COMMENSURATE-INCOMMENSURATE MAGNETIC PHASE-TRANSITIONS OF CEPTAL

The magnetic properties of CePtAl with orthorhombic TiNiSi-type chemical structure were investigated. Single-crystal experiments of electrical resistivity, specific heat and magnetic susceptibility reveal the existence of three magnetic phase transitions at T-1 = 5.9 K (second order), T-2 = 4.3 K (first order) and T-3 = 2.2 K (first order). Powder neutron diffraction measurements detected complex magnetic structures consisting of two coexisting, partly incommensurate magnetic propagation vectors: k(1) = 0 and k(3) = [0,0.463(8), 0] for T-2 less than or equal to T less than or equal to T-1; and k(1) = 0 and k(2) = [0, 1/2, 0] for T less than or equal to T-2. The predominantly ferromagnetic k(1) component and the antiferromagnetic k(2) component of the magnetic low-temperature structure of CePtAl were refined at 1.5 K. Magnetism in CePtAl is dominated by a ferromagnetic component (1.35(4)mu(B) at 1.5 K) oriented parallel to the magnetically easy a axis. The complexity of magnetic structures in CePtAl appears to be caused by competing anisotropic magnetic exchange interactions in a network, where Ce-Ce nearest neighbours (3.56 Angstrom) form chains parallel to the b axis and Ce-Ce second-nearest neighbours (3.77 Angstrom) form chains along the a axis.