Magnetic and superconducting properties of holmium-rich (Er1-xHox)Rh4B4

Neutron scattering, susceptibility, and resistivity measurements have been carried out on (Er1-xHox)Rh4B4 for concentrations x=1.0, 0.89, 0.84, and 0.75, which span the multicritical point (xc0.9) where the magnetic and superconducting phase boundaries meet. For all concentrations studied a transition to long-range ferromagnetic order is observed, with the Ho moments ordering along the tetragonal axis. The spins are in fact locked along the c axis by strong crystal-field anisotropies, yielding Ising-like magnetic behavior in this concentration region. The temperature dependence of the order parameter is mean-field-like, suggesting that the range of the magnetic interactions is large, and we attribute this behavior to the importance of dipolar interactions. In the concentration range x<xc where superconductivity is observed, the magnetic transition appears to be continuous and reversible, and occurs within the nominal superconducting interval Tc2(x)<T<Tc1(x). Thus there is some kind of coexistence of long-range magnetic order and superconductivity in the samples, but the nature of this coexistence cannot be determined unambiguously. In particular the experimental evidence in this system is not sufficient to decide if there is true microscopic coexistence, or whether some portions of the sample are ferromagnetic and normal while other regions are superconducting and paramagnetic (or magnetically ordered with a very-long-wavelength sinusoidal periodicity). Below TM strongly-temperature-dependent small-angle scattering is observed, characterized by a cross section of the Porod form S(Q)=A(T)/Q4, where A(T) increases monotonically with decreasing temperature. This wave-vector dependence is consistent with the assumption that the scattering originates from domain walls whose widths are large compared to the inverse-Q range explored (300 A). In particular, no oscillatory component to the magnetization was observed for any concentration or temperature within the experimentally accessible wave-vector range of Q>0.003 A-1, in contrast to the behavior of ErRh4B4. At the lowest concentration studied (75 at. % Ho) the magnetization was found to be reduced in the temperature region where superconductivity appeared. The magnetic order parameter, however, was still found to be continuous and reversible.