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Exchange Anisotropy of Polycrystalline Ferromagnetic / Antiferromagnetic Bilayers
Masakiyo Tsunoda,Migaku Takahashi 한국자기학회 2002 Journal of Magnetics Vol.7 No.3
The role of magnetic anisotropy of the antiferromagnetic layer on the magnetization process of exchange coupled polycrystalline ferromagnetic/antiferromagnetic bilayers is discussed. In order to elucidate the magnetic torque response of Ni-Fe/Mn-Ir bilayers, the single spin ensemble model is newly introduced, taking into account the two-dimensionally random distribution of the magnetic anisotropy axes of the antiferromagnetic grains. The mechanism of the reversible inducement of the exchange anisotropy along desirable directions by field cooling procedure is successfully explained with the new model. Unidirectional anisotropy constant, J_K, of polycrystalline Ni-Fe/Mn-Ir and Co-Fe/Mn-Ir bilayers is investigated as functions of the chemical composition of both the ferromagnetic layer and the antiferromagnetic layer. The effects of microstructure and surface modification of the antiferromagnetic layer on J_K are also discussed. As a notable result, an extra large value of J_K, which exceeds 0.5 erg/㎠, is obtained for Co_(70)Fe_(30)/Mn_(75)Ir_(25) bilayer with the ultra-thin (50 Å~100 Å) Mn-Ir layer. The exchange anisotropy of Co_(70)Fe_(30) 40Å/Mn_(75)Ir_(25) 100 Å bilayer is stable for thermal annealing up to 400℃, which is sufficiently high for the application of spin valve magnetoresistive devices.
마이크로파 여기 프라즈마법으로 제조한 강자성 터널링 접합의 국소전도특성
윤대식(Tae Sick Yoon),김철기(Cheol Gi Kim),김종오(Chong-Oh Kim),Masakiyo Tsunoda(Masakiyo Tsunoda),Migaku Takahashi(Migaku Takahashi),Ying Li(Ying Li) 한국자기학회 2003 韓國磁氣學會誌 Vol.13 No.2
Ferromagnetic tunnel junctions were fabricated by dc magnetron sputtering and plasma oxidation process. The local transport properties of the ferromagnetic tunnel junctions were studied using contact-mode Atomic Force Microscopy (AFM) and the local current-voltage analysis. Tunnel junctions with the structure of sub./Ta/Cu/Ta/NiFe/Cu/Mn_(75)Ir_(25)/Co_(70)Fe_(30)/Al-oxide were prepared on thermally oxidized Si wafers. Al-oxide layers were formed with microwave excited plasma using radial line slot antenna (RLSA) for 5 and 7 sec. Kr gas was used as the inert gas mixed with O₂ gas for the plasma oxidization. No correlation between topography and current image was observed while they were measured simultaneously. The local current distribution was well identified with the distribution of local barrier height. Assuming the gaussian distribution of the local barrier height, the ferromagnetic tunnel junction with longer oxidation time was well fitted with the experimental results. As contrast, in the case of the shorter time oxidation junction, the current mainly flow through the low barrier height area for its insufficient oxygen. Such leakage current might result in the decrease of tunnel magnetoresistance (TMR) ratio.
Local Variation of Magnetic Parameters of the Free Layer in TMR Junctions
CheolGi Kim,Toshihiro Shoyama,Masakiyo Tsunoda,Migaku Takahashi,Tae Hyo Lee,Chong-Oh Kim 한국자기학회 2002 Journal of Magnetics Vol.7 No.3
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 ㎛ diameter. Tunnel junctions were deposited using the DC magnetron sputtering method in a chamber with a base pressure of 3 × 10^(-9) Torr. The relatively irregular variations of coercive force Hc (~17.5 Oe) and unidirectional anisotropy field Hua (~7.5 Oe) in the as-deposited sample are revealed. After 200 ℃ annealing, Hc decreases to 15 Oe but Hua increases to 20 Oe with smooth local variations. Two-dimensional plots of Hc and Hua 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 Å near the edge and 5.8 Å at the junction center. Comparison of surface roughness with the variation of Hua suggests that the Hua 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.
Magnetic Tunnel Junctions with AlN and AlO Barriers
Tae Sick Yoon,Satoru Yoshimura,Masakiyo Tsunoda,Migaku Takahashi,Bum Chan Park,Young Woo Lee,Ying Li,Chong Oh Kim 한국자기학회 2004 Journal of Magnetics Vol.9 No.1
We studied the magnetotransport properties of tunnel junctions with AlO and AlN barriers fabricated using microwave-excited plasma. The plasma nitridation process provided wider controllability than the plasma oxidization for the formation of MTJs with ultra-thin insulating layer, because of the slow nitriding rate of metal Al layers, comparing with the oxidizing rate of them. High tunnel magnetoresistance (TMR) ratios of 49 and 44% with respective resistance-area product (R × A) of 3 × 10⁴ and 6 × 10³ Ωμ㎡ were obtained in the Co-Fe/Al- N/Co-Fe MTJs. We conclude that AlN is a hopeful barrier material to realize MTJs with high TMR ratio and low R × A for high performance MRAM cells. In addition, in order to clarify the annealing temperature dependence of TMR, the local transport properties were measured for Ta 50 Å/Cu 200 Å/Ta 50 Å/Ni_(76)Fe₂₄ 20 Å/Cu 50 Å/Mn_(75)Ir_(25) 100 Å/Co_(71)Fe_(29) 40 Å/Al-O junction with d_(Al) = 8 Å and P_(O2) × t_(OX) = 8.4 × 10⁴ L at various temperatures. The current histogram statistically calculated from the electrical current image was well in accord with the fitting result considering the Gaussian distribution and Fowler-Nordheim equation. After annealing at 340 ℃, where the TMR ratio of the corresponding MTJ had the maximum value of 44%, the average barrier height increased to 1.12 eV and its standard deviation decreased to 0.1 eV. The increase of TMR ratio after annealing could be well explained by the enhancement of the average barrier height and the reduction of its fluctuation.