Ultra-high sensitivity magnetic field and magnetization measurements with an atomic magnetometer

We describe an ultra-sensitive atomic magnetometer using optically-pumped potassium atoms operating in spin-exchange relaxation free (SERF) regime. We demonstrate magnetic field sensitivity of 160 aT/Hz${}^{1\mathrm{/}2}$ in a gradiometer arrangement with a measurement volume of 0.45 cm${}^3$ and energy resolution per unit time of $44\mathrm{\hslash }$. As an example of a new application enabled by such a magnetometer we describe measurements of weak remnant rock magnetization as a function of temperature with a sensitivity on the order of 10${}^{-10}$ emu/cm${}^3$/Hz${}^{1\mathrm{/}2}$ and temperatures up to 420${}^{\circ }$C.