Full insight into the dynamics of a coupled quantum system depends on the ability to follow the effect of a local excitation in real-time. Here, we trace the free coherent evolution of a pair of coupled atomic spins by means of scanning tunneling microscopy. Rather than using microwave pulses, we use a direct-current pump-probe scheme to detect the local magnetization after a current-induced excitation performed on one of the spins. By making use of magnetic interaction with the probe tip, we are able to tune the relative precession of the spins. We show that only if their Larmor frequencies match, the two spins can entangle, causing angular momentum to be swapped back and forth. These results provide insight into the locality of electron spin scattering and set the stage for controlled migration of a quantum state through an extended spin lattice.

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(151)Eu Mössbauer spectroscopy and neutron powder diffraction are combined to show that the tetragonal (I4mm #107) compound EuNiGe3 orders magnetically below [Formula: see text] K and adopts a complex incommensurate helicoidal magnetic structure at 3.6 K, with a propagation vector [Formula: see text] and a Eu moment of 7.1(2) [Formula: see text]. On warming through 6 K an incommensurate sinusoidal modulation develops and dominates the magnetic order by 12 K.

The orbital angular moment of magnetic atoms adsorbed on surfaces is often quenched as a result of an anisotropic crystal field. Due to spin-orbit coupling, what remains of the orbital moment typically delineates the orientation of the electron spin. These two effects limit the scope of information processing based on these atoms to essentially only one magnetic degree of freedom: the spin. In this work, we gain independent access to both the spin and orbital degrees of freedom of a single atom, inciting and probing excitations of each moment. By coordinating a single Fe atom atop the nitrogen site of the Cu 2 N lattice, we realize a singleatom system with a large zero-field splitting-the largest reported for Fe atoms on surfaces-and an unquenched uniaxial orbital moment that closely approaches the free-atom value. We demonstrate a full reversal of the orbital moment through a singleelectron tunneling event between the tip and Fe atom, a process that is mediated by a charged virtual state and leaves the spin unchanged. These results, which we corroborate using density functional theory and first-principles multiplet calculations, demonstrate independent control over the spin and orbital degrees of freedom in a single-atom system.

Room temperature neutron powder diffraction has been used to investigate the chemical structure and magnetic ordering of a series of tetragonal (I4/mmm #139) MnxGa (1.15 ≤ x ≤ 2.0) alloys. Initially (x < 1.5) the excess Mn goes on the 2b site with vacancies appearing at the 2a site. For x > 1.5 Mn also appears on the 2a site. The manganese atoms on the 4d site carry an almost constant moment of 2.16(6) μB/Mn. The loss of magnetisation seen with increasing Mn content is shown to be the result of large (∼3 μB/Mn), antiparallel Mn moments on the 2b, and later 2a sites, and not to a reduction of the Mn moment on the 4d sites.