Chengtao Yu

Dipolar induced, spatially localized resonance in magnetic antidot arrays

Dipole induced, spatially localized ferromagnetic resonances (at 35 GHz) are observed in micron-sized antidot arrays in permalloy films fabricated with photolithography. All square (3 μm×3 μm) and rectangular (3 μm×4, 5, and 7 μm) array samples exhibit double resonances, with each resonance possessing uniaxial in-plane anisotropy. Interestingly, the easy axes of the two resonances are orthogonal in all cases. The magnitude of the induced dipolar anisotropy decreases with increasing rectangular aspect ratio for one of the resonances, but remains essentially constant for the other. Micromagnetic simulations reveal that the two resonance peaks are the consequence of a dipole field distribution producing two areas with distinctly different demagnetizing field patterns.

Direct measurement of spatially localized ferromagnetic-resonance modes in an antidot lattice (invited)

Recent ferromagnetic-resonance (FMR) measurements and related simulations on antidot structures suggested the existence of spatially localized modes. In this report we confirm the existence of these modes using time-resolved Kerr microscopy (TRKM) as a local probe of the magnetodynamics. FMR measurements on an antidot array (a 40-nm-thick permalloy film with a hole size of 1.5μm and a hole lattice spacing of 3μm×5μm) at frequencies between 10 and 35GHz reveal two main resonances, whose relative amplitudes and orthogonal uniaxial in-plane anisotropies suggest the existence of modes localized between holes along each of the principal axes. TRKM measurements in applied fields ranging from 100to600Oe show explicitly the existence of these two modes—one at low frequency between the holes along the short axis and one at higher frequency between the holes along the long axis. TRKM also reveals additional mode structure, most notably a low-frequency mode localized along the edges of the antidots, similar to the e...

Magnetization reversal and nanostructure refinement in magnetically annealed Nd2Fe14B∕α-Fe-type nanocomposites

Nanostructure refinement, magnetic anisotropy and hard magnetic property enhancement have been observed in melt-spun Nd2.4Pr5.6Dy1Fe84Mo1B6 nanocomposites annealed in an in-plane or out-of-plane field of 1.2T. The magnetic annealing results in an enhancement of an out-of-plane (110) crystal texture of α-Fe and an in-plane uniaxial magnetic anisotropy of the 2:14:1 phase. Magnetic annealing also introduces finer, less angular and more homogeneously distributed soft and hard nanograins. Field dependent torque measurement indicates a complex magnetization reversal mechanism in these nanocomposites. Compared with the sample annealed without a field, there is a noticeable improvement in the hard magnetic properties for the magnetically annealed samples. Especially, the energy product (BH)max was enhanced by 26.6% (from 94to119kJ∕m3). The improvement in the magnetic properties is a result of the enhanced crystallographic texture, nanostructure refinement, and in-plane uniaxial magnetic anisotropy enhancement.

Lateral standing spin waves in permalloy antidot arrays

Spin wave modes in permalloy antidot arrays have been investigated with ferromagnetic resonance at 9.7 GHz. In contrast to a quadratic dispersion expected for exchange standing spin waves, nearly linear relationship exists between the resonance field and mode index, and it can be approximately described by the dipole-dipole and exchange theory. Time-dependent micromagnetic simulations show that the spin wave modes are a result of lateral confinement from the vacant holes. Furthermore, the simulations visually reveal the existence of localized spin waves due to localized boundary conditions in antidots arrays.

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.

Magnetic properties of a noninterpenetrating chiral porous cobalt metal-organic framewok

Noninterpenetrating chiral porous cobalt metal-organic framework built from the coplanar triangular H3TATB ligands has been synthesized under solvothermal conditions. X-ray analysis revealed the compound crystallized in the chiral space group either P4132 or P4332, and adopted a trimetallic “hourglass” secondary building unit, in which there are two terminal Td-Co atoms and one central Oh-Co with Td-Co and Oh-Co distance about 3.604A. Paramagnetic behavior is observed throughout the measured temperature range of 2–300K. Assuming a uniform isotropic interaction between the nearest neighbor Co atoms, susceptibility of the trinuclear Co2+ linear chain has been calculated by following Van Vleck’s quantum model for spin-spin couplings. The resultant fit to the data indicates an antiferromagnetic Co–Co interaction in the molecule, with coupling strength J∼−6.2×10−16erg, θ∼−16K.

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.

Ferromagnetic resonance in ferromagnetic/ferroelectric Fe∕BaTiO3∕SrTiO3(001)

Single Fe(001) films (30nm thick) have been epitaxially grown on ferroelectric BaTiO3∕SrTiO3(001) substrates at different growth temperatures to study the mutual interaction between the multiferroic components. This paper reports on the as-grown magnetic properties of the structures as a precursor to a full investigation of the multiferroic interactions. Ferromagnetic resonance measurements were carried out at 36GHz cavity and variable frequency microstrip resonators. Dual resonance modes are observed in the film, which are attributed to relaxed Fe in the film interior and strained Fe at the interface. Fourfold anisotropy is present for both modes with energy density consistent with that of bulk Fe. The interface mode is characterized by a large out-of-plane anisotropy comparable and opposite in sign to the shape anisotropy. This strained interfacial Fe should serve to couple the multiferroic components in this system. Dispersion curves show both optic and acoustic branches along the hard axis [110], with...

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.