Implantation of bubble garnets

Thin Solid Films, 114 (1984) 3-31 ELECTRONICS AND OPTICS

IMPLANTATION OF BUBBLE GARNETS P. GERARD Laboratoire d'Electronique et de Technologie de l 'Informatique, Centre d'Etudes Nucl~aires de Grenoble, Avenue des Martyrs 85)(, 38041 Grenoble C~dex (France)

A review is given of ion implantation in bubble garnets. After a short introduction to the material, the implantation effects and the most frequently used characterization techniques are related briefly. Then a general survey is made of three different kinds of implantation, which are neon, hydrogen and high dose implantations. Neon implantation only requires a very low dose, and therefore the effects are restricted to pure damage. Hydrogen implantation, which gives rise to an abnormally high anisotropy, is considered separately on account of its sustained interest for small bubble devices. Last but not least, garnets implanted with high doses of heavy ions and successively annealed at high temperature are dealt with. The last subject is new and promising as it can be applied to alter locally the garnet properties through doping.

1. INTRODUCTION

During the last 10 years since the excellent review article of North and Wolfe1 showing the effects of ion implantation in magnetic bubble materials, many research groups interested in this field have contributed to this subject. The goal of ion implantation is to suppress hard bubble formation and to make propagation patterns for memory displays by the creation of an in-plane magnetization in a thin film above the bulk layer supporting the magnetic bubbles. Research on implantation effects in ferrimagnetic garnets is tending towards neon, hydrogen and high dose implantation. Until recently work has focused on neon implantation, the energy for this being compatible with industrial implantation systems and the dose required to obtain the in-plane magnetization being low. Moreover, neon is inert and does not involve complications with the garnet as other chemically active elements do. The ideal dose to suppress hard bubbles is around 2 x 1014 Ne ÷ ions cm -2 (refs. 2-6). Komenou e t al. 7 have shown, by progressively etching the damaged layer, that part of this very implantation is paramagnetic. This observation has given rise to great interest in the formation of paramagnetism in garnets. Another important observation made by Hirko and Ju s has also oriented a whole series of current research activities. It is the abnormally high variation in anisotropy resulting from the hydrogen implantation, this peculiar 0040-6090/84/$3.00

© Elsevier Sequoia/Printed in The Netherlands

4

P. GERARD

behaviour being beneficial to the new generation of high density displays. A third tendency has proceeded from studies of paramagnetism by progressively increasing doses 9'1° until the implanted layer is saturated with defects and has become amorphous. This highly damaged garnet layer is then recrystallized by means of high temperature annealing 9-12. By using this implantation technique it is possible to introduce into the garnet a wide range of atomic species, thus making it possible to obtain a specific dopant effect 11 for new applications. The conjunction of complexities on the one hand of the crystalline structure I 3 and on the other hand of radiation damage effects demands the association of several well-adapted and complementary techniques to obtain a better insight into ion implantation in ferrimagnetic garnets. The literature is filled with contributions aiming to correlate either directly or indirectly the lattice disorder, the implanted impurity distribution and the magnetic properties. Among these are the following" (1) for lattice disorder measurements, X-ray diffraction 1'7'1° 12,14-23, relative etching rates 4'6'24"25, stress 5'26'27, Rutherford backscattering and channelling 2s-31, transmission electron microscopy (TEM) and electron diffraction32'33 and optical absorption34; (2) for impurity profile measurements, secondary ion mass spectrometry (SIMS) 11 and nuclear reaction31'35; (3) for magnetic measurements, bubble collapse field 2'3"5 7,30,36, drive field4, ferromagnetic resonance (FMR)8,10-12,17,19,23,35,37 4s, a.c. susceptibility49-51, vibrating-sample magnetometry (VSM) 37'52 35, conversion electron M6ssbauer spectroscopy (CEMS) 9'2s'56-6° and magneto-optics 55'61'62. The ideal study would be to characterize completely implantation effects in a garnet of a given composition. It would be to determine factors such as the range distribution of the implanted ions, the concentration and the nature of the structural disorder as functions of the depth. It would be to define their respective influences on the magnetic properties and also to follow the evolution of these parameters after subsequent annealing treatments. Unfortunately, this outcome has seldom been possible, mostly on account of slight modifications of technical (or technological) aims in the course of the study. In order to find the common features among the studies made with garnets of different compositions, it is necessary to spend a little time on bubble garnet material, stressing the parameters concerning implantation effects. It is also important to spend some time on ion implantation, confining ourselves to generalities of implantation in garnets. First we shall consider the effects desired for making displays. Next we shall look at all the secondary effects due to structural damage or to specific actions of the implanted impurity before or after annealing treatments. The separate sections are presented as follows. In Section 2 general remarks on bubble garnet materials are given. Section 3 is on implantation effects in bubble garnets. A quick description of the information obtained by the different tools used to study damaged garnet layers is given in Section 4. In Section 5, only defects with no influence of the implanted impurity are considered; this relates back to earlier studies on neon implantation. In Section 6 we take bearings on the peculiar behaviour of the hydrogen implantation. In Section 7 we look at annealed garnet layers which have previously been highly damaged with heavy ions, laying stress on the dopant effects.

IMPLANTATION OF BUBBLE GARNETS

2.

5

BUBBLE GARNET MATERIAL

Implantation in garnets has mainly been applied to the bubble memory field, so all the experiments described hereafter will be limited to magnetic bubble material 63. This is a rare earth (RE) iron garnet of formula {RE3}[Fe2](Fe3)OI2 where { }, [] and 0 stand for dodecahedral (c), octahedral (a) and tetrahedral (d) sites respectively. The ferrimagnetic garnet film is grown by liquid phase epitaxy on a "non-magnetic" Gd3GasOi2 (GGG) substrate. Apart from one recent exception 64 the film orientation is (111). The film thickness h for a given bubble diameter d and the quality factor Q are defined by Thiele's criteria 65: h = d = 91 = K~ Q= ~

9(AK~)1/2 riM2

HK = 4n-----~~> 1

(la) (lb)

where I is the characteristic length, K u the uniaxial magnetic anisotropy, 2riMs2 the demagnetization energy, HK = 2Ku/Ms the uniaxial anisotropy field and 4riMs the saturation magnetization. Q expresses the necessary condition for there to be a unique easy axis of magnetization perpendicular to the plane of the film. A depends on the relative concentrations of Fe a ÷ ions in a and d sites and on the angle formed between these sites. Above the compensation temperature 4riM,, because of the weaker coupling of c sites, is essentially the algebraic sum of the two iron sublattices. For reducing the magnetization of iron garnets non-magnetic diluents such as gallium or germanium are added which replace preferentially iron ions on d sites. Ku is derived from two mechanisms: a growth-induced anisotropy KG and a stressinduced anisotropy K s. The properties of K , are monitored by mixing, in a suitable proportion, several well-chosen REs on c sites. Bubble materials have to be "implantable". This condition requires the presence of ions such as samarium, dysprosium and thulium that impart to the garnet a large negative magnetostriction 2111. KG is the predominant factor for perpendicular magnetization. Although its origin is still poorly understood 66, it seems to be correlated with ion pair formation between REs of different atomic radii, e.g. Sm-Lu. In looking at implantation effects in garnets, it is of paramount importance to take into account interactions between ions on different sublattices. As well as the interactions between a and d sites already mentioned above, atoms such as samarium have a large orbital contribution inducing, as a result of interactions with d sites, a substantial magnetic damping ~tto the garnet, in addition to magnetostriction. However, these parameters are smaller or null for ions with little or no angular momentum (yttrium, gadolinium or lutetium). 3.

IMPLANTATION EFFECTS

Here, the ion beam is always chosen to be unaligned to any low index direction of the crystal. Hence there is no channelling effect, and from the ion trajectory point of view the target can be dealt with as if it were amorphous. The ion penetrating into the target slows down progressively until it comes to rest at a certain distance from

6

P. GERARD

the surface 67. In its path it has interacted with the atoms of the solid creating a certain number of different defects. The local concentrations and the kind of implanted impurities and defects in a given material are both dependent on the implanted species and on its energy and its dose. 3.1. Implanted ion distribution 67

The description of the slowing down of the ion is based on the theory of Lindhard, Scharffand Schiett ( L S S ) 68. LOW energy ions are mainly stopped through nuclear collisions. These are elastic interactions (nuclear stopping power). High energy ions lose their energy mainly via electronic collisions in which the moving ion excites or ejects electrons. These are inelastic interactions (electronic stopping power) involving little energy loss per collision, little deflection and negligible lattice damage. The relative importance of the two energy loss mechanisms, considered in the theory as being independent, changes rapidly with the energy E and the atomic number Z t of the incident ion. Electronic stopping predominates for high E and low Z 1, whereas nuclear stopping takes over for low E and high Z r The distance travelled between collisions and the energy transferred per collision of an implanted ion are random variables. Hence a given ion implanted at a given incident energy does not have the same range. There is a broad statistical distribution or straggling effect of the implanted ions in the target. This is referred to as the range distribution which for heavy ions or low incident energies is roughly gaussian and was characterized by LSS 6s in terms of a mean projected range Rp and a projected standard deviation ARp. For light ions 2~ the distributions are skewed towards the surface and then the implant profile is represented by Pearson type IV functions 69. These calculated values as well as the energy losses of most implanted ion species in all types of monoatomic targets corresponding to a large incident energy scale (0 MeV < E < 1 MeV) are given in tablesT°'TL F r o m these tables it is possible to define the range distribution of an implanted ion in a polyatomic target (the number of different elements has to be rather small) knowing the energy and dose of the incident ions and the relative abundances of the elements in the compound. With bubble garnets this is quite a complicated matter because they contain many different atomic species. For simplicity a calculation program can be devised so as to treat the garnet as a ternary compound of formula RE3XsO~2. This modelling has been done in our laboratory with a garnet often used to study implantation. Its composition determined by microprobe analysis and some of its basic characteristics are shown in Table I. The heavy elements of this garnet, yttrium, samarium, lutetium and lead, are averaged in RE and the elements iron, germanium and calcium are averaged in X of TABLE I SOME BASIC CHARACTERISTICS OF THE AS-GROWN FILM OF COMPOSITION YI 765mo.24LUo, lsCao.96Feg.06Geo.sPbo.02Ol 2

Thickness h (p.m)

3.5

Bubble diameter d

Collapse .field Hco

Characteristic length l

(pm)

(Oe)

(pm)

3.3

158

0.37

4nM~

Curie

(G)

temperature Tc ('C)

300

205

7

IMPLANTATION OF BUBBLE GARNETS

the ternary model. The characteristics of this model necessary for the determination of statistical moments and for the nuclear energy loss rates obtained from LSS tables are given in Table II. TABLE II SPECIFICATIONSOF RE3XsOI2 REPRESENTATIVEOF THE COMPOUNDOF TABLE I

Atom

Number of atoms per formula unit

Atomic mass M

Atomic number Z

Specific gravity p

Number of atoms per unit volume

RE X O

3 5 12

83.6 58.6 16

36.78 26.98 8

1.748 2.042 1.338

1.26 × 1022 2.1 × 1022 5.03 X 1022

3.2. Damage and strain distribution 72 It has been found that strain and damage are proportional to the nuclear energy loss 1. An implanted ion makes on its trajectory a number of collisions with the lattice atoms. If the transferred energy during the nuclear collision is sufficient, i.e. greater than the displacement threshold energy Eo which is around 25 eV, it may be possible for an atom to be displaced from its site. The displaced atom can in turn displace other atoms thus creating a cascade of atomic collisions. The size and character of a cascade depends on a large number of factors, including the mass of the incident ion and its energy, the mass of the target atoms and the temperature of the target. This leads to a distribution of vacancies, interstitial atoms and other types of lattice disorder around the ion tracks. The number of displaced target atoms per unit volume has been calculated by Kinchin and Pease 73. Sigmund and Sanders TM have produced a damage theory from which it is possible to estimate the distribution energy deposited into atomic processes. Their results combined with LSS computations provide reasonable estimates of the mean and standard deviations of the actual damage distribution when annealing is inhibited while implanting. MacNeal and Speriosu 21 have determined a strain distribution by a calculation based on known stopped ion distributions, making the assumption that the resulting strain is proportional to the energy lost during nuclear collisions. The calculated strain is then matched to measured strain distributions. The result of this procedure is shown in Fig. 1 for implantation with 200 keV Ne + ions to a dose of 2 x 1014 ions c m - 2 . It can be verified that the measured strain profile conforms very closely to the theory. The calculated peak strain position from the surface, using the Sigmund and Sanders relationship, is 0.19 I~m compared with 0.17 ~tm obtained experimentally. 3.3. Definition of dose levels Defects are progressively produced by slowly increasing the implantation dose. This process involves displacements of atoms from their position in the crystalline structure and/or the formation of broken bonds. In the case of relatively low doses, implantation produces internal stresses. These are at first proportional to the nominal dose. As the number of defects increases, a certain relaxation takes place which tends to a saturation value corresponding to amorphousness after a certain dose level is reached. In Figs. 2(a), 2(b) and 2(c) damage band diagrams of the

8

P. G E R A R D

1,5 A .~ 1,0

o,s

0,0 0,0

0,1

0,2

0,3

0,4

0,5

Distance from surface (rUm)

Fig. 1. Calculated ( ) and X-ray (O) strain distributions as a function of the depth below the surface (implantation with 200 keV neon ions to a dose of 2.0 × 10~'*ions cm - 2). The stopped ion distribution is indicated by a broken curve and the arrow indicates the average projected range Rp 2

Amorphousness

Paramagnetism

KI>

Ku

g

i Depth

Depth

~n

Depth

to) (a) (b) Fig. 2. Effect of(a) a low dose, (b) a medium dose and (c) a high dose on the damage as a function of the depth.

influence of the dose on the d a m a g e profile related to low, m e d i u m a n d high doses respectively are shown. T h e idea of these d i a g r a m s is t a k e n from E n g e m a n n et al. 4, m o d i f i e d s o m e w h a t as r e g a r d s the d a m a g e levels. There are four d a m a g e bands. In the lowest b a n d the degree of d a m a g e is still insufficient to convert the direction of m a g n e t i z a t i o n . This h a p p e n s in the b a n d j u s t above, where, as we shall see, the i n d u c e d stress due to defects has g e n e r a t e d an a n i s o t r o p y which exceeds the initial uniaxial a n i s o t r o p y . In the third b a n d , p a r a m a g n e t i s m at the m e a s u r e d t e m p e r a t u r e takes over a n d finally in the u p p e r m o s t b a n d the layer b e c o m e s a m o r p h o u s . The d a m a g e due to an oxygen i m p l a n t a t i o n at 100 keV has recently been s t u d i e d aa as a function of the dose by T E M . It was shown that at s o m e critical value a thin a m o r p h o u s slab a p p e a r s in the i m p l a n t e d layer which increases in width with i n c r e a s i n g dose. This b r o a d e n i n g process of the a m o r p h o u s thickness is quite s t r a i g h t f o r w a r d from Fig. 2(c), where a slight increase in the dose will have a m a r k e d influence on the s - k value w i t h o u t c h a n g i n g m u c h the total thickness of the i m p l a n t e d layer. At very high doses, the i n c r e m e n t in s - k will tend to s a t u r a t e because the r a n g e of a c t i o n of cascades is limited n e a r the surface by the i m p l a n t energy. A g o o d technique to o b t a i n the dose c o r r e s p o n d i n g to this d a m a g e

9

IMPLANTATION OF BUBBLE GARNETS

saturation value is the cantilever beam method. It consists in measuring the induced flection of a garnet beam during implantation and, with the help of the elasticity relationship, in determining the integrated stress in the damaged layer. This technique has previously been applied to neon implantation s and more recently to arsenic implantation at 120 keV 11. The integrated stress S (dyn cm-1) measurements as a function of the arsenic dose are given in Fig. 3. This curve permits the overall effect of the dose on the stress to be defined. Low (a), medium (b) and high (c) doses correspond to the initial linear part, to the departure from linearity up to saturation and to the saturation effect respectively. For medium doses there starts to be a certain overlapping of cascades that increases with dose. The above-mentioned amorphous layer should appear somewhere in that interval and could correspond to a dose of about 1014 As + ions c m - 2. From there on the amorphous thickness should increase giving rise to a stress relaxation which tends towards a plateau at a dose of approximately 6 x 1014 As + ions cm-2.

'~ 1 0 4

a

10 3

b 10 2 1 0 ~2

1 0 la

, ÷~+I ~ +c 10

TM

1~ 1 0 is

Dose

Fig. 3. Integrated stress as a functionofthe dose for implantation of arsenicat 120 keV.

3.4. Pure magnetostriction implantation effect Damage caused by ion bombardment in a garnet film gives rise to an increase Aa±/a in the lattice parameter. In Fig. 4 are shown a schematic diagram of an implanted bubble material as well as the stress and strain configuration in an implanted unit cell. This increase in lattice parameter is only possible along the z direction perpendicular to the film plane as measured by an X-ray technique 2°. Indeed, such an expansion cannot take place laterally because of the relative importance of the underlying sample with respect to the thickness of the implanted layer. The damaged layer is therefore placed in a state of lateral compression. In garnets with negative magnetostriction this compressive stress induces an in-plane magnetic anisotropy which has to overcome the growth- and the stress-induced uniaxial anisotropies K G and Ks respectively Of the as-grown material 36. The components of the strain are exx = eyy = 0 and e~ = Aa±/a. This strain produces a compressive stress o H= oxx = o , parallel to the film plane with no contribution in the perpendicular direction tr== = 0.

l0

P.

Implanted layer Ferrimagnetic Garnet

GERARD

Non magnetic substrate

f

I Ha

1 i

I

G.G.G.

(a)

/

I

i I

i

(b) Fig. 4. Implantation effectin a bubble material: (a) in-plane magnetic layer in an epitaxial film;(b) unit cell in the damaged layer. The relationship between the stress and the strain is as followsV5 : E Aa± or//- l + v a

(2)

where E is Young's modulus and v Poisson's ratio. The effect of this stress is to produce an induced anisotropy energy density Ksi or a stress-induced anisotropy field Hsi given by Ksi =

3 22111a//

(3a)

Hsi -

32111 --~rj/

(3b)

Ms

This implantation-induced anisotropy Ksl creates an easy axis of magnetization parallel to the film plane (211 x being negative). Thus the magnetization in the implanted layer can be converted from perpendicular to parallel to the film plane if the following condition holds: K , + Ksi < 2rtMs 2

(4)

This condition is only fulfilled with garnet films which have large negative 21 ~~; then implantation doses can be limited to produce m a x i m u m lattice strain without causing too much damage to the film. In such a case an ideal uniform profile can be achieved by using several implantations and by suitably adjusting doses and energies 76 to obtain a constant Aa/a value throughout the implanted layer. 4. CHARACTERIZATION The descriptions are focused especially on what piece of information each technique can add to the general understanding of implantation effects. We shall only take into account methods that have been used sufficiently frequently in the analysis of the evolution of damage, e.g. as a function of energy, dose or annealing procedure.

IMPLANTATION OF BUBBLE GARNETS

11

4.1. Determination of strain and damage profiles by X-ray double-crystal diffraction Apart from some investigations by the dynamical theory in Japan 1a, most of the strain and damage distributions 12' t4. t 6.20-23 have been studied by the kinematic theory, which has given proof of its consistency although the mathematics pertaining to it are much simpler. Strain and damage profiles are obtained by fitting a model to the X-ray rocking curves (see ref. 20 for details). Strain is the uniform deformation of crystal planes parallel to the surface and damage is the random displacement of atoms from their original sites. With this X-ray diffraction method it is possible to differentiate point defects, strain and extended defects. The sensitivity of the rocking curve to the strain, the layer thickness and the damage depend on the relative importance of the implanted effects. In summary, the accuracy for the strain and layer thickness is very good (better than _+5~) while that for the damage is considerably less. Even so, results are accurate enough to assert that strain and damage are linearly correlated 2°.

4.2. Differential chemical etching This technique is based on the fact that the enhancement in chemical etching rate is proportional to the damage concentration 24. The etchant is usually hot orthophosphoric acid. The differential etching profile is obtained by measuring as a function of depth the ratio VdVu~, V~and Vu~being the etching rates of implanted and unimplanted garnets respectively. This method is of interest in determining the thickness of the implanted layer and the damage profile. Sometimes, to study certain properties as a function of depth, stripping of the damaged layer to well-defined thicknesses is a necessary process. Although there is some lateral uneven etching 23, this chemical technique is often preferred to ion milling in order to avoid annealing or diffusion effects. So before this stripping experiment, it is necessary to determine the relative etching rate profile. The accuracy here is also dependent on the extent of the damage, but on an average it is about + 10~.

4.3. Rutherford backscattering and channelling29'3° The channelling curves are obtained by measuring the channelling backscattering yield of a well-collimated incident beam of energetic He ÷ ions. This technique permits the amount of disorder in ion-implanted films to be measured. In order to obtain sound results, the best procedure consists in successively stripping thin layers of the sample away. Since the depth analysis is only a few hundred ~ngstr6ms it is possible to measure the minimum backscattering yield as a function of depth into the implanted layer. It is important to note that Rutherford backscattering of channelled ions provides different information than does X-ray diffraction. It is only sensitive to displacements of heavy atoms or to strain in a direction perpendicular to the channel axis. Therefore in the case of the (111) axis it will give information on modifications parallel to the surface. This technique is especially reliable in the study of amorphous or highly damaged layers. Its accuracy is then better than a few per cent. These channelling effect measurements have also been used with nuclear reactions to determine lattice locations of displaced oxygen atoms 31.

12

P. GERARD

4.4. Conversion electron M6ssbauer spectroscopy CEMS 9'77 is a powerful method to characterize the different states of the ion in implanted layers. M6ssbauer spectra of 57Fe nuclei using the CEMS technique only probe a layer of 1000~, at the surface of the sample. The technique provides information on both the extent of magnetization and its direction relative to the ~, ray beam. The former is obtained from the magnitude of the hyperfine field and the latter from the intensity ratio between the various peaks of the magnetic ion sextuplets. Another interesting feature is the possibility of analysing paramagnetism. Indeed, the appearance of paramagnetism with increasing dose is clearly shown by a widening of the various peaks of the sextuplets and by the growth of a non-magnetic component in the form ofa quadrupole doublet. The accuracy on the hyperfine field is fairly good, of the order of a few per cent, and is a little worse for values necessitating peak integrations. It is a strong function of the counting statistics and of the degree of substitution of the non-magnetic component in the garnet.

4.5. Ferrimagnetic resonance F M R is the most used tool to measure magnetic properties in garnets. The F M R spectra of relatively thick implanted films can be interpreted by using a model of layers with negligible exchange coupling so that the main mode in each layer will be close to uniform resonance. Computations have been developed by Wilts and coworkers 12'23'3a'42 for a surface layer with either a unimodal or a polymodal magnetic profile. In the first case, the dose has to be limited to a low level. The process consists in calculating a spectrum corresponding to a trial distribution of the magnetic properties such as the magnetization M, the uniaxial and crystalline anisotropies K u and K ~, the exchange constant A and the g values, as functions of the depth, and to compare it with the experimental spectrum. The accuracy of this information depends on the number of observable surface stationary spin wave (SSW) modes, the narrowness of the uniform mode linewidth and also on the exact determination of the implanted thickness. The information so obtained by F M R is the local field H u for uniform resonance given by

Hu = °)-HK+47tM +~H1 7 where HK is the uniaxial anisotropy field and the other terms have their usual meanings. The accuracy claimed for the profile Hu(z) of the uniform fields is a few per cent and the change in M is only about 10~ or 20~o of the original value. In the second case, part of the implanted layer has become "non-magnetic". It is now necessary to have recourse to etching in order to reconstruct the profile. When materials of large linewidth are investigated, which often happens with bubble garnets, Wilts's method gives relatively poor results. There are too few resonances and too close a spacing between the bulk and the uniform mode for reliable resolution. Nevertheless, under these circumstances what can be obtained from spectra taken with the applied field perpendicular to the film plane are parameters which are important in defining the implanted layer and its evolution with annealing treatments, as follows. (1) 8Ht is the variation in the ion-implantation-induced anisotropy field. It can be obtained from the resonance conditions corresponding to each separate layer. It

IMPLANTATION OF BUBBLE GARNETS

13

is given with good accuracy from the relation HK = n ± l - / - / 1 o

(5)

where H±I and H±o are the main modes of the implanted and the bulk layers respectively. (2) AH1 is the peak-to-peak linewidth of the derivative of the absorption of the implanted layer. In certain RE-substituted garnets the magnetic loss is due to a longitudinal relaxation mechanism. It can be shown that for a given measured temperature 7s H 1 = C stNRE M

(6)

if the relaxing RE concentration NRE is not too high. Usually in an implanted layer, AH 1 is smaller than the bulk linewidth AHo. This shows that in this layer the RE is preferentially decoupled from the garnet system. In the case of samarium, at room temperature, its contribution to M and the g factor is negligible, so a reduction in the magnetically active Sm 3 ÷ will affect K u and 2111 which will both be reduced in the implanted layer with respect to the bulk. (3) ia is the intensity ratio of the main surface mode to the bulk mode. It gives information on the magnetization and on the homogeneity of the implanted layer. When both layers are magnetically homogeneous it is given by II(AH1) 2

LIM 1

is = lo(AHo)----~ = Lo---~o

(7)

where 11 and I o are the mode amplitudes and L 1 and L o the thicknesses of implanted and bulk layers respectively. The magnetic homogeneity of the implanted layer can be studied from an etching experiment. (4) Tc is the Curie temperature. For a particular layer Tc can be deduced from the temperature dependence of the mode amplitude corresponding to this layer. At Tc the magnetization and consequently the intensity of the resonance line rapidly approach zero, so Tc can be determined by extrapolation. 5. NEON IMPLANTATION

Neon is a perfect candidate for the study of pure implantation effects such as the influence of strain and damage on the structural and magnetic properties and for following the evolution of these parameters after subsequent thermal treatments. This study can be undertaken without its being subject to an influence attributable to the dopant in the form of chemical or pressure effects. The latter are liable to cause blistering phenomena during annealings as was shown for helium 2'23. Here we propose to consider only low and medium neon doses.

5.1. Influence of doses By combining their complementary and accurate methods using X-ray rocking curves and F M R respectively, Speriosu and Wilts 23 have determined structural and magnetic properties as functions of the depth. They made a comparison showing on the one hand the analogies of He + and Ne ÷ implantations and on the other hand

14

P. GERARD

the singularities of H + implantation. In Fig. 5 their curve of the change in the local uniform resonance field in the region of m a x i m u m strain is presented as a function of strain in the gadolinium-, thulium- and gallium-substituted yttrium indium garnet (Gd, Tm, Ga: YIG). This material has almost no magnetic loss, i.e. it is a narrow linewidth material.

~

3

2

I

Paramagnetic I=

1

Amorphous

o

Strain (%1

Fig. 5. AH, ( = - AHK+ A(4nMs)+ 2AH0 vs. t he strain for neon implantation at 190 keV t 2,23 The magnetic anisotropy is initially proportional to the strain. The shape is in agreement with the magnetostriction effect estimated from the composition. There is a m a x i m u m in AHu (of - 2 . 8 kOe) for a strain of 1.1~o (2 x 1014Ne + ions cm 2). At strains near 1.8~o the anisotropy drops to zero and so does 4nM s, A/Ms and H~. The garnet becomes amorphous for strains of about 2.5~o (5 x 1014 Ne + ions cm 2). This m a x i m u m strain of 2.5~o has been found with an Sm, Lu, Ca, G e : Y I G composition 52. Satoh et al. found an identical feature for He +, except that the values of AHu ( - 3.6 kOe) at saturation, of the strain (2.3~o) for which AH, became zero and of the strain (3.4~o) at amorphousness were all higher, These discrepancies are likely to be explained by the larger extent of damage due to Ne + cascades. We think that this fact is also emphasized in their experiment by the higher rate of change in the linewidth of the uniform mode with increasing strain for Ne + than for He +. The reduction in magnetization within the implanted layer has been mentioned previously7'9'19'39"42''1"3"50'56'57'62 and measured directly by using VSM 37'52-55. There is a correlated decrease in the Curie temperature 19.43, 50, 51, as expected. 5.2. Annealing treatments

Ion implantation displaces lattice atoms at random from their equilibrium position in the crystal. Strain relief on annealing involves a progressive reordering of the displaced atoms back to their original sites. This process involves complicated features as shown in a study by de Roode and Algra 1° and presented in Fig. 6. It can be seen that there are different healing mechanisms which have definite activation energies in certain temperature intervals. These annealing processes also depend on the composition of the garnet, de Roode and Algra have shown that for both films

IMPLANTATION OF BUBBLE GARNETS

15

0.4

BOO z ~

.¢

,¢

0.3

'~'~'~

0.2

-

,

600

~

\,~

4o0

0.1

0.0

0

200

~

~

200

400

600

800

~

0

1000

TA(°C)

Fig. 6. Variation in the strain and in the induced anisotropy field with the annealing temperature for two garnets1° (implantation with 300 keV neon ions to a dose of 1 x 1014 ions cm- 2): O, (3, Tm, Ca, Ge: YIG; A, La, Ga: YIG.

the uniaxial anisotropy is proportional to the strain. Speriosu and Wilts 23 have likewise found different stages of strain variation during the healing of implanted Gd, Tm, Ga:YIG. This is shown in Fig. 7 where variations in the normalized strain with the annealing temperature are given for neon at two nominal doses (5 x 1013 and 101+ Ne + ions cm -2) and for hydrogen at 5 x 1015 H2 + ions cm -2. They found that at 600 °C the magnetization is completely annealed while A I M is some 20% below the bulk value. At this temperature the strain is 40% of the as-implanted value so this discrepancy in A can probably be explained as due to some distortion of the structure occurring between iron atoms on a and d sites. Extending the measurements to 1100 °C, Awano e t al. 12 have observed that at a very low dose the variation in the anisotropy corresponds to a pure magnetostrictive effect except at high annealing temperatures. This is not strictly true even at a dose of 1014 Ne + ions c m - 2 where they found that the strain change decreases a little faster than Hu does. At 1100 °C the strain becomes zero but there is still a residual anisotropy that could be accounted for by a random diffusion of the REs back to the c sites. Satoh ~t al. 52 have also observed that there are variations in the rate of strain relief with the annealing temperature. They also measured directly that a 400 °C treatment was sufficient to anneal more than 90% of the average magnetization of a layer damaged by a neon implantation at a dose of 2 × 1014 Ne ÷ ions cm -2 and an energy of 270 keV. There are further examples studied by other methods of the different rebuilding mechanisms. Seman et al. 34 have observed an increase, with increasing neon dose, in the optical absorption associated with the 2p ---, 3d charge transfer bands, which seems to be correlated with the number of dangling bonds between F e - - - O - - F e groups. At TA = 600 °C, where as seen above 23 M is completely healed, the absorption is about that for the as-grown garnet. They have also shown that annealing at a low temperature (200 °C) gives rise to an abnormal increase in the absorption. This effect was also observed by Picone and Morrish 9 using CEMS. By looking at the variation in the collapse field Washburn and GaUi 6 have found that

16

P. GERARD

the annealing behaviour of this parameter is different below and above 400°C. Figure 8 shows the linewidths of the main bulk (AHo) and implanted (AHt) modes determined by F M R in a perpendicular configuration v e r s u s the annealing temperature. The full curves are for a wide linewidth Sm, Lu, Ca, G e : Y I G garnet whose characteristics are given in Table I. The broken curves are drawn approximately from certain values given in the text of ref. 23. They correspond to the narrow linewidth garnet Gd, Tm, Ga: YIG. As previously mentioned (Section 5.5), longitudinal relaxation of samarium is a source of linewidth broadening AHsm. There are other sources such as voids, heterogeneous stresses, inhomogeneous regions 79 or lateral non-uniformities 23. Implantation in wide linewidth garnets decouples certain active REs. This effect narrows the resonance. At the same time certain defects are formed, resulting in a linewidth broadening AHd; hence AH1 total

=

AH1 Sm-~-AH1 d

(8)

AH~ total may be either wider or narrower than AHo depending on the relative variations due to defects or on the damping coefficient ~. Here for the as-implanted Sm, Lu, Ca, G e : Y I G the decrease in ~ is preponderant. In passing, let us recall that Vella-Coleiro e t al. 19 have reported that implantation suppresses a large amount of the growth-induced anisotropy. The origin of this suppression of K~ is probably related to the preferential decoupling of RE ions. During the annealing to 600 °C defects causing the resonance broadening are healed and above this temperature the concentration of active samarium atoms increases. For Gd, Tm, G a : Y I G ct is very small so the linewidth AH 1 > AHo. Above 400 °C AH1 < AHo so there should be some contribution due to ~. From these combined studies it can be concluded that the radiation damage is annealed in two stages. The first stage corresponds to the low temperature repairing of the F e - - O - - F e bonds (a and d sites) via short-range

100

-1 80

600 "

•

~__A

•

•

t~A

E ~ 60 Z

40

-~HI

\

AN °

20

0

0

200

400

600

2 0 T A (°C)

0

600

8 0 T~(°C)

Fig. 7. Normalized strain vs. the annealing temperature 23 for implantation with 190 keV neon ions to doses of 5 x 1013 ionscm -2 (©) and I x 10 TM ions cm -2 (O) and with 120 keV H2 + ions to a dose of 5 × 1015 ions cm -2 (A). Fig. 8. Linewidth vs. T A for neon implantation in two garnets: - - , Sm, Lu, Ca, G e : Y I G (200 keV; 2 x 1 0 1 4 i o n s c m - 2 ) ; - - - , G d , Tm, G a : Y I G ( 1 9 0 k e V ; 2 × 1014ionscm 2).

IMPLANTATION OF BUBBLE GARNETS

17

diffusion of mainly oxygen atoms. This process leads to a rise in M, in the exchange coupling A and in the Curie temperature. This repairing is probably not governed by a single activation energy (Fig. 7). Above 600 °C there is a second stage which involves long-range diffusion requiring a higher activation energy. During this longrange rebuilding process the RE interaction with the iron atoms increases, as do the quality of the crystal structure, 2111 and 0t. Recoupling of the RE ion manifests itself in a partial reconstruction of the growth-induced anisotropy 19'45. 5.3. Study of the "ideal dose" (2 x 1014Ne + ions cm-2) It has been shown 6"7 that the bubble collapse field of garnets implanted with neon at this dose is increased appreciably by a 400 °C annealing. This fact has led us to look at this neon implantation (200 keV; 2 x 10~4 Ne + ions cm -2) in a given garnet (see Table I). Generally the study was made on the as-implanted sample and after a 400 °C annealing. Various techniques were used to obtain information on heterogeneities within the thickness of the implanted layer. The method consisted in successively stripping away thin layers of the damaged film by chemical etching and then measuring parameters of the remaining film. The results are given in Fig. 9, which shows the variations in different parameters as functions of the depth. The full and broken curves correspond to the implanted sample before and after a 400 °C annealing respectively. Figure 9(a) shows the differential etching rate 27 and Fig. 9(b) the Rutherford backscattering yield Zm~n(%)30. The latter gives an idea of the displacement of heavy atoms from their equilibrium positions in the lattice. Figure 9(c) shows the excess oxygen atoms along the (111) axis with respect to the unimplanted crystal31. The stress tr (dyncm-2) 27 is given in Fig. 9(d) and the intensity ratio 43 (for details see Section 4.5.3) in Fig. 9(e). The anisotropy and the magnetization profiles of our as-implanted samples, determined by Wiltss°, are presented in Fig. 9(f); these results are in good agreement with Kerr measurements 62. From these different results it is possible to draw certain conclusions. From Fig. 1 and Fig. 9(a) we can say that the strain is correlated to the etching rate, i.e. to broken bonds. It has also been shown 2° that the damage is proportional to the strain which is proportional to the etching rate16. The maximum number of broken bonds coincides with the maximum number of displaced oxygen atoms and with the minimum of the magnetization and of A so. This behaviour is quite normal, because when there are broken oxygen bonds there should be a decrease in the superexchange between a and d sites and therefore a decrease in the magnetization. The stress measured from the flection relief as the damaged layer is progressively etched seems to be correlated with the formation of broken bonds and with the Rutherford backscattering results, but not with the anisotropy profile. Where there are broken oxygen bonds a certain proportion of heavy garnet atoms which are under stress, as can be seen near the surface, are relaxed somewhat back to their equilibrium position. Concerning the AHK profile, there is some semblance of an explanation which seems to be quite sound. As long as the maximum of the strain corresponds to the maximum of the induced anisotropy (when the magnetic profile is still monomodal), the stress causing the magnetostrictive mechanism is some other parameter which acts locally in the strained region. A 400 °C annealing reduces the relative etching rate, i.e. the number of broken

18

P. GERARD

Vl o

Vu-~ y° 30

E

30~

20

20

10

10'

0

1000

2000

(a)

3000 Depth (A)

1000 (b)

2000 3000 Depth ( ,4 )

-n

E

1011

c

e~

O

"~ 101c

;, .-.,/

10 9

0

0

1000

(c)

2000

3000 Depth ( A, )

10 a (d)

1000

2000

3000 Depth ( A )

~ 3000

300~

~.~=2000

200

1000

100

0 1000 20'00 3000 2000 3000 Deoth ( A ) Depth ( A ) (f) Fig. 9. (a) Differential etching rate, (b) Rutherford backscattering yield Zmi., (C) number of oxygen defects, (d) stress, (e) intensity ratio and (f) anisotropy and magnetization profiles for a neon implantation (200 keV; 2 x 1014 cm-2): , as implanted; - - -, annealed at 400 °C.

0

1000

(e)

bonds. 4riMs and A must increase and so must the stress within the strained region with a tendency to become more uniform over the entire damaged film. 6. HYDROGEN IMPLANTATION Hydrogen has a peculiar effect with respect to other implanted ions. Its behaviour can be assigned to the facts that (1) the dose necessary to reach the proper damage level is one to several orders of magnitude higher than that for any other implanted ion, (2) its chemical activity and its mobility have unrivalled effects and

IMPLANTATION OF BUBBLE GARNETS

19

(3) the damage caused by H ÷ collisions with garnet nuclei produces a more homogeneous strain 81. It is the purpose of this section to lay stress on the discrepancies relevant to both the dopant effect due to the presence of hydrogen in the garnet and the type of defect created by H ÷ implantation. The influence of some sort of chemical interaction with the garnet has never been detected with any other ion because the dose required would have rendered the implanted garnet layer paramagnetic or even amorphous before the appearance of such an effect. Further, it will be seen that this phenomenon is only possible through a high temperature annealing treatment. 6.1. H + implantation effect

Hirko and Ju 8 have found that the anisotropy field change 6HK of an H +implanted layer is very different from that for other ion species. ~H~ saturates as a function of the damage level for most ions (i.e. He +, B ÷, Ne ÷ etc.) while for H ÷ there is no such saturation effect as can be seen in Fig. 10. This discovery has given rise to great technical interest for the manufacture of high density bubble memories. Hence this experiment has been reproduced in many laboratories 23,52,53, s2 and the same qualitative results have been found with garnets of different compositions. Makino et al. 47 have emphasized the abnormal action of hydrogen by comparing it with a helium implantation which was set up so as to produce an identical strain in Eu3FesOx2 whose 2111 is very low. In the former case they found a very large 8 H K whereas for helium the induced anisotropy was almost zero as expected. Kryder et al. 46 have determined for a deuterium implantation that part of the anisotropy field change is attributable to some mechanism other than pure magnetostriction. In a comparative studyaa on H +- and Ar ÷-implanted garnets by electron spectroscopy for chemical analysis it has been shown by heating both samples under vacuum that for H + implantation the reduction of Fe 3÷ to Fe 2 ÷ takes place at about 50 °C while for Ar ÷ it only starts above 300°C. Algra and Robertson s4 have found that in H ÷-implanted (Y, La)3(Fe, Ga)50~ 2 thin films 6HK had the opposite sign to that for rare gas implantation. In the iron gallium garnet system the magnetostriction is essentially due to the iron ions. Measurements on a YIG s5 showed that 2111 changes sign when a small proportion of Fe 3+ is reduced to Fe 2 4. This probably explains the above result. CEMS spectra of H2 ÷-implanted garnet bubble films investigated by Morrish et al. 59 show a different feature in comparison with those for neon implantation2S. 56-5s. On the one hand they draw attention to the existence of a new site designated d' and on the other hand they indicate that even when the hydrogen dose is large there is almost no evidence of paramagnetic absorption. The appearance of this new d' site has been confirmed for another bubble garnet filma6 and also for YIG sT. From the last study it is quite obvious, on account of either the values of the isomer shift or the marked decrease in the relative ratio of d sites after implantation, that this d' site arises from a significant modification of the d site. The influence on the strain of hydrogen implantation has been measured 23 below the damaged region. For the highest doses investigated the hydrogen,induced strain is located within the bulk region, i.e. in the lowest region of the damage profile as expected from the theory21'67'7°'sl. The induced strain region varies in direct proportion to the hydrogen dose. Amongst the factors that differentiate hydrogen from other implanted species,

20

P. GERARD

the damage is an important feature. Matsutera et al. 81 have calculated, using a Monte Carlo simulation technique, that more than 90~o of the H ÷ collisions bring about an energy transfer of less than 10 eV. This energy hardly corresponds to the binding energy between garnet atoms. In contrast, more than 40~o of the neon collisions involve energy transfers of more than 100 eV. Therefore Ne + implantation induces severe injuries to the garnet lattice whereas H + implantation causes only mild damage effects in the form of broken bonds or of relatively small displacements of oxygen nuclei. All these observations seem to show that the properties of hydrogen-implanted layers are determined by some chemical effect associated with the amount of incorporated H ÷ and by the particular radiation-induced defect caused by this small mobile ion. In order to derive the respective influences of these two parameters, it is convenient to follow the evolution of the magnetic as well as the structural properties as functions of the progressive annealings of H ÷-implanted layers. 6.2. A n n e a l i n g t r e a t m e n t s Suran et al. 45 have studied the effect of a triple H + implantation whose conditions are (a) 100keV, 2× 1016 H + ions cm 2, (b) 60keV, 1 × 1016 H + ions cm -2 and (c) 30 keV, l x 1016 H + ions cm 2 respectively in (Y, Sm, Lu, Ca)3-

(Fe, Ge)5012. It can be inferred from an etching experiment that the as-implanted sample is built up of three uncoupled sublayers, each having different magnetic properties governed by H ÷ interactions with the lattice. On annealing, the strain relief as well as the magnetic behaviour evolve in three stages. Stage I occurs below an annealing temperature TA of about 300 °C. Stage II occurs for 300 °C < TA < 600 °C and stage I I | corresponds to TA > 600 °C. The behaviour of stage III samples is identical with the high temperature stage for neon, so we shall not consider it

80001 /

o.o

o'.4

o'.8

C

122

2:0

2:4

-lc

0

100

Damage level (eV/~. 3)

200

TMeasured

Fig. 10. ~HK vs. the damage level s for implantation with H + (e), He + (O) and B + (A). Fig. 11. Effective anisotropy vs. the measured temperature: + , 200 °C, 15 min; (D, 200 °C, 30 min.

21

IMPLANTATION OF BUBBLE GARNETS

further. In stage I, it is known that the magnetic parameters of the different sublayers and their evolution with TAare connected with the different H ÷ distributions. What remains to be understood is the specific modification of the garnet caused by H ÷. Let us consider the peculiarities of these magnetic characteristics. Figure 11 displays the variation in the effective anisotropy field H~:' with the measured temperature corresponding to TA = 200 °C after durations of 15 and 30 min. Curves A, B and C correspond to the three ion distributions (a), (b) and (c) respectively. It should be noted that after an annealing of 15 min the values of HK' are very large at room temperature but decrease very steeply with increasing measured temperature. Another important characteristic of this annealing at 200 °C is that IlK' measured at room temperature varies extremely rapidly with the annealing time. The different characteristics of these two annealing times at TA = 200 °C are shown in Table III. TABLE III EFFECT OF THE ANNEALINGTIME t A ON THE MAGNETICPARAMETERSFOR TA = 200 °C Magnetic parameter

Valuefor thefollowing annealingtimes tA

Ii A x 10-2 AH1A (Oe) (HK')^ (Oe) I1B x 10 -2 AHI B (Oe) (HK')B (Oe) ( H g ' ) ^ - (HK')a (Oe) (To)A (°C)

=

7.5 450 8990 3.2 540 7670 1320 130

15 min

t A = 30 min

9.5 325 4470 2.9 250 3640 830 145

A comparison of the two sets of results shows that (a) the intensity of the surface mode A increases while those of modes B and C decrease, (b) the various HK' values are reduced to roughly one-half of their initial values, (c) the difference in the HK' values corresponding to the sublayers decreases and (d) the Curie temperatures Tc of the three sublayers increase greatly. The intensity ratio iR (i.e. M1) is much smaller than the expected value corresponding to radiation-induced defects. A possible explanation for the high H~:' values which seems to be related to the amount of absorbed H ÷ is that the chemical perturbation greatly reduces the value of M 1 without having much effect on the Fe-Sm exchange field. Under these circumstances the ratio 21111/M1 (see eqn. (3)) increases greatly with respect to the ordinary magnetostrictive phenomenon. As we shall soon see, at room temperature AH~ in the present case is larger than it is in type II samples. This result could be related to an increase in both AHa and AHRE(see eqn. (8)). The value of AHd is higher because of the excess local disorder induced by the absorbed H ÷ and the increase in AHREcould be due to a decrease in MI (eqn. (6)). In perpendicular FMR as the annealing temperature is progressively increased from the ambient value to above 300 °C it can be observed that the peculiar modes (broad AH1 and small intensity) progressively shift towards low field values while the hydrogen concentration decreases. Around 250 °C they finally merge into a high intensity uniform mode and four to five very weakly excited surface modes that are

22

P. G E R A R D

SSW modes that follow a quadratic dispersion law. From the general aspect of these spectra corresponding to stage II it can be asserted that the implanted layer possesses uniform magnetic properties along its thickness which are determined only by the radiation-induced lattice damage. This uniform distribution of magnetic parameters can be attributed on the one hand to the homogeneity of the damage profile (i.e. triple implantation) and on the other hand to the type of defect due to H ÷ (ref. 81). In Fig. 12 the variation in the implanted mode linewidth AH 1, measured at room temperature, is shown as a function of TA. At low TA the values reported belong to mode A.

~ 600

400

200

i

0

i

i

200

Fig. 12. Linewidthvs.

i

400

i

I

600

I

I

800 T~. (°C)

T A for H + implantation.

A comparison of these results with the AH~ behaviour for neon, also measured on a large linewidth garnet, presented in Fig. 8 gives an idea of the modifications brought about by the H ÷ implantation. Below 300°C, as mentioned earlier, hydrogen is the cause of AH~, a broadening which gets narrower as hydrogen diffuses out of the sample 35. In the range 300 °C < TA < 600 °C, at variance with the neon results, AH~ remains constant. The plausible explanation for this plateau must again be the small cascades resulting from hydrogen collisions 8x. Speriosu and Wilts23 have recently confirmed some of the previous conclusions, bringing forward other interesting parts to the hydrogen puzzle. They have determined that in the case of a relatively low dose (120keV; 5 x 10 is H2 ÷ ions cm -2) where the strain maximum is only 0.6~o, there is as marked a reduction as 60~o in 4r~Ms localized below the damaged region corresponding to the position of their measured hydrogen-induced strain. This interesting point, which lays stress on the particular influence of the hydrogen distribution on the magnetic properties of the garnet, is also accentuated in Fig. 13, also taken from ref. 23. Figure 13 shows the AH, profile (full curves) and the variation in the anisotropy field profile determined from the strain profile, assuming that it is due only to a pure magnetostrictive effect (broken curves), after various annealing treatments for a nominal dose of 5 x I0 ~5 H2 ÷ ions c m - 2 at an energy of 120 keV. Before annealing, AH, seems to be correlated to the

23

IMPLANTATION OF BUBBLE GARNETS

6 (a)'

(d)'

2

z=

<12 0

4

(c)

(f)

2000

4000

0

2000

4000

6000

Depth (A }

Fig. 13. Changes in the profiles of the local uniform resonance field and the anisotropy field if it were purely magnetostrictive for implantation with 120 keV H2 + ions to a dose of 5 x 1015 ions crn- 2 (ref. 23) for (a) an as-implanted sample and after annealing to the following temperatures: (b) 200 °C; (c) 300 °C; (d) 400 °C; (e) 500 °C; (f) 600 °C.

position of hydrogen. This connection is further confirmed by looking at the different profiles after annealing at successively higher temperatures. At 400 °C there is no more hydrogen and it can be seen that from 400 to 600 °C AH, is governed by a pure magnetostrictive mechanism throughout the whole profile. At 600 °C they found that not only 4riMs but also A reached their respective bulk values. The last result for A, contrary to the case of the neon implantation, probably arises because there is less lattice distortion. Having drawn attention to the differences between H ÷ and Ne ÷ implantations we should point out that there are similarities such as the normalized strain relief as a function of annealing temperature presented in Fig. 7 23. It can be seen that the general trend of the curves have roughly the same feature. 7. HIGH DOSE IMPLANTATION The formation of an amorphous layer in heavily implanted garnets has been shown directly by TEM 33'88 and indirectly using CEMS 9'28'56-58'6°, X-ray double-crystal diffraction, FMR 1°-12 and a susceptibility method 51. Annealing studies on samples implanted with a high dose of neon have revealed a behaviour other than that known with low dose implantation9-12'6°. A garnet layer made amorphous by a high dose implantation recovers good crystalline qualities 6°'88 after an 800 °C thermal treatment. This fact opens a new area for ion implantation. It is the behaviour of the dopant in implanted garnets. Apart from the interest in modifying locally the properties of the material using all sorts of masking techniques as commonly applied in microelectronics, there are other advantages over doping by growth processes. For example, dopants can be implanted at temperatures at which normal diffusion is negligible. The dopant concentration is not limited by ordinary thermodynamic equilibrium conditions and so a much wider variety of dopants may be used. Here we propose to show (1) the difference between high and low doses on

24

P. G E R A R D

the annealing processes, (2) the distribution of the implantation effects with depth of an iron-implanted garnet (as prepared), (3) the epitaxial regrowth mechanism of such an implantation and (4) the specific dopant effect by comparing the properties during the annealing process of layers implanted with iron and with two other dopants.

7.1. Annealing compar&on between high and low dose effects In the case of a low dose implantation, the variation 6H~ in the induced anisotropy field always decreases as a function of the annealing temperature. There are no exceptions: see for example ref. 19 or Fig. 6 taken from ref. 10. 6HK behaves differently in the case of high doses as shown in Fig. 14 where 6H~ is displayed as a function of Ta for an Fe + implantation at 120 keV and at doses of 1 × 1016 and 6 × 1016 ions cm z. Such an increase in 6HK was previously observed for neon implantation at a dose of 1016 Ne + ions c m - z (ref. 10) and was then considered to be rather surprising. Three different annealing processes stand out from Fig. 14, as follows: (a) between 400 and 500 °C 6HK increases with the dose; (b) between 500 and 650 °C the slope of 6HK is independent of the dose; (c) between 650 and 900 °C 6HK decreases in inverse proportion to the dose.

2#

4

3

2

1

oi

i

I

I

I

I

400

500

600

700

800 T A (°C)

Fig. 14. 6 H K v s . the a n n e a l i n g t e m p e r a t u r e for i m p l a n t a t i o n with 120 keV Fe ÷ ions to doses of 1 x 1016 ions cm -2 ((2)) a n d 6 x 1016 ions cm -2 (A).

Another unexpected evolution of the annealing processes between low and high dose implantations is given in Fig. 15 which shows the logarithm of the Curie temperature Tc as a function of the inverse annealing temperature 1/TA. The continuous line relates to an iron implantation (120 keV; 1 x 1016 Fe ÷ ions c m - 2)11 and the broken curve corresponds to a neon implantation (270 keV; 2 x 1014 Ne ÷ ions cm-2)19; both types of implanted sample were annealed in oxygen for 1 h at each temperature. Since these curves are not consistent with equilibrium rate measurements they do not really respond to Arrhenius plots. In any case a value for a thermal activation energy in such damaged systems does not make much sense a9. However, some activated process should exist for the restoration of the layer

25

IMPLANTATION OF BUBBLE GARNETS

oc 900

800

700

600

500 I

2300

2200

-i

21 O0

2°°°o.,

oi,

,Io

I

,I, 1/T A

.

,.2

I

,.3

10 a

Fig. 15. Plot of log Tc vs. the inverse annealing temperature 1/TA for implantations with Fe + (120 keY; l x 1016 ions cm-2)11 ( ) and Ne+ (270 keY; 2 x 10f4 ions cm-2)19 ( - - - ) .

magnetization given by Tc. The most striking feature in Fig. 15 is that for high doses there is only one annealing process whereas for low doses there are two different mechanisms. In the former case the.damaged layer is so broken up that there seems to be no hindrance to diffusion. This is not so for low doses at relatively low temperatures (TA < 650 °C) where what is left of the crystal structure can prevent some structural rearrangement. Above 650 °C, because of the increase in the thermal activation, the annealing process is then more or less identical with that for the high dose implantation.

7.2. Profile of implantation effects The comments in this and the next section are based on combined studies that will only be briefly mentioned. More often than not the study was limited to an iron (120 keV; 1 x 1016 Fe + ions cm -2) implantation, but sometimes, for experimental reasons, the implantation conditions were a little different. Nevertheless, still unpublished results permit us to assert that these variations have not affected the general conclusions of the work. A schematic diagram of the implantation effects is shown in Fig. 16. The accuracy of the thicknesses given in Fig. 16 for the various zones is more or less 100 A. Zone A - C is amorphous as shown by TEM ss and by ion channelling measurements 6°. It is subdivided into two parts which are as follows. (a) In zone A-B is to be found the implanted ion distribution whose thickness has been defined in the above section and confirmed by F M R on stripped samples annealed at suitably chosen temperatures 11. (b) Zone B - C does not contain any implanted impurity. The cause of the amorphousness is explained by recoils of the matrix atoms. Zone A - C corresponds to s-k in Fig. 2(c). It is paramagnctic down to the temperature of liquid nitrogen 6° and measurements at lower temperatures are planned for the near future.

26

p. GERARD Amorphous 900

Surface

"1 =14

300 ~.

300 A

,

300 A

~A O.e~

Bulk

_EE A

B

C

D

1800 A, Fig. 16. Schematic diagram of the implantation effects for an Fe ÷ implantation (120 keV; 1 × 10 TM ions cm - 2).

C - D is a paramagnetic zone at room temperature as measured by FMR. It corresponds to k-1 in Fig. 2(c). D - E is ferrimagnetic, the magnetization lying in the plane of the layer. It corresponds to 1-m in Fig. 2(c). E - F should also exist (m-n in Fig. 2(c)) but it is too thin to be measurable. The total thickness A-E of the implanted layer is determined by chemical etching. Indeed, on account of the high density of defects, the entire damaged layer is completely dissolved after the sample has been dipped for 3 s in orthophosphoric acid at room temperature.

7.3. Epitaxial regrowth mechanism From F M R xt the evolution of the magnetic properties is analysed within the thickness of the implanted layer on samples annealed for 1 h in oxygen at temperatures of 500, 650 and 800 °C, chosen after Fig. 14. From the 500 °C magnetic profile it was shown that the surface layer referred to as A-C in Fig. 16 is still amorphous. From the evolution of 6H Kand AH1 (defined in Section 4.5) with depth for the three annealed samples, it can be stated that (1) the crystalline regrowth mechanism starts in the back of the damaged layer, i.e. it is a regrowth epitaxy, and (2) the surface layer containing the implanted species down to 900 ~, behaves differently from the rest of the implanted layer. In Fig. 17 the annealing behaviour of the channelling spectra is presented 6°. The details of the implantation conditions are given in the caption of the figure. It can be seen that the implanted layer is fully damaged and remains so after annealing at temperatures of 400 and 550 °C. The crystal recovery is partially achieved after a 650 °C annealing and is completed after an 800 °C annealing. From the channelling measurements, it can be concluded that a solid phase epitaxial regrowth of the implanted layer takes place in the temperature range 550-800 °C. It is obvious that this recrystallization process starts from the lowest region of the implanted zone and propagates towards the surface with increasing temperature. Results obtained from CEMS 6° are only concerned with a layer of about 1000 ~ below the surface so this technique is limited to the study of the layer containing the implanted ion distribution. CEMS shows that just after implantation the layer is amorphous and that the iron is present in three different forms: metallic precipitates, Fe 2 + ions and Fe 3 + ions. After a 400 °C annealing all the iron species

27

IMPLANTATION OF BUBBLE GARNETS "~ lOOO

i

800 600

"~-%-

i!iOOo

Depth (A.) ~ • ~ m .

~

400 F " , ~ : . a : , . . , f " ' e l

l

ol

230

.Ge

.......

.

.

.

330

.

.

.

--

%

t

.......:""':"'::.:'-;.'.'..-.:.-._ .""'- ~'¢.-,..X ....'~

430

530 630 730 Channel number (1.9 keV/ch.)

Fig. 17. Ion channelling measurements (He +; 1.5 MeV; 165°) in the (111) direction of a garnet film subjected to Fe + implantation (120 keV; 6 x 101e ions ern - 2) and subsequently annealed. ( 111 )-aligned spectra:---, 400 °C, I h; . . . . ,550 °C, 1 h ; - - - , 650 °C, 1 h ; - - - - - , 800 °C, 1 h ; . . . , unimplanted~ - - , unaligned or random spectrum.

have been oxidized to the Fe 3 ÷ state, in a matrix which remains paramagnetic down to liquid nitrogen temperature. Annealing between 500 and 650 °C leads to a partial recrystallization of the layer. At 800°C the recrystallization is complete and the majority of iron ions are located at a and d sites of the garnets. However, a small fraction is precipitated in the form of 0t-Fe20 3 particles. From the combined observation it is possible to assert the following. (1) The regrowth mechanism is a solid phase epitaxial process. (2) The crystalline rearrangement starts to be measurable at 400 °C. (3) Between 400 and 500 °C the front of this rearrangement moves from D to C (Fig. 16). The way it moves depends on the dose as does the thickness of this layer, because this layer is a damaged paramagnetic crystal at room temperature and its damage level is directly proportional to the dose. (4) Between 500 and 600 °C the front travels from C to B. This layer is saturated with defects, so the rebuilding is independent of the dose. (5) Between 600 and 650 °C the front moves from B to A. Naturally, in this layer all the properties must be dependent on the dose. (6) Above 650 °C there is a progressive relaxation of the strain, and the iron oxide contained in the aggregates is transformed into 0t-Fe203. It seems that during these processes a simultaneous decrease occurs in the magnetization as shown in Section 7.4.

7.4. Specific dopant effect We propose to compare the implantation effects of Fe + with those of two other implants, namely As + and Ga +, and to follow the effect of subsequent annealing treatmerits 1z for the three implantations. The implantation conditions were the same for the three ions (120 keV; 1 x 1016 ions era- 2) and the implanted material is the garnet described in Table I. The implants were chosen because they have almost identical atomic numbers Z but different chemical affinities with the garnet components.

28

P. GERARD

Calculation of the ionic distributions 7°'71 using the modelling of Table II for the three ions and the ionic profile distribution of arsenic measured by SIMS are in good agreement. In all three cases the extent of the implanted ion distribution is limited to a surface layer of about 900 A. The specific effects are shown in Fig. 18. Figure 18(a) displays the relative increment AHc/H,oin the bubble collapse field as a function of annealing temperatures for iron, arsenic and gallium. It can be seen that there is a certain similarity in the evolution of the phenomenon except for the position and for the relative value of the maximum for iron compared with those for arsenic and gallium implants. More direct evidence of this specificity is given in Fig. 18(b) showing the relative intensity iR which is proportional to 4riMs of the implanted layer for iron and gallium implantations after various thermal treatments. At 750 °C, iR for the iron implantation is greater. AHclO

/

IR 10

Hco

/

8

6 w

4

4 / /

o:.<' 400

i

6 0

(a)

800

I

0

1000 T A (°C)

400

i

i

600

i

8;0

1000 T A (°C)

(b)

Fig. 18. (a) AHc/Hcoas a function of TA for iron (0), gallium ( + ) and arsenic (A) implantation (120 keV; 1 x i016 ions c m - 2); (b) iR as a function of TAfor iron (V) and gallium (0) implantation (120 keV; 1 × 1016 ions cm -2) and for xenon implantation (O) (200 keV; 3 x 1015 ions cm - 2).

This discrepancy is probably due to the fact that some iron excess is replacing germanium atoms preferentially on d sites. We think that the reduction in ia above 750°C arises from interactions between implanted impurities and the matrix elements. This hypothesis is verified by applying the same annealing treatment to a xenon implantation (200 keV; 3 x 1015 Xe ÷ ions cm-2), xenon being an inert element. In this case, as shown by the broken curve, iR increases monotonically 9°. 8. CONCLUSION

Firstly it is important to notice, from all the published work, that the general trends of the implantation effects can be deduced from studies made with garnets of any composition.

IMPLANTATION OF BUBBLE GARNETS

29

The magnetic properties for neon implantation exhibit many common features with those for other inert ions (boron, helium, nitrogen etc.), both in their evolution with the dose and their annealing behaviour. The general description of the phenomena seems to be fairly well understood even though many of the details are not resolved and will probably never be, on account of the complexity of garnets. When the ion dose of an inert element is progressively increased, on the one hand a strain is set up, which is initially proportional to the stress and gives rise to a magnetostrictive mechanism, and on the other hand damage (i.e. broken bonds and displaced atoms) is produced. The latter process tends to cause relaxation of the structure and to change little by little the ferrimagnetic implanted film into a paramagnetic layer and then into an amorphous layer. The annealing behaviour of a damaged crystalline layer is markedly different from that of an amorphous layer. The damaged crystalline film starts to reorder itself at a much lower temperature than that for the epitaxial regrowth of the amorphous layer. It has been observed that the type and extent of crystalline damage due to an incident ion or cascade depends greatly on the atomic number of the ion and that this type of defect has a direct influence on the magnetic characteristics of the garnet. Thus, if the technological interest is sustained, further investigations on the annealing mechanisms of these different damage clusters as a function of the implanted species would be a matter of great fundamental as well as practical importance. For H + implantation the problem is still pending. It appears to be quite clear that the presence of hydrogen interferes chemically with some elements of the garnet, having a specific action on the magnetization. Hints of interaction between hydrogen and iron on d sites have been found. Nevertheless the exact mechanism of the particular hydrogen effects requires further experiments. The annealing behaviour of highly implanted garnet layers indicates that (a) an epitaxial regrowth mechanism starts above 600 °C and (b) the rebuilt layer has characteristics specific to the implanted species. This new procedure affords a means of altering the chemical composition of a garnet by implanting a chosen element at a given depth and at a given concentration. In the present case this concentration can often exceed the equilibrium solid solubility value. Therefore, because of this the implanted layer is a metastable state. The next step in the research should probably be to keep the system out of equilibrium by annealing the implanted layer with a laser or an electron beam rather than with an ordinary furnace, as has been done for H ÷ implantation 17. REFERENCES 1 J.C. North and R. Wolfe, Ion implantation effects in bubble garnets. In B. L. Crowder (ed.), Ion Implantation in Semiconductors andother Materials, Plenum, New York, 1973, p. 505. 2 R. Wolfe, J. C. North and Y. P. Lai, Appl. Phys. Lett., 22 (1973) 683. 3 J.E. Davies, IBMJ. Res. Dev., 21 (1977) 522. 4 J. Engemann and T. Hsu, Appl. Phys. Lett., 30 (1977) 125. 5 H. Jouve, J. Appl. Phys., 50 (1979) 2246. 6 H.A. Washburn and G. Galli, J. Appl. Phys., 50 (1979) 2267. 7 K. Komenou, I. Hirai, K. Asama and M. Sakai, J. Appl. Phys., 49 (1978) 5816. 8 R. Hirko and K. Ju, IEEE Trans. Magn., 16 (1980) 958. 9 P.J. Picone and A. H. Morrish, J. Appl. Phys., 53 (1982) 2471. 10 W.H. de Roode and H. A. Algra, J. Appl. Phys., 53 (1982) 2507.

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