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FM bilayer films
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) e849–e851
Rotational relaxation of pinning field in AFM/FM bilayer films Taras Pokhila,*, Roy Chantrellb, Chunhong Houa, Eric Singletona a
Seagate Technology, RHO, 7801 Computer Avenue South, Minneapolis, MN 55435, USA b Seagate Research, Pittsburgh, PA, USA
Abstract Rotational relaxation of the pinning field was studied in PtMn/CoFe and IrMn/CoFe exchange coupled bilayer films using a vector vibrating sample magnetometer (VVSM). The measurement allows determination of the pinning field direction and magnitude as well as the easy axis direction. The experimental results can be explained using the model of thermally activated switching of AFM grains taking into account the distribution of the activation energies of the grains, in-plane exchange in the FM layer, and the difference in the origin of the uniaxial anisotropy. r 2004 Elsevier B.V. All rights reserved. PACS: 75.60.Nt; 75.50.Ee Keywords: Thin film; Exchange coupling; Magnetic relaxation
Samples of PtMn/CoFe and IrMn/CoFe bilayer films were deposited on oxidized Si(0 0 1) substrates using DC magnetron sputtering. The samples were annealed in a magnetic field to maximize the exchange field. The pinning field rotational relaxation was measured using a vector vibrating sample magnetometer (VVSM) (Digital Measurement Systems VSM model 10). The VVSM simultaneously measures two components of the magnetic moment. Examples of hysteresis loops of two components of magnetization in IrMn/CoFe sample are shown in Fig. 1. The first loop (Fig. 1(a)) was measured in the sample in which the pinning field was parallel to the uniaxial anisotropy axis. If the hysteresis loop measurement field is applied parallel to the easy axis and pinning field, then the component of magnetization transverse to the applied field remains close to zero within the entire measurement range. This happens because nothing breaks the symmetry of the magnetization reversal, and, therefore, equal parts of sample magnetic moment rotate clockwise and counterclockwise (see spin diagram in Fig. 1(a)). If the easy axis makes an angle with pinning field (Fig. 1(b)), then the *Corresponding author. E-mail address: taras g [email protected] (T. Pokhil).
component of magnetization transverse to the applied field peaks in opposite directions when the applied field is changed from plus to minus and back, because the largest part of the moment rotates through the closest direction along the easy axis. Finally, if the field is applied parallel to the easy axis and at an angle to the pinning direction (Fig. 1(c)), almost the entire moment rotates through the same direction (pinning direction) during positive and negative field excursions. The rotational pinning field relaxation measurement was done in the following way. The AFM/FM samples were annealed at elevated temperature in the VVSM in a 10 kOe field applied perpendicular to the initial pinning direction for a certain time. The field was sufficiently strong to saturate the sample in the field direction. Then the temperature was reduced to room temperature. The new pinning direction was found by adjusting the measurement field direction so that the component of magnetization transverse to the field remained zero or zero on average within the measurement field limits. The magnitude of the pinning was measured in this direction. Then annealing and measurement steps were repeated. This procedure was used to investigate the dependence of the pinning field direction and magnitude on the annealing time. After a certain time of annealing in a
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.177
ARTICLE IN PRESS T. Pokhil et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e849–e851
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Fig. 1. Magnetization vector hysteresis loops of 100 A IrMn/50 A CoFe films with tree different relative orientations of easy axis, pinning field and applied field.
380 40 340 20
300 260
0 0
500
1000
1500
2000
Anneal time, min.
Pin. Field Direction, deg.
60
Field direction changed from perpendicular to parallel to the initial pinning direction
80
420
170
60
150
40
130
20
110
Pinning Field, Oe.
External field turned OFF
Pinning Field, Oe.
Pin. Field Direction, deg.
80
90
0
0
500
1000
1500
2000
Anneal time, min.
Fig. 2. The pinning field magnitude (open symbols) and direction (solid symbols) as a function of annealing time at 150 C (triangles) and 180 C (squares) in 10 kOe field applied perpendicularly to the initial pinning direction in 200 A PtMn/ 40 A CoFe film.
Fig. 3. The pinning field magnitude (open symbols) and direction (solid symbols) as a function of annealing time at 100 C and 150 C in 10 kOe field applied perpendicularly to the initial pinning direction in 100 A IrMn/50 A CoFe film. Arrows show the time at which the external field direction was changed from perpendicular to parallel to the initial pinning direction.
field of 10 kOe applied normally to the initial pinning, the samples were further annealed in zero field or in 10 kOe field applied in the initial pinning direction. Figs. 2 and 3 show the dependence of pinning field direction and magnitude in PtMn/CoFe and IrMn/CoFe on annealing time at various temperatures. During the field annealing the pinning field direction rotated away from the initial direction towards the direction of the field applied during annealing, and the magnitude of the pinning decreased. The pinning field rotation and decrease in magnitude was faster at higher temperatures. During further annealing in zero field (Fig. 2), the
pinning field rotated back to the initial direction and regained its initial value. Vector hysteresis loops show that both unidirectional and uniaxial anisotropies rotated in the PtMn/CoFe sample. In IrMn/CoFe samples only unidirectional anisotropy rotated, while the uniaxial anisotropy remained at its initial orientation. This results from the different origin of the induced uniaxial anisotropy in the samples. In the PtMn/CoFe sample the induced uniaxial anisotropy is an interfacial anisotropy [1], or anisotropy of the AFM grains [2–6], and therefore it rotates with the pinning field. In the IrMn/CoFe sample the uniaxial anisotropy is a bulk
ARTICLE IN PRESS T. Pokhil et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e849–e851
(structural) anisotropy in CoFe layer induced by its growth on IrMn or by the seed layer underlying IrMn. The experimental results have been explained using the model of thermally activated switching of AFM grains. The model, which will be described separately [6] takes into account the distribution of the activation energies of the grains, magnetostatic and exchange interactions within the FM layer, exchange coupling within the AFM layer and the exchange coupling between FM and AFM layers. The easy axis direction of AFM grains is isotropically distributed in the plane of the film. The initial annealing of the sample sets the spin structure of the AFM grains in such a way that the average of the ‘‘individual pinning direction’’ of the grains points in the direction of the field applied during the anneal. During annealing in the field, applied perpendicular to the initial pinning direction, the spin structure in some of the AFM grains reverse due to the torque applied to the grains by the saturated FM layer. This results in a shift and widening of the distribution of ‘‘individual pinning directions’’ of the grains. The shift leads to rotation of the average pinning direction, and widening of the distribution decreases the magnitude of the pinning field. After a certain annealing time the distribution width starts to decrease, because of the continuing increase of the number of switched AFM grains. This slightly increases the overall pinning. This
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process is slow because it involves switching of more stable AFM grains (grains with higher activation energy). The effect shows as a shallow minimum in the left part of the dependencies of pinning field on annealing time in Figs. 2 and 3 (180 C curve in Fig. 2, and 150 C curve in Fig. 3). During further annealing in zero field the switched AFM grains start switching back. This occurs because the most stable AFM grains retained their initial orientation during the field anneal. The less stable grains that switched during the field anneal are now forced to switch back by the exchange interaction with the unswitched (stable) grains through the FM layer.
References [1] T.C. Schulthess, W.H. Butler, J. Appl. Phys. 85 (1999) 5510. [2] E. Fulcomer, S.H. Charap, J. Appl. Phys. 43 (1972) 4190. [3] C. Hou, H. Fujiwara, F. Ueda, J. Magn. Magn. Mater. 198–199 (1999) 450. [4] M.D. Stiles, R.D. McMichael, Phys. Rev. B 59 (1998) 3722. [5] M.D. Stiles, R.D. McMichael, Phys. Rev. B 60 (1999) 12950. [6] T. Pokhil, R.W. Chantrell, C. Hou, E. Singleton, in preparation.
Journal of Magnetism and Magnetic Materials 272–276 (2004) e849–e851
Rotational relaxation of pinning field in AFM/FM bilayer films Taras Pokhila,*, Roy Chantrellb, Chunhong Houa, Eric Singletona a
Seagate Technology, RHO, 7801 Computer Avenue South, Minneapolis, MN 55435, USA b Seagate Research, Pittsburgh, PA, USA
Abstract Rotational relaxation of the pinning field was studied in PtMn/CoFe and IrMn/CoFe exchange coupled bilayer films using a vector vibrating sample magnetometer (VVSM). The measurement allows determination of the pinning field direction and magnitude as well as the easy axis direction. The experimental results can be explained using the model of thermally activated switching of AFM grains taking into account the distribution of the activation energies of the grains, in-plane exchange in the FM layer, and the difference in the origin of the uniaxial anisotropy. r 2004 Elsevier B.V. All rights reserved. PACS: 75.60.Nt; 75.50.Ee Keywords: Thin film; Exchange coupling; Magnetic relaxation
Samples of PtMn/CoFe and IrMn/CoFe bilayer films were deposited on oxidized Si(0 0 1) substrates using DC magnetron sputtering. The samples were annealed in a magnetic field to maximize the exchange field. The pinning field rotational relaxation was measured using a vector vibrating sample magnetometer (VVSM) (Digital Measurement Systems VSM model 10). The VVSM simultaneously measures two components of the magnetic moment. Examples of hysteresis loops of two components of magnetization in IrMn/CoFe sample are shown in Fig. 1. The first loop (Fig. 1(a)) was measured in the sample in which the pinning field was parallel to the uniaxial anisotropy axis. If the hysteresis loop measurement field is applied parallel to the easy axis and pinning field, then the component of magnetization transverse to the applied field remains close to zero within the entire measurement range. This happens because nothing breaks the symmetry of the magnetization reversal, and, therefore, equal parts of sample magnetic moment rotate clockwise and counterclockwise (see spin diagram in Fig. 1(a)). If the easy axis makes an angle with pinning field (Fig. 1(b)), then the *Corresponding author. E-mail address: taras g [email protected] (T. Pokhil).
component of magnetization transverse to the applied field peaks in opposite directions when the applied field is changed from plus to minus and back, because the largest part of the moment rotates through the closest direction along the easy axis. Finally, if the field is applied parallel to the easy axis and at an angle to the pinning direction (Fig. 1(c)), almost the entire moment rotates through the same direction (pinning direction) during positive and negative field excursions. The rotational pinning field relaxation measurement was done in the following way. The AFM/FM samples were annealed at elevated temperature in the VVSM in a 10 kOe field applied perpendicular to the initial pinning direction for a certain time. The field was sufficiently strong to saturate the sample in the field direction. Then the temperature was reduced to room temperature. The new pinning direction was found by adjusting the measurement field direction so that the component of magnetization transverse to the field remained zero or zero on average within the measurement field limits. The magnitude of the pinning was measured in this direction. Then annealing and measurement steps were repeated. This procedure was used to investigate the dependence of the pinning field direction and magnitude on the annealing time. After a certain time of annealing in a
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.177
ARTICLE IN PRESS T. Pokhil et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e849–e851
e850
Fig. 1. Magnetization vector hysteresis loops of 100 A IrMn/50 A CoFe films with tree different relative orientations of easy axis, pinning field and applied field.
380 40 340 20
300 260
0 0
500
1000
1500
2000
Anneal time, min.
Pin. Field Direction, deg.
60
Field direction changed from perpendicular to parallel to the initial pinning direction
80
420
170
60
150
40
130
20
110
Pinning Field, Oe.
External field turned OFF
Pinning Field, Oe.
Pin. Field Direction, deg.
80
90
0
0
500
1000
1500
2000
Anneal time, min.
Fig. 2. The pinning field magnitude (open symbols) and direction (solid symbols) as a function of annealing time at 150 C (triangles) and 180 C (squares) in 10 kOe field applied perpendicularly to the initial pinning direction in 200 A PtMn/ 40 A CoFe film.
Fig. 3. The pinning field magnitude (open symbols) and direction (solid symbols) as a function of annealing time at 100 C and 150 C in 10 kOe field applied perpendicularly to the initial pinning direction in 100 A IrMn/50 A CoFe film. Arrows show the time at which the external field direction was changed from perpendicular to parallel to the initial pinning direction.
field of 10 kOe applied normally to the initial pinning, the samples were further annealed in zero field or in 10 kOe field applied in the initial pinning direction. Figs. 2 and 3 show the dependence of pinning field direction and magnitude in PtMn/CoFe and IrMn/CoFe on annealing time at various temperatures. During the field annealing the pinning field direction rotated away from the initial direction towards the direction of the field applied during annealing, and the magnitude of the pinning decreased. The pinning field rotation and decrease in magnitude was faster at higher temperatures. During further annealing in zero field (Fig. 2), the
pinning field rotated back to the initial direction and regained its initial value. Vector hysteresis loops show that both unidirectional and uniaxial anisotropies rotated in the PtMn/CoFe sample. In IrMn/CoFe samples only unidirectional anisotropy rotated, while the uniaxial anisotropy remained at its initial orientation. This results from the different origin of the induced uniaxial anisotropy in the samples. In the PtMn/CoFe sample the induced uniaxial anisotropy is an interfacial anisotropy [1], or anisotropy of the AFM grains [2–6], and therefore it rotates with the pinning field. In the IrMn/CoFe sample the uniaxial anisotropy is a bulk
ARTICLE IN PRESS T. Pokhil et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e849–e851
(structural) anisotropy in CoFe layer induced by its growth on IrMn or by the seed layer underlying IrMn. The experimental results have been explained using the model of thermally activated switching of AFM grains. The model, which will be described separately [6] takes into account the distribution of the activation energies of the grains, magnetostatic and exchange interactions within the FM layer, exchange coupling within the AFM layer and the exchange coupling between FM and AFM layers. The easy axis direction of AFM grains is isotropically distributed in the plane of the film. The initial annealing of the sample sets the spin structure of the AFM grains in such a way that the average of the ‘‘individual pinning direction’’ of the grains points in the direction of the field applied during the anneal. During annealing in the field, applied perpendicular to the initial pinning direction, the spin structure in some of the AFM grains reverse due to the torque applied to the grains by the saturated FM layer. This results in a shift and widening of the distribution of ‘‘individual pinning directions’’ of the grains. The shift leads to rotation of the average pinning direction, and widening of the distribution decreases the magnitude of the pinning field. After a certain annealing time the distribution width starts to decrease, because of the continuing increase of the number of switched AFM grains. This slightly increases the overall pinning. This
e851
process is slow because it involves switching of more stable AFM grains (grains with higher activation energy). The effect shows as a shallow minimum in the left part of the dependencies of pinning field on annealing time in Figs. 2 and 3 (180 C curve in Fig. 2, and 150 C curve in Fig. 3). During further annealing in zero field the switched AFM grains start switching back. This occurs because the most stable AFM grains retained their initial orientation during the field anneal. The less stable grains that switched during the field anneal are now forced to switch back by the exchange interaction with the unswitched (stable) grains through the FM layer.
References [1] T.C. Schulthess, W.H. Butler, J. Appl. Phys. 85 (1999) 5510. [2] E. Fulcomer, S.H. Charap, J. Appl. Phys. 43 (1972) 4190. [3] C. Hou, H. Fujiwara, F. Ueda, J. Magn. Magn. Mater. 198–199 (1999) 450. [4] M.D. Stiles, R.D. McMichael, Phys. Rev. B 59 (1998) 3722. [5] M.D. Stiles, R.D. McMichael, Phys. Rev. B 60 (1999) 12950. [6] T. Pokhil, R.W. Chantrell, C. Hou, E. Singleton, in preparation.