Magnetization plateau of two-dimensional antiferromagnetic solid 3He on a triangular lattice.
3He film adsorbed on a graphite surface offers an ideal two-dimensional antiferromagnetic S=1/2 quantum spin system on a triangular lattice. In addition, competition between various multiple spin exchange interactions makes it strongly frustrated. The ground state in such a frustrated quantum spin system is one of the most interesting issues in condensed matter physics. Recent experiments on the antiferromagnetic solid 3He in the second layer indicate the ground state is a spin liquid, whereas there still exists a controversy on whether or not the spin gap is finite. Here we report the first precise magnetization measurement below 1 mK over the wide magnetic field range up to 11 T. The magnetization curve is found to show a plateau at half of the saturation magnetization, being followed by the full saturation at the unexpectedly high field of around 10 T.