• Journal of Magnetics
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• 제12권1호
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• pp.17-20
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• 2007

We performed magnetic force microscopy (MFM) observation on array of Co dots in order to understand magnetic state and magnetization behavior of submicron sized Co dots patterned on GaMnAs bridge. MFM observations showed the magnetization reversal and processes of local magnetization of individual ferromagnetic Co nanodots. Magnetic state of Co dots either single domain or vortex is dependent on geometrical size and thickness. Transition from single domain to vortex state can be realized with MFM tip assisted local field. Magnetization reversal process takes place through sequential reversal of individual dots. Localized inhomogeneous magnetic field can be manipulated by controlling magnetic state of individual Co dot in the array structure.

• Journal of Magnetics
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• 제11권2호
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• pp.66-69
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• 2006

Local magnetization reversal in the exchange-biased NiFe/FeMn and $[Pd/Co]_5/FeMn$ multilayers with in-plane and out-of-plane magnetic anisotropy was achieved by using laser annealing. The local annealed NiFe/FeMn film under the opposite magnetic field shows a magnetoresistance (MR) curve having two symmetric peaks at the positive and negative exchange biasing field (${\pm}H_{ex}$). The intensity of the nucleated MR peak rises as the exposed area extends during the laser annealing process, and the peak disappears under the reverse magnetic field. In the case of [Pd/Co]/FeMn films, the local magnetization reversal increased gradually as the laser power increases. The locally reversed magnetization was restored under the opposite magnetic field.

• Journal of Magnetics
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• 제5권1호
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• pp.4-8
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• 2000

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 $(2-{\AA}\;Co/ll-{\AA} Pd)_10$ sample showed a smooth variation of the local coercivity, while $(2-{\AA}\;Co/ll-{\AA} Pd)_18$ showed a large fluctuation. From micromagnetic considerations based on a thermally activated relaxation odel, 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 far the former sample and dominant nucleation for the latter one.

• Journal of Magnetics
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• 제11권2호
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• pp.61-65
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• 2006

The physical origin of complex dynamic domain configuration in nonequilibrium magnetic systems with mesoscopic length scales has been studied. An increasing complexity in the spatial feature of the evolution is found to accompany the increasing reversal speed, when a ferromagnetic element is driven by progressively faster switching fields applied antiparallel to the initial magnetization direction. As reversal rates approach the characteristic precession frequencies of spin fluctuations, the thermal energy can boost the magnetization into local configurations which are completely different from those experienced during quasistatic reversal. The sensitive dependence of the spatial pattern on switching speed can be understood in terms of a dynamic exchange interaction of thermally excited spins; the coherent modulation of the spins is strongly dependent on the rise time of switching pulses.

• 한국자기학회지
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• 제14권6호
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• pp.224-227
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• 2004

교환결합된 NiFe/FeMn 이중박막에서 Laser 열처리를 통하여 국소적인 자화반전을 연구하였다. 600 G의 외부 자기장을 시편에 가하면서 DPSS Laser로 자화 반전시켰으며, 300 mW에서 15분 동안 국소적인 부분을 조사하였다. 국소적으로 자화반전 시킨 박막을 다시 원래 방향으로 역 반전시켜 자기저항(MR)곡선 피크가 감소된 것을 관찰하였다. 이는 직접적으로 Laser를 가한 박막의 손상과 계면에서 내부 확산에 의한 것으로 생각된다 국소적인 반전 자화의 자구구조는 MU을 통하여 관찰하였다. 자화반전 영역에서 새로운 자구벽을 형성하였으며, Laser가 조사된 영역근처에서 자구벽이 형성된 것을 관찰하였다.

• 한국자기학회:학술대회 개요집
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• 한국자기학회 2000년도 International Symposium on Magnetics The 2000 Fall Conference
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• pp.200-206
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• 2000

We have investigated the activation magnetic moment, which characterizes the basis 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 submicrometer scale; the fluctuation increased with increasing the number of layers in Co/Pd multilayers. The distribution could be well analyzed by exp($\delta$m$\^$3/2/), where $\delta$m is the deviation of the activation magnetic moment from the mean value.

• Journal of Magnetics
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• 제6권2호
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• pp.70-72
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• 2001

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($\delta m^{3/2}$), where $\delta$m is the deviation of the activation magnetic moment from the mean value.

• 한국자기학회:학술대회 개요집
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• 한국자기학회 2001년도 춘계연구발표회 논문개요집
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• pp.112-113
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• 2001

• 한국자기학회지
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• 제14권6호
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• pp.228-231
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• 2004

Ta(5 nm)/NiFe(11 nm)/FeMn(16 nm) Ta(5 nm)과 Ta(5 nm)/NiFe(11 run)/FeMn(16 nm)/NiFe(7 nm)/Ta(5 nm) 다층박막구조를 제작하여, Laser의 power, 조사 위치, 자장의 방향에 따라 자기저항(MR)비와 교환결합력(H$_{ex}$) 자기적 특성을 연구하였다. Laser 열처리는 power를 440 mW까지 증가시키면서 FeMn/NiFe 이중박막을 15분 씩 Laser로 조사하여, 200 mW에서부터 음의 자기장에서 자기저항곡선의 피크가 발생하기 시작하였다. 400 mW까지 증가시키면서 양의 자기장에서 자기저항곡선의 MR비와 H$_{ex}$:가 각각 0.9%에서 0.1%로, 87 Oe에서 76 Oe로 감소하였고, 음의 자기장 피크에서는 MR비와 H$_{ex}$가 모두 증가하였다. 또한 NiFe/FeMn/NiFe삼중박막에서도 같은 결과를 얻었다. 300 mA로 laser열처리한 면적을 증가시켰을 때 자기저항곡선이 positive 피크는 감소하고 negative 피크는 증가하였다. 이러한 Laser 열처리효과에 의해 결과적으로 교환결합된 박막구조의 국소적 자화반전을 가능하게 하였다.

• Journal of Magnetics
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• 제7권3호
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• pp.72-79
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• 2002

Local M-H loops have been measured on the free layer of a tunneling magnetoresistance (TMR) junction using the magneto-optical Kerr effect (MOKE) system, with an optical beam size of about 2 $\mu$m diameter. Tunnel junctions were deposited using the DC magnetron sputtering method in a chamber with a base pressure of 3$\times$10$^{-9}$ Torr. The relatively irregular variations of coercive force H$_c$(∼17.5 Oe) and unidirectional anisotropy field H$_{ua}$(∼7.5 Oe) in the as-deposited sample are revealed. After $200{^{\circ}C}$ annealing, He decreases to 15 Oe but H$_{ua}$ increases to 20 Oe with smooth local variations. Two-dimensional plots of H$_c$ and H$_{ua}$ show the symmetric saddle shapes with their axes aligned with the pinned layer, irrespective of the annealing field angle. This is thought to be caused by geometric effects during deposition, together with a minor annealing effect. In addition, the variation of root mean square (RMS) surface roughness reveals it to be symmetric with respect to the center of the pinned-layer axis, with the roughness of 2.5 $\AA$ near the edge and 5.8 $\AA$ at the junction center. Comparison of surface roughness with the variation of H$_{ua}$ suggests that the H$_{ua}$ variation of the free layer is well described by dipole interactions related to surface roughness. As a whole, the reversal magnetization is not uniform over the entire junction area and the macroscopic properties are governed by the average sum of local distributions.