Disorder-induced damping of spin excitations in Cr-doped <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>BaFe</mml:mi> <mml:mn>2</mml:mn> </mml:msub> <mml:msub> <mml:mi>As</mml:mi> <mml:mn>2</mml:mn> </mml:msub

In doped Hund's metals, such as the iron-based superconductors, effects like charge doping and chemical pressure are often considered the dominant factors. Partial chemical substitution, however, inevitably introduces disorder. Here, we investigate spin excitations in Ba( <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:msub> <a:mi>Fe</a:mi> <a:mrow> <a:mn>1</a:mn> <a:mo>−</a:mo> <a:mi>x</a:mi> </a:mrow> </a:msub> <a:msub> <a:mi>Cr</a:mi> <a:mi>x</a:mi> </a:msub> <a:msub> <a:mrow> <a:mo>)</a:mo> </a:mrow> <a:mn>2</a:mn> </a:msub> <a:msub> <a:mi>As</a:mi> <a:mn>2</a:mn> </a:msub> </a:mrow> </a:math> (CrBFA) by high-resolution resonant inelastic x-ray scattering for samples with <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:mrow> <b:mi>x</b:mi> <b:mo>=</b:mo> <b:mn>0</b:mn> <b:mo>,</b:mo> <b:mn>0.035</b:mn> <b:mo>,</b:mo> </b:mrow> </b:math> and <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:mrow> <c:mn>0.085</c:mn> </c:mrow> </c:math> . In CrBFA, Cr acts as a hole dopant, but also introduces localized spins that compete with Fe-derived magnetic excitations. We found that the Fe-derived magnetic excitations are softened and damped, becoming overdamped for <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"> <d:mrow> <d:mi>x</d:mi> <d:mo>=</d:mo> <d:mn>0.085</d:mn> </d:mrow> </d:math> . At this doping level, complementary angle-resolved photoemission spectroscopy measurements show increased electronic localization and a suppression of the nematic <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"> <e:mrow> <e:msub> <e:mi>d</e:mi> <e:mrow> <e:mi>x</e:mi> <e:mi>z</e:mi> </e:mrow> </e:msub> <e:mo>/</e:mo> <e:msub> <e:mi>d</e:mi> <e:mrow> <e:mi>y</e:mi> <e:mi>z</e:mi> </e:mrow> </e:msub> </e:mrow> </e:math> band splitting present in the parent compound. We thus propose a localized spin model that explicitly incorporates substitutional disorder and Cr local moments, successfully reproducing our key observations. Our findings reveal a case where disorder dominates over charge doping in the case of a Hund's metal.