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-dimension
al, {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.
We have grown single crystal samples of Co substituted CaFe2As2 using an FeAs flux and systematically studied the effects of annealing/quenching temperature on the physical properties of these samples. Whereas the as-grown samples (quenched from 960C
) all enter the collapsed tetragonal phase upon cooling, annealing/quenching temperatures between 350C and 800C can be used to tune the system to low temperature antiferromagnetic/orthorhomic or superconducting states as well. The progression of the transition temperature versus annealing/quenching temperature (T-T$_{anneal}$) phase diagrams with increasing Co concentration shows that, by substituting Co, the antiferromagnetic/orthorhombic and the collapsed tetragonal phase lines are separated and bulk superconductivity is revealed. We established a 3D phase diagram with Co concentration and annealing/quenching temperature as two independent control parameters. At ambient pressure, for modest x and T$_{anneal}$ values, the Ca(Fe1-xCox)2As2 system offers ready access to the salient low temperature states associated with Fe-based superconductors: antiferromagnetic/orthorhombic, superconducting, and non-magnetic/collapsed tetragonal.
The low temperature, magnetic phase transition in LuFe2Ge2 is thought to be associated with itinerant magnetism. The effects of Y and Sc substitutions on the Lu site, as well as Ru and Co substitutions on the Fe site, on the low temperature magnetic
phase transition of LuFe2Ge2 compound have been studied in single crystals via microscopic, thermodynamic and transport measurements. On one hand, Co substitution suppresses the transition below our base temperature of 2 K even at our lowest substitution level. On the other hand, Sc substitution enhances the transition temperature, and Y or Ru substitution suppresses the transition to lower temperature. Phase diagrams for Y, Sc and Ru substitutions have been constructed and the possibility of a unifying, composite diagram is discussed.
