Pavel Kabos

Control of magnetization dynamics in Ni/sub 81/Fe/sub 19/ thin films through the use of rare-earth dopants

We show that the magnetization dynamics of soft ferromagnetic thin films can be tuned using rare-earth (RE) dopants. Low concentrations (2 to 10%) of Tb in 50 nm Ni/sub 81/Fe/sub 19/ films are found to increase the Gilbert magnetic damping parameter /spl alpha/ over two orders of magnitude without great effect on easy axis coercivity or saturation magnetization. Comparison with Gd dopants indicates that the orbital character of the Tb moment is important for transferring magnetic energy to the lattice. Structural transformations from the crystalline to the amorphous state, observed over the first 2%-10% of RE doping, may play a contributing but not sufficient role in damping in these films. The approach demonstrated here shows promise for adjusting the dynamical response, from underdamped to critically damped, in thin film materials for magnetic devices.

Calibrated nanoscale dopant profiling using a scanning microwave microscope

The scanning microwave microscope is used for calibrated capacitance spectroscopy and spatially resolved dopant profiling measurements. It consists of an atomic force microscope combined with a vector network analyzer operating between 1–20 GHz. On silicon semiconductor calibration samples with doping concentrations ranging from 1015 to 1020 atoms/cm3, calibrated capacitance-voltage curves as well as derivative dC/dV curves were acquired. The change of the capacitance and the dC/dV signal is directly related to the dopant concentration allowing for quantitative dopant profiling. The method was tested on various samples with known dopant concentration and the resolution of dopant profiling determined to 20% while the absolute accuracy is within an order of magnitude. Using a modeling approach the dopant profiling calibration curves were analyzed with respect to varying tip diameter and oxide thickness allowing for improvements of the calibration accuracy. Bipolar samples were investigated and nano-scale de...

Frequency-selective contrast on variably doped p-type silicon with a scanning microwave microscope

We report on frequency-dependent contrast in d(S11)/dV measurements of a variably doped p-type silicon sample in the frequency range from 2 GHz to 18 GHz. The measurements were conducted with a scanning microwave microscope. The measurements were done at selected frequencies while varying the DC tip voltage. The measured d(S11)/dV signal shows a maximum for doping concentrations (NA) of 1015 cm−3−1016 cm−3 at 2.3 GHz. As the microscope operating frequency is increased, this maximum sequentially “switches” through the regions of increasing dopant concentration, displaying a maximum for NA of 1017 cm−3−1018 cm−3 at 17.9 GHz. The frequency dependent “switching” is attributed to the physics of tip-to-sample interaction, particularly as related to the frequency-dependent local surface resistance and the depletion capacitance that control the RC time constant of tip-to-sample interaction. This provides a unique platform for local, frequency-selective, spatially resolved microwave spectroscopy of semiconducting ...

Angle dependence of the ferromagnetic resonance linewidth and two magnon losses in pulsed laser deposited films of yttrium iron garnet, MnZn ferrite, and NiZn ferrite

Shorted waveguide ferromagnetic resonance (FMR) measurements were made at 9.5 GHz for pulsed laser deposited yttrium iron garnet (YIG), MnZn ferrite, and NiZn ferrite films with thicknesses of 1.8, 0.5, and 1.5 μm, respectively. The FMR field versus the field angle confirmed the operational assumption of a uniform mode response. For the YIG, the linewidth was 10 Oe when the external field and magnetization vectors were perpendicular to the disk (perpendicular FMR) and increased smoothly to a maximum value of 27 Oe when the field and magnetization were in the film plane (parallel FMR). For the MnZn, the linewidth was 49 Oe at perpendicular FMR, increased with the angle between the film normal and the external field to a broad maximum of 80 Oe at 35°, and then dropped to 65 Oe at parallel FMR. For the NiZn, the linewidth was 310 Oe at perpendicular FMR, increased with the field angle to a broad maximum of 1530 Oe at 45°, and then dropped to 960 Oe at parallel FMR. The linewidths were larger than predicted f...

High frequency measurements of CoFeHfO thin films

High-frequency measurements of the transverse susceptibility and damping constant of CoFeHfO thin films have been made over a frequency range of 0.1 GHz to 6 GHz as a function of film resistivity, thickness, and temperature. The film resistivity varied from 250 /spl mu//spl Omega/cm to 2100 /spl mu//spl Omega/cm. The films show relatively low damping at high frequencies with the damping constant /spl alpha/ ranging from 0.01 to 0.06. The damping constant increases with film resistivity and, for the highest resistivity films, the damping constant decreases as the thickness increases. The damping constant, induced anisotropy, and film resistivity show weak temperature dependence over a temperature range from 4 K to 300 K. The low damping constant, in conjunction with the high anisotropy, large resistivity, and large spin-dependent-tunneling magnetoresistance, makes this material attractive for several high-frequency magnetic device applications.

Phase profiles of microwave magnetic envelope solitons

A new method for the analysis of microwave magnetic envelope (MME) solitons has been developed. This method is based on the determination and analysis of output microwave pulse phase profiles. Simple analytical results based on the nonlinear Schrodinger equation show that MME soliton phase profiles contain the necessary and sufficient information needed to define a particular pulse as a linear dispersive pulse or a fully formed soliton. The effects are demonstrated both theoretically and experimentally for magnetostatic backward volume wave and magnetostatic surface wave pulse signals. Theoretical phase profiles are considered for Gaussian, hyperbolic secant, and rectangular pulse shapes. Experimental profiles are obtained for rectangular input pulses. The measured phase profiles compare favorably with the numerical results. Both the data and the theory show that a constant phase profile across the pulse provides a consistent and quantitative criterion for an MME soliton.

A Framework for Broadband Characterization of Individual Nanowires

A framework for broadband characterization of individual nanowires (NWs) is discussed in this letter. Specifically, on-wafer multiline thru-reflect-line (TRL) measurements, finite element modeling, and specially fabricated test structures with both extremely high and low impedances are jointly used to validate the feasibility of both measurements and modeling for characterizing small components. The test structures are designed as coplanar waveguide (CPW) devices with 100 nm and 250 nm diameter platinum (Pt) NWs. Though it is not possible to distinguish between the conductivity of the wire and contact resistances, we determine a range for conductivity and contact resistance over wide microwave bandwidth by minimizing the standard deviation between the measurements and full-wave modeling.

High‐field effective linewidth and eddy current losses in moderate conductivity single‐crystal M‐type barium hexagonal ferrite disks at 10–60 GHz

The losses associated with the high‐field tail region of the ferromagnetic resonance (FMR) absorption curve were investigated at 10, 19, 35, and 60 GHz for 0.10–1.75‐mm‐thick c‐plane circular disks of flux‐grown single‐crystal M‐type barium ferrite materials. A conventional high‐field effective linewidth analysis of the data yielded an effective linewidth which increased with the square of the disk thickness and linearly with frequency, dependencies which indicate a predominant eddy current loss process. Based on these results, an eddy current loss analysis of the tail region was done, based on the insulator FMR response and eddy current losses driven by the FMR response. This analysis leads to a new noninvasive technique for the determination of the microwave conductivity in moderate conductivity ferrites. One obtains the conductivity from an appropriate analysis of the FMR absorption tail in the same way that analysis of the magnetic loss tail yields a high‐field effective linewidth. Based on this techn...

A wave‐vector selective light scattering magnon spectrometer

A new type of magnon spectrometer has been developed, based on inelastic light scattering from magnetic excitations. This spectrometer enables one to study magnetic excitations of a specific frequency and wave vector as a function of static field, microwave power, temperature, or any other external experimental parameter. The sample is excited by an applied microwave field and simultaneously probed by laser light. The inelastically scattered light corresponding to a particular magnon wave vector k is selected and its intensity at a specific frequency ωk is recorded. The intensity of this light is a direct measure of the strength of that particular magnetic excitation at k and ωk. Spectra taken in this way present a direct and localized measurement of specific, selectable magnons, in contrast with standard microwave measurements which give only a measure of the total response of all excited modes over the entire sample.

Active magnetostatic wave delay line

Two configurations for an active microwave delay line with feedback to improve the loss per unit delay time have been made and characterized. The lines utilized thin yttrium iron garnet (YIG) film, magnetostatic wave (MSW) passive elements, a microwave amplifier, and a variable attenuator connected in a feedback loop. The two delay line setups used a surface wave (MSSW) configuration with a single MG element and a nondispersive configuration with MSSW and backward volume wave (MSBVW) delay lines in series. For each setup, a sequence of delayed pulses at the output is observed when a single microwave pulse is fed to the input. The one-pass delay time T of the pulse through the MSW delay line and the number n of circulations in the feedback loop defines the total delay time nT for the nth pulse. The feedback resulted in a reduction in the output pulse decay rate and longer useful delay times. For a passive delay line with no feedback, typical values of the loss p.u. delay time L and useful delay time (nT)/sub U/ were -110 dB//spl mu/s and 0.1-0.2 /spl mu/s, respectively. With feedback for the MSSW delay line, a 4.5-GHz operating frequency, and 40 ns input pulses, L and (nT)/sub U/ were -19 dB//spl mu/s and 0.6 /spl mu/s, respectively. Feedback with the nondispersive delay line gave L and (nT)/sub U/ values of -12 dB//spl mu/s and 3 /spl mu/S, respectively, for 4.5-GHz, 40-ns input pulses.