Berlin 2012 – scientific programme

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MA: Fachverband Magnetismus

MA 52: Poster II - Magnetic Heusler compounds, Magnetic shape memory alloys, Thin Films, Micro-/Nano-structured magnetic materials, Graphene, Spins in organics, Magnetic imaging, Surface Magnetism, Spin excitations/Torque, Spincaloric transport

MA 52.49: Poster

Friday, March 30, 2012, 11:00–14:00, Poster A

Quantum-Mechanical Model of Spin Polarized STM — •Kolja Them, Thim Stapelfeldt, Elena Y. Vedmedenko, and Roland Wiesendanger — Institute for Applied Physics University of Hamburg Jungiusstr. 11 20355 Hamburg

Spin sensitive studies of individual magnetic ad-atoms and atomic ensembles on surfaces with spin-polarized scanning tunneling microscopy (SP-STM) have raised the necessity of a quantum-mechanical description of spin dynamics during SP-STM experiments. The quantum-mechanical treatments of STM experiments typically deal with the expectation values of observables using Gibbs ensemble averages. An SP-STM measurement, however, is a time-average of the expectation values. The lack of the time-averaging might be a reason for the fact that the theoretically predicted relaxation time on the nanosecond scale for a single spin of a magnetic adatom is at odd with the femtosecond rates measured for Fe adatoms on semiconductor surfaces. Another still unexplained finding is the extremely high switching frequency of Co atoms on Pt(111) at zero magnetic field. In the present paper we use the algebraic formulation of quantum statistical mechanics to clearly separate the thermal equilibrium Gibbs states and the time evolution of the system during SP-STM experiments. Using elaborated techniques we calculate dynamics of single quantum spins as well as magnetic ensembles at finite temperatures. We demonstrate that the relaxation times of those quantum objects on different substrates lie in the femto- or pico-second regime.