Onset of Ferromagnetism in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Eu</mml:mi></mml:mrow><mml:mrow><mml:mi>x</mml:mi></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Sr</mml:mi></mml:mrow><mml:mrow><mml:mn>1</mml:mn><mml:mo>−</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub></mml:mrow><mml:mi mathvariant="normal">S</mml:mi></mml:math>near<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>x</mml:mi><mml:mo>=</mml:mo><mml:mn>0.5</mml:mn><mml:mn/></mml:math>

In a neutron-diffraction study of insulating ${\mathrm{Eu}}_{x}{\mathrm{Sr}}_{1\ensuremath{-}x}\mathrm{S}$ with $x=0.40, 0.50, 0.52, \mathrm{and} 0.54$, the critical concentration for the spin-glass-to-ferromagnetism transition is determined to be as high as ${x}_{c}=0.51$. For $x$ just above ${x}_{c}$, magnetic microdomains coexist with longrange ferromagnetic order, decoupled from one another by competing antiferromagnetic bonds. For the first time the breakdown of ferromagnetic order into microdomains is directly observed by decreasing the temperature, and it is interpreted as a ferromagnetism-to-spin-glass transition.