Lorentz Microscopy of nano-scale magnetic particles embedded in metal matrices

采用明场透射电镜、高分辨透射电镜、洛伦兹显微镜、VSM和SQUID测量等方法研究了纳米磁性颗粒在铜基体中的畴结构。BFTEM表明共格的钴颗粒转变为尺寸为50 nm的非共格沉淀。八面体析出相在失去共格性后，进入快速粗化阶段。本洛伦兹显微镜检测由于钴粒子的磁化的入射电子束的偏移。偏离角的大小估计为从简单的洛伦兹力模型导出的值的两倍大。Lorentz EM还表明非相干沉淀物由四个磁畴组成，每个磁畴的磁化方向都是[100]。SQUID测量表明样品的磁性遵循超顺磁性M-T曲线。但颗粒的性质逐渐转变为铁磁性。在短暂的退火后，磁化曲线一度下降。然后M曲线随着退火而增大。在两个相干粒子耦合时，矫顽力达到最大值。

Domain structure of nano-scale magnetic particles embedded in copper matrices were studied by bright-field TEM, HRTEM and Lorentz microscopy, together with VSM and SQUID measurements. BFTEM showed that the coherent cobalt particles were transformed into incoherent precipitates at 50nm in size. After loosing the coherency, the octahedral precipitates entered into rapid coarsening. The present Lorentz microscopy detected the deviation of the incident electron beam due to the magnetization of cobalt particles. The magnitude of the deviating angles was estimated as twice as large as the value derived from a simple Lorentz force model. Lorentz EM also shows that incoherent precipitates were composed of four magnetic domains, each of which has a magnetization oriented to [100].SQUID measurements show that the magnetic properties of the specimens followed in a superparamagnetic M-T curve. But the property of the particles gradually changed into ferromagnetic. The magnetization curve once decreased just after a short annealing. Then the M curve increased with the annealing. The highest coersivity attained at the period that two coherent particles were coupled.