# Helical magnetic ordering in Sr(Co1-xNix)2As2

## TL;DR

This study reveals that Ni-doping in SrCo2As2 induces a transition from non-magnetic or stripe-type AF fluctuations to a long-range helical AF order, highlighting complex magnetic interactions influenced by electronic structure.

## Contribution

It demonstrates the emergence of incommensurate helical AF order in Sr(Co1-xNix)2As2 with Ni-doping, contrasting previous stripe-type AF fluctuations and indicating competing magnetic interactions.

## Key findings

- Ni-doping induces long-range AF order
- The AF order is helical, not stripe-type
- No structural phase transition observed

## Abstract

SrCo2As2 is a peculiar itinerant magnetic system that does not order magnetically, but inelastic neutron scattering experiments observe the same stripe-type antiferromagnetic (AF) fluctuations found in many of the Fe-based superconductors along with evidence of magnetic frustration. Here we present results from neutron diffraction measurements on single crystals of Sr(Co1-xNix)2As2 that show the development of long-range AF order with Ni-doping. However, the AF order is not stripe-type. Rather, the magnetic structure consists of ferromagnetically-aligned (FM) layers (with moments laying in the layer) that are AF arranged along c with an incommensurate propagation vector of (0 0 tau), i.e. a helix. Using high-energy x-ray diffraction, we find no evidence for a temperature-induced structural phase transition that would indicate a collinear AF order. This finding supports a picture of competing FM and AF interactions within the square transition-metal layers due to flat-band magnetic instabilities. However, the composition dependence of the propagation vector suggests that far more subtle Fermi surface and orbital effects control the interlayer magnetic correlations.