Anomalous superconducting spin-valve effect in NbN/FeN/Cu/FeN/FeMn multilayers

황태종,김동호 한국초전도.저온공학회 2017 한국초전도저온공학회논문지 Vol.19 No.3

We have studied magnetic and transport properties of NbN/FeN/Cu/FeN/FeMn spin-valve structure. In-plane magnetic moment exhibited typical hysteresis loops of spin valves in the normal state of NbN film at 20 K. On the other hand, the magnetic hysteresis loop in the superconducting state exhibited more complex behavior in which exchange bias provided by antiferrmagnetic FeMn layer to adjacent FeN layer was disturbed by superconductivity. Because of this, the ideal superconducting spin-valve effect was not detected. Instead the stray field originated from unsaturated magnetic states dominated the transport properties of NbN/FeN/Cu/FeN/FeMn multilayer.

Nb/Permalloy 다층막 구조에서 비등방성 자기저항 특성

황태종,김동호 한국물리학회 2006 새물리 Vol.53 No.6

We study the magnetoresistance of Nb/Py bilayers and Py/Nb/Py trilayers to investigate the variations of the magnetoresistance. The directional magnetoresistance was measured by applying a magnetic field parallel to the layer plane. Above the superconducting transition temperature of Nb, the magnetoresistance of both bilayers and trilayers showed the standard behavior of an anisotropic magnetoresistance shaped with mirror symmetry. However, in both bilayers and trilayers, we found a strong increase in the MR ratio of more than an order of magnitude with different field-dependent behaviors in the middle of the superconducting transition from the normal state MR. We describe the possible mechanisms for this unusual resistance increase in the middle of the superconducting transition in terms of effects due to domain wall dynamics and a rotation of the magnetic moment of the domain. Nb와 Py로 구성된 다층막 구조에서 자기장 쓸기를 하면서 Nb 층의 상태에 따른 자기저항을 연구하였다. 자기저항은 박막 면에 평행하게 자기장을 가하면서 측정하였다. 초전도체/자성체 복합구조에서 자구벽의 생성과 전파에 의한 자기저항의 변화를 조사하기 위해 Nb/Py 이중막의 시료를 제작하였고, 자성층 간 자기모멘트의 방향에 따라 자기저항의 변화를 조사하기 위해 Py/Nb/Py 삼층막으로 제작하였다. Nb 층이 상전도 상태에서 측정한 이중막과 삼층막의 자기저항은 비등방성 자기저항의 특징을 보였으며, 거울대칭을 가지는 자기저항의 이력 형태를 보였다. 초전도 전이구간의 온도에서 자기저항의 저항변화율이 상전도 상태에서보다 수백 배 이상으로 증가함을 볼 수 있었으며 자기저항의 경향성도 바뀌어졌다. 초전도 전이구간에서 자기저항을 증가시키는 요인으로 자성층의 자구벽 운동과 자구의 자기모멘트 회전으로 설명하였다.

사파이어 기판을 사용한 병렬 검출코일 구조의 계단형 모서리 접합 SQUID 자력계

황태종,김인선,김동호,박용기 한국초전도학회 2004 Progress in superconductivity Vol.5 No.2

Step-edge Josephson junctions (SEJ) have been fabricated on sapphire substrates with in situ deposited films of CeO$_2$ buffer layer and YBa$_2$Cu$_3$O$_{7}$ films on the low angle steps. Direct coupled SQUID magnetometers with the SEJ were formed on 1 cm X 1 cm R-plane sapphire substrates. Typical 5-${\mu}{\textrm}{m}$-wide Josephson junctions have R$_{N}$ of 3 Ω and I$_{c}$ of 50 $mutextrm{A}$ at 77 K. The direct coupled SQUID magnetometers were designed to have pickup coils of 50-${\mu}{\textrm}{m}$-wide 16 parallel loops on the 1 cm X 1 cm substrates with outer dimension of 8.8 mm X 8.8 mm. The SEJ SQUID magnetometers exhibit relatively low 1/f noise even with dc bias control, and could be stably controlled by flux-locked loops in the magnetically disturbed environment. Field noise of the do SQUID was measured to be 200∼300 fT/Hz$^{1}$2/in the white noise region and about 2 pT/Hz$^{1}$2/ at 1 Hz when measured with dc bias method.hod.d.

Switching behavior in Peramlloy/Niobium/Permalloy trilayer

황태종,김동호 한국초전도.저온공학회 2014 한국초전도저온공학회논문지 Vol.16 No.4

We have investigated the effect of temperature and bias current on the stability of the inverse spin-switch effect inPermalloy(Py)/Nb/Permalloy pseudo spin-valves. The inverse spin-switch operates between two orientations of the ferromagneticmoments of Py layers; parallel (ON) and antiparallel-domain (OFF) state. Measuring time scans of the resistance changes betweenthe ON and OFF state, ΔRON-OFF, while alternating magnetic fields between the two states at various temperatures and bias currents,revealed that enhancement of ΔRON-OFF is a key factor to achieve successful operation of superconducting spin switch.

Influence of Stray Fields and the Proximity Effect in Ferromagnet/Superconductor/Ferromagnet Spin Valves

황태종,김동호 한국물리학회 2012 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.61 No.10

We have investigated two switching mechanisms, the proximity effect and the influence of stray fields, in Permalloy (Py)/Nb/Py spin valves. Samples were grown by dc magnetron sputtering and patterned to form a bar shape. The coexistence of proximity effect and stray-field effect in Py/Nb/Py has been found and could be explained by three resistance states; low-resistance antiparallel state, intermediate-resistance parallel state, and high-resistance domain state with multiple domains in both Py layers. In order to compare the stray-field effect with the proximity effect, we increased the bias current density to enhance the Lorentz force on flux lines in the Nb layer in which flux lines are induced by stray fields from the domain walls of the Py layers. Increase of the magnetoresistance due to stray-field-induced flux flow in the domain state with increasing bias current density has been observed as expected, meanwhile a resistance difference between the parallel and antiparallel states due to the proximity has also been observed. Between two switching mechanisms, we found that a switch operating on stray field effect under high bias current density on the order of 105 A/cm2 is more advantageous for the usage of a superconducting spin-valve as a resistance switch. Finally, we show that the stray-field-induced flux flow can be stabilized by orienting the magnetization easy axis perpendicular to the bar axis.

강자성체?초전도체 다층박막에서 강자성체의 자구상태에 따른 근접효과의 특성 변화

황태종,김동호,김혜영,윤원식,이형철 한국물리학회 2007 새물리 Vol.55 No.5

강자성체 (ferromagnet, F)/초전도체 (superconductor, S)/강자성체인 삼층막의 기본구조와 반강자성체 (antiferromanget, AF)층이 추가되어 있는 다층막 구조의 자기저항과 초전도 전이온도의 변화를 자성층의 상태 변화와 관련하여 연구하였다. 상전도 상태에서 F/S/F 삼층막의 자기저항의 특성은 두 자성층 중 한 층의 자화 방향이 바뀔 때 자기저항의 급격한 변화를 보이며 두 자성층이 반평행 상태에서 음의 자기저항을 보였다. AF/F/S/F다층막인 경우 두 자성층의 자화 변화에 따른 급격한 변화를 볼 수 없었고 두 자성층이 반평행 상태의 자기저항은 평행한 상태의 자기저항과 거의 동일하였다. 그리고 F/S/F 구조에서는 두 자성층이 반평행 상태에서 초전도 전이온도의 감소를 보이나, AF/F/S/F 구조에서는 전이온도의 증가를 보였다. 이 원인 규명을 위하여 단일 자성층의 박막에서 자벽의 생성과 소멸을 관측하면서 동시에 자기저항의 변화를 실시간으로 관측한 결과, 자벽의 생성과 소멸시에 급격한 자기저항의 변화를 관측할 수 있었다. 이 결과를 바탕으로 두 구조에서 나타나는 자기저항과 초전도 전이온도의 차이를 자성층에 발생하는 자벽의 영향으로 설명하였다. Suppression of the superconducting transition temperatures ($T_c$) by proximity effect in trilayer structure of ferromagnet/superconductor/ferromagnet has been investigated. We observed opposite trends of $T_c$ variation depending on whether the magnetic pinning layer was included or not; higher $T_c$ in multilayers with pinning layer when the magnetic moments of two ferromagnetic layers are antiparallel (AP state), but lower $T_c$ without pinning layer. It was found that stabilization of the magnetization state by pinning layer is a necessity to achieve higher $T_c$ in the AP state, otherwise without pinning layer, domain nucleation in the AP state disturbs superconductivity.

안정된 역스핀 스위치 효과를 위한 새김눈에 의한 자구벽 고정

황태종,김동호 한국물리학회 2011 새물리 Vol.61 No.1

Abstract We studied the domain-wall pinning effect for a stable inverse spin-switch behavior in Py/Nb/Py trilayers. For the domain wall pinning, local distortions produced by adding notches and T-shape arms to the bar-shape specimens were used as pinning centers. The pinning of the domain walls to the artificial structures was investigated by measuring the magnetoresistance. The pinning effect was first confirmed in single Py layers and later compared with the results for trilayer samples. Notch-type pinning sites were found to be more effective than the T-shape arms for pinning the domain walls not only in single Py layers but also in Py/Nb/Py trilayers. As a result, the inverse-spin switch effect was observed in a broader magnetic field range in the sample with the notch-type pinning centers. Py/Nb/Py 삼층막에서 나타나는 역스핀 스위치의 안정화를 위하여자구벽을 고정시키는 방법에 대하여 연구하였다. 자구벽을 고정시키는방법으로 미세 구조화된 시편에 새김눈 (notch) 형태와 T형태의 국소적변형을 주었고 이들 시편의 자기저항을 측정하여 고정점에 의한자구벽의 고정 효과를 관측하였다. 고정점의 역할은 단층인 퍼멀로이박막 시편에서 자기저항의 변화를 통하여 관측하였고 이를 삼층막의경우와 비교하였다. 단일막에서 새김눈 형태인 고정점은 T 형인고정점보다 넓은 자기장 영역에서 자구벽을 고정시켰으며,삼층막에서도 새김눈 형태의 고정점이 포함된 시료에서 역스핀 스위치현상을 보이는 자기장 영역의 범위가 더 크게 나타났다.

자화 용이축의 방향 변화에 따른 Py/Nb/Py 다층막의 자기저항과 초전도 전이온도의 변화

황태종,차유정,김동호 한국물리학회 2009 새물리 Vol.58 No.2

We studied the effect of magnetic state variations on superconductivity by fabricating Py/Nb/Py trilayer samples in which the orientation of the magnetization easy axis, of the Py layers with respect to the direction of the applied current was at various angles. We measured the magnetoresistance when the Nb layer was in the normal state and when it was in the superconducting transition state. We also measured the superconducting transition temperature as a function of the magnetic states of both Py layers. In the normal state, the magnetoresistance properties of the trilayer were governed mostly by the anisotropic magnetoresistnace of the Py layers and could be explained by using a single domain model with different uniaxial orientations. In the superconducting transition region, the magnetoresistance showed an upward resistance jump when the magnetic state changed from the antiparallel state to the parallel state, which was accompanied by an increase in the superconducting transition temperature of a few mK. These results could be explained by using the effect of magnetic fluctuations the domain stability. 자성층의 자화상태 변화와 초전도성 변화와의 관계를 조사하기 위하여 Py/Nb/Py 다층막의 자화 용이축이 전류방향에 대하여 다양한 각도가 되게 시편을 제작하였다. 시편의 길이 방향으로 자기장을 인가하면서 Nb 층의 특성이 상전도 상태일 때와 초전도 전이 상태의 온도에서 각각 자기저항을 측정하였다. 그리고 Py 층의 자화 상태의 변화에 따른 초전도 전이온도의 변화를 측정하였다. Nb 층이 상전도 상태일 때 다층막의 자기저항은 Py 층의 비등방성 자기저항 특성을 보였으며, 자화 용이축에 대한 전류방향의 변화로 나타나는 자기저항은 일축 이방성을 가진 단일자구 모델로 설명하였다. 초전도 전이구간의 자기저항은 두 자성층 중 한 층의 자화 방향이 바뀔 때 급격한 저항의 증가를 보였고, 수 mK 정도인 초전도 전이온도의 변화를 보였다. 초전도 전이구간에서 나타나는 자기저항과 전이온도의 변화를 자기요동에 의한 자구의 안정성으로 설명하였다.

YBCO SQUID gradiometers의 저주파 잡음 특성 연구

황태종,김인선,김동호,박용기 한국초전도학회 2002 Progress in superconductivity Vol.4 No.1

We have fabricated YBCO SQUID 1st order gradiometers on $30^{\circ}$STO bicrystal substrate. The pickup coil size was 3.8mm$\times$3.8mm and baseline was 5mm. Three types of SQUID gradiometer were designed and tested for unshielded operation; solid pickup coil, pickup coil consisting of 4 parallel $ 50\mu\textrm{m}$-wide loops, and solid pickup coil with flux dam. We have investigated external magnetic field dependence of the SQUID gradiometers on the magnetic field noiseproperties. Significant increase of low frequency noise with the application of static field has been observed in the case of parallel and flux dam type pickup coil above threshold field of $1.3 \mu$T. Magnetic field noise at 1 Hz measured in the magnetically shielding room was 30, 165, 480 fTcm/sup -1/Hz/sup -1/2/ for solid type and slot type and parallel loops type, respectively.

Magnetoresistance of Nb/Permalloy Bilayer Films

황태종,김동호 한국물리학회 2006 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.48 No.5I

The magnetoresistance of dc magnetron sputtered Nb/permalloy bilayer films has been studied. The bilayer films were patterned to measure the resistance by using conventional photolithography and Ar ion milling. The widths of the patterns were varied to study the effect of the magnetic domains in the permalloy layer on the superconducting properties of the Nb films. Above the superconducting transition temperature of Nb, the magnetoresistance for magnetic fields applied parallel to the plane showed a resistance dip accompanied by a subsequent resistance jump due to magnetic domain rearrangement in the permalloy film. However, in the middle of the transition where the Nb resistance shunts the permalloy resistance almost completely, a resistance peak was observed at slightly higher magnetic fields. We discuss the possible origins of this unusual resistance increase in terms of ferromagnet-superconductor interface scattering.