Sputter deposited, equiatomic Pt-Mn thin films used in giant magnetoresistive spin valves are found not to exist in the antiferromagnetic state required for device operation. Therefore, an annealing step is needed to induce a phase transformation from the as-deposited, paramagnetic A1 ͑fcc͒ phase to the antiferromagnetic L1 0 phase. The L1 0 phase is the thermodynamically stable configuration, but favorable kinetics for the transformation were only found above 260°C. The A1 to L1 0 phase transformation was studied by x-ray diffraction, transmission electron microscopy and differential scanning calorimetry ͑DSC͒. The nucleation and growth conditions were evaluated and an exothermic transformation enthalpy of Ϫ12.1 kJ/mol of atoms was determined. The kinetics of the reaction were simulated using the Johnson-Mehl-Avrami analysis, where the necessary parameters were determined by the Kissinger and Ozawa methods from constant scanning rate DSC experiments ͓H. Yinnon and D. R. Uhlmann, J. Non-Cryst. Solids 54, 253 ͑1983͔͒. The resulting simulations were compared to DSC data as well as isothermal x-ray peak shift data and a reasonable agreement was obtained.
Spin tunneling recording heads above 20 Gb/in 2 have been fabricated using a bottom tunneling junction stack. The spin tunneling stack is made of Ta/PtMn/CoFe/Ru/CoFe/AlO/NiFe/Ta and stabilized by a permanent magnet abutted junction. The effective junction width is about 0.4 m wide and lapped to the junction with an optimum stripe height. The barrier has resistance area product of 15-20 m 2 , leading to a typical head resistance of around 50 . Isolated pulses during the spin-stand test shows large signal up to 10 mV. On track error rate floor is better than 10 9 and the head signal-to-noise ratio is also better than that of a conventional spin valve GMR head. The areal density estimated (using BER of 10 5 ) is above 20 Gb/in 2 .Index Terms-Areal density, micromagnetic modeling, spinstand test, spin tunneling heads, TGMR.
Magnetic microstructure, exchange induced uniaxial and unidirectional anisotropy and structural transformation have been studied in PtMn/NiFe bilayer films and small elements as a function of annealing time. The relationship between the fcc-fct ordering phase transformation in PtMn and the development of exchange induced magnetic properties in PtMn/NiFe bilayers is complicated by the fact that the transformation occurs throughout the entire volume of the PtMn film, while the exchange between the layers is predominantly an interface effect. Consequently, the development of the exchange anisotropy should depend primarily on the character of the structural transformation at the interface between PtMn and NiFe. The purpose of this article is to correlate the volume phase transformation in PtMn to the development of exchange anisotropy and micromagnetic behavior in PtMn/NiFe bilayers. The interface structure can be inferred from the anisotropy and micromagnetic measurements, leading to a model that explains the relationship between the volume and interface transformation structures in PtMn, and magnetic properties of the bilayers. The structure and magnetic properties were characterized by x-ray diffraction, vibrating sample magnetometry, and magnetic force microscopy.
Tunneling magnetoresistive (TMR) readers capable of 150 Gb/in 2 of areal density magnetic recording for hard disk drive were demonstrated with bit-error-rate performance. The head design used is basically a bottom type stack of Ta/PtMn/CoFe/ Ru/CoFe/oxide barrier/CoFe/NiFe/Ta cap with abutted hard bias stabilization. The electrical reader width is about 4 to reach a very high track density and shield-to-shield spacing is about 700 A for high linear density. On-track bit error floor is better than 10 5 at a linear density of 900 KBPI and the recording system noise is dominated by the media. The best areal density achieved (using 4 OTC reference level) is 143 Gb/in 2 using symmetric squeeze and 152 Gb/in 2 using asymmetric squeeze method, respectively. It was found that the TMR head has several decibels more signal-to-noise ratio gain over spin valve readers at 150 Gb/in 2 and beyond. The TMR head is also suitable for perpendicular recording application.Index Terms-Areal density, current-perpendicular-to-plane (CPP) geometry, perpendicular recording, signal-to-noise ratio (SNR), tunneling magnetoresistive (TMR) heads.
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