Transverse susceptibility of Ising spins

The ac vector magnetic susceptibility of the Ising magnet LiHo$_x$Y$_{1-x}$F$_4$ is studied as a function of frequency, excitation amplitude, transverse field and temperature. In the spin liquid ($x=4.5$\%), application of a dc magnetic field projects the linear susceptibility transverse to the Ising axis. In addition, pumping the system along the Ising axis produces non-linear excitations in the transverse plane; these excitations can encode information and are potentially useful for quantum computation. In the ferromagnet ($x=65$\%), measurements of the transverse susceptibility are used to probe the dynamics of domain wall tunneling and to estimate the mass of the domain walls.