Anomalous Nernst Effect in Epitaxially Grown Mn3Sn Thin Films

Abstract

New trends in next-generation magnetic information storage focus on controlling the magnetic domain wall propagation and positioning. Ferromagnetic as well as antiferromagnetic materials have gained attention for energy harvesting and memory devices, particularly in the field of spin caloritronic. The Seebeck effect, on which the conventional thermoelectric devices are working generate thermoelectric voltage longitudinally to the temperature gradient. However, for the application, this effect has a shortcoming of the heat source part of the electrical circuit. To surmount this challenge, multiterminal thermoelectric devices have freedom of spatial separation of heat reservoir from the electrical circuit.

Our study explores the potential implementation of the Anomalous Nernst Effect, in which a temperature gradient (∇T) and magnetic field (B) perpendicular to each other produces a transverse thermoelectric voltage signal. Epitaxially grown Mn3Sn thin films (30 nm) on MgO (110) substrates with a Tungsten (W) seed layer (7 nm) and Tantalum (Ta) capping layer (3 nm) serve as the device structure. Hall bar-shaped devices were fabricated using photolithography and ion milling processes. Then, a separate 50 nm thick tungsten heater was fabricated by W sputtering using DC magnetron sputter method and lift-off process across the longitudinal contact of the Hall bar at each end. The sample layout and geometry of the on-chip heaters enables the direct measurements of the Anomalous Hall Effect and Anomalous Nernst Effect. By applying out-of-plane magnetic field and current through the heater line, we measure the Nernst voltage signals from the nearest voltage contacts. All the measurements were performed at room temperature using the Keithly 6221 source meter, Keithley 2182A as a voltmeter and an electromagnetic.

Antiferromagnetic materials, having non-collinear spin configuration, lead to generation of large Berry curvature via spin-orbit coupling gained interest due to their thermoelectric magneto-transport effects. This Berry curvature arising from the magnetic multipole plays an important role not only in Anomalous Hall effect but also in Anomalous Nernst effect. Breaking of time reversal and inversion symmetry along with the translation and time reversal leads to the Integral nonzero value, these are the intrinsic contributors to the AHE and ANE. However, the AHE arises from all the occupied bands while the ANE originates only from the occupied bands of the Fermi level.

Unlike conventional thermoelectric devices, Anomalous Nernst devices eliminate the need for p-type and n-type materials, simplifying technological integration. This work not only highlights the development of multi-terminal devices but also expands the understanding of Anomalous Nernst Effect in antiferromagnetic materials which leads to the future innovations and applications in energy harvesting and memory devices.