D. Spoddig

Spin–orbit-coupling effects on g-value and damping factor of the ferromagnetic resonance in Co and Fe films

The spectroscopic splitting factor g and the Gilbert damping constant G are magnetic parameters accessible to ferromagnetic resonance (FMR) measurements, which apart from the magneto-crystalline anisotropy energy can provide information on the spin–orbit coupling in magnetically ordered material. Whereas the effect of spin–orbit coupling has been thoroughly investigated and is well understood in insulating transition metal compounds, in 3d-metallic magnetic compounds the microscopic mechanism still needs further clarification. Particularly in thin films and multilayers interface effects and interaction between layers can modify both spin and orbital moments leading to changes of the g-value and the Gilbert damping constant. Experimental results are presented from frequency dependent FMR measurements on Co epitaxial films grown on Cr(001) and on films of the alloy Co1−xFex(100) deposited on MgO(001), and from recent studies on Fe(100) films grown on InAs(001). The experimental data yield clear evidence of the importance of surfaces or interfaces of the films on the magnitude of orbital and spin moment.

Shape, orientation, and crystalline composition of silver islands on Si(111)

Photoemission electron microscopy and spot profile analyzing low-energy electron diffraction have been used to study the temperature-dependent growth of Ag islands on a Si(111) surface. Depending on growth temperature, various island shapes can be formed. At low temperatures, polygonic islands are formed, consisting of both Ag(001) and Ag(111) crystal orientations. At higher temperatures, islands consist mostly of Ag(111) orientation and are predominantly of triangular shape. As the islands grow, it is possible that the crystalline composition of an island changes. We observed that Ag(001)-oriented areas convert into areas of Ag(111) orientation. The rotational orientation of the Ag islands with respect to the substrate is explained by a modified coincidence-site lattice approach.

Locally resolved ferromagnetic resonance in Co stripes

Microwave excitations of Co stripes of 100×1.5×0.025μm3 were investigated by angular dependent ferromagnetic resonance (FMR) and by locally resolved scanning thermal microscopy based (SThM) FMR, offering a lateral resolution of <100nm and a sensitivity of 106 spins. Besides the uniform excitation, backward volume modes and a rim resonance were identified by SThM-FMR imaging. Micromagnetic simulations (OOMMF) confirm the experimentally observed lateral confinement of these modes. The magnetic parameters of the Co stripes correspond to the ones of Co bulk with a surface anisotropy Ks=0.5mJ∕m2.

Imaging of ferromagnetic-resonance excitations in Permalloy nanostructures on Si using scanning near-field thermal microscopy

The investigated structures were Permalloy (Py) stripes of 3×0.3μm2 and a thickness of 24nm. Two samples with lattices of stripes of 2μm distance between the stripes and a lattice constant of 5μm are reported in this paper. One of the samples had crossed perpendicular lattices. The samples were prepared using standard lithography lift-off processes and comprised large arrays of at least 1000 stripes, such that enough material was available to perform conventional ferromagnetic-resonance (FMR) measurements. The overall anisotropy and resonance line behavior of the samples were determined by conventional FMR measurements. The origin of the FMR modes was then deduced locally resolved in single stripes using a scanning thermal microscope (SThM), which features a lateral resolution of 100nm and a temperature resolution of the order of a few millikelvins, mounted on a conventional FMR setup. The technique is based on the detection of the dissipated heat due to microwave absorption while in FMR. This setup provi...

Kerr detected time average of magnetization precession in ferromagnetic resonance

In ferromagnetic resonance (FMR), the magnetization precesses around its equilibrium orientation. A conventional magneto-optical Kerr effect (MOKE) setup, was used to detect the time average of the precession of the magnetization. The microwave power was adjusted to the linear precession regime. The different components of the high-frequency magnetization can be discriminated by the choice of the plane of incidence of the light. The use of a conventional FMR cavity setup ensures well defined homogeneous excitation of the magnetization and allows angle dependent measurements. A further benefit of the MOKE detection is the spatial resolution for FMR on a micrometer scale. The samples under investigation were (001)-Fe films on ZnSe covered by ZnSe, which were characterized by conventional angle and frequency dependent FMR as well.

Scanning thermal microwave resonance microscopy of Ni nanodots

Ni nanodots of random shape were deposited electrolytically on a mica substrate covered by Au with [111] texture. The dots had a maximum size of 260 nm in height and 150 nm in diameter. These dots were investigated by conventional and locally resolved ferromagnetic resonance (FMR). We present for the first time FMR spectra taken from a single nanodot by detection of the thermoelastic response of the microwave absorption in the dot by a scanning tunneling microscope (STM). The single Ni dots show sharp separate resonance lines, governed by the influence of shape anisotropy. We could prove that the expansion of the sample detected by the z-piezo voltage of the STM signal in the FMR line is due to the thermoelastic effect caused by heating through the absorbed microwave power.

Microwave soft x-ray microscopy for nanoscale magnetization dynamics in the 5–10 GHz frequency range

We present a scanning transmission x-ray microscopy setup combined with a novel microwave synchronization scheme for studying high frequency magnetization dynamics at synchrotron light sources. The sensitivity necessary to detect small changes in the magnetization on short time scales and nanometer spatial dimensions is achieved by combining the excitation mechanism with single photon counting electronics that is locked to the synchrotron operation frequency. Our instrument is capable of creating direct images of dynamical phenomena in the 5-10 GHz range, with high spatial resolution. When used together with circularly polarized x-rays, the above capabilities can be combined to study magnetic phenomena at microwave frequencies, such as ferromagnetic resonance (FMR) and spin waves. We demonstrate the capabilities of our technique by presenting phase resolved images of a ∼6 GHz nanoscale spin wave generated by a spin torque oscillator, as well as the uniform ferromagnetic precession with ∼0.1° amplitude at ∼9 GHz in a micrometer-sized cobalt strip.

Magnetic properties of nanolaminated (Mo0.5Mn0.5)2GaC MAX phase

The magnetic properties of hexagonal (Mo0.5Mn0.5)2GaC MAX phase synthesized as epitaxial films on MgO (111) substrates with the c-axis perpendicular to the film plane are presented. The analysis of temperature-dependent ferromagnetic resonance (FMR) and magnetometry data reveals a ferro- to paramagnetic phase transition at 220 K. The electrical transport measurements at 5 K show a negative magnetoresistance of 6% in a magnetic field of 9 T. Further analysis confirms the spin-dependent scattering of charge carriers in this layered material. A small perpendicular (c-axis) magnetocrystalline anisotropy energy density (MAE) of 4.5 kJ/m3 at 100 K was found using FMR. Accordingly, (Mo0.5Mn0.5)2GaC behaves similar to the (Cr0.5Mn0.5)2GaC MAX phase as a soft magnetic material. The density functional theory calculations reveal that the sign and the amplitude of the MAE can be very sensitive to (Mo0.5Mn0.5)2GaC lattice parameters, which may explain the measured soft magnetic properties.

Epitaxial Ag wires with a single grain boundary for electromigration

Test structures for electromigration with defined grain boundary configurations can be fabricated using focused ion beam (FIB). We present a novel approach of combining epitaxial growth of Ag islands with FIB milling. Depending on the growth parameters, bi-crystalline Ag islands can be grown on Si(111) surfaces and can be structured into wires by FIB. To avoid doping effects of the used Ga FIB, silicon on insulator (SOI) substrates are used. By cutting through the device layer of the SOI substrate with deep trenches, the Ag wire can be electrically separated from the rest of the substrate. In this way, Ag wires with one isolated grain boundary of arbitrary direction can be assembled. Using scanning electron microscopy we demonstrate the feasibility of our approach.

Template-Assisted Direct Growth of 1 Td/in2 Bit Patterned Media

We present a method for growing bit patterned magnetic recording media using directed growth of sputtered granular perpendicular magnetic recording media. The grain nucleation is templated using an epitaxial seed layer, which contains Pt pillars separated by amorphous metal oxide. The scheme enables the creation of both templated data and servo regions suitable for high density hard disk drive operation. We illustrate the importance of using a process that is both topographically and chemically driven to achieve high quality media.