Characteristics of the magnetic behavior in magnetic multilayers with geometrical and structural symmetry breaking

Abstract

In this thesis, the magnetic behavior in magnetic multilayers with geometrical or structural broken symmetry was investigated for application to real devices such as logic devices, sensors and racetrack memory. In magnetic logic devices, the magnetization of domains in magnetic nanowires indicates information as 1 or 0, so control of domain wall motion has consistently received attention. In the second chapter, the chirality-dependent domain wall motion in Y-shaped magnetic nanowires asymmetrically attached to a nucleation pad was studied. As a result, in this geometry, clockwise transverse walls were found to be stably formed in a relatively low transverse field range and easily injected into the nanowire compared to the case of counterclockwise walls. This result is due to the tendency that one edge, the one with higher magnetostatic energy, of the transverse wall avoids a pinning site, which offers selectivity for chirality. The chirality of the wall was detected by magnetoresistance measurement of the Y-shaped spin-valve nanowire with the structure of Ta/NiFe/Cu/CoFe/IrMn/Ta. To date, the chirality of the domain wall has been electrically confirmed by magnetoresistance measurement in notched nanowires because the domain wall motion is dependent on the chirality at the notch. However, due to the difficulty in patterning a notch in a nanowire with sub-200 nm width, optical measurement has been considered convenient for transverse wall detection rather than electrical measurement. The Y-shaped spin-valve nanowire used here would be a convenient tool to detect the transverse wall. An object oriented micromagnetic framework (OOMMF) was implemented for the investigation of domain wall nucleation in the suggested geometry and supported the experimental results. A few years ago, STT was the sole method to reverse the magnetization by the current. Many research groups have focused on improving STT efficiency, resulting in commercialization of STT-magnetoresistive random access memory (STT-MRAM), where information is written by STT and read by the tunnel magnetoresistance effect (TMR). Recently, to overcome the limits from STT, such as the magnetization reversal speed, relation of retention time with switching current, and careful design considering the breakdown voltage, the control of magnetization with spin-orbit torque (SOT) has been intensively investigated. In heavy metal/ferromagnet bilayers, the in-plane current generates two SOTs of Slonczewski-like torque and field-like torque. Although both torques do not have a preference for magnetic states in ferromagnets with perpendicular magnetic anisotropy, the SLT could give preference for magnetic states corresponding to the current polarity if the magnetization has the same component as the current flow direction. In most experiments, to break symmetry, the external magnetic field is constantly applied along the current axis during current switching. However, the external magnetic field can cause detrimental effects on neighboring circuits, which become more serious as the spin device increases in density. In the third chapter, magnetization switching with SOT was investigated using interlayer coupling via a Ta spacer without an external magnetic field. The external magnetic field was replaced with an effective field from interlayer coupling between a Co layer with perpendicular magnetic anisotropy and a CoFe layer with in-plane magnetic anisotropy. The magnetization state was confirmed by anomalous Hall measurement of a Hall bar where the structure was Ta/Pt/Co/Pt/Ta/CoFe/IrMn/Ta. As a result, the magnetization of the Co layer was successfully reversed only with the in-plane current, and the switching sign at the zero field was kept until an external magnetic field of 600 Oe was applied along the opposite direction of the interlayer coupling. In comparison with the structure using a Ru spacer layer in a recent report, it was found that ultrathin thickness allows for stable magnetization switching, although tantalum is a material known to induce weak interlayer coupling. Therefore, the choice of the proper material for a spacer layer regardless of the interlayer coupling strength holds great potential for both enhancement of zero-field switching and optimization of junction functionality.