Discovery of a second type of high-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>T</mml:mi><mml:mi>S</mml:mi></mml:msub></mml:math> magnetic spiral in layered <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:

Layered Cu/Fe perovskites with a <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:msub><a:mi>YBaCuFeO</a:mi><a:mn>5</a:mn></a:msub></a:math> structure have attracted significant attention owing to the high thermal stability (<b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:msub><b:mi>T</b:mi><b:mi>S</b:mi></b:msub></b:math>) of their incommensurate spiral magnetic phase. The intrinsic coupling between spin and ferroelectric orders in these cycloidally ordered multiferroics makes them among the most promising spin-driven magnetoelectric multiferroic candidates. In this system, strong in-plane antiferromagnetic couplings lead to collinearly ordered magnetic planes with spin directions alternating with periodicity (<c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:msub><c:mi>k</c:mi><c:mi>x</c:mi></c:msub><c:mo>,</c:mo><c:mo> </c:mo><c:msub><c:mi>k</c:mi><c:mi>y</c:mi></c:msub></c:math>) = (0.5, 0.5), while spin propagation along the perpendicular direction is governed by competing frustrated magnetic couplings of varying signs and strengths. In compounds of this family studied to date, the incommensurate spiral order emerges below <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:msub><d:mi>T</d:mi><d:mi>S</d:mi></d:msub></d:math> due to correlated canting of ferromagnetically coupled spins in bipyramidal layers (<e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mrow><e:msub><e:mi>k</e:mi><e:mi>z</e:mi></e:msub><e:mo>=</e:mo><e:mn>0.5</e:mn><e:mo>→</e:mo><e:mn>0.5</e:mn><e:mo>±</e:mo><e:mspace width="4pt"/><e:msub><e:mi>q</e:mi><e:mi>S</e:mi></e:msub></e:mrow></e:math>). Here, we demonstrate that a spiral magnetic ground state can also arise from a distinct precursor collinear spin configuration, where, above <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"><g:msub><g:mi>T</g:mi><g:mi>S</g:mi></g:msub></g:math>, spins in the bipyramids are initially antiferromagnetically coupled (<h:math xmlns:h="http://www.w3.org/1998/Math/MathML"><h:mrow><h:msub><h:mi>k</h:mi><h:mi>z</h:mi></h:msub><h:mo>=</h:mo><h:mn>0</h:mn><h:mo>→</h:mo><h:mo>±</h:mo><h:mspace width="4pt"/><h:msub><h:mi>q</h:mi><h:mi>S</h:mi></h:msub></h:mrow></h:math>). This newly observed spiral magnetic transition, identified and investigated in <j:math xmlns:j="http://www.w3.org/1998/Math/MathML"><j:mrow><j:msub><j:mi>YBaCu</j:mi><j:mrow><j:mn>1</j:mn><j:mo>−</j:mo><j:mi>x</j:mi></j:mrow></j:msub><j:msub><j:mi mathvariant="normal">M</j:mi><j:mi>x</j:mi></j:msub><j:msub><j:mi>FeO</j:mi><j:mn>5</j:mn></j:msub></j:mrow></j:math> compounds (M: Co, Zn), exists beyond the triple magnetic point, previously considered the critical limit for the ordering of the magnetic spiral in <l:math xmlns:l="http://www.w3.org/1998/Math/MathML"><l:msub><l:mi>YBaCuFeO</l:mi><l:mn>5</l:mn></l:msub></l:math>. In the compounds investigated, the new spiral state persists in this region, stabilizing at temperatures up to 365 K.