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NiFe(t) bilayers
Journal of Magnetism and Magnetic Materials 215}216 (2000) 585}588
Thickness dependence of planar Hall resistance and "eld sensitivity in NiO(30 nm)/NiFe(t) bilayers D.Y. Kim , C.G. Kim *, B.S. Park, C.M. Park Telecommunication System Lab. 2, Hyundai Electronics Industrials Co. Ltd, Kyonggi 476-860, South Korea Department of Physics, Sun Moon University, Chung-nam 336-840, South Korea Department of Physics, Sangji University, Wonju 220-702, South Korea
Abstract We measured the planar Hall resistance (PHR) pro"les in NiO (30 nm)/NiFe (t) bilayers for t"5, 10, 20 and 30 nm and analyzed its "eld sensitivity in terms of exchange-coupling "eld and anisotropy constant. The measured PHR shows linear "eld dependence at near H"0 as well as small hysteresis. The linear "eld range *H and resistance change, *R"R !R , decrease with the NiFe thickness, where *H is calculated to be proportional to the anisotropy constant , , K and exchange-coupling "eld H . However, the "eld sensitivity *R/*H shows a maximum value at t"20 nm; where K is the minimum. The PHR has the advantage of a linear response at the operating "eld range and can be used for a recording read-out head and related applications. 2000 Elsevier Science B.V. All rights reserved. Keywords: AMR; PHR; Field sensitivity; Exchange-coupling "eld; Anisotropy constant
1. Introduction Recently, the magnetoresistance (MR) e!ect in thin "lms has stimulated a continuously increasing interest, and an e!ort on anisotropic magnetoresistance (AMR) and giant magnetoresistance (GMR) in multilayers has been made for magnetoresistive read-out heads [1]. A majority of the studies in the "eld are devoted to the research of multilayered structures showing the largest possible sensitivity of the resistivity for the magnetic "eld, and a large number of transition metal-based multilayered structures exhibiting large MR ratios have been found [2]. In connection with the technological problems to be solved, a number of MR sensor designs have been suggested and tested to linearize the transducer, to enhance the resolution limited by the MR ratio. The planar Hall resistance (PHR) has the advantage of a linear response in "rst order and a larger PHR ratio [3}5]. * Corresponding author. Tel.: #82-418-530-2237; fax: #82418-41-7425. E-mail address: [email protected] (C.G. Kim).
The exchange coupling between constituent layers in the NiO/NiFe bilayer is known to be a signi"cant parameter in the sensitivity of magnetoresistance because the bias "eld in the NiO layer causes pinning e!ects on the magnetic domain in the NiFe layer [6]. Because of their structural stability, AF NiO "lms have received some attention for applications to MR heads for high-density hard disk drive [7]. In the present paper, we have carried out PHR measurements in NiO(30 nm)/NiFe(t) bilayers for t"5, 10, 20 and 30 nm. The linear "eld range and the "eld sensitivity are explained in terms of the dependence of exchange-coupling "eld and anisotropy "eld on NiFe thickness.
2. Analysis of PHR and 5eld sensitivity Fig. 1 shows the coordinates used to describe the rotational magnetization process under the applied magnetic "eld in NiO/NiFe samples. H is the exchange coupling "eld due to the NiO antiferromagnetic layer, and it shows a biasing "eld e!ect. K is the e!ective
0304-8853/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 2 2 9 - 8
586
D.Y. Kim et al. / Journal of Magnetism and Magnetic Materials 215}216 (2000) 585}588
The "eld sensitivity of PHR in Eq. (3) is written as *R *
.&0 "(R !R )cos 2
. , , *H *H
(4)
Calculating * /*H from Eq. (1), and substituting into Eq. (4), the "eld sensitivity can be obtained as *R (R !R )cos 2 cos
.&0 " , , , *H H cos 2( !c)#H sin !H cos
)
Fig. 1. The coordinates for domain rotation process. Here, c and
are the angles of the anisotropy constant, and magnetization from the exchange-coupling "eld, H , respectively, and I is the measuring current.
in-plane anisotropy constant with an angle c from H . The applied magnetic "eld H is directed perpendicular to H , and forces the magnetic moment to rotate by an angle towards H. We introduce the magnetic energy density for the NiFe layer in the NiO/NiFe sample, which can be written in the following simple form E"K sin( !c)!HM sin !H M cos , (1) where M is the saturation magnetization. In Eq. (1), the demagnetizing "eld is neglected because it was calculated to be less than 0.5 Oe for our sample geometry and the "eld variations in the PHR were measured in the range of !1.25 to 1.25 kOe. The angle determines the orientation of the magnetization in an equilibrium state with minimum total energy, whose values are calculated under the conditions of *E/* "0. The MR and PHR in conductive ferromagnetic "lms are directly related to the AMR e!ect, which is expressed as [5] R "R #(R !R ) sin , (2) +0 , , , R "(R !R )sin cos , (3) .&0 , , where R and R are the resistances of the magnetoresis, , tive elements parallel and perpendicular to the magnetization vector, respectively, and is the angle of the rotation of the magnetization vector with respect to the exchange-coupling "eld. The MR and PHR in Eqs. (2) and (3) vary with the angle . The MR has an o!set resistance R , while PHR does not impose the o!set , resistance. Therefore, the PHR has the advantage of a large PHR ratio and linear response characteristics with angle having a small value.
(5)
where H "2K /M is the anisotropy "eld. In Eq. (5), ) the "eld sensitivity depends not only on the intrinsic parameters (R !R ), H , and H , but also on extrinsic , , ) parameters such as the angle and the applied magnetic "eld H.
3. Experimental The NiO "lms were deposited at a rate of 0.5 nm/min on Corning glass 7059 by RF-magnetron sputtering by using a 3-in-diameter sintered NiO target at an Ar partial pressure of 5.0;10\ Torr without introducing oxygen gas. NiFe(t) "lms for t"5, 10, 20 and 30 nm were deposited at about 0.2 nm/s on NiO(30 nm). A uniaxial deposition "eld of 350 Oe was applied in the plane of the "lm surface to induce the exchange-coupling "eld. The PHR was measured by the four-probe method as a function of the magnetic "eld applied by the electromagnets in the range of !1.25 to 1.25 kOe in the direction of exchange-coupling "eld. The measuring current was perpendicular to the exchange-coupling "eld.
4. Results and discussion Fig. 2(a) shows the measured PHR pro"les in NiO(30 nm)/NiFe(t) for t"5, 10, 20 and 30 nm samples. The PHR pro"les show a linear "eld dependence near H"0 as well as a small magnetic hysteresis. The linear region can be used for a recording read-out head and related applications. The "eld range *H of linear response between minimum and maximum resistances, and *R"R !R decrease with NiFe thickness. From the , , measured PHR pro"les in Fig. 2(a) and the measured MR pro"les in Ref. [7], we obtain the material parameters of *H, *R, H , H , and c, which are summarized ) in Table 1. We can obtain *H from the intervals between the minimum and maximum resistances in Fig. 2(a), where the "eld sensitivity is zero and the magnetization angle
is 453C. These two conditions are substituted into Eq. (5), and *H is obtained as follows: *H"2(H #(2H sin 2c), )
(6)
D.Y. Kim et al. / Journal of Magnetism and Magnetic Materials 215}216 (2000) 585}588
587
Table 1 Material parameters for *H, *R, *R/*H, H , and H ) t (nm)
*H (Oe) *R (X)
H (Oe) H (Oe) c (deg.) )
5 10 20 30
450 228 60 63
192 94 28 27
0.992 0.570 0.232 0.195
96 35 22 33
41 28 12 60
Fig. 3. The "eld sensitivity versus NiFe thickness. The squares and circles are the measured and calculated "eld sensitivities, respectively. Fig. 2. The measured (a) and calculated (b) PHR pro"les in NiO(30 nm)/NiFe(t) for t"5, 10, 20, and 30 nm, respectively.
where *H depends on H , H and c. However, H is ) larger than (2H sin 2c as shown in Table 1, therefore, ) the "eld range of linear response is dominantly dependent on the exchange-coupling "eld in the NiO/NiFe bilayer. Fig. 2(b) shows the calculated PHR during a half-cycle of magnetization using Eqs. (1) and (3) on the basis of the material parameters given in Table 1. The calculated results are in good agreement with the measured ones, demonstrating the soundness of the PHR analysis. Therefore, the "eld sensitivity is approximately expressed as *R (R !R ) , , " . *H 2(H #(2H sin 2c) )
(7)
The variation of the "eld sensitivity *R/*H with NiFe thickness is shown in Fig. 3, where the square and circle symbols represent the measured and calculated results,
respectively. The calculated results, which is obtained by Eq. (7) using the material parameters given in Table 1, is larger than the measured one, however, the trend of thickness dependence is similar. The "eld sensitivity in Fig. 3 shows a maximum value at t"20 nm. *R and H decrease with NiFe thickness, but the anisotropy "eld shows a minimum value at t"20 nm in Table 1. Thus, the "eld sensitivity dominantly depends on the anisotropy "eld and angle c. In conclusion, the PHR has the advantage of the linear response at operating "eld range and small hysteresis, which can be used for a magnetic "eld sensor and other related applications. The "eld sensitivity *R/*H of PHR shows a maximum value at t"20 nm in NiO/NiFe bilayer, and it dominantly depends on the anisotropy "eld characteristics.
Acknowledgements This work was supported by the Korea Research Foundation under Grant No. KRF-99-042-E00106E5105.
588
D.Y. Kim et al. / Journal of Magnetism and Magnetic Materials 215}216 (2000) 585}588
References [1] C.H. Tsang, R.E. Fontana Jr., T. Lin, D.E. Heim, B.A. Gurney, M.L. Williams, IBM J. Res. Dev. 42 (1998) 104. [2] B. Dieny, J. Magn. Magn. Mater. 136 (1994) 335. [3] F.G. West, J. Appl. Phys. 34 (1963) 1171.
[4] T.R. Mcguire, R.I. Potter, IEEE Trans Mag 11 (1975) 1018. [5] J.H. Fluitman, J. Appl. Phys. 52 (1981) 2468. [6] M.J. Carey, A.E. Berkowitz, Appl. Phys. Lett. 60 (1992) 3060. [7] D.Y. Kim, C.G. Kim, B.S. Park, D.G. Hwang, S.S. Lee, J. Appl. Phys. 85 (1999) 5783.
Thickness dependence of planar Hall resistance and "eld sensitivity in NiO(30 nm)/NiFe(t) bilayers D.Y. Kim , C.G. Kim *, B.S. Park, C.M. Park Telecommunication System Lab. 2, Hyundai Electronics Industrials Co. Ltd, Kyonggi 476-860, South Korea Department of Physics, Sun Moon University, Chung-nam 336-840, South Korea Department of Physics, Sangji University, Wonju 220-702, South Korea
Abstract We measured the planar Hall resistance (PHR) pro"les in NiO (30 nm)/NiFe (t) bilayers for t"5, 10, 20 and 30 nm and analyzed its "eld sensitivity in terms of exchange-coupling "eld and anisotropy constant. The measured PHR shows linear "eld dependence at near H"0 as well as small hysteresis. The linear "eld range *H and resistance change, *R"R !R , decrease with the NiFe thickness, where *H is calculated to be proportional to the anisotropy constant , , K and exchange-coupling "eld H . However, the "eld sensitivity *R/*H shows a maximum value at t"20 nm; where K is the minimum. The PHR has the advantage of a linear response at the operating "eld range and can be used for a recording read-out head and related applications. 2000 Elsevier Science B.V. All rights reserved. Keywords: AMR; PHR; Field sensitivity; Exchange-coupling "eld; Anisotropy constant
1. Introduction Recently, the magnetoresistance (MR) e!ect in thin "lms has stimulated a continuously increasing interest, and an e!ort on anisotropic magnetoresistance (AMR) and giant magnetoresistance (GMR) in multilayers has been made for magnetoresistive read-out heads [1]. A majority of the studies in the "eld are devoted to the research of multilayered structures showing the largest possible sensitivity of the resistivity for the magnetic "eld, and a large number of transition metal-based multilayered structures exhibiting large MR ratios have been found [2]. In connection with the technological problems to be solved, a number of MR sensor designs have been suggested and tested to linearize the transducer, to enhance the resolution limited by the MR ratio. The planar Hall resistance (PHR) has the advantage of a linear response in "rst order and a larger PHR ratio [3}5]. * Corresponding author. Tel.: #82-418-530-2237; fax: #82418-41-7425. E-mail address: [email protected] (C.G. Kim).
The exchange coupling between constituent layers in the NiO/NiFe bilayer is known to be a signi"cant parameter in the sensitivity of magnetoresistance because the bias "eld in the NiO layer causes pinning e!ects on the magnetic domain in the NiFe layer [6]. Because of their structural stability, AF NiO "lms have received some attention for applications to MR heads for high-density hard disk drive [7]. In the present paper, we have carried out PHR measurements in NiO(30 nm)/NiFe(t) bilayers for t"5, 10, 20 and 30 nm. The linear "eld range and the "eld sensitivity are explained in terms of the dependence of exchange-coupling "eld and anisotropy "eld on NiFe thickness.
2. Analysis of PHR and 5eld sensitivity Fig. 1 shows the coordinates used to describe the rotational magnetization process under the applied magnetic "eld in NiO/NiFe samples. H is the exchange coupling "eld due to the NiO antiferromagnetic layer, and it shows a biasing "eld e!ect. K is the e!ective
0304-8853/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 2 2 9 - 8
586
D.Y. Kim et al. / Journal of Magnetism and Magnetic Materials 215}216 (2000) 585}588
The "eld sensitivity of PHR in Eq. (3) is written as *R *
.&0 "(R !R )cos 2
. , , *H *H
(4)
Calculating * /*H from Eq. (1), and substituting into Eq. (4), the "eld sensitivity can be obtained as *R (R !R )cos 2 cos
.&0 " , , , *H H cos 2( !c)#H sin !H cos
)
Fig. 1. The coordinates for domain rotation process. Here, c and
are the angles of the anisotropy constant, and magnetization from the exchange-coupling "eld, H , respectively, and I is the measuring current.
in-plane anisotropy constant with an angle c from H . The applied magnetic "eld H is directed perpendicular to H , and forces the magnetic moment to rotate by an angle towards H. We introduce the magnetic energy density for the NiFe layer in the NiO/NiFe sample, which can be written in the following simple form E"K sin( !c)!HM sin !H M cos , (1) where M is the saturation magnetization. In Eq. (1), the demagnetizing "eld is neglected because it was calculated to be less than 0.5 Oe for our sample geometry and the "eld variations in the PHR were measured in the range of !1.25 to 1.25 kOe. The angle determines the orientation of the magnetization in an equilibrium state with minimum total energy, whose values are calculated under the conditions of *E/* "0. The MR and PHR in conductive ferromagnetic "lms are directly related to the AMR e!ect, which is expressed as [5] R "R #(R !R ) sin , (2) +0 , , , R "(R !R )sin cos , (3) .&0 , , where R and R are the resistances of the magnetoresis, , tive elements parallel and perpendicular to the magnetization vector, respectively, and is the angle of the rotation of the magnetization vector with respect to the exchange-coupling "eld. The MR and PHR in Eqs. (2) and (3) vary with the angle . The MR has an o!set resistance R , while PHR does not impose the o!set , resistance. Therefore, the PHR has the advantage of a large PHR ratio and linear response characteristics with angle having a small value.
(5)
where H "2K /M is the anisotropy "eld. In Eq. (5), ) the "eld sensitivity depends not only on the intrinsic parameters (R !R ), H , and H , but also on extrinsic , , ) parameters such as the angle and the applied magnetic "eld H.
3. Experimental The NiO "lms were deposited at a rate of 0.5 nm/min on Corning glass 7059 by RF-magnetron sputtering by using a 3-in-diameter sintered NiO target at an Ar partial pressure of 5.0;10\ Torr without introducing oxygen gas. NiFe(t) "lms for t"5, 10, 20 and 30 nm were deposited at about 0.2 nm/s on NiO(30 nm). A uniaxial deposition "eld of 350 Oe was applied in the plane of the "lm surface to induce the exchange-coupling "eld. The PHR was measured by the four-probe method as a function of the magnetic "eld applied by the electromagnets in the range of !1.25 to 1.25 kOe in the direction of exchange-coupling "eld. The measuring current was perpendicular to the exchange-coupling "eld.
4. Results and discussion Fig. 2(a) shows the measured PHR pro"les in NiO(30 nm)/NiFe(t) for t"5, 10, 20 and 30 nm samples. The PHR pro"les show a linear "eld dependence near H"0 as well as a small magnetic hysteresis. The linear region can be used for a recording read-out head and related applications. The "eld range *H of linear response between minimum and maximum resistances, and *R"R !R decrease with NiFe thickness. From the , , measured PHR pro"les in Fig. 2(a) and the measured MR pro"les in Ref. [7], we obtain the material parameters of *H, *R, H , H , and c, which are summarized ) in Table 1. We can obtain *H from the intervals between the minimum and maximum resistances in Fig. 2(a), where the "eld sensitivity is zero and the magnetization angle
is 453C. These two conditions are substituted into Eq. (5), and *H is obtained as follows: *H"2(H #(2H sin 2c), )
(6)
D.Y. Kim et al. / Journal of Magnetism and Magnetic Materials 215}216 (2000) 585}588
587
Table 1 Material parameters for *H, *R, *R/*H, H , and H ) t (nm)
*H (Oe) *R (X)
H (Oe) H (Oe) c (deg.) )
5 10 20 30
450 228 60 63
192 94 28 27
0.992 0.570 0.232 0.195
96 35 22 33
41 28 12 60
Fig. 3. The "eld sensitivity versus NiFe thickness. The squares and circles are the measured and calculated "eld sensitivities, respectively. Fig. 2. The measured (a) and calculated (b) PHR pro"les in NiO(30 nm)/NiFe(t) for t"5, 10, 20, and 30 nm, respectively.
where *H depends on H , H and c. However, H is ) larger than (2H sin 2c as shown in Table 1, therefore, ) the "eld range of linear response is dominantly dependent on the exchange-coupling "eld in the NiO/NiFe bilayer. Fig. 2(b) shows the calculated PHR during a half-cycle of magnetization using Eqs. (1) and (3) on the basis of the material parameters given in Table 1. The calculated results are in good agreement with the measured ones, demonstrating the soundness of the PHR analysis. Therefore, the "eld sensitivity is approximately expressed as *R (R !R ) , , " . *H 2(H #(2H sin 2c) )
(7)
The variation of the "eld sensitivity *R/*H with NiFe thickness is shown in Fig. 3, where the square and circle symbols represent the measured and calculated results,
respectively. The calculated results, which is obtained by Eq. (7) using the material parameters given in Table 1, is larger than the measured one, however, the trend of thickness dependence is similar. The "eld sensitivity in Fig. 3 shows a maximum value at t"20 nm. *R and H decrease with NiFe thickness, but the anisotropy "eld shows a minimum value at t"20 nm in Table 1. Thus, the "eld sensitivity dominantly depends on the anisotropy "eld and angle c. In conclusion, the PHR has the advantage of the linear response at operating "eld range and small hysteresis, which can be used for a magnetic "eld sensor and other related applications. The "eld sensitivity *R/*H of PHR shows a maximum value at t"20 nm in NiO/NiFe bilayer, and it dominantly depends on the anisotropy "eld characteristics.
Acknowledgements This work was supported by the Korea Research Foundation under Grant No. KRF-99-042-E00106E5105.
588
D.Y. Kim et al. / Journal of Magnetism and Magnetic Materials 215}216 (2000) 585}588
References [1] C.H. Tsang, R.E. Fontana Jr., T. Lin, D.E. Heim, B.A. Gurney, M.L. Williams, IBM J. Res. Dev. 42 (1998) 104. [2] B. Dieny, J. Magn. Magn. Mater. 136 (1994) 335. [3] F.G. West, J. Appl. Phys. 34 (1963) 1171.
[4] T.R. Mcguire, R.I. Potter, IEEE Trans Mag 11 (1975) 1018. [5] J.H. Fluitman, J. Appl. Phys. 52 (1981) 2468. [6] M.J. Carey, A.E. Berkowitz, Appl. Phys. Lett. 60 (1992) 3060. [7] D.Y. Kim, C.G. Kim, B.S. Park, D.G. Hwang, S.S. Lee, J. Appl. Phys. 85 (1999) 5783.