Structural, superconducting and magnetic properties of ErNi2-xFexB2C(0 ≤ x ≤ 0.2) and ErNi1.96Fe0.04B2Cy (0.9 ≤ y ≤ 1.05)

Polycrystalline samples with composition ErNi(2-x)Fe(x)B(2)C (0 less than or equal to x less than or equal to 0.2) and ErNi(1.96)Fe(0.04)B(2)Cr (0.9 less than or equal to y less than or equal to 1.05) were synthesized and characterized by powder X-ray diffraction, electron probe microanalysis, magnetic susceptibility and electrical resistivity measurements. Upon iron doping, a solid solution behaviour is followed in the explored concentration range. Compared to their parent compound ErNi(2)B(2)C (T(c) = 10.5 K, T(N) = 6.0 K, T(WFM) = 2.3 K), the Neel temperature (T(N)) and the weak ferromagnetic transition temperature (T(WFM)) of the substituted compounds moderately decrease while their superconducting transition (T(c)) is very strongly depressed (d(Tc)/dx = -150 K/mol, averaged). We attribute the weak diminution of T(N) and T(WFM) with x to the variations of the density of states at the Fermi level (N(epsilon(f))), of the exchange parameter (I(sf)) and of the crystalline electric field parameters which nearly compensates each other. We attribute the concomitant and strong T(c) depression to the decrease of N(epsilon(f)) and the increase of I(sf) with Fe substitution. Varying the carbon concentration (y) reveals a range of homogeneity on the carbon poor side close to stoichiometry. T(N) and T(WFM) in ErNi(1.96)Fe(0.04)B(2)C(y) do not depend on y. In zero applied field, the y = 1 sample is a regular superconductor whereas the y = 0.99 is a re-entrant superconductor between 5.5 and similar to6 K. Anyway, lack or excess of carbon (y < 1 or y > 1) both depress superconductivity. Magneto-transport measurements in ErNi(1.96)Fe(0.04)B(2)C(y) (y = 0.99, 1.0) show that H(c2) (T) is uniformly depressed by iron substitution. In portions of the ErNi(1.96)Fe(0.04)B(2)C(0.99) sample, H(c2) (T) is further depressed by carbon understoichiometry, strongly enough to allow these portions to re-enter the normal state in zero applied field. (C) 2004 Elsevier B.V. All rights reserved.