Interfacial perpendicular magnetic anisotropy, which is characterized by first-order (K₁) and second-order (K₂) anisotropy, 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, the previously proposed origins of K₂ cannot fully account for the experimental results. Here, we experimentally show 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.

以一阶（K ₁）和二阶（K ₂）各向异性为特征的界面垂直磁各向异性是非易失性磁性器件的核心现象。满足特定条件的较大的K ₂稳定易锥状态，在该状态下，与膜法线成一定角度形成平衡磁化强度。易锥状态为先进的自旋电子器件和独特的自旋纹理（如自旋超流体和易锥畴壁）提供了诱人的可能性。通过实验实现易锥状态需要了解K ₂的起源，从而增强K ₂。但是，以前建议的起源K ₂不能完全说明实验结果。在这里，我们通过实验证明，K ₂随Pt / Co / X异质结构中的Co和X层之间的功函数差（X = Pd，Cu，Pt，Mo，Ru，W和Ta）几乎成线性比例变化，表明反演不对称在K _2中的重要作用。我们的结果为基于易锥状态增强K ₂和实现磁性应用提供了指导。