Supporting Online Material for Measurement of Fast Electron Spin Relaxation Times with Atomic Resolution

1) IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, California 95120, USA 2) Institute of the Physics of Nanostructures, Ecole Polytechnique Federale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland Experimental Setup: We use an ultra-high-vacuum scanning tunneling microscope (STM) operating at an adjustable temperature of 0.6 K to 10 K. Magnetic fields up to 7 T were applied perpendicular to the sample surface. The fast voltage pulses are applied to the STM tip, but throughout this report we use the commonly employed convention of specifying the voltage of the sample with respect to the tip. For pump-probe measurements the tip-sample distance was fixed by setting the tunnel current to 1 nA at +10.0 mV sample voltage prior to opening the feedback loop. The pump and probe voltage pulses were generated as continous pulse trains by a pulse pattern generator (Agilent 81110A). The pump-probe cycle was repeated every 2 µs and the probe pulse was chopped at 810 Hz. The pump and probe pulses were summed, attenuated by 20 dB and applied to the tip of the STM. The tunnel current is detected at the sample and fed to a current preamplifier (Femto DLPCA-200) with ~1 kHz bandwidth and from there to a lockin amplifier to selectively detect the 810 Hz component of the current corresponding to the tunnel current of the probe pulse. For the measurements shown in Fig. 2C, Fig. 4 and Fig. S2 the pump and probe pulses were 100 ns FWHM long with 50 ns linear-ramp rise and fall times. The amplitude of the pump pulse was −36.5 mV and thereby well above the −16.7 mV threshold for spin excitation. The probe pulse voltage was −4.0 mV leading to a baseline of N = 341 electrons per probe pulse as measured when the probe precedes the pump. The pulse parameters used in Fig. 3A are the same except for a lowered probe-pulse voltage of −1.2 mV and the variable pump-pulse voltage. Spin-polarized STM measurements on Fe-Cu dimers: The spin-polarized tip used in this work has one magnetic atom, Mn, attached to the otherwise non-magnetic Cu-coated apex. The magnetic atom that was used to create spin-polarization in the tip was picked up after the Fe-Cu dimers were assembled by vertical atom manipulation (S1, S2). The magnetic moment of the attached atom is aligned parallel to the external magnetic field. This also determines the direction of the tip's spin-polarization (S3). Since the magnetic atom at the tip apex is adsorbed directly on the metal surface of the tip (without a decoupling layer such as Cu