Single crystal Fe films grown on GaAs substrates

Journal of Magnetism and Magnetic Materials 28 (1982) 299-304 North-Holland Publishing Company

299

SINGLE CRYSTAL Fe FILMS GROWN ON GaAs SUBSTRATES W. WETTLING, R.S. SMITH, W. JANTZ and P.M. GANSER Fraunhofer-Institut fiir Angewandte Festk6rperphysilc, Eckerstrasse 4, D-7800 Freiburg, Fed. Rep. Germany Received 16 April 1982

Fe films epitaxially grown on GaAs substrates have been prepared and investigated. On { 110} surfaces, single crystal films are obtained. The magnetic properties are investigated using ferromagnetic resonance and inelastic light scattering from surface magnons. A satisfactory interpretation is obtained by assuming cubic and growth-induced tilted uniaxial anisotropy.

1. Introduction

2. Sample preparation

It has recently been demonstrated [1,2], that epitaxial growth of iron films on GaAs substrates is possible. Preliminary ferromagnetic resonance (FMR) data [2] demonstrate excellent single crystal quality of these films, as indicated by a very small F M R linewidth and the anisotropic variation of the field for resonance, as is typical for single crystal specimens. Therefore, this new compound material has great appeal for possible monolithic integration of magnetic device components into microwave circuitry. But even without taking into account the ferromagnetic properties of Fe, the feasibility of epitaxial metal deposition on GaAs is quite interesting for the study of metal-semiconductors interfaces [3-7]. Stimulated by these recent achievements, we have prepared Fe films on GaAs by molecular beam epitaxy (MBE). In this first report, we only briefly refer to the fabrication techniques. An experimental analysis using FMR and Brillouin Scattering (BS) is presented. The data are interpreted in terms of magnetostatic theory, yielding a set of parameters that describe the magnetic properties of iron films on GaAs.

Fig. 1 shows the {110} surfaces of GaAs and Fe, demonstrating the good, although not quite perfect, lattice match. The deviation of the lattice constants is 1.4%. Fe single crystal films with different thicknesses ranging from 10 to 200 nm were grown on both (100} and { 110} GaAs substrate surfaces. The following procedures and 0 Ga

[] 0

[] r~

[]

[]

[]

[]

[]

[]

[]

[]

[]

[]

13

[]

[]

[] r',l

[] 0

13

Ill

[] []

[] 0

[]

[]

" Fe

[] r'l

13

I~I

~l

[] Am

[] 13

[] I-,I

r,I

Fig. 1. {i10} surfaces of the GaAs and the bcc Fe lattice.

0304-8853/82/0000-0000/$02.75 © 1982 North-Holland

300

W. Wettling et aL / Fe films grown on GaAs substrates

parameters were maintained during fabrication: a) Sub~trate surface preparation by chemical etching with H2SO4:H202:H20 (5:1:1) to form a protective oxide layer. b) Heating of the sample mounted in the vacuum chamber to 550°C for 5 min to remove the oxide layer. c) Film deposition, with a Fe source temperature about 1200°C and a substrate temperature between 200 and 300°C. d) Crystal quality control during growth using insitu reflection high energy electron diffraction (RHEED). Rotating crystal X-ray analysis confirmed that Fe deposited on {110} surfaces forms bcc single crystal films, with only a small misalignment of the Fe (110} axis with respect to the GaAs (110} axis. By contrast, films grown on (100} substrates were found to be polycrystalline. The (001} axes in the film plane of the individual crystal islands were found to be rotated -+45 ° in the film plane with respect to the (001} axis of the substrate. Such samples did not exhibit F M R lines and were not investigated any further. Small film thicknesses on the order of 10 nm were measured using the intensity of X-ray fluorescence. All investigations described were done on sample 535b, with thickness d = 11 nm ± 10%, grown on a { 110} Cr doped semi-insulating substrate.

3. FMR investigations A commercial Varian EPR spectrometer, operating at Q-band ( ~ 35 GHz) was used for F M R studies. Thus far, only measurements at room temperature were performed. The sample was mounted such that the external magnetic field H could be rotated in the plane of the sample. The orientation ~k, together with 0 defining the resulting orientation of the saturated magnetization 4~rM, is illustrated in fig. 2. To be specific the film plane is defined (110). It is assumed that, as long as H remains in the sample plane, M will do so too, because the strong surface demagnetization overdominates cubic anisotropy.

C001]

CO/O]

(

)

COlO]

cli0] [100

1110] Fig. 2. Coordinate system defining film plane, magnetic field and magnetization direction with respect to the cubic crystallographic axes.

F M R data, as presented below, can be interpreted by generalizing the calculations, first given by Artmann [8] for the case of cubic anisotropy and spherical samples, by additionally including surface demagnetization and noncubic anisotropy contributions. At low microwave frequencies (e.g. 9 GHz as reported [2]), the small external field needed for F M R results in large orientational differences between M and H (0 # ~k)- Therefore, the approximate analytic expressions valid for quasi-lineup of M and H must not be used and a numerical evaluation is necessary. Such complications are largely absent at the higher Q-band resonance frequency. Here, the uniform precession F M R requires H ~ 6 kOe (see fig. 4), such that, for H in the film plane, H [IM is satisfied well enough to use quasi-alignment formulas. Then, for thin films, p =H~ (H;

+4qrM),

(1)

where ~ is the frequency, y = 1.85 × l07 (Oe s)- ' is the gyromagnetic ratio, and

H~i):H+H(i)+H(~i);

i:1,2.

(2)

Here we anticipate a growth-induced anisotropy H (i) to be discussed below. The contributions H~(~) of the cubic crystalline anisotropy, to first order,

W. Wettling et aL / Fe films grown on GaAs substrates

301

3t..97 5 GHz 6t

A

Fe on gaAs

ii/i/

x

34. 970 GHz

6,~

~// [111]

AH = 101 0e

6~

62 ~r

6.5

6.6

6.7

6.8

6.9

MAGNETIC F I E L D

60 0

.

I

30

,

•

/

.

.

I

7.1

Fig. 4. Q-band ferromagnetic resonance line (derivative).

[llO] •

7.0 (kOo)

,

.

I

,

.

I

60 90 120 150 ANGLE 8 (Degrees)

.

,

180

Fig. 3. Ferromagnetic resonance field dependence on the orientation of the external magnetic field in the Fe film plane. The solid and dashed lines are calculated, as explained in the text.

may be written [8]: HoD)= -~K] (2 - sin 0 - 3 sin22 0),

(3a)

H~2) = ~K1 - ( 2 - 4 s i n 0 - ~ 3- sin22 0).

(3b)

The experimental angular variation is shown in fig. 3. The dashed line, calculated from the above equations assuming H,°) = 0, is seen to explain the data reasonably well. Nevertheless, a significantly improved fit is obtained by assuming an additional uniaxial anisotropy with its easy axis pointing along a direction tilted away from [001] towards [111] by an angle q,. The contributions to the effective magnetic fields read: H~,) = 2Ko [ 1 - 2 sin2(0- ¢)], M

(4a)

H~2) = ~Ku - [ 2 - 2 sin2(O- 4,)].

(4b)

The solid line in fig. 3 is based on these generalized relations. The resulting parameters f mgnetization, cubic and tilted uniaxial anisotropy are listed in table 1. The oblique orientation of the uniaxial anisotropy is necessary to reproduce the different resonance field values observed for HIP [111] and HJl [111]. Fig. 4 demonstrates the FMR linewidth AH obtained at Q-band. It is important to realize that, from the above relations, one has 8qr2pAp

',

A H - - m y2

(5)

i.e. the apparent AH, for a given relaxation rate Av, increases with frequency and is reduced by the surface demagnetization term, unless H ~ , 4 , r M . These circumstances must be kept in mind when

Table 1 Material parameters, as discussed in the text, of an iron film grown epitaxially on semi-insulating GaAs substrate Method

Fig.

4*rm (kG)

K I / M (kG)

K u / M (kG)

e# (deg)

FMR BS BS

3 6 7

15.6

0.21 0.21 0.2

0.04 0.14

8 8

15.6 16

d (rim)

I1 12

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W. Wettling et al. / Fe films grown on GaAs substrates

comparing data obtained at different frequencies or with differently shaped samples. For instance, iron whiskers whose shape is approximately a long cylinder, simply have Ap = (y/2~r)AH. Eq. (5) is again based on the quasi-alignment condition, thus a direct comparison with AH observed at X-band is not adviseable.

The Brillouin Scattering (BS) technique has been applied successfully to the study of magnetic excitations [9] after the development of high contrast multipass Farbry Perot spectrometers [10]. If the Faraday rotation and hence the scattering efficiency is high, even strongly absorbing materials, such as metals, may be investigated [11]. In this case, the most intensive inelastic scattering arises from surface magnons [12]. As a rule, by BS one does not observe the uniform precession, but excitations with finite wavevector kin, because the inelastic light scattering involves changes in energy and momentum of the scattered photon that must both be compensated by the scattering magnon. We used the backscattering configuration with adjustable angle of incidence 6, as illustrated in fig. 7, insert. The vector diagram shows that the magnon wavevector k m equals twice the component k~'nc(parallel to the film plane) of the inealstically scattered photon.

H=0,5

18

r \

/

,s[

4. Inelastic light scattering investigations

~00

z

kOe // [ 0 0 1 ]

0

30

60 ANGLE

90 120 0 (Degrees)

150

180

Fig. 6. Surface magnon frequency dependence on the orientation of the external magnetic field ( H = k O e ) in the film plane. The angle of incidence of the laser beam in 45 ° , corresponding to k m = l . 7 3 X 1 0 5 cm - I .

Thus, the scattering geometry determines k m. The magnon energy is obtained from the photon frequency shift as recorded in the BS spectrum shown in fig. 5. Both surface magnon (SM) and phonon (Ph) lines are recorded. The latter will not be discussed here. The appearance of the SM line only at the Stokes (or Antistokes) side of the spectrum is a characteristic feature of surface magnons [12]. The angular variation of the surface magnon frequency Pm upon rotating H in the (110) film plane is shown in fig. 6. H = 1 kOe was kept constant as well as ~ = 45 °, corresponding t o k m = 1.73 × 105 cm -1. As in the previous section, a pattern suggesting uniaxial anisotropy tilted away from [001] is observed. A quantitative interpretation of these data is obtained using the magnetostatic surface wave theory of Damon and Eshbach [13], giving

(~_)2 t'm2=H~l)(H}2)+4"rrM)

tn 300 1-

SH

o 200 (_l

Ph

Ph

+ (4~rM)ZA(kd),

100

r~

!l'

i -S0

i -t.0

- 3 0 -20 -10 FREQUENCY

0 +10 +20 SHIFT (GHz)

.

+30

.

+t.0

A(x)

i +50

Fig. 5. Inelastic light scattering spectrum. The free spectral range is indicated by the central and two neighboring Rayleigh lines. Inelastic light scattering arises from phonons (Ph) and surface m a g n o n s (SM).

(6)

where = ¼[1 - exp(--2x)] and Hr~1)= H~2) = H for isotropic materials. Without rigorous justification, by way of analogy with the F M R case, we introduce crystalline anisotropy into this formula by using eqs. (3a, b) and (4a, b). The solid line in fig. 6 is thus calculated, using parameters as given in table 1.

W. Wettling et aL / Fe films grown on GaAs substrates

50

o

t

[00!:] I

i~

HII

i~ k i nc .

km

25°

'

~

I0°

> 30

L3 Z UJ

o 20 w IIC u.

l0 I

I

I

I

I

0.5

1

1.5

2

2.5

MAGNETIC

FIELD H(kOe)

Fig. 7. Magnetic field dependence of the surface magnon frequency for three different magnon wavevectors kin= (1.73,0.77,0.35)×105 cm -1 (top to bottom). The insert illustrates the scattering geometry that allows to select a specific k m•

Upon varying ~, BS from surface magnons with different k m is obtained and may be compared with the theoretical wavevector dependence given by eq. (6). Experimental and theoretical results are given in fig. 7, displaying the field dependence of ~'m for three different wavevectors. We emphasize that all data are well described simultaneously by one set of parameters, as listed in table 1. Hence, these measurements allow an independent determination of d in good agreement with the result obtained from X-ray fluorescence, see section 2.

303

fluence of the substrate material on the magnetic properties of the film, e.g. originating from the compressive strain caused by the 1.4% mismatch (fig. 1). In addition, or alternatively, a further growth-induced anisotropy with easy axis perpendicular to the film, not considered in the above analysis, might be responsible. Supporting magnetometer experiments with films of different d, including temperature variation, are necessary to clarify these aspects. At present, we also do not have a satisfactory explanation for the reduction of K~ (2.6 × 105 erg cm -3) as compared to the bulk room temperature value (4.8 × 105 erg cm-3). While both FMR and BS data are well interpreted by the same 4~rM and K~ parameters, the deviation from cubic anisotropic behaviour as determined by BS is much more pronounced than has been found, with the same sample, by Q-band FMR. Consequently, the K u values needed to fit the data on the basis of a tilted uniaxial anisotropy model differ rather strongly. One obvious explanation for this discrepancy might be that BS and FMR observe different magnetic excitations (surface magnon and uniform precession, respectively). Also, the heuristic method used to introduce anisotropy into eq. (6) might well be inadequate. However, X-band FMR data, to be reported in a forthcoming paper, also yield a K , value very different from that determined by Q band FMR. More detailed experimental and theoretical investigations are required to resolve these discrepancies. Acknowledgements

5. Discussion The interpretation of FMR and BS data using magnetostatic theory has given a set of material parameters collected in table 1. It is seen that all data yield a saturation magnetization 4~rM~< 16 kG. This value is considerably smaller than that reported for bulk iron ( ~ 21 kG) and is roughly in agreement with quoted findings [14]. On the other hand, recent investigations of Fe films grown on Ag [15] exhibited comparable.reduction of 4¢rM only for films substantially thinner than our sample. These discrepancies indicate a possible in-

We are indepted to J. Schneider for proposing this investigation and many stimulating discussions. The X-ray investigations have been performed by G. Brandt. References [1] J.R. Waldrop and R.W. Grant, Appl. Phys. Lett. 34 (1979) 630. [2] G.A. Prinz and J.J. Krebs, Appl. Phys. Lett. 39 (1981)397. [3] A.Y. Cho and P.D. Dernier, J. Appl. Phys. 49 (1978) 3328. [4] R. Ludeke, L.L. Chang and L. Esaki, Appl. Phys. Lett. 23 (1973) 201.

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W. Wettling et al. / Fe films grown on GaAs substrates

[5] J. Massies and N.T. Link, Surface Sci. 114 (1982) 147. [6] J. Massies and N.T. Link, J. Crystal Growth 56 (1982) 25. [7] G.A. Prinz, J.M. Ferrari and M. Goldenberg, Appl. Phys. Lett. 40 (1982) 155. [8] J.O. Artmann, Phys. Rev. 105 (1957) 62. [9] J.R. Sandercock and W. Wettling, Solid State Commun. 13 (1973) 1732. [10] J.R. Sandercock, In: Light Scattering in Solids, ed. M. Balkanski (Flammarion, Paris, 1971) p.9. [11] J.R. Sandercock and W. Wettling, IEEE Trans. Magn. MAG-14 (1978) 442.

[12] P. Grtinberg and F. Metawe, Phys. Rev. Lett. 39 (1977) 1571. [13] R.W. Damon and J.R. Eshbach, J. Phys. Chem. Solids 19 (1961) 308. [14] G.A. Prinz, G.T. Rado and J.J. Krebs, 27th Annual Conf. on Magnetism and Magnetic Materials, Atlanta (1981) paper CB-3. [15] J. Tyson, A.H. Owens, J.C. Walker and G. Bayreuther, J. Appl. Phys. 52 (1981) 2487.