Microscopic and macroscopic inhomogeneity of magnetization and anistropy in amorphous rare earth/transition metal films

D. HAFNER and H. HOFFMANN: Inhomogeneity of Magnetization in Amorphous Films 549

phys. stat. sol. (a) 52, 549 (1979)

Subject classification: 18.2.1 and 20.1; 2; 21.1; 21.4

Institut fu r Angewandte Physik der Universitut Reyensburgl)

Microscopic and Macroscopic Inhomogeneity of Magnetization and Anistropy in Amorphous Rare Earth/Transition Metal Films

BY D. HAFNER and H. HOFFMANN

Amorphous Gd/Co and Gd/Co/Mo-films are investigated by measuring the field dependence of the susceptibility. This allows a determination of the value and sign of the perpendicular uniaxial anisotropy as well as the value of the effective ripple stray field. The measurements are made at spots of 50 to 100 ym diameter, allowing one to scan the film surface. Measurements from a spot on the film surface and at the opposing spot on the film-glass substrate interface are performed simul- taneously. I n this way the anisotropy at two related points on both surfaces of the film can be compared. I n general the results show the existence of a ripple stray field which can be accounted for by inhomogeneities in the amorphous films. The perpendicular anisotropy at the free film surface is always lower than the perpendicular anisotropy at the film-substrate interface. I n some cases the magnetization at the film surface is in-plane, while a t the substrate the magnetization is out-of- plane. The reduction of the perpendicular anisotropy is a n ageing effect due to oxidation.

dmorphe Gd/Co- und Gd/Co/Mo-Schichten werden uber die Feldabhangigkeit der Suszeptibilitat untersucht. Auf diese Weise konnen Vorzeichen und Betrag der senkrechten Anisotropie wie auch das effektive Ripple-Streufeld bestimmt werden. Die Messungen werden mit Hilfe des magneto- optischen Kerr-Effekts auf Flecken von 50 bis 100 pm Durchmesser durchgefuhrt. Die MeDanord- nung erlaubt die gleichzeitige Untersuchung zweier auf Vorder- und Ruckseite der Schicht gegen- uberliegender Flecken. Die Ergebnisse zeigen auf der Schichtoberseite grundsatzlich eine kleinere Snisotropie als auf der Substratseite. Sie zeigen ferner, daD ein betrachtliches Ripple-Streufeld in den amorphen Schichten existiert.

1. Introduction Amorphous ferro- or ferrimagnetic fiinis are of interest for both basic research and magnetic device applications. Alloy films of rare earth and transition metals evaporated or sputtered under certain conditions, or treated by annealing after deposition, exhibit a uniaxial anisotropy perpendicular to the film surface. The sources of this anisotropy have been discussed in a number of papers. It is assumed by one of the authors [ l ] that short-range pair ordering, which leads to dipolar energy or pseudo-dipolar exchange energy, can explain all the observations mentioned in the literature.

Besides this problem of the sources of the anisotropy, the absolute value of the uniaxial anisotropy constant

is of interest, especially for device applications. The homogeneity of this constant as well as its behaviour with ageing is of further fundamental interest.

The numerical values for the anisotropy as published in the literature result from integral nieasurements over the whole sample, thus giving effectively a mean value.

I ) 84 Regensburg, BRD.

3K*

550 D. HAFNER and H. HOFFMANN

Fig. 1. The disc-shaped film F A (diameter = 1 cm) with the investigated spots F, of about 50 pm diameter including the small scale inhomogeneities (points) of about 10 nm diameter

The methods discussed to date are ferromagnetic resonance [2 , 31, vibrating sample magnetometer [4 to 61, loop traces [7], and domain observations [8]. Only the last one gives something like local values.

I n practice a bubble with a diameter of a few microns will experience the anisotropy on a similar scale and not the mean value for the film. By being shifted over the film it will experience different values a t different sites. It thus seems useful to investigate the anisotropy on as small a scale as possible, in order to measure the homogeneity of this small-scale anisotropy as well as the ripple stray field due to the local anisotropy. The latter gives the contribution of the inhomogeneous part of the anisotropy within this small scale (Fig. 1) . For these purposes the usual integral methods of determining anisotropies are inadequate.

Apart from the absolute value of the uniaxial anisotropy and its distribution on scales larger than 50 pm spot diameter the “local” anisotropy in the amorphous films is of interest. This anisotropy is caused by inhomogeneities on a scale much smaller than the 50 pm spot diameter. For instance this local anisotropy will result from density fluctuations as revealed by the existence of columns [l, 7, 9 to 121 of about 10 nni diameter in the films. Even though all the authors try to neglect this fluctuating local anisotropy it gives rise to divergences of the magnetization leading to the internal ripple stray field, which for soft magnetic films is a well-known fact [13]. One would expect a ripple stray field to exist in the amorphous RE/TM films also. I t s absolute value is of fundamental importance for the application of the films as bubble material.

2. The Method The anisotropy and the ripple stray field are investigated by measuring the dependence of the susceptibility on an applied dc field. The method has also been used for the investigation of soft magnetic transition metal films [14].

The field arrangement is given in Fig. 2 . An in-plane dc field which could be varied between 0 and 16 kOe was applied to saturate the magnetization in the film plane. Perpendicular to it an ac tickle field of a frequency 200 to 400 Hz and an amplitude 5 to 15 Oe was applied, which causes an ac perpendicular component of the magnetization. This component is observed by means of the polar Kerr effect.

The optical arrangement for observing the magneto-optic Kerr effect is given in Fig. 3. The laser light passes through a polarizer. The beam 1 + 2 is then split into two beams 1 and 2 . Beam 1 is focussed by an adjustable lens system through a glass substrate plate onto the film surface into a spot of about 50 p i diameter. Beam 2

Fig. 2. Field arrangement for susceptibility measurement

TEJI and Auger Electron Spectroscopy Studies of Boron Implanted Silicon 551

- - a-" f-0min

$6- ? - 6

% f=Omin

G 4 -

\

,,' '\ I

1 32 04 06 08 0 02 04 06 08 depth hm I - depth kml-

Fin. 6 Fie. 7

Fig. 6. Experimental determined B depth concentration profile (solid curve) compared with theoretical expected distribution after an anneal of 300 min a t 1135 "C

Fig. 7. Experimental determined variation in B depth concentration profile with time of anneal, t , a t 1135 "C

where to is time parameter which describes the initial distribution a t time, t = 0. The diffusion coefficient! D = Do e-QIkT was calculated from the values Do = 5.1 cm2 s-l and an activation energy Q = 3.7 eV for B in Si [8]. The parameter OL is determined from the initial conditions and would correspond to a surface concentration a t the centre of the distribution at time t, = 0. For temperature up to 1050 "C, anneals as long as one hour gave no deviation from the experimental curve for an unannealed sample shown in Fig. 6.

The experimental results for B distribution following anneal a t 1135 "C were very different from that predicted by the above theoretical expression as seen in Fig. 7. Four notable differences are observed. Firstly, the maximum concentration of boron decreases a t an order of magnitude more rapidly than theory predicts. Secondly, the shape of the curve does not change appreciably from its original bell-shape with time. Thirdly, the area under the curve, which represents the total implanted boron present, decreases rapidly in the originally damaged region with time of anneal, indicating a loss of boron and, finally, a build-up of B becomes detectable a t the surface of im- plantation.

4. Discussion

The implanted boron atoms produce most of its damage a t the end of their trajectories in the vicinity of the theoretically predicted projected range. I n this region elastic displacement damage is the dominant collision process and is responsible for the creation of a large number of vacancies and interstitials. These defects together with the high concentration of implanted boron atoms produce a volume expansion in the damage zone which generates a tensile stress in the surrounding material. On heating the sample to temperatures of about 800 "C dislocation sources are activated in the damage zone and spread into the material, initially only towards the region away from the implant surface in order to relieve the stress set up between the good and bad material. The micrograph shown in Fig. 8 shows how such dislocations, in the form of half-loops, spread out into the good material to eventually form dislocation lines which align themselves roughly along the (110) directions parallel to the damage zone. An analysis of these dislocations show that they are of the (110) type which glide on the (111) surfaces inclined to the bombarded surface. They predominantly

552 D. HAFNER and H. HOFBMANN

The effective field consists of the applied field, Ha, the resulting anisotropy field,

(4)

H i , and the effective demagnetizing ripple field, H,:

Heff == H a - Hl'L + H , where

H' = H a - H i is the ideal effective field of a total homogeneous (ideal) film and

H , = B H ' - " . (6) Relation (6) follows from a two-dimensional ripple theory, which assumes the applied field is strong enough so that we can neglect ripple components perpendicular to the film plane. The general calculation follows the line of ripple field discussion as given by one of the authors [15] for permalloy films. I n (6) B is a material constant depend- ing on the anisotropy, exchange constant, magnetization and in particular on the local anisotropy. B = 0 describes an ideal film without any small scale inhomogeneity. Thus the numerical value of B is a measure of the film quality with respect to the homogeneity. The power, --a, of H' depends on the coupling volume of the ripple and on the small scale volume of the homogeneity, such as the column volume in the present case. The value of a for the measurements in the field interval H i < H a < 10 kOe is calculated to be a = 1. The calculation will be given elsewhere.

The total effective field then is given by

Heff z= H , - H;, + B ( H , - Hi)-' . (7 1 For the described field arrangement the transverse biased susceptibility x is related to the effective field by

The reciprocal of the susceptibility 1 1 - = - { ( H a - x Ms

plotted versus the applied field allows by proper fitting the determination of H i and the sample constant B. This, accomplished spot by spot, allows an exhaustive investigation of the sample quality.

3. The Films

Evaporated as well as sputtered RE/TM films have been investigated. The purpose of this paper is to demonstrate the existence of inhomogeneities and ripple stray field rather than to show their dependence on film composition and film production. Therefore, only five typical films are discussed as examples in detail. Film 1 : evaporated Gd/Co,

composition 28.9 atyo Gd, 71.1 atyo Co, film thickness d = 0.5 pm, oxygen partial pressure during evaporation po, = 6 x lo-* Tom.

composition: 17 atyo Gd, 83 atyo Co, bias voltage -85 V, argon atmosphere p~~ = 1.8 x

Film 2 : sputtered Gd/Co,

Torr,

Inhomogeneity of Magnetization and Anisotropy in Amorphous Films 553

substrate temperature 70 "C, film thickness d 1: 0.45 pm.

composition 17.1 atyo Gd, 82.9 a t% Co, bias voltage -85 V, argon atmosphere par = 1.8 x substrate temperature 70 "C, film thickness d ;r 0.24 pm.

composition 18 atyo Gd, 69 atyo Co, 9 atyo Mo, 4 atyo Ar, film thickness d = 2.0 pm.

composition 18 atyo Gd, 69 atyo Co, 9 atyo Mo, 4 atyo Ar, film thickness d = 3.2 pm.

Film 3 : sputtered Gd/Co,

Torr,

Fliin 4 : sputtered Gd/Co/Mo,

Film 5 : sputtered Gd/Co/Mo,

9 1 1 of these films are representative of amorphous uniaxial films with resulting perpendicular anisotropy, i.e. they showed stripe domains. None of the films were investigated in the as-deposited state, but only some weeks after deposition. If there exist ageing effects, then the films were aged.

4. Measurements The transverse biased susceptibility was measured a t decreasing in-plane field H , after the magnetization had been saturated in the film plane with fields of 16 kOe. At decreasing field Ha the susceptibility x increases, as demonstrated in Fig. 4. A t the ordinate the output signal of the lock-in amplifier Y N x is plotted as scale.

o r

The reciprocal of the susceptibility increases with increasing field. It follows a straight line a t large fields, which gives the reciprocal of the susceptibility of an ideal sample, and deviates from the straight line at lower fields.

The extrapolation of the straight line to the abscissa gives a first approximation for the resulting anisotropy field H1; after (3). The better results can be achieved by a computer fit of the measured susceptibility curve to (9) or (9a). The fit then also gives the value of B, which characterizes the quality of the local homogeneity of the film magnetization. A straightforward way of obtaining approximate values of

40 t - L

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20

10

0

Fig. 4. (1) Susceptibility (x = M,/(H' + ; 2 . 2 W - I ) ) , ( 2 ) reciprocal of the susceptibility (1/x = a(W f + 2.2H'-')), (3) reciprocal of the ideal susceptibility (1/x = a{H, - ( H k - 4n%,)} = aH') and (4) losses vs. applied in-plane field Haatthefree surface of film 1

8 12 16 H, ik0el- - p i 4

iik -05kOe

5 54 D. HAFNER and H. HOFFMANN

B is to plot the reciprocal output signal divided by the ideal effective field (Ha - H i ) versus the reciprocal of the square of the ideal effective field:

= u + aB(H, - Hi)- ' . (10) 1 1 Y (Ha - H i )

The plot should give a straight line. It can be used to control how accurate H i has been determined by the extrapolation of the 1/x curve. The apparatus constant a and the film parameter B can be determined if the plot gives approximately a straight line.

The dash-dotted curve of Fig. 4 was measured by the lock-in amplifier, phase shifted by 90" against the susceptibility curve. This curve gives the losses, which are very low a t larger applied field and which increase rapidly when the field approaches the resulting anisotropy field Hi<.

5. Results The measured curves taken a t the surface for film 1 are given in Fig. 4. The reciprocal of the susceptibility curve (open circles and dashed curve) fits the relation of (8a) exactly, including the (Ha - H i ) - 1 term for the ripple stray field.

The extrapolation of the straight line fit gives H i = 0.5 x lo3 Oe. The horizontal deviation of the dashed curve from the straight line gives the mean value of the ripple stray field, which for instance in the case of film 1 equals 1.3 kOe a t an in-plane field H a = 2 kOe. The effective demagnetizing field in this situation is of the same order of magnitude as the applied field.

I n Fig. 5 the measured and calculated curves for film 2 which belong to the free surface of this film are plotted. For H a < 6 kOe the open-circle dashed curve fits (9a) well, whereas above 6 kOe the measured values deviate from the expected curve. The deviation is open to further investigation.

The resulting anisotropy field HL = -0.9 x lo3 Oe is smaller than zero. The niag- netization a t the film surface has an in-plane easy axis. The perpendicular anisotropy K , is too small to overcome the demagnetizing anisotropy 27cM;.

I n Fig. 6 measured and calculated curves for film 2, the same film as in Fig. 5, are plotted. This figure gives the values a t a spot a t the surface-film interface just. oppo- site to the spot which was investigated in Fig. 5. The amount of theresultinganisotropy

Fig. 5 Fig. 6 Fig. 5 . (1) Susceptibility, ( 2 ) reciprocal of the susceptibility, and (3) reciprocal of the ideal susceptibility vs. applied field Ha a t the free surface of film 2 Fig. 6. (1) Susceptibility, (2) reciprocal of the susceptibility, and (3) reciprocal of the ideal 8us- ceptibility vs. applied in-plane field Ha a t the film-substrate interface of film 2


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field HL is quite different from the value a t the film surface (Fig. 5). I n contrast to the film surface, a t the substrate-film interface the easy axis of magnetization isperpen- dicular to the film plane.

The 1/x curve fits perfectly to @a).

Fig. 7 represents the measured and calculated curves for film 3. Two opposing spots in the centre of the two surfaces were investigated, leading to two very different values of HI;. A t the free surface again the easy axis of magnetization is in- plane, while a t the film-substrate interface i t is perpendicular to the film.

Fig. 8 gives the same measure ments for a Gd/Co/Mo film (film 4). Again strong differences in the HI; values a t both surfaces occur even though in the case of Fig. 8 the easy axis of magnetization a t both surfaces is perpendicular t o the film plane.

Table 1 summarizes the results of the measurements. I n general, the uniaxial perpendicular anisotropy constant K , a t the film surface is smaller than a t the film-substrate interface. This indicates a strong anisotropy gradient through the film thickness. The decrease of the perpendicular anisotropy in Gd/Co/Mo films is much smaller than in Gd/Co films. There may also be an influence of the film thickness. More investigations are needed to clarify this point.

The small-scale inhomogeneity parallel t o the film plane is expres- sed by the value of B in Table 1. This inhomogeneity leads to the ripple stray field. I t s value in units of the anisotropy field is given by B when the applied field H , = 2111;. One realizes that a t the surface of the evaporatedand sputtered Gd/Co films the ripple stray field

556 D. HAFNER and H. HOFFMANN

4 I-

Fig. 7. (1) and (1') Susceptibility, (2) and (2') reciprocal of the susceptibility, and (3) and (3') reciprocal of the ideal susceptibility vs. applied in-plane field Ha. Curves 1, 2, and 3 belong to the free surface of film 3, while curves l', 2', and 3' was measured a t the film-substrate interface of the same film Fig. 8. Curves like in Fig. 7 for film 4

parallel to t'he film plane is of the same order of magnitude as the applied field A number of propert'ies are expected to be influenced by this fact'. The ripple stray field in the sput.tered Gd/Co/Mo filiiis is one order of magnit,ude smaller than in t.he Gd/Co films indicating that these films are more homogeneous.

In all cases, except' film 4, the homogeneity a t the film-substrate interface is better t'han a t the film surface.

The long-range homogeneity was tested by the spot to spot scanning of the snscep- t,ibility measurement,. These preliminary investigat,ions were carried out at. the free surface with film 5. At' five spots either in the film centre or not very far from it the values of Table 2 were found. I n the film centre the H i values were scat'tered by about 10% around the mean value. The scatter of the values increases strongly close to the film edge. This film has also relatively low absolute values B of the small-range inhomogeneity, but their scattering is much larger than that of the Hi<-values.

Such tests of homogeneity can be carried out without first making bubble devices and then looking for bubble movements. Filnis with good homogeneity will be the best bubble films.

Table 2 Fluctuation of the perpendicular anisotropy field H i and ripple stray field constant B. Spots 1 to 4 were located in the film centre, while spot 5 was out-of centre

1 1 .0 0.066 2 1 .o 0.17 3 1.1 0.15 4 1.2 0.034 5 -0.6 large

-~ ~ ~ ~ _ _ _ ~~

surhce

$;fi' Fig. 9. Model of oxygen distribution in films causing and destroying the uniaxial &, lar anisotropy

the Gd/Co perpendicu-


b

.'\ n I 1

, l a ~ 01 02 03 04 I

I D substrate I 07 02 ~

film pi& deFA il...i- film profile depthbm)- : J r h siihstrait.

Fig. 10. Depth profiling and elemental analysis of a ) film 2 and b) film 3

6. Discussion

The main result of the investigation is to he seen in the existence of AK, -t 0 in Table 1. The uniaxial perpendicular anisotropy a t the free film surface is always much smaller than a t the film-substrate interface. It is assumed that this decrease of the anisotropy is caused by ageing effects as can be explained by the model for forniation of the perpendicular anisotropy, given by one of the authors [l]. Oxygen, which causes the anisotropic pair ordering of excess Co moments along certain paths perpendicular t o the film plane, in the ageing effect diffuses into the whole surface, deactivating the Gd moments by forming gadolinium oxide. The oxygen diffusion into the surface layer now causes randoni spattering of excess Co moments which is now isotropic and and which cancels the original anisotropic distribution of exaess moments in the surface layer, as schematically drawn in Fig. 9. This interpretation is supported by Auger elemental analysis and ion etching depth profiling. Fig. 10a gives for film 2 the coin- position through the film thickness, and Fig. 10b gives the same for film 3. I n both cases oxygen enrichment is to be seen within the first 50 nm surface layer. This oxygen enrichmefit in many cases is accompanied by an enrichment of gadolinium as shown in Fig. 10 b.

The diffusion of oxygen into the surface layer also increases the small scale inhonio- geneities which can be caused for instance by local stresses due to the forniation of gadolinium oxide.

The results very clearly show the effect of oxygen on the perpendicular uniaxial anisotropy. Used just in the right concentration it causes the anisotropy, but if there is excess oxygen i t decreases or destroys the anisotropy. It is responsible for unwanted ageing effects. However once the role of oxygen is known, its disadvantage can be turned to an advantage by proper oxygen treatment of the RE/TM films.

Acknowledgements

This work has been supported by the Deutsche Forschungsgemeinschaft.

film 1 (and similar films). The authors are indebted to Dr. Brunsch, IBM Germany, Sindelfingen for preparing

References [l] H. HOFFMANN, A. J. OWEN, and F. SCHROPF, phys. stat. sol. (a) 52, 161 (1979). [2] D. C. CRONEMIEYER, AIP Conf. Proc. 18, 85 (1974). [3] R. C. TAYLOR and A. GANGULEE, J. appl. Phys. 47,4666 (1976). [4] A. BRUNSCR and J. SCRNEIDER, J. appl. Phys. 48, 2641 (1977). [5] R. C. TAYLOR, J. appl. Phys. 4i, 1164 (1976). [6] NEIL HEIMAN, N. KAZAMA, D. F. KYSER, andV. J. MINKIEWICZ, J. appl. Phys. 49,366 (1978).

558 D. HAFNER and H. HOFFMANB: Inhomogeneity of Magnetization in Amorphous Films

[7] S. R. HERD, MMM, Minneapolis 1977. [8] S. S. NANDRA and P. J. GRUNDY, phys. stat. sol. (a) 41, 65 (1977). [9] J. H. LEAMY and A. G. DIRKS, J. Phys. D 10, L95 (1977).

[lo] S. R. HERD and P. CHAUDHARI, phys. stat. sol. (a) 18, 603 (1973). [ll] A. G. DIRKS and H. J. LEAMY, Thin Solid Films 47, 219 (1977). [12] S. R. HERD, phys. stat. sol. (a) 44, 363 (1977). [13] For instance H. Ic3. WIEDEMANN and H. HOFFMANN, Z. angew. Phys. 30, 117 (1970).

K. KEMPTER and H. HOFFMANN, J. Physique 32, 396 (1970). [14] K. KEMPTER and H. HOFBMANN, 2. angew. Phys. 28,251 (1970). [15] H. HOFFMANN, phys. stat. sol. 33, 175 (1969).

(Received J a n u a r y 31, 1979)

Note added in proof: The values of 432 M , are mean values for the whole film. Due to oxidation a t the surface one has to expect 432 M , valuet a t the surface to deviate from the mean values. This influences the amount of the calculated values of K , a t the surface.

Documents

Microscopic and macroscopic inhomogeneity of magnetization and anistropy in amorphous rare earth/transition metal films