Characterization of the magnetic phase transitions in double perovskite <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>Nd</mml:mi> <mml:mn>2</mml:mn> </mml:msub> <mml:msub> <mml:mi>NiMnO</mml:mi> <mml:mn>6</mm

Through detailed analysis of neutron powder diffraction data, we confirm that the double perovskite <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:msub> <a:mi>Nd</a:mi> <a:mn>2</a:mn> </a:msub> <a:msub> <a:mi>NiMnO</a:mi> <a:mn>6</a:mn> </a:msub> </a:mrow> </a:math> adopts a monoclinic <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:mrow> <b:mi>P</b:mi> <b:msub> <b:mn>2</b:mn> <b:mn>1</b:mn> </b:msub> <b:mo>/</b:mo> <b:mi>n</b:mi> </b:mrow> </b:math> structure with nearly complete <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:mrow> <c:mi>B</c:mi> </c:mrow> </c:math> -site ordering of <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"> <d:msup> <d:mrow> <d:mi>Ni</d:mi> </d:mrow> <d:mrow> <d:mn>2</d:mn> <d:mo>+</d:mo> </d:mrow> </d:msup> </d:math> and <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"> <e:msup> <e:mrow> <e:mi>Mn</e:mi> </e:mrow> <e:mrow> <e:mn>4</e:mn> <e:mo>+</e:mo> </e:mrow> </e:msup> </e:math> cations. Below <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"> <f:mrow> <f:msub> <f:mi>T</f:mi> <f:mn>1</f:mn> </f:msub> <f:mo>=</f:mo> <f:mn>198</f:mn> <f:mspace width="0.16em"/> <f:mi mathvariant="normal">K</f:mi> </f:mrow> </f:math> , magnetic susceptibility measurements and further analysis of neutron diffraction data reveal that the <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"> <i:msup> <i:mi>Ni</i:mi> <i:mrow> <i:mn>2</i:mn> <i:mo>+</i:mo> </i:mrow> </i:msup> </i:math> and <j:math xmlns:j="http://www.w3.org/1998/Math/MathML"> <j:msup> <j:mi>Mn</j:mi> <j:mrow> <j:mn>4</j:mn> <j:mo>+</j:mo> </j:mrow> </j:msup> </j:math> sublattices undergo ferromagnetic ordering, as expected of strong <k:math xmlns:k="http://www.w3.org/1998/Math/MathML"> <k:mrow> <k:mn>3</k:mn> <k:mi>d</k:mi> <k:mtext>−</k:mtext> <k:mn>3</k:mn> <k:mi>d</k:mi> </k:mrow> </k:math> exchange interactions. Upon cooling through <l:math xmlns:l="http://www.w3.org/1998/Math/MathML"> <l:msub> <l:mi>T</l:mi> <l:mn>2</l:mn> </l:msub> <l:mo> </l:mo> <l:mrow> <l:mo>=</l:mo> <l:mn>22</l:mn> <l:mspace width="0.16em"/> <l:mi mathvariant="normal">K</l:mi> </l:mrow> </l:math> , a secondary magnetic transition is observed, below which we have discovered an additional noncollinear symmetry-breaking order of <o:math xmlns:o="http://www.w3.org/1998/Math/MathML"> <o:msup> <o:mi>Nd</o:mi> <o:mrow> <o:mn>3</o:mn> <o:mo>+</o:mo> </o:mrow> </o:msup> </o:math> moments. We argue that the canting of the rare-earth moments naturally arises through the competition of <p:math xmlns:p="http://www.w3.org/1998/Math/MathML"> <p:mrow> <p:mi>f</p:mi> <p:mtext>−</p:mtext> <p:mi>d</p:mi> </p:mrow> </p:math> and <q:math xmlns:q="http://www.w3.org/1998/Math/MathML"> <q:mrow> <q:mi>f</q:mi> <q:mtext>−</q:mtext> <q:mi>f</q:mi> </q:mrow> </q:math> Heisenberg exchange interactions, which may be finely balanced in the double perovskite framework. Furthermore, the symmetry of the ground state magnetic structure implies significant <r:math xmlns:r="http://www.w3.org/1998/Math/MathML"> <r:msup> <r:mrow> <r:mi>Nd</r:mi> </r:mrow> <r:mrow> <r:mn>3</r:mn> <r:mo>+</r:mo> </r:mrow> </r:msup> </r:math> easy plane anisotropy, and an effective decoupling of the antiferromagnetic spin canting from the transition metal sublattice. Between <s:math xmlns:s="http://www.w3.org/1998/Math/MathML"> <s:msub> <s:mi>T</s:mi> <s:mn>1</s:mn> </s:msub> </s:math> and <t:math xmlns:t="http://www.w3.org/1998/Math/MathML"> <t:msub> <t:mi>T</t:mi> <t:mn>2</t:mn> </t:msub> <t:mo>,</t:mo> <t:mo> </t:mo> <t:mrow> <t:msub> <t:mi>Nd</t:mi> <t:mn>2</t:mn> </t:msub> <t:msub> <t:mi>NiMnO</t:mi> <t:mn>6</t:mn> </t:msub> </t:mrow> </t:math> shows anomalous behavior in the frequency-dependent ac magnetic susceptibility that is characteristic of reentrant spin-glass-like properties attributed to antisite disorder and competing interactions. Finally, analysis of isothermal magnetization yields magnetic entropy changes that suggest <u:math xmlns:u="http://www.w3.org/1998/Math/MathML"> <u:mrow> <u:msub> <u:mi>Nd</u:mi> <u:mn>2</u:mn> </u:msub> <u:msub> <u:mi>NiMnO</u:mi> <u:mn>6</u:mn> </u:msub> </u:mrow> </u:math> and related compounds have potential for magnetic refrigeration, showing a peak in the magnetic entropy change of <v:math xmlns:v="http://www.w3.org/1998/Math/MathML"> <v:mrow> <v:mn>2.25</v:mn> <v:mspace width="0.28em"/> <v:mi mathvariant="normal">J</v:mi> <v:mspace width="0.16em"/> <v:msup> <v:mi>kg</v:mi> <v:mrow> <v:mo>−</v:mo> <v:mn>1</v:mn> </v:mrow> </v:msup> <v:msup> <v:mi mathvariant="normal">K</v:mi> <v:mrow> <v:mo>−</v:mo> <v:mn>1</v:mn> </v:mrow> </v:msup> </v:mrow> </v:math> at <ab:math xmlns:ab="http://www.w3.org/1998/Math/MathML"> <ab:msub> <ab:mi>T</ab:mi> <ab:mn>1</ab:mn> </ab:msub> </ab:math> under a 7 T field. The scaling behavior of the magnetic entropy, paired with analysis of other critical exponents, shows that the ferromagnetic transition at <bb:math xmlns