Liesl Folks

Localized magnetic modification of permalloy using Cr+ ion implantation

Doping ferromagnetic nickel–iron alloys with chromium causes the Curie temperature to be reduced. We have demonstrated that solid state solutions of Ni–Fe–Cr can be formed by implanting Cr ions into ferromagnetic NiFe alloy films, thus creating paramagnetic films. We find that the magnetic moment and coercivity decrease steadily with Cr+ dose, reaching zero at room temperature with the onset of paramagnetism. Using Cr+ implantation in conjunction with a lithographic mask we have patterned continuous Ni80Fe20 films into separate regions that are ferromagnetic and paramagnetic at room temperature. This magnetic patterning process may have applications for the manufacture of magnetic write heads, such as for the notching process used to constrain the stray field from the write gap.

Effects of radio-frequency current on spin-transfer-torque-induced dynamics

In this work we investigate the highly unexplored PS dynamical regime as well as the effects of additional rf currents on the PS and switching modes. The PS modes are found to interact with the applied rf currents having frequencies in the range of the dc-only driven modes, and under certain conditions are found to frequency lock. In contrast to previous frequency locking results [7, 8, 9], by looking at these effects in the PS regime, we find a measurable dependence of the dc switching threshold on the external rf frequency, finj. In the locking range the rf appears to have a stabilizing effect which hinders switching, however, at frequencies just below this range, we find a measurable reduction in the direct critical current for switching, Ic. While we have already reported this effect [10], an explanation for the origin of this effect has not been given. In this work, by studying the behavior of this system in frequency domain and through macrospin simulations (MS) (using the Landau-Lifshitz-Gilbert equation with Slonczewski STT [2]) we provide a description of the magnetization dynamics originating from the interaction between the dc driven switching modes and the applied rf. The results show good agreement with our experimental data. EXPERIMENT

Vortex magnetodynamics: Ferromagnetic resonance in permalloy dot arrays

Permalloy dot arrays (square lattices of 100, 200, 500, and 1000 nm dots, 40 nm thick) have been fabricated using e-beam lithography. Hysteresis loops show vortex states at remanence in all samples. Variable frequency ferromagnetic resonance was used to probe the dynamics of both vortex and single domain states. The uniform resonance mode is present at fields above the vortex annihilation when field sweeping up and vortex nucleation field when field sweeping down. In the vortex state multiple resonances are observed in the 500 and 1000 nm samples, but interestingly, not in the 100 and 200 nm. In the frequency/field dispersion curve, these vortex resonances exhibit optical mode characteristics, whereas the uniform mode exhibits the expected acoustic mode behavior. Resonance modes corresponding to the annihilation and creation field (transition from vortex to single domain or verse versa) are also observed and discussed.

Probing activation energy barrier distribution for reversal of strongly exchange-coupled magnetic multilayer thin films

We demonstrate a measurement technique with zero-applied magnetic field to deduce and spatially map the activation energy barrier distribution of strongly exchange-coupled magnetic-multilayer thin films, which is otherwise inaccessible with conventional methods in the presence of an applied magnetic field. Our technique involves the analysis of magnetic force microscopy images of magnetic microwires, whose magnetizations have been subject to thermal decay due to Joule heating from applied nanosecond scale current pulses. Fitting the results of such measurements on CoNi/Pd magnetic-multilayer microwires to a modified Arrhenius–Neel formalism yields an energy barrier distribution with 8% sigma, in good agreement with complementary fits of the switching-field-distribution measurements on patterned CoNi/Pd magnetic-multilayer islands.

Exchange-coupling modified spin wave spectra in the perpendicularly magnetized Permalloy nanodot chain arrays

Spin wave spectra in exchange-coupled nanoscale dot chain arrays were studied using ferromagnetic resonance. The dot chain arrays, with dot diameters of 300 nm and thicknesses of 40 nm, coupled via Permalloy bridges of width ranging from 0 (no bridge) to 60 nm, were fabricated using e-beam lithography. In the perpendicularly magnetized isolated dots, multiple sharp ferromagnetic resonant peaks were observed, which are associated with the quantized in-plane wave vector due to the finite dot radius. These spectrum lines shift to higher fields for samples with wider bridges due to the increasing effective demagnetizing factor. For coupled dots, additional satellite peaks were observed at both sides of higher order spin wave modes and at the lower field side of the uniform mode. We show that these satellite peaks can be attributed to the excitation in each bridged dot and the interdot exchange coupling, respectively.

Exchange-coupled suppression of vortex formation in permalloy nanodot chain arrays

The effect of interdot exchange coupling on the magnetization reversal processes in nanodots has been investigated in a study on permalloy dot arrays with dot diameters of 300 nm and thicknesses of 40 nm. The dots, fabricated using e-beam lithography and ion beam deposition, are exchange coupled via 50 nm long permalloy bridges of widths ranging from 0 to 60 nm. Chains of five collinear coupled dots form the unit cell of the array structure. Magneto-optical Kerr effect hysteresis loops are reported with comparison to simulations. With field applied along the coupling direction, vortex nucleation occurs sequentially from the outer to the inner dots of the chain. Compared with uncoupled dots, nucleation is suppressed by the interdot exchange coupling resulting in highly correlated magnetization patterns. Buildup of exchange energy in the inner dots ultimately drives the bridge magnetization to switch, followed by abrupt, simultaneous vortex nucleation in these dots. Along the perpendicular-to-the-coupling direction, the exchange interaction has little effect and magnetic reversal is almost identical to arrays of isolated dots of the same geometry.The effect of interdot exchange coupling on the magnetization reversal processes in nanodots has been investigated in a study on permalloy dot arrays with dot diameters of 300 nm and thicknesses of 40 nm. The dots, fabricated using e-beam lithography and ion beam deposition, are exchange coupled via 50 nm long permalloy bridges of widths ranging from 0 to 60 nm. Chains of five collinear coupled dots form the unit cell of the array structure. Magneto-optical Kerr effect hysteresis loops are reported with comparison to simulations. With field applied along the coupling direction, vortex nucleation occurs sequentially from the outer to the inner dots of the chain. Compared with uncoupled dots, nucleation is suppressed by the interdot exchange coupling resulting in highly correlated magnetization patterns. Buildup of exchange energy in the inner dots ultimately drives the bridge magnetization to switch, followed by abrupt, simultaneous vortex nucleation in these dots. Along the perpendicular-to-the-coupling d...

Ferromagnetic resonance in the proximity of the unstable perpendicular equilibrium: A study in Permalloy thin films and nanoscale dots

We are investigating the magnetization dynamics of a ferromagnetic system in the proximity of the unstable perpendicular equilibrium. The test system utilized is Permalloy in thin film and dot arrays with the applied field along a small angle off the film normal. Frequency∕field dispersion curves (up to 15GHz) obtained at different small angles (θH) reveal significant deviations from the perpendicular linear dispersion at low frequencies, and they form almost frequency independent flat plateau with a local minimum near the demagnetization field. The existence of the local minimum results in three resonances in a constant frequency∕swept field scans. Such behavior is modeled successfully, in particular, in the film case, by using an approach for oblique angle resonance [P. E. Tannenwald, Phys. Rev. 105, 377 (1957)]. For the dots, additional influence from either induced in-plane anisotropy or vortex formation is discussed.