  We investigate the dynamics of ion crystals in zigzag configuration in
transverse magnetic field gradients. A surface-electrode Paul trap is employed
to trap $^{40}$Ca$^+$ ions and features submerged wires to generate magnetic
field gradients of up to 16.3(9) T/m at the ions position. With the gradient
aligned in the direction perpendicular to the axis of weakest confinement,
along which linear ion crystals are formed, we demonstrate magnetic field
gradient induced coupling between the spin and ion motion. For crystals of
three ions across their linear-to-zigzag structural transition we perform
sideband spectroscopy upon directly driving the spins with a radiofrequency
field. Furthermore, we observe the rich excitation spectrum of vibrational
modes in a planar crystal comprised of four ions.
