Anomalous magnetism in the 3 d − 4 f double perovskite oxide Nd 2 FeCrO 6 with the Kramers ion Nd 3 +

The complex magnetic behavior of double-perovskite oxides (



A
2



BB

′


O
6



) with rare-earth (RE) element A and transition metal (TM) elements B-



B

′


is determined by the interactions between intra- and interatomic magnetic moments. The peculiar magnetism in these systems stems from the interplay between spin, orbit, and lattice degrees of freedom. This study comprehensively investigates the role of spin-orbit entangled



J
eff

=
1
/
2


moments of the Kramers ion



Nd


3
+



on the magnetic ground state of a B-site ordered ferrimagnetic (FiM) double-perovskite oxide,



Nd
2


FeCrO
6



. Furthermore, employing microscopic techniques like


μ
SR


(muon spin rotation and relaxation) and neutron diffraction, we gained insight into the origin of low-temperature anomalies in the magnetic ground state of the system. The DC magnetization data encompass the first transition at critical temperature



T
N

=
250


 K, followed by a negative magnetization below 15 K. The temperature-dependent neutron diffraction shows a commensurate magnetic ordering below 250 K, forming a ferrimagnetic ground state, supported by theoretical calculations. In addition, a sharp drop in initial muon asymmetry confirms the transition from a disordered state to a long-range-ordered state at 250 K. Interestingly, the thermal evolution of the dynamic muon spin-relaxation rate (


λ
L


) reveals a transition at two temperatures, at 250 K and


≈

11


 K, suggesting a low-

T

ordering at the A site. The heat-capacity data show that



Nd


3
+



hosts the ground-state doublet (pseudospin-1/2) at low temperatures. Our analysis of neutron diffraction, heat capacity, and


μ
SR


results suggests that the correlation between the



Nd


3
+



doublet and the complex interaction between Nd-TM moments gives rise to the observed low-temperature anomaly.