Quantum critical point followed by Kondo-like behavior due to Cu substitution in the itinerant antiferromagnet La 2 ( Cu x Ni 1 − x ) 7

Abstract

La2⁢Ni7 is an itinerant magnetic system with a small ordered moment of ∼ 0.1⁢µ𝐵/Ni and a series of antiferromagnetic (AFM) transitions at 𝑇1=61.0K, 𝑇2=56.5K, and 𝑇3=42.2K. 𝑀⁡(𝐻), and 𝜌⁡(𝐻) isotherms as well as constant field 𝑀⁡(𝑇) and 𝜌⁡(𝑇) measurements on single-crystalline samples manifest a complex, anisotropic 𝐻−𝑇 phase diagram with multiple phase lines. Here we present the growth and characterization of single crystals of the La2⁢(Cu𝑥⁢Ni1−𝑥)7 series for 0 ≤𝑥≤ 0.181. We measured powder x-ray diffraction and composition, as well as anisotropic temperature- and field-dependent resistivity, temperature- and field-dependent magnetization, and temperature-dependent heat capacity on these single crystals. Using the measured data we infer a transition temperature-composition (𝑇−𝑥) phase diagram for this system to study the evolution of the AFM ordering upon Cu substitution. For 0≤𝑥≤0.097, the system remains magnetically ordered at base temperature with 𝑥≤ 0.012, showing signs of multiple AFM ordering temperatures. For the higher substitution levels 0.125≤𝑥≤0.181, there are no signatures of magnetic ordering, but anomalous features in resistance and heat capacity data are observed which are consistent with the Kondo effect in this system. The intermediate 𝑥=0.105 sample lies between the magnetic ordered and the Kondo regime and is in the vicinity of the AFM quantum critical point (QCP). Thus, La2⁢(Cu𝑥⁢Ni1−𝑥)7 is an example of a small moment system that can be tuned through a QCP. Given these data combined with the fact that the La2⁢Ni7 structure has kagomelike, Ni sublattices running perpendicular to the crystallographic 𝑐 axis, and a predicted 3⁢𝑑-electron flat band that contributes to the density of states near the Fermi energy, La2⁢(Cu𝑥⁢Ni1−𝑥)7 becomes a promising system to host and study exotic physics.