Byong Sun Chun

Probing One Antiferromagnetic Antiphase Boundary and Single Magnetite Domain Using Nanogap Contacts

We have probed one antiferromagnetic (AF) antiphase boundary (APB) and a single Fe(3)O(4) domain using nanogap contacts. Our experiments directly demonstrate that, in the case of probing one AF-APB, a large magnetoresistance (MR), high resistivity, and a high saturation field are observed as compared with the case of probing a single Fe(3)O(4) domain. The shape of the temperature-dependent MR curves is also found to differ between the single domain and one of the AF-APB measurements, with a characteristic strong temperature dependence for the single domain and temperature independence for the one AF-APB case. We argue that these observations are indicative of profound changes in the electronic transport across APBs. The investigated APB defects increase the activation energy and disturb the long-range charge ordering of monodomain Fe(3)O(4).

Magnetic bubblecade memory based on chiral domain walls

Unidirectional motion of magnetic domain walls is the key concept underlying next-generation domain-wall-mediated memory and logic devices.

Magnetization switching and tunneling magnetoresistance effects of synthetic antiferromagnet free layers consisting of amorphous NiFeSiB

MTJs with the synthetic antiferromagnetic (SAF) free layer consisting of CoFeSiB/Ru/CoFeSiB were prepared because a SAF structure can reduce the magnetostatic coupling. Magnetic switching and tunneling magnetoresistance (TMR) ratio of MTJs were investigated by experiment and by simulation. SAF structures show lower TMR ratio and coercivity (H/sub c/) than single free layer due to low net magnetic moment. The CoFeSiB SAF structure was found to have lower exchange energy.

Thickness dependence of the effective damping in epitaxial Fe3O4/MgO thin films

The precessional magnetization dynamics of high quality epitaxial magnetite (Fe3O4) thin films growth on MgO are investigated by inductive magnetization dynamic measurements in time and frequency domain. An upper bound for the intrinsic Gilbert damping parameter of α0=0.037±0.001 is derived, which is significantly lower than previously reported for epitaxial Fe3O4 on GaAs. With increasing film thickness from 5 up to 100 nm a strong increase in the effective damping up to 0.2 is observed which cannot be explained by simple nonuniform spin wave excitations. Possible origins of this effect are discussed.

Domain wall width and velocity behaviors in notched magnetic devices

Current-induced domain wall motion on a magnetic strip has been studied for its potential application to magnetic logic and memory devices. In this study, we carried out a series of calculations to investigate the changes of the domain wall velocity and width as a function of the notch shape using the Landau-Lifschitz-Gilbert solver. The domain wall had a uniform velocity when the notches were shallow and wide, while the width of the domain wall became narrow when the notches were deep and narrow. To produce a domain wall structure with both a uniform velocity and a narrow width, an asymmetric notch shape was devised.

Magnetoresistance and magnetization switching characteristics of magnetic tunnel junctions with amorphous CoFeSiB single and synthetic antiferromagnet free layers

To obtain low switching field (Hsw) we introduced amorphous ferromagnetic Co70.5Fe4.5Si15B10 single and synthetic antiferromagnet (SAF) free layers in magnetic tunnel junctions (MTJs). The switching characteristics for MTJs with structures Si∕SiO2∕Ta 45/Ru 9.5/IrMn 10/CoFe 7∕AlOx∕CoFeSiB 7 or CoFeSiB (t)∕Ru 1.0/CoFeSiB (7−t)∕Ru 60 (in nanometer) were investigated and compared to MTJs with Co75Fe25 and Ni80Fe20 free layers. CoFeSiB showed a lower saturation magnetization of 560emu∕cm3 and a higher anisotropy constant of 2800erg∕cm3 than CoFe and NiFe, respectively. An exchange coupling energy (Jex) of −0.003erg∕cm2 was observed by inserting a 1.0nm Ru layer in between CoFeSiB layers. In the CoFeSiB single and SAF free layer MTJs, it was found that the size dependence of the Hsw originated from the lower Jex experimentally and by micromagnetic simulation based on the Landau-Lisfschitz-Gilbert equation. The CoFeSiB SAF structures showed lower Hsw than that of NiFe, CoFe, and CoFeSiB single structures. The Co...

Characteristics of magnetic tunnel junctions consisting of amorphous CoNbZr layers

Magnetic tunnel junctions comprising amorphous CoNbZr layers have been investigated. Co85.5Nb8Zr6.5 (in at. %) layers were employed to substitute traditionally used Ta layers with an emphasis given on understanding underlayer effect. The typical junction structure was SiO2/CoNbZr or Ta 2/CoFe 8/IrMn 7.5/CoFe 3/Al 1.6+oxidation/CoFe 3/CoNbZr or Ta 2 (nm). For both as-deposited state and after annealing, the CoNbZr-underlayered structure showed superior surface smoothness up to the tunnel barrier than Ta-underlayered one (0.16 vs 0.34 nm). Ultrasmooth layer structure was proven beneficial for reducing temperature-dependent magnetoresistance properties. A longer annealing degraded the properties most likely due to Mn interdiffusion toward the bottom ferromagnetic electrode as confirmed by Auger electron spectroscopy. By a slight change in the bottom electrode thickness (CoNbZr 4/CoFe 10), the tunneling magnetoresistance ratio can be increased up to 32% after 10 min annealing at 300 °C.

Suppression of bias voltage dependence in double-barrier magnetic tunnel junctions comprised of freelayers with an amorphous layer insertion

A double-barrier magnetic tunnel junction (DMTJ) comprising an amorphous freelayer was investigated. The typical sample structure consists of Ta42∕CoZrNb9.2∕IrMn15.5∕CoFe3.3∕AlOx2∕freelayer∕AlOx2∕CoFe7.5∕IrMn15.5∕Ru60(nm). The freelayer includes CoFe2.3∕CoZrNb3.4∕CoFe3. A CoFe 5nm single freelayer was also prepared for reference purpose. Although TMR ratio and RA product appear similar, less attenuation in the TMR ratio was observed at 0.4V for the junction with CoZrNb layer. Coercivity and interlayer coupling were reduced while squareness was improved. The top portion of the DMTJ including top barrier became more uniform after amorphous layer insertion.

Skyrmion motion driven by oscillating magnetic field

The one-dimensional magnetic skyrmion motion induced by an electric current has attracted much interest because of its application potential in next-generation magnetic memory devices. Recently, the unidirectional motion of large (20 μm in diameter) magnetic bubbles with two-dimensional skyrmion topology, driven by an oscillating magnetic field, has also been demonstrated. For application in high-density memory devices, it is preferable to reduce the size of skyrmion. Here we show by numerical simulation that a skyrmion of a few tens of nanometres can also be driven by high-frequency field oscillations, but with a different direction of motion from the in-plane component of the tilted oscillating field. We found that a high-frequency field for small skyrmions can excite skyrmion resonant modes and that a combination of different modes results in a final skyrmion motion with a helical trajectory. Because this helical motion depends on the frequency of the field, we can control both the speed and the direction of the skyrmion motion, which is a distinguishable characteristic compared with other methods.

Magnetotransport properties of dual MgO barrier magnetic tunnel junctions consisting of CoFeB/FeNiSiB/CoFeB free layers

We report the transport properties of a dual MgO barrier magnetic tunnel junction (DMTJ) where a FeNiSiB layer was inserted in a CoFeB free layer. Upon post-deposition annealing at 330 °C, the tunneling magnetoresistance (TMR) ratio of the DMTJ with a hybrid CoFeB/FeNiSiB/CoFeB free layer reached 195% which is higher than the TMR ratio of 121% from the DMTJ with the single CoFeB free layer. The bias voltage dependence profile was more symmetric for the hybrid case. Boron depth profiling result suggests that the FeNiSiB layer dragged boron atoms more to it rather than letting them diffuse toward CoFeB/MgO interfaces, resulting in improved MTJ performances.