  The nonreciprocal magnetoelectric effect, also known as the Tellegen effect,
promises a number of groundbreaking phenomena connected to fundamental (e.g.,
electrodynamics of axion and relativistic matter) and applied physics (e.g.,
magnetless isolators). We propose a three-dimensional metamaterial with an
isotropic and resonant Tellegen response in the visible frequency range. The
metamaterial is formed by randomly oriented bi-material nanocylinders in a host
medium. Each nanocylinder consists of a ferromagnet in a single-domain magnetic
state and a high-permittivity dielectric operating near the magnetic Mie-type
resonance. The proposed metamaterial requires no external magnetic bias and
operates on the spontaneous magnetization of the nanocylinders. By leveraging
the emerging magnetic Weyl semimetals, we further show how a giant bulk
effective magnetoelectric effect can be achieved in a proposed metamaterial,
exceeding that of natural materials by almost four orders of magnitude.
