N . H ~ journal of

magnetism and magnetic materials

ELSEVIER Journal of Magnetism and Magnetic Materials I66 (1997) 261-266

Oscillation period of magnetoresistance and texture in Co/Cu-Ni and Ni -Fe /Cu-Ni multilayers

Ryoichi Nakatani *, Katsumi Hoshino, Hiroyuki Hoshiya, Yutaka Sugita Central Research Laboratory, Hitachi, Ltd., 1-280 Higashi-koigakubo, Kokubunji, To~'o 185, Japan

Received 31 May 1996; revised 30 July 1996

Abstract

We have investigated oscillations of magnetoresistance ratio and film texture in Co/Cu-Ni and Ni-Fe/Cu-Ni multilayers. As the non-magnetic layer thickness was changed, the magnetoresistance ratio oscillated in both the Co/Cu-Ni and the Ni-Fe/Cu-Ni multilayers. The film texture also oscillated between (100) and (111). As the Ni concentration of the Cu-Ni non-magnetic layers increased, the oscillation period of the magnetoresistance ratio became longer in both the multilayers. The oscillation period of the texture also became longer. These results indicate that the changes of the neck diameter of the Fermi surface of the Cu-Ni iayers vary the periods of the oscillations in both the magnetoresistance ratio and the texture.

PACS: 75.70.-i; 73.50,Jt

Keywords: Multilayer; Giant magnetoresistance; Film texture; Fermi surface; X-ray diffraction

1. Introduct ion

Giant magnetoresistive effects and oscillatory magnetoresistance changes have been found in many magnetic multilayers [1-3]. It has also been reported that the period of the oscillation of the magnetoresis- tance ratio becomes longer as the Ni concentration of the C u - N i non-magnetic layers increases in C o / C u - N i multilayers [4]. The changes in the pe- riod of the oscillation of the magnetoresistance ratio are accompanied by the changes in the neck diameter of the Fermi surface of the C u - N i layers.

On the other hand, the oscillation of the film texture between (100) and (111) has been found in

* Corresponding author. Fax: + 81-423-27-7724.

N i - F e / C u multilayers [5]. The X-ray diffraction intensities of (200) diffraction peak I200 and (111) diffraction peak I1~1 oscillate as the thickness of the Cu layer changes. The periods of the oscillations of the /200 and the I1~ t are the same. The /2oo and the Iii ~ become strong alternately. The peaks of the X-ray intensity ratio I2oo/(I11 t +/2o0) exist at the Cu layer thicknesses that also cause high magnetore- sistance ratios. It is thought that the film textures strongly correlate to the giant magnetoresistive ef- fects. However, the relationship between the texture and the magnetoresistance has not been clarified yet.

In this study, we investigate the oscillations of the magnetoresistance ratio and the film texture in the C o / C u - N i and the N i - F e / C u - N i multilayers. The changes in the oscillation period of the texture with

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262 R. Nakatani et al. / Jour~al of Magnetism and Magnetic Materials 155 (1997) 261-265

varying the Ni concentration of the Cu-Ni non-mag- netic layers in the C o / C u - N i and the N i - F e / C u - N i multilayers are found. The shifts of the peak posi- tions of the magnetoresistance ratio and the X-ray diffraction intensity 120 o due to the changes of the Ni concentration of the Cu-Ni non-magnetic layers show about the same tendencies, each other.

2. Experimental procedures

We used Si(100) wafers of ~ 7 mm square as substrates. Fe buffer layers 5 nm thick, were formed on the Si substrates. The multilayer consists of 20 layers of (Co or Ni-Fe) and 19 layers of (Cu or Cu-Ni). The first Co or Ni-Fe layer was directly deposited on the Fe buffer layer. The thicknesses of the Co or Ni-Fe layers were 1.0 nm, and those of the Cu or Cu-Ni layers were changed. The thick- nesses were detected using a quartz oscillating thick- ness monitor situated near the substrates. The con- centrations of the non-magnetic layers are Cu, Cu- 7.5at%Ni, Cu-15.5at%Ni and Cu-34.7at%Ni.

The multilayers were prepared using an ion beam sputtering apparatus. The Ni-Fe layers were formed by using the Ni-20at%Fe alloy plate target. The Cu-Ni layers of the Cu-7.5at%Ni and the Cu- 15.5at%Ni were formed by using the composite tar- gets made of the Cu plates and the Ni chips. The Cu-Ni layers of the Cu-34.7at%Ni were formed by using an alloy plate target. The acceleration voltage of the ion gun was 300 V with an ion current of 30 mA, for the depositions of the Co, Ni-Fe, Cu and Cu-Ni layers. The ion current was changed to 60 mA for the depositions of the Fe layers. The deposi- tion rates were 0.02-0.03 nm/s . The Argon pressure during sputtering was 0.02 Pa.

The magnetoresistance ratios of the muttilayers were measured using the 4-terminal method at room temperature. The maximum magnetic field was 800 kA/m. The magnetic field was applied in-plane and perpendicular to the current. The magnetoresistance ratio is defined as the ratio of the total resistivity change to the resistivity at a magnetic field of 800 kA/m. The textures of the multilayers were investi- gated using a diffractometer with Cu Kc~ X-rays. The concentrations of the Cu-Ni layers were mea- sured by inductively coupled plasma spectroscopy.

3. Experimental results

3.1. Co / Cu-Ni muttilayers

Fig. 1 shows the oscillatory magnetoresistance changes due to the Cu-Ni non-magnetic layer thick- nesses in the C o / C u - N i multilayers, where the Ni concentration of the Cu-Ni layers is varied. As shown in the figure, the first peak exists at about the same positions when the Ni concentration of the Cu-Ni layers is below 15.5 at%, but moves to 1.7 nm when the Ni concentration is 34.7 at%. The second peak slightly moves to thicker directions as the Ni concentration increases at the Ni concentra- tions below 15.5 at%. The second peak moves to 3.4 nm at the Ni concentration of 34.7 at%.

Fig. 2 shows the changes in the positions of the 1st peak and the 2nd peak of the magnetoresistance ratio with changing Ni concentration of the Cu-Ni non-magnetic layers in the C o / C u - N i multitayers. The figure also shows the results of the C o / C u - N i multilayers which have the ( I i0) preferred orienta- tions reported by Okuno and Inomata [4]. In the present work, the texture of the multilayer oscillates as the thickness of non-magnetic layers changes, as

20 . . . . . , •

~:~0 .=

'T'f ~ , , , i , 3 4

Thickness of Cu Layers t (nm) s (a)Ni 0 a t ~

20 , . . . . .

i s

cg

2 3 4 Thickness of Cu-Nl Layers t (nm 5)

(c)Nil 5,SAWN

, , , , , , , t ,

I 2 3 TNckness of Cu-Ni Layers t 4 (nm) s

(b)Ni 7 , 5 a t ~

0.6

's t ~o

= o~lo,~qo ,.5 ,,o

CO 2 3 4 Thickness of Cu-Ni Layers t (nrn)

(d)Ni 34.7at%

Fig. 1. Oscillations of magnetoreslstance ratios due to non-mag- netic layer thicknesses in the C o / C u - N i multilayers, where the Ni concentration of the Cu-Ni layers is varied.

R. Nakatani et al. / Jounzal of Magnetism and Magnetic Materials 155 (1997) 251-255 263

mentioned later. However, the positions of the 1st peak and the 2rid peak move with about the same tendencies as the Ni concentration increases in the present work and the reported results.

Fig. 3 shows the X-ray diffraction patterns of the C o / C u multilayers with various Cu layer thick- nesses. The diffraction peaks of the Co and Cu layers can not be distinguished because they have identical crystal structures and almost the same lattice parame- ters. When the thickness of the Cu layers is 0.9 nm, we can observe only (200) diffraction peaks of the Co and Cu layers. When the thickness of the Cu layers is 1.4 nm, the (111) peaks appear and the (200) peaks become very weak. When the thickness of the Cu layers is 2.0 nm, the (111) peaks become very weak and the (200) peaks become strong again. As shown in the figure, the texture also oscillates as the thickness of the Cu layers increases in the C o / C u multilayers.

Fig. 4 shows the oscillations of the X-ray diffrac- tion intensities of the (200) peaks ]200 and the (111) peaks Itt~ in the C o / C u - N i multilayers, where the Ni concentration of the Cu-Ni layers is varied. Both the /20 o and the Ilt l oscillate as the thicknesses of non-magnetic layers change. The Z2o 0 and the I ,~ become strong alternately. When the Ni concentra- tion of the Cu-Ni non-magnetic layers is 0 at%, the first /200 peak exists at 0.9 nm. As the Ni concentra- tion of the Cu-Ni non-magnetic layers increases, the first I200 peak moves to thicker directions. The first _To 0 peak exists at 2.0 nm when the Ni concentration is 34.7 at%. Other peaks also move to thicker direc-

Nonmagnetic Layer: Cu-xa~l, Ni

o 4 O ~

1~ Present Work 8 J

a. Okune & lnemata I

0 1 0 2to 3'0 4 0 Ni Concentration of Nonmagnetic Layer (at%)

Fig. 2. Changes in the positions of the tst peak and the 2nd peak of magnetoresistance ratios with changing the Ni concentration of the Cu-Ni non-magnetic layers in the Co /Cu-Ni multilayers.

1.0 ' ~ (a)

t=0.9 nm

0 t t t

2.0 ]]/c°' Cu (111) ' t t=l(b) • ~ / Co, Cu (200)

p4 nm

_..J "~ 0 , r 7 ~" 1.5 co, cu I20;) (c)

, co, c~(111) \ A . t=2.0nm

, -_a ._ ._J L._ " 3.0 ,co.' 0u(111) ' (d)

~ . / ' t=2,6 nm i i

0 t t

1.5 Co, Cu {20~) ' (e) t=3.1 nm

0 r

40 45 50 55 2 O (deg)

Fig. 3. X-ray diffraction patterns of the Co/Cu multilayers with various Cu layer thicknesses•

tions as the Ni concentration of the Cu-Ni non-mag- netic layers increases.

3.2. Ni-Fe / Cu-Ni multilayers

Fig. 5 shows the oscillatory magnetoresistance changes due to the Cu-Ni non-magnetic layer thick- nesses in the N i - F e / C u - N i multilayers, where the Ni concentration of the Cu-Ni layers is varied. As shown in the figure, the first peak of the magnetore- sistance ratio slightly moves to thicker directions as the Ni concentration of the Cu-Ni layers increases at the Ni concentrations below 15.5 at%. The first peak exists at 1.8 nm when the Ni concentration is 34.7 at%. The second peak slightly moves to thicker directions as the Ni concentration increases at the Ni concentrations below 15.5 at%. The second peak moves to 3.6 nm when the Ni concentration is 34.7 at%.

We compare the peak positions of the magnetore- sistance ratios between the C o / C u - N i and the N i - F e / C u - N i multilayers. Fig. 6 shows the relationship between the peak position and the Ni concentration of the Cu-Ni non-magnetic layers in the C o / C u - N i and the N i - F e / C u - N i multilayers. The white circles

264 R. Nakatani et at./Journal of 34agnetism and Magnetic Materials 155 (I997) 251-255

1.5 . ,

"[111

,~1,0~0.5 ~ ~ 1 2 o ~ o ~ ~" " ,

0,0 ~ 1 "2 3 4 Thickness of Cu Layers

(a)Ni 0atN

I2oo

1 ~ - 2 ~3 4 Thickness of Cu-Ni Layers t

(c)Ni15.SatN

Fig. 4. Oscillations of X-ray diffraction intensities of (200) Cu-Ni layers is varied.

5 t (nrn)

S , , . , •

2 ~ 2

&& o o

0 0

. . . . 3

2 ~ &

_

; i ;o Thickness of Cu-Ni Layers t (nm)

(b)Ni 7.SatN % . , . , . , , , ,

t

t • 0 0 ~

[1tl

r I

4

3

2 ~

1

0 4

(rim) Thickness of Cu-Ni Layers t (nm) (d)Ni34.7at°~

and (111) in the Co/Cu-Ni multilayers where the Ni concentration of the

and squares are peak positions of the first peaks and the second peaks, respectively, for the C o / C u - N i multilayers. And the black circles and squares are for

2O 20 I

' ' ~ ' ~ ' i . . . . Thickness of Cu Layers t (nm) "~h[cknelss of ~u-Ni ~.~ayers ~ (nm¢

(a)Ni 0at~ (13) NW,Sat~ 20 . , , , . 20

o o ~0. '- ~o

Thickness t (rim) 5

1 2 3 4 1 S 3 4

Thickness o( Cu-Ni Layers t (nm) Thickness of Cu-Ni Layers t (nm) (o)Ni 15,5at% (d)Ni 34.7at~

Fig. 5. Oscillations of magnetoreststance ratios due to non-mag- netic layer thicknesses in the Ni-Fe/Cu-Ni muttilayers where the Ni concentration of the Cu-Ni layers is varied.

the N i - F e / C u - N i multilayers. As the Ni concentra- tion of the Cu- N i layers increases, the peaks move to thicker directions with the same tendencies.

Fig. 7 shows the oscillations of the X-ray diffrac- tion intensities of the (200) peaks I200 and the (111) peaks It1 t in the N i - F e / C u - N i multilayers, where the Ni concentration of the C u- N i layers is varied. Both the /200 and the It x ~ oscillate as the thicknesses

4

I N '°nrn a'gne"c Layer*' Cu~xat%'~

~5

0[ , f , 1 , I

0 1 0 20 30 4 0

Ni Concentration of Nonmagnetic Layer (at%)

Fig. 6. Relationship between peak positions of the magnetoresis- tance ratio and the Ni concentration of the Cu-Ni non-magnetic layers in the Co/Cu-Ni and the Ni-Fe/Cu-Ni mukilayers.

R. Nakatani et al. ~Journal of Magnetism and Magnetic Materials 166 (1997) 261-266 265

0 1,5r

I m 0.5 8 ~

& o

~0.4 o 4= ~ = - - 1,0 ~

0.2 2

0,~ O q i " 2 3

Thickness of Cu Layers t (nm) Thickness of Cu-Ni Layers t (nm) (b) l~ 7 . h a t ~ 1.~ (a)N40at~ - - !5 1.ot

0~2 0 ,~ 1

0.O0 0 ( " I - 2 - 3 4

Thickness of Cu-Ni Layers t (nm) Thickness of Cu-Ni Layers t (rim) (c)Ni 15 ,5a t% (d)Ni 3 4 . 7 a t ~

Fig. 7. Oscillations of X-ray diffraction intensities of (200) and (I 11) in the N i - F e / C u - N i multilayers where the Ni concentra- tion of the Cu-Ni layers is varied.

of non-magnetic layers change. When the Ni concen- tration of the Cu-Ni non-magnetic layers is 0 at%, the first I200 peak exists at 1.0 nm. As the Ni concentration of the Cu-Ni non-magnetic layers in- creases, the first /2oo peak moves to thicker direc- tions. The first I200 peak exists at 2.2 nm when the Ni concentration is 34.7 at%. Other peaks also move to thicker directions as the Ni concentration in- creases.

We compare the peak positions of the /2o 0 be-

4 I Nonmagnetic/ayer:'Cu.xat%N i --

"~ i" Ni-Fe/Ou-Ni f

[ / ~ . . . . . . Ni-Fe/Cu-Ni

g2

~ . ~ Co/Cu-Ni

#_

O [ ' I i I i I i

0 10 20 30 40 Ni Concentration of Nonmagnetic Layer (at%)

Fig. 8. Relationship between peak positions of I200 and the Ni concentration of the Cu-Ni non-magnetic layer in the Co/Cu-Ni and the Ni-Fe/Cu-Ni multilayers.

tween the C o / C u - N i and the N i - F e / C u - N i multi- layers. Fig. 8 shows the relationship between the peak position of I200 and the Ni concentration of the Cu-Ni non-magnetic layer in the C o / C u - N i and the N i - F e / C u - N i multilayers. The white circles and squares are peak positions of the first peaks and the second peaks, respectively, for the C o / C u - N i multi- layers. And the black circles and squares are for the N i - F e / C u - N i multilayers. As the Ni concentration increases, the peaks move to thicker directions with the same tendencies. It indicates that the oscillation of the texture does not relate to the magnetic layer materials. It relates to the non-magnetic layer materi- als.

4. Discussion

It has been already reported that the peaks of magnetoresistance ratios move to thicker directions as the Ni concentration of the Cu-Ni non-magnetic layer increases in the C o / C u - N i multilayers which have the ( i10) preferred orientations [4]. The changes in the period of the oscillation of the magne- toresistance ratio are accompanied by the changes in the neck diameter of the Fermi surface of the Cu-Ni layers. It has been also known that the preferred orientation of the multilayer varies the periodicity of the change in the magnetoresistance ratio [6,7].

In this study, though the preferred orientation of the multilayers is not fixed, the peaks of magnetore- sistance ratios move to thicker directions as the Ni concentration of the Cu-Ni non-magnetic layers in- creases in the C o / C u - N i and the N i - F e / C u - N i multilayers, as shown in Fig. 6. Moreover, as shown in Fig. 2, there is only a little difference between the peak positions of the magnetoresistance ratios of the present work and those of the previous report [4]. The similarity of the peak positions of the magne- toresistance ratios indicates that the changes in the neck diameter due to the Ni concentration of the Cu-Ni non-magnetic layer also cause the shifts of the peaks of the magnetoresistance ratios in the C o / C u - N i and the N i - F e / C u - N i multilayers in this study.

On the other hand, the peaks of the X-ray diffrac- tion intensity I20o also move to thicker directions as

266 R. Nakatani et al. / Journal of Magnetism and Magnetic Materials 166 (1997) 261-266

the Ni concentration of the Cu-Ni non-magnetic layer increases, as shown in Fig. 8. There is a similarity between the periodicities of the changes in the magnetoresistance ratios and the X-ray diffrac- tion intensities I200 as shown in Figs. 6 and 8. Of course the oscillation of the magnetoresistance ratio has been observed in the C o / C u - N i multilayers with the fixed preferred orientations [4]. Therefore, the oscillations of the textures of the multilayers do not cause the oscillations of the magnetoresistance ratios. However, the similarity between the peaks positions of the magnetoresistance ratios and the X-ray diffraction intensity I200 seems to indicate that the origin of the oscillations of the textures is similar to that of the oscillations of the magnetoresistance ratios.

5. Conclusions

the changes of the Ni concentration of the Cu-Ni non-magnetic layers show the same tendencies.

Acknowledgements

The authors are grateful to Professor Soshin Chikazumi of Edogawa University, Professor Susumu Uchiyama of Aichi Institute of Technology, Profes- sor Shigeru Tsunashima and Associate Professor Satoshi Iwata of Nagoya University, Dr. Mutuko Jimbo of Daido Institute of Technology, Dr. Teruaki Takeuchi of Hitachi, Ltd. for their valuable discus- sions. They also thank Hisao Kojima and Atsuko Matsubara of Hitachi, Ltd. for collaboration on in- ductively coupled plasma spectroscopy, and Drs. Masaaki Futamoto and Yoshihiro Shiroishi of Hi- tachi, Ltd. for their helpful discussions and encour- agements.

We investigated the magnetoresistance effects and the textures of the C o / C u - N i and the N i - F e / C u - N i multilayers. The following results were obtained.

(1) As the thickness of the Cu-Ni non-magnetic layer changes, the texture of the multilayers oscil- lates between (100) and (111) in both the multilay- ers.

(2) The peaks of the X-ray diffraction intensities I200 move to thicker directions as the Ni concentra- tion of the Cu-Ni non-magnetic layers increases in both the multilayers.

(3) The shifts of the peaks of magnetoresistance ratio and the X-ray diffraction intensity I200 due to

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