Structural, superconducting and magnetic properties of ErNi 2− xFe xB 2C (0⩽ x⩽0.2) and ErNi 1.96Fe 0.04B 2C y (0.9⩽ y⩽1.05)

Abstract

Polycrystalline samples with composition ErNi 2− x Fe x B 2C (0⩽ x⩽0.2) and ErNi 1.96Fe 0.04B 2C y (0.9⩽ y⩽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 2B 2C ( T c=10.5 K, T N=6.0 K, T WFM=2.3 K), the Néel 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 T c/d x=−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( ε 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( ε 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 1.96Fe 0.04B 2C 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 ∼6 K. Anyway, lack or excess of carbon ( y<1 or y>1) both depress superconductivity. Magneto-transport measurements in ErNi 1.96Fe 0.04B 2C y ( y=0.99, 1.0) show that H c2( T) is uniformly depressed by iron substitution. In portions of the ErNi 1.96Fe 0.04B 2C 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.