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nlugolU¢ materials ELSEVIER Journal of Magnetism and Magnetic Materials 145 (1995) L268-L272

Letter to the Editor

Anisotropic low-field giant magnetoresistance in [Ni-Fe-Co/eu] multilayers

Hiroshi Sakakima a, Mitsuo Satomi a, Kazuhiro Onaka b, Shigeru Yamamoto b a Central Research Laboratory, Matsushita Electric Industrial Co., Ltd., 3-4 Hikaridai, Seika-cho, Souraku-gun, Kyoto 619-02, Japan

Matsushita Electric Component Co., Ltd., 1006 Kadoma, Osaka 571, Japan

Received 16 November 1994; in revised form 12 January 1995

Abstract

[Ni-Fe-Co(3 nm)/Cu(2 nm)] multilayers having in-plane uniaxial magnetic anisotropy were prepared by using a carousel type sputtering apparatus. The MR (magnetoresistance) ratio was about 8% and the MR curve along the easy axis showed a sharp split hysteresis with a very small transient field, AH ~ 3 Oe, though the switching is irreversible. The experimental results are discussed by using magnetic phase diagrams calculated by Dieny et al. (1990) and Folkerts (1991).

1. Introduction

Since the discovery of giant magnetoresistance (GMR) in [Fe/Cr] multilayers [3], many studies have been conducted on the phenomenon, which has been attributed to interlayer antiferromagnetic cou- pling between the magnetic layers across the non- magnetic layer [4]. The magnetoresistance and the magnitude of the interlayer exchange coupling were found to oscillate with nonmagnetic layer thickness in [Co/Ru], [Co/Cr] and [Fe/Cr] multilayers [5]. However, these multilayers show GMR only when a large magnetic field (several kOe) is applied.

It is desirable from the applicational point of view that the multilayers show a low-field GMR at RT. We reported that [ N i - F e - C o / C u ( / C o / C u ) ] multi- layers [6,7] prepared by sputtering showed a large MR ratio, 10-15% with small saturation fields, H~ = 0.1-0.15 kOe, when the Cu layer thickness tc~ was about 2 nm. These multilayers are applicable for

MR sensors but not for MR heads as the H s values are not sufficiently small.

We have studied spin-valve [8] multilayers using antiferromagnetic layers such as [ { A F / M / C u / M } / Cu] [9] and those using hard magnetic layers such as [{HM/Cu/M}/Cu] [10], where A F - - F e - M n , HM = Co-Pt, M = Ni-Fe-Co, Co-Fe. These spin valves show very small switching field, H t < 0.01 Oe, but the MR changes are not so large as those of ex- change-coupled-type GMR multilayers.

[Co/Cu] [11] and [Ni -Co-Fe -Cu /Cu] [12] mul- tilayers having in-plane uniaxial magnetic anisotropy were studied to obtain low-field GMR. These multi- layers shows wall motion type [2] MR curves with a plateau near zero field and the MR changes occur near Hs. Therefore, the MR changes could not ob- tained near zero field.

We have also studied GMR multilayers having in-plane uniaxial anisotropy. These multilayers showed spin rotation type [2] MR curves with split

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H. Sakakima et al. /Journal of Magnetism and Magnetic Materials 145 (1995) L268-L272 L269

hysteresis. The MR change near zero field was about 8% and the switching field was about 3 0 e , though the switching is not reversible.

10 4 A / m ) in the sample plane. AC MR curves and M - H loops at 60 Hz were also measured at RT.

2. Experimental

The anisotropic [Ni-Fe-Co/Cu] N multilayers were prepared onto glass substrates fixed on a rotat- ing carousel holder by an rf sputtering apparatus equipped with 5" × 15" sized Cu and Ni80Fel0Cot0 alloy targets. The distance between the substrates and the targets was about 50 mm. So the films were deposited with oblique incident angles, resulting in the uniaxial magnetic anisotropy. The sputtering Ar gas pressure was kept at 0.8 Pa (6 mTorr). The deposition rates of Ni-Fe-Co and Cu were adjusted to be around 3 and 2 nm per rotation of the carousel respectively, by controlling the rotation speed of the carousel and the sputtering power. The repetition number, N, was 30, so the sample thickness was about 150 nm.

The film structure was studied with transmission electron microscope (TEM) and X-ray diffraction (XRD) with CuK,. The magnetoresistance was mea- sured at RT using the four-point probe method with application of a dc magnetic field of 500 Oe ( ~ 4 ×

3. Results

The obtained [Ni-Fe-Co/Cu] films were con- firmed to have clear layered structure by small-angle XRD and TEM, though the films were prepared by simply rotating the carousel in front of the large targets. Fig. 1 shows the cross-sectional TEM image of the multilayers. The interfaces are considerably smooth and continuous, though they are not so smooth and clear as those in isotropic [Ni-Fe- Co/Cu] multilayers [6,7] prepared by rf diode sput- tering equipped with 80 mm-diameter targets.

The obtained [Ni-Fe-Co/Cu] multilayers showed in-plane uniaxial anisotropy with easy axis parallel to the rotation direction of the carousel and showed GMR of ~ 8% when the Cu layer thickness, tcu, was 2 nm, like the 2nd MR peak [7] found in the isotropic [Ni-Fe-Co/Cu] multilayers prepared by rf diode sputtering (where the 1st MR peak was found at tc~ = 0.9 nm). Fig. 2 shows the dc MR curves measured with applying magnetic field parallel to the hard axis or parallel to the easy axis of the [Ni-Fe- Co(3 nm)/Cu(2 rim)] multilayer. The MR curve

Fig. 1. Cross sectional TEM image of [Ni-Fe-Co(3 nm)/Cu(2 nm)].

L270 H. Sakakima et al. / Journal of Magnetism and Magnetic Materials 145 (1995) L268-L272

: (a)

0 8

£o)

0

-300 300 0

APPU~ ~ (Oe)

Fig. 2. DC MR curves of [Ni-Fe-Co(3 nm)/Cu(2 nm)]. (a) H ± easy axis, (b) H 1] easy axis.

along the hard axis does not so much differ from those reported for the isotropic [ N i - F e - C o / C u ( / C o / C u ) ] multilayers [6], but the MR curve along the easy axis is quite different and shows sharp split hysteresis with very small switching field, H t ~ 5 Oe, and with small saturation field, H s ~ 20 Oe.

Fig. 3 shows the ac MR curves measured with maximum field strength of 30 Oe. The shape of the MR curve along the hard axis is similar to those reported for the isotropic GMR multilayers. The MR curve along the easy axis is similar to those reported for spin-value multilayers with a small transient field, A H ~ 3Oe, though the multilayers of the present study have only one kind of magnetic film, N i - F e - Co. The magnetization switching is, however, irre- versible and the MR curve changes to a square minor loop when the field is swept between - 5 to 5 0 e . Fig. 4 shows the ac M - H loops for the sample. The

t~

- 3 0 Ht Hs 30

APPLIED FIELD (Oe)

Fig. 3. AC MR curves of [Ni-Fe-Co(3 nm)/Cu(2 nm)]. (a) H _L easy axis, (b) H II easy axis.

H. Sakakima et al. / Journal o f Magnetism and Magnetic Materials 145 (1995) L268-L272 L271

i ~ i ~¸

g

Z~

-20 20

APPLIED FIELD (Oe)

Fig. 4. M - H loops of [Ni-Fe-Co(3 nm)/Cu(2 nm)]. (a) H 2. easy axis, (b) H IJ easy axis.

coercivity, H c, for the easy axis seems to coincide with the H t in the MR curve for the easy axis. This implies that an antiparallel spin configuration of the magnetic layers are realized around H t.

4. Discussion

We would like to discuss our experimental results by using the magnetic phase diagram calculated by Dieny et al. [1] and Folkerts [2]. The total energy of the system having in-plane uniaxial anisotropy, K, composed of two magnetic layers with equal layer thickness, t, and magnetization, M, coupled antifer- romagnetically with exchange coupling energy, J, through a non-magnetic layer is given by:

E = -½MH(cos 0 t + cos 02) - J cos(01 - 02)

+ E K ,

where

E K = ½K(sin201 + sine02) for , Ileasy axis,

Er = ½K(cos201 + cos202) for H _1_ easy axis.

The stable configurations for magnetization are found by equating OE/O 1 and OE/O 2 to zero and the following phases are obtained [2].

(I) H IJeasy axis: (a) 0 1 = 0 2 = 0 ; e = p - 2 h . (b) 0 1 = - 0 2 with cos 0 l = h / ( 2 p - 1 ) > 0 ;

e = l - p - c o s 01 . (c) 0 l = 0 , 02='rr; e = - p . (d) 0 1 = - 0 2 with cos 0 l = h / ( 2 p - 1 ) < 0 ;

= l - p - c o s 01 . (e) 01=02=7r; e = p + 2h. (II) H _1_ easy axis: (f) 0 1 = 0 2 = 0 ; s = l + p - 2 h .

L272 H. Sakakima et al. /Journal of Magnetism and Magnetic Materials 145 (1995) L268-L272

(g) 01 = - 0 2 with cos 01 = h / ( 2 p + 1); 8 = - p - h cos 0 l .

(h) 0 1 = 0 2='rr; e = l + p + 2 h . Here e = E l K , p = - J / K , and h = H M / 2 K .

MR curves corresponding to the transitions be- tween the magnetic phases above mentioned are calculated by Folkerts [2] on examining the total energy of the system and d2E where

dZE=(O2E/~012) d012 + 2(OZE/aOlO02) d01 d02

+ (02e/ 022) d0?. Spin rotation with dO z = CdO 1 can only occur for d2E < 0, resulting in hysteretic MR curves.

Comparing our experimental results with the cal- culted ones, the MR curve for H I] easy axis with split hysteresis shown in Fig. 2 or in Fig. 3 seems to correspond to the spin rotation type MR curve [2] with following transitions:

1 1 (a) ~ ( c ) for 3 < P < 7 and h = 2 p - 1 .

( c ) ~ ( e ) for p < 2 and h = - p - 1 .

The MR curve for H _1_ easy axis shown in Fig. 2 or in Fig. 3 may be explained with the transitions (f) ~ (g) and (g)--* (h) that occur when h = + ( 2 p + 1) [2].

From the comparison between the calculated MR curve and experimental one, p is considered to be close to 0.5, and K and J are estimated to be about 0.7 × 10 4 erg/cc and - 3 . 5 × 10 3 erg /cc ( j = tJ ~ - -1 )< 10 -3 e r g / c m 2 ) , respectively. The small J value in the anisotropic multilayers probably origi- nated in the interface roughness caused by the depo- sition with oblique incident angles.

4. Conclusions

Anisotropic low-field GMR was found in [Ni- Fe-Co(3 nm)/Cu(2 nm)] multilayers having in-plane uniaxial magnetic anisotropy prepared by using a

carousel type sputtering apparatus. The MR ratio was about 8% and the MR curve along the easy axis showed a sharp split hysteresis with a very small transient field, AH ~ 3 Oe, though the switching is not reversible. The experimental results can be ex- plained qualitatively by using magnetic phase dia- gram calculated by Folkerts.

Acknowledgments

The authors wish to thank Mr. Okano, Matsushita Techno-Research for obtaining cross-sectional TEM images of our samples.

References

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