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Magnetic properties of PdNi alloys and multilayers
Journal of Magnetism and Magnetic Materials 93 (1991) 469-471 North-Holland
469
Magnetic properties of PdNi alloys and multilayers H. T a k a h a s h i , S. T s u n a s h i m a , S. F u k a t s u and S. U c h i y a m a Dept. of Electric Engineering, Nagoya Unic., Nagoya 464-01, Japan
Magnetic properties of PdNi alloys and multilayers (MLs) were investigated. The alloy films sputtered at higher Ar pressure exhibit uniaxial magnetic anisotropy K u with the easy axis perpendicular to the film plane. This anisotropy is ascribed to the stress due to substrate constraint. Perpendicular magnetic anisotropy of the MLs sputtered under nearly stress-free conditions increases with decreasing bilayer period and the perpendicular magnetization is realized for the bilayer period of 15 A. The surface anisotropy is estimated to be about 0.05 erg/cm 2.
1. Introduction
Many kinds of noble metal ( N M ) / C o MLs, such as C o / P d [1], C o / P t [1, 2] and C o / A u [3] exhibit magnetic anisotropy with the easy axis perpendicular to the film plane. This anisotropy seems to originate from the interface of sublayers. In case of P d / C o MLs, we have pointed out that the lattice misfit between Pd and highly magnetostrictive PdCo alloy at the interface could induce the surface anisotropy [4]. For further understanding of the anisotropy, we have investigated the magnetic properties of PdNi alloys and MLs, which are similar to PdCo in both crystal structure and magnetostriction.
and the rotating speed of the substrate table, respectively. The composition of alloy films ranged from 20 to 70at% Pd. The total thickness is around 4000 A for alloys and 2000 A for MLs. The periodicity was examined by X-ray diffraction, the composition by EPMA, and magnetic properties were measured using a VSM and a torque magnetometer. The saturation magnetostriction constant All 1 was derived from the magnetoelastic coupling constant B, which was determined from the anisotropy Kue induced by the application of the in-plane uniaxial strain [5]. The planar stress or was estimated from the change of substrate curvature before and after the deposition of films. Polar magneto-optical Kerr rotation was measured by Faraday modulation method from the film surface side.
2. Experimental
Samples were prepared on water-cooled glass substrate of 0.15 mm in thickness by rf sputtering method. A composite type target was used for making alloy films and two separate targets for MLs. Substrates for MLs were placed on a rotating table. Composition and periodicity of MLs were controlled by adjusting the deposition rate
3. Results and discussion
Fig. l(a) shows examples of the magnetization curves for Pd35Ni65 alloys measured with a field parallel (11) and perpendicular ( ± ) to the film plane. It is seen that perpendicular anisotropy increases with increasing Ar pressure.
0304-8853/91/$03.50 © 1991- Elsevier Science Publishers B.V. (North-Holland)
H. Takahashi et al. / Magnetic properties of PdNi alloys and multilayers
470
M(emu/cm3) pd(A)/Ni(A)200I =22.8/ii.2, I00F ,
M (emu/cm 3) 400T y
PAr= 15mTorr -io
-;
o
"2~
xlO5
o
i0
. . - ' ' ~ A r = 25mTorr
i
///-20°
5
*-400 ~;;
(kOe)
-I0 -5 A
1100
I'0
--~-200
\ \ AX
\,
~5
A%
\,
v
m
20mTor,r
5
i - "~
16.7/8.2 ( - l
J
v: 0
)
-5
I
0
[
20
40
1%%
60
_1
80
100
Pd CONTENT ( a t % )
25mTorr
~ti""
10.1/5.0
~;.',"",. t
alloys
(a) Pd35Ni65
Fig. 2. C o m p o s i t i o n a l d e p e n d e n c e o f uniaxial a n i s o t r o p y cons t a n t K~. D o t t e d c u r v e s a r e stress i n d u c e d a n i s o t r o p y K'~f c a l c u l a t e d f r o m - ~A 1i i O'. i
(b) PdlNiMks
Fig. 1. M a g n e t i z a t i o n c u r v e s o f P d N i alloys a n d M L s m e a s u r e d at r o o m t e m p e r a t u r e •
In fig. 2, solid curves show compositional dependence of uniaxial magnetic anisotropy Ku, w h e r e K u = K~ff + 2 v M 2 (Keff: effective anisotropy estimated from the amplitude of the torque curve). When the Ar pressure PAr is 25 mTorr, K u is positive and has a maximum at a Pd content of about 40at%, and it decreases sharply with increasing Pd content. The maximum K u is about 1.2 × 1 0 6 e r g / c m 3. When PAr is reduced to 10 mTorr, where the planar stress ~r is very small, K u becomes negative and decreases slightly with increasing Pd content. This anisotropy is ascribed to the stress due to the substrate constraint since both the magnitude and composition dependence agree well with the dotted curves, which are calculated from K~
=
3
--~1110",
(1)
where the magnetostriction constant h l~ and the stress ~r were measured separately.
Fig. l(b) shows examples of the magnetization curves for MLs, where the average composition of sublayers is about 37 at% Ni. It is seen that the perpendicular anisotropy increases with decreasing bilayer period, and that when the bilayer o period becomes as small as 15A, the easy axis becomes perpendicular to the film plane. Fig. 3 shows the effective anisotropy Ken. times bilayer period A versus Ni layer thickness tNi, where Kerr is the energy density per unit volume of the multilayers. From the extrapolation to tN~ = 0 , surface anisotropy is estimated at around 0.05 e r g / c m 2. Fig. 4 shows Kerr rotation spectra of P d / N i MLs with various bilayer periods. The Kerr rotation increases with decreasing bilayer period, and when the bilayer period become as small as 7.6.A, the Kerr rotation spectrum of MLs becomes almost the same as that of alloy films with corresponding composition. This suggests the occurrence of alloying a n d / o r the change of electronic states at the interface• Since the magnetostriction of PdNi alloys is as large as that of PdCo at low temperature [6], the stress-induced anisotropy due to lattice misfit is thought to be one of origins of perpendicular magnetic anisotropy in P d / N i MLs as it is in P d / C o MLs. However, the magnitude of surface
H. Takahashi et al. / Magnetic properties of PdNi alloys and muhilayers 0.2,
,
i
471
fact, the magnetostriction of P d / N i (3.5 A/1.7.~l) MLs at room temperature is about one third of a P d / C o ML with a similar layer structure.
i
31 a t % Ni 3, 0.1
4. Conclusion )<
I
0
•
5
O dl' II
I0
an...
15
20
tNi ( R )
Fig. 3. Ni layer thickness tNi dependence of effective anisotropy times bilayer period K~jIA of Pd/Ni MLs with various average composition.
anisotropy is much smaller in P d / N i MLs than in P d / C o . A possible reason for this difference is as follows: With decreasing bilayer period, the magnetic properties of P d / N i MLs become similar to those of PdNi alloys, and the Curie temperature becomes lower. Then, in general, magnetostriction and magnetic anisotropy become smaller. In -0.20 o
-'-Pd(A)/Ni(A)=5.1/2.5 - - - 10.I/5.1 -o-19.3/9.4
-0.15
RF sputtered PdNi alloy films have uniaxial anisotropy with the easy axis perpendicular to the film plane. At high Ar pressure of 25reTort, a large K u up to 1 0 6 e r g / c m 3 is induced. The origin of the anisotropy is ascribed to the stress-magnetostriction mechanism. In P d / N i MLs with a short bilayer period of about 15 A, the easy axis becomes perpendicular to the film plane, and the estimated surface anisotropy is around 0.05 e r g / c m 2.
Acknowledgements This work was supported by Grant-in-Aid for Special Project Research from the Ministory of Education, Science and Culture of Japan. The authors are grateful to Mr. Adachi for EPMA measurement. The authors would like to thank Dr. S. Iwata, Dr. M. Nawate and Mr. K. Nakamura for valuable discussion.
--Pd63Ni37 ALLOY
v -0. I0
References -0.05 °
+0.05
e
200
400
J
i
600
800
WAVELENGTH (nm ) Fig. 4. Kerr rotation spectra of Pd/Ni MLs with various bilayer period, where the average composition is about 37 at% Ni. Dotted curve is the spectrum of Pd~3Ni37 alloy film.
[1] P.F. Carcia, J. Appl. Phys. 63 (1988) 5066. [2] H.J.G. Draaisma and W.J.M. de ,longe, J. Appl. Phys. 62 (1987) 3318. [3] F.J.A. den Broeder, D. Kuiper, A.P. van de Mosselaer and W. Hoving, Phys. Rev. Lett. 60 (1988) .2769. [4] S. Tsumashima, K. Nagase, K. Nakamura and S. Uchiyama, IEEE Trans. Magn. MAG-25 (1989) 3761. [5] S. Tsunashima, H. Takagi, K. Kamegaki, T. Fujii and S. Uchiyama, IEEE Trans. Magn. MAG-14 (1978) 844. [6] T. Tokunaga, M. Kohri, H. Kadomatsu and H. Fujiwara, J. Phys. Soc. Japan 50 (1981) 1411.
469
Magnetic properties of PdNi alloys and multilayers H. T a k a h a s h i , S. T s u n a s h i m a , S. F u k a t s u and S. U c h i y a m a Dept. of Electric Engineering, Nagoya Unic., Nagoya 464-01, Japan
Magnetic properties of PdNi alloys and multilayers (MLs) were investigated. The alloy films sputtered at higher Ar pressure exhibit uniaxial magnetic anisotropy K u with the easy axis perpendicular to the film plane. This anisotropy is ascribed to the stress due to substrate constraint. Perpendicular magnetic anisotropy of the MLs sputtered under nearly stress-free conditions increases with decreasing bilayer period and the perpendicular magnetization is realized for the bilayer period of 15 A. The surface anisotropy is estimated to be about 0.05 erg/cm 2.
1. Introduction
Many kinds of noble metal ( N M ) / C o MLs, such as C o / P d [1], C o / P t [1, 2] and C o / A u [3] exhibit magnetic anisotropy with the easy axis perpendicular to the film plane. This anisotropy seems to originate from the interface of sublayers. In case of P d / C o MLs, we have pointed out that the lattice misfit between Pd and highly magnetostrictive PdCo alloy at the interface could induce the surface anisotropy [4]. For further understanding of the anisotropy, we have investigated the magnetic properties of PdNi alloys and MLs, which are similar to PdCo in both crystal structure and magnetostriction.
and the rotating speed of the substrate table, respectively. The composition of alloy films ranged from 20 to 70at% Pd. The total thickness is around 4000 A for alloys and 2000 A for MLs. The periodicity was examined by X-ray diffraction, the composition by EPMA, and magnetic properties were measured using a VSM and a torque magnetometer. The saturation magnetostriction constant All 1 was derived from the magnetoelastic coupling constant B, which was determined from the anisotropy Kue induced by the application of the in-plane uniaxial strain [5]. The planar stress or was estimated from the change of substrate curvature before and after the deposition of films. Polar magneto-optical Kerr rotation was measured by Faraday modulation method from the film surface side.
2. Experimental
Samples were prepared on water-cooled glass substrate of 0.15 mm in thickness by rf sputtering method. A composite type target was used for making alloy films and two separate targets for MLs. Substrates for MLs were placed on a rotating table. Composition and periodicity of MLs were controlled by adjusting the deposition rate
3. Results and discussion
Fig. l(a) shows examples of the magnetization curves for Pd35Ni65 alloys measured with a field parallel (11) and perpendicular ( ± ) to the film plane. It is seen that perpendicular anisotropy increases with increasing Ar pressure.
0304-8853/91/$03.50 © 1991- Elsevier Science Publishers B.V. (North-Holland)
H. Takahashi et al. / Magnetic properties of PdNi alloys and multilayers
470
M(emu/cm3) pd(A)/Ni(A)200I =22.8/ii.2, I00F ,
M (emu/cm 3) 400T y
PAr= 15mTorr -io
-;
o
"2~
xlO5
o
i0
. . - ' ' ~ A r = 25mTorr
i
///-20°
5
*-400 ~;;
(kOe)
-I0 -5 A
1100
I'0
--~-200
\ \ AX
\,
~5
A%
\,
v
m
20mTor,r
5
i - "~
16.7/8.2 ( - l
J
v: 0
)
-5
I
0
[
20
40
1%%
60
_1
80
100
Pd CONTENT ( a t % )
25mTorr
~ti""
10.1/5.0
~;.',"",. t
alloys
(a) Pd35Ni65
Fig. 2. C o m p o s i t i o n a l d e p e n d e n c e o f uniaxial a n i s o t r o p y cons t a n t K~. D o t t e d c u r v e s a r e stress i n d u c e d a n i s o t r o p y K'~f c a l c u l a t e d f r o m - ~A 1i i O'. i
(b) PdlNiMks
Fig. 1. M a g n e t i z a t i o n c u r v e s o f P d N i alloys a n d M L s m e a s u r e d at r o o m t e m p e r a t u r e •
In fig. 2, solid curves show compositional dependence of uniaxial magnetic anisotropy Ku, w h e r e K u = K~ff + 2 v M 2 (Keff: effective anisotropy estimated from the amplitude of the torque curve). When the Ar pressure PAr is 25 mTorr, K u is positive and has a maximum at a Pd content of about 40at%, and it decreases sharply with increasing Pd content. The maximum K u is about 1.2 × 1 0 6 e r g / c m 3. When PAr is reduced to 10 mTorr, where the planar stress ~r is very small, K u becomes negative and decreases slightly with increasing Pd content. This anisotropy is ascribed to the stress due to the substrate constraint since both the magnitude and composition dependence agree well with the dotted curves, which are calculated from K~
=
3
--~1110",
(1)
where the magnetostriction constant h l~ and the stress ~r were measured separately.
Fig. l(b) shows examples of the magnetization curves for MLs, where the average composition of sublayers is about 37 at% Ni. It is seen that the perpendicular anisotropy increases with decreasing bilayer period, and that when the bilayer o period becomes as small as 15A, the easy axis becomes perpendicular to the film plane. Fig. 3 shows the effective anisotropy Ken. times bilayer period A versus Ni layer thickness tNi, where Kerr is the energy density per unit volume of the multilayers. From the extrapolation to tN~ = 0 , surface anisotropy is estimated at around 0.05 e r g / c m 2. Fig. 4 shows Kerr rotation spectra of P d / N i MLs with various bilayer periods. The Kerr rotation increases with decreasing bilayer period, and when the bilayer period become as small as 7.6.A, the Kerr rotation spectrum of MLs becomes almost the same as that of alloy films with corresponding composition. This suggests the occurrence of alloying a n d / o r the change of electronic states at the interface• Since the magnetostriction of PdNi alloys is as large as that of PdCo at low temperature [6], the stress-induced anisotropy due to lattice misfit is thought to be one of origins of perpendicular magnetic anisotropy in P d / N i MLs as it is in P d / C o MLs. However, the magnitude of surface
H. Takahashi et al. / Magnetic properties of PdNi alloys and muhilayers 0.2,
,
i
471
fact, the magnetostriction of P d / N i (3.5 A/1.7.~l) MLs at room temperature is about one third of a P d / C o ML with a similar layer structure.
i
31 a t % Ni 3, 0.1
4. Conclusion )<
I
0
•
5
O dl' II
I0
an...
15
20
tNi ( R )
Fig. 3. Ni layer thickness tNi dependence of effective anisotropy times bilayer period K~jIA of Pd/Ni MLs with various average composition.
anisotropy is much smaller in P d / N i MLs than in P d / C o . A possible reason for this difference is as follows: With decreasing bilayer period, the magnetic properties of P d / N i MLs become similar to those of PdNi alloys, and the Curie temperature becomes lower. Then, in general, magnetostriction and magnetic anisotropy become smaller. In -0.20 o
-'-Pd(A)/Ni(A)=5.1/2.5 - - - 10.I/5.1 -o-19.3/9.4
-0.15
RF sputtered PdNi alloy films have uniaxial anisotropy with the easy axis perpendicular to the film plane. At high Ar pressure of 25reTort, a large K u up to 1 0 6 e r g / c m 3 is induced. The origin of the anisotropy is ascribed to the stress-magnetostriction mechanism. In P d / N i MLs with a short bilayer period of about 15 A, the easy axis becomes perpendicular to the film plane, and the estimated surface anisotropy is around 0.05 e r g / c m 2.
Acknowledgements This work was supported by Grant-in-Aid for Special Project Research from the Ministory of Education, Science and Culture of Japan. The authors are grateful to Mr. Adachi for EPMA measurement. The authors would like to thank Dr. S. Iwata, Dr. M. Nawate and Mr. K. Nakamura for valuable discussion.
--Pd63Ni37 ALLOY
v -0. I0
References -0.05 °
+0.05
e
200
400
J
i
600
800
WAVELENGTH (nm ) Fig. 4. Kerr rotation spectra of Pd/Ni MLs with various bilayer period, where the average composition is about 37 at% Ni. Dotted curve is the spectrum of Pd~3Ni37 alloy film.
[1] P.F. Carcia, J. Appl. Phys. 63 (1988) 5066. [2] H.J.G. Draaisma and W.J.M. de ,longe, J. Appl. Phys. 62 (1987) 3318. [3] F.J.A. den Broeder, D. Kuiper, A.P. van de Mosselaer and W. Hoving, Phys. Rev. Lett. 60 (1988) .2769. [4] S. Tsumashima, K. Nagase, K. Nakamura and S. Uchiyama, IEEE Trans. Magn. MAG-25 (1989) 3761. [5] S. Tsunashima, H. Takagi, K. Kamegaki, T. Fujii and S. Uchiyama, IEEE Trans. Magn. MAG-14 (1978) 844. [6] T. Tokunaga, M. Kohri, H. Kadomatsu and H. Fujiwara, J. Phys. Soc. Japan 50 (1981) 1411.