Yuji Uehara

Inverse tunnel magnetoresistance in magnetic tunnel junctions with an Fe4N electrode

The magnetotransport properties of Fe4N∕MgO∕CoFeB and Fe/MgO/CoFeB magnetic tunnel junctions (MTJs) were investigated at room temperature. In the Fe/MgO/CoFeB-MTJ, normal tunnel magnetoresistance (TMR) effect and roughly symmetric bias voltage (VB) dependence were observed, similar to the MTJs exhibiting coherent tunneling such as Fe/MgO/Fe. On the other hand, the inverse TMR effect, showing higher tunnel resistance for parallel magnetization configuration than for antiparallel configuration, and strong asymmetric VB dependence of TMR ratio were observed in the Fe4N∕MgO∕CoFeB-MTJ. The maximum TMR magnitude of 18.5% was obtained at VB=−200 mV, where the current flows from Fe4N to CoFeB. The enhancement of the inverse TMR ratio around VB=−200 mV is due to the broad peak of tunnel conductance in antiparallel configuration of Fe4N and CoFeB magnetizations. A large peak of the density of state at +300 meV from the Fermi level for minority spin electrons of bulk Fe4N might be an origin of this phenomenon.

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 ...

Microwave-Assisted Magnetization Reversal in a Perpendicularly Magnetized Film

Microwave-assisted magnetization reversal in a perpendicularly magnetized film was investigated using the anomalous Hall effect. Although the reversal process of the sample in this work was governed by nucleation and domain wall propagation, a large decrease in the reversal field, as much as 75% of the coercive field, was observed when the optimum rf magnetic field was applied. Our results indicate that the microwave-assisted effect promotes nucleation of the reversed domain leading to a reduction of the reversal field in a nucleation-type perpendicularly magnetized system.

Spin-valve sensors with domain control hard magnet layers

We have fabricated unshielded narrow-track NiFe/Cu/NiFe/FeMn spin-valve sensors with a height of 2 /spl mu/m and a track-width of less than 2 /spl mu/m using CoCrPt hard magnets (4/spl pi/Mr=5500 G, Hc=1000 Oe) as a domain control layer. Barkhausen noise was completely suppressed by a longitudinal biasing field from the CoCrPt layers. The bias state was improved by applying a strong ferromagnetic exchange coupling field of 30 Oe between the free and pinned NiFe layers through a 14 /spl Aring/-thick Cu interlayer. We have also fabricated shielded CoCrPt magnet-biased spin-valve read heads with a track-width of 1.7 /spl mu/m. The heads had no Barkhausen noise and very low crosstalk noise.

Micromagnetic Simulations of Magnetization Reversal in Misaligned Multigrain Magnets With Various Grain Boundary Properties Using Large-Scale Parallel Computing

This paper reports on micromagnetic simulations of the magnetization reversal behavior in polycrystalline Nd-Fe-B sintered magnets using large-scale parallel computing. A multigrain model is introduced to calculate the grain alignment dependence of the coercivity of polycrystalline Nd-Fe-B magnets with various magnetic characteristics at grain boundaries (GBs). The magnetic domain wall motion is accurately treated by dividing the analyzed object into extremely small elements. The multigrain model with a soft magnetic GB phase and a reverse domain at the initial state well reproduces experimental results. The calculations of coercivity with several GB widths are also carried out to seek for the origin of the sudden decrease of coercivity with nearly perfect grain alignment.

Write heads with pole tip consisting of high Bs FeCoAlO films

We investigated the magnetic properties, corrosion resistance, and microstructure for the sputtered FeCoAlO films, and the properties of write heads that use them for pole tips. The FeCoAlO films have soft magnetic properties as well as magnetic induction Bs /spl sim/ 2.2 T, which is equal to the Bs of pure-Fe. In-plane uniaxial anisotropy for the as-made films is almost unchanged though a magnetic field is applied during annealing, i.e., the magnetic annealing effect is negligible. The films have sufficient magnetic thermal stability and corrosion resistance to withstand magnetic head fabrication processes. For write heads with pole tips which consist of FeCoAlO films, the overwrite (O/W) and the nonlinear transition shift (NLTS) show about -35 dB and 13%, respectively. Therefore, these FeCoAlO films can be advantageously applied to writing heads.

Analysis of thermal magnetic noise in spin-valve GMR heads by using micromagnetic simulation

The size of the sensor in spin-valve GMR heads has been reduced to increase the areal magnetic recording density. Thermal magnetic noise, which arises from thermal fluctuation, becomes the main source of head noise and a limitation on recording density. Insufficient abutted permanent magnetic biasing yields an asymmetrical waveform both for signal output and thermal magnetic noise. This is due to the same mechanism as that of Barkhausen noise. In contrast, it has been found that small Hex, which is the exchange coupling strength between the bottom pinned layer (Pin1) and the antiferromagnetic biasing layer, emphasizes thermal magnetic noise without affecting signal output. In a head with small Hex, the magnetization near the air bearing surface and the top of the Pin2 layer tilts in the direction of the track width and randomly flips in the opposite direction. In synthetic ferrimagnetic heads, thermal magnetic noise chiefly depends on Hex rather than unidirectional anisotropy, Hua. The value of Hua does n...

Ultra-high magnetic moment films for write head

Novel magnetic materials with ultra-high magnetic moment are presented. The Fe/sub 70/Co/sub 30/ alloy is known to have the highest saturation magnetization of 2.45 T in nature. On the other hand, sputtered Fe/sub 70/Co/sub 30/:Pd and [Fe/sub 70/Co/sub 30//Pd]/sub n/ super-lattice films revealed higher saturation magnetization of 2.49 and 2.57 T at room temperature, respectively, while that of a Fe/sub 70/Co/sub 30/ single-layered film was 2.38 T. These values seemed to be acceptable of a main pole material of the write head in the hard-disk drive with an areal recording density above 160 Gbits/in/sup 2/. In addition, these films can be synthesized using a conventional sputtering apparatus as same as a Fe/sub 70/Co/sub 30/ films. Therefore, these films will promise a feasible way for the increase of an areal recording density without any additional technologies, such as a heat assisted recording, contact recording, or further development of the media.

Influence of Diffused Boron Into MgO Barrier on Pinhole Creation in CoFeB/MgO/CoFeB Magnetic Tunnel Junctions

A relationship between boron (B) diffusion into the MgO barrier and pinhole creation in CoFeB/MgO/CoFeB-magnetic tunnel junctions (MTJs) was investigated. The diffused B in the MgO layer was identified by secondary ion mass spectrometry for the MTJs annealed at 350degC , which provide the giant magnetoresistance (TMR) ratio. The pinhole density, estimated from the statistic distribution of breakdown voltage of the TMR properties, increased as either the thickness or the B content of the CoFeB layer became thicker or higher. These experimental findings imply that the diffused B into the MgO barrier creates pinholes to short-circuit the tunnel conduction, since the amount of diffused B into the MgO barrier might be related to the total amount of the B content in the CoFeB layer. Three different techniques were found to be useful for the reduction of diffused B into the MgO barrier layer; usage of materials having boron affinity for capping layer, decrease of the total amount of B-content in CoFeB layer, and reduction of grain boundaries in the MgO barrier layer.

Large Exchange Bias and High Blocking Temperature of MgO-Barrier-MTJs With L1

We examined the magnetic properties of CoFeB/MgO/CoFeB based magnetic tunnel junctions (MTJs) with L1₂₍ordered)-Mn₃Ir or gamma(disordered)-Mn₇₅Ir₂₅ as an antiferromagnetic (AFM) layer. Both of them showed tunnel magnetoresistance (TMR) ratio of about 160% after 350degC annealing. The exchange bias field (H_ex) of the MTJs with L1₂-Mn₃ Iris significantly larger than that with gamma-MnIr. The blocking temperature (T_B) of the MTJ film with L1₂-Mn₃Ir is about 50degC higher than that with gamma-MnIr. These results prove that L1₂-Mn₃Ir is a great candidate as AFM in MgO-barrier-MTJs and makes them improve in exchange bias properties.