Large anomalous Nernst conductivity of <i>L</i>1<sub>0</sub>-ordered CoPt in CoPt composition-spread thin films
Ryo Toyama, Keisuke Masuda, Kodchakorn Simalaotao, Weinan Zhou, Varun K. Kushwaha, Yuya Sakuraba
Abstract
Abstract We demonstrate a high-throughput experimental characterization of anomalous Nernst conductivity ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mi>α</mml:mi> <mml:mrow> <mml:mi>x</mml:mi> <mml:mi>y</mml:mi> </mml:mrow> <mml:mi>A</mml:mi> </mml:msubsup> </mml:mrow> </mml:math> ) of L 1 0 -ordered CoPt using Co 1– x Pt x composition-spread thin films on MgO(100) substrates. The compositional dependence of the anomalous Nernst effect, anomalous Hall effect (AHE) and Seebeck effect is systematically measured. As increasing the Pt concentration, the crystal structure in the composition-spread film grown at 500 °C changes from face-centered cubic (fcc) Co, A 1-disordered CoPt, L 1 0 -ordered CoPt, A 1-CoPt to fcc Pt. The largest <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mi>α</mml:mi> <mml:mrow> <mml:mi>x</mml:mi> <mml:mi>y</mml:mi> </mml:mrow> <mml:mi>A</mml:mi> </mml:msubsup> </mml:mrow> </mml:math> of 2.52 A m –1 K –1 is obtained in L 1 0 -CoPt for Pt-rich composition of x = 70%, which is larger than that for an additionally fabricated nearly stoichiometric L 1 0 -Co 48 Pt 52 reference uniform film. The contribution from direct conversion of a temperature gradient to a transverse charge current through <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mi>α</mml:mi> <mml:mrow> <mml:mi>x</mml:mi> <mml:mi>y</mml:mi> </mml:mrow> <mml:mi>A</mml:mi> </mml:msubsup> </mml:mrow> </mml:math> is dominant to the total anomalous Nernst coefficient compared to the AHE-related contribution. From a scaling analysis of the AHE, the intrinsic contribution is found to be dominant for x = 70%. A theoretical calculation for <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mi>α</mml:mi> <mml:mrow> <mml:mi>x</mml:mi> <mml:mi>y</mml:mi> </mml:mrow> <mml:mi>A</mml:mi> </mml:msubsup> </mml:mrow> </mml:math> of L 1 0 -Co 50 Pt 50 agrees with the experimental <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mi>α</mml:mi> <mml:mrow> <mml:mi>x</mml:mi> <mml:mi>y</mml:mi> </mml:mrow> <mml:mi>A</mml:mi> </mml:msubsup> </mml:mrow> </mml:math> value for the nearly stoichiometric reference film by considering on-site Coulomb interaction for Co atoms. We also point out the possible electron doping effect by the addition of Pt in L 1 0 -CoPt, which could explain the larger <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mi>α</mml:mi> <mml:mrow> <mml:mi>x</mml:mi> <mml:mi>y</mml:mi> </mml:mrow> <mml:mi>A</mml:mi> </mml:msubsup> </mml:mrow> </mml:math> for the off-stoichiometric Pt-rich composition than that for the nearly stoichiometric one. Our experimental and theoretical results suggest the potential of L 1 0 -CoPt with a large <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mi>α</mml:mi> <mml:mrow> <mml:mi>x</mml:mi> <mml:mi>y</mml:mi> </mml:mrow> <mml:mi>A</mml:mi> </mml:msubsup> </mml:mrow> </mml:math> originating from the intrinsic mechanism for future thermoelectric applications.