Authors: Elizabeth Donoway | Thaís V. Trevisan | Alex Liebman‐Peláez | Ryan Day | K. Yamakawa | Yuping Sun | Jian-Rui Soh | D. Prabhakaran | A. T. Boothroyd | Rafael M. Fernandes | James G. Analytis | Joel E. Moore | J. Orenstein | Veronika Sunko
Source: Physical Review X (article, 2024-07-22)
Field: Materials Science — Electronic, Optical and Magnetic Materials

Multimodal Approach Reveals the Symmetry-Breaking Pathway to the Broken Helix in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>EuIn</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi>As</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math> Understanding and manipulating emergent phases, which are themes at the forefront of quantum-materials research, rely on identifying their underlying symmetries. This general principle has been particularly prominent in materials with coupled electronic and magnetic degrees of freedom, in which magnetic order influences the electronic band structure and can lead to exotic topological effects. However, identifying symmetry of a magnetically ordered phase can pose a challenge, particularly in the presence of small domains. Here we introduce a multimodal approach for determining magnetic structures, which combines symmetry-sensitive optical probes, scattering, and group-theoretical analysis. We apply it to <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:msub><a:mrow><a:mi>EuIn</a:mi></a:mrow><a:mrow><a:mn>2</a:mn></a:mrow></a:msub></a:mrow><a:mrow><a:msub><a:mrow><a:mi>As</a:mi></a:mrow><a:mrow><a:mn>2</a:mn></a:mrow></a:msub></a:mrow></a:math>, a material that has received attention as a candidate axion insulator. While first-principles calculations predict this state on the assumption of a simple collinear antiferromagnetic structure, subsequent neutron-scattering measurements reveal a much more intricate magnetic ground state characterized by two coexisting magnetic wave vectors reached by successive thermal phase transitions. The proposed high- and low-temperature phases are a spin helix and a state with interpenetrating helical and Néel antiferromagnetic order termed a “broken helix,” respectively. Employing a multimodal approach, we identify the magnetic structure associated with these two phases of <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mrow><c:msub><c:mrow><c:mi>EuIn</c:mi></c:mrow><c:mrow><c:mn>2</c:mn></c:mrow></c:msub></c:mrow><c:mrow><c:msub><c:mrow><c:mi>As</c:mi></c:mrow><c:mrow><c:mn>2</c:mn></c:mrow></c:msub></c:mrow></c:math>. We find that the higher-temperature phase is characterized by a variation of the magnetic moment amplitude from layer to layer, with the moment vanishing entirely in every third Eu layer. The lower-temperature structure is similar to the broken helix, with one important difference: Because of local strain, the relative orientation of the magnetic structure and the lattice is not fixed. Consequently, the symmetry required to protect the axion phase is not generically protected in <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mrow><e:msub><e:mrow><e:mi>EuIn</e:mi></e:mrow><e:mrow><e:mn>2</e:mn></e:mrow></e:msub></e:mrow><e:mrow><e:msub><e:mrow><e:mi>As</e:mi></e:mrow><e:mrow><e:mn>2</e:mn></e:mrow></e:msub></e:mrow></e:math>, but we show that it can be restored if the magnetic structure is tuned with uniaxial strain. Finally, we present a spin Hamiltonian that identifies the spin interactions that account for the complex magnetic order in <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:mrow><g:msub><g:mrow><g:mi>EuIn</g:mi></g:mrow><g:mrow><g:mn>2</g:mn></g:mrow></g:msub></g:mrow><g:mrow><g:msub><g:mrow><g:mi>As</g:mi></g:mrow><g:mrow><g:mn>2</g:mn></g:mrow></g:msub></g:mrow></g:math>. Our work highlights the importance of a multimodal approach in determining the symmetry of complex order parameters. Published by the American Physical Society 2024 Antiferromagnetism ; Homogeneous space ; Physics ; Symmetry breaking ; Neutron scattering ; Condensed matter physics ; Scattering ; Quantum mechanics ; Mathematics ; Geometry Magnetic and transport properties of perovskites and related materials ; Advanced Condensed Matter Physics ; Physics of Superconductivity and Magnetism