Journal of Magnetism and Magnetic Materials 165 (1997) 108-110 journal of magnetism

~ i ~ and magnetic

J R materials ELSEVIER

Magnetoresistance in CoMnB/Co(Fe)/Cu/Co(Fe) spin-valves H. Sakakima *, M. Satomi, Y. Irie, Y. Kawawake

Central Research Laboratory, Matsushita Electric Industrial Co., Ltd., 3-4 Hikaridai, Seika-cho, Souraku-gun, Kyoto 619-02, Japan

Abstract Magnetoresistance in A M / M ' / C u / M spin-valves was studied, where AM = CoMnB or CoNbZr and M',M = Co or

CoFe. M' is a thin magnetic layer interposed between an AM and a Cu layer to enhance the MR ratio, and M is a magnetic layer having a larger H c than the AM layer. A considerably large MR ratio, 7.6%, with small switching field was obtained for the spin-valves with AM = CoMnB and M ' ( = M) = Co. Surface oxidation of the M layer was effective in increasing the saturation field of the spin-valves. These spin-valves are very thin ( < l0 nm) and are advantageous for MR heads having narrow shield gaps.

Keywords: Spin-valve; Magnetoresistance; Multilayers; Coercivity

1. Introduction

Giant magnetoresistive multilayers composed of mag- netic layers having different coercivities were proposed by Shinjo et al. [1] and spin-valves using antiferromagnetic layers were proposed by Dieny et al. [2]. These authors studied spin-valves having high sensitivity, d (MR) /dH, without using antiferromagnetic layers such as N iFeCo/Cu /CoPt [3] or N iFeCo/Cu /CoFe [4].

Spin-valves with amorphous alloy films for the soft magnetic layers such as C o F e B / C u / C o were proposed by Jimbo et al. [5]. We report in this paper on the MR properties of A M / M ' / C u / M spin-valves, where AM is an amorphous magnetic layer such as CoMnB or CoNbZr, M' is a thin magnetic layer interposed between the AM and the Cu layer to enhance the MR ratio [6], without deteriorating the soft magnetic property of the A M / M ' layer, and M is a magnetic layer having larger coercivity, H c, than that of the AM layer.

2. Experimental

A M / M ' / C u / M spin-valves were sputtered onto glass substrates fixed on a water cooled sample holder located 60 mm above the cathodes. 80 mm diameter alloy targets of C068Mn6B26, C083Nbll.sZrs.5 and CovsFe2Nb10B]o, of Cu, and of Co, C090Felo and C05oFeso were used to form the AM layer, Cu layer, and M' and M layer, respectively. The sputtering apparatus used for this study

* Corresponding author.

has three cathode targets. Therefore, the spin-valves had to be prepared with M = M' for this experiment. The magne- toresistance was measured at RT using the four-point probe method with application of a 0.5 kOe dc magnetic field in the film plane.

3. Results and discussion

3.1. C o M n B / M ' / C u / M

We first studied the MR properties of C o M n B / M ' / C u / M spin-valves. Fig. 1 shows the M ' ( = Co) layer thickness ( t ) dependence of the MR ratio for CoMnB(2)/Co(tco)/Cu(2.4)/Co(2), where the value in parentheses indicates the layer thickness in nm. The MR ratio increases from 1.7 to 7% when tco increases from 0 to 1.2 nm. The soft magnetic properties of the C o M n B / M ' layer were not deteriorated for t co< 1.2 nm. The increase in the MR ratio on interposing the M ' ( = Co) layer is probably due to the enhancement of the spin scattering at the interface between Cu and the magnetic layer, as the C o / C u interface is effective in increasing the MR ratio.

Fig. 2 shows the CoMnB layer thickness dependence of the MR ratio for CoMnB(t)/Co(1.2)/Cu(2.4)/Co(2). The MR ratio exhibits a maximum around t = 1 nm. The decrease in the MR ratio for t > l nm is due to the increase in resistivity, as CoMnB has a high resistivity. The decrease for t < 1 nm is probably due to the deteriora- tion of the magnetic property of CoMnB.

The Cu layer thickness dependence of the MR ratio was also studied for CoMnB(2)/Co(1.2)/Cu(tcu)/Co(2). The MR ratio showed a broad maximum around tc, ~ 2.1-2.3 nm as for other spin-valves composed of a soft

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H. Sakakima et al. / Journal of Magnetism and Magnetic Materials 165 (1997) 108-110 109

0

[]

[]

O [] []

O O

I I I

0 0.5 1 1.5

Co Layer Thickness t(nm)

Fig. 1. M'( = Co) layer thickness dependence of the MR ratio for CoMnB(2)/Co(tco )/Cu(2.4)/Co(2).

8.00

6.00

O

4.00 n"

2.00

O.O0 I I

-40 40

| | i |

J I I I I

-20 0 20 H (kA/m)

Fig. 3. MR curve for CoMnB(1.5)/Co(1.2)/Cu(2.1)/Co(2).

magnetic layer and a hard magnetic layer separated by a nonmagnetic layer [3]. By optimizing the layer thickness of the AM, M' and Cu layer, a considerably large MR ratio of 7.6% with a small switching field, < 10 Oe, was obtained for CoMnB(1.5)/Co(1.2)/Cu(2.1)/Co(2), as shown in Fig. 3. These MR curves are desirable for spin-valve memory [7] as they have small switching and saturation fields.

The dependence of the MR curves on the composition of the M ( = M') layer was also studied. By substituting CosoFeso for Co in the M(= M') layer, the saturation fields of the MR curves increased due to the increase in the H c of the M layer, but the sensitivity decreased due to the deterioration of the soft magnetic properties of the M' layer. MR ratios of 4.7% were obtained for CoMnB(2)/CosoFeso(0.8)/Cu(2.1)/CosoFeso(2). By us- ing Co9oFeio as the M ( = M') layer, the MR ratios de- creased drastically to 0.5%, because of the decrease in the H c of the M layer, which deteriorates the anti-parallel spin configuration of the M and the CoMnB/M' layers.

We had previously studied the MR properties of

[NiFeCo/Cu/Coloo_xFex/CU] multilayers and found that the MR maximum was obtained with x = 50 (i.e. CosoFeso ) [4], as expected from the theory of Inoue et al. [8]. This is not realized for the present system, probably due to the undesirable condition of M ' = M. A further study will be performed on ideal spin-valves having a hard magnetic CosoFeso layer as the M layer and a thin Co-rich soft magnetic layer such as Co9oFelo as the M' layer by using a sputtering apparatus having four (or more) cathode targets.

3.2. CoMnB / M' / Cu / M / MO

Surface oxidation of the M layer is effective in increas- ing the H e of the M layer. We studied the MR properties of C o M n B / M ' / C u / M / M O spin-valves. The MO layer was formed by reactive sputtering using 0 2 mixed Ar gas. Fig. 4 shows the dependence of the MR ratios and the saturation fields, H s, on the MO(= CoO) layer thickness in CoMnB(2)/Co(1.2)/Cu(2.4)/Co(2)/CoO(t). H s in- creases with increasing t, because of the increase in the H c of the C o / C o O layer, and saturated for t > 3 nm.

6

o

4

2

0 [ ]

I f I I I

1 2 3 4 5

CoMnB Layer Thickness t(rm)

8

i

B'

~4

2

[ ] [ ]

[ ]

i I i I I

0.2 0,4 0.6 0,8 1

CoO Leyer Tkicknesl tCnm)

15

E

10

5

0 1,

Fig. 2. CoMnB layer thickness dependence of the MR ratio for Fig. 4. Dependence of MR ratios and H s on the MO( = CoO) CoMnB(t)/Co(1.2)/Cu(2.4)/Co(2). layer thickness for CoMnB(2)/Co(1.2)/Cu(2.4)/Co(2)/CoO(t).

110 H. Sakakima et al. / Journal of Magnetism and Magnetic Materials 165 (1997) 108-110

8 . 0 0 ' ' ' ' ' ' '

6 . 0 0

0 4 . 0 0

r r

2 . O 0

0 . 0 0 I I I I I I I

-40 -20 0 20 40 H (kA/m)

Fig. 5. MR curves for CoMnB(2)/ /Co(I.2)/Cu(2.4)/Co(2)/ COO(0.3).

The MR ratio gradually decreases with increasing t, although the increase of the resistivity was considerably small (few %) for 0 < t < 0.5 nm. The reason for this is uncertain at present. The MR curves of these C o M n B / C o / C u / C o / C o O spin-valves (Fig. 5) having small switching fields and larger H s than those of the C o M n B / C o / C u / C o spin-valves as shown in Fig. 3, are desirable for MR sensors and heads.

The M R rat ios of C o M n B / C o 9 o F e ] o ( = M ' ) /Cu /Co9oFe lo (= M) spin-valves were very small, as mentioned above. The values increase due to surface oxi- dation of the Co9oFelo( = M) layer, as the H c of the CoFe/CoFeO layer increases and the difference in H e between the CoMnB/CoFe and the CoFe/CoFeO layer becomes larger, which enables an anti-parallel spin config- uration between the magnetic layers. An MR ratio of 5.7% was obtained for C o M n B ( 2 ) / C o F e ( 1 . 2 ) / C u ( 2 . 4 ) / CoFe(2)/CoFe0(0.3).

10

6 0

= 4

0 0 0 0 0 •

• 0

I Co~r~B/Co/Cu/Co CoUnB/Co/Cu/Co/Co0 L O • 0 , I , I , I T

0 100 200 300 400

Annealing Temperature ( ~ )

Fig. 6. Annealing temperature dependence of the MR ratios for C o M n B ( 2 ) / C o ( 1 . 2 ) / C u ( 2 . 4 ) / C o ( 2 ) / C o O ( 0 . 5 ) and CoMnB(2)/Co(1.2)/Cu(2.4)/Co(2).

3.3. Thermal stability

The thermal stability of the MR ratio of C o M n B / C o / C u / C o and C o M n B / C o / C u / C o / C o O was studied. The samples were annealed for 30 min at 100- 350°C in vacuum. Fig. 6 shows the annealing temperature dependence of the MR ratios. The MR ratio was stable up to 300°C for C o M n B ( 2 ) / C o ( 1 . 2 ) / C u ( 2 . 4 ) / C o ( 2 ) / COO(0.5) and was stable up to 250°C for CoMnB(2) / Co(1.2)/Cu(2.4)/Co(2). The increase of the MR ratio by annealing at 100-300°C and the decrease by annealing above 300°C for C o M n B / C o / C u / C o / C o O is attributed to the change in the H c of the C o / C o O layer, but the reason for the deterioration of the MR ratios by annealing above 300°C for C o M n B / C o / C u / C o is uncertain at pre- sent, as the [ N i F e C o / C u / C o / C u ] multilayers are ther- mally stable up to 300°C [9]. It may be that these spin- valves have a considerably high thermal stability, when we consider that they are thinner than 10 nm and have no buffer and capping layers.

3.4. Other spin-valves

We have studied other spin-valves by substituting Co83Nbl].sZrs.5 or Co78Fe2Nb10B]o for CoMnB. Gener- ally speaking, the MR ratios of the spin-valves having an AM layer with no B were smaller than those of the spin-valves having an AM layer containing B. The MR ratio of CoNbZr(2)/Co(1.2) /Cu(3)/Co(2) was 2.7% and that of CoFeNbB(2)/Co(1.5)/Cu(2.4)/Co(2) was 5.1%. Details of the results will be published elsewhere.

4. Conclusion

The MR properties of A M / M ' / C u / M ( / M O ) (AM = CoMnB, CoNbZr, CoFeNbB; M = M' = C o , Co9oFe]o, CosoFe5o) spin-valves were studied. C o M n B / C o / C u / C o ( / C o O ) showed considerably large MR ratios, 5 - 7.6%, and were thermally stable up to 250°C. These spin-valves were prepared without buffer layers and are composed of very thin ( < 2 nm) AM, M', Cu and M (and MO) layers, resulting in a total thickness of < 10 nm and are advantageous for MR heads having narrow shield gaps.

References

[1] T. Shinjo et al., J, Phys. Soc. Jpn. 59 (1990) 3061. [2] B. Dieny et al., J. Magn. Magn. Mater. 93 (1991) 101. [3] H. Sakakima et al., Jpn. J. Appl. Phys. 33 (1994) L1668. [4] M. Satomi et al., Digests of 19th Ann. Conf. on Magnetism in

Japan (1995) p. 346. [5] M. Jimbo et al., Jpn. J. Appl. Phys. 34 (1995) Ll12. [6] S.S.P. Parkin et al., Appl. Phys. Lett. 61 (1992) 1358. [7] Y. lrie et al., Jpn. J. Appl. Phys. 34 (1995) L415. [8] J. Inoue et al., J. Magn. Magn. Mater. 121 (1993) 344. [9] M. Satomi et al., J. Magn. Magn. Mater. 126 (1993) 504.