Probing cluster dynamics in Ni-Cr alloys close to the ferromagnetic quantum critical point by μSR

Ferromagnetic alloys exhibit a quantum critical point (QCP) driven by disorder accompanied by a quantum Griffiths phase (QGP), a region of dynamic clusters. This has been previously studied in Ni_1-xV_x alloys around a critical concentration x_c with muon spin relaxation (μSR) measurements [1] supporting the signatures of a QGP. Ni_1-xCr_x alloys should show similar phenomenology but with significantly smaller nuclear magnetic moments that mean muons are able to probe even smaller electronic magnetic fields. Muon measurements on two polycrystalline samples of Ni_1-xCr_x alloys close to x_c have been performed using the MuSR instrument at ISIS. Ni_0.88Cr_0.12 muon data show a broad static magnetic field distribution indicating an ordered ferromagnetic state and signs of coexisting dynamic clusters below 2.5K. The presence of long-range ferromagnetic order with short-range fluctuations is similar to Ni_0.89V_0.11 confirmed by μSR [1] and small-angle neutron scattering measurements [2]. For Ni_0.873Cr_0.127, the muon data defy simple descriptions indicating more complex inhomogeneities in space and multiple timescales and suggest a cluster glass toward low temperatures.

[1] R. Wang et al.  Phys. Rev. Lett. 118, 267202 (2017)

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