Motonobu Nagai

230% room-temperature magnetoresistance in CoFeB∕MgO∕CoFeB magnetic tunnel junctions

The magnetoresistance ratio of 230% at room temperature is reported. This was achieved in spin-valve type magnetic tunnel junctions using MgO barrier layer and amorphous CoFeB ferromagnetic electrodes fabricated on thermally oxidized Si substrates. The amorphous CoFeB electrodes are of great advantage to the polycrystalline FeCo electrodes in achieving a high homogeneity in small 100 nm-sized MTJs.

Giant tunneling magnetoresistance effect in low-resistance CoFeB∕MgO(001)∕CoFeB magnetic tunnel junctions for read-head applications

The giant tunneling magnetoresistance effect has been achieved in low-resistance CoFeB∕MgO(001)∕CoFeB magnetic tunnel junctions (MTJs) at room temperature. A magnetoresistance (MR) ratio as high as 138%, seven times that of state-of-the-art MTJs for magnetic sensor application, was obtained at room temperature in MTJs with a resistance-area product (RA) as low as 2.4Ωμm2. Such a high MR ratio at such a low resistance was made possible by introducing an ultrathin Mg metal layer with a thickness of 4 A between the CoFeB bottom electrode layer and the MgO(001) tunnel barrier layer. The Mg layer was slightly but not fully oxidized, which resulted in a reduction in MR for a thicker MgO barrier (high RA) region and in an increase in MR for a thinner barrier (low RA) region. The Mg layer improves the crystalline orientation of the MgO(001) layer when the MgO(001) layer is thin. These MTJs will accelerate the realization of highly sensitive read heads for ultrahigh-density hard-disk drives.

Huge magnetoresistance and low junction resistance in magnetic tunnel junctions with crystalline MgO barrier

Inserting a 4 /spl Aring/-Mg metal layer between the amorphous CoFeB bottom electrode layer and the MgO barrier layer was found to be effective in realizing huge magnetoresistance effect in low-resistance CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs). As a result, magnetoresistance (MR) ratio as high as 138% at resistance-area product (RA) of about 2.4 /spl Omega//spl middot//spl mu/m/sup 2/ was obtained. This value is about seven times that of state-of-the-art MTJs for magnetic sensor application. X-ray diffraction analysis clarified that crystal orientation of the poly-crystalline MgO(001) barrier layer was improved by the Mg layer. It is suggested that the higher crystalline orientation of the MgO(001) barrier layer could have enhanced the coherent tunneling of /spl Delta//sub 1/ electrons, resulting in an increase of MR ratio at the low RA (thin MgO thickness) region. The annealing temperature and free layer materials have also been optimized to satisfy the requirements for practical read head application. Although this optimization resulted in a reduction in the MR ratio to about 45%-53%, this value is still more than twice the highest MR ratio of conventional MTJs. The currently developed fabrication process will accelerate the development of highly sensitive read heads for ultrahigh-density hard-disk drives.

CoFeB/MgO/CoFeB magnetic tunnel junctions with high TMR ratio and low junction resistance

A successful attempt in obtaining magnetic tunnel junctions (MTJs) with resistance-area (RA) of 4.7 /spl Omega/m/sup 2/ and magneto-resistance (MR) ratio of about 150% at room temperature is reported, in MTJs using CoFeB/MgO/CoFeB. The MTJs consisting of Ta/CuN/Ta/Pt/sub 50/Mn/sub 50//Co/sub 70/Fe/sub 30//Ru/Co/sub 60/Fe/sub 20/B/sub 20//MgO (x nm)/Co/sub 60/Fe/sub 20/B/sub 20//Ta/Cu/Ta/Ru were prepared on a thermally oxidized silicon wafer using an ANELVA C-7100 sputtering system. All the metallic films were deposited by using dc magnetron method. The insulating MgO layer was deposited by rf sputtering directly from a sintered MgO target, and the thickness was varied from 6 to 24 /spl Aring/. After the film deposition, the MTJs were annealed at 360/spl deg/C for 2 hours in a magnetic field of 8 kOe. It was found that as MgO thickness decreases, RA decreases exponentially, but MR ratio remains constant of about 230% until around 12 /spl Aring/ and gradually decreases for MgO thickness below 12 /spl Aring/.

Effect of capping layer material on tunnel magnetoresistance in CoFeB-MgO-CoFeB magnetic tunnel junctions

The effect of capping layer (CL) material on tunnel magnetoresistance (TMR) and magnetic properties in CoFeB/MgO/CoFeB magnetic tunnel junctions has been studied. The use of CL materials such as Ta, Ru, PtMn, Mg and IrMn yielded high MR ratio of more than 100%. The use of ferromagnetic materials, on the other hand, such as NiFe, CoFe or Fe decreased the MR ratio, as well as using Al, Cu, NiFeCr or Ir. The dependence of coercivity on CL materials for the same samples are also studied. It was speculated from the results that the degradation of the TMR and the magnetic properties when particular capping layer materials were used, is due to the formation of a reactive layer between the free layer and the capping layer.

Giant tunneling magnetoresistance in MgO-based magnetic tunnel junctions and its industrial applications

First-principle theories predicted an extremely high magnetoresistance (MR) ratio over 1000% in epitaxial Fe(001)/MgO(001)/Fe(001) MTJs. We have fabricated fully epitaxial Fe-Co(001)/MgO(001)/Fe-Co(001) MTJs and textured CoFeB/MgO(001)/CoFeB MTJs and achieved giant MR ratios above 400% at room temperature. An ultra-low resistance-area (RA) product indispensable for magnetic sensor application has also been achieved in CoFeB/MgO(001)/CoFeB MTJs. The giant TMR effect in MgO-based MTJs is the key for next-generation spintronic devices.