Cu] multilayers

Journal of Magnetism and Magnetic Materials 126 (1993) 437-439 North-Holland

Magnetoresistance of non-coupled [NiFe/Cu/Co/Cu] multilayers H. Y a m a m o t o a, y . M o t o m u r a

a T. A n n o b,1 and T. Shinjo b

a Functional Devices Research Laboratories, NEC Corporation, Kawasaki 216, Japan b Institute for Chemical Research, Kyoto University, Uji, Kyoto-fu 611, Japan

Non-coupled-type [NiFe/Cu/Co/Cu] multilayers prepared on Cr buffer layers showed enhanced magnetoresistance (MR) ratios and sharp field dependences. The effect of interface doping with impurity layers is systematicallystudied. 1. Introduction Baibich et al. discovered the giant magnetoresistance (MR) effect in F e / C r multilayers in 1988 [1]. The reduction of resistivity in a [Fe(30 ,~)/Cr(9 "~)]60 multilayer is 46% at 4.2 K. The origin of this effect is believed to be the scattering of conduction electrons, depending on the spins at the magnetic layers. The authors have explored the MR effect in noncoupled-type multilayers consisting of two magnetic component with different coercive forces, and relatively thick intervening nonmagnetic layers. An antiparallel state is formed in the magnetization process as a result of the different coercive forces. The interaction between adjacent magnetic layers is relatively small, so that a large MR change is obtained in a small field. The MR ratio of 10% in a [NiFe(30 ,~)/ Cu(50 ,~)/Co(30 ,~)/Cu(50 ~')]15 multilayer was observed with a sweeping magnetic field of +500 Oe at RT [2]. The MR properties of non-coupled-type sandwich films have been studied by Dieny et al. [3]. For technical applications such as MR sensors and magnetic recording MR heads, investigations into the soft MR effect are important. In this paper, the improvement of MR properties using Cr buffer layers is described, and the effects of interface doping with impurity layers are discussed. 2. Experimental The multilayers were prepared by the ultrahigh vacuum evaporation method, by successively depositing a Cr buffer layer, NiFe alloy, Cu, Co and Cu layers on glass substrates. The four-layer unit (NiFe, Cu, Co and

Correspondence to: Dr H. Yamamoto, Functional Devices Research Laboratories, NEC Corporation, 4-1-1, Miyazaki, Miyamae-ku, Kawasaki 216, Japan. Tel: 044-856-2086; fax 044-856-2220. ~ Present address: Ube Industries, Ube, Yamaguchi 755, Japan.

Cu) was repeated 5 times or more. The base pressure was about 10 10 Torr, the deposition rate was 0.3-0.5 A / s , and the substrate was at room temperature (RT) during the evaporation. The multilayers were analyzed by X-ray diffraction (XRD). Cross-sectional images of the multilayers were observed by transmission electron microscopy (TEM). The resistance was measured using the conventional four-terminal method at RT. The direction of the current (1 × 10 4 A) was in-plane, perpendicular to the external field, which was also in-plane. The MR ratio was defined as A p / p o × 100%, where P0 is the sheet resistivity at 500 Oe. The magnetic hysteresis curves were obtained by a vibratingsample magnetometer (VSM). 3. Result and discussion 3.1. Effect of the Cr buffer layer

The MR ratio for an evaporated N i F e / C u / C o / C u system has been enhanced by a Cr buffer layer. Figs. I(A) and (B) show the MR and M - H curves at RT for the Cr(30 _A)/[NiFe(20 ,~)/Cu(40 ,~)/Co(20 ,~)/ Cu(40 A)]50 sample. The observed MR ratios are 14% at RT, and over 50% at 4.2 K, almost the same as the values obtained for antiferromagnetically coupled F e / C r multilayers. In the following, samples A and B denote multilayers with and without Cr buffer layers, respectively. The small-angle X-ray diffraction patterns for samples A and B are quite different. The second Bragg peak, corresponding to the multilayer periodicity in sample B was observed, whereas in sample A, up to 6 peaks could be observed, eventhough the X-ray diffraction intensity was reduced due to X-ray absorption at the Cr buffer layer. This result shows that sample A has a better flatness in each layer than sample B. A crosssectional image of sample A, by TEM observation, also indicate sharp interfaces and good flatness for individual layers. In summary, the flatness of the multilayer is improved by a Cr buffer layer, and the MR ratio of the o

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

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was studied systematically. 1 ,~ transition metals (TM = Cr, Mn, Fe, Co and Nis0F%0) were inserted at the m a g n e t i c / n o n m a g n e t i c interfaces in Cr(50 ,~)/ [NiFe(20 A ) / C u ( 3 5 A ) / C o ( 1 0 A ) / C u ( 3 5 '~)]5 multilaycrs. The MR ratio, the sheet resistivity Po and Ap arc shown in order of the atomic n u m b e r of the inserted TMs in fig. 3. In the case of multilayers with Cr or Mn impurity layers, the Po is almost a factor of 2, because of the crystallographic randomness at the interfaces. The MR change Ap depends on the TM. As mentioned above, the MR ratio for the N i F e / C u / C o / C u system with a Cr layer is reduced dramatically. The (Co)/NiFe/(Co)/Cu/Co/Cu system shows a larger MR ratio than a conventional N i F e / C u / C o / C u system. Inoue et al. [7] calculated Ap in T M / C u multilaycrs using the Anderson model with the H a r t r e e - F o c k approximation, and found that the virtual band states (VBS) appear in the s-band, since the s- and d-band positions are at almost the same energy level. The majority ( + ) and minority ( - ) spin VBS split energetically, which results in the difference in the + and spin density of states (DOS) at the Fermi level (EF). The relative positions of the + / spin VBS and E~:

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It has been found that the MR change, Ap, is sensitive to the state of the m a g n e t i c / n o n m a g n e t i c interfaces in magnetic multilayers [4-6]. Thus MR properties should depend strongly on the insertion impurities at the interfaces. Figs. 2(A) and (B) show the M - H and MR curves for Cr(50 ,~)/[NiFe(20 ,~)/Cu(35 A ) / C r ( t ) / C o ( 1 0 A) / C r ( t ) / C u ( 3 5 A)]s multilayers with Cr impurity layers inserted at the C o / C u interfaces, where tCr = 0 and 0.5 * , respectively. Good antiparallel orientations between NiFe and Co magnetizations were obtained in both cases, but Ap abruptly decreased by approximately 1/10 in the case of the 0.5 * thick Cr impurity layers. In the case tc~ = 2 ,~, the MR change was almost zero. This reduction in Ap is noteworthy, even though the 0.5 A thick Cr layer is not formed in an even monolayer, but the Cr atoms are dispersed at the C o / C u interfaces. The conclusion is that spin-dependent scattering occurs predominantly at the interface. The MR ratio dependence on the inserted materials

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This paper has shown the M R change Ap depends on the Cr buffer layer and on thin impurity layers inserted at the m a g n e t i c / n o n m a g n e t i c interfaces in non-coupled N i F e / C u / C o / C u multilayers. For technical applications, it is important that the M R change is enhanced by improving the flatness of the interface using Cr buffer layers. The multilayers with a thin Cr layer inserted at m a g n e t i c / n o n m a g n e t i c interfaces show almost no change in MR. On the other hand, an intervening thin Co layer enhanced the M R ratio in the ( C o ) / NiFe/(Co)/Cu/Co/Cu system. This result indicates that spin-dependent scattering occurs primarily at the interfaces.

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References

Ni(Fe)

'I?M Fig. 3. MR ratios for Cr(50 ,~)/[TM(1 ,~)/NiFe(20 ,~)g/TM(1 A)/Cu(35 ,~)/TM(1 A)/Co(10 ,~)/TM(1 ,~)/Cu(35 A)] 5, as a function of atomic number order for inserted magnetic transition metals (TM = Cr, Mn, Fe, Co and NisoFe20). depend on the magnetic T M in T M / C u multilayers. The maximum Ap value was obtained in a C o / C u multilayer by this model calculation. This experimental result on the M R properties of an interface-doped NiFe/Cu/Co/Cu multilayer is consistent with this calculated result. It is certain that the interfaces play an essential role in the M R effect of multilayers.

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