Two-dimensional surface magnetism in the bulk paramagnetic intermetallic alloy CoAl(100).

Utilizing a combination of the in situ magneto-optical Kerr effect and scanning tunneling microscopy and spectroscopy measurements, we show that the (100) surface of the B2 bulk paramagnetic CoAl is an excellent representation of a two-dimensional ferromagnet. The order-parameter critical exponent beta=0.22+/-0.02 is determined, which is the universal signature of a finite-size two-dimensional XY behavior. The Curie temperature is found to be T(c)=90 K. The magnetism can be explained by the appearance of Co antisite atoms at the surface.