arXiv subjects

Nazma Firdosh

Publications and source records attributed to Nazma Firdosh.

3 recordsLinked to original sources

arXiv2026

Magnetic criticality and magnetocaloric response in MnBi$_2$Te$_4$ and MnBi$_4$Te$_7$

MnBi$_2$Te$_4$ and MnBi$_4$Te$_7$ are antiferromagnetic topological insulators belonging to the MnBi$_{2n}$Te$_{3n+1}$ series, where structural layering provides a natural route to tune magnetic interaction in van der Waals magnets. Despite extensive interest in their topological properties, how the insertion of Bi$_2$Te$_3$ quintuple layers modifies magnetic critical fluctuations near the antiferromagnetic transition remains unresolved. Here, we combine scanning tunneling microscopy (STM), critical scaling analysis, and magnetocaloric measurements to directly correlate real-space structures with magnetic criticality. STM reveals atomically flat septuple-layer terraces in MnBi$_2$Te$_4$ whereas MnBi$_4$Te$_7$ displays coexisting septuple and quintuple layer terminations reflecting its alternating stacking sequence. MnBi$_2$Te$_4$ exhibits robust three-dimensional Ising-like critical behavior together with a distinct low-temperature first-order transition. In contrast, MnBi$_4$Te$_7$ displays crossover-dominated criticality arising from weakened interlayer exchange and competing magnetic phases. Correspondingly, the magnetocaloric response differs significantly between the two compounds. MnBi$_2$Te$_4$ shows dual-type magnetocaloric behavior with a sharp field-induced sign reversal of the isothermal magnetic entropy change ($-\Delta S_M$). It exhibits both inverse ($-\Delta S_M < 0$) and conventional ($-\Delta S_M > 0$) magnetocaloric effects. In contrast, MnBi$_4$Te$_7$ shows only conventional magnetocaloric response with a broad positive entropy peak. These results establish structural layering as a key parameter governing magnetic critical fluctuations and magnetocaloric behavior in MnBi$_{2n}$Te$_{3n+1}$ topological magnets.

cond-mat.mtrl-sci↗

arXiv2025

Intertwined magnetic phase driven exchange bias and its impact on the anomalous Hall effect in MnBi$_4$Te$_7$

We report on the interplay between atomic scale inhomogeneity and competing magnetic phases and its effect on the anomalous Hall effect in the layered antiferromagnet MnBi$_4$Te$_7$, a natural superlattice hosting coexisting ferromagnetic and antiferromagnetic phases. Using a combination of scanning tunneling microscopy (STM), DC and AC magnetization, and magneto-transport measurements, we reveal that intrinsic Mn Bi antisite defects induce strong interlayer exchange coupling, giving rise to a robust exchange bias observed in both magnetic and Hall responses. The exchange bias undergoes a transition from asymmetric to symmetric behavior between 2 K and 6 K, indicating a temperature driven dynamical reconfiguration of interfacial spin structures. The training effect analysis revealed a stronger contribution of frozen spins at 2 K compared to 6 K, with relaxation amplitude shift from -264 Oe to 306 Oe. This sign reversal indicates a field-induced change in interfacial coupling. The temperature dependence of longitudinal resistivity and magnetization reveals complementary behavior, indicating the coexistence of two distinct spin states near the magnetic transition temperature. The phase fraction based resistivity model captures the distinct scattering mechanisms that govern electronic transport across different magnetic regimes. Our findings offer a direct link between microscopic disorder, interfacial magnetism and macroscopic topological phenomena in magnetic topological insulators.

cond-mat.str-el↗

arXiv2025

Anomalous Hall effect in highly c-plane oriented Mn$_{3}$Ge/Si(100) thin films grown by pulsed laser deposition

Antiferromagnetic Mn$_{3}$Ge with a non-collinear Kagome structures present exciting prospects for exploring Berry curvature driven anomalous Hall effects (AHE). Despite substantial progress in bulk systems, the synthesis of crystalline thin films directly on silicon with a hexagonal phase presents a particular challenge unless a buffer layer is employed. In this study, we report the synthesis of single phase c-plane oriented hexagonal Mn$_{3}$Ge(0001) films on Si(100) using pulsed laser deposition. Under suitable growth conditions, we obtain layer-by-layer films with atomically flat surfaces and interfaces. High-resolution scanning tunneling microscopy study reveals the detail surface atomic structures, where the surface Mn atoms spontaneously arrange into a Kagome lattice. Tunneling spectroscopy (dI/dV) measurement on the atomically resolved Kagome surface show a minima in local density of states near the Fermi level, likely originated from the Weyl crossings near K points. Despite the nearly vanishing magnetization, magnetotransport measurements in 30 nm $Mn_{3}$Ge(0001) films show anomalous Hall resistivity up to 0.41 ($\mu\Omega\cdot\text{cm}$) at 2 K. Our \textit{ab initio} calculations shed further light on the existence of topological features and the band structures in Mn$_{3+x}$Ge$_{1-x}$ with increasing Mn concentration $x$. The anomalous Hall response at room temperature in crystalline Mn$_{3}$Ge films on Si(100) offer promising potential for the development of antiferromagnetic spintronics.

cond-mat.mes-hall↗