纳米结构物理学—6.pptVIP

* 磁性纳米结构及自旋电子学 High–density hard drive: at 100 Gb/in2 or 20 Gb/cm2, area of a bit: ~ 25 nm ? 160 nm. media stability? How to read and write? Information storage and processing using spin? How far can this go? Longitudinal magnetic recording Magnetism of Materials Magnetic moment of electron: Bohr magnetron ?B = e?/(2me) = 9.3 × 10-24 J/T, g ? 2, spin vector, |s| = ? ? Magnetic moments of atom: L, S, J: quantum numbers of orbital, spin, total electronic angular momenta; g(L,S,J) = Lande g factor Magnetization M = total magnetic moment per unit volume When M = 0 ? atomic moment = 0, or no order Spontaneous magnetic order exists in some materials Magnetic order of materials is determined by various interactions Quantum exchange interaction: Jij: exchange constant between electrons at atoms i and j Jij 0: ferromagnetic magnetic moments all aligned, Fe, Co, Ni Jij 0: anti-ferromagnetic neighboring moments anti-parallel Ferromagnetic order is destroyed above Curie temperature Anti-ferromagnetic order is destroyed above Néel temperature But bulk ferromagnetic material often shows no net magnetization It consists of many “random” magnetic domains Magnetostatic energy: (2?0)-1?B2dV Minimum domain size ~ 0.1 ?m Domain walls: gradual orientation transition between two domains Domain wall width: 100 -102 nm Minimize total energy (exchange + magnetostatic +…) Magnetic “charges” or poles on surface: n ? M Magnetostatic energy is reduced in multi-domain configuration Domain walls cost energy, and cannot be abrupt Magnetization in external field H: domains aligned with H grow at expense of others Domain walls move coercivity remanence Magnetization induction: B = ?0(H + M) Small ferromagnetic particle spontaneously magnetize into a single-domain magnet Magnetostatic energy ? L3 if single domain Domain wall energy ? L2 if multi-domain Critical size ~100-300 nm Shape anisotropy: ground-state magnetization direction determined by m