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GaAs heterostructures studied by NMR
Journal of Magnetism and Magnetic Materials 226}230 (2001) 1588}1590
Microscopic magnetism in MnAs/GaAs heterostructures studied by NMR M. WoH jcik , E. Jedryka *, S. Nadolski , Z. Liu , K. Dessein , G. Borghs, J. De Boeck Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, 02 668 Warsaw, Poland IMEC, MCP/ME, Kapeldreef 75, B-3001 Leuven, Belgium K.U. Leuven, Physics Department, Celestijnenlaan 200D, B-3001 Leuven, Belgium K.U. Leuven, Applied Materials Science (MTM) Department, de Croylaan 2, B-3001 Leuven, Belgium
Abstract The "rst NMR study of MnAs epitaxial thin "lms grown on (0 0 1)GaAs is reported. Based on the invariance of Mn and As hyper"ne "elds it is shown that the local magnetic properties of MnAs thin "lms remain the same as in the bulk material. A hysteresis of the domain pattern upon applying the magnetic "eld is inferred from the evident reversible changes of the NMR spectrum shape. 2001 Elsevier Science B.V. All rights reserved. Keywords: NMR-spin echo; Hyper"ne "elds; Semiconductor*ferromagnetic; Thin "lms*epitaxial; Thin "lms*ferromagnetic; Magnetic moments distribution; Domain wall
Epitaxial ferromagnetic thin "lms grown directly on a semiconductor substrate attract a lot of interest due to potential applications in novel electronic devices [1]. An example of such heterostructures are high-quality ferromagnetic thin "lms of MnAs grown on GaAs substrate using MBE deposition. The magnetic properties of those "lms (saturation magnetization and coercivity) have been shown to depend on preparation conditions [2,3]. In addition, they proved to vary drastically with the sample thickness [2]. The origin of these sensitive changes remains unknown and calls for a study of microscopic magnetic properties with a local probe, such as NMR. While the studies of bulk MnAs at 77 K have been carried out already in the early times of ferromagnetic NMR [4,5], we report here the "rst NMR results on MnAs in the form of thin "lms. Three samples of hexagonal MnAs "lms have been epitaxially grown on (0 0 1) GaAs and magnetically characterized at room temperature using AGFM magnetometer. Their parameters are
* Corresponding author. Tel.: #48-22-8435212; fax: 48-228430926. E-mail address: [email protected] (E. Jedryka).
listed in Table 1. All measured samples reveal a strong in-plane anisotropy. An almost perfectly square hysteresis loop has been recorded for magnetic "eld applied in sample's plane along the easy axis, similarly to previous reports [2,3]. From Table 1 a considerable reduction of saturation magnetization with respect to M in the bulk material is obvious, as well as a considerable scatter of M values within the thin "lm samples. NMR experiment has been carried out in such a way that the radio frequency (RF) "eld produced in the NMR coil was parallel to sample's easy axis. Any attempt to observe NMR with the RF "eld oriented perpendicular to the easy axis has been unsuccessful. This observation proves that only the nuclei located within the domain walls contribute to the NMR signal. Fig. 1 presents the NMR spectra recorded at 4.2 K from the three studied samples. They consist of two groups of resonance lines centered at 220 and 239 MHz, which are readily interpreted as resonance of As and Mn, respectively * in agreement with the previously reported spectra for bulk MnAs [4,5]. A regular substructure consisting of 3 satellite lines for As (nuclear spin I"3/2) and "ve satellites for Mn (I"5/2) is due to quadrupolar interaction with the electric "eld gradient
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 1 0 3 0 - 1
M. Wo& jcik et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1588}1590
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Table 1 The growth conditions, magnetic properties and hyper"ne "elds for the three studied MnAs thin "lms and the literature reference data for bulk MnAs
Sample I Sample II Sample III Bulk MnAs
Layer thickness (nm)
Growth temperature (3C)
M (emu/cm) (Room temperature)
H (Oe) (Room temperature)
Mn HF (kOe) (4.2 K)
As HF (kOe) (4.2 K)
100 250 275
250 250 275
460 410 208 670 [3]
100}140 30 70
226.5 226.0 225.7 226.5 [4,5]
302.2 302.1 300.9 285}288 [4,5]
Fig. 2. As and Mn NMR spectra recorded at a constant RF power from 100 nm thick MnAs layer for two states of the domain structure: (a) initial state, (b) following the application of strong RF pulses, (c) after demagnetizing by heating the sample above ¹ . !
Fig. 1. As and Mn NMR spectra at 4.2 K from MnAs thin "lms: (a) 100 nm, (b) 250 nm, (c) 275 nm thick. The spectrum intensity has been corrected for the intrinsic NMR enhancement factor at each frequency point, computed from intensities obtained at di!erent levels of RF excitation. The vertical lines are eye-guides indicating the characteristic frequency positions.
in a sample and is a "ngerprint of a good crystal quality. The quadrupolar splittings are almost identical in all studied samples: (Mn)"1.6 MHz and (As)" / / 3.2 MHz. The central NMR frequency corresponding to the ! nuclear spin transition is de"ned by the hyper"ne "eld (HF), which is proportional to the on-site magnetic moment and to the magnetic moments of the neighbouring atoms. The experimentally observed Mn NMR frequencies (hyper"ne "elds) are constant for all studied samples, proving that the local magnetic moment of Mn atoms is preserved. In the same way, the lack of variation of As NMR frequency (determined by the
transferred hyper"ne "eld) is an additional proof of stability of Mn magnetic moment. Moreover, hyper"ne "elds in studied thin "lms are very close to the values reported for bulk MnAs (see Table 1). Thus, the observed reduction of saturation magnetization in thin MnAs "lms is not related to the change of Mn local magnetic moment and must have another source, e.g. presence of nonmagnetic inclusions in the sample [3]. The local magnetic properties of MnAs phase remain the same in thin "lm as in the bulk material. While the hyper"ne "elds remained unchanged, a hysteresis of the NMR spectrum shape and intensity in studied thin "lms has been observed as a function of excitation conditions, as shown in Fig. 2. After the sample has been subject to RF "eld of high amplitude, the spectrum changed from that shown in Fig. 2a to that in Fig. 2b. After heating above ¹ and demagnetizing the sample, the NMR spectrum (Fig. 2c) came back to its original shape from Fig. 2a. This behaviour can be interpreted as instabilities of the domain structure. The growth of hexagonal MnAs on the cubic (0 0 1) GaAs is expected to be characterized by the following epitaxial
1590
M. Wo& jcik et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1588}1590
relationship [6]: !(1 1 0 0)MnAs(0 0 1)GaAs and
domain pattern upon applying the magnetic "eld is inferred from the evident reversible changes of the NMR spectrum shape.
[0 0 0 1]MnAs[!1 1 0]GaAs. For such growth mode of MnAs, both the easy axis and the hard magnetization direction lie in sample's plane. Since the material is highly anisotropic, one can expect that the domain wall is formed by magnetization rotation out of sample's plane, leading to a considerable magnetic charge on a surface. The reported stripe domain structure observed with the atomic force microscopy [7] suggests indeed this kind of domain walls. The actual domain structure may be even more complicated and other kind of domains may also exist in order to reduce the magnetic energy on sample surface. Application of a strong RF pulse may result in changing the domain pattern which consequently leads to the observed reversible change of the NMR spectrum shape. In conclusion, the present study shows that the local magnetic properties of MnAs phase remain the same in thin "lms as in the bulk material and cannot account for the reported reduction of saturation magnetization in thin "lms. At the same time, a certain hysteresis in the
This work was partly supported by the Bilateral Agreement between Flanders and Poland (BIL99/22) and by the grant from Ford Motor Company.
References [1] G.A. Prinz, Science 250 (1990) 1092. [2] M. Tanaka, J.P. Harbison, M.C. Park, Y.S. Park, T. Shin, G.M. Rothberg, J. Appl. Phys. 76 (1994) 6278. [3] T. Uragami, K. Ono, M. Mizuguchi, H. Fijioka, M. Tanaka, M. Oshima, J. Crystal Growth 209 (2000) 561. [4] T. Hihara, Y. Koi, A. Tsujimura, J. Phys. Soc. Jpn 17 (1962) 1320. [5] S.L. Pinjare, K.V.S. Rama Rao, Magn. Magn. Mater. 30 (1982) 27. [6] F. Schippan, A. Trampert, L. Daweritz, K.H. Ploog, J. Vac. Sci. Technol. B17 (1999) 1716. [7] M.C. Park, Y. Park, T. Shin, G.M. Rothberg, M. Tanaka, J.P. Harbison, J. Appl. Phys. 79 (1996) 4967.
Microscopic magnetism in MnAs/GaAs heterostructures studied by NMR M. WoH jcik , E. Jedryka *, S. Nadolski , Z. Liu , K. Dessein , G. Borghs, J. De Boeck Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, 02 668 Warsaw, Poland IMEC, MCP/ME, Kapeldreef 75, B-3001 Leuven, Belgium K.U. Leuven, Physics Department, Celestijnenlaan 200D, B-3001 Leuven, Belgium K.U. Leuven, Applied Materials Science (MTM) Department, de Croylaan 2, B-3001 Leuven, Belgium
Abstract The "rst NMR study of MnAs epitaxial thin "lms grown on (0 0 1)GaAs is reported. Based on the invariance of Mn and As hyper"ne "elds it is shown that the local magnetic properties of MnAs thin "lms remain the same as in the bulk material. A hysteresis of the domain pattern upon applying the magnetic "eld is inferred from the evident reversible changes of the NMR spectrum shape. 2001 Elsevier Science B.V. All rights reserved. Keywords: NMR-spin echo; Hyper"ne "elds; Semiconductor*ferromagnetic; Thin "lms*epitaxial; Thin "lms*ferromagnetic; Magnetic moments distribution; Domain wall
Epitaxial ferromagnetic thin "lms grown directly on a semiconductor substrate attract a lot of interest due to potential applications in novel electronic devices [1]. An example of such heterostructures are high-quality ferromagnetic thin "lms of MnAs grown on GaAs substrate using MBE deposition. The magnetic properties of those "lms (saturation magnetization and coercivity) have been shown to depend on preparation conditions [2,3]. In addition, they proved to vary drastically with the sample thickness [2]. The origin of these sensitive changes remains unknown and calls for a study of microscopic magnetic properties with a local probe, such as NMR. While the studies of bulk MnAs at 77 K have been carried out already in the early times of ferromagnetic NMR [4,5], we report here the "rst NMR results on MnAs in the form of thin "lms. Three samples of hexagonal MnAs "lms have been epitaxially grown on (0 0 1) GaAs and magnetically characterized at room temperature using AGFM magnetometer. Their parameters are
* Corresponding author. Tel.: #48-22-8435212; fax: 48-228430926. E-mail address: [email protected] (E. Jedryka).
listed in Table 1. All measured samples reveal a strong in-plane anisotropy. An almost perfectly square hysteresis loop has been recorded for magnetic "eld applied in sample's plane along the easy axis, similarly to previous reports [2,3]. From Table 1 a considerable reduction of saturation magnetization with respect to M in the bulk material is obvious, as well as a considerable scatter of M values within the thin "lm samples. NMR experiment has been carried out in such a way that the radio frequency (RF) "eld produced in the NMR coil was parallel to sample's easy axis. Any attempt to observe NMR with the RF "eld oriented perpendicular to the easy axis has been unsuccessful. This observation proves that only the nuclei located within the domain walls contribute to the NMR signal. Fig. 1 presents the NMR spectra recorded at 4.2 K from the three studied samples. They consist of two groups of resonance lines centered at 220 and 239 MHz, which are readily interpreted as resonance of As and Mn, respectively * in agreement with the previously reported spectra for bulk MnAs [4,5]. A regular substructure consisting of 3 satellite lines for As (nuclear spin I"3/2) and "ve satellites for Mn (I"5/2) is due to quadrupolar interaction with the electric "eld gradient
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 1 0 3 0 - 1
M. Wo& jcik et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1588}1590
1589
Table 1 The growth conditions, magnetic properties and hyper"ne "elds for the three studied MnAs thin "lms and the literature reference data for bulk MnAs
Sample I Sample II Sample III Bulk MnAs
Layer thickness (nm)
Growth temperature (3C)
M (emu/cm) (Room temperature)
H (Oe) (Room temperature)
Mn HF (kOe) (4.2 K)
As HF (kOe) (4.2 K)
100 250 275
250 250 275
460 410 208 670 [3]
100}140 30 70
226.5 226.0 225.7 226.5 [4,5]
302.2 302.1 300.9 285}288 [4,5]
Fig. 2. As and Mn NMR spectra recorded at a constant RF power from 100 nm thick MnAs layer for two states of the domain structure: (a) initial state, (b) following the application of strong RF pulses, (c) after demagnetizing by heating the sample above ¹ . !
Fig. 1. As and Mn NMR spectra at 4.2 K from MnAs thin "lms: (a) 100 nm, (b) 250 nm, (c) 275 nm thick. The spectrum intensity has been corrected for the intrinsic NMR enhancement factor at each frequency point, computed from intensities obtained at di!erent levels of RF excitation. The vertical lines are eye-guides indicating the characteristic frequency positions.
in a sample and is a "ngerprint of a good crystal quality. The quadrupolar splittings are almost identical in all studied samples: (Mn)"1.6 MHz and (As)" / / 3.2 MHz. The central NMR frequency corresponding to the ! nuclear spin transition is de"ned by the hyper"ne "eld (HF), which is proportional to the on-site magnetic moment and to the magnetic moments of the neighbouring atoms. The experimentally observed Mn NMR frequencies (hyper"ne "elds) are constant for all studied samples, proving that the local magnetic moment of Mn atoms is preserved. In the same way, the lack of variation of As NMR frequency (determined by the
transferred hyper"ne "eld) is an additional proof of stability of Mn magnetic moment. Moreover, hyper"ne "elds in studied thin "lms are very close to the values reported for bulk MnAs (see Table 1). Thus, the observed reduction of saturation magnetization in thin MnAs "lms is not related to the change of Mn local magnetic moment and must have another source, e.g. presence of nonmagnetic inclusions in the sample [3]. The local magnetic properties of MnAs phase remain the same in thin "lm as in the bulk material. While the hyper"ne "elds remained unchanged, a hysteresis of the NMR spectrum shape and intensity in studied thin "lms has been observed as a function of excitation conditions, as shown in Fig. 2. After the sample has been subject to RF "eld of high amplitude, the spectrum changed from that shown in Fig. 2a to that in Fig. 2b. After heating above ¹ and demagnetizing the sample, the NMR spectrum (Fig. 2c) came back to its original shape from Fig. 2a. This behaviour can be interpreted as instabilities of the domain structure. The growth of hexagonal MnAs on the cubic (0 0 1) GaAs is expected to be characterized by the following epitaxial
1590
M. Wo& jcik et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1588}1590
relationship [6]: !(1 1 0 0)MnAs(0 0 1)GaAs and
domain pattern upon applying the magnetic "eld is inferred from the evident reversible changes of the NMR spectrum shape.
[0 0 0 1]MnAs[!1 1 0]GaAs. For such growth mode of MnAs, both the easy axis and the hard magnetization direction lie in sample's plane. Since the material is highly anisotropic, one can expect that the domain wall is formed by magnetization rotation out of sample's plane, leading to a considerable magnetic charge on a surface. The reported stripe domain structure observed with the atomic force microscopy [7] suggests indeed this kind of domain walls. The actual domain structure may be even more complicated and other kind of domains may also exist in order to reduce the magnetic energy on sample surface. Application of a strong RF pulse may result in changing the domain pattern which consequently leads to the observed reversible change of the NMR spectrum shape. In conclusion, the present study shows that the local magnetic properties of MnAs phase remain the same in thin "lms as in the bulk material and cannot account for the reported reduction of saturation magnetization in thin "lms. At the same time, a certain hysteresis in the
This work was partly supported by the Bilateral Agreement between Flanders and Poland (BIL99/22) and by the grant from Ford Motor Company.
References [1] G.A. Prinz, Science 250 (1990) 1092. [2] M. Tanaka, J.P. Harbison, M.C. Park, Y.S. Park, T. Shin, G.M. Rothberg, J. Appl. Phys. 76 (1994) 6278. [3] T. Uragami, K. Ono, M. Mizuguchi, H. Fijioka, M. Tanaka, M. Oshima, J. Crystal Growth 209 (2000) 561. [4] T. Hihara, Y. Koi, A. Tsujimura, J. Phys. Soc. Jpn 17 (1962) 1320. [5] S.L. Pinjare, K.V.S. Rama Rao, Magn. Magn. Mater. 30 (1982) 27. [6] F. Schippan, A. Trampert, L. Daweritz, K.H. Ploog, J. Vac. Sci. Technol. B17 (1999) 1716. [7] M.C. Park, Y. Park, T. Shin, G.M. Rothberg, M. Tanaka, J.P. Harbison, J. Appl. Phys. 79 (1996) 4967.