Breakdown of the field-induced superconductivity by dynamical spin reversal

The quasi-two-dimensional molecular conductor $\lambda$-(BETS)${}_2$FeCl${}_4$, where BETS is bis(ethylenedithio)tetraselenafulvalene, shows a superconductivity in the high magnetic field environment. This is due to the compensation effect between the external field and the internal field created by the Fe${}^{3+}$ magnetic moment. Here, we report the simultaneous transport and electron spin resonance (ESR) measurements in the field-induced superconducting state down to 1.65 K. We find that the field-induced superconductivity is partially destroyed by ESR transitions, namely, by dynamical spin reversals of Fe${}^{3+}$. Moreover, it is found from our ESR results that the field-induced superconducting state is inhomogeneous, coexisting with the residual paramagnetic metal state.

Figure 1

(Left) Crystal structure of $\lambda$-(BETS)${}_2$FeCl${}_4$ salts. For mixed compound, the Fe is partially substituted with Ga. (Right) Schematic phase diagram of mixed compounds, $\lambda$-(BETS)${}_2$Fe${}_x$Ga${}_{1-x}$Cl${}_4$ ($x>0.33$). AFI and FISC denote antiferromagnetic insulator and field-induced superconductor, respectively. The FISC phase appears only for $B\parallel \mathit{ac}$ plane.

Figure 2

Schematic diagram of the experimental setup.

Figure 3

(a) The magnetoresistance and (b) the typical ESR spectra of $\lambda$-(BETS)${}_2$Fe${}_x$Ga${}_{1-x}$Cl${}_4$ ($x=0.6$) for $B\perp$ and $B\parallel \mathit{ac}$ plane at 1.7 K.

Figure 4

The temperature dependence of (a) the magnetoresistance and (b) the ESR spectra for the $x=0.6$ compound ($B\parallel \mathit{ac}$ plane). The black arrows in (a) show the resistance anomalies at the resonance field

Figure 5

The magnetoresistance of $\lambda$-(BETS)${}_2$Fe${}_x$Ga${}_{1-x}$Cl${}_4$($x=0.5$) at 1.7 K for $B\parallel \mathit{ac}$ plane.

Figure 6

The temperature dependence of (a) the magnetoresistance and (b) the ESR spectra for the $x=0.5$ compound in the FISC phase ($B\parallel \mathit{ac}$ plane). The open circles in (a) represent the observed resistance peaks at the resonance field.

Figure 7

(a) The temperature dependence of resistance peak for the $x=0.5$ compound. The actual resistance is offset for each temperature for clarity. (b) The power dependence of the irradiated millimeter wave for 270 GHz at 3.6 K, which shows the maximum resistance peak $\Delta R$. The inset is the attenuation diagram of $\Delta R$.

Figure 8

The temperature dependence of the magnetoresistance for the $x=0.34$ compound ($B\parallel \mathit{ac}$ plane). The open circles represent the observed resistance peaks at the resonance field.