IEEE TRANSACTIONS ON MAGNETICS, VOL. 37, NO. 4, JULY 2001
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Magnetic Properties of Specular Spin-Valves Containing Nano-Oxide Layers J. C. S. Kools, S. B. Sant, K. Rook, W. Xiong, Faiz Dahmani, W. Ye, J. Nuñez-Regueiro, Y. Kawana, M. Mao, K. Koi, H. Iwasaki, and M. Sahashi
Abstract—Nano-Oxide Layers (NOL) have been introduced recently as a means to enhance the degree of specular scattering at the outer surfaces in a spin-valve, while still using metallic antiferromagnets such as PtMn or IrMn. A key requirement of the NOL layer in the pinned layer is to be able to provide strong ferromagnetic coupling between two ferromagnetic layers. In this paper, a quantitative study of the ferromagnetic interlayer coupling over the NOL-layer is presented. A Stoner–Wohlfarth model is developed to allow for quantitative analysis. The effect of different metal deposition methods (PVD or IBD), different pinning structures (PtMn and IrMn; simple and synthetic) and different oxidation methods on the coupling is compared. Index Terms—Magnetoresistance, spin-valve.
specular
ferromagnetic layers P and P for them to act as a single layer. A key requirement of the NOL layer is not to affect the soft magnetic properties of the free layer. In this paper, a study of the ferromagnetic interlayer coupling over the NOL-layer is presented. A Stoner–Wohlfarth model is developed to allow for quantitative analysis. The effect of different metal deposition methods (PVD or IBD), different pinning structures (PtMn and IrMn; simple and synthetic) and different oxidation methods on the coupling is compared.
reflection,
I. INTRODUCTION
I
T HAS been well established that spin-valves using an oxidic antiferromagnet such as NiO display a significantly higher MR-ratio than full-metal spin-valves. MR-ratios around 19%, or roughly twice as large as those regularly found in full metal spin-valves have been obtained in NiO/CoFe/Cu/CoFe/TaO spin-valves [1]. The TaO layer is deposited as a thin Ta layer, and oxidized upon exposing the sample to oxygen. This is explained [2] by the occurrence of specular electron scattering at the metal–oxide interfaces between pinned layer and antiferromagnet. Unfortunately, the limited thermal stability and exchange field of NiO, in comparison with metallic materials such as NiMn or PtMn hampers application in high density recording heads. The introduction [3] of Nano-Oxide Layers (NOL) in the pinned layer allows to combine the superior magnetic performance of the metal biased spin-valve with the MR-enhancement of specular scattering at a metal–oxide interface. A generic configuration of such a spin-valve would for simple be: Substr/seed/AF/P /NOL /P /Cu/F/NOL bottom specular spin-valves, and Substr/seed/AF/P /Ru/P / NOL /P /Cu/F/NOL for synthetic bottom spin-valves. In order to fully exploit the superior pinning properties of the metallic AF layer, the NOL layer has to be able to provide sufficiently strong ferromagnetic coupling between the two Manuscript received October 13, 2000. J. C. S. Kools, S. B. Sant, K. Rook, W. Xiong, F. Dahmani, W. Ye, J. NuñezRegueiro, and Y. Kawana are with Veeco Instruments, San Jose, CA 95119 USA. They are also with Read-Rite Corporation, Fremont, CA 94123 USA (e-mail:
[email protected]). M. Mao is with Lawrence Livermore National Laboratories, CA 94550 USA. K. Koi, H. Iwasaki, and M. Sahashi are with the Corporate Research and Development Center, Toshiba Corp., Kawasaki 212-8552, Japan. Publisher Item Identifier S 0018-9464(01)05878-2.
II. EXPERIMENTAL Simple bottom spin-valves with the structure: SiO /50 Å Ta/20 Å Ni Fe /70 Å Ir Mn /20Å Co Fe /Oxidation/ 20 Å Co Fe /20 Å Cu/20 Å Co Fe /10 Å Cu/10 Å Ta were formed in a number of different ways: The effect of different metal deposition techniques was compared by deposition of nominally identical structures by ion beam deposition (IBD) and PVD in a planetary or static mode, while keeping the type of oxidation process constant, i.e., natural oxidation (exposure to varying doses of O -gas). The effect of different spin-valve configuration was investigated by fabricating structures such as the one given above by PVD and natural oxidation, but replacing the Ir Mn layer by 150 Å Pt Mn , 150 Å Pt Mn /20 Å Co Fe /8 Å Ru, or, 70 Å Ir Mn /20 Å Co Fe /8 Å Ru. The latter two structures are referred to as synthetic structures. The effect of different oxidation methods was evaluated by deposition of nominally identical structures, while keeping the metal deposition constant (IBD) and varying the oxidation conditions. Three oxidation methods were compared, namely natural oxidation (exposure to molecular O ), atom beam oxidation (ABO) and ion beam oxidation (IBO). The oxidation is characterized by the total dose [ partial pressure time; measured Torrsec)]. ABO and IBO were perin Langmuir ( formed by exposing the substrate to the downstream flow of an RF-ICP plasma. In the case of ABO, no acceleration takes place, in the case of IBO, we studied acceleration voltages in the range 20–200 eV. MagnetoResistance (MR) and Magnetization (MH) loops were measured for all samples. III. MODEL In order to allow a quantitative study of the ferromagnetic coupling over the NOL layer, we developed a simple Stoner Wohlfarth type total energy model, both for synthetic and simple
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IEEE TRANSACTIONS ON MAGNETICS, VOL. 37, NO. 4, JULY 2001
spin-valves. We will describe below the synthetic spin-valve. The total energy contains the following eight energy terms: • Four Zeeman terms:
where is the saturation magnetization of layer , its thickness and the angle between the magnetization and the external field . • Three Interlayer coupling terms:
where the coupling over the Ru is strongly antiferromag), the coupling over the NOL strongly ferronetic ( ), and the coupling over the Cu layer magnetic ( ) weakly ferromagnetic or antiferromagnetic. ( • One Exchange bias term:
Since we are mostly concerned with the high field loop, the uniaxial anisotropy in the free layer was not included. and Energy minimization then leads to curves in the usual way [4]. The case for simple (i.e., nonsynthetic) spin-valves can be obtained as a straightforward simplification. on calculated Fig. 1 shows the effect of a variation of magnetization and magnetoresistance loops for a simple spinvalve, using the parameters as given in the caption. Roughly comthree regimes can be distinguished. For values of , the two sublayers of the pinned layer parable or larger than switch in unison, essentially acting as a single pinned layer. For which are similar to, or slightly lower than values of (e.g., the curve for erg/cm in the figure), both layers rotate in approximately the same field range, but their angle is different. This will manifest itself as a slanting of the and loops, which could be interpreted mistakenly . Finally, for smaller than about half as an increase of , the two layers will switch essentially independent. In of the latter regime, the switching field of , and therefore the right hand slope of the MR loop quickly reduces with increasing . From an application point of view, it is desirable to obtain the . situation where Some results for the synthetic spin-valve are shown in Fig. 2, which shows the simulated MR-loop shape. For small values ( erg/cm ), is found to switch at relatively of and remain unchanged until low field values, while significantly higher field values are reached. For higher values , all three constituent layers of the pinned layer are of switching in the same field interval. One can thus see that the for application in a synthetic spin-valve minimal value of is in this case approximately 0.5 erg/cm . This value depends , and . on the
Fig. 1. Calculated magnetization and magnetoresistance loops for simple spin-valves with the parameters: , , (In Å Co Fe equivalent) : erg/cm , = : %, : erg/cm and varying values of .
J = 0 0065
J
M t = 20 M t = 20 M t = 10 = 0 16 1R R = 11 8 J
Fig. 2. Calculated magnetoresistance loops for synthetic spin-valves with the parameters: , , (In Å Co Fe equivalent) : erg/cm , : erg/cm , : erg/cm .
J = 0 0065
M t = 20 M t J = 0 16
= 20 M t = 10 J = 00 65
IV. EXPERIMENTAL RESULTS AND DISCUSSION The effect of increasing oxidation dose on IrMn bottom spinvalves deposited by PVD is shown in Fig. 3. It is found that increasing the O dose leads to an enhancement of the MR-ratio from the 12% range for the unoxidized sample (not shown) to with the 15.5% range, indicative of the formation of an
KOOLS et al.: MAGNETIC PROPERTIES OF SPECULAR SPIN-VALVES CONTAINING NOL
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Fig. 3. Measured loops for IrMn bottom spin-valves deposited by PVD and oxidized by natural oxidation. The O dose is indicated in Langmuir (1 Torrsec).
10
L=
an increasing degree of specular scattering. One also sees that the right hand slope of the MR-curve becomes slanted with increasing oxygen dose, similar to the loops shown in Fig. 1. The as the oxide latter is a clear indication of the decrease of layer gets thicker. Similar behavior was observed when the structure of the underlying pinning layers, or the deposition method was changed, albeit that the characteristic oxygen dose required to form an oxide layer with a certain degree of specularity and value of varies. The latter is interpreted as an indication that the oxidation kinetics are dependent on the microstructure of the initial metal layers. The MR-ratio of the IBD-deposited spinvalves varies from about 8.5% to 12%. It is well established that IBD-spin-valves tend to give somewhat lower MR-ratios due to stronger interface mixing. The oxidation method was found to have a dramatic effect on the properties of the NOL, as illustrated in Fig. 4. This figure shows measured VSM and MR-loops for nominally identical stacks, except for that the mode of oxidation has been varied. In the case of Atomic Beam Oxidation (ABO), the two pinned layers switch in unison. Fitting the curves only allows to oberg/cm . tain a lower limit for the coupling, namely The total magnetization of the two pinned layers is equivalent to 35 Å. Since the initial nominal CoFe thickness was 20 Å 20 Å, and some magnetic dead layers could occur near the CoFe/Cu and IrMn/CoFe interfaces, one can conclude that 5 Å is an upper limit for the amount of CoFe transformed in a nonmagnetic oxide in the case of ABO. In contrast, in the case of Ion Beam Oxidation (IBO; Ion energy 40 eV), the coupling is much weaker (0.015 erg/cm ), and the amount of oxidized CoFe is much larger (11 Å), indicating the formation of a thicker oxide. This difference can be attributed to the larger penetration depth of the high energy ions in IBO.
Fig. 4. Measured VSM and MR-loops for IBD-deposited IrMn spin-valves oxidized by ABO and 40 eV IBO. The data have been fitted to models with the parameters: = : %, , : erg/cm : erg/cm ; , , : erg/cm for IBO; , : erg/cm for ABO. The oxygen dose is kept constant.
1R R 11 7 M t = 20 J = 0 16 0 0065 M t = 20 M t = 9 J = 0 015 M t + M t = 35 J 0 3
J =
V. CONCLUSIONS The ferromagnetic coupling over Nano-Oxide Layers formed by oxidation of Co Fe has been investigated. A model for the switching behavior has been constructed. The effect of spin-valve configuration, metal deposition method, and oxidation method are investigated. It has been found that for this material system, Atomic Beam Oxidation (ABO) gives the highest coupling. ACKNOWLEDGMENT The authors would like to thank E. Lakios for his continuing support of this project. REFERENCES [1] W. F. Egelhoff, Jr., P. J. Chen, C. J. Powell, M. D. Stiles, R. D. McMichael, J. H. Judy, K. Takano, and A. E. Berkowitz, “Oxygen as a surfactant in the growth of giant magnetoresistance spin-valves,” J. Appl. Phys., vol. 82, pp. 6142–6151, 1997. [2] H. Swagten, G. J. Strijkers, R. H. J. N. Bitter, W. J. M. de Jonge, and J. C. S. Kools, “Specular reflection in spin-valves bounded by NiO layers,” IEEE Trans. Magn., vol. 34, pp. 948–953, 1998. [3] Y. Kamiguchi, H. Yuasa, H. Fukuzawa, K. Koi, H. Iwasaki, and M. Sahashi, Digests of InterMag, 1999, DB-1. [4] M. R. Parker, H. Fujiwara, S. Hossain, and W. E. Webb, “Magnetic vector model of spin-valves,” IEEE Trans. Magn., vol. 31, pp. 2618–2620, 1994.
