Resistance fluctuations probing the magnetic polaron state in the antiferromagnetic insulator Eu₅In₂Sb₆

We investigate the nonsymmorphic Zintl semiconductor Eu₅In₂Sb₆. Zintl phases are valence precise intermetallic phases formed by cations and covalently bonded (poly)anionic structures containing post-transition metals. The electron transfer between those two entities gives rise to an insulating state, whereas the inclusion of rare-earth elements allows for magnetism that may promote new quantum ground states. Eu₅In₂Sb₆ is a rare example of an antiferromagnet exhibiting a colossal magnetoresistance (CMR) effect, which may be driven by the presence of magnetic polarons. Upon cooling, a magnetic transition occurs at the Neel temperature T_N1 = 14 K, which can be suppressed by a magnetic field. While the negative MR is small at room temperature, it rapidly increases with decreasing temperature and ultimately peaks at -99.999% for B  = 9 T at T_N1, which is one of the largest observed CMR in a stoichiometric antiferromagnet [1]. The magnetic ground state below another transition temperature T_N2 = 7 K, however, is complex and yet to be fully understood. As a powerful probe to test possible scenarios for explaining the CMR effect, we employ resistance fluctuation (noise) spectroscopy and analyze the observed generic 1/f-type noise in terms of a model of magnetic polarons.

[1] P. Rosa et al., npj Quantum Mater. 5, 52 (2020)