Quantum criticality in Ta(Fe1-xVx)2 probed by 51V NMR and magnetization

Itinerant 3d weak ferromagnets like e.g. NbFe2 or MnSi are tunable towards a (quantum) critical regime by doping, external pressure or magnetic field. The vicinity of a QCP leads to non Fermi liquid (NFL) behavior manifested by scaling laws in bulk properties like magnetization M(T), resistivity ρ(T) and specific heat C(T). In NMR, NFL phenomena are associated with non-Korringa like features in spin lattice relaxation rate (1/T1) for T →0. We focus on NMR investigations on polycrystalline Ta(Fe1-xVx)2 (x=0.02,0.05,0.2,0.3) samples. For x=0.02, the magnetic properties are reminiscent of itinerant FM at the verge of a FM instability with enhanced M(T →0) indicating FM fluctuations. 51V NMR spectra are broadened inhomogeneously at low temperatures with considerable shift which reflects the hyperfine coupling of itinerant 3d moments with the 51V nuclei. 51(1/T1T) violates the Korringa law and exhibits a T-0.8 dependence for small fields which is a signature of critical fluctuations. The value of the Korringa product (< 1) indicates FM correlations between itinerant moments. Further doping the system undergoes AFM ordering up to x=0.3, where a simple paramagnetic behavior is observed. Ta(Fe1-xVx)2 is of special interest because FM critical fluctuations evolve in the proximity of an AFM state by nominal V doping.