Multiple synchronization attractors of serially connected spin-torque nanooscillators

Spin-torque nanooscillators (STNOs), which have both the common properties of nanosized oscillators (small size, tunable operating frequency) and some particular ones (wide operating range, easy on-chip integration, etc.), have received a great deal of attention due to their high potential in applications. Yet synchronization of serially connected STNOs has been considered essential for applications. In this paper, we present findings concerning the following properties of synchronized serially connected STNOs: (i) multiple synchronization attractors coexist, and the attracting basins are entangled in a complicated manner; (ii) these attractors have different synchronized frequencies and output powers; and (iii) switching among these attractors can be induced by a small noise, which causes a resonance peak in the power spectra to vanish. These characteristics can be understood using saddle-node bifurcations and have direct impact on laboratory experiments and the potential applications of STNO-based devices. © 2012 American Physical Society.published_or_final_versio

Antiferromagnetic Bloch line driven by spin current as room-temperature analog of a fluxon in a long Josephson junction

Antiferromagnets (AFMs) are promising materials for future high-frequency field-free spintronic applications. Self-localized spin structures can enhance their capabilities and introduce new functionalities to AFM-based devices. Here we consider a domain wall (DW), a topological soliton that bridges a connection between two ground states, similar to a Josephson junction (JJ) link between two superconductors. We demonstrate the similarities between DWs in bi-axial AFM with easy-axis primary anisotropy, driven by a spin current, and long Josephson junctions (LJJs). We found that the Bloch line (BL) in DWs resembles the fluxon state of JJs, creating a close analogy between the two systems. We propose a scheme that allows us to create, move, read, and delete such BLs. This transmission line operates at room temperature and can be dynamically reconfigured in contrast to superconductors. Results of a developed model were confirmed by micromagnetic simulations for Cr$_2$O$_3$ and DyFeO$_3$, i.e., correspondingly with weak and strong in-plane anisotropy. Overall, the proposed scheme has significant potential for use in magnetic memory and logic devices.Comment: 8 pages, 5 figures; Published in Physical Review Applie

Pinholes May Mimic Tunneling

Interest in magnetic-tunnel junctions has prompted a re-examination of tunneling measurements through thin insulating films. In any study of metal-insulator-metal trilayers, one tries to eliminate the possibility of pinholes (small areas over which the thickness of the insulator goes to zero so that the upper and lower metals of the trilayer make direct contact). Recently, we have presented experimental evidence that ferromagnet-insulator-normal trilayers that appear from current-voltage plots to be pinhole-free may nonetheless in some cases harbor pinholes. Here, we show how pinholes may arise in a simple but realistic model of film deposition and that purely classical conduction through pinholes may mimic one aspect of tunneling, the exponential decay in current with insulating thickness.Comment: 9 pages, 3 figures, plain TeX; submitted to Journal of Applied Physic

Zero Field precession and hysteretic threshold currents in spin torque oscillators with tilted polarizer

Using non-linear system theory and numerical simulations we map out the static and dynamic phase diagram in zero applied field of a spin torque oscillator with a tilted polarizer (TP-STO).We find that for sufficiently large currents, even very small tilt angles (beta>1 degree) will lead to steady free layer precession in zero field. Within a rather large range of tilt angles, 1 degree< beta <19 degree, we find coexisting static states and hysteretic switching between these using only current. In a more narrow window (1 degree<beta<5 degree) one of the static states turns into a limit cycle (precession). The coexistence of static and dynamic states in zero magnetic field is unique to the tilted polarizer and leads to large hysteresis in the upper and lower threshold currents for TP-STO operation.Comment: 5 pages, 4 figure

Probing vertically graded anisotropy in FePtCu films

Field-dependent polarized neutron reflectivity (PNR) and magnetometry are employed to study the magnetic properties of compositionally uniform and graded FePtCu films as a function of annealing temperature (TA). The PNR results are able to directly probe the compositional and anisotropy variations through the film thickness. Further details about how the reversal mechanisms evolve are then elucidated by using a first-order reversal curve technique. The reversal of the graded sample annealed at 300º C occurs by an initial rapid switching of the dominant soft A1 phase toward the surface of the film, followed by the gradual reversal of the residual hard phase components toward the bottom. This indicates that the anisotropy gradient is not well established at this low TA. A fundamentally different mechanism is found after annealing at 400ºC, where the rapid switching of the entire film is preceded by a gradual reversal of the soft layers. This suggests that the anisotropy gradient has become better established through the film thickness. The field-dependent PNR measurements confirm the existence of an anisotropy gradient, where the lower (higher) anisotropy portions are now toward the bottom (top) of the film because of the Cu compositional gradient. However, after annealing at 500º C,a single rapid reversal is found, indicating the formation of a uniform hard film. In this case, PNR demonstrates a more uniform magnetic depth profile that is consistent with a uniform reference sample, suggesting significant interdiffusion of the Cu is degrading the compositional and induced anisotropy gradient at this elevated TA

Prognostic implications of various models for calculation of S-phase fraction in 259 patients with soft tissue sarcoma

The S-phase fraction (SPF) in flow cytometric DNA histograms in soft tissue sarcoma (STS) can be calculated in various ways. The traditional planimetric method of Baisch has been shown to be prognostic, but is hampered by a failure rate of around 40%. We therefore tested other models to see if this rate could be decreased with retained prognostic value. In 259 STS of the locomotor system the SPF was calculated according to Baisch and with commercial parametric MultiCycle software using different corrections for background. Using the Baisch model, 159 histograms could be evaluated for SPF. The 5-year metastasis-free survival rate (MFSR) was 0.94 for the low-risk group (defined with SPF), and 0.53 for the high-risk group. In the low-risk group, four of the seven patients who developed metastasis did so after 5 years. Using the MultiCycle software, SPF could be calculated in 253 tumours. Depending on type of background correction used, the 5-year MFSR varied between 0.67 and 0.82 for the low-risk group, and between 0.47 and 0.53 for the high-risk group. The late metastasis pattern in the low-risk group was never seen using the MultiCycle software. We conclude that in paraffin archival material, calculation of SPF according to Baisch is preferable in clinical use due to better separation between low-risk and high-risk groups, and also the possibility to identify patients who metastasize late. © 1999 Cancer Research Campaig

Magnetoplasmonic design rules for active magneto-optics

Light polarization rotators and non-reciprocal optical isolators are essential building blocks in photonics technology. These macroscopic passive devices are commonly based on magneto-optical Faraday and Kerr polarization rotation. Magnetoplasmonics - the combination of magnetism and plasmonics - is a promising route to bring these devices to the nanoscale. We introduce design rules for highly tunable active magnetoplasmonic elements in which we can tailor the amplitude and sign of the Kerr response over a broad spectral range