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 La1-xR'xCo2P2 phases also forces the R 4f moments to adopt FM arrangement, although antiparallel to the Co moments. As an example, in La0.75Pr0.25Co2P2 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 Ca1-xEuxCo2As2 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.