Bulk magnetic properties of distorted square lattice compounds <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msup> <mml:mi>M</mml:mi> <mml:mo>′</mml:mo> </mml:msup> <mml:mtext>−</mml:mtext> <mml:mi>L</mml:mi> <mml:mi>n</mm

We report bulk magnetic properties of the monoclinic lanthanide tantalates, <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:msup> <a:mi>M</a:mi> <a:mo>′</a:mo> </a:msup> <a:mtext>−</a:mtext> <a:mrow> <a:mi>L</a:mi> <a:mi>n</a:mi> </a:mrow> <a:msub> <a:mi>TaO</a:mi> <a:mn>4</a:mn> </a:msub> </a:mrow> </a:math> ( <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:mrow> <b:mi>L</b:mi> <b:mi>n</b:mi> </b:mrow> </b:math> = Tb, Dy, Ho, Er), where the magnetic <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:mrow> <c:mi>L</c:mi> <c:msup> <c:mi>n</c:mi> <c:mrow> <c:mn>3</c:mn> <c:mo>+</c:mo> </c:mrow> </c:msup> </c:mrow> </c:math> ions are arranged on a distorted 2D square lattice. The heavier analog <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"> <d:mrow> <d:msup> <d:mi>M</d:mi> <d:mo>′</d:mo> </d:msup> <d:mtext>−</d:mtext> <d:msub> <d:mi>YbTaO</d:mi> <d:mn>4</d:mn> </d:msub> </d:mrow> </d:math> has been investigated as a spin-orbit-coupled, quasi-two-dimensional frustrated magnet, and the properties of the other <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"> <e:mrow> <e:msup> <e:mi>M</e:mi> <e:mo>′</e:mo> </e:msup> <e:mtext>−</e:mtext> <e:mrow> <e:mi>L</e:mi> <e:mi>n</e:mi> </e:mrow> <e:msub> <e:mi>TaO</e:mi> <e:mn>4</e:mn> </e:msub> </e:mrow> </e:math> are expected to vary depending on the electronic configuration of the <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"> <f:mrow> <f:mi>L</f:mi> <f:mi>n</f:mi> </f:mrow> </f:math> ion, namely, Kramers vs non-Kramers behavior and different crystal electric field parameters. In this work, powder neutron diffraction is used to confirm the crystal structure for <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"> <g:mrow> <g:mi>L</g:mi> <g:mi>n</g:mi> <g:mo>=</g:mo> </g:mrow> </g:math> Tb, Ho, Er, and to determine the magnetic structure of <h:math xmlns:h="http://www.w3.org/1998/Math/MathML"> <h:mrow> <h:msup> <h:mi>M</h:mi> <h:mo>′</h:mo> </h:msup> <h:mtext>−</h:mtext> <h:msub> <h:mi>TbTaO</h:mi> <h:mn>4</h:mn> </h:msub> </h:mrow> </h:math> , which displays long-range antiferromagnetic (AFM) order below <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"> <i:mrow> <i:msub> <i:mi>T</i:mi> <i:mi mathvariant="normal">N</i:mi> </i:msub> <i:mo>=</i:mo> <i:mn>2.1</i:mn> <i:mspace width="0.16em"/> <i:mi mathvariant="normal">K</i:mi> </i:mrow> </i:math> . The <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"> <m:msup> <m:mrow> <m:mi>Tb</m:mi> </m:mrow> <m:mrow> <m:mn>3</m:mn> <m:mo>+</m:mo> </m:mrow> </m:msup> </m:math> moments are aligned primarily along the <n:math xmlns:n="http://www.w3.org/1998/Math/MathML"> <n:mi>c</n:mi> </n:math> axis with AFM nearest-neighbor interactions. Susceptibility data suggest that <o:math xmlns:o="http://www.w3.org/1998/Math/MathML"> <o:mrow> <o:msup> <o:mi>M</o:mi> <o:mo>′</o:mo> </o:msup> <o:mtext>−</o:mtext> <o:msub> <o:mi>DyTaO</o:mi> <o:mn>4</o:mn> </o:msub> </o:mrow> </o:math> may display short-range ordering around 2.7 K, while <p:math xmlns:p="http://www.w3.org/1998/Math/MathML"> <p:mrow> <p:msup> <p:mi>M</p:mi> <p:mo>′</p:mo> </p:msup> <p:mtext>−</p:mtext> <p:msub> <p:mi>HoTaO</p:mi> <p:mn>4</p:mn> </p:msub> </p:mrow> </p:math> and <q:math xmlns:q="http://www.w3.org/1998/Math/MathML"> <q:mrow> <q:msup> <q:mi>M</q:mi> <q:mo>′</q:mo> </q:msup> <q:mtext>−</q:mtext> <q:msub> <q:mi>ErTaO</q:mi> <q:mn>4</q:mn> </q:msub> </q:mrow> </q:math> show AFM correlations but do not order above 1.8 K. Measurements of the magnetic specific heat provide evidence for a Kramers doublet ground state in <r:math xmlns:r="http://www.w3.org/1998/Math/MathML"> <r:mrow> <r:msup> <r:mi>M</r:mi> <r:mo>′</r:mo> </r:msup> <r:mtext>−</r:mtext> <r:msub> <r:mi>ErTaO</r:mi> <r:mn>4</r:mn> </r:msub> </r:mrow> </r:math> , similar to its heavier analog <s:math xmlns:s="http://www.w3.org/1998/Math/MathML"> <s:mrow> <s:msup> <s:mi>M</s:mi> <s:mo>′</s:mo> </s:msup> <s:mtext>−</s:mtext> <s:msub> <s:mi>YbTaO</s:mi> <s:mn>4</s:mn> </s:msub> </s:mrow> </s:math> .