Title: Static and dynamic magnetic properties of layered honeycomb A2IrO3 and magnetic topological insulator (MnBi2Te4)(Bi2Te3)n
Speaker: Dr. Kavita Mehlawat, Venue: C V Raman Hall, Time: 11:30 AM (6th June 2025)
Abstract: This talk will consist of two subparts:
1) Kitaev-like bond-directional exchange interactions are novel and quite different from the ubiquitous Heisenberg interactions found in most magnets. These kinds of interactions open up new possibilities in the exploration and designing of new quantum magnets, which can host novel quantum ground states like spin-liquids. We have synthesized single crystals of A2TO3 (A = Na, Li, and T = Ir, Ru) and studied their electrical transport, magnetic, and thermal properties. Our work provides several new results: (i) the first thermodynamic evidence of possible fictionalization of electrons in Na2IrO3 because of proximity to the Kitaev spin-liquid state, (ii) a novel method (reactive ion etching) of surface doping Na2IrO3 and possibly other layered oxides has been discovered. The surface conductivity of Na2IrO3 crystals could be increased by 11 orders of magnitude by varying etching times. The samples that turned metallic show transport anomalies consistent with charge density wave or structural instabilities.
2) We report a high-frequency/high-magnetic field electron spin resonance (HF-ESR) spectroscopy study in the sub-THz frequency domain of the two representatives of the family of magnetic topological insulators (MnBi2Te4 )(Bi2Te3 )n with n = 0 and 1. The HF-ESR measurements in the magnetically ordered state at a low temperature of T = 4K, combined with the calculations of the resonance modes, showed that the spin dynamics in MnBi4Te7 is typical for an anisotropic easy-axis type ferromagnet, whereas MnBi2Te4 demonstrates excitations of an anisotropic easy-axis type antiferromagnet. However, by applying the field stronger than a threshold value ∼6 T, we observed in MnBi2Te4 a crossover from the antiferromagnetic (AFM) resonance modes to the ferromagnetic (FM) modes, whose properties are very similar to the FM response of MnBi4Te7.
Talk 2:
Title: Exploratory Synthesis and Novel Magnetism in Pnictides and Topological Zintl Phase Compounds
Speaker: Dr. Santanu Pakhira, Venue: C V Raman Hall, Time: 15:00 hrs (6th June 2025)
Abstract: The interplay between magnetism and flat-band (FB) instability is a central theme in quantum materials research. A striking example is the emergence of magnetic order in a nominally nonmagnetic compound when a flat band is tuned near the Fermi level (EF). ACo2As2 (A = Ca, Sr) are two unique itinerant frustrated magnetic systems having electronic FB close to the Fermi energy. CaCo2-yAs2 exhibits A-type antiferromagnetic (AFM) ordering below TN ~ 52 K. Both electron- and hole-doping onto the Co-site through Fe and Ni substitutions strongly suppress the AFM ordering in the system followed by a carrier tuned Stoner transition. In the absence of long-range magnetic ordering, frustration-driven strong quasi-1D ferromagnetic quantum spin-fluctuations develop in both Fe- and Ni-doped CaCo2-yAs2 along with nonFermi-liquid behavior. Isostructural analogue SrCo2As2 does not order magnetically down to at least 50 mK, but stripe AFM spin fluctuations exist. We found that minimal nonmagnetic Pd substitutions trigger long-range AFM order in the system. The observed magnetic behavior in both the compounds reveal the central role of FB instability in tuning the magnetic order in itinerant magnetic systems via the Stoner mechanism, a prototypical quantum phase transition. On the other hand, a variety of Eu-based geometrically frustrated Zintl-phase compounds have been recently reported to host novel topological electronic states with ultrahigh carrier mobility. Gaining insight into the intricate interplay between magnetism and band topology is essential for interpreting the observed diverse physical phenomena in these materials. Notably, many of these Eu-based compounds exhibit a nonlinear magnetization response along the easy-plane direction below their A-type antiferromagnetic (AFM) ordering temperature. Our study demonstrates that this anomalous behavior arises from a low-field induced spin reorientation within the threefold AFM domain structure, driven by weak in-plane magnetic anisotropy.
Narayan Pradhan
Chair- School of Materials Sciences
