Stacking-engineered tunability of electronic and magnetic states in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>Nb</mml:mi> <mml:mn>3</mml:mn> </mml:msub> <mml:msub> <mml:mi mathvariant="normal">I</mml:mi>

Both monolayer and bilayer <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:msub> <a:mi>Nb</a:mi> <a:mn>3</a:mn> </a:msub> <a:msub> <a:mi mathvariant="normal">I</a:mi> <a:mn>8</a:mn> </a:msub> </a:mrow> </a:math> were theoretically predicted to be magnetic, whereas bulk <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:mrow> <c:msub> <c:mi>Nb</c:mi> <c:mn>3</c:mn> </c:msub> <c:msub> <c:mi mathvariant="normal">I</c:mi> <c:mn>8</c:mn> </c:msub> </c:mrow> </c:math> was experimentally observed to exhibit a nonmagnetic (NM) ground state. In this work, we pursue the origin of bulk NM behavior and the evolution of magnetism in <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"> <e:mrow> <e:msub> <e:mi>Nb</e:mi> <e:mn>3</e:mn> </e:msub> <e:msub> <e:mi mathvariant="normal">I</e:mi> <e:mn>8</e:mn> </e:msub> </e:mrow> </e:math> nanofilms using first-principles calculations. We identified three distinct stacking types in bilayer <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"> <g:mrow> <g:msub> <g:mi>Nb</g:mi> <g:mn>3</g:mn> </g:msub> <g:msub> <g:mi mathvariant="normal">I</g:mi> <g:mn>8</g:mn> </g:msub> </g:mrow> </g:math> , further categorized into nine specific configurations. Strikingly, the HH-AA stacking configuration adopts an NM ground state, while the other eight configurations stabilize as antiferromagnetic (AFM) states. The magnetization loss in HH-AA stacking originates from charge transfer from interfacial I atoms to Nb atoms, driving the <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"> <i:msub> <i:mi>Nb</i:mi> <i:mn>3</i:mn> </i:msub> </i:math> trimer's transition from <j:math xmlns:j="http://www.w3.org/1998/Math/MathML"> <j:mrow> <j:msub> <j:mrow> <j:mo>[</j:mo> <j:mi>Nb</j:mi> </j:mrow> <j:mn>3</j:mn> </j:msub> <j:msup> <j:mrow> <j:mo>]</j:mo> </j:mrow> <j:mrow> <j:mn>8</j:mn> <j:mo>+</j:mo> </j:mrow> </j:msup> </j:mrow> </j:math> ( <k:math xmlns:k="http://www.w3.org/1998/Math/MathML"> <k:mrow> <k:mi>S</k:mi> </k:mrow> </k:math> = 1/2) to <l:math xmlns:l="http://www.w3.org/1998/Math/MathML"> <l:mrow> <l:msub> <l:mrow> <l:mo>[</l:mo> <l:mi>Nb</l:mi> </l:mrow> <l:mn>3</l:mn> </l:msub> <l:msup> <l:mrow> <l:mo>]</l:mo> </l:mrow> <l:mrow> <l:mn>7</l:mn> <l:mo>+</l:mo> </l:mrow> </l:msup> </l:mrow> </l:math> ( <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"> <m:mrow> <m:mi>S</m:mi> </m:mrow> </m:math> = 0). Furthermore, multilayer <n:math xmlns:n="http://www.w3.org/1998/Math/MathML"> <n:mrow> <n:msub> <n:mi>Nb</n:mi> <n:mn>3</n:mn> </n:msub> <n:msub> <n:mi mathvariant="normal">I</n:mi> <n:mn>8</n:mn> </n:msub> </n:mrow> </n:math> films with bulk stacking exhibit layer-dependent oscillations between ferromagnetic (FM) and NM states, alternating with odd and even layer counts. The NM ground state of bulk <p:math xmlns:p="http://www.w3.org/1998/Math/MathML"> <p:mrow> <p:msub> <p:mi>Nb</p:mi> <p:mn>3</p:mn> </p:msub> <p:msub> <p:mi mathvariant="normal">I</p:mi> <p:mn>8</p:mn> </p:msub> </p:mrow> </p:math> , which consists of alternating HH-AA and TT-AC stacking sequences, results from the moment quenching in HH-AA stacking over the AFM coupling in TT-AC stacking. Our findings establish <r:math xmlns:r="http://www.w3.org/1998/Math/MathML"> <r:mrow> <r:msub> <r:mi>Nb</r:mi> <r:mn>3</r:mn> </r:msub> <r:msub> <r:mi mathvariant="normal">I</r:mi> <r:mn>8</r:mn> </r:msub> </r:mrow> </r:math> as a promising platform for stacking-engineered functionalities. Its stacking diversity not only broadens the functional landscape of two-dimensional materials but also reveals hidden correlations between structural degrees of freedom and electronic/magnetic responses.