CEMS study of H2 annealed YIG films

Journal of Magnetism and Magnetic Materials 62 (1986) 103-107 North-Holland, Amsterdam

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CEMS S T U D Y OF H 2 A N N E A L E D YIG FILMS G. BALESTRINO, S. LAGOMARSINO, A. TUCCIARONE and P. GERARD + Istituto di Elettronica dello Stato Solido, Via Cineto Romano 42, 00156 Roma, Italy + LE. T.L, Commissariat ~ l'Energie Atomique, 85,1(-38041 Grenoble, France Received 11 November 1985; in final form 19 June 1986

We report on a CEMS investigation of the surface layer obtained in pure YIG film after an annealing of 20 h in H 2 at 480°C. We show that the surface layer has a garnet structure with the hyperfine fields coinciding with those of pure YIG. The magnetization in the surface layer is found to be parallel to that of the underlying film bulk. Furthermore we analyze the line widths of the M~Sssbauer peaks pertaining to such surface layer and compare the results with those obtained for a YIG film implanted with two different doses of Ne +, namely 7×1013 Ne + cm -2 at 50 keV and 2.7×10 vt Ne + cm -2 at 50 keV. From this comparison it results that the peaks for the annealed film do not show any broadening while those for the implanted film are broadened. This clearly indicates that, within the experimental error, there is no damage in the surface layer obtained by annealing in H2 and, in any case, the damage is much lower than found in the surface layers of YIG films implanted with doses of practical interest.

1. Introduction

The possibility of creating in garnet films surface layers with magnetic and structural properties different from those of the fiLm bulk is of relevant interest in bubble garnet technology [1]. Such surface layers are currently obtained by ion implantation. In fact ion implantation changes the lattice constant in a thin surface layer. The lattice mismatch between the surface and the bulk of the film gives rise to a stress which through the magnetoelastic effect can turn the magnetization of the surface layer from perpendicular to in-plane. This effect has the useful consequences of eliminating the hard bubbles and creating patterns of easy propagation. Recently a similar change of the lattice constant in the surface layer has been obtained in yttrium-iron garnet films doped with Ca CYIG:Ca) by annealing in hydrogen atmosphere at temperatures higher than 450°C [2]. Subsequently such effect has also been found in pure YIG films [3]. In this case a YIG film grown by epitaxy from liquid phase on a gadolinium-gallium garnet substrate was annealed at 480°C in H 2. The surface

layer was then investigated by double axis X-ray diffractometry: in particular the strain profile in such layer was deduced from the analysis of the X-ray rocking curves [4]. The central result of the investigation carried out in ref. [3] was that, at variance with the implanted case, the surface layer has a constant strain. However, the method based on the analysis of the rocking curves, while very sensitive to the strain profile, is not very sensitive to the lattice damage level. On the other hand it has been shown by measurement on implanted YIG films that conversion electron MSssbauer spectroscopy is a sensitive tool to investigate the lattice damage within a surface layer about 1000 A thick [5]. In fact damage corresponds on microscopic scale to a change of the local environments around the iron ions. This effect causes some changes in the values of the hyperfine fields which show up in the CEMS spectra. One effect of damage is the decrease of the average magnetic internal field Him: eventually if the damage is high enough the film can become paramagnetic w i t h Hin t - - 0 . A n o t h e r effect which is even more sensitive to the damage is the broadening of the M/Sssbauer peaks: in fact disorder on a micro-

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104

G. Balestrino et aL / CEMS study of H 2 annealed Y1G films

scopic scale gives rise no longer to a well defined value of the hyperfine fields, both H i n t and quadrupole splitting QS, but rather to a whole range of values according to the local environments surrounding the 57Fe ions. This effect causes a broadening of the peaks which is of course larger the larger is the damage. Therefore the broadening of the MSssbaner peaks can be used as a sensitive probe of the damage level. In this paper we have used the CEMS technique to investigate the magnetic structure and the damage in the surface layer, obtained by annealing in H 2 in a pure YIG epitaxial film grown on a G G G substrate. The line widths of MiSssbauer peaks obtained for such layer have been compared both with those of pure Y I G and with those obtained for Y I G films implanted with Ne ÷. 2. Experimental

The Y I G film 2.5 I~m thick, was grown at Philips Eindhoven, by liquid phase epitaxy on a (111) oriented gadolinium-gallium garnet ( G G G ) substrate. The 57Fe content in the film was enriched up to 25% to improve the statistics in the CEMS measurements. The X-ray diffraction data have been taken with a double-crystal diffractometer. The (444) C u K a has been utilized. Fig. l a shows the diffraction curve of the as-grown film. Both the film and a)

substrate /~ 50"~ ~ grown

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Fig. 1. Rocking curves of the as-grown film (a) and of the film after the annealing in H 2 (b).

the substrate peaks are visible, therefore the lattice mismatch perpendicular to the surface Aa . could be directly measured, leading to A a . = - 8 . 6 7 × 10 -3 .~. Applying the necessary correction for the strain [6] the "relaxed" lattice parameter afoWaS obtained. The value a t = (12.3770 + 0.0001) A indicates that the Y I G film is highly stoichiometric. The film was then cut in two equal parts: the first half was used for the H 2 annealing experiments, the second half was ion implanted. The first half of the film was annealed in H 2 atmosphere at 480°C for 20 h. After this annealing treatment a new phase, analogous to that discussed in ref. [3], appeared with a contracted lattice parameter with respect to the as-grown film. The new phase shows up in the diffraction pattern (fig. lb) as a new peak close to the film peak on its high angle side. In the present case, because of the quite long annealing time (20 h) and the small thickness of the film, the surface layer has a thickness comparable with that of the film bulk. The relaxed lattice parameter of the surface layer was measured from the diffraction pattern and was found to be equal to: (12.3760 + 0.0001) A. The second half of the film was implanted with Ne + in two steps: first it was implanted with a dose of 7 × 1013Ne + cm -2 at 50 keV and then subjected to a further implantation with 2 × 1014Ne + cm -2 at 50 keV (the total implanted dose was, in this second case, equal to 2.7 × 10i4Ne + cm-2). As is well known [1], the ion implantation leads to the formation of a surface layer with an expanded lattice parameter with respect to the underlying as-grown film. We checked by X-ray double crystal diffraction the presence of the peak characteristic of this surface layer on the low-angle side of the diffraction curve [4]. The as-grown film as well as the annealed part and the implanted part were finally investigated by CEMS. A 2~r gas flow detector utilizing a mixture of 90% He and 10% CH 4 was constructed. In fig. 2a the CEMS spectrum is shown after the annealing treatment of H 2, whereas fig. 2b and c show the CEMS spectra after the implantation with a dose of 7 × 1 0 1 3 N e + cm -2 and 2 . 7 x 1014Ne + c m - 2, respectively.

G. Balestrino et al. / C E M S study o f H z annealed Y I G f i l m s

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VELOCITY fmmls) Fig. 2. CEMS spectra of the YIG film after the annealingin H 2 (a) and after the implantationwith 7 × 1013Ne+ at 50 keY (b) and 2.7× 1014Ne+ at 50 keV (c). 3. Results and discussion

The hyperfine fields pertaining to the surface layer obtained by annealing in hydrogen were deduced [rom the CEMS spectrum shown in fig. 2a. A CEMS spectrum, not shown here, of the

YIG film before the annealing process was also taken for purpose of comparison. The values of the hyperfine fields of the YIG film, both before and after the annealing process, are shown in table 1 together with the known values obtained by transmission MSssbauer technique. The values re-

G. Balestrino et al. / CEMS study of H 2 annealed YIG films

106

Table 1 Hyperfine fields for the surface layer and the bulk of the film

As-grown film After H 2 ann. Pure Y I G

Octahedral sites

Tetrahedral sites

IS * (mm/s)

Hin t (kOe)

IS * (mm/s)

Hin t (kOe)

0.38 + 0.03 0.37+0.03 0.39

487 + 6 487+6 483-491

0.16 5:0.03 0.16+_0.03 0.16

396 + 6 397+_6 396

* Referred to a-Fe.

ported in table 1 all agree within the experimental errors thus showing that the surface layer has a garnet structure. Furthermore from the intensity ratio R between the intermediate and the extreme peaks of the CEMS spectra we were able to calculate the angle O between the easy axis of magnetization and the direction of propagation of the incident ~, rays: in our experiment the direction of propagation of the y ray was perpendicular to the film surface and therefore parallel to the [111] direction. The formula R = 4 sin20/(3 + 3 cos20) was utilized to calculate the angle O. For the as-grown film the angle O was found to be equal to (70.6 + 2) °, thus indicating that the magnetization M is broken into domains along the three O l l ) directions not perpendicular to the film surface. This result can be understood on the basis of the following considerations: the MiSssbauer spectra are taken in zero magnetic external field, therefore the magnetization can be broken into domains. This effect strongly diminishes the effectiveness of the shape anisotropy. Therefore the easy axis is determined by the cubic anisotropy which, in YIG, makes all the (111) directions equally easy. However the shape anisotropy still acts as a perturbation on the cubic anisotropy choosing, among the four possible ( 1 1 i ) directions, those three which are close to the plane. The angle 0 calculated for the surface layer of both H E annealed and low dose Ne + implanted Table 2

F(channels)

As-grown

Annealed in H 2

Implanted (7×1013 Ne + )

Implanted (2.7×1014 Ne+ )

11.1+0.5

10.94-0.5

13.1+0.5

17.7+0.5

films comes out equal to that of the as-grown film: this result is easy to understand because the stresses involved are too small in this case to change the easy axis of magnetization. The easy axis stays along the same (111) directions even after the low dose implantation (fig. 2b)). On the other hand, after implantation with 2.7 × 10laNe ÷ cm -2 the induced stress is high enough to bring the magnetization of the surface layer in plane. The width of the MiSssbauer peaks, as mentioned above, is very sensitive to the damage. Therefore the average full width at half maximum of the peaks has been calculated from the CEMS spectra. The average has been carried only on those peaks which are well resolved, namely the octahedral peaks ( I = 3/2, m t = - 3/2) ~ ( I = 1/2, m t = - 1 / 2 ) , ( I = 1/2, rn I = 1/2) --* ( I = 1/2, r n t = 1/2) and the tetrahedral peaks ( I = 3/2, r n t = 3/2) ~ ( I = 1 / 2 , mr=l/2), (I= 3/2, m I = 1/2) ~ ( I = 1/2, m t = 1/2) and ( I = 3/2, m t - - - 3 / 2 ) ~ ( I = 1 / 2 , r o t = - 1 / 2 ) . The value of F obtained from the CEMS spectrum of the surface layer after the annealing in H E was compared with those obtained for the film as grown and implanted with the two doses of Ne ÷. The values of P are reported in table II: the value of r for the film annealed in hydrogen is the same as for the as-grown film while it is smaller than the values obtained after implantation even w i t h the lower dose of neon. This result shows that the surface layer obtained by annealing in H E is undamaged within the experimental error.

4. Conclusions We have investigated by CEMS technique the surface layer which is obtained in Y I G film after annealing in hydrogen atmosphere at temperatures above 450°C. It has been shown that the hyperfine fields of such layer coincide with those of pure YIG. It has also been demonstrated that the lattice damage in such surface layer is absent or, in any case, very low in comparison with that induced by the implantation processes. Furthermore we have found that the direction of magnetization in the surface layer is parallel to the direction of magnetization in the underlying film bulk:

G. Balestrino et al. / CEMS study of H 2 annealed YIG films

this can be understood because in this case the lattice mismatch between the surface layer and the film bulk is not large enough to change the easy axis of magnetization. However this m a y not be true in general. Hence it seems worthwhile to f u r t h e r investigate the possibility of controlling the lattice mismatch between such surface layer and the film bulk, along with the magnetoelastic constants of the film for the purpose of modifying also the magnetic properties of the surface.

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References [1] P. Gerard, Thin Solid Films 114 (1984) 3. [2] B. Antonini, C.D. Brandle, S. Lagomarsino, A. Paoletti, P. Paroli and A. Tucciarone, L Appl. Phys. 55 (1984) 2179. [3] G. Balestrino, S. Lagomarsino and A. Tucciarone, L Appl. Phys. 59 (1986) 424. [4] V.S. Speriosu, H.L. Glass and T. Kobayashi, Appl. Phys. Lett. 34 (1979) 539. [5] P.H. Smit, H.A. Algra and J.H. Robertson, L Appl. Phys. 52 (1981) 2364. [6] S. Lagomarsino and A. Tucciarone, Thin Solid Films 114 (1984) 45.