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Activation Volumes of Wall - Motion and Nucleation Processes in Co / Pd Multilayers
Sug-Bong Choe,Sung-Chul Shin 한국자기학회 2000 Journal of Magnetics Vol.5 No.2
The correlation between the activation volumes of wall-motion and nucleation processes in Co/Pd multilayers has been investigated. Each activation volume was estimated from the field dependence of the wall-motion speed and the nucleation rate, respectively, based on time-resolved domain patterns grabbed by a MOKE microscope system. Both the activation volumes are changed in the same manner around 0.2~1.1 × 10^(-17) ㎤ with changes in the multilayered structure. Interestingly, the correlation between the activation volumes is sensitive to the multilayered structure; the wall-motion activation volume is smaller than the nucleation activation volume for a sample having a smaller number of repeats and a thinner Co-layer thickness, and vice versa. The correlation is closely related with the contrasting reversal modes; the process having the smaller activation volume dominates.
Interfacial Dzyaloshinskii-Moriya Interaction in Ferromagnetic Thin Films
Sug-Bong Choe 한국자기학회 2021 한국자기학회 학술연구발표회 논문개요집 Vol.31 No.2
Interfacial phenomena play decisive roles in modern science and technology as the scale shrinks down to a few atomic layers. Such minute nanostructures require more comprehensive understanding beyond the conventional concepts of interfaces and interfacial phenomena generated at interfaces. The Dzyaloshinskii–Moriya interaction (DMI) generates intriguing chiral magnetic objects such as magnetic skyrmions and chiral domain walls that can be used as building blocks in emerging magnetic nanodevices. Precise control of the DMI strength is one of the key issues to achieve better stability and functionality of these chiral objects. Here we present two major experimental results on the interfacial DMI and spin-orbit torque (SOT) in ferromagnetic films. First, we report an experimental observation that in magnetic trilayer films, the DMI strength exhibits a noticeable correlation with the work functions of the nonmagnetic layers interfaced to the magnetic layer. Such correlation with the intrinsic material parameters provides a guideline for material selection to engineer the DMI strength. Second, from series of a-few-atomic-layer-thick magnetic films, here we demonstrate experimentally that, contrary to the common notion, interfacial phenomena require finite thickness for their full emergence. The layer thickness dependences revealed that the interfacial DMI begins to appear with thickness and emerges completely in the thickness of 2 to 3 atomic layers, at which the magnitude is maximized. This implies that the DMI is suppressed, when the “bulk” layer adjacent to interfaces is thinner than the threshold thickness. The existence of the threshold thickness indicates the need to refine conventional perspectives on interfacial phenomena and imposes the lowest structural bound and optimum thickness to maximize interfacial effects for technological applications.
Measurement of Activation Magnetic Moment in Ferromagnetic Thin Films
Sug-Bong Choe,Sung-Chul Shin 한국자기학회 2001 Journal of Magnetics Vol.6 No.2
We have investigated the activation magnetic moment, which characterizes the basic magnetic moment acting as a single magnetic particle during magnetization reversal. The activation magnetic moment was measured from each local area on continuous ferromagnetic thin films, by analyzing the magnetic field dependence of magnetization reversal of the corresponding local area, based on a thermally activated relaxation process. It was found that the activation magnetic moment was nonuniform on a submicrometer scale; the fluctuation increased with increasing the number of layers in Co/Pd multilayers. The distribution could be well described by exp(δm^(3/2)), where δm is the deviation of the activation magnetic moment from the mean value.
Influence of Local Coercivity Variation on Magnetization Reversal Dynamics
Sug-Bong Choe,Hyuk-Jae Jang,Sung-Chul Shin 한국자기학회 2000 Journal of Magnetics Vol.5 No.1
Local coercivity variation of Co/Pd nanomultilayers has been investigated by measuring the polar Kerr hysteresis loops of local areas of submicron size using a magnetooptic Kerr microscope system. Interestingly, the local coercivity distribution is very sensitive to an increase in the number of repeats: the (2-Å Co/11-Å Pd)_(10) sample showed a smooth variation of the local coercivity, while (2-Å Co/11-Å Pd)_(18) showed a large fluctuation. From micromagnetic considerations based on a thermally activated relaxation model, we have found that this local coercivity variation has a crucial effect on the contrasting magnetization reversal behavior observed in those samples: dominant wall-motion for the former sample and dominant nucleation for the latter one.
Sug-Bong Choe,Sung-Chul Shin 한국자기학회 2000 Journal of Magnetics Vol.5 No.3
A magneto-optical microscope magnetometer (MOMM) has been developed to simultaneously measure 2-dimensional array hysteresis loops of each local area of 320×320-㎚² spots on ferromagnetic films, in addition to grabbing time-resolved domain evolution patterns. Using the system, spatial distribution of local coercivity can be quantitatively generated and then, compared directly with domain patterns grabbed at precisely the same position of a sample. It is clearly demonstrated that local coercivity distribution governs domain reversal behavior via a thermally activated relaxation process.
Depinning field from notches on Co/Pd multilayer nanowires with perpendicular magnetic anisotropy
Wiley - VCH Verlag GmbH & Co. KGaA 2007 Physica Status Solidi C Vol.4 No.12
<P>Domain-wall pinning and depinning mechanisms of ferromagnetc nanowires with perpendicular magnetic anisotropy were investigated. Micromagnetic simulation results revealed that the depinning field from notches were determined solely by the gap distance between the notches, irrespective to the wire width and the notch size. The double-notch wires exhibited two-fold décalcomanie domain patterns and one half patterns were identical to those of single-notch wires. Therefore, the double-notch wires showed the same depinning field compared to single-notch wires with half width. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)</P>