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In this work we investigate the magnetic field dependence of the precession frequency of vortex states in spin-valve nanocontacts with an amorphous CoFeB free layer and an artificial antiferromagnet as polarizer. The nanocontacts have radii between 70 and 90 nm. We show that the excitation frequency in these devices responds to small, in-plane magnetic fields along the easy and hard axis directions. The characteristics of the frequency response depend on the generated magnetic configuration under the nanocontact. This, in turn, results from the combined effect of the applied magnetic field and the current-generated Oersted field. Taking also into account the relative large nanocontact radii, a variety of vortex excitation modes can arise with distinctive frequency versus field responses, some of which could be considered for magnetic field sensing applications. [ABSTRACT FROM PUBLISHER]

This paper presents a small microcontroller-based sine-wave generator for probe signals of a wide range of amplitudes and frequencies, which requires very little energy so that it can be used for wireless applications in biosensing. The presented realization generates a 10-kHz sine wave with a 100-mV peak-to-peak voltage. Its total harmonic distortion is less than 0.15%, and the spurious free dynamic range excluding harmonics is about 85 dB. The generator's total power consumption at a 3.3-V supply voltage and without output buffers is less than 3.5 mW. This value includes the power requirement of the microcontroller's internal 8-MHz clock generator and is below the power needs of current direct-digital-synthesis-based signal generators. The presented concept bases on commercially available electronic components and is implemented for probe signal generation in wireless batteryless tags for long-term impedance measurements on standard microbiological cell cultures. The included microcontroller enables the use of a phase-controlled trigger signal and other simultaneous control functions, e.g., for peripheral components and data communication. [ABSTRACT FROM AUTHOR]