Juan Francisco Sierra

Spin waves in circular soft magnetic dots at the crossover between vortex and single domain state

We report on linear spin dynamics in the vortex state of the Permalloy dots subjected to stratified (magnetic) field. We demonstrate experimentally and by simulations the existence of two distinct dynamic regimes corresponding to the vortex stable and metastable states. Breaking cylindrical symmetry leads to unexpected eigenmodes frequency splitting in the stable state and appearance of new eigenmodes in the metastable state above the vortex nucleation field. Dynamic response in the metastable state strongly depends on relative orientation of the external rf pumping and the bias magnetic fields. These findings may be relevant for different vortex states in confined and stratified conditions.

Precise probing spin wave mode frequencies in the vortex state of circular magnetic dots

We report on detailed broadband ferromagnetic resonance measurements of azimuthal and radial spin wave excitations in circular Permalloy dots in the vortex ground state. Dots with aspect ratio (β=height over radius) varied from 0.03 to 0.1 were explored. The frequency splitting of two lowest azimuthal modes was observed. The experimentally observed dependence of the frequency splitting on β was reasonably well described by dynamic splitting model accounting the spin waves and vortex gyrotropic mode interaction.

Broadband ferromagnetic resonance linewidth measurement of magnetic tunnel junction multilayers

The broadband ferromagnetic resonance (FMR) linewidth of the free layer of magnetic tunnel junctions is used as a simple diagnostic of the quality of the magnetic structure. The FMR linewidth increases near the field regions of free layer reversal and pinned layer reversal, and this increase correlates with an increase in magnetic hysteresis in unpatterned films, low-frequency noise in patterned devices, and previous observations of magnetic domain ripple by use of Lorentz microscopy. Postannealing changes the free layer FMR linewidth, indicating that considerable magnetic disorder, originating in the exchange-biased pinned layer, is transferred to the free layer.

Plain superconducting films as magnetic field tunable two-dimensional rectifiers

Longitudinal and transversal permanent electric fields generated by an ac current through superconducting Pb and Nb thin strips have been studied as the function of the drive frequency (103 105Hz) the rectification becomes two dimensional. The rectification strongly depends on the magnetic field, temperature and ac drive. The unusual dc electric field topology generated by the ac current in the superconducting strips can be explained by a local rectification due the oppositely directed asymmetric edge (Bean-Livingston type) barriers.

Broadband Magnetic Response of Periodic Arrays of FeNi Dots

We present a study of magnetization dynamics at room temperature in periodic arrays of 50 nm thick FeNi (Py) circular magnetic dots of 500 nm radius and different center to center distance (1200 and 2500 nm), by using a broadband magnetometer based on vector network analyzer which works between 300 kHz and 8.5 GHz. We also present a comparison between the dynamic response, ferromagnetic resonance (FMR) and its linewidth, with static magnetic characteristics such as magnetization curves. The FMR peak appears just above the nucleation field and is perfectly described by Kittel formula taking into account the demagnetizing factor of an individual magnetic dot. In addition to FMR we observed a spin wave resonance below the uniform mode, which could be attributed to spin waves in confined systems. The FMR linewidth shows a significant broadening close to the field region corresponding to nucleation of magnetic vortex.

Temperature dependent dynamic and static magnetic response in magnetic tunnel junctions with Permalloy layers

Ferromagnetic resonance and static magnetic properties of CoFe∕Al2O3∕CoFe∕Py and CoFe∕Al2O3∕CoFeB∕Py magnetic tunnel junctions and of 25nm thick single-layer Permalloy (Py) films have been studied as a function of temperature down to 2K. The temperature dependence of the ferromagnetic resonance excited in the Py layers in magnetic tunnel junctions shows “kneelike” enhancement of the resonance frequency accompanied by an anomaly in the magnetization near 60K. We attribute the anomalous static and dynamic magnetic response at low temperatures to interface stress induced magnetic reorientation transition at the Py interface which could be influenced by dipolar soft-hard layer coupling through the Al2O3 barrier.

Unusual dc electric fields induced by a high frequency alternating current in superconducting Nb films under a perpendicular magnetic field

We report a systematic study of dc electric fields produced by sinusoidal high frequency ac currents in Nb superconducting films subject to a constant magnetic field perpendicular to the film plane. At frequencies in the 100kHz to MHz range appears a new rectification effect which has not been previously observed at lower frequencies. We have observed the dc electric field generated in this regime in films without intentionally created anisotropic pinning centres, i.e. plain films, both in strip geometry as in cross- shape geometry, and also in films with symmetric periodic pinning centres. The electric field appears in both directions along and transverse to the alternating current and is essentially different at opposite film sides. It depends strongly on the intensity of the magnetic field and may exceed by nearly an order of magnitude the rectified electric fields recently reported at lower frequencies (few kHz) in systems with artificially induced anisotropic vortex pinning. The effect has a non-monotonic dependence on the drive current frequency, being maximum around a few 100kHz to MHz, and shows a complicated temperature dependence. It is found to be different in long strips and cross shape samples. In the case of films with symmetric periodic pinning centres the rectified voltage