Masashige Sato

Transmission electron microscopy study on the crystallization and boron distribution of CoFeB/MgO/CoFeB magnetic tunnel junctions with various capping layers

High-resolution transmission electron microscopy and electron energy-loss spectroscopy (EELS) were used to study the microstructural properties of CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) with various capping layers. Crystallization of CoFeB layers was strongly dependent on the capping materials, and was affected by B diffusion. With NiFe-cap MTJs, CoFeB crystallized from the cap interface and formed a fcc structure; on the other hand, with Ta- and Ti-cap MTJs, CoFeB crystallized from the MgO interface and formed a bcc structure. EELS analysis showed that B mainly diffused to the capping layers and rarely to the MgO layers with increasing temperature. With Ti-cap MTJs, B diffusion caused hcp-Ti crystals to form an amorphous structure and CoFeB crystallized at lower temperature.

FERROMAGNETIC TUNNEL JUNCTIONS WITH PLASMA-OXIDIZED AL BARRIERS AND THEIR ANNEALING EFFECTS

Cross-geometrical Ni–Fe/Co/Al–AlO/Co/Ni–Fe/Fe–Mn/Ni–Fe tunnel junctions were fabricated by magnetron sputtering. To form the insulating layer, an Al layer was reverse sputtered in an atmosphere of either oxygen or oxygen–argon mixture at low power after deposition. The oxidization time necessary to form an AlO barrier was much shorter than that by natural oxidization, lasting for only a half to a few minutes. By adding argon to oxygen, the oxidization was slowed down and high MR ratios were obtained for a wide range of time. A magnetoresistance (MR) ratio of 16% was observed in the as-deposited junction when the barrier was oxidized in oxygen plasma for 35 s. In addition, the MR ratio increased to 24% by annealing at 300 °C. In as-deposited junctions, the tunnel resistances were increased by increasing the plasma oxidization time, but the MR ratios gradually decreased. The estimated tunnel barrier width increased and the barrier height decreased with the plasma oxidization time. After annealing, the MR ra...

Spin-valve-like properties and annealing effect in ferromagnetic tunnel junctions

We fabricated ferromagnetic tunnel junctions with naturally oxidized Al barriers, and investigated the effects of Al oxidization time, Al thickness and annealing temperature on the tunnel properties. Changes of between 10 and 15% have been observed in the resistance, and in some junctions, the MR ratios increase by annealing at 250 to 300/spl deg/C. The Ni-Fe/Co/Al-AlO/sub x//Co/Ni-Fe/Fe-Mn/NiFe junction showed a 24% spin-valve-like MR change after annealing at 300/spl deg/C for one hour.

In situ heat treatment of ultrathin MgO layer for giant magnetoresistance ratio with low resistance area product in CoFeB/MgO/CoFeB magnetic tunnel junctions

In order to promote the grain growth of ultrathin MgO barrier deposited on a CoFeB layer, in situ infrared (IR) heat treatment just after the deposition of MgO barrier was examined. In case that IR heat treatment was not applied, tunneling magnetoresistance (TMR) ratio of CoFeB/MgO/CoFeB magnetic tunnel junction (MTJ) was significantly decreased with decreasing resistance area (RA) product to less than 10 Ω μm2. On the other hand, TMR ratio of 206% was achieved at the RA product of 2.1 Ω μm2 when the IR heat treatment was applied. According to the cross sectional transmission electron microscope images for the samples with 0.76-nm-thick (∼4 ML) MgO barrier, the (001) oriented well crystallized structure with smooth interface was observed for the IR heated sample. Moreover, it revealed that the lateral grain size of MgO was significantly enlarged compared to that for the sample without IR heating. The improvement of TMR properties at low RA product region by the heat treatment might be due to the decrease ...

Effect of CoFe composition of the spin-valvelike ferromagnetic tunnel junction

The magnetoresistance (MR) ratio of a ferromagnetic tunnel junction with a spin-valvelike structure of NiFe/CoFex/Al–O/CoFex/NiFe/IrMn/Al is dependent upon CoFe composition. MR ratio increases with increasing Fe content, and shows a maximum of 42% for Fe content 26 at. % after annealing at 225 °C. Before annealing, the bias voltage dependence depend on Fe content, however, the bias voltage dependence did not depend on Fe content after annealing. We think that the increasing defect states in the barrier layer with the increasing Fe content in CoFex layer cause the degradation of the bias voltage dependence. After annealing, the defect states of samples decrease to same level, and the bias voltage dependencies of samples are improved and become same.

Spin-Valve-Like Properties of Ferromagnetic Tunnel Junctions

Ferromagnetic tunnel junctions with naturally oxidized Al barriers were fabricated by magnetron sputtering. Using metal masks, 0.01 mm2 junction areas were patterned. Junctions with slightly oxidized Al barriers showed small tunnel resistances. Of those, some had extraordinarily large MR ratios, others had different tunnel resistances according to different current-flow paths, and some had characteristics of both. In contrast, junctions with well oxidized Al barriers exhibited a large tunnel resistance and good stability. The Ni–Fe/Co/Al–AlOx /Co/Ni–Fe/Fe–Mn/Ni–Fe junction showed spin-valve-like R-H properties. The MR ratio was 10% for a 20 Oe magnetic field.

Lower-temperature crystallization of CoFeB in MgO magnetic tunnel junctions by using Ti capping layer

Effects of capping materials on magnetoresistance (MR) properties of MgO magnetic tunnel junctions (MTJs) with a CoFeB free layer were investigated. MR ratios of samples with various capping materials showed a difference in annealing temperature dependence. MTJ with a Ti capping layer annealed at 270°C showed a MR ratio 1.4 times greater than that with a conventional Ta or Ru capping layer. Secondary ion mass spectroscopy and high-resolution transmission electron microscopy images revealed that crystallization of CoFeB was remarkably affected by adjacent materials and the Ti capping layer adjoining CoFeB acted as a boron-absorption layer. These results suggest that the crystallization process can be controlled by choosing proper capping materials. Ti is one of the effective materials that accelerate the crystallization of CoFeB layers at lower annealing temperature.

Scalability of Spin Accumulation Sensor

In this work, we investigated a scalability of spin accumulation signal for various device sizes. We found that the spin accumulation signal is enhanced by shrinking Cu wire width and thickness. Moreover we estimate the signal to noise ratio (SNR) of a spin accumulation head structure by using the size dependence of spin accumulation signal. The SNR increases with reducing nonmagnetic wire width and thickness, which are corresponding to a track width and a gap width, respectively. And it has the maximum value around 2 ~ 3 Tbit/inch2 resolution. From these results, it has a possibility that spin accumulation effect is applicable to a new read head for high density hard disk drive.

Effects of interface oxidization in ferromagnetic tunnel junctions

We fabricated Ni-Fe/Co/Al-AlO/Co/Ni-Fe/Fe-Mn/Ni-Fe ferromagnetic tunnel junctions in which the bottom Co surface is exposed or not to air. While the MR ratio increased by annealing for the sample in which bottom Co surface was exposed to air, it decreased for the sample in which bottom Co surface was not exposed to air. This suggests that a slightly oxidized layer at the bottom Co/Al interface acts as a diffusion prevention layer. Moreover, the voltage-resistance dependence became asymmetric following annealing of the sample with a non oxidized Co surface and the bias dependence of MR ratio was improved.

Spin-valve - Properties of Ferromagnetic Tunnel Junctions

Recently, large changes in resistance of 18% were observed at room temperature in the ferromagnetic/insulator/ferromagnetic tunnel junction. This effect is theoretically supposed to show an MR ratio larger than those of spin-valves. Therefore, ferromagnetic tunnel junctions will be applicable for magnetic read head sensors. We have fabricated ferromagnetic tunnel junctions having a spin-valve-like layer structure. Thermal stability were also investigated.