Gerrit Eilers

Terahertz spin current pulses controlled by magnetic heterostructures

1. Department of Physical Chemistry, Fritz Haber Institute, Berlin, Germany. 2. Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden. 3. I. Physikalisches Institut, Georg-August-Universität Göttingen, Göttingen, Germany. 4. Helmholtz-Zentrum Berlin fϋr Materialien und Energie, Berlin, Germany. 5. Peter Grünberg Institute and Institute for Advanced Simulation, Forschungszentrum Jülich and JARA, Jülich, Germany.In spin-based electronics, information is encoded by the spin state of electron bunches. Processing this information requires the controlled transport of spin angular momentum through a solid, preferably at frequencies reaching the so far unexplored terahertz regime. Here, we demonstrate, by experiment and theory, that the temporal shape of femtosecond spin current bursts can be manipulated by using specifically designed magnetic heterostructures. A laser pulse is used to drive spins from a ferromagnetic iron thin film into a non-magnetic cap layer that has either low (ruthenium) or high (gold) electron mobility. The resulting transient spin current is detected by means of an ultrafast, contactless amperemeter based on the inverse spin Hall effect, which converts the spin flow into a terahertz electromagnetic pulse. We find that the ruthenium cap layer yields a considerably longer spin current pulse because electrons are injected into ruthenium d states, which have a much lower mobility than gold sp states. Thus, spin current pulses and the resulting terahertz transients can be shaped by tailoring magnetic heterostructures, which opens the door to engineering high-speed spintronic devices and, potentially, broadband terahertz emitters.

Epitaxial growth of MgO and Fe∕MgO∕Fe magnetic tunnel junctions on (100)-Si by molecular beam epitaxy

Epitaxial growth of MgO barrier on Si is of technological importance due to the symmetry filtering effect of the MgO barrier in conjunction with bcc-ferromagnets. We study the epitaxial growth of MgO on (100)-Si by molecular beam epitaxy. MgO matches Si with 4:3 cell ratio, which renders Fe to be 45° rotated relative to Si, in sharp contrast to the direct epitaxial growth of Fe on Si. The compressive strains from Si lead to the formation of small angle grain boundaries in MgO below 5nm, and also affect the transport characteristics of Fe∕MgO∕Fe magnetic tunnel junctions formed on top.

Spin-wave population in nickel after femtosecond laser pulse excitation

The spin-wave relaxation mechanisms after intense laser excitation in ferromagnetic nickel films are investigated with all-optical pump-probe experiments. Uniform precession (Kittel mode), Damon-Eshbach surface modes and perpendicular standing spin waves can be identified by their dispersion f(H). However, different to other ferromagnets f(H) deviates from the expected behavior. Namely, a mode discontinuity is observed, that can be attributed to a non-linear process. Above a critical field the power spectrum reveals a redistribution of the energy within the spin-wave spectrum populated.

Intrinsic and nonlocal Gilbert damping parameter in all optical pump-probe experiments

The study of magnetization dynamics on the femtosecond time scale is an important task for the implementation of future ultrafast spintronics. With the time resolution inherent using femtosecond laser pulses in all optical pump-probe experiments, the basic time constants of magnetic precessional modes as well as the energy dissipation processes, which determine the Gilbert damping, can be studied. The dominant magnetic relaxation modes for the thin Ni films have frequencies in the range of 1.5–13GHz. The corresponding Gilbert damping parameter is found to be dependent on the precession mode. The α values range from 0.05 to 0.8 for highly damped modes. The nonlocal Gilbert damping due to evanescent spin currents and two-magnon scattering is studied for double layers Ni∕Cr∕Si(100) with varied Ni thicknesses. A large increase of the damping parameter for films with a thinner Ni layer is observed.

Electric breakdown in ultrathin MgO tunnel barrier junctions for spin-transfer torque switching

Magnetic tunnel junctions for spin-transfer torque (STT) switching are prepared to investigate the dielectric breakdown. Intact and broken tunnel junctions are characterized by transport measurements prior to transmission electron microscopy analysis. The comparison to our previous model for thicker MgO tunnel barriers reveals a different breakdown mechanism arising from the high current densities in a STT device: instead of local pinhole formation at a constant rate, massive electromigration and heating leads to displacement of the junction material and voids are appearing. This is determined by element resolved energy dispersive x-ray spectroscopy and three dimensional tomographic reconstruction.

Magnetization dynamics in optically excited nanostructured nickel films

In this work, the laser-induced magnetization dynamics of nanostructured nickel films is investigated. The influence of the nanosize is discussed considering the timescale of hundreds of femtoseconds as well as the GHz regime. While no nanosize effect is observed on the short timescale, the excited magnetic mode in the GHz regime can be identified by comparison with micromagnetic simulations. The thickness dependence reveals insight into the dipole interaction between single nickel structures. Also, transient reflectivity changes are discussed.

Direct imaging of the structural change generated by dielectric breakdown in MgO based magnetic tunnel junctions

MgO based magnetic tunnel junctions are prepared to investigate the dielectric breakdown of the tunnel barrier. The breakdown is directly visualized by transmission electron microscopy measurements. The broken tunnel junctions are prepared for the microscopy measurements by focussed ion beam out of the junctions characterized by transport investigations. Consequently, a direct comparison of transport behavior and structure of the intact and broken junctions is obtained. Compared to earlier findings in Alumina based junctions, the MgO barrier shows much more microscopic pinholes after breakdown. This can be explained within a simple model assuming a relationship between the current density at the breakdown and the rate of pinhole formation.

Long-range order on the atomic scale induced at CoFeB/MgO interfaces

The amorphous (a-) CoFeB/crystalline (c-) MgO based tunneling system interface has been studied by means of quantitative high resolution transmission electron microscopy from atomic to micrometer length scales with increasing annealing temperatures. On the micron scale an irregular nucleation is found. On the atomic scale a long-range order is induced by the MgO interface, explaining the high tunnel magnetoresistance values >100% even for not fully crystallized CoFeB/MgO/CoFeB tunnel junctions.

Nonlinear terahertz spectroscopy of magnetically ordered solids

We report on two investigations demonstrating that terahertz spectroscopy is a powerful tool to study the orbital and spin dynamics of electrons in magnetically ordered solids. First, we show that the terahertz pulse emitted from a photo-excited ferromagnetic Fe film is highly sensitive to the motion of spin-polarized electrons along the film interfaces. Second, the magnetic field of an intense terahertz pulse is used to induce a transient magnetization in the antiferromagnet NiO via Zeeman coupling to the NiO spins. A long-lived spin precession at the magnon frequency of 1 THz results.

Transmission Electron Microscopy Analysis of Tunnel Magneto Resistance Elements with Amorphous CoFeB Electrodes and MgO Barrier

Magnetic tunnel-junctions consisting of CoFeB/ MgO/ CoFeB trilayers have been of great interest in research just recently. Due to their high magnetoresistance they are a promising candidate for the fabrication of spintorque MRAM devices. For future writing concepts like current-induced magnetic switching, magnetic tunnel-junctions (MTJs) with thin barriers are necessary to provide sufficient high current densities. In such elements the tunnel magneto resistance (TMR) is strongly dependent on the electron transmission at the metal/oxide interfaces. Therefore the quality of the interfaces is of great significance and has to be optimized on the nanoscale.