Fabrication of epitaxial, magnetic multiple-barrier-structures and development of spin-resonant-tunneling -device basic technology

钴基全Heusler合金薄膜最近引起了人们的极大兴趣，作为自旋电子器件非常理想的铁磁电极，因为理论上预测其中一些合金具有半金属铁磁性质，并且因为它们的居里温度远高于室温(RT)。本研究采用磁控溅射法在含镁缓冲层的MgO(001)衬底上外延生长了Co2Crlt；0.6&Gt；Fe0.4gt；Al(CCFA)和Co2MnGe(CMG)Co基全Heusler合金薄膜。薄膜是在RT条件下沉积的，然后在400-600℃的温度范围内进行原位热处理。退火后的CCFA和CMG薄膜具有足够平坦的表面形貌，表面粗糙度约为0.25 nm RMS。利用这些外延生长的薄膜，我们制备了全外延磁性隧道结(MTJ)，它由CCFA或CMG的钴基全Heusler薄膜作为下电极，MgO隧道势垒和CoFE(CoFe)上电极组成。微细外延CCFA/MgO/CoFe MTJ显示出较高的隧道磁阻(TMR)，在RT时为42%，在55K时为74%。这些结果证实了外延MTJ有望成为利用钴基全Heusler合金薄膜潜在的高自旋极化的关键器件结构。我们还制备了由CCFA/MgO/Co_&lt；50&gt；Fe_&lt；50&gt；/MgO/Co_&lt；50&gt；Fe_&lt；50&gt；.组成的外延双势垒结构制备的双势垒结构的TMR比单势垒结构高一倍，单势垒结构的TMR比单势垒结构高一倍，单势垒结构由CCFA/MgO/Co_&lt；50&gt；Fe_&lt；50&gt；和V_&lt；Half&gt；综上所述，我们开发了用于外延、磁性多势垒结构的基本制造技术。

Cobalt-based full-Heusler alloy thin films have recently attracted much interest as highly desirable ferromagnetic electrodes for spintronic devices because of the half-metallic ferromagnetic nature theoretically predicted for some of these alloys, and because of their high Curie temperatures, which are well above room temperature (RT). In this study, Co-based full-Heusler alloy thin films of Co_2Cr_<0.6>Fe_<0.4>Al (CCFA) and Co_2MnGe (CMG) were epitaxially grown on MgO-buffered MgO (001) substrates using magnetron sputtering. The films were deposited at RT and subsequently annealed in situ at temperatures ranging from 400 to 600℃. The annealed films of CCFA and CMG had sufficiently flat surface morphologies with roughness of about 0.25-nm rms. Using these epitaxially grown thin films, we fabricated fully epitaxial magnetic tunnel junctions (MTJs) consisting of a Co-based full-Heusler thin film of either CCFA or CMG as a lower electrode, a MgO tunnel barrier, and a Co_<50>Fe_<50> (CoFe) upper electrode. The microfabricated epitaxial CCFA/MgO/CoFe MTJs demonstrated relatively high tunnel magnetoresistance (TMR) ratios of 42% at RT and 74% at 55K. These results confirm the promise of epitaxial MTJs as a key device structure for utilizing the potentially high spin-polarization of Co-based full-Heusler alloy thin films. We also fabricated epitaxial double-barrier-structures consisting of CCFA/MgO/Co_<50>Fe_<50>/MgO/Co_<50>Fe_<50>. The fabricated double-barrier-structures showed TMR ratios comparable with that of single-barrier structures consisting of CCFA/MgO/Co_<50>Fe_<50> and V_<half> (the bias voltage at which the TMR ratio fell to half the zero-bias value) twice as high as that of the single-barrier structures. In summary, we have developed basic fabrication technologies for epitaxial, magnetic multiple-barrier-structures.