# Flat band magnetism and helical magnetic order in Ni-doped SrCo$_2$As$_2$

## TL;DR

This study explores how Ni doping in SrCo$_2$As$_2$ induces flat band-driven helimagnetic order, revealing a tunable magnetic phase influenced by electronic structure modifications.

## Contribution

It demonstrates the emergence of helimagnetic order in Ni-doped SrCo$_2$As$_2$ and links it to flat band tuning via combined neutron diffraction and ARPES measurements.

## Key findings

- Helimagnetic order with wavevector q=(0,0,0.56) observed at x=0.1.
- Flat band tuning correlates with the onset of helimagnetism.
- Electronic structure modifications drive magnetic phase transitions.

## Abstract

A series of Sr(Co$_{1-x}$Ni$_x$)$_2$As$_2$ single crystals was synthesized allowing a comprehensive phase diagram with respect to field, temperature, and chemical substitution to be established. Our neutron diffraction experiments revealed a helimagnetic order with magnetic moments ferromagnetically (FM) aligned in the $ab$ plane and a helimagnetic wavevector of $q=(0,0,0.56)$ for $x$ = 0.1. The combination of neutron diffraction and angle-resolved photoemission spectroscopy (ARPES) measurements show that the tuning of a flat band with $d_{x^2-y^2}$ orbital character drives the helimagnetism and indicates the possibility of a quantum order-by-disorder mechanism.