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Spin-torque nano-oscillators (STNOs) typically consist of two single domain ferromagnetic layers separated by a metallic spacer or a tunnel barrier, one with its magnetization fixed (polarizing layer), the other one susceptible to torques (free layer). An electric current traversing the system perpendicular to the layers becomes spin-polarized and exerts torques on the magnetic moments [1-3], thereby inducing switching or steady-state dynamics. The pinning of the polarizing layer can be achieved by exchange coupling to an antiferromagnet [4] or by extending its thickness and lateral dimension [5]. In the absence of pinning, both ferromagnetic layers can be excited. For increasingly symmetric STNOs, this can lead to a dynamic equilibrium state called the Slonczewski-windmill [1,6], with the magnetic moments of both layers rotating in the same direction with a constant relative angle, resulting in a vanishing magnetoresistance (MR) time-dependence. Here we investigate STNOs containing two stacked magnetic vortices, i.e., a system consisting of two ferromagnetic disks, each in a vortex state and separated by a metallic, nonmagnetic spacer. Employing analytical and numerical methods, we study the coupled spin torque-driven motion of the magnetizations in the two disks, which are not pinned by any of the above mentioned mechanisms. The theoretical findings are supported by our experimental data obtained from double-vortex Fe/Ag/Fe STNOs. The motion of the magnetic vortex in each of the disks is governed by the Thiele equation [7] with an additional force expression arising from the transfer of spin angular momentum from the polarised current to the vortex. Assuming that in the double vortex system, each vortex is free to move while at the same time it serves as a polarizing layer for the other, we solve the system of Thiele equations coupled by the spin-polarized current. We use parabolic approximations to the magnetostatic potentials for each vortex, which are chosen to represent our Fe/Ag/Fe nanopillars with ferromagnetic layers with a thickness ratio of 5/3; the uncoupled top and bottom vortices’ eigenfrequencies are set to 1.0 and 1.7 GHz, respectively. The solutions are obtained numerically using Maple's rkf45 implementation. The results can be summarized as follows: While the spin torque induces large orbit vortex gyration in one of the layers, the vortex motion in the other disk is strongly reduced, resulting in a quenching of the Slonczewski-windmill mode. Which of the two layers contributes dominantly to the magnetization dynamics is determined by the direction of the applied current. This effect results from an adaption of the motion of the constricted vortex according to the dominant one. The former acquires a stable phase to the dominant vortex, while the latter determines the frequency and sense of gyration of the whole system: If its core polarity is positive (negative), the system gyrates in the counterclockwise (clockwise) direction. If the dominating vortex is in the top disk, the gyration frequency is 1.0 GHz while for large orbit gyration in the bottom disk, we obtain 1.7 GHz. Figure 1 displays the relations between the phase, frequency and gyration radius of the bottom vortex in the case, where the top vortex is dominant. For an experimental confirmation of the frequency and phase adaption mechanism and the related quenching of the windmill modes, we study the current-induced magnetization dynamics of a Fe/Ag/Fe nanopillar with a Fe layer thickness ratio of 5/3. We apply current densities of 6.1x107 A/cm2 and investigate the resulting double vortex dynamics depending on the current polarity. At low external field, the ratio between the obtained frequencies is close to the ratio of the disk aspect ratios, which strongly supports our numerical findings. References: [1] J. C. Slonczewski, J. Magn. Magn. Mater. 159, L1 (1996). [2] L. Berger, Phys. Rev. B 54, 9353 (1996). [3] J. C. Slonczewski, J. Magn. Magn. Mater. 247, 324 (2002). [4] I. N. Krivorotov et al., Science 307, 228 (2005). [5] I. Kiselev et al., Nature 425, 380 (2003). [6] Y. B. Bazaliy, D. Olaosebikan, and B. A. Jones, J. Nanosci. Nanotechnol. 8, 2891 (2008). [7] A. A. Thiele, Phys. Rev. Lett. 30, 230 (1973).

Owing to their ability of narrow bandwidth operation, magnetic vortex based spin-torque nano-oscillators (STNOs) are promising candidates for future on-chip microwave sources. Typically, these oscillators are nanopillars containing two stacked ferromagnetic disks, one in a vortex state, the other with quasi-homogeneous magnetization. Devices of that kind have been investigated extensively over the past years [1-3]. Only recently, a different type of vortex oscillators has attracted much interest. In these “double vortex oscillators”, both ferromagnetic disks are in a vortex state. Depending on the relative vorticity, the local alignment of the magnetic layers can be either parallel or antiparallel. Thus, the system constitutes an analogue to single domain spin valves, while also retaining the good oscillator properties of magnetic vortices, which makes this type of STNO particularly interesting for studying fundamental aspects of spin-transfer torque. However, only few studies of double-vortex oscillators exist to-date [5,6]. Here we present our results on Fe/Ag/Fe double vortex oscillators. Combining experimental and numerical methods, we address the yet open question of how the magnetostatic interaction of the vortices – their separation is typically in the order of a few nanometers –affects the spin-torque induced dynamics. Our samples are all-metallic nanopillars 150 nm in diameter, containing a Fe(30)/Ag(6)/Fe(15) pseudo spin valve (layer thicknesses given in nm). By applying d.c. currents perpendicular to the plane of the layers, we excite magnetization dynamics corresponding to gyrotropic vortex motion. The sample geometry stabilizes the magnetic vortex state in each Fe disk even if the strong Oersted fields and the vortex magnetization have opposite sense of rotations. This high degree of stability provides a yet unprecedented opportunity to investigate a more exotic class of double vortex states – those with opposed vorticities – under the high current densities necessary to enter the spin-torque precession regime. Figure 1 displays a typical combined magnetoresistance and high frequency measurement. The magnetic field, which has an angle of 30° with the sample plane, is swept from positive values to negative saturation. For each field value, in addition to the d.c. voltage, a spectrum is also recorded. In a field interval about 100 mT wide, the sample is in a double vortex state where the vorticities of the top and bottom vortices are opposed to each other. According to the top panel of Fig. 1, this state exhibits magnetization dynamics. We vary our state preparation procedure in order to create the various vorticity and core polarity combinations. The obtained states are characterized with respect to their d.c. and high frequency behavior using the above described measuring technique where in all measurements, the electron flow is directed from the bottom to the top Fe disk. We obtain a fine structure in the modes, where the frequency splittings are in the order of hundreds of MHz. The micromagnetic simulations are performed with our code TetraMag [7]. The simulations include the Oersted field corresponding to a sample current of 10 mA in magnitude. For a given vorticity and core polarity combination, we find the eigenmodes and frequencies of the system. Comparing the experimentally found set of frequencies to the simulated cases, we find that each measured peak matches the lowest mode of a corresponding computed spectrum in frequency. This suggests that the observed fine structure is caused by two effects. First, the Oersted field lifts the degeneracy of states with the top vortex vorticity parallel or antiparallel to the field’s sense of rotation. On top of this Zeemann-type frequency alteration, the resulting modes are split further depending on the relative alignment of the top and bottom vortex cores. This suggests that the second splitting is due to the magnetostatic interaction of the vortex cores. This remarkable result sheds light on the subtle interplay of forces governing the dynamics of double vortex oscillators, while on the other hand it allows measuring changes in relative core alignment. References: [1] V. S. Pribiag et al., Nature Phys. 3, 498 (2007). [2] A. Dussaux et al., Nat. Commun. 1:8 DOI:10.1038 / ncomms1006 (2010). [3] X. W. Yu et al., Phys. Rev. Lett. 106, 167202 (2011). [5] A. V. Khvalkovskiy et al., Appl. Phys. Lett. 96, 212507 (2010). [6] N. Locatelli et al., Appl. Phys. Lett. 98, 062501 (2011). [7] A. Kákay, E. Westphal, R. Hertel, IEEE Trans. Magn. 46, 2303 (2010).

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Fragestellung: Ziel der prospektiven Multicenterstudie war der Vergleich verschiedener Osteosyntheseverfahren bei Humeruskopf-3-Part-Frakturen hinsichtlich der klinisch/ radiologischen Ergebnisse und Komplikationen Methodik: 227 Patienten (Durchschnittsalter 64,5 Jahre) mit einer instabilen [for full text, please go to the a.m. URL], Deutscher Kongress für Orthopädie und Unfallchirurgie; 74. Jahrestagung der Deutschen Gesellschaft für Unfallchirurgie, 96. Tagung der Deutschen Gesellschaft für Orthopädie und Orthopädische Chirurgie, 51. Tagung des Berufsverbandes der Fachärzte für Orthopädie

We investigate current-perpendicular-plane giant magnetoresistance (CPP-GMR) and current-induced magnetization switching in single-crystalline Fe/Ag/Fe nanopillars of 70 nm diameter. The interplay between the in-plane, fourfold magnetocrystalline anisotropy of the Fe(001) layers and the spin-transfer torque (STT) gives rise to a two-step switching behavior, which allows an investigation of the angular dependences of CPP-GMR and spin-transfer torque. Both behave asymmetrically with respect to the perpendicular alignment of the two Fe layer magnetizations as theoretically predicted due to strong spin accumulation at the Fe/Ag(001) interfaces [M. D. Stiles and D. R. Penn, Phys. Rev. B 61, 3200 (2000)]. The asymmetry parameter determined from the STT data quantitatively agrees with calculated spin-dependent interface resistances, whereas CPP-GMR yields a smaller degree of asymmetry.