Nanophotonic optomechanical devices allow observation of nanoscale vibrations with sensitivity that has dramatically advanced metrology of nanomechanical structures [1][2][3][4][5][6][7][8][9] and has the potential to impact studies of nanoscale physical systems in a similar manner [10,11]. Here we demonstrate this potential with a nanophotonic optomechanical torque magnetometer and radiofrequency (RF) magnetic susceptometer. Exquisite readout sensitivity provided by a nanocavity integrated within a torsional nanomechanical resonator enables observations of the unique net magnetization and RF-driven responses of single mesoscopic magnetic structures in ambient conditions. The magnetic moment resolution is sufficient for observation of Barkhausen steps in the magnetic hysteresis of a lithographically patterned permalloy island [12]. In addition, significantly enhanced RF susceptibility is found over narrow field ranges and attributed to thermally assisted driven hopping of a magnetic vortex core between neighboring pinning sites [13]. The on-chip magneto-susceptometer scheme offers a promising path to powerful integrated cavity optomechanical devices for quantitative characterization of magnetic micro-and nanosystems in science and technology.Torque magnetometry has seen recent resurgence owing to miniaturization of mechanical devices [14]. The high detection sensitivity of resonant nanomechanical torque sensors has allowed for minimally-invasive observations of magnetostatic interactions and hysteresis in a variety of magnetic materials including thin films [15], mesoscale confined geometries that are deposited [16] or epitaxially grown [17], and small aggregates of nanoparticles [18]. Going beyond the static limit, nanomechanical torque magnetometry has been extended to timescales allowing for detection of slow thermally-activated dynamics [12], AC susceptibility [17], and magnetic resonance [19,20].This powerful technique relies upon detection of the deflection of a mechanical element by angular momentum transfer originating from magnetic torques τ = µ 0 m×H, generated as the magnetic moments in the system, m, experience an orthogonally-directed component of the applied magnetic field, H. So far, improvements to torque magnetometers have been driven primarily by enhancements to the response of nanomechanical resonators resulting from their low mass and high mechanical quality factor (Q m ). Readout of magnetically driven motion has involved detection through free-space optical interferometric methods with very low optical quality factor (Q o ≈ 1) Fabry-Perot cavities formed between the nanomechanical resonator its supporting substrate [16]. However, as device dimensions scale down, and the number of magnetic spins become too small or the dynamics too fast, the mechanical deflections become more difficult to detect. Migration to a more sensitive readout scheme is essential. The integration of a nanoscale optical cavity offers a natural path for improvement. Nanocavity-optomechanical devices enhance mechanarXiv:...

A universal, torque-mixing method for magnetic resonance spectroscopy is presented. In analogy to resonance detection by magnetic induction, the transverse component of a precessing dipole moment can be measured in sensitive broadband spectroscopy, here using a resonant mechanical torque sensor. Unlike induction, the torque amplitude allows equilibrium magnetic properties to be monitored simultaneously with the spin dynamics. Comprehensive electron spin resonance spectra of a single-crystal, mesoscopic yttrium iron garnet disk at room temperature reveal assisted switching between magnetization states and mode-dependent spin resonance interactions with nanoscale surface imperfections. The rich detail allows analysis of even complex three-dimensional spin textures. The flexibility of microelectromechanical and optomechanical devices combined with broad generality and capabilities of torque-mixing magnetic resonance spectroscopy offers great opportunities for development of integrated devices.

Nanoelectromechanical systems could have applications in fields as diverse as ultrasensitive mass detection and mechanical computation, and can also be used to explore fundamental phenomena such as quantized heat conductance and quantum-limited displacement. Most nanomechanical studies to date have been performed in the frequency domain. However, applications in computation and information storage will require transient excitation and high-speed time-domain operation of nanomechanical systems. Here we show a time-resolved optical approach to the transduction of ultrahigh-frequency nanoelectromechanical systems, and demonstrate that coherent control of nanomechanical oscillation is possible through appropriate pulse programming. A series of cantilevers with resonant frequencies ranging from less than 10 MHz to over 1 GHz are characterized using the same pulse parameters.

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