Magnetic structure and crystal field states of the heavy fermion system YbPt5B2

Monoclinic compounds <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:msub><a:mi>YbPt</a:mi><a:mn>5</a:mn></a:msub><a:msub><a:mi mathvariant="normal">B</a:mi><a:mn>2</a:mn></a:msub></a:mrow></a:math> and <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mrow><c:msub><c:mi>LuPt</c:mi><c:mn>5</c:mn></c:msub><c:msub><c:mi mathvariant="normal">B</c:mi><c:mn>2</c:mn></c:msub></c:mrow></c:math> [space group <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mrow><e:mi>C</e:mi><e:mn>2</e:mn><e:mo>/</e:mo><e:mi>m</e:mi></e:mrow></e:math> (No. 12), own structure type] have been studied by means of elastic and inelastic neutron scattering experiments. The heavy fermion system <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"><f:mrow><f:msub><f:mi>YbPt</f:mi><f:mn>5</f:mn></f:msub><f:msub><f:mi mathvariant="normal">B</f:mi><f:mn>2</f:mn></f:msub></f:mrow></f:math> orders antiferromagnetically below <h:math xmlns:h="http://www.w3.org/1998/Math/MathML"><h:mrow><h:msub><h:mi>T</h:mi><h:mrow><h:mi mathvariant="normal">N</h:mi><h:mn>1</h:mn></h:mrow></h:msub><h:mo>=</h:mo><h:mn>7.8</h:mn><h:mspace width="0.28em"/><h:mi mathvariant="normal">K</h:mi></h:mrow></h:math> and exhibits an incommensurate magnetic structure with a temperature-dependent propagation vector, <l:math xmlns:l="http://www.w3.org/1998/Math/MathML"><l:mrow><l:msub><l:mover accent="true"><l:mi>k</l:mi><l:mo>⃗</l:mo></l:mover><l:mn>1</l:mn></l:msub><l:mo>=</l:mo><l:mrow><l:mo>(</l:mo><l:mn>0.194</l:mn><l:mo>,</l:mo><l:mn>0</l:mn><l:mo>,</l:mo><l:mo>−</l:mo><l:mn>0.045</l:mn><l:mo>;</l:mo><l:mspace width="0.16em"/><l:mi>T</l:mi><l:mo>=</l:mo><l:mn>6</l:mn><l:mspace width="0.28em"/><l:mi mathvariant="normal">K</l:mi><l:mo>)</l:mo></l:mrow></l:mrow></l:math>. Below <q:math xmlns:q="http://www.w3.org/1998/Math/MathML"><q:mrow><q:msub><q:mi>T</q:mi><q:mrow><q:mi mathvariant="normal">N</q:mi><q:mn>2</q:mn></q:mrow></q:msub><q:mo>=</q:mo><q:mn>4.7</q:mn><q:mspace width="0.28em"/><q:mi mathvariant="normal">K</q:mi></q:mrow></q:math>, a transition into a commensurate structure, <u:math xmlns:u="http://www.w3.org/1998/Math/MathML"><u:mrow><u:msub><u:mover accent="true"><u:mi>k</u:mi><u:mo>⃗</u:mo></u:mover><u:mn>2</u:mn></u:msub><u:mo>=</u:mo><u:mrow><u:mo>(</u:mo><u:mn>0</u:mn><u:mo>,</u:mo><u:mn>0</u:mn><u:mo>,</u:mo><u:mn>0</u:mn><u:mo>)</u:mo></u:mrow></u:mrow></u:math>, takes place. The transition at <w:math xmlns:w="http://www.w3.org/1998/Math/MathML"><w:msub><w:mi>T</w:mi><w:mrow><w:mi mathvariant="normal">N</w:mi><w:mn>2</w:mn></w:mrow></w:msub></w:math> turns out to be of first order, as obvious from commensurate and incommensurate contributions to the magnetic moments around <y:math xmlns:y="http://www.w3.org/1998/Math/MathML"><y:mrow><y:mi>T</y:mi><y:mo>=</y:mo><y:msub><y:mi>T</y:mi><y:mrow><y:mi mathvariant="normal">N</y:mi><y:mn>2</y:mn></y:mrow></y:msub></y:mrow></y:math>. Rather large Yb moments, <ab:math xmlns:ab="http://www.w3.org/1998/Math/MathML"><ab:mrow><ab:mi>m</ab:mi><ab:mrow><ab:mo>(</ab:mo><ab:mi>Yb</ab:mi><ab:mo>)</ab:mo></ab:mrow><ab:mo>></ab:mo><ab:mn>3</ab:mn><ab:msub><ab:mi>μ</ab:mi><ab:mi mathvariant="normal">B</ab:mi></ab:msub></ab:mrow></ab:math>, yet reduced due to the Kondo effect, were obtained as a result of the splitting of the <cb:math xmlns:cb="http://www.w3.org/1998/Math/MathML"><cb:msup><cb:mrow><cb:mi>Yb</cb:mi></cb:mrow><cb:mrow><cb:mn>3</cb:mn><cb:mo>+</cb:mo></cb:mrow></cb:msup></cb:math> eightfold-degenerate ground state into four doublets, owing to crystalline electric field (CEF) effects. Such unusually large moments in Yb systems can be explained only by an appropriate wave function <db:math xmlns:db="http://www.w3.org/1998/Math/MathML"><db:mrow><db:msubsup><db:mo>∑</db:mo><db:mrow><db:mi>i</db:mi><db:mo>=</db:mo><db:mo>−</db:mo><db:mn>7</db:mn><db:mo>/</db:mo><db:mn>2</db:mn></db:mrow><db:mrow><db:mi>i</db:mi><db:mo>=</db:mo><db:mn>7</db:mn><db:mo>/</db:mo><db:mn>2</db:mn></db:mrow></db:msubsup><db:mo>±</db:mo><db:msub><db:mi>α</db:mi><db:mi>i</db:mi></db:msub><db:mrow><db:mo>|</db:mo><db:mi>i</db:mi><db:mo>/</db:mo><db:mn>2</db:mn><db:mo>〉</db:mo></db:mrow></db:mrow></db:math>, <eb:math xmlns:eb="http://www.w3.org/1998/Math/MathML"><eb:mrow><eb:msub><eb:mo>∑</eb:mo><eb:mi>i</eb:mi></eb:msub><eb:msubsup><eb:mi>α</eb:mi><eb:mi>i</eb:mi><eb:mn>2</eb:mn></eb:msubsup><eb:mo>=</eb:mo><eb:mn>1</eb:mn></eb:mrow></eb:math>, constituting the CEF ground state doublet, which indeed was concluded from the present inelastic neutron data, considering the crystalline electric field theory. The overall CEF splitting is on the order of 44 meV, while the first excited level is located <fb:math xmlns:fb="http://www.w3.org/1998/Math/MathML"><fb:mrow><fb:mo>≈</fb:mo><fb:mn>25</fb:mn></fb:mrow></fb:math> meV above the ground state.