Magnetoelastic-coupling-assisted first-order antiferromagnetic transitions in the corrugated honeycomb-lattice compounds (Ca,Sr)Mn2P2

Understanding the origin of unconventional magnetic order is a central theme in the research field of functional quantum materials for spin-dependent applications. In general, a paramagnetic (PM) to antiferromagnetic (AFM) transition is of thermodynamically second order in nature in the absence of a magnetic field. In the magnetically frustrated corrugated-honeycomb lattice compounds <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mo>(</a:mo><a:mrow><a:mi>Ca</a:mi><a:mo>,</a:mo><a:mi>Sr</a:mi><a:mo>)</a:mo></a:mrow><a:msub><a:mi>Mn</a:mi><a:mn>2</a:mn></a:msub><a:msub><a:mi mathvariant="normal">P</a:mi><a:mn>2</a:mn></a:msub></a:mrow></a:math>, electrical resistivity <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mi>ρ</c:mi></c:math> measurements in the <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:mrow><d:mi>a</d:mi><d:mi>b</d:mi></d:mrow></d:math> plane are reported to exhibit sharp discontinuities at the respective PM to AFM ordering temperatures <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mrow><e:msub><e:mi>T</e:mi><e:mi mathvariant="normal">N</e:mi></e:msub><e:mo>=</e:mo><e:mn>69.5</e:mn></e:mrow></e:math> and 53 K, respectively, signifying the unusual first-order nature of the AFM transition. To reveal the origin of such anomalous transitions, high-resolution linear thermal-expansion measurements are reported for both <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"><g:mrow><g:msub><g:mi>CaMn</g:mi><g:mn>2</g:mn></g:msub><g:msub><g:mi mathvariant="normal">P</g:mi><g:mn>2</g:mn></g:msub></g:mrow></g:math> and <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"><i:mrow><i:msub><i:mi>SrMn</i:mi><i:mn>2</i:mn></i:msub><i:msub><i:mi mathvariant="normal">P</i:mi><i:mn>2</i:mn></i:msub></i:mrow></i:math> in the <k:math xmlns:k="http://www.w3.org/1998/Math/MathML"><k:mrow><k:mi>a</k:mi><k:mi>b</k:mi></k:mrow></k:math> plane and along the <l:math xmlns:l="http://www.w3.org/1998/Math/MathML"><l:mi>c</l:mi></l:math> axis using capacitance dilatometry. The thermal expansion measurements show a sharp discontinuity in <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"><m:mrow><m:mi mathvariant="normal">Δ</m:mi><m:mi>L</m:mi><m:mo>/</m:mo><m:mi>L</m:mi></m:mrow></m:math> at <o:math xmlns:o="http://www.w3.org/1998/Math/MathML"><o:msub><o:mi>T</o:mi><o:mi mathvariant="normal">N</o:mi></o:msub></o:math>, yielding a diverging behavior in the thermal expansion coefficient <q:math xmlns:q="http://www.w3.org/1998/Math/MathML"><q:mrow><q:mi>α</q:mi><q:mo>(</q:mo><q:mi>T</q:mi><q:mo>)</q:mo></q:mrow></q:math> associated with strong magnetoelastic coupling, driving the first-order AFM transition in <r:math xmlns:r="http://www.w3.org/1998/Math/MathML"><r:mrow><r:msub><r:mi>CaMn</r:mi><r:mn>2</r:mn></r:msub><r:msub><r:mi mathvariant="normal">P</r:mi><r:mn>2</r:mn></r:msub></r:mrow></r:math>. However, this effect is weaker in <t:math xmlns:t="http://www.w3.org/1998/Math/MathML"><t:mrow><t:msub><t:mi>SrMn</t:mi><t:mn>2</t:mn></t:msub><t:msub><t:mi mathvariant="normal">P</t:mi><t:mn>2</t:mn></t:msub></t:mrow></t:math>, consistent with the magnetic entropy changes <v:math xmlns:v="http://www.w3.org/1998/Math/MathML"><v:mrow><v:mi>S</v:mi><v:mo>(</v:mo><v:mi>T</v:mi><v:mo>)</v:mo></v:mrow></v:math> obtained from the heat-capacity measurements. Temperature-dependent <w:math xmlns:w="http://www.w3.org/1998/Math/MathML"><w:mrow><w:mi>a</w:mi><w:mi>b</w:mi></w:mrow></w:math>-plane x-ray diffraction measurements for <x:math xmlns:x="http://www.w3.org/1998/Math/MathML"><x:mrow><x:msub><x:mi>CaMn</x:mi><x:mn>2</x:mn></x:msub><x:msub><x:mi mathvariant="normal">P</x:mi><x:mn>2</x:mn></x:msub></x:mrow></x:math> near its <z:math xmlns:z="http://www.w3.org/1998/Math/MathML"><z:msub><z:mi>T</z:mi><z:mi mathvariant="normal">N</z:mi></z:msub></z:math> yield a volume thermal expansion coefficient consistent with literature data for other materials. The uniaxial pressure derivatives of <bb:math xmlns:bb="http://www.w3.org/1998/Math/MathML"><bb:msub><bb:mi>T</bb:mi><bb:mi mathvariant="normal">N</bb:mi></bb:msub></bb:math> in the two compounds are found to be respectively opposite in sign, presumably due to the different magnetic structures.