High magnetoresistance in sputtered permalloy thin films through ...

IEEE TRANSACTIONS ON MAGNETICS, VOL. 36, NO. 1, JANUARY 2000

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High Magnetoresistance in Sputtered Permalloy Thin Films Through Growth on Seed Layers of (Ni0.81Fe0.19)1 xCrx W. Y. Lee, M. F. Toney, and D. Mauri

Abstract—The use of a thin (Ni0.81Fe0.19)1 Cr seed layer for obtaining high anisotropic magnetoresistance in Permalloy (Ni0.81Fe0.19) films is reported. The process yields a high of, for example, 3.2% for 120–Å-thick NiFe, without high-temperature deposition or annealing. X-ray diffraction shows that the NiFeCr seed layer causes the formation of large (111) textured grains in the Permalloy film, and that the interface between these two layers is quite smooth. These both increase the and reduce the resistance in the film, which lead to the high . Also discussed is the enhanced and thermal stability trilayer magnetoresistive sensors using this NiFeCr instead of Ta as a spacer.

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Index Terms—Anisotropic magnetoresistance, Permalloy film, seed layer.

I. INTRODUCTION

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HE MAGNETIC properties of sputtered thin films can be dramatically improved through deposition onto appropriate seed or buffer layers. In magnetic media, for example, Cr, Cr-alloy and NiAl seed layers are used to obtain good recording characteristics [1], [2], and the giant magnetoresistance in multilayers and spin valves structures can be significantly improved through the use of an appropriate seed layer [3]–[5]. In chemically ordered alloys, the extent of chemical order is highly dependent on the choice of seed layer [6], [7]. In some cases, there is a good understanding of how the seed layer influences the magnetic properties most often through a change in film structure (for example, in magnetic media where the Cr seed layer induces an (11.0) orientation in the magnetic media). However, the effect that the seed layer has on the film structure and the resulting effect on the magnetic properties is not always clear, which often makes selection of an appropriate seed layer difficult. In this paper, we describe the use of a 30–50-Å-thick Cr ( ) seed layer for obtaining (Ni0.81Fe0.19) high anisotropic magnetoresistance (AMR) in Permalloy (Ni0.81Fe0.19) thin films. We show that the high AMR coeffi) results from the formation of large grains in the cient ( Manuscript received August 19, 1999; revised September 24, 1999. This work was supported in part by National Synchrotron Light Source, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Division of Materials Sciences and Division of Chemical Sciences under Contract DE-AC02-98CH10886. W. Y. Lee and M. F. Toney are with IBM Almaden Research Center, San Jose, CA 95120 USA (e-mail: [email protected]). D. Mauri is with IBM Storage System Division, San Jose, CA 95193 USA. Publisher Item Identifier S 0018-9464(00)00463-5.

Permalloy/NiFeCr films and demonstrate that the Permalloy film restructures the poorly ordered NiFeCr film forming a mutually epitaxial, well-ordered structure. These Permalloy than films deposited on the films have much higher more conventionally used Ta seed layer, and they are thermally stable up to 450 C. Permalloy films fabricated in this manner have potential use in magnetic recording heads that employ read sensors based on the AMR effect (MR heads). Such heads can read data at high areal densities [8], but the extendibility of this MR technology to higher density is limited by the of the Permalloy sensor material, particularly for thickness ≤ 90 Å, which are required for areal density exceeding 5 Gb/in2 [8]. To extend the MR technology to higher areal densities, a for Permalloy in this thickness range is required. larger of Permalloy film can Others have shown that the be increased by as much as 50% through the use of high temperature deposition [9], [10] or postgrowth annealing [11]; however, the high temperature processing required to achieve this enhancement is undesirable in sensor manufacturing. The use of NiFeCr seed layers provides an alternative method to . obtaining suitably high

II. EXPERIMENTAL Thin films of Ni0.81Fe0.19/(Ni0.81Fe0.19) Cr were sputter-deposited sequentially from two separate Ni0.81Fe0.19 and Cr targets in a research magnetron sputtering system with torr. The Cr concentration a base pressure of Cr layer was varied by adjusting in the (Ni0.81Fe0.19) the deposition rate of the Ni0.81Fe0.19 and Cr targets during co-sputtering. Films of 30–1500 Å thickness were deposited in 3 mtorr Ar at 30–150 C on 1 in glass or Si (100) and 47 mm glass or N58 (an Al2O3 and TiC composite) substrates precoated with ≈1000 Å thick Al2O3. A deposition rate of e.g., 50 Å/min. was obtained for Ni0.81Fe0.19 and Cr with 100 and 130 W input power, respectively, from 2 in sputtering targets. To improve the film thickness and composition uniformity, the substrates were rotated at 30 RPM during deposition. The samples were deposited in the presence of permanent magnets which provide ≈150 Oe in-plane magnetic field to induce an in-plane uniaxial easy axis on the films. In addition, bilayer samples of Ni0.81Fe0.19 on Ni0.49Fe0.12Cr0.39 or Ta, were sputter-deposited at ambient temperature from separate targets in a commercial RF or ion-beam sputtering system on the substrates mentioned earlier. Post-deposition annealing of

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some of the films was done in the presence of a magnetic field torr. in a vacuum furnace having a base pressure of The magnetic properties of the films such as saturation magnetization ( ), coercivity ( ), hard axis anisotropy ( ), and ) were measured with a commercial B-H looper [12]. ( values reported in this paper were from films deThe posited on 1 in glass substrates. The values for the same films deposited on 47 mm glass substrates are ≈10% smaller due to the dimension of the four-point probe used. The film composition was determined by Rutherford backward scattering (RBS) and crystal and microstructure by X-ray diffraction (XRD) using and synchrotron radiation and by transmission elecboth Cu tron microscopy (TEM). III. RESULTS AND DISCUSSION The of Permalloy films depends significantly on the Cr seed layer. Cr concentration ( ) in the (Ni0.81Fe0.19) has a flat topped peak at As a function of , and falls off quickly for and . For example, for 80 Å thick Permalloy films was 64 and 55% higher Cr films with for deposition on 50 Å thick (Ni0.81Fe0.19) and 0.3, respectively, than with . Detailed results concerning the effects of Cr concentration on and the crystal structure of 50–500 Å (Ni0.81Fe0.19) Cr films will be published in a separate paper. Here we will focus , since this provides the maximum . These on Cr films become nonmagnetic at , (Ni0.81Fe0.19) cm at 200 and 45 and have a resistivity ( ) of 140 and 175 . Å, respectively, for the film with for Permalloy films deposited with Fig. 1 shows and without a 50 Å thick (Ni0.81Fe0.19)0.6Cr0.4 seed layer as a function of Permalloy thickness. Over the Permalloy thickness range of 80–475 Å, the NiFeCr seed layer is seen to increase by ≈50–80%. For comparison, is 1.8% for a 120 Å thick Permalloy film alone and is 2.1% for the film with a 50 Å Ta seed layer (see Fig. 2(a)), both substantially lower than the 3.0% for the film with the NiFeCr seed layer. The of Permalloy increases with increasing NiFeCr seed layer thickness and reaches a maximum value at a critical thickness of ≈30–45 Å, depending on the film deposition technique. For 120 Å Ni0.81Fe0.19/0–140 Å Ni0.49Fe0.12Cr0.39 bilayer films deposited by RF diode sputtering, the critical thickness is 42 versus the seed layer thickness data Å, as seen from the shown in Fig. 2(a). (The particle size of these permalloy films versus the seed layer thickness is shown in Fig. 2(b); see below.) For deposition by ion beam and dc magnetron sputtering (120 Å Ni0.81Fe0.19/Ni0.49Fe0.12Cr0.39), the critical thickness is ≈30 and 45 Å, respectively. We believe this difference is due to a difference in film microstructure resulting from the different for the sputtering methods. Note that the decrease in films deposited on thicker NiFeCr shown in Fig. 2(a) can be attributed to the higher current shunting expected for these thicker seed layers. The data shown in Fig. 1 are from the films were deposited nominally at 100 C. Comparably large also observed for the films deposited at lower temperatures (down to 30 C). Bilayer Permalloy/NiFeCr films are thermally was stable up to about 450 C, since no degradation in

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Fig. 1. The values of R=R versus the thickness of Permalloy films deposited seed with (dash curve) and without (solid curve) 50 Å (Ni0.81Fe0.19)0.6Cr layer in a DC magnetron sputtering system. For comparison, the values for a 120 Å thick Permalloy film (sputter-deposited in an RF diode system) before (open diamond) and after (open square) annealing at 400 C for 2 h, and sputter-deposited at 350 C in a dc magnetron system (solid diamond) are also given.

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Fig. 2. The values of (a) R=R (top panel) and (b) particle size (bottom panel) versus the thickness of the Ni0.49Fe0.12Cr0.39 seed layer for 120 Å thick Ni0.81Fe0.19 films deposited on Ni0.49Fe0.12Cr0.39 in an RF diode sputtering system. Solid squares are data from the the same 120 Å thick Ni0.81Fe0.19 film deposited on 50 Å thick Ta seed layer instead.

observed until annealing above 450–500 C (depending on for these films generally Permalloy thickness). Indeed, increased by ≈10% after annealing at up to 400 C. As an for a 246 Å Permalloy film deposited on a example,

Lee et al.: HIGH MAGNETORESISTANCE IN SPUTTERED PERMALLOY THIN FILMS

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TABLE I THE VALUES OF R=R(%) AND R = FOR THE AS-DEPOSITED AND POST-ANNEALED MR TRILAYER CONSISTING OF (a) 102 Å NiFeNb/30 Å Ni0.49Fe0.12Cr0.39/120 Å Ni0.81Fe0.19/30 Å Ni0.49Fe0.12Cr0.39 AND (b) 102 Å NiFeNb/50 Å Ta/120 Å Ni0.81Fe0.19/30 Å Ta

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NiFeCr seed layer increases from 3.65% to 3.95% after annealing at 300 C for 2 h, and to 4.10% after further annealing at 450 C for 2 h. The remarkable thermal stability of these films is in contrast to the poor thermal stability of Ta seeded films, where interdiffusion between the Ta and Permalloy is significant at 300 C [13], [18]. were also observed High thermal stability and for the trilayer sensors consisting of 102 Å NiFeNb/30 Å Ni0.49Fe0.12Cr0.39/120 Å Ni0.81Fe0.19/30 Å Ni0.49Fe0.12Cr0.39 deposited by ion-beam sputtering, as can be seen from the and R data obtained before and after post-annealing at 250 and 350 C for ≈7 h, shown in Table I. In these trilayer sensors, the NiFeNb layer is the soft biasing layer similar to that reported previously [12] and the Ni0.49Fe0.12Cr0.39 layers serve both as the spacer layer and overcoat. For comparison, the data for the trilayer sensors using the standard 50 Å Ta spacer layer and 30 Å Ta overcoat were also include in Table I. The trilayer sensor with the NiFeCr spacer layer not only produces in the as-deposited state, but also suffers a 25% higher much less thermal degradation after 250 and 350 C annealing than the corresponding trilayer sensors with the Ta spacer in the as-deposited state was layer. A 25–30% higher also obtained for the trilayer with this NiFeCr spacer than the trilayer with Ta spacer when the thickness of the Permalloy film is 90 or 150 Å which are the thickness required for 5 and 1 Gb/in2 recording density, respectively. For example, the of the trilayer sensor with 90 Å Permalloy is 1.97% with 42 Å Ni0.49Fe0.12Cr0.39 as a spacer layer, compared to values given here 1.51% with 50 Å Ta spacer layer. The are for the trilayers overcoated correspondingly with 35 Å of Ni0.49Fe0.12Cr0.39 or Ta. Results of B-H looper measurements reveal that the presence of Ni0.49Fe0.12Cr0.39 either as a underlayer in a bilayer or as a spacer layer in a trilayer structure does not significantly change the magnetic properties of the Permalloy film. No change in the magnetic moment of the Permalloy films ( 760 emu/cc) due to the presence of the seed layer was observed, since the ratio of the magnetic moment of the Permalloy film deposited with the NiFeCr seed layer to that without the seed layer is 1.001 in± 0.049 over the thickness range studied. The value of creases with the thickness of both Permalloy films deposited with and without the NiFeCr seed layer. For the same thickness, it is only slightly higher for the films deposited with NiFeCr seed layer, e.g., 1.1 versus 0.95 Oe at 130 Å. No significant difand between the trilayer with ference in the value of the Ni0.49Fe0.12Cr0.39 spacer layer and the trilayer with the Ta spacer layer was observed either. For the two trilayers with 90 of 1.4 Å thick Permalloy mentioned earlier, for example, a

Oe and a of 5.2 Oe were obtained for the one with the of 0.9 Oe and Ni0.49Fe0.12Cr0.39 spacer layer, compared to a of 4.6 Oe for the one with the Ta spacer layer. For the a and increase by Ta-trilayer shown in Table I, the values of decreases by 20% a factor of 10 and 3.5, respectively, and after annealing under the conditions shown. Contrary to this, these values remain practically unchanged for the NiFeCr-trilayer after annealing, consistent with the high thermal stability and shown in Table I. The high thermal stability of and the magnetic data for both the bilayer and the trilayer samples suggest no significant interdiffusion occurring between the Permalloy and the NiFeCr seed layer. High temperature annealing has been previously used to sigof Permalloy films [11] with a nificantly improve the of 3.5% reported for 200 Å thick films with annealing at 400 C. As a comparison of this approach with the NiFeCr seed before and after post-annealing at 400 C for 2 h layer, are also included in Fig. 1 for 120 Å, RF diode sputter-deposited for the 120 Å thick Permalloy film Permalloy films. The deposited on 50 Å (Ni0.81Fe0.19)0.6Cr0.4 is nearly the same as that of the annealed film, suggesting that the microstructure of the Permalloy films induced by the seed layer is similar to that achieved by high temperature annealing. To understand the cause of the high AMR in the NiFe/NiFeCr bilayers, in-plane (grazing incidence) [15], [16] and specular ) X-ray diffraction data were obtained for a series of struc( tures with 120 Å Ni0.81Fe0.19 films grown by RF diode sputtering on Ni0.49Fe0.12Cr0.39 seed layers with thickness between 0 and 140 Å, as well as a single 50 Å Ni0.49Fe0.12Cr0.39 film. We first discuss the specular (out-of-plane) diffraction data. For the single NiFeCr film, there is only a single weak peak corÅ, which we interpret as a bcc (110) responding to peak, consistent with the in-plane data (see below). In the single Permalloy film, an fcc (111) peak and a weak (200) peak are observed, suggestive of a relatively unoriented film. However, for all the NiFe films grown on NiFeCr, only the fcc (111) reflection is observed and this is very strong (about two orders of magnitude higher than the single NiFe film). In addition, interference fringes near the (111) peaks were observed for these films. The existence of these interference fringes shows that the interfaces in the films are quite smooth, consistent with the previously described thermal stability of the interface between Permalloy and the NiFeCr seed layer. Modeling of these interference fringe data supports this conclusion and shows that there are two separate NiFe and NiFeCr layers with different out-of-plane spacings. Fig. 3 shows the in-plane diffraction patterns for: (a) 50 Å Ni0.49Fe0.12Cr0.39, (b) 120 Å Ni0.81Fe0.19, and (c) 120 Å

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Fig. 3. The in-plane diffraction patterns for: (a) 50 Å Ni0.49Fe0.12Cr0.39, (b) 120 Å Ni0.81Fe0.19, and (c) 120 Å Ni0.81Fe0.19/50 Å Ni0.49Fe0.12Cr0.39 films deposited by RF diode sputtering. Here Q is the scattering vector with a magnitude of = , where is half the diffraction angle. The very weak peak of Q : Å−1 in film; (c) is due to a thin oxide present on the surface of the 120 Å Ni0.81Fe0.19 film. The R=R value was measured to be 3.0 and 2.1% for film (c) and (b), respectively.

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Ni0.81Fe0.19/50 Å Ni0.49Fe0.12Cr0.39. The diffraction pattern for the single NiFeCr film (a) shows a poorly ordered, but predominately BCC phase with a high stacking fault density, judging from the broad bcc (110) peak and the absence of an fcc (200) peak. The single Permalloy film (b) forms an FCC phase that is unoriented, as evidenced by the powder-like distribution of the intensities for the (111), (200), (220), and (311) reflections. In striking contrast, the diffraction pattern of the Ni0.81Fe0.19/ Ni0.49Fe0.12Cr0.39 film shows only a single, sharp (220) peak. The presence of only the (220) peak indicates a strong (111) texturing of the film, consistent with the specular diffraction data. The observation of only a single diffraction peak for film (c) shows that the Ni0.81Fe0.19 and the Ni0.49Fe0.12Cr0.39 layers have adopted the same structure with the same in-plane lattice constant: they are mutually epitaxial. Consistent with this is the lack of a diffraction peak from Å−1 [compare the single Ni0.49Fe0.12Cr0.39 film at Figs. 3(a) and (c)], and TEM data showing columnar growth. These results are quite surprising since they show that the deposition of only 120 Å of Permalloy causes a restructuring of the 50 Å NiFeCr film from a defective bcc into a well ordered fcc structure. Thus, the important aspect of the seed layer is NOT that it forms a smooth, highly crystalline template upon which the Permalloy film grows, but rather that it is poorly ordered and apparently easily restructured. This is in complete contrast to most other seed layers that form a well-ordered template for the growth of the magnetic thin films [3], [4]. The in-plane particle size of the Permalloy/NiFeCr film is estimated from the (220) peak width in the in-plane diffraction patterns, after approximately accounting for the nonuniform strain

contribution to the peak width by subtracting off the nonuniform strain measured for the Permalloy films without the NiFeCr seed layer. Fig. 2(b) shows this as a function of NiFeCr seed layer thickness for the same set of films shown in Fig. 2(a). As is evident, the particle size increases monotonically with increasing Ni0.49Fe0.12Cr0.39 seed layer thickness, reaching a plateau value of ≈220 Å at a critical thickness of ≈42 Å for the seed layer (see above). Preliminary TEM data is consistent with this picture and shows an average grain size of 250 Å at the same seed layer thickness. The data in Figs. 2(a) and (b) show that enhancement is strongly correlated with the particle the size in the Permalloy/NiFeCr film. Thus, the increase in with NiFeCr thickness is predominantly due to the increase in particle size, as a result of two effects. First, the resistivity of Permalloy films will decrease with increasing particle size due to reduction in the grain boundary scattering of conduction elecdue to high-tempertrons [9], [17], []. The increase in ature deposition [9], [10] or post-annealing [11] is reported to result from the same effect. Second, depending on the film thick( film thickness) of the Permalloy films ness, the .cm when deposited directly on the increases from 0.65–0.76 .cm substrate without this NiFeCr seed layer to 0.76–1.09 for when deposited on this NiFeCr seed layer []. The higher the Permalloy film deposited on the NiFeCr seed layer probably results from its more bulk-like microstructure, similar to observed for the thicker Permalloy films. Thus, the higher observed for the Permalloy we attribute the enhanced film deposited on the NiFeCr seed layer to the reduced and induced by the large grains in the films. increased ACKNOWLEDGMENT The authors would like to thank A. Kellock and R. Beyers for the RBS and TEM results, respectively. They also would like to thank J. Jordan-Sweet for assistance with beamline X20A at the National Synchrotron Light Source. REFERENCES [1] D. E. Laughlin, L. L. Li, T. Li, and D. N. Lambeth, “The control and characterization of the crystallographic texture of longitudinal thin film recording media,” IEEE Trans. Magn., vol. 32, pp. 3632–3637, 1996. [2] J. Li, M. Mirzamaani, X. Bian, M. Doerner, S. Duan, K. Tang, M. Toney, T. Arnoldussen, and M. Madison, “10 Gb/in2 longitudinal media on a glass substrate,” J. Appl. Phys., vol. 85, pp. 4286–4291, 1999. [3] Z. T. Diao, S. Goto, K. Meguro, S. Tsunashima, and M. Jimbo, “Role of the buffer layers in determining the antiferromagnetic coupling and magnetoresistance of NiFeCo/Cu superlattice,” J. Appl. Phys., vol. 81, pp. 2327–2335, 1997. [4] S. S. P. Parkin, Z. G. Li, and D. J. Smith, “Giant magnetoresistance in antiferromagnetic Co/Cu multilayers,” Appl. Phys. Lett., vol. 58, pp. 2710–2712, 1991. [5] T. Mizuguchi, S. Terada, T. Miyauchi, and A. Matsuzono, “Characterization of NiFe/CuNi multilayer GMR sensors for vertical GMR heads,” IEEE Trans. Magn., vol. 34, pp. 1504–1506, 1998. [6] P. Caro, A. Cebollada, F. Briones, and M. F. Toney, “Structure and chemical order in sputtered epitaxial FePd (001) alloys,” J. Crystal Growth, vol. 187, pp. 426–434, 1998. [7] J. U. Thiele, L. Folks, M. F. Toney, and D. Weller, “Perpendicular magnetic anisotropy and magnetic structure in sputtered epitaxial FePt (001) L10 films,” J. Appl. Phys., vol. 84, pp. 5686–5692, 1998. [8] C. Tsang, T. Lin, S. MacDonald, M. Pinarbasi, and N. Robertson, “5 Gb/in2 recording demonstration with conventional AMR heads and thin film disks,” IEEE Trans. Magn., vol. 33, pp. 2866–2871, 1997.

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