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Structural and magnetic study of hard–soft systems with ZnO barrier grown by pulsed laser deposition
ARTICLE IN PRESS Microelectronics Journal 40 (2009) 246–249
Contents lists available at ScienceDirect
Microelectronics Journal journal homepage: www.elsevier.com/locate/mejo
Structural and magnetic study of hard–soft systems with ZnO barrier grown by pulsed laser deposition H. Bieber, G. Versini, S. Barre, J.-L. Loison, G. Schmerber, C. Ulhaq-Bouillet, S. Colis, A. Dinia Institut de Physique et Chimie des Mate´riaux Strasbourg, IPCMS UMR 7504 du CNRS, ULP-ECPM, 23 rue du Lœss, B.P. 43, 67034 Strasbourg Cedex 2, France
a r t i c l e in f o
a b s t r a c t
Available online 16 September 2008
Hard–soft systems with magnetic transition metal electrodes and ZnO barrier of variable thickness have been epitaxially grown by pulsed laser deposition on MgO(1 0 0) substrates. The structural reflection high-energy electron diffraction (RHEED) and X-ray diffraction (XRD) analysis have shown an epitaxial growth of the CoFe2 bottom electrode, the permalloy top electrode and of the ZnO barrier. Magnetic measurements have shown a clear plateau with a separate reversal of both magnetizations of the top and bottom electrodes, which is promising for further tunnel magnetoresistance measurements. A ferromagnetic coupling between the magnetic electrodes through the barrier has been observed. & 2008 Elsevier Ltd. All rights reserved.
Keywords: ZnO tunnel barrier Pulsed laser deposition Thin film Coupling
1. Introduction Ferromagnetism in Co-doped zinc oxide has been intensively studied since the prediction of Dietl et al. [1], foresawing roomtemperature ferromagnetism in transition metal-doped semiconductors. However, since this prediction, the scientific world has not been unanimous on the intrinsic origin of the—when really observed—ferromagnetism. Indeed some groups report roomtemperature ferromagnetism in ZnCoO thin films [2,3], whereas the extrinsic origin like Co-clustering is claimed by others [4–7]. Other magnetic semiconductors such as GaMnAs are far more better understood but their Curie temperature is low (170 K) [8]. As the origin of ferromagnetism in ZnCoO and thus the insertion of a ferromagnetic ZnCoO electrode in a multilayer system are still open ended, only few groups have focused on the use of ZnO as a barrier for such hard–soft architectures [9]. Fully epitaxial (Zn,Co)O/ZnO/(Zn,Co)O junctions showed already a magnetoresistance effect, which is attributed by the authors to a tunnelling phenomenon between two ferromagnetic electrodes [10,11], although single ZnCoO layers have shown similar magnetoresistance [12]. This result has nevertheless to be confirmed since the origin of the magnetism in the ZnCoO system is still not well understood. Le Brizoual et al. [13] observed a TMR signal of 8% at 77 K through a ZnO barrier between two Co electrodes deposited by dc-sputtering. The main problem is the oxidation of the bottom electrode, due to the use of reactive sputtering for the growing of the barrier from a metallic Zn target. To avoid this phenomenon, pulsed laser deposition (PLD) can offer an alternative since it has
Corresponding author.
E-mail address: [email protected] (A. Dinia). 0026-2692/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2008.07.044
already been successfully used to obtain a magnetic hard–soft architecture. Our aim is to study ZnO as an insulating barrier in a hard–soft system with magnetic transition metal electrodes. The system was grown by pulsed laser deposition to deposit the trilayers under vacuum, and thus ZnO from a stoichiometric target in order to avoid the oxidation of the bottom electrode.
2. Experimental procedure The hard–soft systems have been deposited on MgO (1 0 0) substrates. The multilayers are composed of CoFe2(7 nm)/ZnO(X nm) Ni80Fe20(20 nm)/Ta(5 nm) with X ¼ 1, 2, 3, 4 or 5 nm. All the layers are grown by PLD from pure stoichiometric targets at a pressure around 2.107 mbar. The fluence of the laser has been kept at 1 J cm2, the repetition rate at 10 Hz and the substrate– target distance at 5 cm. Prior to deposition, the substrates were heated for 2 h at 500 1C. Afterwards, the CoFe2 and ZnO layers have been deposited at 500 and 250 1C, respectively, to obtain a good crystallinity and a limited roughness. The top NiFe electrode is grown at room temperature to avoid any interdiffusion with the barrier. The average roughness determined by atomic force microscope (AFM) of 0,23 nm has been reached on the system MgO (0 0 1)/CoFe2(7 nm)/ZnO(3 nm). The growth and the crystallinity have been checked by X-ray diffraction (XRD) measurements on a Siemens D500 diffractometer in y/2y configuration with a cobalt source lKa1 ¼ 1,78901 A˚ and by in-situ reflection high-energy electron diffraction (RHEED). In order to have further insight on the structure of the barrier and the magnetic/non-magnetic interfaces, high-resolution
ARTICLE IN PRESS H. Bieber et al. / Microelectronics Journal 40 (2009) 246–249
transmission electron microscopy (HRTEM) analysis has been performed with a TOPCON EM002B microscope. Temperaturedependent magnetic measurements have been performed using a superconducting quantum interference device (SQUID) magnetometer.
3. Results and discussion The growth mechanism has been first in-situ analysed by RHEED. Fig. 1(a) shows RHEED pattern obtained on top of the 7-nm-thick CoFe2 layer. The observation of fine streaks is the evidence of an epitaxial two-dimensional (2D) growth. Such a 2D growth is also observed for the 4-nm-thick ZnO barrier deposited on the top of the CoFe2 layer as evidenced by the pattern reported in Fig. 1(b). However, in this case the streaks are more spotty, which indicates the presence of small roughness in agreement with AFM measurements. The RHEED pattern in Fig. 2(d) after the deposition of the permalloy (Py) layer shows also spotty streaks, which stands for an epitaxial growth of the Py top electrode with a small roughness. Fig. 2(a) shows the XRD pattern of a CoFe2(7 nm)/ZnO(15 nm) multilayer grown on MgO(1 0 0), which gives a hint towards the epitaxial growth of both layers. This is evidenced by the presence of the 2 peaks at 2y ¼ 36,927 and 77,3701 attributed, respectively, to the (1 0 0) and (2 0 0) CoFe2 planes and 2 others at 2y ¼ 39,932 and 86,1061 corresponding to (0 0 0 2) and (0 0 0 4) ZnO planes. The peaks of the substrate have been cut to avoid the saturation of the X-ray detector. The ZnO layer has intentionally been grown thicker in order to make possible the analysis by XRD. Fig. 2(b) shows the diffraction pattern of a complete hard–soft structure and the presence of the (111) peak of Py at 2y ¼ 51,5101, which suggests an epitaxial growth of the Py layer as well. The epitaxy is
a
b
247
rather surprising as Py was deposited at room temperature, but can be understood considering the high energy of the ablated particles arriving on the substrate. Such PLD room-temperature epitaxy has already been reported by Fix et al. [14]. To have more details on the crystalline structure and the morphology of interfaces, cross-sectional and low- and highresolution transmission electron microscopy (TEM) images on the hard–soft structure with the barrier thickness of 3 nm have been made. The low-magnification image in bright field mode in Fig. 3(a) shows that the ZnO barrier (dark contrast) is continuous over the whole surface and presents rather flat interfaces. These observations are promising for tunnelling magnetoresistance. The HRTEM image given in Fig. 3(b) confirms the epitaxial growth of the ZnO barrier with the [0 0 1] direction perpendicular to the film plane. The sharp contrast between CoFe2 and the ZnO layers indicates the absence of an oxidized CoFe2 layer at the CoFe2/ZnO interface. The magnetization major loops at room and low temperature have been measured by SQUID. All samples with barrier thickness of 2 nm or above show magnetization loops similar to the one reported in Fig. 4 i.e. two separated magnetization switching. In contrast, when the ZnO barrier is only 1 nm thick a single magnetization reversal is observed. This suggests the continuity of the barrier in the samples with thicknesses 2, 3, 4 and 5 nm in agreement with TEM observations. The sharp reversal of the soft Py layer appears at 10 Oe and the second one of the hard CoFe2 layer begins progressively at 200 Oe leading to a plateau of more than 100 Oe. This plateau is also visible in Fig. 5, which shows the minor loop. This is a first indication of the possibility of having a magnetoresistance. Furthermore, Fig. 5 shows the minor loop at 295 K for the sample with a barrier thickness of 4 nm. The shift of the minor loop towards the negative fields indicates a negative exchange
c
d
Fig. 1. RHEED patterns along the [0 0 1]MgO direction (a) of the MgO substrate, after the deposition of (b) a 7-nm-thick CoFe2 layer and (c) a 4-nm-thick ZnO layer and (d) a 20-nm-thick Py layer.
Fig. 2. X-ray diffraction pattern of a (a) MgO(1 0 0)/CoFe2(7 nm)/ZnO(15 nm) multilayer and of a (b) MgO(1 0 0)/CoFe2(7 nm)/ZnO(3 nm)/Py(20 nm)/Ta(5 nm).
ARTICLE IN PRESS 248
H. Bieber et al. / Microelectronics Journal 40 (2009) 246–249
200 10K 100K
NiFe CoFe2 MgO
M (µemu)
ZnO 50 nm
100
295K
0
NiFe -100
-50
0 H (Oe)
50
100
Fig. 5. Magnetization minor loops of the multilayer with barrier thickness of 4 nm measured at 10, 100 and 295 K by SQUID.
ZnO peel effect (i), but the present results cannot definitely exclude the other types of interaction.
4. Conclusion
CoFe2 3 nm
Fig. 3. (a) Cross-sectional low-magnification TEM image showing the continuous character of the layers and their homogenous thickness (negative contrast), (b) cross-sectional HRTEM image of the multilayer with the ZnO barrier thickness of 3 nm.
200
Acknowledgement
295K
H.B. wishes to thank the Region Alsace for its financial support.
100 M (µemu)
Hard–soft magnetic system integrating an epitaxial ZnO barrier has been deposited by PLD. TEM indicated no oxidation of the magnetic electrodes adjacent to the barrier. Separate reversal of the magnetization of the soft and hard layers has been observed. Transport measurements through the ZnO barrier will be carried out to evaluate the polarisation of the magnetic/non-magnetic interfaces and the quality of our barrier.
References
0
-100
-200 -2.0
-1.5
-1.0
-0.5
0.0 0.5 H (kOe)
1.0
1.5
2.0
Fig. 4. Magnetization major loop of the multilayer with barrier thickness of 4 nm measured at room temperature by SQUID.
bias field (Hex), and thus a ferromagnetic coupling between the Py and the CoFe2 layers through the ZnO barrier. The values of Hex goes from 56.5 Oe for the 2-nm-thick ZnO layer to 22.0 Oe for 5-nm-ZnO barrier. Hex increases with the decrease of the barrier thickness. As the TEM image and major loop SQUID measurements have shown the continuity of the barrier and the presence of a distinct reversal of the magnetization of both electrodes, respectively, the direct coupling induced by pin-holes can be excluded. Different indirect exchange coupling can be taken into account like (i) orange-peel coupling [15] (ii) quantum interferences due to confinement in the oxide layer [16] and (iii) domainwall dipole coupling [17,18]. Hex is temperature independent as shown in Fig. 4, where the minor loops recorded at 10, 100 and 295 K are superimposed. This observation goes towards an orange
[1] T. Dietl, H. Ohno, F. Matsukara, J. Cibert, D. Ferrand, A. Zener, Model description of ferromagnetism in zinc-blende. Magnetic semiconductors, Science 287 (2000) 1019–1022. [2] K. Ueda, H. Tabata, T. Kawai, Magnetic and electric properties of transitionmetal-doped ZnO films, Appl. Phys. Lett. 79 (2001) 988–990. [3] A. Dinia, G. Schmerber, C. Me´ny, V. Pierron-Bohnes, E. Beaurepaire, Roomtemperature ferromagnetism in Zn1xCoxO magnetix semiconductors prepared by sputtering, J. Appl. Phys. 97 (2005) 123908. [4] J.H. Park, M.G. Kim, H.M. Jang, S. Ryu, Co-metal clustering as the origin of ferromagnetism in Co-doped ZnO thin films, Appl. Phys. Lett. 84 (2004) 1338–1340. [5] J. Antony, S. Pendyala, A. Sharma, X.B. Chen, J. Morrison, L. Bergman, et al., Room temperature ferromagnetism and ultraviolet optical properties of Co-doped ZnO nanocluster films, J. Appl. Phys. 97 (2005) 10D307. [6] J.H. Kim, H. Kim, D. Kim, Y.E. Ihm, W.K. Choo, Magnetic properties of epitaxially grown semiconducting Zn1xCoxO thin films by pulsed laser deposition, J. Appl. Phys. 92 (2002) 6066. [7] A. Barla, G. Schmerber, E. Beaurepaire, A. Dinia, H. Bieber, S. Colis, et al., Paramagnetism of the Co sublattice in ferromagnetic Zn1xCoxO films, Phys. Rev. B 76 (2007) 125201. [8] C. Gould, K. Pappert, G. Schmidt, L.W. Molenkamp, Magnetic anisotropies and (GaMn)As-based spintronic devices, Adv. Mater. 19 (2007) 323–340. [9] L. Le Brizoual, P. Alnot, M. Hehn, F. Montaigne, M. Alnot, A. Schuhl, et al., Magnetic tunnel junctions with a zinc oxide–cobalt oxide composite tunnel barrier, Appl. Phys. Lett. 86 (2005) 112505–112507. [10] C. Song, X.J. Liu, F. Pan, Fully epitaxial (ZnCo)O/ZnO/(Zn,Co)O junction and its tunnel magnetoresistance, Appl. Phys. Lett. 91 (2007) 042106. [11] S. Ramachandran, J.T. Prater, N. Sudhakar, D. Kumar, J. Narayan, Magnetic properties of epitaxial oxide heterostructures, Solid State Commun. 145 (2008) 18. [12] P. Stamenov, M. Venkatesan, L.S. Dorneles, D. Maude, J.M.D. Coey, Magnetoresistance of Co-doped ZnO thin films, J. Appl. Phys. 99 (2006) 08M124. [13] T. Fix, M. Trassin, R. Sayed Hassan, G. Schmerber, N. Viart, C. Me´ny, et al., How to obtain a magnetic hard–soft architecture by pulsed laser deposition, Nanotechnology 18 (2007) 495708.
ARTICLE IN PRESS H. Bieber et al. / Microelectronics Journal 40 (2009) 246–249
[14] T. Fix, C. Ulhaq-Bouillet, S. Colis, A. Dinia, Bertoni, J. Verbeeck, et al., Nanoscale analysis of interfaces in a metal/oxide/oxide trilayer obtained by pulsed laser deposition, Appl. Phys. Lett. 91 (2007) 023106. [15] L. Ne´el, Sur un nouveau mode de couplage entre les aimantations de deux couches minces ferromagne´tiques, C. R. 255 (1962) 1676–1681. [16] P. Bruno, Theory of interlayer magnetic coupling, Phys. Rev. B 52 (1995) 411–439.
249
[17] C.L. Platt, M.R. McCartney, F.T. Parker, A.E. Berkowitz, Magnetic interlayer coupling in ferromagnet/insulator/ferromagnet structures, Phys. Rev. B 61 (2000) 9633–9641. [18] S. Colis, A. Dinia, Domain wall duplication in a hard–soft spin-valve structure using the CoFe/Ir/CoFe artificial antiferromagnetic subsystem, Phys. Rev. B 66 (2002) 174425.
Contents lists available at ScienceDirect
Microelectronics Journal journal homepage: www.elsevier.com/locate/mejo
Structural and magnetic study of hard–soft systems with ZnO barrier grown by pulsed laser deposition H. Bieber, G. Versini, S. Barre, J.-L. Loison, G. Schmerber, C. Ulhaq-Bouillet, S. Colis, A. Dinia Institut de Physique et Chimie des Mate´riaux Strasbourg, IPCMS UMR 7504 du CNRS, ULP-ECPM, 23 rue du Lœss, B.P. 43, 67034 Strasbourg Cedex 2, France
a r t i c l e in f o
a b s t r a c t
Available online 16 September 2008
Hard–soft systems with magnetic transition metal electrodes and ZnO barrier of variable thickness have been epitaxially grown by pulsed laser deposition on MgO(1 0 0) substrates. The structural reflection high-energy electron diffraction (RHEED) and X-ray diffraction (XRD) analysis have shown an epitaxial growth of the CoFe2 bottom electrode, the permalloy top electrode and of the ZnO barrier. Magnetic measurements have shown a clear plateau with a separate reversal of both magnetizations of the top and bottom electrodes, which is promising for further tunnel magnetoresistance measurements. A ferromagnetic coupling between the magnetic electrodes through the barrier has been observed. & 2008 Elsevier Ltd. All rights reserved.
Keywords: ZnO tunnel barrier Pulsed laser deposition Thin film Coupling
1. Introduction Ferromagnetism in Co-doped zinc oxide has been intensively studied since the prediction of Dietl et al. [1], foresawing roomtemperature ferromagnetism in transition metal-doped semiconductors. However, since this prediction, the scientific world has not been unanimous on the intrinsic origin of the—when really observed—ferromagnetism. Indeed some groups report roomtemperature ferromagnetism in ZnCoO thin films [2,3], whereas the extrinsic origin like Co-clustering is claimed by others [4–7]. Other magnetic semiconductors such as GaMnAs are far more better understood but their Curie temperature is low (170 K) [8]. As the origin of ferromagnetism in ZnCoO and thus the insertion of a ferromagnetic ZnCoO electrode in a multilayer system are still open ended, only few groups have focused on the use of ZnO as a barrier for such hard–soft architectures [9]. Fully epitaxial (Zn,Co)O/ZnO/(Zn,Co)O junctions showed already a magnetoresistance effect, which is attributed by the authors to a tunnelling phenomenon between two ferromagnetic electrodes [10,11], although single ZnCoO layers have shown similar magnetoresistance [12]. This result has nevertheless to be confirmed since the origin of the magnetism in the ZnCoO system is still not well understood. Le Brizoual et al. [13] observed a TMR signal of 8% at 77 K through a ZnO barrier between two Co electrodes deposited by dc-sputtering. The main problem is the oxidation of the bottom electrode, due to the use of reactive sputtering for the growing of the barrier from a metallic Zn target. To avoid this phenomenon, pulsed laser deposition (PLD) can offer an alternative since it has
Corresponding author.
E-mail address: [email protected] (A. Dinia). 0026-2692/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2008.07.044
already been successfully used to obtain a magnetic hard–soft architecture. Our aim is to study ZnO as an insulating barrier in a hard–soft system with magnetic transition metal electrodes. The system was grown by pulsed laser deposition to deposit the trilayers under vacuum, and thus ZnO from a stoichiometric target in order to avoid the oxidation of the bottom electrode.
2. Experimental procedure The hard–soft systems have been deposited on MgO (1 0 0) substrates. The multilayers are composed of CoFe2(7 nm)/ZnO(X nm) Ni80Fe20(20 nm)/Ta(5 nm) with X ¼ 1, 2, 3, 4 or 5 nm. All the layers are grown by PLD from pure stoichiometric targets at a pressure around 2.107 mbar. The fluence of the laser has been kept at 1 J cm2, the repetition rate at 10 Hz and the substrate– target distance at 5 cm. Prior to deposition, the substrates were heated for 2 h at 500 1C. Afterwards, the CoFe2 and ZnO layers have been deposited at 500 and 250 1C, respectively, to obtain a good crystallinity and a limited roughness. The top NiFe electrode is grown at room temperature to avoid any interdiffusion with the barrier. The average roughness determined by atomic force microscope (AFM) of 0,23 nm has been reached on the system MgO (0 0 1)/CoFe2(7 nm)/ZnO(3 nm). The growth and the crystallinity have been checked by X-ray diffraction (XRD) measurements on a Siemens D500 diffractometer in y/2y configuration with a cobalt source lKa1 ¼ 1,78901 A˚ and by in-situ reflection high-energy electron diffraction (RHEED). In order to have further insight on the structure of the barrier and the magnetic/non-magnetic interfaces, high-resolution
ARTICLE IN PRESS H. Bieber et al. / Microelectronics Journal 40 (2009) 246–249
transmission electron microscopy (HRTEM) analysis has been performed with a TOPCON EM002B microscope. Temperaturedependent magnetic measurements have been performed using a superconducting quantum interference device (SQUID) magnetometer.
3. Results and discussion The growth mechanism has been first in-situ analysed by RHEED. Fig. 1(a) shows RHEED pattern obtained on top of the 7-nm-thick CoFe2 layer. The observation of fine streaks is the evidence of an epitaxial two-dimensional (2D) growth. Such a 2D growth is also observed for the 4-nm-thick ZnO barrier deposited on the top of the CoFe2 layer as evidenced by the pattern reported in Fig. 1(b). However, in this case the streaks are more spotty, which indicates the presence of small roughness in agreement with AFM measurements. The RHEED pattern in Fig. 2(d) after the deposition of the permalloy (Py) layer shows also spotty streaks, which stands for an epitaxial growth of the Py top electrode with a small roughness. Fig. 2(a) shows the XRD pattern of a CoFe2(7 nm)/ZnO(15 nm) multilayer grown on MgO(1 0 0), which gives a hint towards the epitaxial growth of both layers. This is evidenced by the presence of the 2 peaks at 2y ¼ 36,927 and 77,3701 attributed, respectively, to the (1 0 0) and (2 0 0) CoFe2 planes and 2 others at 2y ¼ 39,932 and 86,1061 corresponding to (0 0 0 2) and (0 0 0 4) ZnO planes. The peaks of the substrate have been cut to avoid the saturation of the X-ray detector. The ZnO layer has intentionally been grown thicker in order to make possible the analysis by XRD. Fig. 2(b) shows the diffraction pattern of a complete hard–soft structure and the presence of the (111) peak of Py at 2y ¼ 51,5101, which suggests an epitaxial growth of the Py layer as well. The epitaxy is
a
b
247
rather surprising as Py was deposited at room temperature, but can be understood considering the high energy of the ablated particles arriving on the substrate. Such PLD room-temperature epitaxy has already been reported by Fix et al. [14]. To have more details on the crystalline structure and the morphology of interfaces, cross-sectional and low- and highresolution transmission electron microscopy (TEM) images on the hard–soft structure with the barrier thickness of 3 nm have been made. The low-magnification image in bright field mode in Fig. 3(a) shows that the ZnO barrier (dark contrast) is continuous over the whole surface and presents rather flat interfaces. These observations are promising for tunnelling magnetoresistance. The HRTEM image given in Fig. 3(b) confirms the epitaxial growth of the ZnO barrier with the [0 0 1] direction perpendicular to the film plane. The sharp contrast between CoFe2 and the ZnO layers indicates the absence of an oxidized CoFe2 layer at the CoFe2/ZnO interface. The magnetization major loops at room and low temperature have been measured by SQUID. All samples with barrier thickness of 2 nm or above show magnetization loops similar to the one reported in Fig. 4 i.e. two separated magnetization switching. In contrast, when the ZnO barrier is only 1 nm thick a single magnetization reversal is observed. This suggests the continuity of the barrier in the samples with thicknesses 2, 3, 4 and 5 nm in agreement with TEM observations. The sharp reversal of the soft Py layer appears at 10 Oe and the second one of the hard CoFe2 layer begins progressively at 200 Oe leading to a plateau of more than 100 Oe. This plateau is also visible in Fig. 5, which shows the minor loop. This is a first indication of the possibility of having a magnetoresistance. Furthermore, Fig. 5 shows the minor loop at 295 K for the sample with a barrier thickness of 4 nm. The shift of the minor loop towards the negative fields indicates a negative exchange
c
d
Fig. 1. RHEED patterns along the [0 0 1]MgO direction (a) of the MgO substrate, after the deposition of (b) a 7-nm-thick CoFe2 layer and (c) a 4-nm-thick ZnO layer and (d) a 20-nm-thick Py layer.
Fig. 2. X-ray diffraction pattern of a (a) MgO(1 0 0)/CoFe2(7 nm)/ZnO(15 nm) multilayer and of a (b) MgO(1 0 0)/CoFe2(7 nm)/ZnO(3 nm)/Py(20 nm)/Ta(5 nm).
ARTICLE IN PRESS 248
H. Bieber et al. / Microelectronics Journal 40 (2009) 246–249
200 10K 100K
NiFe CoFe2 MgO
M (µemu)
ZnO 50 nm
100
295K
0
NiFe -100
-50
0 H (Oe)
50
100
Fig. 5. Magnetization minor loops of the multilayer with barrier thickness of 4 nm measured at 10, 100 and 295 K by SQUID.
ZnO peel effect (i), but the present results cannot definitely exclude the other types of interaction.
4. Conclusion
CoFe2 3 nm
Fig. 3. (a) Cross-sectional low-magnification TEM image showing the continuous character of the layers and their homogenous thickness (negative contrast), (b) cross-sectional HRTEM image of the multilayer with the ZnO barrier thickness of 3 nm.
200
Acknowledgement
295K
H.B. wishes to thank the Region Alsace for its financial support.
100 M (µemu)
Hard–soft magnetic system integrating an epitaxial ZnO barrier has been deposited by PLD. TEM indicated no oxidation of the magnetic electrodes adjacent to the barrier. Separate reversal of the magnetization of the soft and hard layers has been observed. Transport measurements through the ZnO barrier will be carried out to evaluate the polarisation of the magnetic/non-magnetic interfaces and the quality of our barrier.
References
0
-100
-200 -2.0
-1.5
-1.0
-0.5
0.0 0.5 H (kOe)
1.0
1.5
2.0
Fig. 4. Magnetization major loop of the multilayer with barrier thickness of 4 nm measured at room temperature by SQUID.
bias field (Hex), and thus a ferromagnetic coupling between the Py and the CoFe2 layers through the ZnO barrier. The values of Hex goes from 56.5 Oe for the 2-nm-thick ZnO layer to 22.0 Oe for 5-nm-ZnO barrier. Hex increases with the decrease of the barrier thickness. As the TEM image and major loop SQUID measurements have shown the continuity of the barrier and the presence of a distinct reversal of the magnetization of both electrodes, respectively, the direct coupling induced by pin-holes can be excluded. Different indirect exchange coupling can be taken into account like (i) orange-peel coupling [15] (ii) quantum interferences due to confinement in the oxide layer [16] and (iii) domainwall dipole coupling [17,18]. Hex is temperature independent as shown in Fig. 4, where the minor loops recorded at 10, 100 and 295 K are superimposed. This observation goes towards an orange
[1] T. Dietl, H. Ohno, F. Matsukara, J. Cibert, D. Ferrand, A. Zener, Model description of ferromagnetism in zinc-blende. Magnetic semiconductors, Science 287 (2000) 1019–1022. [2] K. Ueda, H. Tabata, T. Kawai, Magnetic and electric properties of transitionmetal-doped ZnO films, Appl. Phys. Lett. 79 (2001) 988–990. [3] A. Dinia, G. Schmerber, C. Me´ny, V. Pierron-Bohnes, E. Beaurepaire, Roomtemperature ferromagnetism in Zn1xCoxO magnetix semiconductors prepared by sputtering, J. Appl. Phys. 97 (2005) 123908. [4] J.H. Park, M.G. Kim, H.M. Jang, S. Ryu, Co-metal clustering as the origin of ferromagnetism in Co-doped ZnO thin films, Appl. Phys. Lett. 84 (2004) 1338–1340. [5] J. Antony, S. Pendyala, A. Sharma, X.B. Chen, J. Morrison, L. Bergman, et al., Room temperature ferromagnetism and ultraviolet optical properties of Co-doped ZnO nanocluster films, J. Appl. Phys. 97 (2005) 10D307. [6] J.H. Kim, H. Kim, D. Kim, Y.E. Ihm, W.K. Choo, Magnetic properties of epitaxially grown semiconducting Zn1xCoxO thin films by pulsed laser deposition, J. Appl. Phys. 92 (2002) 6066. [7] A. Barla, G. Schmerber, E. Beaurepaire, A. Dinia, H. Bieber, S. Colis, et al., Paramagnetism of the Co sublattice in ferromagnetic Zn1xCoxO films, Phys. Rev. B 76 (2007) 125201. [8] C. Gould, K. Pappert, G. Schmidt, L.W. Molenkamp, Magnetic anisotropies and (GaMn)As-based spintronic devices, Adv. Mater. 19 (2007) 323–340. [9] L. Le Brizoual, P. Alnot, M. Hehn, F. Montaigne, M. Alnot, A. Schuhl, et al., Magnetic tunnel junctions with a zinc oxide–cobalt oxide composite tunnel barrier, Appl. Phys. Lett. 86 (2005) 112505–112507. [10] C. Song, X.J. Liu, F. Pan, Fully epitaxial (ZnCo)O/ZnO/(Zn,Co)O junction and its tunnel magnetoresistance, Appl. Phys. Lett. 91 (2007) 042106. [11] S. Ramachandran, J.T. Prater, N. Sudhakar, D. Kumar, J. Narayan, Magnetic properties of epitaxial oxide heterostructures, Solid State Commun. 145 (2008) 18. [12] P. Stamenov, M. Venkatesan, L.S. Dorneles, D. Maude, J.M.D. Coey, Magnetoresistance of Co-doped ZnO thin films, J. Appl. Phys. 99 (2006) 08M124. [13] T. Fix, M. Trassin, R. Sayed Hassan, G. Schmerber, N. Viart, C. Me´ny, et al., How to obtain a magnetic hard–soft architecture by pulsed laser deposition, Nanotechnology 18 (2007) 495708.
ARTICLE IN PRESS H. Bieber et al. / Microelectronics Journal 40 (2009) 246–249
[14] T. Fix, C. Ulhaq-Bouillet, S. Colis, A. Dinia, Bertoni, J. Verbeeck, et al., Nanoscale analysis of interfaces in a metal/oxide/oxide trilayer obtained by pulsed laser deposition, Appl. Phys. Lett. 91 (2007) 023106. [15] L. Ne´el, Sur un nouveau mode de couplage entre les aimantations de deux couches minces ferromagne´tiques, C. R. 255 (1962) 1676–1681. [16] P. Bruno, Theory of interlayer magnetic coupling, Phys. Rev. B 52 (1995) 411–439.
249
[17] C.L. Platt, M.R. McCartney, F.T. Parker, A.E. Berkowitz, Magnetic interlayer coupling in ferromagnet/insulator/ferromagnet structures, Phys. Rev. B 61 (2000) 9633–9641. [18] S. Colis, A. Dinia, Domain wall duplication in a hard–soft spin-valve structure using the CoFe/Ir/CoFe artificial antiferromagnetic subsystem, Phys. Rev. B 66 (2002) 174425.