Spin-dependent tunneling in epitaxial Fe/Cr/MgO/Fe magnetic tunnel junctions with an ultrathin Cr(001) spacer layer
Spin-dependent tunneling in epitaxial Fe/Cr/MgO/Fe magnetic tunnel junctions with an ultrathin Cr(001) spacer layer
复制标题
DOI：
10.1103/physrevb.79.174436
复制
发表时间：
2009-05
影响因子：
3.7
通讯作者：
R. Matsumoto;A. Fukushima;K. Yakushiji;S. Nishioka;T. Nagahama;T. Katayama;Yoshishige Suzuki;K. Ando;S. Yuasa
中科院分区：
文献类型：
--
作者：
R. Matsumoto;A. Fukushima;K. Yakushiji;S. Nishioka;T. Nagahama;T. Katayama;Yoshishige Suzuki;K. Ando;S. Yuasa
我们制造了完全外延的 Fe/Cr/MgO/Fe(001) 磁隧道结 (MTJ)，其原子级平坦的超薄 Cr(001) 层生长在 MgO 势垒层下方，并研究了自旋相关输运以阐明隧道电子的散射过程。由于 Cr 在费米能量 $({E}_{F})$ 处不具有具有 ${\ensuremath{\Delta}}_{1}$ 对称性的布洛赫态，因此 MgO 中主要介导隧道电流的 ${\ensuremath{\Delta}}_{1}$ 倏逝态无法在没有散射过程的情况下与 Cr 布洛赫态耦合。因此，Fe/Cr/MgO/Fe(001) MTJ 是研究非镜面散射过程的模型系统，其中隧道态的轨道对称性不守恒。研究发现，MTJ 的电阻面积 (RA) 乘积不会随 Cr 厚度 $({t}_{\text{Cr}})$ 呈指数增加，这表明尽管 ${E}_{F}$ 处不存在 ${\ensuremath{\Delta}}_{1}$ 状态，但 Cr 层并不能充当完美的隧道势垒。此外，MTJ 的磁阻比随着 ${t}_{\text{Cr}}$ 的函数而振荡，周期为 2 个单层，反映了 Cr(001) 的层状反铁磁结构。令人惊讶的是，MR比在奇数个Cr单层处显示局部最大值，在偶数个Cr单层处显示局部最小值，这表明隧道电流相对于界面磁化强度是相反的自旋极化。这些结果表明，非镜面散射介导了 MgO 中的倏逝态与 Cr 中具有负自旋极化的某些非 ${\ensuremath{\Delta}}_{1}$ Bloch 态之间的耦合，从而即使在低温和小偏置电压下也会产生非镜面隧道电流。作为参考样品，我们还通过在 MgO 阻挡层上生长 Cr(001) 层来研究具有氧化较少的 Cr/MgO 界面的 Fe/MgO/Cr/Fe MTJ，并发现随着 ${t}_{\text{Cr}}$ 的增加，它们的 RA 乘积增加得更快。这表明 Fe/Cr/MgO/Fe MTJ 中界面 Cr 原子的部分氧化是非镜面散射的主要来源之一。人们发现温度的升高和偏置电压的施加都大大增强了电子散射，从而产生了隧道电导。虽然磁振子散射引起的反平行电导的温度依赖性遵循布洛赫 ${T}^{3/2}$ 定律，但非镜面散射引起的电导却偏离布洛赫 ${T}^{3/2}$ 定律。目前的实验结果为我们更清楚地了解 MgO 基 MTJ 的隧道过程提供了重要线索。
We fabricated fully epitaxial Fe/Cr/MgO/Fe(001) magnetic tunnel junctions (MTJs) with an atomically flat ultrathin Cr(001) layer grown below the MgO barrier layer and studied the spin-dependent transport to clarify scattering process of tunneling electrons. Because Cr does not have Bloch states with ${\ensuremath{\Delta}}_{1}$ symmetry at the Fermi energy $({E}_{F})$, ${\ensuremath{\Delta}}_{1}$ evanescent states in MgO, which dominantly mediate the tunneling current, cannot couple with Cr Bloch states without a scattering process. The Fe/Cr/MgO/Fe(001) MTJs are therefore a model system for studying nonspecular scattering processes where the orbital symmetry of tunneling states is not conserved. The resistance-area (RA) product of the MTJs was found to not increase exponentially as a function of the Cr thickness $({t}_{\text{Cr}})$, indicating that the Cr layer does not act as a perfect tunnel barrier despite of the absence of ${\ensuremath{\Delta}}_{1}$ states at ${E}_{F}$. Moreover, the magnetoresistance ratio of the MTJs was seen to oscillate as a function of ${t}_{\text{Cr}}$ with a period of 2 monolayers, reflecting the layered antiferromagnetic structure of Cr(001). Surprisingly, the MR ratio showed local maxima at the odd numbers of Cr monolayers and local minima at the even numbers of Cr monolayers, indicating that the tunneling current is oppositely spin polarized with respect to the interface magnetization. These results suggest that nonspecular scatterings mediate the coupling between evanescent states in MgO and certain non-${\ensuremath{\Delta}}_{1}$ Bloch states in Cr that have negative spin polarization, thereby inducing nonspecular tunneling current even at a low temperature and a small bias voltage. We also investigated, as a reference sample, Fe/MgO/Cr/Fe MTJs with a less-oxidized Cr/MgO interface by growing the Cr(001) layer on the MgO barrier layer and found that their RA product increased much more rapidly with increasing ${t}_{\text{Cr}}$. This indicates that partial oxidation of interface Cr atoms in the Fe/Cr/MgO/Fe MTJs is one of the major origins of nonspecular scatterings. Both an increase in temperature and the application of bias voltage were found to greatly enhance the electron scattering that gives rise to tunneling conductance. While the temperature dependence of the antiparallel conductance due to magnon scatterings follows the Bloch ${T}^{3/2}$ law, the conductance due to nonspecular scatterings deviates from the Bloch ${T}^{3/2}$ law. The present experimental results give us important clues to a clearer understanding of the tunneling process in MgO-based MTJs.