1) and Ce-YIG "lms were measured using a Varian DMS90 visible spectrometer. The transmission was calibrated as 100% at a wavelength of 900 nm with no sample present. Fig. 8 shows the optical transmission spectra (the discontinuity at 345 nm is caused by a changeover in the light sources). The absorption edge (de"ned as 50% of total transmission) of SGGG occurs at &300 nm but even thin YIG or Ce-YIG "lms raised this edge to above 400 nm showing that 308 nm is a suitable wavelength laser to use for PLAD of YIG as it is below the absorption edge. SGGG has a low absorption coe$cient and is almost transparent in the near infrared, the asreceived SGGG curve in Fig. 9 is almost #at down to 400 nm. The reduction in transmission to 88.5% is largely due to re#ection losses. The optical absorption for the YIG and Ce-YIG can be estimated by dividing the transmission coe$cient measured for the YIG and Ce-YIG thin "lm on SGGG

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Fig. 8. Optical transmission spectra of a bare SGGG substrate, S2.1, Ce(O ) and Ce(Ar) samples. 

Fig. 9. Estimated optical absorption coe$cients for S2.1, Ce(O ) and Ce(Ar) samples. 

substrate with the curve for bare substrate. The results shown in Fig. 9 indicate that YIG and CeYIG begin to absorb strongly below 500 nm. 3.5. Laser-induced target surface modixcations Laser-induced surface modi"cations to the ablation targets were studied using optical microscopy, SEM and EDS. The optical micrographs in Fig. 10 show three di!erent stages of ablation cone formation. Shallow ripple-like structures occur at the edge of the ablated area; they become deeper as the laser damage increases and eventually fully developed cones form. Figs. 11a and b show SEM micrographs of the cone structures on the YIG target at ;200 and ;1700 magni"cations, respectively. The surface has cone-like structures approximately 70 lm long

Fig. 10. Optical micrographs (;500) of cone formation stages: (a) shallow ripple like structures at the edge of the ablated area; (b) intermediate laser damage; and (c) fully developed cones with rounded tips.

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Fig. 12. SEM EDS X-ray analysis of YIG target: (a) before ablation; and (b) after ablation (&30 000 shots/site).

Fig. 11. SEM micrographs of YIG target showing cone structures at: (a) ;200; and (b) ;1700 magni"cations.

and 30 lm wide, the surfaces of the structures are smooth. The facetted structure is caused by the resolution of the digital photograph and printer. The higher magni"cation view show the cones have rounded surfaces with some evidence of material #ow indicating that the material has been in a liquid state. The ablation target was hit with &30 000 shots/site during the ablation process. The number of laser shots/site was calculated by multiplying the total number of laser pulses by the ratio of beam area to total exposed area. The cone axes are not normal to the surface; they appear to

be lying at an angle. Several researchers [20}22] have reported similar cone structures on YBCO targets, they observed that the cones point towards the laser beam. As the beam is incident at 453 to the target in the present work they would be expected to form at 453 to the surface of the YIG targets. EDS analysis was carried out on the fresh and ablated targets. Figs. 12a and b show the results for YIG, indicating that the ablated target contains more yttrium and less iron (49% Fe O : 51% Y O ) compared to the fresh target     (67% Fe O : 33% Y O ). This yttrium enrich    ment process may be due to the incongruent melting of YIG. A similar yttrium enrichment process has been observed on YBCO targets, which also melt incongruently [20,21]. Foltyn et al. [20] reported a drop in deposition rate of a factor of 4 after 1000 laser shots/site for YBCO targets. Krajnovich and Vasquez [23] reported that the

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reduction in deposition rate stops when cones have been completely formed on excimer irradiated polymers. YIG melts incongruently to form liquid and yttrium orthoferrite (YFeO ), however this process  requires di!usion of atoms and segregation can only occur if the heating rates are slow. Laser ablation is a very rapid heating process so the ablation plume always tends to have a composition very close to the start material, however the cooling process is not rapid. YFeO is the solid phase with  the higher melting point, this will tend to freeze "rst leading to yttrium enrichment and cone formation. It should be noted that the enrichment process occurs progressively over a large number of laser shots, the Y O enrichment per laser shot is only   4;10\ mol%. As the ablation rate decreases signi"cantly after 10 shots per site [20] the maximum deviation of the composition of the ablated plume from stoichiometry would be only 0.4 mol%. The cones have a larger surface area than the original target; the laser #uence is therefore lowered leading to a reduction in ablation rate. This e!ect could be seen during the ablation, there was a noticeable decrease in the size of the ablation plume after approximately half an hour. The laser beam was moved to di!erent spots whenever this happened during the ablation process. Almost identical cone structures formed on the surface of the Ce-YIG ablation targets, yttrium enrichment was also observed in this case.

193

that there are high levels of stress present in the "lms. The S2-series "lms, which were grown at a greater target to substrate distance, had fewer and smaller droplets. The "lms had saturation magnetisations close to bulk YIG, in-plane VSM results for the S2-series "lms show angular single crystal like hysteresis loops. The out-of-plane VSM measurements also show some evidence of strain in the samples. The (1 1 1) orientated YIG "lms appear to be magnetically isotropic in-plane. Optical spectroscopy of the "lms showed an absorption edge above 400 nm for YIG. Yttrium rich cone formations were observed on both the YIG and Ce-YIG ablation targets.

Acknowledgements The authors would like to thank colleagues in the Physics department for help with the analytical techniques. The Advanced Materials, Glass Ceramic and Crystallography groups assisted with the XRD and SEM analysis. J. Reed and A. Lovejoy provided technical support. Charles Dewhurst and Martin Lees are thanked for their help with the Oxford Instruments VSM. N.B. Ibrahim would like to thank the Malaysian Government and the Universiti Kebangsaan Malaysia for the study leave.

References 4. Summary It is only possible to grow uncracked YIG "lms on SGGG substrates up to 1 lm thick as the lattice mismatch is &1%, Ce-YIG has a better lattice match with SGGG and it is possible to grow thicker uncracked "lms. Ce-YIG "lms grown in argon by PLAD from oxygen rich targets produce greenish "lms indicative of cerium in the 3> oxidation state. Generally, YIG "lms thicker than 1 lm show several overlapping X-ray peaks indicating a layered structure in the "lms with decreasing lattice parameter tending to the value for bulk YIG. The XRD results can only be explained if the substrate/"lm structure bends upwards, this implies

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