Correlating Itinerant Magnetism in RCo2Pn2 Pnictides (R = La, Ce, Pr, Nd, Eu, Ca; Pn = P, As) to Their Crystal and Electronic Structures

Abstract

Rare-earth cobalt pnictides, RCo₂Pn₂ (Pn = P, As), belong to the ThCr₂Si₂ structure type, which is ubiquitous among intermetallic compounds. The structural and magnetic properties of simple ternary RCo₂P₂ phosphides, which combine partially delocalized (itinerant) 3d magnetic moments of cobalt and localized 4f magnetic moments of lanthanides, were investigated extensively in 1980-1990s, predominantly by the Jeitschko group. Those studies established that LaCo₂P₂ shows ferromagnetic (FM) ordering of Co moments, while the other members of the series, with R = Ce, Pr, Nd, or Sm, exhibit antiferromagnetic (AFM) ordering in both R and Co magnetic sublattices. This observation also correlated with the larger separation between the [Co₂P₂] layers in the crystal structure of LaCo₂P₂ as compared to the decreased interlayer distances in the other structures of the RCo₂P₂ series. Our work over the past decade has focused on unraveling the rich magnetic behavior that can be observed in these systems when internal chemical and external physical factors are used to perturb their crystal and electronic structures. We began our foray into these materials by demonstrating that the preservation of FM ordering of Co 3d moments in the mixed La_1-xR'_xCo₂P₂ phases also forces the R 4f moments to adopt FM arrangement, although antiparallel to the Co moments. As an example, in La_0.75Pr_0.25Co₂P₂ such mutual influence of the 3d and 4f moments leads to a cascade of magnetic phase transitions. All these changes were traced back to the modification of the crystal structure and, consequently, the electronic band structure of these materials. The substitution of smaller R³⁺ ions for the La³⁺ ions leads to structural compression along the tetragonal c axis, perpendicular to the [Co₂P₂] layers, and an increase in the Co-Co distances within the layer. This structural effect is translated into more localized Co magnetic moments, stronger magnetic exchange between Co sites, and higher ordering temperatures. A more dramatic change in properties is observed in EuCo₂Pn₂, which exhibit AFM ordering of the localized 4f moments of Eu²⁺ ions and only paramagnetic behavior in the Co sublattice. Under applied pressure, these compounds undergo structural collapse, which causes a dramatic decrease in the separation between the [Co₂Pn₂] layers, an increase in the oxidation state of Eu, and magnetic ordering of Co moments. We further demonstrated that similar effects can be stimulated by chemical compression, which is achieved by doping Eu into the more constrained lattice sites, for example, in PrCo₂P₂ or CaCo₂As₂. In both cases, the induced mixed valence of Eu results in the change from AFM to FM ordering in the Co sublattice. A series of solid solutions Ca_1-xEu_xCo₂As₂ shows a fascinating evolution of magnetic behavior from AFM ordering of Co 3d moments to simultaneous FM ordering of Co 3d and Eu 4f moments to AFM ordering of Eu 4f moments as one proceeds from CaCo₂As₂ to EuCo₂As₂. Importantly, all these changes in magnetic properties are well justified by the analysis of electronic density of states and crystal orbital Hamilton population, providing the understanding of how chemical factors can be leveraged, in general, to modify properties of itinerant magnets.