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Observation of room-temperature ferromagnetism in (Al,Mn)N thin films
ARTICLE IN PRESS
Journal of Crystal Growth 271 (2004) 420–424 www.elsevier.com/locate/jcrysgro
Observation of room-temperature ferromagnetism in (Al,Mn)N thin films Moon-Ho Hama, Sukho Yoonb, Yongjo Parkb, Jae-Min Myounga, a
Information and Electronic Materials Research Laboratory, Department of Materials Science and Engineering, Yonsei University, 134 Shinchon-Dong, Seodaemun-Gu, Seoul 120-749, Republic of Korea b Photonics Laboratory, Samsung Advanced Institute of Technology, P.O.Box 111, Suwon 440-600, Republic of Korea Received 21 June 2004; accepted 5 August 2004 Communicated by D.W. Shaw Available online 25 September 2004
Abstract We present the structural, electrical and magnetic properties in the (Al,Mn)N thin films with various Mn concentrations grown by plasma-enhanced molecular beam epitaxy. The X-ray diffraction patterns reveal that the (Al,Mn)N films have the wurtzite structure without secondary phases. The (Al1xMnx)N films with x ¼ 1:7% and 2.6% were obviously found to exhibit the ferromagnetic ordering with Curie temperature exceeding room temperature while the film with x ¼ 3:0% was observed to show the ferromagnetic ordering at 5 K, indicating that a critical Mn concentration exists. Since all the films exhibit insulating characteristics, the origin of ferromagnetism in (Al,Mn)N might be attributed to indirect exchange interaction caused by virtual electron excitations from Mn acceptor level to the valence band within the samples. r 2004 Elsevier B.V. All rights reserved. PACS: 75.50.Pp; 75.70.i; 73.61.Ey; 68.55.a Keywords: A1. Ferromagnetism; A3. PEMBE; B1. (Al,Mn)N; B2. Diluted magnetic semiconductor
1. Introduction Diluted magnetic semiconductors (DMSs) have attracted great attention due to applications for spintronic semiconductor devices such as spin-polarized Corresponding author. Tel.: +82-2-2123-2843; fax: +82-2-
365-2680. E-mail address: [email protected] (J.-M. Myoung).
light-emitting diodes (spin LEDs) [1], spin-polarized field effect transistors (spin FETs) [2], and quantum computers [3]. The DMSs are semiconductors incorporated with particular transition metals as magnetic dopants, offering novel functionalities by introducing the spin degree of freedom in addition to charge of carriers (electrons or holes) [3,4]. In recent years, (Ga,Mn)As has been intensively studied as a representative DMS, but its Curie
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.08.004
ARTICLE IN PRESS M.-H. Ham et al. / Journal of Crystal Growth 271 (2004) 420–424
temperatures (TC) stay below 140 K [5], being far from room temperature. For practical device applications, it is necessary to discover DMSs which have TC exceeding room temperature. The theoretical studies by Dietl et al. predicted TC above room temperature for Mn-doped GaN and ZnO by the mean-field Zener model of ferromagnetism [6]. On the other hand, Litvinov and Dugaev proposed theoretically that Mn-doped AlN might result in a higher TC than that of Mn-doped GaN on the basis of indirect exchange interaction caused by virtual electron excitations from magnetic impurity acceptor levels to the valence band [7]. In addition to these predictions, AlN plays an important role for optoelectronic devices such as blue-, violet-, and ultraviolet (UV)LEDs and laser diodes (LDs) [8]. Also, it has been used as a tunnel barrier and an insulator for semiconductor-based devices. More recently, the magnetic and magnetotransport properties in (Ga,Mn)N DMSs with TC exceeding room temperature grown by plasmaenhanced molecular beam epitaxy (PEMBE) [9,10] and metalorganic chemical vapor deposition (MOCVD) [11] have been reported in several groups. The (Al,Mn)N with TC higher than room temperature has been successfully synthesized by gas-source MBE but its magnetization was very small [12]. In this paper, we report on the roomtemperature ferromagnetism in the (Al,Mn)N thin films with various Mn concentrations grown by PEMBE. The (Al,Mn)N films were found to exhibit insulating behavior and the ferromagnetic ordering with TC higher than room temperature.
2. Experimental procedure The growth of the (Al,Mn)N thin films with various Mn concentrations was carried out in a PEMBE system under ultrahigh vacuum (UHV) conditions with a base pressure of 1 1010 Torr. The source materials were high-purity Al (6 N) and Mn (5 N) metals that were heated in conventional effusion cells, respectively. As the nitrogen source, high-purity nitrogen (6 N) gas was supplied through a radio-frequency (RF) plasma source. Sapphire (0 0 0 1) was used as a substrate
421
and was degreased using a standard cleaning procedure. Then, the substrates were exposed to the nitrogen plasma of 350 W at 700 1C for 30 min. After the nitridation, the (Al,Mn)N films were grown on the substrate at a substrate temperature of 700 1C. During the growth of the (Al,Mn)N films, the Al cell temperature, the nitrogen flow rate and the RF plasma power were kept at 1200 1C, 2 sccm and 350 W, respectively. Under these conditions, the typical growth rate was 2 nm/ min. Mn concentrations in the films were controlled by changing Mn cell temperatures (650, 700, and 750 1C), and were estimated to be 1.7%, 2.6% and 3.0%, respectively, as measured by secondary ion mass spectrometry (SIMS). The thickness of the (Al,Mn)N films was approximately 400 nm, measured by surface profiler. The crystalline quality of the (Al,Mn)N films was investigated by double-crystal X-ray diffraction (DCXRD) analysis using Ni-filtered Cu Ka source. Magnetic behavior was measured using a superconducting quantum interference device (SQUID) magnetometer and a high-sensitivity (108 emu) alternating gradient magnetometer (AGM) in the temperature range 5–300 K. The electrical properties were determined by van der Pauw Hall measurements applying a magnetic field up to 1.64 T at room temperature.
3. Results and discussion In order to investigate the structural properties for the (Al,Mn)N films with various Mn concentrations, XRD analyses were performed and the results are shown in Fig. 1. Two peaks from wurtzite (Al,Mn)N (0 0 0 2) and (0 0 0 6) were observed in addition to the diffraction peak from sapphire (0 0 0 2). Compared to undoped AlN, no additional peaks were found when Mn was incorporated into the host AlN films. This result obviously indicates that all the (Al,Mn)N films have the wurtzite structure without any secondary phase formation such as Mn3N2, Mn4N, Al3Mn, and Mn clusters. The inset of Fig. 1 illustrates the variation of 2y for (Al,Mn)N (0 0 0 2) peak as a function of Mn concentration. As Mn concentration increases from 1.7% to 2.6%, the peak
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Fig. 1. X-ray diffraction patterns for the (Al1xMnx)N films with (a) x ¼ 1:7%; (b) x ¼ 2:6%; and (c) x ¼ 3:0%: The inset shows the variation of 2y as a function of Mn concentration.
position of (Al,Mn)N (0 0 0 2) shifts from 36.031 to 36.121. According to Bragg’s law, the c-axis lattice constant of films decreases with higher Mn incorporation within the films. This tendency has been found in (Ga,Mn)N [13] and provides the clear evidence that Mn ions were incorporated into the substitutional sites in the (Al,Mn)N structure. However, as Mn concentration increases to a higher concentration of 3.0%, its peak slightly moves back to a lower angle of 36.101 and the caxis lattice constant of the film expands. It is regarded that Mn ions do not effectively replace Al ions located at the substitutional sites but partial Mn ions are incorporated into the interstitial sites rather than the substitutional sites within the film. The magnetic properties of the (Al,Mn)N films with various Mn concentrations were examined using AGM that is appropriate to evaluate small magnetization because of high sensitivity. Fig. 2 shows magnetization versus magnetic field (M–H) curves of the (Al1xMnx)N films with (a) x ¼ 1:7% and (b) x ¼ 2:6% at room temperature. Clear hysteresis loops for both films were obtained, indicating that the films exhibit the ferromagnetic ordering. However, for the (Al1xMnx)N film with x ¼ 3:0%; no hysteresis loop was observed. In particular, for the (Al1xMnx)N film with x ¼ 2:6%; the saturation
Fig. 2. Hysteresis loops for the (Al1xMnx)N films with (a) x ¼ 1:7% and (b) x ¼ 2:6% at room temperature, obtained with magnetic fields up to 5000 Oe applied parallel to the plane of the samples by AGM. For the (Al,Mn)N films, the nonmagnetic contributions of sapphire substrate and experimental apparatus have been subtracted from the data.
magnetization (Ms) at 300 K was 2.9 emu/cm3, and is larger than that for (Al,Mn)N reported in the previous work [12]. The magnetic moment calculated from Ms is 0.25 Bohr magneton (mB)/Mn. More importantly, the (Al,Mn)N films have coercivities (Hc) varying from 40Oe to 50 Oe as Mn concentrations increase from 1.7% to 2.6%. This indicates that the (Al,Mn)N films do not exhibit a superparamagnetic behavior due to nano-clusters [9,14] as secondary phases, e.g., MnSb nano-clusters in (Ga,Mn)Sb, giving rise to no hysteresis [14]. However, if secondary phases being large enough to have large blocking temperature exist within (Al,Mn)N, it might be possible to exhibit the hysteretic feature from such secondary phases. Fig. 3 shows M–H curves of the (Al1xMnx)N films with (a) x ¼ 2:6% and (b) x ¼ 3:0% measured at 5 K using SQUID magnetometer. For the (Al1xMnx)N film with x ¼ 2:6%; Ms and Hc at 5 K were 6 emu/cm3 which corresponded to 0.52 mB/Mn and 150 Oe, respectively, and both at 5 K were larger than those at 300 K shown in Fig. 2. The (Al1xMnx)N film with x ¼ 3:0% exhibited M s ¼ 1:6 emu/cm3 which corresponded
ARTICLE IN PRESS M.-H. Ham et al. / Journal of Crystal Growth 271 (2004) 420–424
Fig. 3. Hysteresis loops for the (Al1xMnx)N films with (a) x ¼ 2:6% and (b) x ¼ 3:0% at 5 K measured by SQUID magnetometer with magnetic fields up to 5000 Oe applied parallel to the plane of the samples.
to 0.12 mB/Mn at 5 K while no hysteresis characteristics were observed at 300 K, indicating that the film has a lower TC than room temperature. It is attributed to increase of antiferromagnetic interactions between the nearest neighbor Mn ions as Mn incorporation increases, resulting in the lower magnetic moment per Mn. This magnetic behavior in the films indicates that a critical Mn concentration exists. It accords that the (Ga,Mn)N films with Mn ¼ 3% rather than higher Mn concentrations had the highest magnetic moment per Mn [9] and is in good agreement with the XRD results. The temperature dependence of magnetization for the (Al1xMnx)N film with x ¼ 2:6% under the field-cooled (FC) mode with a magnetic field of 100 Oe measured by SQUID magnetometer is shown in Fig. 4. The magnetization for the (Al,Mn)N film decreases slowly and continuously with increasing temperature. The ferromagnetic ordering for the (Al,Mn)N film was clearly observed in the temperature range 5–300 K, indicating that TC for the film exceeds at least room temperature. The electrical properties in the (Al,Mn)N films were determined by van der Pauw Hall measurements at room temperature. The Ohmic contact to the (Al,Mn)N films was made using indium. All the films exhibited highly resistive characteristics
423
Fig. 4. Temperature dependence of the magnetization for the (Al1xMnx)N film with x ¼ 2:6% under FC mode with a magnetic field of 100 Oe measured by SQUID magnetometer.
(60 MO), indicating that they had insulating properties. Therefore, the origin of ferromagnetism in the (Al,Mn)N films is not ascribed to carrier-mediated Ruderman–Kittel–Kasuya– Yosida (RKKY) interaction [4]. One possible explanation for this ferromagnetism is the formation of secondary phases such as manganese nitrides and Mn clusters. However, the evidence of such secondary phases in XRD measurements was not observed. Hence, the ferromagnetic ordering in our (Al,Mn)N is believed to be the intrinsic property. Another possibility is that the indirect exchange interaction in (Al,Mn)N films was caused by virtual electron excitations from Mn acceptor level to the valence band within the films [7]. However, it is still unclear and further studies are necessary to clarify the origin of ferromagnetism in the Mn-doped AlN.
4. Conclusion In summary, we have investigated the structural, electrical and magnetic properties in the (Al,Mn)N thin films with various Mn concentrations grown by PEMBE. The (Al1xMnx)N films with x ¼ 1:7% and 2.6% exhibited the ferromagnetic ordering with
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Curie temperature above room temperature while the film with x ¼ 3:0% showed the ferromagnetic ordering at 5 K, indicating that a critical Mn concentration exists. XRD analysis did not identify the presence of any secondary phases within the wurtzite (Al,Mn)N films. Since the (Al,Mn)N films show insulating characteristics, the origin of ferromagnetism in (Al,Mn)N is not likely to carriermediated RKKY interaction but might be due to indirect exchange interaction caused by virtual electron excitations from Mn acceptor level to the valence band within the samples.
Acknowledgements This work was supported by Samsung Advanced Institute of Technology (Grant No. 20032-0799). The authors thank S. J. Oh at Korea Basic Science Institute for his assistance with SQUID magnetometer. References [1] Y. Ohno, D.K. Young, B. Beschoten, F. Matsukura, H. Ohno, D.D. Awschalom, Nature 402 (1999) 790.
[2] S. Datta, B. Das, Appl. Phys. Lett. 56 (1990) 665. [3] 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. [4] H. Ohno, Science 281 (1998) 951. [5] K.W. Edmonds, K.Y. Wang, R.P. Campion, A.C. Neumann, N.R.S. Farley, B.L. Gallagher, C.T. Foxon, Appl. Phys. Lett. 81 (2002) 4991. [6] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 287 (2000) 1019. [7] V.I. Litvinov, V.K. Dugaev, Phys. Rev. Lett. 86 (2001) 5593. [8] See, e.g. S. Nakamura, S. Pearton, G. Fasol, The Blue Laser Diode, Springer, Berlin, 2000. [9] G.T. Thaler, M.E. Overberg, B. Gila, R. Frazier, C.R. Abernathy, S.J. Pearton, J.S. Lee, S.Y. Lee, Y.D. Park, Z.G. Khim, J. Kim, F. Ren, Appl. Phys. Lett. 80 (2002) 3964. [10] M.H. Ham, M.C. Jeong, W.Y. Lee, J.M. Myoung, J.Y. Chang, S.H. Han, Jpn. J. Appl. Phys. 42 (2003) L1372. [11] M.L. Reed, N.A. El-Masry, H.H. Stadelmaier, M.K. Ritums, M.J. Reed, C.A. Parker, J.C. Roberts, S.M. Bedair, Appl. Phys. Lett. 79 (2001) 3473. [12] R. Frazier, G. Thaler, M. Overberg, B. Gila, C.R. Abernathy, S.J. Pearton, Appl. Phys. Lett. 83 (2003) 1758. [13] G. Thaler, R. Frazier, B. Gila, J. Stapleton, M. Davidson, C.R. Abernathy, S.J. Pearton, C. Segre, Appl. Phys. Lett. 84 (2004) 1314. [14] E. Abe, F. Matsukura, H. Yasuda, Y. Ohno, H. Ohno, Physica E 7 (2000) 981.
Journal of Crystal Growth 271 (2004) 420–424 www.elsevier.com/locate/jcrysgro
Observation of room-temperature ferromagnetism in (Al,Mn)N thin films Moon-Ho Hama, Sukho Yoonb, Yongjo Parkb, Jae-Min Myounga, a
Information and Electronic Materials Research Laboratory, Department of Materials Science and Engineering, Yonsei University, 134 Shinchon-Dong, Seodaemun-Gu, Seoul 120-749, Republic of Korea b Photonics Laboratory, Samsung Advanced Institute of Technology, P.O.Box 111, Suwon 440-600, Republic of Korea Received 21 June 2004; accepted 5 August 2004 Communicated by D.W. Shaw Available online 25 September 2004
Abstract We present the structural, electrical and magnetic properties in the (Al,Mn)N thin films with various Mn concentrations grown by plasma-enhanced molecular beam epitaxy. The X-ray diffraction patterns reveal that the (Al,Mn)N films have the wurtzite structure without secondary phases. The (Al1xMnx)N films with x ¼ 1:7% and 2.6% were obviously found to exhibit the ferromagnetic ordering with Curie temperature exceeding room temperature while the film with x ¼ 3:0% was observed to show the ferromagnetic ordering at 5 K, indicating that a critical Mn concentration exists. Since all the films exhibit insulating characteristics, the origin of ferromagnetism in (Al,Mn)N might be attributed to indirect exchange interaction caused by virtual electron excitations from Mn acceptor level to the valence band within the samples. r 2004 Elsevier B.V. All rights reserved. PACS: 75.50.Pp; 75.70.i; 73.61.Ey; 68.55.a Keywords: A1. Ferromagnetism; A3. PEMBE; B1. (Al,Mn)N; B2. Diluted magnetic semiconductor
1. Introduction Diluted magnetic semiconductors (DMSs) have attracted great attention due to applications for spintronic semiconductor devices such as spin-polarized Corresponding author. Tel.: +82-2-2123-2843; fax: +82-2-
365-2680. E-mail address: [email protected] (J.-M. Myoung).
light-emitting diodes (spin LEDs) [1], spin-polarized field effect transistors (spin FETs) [2], and quantum computers [3]. The DMSs are semiconductors incorporated with particular transition metals as magnetic dopants, offering novel functionalities by introducing the spin degree of freedom in addition to charge of carriers (electrons or holes) [3,4]. In recent years, (Ga,Mn)As has been intensively studied as a representative DMS, but its Curie
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.08.004
ARTICLE IN PRESS M.-H. Ham et al. / Journal of Crystal Growth 271 (2004) 420–424
temperatures (TC) stay below 140 K [5], being far from room temperature. For practical device applications, it is necessary to discover DMSs which have TC exceeding room temperature. The theoretical studies by Dietl et al. predicted TC above room temperature for Mn-doped GaN and ZnO by the mean-field Zener model of ferromagnetism [6]. On the other hand, Litvinov and Dugaev proposed theoretically that Mn-doped AlN might result in a higher TC than that of Mn-doped GaN on the basis of indirect exchange interaction caused by virtual electron excitations from magnetic impurity acceptor levels to the valence band [7]. In addition to these predictions, AlN plays an important role for optoelectronic devices such as blue-, violet-, and ultraviolet (UV)LEDs and laser diodes (LDs) [8]. Also, it has been used as a tunnel barrier and an insulator for semiconductor-based devices. More recently, the magnetic and magnetotransport properties in (Ga,Mn)N DMSs with TC exceeding room temperature grown by plasmaenhanced molecular beam epitaxy (PEMBE) [9,10] and metalorganic chemical vapor deposition (MOCVD) [11] have been reported in several groups. The (Al,Mn)N with TC higher than room temperature has been successfully synthesized by gas-source MBE but its magnetization was very small [12]. In this paper, we report on the roomtemperature ferromagnetism in the (Al,Mn)N thin films with various Mn concentrations grown by PEMBE. The (Al,Mn)N films were found to exhibit insulating behavior and the ferromagnetic ordering with TC higher than room temperature.
2. Experimental procedure The growth of the (Al,Mn)N thin films with various Mn concentrations was carried out in a PEMBE system under ultrahigh vacuum (UHV) conditions with a base pressure of 1 1010 Torr. The source materials were high-purity Al (6 N) and Mn (5 N) metals that were heated in conventional effusion cells, respectively. As the nitrogen source, high-purity nitrogen (6 N) gas was supplied through a radio-frequency (RF) plasma source. Sapphire (0 0 0 1) was used as a substrate
421
and was degreased using a standard cleaning procedure. Then, the substrates were exposed to the nitrogen plasma of 350 W at 700 1C for 30 min. After the nitridation, the (Al,Mn)N films were grown on the substrate at a substrate temperature of 700 1C. During the growth of the (Al,Mn)N films, the Al cell temperature, the nitrogen flow rate and the RF plasma power were kept at 1200 1C, 2 sccm and 350 W, respectively. Under these conditions, the typical growth rate was 2 nm/ min. Mn concentrations in the films were controlled by changing Mn cell temperatures (650, 700, and 750 1C), and were estimated to be 1.7%, 2.6% and 3.0%, respectively, as measured by secondary ion mass spectrometry (SIMS). The thickness of the (Al,Mn)N films was approximately 400 nm, measured by surface profiler. The crystalline quality of the (Al,Mn)N films was investigated by double-crystal X-ray diffraction (DCXRD) analysis using Ni-filtered Cu Ka source. Magnetic behavior was measured using a superconducting quantum interference device (SQUID) magnetometer and a high-sensitivity (108 emu) alternating gradient magnetometer (AGM) in the temperature range 5–300 K. The electrical properties were determined by van der Pauw Hall measurements applying a magnetic field up to 1.64 T at room temperature.
3. Results and discussion In order to investigate the structural properties for the (Al,Mn)N films with various Mn concentrations, XRD analyses were performed and the results are shown in Fig. 1. Two peaks from wurtzite (Al,Mn)N (0 0 0 2) and (0 0 0 6) were observed in addition to the diffraction peak from sapphire (0 0 0 2). Compared to undoped AlN, no additional peaks were found when Mn was incorporated into the host AlN films. This result obviously indicates that all the (Al,Mn)N films have the wurtzite structure without any secondary phase formation such as Mn3N2, Mn4N, Al3Mn, and Mn clusters. The inset of Fig. 1 illustrates the variation of 2y for (Al,Mn)N (0 0 0 2) peak as a function of Mn concentration. As Mn concentration increases from 1.7% to 2.6%, the peak
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Fig. 1. X-ray diffraction patterns for the (Al1xMnx)N films with (a) x ¼ 1:7%; (b) x ¼ 2:6%; and (c) x ¼ 3:0%: The inset shows the variation of 2y as a function of Mn concentration.
position of (Al,Mn)N (0 0 0 2) shifts from 36.031 to 36.121. According to Bragg’s law, the c-axis lattice constant of films decreases with higher Mn incorporation within the films. This tendency has been found in (Ga,Mn)N [13] and provides the clear evidence that Mn ions were incorporated into the substitutional sites in the (Al,Mn)N structure. However, as Mn concentration increases to a higher concentration of 3.0%, its peak slightly moves back to a lower angle of 36.101 and the caxis lattice constant of the film expands. It is regarded that Mn ions do not effectively replace Al ions located at the substitutional sites but partial Mn ions are incorporated into the interstitial sites rather than the substitutional sites within the film. The magnetic properties of the (Al,Mn)N films with various Mn concentrations were examined using AGM that is appropriate to evaluate small magnetization because of high sensitivity. Fig. 2 shows magnetization versus magnetic field (M–H) curves of the (Al1xMnx)N films with (a) x ¼ 1:7% and (b) x ¼ 2:6% at room temperature. Clear hysteresis loops for both films were obtained, indicating that the films exhibit the ferromagnetic ordering. However, for the (Al1xMnx)N film with x ¼ 3:0%; no hysteresis loop was observed. In particular, for the (Al1xMnx)N film with x ¼ 2:6%; the saturation
Fig. 2. Hysteresis loops for the (Al1xMnx)N films with (a) x ¼ 1:7% and (b) x ¼ 2:6% at room temperature, obtained with magnetic fields up to 5000 Oe applied parallel to the plane of the samples by AGM. For the (Al,Mn)N films, the nonmagnetic contributions of sapphire substrate and experimental apparatus have been subtracted from the data.
magnetization (Ms) at 300 K was 2.9 emu/cm3, and is larger than that for (Al,Mn)N reported in the previous work [12]. The magnetic moment calculated from Ms is 0.25 Bohr magneton (mB)/Mn. More importantly, the (Al,Mn)N films have coercivities (Hc) varying from 40Oe to 50 Oe as Mn concentrations increase from 1.7% to 2.6%. This indicates that the (Al,Mn)N films do not exhibit a superparamagnetic behavior due to nano-clusters [9,14] as secondary phases, e.g., MnSb nano-clusters in (Ga,Mn)Sb, giving rise to no hysteresis [14]. However, if secondary phases being large enough to have large blocking temperature exist within (Al,Mn)N, it might be possible to exhibit the hysteretic feature from such secondary phases. Fig. 3 shows M–H curves of the (Al1xMnx)N films with (a) x ¼ 2:6% and (b) x ¼ 3:0% measured at 5 K using SQUID magnetometer. For the (Al1xMnx)N film with x ¼ 2:6%; Ms and Hc at 5 K were 6 emu/cm3 which corresponded to 0.52 mB/Mn and 150 Oe, respectively, and both at 5 K were larger than those at 300 K shown in Fig. 2. The (Al1xMnx)N film with x ¼ 3:0% exhibited M s ¼ 1:6 emu/cm3 which corresponded
ARTICLE IN PRESS M.-H. Ham et al. / Journal of Crystal Growth 271 (2004) 420–424
Fig. 3. Hysteresis loops for the (Al1xMnx)N films with (a) x ¼ 2:6% and (b) x ¼ 3:0% at 5 K measured by SQUID magnetometer with magnetic fields up to 5000 Oe applied parallel to the plane of the samples.
to 0.12 mB/Mn at 5 K while no hysteresis characteristics were observed at 300 K, indicating that the film has a lower TC than room temperature. It is attributed to increase of antiferromagnetic interactions between the nearest neighbor Mn ions as Mn incorporation increases, resulting in the lower magnetic moment per Mn. This magnetic behavior in the films indicates that a critical Mn concentration exists. It accords that the (Ga,Mn)N films with Mn ¼ 3% rather than higher Mn concentrations had the highest magnetic moment per Mn [9] and is in good agreement with the XRD results. The temperature dependence of magnetization for the (Al1xMnx)N film with x ¼ 2:6% under the field-cooled (FC) mode with a magnetic field of 100 Oe measured by SQUID magnetometer is shown in Fig. 4. The magnetization for the (Al,Mn)N film decreases slowly and continuously with increasing temperature. The ferromagnetic ordering for the (Al,Mn)N film was clearly observed in the temperature range 5–300 K, indicating that TC for the film exceeds at least room temperature. The electrical properties in the (Al,Mn)N films were determined by van der Pauw Hall measurements at room temperature. The Ohmic contact to the (Al,Mn)N films was made using indium. All the films exhibited highly resistive characteristics
423
Fig. 4. Temperature dependence of the magnetization for the (Al1xMnx)N film with x ¼ 2:6% under FC mode with a magnetic field of 100 Oe measured by SQUID magnetometer.
(60 MO), indicating that they had insulating properties. Therefore, the origin of ferromagnetism in the (Al,Mn)N films is not ascribed to carrier-mediated Ruderman–Kittel–Kasuya– Yosida (RKKY) interaction [4]. One possible explanation for this ferromagnetism is the formation of secondary phases such as manganese nitrides and Mn clusters. However, the evidence of such secondary phases in XRD measurements was not observed. Hence, the ferromagnetic ordering in our (Al,Mn)N is believed to be the intrinsic property. Another possibility is that the indirect exchange interaction in (Al,Mn)N films was caused by virtual electron excitations from Mn acceptor level to the valence band within the films [7]. However, it is still unclear and further studies are necessary to clarify the origin of ferromagnetism in the Mn-doped AlN.
4. Conclusion In summary, we have investigated the structural, electrical and magnetic properties in the (Al,Mn)N thin films with various Mn concentrations grown by PEMBE. The (Al1xMnx)N films with x ¼ 1:7% and 2.6% exhibited the ferromagnetic ordering with
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Curie temperature above room temperature while the film with x ¼ 3:0% showed the ferromagnetic ordering at 5 K, indicating that a critical Mn concentration exists. XRD analysis did not identify the presence of any secondary phases within the wurtzite (Al,Mn)N films. Since the (Al,Mn)N films show insulating characteristics, the origin of ferromagnetism in (Al,Mn)N is not likely to carriermediated RKKY interaction but might be due to indirect exchange interaction caused by virtual electron excitations from Mn acceptor level to the valence band within the samples.
Acknowledgements This work was supported by Samsung Advanced Institute of Technology (Grant No. 20032-0799). The authors thank S. J. Oh at Korea Basic Science Institute for his assistance with SQUID magnetometer. References [1] Y. Ohno, D.K. Young, B. Beschoten, F. Matsukura, H. Ohno, D.D. Awschalom, Nature 402 (1999) 790.
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