Real magnets usually do not have a continuous symmetry, since the spin-orbit coupling of the electrons imposes an anisotropy. For atomic systems like graphene, one can show that monolayers of cosmological (or at least continental) size are necessary to measure a significant size of the amplitudes of fluctuations. A recent discussion about the Hohenberg–Mermin–Wagner theorems and its limitations in the thermodynamic limit is given by Bertrand Halperin. More recently, it was shown that the most severe physical limitation are finite-size effects in 2D, because the suppression due to infrared fluctuations is only logarithmic in the size: The sample would have to be larger than the observable universe for a 2D superconducting transition to be suppressed below ~100 K. For magnetism, there is a similar behaviour where the sample size must approach the size of the universe to have a Curie temperature T c in the mK range. However, because disorder and interlayer coupling compete with finite-size effects at restoring order, it cannot be said a priori which of them is responsible for the observation of magnetic ordering in a given 2D sample.