Yoonsung Han

Injection Molding of Nanopillars for Perpendicular Patterned Magnetic Media with Metallic Nanostamp

In this work, we focus on two important technical issues that need to be overcome before the injection molding technology can be used in the mass production of patterned media, possibly, of small form factor or non-rotating type. Firstly, we describe a fabrication method of a metallic nanostamp through the use of an ultraviolet (UV)-imprinted polymeric nanomaster. Secondly, we examine a method to use a passive heating of the nanostamp to increase the replication quality and to obtain the desired height of the injection-molded nanopillars. Finally, we show that the magnetic film on each injection-molded nanopillars with passive heating has a single magnetic domain.

Effect of Diffused B During Annealing on the Electronic Structure of the MgO Barrier in CoFeB/MgO/CoFeB Magnetic Tunnel Junctions

We investigate the chemical state and electronic structure of boron in the MgO layer in CoFeB/MgO/CoFeB magnetic tunnel junctions by synchrotron radiation X-ray photoelectron spectroscopy and first-principles calculations. In the top CoFeB, we observe three boron 1s peaks corresponding to oxidic, intermediate, and metallic boron. As the annealed films were etched, the B2O3 starts to appear together with the Mg 2p peak, indicating that boron atoms diffuse into the MgO barrier. Our calculations show that B impurities in the MgO barrier do not create any midgap states but reduce the MgO barrier height with the increase of boron concentration.

Angular dependence of the exchange bias direction and giant magnetoresistance on different cooling-field strengths

We studied the effect of different cooling-field strengths on the exchange bias by measuring the angular-dependent sheet resistance and the giant magnetoresistance of exchange-biased spin valves using a PtMn antiferromagnetic and a synthetic antiferromagnetic layer. When we annealed the spin valve at a cooling-field of 100 Oe, the exchange bias was antiparallel to the cooling-field. As we increased the cooling-field to 4000 Oe, the exchange bias direction gradually rotated and it ended up parallel to the cooling-field direction. The giant magnetoresistance also changed with the cooling-field strength. In the cooling-field range between 100 and 4000 Oe, the magnetoresistance ratios measured along the cooling-field direction were significantly reduced. However, the magnetoresistance ratios measured along the exchange bias direction increased, although still remaining smaller than those of the spin valve annealed at 100 or 4000 Oe. On the other hand, the exchange bias strength did not change significantly wi...

Microscopic and electronic roles of B in CoFeB-based magnetic tunnel junctions

The giant tunneling magnetoresistance (TMR) in lattice-matched crystalline magnetic tunnel junctions (MTJs) strongly depends on the majority-spin Δ1 bands of ferromagnetic electrodes. Our synchrotron radiation X-ray photoelectron spectroscopy and first-principles calculations show that B atoms suppress the formation of CoFe–O during CoFeB deposition in CoFeB/MgO/CoFeB MTJs, thereby lessening an effect that significantly degrades the majority-spin Δ1 bands, and that CoFe–B has properties superior to CoFe–O in electron conduction in the majority-spin Δ1 bands. During annealing, some of the B in CoFeB diffuses out, enhancing the valence bands of metallic CoFe, which improves the TMR value even further. Our present work elucidates the microscopic and electronic roles of B in MgO MTJs with CoFeB electrodes.

On-film tunneling resistance measurements of unpatterned magnetic tunnel junctions

We successfully measured not only the tunneling properties but also the magnetic properties of a magnetic tunnel junction without patterning the magnetic tunnel junction stack itself by the current-in-plane tunneling method. Arrays of in-line four-point-probe sets with different spacings defined on the wafer made it possible to evaluate film tunneling properties. The estimated results differed little from those estimated by the commercial instrument. The small standard deviations of measured magnetic properties prove that our method is indeed reliable. Even though the properties for a less than submicron spacing were not available, we could successfully estimate the major characteristics of magnetic tunnel junctions. Our method can be applied in any environment, even in air without any clean-room facilities, and completed in as little as a day.

Anisotropy dispersion in the exchange-biased pinned layer of a spin valve prepared by 550 eV hydrogen-ion irradiation

By investigating angular dependence of resistance and applying the Boltzmann distribution to the anisotropy dispersion of the magnetization in an exchange-biased pinned layer, we quantized the intrinsic anisotropy dispersion σγ of spin valves. The σγ was estimated to be 0.412° for the as-deposited spin valve and 0.183° for the ion-irradiated spin valve. This indicates that the dispersion indeed narrowed when the spin valve was field-annealed or irradiated by 550 eV hydrogen ions under a magnetic field, which is consistent with our previous attribution to the significant improvement in both exchange anisotropy and giant magnetoresistance of spin valves thus treated. Our methodology can be applied for other spin devices characterized by angular dependence of resistance to determine useful device properties such as the intrinsic anisotropy dispersion and the exchange bias of the exchange-biased reference layer.

Improvement in both giant magnetoresistance and exchange bias through hydrogen ion irradiation at low energy

Irradiation of IrMn-based spin valves with 550 eV hydrogen ions increased their giant magnetoresistance and exchange bias by 20% and 60%, respectively. This significant enhancement stems from the strong (111) texture and small mosaic spread of the IrMn antiferromagnet that resulted from the microstructural reconstruction caused by the energy transfer during the bombardment by hydrogen ions, as well as by the narrow dispersion in the exchange bias. Irradiation with the hydrogen ion at low energy can improve the properties of spin valves without resulting in undue degradation in the performance or the microstructure.

Effects of H-ion irradiation on the properties of a spin valve

Irradiation with much less than 1.0keV H ions significantly improved the properties of IrMn-based spin valves. The giant magnetoresistance (GMR) was increased from 8.1% to 9.6% and the exchange bias field from 355to565Oe after irradiation at 550eV. The irradiation achieved even higher GMR than that achieved by field annealing: 9.6% vs 8.7%. We attribute this enhancement to strong (111) textures of the IrMn antiferromagnet and CoFe∕Cu∕CoFe∕NiFe layers, as well as to a narrow mosaic spread of the (111) IrMn, both of which were developed by momentum transferred during the ion bombardment. The irradiated spin valve showed exchange bias as large as and (111) textures as strong as those of the field-annealed spin valve. The significant difference in the GMR values of the irradiated versus the field-annealed spin valves was probably due to different degrees of intermixing between layers. In the case of irradiation, the low energy of the lightest H ion likely resulted in little intermixing and, hence, the interfa...

Magnetization Reversal Behavior of Submicron-sized Magnetic Films in Response to Sub-ns Longitudinal Field Pulses Along the Easy Axis

We simulated the magnetization reversal behavior of submicron-thickness magnetic films by applying pulses of sub-ns-long durations and amplitudes along the easy axis. The films were rectangular and elliptical , and their thickness was 2 nm and 4 nm. We observed different behaviors depending upon the shape and thickness of the films and found a normal non-switching in regions in which we expected complete switching after relaxation. In the elliptical film, the non-switching regions were found to be random and to be widely distributed throughout the switching map. The strong demagnetization field along the z-axis, the film thickness direction, is likely responsible for this abnormal behavior. In the rectangular film, the abnormal non-switching regions were less distributed than they were in the elliptical film due to edge domains resulting from the small or demagnetization field during the switching. Our simulation confirms that large demagnetization is detrimental to the ultra-fast magnetization reversal of magnetic ultra-thin films.

Substantial enhancement in the performance of a spin valve through the ultra-thin insertion of partially oxidized Fe

We found that the ultra-thin insertion of a mix of Fe and Fe oxide in the pinned layer substantially enhanced the performance of a spin valve. With the exchange bias field kept large, the giant magnetoresistance (GMR) reached 11.3%, which corresponded to a 46% increase in GMR compared with the GMR of spin valves without the insertion. We attribute this considerable enhancement to spin-dependent scattering boosted by the combined effect of the modified local electronic structures at the Fe and specular reflections at the α-Fe2O3 of the insertion.