Three-dimensional Ising behavior of antiferromagnetic Bi2CuO4

Abstract

Bi2CuO4 possesses a unique crystal structure that distinguishes it from other cuprates. It is constructed from isolated CuO4 plaquettes forming staggered chains along the c axis of its tetragonal unit cell. Several aspects of antiferromagnetism in this system, observed below T-N similar to 43 K, are poorly understood, including the orientation of the ordered magnetic moments. There is disagreement about whether Bi2CuO4 exhibits easy-axis or easy-plane anisotropy despite numerous attempts to settle the question. In order to better understand its magnetism, thermal expansion and heat capacity were measured on a single crystal of Bi2CuO4, and the critical behavior exhibited about T-N in those measurements was carefully studied. Best-fit values for the critical exponent alpha and the ratio of the leading singularity amplitudes A(+)/A(-) from thermal-expansion data along both a and c are consistent with the three-dimensional Ising universality class. Assignment of the Ising universality class suggests Bi2CuO4 is an easy-axis antiferromagnet with moments ordered along c.