Magnetic structure in the zero-applied-field reentrant superconductorErNi1.96Fe0.04B2C0.99

Abstract

${\mathrm{ErNi}}_{1.96}{\mathrm{Fe}}_{0.04}{\mathrm{B}}_{2}{\mathrm{C}}_{0.99}$ is superconducting at ${T}_{c}=7.3K$ 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 $({\ensuremath{\chi}}_{\mathrm{ac}}^{\ensuremath{'}})$ and resistivity measurements. Specific heat ${(C}_{p})$ measurements give a magnetic ordering temperature ${T}_{\mathrm{N}}=5.7\ifmmode\pm\else\textpm\fi{}0.2K.$ Powder neutron diffraction measurements showed that the ${\mathrm{Er}}^{3+}$ magnetic moments adopt below ${T}_{\mathrm{N}}=5.9\ifmmode\pm\else\textpm\fi{}0.2K$ a transverse modulated structure along the $a$ axis $(\mathbf{q}=0.559{\mathbf{a}}^{*}).$ Observation of third-order and fifth-order harmonics indicates a squaring of the structure at lower temperature $(8.2{\ensuremath{\mu}}_{\mathrm{B}}$ at 1.4 K). Although even-order peaks could not be detected in the neutron data, a weak ferromagnetic transition at ${T}_{\mathrm{WFM}}=2.2K$ 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 ${(I}_{\mathbf{q}}),$ the magnetic coherence length $({\ensuremath{\xi}}_{m}),$ and the magnetic wave vector (q). The magnetic structure of ${\mathrm{ErNi}}_{1.96}{\mathrm{Fe}}_{0.04}{\mathrm{B}}_{2}{C}_{0.99}$ is nearly identical to that of the parent compound ${\mathrm{ErNi}}_{2}{\mathrm{B}}_{2}\mathrm{C}.$ Precise comparison between the temperature variations of ${I}_{\mathbf{q}},$ ${\ensuremath{\xi}}_{m},$ and ${\ensuremath{\chi}}_{\mathrm{ac}}^{\ensuremath{'}}$ shows that the transverse ${\mathbf{a}}^{*}$-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. ${}^{57}\mathrm{Fe}$ M\ossbauer 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.