Andreas Heinrich

Mon, Sep 20, 2010, 1:15 pm to 6:00 pm
PCTS Seminar Room
Title: "Measuring the spin dynamics of single atoms with nanosecond time resolution" Abstract:Spin is a fundamental quantity in quantum mechanics. Its manifestations range from the magnet on our fridge to the magnetic-field dependent splitting of a beam of evaporated atoms in the famous Stern-Gerlach experiment. With the advent of spin-based electronic devices, the behavior of nanostructured magnetic materials draws increasing attention. In most applications of spin systems, time dependent properties play a decisive role. Techniques like pulsed magnetic resonance or time-resolved magneto-optical Kerr effect give insight into the spin dynamics but often lack single spin sensitivity or atomic-scale spatial resolution. Here we show how a Scanning Tunneling Microscope (STM) can be used to study the dynamic evolution of individual spin systems on surfaces on time scales ranging from pico- to nanoseconds. We focus on individual transition metal atoms deposited onto a monolayer thin Cu2N decoupling layer on Cu(100) and study their spin relaxation. In this environment Fe and Mn exhibit a set of quantum spin states with well-defined energy levels. In a strong magnetic field the Mn atom's spin states form a uniform ladder of Zeeman states to higher energy. When such a Mn spin is excited by tunneling electrons into an excited spin state, the relaxation happens on timescales below 1 ns. We obtain access to such fast time scales by comparing low-current with high-current measurements. In the low-current case the system remains near the ground state whereas in the high-current case the system is pumped far away from thermal equilibrium by frequent interactions with the tunneling electrons. For Fe atoms on the other hand, magneto-crystalline anisotropy can strongly modify the energetic configuration of the spin states and drastically change their spin relaxation behavior. We find that placing a Cu atom close to a Fe atom boosts the uniaxial anisotropy energy of an individual Fe atom. This creates a long-lived (meta-stable) spin state with spin relaxation times in excess of 200 ns. By employing an all-electronic pump-probe measurement scheme we can directly follow the time-dependent spin properties of such systems down to the nanosecond time scale. In this new measurement scheme a strong voltage pulse creates spin excitations in the systems. At the end of this pump pulse, the spin system can be in any spin state, including highly excited ones. A time-delayed and weaker probe pulse then senses the magnetic orientation of the spin system via spin-polarized tunneling from the tip. The resultant time-dependence is a function solely of the voltages pulses at the STM’s tunnel junction and does not require fast current amplifiers to achieve nanosecond time resolution. For potential spintronics applications, these experiments demonstrate the ability to manipulate the environment of individual atomic spins and to directly monitor the resulting effect on the spin dynamics.