Macroscopic Quantum Phenomena in Magnetic Systems

本文将研究磁性系统中宏观自由度的量子力学隧穿，如铁磁粒子中的总矩和大小约为的反铁磁粒子中的尼尔向量。100埃。如果存在这种情况，它将大大扩大叠加原理的已知适用范围。为了指导和理解实验，我们将特别强调对环境自由度引起的耗散的研究。耗散介质的例子有核自旋、涡流和自旋波。本文还将研究自旋路径积分中拓扑相引起的某些量子现象。磁畴壁运动也将从隧道掘进的角度进行研究。路径积分和微扰方法的结合将涉及数值和分析工作。这个建议涉及应用于具有大量自由度的系统的量子力学的理论研究。问题是，如果系统的状态是由宏观上不同状态的叠加决定的(例如：磁矩（及其方向）的行为与那些自由度较小的磁矩的行为类似，即在经典禁止的障碍上表现出隧道效应或表现出共振。这些现象的最佳候选者是小磁性粒子，但是，迄今为止，旨在证明其存在的实验一直存在争议。这项工作旨在指导和批评实验，并研究那些可能掩盖效果的机制（能量损失）。这项工作具有价值，因为它可能与磁性记录介质相关，而这种影响是不可取的。

The quantum mechanical tunneling of macroscopic degrees of freedom will be studied in magnetic systems, such as the total moment in ferromagnetic and the Neel vector in antiferromagnetic particles of size approx. 100 Angstroms. If such exists, it would significantly enlarge the known range of applicability of the superposition principle. Special emphasis will be placed upon studying dissipation due to environmental degrees of freedom with a view to guiding and understanding experiments. Examples of dissipative agents are nuclear spins, eddy currents and spin waves. Certain quantum phenomena resulting from the topological phase in the spin path integral will also be studied. Magnetic domain wall motion will also be studied from a tunneling perspective. A combination of path integral and perturbative methods will be employed involving both numerical and analytical work. %%% This proposal involves the theoretical study of quantum mechanics applied to systems with a large number of degrees of freedom. The question is whether systems whose state is determined by a superposition of macroscopically distinct states (eg. a magnetic moment and its orientation) can behave in a similar way to those with few degrees of freedom - that is exhibit tunneling across classically forbidden barriers or exhibit resonance. Good candidates for these phenomena are small magnetic particles, but, to date, experiments aiming at proof of existence have been controversial. This work aims to guide and critique experiment and to study those mechanisms (energy loss) which could mask the effect. The work has value because of its possible relevance to magnetic recording media where such effects would be undesirable.