Nanocavity optomechanical torque magnetometry and radiofrequency susceptometry

Abstract

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:...

Probing macroscopic quantum superpositions with nanorotors

“…The torque sensitivity of the tenth revival of a CNT (T=100 μK) is illustrated in figure 3, showing the alignment reduction due to an external torque of magnitude N ext orthogonal to the trapping laser polarization. The numerical simulations of the torque-induced dynamics, involving transitions between 360 000 angular momentum states (see Materials and Methods), show that torques on the order of 10 −30 Nm are observable, eight orders of magnitude smaller than the levitated [28,29] or solid-stateintegrated [32] setups considered so far. By attaching single elementary charges to the ends of the SNR discussed above one can measure electrostatic fields at values well below mV m −1 .…”

“…The proposed scheme requires cavity-or feedback-cooling of the nanoparticle rotation [30,31] to below a Kelvin, while it is independent of its center-of-mass temperature. An observation of orientational quantum revivals with nanorotors would substantially advance macroscopic superposition tests, provide the first experimental test of the angular momentum quantization of massive objects, and enable quantum coherent gyroscopic torque sensing with the potential of improving state-of-the-art devices [29,32] by many orders of magnitude.…”

Two-axis cavity optomechanical torque characterization of magnetic microstructures

“…Previously, the high sensitivity of optical nanocavity readouts enabled versatile magnetic torque measurements with a single mechanical mode [11][12][13]. Operation under ambient conditions facilitated adjustment of the field geometry to vary the measurements between conventional net moment hysteresis and a unique setup-one where the same component of torque would additionally manifest off-diagonal susceptibility contributions arising from microstructural inhomogeneity in the sample.…”

Fabrication and magnetic control of Y3Fe5O12 cantilevers

“…In the Einstein-de Haas effect, mechanical rotation is induced by transfer of angular momentum from magnetization to mechanical ones. To detect mechanical effects induced by spins, a cantilever structure provides one of the most suitable tools [5][6][7][8][9][10]. A cantilever is a long rigid plate of which one end is supported tightly but the other end can mount a load.…”

“…A cantilever is a long rigid plate of which one end is supported tightly but the other end can mount a load. Because of their high sensitivity [11,12], cheapness, and ease of fabrication in large areas, cantilever structures have been essential in spin mechanics [1,[5][6][7][8][9][10].…”