- Email: [email protected]
Magnetic anisotropy and magnetization reversal in Fe films grown on GaAs (001) and (110)
Journal of Magnetism and Magnetic Materials 148 (1995) 262-263
Journal of magnolisafl and magnetic malerlalc
N
ELSEVIER
Magnetic anisotropy and magnetization reversal in Fe films grown on GaAs (001) and (110) C. Daboo a, * M. Gester a S.J. Gray ~, J.A.C. Bland a, R.J. Hicken ~, E. Gu a, R. Ploessl b, J.N. Chapman b a Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge, CB30HE, UK Department of Pl~ysws and Astronomy, Umversity of Glasgow, Glasgow ¢712 8QQ, UK b
°
.
Abstract We present the results of a systenmtic study of the evolution of magnetic anisotropies of epitaxial Fe films grown on GaAs (001) and (110) substrates and also on epitaxial Ag buffer layers. The anisotr01ay was studied in-situ with the magneto~optie Ken- effect (MOKE). This provides a qualitative thickness dependence of the in-plane magnetic anisotropy for single films on the same substrate surface, thus avoiding any uncertainties due to different substrate morphologies. These in-situ studies are complemented by ex-situ MOKE, Brillouin Light Scattering (BLS), and Lorentz microscopy measurements which reveal the relation between the magnetization reversal processes and mag-~etic anisotropy behaviour on a macroscopic and microscopic scale.
Previous studies of the F e / G a A s system [1-3] have concentrated on ex-situ measurements of the magnetic anisotropy using techniques such as ferromagnetic resonance (FMR), vibrating sample magnet0metry (VSM) and MOKE. Such ex-situ measurements are carried out on a number of samples o f different thickness, and thus the observed thickness dependence is affected by details of the substrate preparation and morphology. In this study tile growth mechanism for Fe grown directly on GaAs has been investigated by analysing LEED spot broadening as a function of Fe thickness, which we relate to the [n-situ MOKE measurements of the magnetic anisotropies of the films. The benefit of such in-situ studies are that they are carried out on a single sample as a function of its film thickness. This eliminates problems associated with the variation in substrate preparation and morphology which usually occurs. The Fe films were deposited onto commercial GaAswafers, some with As caps and others ~rith etch stop buffer layers used for transmission electron microscopy (TEM) and Lorentz microscopy measurements. The GaAs is first heated in UHV to 600°C for at least half an hour to desorb any surface contaminants. After this step w e were unable to observe any LEED pattern, and Auger spectroscopy revealed that the surface oxide had indeed been l'emoved but the initial carbon contamination remained. Only after
* Corresponding author. Fax: +44-1225-350266; [email protected].
e-mail:
Ax+-ion bombardment and subsequent annealing at 600°C for half an hour could a c(4 × 6) reconstructed LEED pattern be obtained, with ~o carbon contamination. Fe was evaporated ~ora an e-beam source at a rate of 1 A per minute and a pressure below 5 × 1 0 - lo mbar. During growth the substrates were kept at 150°C, the average of temperatures reported to give good epitaxial growth [4,5]. During the deposition on ion bombarded and annealed substrates a LEED pattern of cubic symmetr~ was present for all thicl~esses. Sharp spots were observed for incident energies corresponding to the 3-D Bragg condition. However, for other energies the spots were broadened along the (110) directions giving rise to crossshaped spots indicating a.n irregular distribution of steps parallel to all four <110) directions on the surfac0. Fe films deposited onto Ga~s substrates that had only been heated did not show a LEED pattern until the thicl~ess exceeded ~ 15 A, indicating more disordered growth. For thicker Fe films the L E E ~ spots showed the same features as in the case of films grown on sputtered and annealed substrates. Ag buffers were grown by first depositing a 15 A Fe seed layer in a similar manner to that described above. Then a ~ 1000 ,~ Ag ~ y e r is deposited at room temperature at a rate of ~ 5 A per minute and then annealed at 300"C for one hour. LEED patterns from the Ag layer show sharp spots. The growth of Fe then proceeds as described above, except at room temperature. In this case the LEED spots are broadened along <100) directions instead of <110) directions as observed for growth of Fe
0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 3 0 4 - 8 8 5 3 ( 9 5 ) 0 0 2 2 9 . 4
(7. Daboo et aL / domrnal o f Magnetism and Magnetic Materials 148 (1995) 262-263 [1101
,, . [100] ........
_7 f j jS-- [
-0.5
0
0,5; -0.5
0
-r,
0.5 -0.5
[TlO]
[010]
J J S J J
S
20A
j
26A
S
33A
S
41A
0
_~ 0.5 -0.5
0
ssA 0.5
Field (kOe) Fig, 1. The ataisotropy behaviour of an Fe~GaAs(001) film for thicknesses between 20 A (top row) and 55 .A (bottom row). directly on OaAs. This indicates a 4-5° rotation of the surface morphology" of the Fe films grows on a Ag buffer layer compared ~vitla Fe films grown directly on OaAs. The magnetic anisotropy behaviour fcr growth of Fe directly onto GaAs and onto Ag buffer layers is found to be sensitive to the substrate preparati0m technique and growth conditioms. ]For growth on (001) GaAs surfaces we observe a thiel~ess dependent uniaxiai auisotropy which dominates at an l:e thickness of 20 ~ , but reduces with increasing Fe thickness (see Figa 1), so that the cubic anisotropy d0mSuates above 55 A. The direction of the uniaxial anis0tropy for Fe/OaAs(001) is always the same when the substlates are sputtered and n_nnealed prior to growth, in this ease the hard uniaxiat axis is along the [110] direction ~vhile the easy axis is al0mg the [110]. No significant uniaxial anisotropy is observed when Fe is grown onto a Ag buffer. For growth c f Fe directly onto G a A s ( l l 0 ) we observe the expected easy" axis switch from the [110] direction to the [001] direction at a critical thickness consistent with the competing effects of a uniaxial and cubic anisotropy, and thicker filnas have a uniaxial anisotropy with its hard axis along the [110] direction. For Fe growth on a Ag
263
buffer layer on GaAs(ll0), the Ag grows in the (111) orientation and as a result the F¢ gro~s in a tri-crystalline state, clearly visible in the LEED patterns, and there is therefore no clear anisotropy behaviottr for such films. IEx-sita MOKE vector magnetomet~ measurements allow as to study the detailed switching behaviour of Fe/~3aA_s films as a function of the in-plane orientation of the applied field and the ratio of unimdal to cubic anisotropy obsereedl in films of differing thickn¢ss [6]. These measurements show that the magnetisation reversal proceeds by a coh,ercnt rotation process which iraehtdes either one or two irrewersible jumps between single domain states. The two-jamlp mechanism can be observed as additional steps in the M ' - H loops, such as those observed alonog the [110] directioms for film thicknesses greater than 33 A in Fig. 1. The numtber of jumps depends specifically on the orientation of-the applied field and the anisotropy ratio. The anisotropy' ratio can be determined by MOKE magnetometry xvhich agrees well with Brillouin light scattering results, Lomntz microscopy results show that the coherent rotation is almost a single domain process, but that the jumps a~e due to domain wall motiom. We were able to map out the phase diagram of one- and "two-jump switching experimentally and theoretically, using the standard coherent rotation model.
References [1] G.A. PrirLz, G.T. Rado and JJ. KreE3s, J. Appl. Phys. 53 (1982) 2087. [2] J.J. ICrebs, F.J. Raehford, P. Lubitz and G.A. Prinz, J. AppL Phys. 53 (1982) 8058. [3] JrML 3~lorcza~ and E.D. Dahlberg, Phys. Rev. B 44 (1991) 9338. [4] J.J. Kmbs, B.T. Jonker and G.A. Pri~, J. AppL Phys. 61 (1987) 2596. [5] P. Etienne, S. Lequien, F. Nguyen-Vas-Daa, R. Cabanel, G. Creucet, A. Friederlch, J. Massies, A. Fert, A. Barth616my and F. Pe~roff, J. Appl. Phys. 67 (1990) 54~30. [6] C Daboo, R.J. Hicken, D.E.P. Eley, M. Gester, S.J. Gray, A.J.R. Ires and J.A.C. B!and, J. Appl. Phys. 75 (1994) 5586.
Journal of magnolisafl and magnetic malerlalc
N
ELSEVIER
Magnetic anisotropy and magnetization reversal in Fe films grown on GaAs (001) and (110) C. Daboo a, * M. Gester a S.J. Gray ~, J.A.C. Bland a, R.J. Hicken ~, E. Gu a, R. Ploessl b, J.N. Chapman b a Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge, CB30HE, UK Department of Pl~ysws and Astronomy, Umversity of Glasgow, Glasgow ¢712 8QQ, UK b
°
.
Abstract We present the results of a systenmtic study of the evolution of magnetic anisotropies of epitaxial Fe films grown on GaAs (001) and (110) substrates and also on epitaxial Ag buffer layers. The anisotr01ay was studied in-situ with the magneto~optie Ken- effect (MOKE). This provides a qualitative thickness dependence of the in-plane magnetic anisotropy for single films on the same substrate surface, thus avoiding any uncertainties due to different substrate morphologies. These in-situ studies are complemented by ex-situ MOKE, Brillouin Light Scattering (BLS), and Lorentz microscopy measurements which reveal the relation between the magnetization reversal processes and mag-~etic anisotropy behaviour on a macroscopic and microscopic scale.
Previous studies of the F e / G a A s system [1-3] have concentrated on ex-situ measurements of the magnetic anisotropy using techniques such as ferromagnetic resonance (FMR), vibrating sample magnet0metry (VSM) and MOKE. Such ex-situ measurements are carried out on a number of samples o f different thickness, and thus the observed thickness dependence is affected by details of the substrate preparation and morphology. In this study tile growth mechanism for Fe grown directly on GaAs has been investigated by analysing LEED spot broadening as a function of Fe thickness, which we relate to the [n-situ MOKE measurements of the magnetic anisotropies of the films. The benefit of such in-situ studies are that they are carried out on a single sample as a function of its film thickness. This eliminates problems associated with the variation in substrate preparation and morphology which usually occurs. The Fe films were deposited onto commercial GaAswafers, some with As caps and others ~rith etch stop buffer layers used for transmission electron microscopy (TEM) and Lorentz microscopy measurements. The GaAs is first heated in UHV to 600°C for at least half an hour to desorb any surface contaminants. After this step w e were unable to observe any LEED pattern, and Auger spectroscopy revealed that the surface oxide had indeed been l'emoved but the initial carbon contamination remained. Only after
* Corresponding author. Fax: +44-1225-350266; [email protected].
e-mail:
Ax+-ion bombardment and subsequent annealing at 600°C for half an hour could a c(4 × 6) reconstructed LEED pattern be obtained, with ~o carbon contamination. Fe was evaporated ~ora an e-beam source at a rate of 1 A per minute and a pressure below 5 × 1 0 - lo mbar. During growth the substrates were kept at 150°C, the average of temperatures reported to give good epitaxial growth [4,5]. During the deposition on ion bombarded and annealed substrates a LEED pattern of cubic symmetr~ was present for all thicl~esses. Sharp spots were observed for incident energies corresponding to the 3-D Bragg condition. However, for other energies the spots were broadened along the (110) directions giving rise to crossshaped spots indicating a.n irregular distribution of steps parallel to all four <110) directions on the surfac0. Fe films deposited onto Ga~s substrates that had only been heated did not show a LEED pattern until the thicl~ess exceeded ~ 15 A, indicating more disordered growth. For thicker Fe films the L E E ~ spots showed the same features as in the case of films grown on sputtered and annealed substrates. Ag buffers were grown by first depositing a 15 A Fe seed layer in a similar manner to that described above. Then a ~ 1000 ,~ Ag ~ y e r is deposited at room temperature at a rate of ~ 5 A per minute and then annealed at 300"C for one hour. LEED patterns from the Ag layer show sharp spots. The growth of Fe then proceeds as described above, except at room temperature. In this case the LEED spots are broadened along <100) directions instead of <110) directions as observed for growth of Fe
0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 3 0 4 - 8 8 5 3 ( 9 5 ) 0 0 2 2 9 . 4
(7. Daboo et aL / domrnal o f Magnetism and Magnetic Materials 148 (1995) 262-263 [1101
,, . [100] ........
_7 f j jS-- [
-0.5
0
0,5; -0.5
0
-r,
0.5 -0.5
[TlO]
[010]
J J S J J
S
20A
j
26A
S
33A
S
41A
0
_~ 0.5 -0.5
0
ssA 0.5
Field (kOe) Fig, 1. The ataisotropy behaviour of an Fe~GaAs(001) film for thicknesses between 20 A (top row) and 55 .A (bottom row). directly on OaAs. This indicates a 4-5° rotation of the surface morphology" of the Fe films grows on a Ag buffer layer compared ~vitla Fe films grown directly on OaAs. The magnetic anisotropy behaviour fcr growth of Fe directly onto GaAs and onto Ag buffer layers is found to be sensitive to the substrate preparati0m technique and growth conditioms. ]For growth on (001) GaAs surfaces we observe a thiel~ess dependent uniaxiai auisotropy which dominates at an l:e thickness of 20 ~ , but reduces with increasing Fe thickness (see Figa 1), so that the cubic anisotropy d0mSuates above 55 A. The direction of the uniaxial anis0tropy for Fe/OaAs(001) is always the same when the substlates are sputtered and n_nnealed prior to growth, in this ease the hard uniaxiat axis is along the [110] direction ~vhile the easy axis is al0mg the [110]. No significant uniaxial anisotropy is observed when Fe is grown onto a Ag buffer. For growth c f Fe directly onto G a A s ( l l 0 ) we observe the expected easy" axis switch from the [110] direction to the [001] direction at a critical thickness consistent with the competing effects of a uniaxial and cubic anisotropy, and thicker filnas have a uniaxial anisotropy with its hard axis along the [110] direction. For Fe growth on a Ag
263
buffer layer on GaAs(ll0), the Ag grows in the (111) orientation and as a result the F¢ gro~s in a tri-crystalline state, clearly visible in the LEED patterns, and there is therefore no clear anisotropy behaviottr for such films. IEx-sita MOKE vector magnetomet~ measurements allow as to study the detailed switching behaviour of Fe/~3aA_s films as a function of the in-plane orientation of the applied field and the ratio of unimdal to cubic anisotropy obsereedl in films of differing thickn¢ss [6]. These measurements show that the magnetisation reversal proceeds by a coh,ercnt rotation process which iraehtdes either one or two irrewersible jumps between single domain states. The two-jamlp mechanism can be observed as additional steps in the M ' - H loops, such as those observed alonog the [110] directioms for film thicknesses greater than 33 A in Fig. 1. The numtber of jumps depends specifically on the orientation of-the applied field and the anisotropy ratio. The anisotropy' ratio can be determined by MOKE magnetometry xvhich agrees well with Brillouin light scattering results, Lomntz microscopy results show that the coherent rotation is almost a single domain process, but that the jumps a~e due to domain wall motiom. We were able to map out the phase diagram of one- and "two-jump switching experimentally and theoretically, using the standard coherent rotation model.
References [1] G.A. PrirLz, G.T. Rado and JJ. KreE3s, J. Appl. Phys. 53 (1982) 2087. [2] J.J. ICrebs, F.J. Raehford, P. Lubitz and G.A. Prinz, J. AppL Phys. 53 (1982) 8058. [3] JrML 3~lorcza~ and E.D. Dahlberg, Phys. Rev. B 44 (1991) 9338. [4] J.J. Kmbs, B.T. Jonker and G.A. Pri~, J. AppL Phys. 61 (1987) 2596. [5] P. Etienne, S. Lequien, F. Nguyen-Vas-Daa, R. Cabanel, G. Creucet, A. Friederlch, J. Massies, A. Fert, A. Barth616my and F. Pe~roff, J. Appl. Phys. 67 (1990) 54~30. [6] C Daboo, R.J. Hicken, D.E.P. Eley, M. Gester, S.J. Gray, A.J.R. Ires and J.A.C. B!and, J. Appl. Phys. 75 (1994) 5586.