We describe a systematic study of the synthesis, microstructure and magnetization of hybrid ferromagnet-semiconductor nanomaterials comprised of MnAs nanoclusters embedded in a p-doped GaAs matrix. These samples are created during the in situ annealing of Be-doped (Ga,Mn)As heterostructures grown by molecular beam epitaxy. Transmission electron microscopy and magnetometry studies reveal two distinct classes of nanoclustered samples whose structural and magnetic properties depend on the Mn content of the initial (Ga,Mn)As layer. For Mn content in the range 5% - 7.5%, annealing creates a superparamagnetic material with a uniform distribution of small clusters (diameter around 6 nm) and with a low blocking temperature (T_B approximately 10 K). While transmission electron microscopy cannot definitively identify the composition and crystalline phase of these small clusters, our experimental data suggest that they may be comprised of either zinc-blende MnAs or Mn-rich regions of (Ga,Mn)As. At higher Mn content (> 8 %), we find that annealing results in an inhomogeneous distribution of both small clusters as well as much larger NiAs-phase MnAs clusters (diameter around 25 nm). These samples also exhibit supermagnetism, albeit with substantially larger magnetic moments and coercive fields, and blocking temperatures well above room temperature.
Low Temperature Magnetic Domain Patterns in MnAs Films Grown on GaAs(001)
用超导干涉仪（5 K ~ 350 K）和原子力显微镜/磁力显微镜（20 K ~ 360 K）研究了MnAs的磁性能随温度的变化。MnAs的结构和磁性能取决于膜厚度接近和远低于居里温度。在铁磁性α-MnAs和顺磁性β-MnAs共存的样品中，畴结构受两相分布的影响。低于该混合相的温度范围的磁畴结构类似于沿易磁化轴具有均匀磁化的单畴结构，除了存在沿易磁化轴伸长的区域，该区域嵌入磁化沿第二易磁化轴的位置，即，正常的电影。这些区域的形状和它们的温度依赖性也与MnAs层厚度有关。
Ab-initio transport theory for digital ferromagnetic heterostructures
由$δ$掺杂GaAs和Mn制成的MnAs/GaAs超晶格被称为数字铁磁异质结构。在这里，我们提出了一个理论密度泛函研究的电子，磁性和输运性质的异质结构。在没有本征施主的情况下，这些系统显示出半金属态密度，交换相互作用比具有相同Mn浓度的随机合金强得多。{ \it Ab initio}弹道输运计算表明，能量接近费米能级的载流子被强烈地限制在MnAs平面周围的几个单层内。这种强约束是大交换耦合的原因。因此，该系统可以被描述为一个二维半金属与大电导的MnAs平面和小电导的垂直方向。
Independent Electronic and Magnetic Doping in (Ga,Mn)As Based Digital Ferromagnetic Heterostructures
铁磁半导体有望将基于金属的自旋电子学扩展到一种结合了广泛可调的电子、光学和磁性的材料系统。在这里，我们采取措施实现这一承诺，通过实现独立控制的铁磁半导体（Ga，Mn）As中的电子掺杂。样品由0.5单层（ML）MnAs与20 ML GaAs交替的超晶格组成，并通过低温（230 C）原子层外延（ALE）生长。这允许减少过量的As掺入，从而减少电荷补偿As相关缺陷的数量。我们生长一系列的样品中的GaAs间隔物（p型和n型掺杂剂，分别）与Be或Si掺杂，并验证其结构质量通过原位反射高能电子衍射（RHEED）和非原位X射线衍射。磁化强度测量揭示了在整个掺杂范围内的铁磁行为，并没有表现出任何迹象的MnAs沉淀。最后，磁输运显示了巨大的平面霍尔效应和强（20%）的电阻波动，可能与畴壁运动。
