Magnetic structure in the zero-applied-field reentrant superconductor ErNi1.96Fe0.04B2C0.99 -: art. no. 214518

ErNi1.96Fe0.04B2C0.99 is superconducting at T-c=7.3 K and is shown to partially reenter the normal state between 5.5 and 5.7 K in zero applied field by means of ac-susceptibility (chi(ac)(')) and resistivity measurements. Specific heat (C-p) measurements give a magnetic ordering temperature T-N=5.7+/-0.2 K. Powder neutron diffraction measurements showed that the Er3+ magnetic moments adopt below T-N=5.9+/-0.2 K a transverse modulated structure along the a axis (q=0.559a(*)). Observation of third-order and fifth-order harmonics indicates a squaring of the structure at lower temperature (8.2mu(B) at 1.4 K). Although even-order peaks could not be detected in the neutron data, a weak ferromagnetic transition at T-WFM=2.2 K could be directly evidenced by magnetic susceptibility measurements and C-p measurements and indirectly evidenced in the thermal variations of the intensity of the first-order magnetic peaks (I-q), the magnetic coherence length (xi(m)), and the magnetic wave vector (q). The magnetic structure of ErNi1.96Fe0.04B2C0.99 is nearly identical to that of the parent compound ErNi2B2C. Precise comparison between the temperature variations of I-q, xi(m), and chi(ac)(') shows that the transverse a(*)-modulated magnetic structure is responsible for the reentrant behavior between 5.5 and 5.7 K. Although it is still pair breaking below 5.5 K, it coexists with superconductivity. Fe-57 Mossbauer spectra display no broadening in the temperature range where the reentrant behavior is observed, showing that the transferred hyperfine field at the Fe site, if any, is smaller than 2 kOe.