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Thermal stability of spin-transfer switching in CPP-GMR devices
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 310 (2007) 2026–2028 www.elsevier.com/locate/jmmm
Thermal stability of spin-transfer switching in CPP-GMR devices Y. Otania,b,, H. Kubotaa, A. Fukushimaa, H. Maeharac, S. Yuasaa,d, Y. Suzukia,e a
National Institute of Advanced Industrial Science and Technology, Tsukuba Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan b Graduate School of Pure and Applied Science, Tsukuba University, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan c Canon ANELVA Corporation, 5-8-1 Yotsuya, Fuchu, Tokyo 183-8508, Japan d PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan e Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan Available online 20 November 2006
Abstract Spin-transfer switching (STS) was investigated in CoFe/Cu/CoFe current-perpendicular-to-plane (CPP) giant magnetoresistive (GMR) devices. The devices showed magnetoresistance ratio of 0.6570.04% at room temperature. Abrupt resistance changes due to STS were clearly observed by applying short current pulses. We evaluated thermal stability factors of the small Co–Fe free layer cell from pulse duration dependence of switching current and from static magnetization processes. The thermal stability factors obtained from the two methods were in good agreement. r 2006 Elsevier B.V. All rights reserved. PACS: 75.70.Pa Keywords: Spin-transfer; Thermal activation; Thermal factor; CPP-GMR
1. Introduction Spin-transfer switching (STS) [1] is a potential writing scheme for high-density magnetoresistive random access memories (MRAMs). In MRAM applications, it is necessary to simultaneously attain both low intrinsic switching current density (Jc0) and sufficiently high thermal stability. Thermally activated STS model [2,3] has predicted that Jc0 and the thermal stability factor (E/(kBT)) can be obtained from the dependence of the switching current density (Jc) on current pulse duration. In a single domain model, the thermal stability is proportional to the coercivity (Hc). Therefore, it is interesting to investigate the relation between the thermal stability factors derived from dynamic STS and static magnetization processes. In this study, we have experimentally examined STS and magnetoresistance in current-perpendicular-to-plane (CPP) giant
Corresponding author. National Institute of Advanced Industrial Science and Technology, Tsukuba Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan. Tel.: +81 29 861 3949; fax: +81 29 861 3432. E-mail address: [email protected] (Y. Otani).
0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.930
magnetoresistive (GMR) devices and discussed thermal stability during magnetization reversal. 2. Experiments Multilayers of sub. /Ta5/Cu100/Ta5/PtMn15/CoFe1.8/ Ru0.85/CoFe2.2/Cu6/CoFe2/Ru5 (in nm) were deposited on thermally oxidized silicon wafers using UHV sputtering (Canon ANELVA C-7100). CPP nanopillars with 100 170 nm2 ellipsoidal cross-section were prepared using microfabrication techniques (electron-beam lithography and Ar ion etching). The pillars were etched down to the top surface of the PtMn layer. Magnetoresistance loops (R–H) were measured using AC (1 mVp-p, 97 kHz) four terminal techniques. The magnetoresistance (MR) ratio was defined as (Rap–Rp)/Rp 100 (%), where Rp and Rap respectively represent the resistance in parallel and antiparallel alignments of the magnetizations of the CoFe (2 nm) free layer and the CoFe (2.2 nm) reference layer. STS loops (R–J) were measured at room temperature by applying sequential current pulses with pulse durations (tp) between 10 ms and 1 s, under an external field (Hext) which compensated the shift of the corresponding R–H loop
ARTICLE IN PRESS Y. Otani et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 2026–2028
(induced by interlayer coupling or magnetostatic coupling at cell edges). Positive current is defined as electrons flowing from the top electrode to the bottom electrode.
3. Results and discussions Fig. 1 shows typical R–H and R–J loops. The R–H loop shows Hc of 115 and 58 Oe loop shifts (Hshift) to positive field direction. The MR ratio was 0.61%. The R–J loop was measured at tp ¼ 100 ms under Hext ¼ 58 Oe. J+ c (Jc ), which corresponds to switching current density from P to AP (AP to P), and was 3.6 107 (1.4 107) A/cm2. Abrupt switching of resistance in both loops implies the coherent rotation of the free layer magnetization. Fig. 2 shows tp dependence of Jc for the same sample. Based on
a
b Hshift
Jc−
Jc+
R (ohm)
5.81 5.80 5.79
2Hc
5.78 -200
0
200
-4
-2
0
2
4
J (107 A/cm2)
H(Oe)
Fig. 1. Typical (a) R–H and (b) R–J loops. In R–J measurement, R was measured at the end of an appropriate interval (400 ms) between the pulses to avoid an increase in R due to Joule heating.
is expressed as the thermal activation model [2], J7 c 7 7 J7 c ¼ Jc0 (1(kBT/E ) ln(tp/t0)), where kB, T, and t0 indicate Boltzmann constant, temperature, and the inverse of the attempt frequency (1 ns), respectively. E7 is defined as (1/2) MsVHk(17Heff/Hk)2, Heff ¼ Hext–Hshift, where V and Ms represent the volume and the saturation magnetization of the free layer, respectively. Heff is the effective field acting on the free layer. From y-intercepts and slopes of the lines in Fig. 2, J+() ¼ 7.2 107 A/cm2 c0 7 2 +() (4.8 10 A/cm ) and E /(kBT) ¼ 49 (38) were obtained. We have neglected the effect of Joule heating as the temperature increase estimated from the resistance change was 7 K even for 5.9 107 A/cm2. We measured 12 samples on the same wafer, which showed small variations of the MR ratio (0.6570.04%). However, both Hc and E/(kBT) values exhibited a rather large distribution. Average values and standard deviation of Hc, E+()/(kBT), and J+() were c0 122733 Oe, 63711 (4779), and 8.671.5 107 A/cm2 (4.770.7 107 A/cm2), respectively. The large distribution of Hc is probably due to the variation of cell shape caused by distorted resistance pattern or redeposition at the cell edges during etching process. Based on the single domain model, the thermal stability factor is also obtained as ESDM/(kBT) ¼ (1/2) MsVHc/(kBT), where Hc is assumed to be anisotropy field (Hk). In all the samples, we examined the relation between the thermal stability obtained from the Jc–tp dependence of STS (ESTS/(kBT)) and that obtained from Hc values in R–H measurement (ESDM/ (kBT)) (Fig. 3). It is confirmed that ESDM/(kBT) values agree with ESTS/(kBT) values. E+ STS/(kBT) shows higher stability and larger distribution than E STS/(kBT). The difference between E+ STS/(kBT) and ESTS/(kBT) could be due to the different effective fields in P and AP states. In MRAM applications, thermal stability of larger than 40 is required for both E7/(kBT) to maintain stored data for about 10 years. Therefore, for Ms ¼ 1600 emu/cm3 and
10
100
Hext = 58 Oe
8
80
4 2
ESTS/(kBT )
Jc (107 A/cm2)
6
+
: Jc
0
: Jc-
-2
60 40 +
: ESTS /(kBT )
20
-4 -6
2027
0
5
10
15 20 ln(tp/ τ0)
25
30
Fig. 2. Pulse duration dependence of switching current density. Open circles and closed triangles represent experimental results J+ c and Jc , respectively. Broken lines are fittings to experimental data based on thermally activated STS model.
0
: ESTS−/(kBT ) 0
20
40
60
80
100
ESDM /(kBT ) Fig. 3. The relation between thermal stability estimated from STS experiment (ESTS/(kBT)) and that calculated from single domain model (ESDM/(kBT)). Open circles and closed triangles represent STS for P to AP and AP to P, respectively. T ¼ 300 K is assumed.
ARTICLE IN PRESS 2028
Y. Otani et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 2026–2028
V ¼ 2 100 170 nm3, Hc of larger than 80 Oe is required for such samples. If CoFeB with Ms ¼ 1200 emu/cm3 [4,5] is used, Hc of larger than 110 Oe is necessary. These coercivities are easily achieved in elongated cell shapes. Therefore, it is possible to obtain high thermal stability in MRAM devices. In summary, we have investigated the relation between thermal stability estimated from STS experiments and that calculated from coercivity based on single domain model. We confirm that thermal stability obtained from the two methods coincides well.
References [1] J.C. Slonczewski, J. Magn. Magn. Mater. 152 (1996) L1. [2] R.H. Koch, J.A. Katine, J.Z. Sun, Phys. Rev. Lett. 92 (2004) 088302. [3] K. Yagami, A.A. Tulapurkar, A. Fukushima, Y. Suzuki, Appl. Phys. Lett. 85 (2004) 5634. [4] D.D. Djayaprawira, K. Tsunekawa, M. Nagai, H. Maehara, S. Yamagata, N. Watanabe, S. Yuasa, K. Ando, Appl. Phys. Lett. 86 (2005) 092502. [5] H. Kubota, A. Fukushima, Y. Ootani, S. Yuasa, K. Ando, H. Maehara, K. Tsunekawa, D.D. Dyayaprawira, N. Watanabe, Y. Suzuki, Jpn. J. Appl. Phys. 44 (2005) L1237.
Journal of Magnetism and Magnetic Materials 310 (2007) 2026–2028 www.elsevier.com/locate/jmmm
Thermal stability of spin-transfer switching in CPP-GMR devices Y. Otania,b,, H. Kubotaa, A. Fukushimaa, H. Maeharac, S. Yuasaa,d, Y. Suzukia,e a
National Institute of Advanced Industrial Science and Technology, Tsukuba Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan b Graduate School of Pure and Applied Science, Tsukuba University, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan c Canon ANELVA Corporation, 5-8-1 Yotsuya, Fuchu, Tokyo 183-8508, Japan d PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan e Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan Available online 20 November 2006
Abstract Spin-transfer switching (STS) was investigated in CoFe/Cu/CoFe current-perpendicular-to-plane (CPP) giant magnetoresistive (GMR) devices. The devices showed magnetoresistance ratio of 0.6570.04% at room temperature. Abrupt resistance changes due to STS were clearly observed by applying short current pulses. We evaluated thermal stability factors of the small Co–Fe free layer cell from pulse duration dependence of switching current and from static magnetization processes. The thermal stability factors obtained from the two methods were in good agreement. r 2006 Elsevier B.V. All rights reserved. PACS: 75.70.Pa Keywords: Spin-transfer; Thermal activation; Thermal factor; CPP-GMR
1. Introduction Spin-transfer switching (STS) [1] is a potential writing scheme for high-density magnetoresistive random access memories (MRAMs). In MRAM applications, it is necessary to simultaneously attain both low intrinsic switching current density (Jc0) and sufficiently high thermal stability. Thermally activated STS model [2,3] has predicted that Jc0 and the thermal stability factor (E/(kBT)) can be obtained from the dependence of the switching current density (Jc) on current pulse duration. In a single domain model, the thermal stability is proportional to the coercivity (Hc). Therefore, it is interesting to investigate the relation between the thermal stability factors derived from dynamic STS and static magnetization processes. In this study, we have experimentally examined STS and magnetoresistance in current-perpendicular-to-plane (CPP) giant
Corresponding author. National Institute of Advanced Industrial Science and Technology, Tsukuba Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan. Tel.: +81 29 861 3949; fax: +81 29 861 3432. E-mail address: [email protected] (Y. Otani).
0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.930
magnetoresistive (GMR) devices and discussed thermal stability during magnetization reversal. 2. Experiments Multilayers of sub. /Ta5/Cu100/Ta5/PtMn15/CoFe1.8/ Ru0.85/CoFe2.2/Cu6/CoFe2/Ru5 (in nm) were deposited on thermally oxidized silicon wafers using UHV sputtering (Canon ANELVA C-7100). CPP nanopillars with 100 170 nm2 ellipsoidal cross-section were prepared using microfabrication techniques (electron-beam lithography and Ar ion etching). The pillars were etched down to the top surface of the PtMn layer. Magnetoresistance loops (R–H) were measured using AC (1 mVp-p, 97 kHz) four terminal techniques. The magnetoresistance (MR) ratio was defined as (Rap–Rp)/Rp 100 (%), where Rp and Rap respectively represent the resistance in parallel and antiparallel alignments of the magnetizations of the CoFe (2 nm) free layer and the CoFe (2.2 nm) reference layer. STS loops (R–J) were measured at room temperature by applying sequential current pulses with pulse durations (tp) between 10 ms and 1 s, under an external field (Hext) which compensated the shift of the corresponding R–H loop
ARTICLE IN PRESS Y. Otani et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 2026–2028
(induced by interlayer coupling or magnetostatic coupling at cell edges). Positive current is defined as electrons flowing from the top electrode to the bottom electrode.
3. Results and discussions Fig. 1 shows typical R–H and R–J loops. The R–H loop shows Hc of 115 and 58 Oe loop shifts (Hshift) to positive field direction. The MR ratio was 0.61%. The R–J loop was measured at tp ¼ 100 ms under Hext ¼ 58 Oe. J+ c (Jc ), which corresponds to switching current density from P to AP (AP to P), and was 3.6 107 (1.4 107) A/cm2. Abrupt switching of resistance in both loops implies the coherent rotation of the free layer magnetization. Fig. 2 shows tp dependence of Jc for the same sample. Based on
a
b Hshift
Jc−
Jc+
R (ohm)
5.81 5.80 5.79
2Hc
5.78 -200
0
200
-4
-2
0
2
4
J (107 A/cm2)
H(Oe)
Fig. 1. Typical (a) R–H and (b) R–J loops. In R–J measurement, R was measured at the end of an appropriate interval (400 ms) between the pulses to avoid an increase in R due to Joule heating.
is expressed as the thermal activation model [2], J7 c 7 7 J7 c ¼ Jc0 (1(kBT/E ) ln(tp/t0)), where kB, T, and t0 indicate Boltzmann constant, temperature, and the inverse of the attempt frequency (1 ns), respectively. E7 is defined as (1/2) MsVHk(17Heff/Hk)2, Heff ¼ Hext–Hshift, where V and Ms represent the volume and the saturation magnetization of the free layer, respectively. Heff is the effective field acting on the free layer. From y-intercepts and slopes of the lines in Fig. 2, J+() ¼ 7.2 107 A/cm2 c0 7 2 +() (4.8 10 A/cm ) and E /(kBT) ¼ 49 (38) were obtained. We have neglected the effect of Joule heating as the temperature increase estimated from the resistance change was 7 K even for 5.9 107 A/cm2. We measured 12 samples on the same wafer, which showed small variations of the MR ratio (0.6570.04%). However, both Hc and E/(kBT) values exhibited a rather large distribution. Average values and standard deviation of Hc, E+()/(kBT), and J+() were c0 122733 Oe, 63711 (4779), and 8.671.5 107 A/cm2 (4.770.7 107 A/cm2), respectively. The large distribution of Hc is probably due to the variation of cell shape caused by distorted resistance pattern or redeposition at the cell edges during etching process. Based on the single domain model, the thermal stability factor is also obtained as ESDM/(kBT) ¼ (1/2) MsVHc/(kBT), where Hc is assumed to be anisotropy field (Hk). In all the samples, we examined the relation between the thermal stability obtained from the Jc–tp dependence of STS (ESTS/(kBT)) and that obtained from Hc values in R–H measurement (ESDM/ (kBT)) (Fig. 3). It is confirmed that ESDM/(kBT) values agree with ESTS/(kBT) values. E+ STS/(kBT) shows higher stability and larger distribution than E STS/(kBT). The difference between E+ STS/(kBT) and ESTS/(kBT) could be due to the different effective fields in P and AP states. In MRAM applications, thermal stability of larger than 40 is required for both E7/(kBT) to maintain stored data for about 10 years. Therefore, for Ms ¼ 1600 emu/cm3 and
10
100
Hext = 58 Oe
8
80
4 2
ESTS/(kBT )
Jc (107 A/cm2)
6
+
: Jc
0
: Jc-
-2
60 40 +
: ESTS /(kBT )
20
-4 -6
2027
0
5
10
15 20 ln(tp/ τ0)
25
30
Fig. 2. Pulse duration dependence of switching current density. Open circles and closed triangles represent experimental results J+ c and Jc , respectively. Broken lines are fittings to experimental data based on thermally activated STS model.
0
: ESTS−/(kBT ) 0
20
40
60
80
100
ESDM /(kBT ) Fig. 3. The relation between thermal stability estimated from STS experiment (ESTS/(kBT)) and that calculated from single domain model (ESDM/(kBT)). Open circles and closed triangles represent STS for P to AP and AP to P, respectively. T ¼ 300 K is assumed.
ARTICLE IN PRESS 2028
Y. Otani et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 2026–2028
V ¼ 2 100 170 nm3, Hc of larger than 80 Oe is required for such samples. If CoFeB with Ms ¼ 1200 emu/cm3 [4,5] is used, Hc of larger than 110 Oe is necessary. These coercivities are easily achieved in elongated cell shapes. Therefore, it is possible to obtain high thermal stability in MRAM devices. In summary, we have investigated the relation between thermal stability estimated from STS experiments and that calculated from coercivity based on single domain model. We confirm that thermal stability obtained from the two methods coincides well.
References [1] J.C. Slonczewski, J. Magn. Magn. Mater. 152 (1996) L1. [2] R.H. Koch, J.A. Katine, J.Z. Sun, Phys. Rev. Lett. 92 (2004) 088302. [3] K. Yagami, A.A. Tulapurkar, A. Fukushima, Y. Suzuki, Appl. Phys. Lett. 85 (2004) 5634. [4] D.D. Djayaprawira, K. Tsunekawa, M. Nagai, H. Maehara, S. Yamagata, N. Watanabe, S. Yuasa, K. Ando, Appl. Phys. Lett. 86 (2005) 092502. [5] H. Kubota, A. Fukushima, Y. Ootani, S. Yuasa, K. Ando, H. Maehara, K. Tsunekawa, D.D. Dyayaprawira, N. Watanabe, Y. Suzuki, Jpn. J. Appl. Phys. 44 (2005) L1237.