Journal Article

Anomalous Nernst effect beyond the magnetization scaling relation in the ferromagnetic Heusler compound Co2MnGa

Guin, Satya N., Manna, Kaustuv, Watzman, Sarah J., Shekhar, Chandra, Noky, Jonathan, Fu, Chenguang, Kumar, Nitesh, Schnelle, Walter, Sun, Yan, Gooth, Johannes, Felser, Claudia

2019

Overview

Applying a temperature gradient in a magnetic material generates a voltage that is perpendicular to both the heat flow and the magnetization. This phenomenon is the anomalous Nernst effect (ANE), which was long thought to be proportional to the value of the magnetization. However, more generally, the ANE has been predicted to originate from a net Berry curvature of all bands near the Fermi level ( E F ). Subsequently, a large anomalous Nernst thermopower ( S yx A ) has recently been observed in topological materials with no net magnetization but a large net Berry curvature [Ω n ( k )] around E F . These experiments clearly fall outside the scope of the conventional magnetization model of the ANE, but a significant question remains. Can the value of the ANE in topological ferromagnets exceed the highest values observed in conventional ferromagnets? Here, we report a remarkably high S yx A -value of ~6.0 µV K −1 in the ferromagnetic topological Heusler compound Co 2 MnGa at room temperature, which is approximately seven times larger than any anomalous Nernst thermopower value ever reported for a conventional ferromagnet. Combined electrical, thermoelectric, and first-principles calculations reveal that this high-value of the ANE arises from a large net Berry curvature near the Fermi level associated with nodal lines and Weyl points. Energy conversion: Heat- recovery magnets identified Thermoelectric devices that convert heat into electricity may benefit from the unusual temperature sensitivity of cobalt–manganese–gallium (Co 2 MnGa) ferromagnets. When one end of a magnetized metal is made hot and the other cold, redistribution of electrons creates an electric voltage perpendicular to the temperature gradient. Satya N. Guin from the Max Planck Institute for Chemical Physics of Solids in Dresden, Germany, and colleagues now report how certain class of material can boost the electrical power produced from “waste heat” source using transverse thermoelectric effect. When the team applied magnetic fields to Co 2 MnGa and characterized its transverse electrical response to temperature gradient, they saw voltage generation several times higher than expected. Computer simulations indicated that the crystal geometry distorted the energy levels available to electron making it easier for electrons to move when thermally excited. We report a high anomalous Nernst thermopower ( S y x A ) -value of ~6.0 µV K −1 at room temperature in the ferromagnetic topological Heusler compound Co 2 MnGa. The measured value is seven-times larger than any anomalous Nernst thermopower value ever reported for a conventional ferromagnet. The high anomalous Nernst effect originates from a large net Berry curvature near the Fermi level associated with nodal lines and Weyl points.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 639/301/119/2792 / 639/301/299/2736 / Biomaterials / Chemistry and Materials Science / Curvature / Electric potential / Electrons / Energy conversion / Energy levels / Energy recovery / Energy Systems / Fermi level / Ferromagnetic materials / First principles / Gallium / Heat / Heat recovery / Heat transmission / Magnetic materials / Magnetism / Magnetization / Magnets / Manganese / Materials Science / Nernst-Ettingshausen effect / Optical and Electronic Materials / Room temperature / Structural Materials / Surface and Interface Science / Temperature gradients / Thermoelectricity / Thin Films / Topology / Voltage