Entropy in Spin Relaxation, Spintronics, and Magnetic Resonance

• Published in: physica status solidi (b) 2020-12, Vol.257 (12), p.n/a
• Main Authors: Csősz, Gábor, Dóra, Balázs, Simon, Ferenc
• Format: Article
• Language: English
• Subjects: dephasing; entropy; magnetic resonance; reversible effects; spin relaxation
• ISSN: 0370-1972; 1521-3951
• DOI: 10.1002/pssb.202000301

The entropy change during spin relaxation for a realistic model system is studied, whose spin dynamics can be handled with the Boltzmann equation. The time evolution of the von Neumann entropy is monitored during the process and is compared with the recently introduced concept of the Loschmidt echo envelope. The time evolution of the two quantities is remarkably similar which helps to distinguish reversible and irreversible changes to the ensemble spin state. The method is also demonstrated for a toy model of nuclear magnetic resonance, where the usual π spin echo is performed numerically, and the echo envelope also follows the time evolution of the von Neumann entropy. The numerical approach highlights the utility of the entropy concept in analyzing various processes which occur during spin relaxation. The concept of “spin entropy” is introduced for the problem of spin relaxation in solids. The approach treats the spins quasiclassically, i.e., the momentum is considered as a classical variable, whereas the spin is treated quantum mechanically. The approach allows to distinguish reversible and irreversible spin relaxation processes. Its utility is also demonstrated for a conventional nuclear magnetic resonance spin echo experiment.