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Enhanced Exchange Pinning Field For Fe Mn Spin Valves
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Enhanced Exchange Pinning Field For Fe Mn Spin Valves



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  • 1. ~ H Journalof magnetism N ELSEVIER Journal of Magnetism and Magnetic Materials 168 (1997) L233 L236 and magnetic materials L e t t e r to t h e E d i t o r Enhanced exchange pinning field for FeMn spin-valves S.L. B u r k e t t * , J.C. L u s t h , J.L. B r e s o w a r 1, M . R . P a r k e r z The Center for Materials for Information Technology, The University of Alabama, Tuscaloosa, ilL 35487-0209, USA Received 2 December 1996 Abstract Enhanced exchangepinning is observed in spin-valves with ultra-thin magnetic pinned layers and FeMn antiferromag- netic layers. In this study, three magnetic material multilayer systems, CoNiFe/Cu, NiFe/Cu, and CoFe/Cu, are investigated. Spin-valves with ultra-thin pinned layers (~0.5 nm) exhibit exchange pinning fields as high as 800-900 Oe in CoNiFe/Cu and CoFe/Cu multilayers while a pinning field of approximately 500 Oe is obtained for NiFe/Cu multilayers. PACS: 75.30; 75.50; 75.70 Keywords: Spin-valve; Multilayer systems; Giant magnetoresistance;FeMn; Antiferromagneticlayers 1. I n t r o d u c t i o n sponse to small magnetic fields. They are also becoming potentially important media candidates Spin-valve thin-film multilayer structures are for magnetic solid state memory (MRAM) [1]. promising candidates for next generation read In these material structures, the magnetic orienta- heads in high-density magnetic recording applica- tion of one layer is controlled by the presence of tions due to their high sensitivity and linear re- an antiferromagnetic material, while another layer is essentially free to rotate when exposed to an applied magnetic field. The change in relative orientation of the two layers will produce a * Correspondence address: Department of Electrical Engin- magnetoresistive (MR) effect [1]. This effect is eering, University of Alabama, Tuscaloosa, AL 35487-0286, achieved in a spin-valve with the 'free' and 'pinned' USA. Fax: (205) 348-6959; e-mail: sburkett@coe.eng.ua.edu. magnetic layers separated by a nonmagnetic Current address: School of Physics, Georgia Institute of Technology, Atlanta, GA 30332-0430, USA. metallic spacer, such as Cu. Spin-valves exhibit 2 Current address: The Department of Electrical Engineering, a magnetoresistance substantially larger than The University of South Alabama, Mobile, AL 36688, USA. the current-generation read heads that employ 0304-8853/97/$17.00 O 1997 Elsevier Science B.V. All rights reserved PII S 0 3 0 4 - 8 8 5 3 ( 9 6 ) 0 0 7 3 0 - 5
  • 2. LETTER TO THE EDITOR L234 S.L. Burkett et al. / Journal of Magnetism and Magnetic Materials 168 (1997) L233-L236 anisotropic magnetoresistance (AMR) [2]. This with a Dektak IIa profilometer. The spin-valves are parameter is highly dependent upon the selection of composed of a 10 nm Ta seed layer followed by materials and the thickness of the thin films making a 10 nm magnetic free layer, a 2.4 nm Cu spacer, up the multilayer stack. An excellent overview ex- a variable (~0.5-4.0 rim) magnetic pinned layer, plaining the advantages and disadvantages for ma- 15 nm F%0Mnso pinning layer, and a 5.0 nm Ta terials choice and various multilayer layouts in capping layer. Three magnetic multilayer systems a spin-valve structure is given by Kools [1]. are used: Co45Ni30Fezs/Cu, NisoFe20/Cu, and Practical spin-valve devices for ultra-high den- Co90Fel0/Cu. It has been shown by other re- sity record heads can involve patterned rectangular searchers that the Co-rich ternary alloy gives en- mesoscopic elements with 'heights' (i.e. lateral di- hanced MR ratios and low coercivity at this mensions) as small as 0.5 gm. Given that Hp is composition [6]. In the case of CoFe/Cu spin- required to be directed orthogonal to the long axis valves, a single layer of CoFe makes up the mag- of the element, it follows that Hp values must be of netic pinned layer as in the other material systems, sufficient magnitude (>>250 Oe) [3] to resist de- however, a multilayer of CoFe 1.7 nm/(Cu 0.8 nm/ magnetizing effects. The latter are approximately CoFe 1.7 nm)s is used as the free layer. This multi- proportional to film thickness while Hp values fol- layered free magnetic layer has been effective in low a 1/thickness dependence rather closely. producing spin-valves with higher G M R ratios and Hamakawa et al., for example, have illustrated this lower coercivities compared to spin-valves fab- thickness dependence for spin-valves utilizing anti- ricated with a single layer of CoFe as the free ferromagnetic NiO layers [4]. Kim et al. have demon- layer [7]. strated high-exchange anisotropy fields for bilayers For evaluation of spin-valve performance, mag- of NiFe/FeMn deposited on silicon substrates netic properties are measured using a four-point when the thickness of the NiFe film is 3.0 nm [5]. linear spring-loaded probe in a variable magnetic Accordingly, ultra-thin pinned layers have, in addi- field. An electromagnet and power supply provide tion to reduced demagnetizing effects, the added the applied field and all measurements are made at advantage of improved spin-valve performance in room temperature. terms of linearity. In this study, we fabricate (mac- roscopic) FeMn-based spin-valves with ultra-thin pinned layers using three different materials 3. Experimental results systems, CoNiFe/Cu, NiFe/Cu, and CoFe/Cu to determine whether enhanced Hp values are sus- MR versus applied field curves are shown in tained as pinned-layer thicknesses are reduced Fig. 1 for a spin-valve fabricated with CoNiFe as to one or two atomic layers. the magnetic material. For this particular set of data, the pinned layer is 0.5, 1.5 and 3.0nm, in Fig. la-Fig, lc, respectively. The relationship be- 2. Experimental techniques tween/-/p and pinned-layer thickness is shown in Fig. 2 for all material systems. It is clear that Hp is Spin-valves are fabricated by DC magnetron inversely related to pinned-layer thickness with sputter deposition on Corning 7059 glass slides Hp approaching 800-900 Oe for ultra-thin (~0.5 nm) (2 x 2 in2). The Vac-Tec Model 250 sputtering sys- magnetic pinned layers. In previous work, we tem is located in a Class-1000 clean room. have observed a further increase in Hp when Predeposition base pressure of the system is less the spin-valves are slow-annealed at (~60°C) than 2 x 10 -7 Torr and samples are deposited un- for extended periods of time [8]. The largest der an Argon pressure of ~ 5.0 mTorr. Films are increase in pinning field is observed when the deposited at ambient temperature with an aligning as-deposited pinning field is high. Structures field of 80 Oe parallel to the substrate surface dur- enhanced in all spin-valves used in our current ing deposition. Sputtering rates are calculated from study: However, the magnitude of G M R is compro- thickness measurements of the reference films made mised somewhat for these samples as shown in
  • 3. S.L. Burke# et al. / Journal of Magnetism and Magnetic Materials 168 (1997) L233-L236 L235 1000 • CoNiFe 900 • CoFe • NiFe 800 3 700 600 2 o 50O 1 400 300 0 6 • -1000 -800 -600 -400 -200 200 2O0 (a ) F.xteman ~eld (Oe) 100 0 I I I [ ] 0.0 0.5 1,0 1.5 2,0 2.5 3.0 _f Pinned Layer Thickness (nm) 3. Fig. 2. Pinning field versus pinned-layer thickness. i 2- 1- • CoNiFe • CoFe A NJFe 0 -lo00 -8OO -6O0 400 -~0 200 (b) e.,a,,',mn v i ~ (c~) n" (.9 1 0 0.0 0.5 1,0 1.5 2.0 2.5 3.0 o1000 400 -600 -400 -200 0 200 P i n n e d L a y e r T h i c k n e s s (nm) (c) Exlemal Field (Oe) Fig. 3. G M R versus pinned-layer thickness. Fig. 1. G M R versus applied field. less dependence on pinned-layer thickness with Fig. 3. G M R for CoNiFe and CoFe material sys- magnitudes ranging from 1.5-2.2% for a corre- tems decrease from 3 - 4 % to less than 2% when the sponding thickness range of 0.8-2.0 nm. pinned layer is below approximately 0.8 nm. G M R Coercivity (He) is also monitored for these in spin-valves utilizing NiFe films appear to exhibit spin-valves. Fig. 4 shows Hp/Hc versus pinned
  • 4. LETTER TO THE EDITOR L236 S.L. Burkett et al. / Journal o f Magnetism and Magnetic Materials 168 (1997) L233-L236 60 (~0.5 nm). Measured values for H p range from 800-900 Oe in CoNiFe/Cu and CoFe/Cu multi- • CoNiFe • CoFe layer material systems and approximately 500 Oe is 50 • NiFe obtained in NiFe/Cu multilayers. Current research efforts are focused on studying the uniformity and 40 microstructure of ultra-thin pinned layers, as well as different annealing schemes for investigating thermal stability in these materials. 3O "1- 20 Acknowledgements This research was supported in part by NSF 10 MRSEC Cooperative Agreement No. DMR- 9400399. The authors would like to thank Profes- I I [ I I sor H. Fujiwara for valuable discussions. One of us 0.0 0.5 1.0 1.5 2.0 2.5 3.0 (M.R.P) would like to acknowledge the support Pinned Layer Thickness (nm) for this work through a research grant from JVC Corporation. Fig. 4. Hp/H c versus pinned-layer thickness. layer thickness for the three multilayer material References systems. It is well-established that Hp/Hc>> 1 is a prerequisite for pinned-layer spins returning to [1] J.C.S. Kools, IEEE Trans. Magn. 32 (1996) 3165. a well-defined quiescent state immediately follow- [2] H.N. Bertram, Theory of Magnetic Recording (Cambridge ing magnetic excitation [9]. Finally, one intriguing University Press, Cambridge, 1994). [3] T.R. Koehler and M.L. Williams, IEEE Trans. Magn. 32 aspect of Fig. 1 deserves mention. In spin-valves (1996) 3446. with ultra-thin pinned layers, the coercivity of the [4] Y. Hamakawa, H. Hoshiya, T. Kawabe, Y. Suzuki, R. Arai, free layer appears to fall to values less than those K. Nakamoto, M. Fuyama and Yi Sugita, IEEE Trans. observed in conventional designs. Fujiwara [10] Magn. 32 (1996) 149. has suggested that reduced magnetostatic coupling [5] Y.K. Kim, K. Ha and L.L. Rea, IEEE Trans. Magn. 31 (1995) 3823. in ultra-thin pinned layers could account for this. [6] Y. Kitade, H. Kikuchi, H. Kishi, M. Otagiri and K. This particular aspect is being investigated further. Kobayashi, IEEE Trans. Magn. 31 (1995) 2600. [7] K. Nishioka, T. Iseki, H. Fnjiwara and M.R. Parker, J. Appl. Phys. 79 (1996) 4970. 4. Conclusions [8] S i . Burkett, S. Kora, J.C. Lusth and M.R. Parker, IEEE Trans. Magn., submitted. [9] T. Lin, C. Tsang, R.E. Fontana and J.K. Howard, IEEE Enhanced exchange pinning field is observed in Trans. Magn. 31 (1995) 2585. FeMn spin-valves with ultra-thin pinned layers [10] H. Fujiwara, private communication.