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Magnetic properties of MnAs thin films grown on GaAs (0 0 1) by MOVPE
ARTICLE IN PRESS
Physica B 388 (2007) 370–373 www.elsevier.com/locate/physb
Magnetic properties of MnAs thin films grown on GaAs (0 0 1) by MOVPE G.E. Sterbinsky, S.J. May, P.T. Chiu, B.W. Wessels Department of Materials Science and Engineering, Northwestern University, 2220 Campus Drive, Evanston, IL 60208-3108, USA Received 11 February 2006; received in revised form 14 June 2006; accepted 15 June 2006
Abstract The thickness dependence of the in-plane uniaxial anisotropy and coercive field of epitaxial MnAs thin films on GaAs (0 0 1) substrates has been determined from the magneto-optic Kerr effect. The metalorganic vapor phase epitaxy grown films are single a phase at room temperature with a B-type variant orientation. The coercive field of these films increases to a maximum for a film 35 nm thick and then decreases in thicker films. An increase in magnetic anisotropy field with increasing thickness is observed and is attributed to an increasing volume contribution to the anisotropy constant. r 2006 Elsevier B.V. All rights reserved. PACS: 75.30.Gw; 75.50.Cc; 75.70.Ak; 81.15.Kk Keywords: Anisotropy; Coercive field; Magneto-optic Kerr effect (MOKE); Manganese arsenide (MnAs); Metal organic vapor phase epitaxy (MOVPE); Thin films
1. Introduction Epitaxial ferromagnetic materials are of interest for spintronic devices, which offer potential advantages of greater processing speed and lower power consumption than current charge-based solid-state devices [1,2]. One potential material for spin injection devices is MnAs, which is a ferromagnetic metal with a Curie temperature (TC) of 318 K [3]. It can be grown epitaxially on semiconductor substrates by molecular beam epitaxy (MBE) [4,5] and metalorganic vapor phase epitaxy (MOVPE) [6,7]. Below TC, bulk MnAs exists as a single hexagonal ferromagnetic a phase. Both the a phase and the paramagnetic b phase, however, are stabilized in MBE films by residual strain [8,9]. Furthermore, when MnAs thin films are deposited on GaAs (0 0 1), a phase layers can be stabilized with two variants, A-type, with the [1¯ 1¯ 2 0] parallel to the GaAs [1 1 0] and the [0 0 0 1] parallel to the GaAs [1¯ 1 0] or B-type with the [1¯ 1¯ 2 0] parallel to the GaAs [1¯ 1 0] and the [1 1¯ 0 2] parallel to the GaAs [1 1 0] [10]. Corresponding author. Tel.: +1 847 491 3219; fax: +1 847 491 7820.
E-mail address: [email protected] (B.W. Wessels). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.06.159
Studies of the magnetic properties of MBE-grown MnAs thin films have shown strong in-plane uniaxial magnetic anisotropy with the easy axis of magnetization along the MnAs [1¯ 1¯ 2 0] for both A and B type films [10,11]. Coercive fields (Hc) varying from 65 to 930 Oe with thickness were measured along the easy axis of films containing both A-type a phase and b phase MnAs [11,12]. An increase in the anisotropy with increasing thickness was also observed [12]. In contrast, only single phase a MnAs with a B-type variant has been previously stabilized in MOVPE films [6]. In this study we have stabilized single a phase, single variant B-type, epitaxial MnAs on GaAs (1 0 0) substrates by MOVPE at deposition temperatures from 400 to 480 1C. The thickness dependence of the coercive field and the in-plane uniaxial anisotropy field (Hu) were measured for films ranging from 9 to 110 nm using the magneto-optic Kerr effect (MOKE) and compared to values for MBE-grown MnAs films. The magnetic properties of the films including coercivity and anisotropy are strongly dependent on layer thickness. The surface morphology of the films was examined using atomic force microscopy (AFM).
ARTICLE IN PRESS G.E. Sterbinsky et al. / Physica B 388 (2007) 370–373
2. Experimental MnAs films were deposited on GaAs (0 0 1) substrates by atmospheric pressure MOVPE in a system previously described [13]. Tricarbonyl (methylcyclopentadienyl) manganese (TCMn) and arsine (0.3% in hydrogen) were used as precursors with palladium-purified hydrogen used as the carrier gas. A V/III molar ratio of 6 to 1 was maintained during growth of all films. Prior to growth, a 10 min anneal was performed under an arsenic overpressure at 600 1C to remove the native oxide from the GaAs substrates. Deposition rates were determined using secondary ion mass spectroscopy (SIMS) depth profiling. The growth rate increased from 3775 to 105715 nm/h as the growth temperature was increased from 400 to 480 1C, respectively. Structural properties of the MnAs films were measured by double-crystal X-ray diffraction (XRD) using Cu Ka1 radiation. The surface morphology was investigated with a Digital Instruments Nano Scope IIIa AFM. Longitudinal MOKE in the MnAs films was measured at room temperature using a system described previously [14]. In this study a tungsten halogen lamp was used as a light source and a Hamamatsu R428 photo-multiplier tube was used as a detector. The Kerr rotation was determined using the same procedure as described elsewhere [14]. 3. Results and discussion Fig. 1 shows the y–2y XRD pattern obtained from a 105 nm thick MnAs film grown at 480 1C. Similar diffraction patterns were observed for all films deposited in this study. The MnAs films show a strong peak at 2y ¼ 31.91 due to the MnAs (1¯ 1 0 1) planes that are parallel to GaAs (0 0 1), indicating that the films are the B-type variant of the a phase [15]. Films were single a phase over the entire deposition temperature range. The same variant was observed for a
371
MnAs film grown at 400 1C on GaAs (0 0 1). The reduction in growth temperature from 480 to 400 1C leads to an improvement in the epitaxial quality of the films as evidenced by the decrease in the (1¯ 1 0 1) rocking curve FWHM from 0.39 to 0.21. The observation of the B-type variant is consistent with MBE studies by Park et al. [16], who reported B-type growth for films at temperatures between 380 and 550 1C. A weak (1¯ 1 0 2) diffraction peak is also observed in the MOVPE MnAs films. The (1¯ 1 0 2) peak intensity is lower than that of the (1¯ 1 0 1) peak by a factor of 200. The (1¯ 1 0 2) peak has also been observed in B-type films grown by MBE [15,16]. The inset of Fig. 1 shows the out-of-plane d-spacing determined by XRD as a function of thickness. The d–spacing does not depend on thickness indicating that films greater than 17 nm thick are fully relaxed. All films exhibited in-plane uniaxial magnetic anisotropy. This can be seen in the plots of the Kerr intensity versus magnetic field (H) shown in Fig. 2 for films of varying thickness grown at 480 1C. The two magnetic hysteresis loops given for each sample were measured with the [1 1 0] and the [1¯ 1 0] directions parallel to the magnetic field. The plots reveal that with increasing thickness, hysteresis loops taken along the easy axis become squarer, while hysteresis loops taken along the hard axis become elongated, indicating an increase in anisotropy. Furthermore, there is an increase in the measured saturation Kerr rotation angle with thickness. The anisotropy field was determined from the Kerr data in Fig. 2 using the following equation: Hu ¼
2ðAE AH Þ , yS
(1)
where yS is the saturation Kerr rotation, and AE and AH are defined as Z
yS
AE;H ¼
H dyE;H .
(2)
0
105 H (T)
GaAs (002)
103
2.85
0
20
40
60
80
-0.6 -0.3 0.0 0.3 0.6
8.8 nm
17.5 nm
9 6 3 0 -3 -6 -9
53 nm
105 nm
20
2
MnAs (1101)
2.75 2.70
102
4
2.80
Kerr Rotation (mdeg)
Intensity (au)
104
d spacing (Å)
-0.6 -0.3 0.0 0.3 0.6
100
thickness (nm)
101
0 -2 -4 20 10
10
0
0
-10 100
26
-10
-20 28
30
32
34
2θ (degrees) Fig. 1. y–2y XRD pattern of a 105 nm thick MnAs film grown on GaAs (0 0 1) deposited at 480 1C. Inset: Out-of-plane d spacing measured by XRD for films ranging from 17.5 to 105 nm in thickness.
Kerr Rotation (mdeg)
2.90
-20 -0.6 -0.3 0.0 0.3 0.6
-0.6 -0.3 0.0 0.3 0.6
H (T) Fig. 2. Kerr rotation versus magnetic field for MnAs films of various thicknesses measured in plane along the [1¯ 1¯ 2 0] and [1 1¯ 0 2] MnAs directions.
ARTICLE IN PRESS G.E. Sterbinsky et al. / Physica B 388 (2007) 370–373
dyE and dyH are differential changes in the Kerr rotations in the direction of magnetization of the easy and hard axis respectively. The values of AH and AE were determined by graphical integration. Hu versus thickness is plotted in Fig. 3, and shows a gradual increase in Hu with thickness. Surfaces and interfaces of materials may have different anisotropies from their bulk form due to different symmetries [17]. In very thin MnAs films, where these effects will be prevalent, we see a suppression of the uniaxial anisotropy. Bulk MnAs has a uniaxial anisotropy field of 1.54 T with the MnAs [0 0 0 1] as the hard axis [18]. With increasing thickness the films become more like bulk MnAs, exhibiting stronger uniaxial anisotropy. Increasing anisotropy with thickness is also seen for films containing A-type MnAs [12]. Saturation of the biaxial magnetic anisotropy to the bulk value with thickness has been observed in cubic thin film systems, including Fe thin films [19]. The dependence of the coercive field measured along the easy axis on thickness can be seen in Fig. 4 We observe an increase in coercive field from 320 to 853 Oe as the film thickness increases from 8.8 to 35 nm. A decrease in coercive field from this point to 496 Oe for a 105 nm thick film then occurs. For comparison, the data measured by Tanaka et al. [11] on mixed A-type a phase and b phase MnAs films grown by MBE is also shown in Fig. 4 [11]. A trend similar to that observed for MOVPE deposited films has been reported for MBE grown films with HC increasing for films from 1 to 10 nm thick and then decreasing as thickness is further increased [11]. The coercive field of a magnetic thin film and its thickness dependence can be described by considering the effects of domain wall motion, 1 dew ew dt ew dl HC ¼ þ þ , (3) 2M S dx t dx l dx where x is the direction of domain wall motion, which is assumed to be perpendicular to the applied magnetic field 0.4
Hu (T)
0.3
0.2
0.1
0.0
0
20
40
60
80
t (nm) Fig. 3. Hu versus thickness for MnAs films.
100
1000
800
Hc (Oe)
372
600
400
200 MOVPE MBE (Ref. 11) 0
0
20
40
60
80
100
120
t (nm) Fig. 4. HC versus thickness for MOVPE-grown MnAs films and MBEgrown MnAs films, as reported by Tanaka et al. [11].
direction, ew the surface energy of the domain wall, Ms the saturation magnetization, t the film thickness, and l is the domain wall length in the direction parallel to the spins on either side of the wall [20]. The surface energy of the domain wall is ew ¼ eex þ
K 1D þ pNDM S , 2
(4)
where eex is the exchange energy, K1 is the in-plane anisotropy constant, N is the demagnetization factor, and D is the domain wall width in the direction perpendicular to the spins on either side of the wall. K1 is the same for Ne´el and Bloch walls as is eex for walls of the same width [20,21]. For a Bloch wall, the demagnetization factor is NBloch ¼ D/(t+D), whereas for a Ne´el wall NNeel ¼ t/ (t+D) [20,21]. Thus NBloch will decrease with thickness whereas NNeel will increase with thickness. For toD, The wall energy of a Ne´el wall will be smaller than that of a Bloch wall and initially the wall energy will increase with thickness. Eventually the surface energy of a Bloch wall, which decreases with thickness, will become less than that of the Ne´el wall and a transition from Ne´el to Bloch domain walls will occur. The wall surface energy will then decrease with thickness. This same trend is observable in the coercive field, being proportional to domain wall surface energy, so an increase in coercive field with increasing thickness in the Ne´el wall regime (small thickness) and a decrease in coercive field with increasing thickness in the Bloch wall regime (large thickness) are observed [22]. AFM images of the film surfaces show island formation as seen in Fig. 5(a) and (b). Similar surface morphologies have been reported for other MOVPE-deposited MnAs thin films [6]. The root mean square surface roughness (Rrms) increases with thickness as shown in Fig. 5(c). The average island size increases from 46711 nm for an 8.8 nmthick film to 173761 nm for a 105 nm-thick film. The increase in island size and surface roughness with thickness
ARTICLE IN PRESS G.E. Sterbinsky et al. / Physica B 388 (2007) 370–373
373
by MOKE. The coercive field increases with layer thickness for films of up to 35 nm and then decreases with increasing thickness. This trend has been attributed to the thickness dependence of the domain wall energy. The uniaxial magnetic anisotropy increases with film thickness, as would be expected for a decreasing contribution of a reconstructed surface or a heterojunction interface to the magnetic properties of the film. (a)
(b)
20 nm
Acknowledgements 100 nm
14
(a)
Rrms (nm)
12 10
This work was supported by the NSF under the spin electronics program ECS-0224210. Support of Argonne National Labs under grant W-31-109-ENG38 is also acknowledged.
8 6
(b)
References
4 2 0 nm
0 (c)
20
40
60
80
100
120
0 nm
t (nm)
Fig. 5. AFM images of an 8.8 nm (a) and a 105 nm (b) thick MnAs samples. The scan size for both images is 2 2 mm. (c) RMS roughness versus thickness for MnAs samples. The line is included as a guide to the eye.
is consistent with an island growth mechanism. In contrast, in MnAs films grown by MBE at 250 1C, the a and b phases separate into a striped pattern where adjoining stripes are of different phases [8,9]. The stripes are the only features visible on the surface of the film and have an average difference in height of 1 nm as measured by AFM [8]. It has been shown that for two films with the same thickness but differing surface roughness, the larger coercive field would belong to the rougher film [21]. That MBE-grown films are relatively smooth while our films have a Rrms on the order of several nanometers is consistent with the lower coercive field observed in MBE films of the same thickness. It is also possible that differences observed in the magnetic properties of MBE and MOVPE films are due to the difference in the phases stabilized. The coupling between the striped ferromagnetic domains and the magnetic properties of the films depend on the widths of paramagnetic domains, which increase with the thickness of the films [9]. 4. Conclusion We have grown MnAs thin films by MOVPE, yielding single, a phase, epitaxial films with a single B-type variant. A strong in-plane uniaxial magnetic anisotropy is observed
[1] S.D. Sarma, J. Fabian, X. Hu, I. Zutic, Solid State Commun. 119 (2001) 207. [2] S.A. Wolf, D.D. Awschalom, R.A. Buhrman, J.M. Daughton, S. von Molnar, M.L. Roukes, A.Y. Chtchelkanova, D.M. Treger, Science 294 (2001) 1488. [3] C. Kittel, Introduction to Solid State Physics, Wiley, 1996. [4] K. Akeura, M. Tanaka, M. Ueki, T. Nishinaga, Appl. Phys. Lett. 67 (1995) 3349. [5] M. Tanaka, J.P. Harbison, M.C. Park, Y.S. Park, T. Shin, G.M. Rothberg, Appl. Phys. Lett. 65 (1994) 1964. [6] P.A. Lane, B. Cockayne, P.J. Wright, P.E. Oliver, M.E.G. Tilsley, N. Smith, I.R. Harris, J. Cryst. Growth 143 (1994) 237. [7] M.E.G. Tilsley, N. Smith, B. Cockayne, I.R. Harris, P.A. Lane, P.E. Oliver, P.J. Wright, J. Alloys Compounds 248 (1997) 125. [8] A.K. Das, C. Pampuch, A. Ney, T. Hesjedal, L. Daweritz, R. Koch, K.H. Ploog, Phys. Rev. Lett. 91 (2003) 087203-1. [9] L. Da¨weritz, C. Herrmann, J. Mohanty, T. Hesjedal, K.H. Ploog, E. Bauer, A. Locatelli, S. Cherifi, R. Belkhou, A. Pavlovska, S. Heun, J. Vac. Sci. Technol. B. 23 (2005) 1759. [10] M. Tanaka, Semicond. Sci. Technol. 17 (2002) 327. [11] M. Tanaka, J.P. Harbison, M.C. Park, Y.S. Park, T. Shin, G.M. Rothberg, J. Appl. Phys. 76 (1994) 6278. [12] M.C. Park, Y. Park, T. Shin, G.M. Rothberg, M. Tanaka, J.P. Harbison, J. Appl. Phys. 79 (1996) 4967. [13] A.J. Blattner, J. Lensch, B.W. Wessels, J. Electron. Mater. 30 (2001) 1408. [14] P.T. Chiu, S.J. May, B.W. Wessels, Appl. Phys. Lett. 85 (2004) 780. [15] M. Tanaka, J.P. Harbison, G.M. Rothberg, J. Cryst. Growth 150 (1995) 1132. [16] J.B. Park, K.H. Kim, K.J. Lee, D.J. Kim, H.J. Kim, Y.E. Ihm, J. Cryst. Growth 273 (2005) 396. [17] D. Sander, J. Phys.: Condens. Matter. 16 (2004) R603. [18] R.W. De Blois, D.S. Rodbell, Phys. Rev. 130 (1963) 1347. [19] N.A. Morley, M.R.J. Gibbs, E. Ahmad, I.G. Will, Y.B. Xu, J. Phys.: Condens. Matter. 17 (2005) 1201. [20] R.F. Soohoo, Magnetic Thin Films, Harper and Row, New York, 1965. [21] Y.-P. Zaho, R.M. Gamache, G.-C. Wang, T.M. Lu, G. Palasantzas, J.Th.M. De Hosson, J. Appl. Phys. 89 (2001) 1325. [22] Y. Park, Y. Shin, J. Kor. Phys. Soc. 37 (2000) 569.
Physica B 388 (2007) 370–373 www.elsevier.com/locate/physb
Magnetic properties of MnAs thin films grown on GaAs (0 0 1) by MOVPE G.E. Sterbinsky, S.J. May, P.T. Chiu, B.W. Wessels Department of Materials Science and Engineering, Northwestern University, 2220 Campus Drive, Evanston, IL 60208-3108, USA Received 11 February 2006; received in revised form 14 June 2006; accepted 15 June 2006
Abstract The thickness dependence of the in-plane uniaxial anisotropy and coercive field of epitaxial MnAs thin films on GaAs (0 0 1) substrates has been determined from the magneto-optic Kerr effect. The metalorganic vapor phase epitaxy grown films are single a phase at room temperature with a B-type variant orientation. The coercive field of these films increases to a maximum for a film 35 nm thick and then decreases in thicker films. An increase in magnetic anisotropy field with increasing thickness is observed and is attributed to an increasing volume contribution to the anisotropy constant. r 2006 Elsevier B.V. All rights reserved. PACS: 75.30.Gw; 75.50.Cc; 75.70.Ak; 81.15.Kk Keywords: Anisotropy; Coercive field; Magneto-optic Kerr effect (MOKE); Manganese arsenide (MnAs); Metal organic vapor phase epitaxy (MOVPE); Thin films
1. Introduction Epitaxial ferromagnetic materials are of interest for spintronic devices, which offer potential advantages of greater processing speed and lower power consumption than current charge-based solid-state devices [1,2]. One potential material for spin injection devices is MnAs, which is a ferromagnetic metal with a Curie temperature (TC) of 318 K [3]. It can be grown epitaxially on semiconductor substrates by molecular beam epitaxy (MBE) [4,5] and metalorganic vapor phase epitaxy (MOVPE) [6,7]. Below TC, bulk MnAs exists as a single hexagonal ferromagnetic a phase. Both the a phase and the paramagnetic b phase, however, are stabilized in MBE films by residual strain [8,9]. Furthermore, when MnAs thin films are deposited on GaAs (0 0 1), a phase layers can be stabilized with two variants, A-type, with the [1¯ 1¯ 2 0] parallel to the GaAs [1 1 0] and the [0 0 0 1] parallel to the GaAs [1¯ 1 0] or B-type with the [1¯ 1¯ 2 0] parallel to the GaAs [1¯ 1 0] and the [1 1¯ 0 2] parallel to the GaAs [1 1 0] [10]. Corresponding author. Tel.: +1 847 491 3219; fax: +1 847 491 7820.
E-mail address: [email protected] (B.W. Wessels). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.06.159
Studies of the magnetic properties of MBE-grown MnAs thin films have shown strong in-plane uniaxial magnetic anisotropy with the easy axis of magnetization along the MnAs [1¯ 1¯ 2 0] for both A and B type films [10,11]. Coercive fields (Hc) varying from 65 to 930 Oe with thickness were measured along the easy axis of films containing both A-type a phase and b phase MnAs [11,12]. An increase in the anisotropy with increasing thickness was also observed [12]. In contrast, only single phase a MnAs with a B-type variant has been previously stabilized in MOVPE films [6]. In this study we have stabilized single a phase, single variant B-type, epitaxial MnAs on GaAs (1 0 0) substrates by MOVPE at deposition temperatures from 400 to 480 1C. The thickness dependence of the coercive field and the in-plane uniaxial anisotropy field (Hu) were measured for films ranging from 9 to 110 nm using the magneto-optic Kerr effect (MOKE) and compared to values for MBE-grown MnAs films. The magnetic properties of the films including coercivity and anisotropy are strongly dependent on layer thickness. The surface morphology of the films was examined using atomic force microscopy (AFM).
ARTICLE IN PRESS G.E. Sterbinsky et al. / Physica B 388 (2007) 370–373
2. Experimental MnAs films were deposited on GaAs (0 0 1) substrates by atmospheric pressure MOVPE in a system previously described [13]. Tricarbonyl (methylcyclopentadienyl) manganese (TCMn) and arsine (0.3% in hydrogen) were used as precursors with palladium-purified hydrogen used as the carrier gas. A V/III molar ratio of 6 to 1 was maintained during growth of all films. Prior to growth, a 10 min anneal was performed under an arsenic overpressure at 600 1C to remove the native oxide from the GaAs substrates. Deposition rates were determined using secondary ion mass spectroscopy (SIMS) depth profiling. The growth rate increased from 3775 to 105715 nm/h as the growth temperature was increased from 400 to 480 1C, respectively. Structural properties of the MnAs films were measured by double-crystal X-ray diffraction (XRD) using Cu Ka1 radiation. The surface morphology was investigated with a Digital Instruments Nano Scope IIIa AFM. Longitudinal MOKE in the MnAs films was measured at room temperature using a system described previously [14]. In this study a tungsten halogen lamp was used as a light source and a Hamamatsu R428 photo-multiplier tube was used as a detector. The Kerr rotation was determined using the same procedure as described elsewhere [14]. 3. Results and discussion Fig. 1 shows the y–2y XRD pattern obtained from a 105 nm thick MnAs film grown at 480 1C. Similar diffraction patterns were observed for all films deposited in this study. The MnAs films show a strong peak at 2y ¼ 31.91 due to the MnAs (1¯ 1 0 1) planes that are parallel to GaAs (0 0 1), indicating that the films are the B-type variant of the a phase [15]. Films were single a phase over the entire deposition temperature range. The same variant was observed for a
371
MnAs film grown at 400 1C on GaAs (0 0 1). The reduction in growth temperature from 480 to 400 1C leads to an improvement in the epitaxial quality of the films as evidenced by the decrease in the (1¯ 1 0 1) rocking curve FWHM from 0.39 to 0.21. The observation of the B-type variant is consistent with MBE studies by Park et al. [16], who reported B-type growth for films at temperatures between 380 and 550 1C. A weak (1¯ 1 0 2) diffraction peak is also observed in the MOVPE MnAs films. The (1¯ 1 0 2) peak intensity is lower than that of the (1¯ 1 0 1) peak by a factor of 200. The (1¯ 1 0 2) peak has also been observed in B-type films grown by MBE [15,16]. The inset of Fig. 1 shows the out-of-plane d-spacing determined by XRD as a function of thickness. The d–spacing does not depend on thickness indicating that films greater than 17 nm thick are fully relaxed. All films exhibited in-plane uniaxial magnetic anisotropy. This can be seen in the plots of the Kerr intensity versus magnetic field (H) shown in Fig. 2 for films of varying thickness grown at 480 1C. The two magnetic hysteresis loops given for each sample were measured with the [1 1 0] and the [1¯ 1 0] directions parallel to the magnetic field. The plots reveal that with increasing thickness, hysteresis loops taken along the easy axis become squarer, while hysteresis loops taken along the hard axis become elongated, indicating an increase in anisotropy. Furthermore, there is an increase in the measured saturation Kerr rotation angle with thickness. The anisotropy field was determined from the Kerr data in Fig. 2 using the following equation: Hu ¼
2ðAE AH Þ , yS
(1)
where yS is the saturation Kerr rotation, and AE and AH are defined as Z
yS
AE;H ¼
H dyE;H .
(2)
0
105 H (T)
GaAs (002)
103
2.85
0
20
40
60
80
-0.6 -0.3 0.0 0.3 0.6
8.8 nm
17.5 nm
9 6 3 0 -3 -6 -9
53 nm
105 nm
20
2
MnAs (1101)
2.75 2.70
102
4
2.80
Kerr Rotation (mdeg)
Intensity (au)
104
d spacing (Å)
-0.6 -0.3 0.0 0.3 0.6
100
thickness (nm)
101
0 -2 -4 20 10
10
0
0
-10 100
26
-10
-20 28
30
32
34
2θ (degrees) Fig. 1. y–2y XRD pattern of a 105 nm thick MnAs film grown on GaAs (0 0 1) deposited at 480 1C. Inset: Out-of-plane d spacing measured by XRD for films ranging from 17.5 to 105 nm in thickness.
Kerr Rotation (mdeg)
2.90
-20 -0.6 -0.3 0.0 0.3 0.6
-0.6 -0.3 0.0 0.3 0.6
H (T) Fig. 2. Kerr rotation versus magnetic field for MnAs films of various thicknesses measured in plane along the [1¯ 1¯ 2 0] and [1 1¯ 0 2] MnAs directions.
ARTICLE IN PRESS G.E. Sterbinsky et al. / Physica B 388 (2007) 370–373
dyE and dyH are differential changes in the Kerr rotations in the direction of magnetization of the easy and hard axis respectively. The values of AH and AE were determined by graphical integration. Hu versus thickness is plotted in Fig. 3, and shows a gradual increase in Hu with thickness. Surfaces and interfaces of materials may have different anisotropies from their bulk form due to different symmetries [17]. In very thin MnAs films, where these effects will be prevalent, we see a suppression of the uniaxial anisotropy. Bulk MnAs has a uniaxial anisotropy field of 1.54 T with the MnAs [0 0 0 1] as the hard axis [18]. With increasing thickness the films become more like bulk MnAs, exhibiting stronger uniaxial anisotropy. Increasing anisotropy with thickness is also seen for films containing A-type MnAs [12]. Saturation of the biaxial magnetic anisotropy to the bulk value with thickness has been observed in cubic thin film systems, including Fe thin films [19]. The dependence of the coercive field measured along the easy axis on thickness can be seen in Fig. 4 We observe an increase in coercive field from 320 to 853 Oe as the film thickness increases from 8.8 to 35 nm. A decrease in coercive field from this point to 496 Oe for a 105 nm thick film then occurs. For comparison, the data measured by Tanaka et al. [11] on mixed A-type a phase and b phase MnAs films grown by MBE is also shown in Fig. 4 [11]. A trend similar to that observed for MOVPE deposited films has been reported for MBE grown films with HC increasing for films from 1 to 10 nm thick and then decreasing as thickness is further increased [11]. The coercive field of a magnetic thin film and its thickness dependence can be described by considering the effects of domain wall motion, 1 dew ew dt ew dl HC ¼ þ þ , (3) 2M S dx t dx l dx where x is the direction of domain wall motion, which is assumed to be perpendicular to the applied magnetic field 0.4
Hu (T)
0.3
0.2
0.1
0.0
0
20
40
60
80
t (nm) Fig. 3. Hu versus thickness for MnAs films.
100
1000
800
Hc (Oe)
372
600
400
200 MOVPE MBE (Ref. 11) 0
0
20
40
60
80
100
120
t (nm) Fig. 4. HC versus thickness for MOVPE-grown MnAs films and MBEgrown MnAs films, as reported by Tanaka et al. [11].
direction, ew the surface energy of the domain wall, Ms the saturation magnetization, t the film thickness, and l is the domain wall length in the direction parallel to the spins on either side of the wall [20]. The surface energy of the domain wall is ew ¼ eex þ
K 1D þ pNDM S , 2
(4)
where eex is the exchange energy, K1 is the in-plane anisotropy constant, N is the demagnetization factor, and D is the domain wall width in the direction perpendicular to the spins on either side of the wall. K1 is the same for Ne´el and Bloch walls as is eex for walls of the same width [20,21]. For a Bloch wall, the demagnetization factor is NBloch ¼ D/(t+D), whereas for a Ne´el wall NNeel ¼ t/ (t+D) [20,21]. Thus NBloch will decrease with thickness whereas NNeel will increase with thickness. For toD, The wall energy of a Ne´el wall will be smaller than that of a Bloch wall and initially the wall energy will increase with thickness. Eventually the surface energy of a Bloch wall, which decreases with thickness, will become less than that of the Ne´el wall and a transition from Ne´el to Bloch domain walls will occur. The wall surface energy will then decrease with thickness. This same trend is observable in the coercive field, being proportional to domain wall surface energy, so an increase in coercive field with increasing thickness in the Ne´el wall regime (small thickness) and a decrease in coercive field with increasing thickness in the Bloch wall regime (large thickness) are observed [22]. AFM images of the film surfaces show island formation as seen in Fig. 5(a) and (b). Similar surface morphologies have been reported for other MOVPE-deposited MnAs thin films [6]. The root mean square surface roughness (Rrms) increases with thickness as shown in Fig. 5(c). The average island size increases from 46711 nm for an 8.8 nmthick film to 173761 nm for a 105 nm-thick film. The increase in island size and surface roughness with thickness
ARTICLE IN PRESS G.E. Sterbinsky et al. / Physica B 388 (2007) 370–373
373
by MOKE. The coercive field increases with layer thickness for films of up to 35 nm and then decreases with increasing thickness. This trend has been attributed to the thickness dependence of the domain wall energy. The uniaxial magnetic anisotropy increases with film thickness, as would be expected for a decreasing contribution of a reconstructed surface or a heterojunction interface to the magnetic properties of the film. (a)
(b)
20 nm
Acknowledgements 100 nm
14
(a)
Rrms (nm)
12 10
This work was supported by the NSF under the spin electronics program ECS-0224210. Support of Argonne National Labs under grant W-31-109-ENG38 is also acknowledged.
8 6
(b)
References
4 2 0 nm
0 (c)
20
40
60
80
100
120
0 nm
t (nm)
Fig. 5. AFM images of an 8.8 nm (a) and a 105 nm (b) thick MnAs samples. The scan size for both images is 2 2 mm. (c) RMS roughness versus thickness for MnAs samples. The line is included as a guide to the eye.
is consistent with an island growth mechanism. In contrast, in MnAs films grown by MBE at 250 1C, the a and b phases separate into a striped pattern where adjoining stripes are of different phases [8,9]. The stripes are the only features visible on the surface of the film and have an average difference in height of 1 nm as measured by AFM [8]. It has been shown that for two films with the same thickness but differing surface roughness, the larger coercive field would belong to the rougher film [21]. That MBE-grown films are relatively smooth while our films have a Rrms on the order of several nanometers is consistent with the lower coercive field observed in MBE films of the same thickness. It is also possible that differences observed in the magnetic properties of MBE and MOVPE films are due to the difference in the phases stabilized. The coupling between the striped ferromagnetic domains and the magnetic properties of the films depend on the widths of paramagnetic domains, which increase with the thickness of the films [9]. 4. Conclusion We have grown MnAs thin films by MOVPE, yielding single, a phase, epitaxial films with a single B-type variant. A strong in-plane uniaxial magnetic anisotropy is observed
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