We apply symmetry considerations to study the possible superconducting order parameters in ferromagnetic ${\mathrm{ZrZn}}_2.$ We predict that the presence and the location of the superconducting gap nodes depend on the direction of magnetization $\mathit{M}.$ In particular, if $\mathit{M}$ is directed along the z axis, then the order parameter should always have zeros. We also discuss how to determine the gap symmetry in ${\mathrm{ZrZn}}_2$ using ultrasound attenuation measurements.

• Received 15 March 2002

DOI:https://doi.org/10.1103/PhysRevB.66.024512