Critical phenomena at the antiferromagnetic transition in MnO

The specific heat of a polycrystalline sample of MnO was measured from T ap proximate to 1 K to T approximate to 400 K using two different experimental apparatuses at zero applied pressure. Features revealed by the data includ e a hyperfine contribution due to the Mn nuclei, a T-2 temperature dependen ce at low temperatures due to the type-II antiferromagnetic magnon contribu tion, and a sharp but well defined antiferromagnetic transition (T-N = 117. 7095 K) that is clearly second order in nature. The critical exponent cu de duced from the transition is consistent with a two-dimensional Ising model. The specific heat of MnO is also compared with recent results on the type- A antiferromagnet LaMnO3. [S0163-1829(99)08533-1].