Penetration depth measurements in HoNi 2B 2C: competition between superconducting and magnetic order

Abstract

We report measurements of the magnetic penetration depth in single crystals of HoNi 2B 2C down to 100 mK using a tunnel-diode-based, self-inductive technique at 28 MHz. We observe a sharp decrease in the RF penetration depth λ( T) at the superconducting transition near 9 K. A pronounced shoulder appears at approximately 8 K, followed by dramatic increases in λ( T) at 5.7 and 5.2 K. These increases correspond approximately to the onsets of previously reported magnetic structures. Just below the 5.2 K peak, λ( T) drops abruptly, suggesting a reentrant superconducting state. Below 3 K, λ( T) increases gradually to a broad maximum near 1 K, then decreases again. In addition, three previously unreported, smaller transitions are observed between 1.7 and 2.5 K. It appears that the c-axis helical phase inhibits the increase in the density of Cooper pairs, but does not destroy superconductivity. Magnetization data of HoNi 2B 2C are also presented for comparison.