Combined effects of transition metal (Ni and Rh) substitution and annealing/quenching on physical properties of CaFe$_{2}$As$_{2}$

TL;DR

This study systematically explores how Ni and Rh substitutions, combined with annealing/quenching, influence the phase diagrams and physical properties of CaFe$_{2}$As$_{2}$, revealing distinct effects on magnetic, structural, and superconducting states.

Contribution

It provides detailed phase diagrams showing the effects of Ni and Rh substitution and annealing on CaFe$_{2}$As$_{2}$, highlighting differences from Co substitution and uncovering reversible annealing effects.

Findings

Ni substitution shrinks superconducting region compared to Co.

Rh substitution promotes collapsed tetragonal phase, suppressing superconductivity.

Reversible effects of annealing observed for Ni and Rh substitutions.

Abstract

We performed systematic studies of the combined effects of annealing/quenching temperature ({\itshape T}$_{A/Q}$) and T = Ni, Rh substitution ({\itshape x}) on the physical properties of Ca(Fe$_{1-x}$T$_{x}$)$_{2}$As$_{2}$. We constructed two-dimensional, {\itshape T}$_{A/Q}$-{\itshape x} phase diagrams for the low-temperature states for both substitutions to map out the relations between ground states and compared them with that of Co-substitution. Ni-substitution, which brings one more extra electron per substituted atom and suppresses the {\itshape c}-lattice parameter at roughly the same rate as Co-substitution, leads to a similar parameter range of antiferromagnetic/orthorhombic in the {\itshape T}$_{A/Q}$-{\itshape x} space as that found for Co-substitution, but has the parameter range for superconductivity shrunk (roughly by a factor of two). This result is similar to what is found when Co- and Ni-substituted BaFe$_{2}$As$_{2}$ are compared. On the other hand, Rh-substitution, which brings the same amount of extra electrons as does Co-substitution, but suppresses the {\itshape c}-lattice parameter more rapidly, has a different phase diagram. The collapsed tetragonal phase exists much more pervasively, to the exclusion of the normal, paramagnetic, tetragonal phase. The range of antiferromagnetic/orthorhombic phase space is noticeably reduced, and the superconducting region is substantially suppressed, essentially truncated by the collapsed tetragonal phase. In addition, we found that whereas for Co-substitution there was no difference between phase diagrams for samples annealed for one or seven days, for Ni- and Rh- substitutions a second, reversible, effect of annealing was revealed by seven-day anneals.