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antiferromagnetic bilayers
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 290–291 (2005) 1290–1293 www.elsevier.com/locate/jmmm
Perpendicular exchange bias in nickel/antiferromagnetic bilayers Sebastiaan van Dijken, Matthew Crofton, J.M.D. Coey SFI Trinity Nanoscience Laboratory, Department of Physics, Trinity College, Dublin 2, Ireland Available online 14 December 2004
Abstract A study of exchange-biased Ni films with perpendicular magnetic anisotropy is presented. Polar magneto-optic Kerr effect measurements on unbiased Si(0 0 1)/Cu/Ni/Cu films reveal high out-of-plane remanence and a loop squareness close to unity. After deposition of an antiferromagnetic layer on top of the Ni film the magnetization direction remains perpendicular to the film plane and for FeMn, IrMn, and NiMn a shift in the hysteresis loop is observed. The perpendicular exchange bias is largest for FeMn, considerably smaller for IrMn and NiMn, and zero for PtMn. All antiferromagnetic films exhibit a (0 0 1) oriented texture. r 2004 Elsevier B.V. All rights reserved. PACS: 75.50.Ee; 75.70.Cn; 75.70.I; 75.30.Gw Keywords: Exchange bias; Perpendicular anisotropy; Ferromagnetism; Antiferromagnetism
1. Introduction Exchange bias phenomena in ferromagnetic (FM)/ antiferromagnetic (AFM) bilayers have been extensively studied in the last decades [1]. In most of these bilayers the magnetization of the FM film is oriented in the film plane. More recently, several studies on exchange bias effects in systems with out-of-plane magnetization have been published. Among them are Co/Pt multilayers with CoO [2,3] and CoFe/Pt, Co/Pt, and Co/Pd multilayers with FeMn [4–6]. The study of perpendicular exchange bias is relevant in the quest for a better understanding of the microscopic origin of the exchange bias phenomenon and it might lead to applications in magnetic sensors and perpendicular magnetic media. In this paper we report Corresponding author. Tel.: +353 1 6083034; +353 1 6083037. E-mail address: [email protected] (S. van Dijken).
fax:
on perpendicular exchange bias effects in Ni/AFM bilayers, where the AFM is FeMn, IrMn, NiMn, or PtMn.
2. Experiment and results The films were grown by dc magnetron sputtering on HF-etched Si(0 0 1) substrates in a Shamrock deposition tool. The magnetization reversal was studied by magneto-optic Kerr effect (MOKE) and SQUID measurements. The MOKE set-up was operated at 50 Hz and the maximum applied magnetic field was limited to about 170 mT. X-ray diffraction (XRD) was used to characterize the film structure. The thickness of the AFM layers was chosen to be larger than the critical thickness, i.e., the thickness above which exchange bias has been obtained in systems with in-plane magnetization [7].
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.11.452
ARTICLE IN PRESS S. van Dijken et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 1290–1293
t=5
1.0
t = 10
0.5
0.0
0.0
Normalized Polar MOKE
0.5
-0.5 -1.0 1.0
t = 20
t = 50
0.5 0.0
1.0
-100
0 100 µ0H (mT)
Fig. 1. Normalized polar MOKE hysteresis loops for Si(0 0 1)/ t nm Cu/4 nm Ni/5 nm Cu films.
Fig. 1 shows polar MOKE hysteresis loops for Si(0 0 1)/t nm Cu/4 nm Ni/5 nm Cu films. The Cu seed layer grows with a (0 0 1) texture on the Si(0 0 1) substrate. Subsequent deposition of Ni on this film leads to a tetragonal distortion of the Ni lattice. This lattice distortion results in a magnetoelastic contribution to the magnetic anisotropy energy of the system favoring an out-of-plane film magnetization [8–10]. For tCu ¼ 50 nm and a Ni film thickness of 4 nm, the magnetoelastic anisotropy energy is larger than the sum of the magnetostatic and interface anisotropy energies (both favoring in-plane magnetization). The result is an out-of-plane film magnetization with full remanence. The deposition of an AFM layer on top of the Ni film does not alter the easy magnetization axis and for FeMn, IrMn, and NiMn it leads to a shift in the polar MOKE hysteresis loop (see Fig. 2). The perpendicular exchange bias field is largest for FeMn. For this film an applied magnetic field of 170 mT is not enough to saturate the magnetization. Fig. 3 shows SQUID magnetization curves for Ni/FeMn and Ni/IrMn bilayers. The exchange bias field for these films is 30 and 5 mT, respectively. The results for the coercive and exchange bias fields are summarized in Table 1. This table also shows the interfacial exchange energy (s) which is calculated from the SQUID data using s ¼ m0 H eb M s tNi ; where Ms is the saturation magnetization of Ni and tNi is the Ni layer thickness. The interfacial exchange energy is large for FeMn, relatively small for IrMn and NiMn, and zero for PtMn. The differences between the MOKE and SQUID data in Table 1 are attributed to dynamic magnetic switching effects. The
PtMn
0.0
-1.0 -100
NiMn
0.5
-1.0 0 100 µ0H (mT)
IrMn
-1.0
-0.5
-100
FeMn
-0.5
-0.5
0 100 µ0H (mT)
-100
0 100 µ0H (mT)
Fig. 2. Normalized polar MOKE hysteresis loops for Si(0 0 1)/ 50 nm Cu/4 nm Ni/t nm AFM/5 nm Cu films. The thickness t of the AFM layers is 10, 10, 25, and 15 nm for FeMn, IrMn, NiMn, and PtMn, respectively.
1.0
IrMn
FeMn
0.5 M/Ms
Normalized Polar MOKE
1.0
1291
0.0 -0.5 -1.0 -50
0 µ0H (mT)
50
-50
0 µ0H (mT)
50
Fig. 3. SQUID magnetization curves for Si(0 0 1)/50 nm Cu/ 4 nm Ni/10 nm FeMn/5 nm Cu and Si(0 0 1)/50 nm Cu/4 nm Ni/ 10 nm IrMn/5 nm Cu.
field sweep rate in the MOKE set-up is about 8.5 103 mT/s, which is about four orders of magnitude larger than that used in the SQUID measurements. A more detailed discussion on dynamic effects in perpendicular exchange-biased systems will be published elsewhere [11]. Exchange-bias effects in FM/AFM bilayer systems strongly depend on crystallinity, grain size, and interface roughness [1]. Fig. 4 shows XRD scans for Si(0 0 1)/ 50 nm Cu/4 nm Ni/AFM/5 nm Cu films. Besides the dominant (0 0 2) Cu reflection, weaker fcc (0 0 2) reflections are also visible for IrMn, NiMn, and PtMn.
ARTICLE IN PRESS S. van Dijken et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 1290–1293
1292
Table 1 Coercive and exchange bias fields measured on Si(0 0 1)/50 nm Cu/4 nm Ni/AFM/5 nm Cu films
No AFM FeMn IrMn NiMn PtMn
m0Hc MOKE
m0Heb MOKE
m0Hc SQUID
m0Heb SQUID
s (mJ/m2)
60 mT 20 mT 57 mT 70 mT 60 mT
0 mT 110 mT 16 mT 10 mT 0 mT
24 mT 3 mT 15 mT 24 mT 24 mT
0 mT 30 mT 5 mT 3 mT 0 mT
0 0.059 0.010 0.006 0
The last column displays the interfacial exchange energy (calculated from the SQUID data).
600
hand, grain boundaries and interface defects will pin uncompensated spins at the AFM interface. Since these spins are responsible for the exchange bias effect, the dissimilar biasing results are more likely due to a different AFM grain size or FM/AFM interface roughness in the Ni/FeMn and Ni/IrMn bilayers. For FM/AFM bilayers with NiMn and PtMn it is normally necessary to anneal the films in order to obtain exchange bias. In the as-deposited state the films have a nonmagnetic fcc structure which upon annealing changes into the AFM L10 structure. The very low (NiMn) and zero (PtMn) perpendicular exchange bias measured on the Ni/AFM bilayer is due to the fcc film structure. An attempt to anneal the Si(0 0 1)/50 nm Cu/ 4 nm Ni/AFM/5 nm Cu films at 200 1C resulted in severe mixing between the Cu and Ni layers which destroyed the perpendicular magnetic anisotropy.
Cu (002)
no AFM
300 0 600
FeMn
Intensity (cps)
300 0 600
IrMn
IrMn (002)
300 0 600
NiMn
300 0 600 300
NiMn (002) PtMn PtMn (002)
0 46
48
50
52
54
Fig. 4. XRD scans for Si(0 0 1)/50 nm Cu/4 nm Ni/t nm AFM/ 5 nm Cu films. The thickness t of the AFM layers is 10, 10, 25, and 15 nm for FeMn, IrMn, NiMn, and PtMn, respectively.
The absence of any FeMn reflections indirectly indicates the growth of the fcc g phase, for which the vertical interlayer distance is very similar to that of Cu(0 0 1) [12]. Exchange bias effects in bilayers with a (0 0 1) texture have not been studied extensively. However, for both FeMn and IrMn the exchange bias of (0 0 1) oriented bilayers with in-plane magnetization is reported to be a factor 1–2 smaller than that of (1 1 1) oriented films [13,14]. We have measured similar exchange bias fields for FeMn and IrMn on (1 1 1)-oriented Co/Pt multilayers with out-of-plane magnetization and for these films we have found no correlation between exchange bias and film texture perfection [15]. It is therefore unlikely that the large difference between the perpendicular exchange bias of Ni/FeMn and Ni/IrMn bilayers is related to the AFM film texture. On the other
3. Summary We have shown that Ni(0 0 1) films with out-of-plane magnetization can exhibit perpendicular exchange bias when covered with a (0 0 1)-textured AFM layer. The largest exchange bias is obtained for FeMn, while considerably smaller perpendicular exchange bias fields are measured for IrMn and NiMn.
Acknowledgements This work was supported by Science Foundation Ireland as part of the CINSE program.
References [1] J. Nogue´s, I.K. Schuller, J. Magn. Magn. Mater. 192 (1999) 203. [2] S. Maat, K. Takano, S.S.P. Parkin, E.E. Fullerton, Phys. Rev. Lett. 87 (2001) 087202. [3] O. Hellwig, S. Maat, J.B. Kortright, E.E. Fullerton, Phys. Rev. B 65 (2002) 144418.
ARTICLE IN PRESS S. van Dijken et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 1290–1293 [4] F. Garcia, G. Casali, S. Auffret, B. Rodmacq, B. Dieny, J. Appl. Phys. 91 (2002) 6905. [5] F. Garcia, J. Sort, B. Rodmacq, S. Auffret, B. Dieny, Appl. Phys. Lett. 83 (2003) 3537. [6] C.H. Marrows, Phys. Rev. B 68 (2003) 012405. [7] E. Hirota, H. Sakakima, K. Inomata, Giant MagnetoResistance Devices, Springer, Berlin, 2002. [8] R. Jungblut, M.T. Johnson, J. aan de Stegge, A. Reinders, F.J.A. den Broeder, J. Appl. Phys. 75 (1994) 6424. [9] B. Schulz, K. Baberschke, Phys. Rev. B 50 (1994) 13467.
1293
[10] S. van Dijken, R. Vollmer, B. Poelsema, J. Kirschner, J. Magn. Magn. Mater. 210 (2000) 316. [11] J. Moritz, S. van Dijken, J.M.D. Coey, unpublished. [12] F. Offi, W. Kuch, J. Kirschner, Phys. Rev. B 66 (2002) 064419. [13] R. Jungblut, R. Coehoorn, M.T. Johnson, J. aan de Stegge, A. Reinders, J. Appl. Phys. 75 (1994) 6659. [14] T. Sato, M. Tsunoda, M. Takahashi, J. Appl. Phys. 95 (2004) 7513. [15] S. van Dijken, J. Moritz, J.M.D. Coey, unpublished.
Journal of Magnetism and Magnetic Materials 290–291 (2005) 1290–1293 www.elsevier.com/locate/jmmm
Perpendicular exchange bias in nickel/antiferromagnetic bilayers Sebastiaan van Dijken, Matthew Crofton, J.M.D. Coey SFI Trinity Nanoscience Laboratory, Department of Physics, Trinity College, Dublin 2, Ireland Available online 14 December 2004
Abstract A study of exchange-biased Ni films with perpendicular magnetic anisotropy is presented. Polar magneto-optic Kerr effect measurements on unbiased Si(0 0 1)/Cu/Ni/Cu films reveal high out-of-plane remanence and a loop squareness close to unity. After deposition of an antiferromagnetic layer on top of the Ni film the magnetization direction remains perpendicular to the film plane and for FeMn, IrMn, and NiMn a shift in the hysteresis loop is observed. The perpendicular exchange bias is largest for FeMn, considerably smaller for IrMn and NiMn, and zero for PtMn. All antiferromagnetic films exhibit a (0 0 1) oriented texture. r 2004 Elsevier B.V. All rights reserved. PACS: 75.50.Ee; 75.70.Cn; 75.70.I; 75.30.Gw Keywords: Exchange bias; Perpendicular anisotropy; Ferromagnetism; Antiferromagnetism
1. Introduction Exchange bias phenomena in ferromagnetic (FM)/ antiferromagnetic (AFM) bilayers have been extensively studied in the last decades [1]. In most of these bilayers the magnetization of the FM film is oriented in the film plane. More recently, several studies on exchange bias effects in systems with out-of-plane magnetization have been published. Among them are Co/Pt multilayers with CoO [2,3] and CoFe/Pt, Co/Pt, and Co/Pd multilayers with FeMn [4–6]. The study of perpendicular exchange bias is relevant in the quest for a better understanding of the microscopic origin of the exchange bias phenomenon and it might lead to applications in magnetic sensors and perpendicular magnetic media. In this paper we report Corresponding author. Tel.: +353 1 6083034; +353 1 6083037. E-mail address: [email protected] (S. van Dijken).
fax:
on perpendicular exchange bias effects in Ni/AFM bilayers, where the AFM is FeMn, IrMn, NiMn, or PtMn.
2. Experiment and results The films were grown by dc magnetron sputtering on HF-etched Si(0 0 1) substrates in a Shamrock deposition tool. The magnetization reversal was studied by magneto-optic Kerr effect (MOKE) and SQUID measurements. The MOKE set-up was operated at 50 Hz and the maximum applied magnetic field was limited to about 170 mT. X-ray diffraction (XRD) was used to characterize the film structure. The thickness of the AFM layers was chosen to be larger than the critical thickness, i.e., the thickness above which exchange bias has been obtained in systems with in-plane magnetization [7].
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.11.452
ARTICLE IN PRESS S. van Dijken et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 1290–1293
t=5
1.0
t = 10
0.5
0.0
0.0
Normalized Polar MOKE
0.5
-0.5 -1.0 1.0
t = 20
t = 50
0.5 0.0
1.0
-100
0 100 µ0H (mT)
Fig. 1. Normalized polar MOKE hysteresis loops for Si(0 0 1)/ t nm Cu/4 nm Ni/5 nm Cu films.
Fig. 1 shows polar MOKE hysteresis loops for Si(0 0 1)/t nm Cu/4 nm Ni/5 nm Cu films. The Cu seed layer grows with a (0 0 1) texture on the Si(0 0 1) substrate. Subsequent deposition of Ni on this film leads to a tetragonal distortion of the Ni lattice. This lattice distortion results in a magnetoelastic contribution to the magnetic anisotropy energy of the system favoring an out-of-plane film magnetization [8–10]. For tCu ¼ 50 nm and a Ni film thickness of 4 nm, the magnetoelastic anisotropy energy is larger than the sum of the magnetostatic and interface anisotropy energies (both favoring in-plane magnetization). The result is an out-of-plane film magnetization with full remanence. The deposition of an AFM layer on top of the Ni film does not alter the easy magnetization axis and for FeMn, IrMn, and NiMn it leads to a shift in the polar MOKE hysteresis loop (see Fig. 2). The perpendicular exchange bias field is largest for FeMn. For this film an applied magnetic field of 170 mT is not enough to saturate the magnetization. Fig. 3 shows SQUID magnetization curves for Ni/FeMn and Ni/IrMn bilayers. The exchange bias field for these films is 30 and 5 mT, respectively. The results for the coercive and exchange bias fields are summarized in Table 1. This table also shows the interfacial exchange energy (s) which is calculated from the SQUID data using s ¼ m0 H eb M s tNi ; where Ms is the saturation magnetization of Ni and tNi is the Ni layer thickness. The interfacial exchange energy is large for FeMn, relatively small for IrMn and NiMn, and zero for PtMn. The differences between the MOKE and SQUID data in Table 1 are attributed to dynamic magnetic switching effects. The
PtMn
0.0
-1.0 -100
NiMn
0.5
-1.0 0 100 µ0H (mT)
IrMn
-1.0
-0.5
-100
FeMn
-0.5
-0.5
0 100 µ0H (mT)
-100
0 100 µ0H (mT)
Fig. 2. Normalized polar MOKE hysteresis loops for Si(0 0 1)/ 50 nm Cu/4 nm Ni/t nm AFM/5 nm Cu films. The thickness t of the AFM layers is 10, 10, 25, and 15 nm for FeMn, IrMn, NiMn, and PtMn, respectively.
1.0
IrMn
FeMn
0.5 M/Ms
Normalized Polar MOKE
1.0
1291
0.0 -0.5 -1.0 -50
0 µ0H (mT)
50
-50
0 µ0H (mT)
50
Fig. 3. SQUID magnetization curves for Si(0 0 1)/50 nm Cu/ 4 nm Ni/10 nm FeMn/5 nm Cu and Si(0 0 1)/50 nm Cu/4 nm Ni/ 10 nm IrMn/5 nm Cu.
field sweep rate in the MOKE set-up is about 8.5 103 mT/s, which is about four orders of magnitude larger than that used in the SQUID measurements. A more detailed discussion on dynamic effects in perpendicular exchange-biased systems will be published elsewhere [11]. Exchange-bias effects in FM/AFM bilayer systems strongly depend on crystallinity, grain size, and interface roughness [1]. Fig. 4 shows XRD scans for Si(0 0 1)/ 50 nm Cu/4 nm Ni/AFM/5 nm Cu films. Besides the dominant (0 0 2) Cu reflection, weaker fcc (0 0 2) reflections are also visible for IrMn, NiMn, and PtMn.
ARTICLE IN PRESS S. van Dijken et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 1290–1293
1292
Table 1 Coercive and exchange bias fields measured on Si(0 0 1)/50 nm Cu/4 nm Ni/AFM/5 nm Cu films
No AFM FeMn IrMn NiMn PtMn
m0Hc MOKE
m0Heb MOKE
m0Hc SQUID
m0Heb SQUID
s (mJ/m2)
60 mT 20 mT 57 mT 70 mT 60 mT
0 mT 110 mT 16 mT 10 mT 0 mT
24 mT 3 mT 15 mT 24 mT 24 mT
0 mT 30 mT 5 mT 3 mT 0 mT
0 0.059 0.010 0.006 0
The last column displays the interfacial exchange energy (calculated from the SQUID data).
600
hand, grain boundaries and interface defects will pin uncompensated spins at the AFM interface. Since these spins are responsible for the exchange bias effect, the dissimilar biasing results are more likely due to a different AFM grain size or FM/AFM interface roughness in the Ni/FeMn and Ni/IrMn bilayers. For FM/AFM bilayers with NiMn and PtMn it is normally necessary to anneal the films in order to obtain exchange bias. In the as-deposited state the films have a nonmagnetic fcc structure which upon annealing changes into the AFM L10 structure. The very low (NiMn) and zero (PtMn) perpendicular exchange bias measured on the Ni/AFM bilayer is due to the fcc film structure. An attempt to anneal the Si(0 0 1)/50 nm Cu/ 4 nm Ni/AFM/5 nm Cu films at 200 1C resulted in severe mixing between the Cu and Ni layers which destroyed the perpendicular magnetic anisotropy.
Cu (002)
no AFM
300 0 600
FeMn
Intensity (cps)
300 0 600
IrMn
IrMn (002)
300 0 600
NiMn
300 0 600 300
NiMn (002) PtMn PtMn (002)
0 46
48
50
52
54
Fig. 4. XRD scans for Si(0 0 1)/50 nm Cu/4 nm Ni/t nm AFM/ 5 nm Cu films. The thickness t of the AFM layers is 10, 10, 25, and 15 nm for FeMn, IrMn, NiMn, and PtMn, respectively.
The absence of any FeMn reflections indirectly indicates the growth of the fcc g phase, for which the vertical interlayer distance is very similar to that of Cu(0 0 1) [12]. Exchange bias effects in bilayers with a (0 0 1) texture have not been studied extensively. However, for both FeMn and IrMn the exchange bias of (0 0 1) oriented bilayers with in-plane magnetization is reported to be a factor 1–2 smaller than that of (1 1 1) oriented films [13,14]. We have measured similar exchange bias fields for FeMn and IrMn on (1 1 1)-oriented Co/Pt multilayers with out-of-plane magnetization and for these films we have found no correlation between exchange bias and film texture perfection [15]. It is therefore unlikely that the large difference between the perpendicular exchange bias of Ni/FeMn and Ni/IrMn bilayers is related to the AFM film texture. On the other
3. Summary We have shown that Ni(0 0 1) films with out-of-plane magnetization can exhibit perpendicular exchange bias when covered with a (0 0 1)-textured AFM layer. The largest exchange bias is obtained for FeMn, while considerably smaller perpendicular exchange bias fields are measured for IrMn and NiMn.
Acknowledgements This work was supported by Science Foundation Ireland as part of the CINSE program.
References [1] J. Nogue´s, I.K. Schuller, J. Magn. Magn. Mater. 192 (1999) 203. [2] S. Maat, K. Takano, S.S.P. Parkin, E.E. Fullerton, Phys. Rev. Lett. 87 (2001) 087202. [3] O. Hellwig, S. Maat, J.B. Kortright, E.E. Fullerton, Phys. Rev. B 65 (2002) 144418.
ARTICLE IN PRESS S. van Dijken et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 1290–1293 [4] F. Garcia, G. Casali, S. Auffret, B. Rodmacq, B. Dieny, J. Appl. Phys. 91 (2002) 6905. [5] F. Garcia, J. Sort, B. Rodmacq, S. Auffret, B. Dieny, Appl. Phys. Lett. 83 (2003) 3537. [6] C.H. Marrows, Phys. Rev. B 68 (2003) 012405. [7] E. Hirota, H. Sakakima, K. Inomata, Giant MagnetoResistance Devices, Springer, Berlin, 2002. [8] R. Jungblut, M.T. Johnson, J. aan de Stegge, A. Reinders, F.J.A. den Broeder, J. Appl. Phys. 75 (1994) 6424. [9] B. Schulz, K. Baberschke, Phys. Rev. B 50 (1994) 13467.
1293
[10] S. van Dijken, R. Vollmer, B. Poelsema, J. Kirschner, J. Magn. Magn. Mater. 210 (2000) 316. [11] J. Moritz, S. van Dijken, J.M.D. Coey, unpublished. [12] F. Offi, W. Kuch, J. Kirschner, Phys. Rev. B 66 (2002) 064419. [13] R. Jungblut, R. Coehoorn, M.T. Johnson, J. aan de Stegge, A. Reinders, J. Appl. Phys. 75 (1994) 6659. [14] T. Sato, M. Tsunoda, M. Takahashi, J. Appl. Phys. 95 (2004) 7513. [15] S. van Dijken, J. Moritz, J.M.D. Coey, unpublished.