Anthony B. Kos

Nanoscale elastic-property measurements and mapping using atomic force acoustic microscopy methods

We describe a dynamic atomic force microscopy (AFM) method for measuring the elastic properties of surfaces, thin films and nanostructures at the nanoscale. Our approach is based on atomic force acoustic microscopy (AFAM) techniques and involves the resonant modes of the AFM cantilever in contact mode. From the frequencies of the resonant modes, the tip–sample contact stiffness k* can be calculated. Values for elastic properties such as the indentation modulus M can be determined from k* with appropriate contact-mechanics models. We present the basic principles of AFAM and explain how it can be used to measure local elastic properties with a lateral spatial resolution of tens of nanometres. Quantitative results for a variety of films as thin as 50 nm are given to illustrate our methods. Studies related to measurement accuracy involving the effects of film thickness and tip wear are also described. Finally, we discuss the design and use of electronics to track the contact-resonance frequency. This extension of AFAM fixed-position methods will enable rapid quantitative imaging of nanoscale elastic properties.

Time domain measurement of phase noise in a spin torque oscillator

We measure oscillator phase from the zero crossings of the voltage vs. time waveform of a spin torque nanocontact oscillating in a vortex mode. The power spectrum of the phase noise varies with Fourier frequency

Nanomechanical mapping with resonance tracking scanned probe microscope

f

Pulsed inductive microwave magnetometer

as

Mapping substrate/film adhesion with contact-resonance-frequency atomic force microscopy

1/f^2

Validity of the thermal activation model for spin-transfer torque switching in magnetic tunnel junctionsa)

, consistent with frequency fluctuations driven by a thermal source. The linewidth implied by phase noise alone is about 70 % of that measured using a spectrum analyzer. A phase-locked loop reduces the phase noise for frequencies within its 3 MHz bandwidth.

Large-angle magnetization dynamics measured by time-resolved ferromagnetic resonance

We present a new digital-signal-processor-based resonance tracking system for scanned probe microscopy (SPM) imaging. The system was developed to enable quantitative imaging of mechanical properties with nanoscale spatial resolution at practical data acquisition rates. It consists of a 32-bit floating-point digital signal processor connected to a high-resolution audio coder/decoder subsystem, an rms-to-dc converter and a voltage-controlled oscillator. These components are used in conjunction with a commercial atomic force microscope to create a versatile platform for SPM mechanical mapping. Images of a glass-fibre/polymer matrix composite sample are presented to demonstrate system performance.

Dynamic anisotropy of thin Permalloy films measured by use of angle-resolved pulsed inductive microwave magnetometry

We describe the apparatus, software, and measurement procedures for a pulsed inductive microwave magnetometer (PIMM). PIMM can measure the dynamical properties of materials used in recording heads for magnetic storage applications, and it can be used as a general magnetodynamics diagnostic tool. PIMM uses a coplanar waveguide as both a source of fast pulsed magnetic fields and as an inductive flux sensor.Magnetic field pulses are provided by a 10 V, 55 ps risetime pulse generator; a 20 GHz digital sampling oscilloscope is used to acquire the fast pulse data; and orthogonal Helmholtz pairs provide the bias and saturating fields required for the measurement. The system can measure dynamical behavior as a function of several variables, including applied magnetic bias field, magnetic pulsed field amplitude and width, and sample orientation. Using a fast Fourier transform, PIMM can determine the frequency dependence of the complex magnetic permeability, as well as the step and impulse responses of the magnetic system. Data from 50 nm Ni–Fe and rare-earth-doped Ni–Fe thin films are presented.

Finite coplanar waveguide width effects in pulsed inductive microwave magnetometry

We have used contact-resonance-frequency atomic force microscopy techniques to nondestructively image variations in adhesion at a buried interface. Images were acquired on a sample containing a 20nm gold (Au) blanket film on silicon (Si) with a 1nm patterned interlayer of titanium (Ti). This design produced regions of very weak adhesion (Si∕Au) and regions of strong adhesion (Si∕Ti∕Au). Values of the contact stiffness were 5% lower in the regions of weak adhesion. The observed behavior is consistent with theoretical predictions for layered systems with disbonds. Our results represent progress towards quantitative measurement of adhesion parameters on the nanoscale.

Gyromagnetic damping and the role of spin-wave generation in pulsed inductive microwave magnetometry

We have performed spin-transfer torque switching experiments with a large number of trials (up to 107 switching events) on nanoscale MgO magnetic tunnel junctions in order to test the validity and the limits of the thermal activation model for spin-torque-assisted switching. Three different methods derived from the model (“read disturb rate,” “switching voltage versus pulse duration,” and “switching voltage distribution” measurements) are used to determine the thermal stability factor and the intrinsic switching voltage. The results obtained from the first two methods agree well with each other as well as with values obtained from quasistatic measurements, if we use only the data for which the voltage is smaller than approximately 0.8 of the intrinsic switching voltage. This agreement also shows that, in our samples, in the low voltage region, the influence from other factors contributing to the switching (such as current-induced heating and field-like torque) is negligible. The third method (switching vo...