Shinji Isogami

75% inverse magnetoresistance at room temperature in Fe4N/MgO/CoFeB magnetic tunnel junctions fabricated on Cu underlayer

Pseudo-single-crystal Cu underlayer (UL) with thermal tolerance was obtained on bare Si wafer by employing a diffusion-blocking layer. Fe4N layer fabricated on the Cu UL has an epitaxial relationship and a large grain diameter. Magnetic tunnel junctions in a stacking sequence of Fe4N/MgO/CoFeB exhibited an inverse tunnel magnetoresistance (TMR) effect at room temperature. The largest magnitude of the TMR ratio, −75.1%, was obtained at bias voltage Vb=−250 mV, where electrons flow from CoFeB to Fe4N. The large inverse TMR ratio is due to the improvement of the crystallinity of the Fe4N film because of the small lattice misfit between Fe4N and Cu.

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 ...

Negative Anisotropic Magnetoresistance in Fe4N Film

Negative anisotropic magnetoresistance (AMR) is observed in Fe4N film from 4.2 to 300 K. The AMR ratio rises with temperature in a stepwise fashion near 50 K, and is accompanied by a change in the magnetization hysteresis. The Campbell and Fert model is extended to investigate the influence of spin-polarization of conduction electrons on the AMR, and it is found that the negative AMR is not observed for majority spin conduction in ferromagnets. Consequently, it is concluded that the negative AMR that observed in the present study is possibly clear evidence of minority spin conduction in Fe4N film, as is predicted theoretically.

Inverse Current-Induced Magnetization Switching in Magnetic Tunnel Junctions with Fe4N Free Layer

Current-induced magnetization switching (CIMS) in CoFeB (pinned layer)/MgO/Fe4N (free layer) magnetic tunnel junctions (MTJs) was investigated at room temperature and an inverse CIMS phenomenon was clearly identified. From the current-field magnetic phase diagram, determined from the dependence of the switching field on the bias current, the direction of spin-transfer torque was found to be opposite to that observed in CoFeB/MgO/CoFeB-MTJs. The inverse CIMS observed in this study cannot be explained only by the conventional theory, which states that the direction of the spin-transfer torque is determined by the sign of the spin polarization of the pinned layer.

Soft X-ray Magnetic Circular Dichroism of a CoFe/MnIr Exchange Bias Film under Pulsed High Magnetic Field

Soft X-ray magnetic circular dichroism (XMCD) under pulsed high magnetic fields of up to 21 T has been measured using a nondestructive pulse magnet. XMCD effects at the Co and the Mn L2,3-edges of a Co70Fe30/Mn75Ir25 bilayer film have been investigated as an example of a high-magnetic-field XMCD measurement. A total electron yield method is adopted to detect absorption. Absorption is recorded by a 1-MHz time-resolved detection synchronized with a 50-ms high-magnetic-field pulse. Using this method, the XMCD effect is represented by µm (H), a function of magnetic field.

Linear correlation between uncompensated antiferromagnetic spins and exchange bias in Mn–Ir/Co100−xFex bilayers

A correlation between uncompensated (UC) antiferromagnetic (AF) spins and exchange bias (EB) strength was investigated for Mn74Ir26/Co100−xFex bilayers. The EB strength had a maximum at x=25 at. %, and the x-ray magnetic circular dichroism of Mn, representing the UC-AF spins, changed systematically in sign and magnitude with respect to x. A linear correlation was found between the EB strength and the root mean square magnitude of the UC-Mn spins. The hidden parameter connecting these two quantities might be exchange coupling energy at the heterointerface, which varies as a function of the ferromagnetic layer composition.

Investigation on the Magnetization Reversal of Nanostructured Magnetic Tunnel Junction Rings

Nanostructured magnetic tunnel junction rings (MTJ) based on CoFeB free layer were fabricated to investigate the corresponding magnetization reversal processes. The dimensions of the MTJ rings were with outer diameter/linewidth of 1 mum/100 nm and 0.7 mu m/100 nm , respectively, in which the fabrication was carried out by a top-down technique in combination with electron beam lithography, ion-milling and etch-back processes. Double transition processes in magnetoresistance (MR) responses are observed in these nanostructured MTJ rings. With considering the surface roughness, an OOMMF simulation was employed to explore the magnetization evolution. The simulated hysteresis loops are consistent with the MR behaviors, unfolding that the magnetic switching process initiates from the forward to reversed onion states through an incomplete vortex state. This two-step switching behavior is reasonably assumed to be due to a pinning site resulting from the surface roughness in a region of the ring. In addition, a series of in situ magnetic force microscopy images extracted from nanostructured MTJ rings reveals the similar magnetization evolution with the simulated results.

30-nm scale fabrication of magnetic tunnel junctions using EB assisted CVD hard masks

30-nm scale fabrication of magnetic tunnel junctions (MTJs) was demonstrated. A scanning electron microscope (SEM) was used for chemical-vapor deposition (CVD) of carbon hard masks. Using electron beam (EB)-CVD, less than several 10-nm scale carbon pillar could be formed on MTJ films. Argon ion milling, of which incident angle from the normal of the film plane was determined 45/spl deg/ and 75/spl deg/, was utilized to pattern the MTJs. TMR properties of 80-nm scale MTJs were successfully measured using DC-four-probe at room temperature.

Current-Induced Magnetization Switching and CPP-GMR in 30 nm

In this study, current-perpendicular-to-plane great magnetoresistance (CPP-GMR) spin valves with the minimum pillar width of 34 nm phi were successfully fabricated using EB-assisted chemical-vapor-deposition (CVD) hard masks. An area of the obtained magnetic cell is about one order smaller compared with those fabricated with normal EB or photo lithography technique. Measurement of transport properties such as current-induced magnetization switching (CIMS) and MR were demonstrated in such spin valves with various pillar widths. Dependence of the CPP-MR properties on the milled pillar width was discussed. In the case of 66 nm phi width in particular, the MR by external magnetic field switching and CIMS were 0.4% and 0.3%, respectively. The critical switching current Ic was ~40 mA (J c~9times108 A/cm2). In the case of smaller width, only MR by external magnetic field was observed

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A DC electric voltage was generated on a Pt-capped Fe4N bilayer film (Fe4N/Pt) by inverse spin-Hall effect (ISHE) under ferromagnetic resonance (FMR) conditions at room temperature. Sign reversal of the electric voltage was observed with the application of an external DC field with opposite direction, and the magnitude of the voltage was proportional to the applied microwave power. The spin current was pumped out of the Fe4N film into the Pt capping film. The real part of the spin mixing conductance (gr↑↓) was quantified for Fe4N/Pt and Ni78Fe22/Pt bilayer films to investigate the spin pumping efficiency at the interface. The gr↑↓ value was larger for the Fe4N/Pt film than for the Ni78Fe22/Pt film. Such a large gr↑↓ could reflect the large population of conduction electrons with minority spins in the Fe4N film compared with that with majority spins.