Crystal Acts Like a Two‐Dimensional Antiferromagnet

A single crystal of $K2NiF4$ acts like a two‐dimensional antiferromagnet above a critical temperature $Tn = 97.1 K,$ according to a recent experiment at Brookhaven. At the critical temperature the crystal undergoes an extremely sharp phase transition to long‐range order in three dimensions. Then below that temperature the magnetization varies as $(T−Tn)β$ where $β = 0.15,$ again suggesting that the magnetic behavior is two dimensional.