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Register a new userMagnetic anisotropy in surface-supported single-ion lanthanide complexes
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Single-ion lanthanide-organic complexes can provide stable magnetic moments with well-defined orientation for spintronic applications on the atomic level. Here, we show by a combined experimental approach of scanning tunneling microscopy and X-ray absorption spectroscopy that dysprosium-tris(1,1,1-trifluoro-4-(2-thienyl)-2,4butanedionate) (Dy(tta)$_3$) complexes deposited on a Au(111) surface undergo a molecular distortion, resulting in distinct crystal field symmetry imposed on the Dy ion. This leads to an easy-axis magnetization direction in the ligand plane. Furthermore, we show that tunneling electrons hardly couple to the spin excitations, which we ascribe to the shielded nature of the $4f$ electrons.
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We investigate the effect of an external magnetic field on the physical properties of the acceptor hole states associated with single Mn acceptors placed near the (110) surface of GaAs. Crosssectional scanning tunneling microscopy images of the acceptor local density of states (LDOS) show that the strongly anisotropic hole wavefunction is not significantly affected by a magnetic field up to 6 T. These experimental results are supported by theoretical calculations based on a tightbinding model of Mn acceptors in GaAs. For Mn acceptors on the (110) surface and the subsurfaces immediately underneath, we find that an applied magnetic field modifies significantly the magnetic anisotropy landscape. However the acceptor hole wavefunction is strongly localized around the Mn and the LDOS is quite independent of the direction of the Mn magnetic moment. On the other hand, for Mn acceptors placed on deeper layers below the surface, the acceptor hole wavefunction is more delocalized and the corresponding LDOS is much more sensitive on the direction of the Mn magnetic moment. However the magnetic anisotropy energy for these magnetic impurities is large (up to 15 meV), and a magnetic field of 10 T can hardly change the landscape and rotate the direction of the Mn magnetic moment away from its easy axis. We predict that substantially larger magnetic fields are required to observe a significant field-dependence of the tunneling current for impurities located several layers below the GaAs surface.
Anisotropy effects can significantly control or modify the ground-state properties of magnetic systems. Yet the origin and the relative importance of the possible anisotropy terms is difficult to assess experimentally and often ambiguous. Here we propose a technique which allows a very direct distinction between single-ion and two-ion anisotropy effects. The method is based on high-resolution neutron spectroscopic investigations of magnetic cluster excitations. This is exemplified for manganese dimers and tetramers in the mixed compounds CsMnxMg1-xBr3 (0.05leqxleq0.40). Our experiments provide evidence for a pronounced anisotropy of the order of 3% of the dominant bilinear exchange interaction, and the anisotropy is dominated by the single-ion term. The detailed characterization of magnetic cluster excitations offers a convenient way to unravel anisotropy effects in any magnetic material.
The nature of the magnetism brought about by Fe adatoms on the surface of the topological insulator Bi2Se3 was examined in terms of density functional calculations. The Fe adatoms exhibit strong easy-axis magnetic anisotropy in the dilute adsorption limit due to the topological surface states (TSS). The spin exchange J between the Fe adatoms follows a Ruderman-Kittel-Kasuya-Yosida (RKKY) behavior with substantial anisotropy, and the Dzyaloshinskii-Moriya (DM) interaction between them is quite strong with |D/J|~0.3 under the mediation by the TSS, and can be further raised to ~0.6 by an external electric field. The apparent single-ion anisotropy of a Fe adatom is indispensable in determining the spin orientation.
Energy relaxation in light-harvesting complexes has been extensively studied by various ultrafast spectroscopic techniques, the fastest processes being in the sub-100 fs range. At the same time much slower dynamics have been observed in individual complexes by single-molecule fluorescence spectroscopy (SMS). In this work we employ a pump-probe type SMS technique to observe the ultrafast energy relaxation in single light-harvesting complexes LH2 of purple bacteria. After excitation at 800 nm, the measured relaxation time distribution of multiple complexes has a peak at 95 fs and is asymmetric, with a tail at slower relaxation times. When tuning the excitation wavelength, the distribution changes in both its shape and position. The observed behaviour agrees with what is to be expected from the LH2 excited states structure. As we show by a Redfield theory calculation of the relaxation times, the distribution shape corresponds to the expected effect of Gaussian disorder of the pigment transition energies. By repeatedly measuring few individual complexes for minutes, we find that complexes sample the relaxation time distribution on a timescale of seconds. Furthermore, by comparing the distribution from three long-lived complexes with the whole ensemble, we demonstrate that the ensemble can be considered ergodic. Our findings thus agree with the commonly used notion of an ensemble of identical LH2 complexes experiencing slow random fluctuations.
First-principles calculations of the magnetic anisotropy energy for Mn- and Fe-atoms on CuN/Cu(001) surface are performed making use of the torque method. The easy magnetization direction is found to be different for Mn and Fe atoms in accord with the experiment. It is shown the magnetic anisotropy has a single-ion character and mainly originates from the local magnetic moment of Mn- and Fe-atoms. The uniaxial magnetic anisotropy constants are calculated in reasonable agreement with the experiment.
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