Unusual effects in transport and magnetism of layered AFM semimetals. The emergence of ferromagn...

on 5-th November 2025 at 11.00, Moscow time, ACTP will hold a seminar "Unusual effects in transport and magnetism of layered AFM semimetals. The emergence of ferromagnetism due to nanodefects" by Academician of the Russian Academy of Sciences, Prof. Vladimir M. Pudalov (Lebedev Physical Institute RAS).

Abstract

In the stoichiometric semimetalic layered antiferromagnets (MnBi_2 Te_4 , EuRbFe_4 As_4 , EuIn_2 As_2 , EuCd_2 As_2 , EuSn_2 As_2 , EuFe_2 As_2 , EuSn_2 P_2 , EuMg_2 Bi_2 , etc.) the electrons or Cooper pairs propagate in alternating magnetic field of the AFM-ordered host lattice. Besides conventional AFM-properties, these layered vdW materials manifest a number of unusual features, which remained puzzling until now. For instance, all the non-Weyl semimetals exhibit isotropic negative magnetoresistance (NMR) correlated with magnetization. We proposed recently a novel mechanism based on the exchange splitting, that explains qualitatively the experimental observations.

In the AFM state, the DC-magnetization shows a weak nonlinearity and even a hysteresis in low fields, whereas AC susceptibility upturns towards lowest temperatures. In the ESR measurements with EuSn_2 As_2 , an additional resonance was observed, atypical for the AFM state. In order to shed a light on these puzzling features, we performed TEM investigation of EuSn_2 As_2 single crystals and identified rarely scattered, monolayer thick planar defects. The defects have a local composition of EuSnAs_2 , with an extra row of Eu atoms and missing Sn-atoms. The supercell of the defective transition layer is identified as Eu_7 Sn_12 As_14 . The latter one contains an odd number of Eu atoms and has a non-zero magnetic moment. The DFT calculations of the band structure and magnetization confirm the ferromagnetic order of the defect layer. Using these results, we proposed the two-phase model where the FM-defects are imbedded into the AFM host lattice. The model with no fitting parameters reasonably describes all the above features, including the temperature and magnetic field dependences of the ESR.