  The magnetic properties of the weak itinerant ferromagnet ZrZn_2 are analyzed
using Landau theory based on a comparison of density functional calculations
and experimental data as a function of field and pressure. We find that the
magnetic properties are strongly affected by the nearby quantum critical point,
even at zero pressure; LDA calculations neglecting quantum critical spin
fluctuations overestimate the magnetization by a factor of approximately three.
Using renormalized Landau theory, we extract pressure dependence of the
fluctuation amplitude. It appears that a simple scaling based on the
fluctuation-dissipation theorem provides a good description of this pressure
dependence.
