Cubic magnetic anisotropy in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>B</mml:mi> <mml:mn>20</mml:mn> </mml:mrow> </mml:math> magnets: Interplay of anisotropy and magnetic order in <mml:math xmlns:mml="http://www.w3

The metallic systems MnSi and <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>Co</a:mi> <a:mi>x</a:mi> </a:msub> <a:mi>Si</a:mi> </a:mrow> </a:math> are known to feature a generic magnetic phase diagram primarily determined by the isotropic exchange and Dzyaloshinskii-Moriya interactions. However, additional weaker anisotropies, lowest in the hierarchy of energy scales, play a crucial role: they determine the relative order of phases in the phase diagram and may even enable skyrmion stability far below the ordering temperature. Among cubic B20 helimagnets, the insulator <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:mrow> <b:msub> <b:mi>Cu</b:mi> <b:mn>2</b:mn> </b:msub> <b:msub> <b:mi>OSeO</b:mi> <b:mn>3</b:mn> </b:msub> </b:mrow> </b:math> is currently the only known example exhibiting a low-temperature, anisotropy-induced skyrmion pocket. In this manuscript, we present a systematic study of cubic magnetocrystalline anisotropy by means of angle-resolved SQUID magnetization measurements in MnSi and <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:mrow> <c:msub> <c:mi>Fe</c:mi> <c:mrow> <c:mn>1</c:mn> <c:mo>−</c:mo> <c:mi>x</c:mi> </c:mrow> </c:msub> <c:msub> <c:mi>Co</c:mi> <c:mi>x</c:mi> </c:msub> <c:mi>Si</c:mi> </c:mrow> </c:math> <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"> <d:mrow> <d:mo>(</d:mo> <d:mn>0.08</d:mn> <d:mo>≤</d:mo> <d:mi>x</d:mi> <d:mo>≤</d:mo> <d:mn>0.70</d:mn> <d:mo>)</d:mo> </d:mrow> </d:math> single crystals and provide quantitative values of the anisotropy constants. For <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"> <e:mrow> <e:msub> <e:mi>Fe</e:mi> <e:mrow> <e:mn>1</e:mn> <e:mo>−</e:mo> <e:mi>x</e:mi> </e:mrow> </e:msub> <e:msub> <e:mi>Co</e:mi> <e:mi>x</e:mi> </e:msub> <e:mi>Si</e:mi> </e:mrow> </e:math> , the cubic anisotropy is found to be strongly dependent on the Co concentration <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"> <f:mi>x</f:mi> </f:math> . From a theoretical point of view, for low Co concentrations <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"> <g:mrow> <g:mo>(</g:mo> <g:mi>x</g:mi> <g:mo>∼</g:mo> <g:mn>0.10</g:mn> <g:mo>)</g:mo> </g:mrow> </g:math> , the anisotropy is expected to be sufficiently strong to stabilize a low-temperature skyrmion lattice. Therefore, <h:math xmlns:h="http://www.w3.org/1998/Math/MathML"> <h:mrow> <h:msub> <h:mi>Fe</h:mi> <h:mrow> <h:mn>1</h:mn> <h:mo>−</h:mo> <h:mi>x</h:mi> </h:mrow> </h:msub> <h:msub> <h:mi>Co</h:mi> <h:mi>x</h:mi> </h:msub> <h:mi>Si</h:mi> </h:mrow> </h:math> may represent the first chiral metallic system to exhibit a low-temperature skyrmion phase controllably stabilized by cubic anisotropy for specific directions of the magnetic field.