Multilevel Magnetoresistance in a Structure Consisting of two spin-valves
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Multilevel Magnetoresistance in a Structure Consisting of two spin-valves Kebin Li, Yihong Wu1, Jinjun Qiu, Guchang Han, Zaibing Guo, and Towchong Chong Data Storage Institute, DSI Building, 5 Engineering Drive 1, Singapore 117608 1
Department of Electrical and Computer Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 ABSTRACT The magnetic and electrical properties as well as the structural characteristics have been studied on a series of samples with structure substrate (Sub)/SV(1)/Al2O35nm/SV(2). Here, SV(1) is either CoFe/IrMn based spin-valve (SV) such as Ta5/NiFe2/IrMn8/CoFe2/Cu2.6/CoFe2/Ta5 (thicknesses are in nanometers) bottom SV or Ta5/NiFe2/CoFe1.5/Cu2.6/CoFe2/FeMn10/Ta5 top SV and SV(2) is Ta5/NiFe2/CoFe1.5(or 2)/Cu2.6/CoFe2/IrMn8/Ta5 top SV. SV(1) and SV(2) in the structure are decoupled by a Al2O3 layer with 5nm in the magnetic properties, however, they are in parallel connection in the electrical properties. In a sample with structure Sub/Ta5/NiFe2/IrMn8/CoFe2/Cu2.6/CoFe2/Ta5 /Al2O35/Ta5/NiFe2/CoFe2/Cu2.6/CoFe2/IrMn8/Ta5, five magnetoresistance states which are related to five magnetization states have been observed after the sample was annealed at T=220 o C with a field strength of 1T under high vacuum because of different interlayer coupling fields (Hint) in the top and bottom CoFe/IrMn based SVs (Hint is about 12.21 Oe in the top CoFe/IrMn SV and 29.3 Oe in the bottom CoFe/IrMn based SV). In a sample with structure Sub/Ta5/NiFe2/CoFe1.5/Cu2.6/CoFe2/FeMn10/Ta5/Al2O35/Ta5/NiFe2/CoFe1.5/Cu2.6/CoFe2 /IrMn8/Ta5, since the blocking temperature of the CoFe/FeMn based SV (Tb is about 150 oC) is lower than that of CoFe/IrMn based SV (Tb is about 230 oC), the spins can be easily engineered and therefore various magnetoresistance states can be obtained when the sample is magnetically annealed at different temperatures in a proper annealing sequence. By properly selecting materials and controlling the magnetically annealing conditions, multilevel giant magnetoresistance (MR) magnetic random access memory (MRAM) cell can be realized, which will significantly improve the MRAM data storage density without increasing any additional processing complexity. INTRODUCTION Multiple value storage is considered as a promising technique for increasing the storage density of magnetic random access memories (MRAMs). In order to implement this technique, one must first identify a suitable cell structure featuring a multi-step response in the magnetoresistance (MR) versus magnetic field curve. Such a structure has been once reported in a NiFe/Cu/NiFeCo/Cu/NiFeCo/Cu/Co pseudo-spin-valve (PSV).[1] However, the MR ratio of the PSV is rather small which may limit its applications in large scale MRAMs. In this paper, we report a new structure that consists of two electrically isolated and magnetically decoupled CoFe/IrMn spin-valves or CoFe/FeMn spin-valves. Multiple distinctive steps associated with different magnetization states have been successfully observed in the MR response curves. The
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