The self-generated magnetic field coupled with the growth of multimode ablative Rayleigh–Taylor instability (ARTI) is investigated by numerical simulations with Nernst effects, resistivity and magnetized heat conduction taken into account. It is found that intense magnetic fields about 1 kT can be generated in multimode ARTI via the Biermann-battery source and the Nernst amplification. The enhancement of the self-similar growing coefficient αb caused by self-generated magnetic fields is attributed to the reinforcement of the vorticity inside the bubbles. It is also found that αb is enhanced more significantly in cases where the perturbation spectra contain more short-wavelength components. The formula for the dependence of αb on the initial perturbation amplitude proposed in Zhang et al (2018 Phys. Rev. Lett.121 185002) remains applicable, but requires a modified coefficient to account for the attenuation of ablative stabilization due to self-generated magnetic fields.