Shinji Yamagata

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.

Ferromagnetism in II–VI diluted magnetic semiconductor Zn1−xCrxTe

Magnetic and transport properties of an epitaxial film of ferromagnetic II–VI diluted magnetic semiconductor (DMS) Zn1−xCrxTe (x=0.035) were investigated. The Curie temperature TC of the film was about 15 K, which is the highest among the reported ferromagnetic II–IV DMS. Hall effect measurements at room temperature showed a hole concentration p of about 1×1015 cm−3, which is several orders lower than that reported for carrier-induced ferromagnetic Zn1−xMnxTe. These results suggest that a ferromagnetic superexchange interaction between Cr ions is responsible for the observed ferromagnetism in Zn1−xCrxTe.

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.

Room-temperature ferromagnetism in highly Cr-doped II–VI diluted magnetic semiconductor Zn1−xCrxTe

Highly Cr-doped II–VI diluted magnetic semiconductor (DMS) Zn1−xCrxTe films with a ferromagnetic long-range order have been grown. A phase diagram of Zn1−xCrxTe in relation to the growth temperature and Cr concentration was determined. Magnetic circular dichroism measurements revealed that a magnetically single phase of DMS Zn1−xCrxTe is obtained in the films with Cr concentration up to x=0.20. Spontaneous magnetization of the film with x=0.20 disappears around 300 K, indicating that the Zn1−xCrxTe is a DMS with room-temperature ferromagnetism.

Novel Stack Structure of Magnetic Tunnel Junction with MgO Tunnel Barrier Prepared by Oxidation Methods: Preferred Grain Growth Promotion Seed Layers and Bi-layered Pinned Layer

Despite superior compatibility to mass-production, magnetic tunnel junction (MTJ) with MgO barrier prepared by oxidation process (MgOx) has shown unacceptable magnetotransport properties for proper operation of spintronics devices because poor crystalline MgOx cannot properly provide a template for crystallization of amorphous CoFeB layers, thus lack of pseudo-epitaxy in overall. We report novel stack structure for MgOx-based MTJ to assure acceptable magnetotransport properties: insertion of preferred-grain-growth-promotion (PGGP) seed layer and bi-layered ferromagnetic pinned layer (bi-PL) to induce preferred grain growth in MgOx and crystallization of CoFeB layers at higher temperature annealing. Microstructure analysis confirms highly crystalline MgOx in pseudo-epitaxy with fully crystallized CoFeB via PGGP by high temperature annealing, attributed to enhanced thermal stability of bi-PL. Tunneling magnetoresistance (TMR) 132.6% at resistance-area product (RA) 1.2 Ω µm2 and 253% at 5.9 Ω µm2 from novel MTJ stack successfully satisfy specifications for spintronics devices.

Magnetoresistance in a room temperature ferromagnetic diluted magnetic semiconductor Zn1−xCrxTe

We observed negative magnetoresistance (MR) in a ferromagnetic diluted magnetic semiconductor Zn1−xCrxTe (x=0.20). At a temperature of 20 K, the MR ratio was −26% (Δρ=−4100 Ωcm) in a magnetic field of 10 kOe. The MR showed a clear hysteresis loop. Negative MR was observed up to about room temperature, corresponding to a Curie temperature. These behaviors indicate that the electrical properties of Zn1−xCrxTe are strongly affected by its magnetic properties through a strong carrier–spin interaction.

Evaluation of barrier uniformity in magnetic tunnel junctions prepared using natural oxidation of thin Mg layers

We fabricated submicron magnetic tunnel junctions (MTJs) using natural oxidation of thin Mg layers deposited by dc sputtering. The MTJs exhibited magnetoresistance (MR) ratios of up to about 150% with a low resistance-area product (RpA) of 8 Ω μm2, which are comparable to those for radio-frequency-sputtered MgO barriers. The submicron MTJs had highly variable MR and RpA values due to a high pinhole density (20 μm−2) in the barriers, whereas current-in-plane-tunneling (CIPT) measurements on the same MTJ films revealed highly reproducible MR and RpA values. This indicates that reproducible CIPT measurements do not necessarily give accurate results of MR and RpA at high pinhole densities.

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.