Ferromagnetic moment in the magnetoelectric antiferromagnet <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>Co</mml:mi> <mml:mn>4</mml:mn> </mml:msub> <mml:msub> <mml:mi>Ta</mml:mi> <mml:mn>2</mml:mn> </mml:msu

Exploring ferromagnetic moments in magnetoelectric materials is of both fundamental and technological importance. It enables the control of the correlated state using an uncompensated moment. However, its material realization is challenging owing to its exclusive microscopic mechanisms. Here, we report a nearly isotropic ferromagnetic moment in the magnetoelectric antiferromagnet <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:msub> <a:mi>Co</a:mi> <a:mn>4</a:mn> </a:msub> <a:msub> <a:mi>Ta</a:mi> <a:mn>2</a:mn> </a:msub> <a:msub> <a:mi mathvariant="normal">O</a:mi> <a:mn>9</a:mn> </a:msub> </a:mrow> </a:math> , by performing dc and ac magnetic susceptibility, magnetization, and neutron diffraction measurements on identical single crystals. Combined with the group theory analysis, we demonstrate a <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:mrow> <c:mi>P</c:mi> <c:mn>1</c:mn> </c:mrow> </c:math> magnetic space group. This implies that physical activities can occur along any crystallographic direction, which explains the unusual magnetoelectric property of <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"> <d:mrow> <d:msub> <d:mi>Co</d:mi> <d:mn>4</d:mn> </d:msub> <d:msub> <d:mi>Ta</d:mi> <d:mn>2</d:mn> </d:msub> <d:msub> <d:mi mathvariant="normal">O</d:mi> <d:mn>9</d:mn> </d:msub> </d:mrow> </d:math> . The ground state is classified as the M-type altermagnet based on the magnetic point group. Our study provides essential information to resolve the debate on the magnetic ground state and understand the magnetoelectric properties of the related compound with the prospect of observing and examining the <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"> <f:mrow> <f:mi>P</f:mi> <f:mn>1</f:mn> </f:mrow> </f:math> magnetic space group or ferromagnet in ferroelectrics.