  The negatively charged nitrogen-vacancy (NV) center in nano- or micro-
diamonds has emerged as a promising magnetic field sensor, as a candidate for
hyper-polarizing paramagnetic species, as well as a tool for spin-mechanics at
the nanoscale. However, NV-doped diamonds are presently not straightforwardly
employable for these applications in a liquid or when levitating under
atmospheric pressures due to the random angular Brownian motion which tends to
rotate the NV quantization axis over the course of the measurments. Here, we
report on angle locking of the crystalline axis of a trapped micro-diamond
along an external magnetic field. Specifically, we use spin population
inversion after a ground state level crossing of the NV center to turn the
diamond into a diamagnet. The diamond crystalline axis naturally aligns to the
magnetic field with high precision and in the absence of micro-wave, offering
bright prospects for applications in biology and spin-mechanical platforms.
