Institute of Experimental and Applied Physics, University of Regensburg, D-93053 Regensburg, Germany.
Phys Rev Lett. 2013 Jun 28;110(26):266101. doi: 10.1103/PhysRevLett.110.266101. Epub 2013 Jun 24.
The spin order of the nickel oxide (001) surface is resolved, employing noncontact atomic force microscopy at 4.4 K using bulk Fe and SmCo tips mounted on a qPlus sensor that oscillates at sub-50 pm amplitudes. The spin-dependent signal is hardly detectable with Fe tips. In contrast, SmCo tips yield a height contrast of 1.35 pm for Ni ions with opposite spins. SmCo tips even show a small height contrast on the O atoms of 0.5 pm within the 2×1 spin unit cell, pointing to the observation of superexchange. We attribute the increased signal-to-noise ratio to the increased magnetocrystalline anisotropy energy of SmCo, which stabilizes the magnetic moment at the apex. Atomic force spectroscopy on the Ni↑, Ni↓, and O lattice site reveals a magnitude of the exchange energy of merely 1 meV at the closest accessible distance with an exponential decay length of λexc=18 pm.
采用非接触原子力显微镜，在 4.4 K 下利用安装在 qPlus 传感器上的块状 Fe 和 SmCo 探针来解析氧化镍 (001) 表面的自旋排列，该传感器以低于 50 pm 的幅度振荡。Fe 探针几乎无法检测到与自旋相关的信号。相比之下，SmCo 探针在相反自旋的 Ni 离子上产生 1.35 pm 的高度对比度。SmCo 探针甚至在 2×1 自旋单元晶格中的 O 原子上显示出 0.5 pm 的小高度对比度，表明观察到超交换。我们将信号噪声比的增加归因于 SmCo 增加的磁晶各向异性能量，这稳定了尖端的磁矩。在 Ni↑、Ni↓和 O 晶格位置上进行的原子力光谱分析表明，在最接近的可及距离处，交换能量的大小仅为 1 meV，其指数衰减长度为 λexc=18 pm。
