- Email: [email protected]
Cr multilayers
Journal of Magnetism and Magnetic Materials 126 (1993) 400-402 North-Holland
Oscillatory interlayer exchange coupling in Ni80Fe20/Cr multilayers Peng Chubing, Dai Chen and Dai Daosheng Department of Physics, Peking Unicersity, Beijing 100871, China
Evidence of antiferromagnetic interlayer exchange coupling in Nisc~Fe2i)/Cr multilayers is reported. In Nisc~Fe~,/('r multilayers, both the magnitude of the interlayer magnetic exchange coupling and the saturation magnetoresistance have been found to oscillate with the Cr spacer layer thickness with a period of about 13 ,~. FMR spectra revealed a weak resonance mode above the regular in-phase mode for the antiferromagnetically coupled films, and only some lower-field resonance modes for ferromagnetically coupled films.
1. Introduction Recently, it was discovered that neighboring iron slabs can couple antiferromagnetically across chromium spacer layers in F e / C r superlattices [1]. Subsequently, a giant saturation magnetoresistance, which depends strongly on the relative orientation of magnetization between the successive 3d ferromagnetic slabs, has been observed in such antiferromagnetic (AF) films [2]. Oscillatory antiferromagnetic interlayer exchange coupling, manifested by the oscillation in the saturation magnetoresistance as a function of the paramagnetic transition spacer layer thickness, was reported by Parkin et al. in several metallic superlattice structures [3]. Motivated by these fascinating properties of interlayer exchange coupling in magnetic multilayer structures, we have prepared a large set of [Ni~0Fe20 (25 , ~ ) / C r ( t "~)]N (tc~ = 5 -- 50, N = 40-20) multilayer structures by electron beam deposition techniques under a vacuum pressure of 3 × 10 7 Torr onto heated glass substrates at 70°C (discussed in ref. [4]), and studied their magnetic and electric properties systematically. Oscillations in the saturation magnetoresistance were also observed, although the magnitude of the magnetoresistance was much smaller than that of F e / C r multilayer structures [5].
MR at 100 K is about twice as high as that at room temperature. From fig. 1 it can be seen that the MR does not change monotonically with the increasing Cr spacer layer thickness. As t~, r is increased beyond 5 ,~, the M R decreases to a very small value for to, ,. = 7-10 •~. As to, r is increased further, the M R becomes larger and reaches its maximum value at to,. = 12 A; it then decreases with further increases in /c'r, and reaches a minimum value at tc:~ = 16 ,~. A further oscillation in the M R is observed for even larger values of t¢,~ with peak values at tc~ = 25 and 38 ,~, but the oscillation in the magnitude is dampened. The oscillation period (about 13 ,~) in p e r m a l l o y / C r multilayers is approximately the same as that for C o / R u multilayers, but is significantly smaller than that for F e / C r multilayers [5]. The smaller oscillation period may partly result from the interdiffusion at p e r m a l l o y / C r interfaces.
l
I
I
I
!
I
i
I
t
I
I
I
l
'~
i
12 q-o,,.--I
2. Results and discussions Fig. 1 shows the saturation magnetoresistance M R ( H = 0)) as a function of the Cr layer thickness (tc~) with a fixed permalloy layers of 25 A at room temperature. The M R versus tcr curve at 100 K is similar to that at room temperature, except that the
l
( = AR/R
Correspondence to." Dr Peng Chubing, Department of Physics,
Peking University, Beijing 100871, China. 0304-8853/93/$06.00 © 1993
10
20
30
|
l
4O
thickness of Cr spacer layer
50
(A)
Fig. 1. Saturation magnetoresistance MR as a function of Cr spacer layer thickness with fixed Nis~jFez~~ layers of 25 f,, al room temperature. The data were taken with the sensing current and the magnetic field in the film plane and arranged orthogonal to one another.
Elsevier Science Publishers B.V. (North-Hollandl
401
Peng Chubing et al. / Exchange coupling in NiFe / Cr
7A Cr
12A Cr
9°
9i7
8.0
A
t___.~ -10
O
10
-10
O
10
magnetic field (Oe) Fig. 2. Examples of hysteresis loops are shown for four representative Nis0Fe20/Cr multilayers at room temperature. The applied magnetic field is in the film plane.
Since Cr is soluble in permalloy, it forms a homogeneous solid solution with an fcc structure in which the Curie point is lowered for small Cr concentrations. At concentrations greater than 10% the Curie point is below room temperature. The interfacial interdiffusion therefore results in a substantial increase in the magnetic separation of the permalloy films for a nominal Cr interlayer deposition thickness. Fig. 2 shows examples of hysteresis loops with fields in the film plane. It is seen that the hysteresis loops for tcr = 12 and 25 ~, (right panels in fig. 2) are 'pinched', and have greatly reduced remanent moment values and large saturation fields Hsat (about 30 Oe), which, in general, is similar in appearance to those of AF coupled films reported in the literature [5] and therefore, regard the samples in the right panels of fig. 2 as being AF cout~led. However, the hysteresis loops for tCr = 7 and 20 A (left panels in fig. 2) are squared with a large remanent magnetization and small H~at (about 5 0 e ) , indicating that the neighboring permalloy layers are ferromagnetically (F) coupled. Furthermore, one might expect to observe two absorption lines in the FMR spectra in an AF film [6]. Indeed, we have observed a weaker line above the main line in the spectra for tcr = 12, 25 ,~ in perpendicular geometry. Examples of FMR spectra for P / C r multilayer structures are shown in fig. 3. In the FMR spectra, the absorption line with maximum intensity is the uniform mode, corresponding to the regular inphase excitation of the magnetization in Nis0Fez0/Cr multilayers; its resonance field shifts to a lower value with increasing Cr spacer layers thickness. Besides the uniform mode, there are some absorption lines of weak intensity in the FMR spectra, except for tc~ = 20 A. Moreover, although two FMR spectra are complicated
8~7
8.8 8'g ~ A
I 9;7 --1
10.6
t ~ 1 2 x .Cr --~'~
Cr
8.6 V 25A Cr .4
HOCOe) Fig. 3. Derivative FMR spectra of Nis0Fe20/Cr multilayer structure with the applied dc magnetic field perpendicular to the film plane at room temperature (the microwave frequency was fixed at 9.8 GHz).
at tcr < 20 .A, all weak absorption lines are located to the left of the uniform mode for tcr = 7 A,, and its resonance fields are lower than that of the uniform mode; however, for tcr = 12 ,~, a weak resonance mode above the uniform mode is clearly identified. At tCr > 20 ,~, the FMR spectra are relatively simple: only a narrow absorption line is observed for tcr = 20 ,~, but for t c r = 2 5 ,~, besides the the uniform mode, a higher-field mode with very weak intensity is also identified. Therefore, from the study of FMR spectra for P / C r multilayer structures, we confirm that the strength of the interlayer exchange coupling oscillates as a function of Cr spacer layer thickness. Having solved the linear equation for the normal modes of the coupled films [6], we get J = A H r e s M d v / 4 , with the definition AHre s =Hre S (weaker l i n e ) - H r e s (main line), where Hr¢,~, J, M s, and t v are the resonance field, interlayer exchange coupling constant for an AF film, the saturation magnetization, and the thickness of permalloy layer ( = 25 A), respectively. J is estimated to be about 0.017 erg/cm 2, which is 50 times lower than that found in F e / C r superlattices (about 1 erg/cm2).
4. Summary We have observed oscillations, as a function of Cr spacer layer thickness, in the magnitude of saturation magnetoresistance. The period of the oscillations is about 13 ,~. In addition, a weaker FMR line above the main FMR line in perpendicular geometry has been observed for the AF films, and therefore, the constant of AF coupling is estimated to be about 0.017 erg/cm z.
402
Peng Chubing et al. / Exchange coupling m NiFe / Cr
Acknowledgements: W e would like to t h a n k Professor F a n g Ruiyi for her helpful discussions a n d encouragement. In addition, this work was s u p p o r t e d by the National Science F o u n d a t i o n of C h i n a a n d the Doctoral P r o g r a m of H i g h e r Education. References [1] P. Griinberg, R. Schreiber, Y. Pang, M.B. Brodsky and C.H. Sowers, Phys. Rev. Lett. 57 (1986) 2442.
[2] M.N. Baibich, J.M. Broto, A. Fert, F. Nguyen Van Dau, F. Petroff, P. Eitenne, G. Creuzet, A. Friederich and J. Chazdas, Phys. Rev. Lett. 61 (1988) 2472. [3] S.S.P. Parkin, R. Bhadra and K.P. Roche, Phys. Rev. Lett. 66 (1991) 2152. [4] Peng Chubing, Dai Chert and Dai Daosheng, J. Appl. Phys. 72 (1992) 4250. [5] S.S.P. Parkin, N. More and K.P. Roche, Phys. Rev. Lett. 64 (1990) 2304. [6] J.J. Krebs, P. Lubiz, A. Chaiken and G.A. Prinz, J. Appl. Phys. 67 (19911) 5920.
Oscillatory interlayer exchange coupling in Ni80Fe20/Cr multilayers Peng Chubing, Dai Chen and Dai Daosheng Department of Physics, Peking Unicersity, Beijing 100871, China
Evidence of antiferromagnetic interlayer exchange coupling in Nisc~Fe2i)/Cr multilayers is reported. In Nisc~Fe~,/('r multilayers, both the magnitude of the interlayer magnetic exchange coupling and the saturation magnetoresistance have been found to oscillate with the Cr spacer layer thickness with a period of about 13 ,~. FMR spectra revealed a weak resonance mode above the regular in-phase mode for the antiferromagnetically coupled films, and only some lower-field resonance modes for ferromagnetically coupled films.
1. Introduction Recently, it was discovered that neighboring iron slabs can couple antiferromagnetically across chromium spacer layers in F e / C r superlattices [1]. Subsequently, a giant saturation magnetoresistance, which depends strongly on the relative orientation of magnetization between the successive 3d ferromagnetic slabs, has been observed in such antiferromagnetic (AF) films [2]. Oscillatory antiferromagnetic interlayer exchange coupling, manifested by the oscillation in the saturation magnetoresistance as a function of the paramagnetic transition spacer layer thickness, was reported by Parkin et al. in several metallic superlattice structures [3]. Motivated by these fascinating properties of interlayer exchange coupling in magnetic multilayer structures, we have prepared a large set of [Ni~0Fe20 (25 , ~ ) / C r ( t "~)]N (tc~ = 5 -- 50, N = 40-20) multilayer structures by electron beam deposition techniques under a vacuum pressure of 3 × 10 7 Torr onto heated glass substrates at 70°C (discussed in ref. [4]), and studied their magnetic and electric properties systematically. Oscillations in the saturation magnetoresistance were also observed, although the magnitude of the magnetoresistance was much smaller than that of F e / C r multilayer structures [5].
MR at 100 K is about twice as high as that at room temperature. From fig. 1 it can be seen that the MR does not change monotonically with the increasing Cr spacer layer thickness. As t~, r is increased beyond 5 ,~, the M R decreases to a very small value for to, ,. = 7-10 •~. As to, r is increased further, the M R becomes larger and reaches its maximum value at to,. = 12 A; it then decreases with further increases in /c'r, and reaches a minimum value at tc:~ = 16 ,~. A further oscillation in the M R is observed for even larger values of t¢,~ with peak values at tc~ = 25 and 38 ,~, but the oscillation in the magnitude is dampened. The oscillation period (about 13 ,~) in p e r m a l l o y / C r multilayers is approximately the same as that for C o / R u multilayers, but is significantly smaller than that for F e / C r multilayers [5]. The smaller oscillation period may partly result from the interdiffusion at p e r m a l l o y / C r interfaces.
l
I
I
I
!
I
i
I
t
I
I
I
l
'~
i
12 q-o,,.--I
2. Results and discussions Fig. 1 shows the saturation magnetoresistance M R ( H = 0)) as a function of the Cr layer thickness (tc~) with a fixed permalloy layers of 25 A at room temperature. The M R versus tcr curve at 100 K is similar to that at room temperature, except that the
l
( = AR/R
Correspondence to." Dr Peng Chubing, Department of Physics,
Peking University, Beijing 100871, China. 0304-8853/93/$06.00 © 1993
10
20
30
|
l
4O
thickness of Cr spacer layer
50
(A)
Fig. 1. Saturation magnetoresistance MR as a function of Cr spacer layer thickness with fixed Nis~jFez~~ layers of 25 f,, al room temperature. The data were taken with the sensing current and the magnetic field in the film plane and arranged orthogonal to one another.
Elsevier Science Publishers B.V. (North-Hollandl
401
Peng Chubing et al. / Exchange coupling in NiFe / Cr
7A Cr
12A Cr
9°
9i7
8.0
A
t___.~ -10
O
10
-10
O
10
magnetic field (Oe) Fig. 2. Examples of hysteresis loops are shown for four representative Nis0Fe20/Cr multilayers at room temperature. The applied magnetic field is in the film plane.
Since Cr is soluble in permalloy, it forms a homogeneous solid solution with an fcc structure in which the Curie point is lowered for small Cr concentrations. At concentrations greater than 10% the Curie point is below room temperature. The interfacial interdiffusion therefore results in a substantial increase in the magnetic separation of the permalloy films for a nominal Cr interlayer deposition thickness. Fig. 2 shows examples of hysteresis loops with fields in the film plane. It is seen that the hysteresis loops for tcr = 12 and 25 ~, (right panels in fig. 2) are 'pinched', and have greatly reduced remanent moment values and large saturation fields Hsat (about 30 Oe), which, in general, is similar in appearance to those of AF coupled films reported in the literature [5] and therefore, regard the samples in the right panels of fig. 2 as being AF cout~led. However, the hysteresis loops for tCr = 7 and 20 A (left panels in fig. 2) are squared with a large remanent magnetization and small H~at (about 5 0 e ) , indicating that the neighboring permalloy layers are ferromagnetically (F) coupled. Furthermore, one might expect to observe two absorption lines in the FMR spectra in an AF film [6]. Indeed, we have observed a weaker line above the main line in the spectra for tcr = 12, 25 ,~ in perpendicular geometry. Examples of FMR spectra for P / C r multilayer structures are shown in fig. 3. In the FMR spectra, the absorption line with maximum intensity is the uniform mode, corresponding to the regular inphase excitation of the magnetization in Nis0Fez0/Cr multilayers; its resonance field shifts to a lower value with increasing Cr spacer layers thickness. Besides the uniform mode, there are some absorption lines of weak intensity in the FMR spectra, except for tc~ = 20 A. Moreover, although two FMR spectra are complicated
8~7
8.8 8'g ~ A
I 9;7 --1
10.6
t ~ 1 2 x .Cr --~'~
Cr
8.6 V 25A Cr .4
HOCOe) Fig. 3. Derivative FMR spectra of Nis0Fe20/Cr multilayer structure with the applied dc magnetic field perpendicular to the film plane at room temperature (the microwave frequency was fixed at 9.8 GHz).
at tcr < 20 .A, all weak absorption lines are located to the left of the uniform mode for tcr = 7 A,, and its resonance fields are lower than that of the uniform mode; however, for tcr = 12 ,~, a weak resonance mode above the uniform mode is clearly identified. At tCr > 20 ,~, the FMR spectra are relatively simple: only a narrow absorption line is observed for tcr = 20 ,~, but for t c r = 2 5 ,~, besides the the uniform mode, a higher-field mode with very weak intensity is also identified. Therefore, from the study of FMR spectra for P / C r multilayer structures, we confirm that the strength of the interlayer exchange coupling oscillates as a function of Cr spacer layer thickness. Having solved the linear equation for the normal modes of the coupled films [6], we get J = A H r e s M d v / 4 , with the definition AHre s =Hre S (weaker l i n e ) - H r e s (main line), where Hr¢,~, J, M s, and t v are the resonance field, interlayer exchange coupling constant for an AF film, the saturation magnetization, and the thickness of permalloy layer ( = 25 A), respectively. J is estimated to be about 0.017 erg/cm 2, which is 50 times lower than that found in F e / C r superlattices (about 1 erg/cm2).
4. Summary We have observed oscillations, as a function of Cr spacer layer thickness, in the magnitude of saturation magnetoresistance. The period of the oscillations is about 13 ,~. In addition, a weaker FMR line above the main FMR line in perpendicular geometry has been observed for the AF films, and therefore, the constant of AF coupling is estimated to be about 0.017 erg/cm z.
402
Peng Chubing et al. / Exchange coupling m NiFe / Cr
Acknowledgements: W e would like to t h a n k Professor F a n g Ruiyi for her helpful discussions a n d encouragement. In addition, this work was s u p p o r t e d by the National Science F o u n d a t i o n of C h i n a a n d the Doctoral P r o g r a m of H i g h e r Education. References [1] P. Griinberg, R. Schreiber, Y. Pang, M.B. Brodsky and C.H. Sowers, Phys. Rev. Lett. 57 (1986) 2442.
[2] M.N. Baibich, J.M. Broto, A. Fert, F. Nguyen Van Dau, F. Petroff, P. Eitenne, G. Creuzet, A. Friederich and J. Chazdas, Phys. Rev. Lett. 61 (1988) 2472. [3] S.S.P. Parkin, R. Bhadra and K.P. Roche, Phys. Rev. Lett. 66 (1991) 2152. [4] Peng Chubing, Dai Chert and Dai Daosheng, J. Appl. Phys. 72 (1992) 4250. [5] S.S.P. Parkin, N. More and K.P. Roche, Phys. Rev. Lett. 64 (1990) 2304. [6] J.J. Krebs, P. Lubiz, A. Chaiken and G.A. Prinz, J. Appl. Phys. 67 (19911) 5920.