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CoFe ferromagnetic tunnel junctions
Journal of Magnetism and Magnetic Materials 226}230 (2001) 922}923
Magnon excitation of CoFe/Al-oxide/CoFe ferromagnetic tunnel junctions J. Murai*, Y. Ando, T. Daibou, K. Yaoita, H.F. Han, T. Miyazaki Departments of Applied Physics, Graduate School of Engineering, Tohoku University, Aoba-yama 08, Sendai, 980-8579, Japan
Abstract Inelastic-electron-tunneling (IET) spectroscopy has been applied to investigate the magnon-induced inelastic tunneling process for Ta/Ni Fe /Cu/Ni Fe /IrMn/Co Fe /Al-oxide/Co Fe /Ni Fe /Ta ferromagnetic tunnel junc tions. For the junction with the oxidation time, t "40 s, which is the exact time to oxidize the 8 As Al, the subtraction spectrum of the IET spectra between the parallel and anti-parallel magnetization con"gurations clearly showed two peaks at $3 and $16 mV showed. On the other hand, the spectrum for the junction with t "120 s showed a broad peak at around 18 mV and a plateau at zero-bias. This was caused by the change of the correlation length of magnon inelastic excitation due to the over oxidation of the bottom electrode. 2001 Elsevier Science B.V. All rights reserved. Keywords: Tunneling; Transport properties; Magnons; Interface structure
Recently, large tunnel magneto-resistance (TMR) of ferromagnetic tunnel junctions has been drawing a great deal of interest. The spin-#ip scattering due to magnon at the interface between a ferromagnetic layer and an insulator causes a large decrease of TMR ratio with an increasing applied bias voltage [1]. The rate of decrease of the TMR ratio with increasing voltage depends on the oxidation and annealing conditions [2]. We would like to clarify the relation between spin-dependent tunneling properties and interface structure of ferromagnetic tunnel junction. In this article, we report on the analysis of ferromagnetic tunnel junctions using inelasticelectron-tunneling (IET) Spectroscopy [3}5,7]. Ta(50 As )/ Ni Fe (30 As )/Cu(200 As )/Ni Fe (30 As )/IrMn(100 As )/ C o F e ( 4 0 As ) / A l ( 8 As ) - o x i d e / C o F e ( 4 0 As ) / Ni Fe (200 As )/Ta(50 As ) spin-valve-type tunnel junc tions were prepared by RF magnetron sputtering with inductive coupled plasma (ICP) and optical lithography methods onto a thermally oxidized Si substrate at Ar pressure of 0.07 Pa. The Al-oxide insulator was formed by ICP oxidation at 0.73 Pa O and 100 W RF power. * Corresponding author. Tel.: #81-22-217-7949; fax: #8122-217-7947. E-mail address: [email protected] (J. Murai).
We fabricated two junctions with di!erent oxidation time, t "40 and 120 s. The oxidation time of 40 s has been previously investigated to be the exact time to oxidize an 8 As Al [6]. t "120 s is too long to oxidize only the Al layer, so that the surface of the bottom ferromagnetic electrode will be over-oxidized. The tunnel resistance, MR ratio, estimated barrier height and barrier thickness obtained by "tting to the current}voltage curves were 3 k m, 15.5%, 1.30 eV, and 9.4 As for the junction with t "40 s and 783 k m, 9.4%, 1.18 eV and 12.5 As for the junction with t "120 s, respectively. dI/d<}< curves and IET spectrum were measured by modulation methods using a home-made electrical circuit [7]. In addition to a dc voltage sweep, a modulation voltage of 2 mV with a frequency of 5.05 kHz in a magnetic "eld up to 100 Oe at 4.2 K was applied to the junctions. Fig. 1(a) shows the IET spectra of the anti-parallel (AP) and parallel (P) magnetization con"gurations for the junction with t "40 s. Two peaks at $3 mV and $16 mV are observed for both curves. Recently, similar IET spectra with two peaks were reported for Ni Fe / Al O /Co Fe /FeMn junction with a low tunnel res istance of 5.2 k m [8]. In order to eliminate the contribution of spin-independent excitation, the subtraction spectrum which is de"ned by the di!erence between the
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 1 1 0 5 - 7
J. Murai et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 922}923
Fig. 1. (a) IET spectra for the spin-valve-type junction of t "40 s. The results for AP (solid line) and P (dashed line) magnetization con"gurations are shown; (b) subtraction spectrum for the spin-valve-type junction with t "40 s.
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previously [3}5,9]. Interestingly, the subtraction spectrum of this junction has a small plateau around zero bias voltage. We have reported the analysis of the IET spectra of ferromagnetic tunnel junctions previously [4,5]. The subtraction spectrum re#ected the magnon density of states of both ferromagnetic electrodes. The correlation lengths of magnons and Curie temperature of the electrode were assumed to provide the lowest and highest cut-o! energy of a spectrum. That is, the variation of the structure of the interface of the ferromagnetic electrode results in the change of the shape of the spectrum. Two peaks of the subtraction spectrum for the junction with t "40 s may be due to two distributions of magnon density of states in both interfaces of CoFe/Al-oxide/CoFe structure. Obscurity of the peak around zero-bias for the junction with t "120 s can be due to the reduction of the correlation length of magnon which comes from the over-oxidation of the bottom electrode. On the other hand, the contribution to the spectrum from zero-bias anomaly cannot be neglected. When there are ferromagnetic impurities at the interface or inside the insulator, IET spectra show a strong negative peak around zero-bias [7], so that the intensity of spectra due to magnon around zero-bias decreases. The small plateau of the spectrum for the junction with t "120 s can be due to the increase of the zero-bias anomaly. This research is supported by the Storage Research Consortium, Regional Consortium Project (NEDO) and Grant-in-Aides for Scienti"c Research from the Ministry of Education, Science, Sports and Culture of Japan.
References
Fig. 2. (a) IET spectra for the spin-valve-type junction of t "120 s. The results for AP (solid line) and P (dashed line) magnetization con"gurations are shown; (b) subtraction spectrum for the spin-valve-type junction of t "120 s.
spectra of both magnetization con"gurations was obtained [3}5] and is shown in Fig. 1(b). The relative intensity of the peak at higher voltage becomes larger in comparison with the original IET spectra. Fig. 2(a) and (b) shows the IET spectra of both magnetization con"gurations and their subtraction spectrum for the junction with t "120 s. The "ne structure of two peaks observed for the junction with t "40 s disappears and a broad peak appears at around 18 mV for these spectra. The shape of these spectra is similar to those of reported
[1] M. Oogane, N. Tezuka, T. Miyazaki, J. Magn. Soc. Jpn. 23 (1999) 1297. [2] S. Kumagai, N. Tezuka, T. Miyazaki, Jpn. J. Appl. Phys. 36 (1997) L1498. [3] J. Murai, Y. Ando, M. Kamijo, H. Kubota, T. Miyazaki, Jpn. J. Appl. Phys. 38 (1999) L1106. [4] J. Murai, Y. Ando, M. Kamijo, T. Daibou, H. Kubota, T. Miyazaki, C. Kim, O. Song, J. Magn. Soc. Jpn. 24 (2000) 615. [5] Y. Ando, J. Murai, H. Kubota, T. Miyazaki, J. Appl. Phys. 87 (2000), in press. [6] Y. Ando, H. Kubota, M. Hayashi, M. Kamijo, K. Yaoita, H.F. Han, T. Miyazaki, Jpn. J. Appl. Phys., submitted for publication. [7] J. Murai, Y. Ando, N. Tezuka, T. Miyazaki, J. Magn. Soc. Jpn. 24 (1998) 573. [8] Y. Aoki, H. Kinoshita, H. Sugawara, H. Sato, H. Tsuge, T. Mitsuzuka, A. Kamijo, Abstracts of ISFMS, 1999, p. 321. [9] J.S. Moodera, J. Nowak, R.J.M. van de Veerdonk, Phys. Rev. Lett. 80 (1998) 2941.
Magnon excitation of CoFe/Al-oxide/CoFe ferromagnetic tunnel junctions J. Murai*, Y. Ando, T. Daibou, K. Yaoita, H.F. Han, T. Miyazaki Departments of Applied Physics, Graduate School of Engineering, Tohoku University, Aoba-yama 08, Sendai, 980-8579, Japan
Abstract Inelastic-electron-tunneling (IET) spectroscopy has been applied to investigate the magnon-induced inelastic tunneling process for Ta/Ni Fe /Cu/Ni Fe /IrMn/Co Fe /Al-oxide/Co Fe /Ni Fe /Ta ferromagnetic tunnel junc tions. For the junction with the oxidation time, t "40 s, which is the exact time to oxidize the 8 As Al, the subtraction spectrum of the IET spectra between the parallel and anti-parallel magnetization con"gurations clearly showed two peaks at $3 and $16 mV showed. On the other hand, the spectrum for the junction with t "120 s showed a broad peak at around 18 mV and a plateau at zero-bias. This was caused by the change of the correlation length of magnon inelastic excitation due to the over oxidation of the bottom electrode. 2001 Elsevier Science B.V. All rights reserved. Keywords: Tunneling; Transport properties; Magnons; Interface structure
Recently, large tunnel magneto-resistance (TMR) of ferromagnetic tunnel junctions has been drawing a great deal of interest. The spin-#ip scattering due to magnon at the interface between a ferromagnetic layer and an insulator causes a large decrease of TMR ratio with an increasing applied bias voltage [1]. The rate of decrease of the TMR ratio with increasing voltage depends on the oxidation and annealing conditions [2]. We would like to clarify the relation between spin-dependent tunneling properties and interface structure of ferromagnetic tunnel junction. In this article, we report on the analysis of ferromagnetic tunnel junctions using inelasticelectron-tunneling (IET) Spectroscopy [3}5,7]. Ta(50 As )/ Ni Fe (30 As )/Cu(200 As )/Ni Fe (30 As )/IrMn(100 As )/ C o F e ( 4 0 As ) / A l ( 8 As ) - o x i d e / C o F e ( 4 0 As ) / Ni Fe (200 As )/Ta(50 As ) spin-valve-type tunnel junc tions were prepared by RF magnetron sputtering with inductive coupled plasma (ICP) and optical lithography methods onto a thermally oxidized Si substrate at Ar pressure of 0.07 Pa. The Al-oxide insulator was formed by ICP oxidation at 0.73 Pa O and 100 W RF power. * Corresponding author. Tel.: #81-22-217-7949; fax: #8122-217-7947. E-mail address: [email protected] (J. Murai).
We fabricated two junctions with di!erent oxidation time, t "40 and 120 s. The oxidation time of 40 s has been previously investigated to be the exact time to oxidize an 8 As Al [6]. t "120 s is too long to oxidize only the Al layer, so that the surface of the bottom ferromagnetic electrode will be over-oxidized. The tunnel resistance, MR ratio, estimated barrier height and barrier thickness obtained by "tting to the current}voltage curves were 3 k m, 15.5%, 1.30 eV, and 9.4 As for the junction with t "40 s and 783 k m, 9.4%, 1.18 eV and 12.5 As for the junction with t "120 s, respectively. dI/d<}< curves and IET spectrum were measured by modulation methods using a home-made electrical circuit [7]. In addition to a dc voltage sweep, a modulation voltage of 2 mV with a frequency of 5.05 kHz in a magnetic "eld up to 100 Oe at 4.2 K was applied to the junctions. Fig. 1(a) shows the IET spectra of the anti-parallel (AP) and parallel (P) magnetization con"gurations for the junction with t "40 s. Two peaks at $3 mV and $16 mV are observed for both curves. Recently, similar IET spectra with two peaks were reported for Ni Fe / Al O /Co Fe /FeMn junction with a low tunnel res istance of 5.2 k m [8]. In order to eliminate the contribution of spin-independent excitation, the subtraction spectrum which is de"ned by the di!erence between the
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 1 1 0 5 - 7
J. Murai et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 922}923
Fig. 1. (a) IET spectra for the spin-valve-type junction of t "40 s. The results for AP (solid line) and P (dashed line) magnetization con"gurations are shown; (b) subtraction spectrum for the spin-valve-type junction with t "40 s.
923
previously [3}5,9]. Interestingly, the subtraction spectrum of this junction has a small plateau around zero bias voltage. We have reported the analysis of the IET spectra of ferromagnetic tunnel junctions previously [4,5]. The subtraction spectrum re#ected the magnon density of states of both ferromagnetic electrodes. The correlation lengths of magnons and Curie temperature of the electrode were assumed to provide the lowest and highest cut-o! energy of a spectrum. That is, the variation of the structure of the interface of the ferromagnetic electrode results in the change of the shape of the spectrum. Two peaks of the subtraction spectrum for the junction with t "40 s may be due to two distributions of magnon density of states in both interfaces of CoFe/Al-oxide/CoFe structure. Obscurity of the peak around zero-bias for the junction with t "120 s can be due to the reduction of the correlation length of magnon which comes from the over-oxidation of the bottom electrode. On the other hand, the contribution to the spectrum from zero-bias anomaly cannot be neglected. When there are ferromagnetic impurities at the interface or inside the insulator, IET spectra show a strong negative peak around zero-bias [7], so that the intensity of spectra due to magnon around zero-bias decreases. The small plateau of the spectrum for the junction with t "120 s can be due to the increase of the zero-bias anomaly. This research is supported by the Storage Research Consortium, Regional Consortium Project (NEDO) and Grant-in-Aides for Scienti"c Research from the Ministry of Education, Science, Sports and Culture of Japan.
References
Fig. 2. (a) IET spectra for the spin-valve-type junction of t "120 s. The results for AP (solid line) and P (dashed line) magnetization con"gurations are shown; (b) subtraction spectrum for the spin-valve-type junction of t "120 s.
spectra of both magnetization con"gurations was obtained [3}5] and is shown in Fig. 1(b). The relative intensity of the peak at higher voltage becomes larger in comparison with the original IET spectra. Fig. 2(a) and (b) shows the IET spectra of both magnetization con"gurations and their subtraction spectrum for the junction with t "120 s. The "ne structure of two peaks observed for the junction with t "40 s disappears and a broad peak appears at around 18 mV for these spectra. The shape of these spectra is similar to those of reported
[1] M. Oogane, N. Tezuka, T. Miyazaki, J. Magn. Soc. Jpn. 23 (1999) 1297. [2] S. Kumagai, N. Tezuka, T. Miyazaki, Jpn. J. Appl. Phys. 36 (1997) L1498. [3] J. Murai, Y. Ando, M. Kamijo, H. Kubota, T. Miyazaki, Jpn. J. Appl. Phys. 38 (1999) L1106. [4] J. Murai, Y. Ando, M. Kamijo, T. Daibou, H. Kubota, T. Miyazaki, C. Kim, O. Song, J. Magn. Soc. Jpn. 24 (2000) 615. [5] Y. Ando, J. Murai, H. Kubota, T. Miyazaki, J. Appl. Phys. 87 (2000), in press. [6] Y. Ando, H. Kubota, M. Hayashi, M. Kamijo, K. Yaoita, H.F. Han, T. Miyazaki, Jpn. J. Appl. Phys., submitted for publication. [7] J. Murai, Y. Ando, N. Tezuka, T. Miyazaki, J. Magn. Soc. Jpn. 24 (1998) 573. [8] Y. Aoki, H. Kinoshita, H. Sugawara, H. Sato, H. Tsuge, T. Mitsuzuka, A. Kamijo, Abstracts of ISFMS, 1999, p. 321. [9] J.S. Moodera, J. Nowak, R.J.M. van de Veerdonk, Phys. Rev. Lett. 80 (1998) 2941.