Nuclear Instruments and Methods 209/210 (1983) 1179-1185

North-Holland Publishing Company

1179

ION IMPLANATION EFFECTS IN BUBBLE GARNET MATERIALS

A. PEREZ ‘) and G. MAREST ‘) I’ Dt!partement de Physique des MatPriaux,

-‘) Institute de Ph_wque Nucl&u-e, Unioersitt! Claude Bernard Lyon-l, 69622 Villeurhanne Cedex, France

P. GERARD, M. MADORE and P. MARTIN L. E.T.I.-Commissariat d I’Energie Atomique, 85X, 38041 Grenoble, France

(YSmLuCa),(FeGe),O,, garnet films grown by liquid phase epitaxy on a GGG substrate have been implanted at room

temperature with 6 X IO ” “Fe+ ions at energy 100 keV. Ion channeling and conversion electron Mossbauer spectroscopy techniques

have been used to characterize the damage and the implanted ions as well as their annealing behaviour at temperatures up to 800°C.

Directly after implantation, the implanted layer is amorphized and the iron atoms are observed in three forms: metallic precipitates,

Fe” and Fe’+ ions. For annealings at temperatures up to 5OO’C. the oxidation of iron into Fe’+ species takes place but the system

remains paramagnetic down to 77 K. At 650°C the crystal recovery is partially achieved and a fraction of the iron ions is observed in

octahedral (a) and tetrahedral (d) sites of the garnet. After annealing at 800°C, a complete recrystallization of the implanted zone is

obtained and the majority of the implanted ions is distributed in a and d sites. However, at this stage a fraction of the implanted iron

is precipitated in the form of wFe,O, particles

1. Introduction

Ion implantation in magnetic bubble garnets has been used up to now to suppress hard bubbles and also to produce propagation patterns for con- tiguous disk memory devices. The implantation process creates damage that expands the lattice, resulting in a lateral compression. This stress, through a phenomenon of magnetostriction in the

garnet, produces a change in the direction of mag- netization from perpendicular to parallel to the implanted surface layer. To achieve this process for a given material, it is important to limit the damage to a reasonable level so as to maintain the crystal in a relatively good state. Excess damage entails a decrease of the exchange constant and thus of the magnetization [l]. Since the beginning [2] most of the studies were limited to low im- planted ion doses ( < lOI ions. cme2) and to the determination of the ideal dose for each species. In the particular case of neon ion implantations, the ideal dose has been determined around 2 X lOI Ne+. cm-2 [3] from comparisons of stress mea- surements and of the variation of the collapse field as a function of the dose. For higher doses evi- dence of the presence of a non-magnetic layer has been provided [4,5]. The formation of this para-

magnetism has recently been investigated using

conversion electron Mossbauer spectroscopy (CEMS) [5,6], X-ray and ferromagnetic resonance (FMR) measurements [7] on heavily implanted garnet layers with neon ions. Another kind of annealing behaviour has been observed for these samples compared with those of low dose im- planted garnets. This new behaviour was the sub- ject of a previous paper [S] in which FMR studies of high dose implanted garnet films with heavy ions (Fe+, Ga+, As+) were presented. From these results, a model of the damaged zone has been proposed as well as the rebuilding mechanism which takes place through a solid phase reepitaxy in the 400-800°C temperature range. Moreover, the specific effect of iron implantations compared to those of Ga and As has been pointed out. In this case a complementary study using the CEMS technique applied to garnet films implanted with high doses (6 X lOI ions. cme2, energy 100 keV) of 57Fe+ ions has been performed. This paper is concerned with these results. In the 57Fe Moss- bauer effect, the conversion electrons emitted have an energy of 7.3 keV and thus the great majority will only emerge from within - 1000 A from the upper surface. This is typically the depth in which 100 keV 57Fe+ ions are distributed. In our case the

0167-5087/83/0000-0000/$03.00 0 1983 North-Holland VIII. INSULATING MATERIALS

1180 A. Perez et al. / Bubble gumet materials

CEMS measurements allow to characterize only the implanted ions and their annealing behaviour during the recrystallization process of the im- planted zone. In parallel with the CEMS study, ion channeling measurements have been carried out in order to follow the regrowth process of the implanted layer. In section 2, the experimental conditions are described. Section 3 concerns the ion channeling and CEMS measurements per-

formed after implantation of the garnet sample with a dose of 6 X lOI ions. cm-* and after an- nealings at some characteristic temperatures (400, 500, 650 and SOO’C). These temperatures were deduced from the previous FMR study [8] and correspond to particular stages of the recrystalliza- tion mechanism.

2. Experimental procedure

2.1. Sample preparation

The garnet films with the composition (YSmLuCa),(FeGe),O,, were grown by liquid phase epitaxy on a gadolinium gallium garnet sub- strate (Gd,Ga,O,,) oriented with a (111) surface plane. The exact composition determined by a microprobe analysis is Y,,,,Sm,,,,Lu,,,,Ca,,,

Fe,.,, Ge0.,Pb0.0,G,2. The basic parameters of the

as grown films are: thickness h = 3.5 pm, bubble diameter d= 3.3 pm, bubble collapse field H,, = 158 Oe, characteristic length I = 0.37 pm, satura- tion magnetization 47rM, = 300 G and NCel tem- perature TN = 205°C.

Implantations with 57Fe+ ions in the dose range

10’h to 10” ions. cmm2 were performed at room

temperature using the isotope separator of the “Institut de Physique Nucltaire de Lyon”. The beam energy was 100 keV and the current density was maintained rather low (- 1 PA. cme2) in order to limit thermal effects during implantation. The samples were scanned to obtain an homoge- neously implanted surface of 1 x 2 cm2.

2.2. Ion channeling and CEMS measurements

The ion channeling measurements were per- formed at room temperature (RT) using a 1.5 MeV alpha particle beam produced in the 2.5 MeV Van de Graaff accelerator of the “Departement de

Recherche Fondamentale de Grenoble”. The back- scattered ions were detected at an angle of 165”. The sample used for this study was implanted with a dose of 6 x 10 ” “Fe+ ions. cm -* at an energy of 120 keV.

The conversion electron Mossbauer spectrome- ter design is similar to that described by Massenet

et al. [9]. The Mossbauer spectra were performed at RT and liquid nitrogen temperature (LNT). A constant-acceleration triangular drive was used and the data folded to provide a constant background. The source was 100 mCi 57Co in a rhodium host.

3. Results

3.1. Expitaxial regrowth of the implanted layer

We know that after implantation in a high dose range (> lOI ions. cme2) the implanted layer is amorphized but subsequent annealings, performed in a pure oxygen atmosphere at temperatures up to 800°C allow a complete recrystallization of the damaged zone. This is well illustrated by the an- nealing behaviour of the channeling spectra. In

fig. 1 are reported the (111) aligned spectra mea- sured on a 6 X lOI 56Fei ions. cm-* implanted sample and subsequently annealed for one hour at temperatures of 400, 550, 650 and 800°C. Let us focus our attention on the high energy part of the spectra extending from the Sm/Lu front to the Y surface front: this region corresponds to an analyzed depth of about 1000 A. The energy- depth conversion is calculated using Ziegler’s data [lo] and Bragg’s rule for compounds. It corre- sponds also nearly to the depth concerned by the implantation with 120 keV 56Fe+ ions: projected range R, = 460 A and AR p = 150 A deduced from the L.S.S. theory [ 1 I]. This implanted layer is fully damaged and it remains so after annealings at temperatures of 400 and 550°C. The crystal re- covery is partially achieved after annealing at 650°C and is completed after annealing at 800°C. Thus from the channeling measurements one can conclude that a solid phase epitaxial regrowth of the implanted layer takes place in the 550 to 800°C temperature range. This recrystallization process starts from the bottom of the implanted zone and propagates towards the surface with increasing temperatures.

A. Perez et al. / Bubble garnet materials

4

t

I

0 230

ie* 1.5 MeV

I 330 430 530 630 i IO

Channel number (1.9 kevlch.)

1181

Fig. 1. Ion channeling measurements in the [ 1111 direction of the garnet films implanted at room temperature with 6x 10” 56Fe+

ions cm -* at an energy of 120 keV and subsequently annealed at temperatures of 400, 550,650 and 800°C. These measurements have

been performed with 1.5 MeV alpha particles.

3.2. Annealing behaviour of implanted iron ions

studied by CEMS

In fig. 2 are reported the conversion electron Mossbauer spectra measured at RT on the garnet sample implanted with 100 keV 57Fet ions and a dose of 6 X 10lh ions. cm-*. The spectra were recorded after implantation and after isochronal (1 h) annealings performed in a pure oxygen atmo- sphere at temperatures of 400, 500,650 and 800°C. The Mossbauer spectra measured at LNT, on the same sample, after each annealing step, are pre- sented in fig. 3. All the spectra have been com- puter fitted and the best fits obtained are repre- sented by the solid lines in fig. 2 and 3. The parameters of these fits are summarized in tables 1 and 2 for the RT and LNT spectra, respectively. All the isomer shifts (IS) are given with respect to metallic iron at RT. These results show that after implantation at RT, iron is observed in three different forms: metallic precipitates, Fe*+ and

Fe3+ ions. The metallic precipitate component which represents about 23% of the spectra is di- vided into two parts. A small part which behaves superparamagnetically down to LNT is responsi- ble for the single line with an isomer shift (IS) close to 0 mm. s- ‘. The second is at the origin of the magnetic sextuplet. In the particular case of the sample implanted with 6 X lOI ions. cm-*, the amplitudes of the lines of the magnetic sextuplet due to metallic iron are quite small. However, recent measurements done on a 10” ions. cm-* implanted crystal confirm undoubtly the presence of this sextuplet. The coexistence of these two components is certainly due to the distri- bution in size of small metallic particles precipi- tated directly during implantation at RT. From these measurements the radius of the largest pre- cipitates can be estimated to be around 40 A. The two paramagnetic components present in the spec- tra in the form of quadrupole doublets with IS,, =0.83 rnrn.s-‘, QS.r= 1.48 mm.s-’ and IS,,

VIII. INSULATING MATERIALS

1182 A. Perez et al. / Bubble garnet mcrreriuls

:il 5o:_,.. A:... ,:

0 5 10 Velocity (mm.&)

Fig. 2. Conversion electron Miissbauer spectra measured at

room temperature with a garnet sample implanted with 6 x 10’” 57Fe+ ions.cm-* at an energy of 100 keV (a) and subsequently

annealed in a pure oxygen atmosphere for one hour at tempera-

tures of 400°C (b). 500°C (c), 65O’C (d) and 800°C (e).

=0.31 mm.s-‘, QS,,= 1.03 mm.s-’ could be attributed to Fe2’ and Fe3+ ions respectively. The Fe3+ doublet can be compared to the one (IS,, = 0.31 mm. s- ‘, QS,, = 1.12 mm . s- ’ ) correspond- ing to Fe 3t ions in splat cooling amorphous YIG as observed by Gyorgy et al. [12]. It can also be compared with the doublet obtained with super- paramagnetic Fe,O, particles at RT [ 131.

After annealing for 1 h at 400°C in a pure oxygen atmosphere, the Mossbauer spectrum is completely modified (figs. 2b and 3b). At this step it can be fitted with only a quadrupole doublet

Annealed 400°C

b

1.05

1

1.1

1.05

1 -10 -5 0 5 10

* I

Velocity (mm.s-I)

d

Fig. 3. Conversion electron Miissbauer spectra measured at

liquid nitrogen temperature with a garnet sample implanted

with 6~ lOI 57Fet ions,cm-*, at an energy of 100 keV (a)

and subsequently annealed in a pure oxygen atmosphere for

one hour at temperatures of 400°C (b). 500°C (c). 650°C (d)

and 800°C (e).

characteristic of Fe3’ ions (IS,, = 0.31 mm. s- ‘. QS,, = 1.20 mm. s-l). Thus the main effect of the annealing at 400°C consists in the oxidation of the metallic precipitates and the Fe*+ ions present after implantations. No significant change is ob- served after annealing at 500°C (figs. 2c and 3~). However, after annealing at 650°C substantial changes appear in the spectra (figs. 2d and 3d). In addition to the paramagnetic quadrupole-split pat- tern located in the middle of the spectra (IS,, = 0.31 mm.s-‘, QS..= 1.21 mm. s-l), we observe two magnetic hyperfine-split patterns with differ-

Tab

le

1 M

ossb

auer

pa

ram

eter

s ob

tain

ed

by

com

pute

r fi

tting

of

th

e sp

ectr

a re

cord

ed

at

room

te

mpe

ratu

re

(fig

. 2)

. IS

: is

omer

sh

ift,

W:

full

wid

th

at

half

-max

imum

of

th

e lin

es,

QS:

quad

rupo

le

split

ting

and

HF:

hy

perf

ine

fiel

d.

Sing

le

line

Qua

drup

ole

doub

lets

M

agne

tic

sext

uple

ts

IS

W

Rel

ativ

e IS

W

Q

S R

elat

ive

IS

W

HF

Rel

ativ

e

(mm

.s-‘

) (m

m.s

K’)

in

tens

ity

(mm

.ss’

) -I

(m

m.s

)

(mm

.s-

‘)

inte

nsity

(m

m.s

s’)

(mm

.s-‘

) (k

Qe)

in

tens

ity

(W)

(%)

As

impl

ante

d -

Ann

eale

d

1 h

- 4O

O’C

Ann

eale

d

I h

-5O

O’C

Ann

eale

d

I h

- 65

0°C

Ann

eale

d

1 h -

80

0°C

0.09

0.

20

1 0.

83

0.48

1.

48

21

- 0.

02

0.98

33

0 22

0.3

1 0.

60

1.03

56

0.31

0.

57

1.20

10

0

0.33

0.

60

1.22

10

0

0.3

1 0.

60

1.21

61

0.

37

0.74

42

5 14

0.16

0.

79

364

2s

0.37

0.

84

440

35

0.15

0.

77

370

52

0.34

0.

44

511

13

Tab

le

2

Mos

sbau

er

para

met

ers

obta

ined

by

com

pute

r fi

tting

of

the

sp

ectr

a re

cord

ed

at l

iqui

d ni

trog

en

tem

pera

ture

(f

ig.

3).

IS:

isom

er

shif

t, W

: fu

ll w

idth

at

ha

lf-m

axim

um

of t

he

lines

, Q

S:

quad

rupo

le

split

ting

and

HF:

hy

perf

ine

fiel

d.

Sing

le

line

Qua

drup

ole

doub

lets

M

agne

tic

sext

uple

ts

IS

W

Rel

ativ

e IS

W

Q

S R

elat

ive

IS

W

Hf

Rel

ativ

e (m

m.s

s’)

(mm

.s-

‘)

inte

nsity

(m

m.s

-‘)

(mm

.s-t

) (m

m.s

K’)

in

tens

ity

(mm

.s-‘

) (m

m-s

-‘)

(kQ

e)

inte

nsity

(%)

(S)

As

impl

ante

d 0.

16

0.43

6

1.08

0.

75

1.61

32

0.

15

0.80

35

2 16

0.49

0.

61 ’

1.

20

47

Ann

eale

d 0.

42

0.70

I .2

9 10

0

1 h

- 40

0°C

Ann

eale

d 0.

44

0.71

1.

31

100

I h

- 50

0°C

Ann

eale

d 0.

42

0.80

1.

30

49

0.48

0.

67

515

17

1 h

- 65

0°C

0.

26

0.89

45

0 34

Ann

eale

d 0.

44

0.48

53

0 35

1 h

- 80

0°C

0.

24

0.67

45

2 47

0.49

0.

56

510

18

ent IS (0.37 and 0.16 mm. s- ’ at RT) and hyper- fine fields (HF) (425 and 364 kOe at RT). These parameters correspond rather well to those of iron ions located in octahedral sites (a sites) and tetra- hedral sites (d sites) of the garnet films [5,14]. In the RT spectrum the relative fractions of these two components are 14% (a sites) and 25% (d sites). Concerning the Fe3+ component which remains

paramagnetic (6 l%), its parameters (IS,, = 0.3 1 mm . s- ‘, QS,, = 1.21 mm. s- ‘) are identical to

those of the component which existed in the spec- tra measured after annealing at 400 and 500°C. The Mossbauer spectra continue to evolve with increasing annealing temperature and at 800°C the

Fe3+ paramagnetic pattern has completely disap- peared (figs. 2e and 3e). At this step the spectra are fitted with a sum of three magnetic sextuplets, the parameters of which are: IS,, = 0.37 mm. s- ’ - HF,, = 440 kOe, IS,, = 0.15 mm. se’ - HF,, = 370 kOe and IS,, = 0.34 mm. s- ’ - HF,, =

511 kOe. The relative intensities of these three components are 35%, 52% and 13% respectively. The first two sextuplets are due to Fe3+ ions in a and d sites but their relative intensities have in- creased by a factor of - 2.3 between 650°C and 800°C anneals. However, the ratio of the fractions in the two sites remains nearly constant (% a/% d = 0.6). Concerning the third sextuplet which ap- pears clearly in the RT spectrum (fig. 2e), its parameters (IS,, = 0.34 mm. sp ‘, HF,, = 511 kOe) are in good agreement with the measured values for Fe3+ in a-Fe,O, (IS,, = 0.38 mm. sp ‘, HF,, = 515 kOe) [15].

4. Discussion and conclusions

The association of ion channeling, conversion electron Mossbauer spectroscopy and ferromag- netic resonance allows a quite good characteriza- tion of iron implantation effects in bubble garnets. The results reported in this paper confirm the mechanisms proposed previously [8] and give com- plementary information on the implanted iron zone ( - 1000 A) and its annealing behaviour. The ion channeling study clearly indicates that the im- planted layer is amorphized after implantation and recrystallizes through a solid phase epitaxial mechanism in the 500-800°C temperature range. This is corroborated with the CEMS measure- ments.

In amorphous implanted unannealed layer, the

presence of precipitated phases and isolated iron ions can be considered. Concerning the precipi- tated phases, one is clearly identified as being metallic iron aggregates. The Fe3+ ions could also be found in Fe,O, particles or dispersed in an amorphous environment.

From the first annealing steps at 400 and 500°C, all the iron species are oxidized in the form of

Fe3+ ions in a matrix which remains paramagnetic down to LNT. Between 500 and 650°C annealing, the appearance of magnetic-split patterns char- acteristic of iron ions distributed in octahedral and tetrahedral sites confirms the partial recrystalliza-

tion of the layer as observed in channeling and FMR studies. At 8OO”C, the recrystallization is complete and the majority of iron ions is located in a and d sites of the garnet. However a small fraction is precipitated in the form of u-Fe,O, particles sufficiently large to get the magnetic pat- tern at RT (HF = 5 11 kOe). The HF values de- duced for a and d sites are in good agreement with those given by Skrimshire et al. [14] and Picone et al. [5] for different as grown bubble garnets.

As shown in our previous studies [8], the garnets films highly implanted with iron ions and rebuilt thermally at 800°C exhibit very homogeneous magnetic properties and are also characterized by an in-plane magnetization. This last property is very beneficial for magnetic bubble memory de- vices. Future CEMS measurements with different iron ion doses and at liquid helium temperature would be very helpful to improve our characteriza- tion of iron implantation effects in garnet.

We wish to thank Dr. E. Ligeon for his assis- tance in channeling measurements. We are inde- bted to Prof. J. Sawicki (Univ. of Cracow, Poland) for fruitful discussions.

References

[1] C.H. Wilts. IEEE Trans. Mag. 17 (1981) 5.

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(61 P.J. Picone and A.H. Morrish, J. Appl. Phys. 53 (1942) 2471.

A. Perez et al. / Bubble garnet materials 1185

[7] W.H. De Roode and H.A. Algra, J. Appl. Phys. 53 (1982)

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[ 1 l] J. Lindhard, M. Scharff and H.E. Schiott, Kg1 Dan. Vid.

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VIII. INSULATING MATERIALS