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.
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 f as 1/f 2 , 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.In a spin torque oscillator (STO), dc current transfers spin angular momentum from a thick "fixed" ferromagnetic layer to a thin "free" ferromagnetic layer. For sufficient current in the appropriate direction, the spin torque counteracts the intrinsic damping torque in the free layer, giving rise to coherent oscillations above a threshold current. As the magnetization precesses along a stable trajectory it produces an oscillating voltage, which in all-metallic devices is due to the giant magnetoresistance effect. The STO frequency can be tuned over a wide range by varying the dc current and a (static) applied magnetic field. Because of their small size (∼ 100 nm), frequency agility, and compatibility with silicon CMOS processing, STOs may be used for applications such as mixing and active phase control in integrated microwave circuits. Further details about spin transfer torque and STOs can be found in recent reviews [1, 2].STOs differ from classical electronic oscillators in several ways, but most important is the essential dependence of frequency on oscillation amplitude [3]. Because of this, an STO cannot be described by standard circuit models comprising a linear resonator and a feedback amplifier. In particular, Leeson's model for phase noise [4] does not apply to STOs. Although oscillator properties such as frequency modulation [5] and phase locking [6] have been demonstrated in STOs, phase noise has not been directly addressed by previous experiments. The time domain measurement of phase is particularly important because all other quantities that characterize the precision of an oscillator can be derived from it [7]. When the output voltage of an STO is measured in the frequency domain using a spectrum analyzer, the linewidth is determined by both phase noise and amplitude noise. Although a recent time domain study [8] provided direct evidence for effects previously inferred from frequency domain data, it did not address phase noise. In this paper we report measurements of phase vs. time for an STO, compare the linewidth due to phase noise alone with that measured using a spectrum analyzer, and demonstrate the reduction of STO phase noise using a phaselocked loop.We studied a nanocontact device from the same batch as those used for a previous study [9]. The magnetic layer structure is Ta (3 nm) / Cu (15 nm) / Co 90 Fe 10 (20 nm) / Cu (4 nm) / Ni 80 Fe 20 (5 nm) and the region of electrical contact to the layers is a circle of 60 nm nominal diameter. As described previously [9,10], spin torque can excite oscillations in these device...
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.
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 voltage distribution measurements) yields incorrect values for the time-scales (<1μs) at which the experiments are performed. Macrospin simulations confirm our findings that the model must be applied only in the low voltage limit, and that in certain devices this limit can extend up to about 0.9 of the intrinsic switching voltage.
We describe the apparatus, software, and measurement procedures for a pulsed inductive microwave magnetometer ͑PIMM͒. 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.
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