Topological Hall Effect in Free-standing NiCo₂O₄ Membranes

We report the observation of topological Hall effect (THE) in 30 nm suspended NiCo₂O₄ (NCO) membranes. Free-standing NCO (001) membranes are achieved via epitaxial growth on Sr₃Al₂O­₆ (SAO) buffered (001) LaAlO₃ (LAO) substrates via off-axis RF magnetron sputtering followed by water etching. X-ray diffraction measurements confirm the single crystalline phase of the membranes. The NCO membranes are then transferred on SiO₂/Si substrate and patterned into Hall bar devices. We have studied the magnetoresistance (MR) and Hall effect in both NCO/SAO/LAO films and NCO membranes. The samples exhibit insulating temperature dependence, and the Curie temperature T_C are above 300 K. Below T_C, we observe anomalous Hall effect (AHE), which shows a nonmonotonic temperature dependence and reverses sign at about 240 K. For NCO membranes, topological Hall effect (THE) signal emerges below 10 K upon magnetic field cooling, a phenomenon that is absent in epitaxial NCO thin films. The THE signal changes sign when the magnetic field is reversed. The MR, AHE, and THE in suspended NCO membranes point to the intricate roles of strain and disorder in determining the magnetic properties of NCO.