Compensation of Effective Field in the Field-Induced Superconductor k-(BETS)2FeBr4 Observed by 77Se NMR

TL;DR

This study uses 77Se NMR to analyze how the exchange field from Fe spins in k-(BETS)2FeBr4 is compensated by an external magnetic field, elucidating the field-induced superconductivity mechanism.

Contribution

It provides the first detailed NMR analysis of the exchange field compensation mechanism in a field-induced superconductor, confirming the Jaccarino-Peter model.

Findings

Fe spins are antiferromagnetically coupled to pi electrons

Exchange field J is -2.3 T/mu_B

Compensation occurs at an external field of 12 T

Abstract

We report results of 77Se NMR frequency shift in the normal state of the organic charge-transfer-salt k-(BETS)2FeBr4 which shows magnetic field-induced superconductivity (FISC). From a simple mean field analysis, we determined the field and the temperature dependences of the magnetization m_{pi} of the \pi conduction electrons on BETS molecules. We found that the Fe spins are antiferromagnetically coupled to the pi electrons and determined the exchange field to be J = -2.3T/mu_B. The exchange field from the fully saturated Fe moments (5 mu_B) is compensated by an external field of 12T. This is close to the central field of the FISC phase, consistent with the Jaccarino-Peter local field-compensation mechanism for FISC (Phys. Rev. Lett. 9, 290 (1962)).