Journal of Magnetism and Magnetic Materials 126 (1993) 425-429 North-Holland

Giant magnetoresistance and low saturation fields in Co-Fe/Cu multilayers

K. Inomata and Y. Saito Toshiba Corporation, Research and Deuelopment Center, Kawasaki 210, Japan

We have investigated the magnetoresistive properties of CoxFe 1 x/Cu multilayers with different CoxFe 1 x and Cu thicknesses, prepared on MgO(ll0) substrates using ion beam sputtering. The MR ratio for Co9Fe/Cu multilayers was significantly larger than that for Co/Cu multilayers. The MR ratio oscillates for Cu layer thickness with a 12 ,~ period for all the multilayers investigated. The Cu intervening magnetic layer coupling depends on the Co concentration and has a broad maximum around x = 0.8. The uniaxial anisotropy induced by epitaxial growth in the multilayers on MgO(ll0) substrates led to a low saturation field and quite a small field change 2~H needed for giant magnetoresistance. An MR ratio of 16% was attained for A H = 50 Oe at room temperature.

I. Introduction 2. Experiments

In the last few years, multilayers formed by ferro- magnetic and nonmagnetic metals have received con- siderable attention due to the giant magnetoresistance (GMR) observed in different systems. Following the discovery in F e / C r multilayers [1], GMR has been observed in multilayers such as C o / C u [2,3], N i - F e / C u [4] and C o / A g [5]. We recently found Co9Fe /Cu [6,7] multilayers possessing a larger MR than that of C o / C u in ur experiment. In these systems, due to antiferro- magnetic coupling, the in-plane magnetizations of the adjacent ferromagnetic layers are antiparallel. Because this coupling is very high (0.15-0.3 e rg /cm 2) a large external field is needed (> 1 kOe) to observe the GMR. The field required for GMR is several tens of Oe for magnetic sensor applications such as MR heads. Several systems, such as multilayers with two magnetic components [9] and spin valves [10], have been devel- oped to obtain a low field in which the magnetic interlayer is uncoupled. We have demonstrated that the in-plane uniaxial anisotropy, induced in C o - F e / C u and Nis0Fez0/Cu multilayers grown on MgO(ll0) sub- strates prepared using ion beam sputtering (IBS), led to the low fields needed for GMR [7,8].

This paper reports in more detail on C o - F e / C u multilayers with in-plane uniaxial anisotropy, prepared on MgO(l l0) substrates. Both the magnetic and Cu layer thickness dependences on GMR and the fields needed for GMR were investigated. Quite a low field for obtaining GMR was obtained and its mechanism is discussed.

Correspondence to: Dr K. Inomata, Toshiba Corporation, Re- search and Development Center, Saiwai-ku, Kawasaki 210, Japan. Fax: 044-549-2260.

CoxFe 1 J C u multilayers were prepared by IBS on MgO(ll0) single-crystal substrates. The base pressure was 5 × 10 -7 Torr and sputtering was carried out using 1.3 × 10 4 Torr Ar ions. V B for the sputtering was optimized according to the CoxFe 1 x composition in order to obtain the respective maximum GMR values. The substrate temperature was fixed at room tempera- ture. The structures were investigated by X-ray and electron diffraction. The magnetic properties were in- vestigated by VSM and a torque magnetometer, and MR measurements were made using the four-point probe method with a field applied in the film plane.

3. Experimental results

3.1. Cu thickness dependence

Fig. 1 shows the MR ratios as a function of Cu layer thickness for (10 ,~ CoxFe l_x/tcu.~ Cu)16 multilayers measured at (a) RT and (b) liquid N 2 temperature. The magnetic field was applied perpendicular to the cur- rent in the film plane. The MR ratio is defined as the ratio of the total resistivity change 2tp to the resistivity at saturation Ps. The MR ratios are for the multilayers prepared with V B optimized according to the CoxFe 1 x compositions. The MR oscillates for Cu layer thickness with a 12 A period for all multilayers. The maximum MR ratio was attained for x = 0.9. The value at the first peak for x = 0.9 is higher than that for C o / C u multilayers by about 5% and 20% at RT and liquid N 2 temperature, respectively; The low MR ratio at the first peak with tc , = 10 A for x = 0.5 may be due to film imperfections. Fe-rich C o - F e / C u multilayers are

0304-8853/93/$06.00 © 1993 - Elsevier Science Publishers B.V. (North-Holland)

40

10

30

n.- -<3 20

P 10

(a) I I I

• (Co9Fe IOA/Cu tcu A),o o (Co 10AICu tcu ,&,),o • (Co0Fe210A/Cu tcu A)~6 ~ (CoFe loA/Cu tcu A)m

R.T.

20 30

teu ( h )

426 K. lnomata, Y. Saito / Giant magnetoresistance in Co-Fe / Cu

40

100

80

60

n-

4O

20

(b)

1.1

I 10

t I

• (Co9Fe IOA/Cu tcu A)m o (Co lOA/Cu too A),+ • (CooFe210A/Cu tc. A)m ,~ (CoFe 10A/Cu tc~ A),6

Liq. N2

20 30

tcu (h)

4 0

Fig. l. MR ratio as a function of Cu layer thickness for Co~Fe I ~ / C u multilayers at (a) RT and (b) liquid N 2 temperature.

difficult to grow epitaxially because of the competit ion of fcc Co and bcc Fe.

Fig. 2 shows the 5 p values as a function of Cu layer thicknesses for (10 A Co~Fe 1 ~/ tc~ A Cu)16 multilay- ers measured at liquid N 2 temperature. The Ap differ- ence at the first peak for x = 0 and 0.9, about 70% higher for x = 0.9, is significantly larger than the MR ratio difference. This is attributed to the higher satura- tion resistivity in CogFe alloy. The Co concentration dependences at liquid Ng temperature on (a) the MR ratio and (b) Ap for (10 A Co~Fe I +/to: . A Cu)16 with

? lo

• °If 10

t • (CogFe l o A / c u tc~ A)~6

o (Co IoA /cu tc, A),, -

• (CosFe210A/Cu tc. A)~6

(CoFe IOA/Cu tCu A)+6

Liq. N 2

20 30 40

t c , (h)

Fig. 2. Ap as a function of Cu layer thickness for Co~Fe[ ~,/Cu multilayers at liquid N 2 temperature.

tcu = 10 and 21 ,~ are shown in fig. 3. It can bc seen that the MR ratio and Ap have maxima at the compo- sition CogFe for both Cu layer thicknesses. In particu- lar, Ap for CogFe with tcu = 10 A is significantly larger than that for the 10 A C o / 1 0 ,~ Cu multilayers. The difference in Ap is larger than that of the MR ratio between both multila~cers, which is based on larger Ps for (10 A C o ~ F e / 1 0 A Cu)]~. These observations confirm that the MR ratio increases essentially with

80 (a) i n I / O ~ / \ I

ICoxFe,.x/eu)+/1st..k\l I tCoFe=lOA /. lSI- P .'aK,, ~1 / l iq N 2 ? (tcu=lOA) •

240

l 20

i J I J I 00.5 0.8 0.9 1.0

X

15

~.10

5 0

0 I 0.5

(b) I I I i

"°,", / \

I J 0.8 0.9 1.0

X

Fig. 3. Co concentration dependence on MR ratio (a) and Ap (b) at liquid N z temperature in (10 ,~ Cox Fe I ~ / t (u ,~ Cu)]+, with to, u = 10 and 21 A, prepared with appropriately opti-

mized V B.

I~ Inomata, Y. Saito / Giant magnetoresistance in Co-Fe / Cu 427

0.3

~. 0.2

O3

0.1

i i ...---O~O~O (CoxFel.xtCU)16 1st. peak tCoFe = 10 A (tcu = 10 A) R.T.

2nd. peak (tcu = 21 A

O ~ , . ~ O - - O ~ o

0 h I 0.5 0.8 0.9 1.0

x Fig. 4. Co concentration dependence on J at RT in (10 ,~

Co~Fel -x / t cu "~ Cu)16 with tc~ = 10 and 21 A,.

the addition of small amount of Fe to C o / C u multilay- ers. This is very important for determining the G M R mechanism in the multilayers. Our results support the theory proposed by Inoue et al. [11].

The saturation field Hs in a M R curve with in-plane uniaxial anisotropy can be described as H~ = 2J / t M. Ms when the field is applied parallel to the easy axis, where J is the interlayer coupling, t M is the magnetic layer thickness and Ms is the saturation magnetization. Thus, J can be est imated using the experimental values of H s, M s and t M, which is shown in fig. 4 for the first and second peaks in the Cu thickness-dependent MR. J seems to take a broad maximum around x = 0.8-0.85 for both Cu layer thicknesses. The value for C o / C u with tc~ = 10 ,~ is nearly the same as that est imated by Parkin et al. [3] using by another method.

3.2. Magnetic layer thickness dependence

Fig. 5 shows the magnetic layer thickness depen-

dence on P0, Ps, Ap and A p / p s in (/CogF e .~ C o 9 F e / 1 0 Cu)16 as a function of /CogF e. In the range of tCogF e

investigated, Ap does not greatly change above t c o ~ e = 10 A, below which it decreases with decreasing tCogFe, presumably due to film imperfections. Ap/ps, however, depends strongly, o n tCogF e and exhibits a maximum at tCogF e = 15 A. This is attributed to P0 behavior. P0 decreases with increasing tCogF e above 15 A, while p~ exhibits a saturation of the decrease, when tcogF e 15 A. This P0 reduction induces the decrease in Ap/ps with increasing tCogF e. The cause of the P0 reduction remains unclear, but may be related to the magnetic domain configuration at zero field. For thicker magnetic layers mult idomain structure may oc- cur at zero field, thus bringing the antiphase domain boundaries [12] in multilayers, which leads to lower P0,

20 H//MgO[lO0] I (tCo, Fe A[CogFe/10 ACu)l~ o ~

R. T. e~e__o_o.......~_e o_ e~e" °e*""°~,e" p o.~

15 50

? °'-~-o-.o_.~_o o_ ~

10 ,,~-.., AP/P s 40

o . \< / 5 n/~-'O--O--O--D-_D 30

0 I I 20 10 20 30

tC%Fe (A)

Fig. 5. Po, Ps, 4p/Ps at RT as functions of CogFe layer thickness for (tCogF e A CogFe/10 ,~ Cu)16 multilayers.

resulting in lower Ap/ps for larger /CogF e. This situa- tion can be seen in the M R curves for multilayers with different /CogFe and the same tcu(21 ,~) shown in fig. 6. For tCogF e = 25 A, the resistivity exhibits a maximum at zero field, but not for thicker tCogFe(40 ,~). The latter suggests the imperfect antiferromagnetic interlayer coupling at zero field and also at maximum Ap/ps state.

(a)

(25A CogFe/21ACu)30

7 Ap/ps= 23.5% "-~ AH = 80 O e ~

, ]J ' , , , ' i f , , , ' t l 1 -0.3 -0.2 -0.1 I 0.1 0.2 0'.3

H(kOe)

(b) (40A Co9Fe/21ACu)20

Ap/ps= 15.6% o~ AH = 50 Oe ~

/ -0.3 -0.2 -0.1 I 0.1 0.2 0.3

H(kOe)

Fig. 6. MR curves at RT for (tco~F e ,~ Co9Fe/21 ,~ Cu)16 multilayers with lCogFe = 25 and 40 A.

428 K. Inomata, Z Saito / Giant magnetoresistance in Co-Fe / Cu

5

E o 4 o~

3

2

1

0

I I I I (tCogF e A.Co9Fe / tCu A. Cu)16 R.T.

- e - tCo9Fe(tCu = IOA) NiX, -o- tcu = 10A) (tCo9Fe

.o~_O~ ~ o r~.~o, o

10 20 0 40 50

tC%Fe / tcu (A)

Fig. 7. In-plane uniaxial anisotropy in Co9Fe/Cu multilayers as a function of CogFe and Cu layer thicknesses.

3.3. Saturation fields

It has been demonstrated that the in-plane uniaxial magnetic anisotropy K~ can be induced in the Co~Fe 1 _ J C u multilayers on a MgO(ll0) substrate. Fig. 7 shows K u as a function of magnetic and Cu layer thicknesses in (tCogF e A CogFe/tc~ ,~ Cu)16. The K u hardly changes with tc~ but decreases drastically with increasing tcogFe , which suggests that the anisotropy can be induced through magnetoelastic energy coupled with the magnetostriction and epitaxial strain of Co-Fe films. This anisotropy leads to a metamagnetic transi- tion from antiferromagnetic to ferromagnetic with an increasing field when K u is larger than J / t M [8]. This metamagnetic transition realizes a low saturation field

o "~" (Co9Fe/Cu)n " r

6.0 RT tCu = I O A /

5 .0 n= 1 6 / e

-;/L 4.0 • tcu=21A

~ !~I //" ' -

2.0 ~ 0.2 . /

/ 1.0 * 05 1/tcoFe(nm 1 )

0 I P i i i P I I I I I I I 0 1.0 2.0

1/tCoFe(nm'l)

Fig. 8. Saturation fields for (tcogF e ,~ Co9Fe/tc. A Cu)16 with t = 10 and 21 A as functions of inverse magnetic layer

thickness.

o ~ RT

60 (IOAC%Fe/IcuA.Cu)I 6

5 0 - - ° - - ( tC%FeA'C%Fe/21A'Cu)2o

40 IC°gVe/tCu~ ° 10/10

30 / /25/21 A / 2O / .o 10/21

/ e z 4 0 / 2 1 . . ~

10 / ~ 10/33

0 i i i r i l i J I I i i , , J l l i 102 103

AH(Oe)

Fig. 9. MR ratio versus field change needed for GMR, H for Co Fe/Cu multilayers with various Co~Fe and Cu layer

thicknesses.

H S and a small applied field change AH required to obtain GMR when the field is applied parallel to the easy axis, as shown in fig. 6, in which AH = 80 and 50 Oe for MR ratios of 24% and 16%, respectively.

The magnetic layer thickness dependence on thc saturation field H S was investigated for (tc, A Cu0/ tCogF e .~ Co9Fe)16 multilayers with tcu = 10 and 21 A. All the samples revealed antiferromagnetic interlayer coupling. Hs was proportional to t CogFe~ as shown in fig. 8, which verifies the theoretical relation H~ = 2 J / t M ' M S [8]. Fig. 8 also demonstrates the invariance of J for magnetic layer thickness. The inset in fig. 8 shows the case of tc, = 21 A, in which AH is also

t-~ Fig. 9 summarizes the relations proportional to CogF e. of MR ratio versus AH for various magnetic and Cu layer thicknesses. This demonstrates the effectiveness of thicker magnetic layers for obtaining AH reduction with large MR ratios. In future, by controlling mag- netic and Cu layer thicknesses an MR ratio of 20% is expected to be attainable for a field of several tens of Oe in C o - F e / C u multilayers with in-plane uniaxial anisotropy.

4. S u m m a r y

The magnetoresistance was investigated for CoxFel_x/Cu multilayers on MgO(ll0) substrates prepared with various V B by IBS. The Cu intervening magnetic layer coupling depended slightly on the CoxFel_ x composition. The MR ratio for C o - F e / C u multilayers with high Co concentrations was shown to be significantly larger than that for Co/Cu multilayers. The in-plane uniaxial anisotropy induced by epitaxial growth on MgO(ll0) leads to a low saturation field and quite a small field change required to obtain

K. Inomata, Y Saito / Giant magnetoresistance in Co-Fe / Cu 429

G M R , which is indispensable for magnet ic field sen- sors such as M R heads.

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