Magnetic field tuned superconducting and normal phase magnetism in CeCo${}_{0.5}$Rh${}_{0.5}$In${}_5$

Abstract

By tuning superconductivity with an applied magnetic field, we use neutrons to compare the magnetic ordered phases in superconducting and normal states of CeCo${}_{0.5}$Rh${}_{0.5}$In${}_5$. At zero field, CeCo${}_{0.5}$Rh${}_{0.5}$In${}_5$ displays both superconductivity ($T_c$=1.3 K) and spatially long-ranged commensurate $\uparrow \downarrow \uparrow \downarrow$ antiferromagnetism ($T_N$=2.5 K, ${\vec{Q}}_0=(\frac{1}{2},\frac{1}{2},\frac{1}{2})$). Neutron spectroscopy fails to measure propagating magnetic excitations with only temporally overdamped fluctuations observable. On applying a magnetic field we find anisotropic behavior in the static magnetism. When the field is along the crystallographic $c$-axis, no change in the static magnetic response is observable. However when the field is oriented within the $a-b$ plane, an increase in $T_N$ and change in the critical response are measured. At low temperatures in the superconducting phase, the elastic magnetic intensity increases linearly ($\propto |H|$) with small $a-b$ oriented fields. However, this trend is interrupted at intermediate fields where commensurate block $\uparrow \uparrow \downarrow \downarrow$ magnetism with propagation vector $\vec{Q}=(\frac{1}{2},\frac{1}{2},\frac{1}{4})$ forms. For large applied fields in the [1 $\overline{1}$ 0] direction which completely suppresses superconductivity, weakly incommensurate magnetic order along $L$ is observed to replace the commensurate response present in the superconducting and vortex phases. We suggest field-induced incommensurate static magnetism, present in the normal state of superconducting and antiferromagnetic CeCo${}_{0.5}$Rh${}_{0.5}$In${}_5$ for $a-b$ plane oriented magnetic fields. We speculate that these field dependent properties are tied to the field induced anisotropy associated with the local Ce${}^{3+}$ crystal field environment of the tetragonal `115' structure.

(13 pages, 7 figures, to be published in Phys. Rev. B)