Evolution of short-range magnetic fluctuations in disordered ferromagnetic alloys.*

We present small-angle neutron scattering (SANS) results on NiV to trace the evolution of the short-range magnetic clusters in a ferromagnetic alloy. NiV is a simple ferromagnetic (FM) alloy with random atomic distribution that undergoes a quantum phase transition from an FM to a paramagnetic state with sufficient substitution of Ni by V. The onset of FM order below transition temperature (Tc) vanishes towards x_c=0.116, indicating a quantum critical point (QCP) with signatures of the disorder [1]. We collected full polarized SANS data on different polycrystalline NiV samples close to the QCP at NG7SANS, NCNR, NIST. Through the angular dependence (of the spin-flip (SF) and DIF signal, the non-SF flipper contrast) with the magnetic field transverse to the beam, we succeed in resolving the small magnetic scattering at diverse lengths scales within the FM state well below Tc<50K [2]. Besides isotropic magnetic short-range correlations that remain at very low temperatures, we find an anisotropic magnetic contribution that reveals large-scale magnetic domains below T_c.

We present how the clusters within the FM phase are affected by an external magnetic field. The SF data (at higher Q>0.1nm^-1) reveal gradual freezing of fluctuations, while the DIF data (at the lower Q<0.2nm^-1) show an increase in domain scattering with increasing field. Getting closer to x_c, the magnetic clusters become more dominant than the ordered phase, but the small magnetic signals are more challenging to resolve for more specific details. However, the data agree with qualitative expectations of a disordered QCP. They reveal magnetic clusters in NiV with signatures of a range of dynamics that mark a quantum Griffith's phase.

[1] R. Wang et al, Phys. Rev. Lett. 18, 267202 (2017)

[2] A. Schroeder et al, AIP Adv.10,015036 (2020)