Penetration depth measurements in HoNi₂B₂C: Competition between superconducting and magnetic order

We report measurements of the magnetic penetration depth in single crystals of HoNi₂B₂C down to 100mK 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 1K, 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 HoNi₂B₂C are also presented for comparison.