Anisotropy at twin interfaces in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mrow><mml:mi>R</mml:mi><mml:mi>T</mml:mi></mml:mrow><mml:mn>12</mml:mn></mml:msub></mml:math> (<mml:math xmlns:mml="http://www.w3.org/1998/Math/M

<a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:msub><a:mrow><a:mi>R</a:mi><a:mi>T</a:mi></a:mrow><a:mn>12</a:mn></a:msub></a:math> materials continue to attract attention due to their potential use as “rare-earth-lean” permanent magnets, but converting their promising intrinsic properties into practical high performance remains an elusive goal. Sophisticated experimental characterization techniques are providing unprecedented insight into the structure of these materials at the atomistic scale. Atomistic spin dynamics or micromagnetics simulations could help unravel the links between these structures and resultant magnet performance, but require input data describing the intrinsic magnetic properties. Here, first-principles calculations based on density functional theory are used to determine these properties for two model interface structures which have been derived from recently reported high-resolution electron microscopy images. One model structure is a stoichiometric twin formed by mirroring the <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:msub><b:mrow><b:mi>R</b:mi><b:mi>T</b:mi></b:mrow><b:mn>12</b:mn></b:msub></b:math> structure in the (101) plane, and the other model structure is a “stacking fault” involving the insertion of a <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:msub><c:mrow><c:mi>R</c:mi><c:mi>T</c:mi></c:mrow><c:mn>4</c:mn></c:msub></c:math> plane and a displacement along the [100] axis. Magnetic moments and crystal field coefficients have been calculated for the optimized structures. The interfaces modify the magnetic properties at the sub-nm scale. In particular, in the <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:mrow><d:mi>R</d:mi></d:mrow></d:math>-rich region of the “stacking fault,” the local easy axis of magnetization rotates by <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:msup><e:mn>49</e:mn><e:mo>∘</e:mo></e:msup></e:math> from its bulk direction, which may lead to reduced coercivity through the easier nucleation of reverse domains.