- Email: [email protected]
NiFe bilayers
Journal of Magnetism and Magnetic Materials 215}216 (2000) 582}584
Temperature dependence of the AMR curve in NiO/NiFe bilayers Cheol Gi Kim *, B.S. Park , D.Y. Kim, J.S. Song, B.K. Min Department of Physics, Sun Moon University, Chungnam 336-840, South Korea Telecommunication Systems Lab. 2, Hyundai Electronics Industrials Co. Ltd, Kyonggi 476-860, South Korea Division of Elect. Mater., KERI, Changwon 641-120, South Korea
Abstract The temperature dependence of the anisotropic magnetoresistance (AMR) curves was measured as a function of the angle between the applied "eld H and the exchange coupling "eld H in NiO(30 nm)/NiFe(t) (t"5, 10 and 30 nm) bilayers, and the data were compared with calculated results using the single-domain model with the crystalline anisotropy "eld as "tting parameter. The AMR curves for H)H are asymmetrical with respect to h"03 in the t"10 nm sample, and the angles for maximum resistance, h , are located at the negative "eld region for the
measurement temperature ¹"300 K. As temperature decreases, the AMR curve becomes nearly symmetric at ¹"100 K, and then again asymmetric with positive h . A comparison of these results with the calculated AMR has
suggested that the crystalline anisotropy "eld increases with decreasing temperature, but its angle decreases. 2000 Elsevier Science B.V. All rights reserved. Keywords: Anisotropic Magnetoresistance; Temperature; Crystalline anisotropy "eld; Single domain
1. Introduction Exchange coupling between an antiferromagnet (AF) and a ferromagnet (FM) is known to be a signi"cant parameter in the "eld sensitivity of magnetoresistance because the bias "eld in the AF layer causes pinning e!ects on the magnetic domain in the FM layer [1]. Because of their structural stability, AF NiO "lms have received some attention, from the standpoint of giant and anisotropic magnetoresistance (GMR, AMR), for applications to magnetoresistance (MR) heads for highdensity hard disk drive [2]. The magnetotransport measurements of Hall e!ect and AMR have been related to the surface, perpendicular or in-plane anisotropies in magnetic thin "lms [3]. The advantage of magnetotransport measurements over
* Corresponding author. Tel.: #82-418-530-2237; fax: #82418-41-7425. E-mail address: [email protected] (C.G. Kim).
more traditional measurement techniques used to determine the anisotropies is the ease of the fewer technique. More recently, MR pro"les measured in easy and hard directions have been interpreted in terms of crystalline anisotropy using a single-domain model [4,5]. In this work, we measured the temperature dependence of the AMR curves in NiO(30 nm)/NiFe(t) (t"5, 10 and 30 nm) samples and compared them with calculations using a single-domain model. It is the purpose of this paper to demonstrate the in#uence of crystalline anisotropy on AMR and to suggest a technique which measures the characteristics of crystalline anisotropy.
2. AMR curve using single-domain model Fig. 1 shows the coordinates used to describe the single-domain model (SDM) for AMR; these coordinates were introduced in a previous paper [4,5]. With these coordinates, the magnetic energy density can be expressed as the sum of the uniaxial anisotropy energy density E , the magnetostatic energy density E , and the
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 8 - 6
C.G. Kim et al. / Journal of Magnetism and Magnetic Materials 215}216 (2000) 582}584
Fig. 1. The coordinate of the single-domain model. c, h and
are the angle of crystalline easy axis, applied "eld, and magnetization of the single-domain from exchange coupling "eld, H , respectively.
exchange coupling energy density E and can be written in the following simple form: E"E #E #E
"K sin( !c)!HM cos(h! )!H M cos , (1) where M is the saturation magnetization, and K is the uniaxial crystalline anisotropy constant. In the above equation, the demagnetizing "eld is neglected because it was calculated to be less than 0.5 Oe for our sample geometry and the variations in the MR ratio were observed in the range of !1 to 1 kO. Angle determines the orientation of the magnetization whose value is in an equilibrium state with minimum total energy. The AMR varies with the angle between the directions of the measuring current and the magnetization as sin in the geometry of Fig. 1 and is given by [4] R !R , sin (H, h), AMR(H, h) (%)" , R ,
(2)
where R and R are the resistances to the measuring , , current in directions parallel and perpendicular to the magnetization, respectively. Fig. 2(a)}(d) show the calculated AMR curves from Eq. (2) for H/H "0.7. The AMR curves are normalized to the maximum AMR ratio. The symmetric AMR curves with respect to h"03 are shown for the parameters of H "0 Oe in Fig. 2(a), and of H /H "0.7 ) ) and c"03 in Fig. 2(b). Therefore, we know that if at least one of H and c is zero then the AMR curve is symmetric ) with respect to h"03. But if both anisotropy parameters di!er from zero, then the AMR curve becomes asymmetric as in Fig. 2(c) and (d). When we de"ne the angle for maximum resistance as h , it is negative for c"203 in
Fig. 2(c) and positive for c"!203 in Fig. 2(d).
583
Fig. 2. The calculated AMR curves by using the single-domain model as functions of the anisotropy "eld H , and angle c. The ) AMR curves are normalized to the maximum AMR ratio: H/H "0.7, (a) H " 0 Oe; H /H "0.7, (b) c"0, (c) 20 ) ) and (d) !20.
3. Experiment 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) (t"5, 10 and 30 nm) "lms were deposited at about 0.2 nm/s on the NiO (30 nm). A uniaxial deposition "eld of 350 Oe was applied to the plane of the "lm surface to induce the exchange coupling "eld. The AMR curves were measured by the four-probe method as a function of the angle h between the applied and the exchange "elds, where the direction of h was taken as negative for a clockwise rotation of the "eld. The measuring current was perpendicular to the exchangecoupling "eld. Measurement was performed in the temperature range between 30 and 300 K.
4. Results and discussion Fig. 3(a) shows the AMR curves for t"10 nm sample at room temperature. The AMR curves for H/H "0.7 and 1.0 are asymmetric with respect to h"03, where the exchange-coupling "eld, H , was measured as 79.5 Oe. The angles for maximum resistance, h , have negative
values, and h values are !1133 and !1343 for
H/H "0.7 and 1.0, respectively. As the applied "eld increases to H/H "1.5, the AMR curve becomes sym metric, and h is about $1303. Eventually, for
H/H "8.0, at completely saturated state, the AMR curve exhibits the typical one that agrees well with the equation of sinh [6]. The temperature dependence of AMR curves for H/H "0.7 is shown in Fig. 2(b). At ¹"300 and 200 K, h is negative, and its values are !1133 and !1083,
584
C.G. Kim et al. / Journal of Magnetism and Magnetic Materials 215}216 (2000) 582}584
Fig. 3. The AMR curves for NiO(30 nm)/NiFe(10 nm). (a) ¹"300 K and H/H "0.7, 1.0, 1.5, 8.0, (b) H/H "0.7 and ¹"300, 200, 100, 50 K.
respectively. As the temperature decreases, these asymmetric AMR curves become nearly symmetric at ¹"100 K and h is $1103. At ¹"50 K, the curve is
again asymmetric and obviously exhibits the change of sign of h as 1253.
By comparing this result with the calculated one with good "tting by using the single-domain model, as in Fig. 2, we "nd that the anisotropy parameters change with temperature. That is, for NiO(30 nm)/NiFe(10 nm) sample, the anisotropy angle, c, is positive at ¹"300 and 200 K, the same as the curve in Fig. 2(c). At 100 K the curves are nearly symmetric, the same as the curve in Fig. 2(b), and c is nearly zero. At 50 K, the sign of h changes
from negative to positive, and the sign of parameters is the same as the curve in Fig. 2(d), that is, c is negative. Fig. 4(a)}(c) show the estimated parameters of H and ) c for the good-"tting curve in NiO(30 nm)/NiFe(t), t"5, 10, 30, respectively. As a whole, H increases with de) creasing temperature, where H is maximum at ) ¹"70 K for t"5, 10 nm sample, and maximum at ¹"50 K for t"30 nm sample. The angle c monotonically decreases with decreasing temperature. Generally, in the case of thin "lms the contribution of the surface anisotropy energy to total anisotropy energy is signi"cant compared to bulk anisotropy energy [7]. The increase in anisotropy "eld with the decrease in temperature may be ascribed for the increase of surface anisotropy. In conclusion, the asymmetric AMR curve can be well described by varying the "tting parameters of the anisotropy "eld, H and angle c from exchange coupling "eld, ) in the single-domain model. The value of H increases ) with decreasing temperature, but the angle c decreases.
References [1] B. Dieny, J. Magn. Magn. Mater. 136 (1994) 335. [2] M.J. Carey, A.E. Berkowitz, Appl. Phys. Lett. 60 (1992) 3060. [3] E.D. Dahlberg, K. Riggs, G.A. Printz, J. Appl. Phys. 63 (1996) 4270. [4] D.Y. Kim, C.G. Kim, B.S. Park, D.G. Hwang, S.S. Lee, J. Appl. Phys. 85 (1999) 5783. [5] C.G. Kim, H.C. Kim, B.S. Park, D.G. Hwang, S.S. Lee, D.Y. Kim, J. Magn. Magn. Mater. 198 (1999) 33. [6] T.R. McGuire, R.I. Potter, IEEE Trans. Magn. MAG-11 (1975) 1018. [7] L.J. Maksymowicz, M. Lubecka, R. Jablonki, J. Magn. Magn. Mater. 192 (1999) 1. Fig. 4. The temperature dependence of the anisotropy "eld and angle in NiO(30 nm)/NiFe(t), where t"(a) 5 nm, (b) 10 nm and (c) 30 nm.
Temperature dependence of the AMR curve in NiO/NiFe bilayers Cheol Gi Kim *, B.S. Park , D.Y. Kim, J.S. Song, B.K. Min Department of Physics, Sun Moon University, Chungnam 336-840, South Korea Telecommunication Systems Lab. 2, Hyundai Electronics Industrials Co. Ltd, Kyonggi 476-860, South Korea Division of Elect. Mater., KERI, Changwon 641-120, South Korea
Abstract The temperature dependence of the anisotropic magnetoresistance (AMR) curves was measured as a function of the angle between the applied "eld H and the exchange coupling "eld H in NiO(30 nm)/NiFe(t) (t"5, 10 and 30 nm) bilayers, and the data were compared with calculated results using the single-domain model with the crystalline anisotropy "eld as "tting parameter. The AMR curves for H)H are asymmetrical with respect to h"03 in the t"10 nm sample, and the angles for maximum resistance, h , are located at the negative "eld region for the
measurement temperature ¹"300 K. As temperature decreases, the AMR curve becomes nearly symmetric at ¹"100 K, and then again asymmetric with positive h . A comparison of these results with the calculated AMR has
suggested that the crystalline anisotropy "eld increases with decreasing temperature, but its angle decreases. 2000 Elsevier Science B.V. All rights reserved. Keywords: Anisotropic Magnetoresistance; Temperature; Crystalline anisotropy "eld; Single domain
1. Introduction Exchange coupling between an antiferromagnet (AF) and a ferromagnet (FM) is known to be a signi"cant parameter in the "eld sensitivity of magnetoresistance because the bias "eld in the AF layer causes pinning e!ects on the magnetic domain in the FM layer [1]. Because of their structural stability, AF NiO "lms have received some attention, from the standpoint of giant and anisotropic magnetoresistance (GMR, AMR), for applications to magnetoresistance (MR) heads for highdensity hard disk drive [2]. The magnetotransport measurements of Hall e!ect and AMR have been related to the surface, perpendicular or in-plane anisotropies in magnetic thin "lms [3]. The advantage of magnetotransport measurements over
* Corresponding author. Tel.: #82-418-530-2237; fax: #82418-41-7425. E-mail address: [email protected] (C.G. Kim).
more traditional measurement techniques used to determine the anisotropies is the ease of the fewer technique. More recently, MR pro"les measured in easy and hard directions have been interpreted in terms of crystalline anisotropy using a single-domain model [4,5]. In this work, we measured the temperature dependence of the AMR curves in NiO(30 nm)/NiFe(t) (t"5, 10 and 30 nm) samples and compared them with calculations using a single-domain model. It is the purpose of this paper to demonstrate the in#uence of crystalline anisotropy on AMR and to suggest a technique which measures the characteristics of crystalline anisotropy.
2. AMR curve using single-domain model Fig. 1 shows the coordinates used to describe the single-domain model (SDM) for AMR; these coordinates were introduced in a previous paper [4,5]. With these coordinates, the magnetic energy density can be expressed as the sum of the uniaxial anisotropy energy density E , the magnetostatic energy density E , and the
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 8 - 6
C.G. Kim et al. / Journal of Magnetism and Magnetic Materials 215}216 (2000) 582}584
Fig. 1. The coordinate of the single-domain model. c, h and
are the angle of crystalline easy axis, applied "eld, and magnetization of the single-domain from exchange coupling "eld, H , respectively.
exchange coupling energy density E and can be written in the following simple form: E"E #E #E
"K sin( !c)!HM cos(h! )!H M cos , (1) where M is the saturation magnetization, and K is the uniaxial crystalline anisotropy constant. In the above equation, the demagnetizing "eld is neglected because it was calculated to be less than 0.5 Oe for our sample geometry and the variations in the MR ratio were observed in the range of !1 to 1 kO. Angle determines the orientation of the magnetization whose value is in an equilibrium state with minimum total energy. The AMR varies with the angle between the directions of the measuring current and the magnetization as sin in the geometry of Fig. 1 and is given by [4] R !R , sin (H, h), AMR(H, h) (%)" , R ,
(2)
where R and R are the resistances to the measuring , , current in directions parallel and perpendicular to the magnetization, respectively. Fig. 2(a)}(d) show the calculated AMR curves from Eq. (2) for H/H "0.7. The AMR curves are normalized to the maximum AMR ratio. The symmetric AMR curves with respect to h"03 are shown for the parameters of H "0 Oe in Fig. 2(a), and of H /H "0.7 ) ) and c"03 in Fig. 2(b). Therefore, we know that if at least one of H and c is zero then the AMR curve is symmetric ) with respect to h"03. But if both anisotropy parameters di!er from zero, then the AMR curve becomes asymmetric as in Fig. 2(c) and (d). When we de"ne the angle for maximum resistance as h , it is negative for c"203 in
Fig. 2(c) and positive for c"!203 in Fig. 2(d).
583
Fig. 2. The calculated AMR curves by using the single-domain model as functions of the anisotropy "eld H , and angle c. The ) AMR curves are normalized to the maximum AMR ratio: H/H "0.7, (a) H " 0 Oe; H /H "0.7, (b) c"0, (c) 20 ) ) and (d) !20.
3. Experiment 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) (t"5, 10 and 30 nm) "lms were deposited at about 0.2 nm/s on the NiO (30 nm). A uniaxial deposition "eld of 350 Oe was applied to the plane of the "lm surface to induce the exchange coupling "eld. The AMR curves were measured by the four-probe method as a function of the angle h between the applied and the exchange "elds, where the direction of h was taken as negative for a clockwise rotation of the "eld. The measuring current was perpendicular to the exchangecoupling "eld. Measurement was performed in the temperature range between 30 and 300 K.
4. Results and discussion Fig. 3(a) shows the AMR curves for t"10 nm sample at room temperature. The AMR curves for H/H "0.7 and 1.0 are asymmetric with respect to h"03, where the exchange-coupling "eld, H , was measured as 79.5 Oe. The angles for maximum resistance, h , have negative
values, and h values are !1133 and !1343 for
H/H "0.7 and 1.0, respectively. As the applied "eld increases to H/H "1.5, the AMR curve becomes sym metric, and h is about $1303. Eventually, for
H/H "8.0, at completely saturated state, the AMR curve exhibits the typical one that agrees well with the equation of sinh [6]. The temperature dependence of AMR curves for H/H "0.7 is shown in Fig. 2(b). At ¹"300 and 200 K, h is negative, and its values are !1133 and !1083,
584
C.G. Kim et al. / Journal of Magnetism and Magnetic Materials 215}216 (2000) 582}584
Fig. 3. The AMR curves for NiO(30 nm)/NiFe(10 nm). (a) ¹"300 K and H/H "0.7, 1.0, 1.5, 8.0, (b) H/H "0.7 and ¹"300, 200, 100, 50 K.
respectively. As the temperature decreases, these asymmetric AMR curves become nearly symmetric at ¹"100 K and h is $1103. At ¹"50 K, the curve is
again asymmetric and obviously exhibits the change of sign of h as 1253.
By comparing this result with the calculated one with good "tting by using the single-domain model, as in Fig. 2, we "nd that the anisotropy parameters change with temperature. That is, for NiO(30 nm)/NiFe(10 nm) sample, the anisotropy angle, c, is positive at ¹"300 and 200 K, the same as the curve in Fig. 2(c). At 100 K the curves are nearly symmetric, the same as the curve in Fig. 2(b), and c is nearly zero. At 50 K, the sign of h changes
from negative to positive, and the sign of parameters is the same as the curve in Fig. 2(d), that is, c is negative. Fig. 4(a)}(c) show the estimated parameters of H and ) c for the good-"tting curve in NiO(30 nm)/NiFe(t), t"5, 10, 30, respectively. As a whole, H increases with de) creasing temperature, where H is maximum at ) ¹"70 K for t"5, 10 nm sample, and maximum at ¹"50 K for t"30 nm sample. The angle c monotonically decreases with decreasing temperature. Generally, in the case of thin "lms the contribution of the surface anisotropy energy to total anisotropy energy is signi"cant compared to bulk anisotropy energy [7]. The increase in anisotropy "eld with the decrease in temperature may be ascribed for the increase of surface anisotropy. In conclusion, the asymmetric AMR curve can be well described by varying the "tting parameters of the anisotropy "eld, H and angle c from exchange coupling "eld, ) in the single-domain model. The value of H increases ) with decreasing temperature, but the angle c decreases.
References [1] B. Dieny, J. Magn. Magn. Mater. 136 (1994) 335. [2] M.J. Carey, A.E. Berkowitz, Appl. Phys. Lett. 60 (1992) 3060. [3] E.D. Dahlberg, K. Riggs, G.A. Printz, J. Appl. Phys. 63 (1996) 4270. [4] D.Y. Kim, C.G. Kim, B.S. Park, D.G. Hwang, S.S. Lee, J. Appl. Phys. 85 (1999) 5783. [5] C.G. Kim, H.C. Kim, B.S. Park, D.G. Hwang, S.S. Lee, D.Y. Kim, J. Magn. Magn. Mater. 198 (1999) 33. [6] T.R. McGuire, R.I. Potter, IEEE Trans. Magn. MAG-11 (1975) 1018. [7] L.J. Maksymowicz, M. Lubecka, R. Jablonki, J. Magn. Magn. Mater. 192 (1999) 1. Fig. 4. The temperature dependence of the anisotropy "eld and angle in NiO(30 nm)/NiFe(t), where t"(a) 5 nm, (b) 10 nm and (c) 30 nm.