Magnetic Pitch as a Tuning Fork for Superconductivity

TL;DR

This paper explores how variable magnetic pitch in helical spin arrangements can tune the transition between ferromagnetic and antiferromagnetic states, influencing superconductivity, with experimental evidence from MnP.

Contribution

It introduces the concept that magnetic pitch length in helical magnets can be used to continuously tune superconductivity, supported by experimental data on MnP.

Findings

Reduced-moment helical spin order observed at high pressure in MnP

Tighter magnetic pitch correlates with the emergence of superconductivity

Magnetic pitch length influences the transition temperature in related compounds

Abstract

Magnetism and superconductivity often compete for preeminence as a material's ground state, and in the right circumstances the fluctuating remains of magnetic order can induce superconducting pairing. The intertwining of the two on the microscopic level, independent of lattice excitations, is especially pronounced in heavy fermion compounds, rare earth cuprates, and iron pnictides. Here we point out that for a helical arrangement of localized spins, a variable magnetic pitch length provides a unique tuning process from ferromagnetic to antiferromagnetic ground state in the long and short wavelength limits, respectively. Such chemical or pressure adjustable helical order naturally provides the possibility for continuous tuning between ferromagnetically and antiferromagnetically mediated superconductivity. At the same time, phonon mediated superconductivity is suppressed because of the local ferromagnetic spin configuration. We employ synchrotron-based magnetic x-ray diffraction techniques to test these ideas in the recently discovered superconductor, MnP. This sensitive probe directly reveals a reduced-moment, helical spin order at high pressure proximate to the superconducting state, with a tightened pitch in comparison to that at ambient pressure where superconductivity is absent. The correlation between magnetic pitch length and superconducting transition temperature in the (Cr/Mn/Fe)(P/As) family suggests a strategy for using spiral magnets as interlocutors for spin fluctuation mediated superconductivity.