T. D. Lee

The effect of casting method and heat treating condition on cold workability of high-Si electrical steel

Abstract The cold workability of Fe–(4.8–6.5)%Si was investigated by varying heat treatment conditions. The effects of casting methods such as ingot casting and spray forming on the cold workability were compared. Amount of B2 ordered phase, Si content, working temperature and Si-oxide were shown to have an effect on the cold workability of high-Si steel.

Role of a paramagnetic amorphous CoZr seed layer in CoCrPt/Ti perpendicular recording media

A fresh CoZr45 paramagnetic amorphous layer of 20 nm thickness is introduced as a seed layer prior to the Ti underlayer deposition on glass substrate in CoCrPt/Ti perpendicular recording media. By the fresh layer introduction, the thickness of Ti underlayer could be reduced to 100 A to obtain good perpendicular magnetic properties. The cause of this reduction of the critical Ti underlayer thickness is the formation of a more smooth and absorbed impurity free surface by the CoZr layer deposition. This ensures better aligned and finer Ti grain formation at an early stage. Details of the improvements of magnetic properties are discussed. In the second half, we are reporting high Ku behaviors of the CoCrPt/Ti films when the magnetic layers are thinner than 200 A. The high Ku behavior was associated with the (10.0) lattice expansion of the Co alloy to maintain coherency with (10.0) lattice of the Ti film at the interfaces. These will be discussed.

Optimum design rule of hysteresis loop for ultrahigh density recording in single-layered perpendicular media

Micromagnetic studies have been performed to find optimum shape of hysteresis loop in a single-layered perpendicular medium for ultrahigh density recording. A good index that was inversely proportional to signal-to-noise ratio (SNR) was developed. By using this index, we found the optimum loop shape should have terms of the ratio of nucleation field to coercivity within -0.4 to -0.6, the loop slope within 2 to 4, and the ratio of grain thickness to grain diameter larger than 2.5.

Failure of exchange-biased low resistance magnetic tunneling junctions upon thermal treatment

Transmission electron microscopy (TEM) and Rutherford backscattering spectroscopy (RBS) were used to characterize low resistance (100–1000 Ω μm2) tunneling junctions consisting of Ta/NiFe/Cu/NiFe/IrMn/CoFe/Al (6.6 and 7.7 A)–oxide/CoFe/NiFe/Ta multilayers after annealing at temperatures ranging from 250 to 500 °C. The Al (7.7 A) junction showed continual improvement in the magnetoresistance (MR) ratio when annealed up to 300 °C while the MR ratio of the Al (6.6 A) junction dropped sharply above 250 °C in spite of the only 1 A difference in the deposited thickness of aluminum metal prior to plasma oxidation. TEM measurement provided evidence that the annealing process improves, in general, structural uniformity in the insulation layer, but thermal treatment can also degrade junction performance at a relatively low temperature due to current leakage through the electrodes. Current leakage can be problematic for a junction whose insulation barrier may be too thin (less than ∼10 A). Both RBS and TEM analyses ...

Magnetic tunnel junctions with Hf oxide and modified Hf oxide tunnel barriers

Magnetic tunnel junctions (MTJ’s) with Hf oxide and modified Hf oxide barriers were fabricated by ozone oxidation. The tunnel magnetoresistance (TMR) ratio in Hf oxide junction was 13% at room temperature and 21% at 77 K. In order to understand the low TMR ratio in MTJ’s with Hf oxides compared to those with Al oxides, tunnel barriers were modified by inserting a thin Al oxide layer of 0.3 nm at the interfaces between ferromagnetic electrodes and Hf oxide insulating layers. As the Al layer of 0.3 nm was inserted at top and bottom interfaces, the TMR ratio was restored to the value of the junctions with Al oxides. This implies that the polarization of CoFe contacted with Al oxide is larger than that of CoFe contacted with Hf oxide and the low TMR ratio in MTJ’s with Hf oxides may be attributed to the reduction of spin polarization of the CoFe electrodes due to CoFe/Hf oxide interface interaction.

Novel simulation model for perpendicular magnetic recording

Novel simulation model for perpendicular recording is developed. The Preisach model merged with the micromagnetic model is used to simulate the perpendicular media. Moreover, a simple method to calculate the MR sensitivity function is also provided. Simulation results of the model are well agreed with experimental measurements. Precise modeling method and recording mechanism of the perpendicular recording with a ring head writing is discussed in detail.

Effect of ac on current-induced domain wall motion

Saitoh et al. [Nature (London) 432, 203 (2004)] have reported the experimental result showing the interplay of a transverse domain wall with an electrical ac of megahertz-range frequencies. They observed a single peak of resistance in the frequency range and interpreted it with a nonadiabatic spin torque. It was argued that an ac current can induce a micrometer-range displacement of domain wall. We reconstructed the experiment in micromagnetic simulations considering the local nonzero nonadiabatic spin torque. We could not observe either an explicit single peak in the frequency-dependent resistance or an eventual displacement of domain wall by use of an ac. It indicates the local nonadiabatic torque is inappropriate to explain the experimental results of ac-induced domain wall motion. Other approaches such as the nonlocal nonadiabatic spin torque may be needed.

Substrate Biasing Effect during MgO Deposition in CoFeB/MgO/CoFeB MTJs

High tunneling magnetoresistance (TMR) ratio and low RA in magnetic tunnel junctions are necessary condition for application in magnetic random access memory. To get high TMR ratio and low RA, good quality of MgO (002) insulating layer is important. To increase crystalline quality of MgO (002) layer, we applied negative bias on the substrate during MgO deposition. We report the results of the tunneling magnetoresistance (TMR) ratio and the resistance-area product (RA) for CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) with substrate bias voltage, and showed TMR increase and RA decrease with substrate bias. The effects of substrate bias voltage on the TMR ratio, RA and MgO (002) peak intensity are discussed.

Fabrication of Magnetic Tunnel Junctions With Co

Tunneling magnetoresistance (TMR) characteristics of magnetic tunnel junctions with a Co₂ FeSi electrode and an MgO crystalline barrier have been investigated. Co₂ FeSi Heusler alloy electrode grown on Cr-buffered MgO(100) substrate starts to have an <i>L</i>2₁ structure when annealed above 420degC. In the cases of CoFeB/MgO/Co₂ FeSi junctions, a high TMR ratio of 158% has been achieved after annealing at 350degC, which is obtained by coherent tunneling between the electrodes and barrier, not by the half-metallic nature of Co₂ FeSi Heusler alloy. However, the Co₂ FeSi Heusler alloy electrode degrades the TMR ratio when compared with an amorphous CoFeB electrode and the bottom Co₂ FeSi electrode makes more degradation of the TMR ratio than the top Co₂ FeSi electrode. The major reason for the low TMR ratio in Co₂ FeSi-based junctions is the broken epitaxial relationship between the bottom Co₂ FeSi electrodes and the MgO crystalline barrier, which is investigated by a cross-sectional transmission electron micrograph.

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The magnetic properties and thermal stability of antiferromagnetically coupled (AFC) media with thin Co interlayers are investigated. Since the thermal stability is strongly dependent on the exchange coupling constant Jex in the AFC media, the thin Co interlayers were inserted on both interfaces of the Ru layer to obtain higher values of Jex. The Jex above 0.6 erg/cm2 was obtained in the AFC media with the Co interlayers above 1 nm in comparison with about 0.1 erg/cm2 in AFC media without the Co interlayers. The thermal stability of the AFC media with Co interlayers was greatly improved over those of the AFC media without Co interlayers.