Laser control of graphite plate tilting on a magnet surface

When a diamagnetic graphite plate is placed on top of a magnet, it is subject to both magnetic levitation and gravitational forces. Computational modeling is used to demonstrate that these forces can balance each other to allow a stable tilt angle θ to be attained when the graphite plate pivots on the magnet surface. θ depends on the magnetic susceptibility χmag, which can be controlled by using laser irradiation to change the temperature of the graphite. Over a limited temperature range, there is a linear relation between the angle change Δθ and the temperature change ΔT. This predicted behavior is confirmed experimentally using a graphite plate (5 × 5 × 0.83 mm) placed on top of a 7.5 cm diameter neodymium iron boride magnet with a surface field strength of ∼5000 G. Using a 532 nm laser, θ could be continuously tuned from 52° to 26° by varying the laser power between 20 and 800 mW. The response time was about 15 s at all powers. Higher laser powers were observed to lead to a complete loss of levitation, followed by recovery and oscillation in and out of the laser beam. The linear dependence of Δθ on power suggests that photothermal tilting of the graphite plate in a magnetic field provides a fatigue resistant way to achieve large angular deviations using an external light source. This effect can be harnessed to function as a laser-controlled beam steering device.