Capping층 재료에 따른 CoFeB/MgO/CoFeB 자기터널접합의 미세구조와 자기저항 특성

정하창,이성래,Chung, Ha-Chang,Lee, Seong-Rae 한국자기학회 2007 韓國磁氣學會誌 Vol.17 No.4

본 연구에서는 CoFeB/MgO/CoFeB 구조를 가지는 자기터널접합에서 capping층 재료의 종류와 열처리 시간에 따른 비정질 top CoFeB 자성층의 결정화 상태 및 자기터널접합의 자기적 특성 변화에 대한 연구결과를 비교 분석 하였다. Hcp(Hexagonal close-packed)의 결정구조를 가지는 Ru(002)를 capping층 재료로 사용한 자기터널접합 박막의 경우에는 열처리 이후 Ru과 인접한 부분의 top CoFeB이 bcc-CoFe(110)로 성장하는 반면, TiAl과 ZrAl을 capping층 재료로 사용한 자기터널접합의 경우는 열처리 이후 top CoFeB이 MgO와 epitaxial하게 bcc-CoFe(002)로 결정성장 하였다. 이로 인해 Ru을 사용한 자기터널접합의 터널자기 저항비(46.7%)보다 약 1.5배 높은 터널자기저항비(TiAl: 71.8%, ZrAl: 72.7%)를 나타내었다. We investigated the effects of the capping layer materials on the crystallization of the amorphous top-CoFeB (t-CoFeB) electrode and the magnetoresistance properties of the magnetic tunnel junctions (MTJs). When the hcp(002)-textured Ru capping layer was used, the amorphous t-CoFeB was crystallized to bcc-CoFe(110). The CoFe(110)/Ru(002) texture relation can be minimized the lattice mismatch down to 5.6%. However, when the fine polycrystalline but almost amorphous TiAl or amorphous ZrAl were used, the amorphous t-CoFeB was crystallized to bcc-CoFe(002). When the amorphous capping materials were used, the evolution of the t-CoFeB texture was affected mainly by the MgO(001) texture. Consequently, the M ratios of the annealed MTJ capped with the ZrAl and TiAl (72.7 and 71.8%) are relatively higher than that of the MTJ with Ru capping layer (46.7%). In conclusions, the texture evolution of the amorphous t-CoFeB during the post deposition annealing could be controlled by the crystallinity of the adjacent capping layer and in turn, it affects the TMR ratio of MTJs.

CoFe/NiFeSiB/CoFe 자유층을 갖는 이중장벽 자기터널접합의 바이어스전압 의존특성

이선영,이장로,Lee, S.Y.,Rhee, J.R. 한국자기학회 2007 韓國磁氣學會誌 Vol.17 No.3

이 연구에서는 Ta 45/Ru 9.5/IrMn 10/CoFe $3/AlO_x$/자유층/$AlO_x$/CoFe 7/IrMn 10/Ru 60(nm) 구조를 갖는 이중장벽 자기터널접합(double-barrier magnetic tunnel junction: DMTJ)를 다루었다. 자유층은 $Ni_{16}Fe_{62}Si_8B_{14}\;7nm$, $Co_{90}Fe_{10}(fcc)$ 7 nm 및 $CoFet_1$/NiFeSiB $t_2$/CoFe $t_1$으로 구성하였으며 두께 $t_1,\;t_2$는 변화시켰다. 즉 TMR비와 RA를 개선하기 위하여 부분적으로 CoFe층을 대체할 수 있는 비정질 NiFeSiB층이 혼합된 자유층 CoFe/NiFeSiB/CoFe을 갖는 DMTJ를 연구하였다. NiFeSiB($t_1=0,\;t_2=7$)만의 자유층을 갖는 DMTJ는 터널자기저항(TMR)비 28%, 면적-저항곱(RA) $86k{\Omega}{\mu}m^2$, 보자력($H_c$) 11 Oe 및 층간 결합장($H_i$) 20 Oe를 나타내었다. $t_1=1.5,\;t_2=4$인 경우의 하이브리드 DMTJ는 TMR비 30%, RA $68k{\Omega}{\mu}m^2$ 및 $H_c\;11\;Oe$를 가졌으나 $H_i$는 37 Oe로 증가하였다. 원자현미경(AFM)과 투과전자현미경(TEM)측정을 통하여 NiFeSiB층 두께가 감소하면 $H_i$가 증가하는 것을 확인하였다. 비정질 NiFeSiB층이 두꺼워지면 보통 계면의 기복을 유도하는 원주형성장(columnar growth)를 지연시키는데 유효하였다. 그러나 NiFeSiB층이 얇으면 표면거칠기는 증가하고 전자기적 Neel 결합 때문에 Hi는 커졌다. The typical double-barrier magnetic tunnel junction (DMTJ) structure examined in this paper consists of a Ta 45/Ru 9.5/IrMn 10/CoFe7/$AlO_x$/free layer/AlO/CoFe 7/IrMn 10/Ru 60 (nm). The free layer consists of an $Ni_{16}Fe_{62}Si_8B_{14}$ 7 nm, $Co_{90}Fe_{10}$ (fcc) 7 nm, or CoFe $t_1$/NiFeSiB $t_2$/CoFe $t_1$ layer in which the thicknesses $t_1$ and $t_2$ are varied. The DMTJ with an NiFeSiB-free layer had a tunneling magnetoresistance (TMR) of 28%, an area-resistance product (RA) of $86\;k{\Omega}{\mu}m^2$, a coercivity ($H_c$) of 11 Oe, and an interlayer coupling field ($H_i$) of 20 Oe. To improve the TMR ratio and RA, a DMTJ comprising an amorphous NiFeSiB layer that could partially substitute for the CoFe free layer was investigated. This hybrid DMTJ had a TMR of 30%, an RA of $68\;k{\Omega}{\mu}m^2$, and a of 11 Oe, but an increased of 37 Oe. We confirmed by atomic force microscopy and transmission electron microscopy that increased as the thickness of NiFeSiB decreased. When the amorphous NiFeSiB layer was thick, it was effective in retarding the columnar growth which usually induces a wavy interface. However, if the NiFeSiB layer was thin, the roughness was increased and became large because of the magnetostatic $N{\acute{e}}el$ coupling.

강자성 비정질 NiFeSiB 자유층을 갖는 자기터널접합의 스위칭 특성

황재연(J. Y. Hwang),이장로(J. R. Rhee) 한국자기학회 2006 韓國磁氣學會誌 Vol.16 No.6

Magnetic tunnel junctions (MTJs), which consisted of amorphous ferromagnetic NiFeSiB free layers, were investigated. The NiFeSiB layers were used to substitute for the traditionally used CoFe and/or NiFe layers with the emphasis being given to obtaining an understanding of the effect of the amorphous free layer on the switching characteristics of the MTJs. Ni₁?Fe?₂Si?B₁₄ has a lower saturation magnetization (M<SUB>s</SUB>: 800 emu/㎤) than Co??Fe₁? and a higher anisotropy constant (K<SUB>u</SUB>: 2,700 erg/㎤) than Ni??Fe₂?. The Si/SiO₂/Ta 45/Ru 9.5/IrMn 10/CoFe 7/AlO<SUB>x</SUB>/NiFeSiB t/Ru 60 (in nanometers) structure was found to be beneficial for the switching characteristics of the MTJ, leading to a reduction in the coercivity (H<SUB>c</SUB>) and an increase in the sensitivity resulted from its lower saturation magnetization and higher uniaxial anisotropy. Furthermore, by inserting a very thin CoFe layer at the tunnel barrier/NiFeSiB interface, the TMR ratio and switching squareness were improved more with the increase of NiFeSiB layer thickness up to 11㎚.

비정질 CoFeSiB 자유층을 갖는 자기터널접합의 스위칭 특성

황재연(J. Y. Hwang),이장로(J. R. Rhee) 한국자기학회 2006 韓國磁氣學會誌 Vol.16 No.6

The switching characteristics of magnetic tunnel junctions (MTJs) comprising amorphous ferromagnetic CoFeSiB free layer have been investigated. CoFeSiB was used for the free layer to enhance the switching characteristics. The typical junction structure was Si/ SiO₂/Ta 45/Ru 9.5/IrMn 10/CoFe 7/AlO<SUB>x</SUB>/CoFeSiB (t)/Ru 60 (in ㎚). CoFeSiB has low saturation magnetization (M<SUB>s</SUB>) of 560 emu/㎤ and high anisotropy constant (K<SUB>u</SUB>) of 2,800 erg/㎤. These properties caused low coercivity (H<SUB>c</SUB>) and high sensitivity in MTJs, and it also confirmed in submicrometer-sized elements by micromagnetic simulation based on the Landau-Lisfschitz-Gilbert equation. By increasing CoFeSiB free layer thickness, the switching characteristics became worse due to increase of the demagnetization field.

Co/Pd 다층막구조가 수직자기터널접합의 자기저항에 미치는 영향

김성동(Sungdong Kim),임동원(Dongwon Lim),이성래(Seong-Rae Lee) 한국자기학회 2006 韓國磁氣學會誌 Vol.16 No.6

We investigated the magnetoresistance of perpendicularly magnetized magnetic tunnel junction composed of Co/Pd multilayers. The magnetoresistance was maximized with Co electrodes of about 5 ㎚ thickness, which evidenced the important role of the interface in tunneling process. Both the change in perpendicular magnetic anisotropy and improvement of junction resistance were observed with changing Co sublayers, while the spin scattering became dominant with increasing Pd sublayers.

자기터널접합을 활용한 고집적 MRAM 소자 기술

전병선(Byong Sun Chun),김영근(Young Keun Kim) 한국자기학회 2006 韓國磁氣學會誌 Vol.16 No.3

Ferromagnetic amorphous Ni₁?Fe?₂Si?B₁₄ and Co<SUB>70.5</SUB>Fe<SUB>4.5</SUB>Si₁?B₁? layers have been devised and incorporated as free layers of magnetic tunnel junctions (MTJs) to improve MRAM reading and writing performance. The NiFeSiB and CoFeSiB single-layer film exhibited a lower saturation magnetization (M<SUB>s</SUB> = 800 emu/㎤, and 560 emu/㎤, respectively) compared to that of a Co??Fe₁?0 (M<SUB>s</SUB> = 1400 emu/㎤). Because amorphous ferromagnetic materials have lower M<SUB>s</SUB> than crystalline ones, the MTJs incorporating amorphous ferromagnetic materials offer lower switching field (H<SUB>sw</SUB>) values than that of the traditional CoFe-based MTJ. The double-barrier MTJ with an amorphous NiFeSiB free layer offered smooth surface resulting in low bias voltage dependence, and high V<SUB>h</SUB> and V<SUB>bd</SUB> compared with the values of the traditional CoFe-based MTJ.

나노 산화층을 사용한 자기터널접합의 특성

추인창(In Chang Chu),전병선(Byong Sun Chun),송민성(Min Sung Song),이성래(Seong Rae Lee),김영근(Young Keun Kim) 한국자기학회 2006 韓國磁氣學會誌 Vol.16 No.2

The tunneling magnetoresistance (TMR) ratios of magnetic tunnel junctions (MTJs), in general, decrease abruptly above 250℃ due to Mn interdiffusion from an antiferromagnet IrMn layer to a ferromagnetic CoFe and/or a tunnel barrier. To improve thermal stability, we prepared MTJs with nano-oxide layers. Using a MTJ structure consisting of underlayer CoNbZr 4/bufferlayer CoFe 10/ antiferromaget IrMn 7.5/pinned layer CoFe 3/tunnel barrier AlO/freelayer CoFe 3/capping CoNbZr 2 (㎚), we placed a nano-oxide layer (NOL) into the underlayer or bufferlayer. Then, the thermal, structural and magneto-electric properties were measured. The TMR ratio, surface flatness, and thermal stability of the MTJs with NOLs were promoted.

차세대 자기저항메모리 MRAM 기술의 특허동향 분석

노수정(S. J. Noh),이지성(J. S. Lee),조지웅(J. U. Cho),김도균(D. K. Kim),김영근(Y. K. Kim),유양미(Y. M. Yoo),하미영(M. Y. Ha),서주원(J. W. Seo) 한국자기학회 2009 韓國磁氣學會誌 Vol.19 No.1

Among the next generation memory, MRAM (Magnetic Random Access Memory) is worthy of notice for substituting the preexisting memory thanks to its non-volatile property and other advantages. Recently perpendicular MRAM and spin transfer torque MRAM techniques are under active investigation to realize a high density and low power consumption. As a result, there are increasing of patents applications for high density, low current density for magnetization switching and high thermal stability. In this paper, we analyze the trend of patent applications and registrations about MRAM and propose a direction of future investigation.

스핀전달토크형 자기저항메모리(STT-MRAM) 기술개발 동향

김도균(D. K. Kim),조지웅(J. U. Cho),노수정(S. J. Noh),김영근(Y. K. Kim) 한국자기학회 2009 韓國磁氣學會誌 Vol.19 No.1

Reduction of the critical current density (Jc) for STT magnetization switching is most important issue of magnetic tunnel junctions (MTJs) based MRAM. This report describes how to decrease the Jc and will introduce the recent research progresses of STT-MRAM devices with material engineering and structural improvement, respectively.

비대칭 자기터널접합에서의 수직 스핀 전달 토크 : 물질 변수에 대한 의존성

한재호(Jae-Ho Han),이현우(Hyun-Woo Lee) 한국자기학회 2011 韓國磁氣學會誌 Vol.21 No.2

Spin-transfer torque is a useful tool to control the magnetic state in nanostructures. In magnetic tunnel junctions, the spin-transfer torque has two components, the in-plane spin torque and the perpendicular spin torque. While properties of the in-plane spin-transfer torque are relatively well understood, properties of the perpendicular spin-transfer torque still remain controversial. A recent experiment demonstrated that in asymmetric magnetic tunnel junctions, the bias voltage dependence of the perpendicular spin-transfer torque contains both linear and quadratic terms in the bias. However it still remains unexplored how the bias voltage dependence changes as a function of material parameters. In this paper, we systematically investigate the perpendicular spin-transfer torque in asymmetric magnetic tunnel junction by varying spin splitting energy, work function difference, and Fermi energy of the ferromagnetic metal leads.