- Email: [email protected]
NiAs magnetic multilayers on GaAs grown by molecular beam epitaxy
Journal of Magnetism and Magnetic Materials 156 (1996) 306-308
~ H Journalof magnetism ELSEVIER
,~
and
~
materials
magnetic
MnAs/NiAs magnetic multilayers on GaAs grown by molecular beam epitaxy M. Tanaka a,b,d, *, j.p. Harbison b, G.M. Rothberg c a Department of Electronic Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan b Bellcore, 331 Newman Springs Road, Red Bank, NJ 07701, USA c Stet,ens Institute of Technology, Hoboken, NJ 07030, USA d PRESTO, Research and Dez.,elopment Corporation of Japan, 4-1-8 Honcho, Kawaguchi, Japan
Abstract We have created a new class of epitaxial magnetic multilayers, consisting of ferromagnetic MnAs and nonmagnetic NiAs, on (001)GaAs substrates by molecular beam epitaxy. It was found that the growth plane of MnAs and NiAs is the (~100) of hexagonal crystal structure, and that the easy axis of magnetization of MnAs is in-plane, along the [~120] axis, which is parallel to the [110] of GaAs. Multi-stepped magnetic hysteresis are controllably realized, making this material promising for the application to multi-level non-volatile recording on semiconductors.
Epitaxial ferromagnetic thin films grown directly on III-V compound semiconductors can lead to the integration of magnetism and III-V electronics/photonics, offering possibilities of hybrid ferromagnetic-semiconductor devices. Despite stringent materials requirements for this purpose, ferromagnetic MnAs, an As-based compound which is thermodynamically stable on GaAs and is very compatible with the III-V growth and processing technology such as molecular beam epitaxy (MBE), is very promising. Recently, we have successfully grown monocrystalline epitaxial MnAs thin films on (001) GaAs substrates by MBE [1,2]. The epitaxial relationship was found to be quite unique; the growth direction is [1100] of the hexagonal unit cell of MnAs, and the [0001] axis (c-axis) and the [~20] axis of MnAs are along the [~10] and [110] of the GaAs substrate, respectively. The easy magnetization direction of the MBE-grown MnAs films is in-plane, along the [1120] of MnAs and the [110] of GaAs. The M - H curves measured at room temperature with the magnetic field applied along the easy magnetization axis show almost perfect squareness with 100% remanence, with the saturation magnetization M~ of 305-601 e m u / c m 3 and coercive field H c of 65-926 Oe, depending on the MnAs thickness. In this report, in order to increase the freedom in materials design and to have more functionality, we have created a new class of magnetic multilayers consisting of
* Corresponding author. Fax: masaaki @ee.t.u-tokyo.ac.jp.
+81-3-3816-4996; email:
ferromagnetic MnAs and nonmagnetic NiAs, both of which are of NiAs-type hexagonal crystal structure, on (001) GaAs substrates by MBE. Here, since all the three materials are As based compounds, we can expect thermodynamical stability with little intermixing at each interface, as well as good compatibility with III-V MBE techniques. Fig. 1 shows typical reflection high energy electron diffraction (RHEED) patterns during the MBE growth of M n A s / N i A s / M n A s / G a A s heterostructures, with the electron beam azimuth along the [110] and [ll0] of GaAs. After growing a 100 nm thick undoped GaAs buffer layer on a (001)GaAs substrate at 580°C, the substrate temperature was cooled to 250°C, and then a 25 nm thick MnAs layer, a 6 nm thick NiAs layer and a 10 nm thick MnAs layer were successively grown. The As 2 flux was kept on throughout the growth. Since we grew the first 25 nm thick MnAs layer on the disordered (4 × 4) (001)GaAs surface (Fig. l(a)), the epitaxial relationship of the M n A s / G a A s was type-A [3], in which the growth plane of MnAs is (T100), and the in-plane relationship is [l~20]MnAs [1[110]GaAs and [0001]MnAs II [ll0]GaAs (see Fig. l(b) and (c)). The same growth direction and epitaxial relationship were observed when NiAs was grown on MnAs, as shown in Fig. l(d) and (e), which indicate that the growth plane of NiAs is (7100) with [1120]NiAs II[1120]MnAs and [0001]NiAs I[[0001]MnAs. Finally, when a 10 nm thick MnAs was grown on NiAs, the epitaxial relationship remained unchanged (Fig. l(f) and (g)). One can see two-fold reconstructions in Fig. l(c) and (d), with the [0001] azimuth of MnAs and with the [1120] azimuth of NiAs, respectively. Note that the dis-
0304-8853/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0 3 0 4 - 8 8 5 3 ( 9 5 ) 0 0 8 7 8 - 0
307
M. Tanaka et al../ Journal o( Magnetism and Magnetic Materials 156 (1996) 306-308
MnAs/NiAs/MnAs/GaAs [110]GaAs azimuth
GaAs buffer layer on GaAs(001)
++ ++
well defined RHEED patterns indicate that monocrystalline M n A s / N i A s multilayers of high structural quality are successfully grown on (001)GaAs. In a previous paper [2], we have reported that the coercive field H c of the MnAs thin films grown on GaAs depends on the MnAs thickness and it decreases l¥om I).93 kOe at 10 nm to 0. l l kOe at 200 nm. This suggests a possibility of changing and controlling the coercive field of the magnetic thin films by changing the thickness. Here, we have grown M n A s / N i A s / M n A s multilayers on GaAs, with two different MnAs thicknesses, in a manner described earlier. The MnAs thickness was varied from 10 nm to 100 nm, while the NiAs thickness was fixed at 6 nm. Magnetization measurements were done using a vibrating sample magnetometer (VSM), and the present multilayers were found to have strong magnetic anisotropy. The easy axis of magnetization was found to be along the [1120] axis of MnAs, which is parallel to the [1120] of NiAs and the [110] of GaAs. An example of the M - H curve is shown in Fig. 2, which shows the result of MnAs(100 nm)/NiAs(5.5 nm)/MnAs(50 nm)/ GaAs(001), measured by VSM at room temperature. Here the magnetic field was applied along the easy magnetization axis. Since the two MnAs layers have different coercive force, two steps are clearly seen in the hysteresis. Even three-step hysteresis in the M - H curve has been successfully realized by growing M n A s / N i A s / M n A s / N i A s / M n A s multilayers with three different MnAs thicknesses. We have found that the coercive force of each MnAs layer in the present multilayer samples is controllable by changing the thickness, and therefore, we can in principle design the magnetic hysteresis characteristics on semiconductors. This phenomenon could be applied to multi-level non-volatile recording coupled with the underlying high speed semiconductor circuitry.
8
'
i
+
i
.
i
.
[0001]MnAs azimuth
Fig. I. Time evolution of RHEED patterns during the MBE growth of MnAs/NiAs/MnAs/GaAs multilayers. (a) Disordered c(4×4)-(001) GaAs surface just prior to the growth of MnAs/NiAs. (b)-(f) After growth of MnAs with the indicated thickness. The electron beam incidence is along the [I 10] (b,d+f) and [110] (c,e,g) direction of GaAs+ For comparison, the integralorder streaks are indicated by arrows in (b), and (d).
E
4
~
2
o
0
i
,
i
,
~ .
+ •
+
.
i ~
.
•~ -2 e-
~4 -6
. +.,.,.,,
-8
-0.8 tance between the integral-order streaks in Fig. l(d) is larger than that in Fig. l(b), resulting from the smaller lattice constant along the c-axis of NiAs (c = 0.5049 rim) than that of MnAs ( c = 0 . 5 7 1 3 nm), while the lattice constant along the a-axis of NiAs ( a = 0.3631 nm) is close to that of MnAs ( a = 0.3713 nm). These streaky and
.
+!
6
[1120]MnAsazimuth
t
-0.4
,--,--,--,-,--,--,--,-
0
0.4
0.8
Magnetic Field (kOe) Fig. 2. M - H characteristics of MnAs(100 nm)/NiAs(5.5 nm)/MnAs(50 nm)/GaAs(001) measured by vibrating sample magnetometry at room temperature. Magnetic field was applied along the easy magnetization axis of MnAs. [I 120], which is parallel to the [I 10] direction of GaAs.
308
M. Tanaka et aL / Journal of Magnetism and Magnetic Materials 156 (1996) 306-308
In summary, we have successfully grown a new Asbased epitaxial magnetic multilayers, consisting of MnAs and NiAs, on (001)GaAs substrates by MBE. It was found that the growth plane of MnAs and NiAs is the (3100) of hexagonal crystal structure, and that the easy axis of magnetization of MnAs is in-plane, along the [1120] axis, which is parallel to the [110] of GaAs. Multi-stepped magnetic hysteresis are controllably realized by changing the thickness of MnAs.
References [1] M. Tanaka, J.P. Harbison, T. Sands, T.L. Cheeks, V.G. Keramidas, G.M. Rothberg, J. Vac. Sci. and Technol. B 12 (1994) 1091. [2] M. Tanaka, J.P. Harbison, M.C. Park, Y.S. Park, T. Shin and G.M. Rothberg, J. Appl. Phys. 76 (1994) 6278. [3] M. Tanaka, J.P. Harbison, M.C. Park, Y.S. Park, T. Shin and G.M. Rothberg, Appl. Phys. Lett. 65 (1994) 1964.
~ H Journalof magnetism ELSEVIER
,~
and
~
materials
magnetic
MnAs/NiAs magnetic multilayers on GaAs grown by molecular beam epitaxy M. Tanaka a,b,d, *, j.p. Harbison b, G.M. Rothberg c a Department of Electronic Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan b Bellcore, 331 Newman Springs Road, Red Bank, NJ 07701, USA c Stet,ens Institute of Technology, Hoboken, NJ 07030, USA d PRESTO, Research and Dez.,elopment Corporation of Japan, 4-1-8 Honcho, Kawaguchi, Japan
Abstract We have created a new class of epitaxial magnetic multilayers, consisting of ferromagnetic MnAs and nonmagnetic NiAs, on (001)GaAs substrates by molecular beam epitaxy. It was found that the growth plane of MnAs and NiAs is the (~100) of hexagonal crystal structure, and that the easy axis of magnetization of MnAs is in-plane, along the [~120] axis, which is parallel to the [110] of GaAs. Multi-stepped magnetic hysteresis are controllably realized, making this material promising for the application to multi-level non-volatile recording on semiconductors.
Epitaxial ferromagnetic thin films grown directly on III-V compound semiconductors can lead to the integration of magnetism and III-V electronics/photonics, offering possibilities of hybrid ferromagnetic-semiconductor devices. Despite stringent materials requirements for this purpose, ferromagnetic MnAs, an As-based compound which is thermodynamically stable on GaAs and is very compatible with the III-V growth and processing technology such as molecular beam epitaxy (MBE), is very promising. Recently, we have successfully grown monocrystalline epitaxial MnAs thin films on (001) GaAs substrates by MBE [1,2]. The epitaxial relationship was found to be quite unique; the growth direction is [1100] of the hexagonal unit cell of MnAs, and the [0001] axis (c-axis) and the [~20] axis of MnAs are along the [~10] and [110] of the GaAs substrate, respectively. The easy magnetization direction of the MBE-grown MnAs films is in-plane, along the [1120] of MnAs and the [110] of GaAs. The M - H curves measured at room temperature with the magnetic field applied along the easy magnetization axis show almost perfect squareness with 100% remanence, with the saturation magnetization M~ of 305-601 e m u / c m 3 and coercive field H c of 65-926 Oe, depending on the MnAs thickness. In this report, in order to increase the freedom in materials design and to have more functionality, we have created a new class of magnetic multilayers consisting of
* Corresponding author. Fax: masaaki @ee.t.u-tokyo.ac.jp.
+81-3-3816-4996; email:
ferromagnetic MnAs and nonmagnetic NiAs, both of which are of NiAs-type hexagonal crystal structure, on (001) GaAs substrates by MBE. Here, since all the three materials are As based compounds, we can expect thermodynamical stability with little intermixing at each interface, as well as good compatibility with III-V MBE techniques. Fig. 1 shows typical reflection high energy electron diffraction (RHEED) patterns during the MBE growth of M n A s / N i A s / M n A s / G a A s heterostructures, with the electron beam azimuth along the [110] and [ll0] of GaAs. After growing a 100 nm thick undoped GaAs buffer layer on a (001)GaAs substrate at 580°C, the substrate temperature was cooled to 250°C, and then a 25 nm thick MnAs layer, a 6 nm thick NiAs layer and a 10 nm thick MnAs layer were successively grown. The As 2 flux was kept on throughout the growth. Since we grew the first 25 nm thick MnAs layer on the disordered (4 × 4) (001)GaAs surface (Fig. l(a)), the epitaxial relationship of the M n A s / G a A s was type-A [3], in which the growth plane of MnAs is (T100), and the in-plane relationship is [l~20]MnAs [1[110]GaAs and [0001]MnAs II [ll0]GaAs (see Fig. l(b) and (c)). The same growth direction and epitaxial relationship were observed when NiAs was grown on MnAs, as shown in Fig. l(d) and (e), which indicate that the growth plane of NiAs is (7100) with [1120]NiAs II[1120]MnAs and [0001]NiAs I[[0001]MnAs. Finally, when a 10 nm thick MnAs was grown on NiAs, the epitaxial relationship remained unchanged (Fig. l(f) and (g)). One can see two-fold reconstructions in Fig. l(c) and (d), with the [0001] azimuth of MnAs and with the [1120] azimuth of NiAs, respectively. Note that the dis-
0304-8853/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0 3 0 4 - 8 8 5 3 ( 9 5 ) 0 0 8 7 8 - 0
307
M. Tanaka et al../ Journal o( Magnetism and Magnetic Materials 156 (1996) 306-308
MnAs/NiAs/MnAs/GaAs [110]GaAs azimuth
GaAs buffer layer on GaAs(001)
++ ++
well defined RHEED patterns indicate that monocrystalline M n A s / N i A s multilayers of high structural quality are successfully grown on (001)GaAs. In a previous paper [2], we have reported that the coercive field H c of the MnAs thin films grown on GaAs depends on the MnAs thickness and it decreases l¥om I).93 kOe at 10 nm to 0. l l kOe at 200 nm. This suggests a possibility of changing and controlling the coercive field of the magnetic thin films by changing the thickness. Here, we have grown M n A s / N i A s / M n A s multilayers on GaAs, with two different MnAs thicknesses, in a manner described earlier. The MnAs thickness was varied from 10 nm to 100 nm, while the NiAs thickness was fixed at 6 nm. Magnetization measurements were done using a vibrating sample magnetometer (VSM), and the present multilayers were found to have strong magnetic anisotropy. The easy axis of magnetization was found to be along the [1120] axis of MnAs, which is parallel to the [1120] of NiAs and the [110] of GaAs. An example of the M - H curve is shown in Fig. 2, which shows the result of MnAs(100 nm)/NiAs(5.5 nm)/MnAs(50 nm)/ GaAs(001), measured by VSM at room temperature. Here the magnetic field was applied along the easy magnetization axis. Since the two MnAs layers have different coercive force, two steps are clearly seen in the hysteresis. Even three-step hysteresis in the M - H curve has been successfully realized by growing M n A s / N i A s / M n A s / N i A s / M n A s multilayers with three different MnAs thicknesses. We have found that the coercive force of each MnAs layer in the present multilayer samples is controllable by changing the thickness, and therefore, we can in principle design the magnetic hysteresis characteristics on semiconductors. This phenomenon could be applied to multi-level non-volatile recording coupled with the underlying high speed semiconductor circuitry.
8
'
i
+
i
.
i
.
[0001]MnAs azimuth
Fig. I. Time evolution of RHEED patterns during the MBE growth of MnAs/NiAs/MnAs/GaAs multilayers. (a) Disordered c(4×4)-(001) GaAs surface just prior to the growth of MnAs/NiAs. (b)-(f) After growth of MnAs with the indicated thickness. The electron beam incidence is along the [I 10] (b,d+f) and [110] (c,e,g) direction of GaAs+ For comparison, the integralorder streaks are indicated by arrows in (b), and (d).
E
4
~
2
o
0
i
,
i
,
~ .
+ •
+
.
i ~
.
•~ -2 e-
~4 -6
. +.,.,.,,
-8
-0.8 tance between the integral-order streaks in Fig. l(d) is larger than that in Fig. l(b), resulting from the smaller lattice constant along the c-axis of NiAs (c = 0.5049 rim) than that of MnAs ( c = 0 . 5 7 1 3 nm), while the lattice constant along the a-axis of NiAs ( a = 0.3631 nm) is close to that of MnAs ( a = 0.3713 nm). These streaky and
.
+!
6
[1120]MnAsazimuth
t
-0.4
,--,--,--,-,--,--,--,-
0
0.4
0.8
Magnetic Field (kOe) Fig. 2. M - H characteristics of MnAs(100 nm)/NiAs(5.5 nm)/MnAs(50 nm)/GaAs(001) measured by vibrating sample magnetometry at room temperature. Magnetic field was applied along the easy magnetization axis of MnAs. [I 120], which is parallel to the [I 10] direction of GaAs.
308
M. Tanaka et aL / Journal of Magnetism and Magnetic Materials 156 (1996) 306-308
In summary, we have successfully grown a new Asbased epitaxial magnetic multilayers, consisting of MnAs and NiAs, on (001)GaAs substrates by MBE. It was found that the growth plane of MnAs and NiAs is the (3100) of hexagonal crystal structure, and that the easy axis of magnetization of MnAs is in-plane, along the [1120] axis, which is parallel to the [110] of GaAs. Multi-stepped magnetic hysteresis are controllably realized by changing the thickness of MnAs.
References [1] M. Tanaka, J.P. Harbison, T. Sands, T.L. Cheeks, V.G. Keramidas, G.M. Rothberg, J. Vac. Sci. and Technol. B 12 (1994) 1091. [2] M. Tanaka, J.P. Harbison, M.C. Park, Y.S. Park, T. Shin and G.M. Rothberg, J. Appl. Phys. 76 (1994) 6278. [3] M. Tanaka, J.P. Harbison, M.C. Park, Y.S. Park, T. Shin and G.M. Rothberg, Appl. Phys. Lett. 65 (1994) 1964.