Pinning of antiferromagnetic domain walls and domain structure in ultrathin <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>La</mml:mi> <mml:mrow> <mml:mn>0.45</mml:mn> </mml:mrow> </mml:msub> <mml:msub> <mml:m

We report the evolution of the antiferromagnetic domain structure of epitaxial <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:msub> <a:mi>La</a:mi> <a:mrow> <a:mn>0.45</a:mn> </a:mrow> </a:msub> <a:msub> <a:mi>Sr</a:mi> <a:mrow> <a:mn>0.55</a:mn> </a:mrow> </a:msub> <a:msub> <a:mi>MnO</a:mi> <a:mn>3</a:mn> </a:msub> </a:mrow> </a:math> (LSMO) ultrathin films with thickness in the range from 5–50 unit cells (uc), using x-ray photoemission electron microscopy (XPEEM). While the 5 uc thick LSMO shows no magnetic contrast down to <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:mrow> <b:mo>∼</b:mo> <b:mn>80</b:mn> <b:mspace width="0.16em"/> <b:mi mathvariant="normal">K</b:mi> </b:mrow> </b:math> , thicker films display a multidomain antiferromagnetic configuration with a nonmonotonic variation in the characteristic domain size from 0.3 <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"> <e:mrow> <e:mi>µ</e:mi> <e:mi mathvariant="normal">m</e:mi> </e:mrow> </e:math> at 10 uc, to 3–5 <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"> <g:mrow> <g:mi>µ</g:mi> <g:mi mathvariant="normal">m</g:mi> </g:mrow> </g:math> at 15–16 uc and 1 <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"> <i:mrow> <i:mi>µ</i:mi> <i:mi mathvariant="normal">m</i:mi> </i:mrow> </i:math> for 50 uc. Post-growth annealing is found not to impact strongly the magnetic domain state. In contrast, for a 15 uc film grown on a substrate characterized by large atomic steps <k:math xmlns:k="http://www.w3.org/1998/Math/MathML"> <k:mrow> <k:mo>(</k:mo> <k:mo>∼</k:mo> <k:mn>1</k:mn> <k:mspace width="0.16em"/> <k:mi>µ</k:mi> <k:mi mathvariant="normal">m</k:mi> <k:mo>)</k:mo> </k:mrow> </k:math> , we observe much larger antiferromagnetic domains and the presence of net magnetic moment in the form of stripes with alternating contrast, assigned to the signal from the top uncompensated spins of the A-type antiferromagnetic state of LSMO. From the combined antiferromagnetic domain structure and the net magnetic moment contrast, we determine the exact orientation of the Néel vector, including at domain walls. We describe the antiferromagnetic domain size distribution in terms of interface and bulk contributions to the density of defects that pin the antiferromagnetic domain walls and which determine the equilibrium domain configuration. Our results demonstrate the impact of thickness and defects on the antiferromagnetic domain size and constitute a stepping stone in controlling the antiferromagnetic domain state required for oxide antiferromagnetic device applications.