  Interfacial perpendicular magnetic anisotropy, which is characterized by the
first-order (K1) and second-order (K2) anisotropies, is the core phenomenon for
nonvolatile magnetic devices. A sizable K2 satisfying a specific condition
stabilizes the easy-cone state, where equilibrium magnetization forms at an
angle from the film normal. The easy-cone state offers intriguing possibilities
for advanced spintronic devices and unique spin textures, such as spin
superfluids and easy-cone domain walls. Experimental realization of the
easy-cone state requires understanding the origin of K2, thereby enhancing K2.
However, previously proposed origins of K2 cannot fully account for
experimental results. Here we show experimentally that K2 scales almost
linearly with the work-function difference between the Co and X layers in
Pt/Co/X heterostructures (X = Pd, Cu, Pt, Mo, Ru, W, and Ta), suggesting the
central role of the inversion asymmetry in K2. Our result provides a guideline
for enhancing K2 and realizing magnetic applications based on the easy-cone
state.
