Conditions for the Invar effect in Fe_(1-x)A_(x) (A={rm Pt},{rm Ni})

We present a necessary condition under which a collinear ferromagnet Fe$_{1-x}A_{x}$ ($A={\rm Pt},{\rm Ni}$) with disordered face-centered-cubic structure exhibits the Invar effect. The condition involves the rate at which the fraction of Fe moments that are antiferromagnetically aligned with the magnetization fluctuates as the system is heated, $dx^{{\rm Fe}\downarrow}/dT$. Another contributing factor is the magnetostructural coupling $\kappa=-(1/V)(\partial V / \partial x^{{\rm Fe}\downarrow})_{T}$, where the volume $V(T, x^{{\rm Fe}\downarrow})$ corresponds to a homogeneous ferromagnetic state, a partially disordered local moment state, or a disordered local moment state depending on the value of $x^{{\rm Fe}\downarrow}$. According to the criterion, the Invar phenomenon occurs only when the thermal expansion arising from the temperature dependence of the fraction of Fe moments which point down $-1/3\,\kappa\,dx^{{\rm Fe}\downarrow}/dT$ compensates for the thermal expansion associated with the anharmonicity of lattice vibrations in a wide temperature interval. Upon further investigation, we provide evidence that only alloys with strong magnetostructural coupling at zero Kelvin can show the Invar effect.