Authors: Roman Rausch | Christoph Karrasch
Source: Physical review. B./Physical review. B (article, 2025-02-19)
Field: Materials Science — Electronic, Optical and Magnetic Materials

Low-spin ground state of the giant single-molecule magnets <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mo>{</mml:mo><mml:msub><mml:mi>Mn</mml:mi><mml:mn>70</mml:mn></mml:msub><mml:mo>}</mml:mo></mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mo>{</mml:mo><mml:msub><mml:mi>Mn</mml:mi><mml:mn>84</mml:mn></mml:msub><mml:mo>}</mml:mo></mml:math> The single-molecule magnets <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mo>{</a:mo><a:msub><a:mi>Mn</a:mi><a:mn>70</a:mn></a:msub><a:mo>}</a:mo></a:math> and <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mo>{</b:mo><b:msub><b:mi>Mn</b:mi><b:mn>84</b:mn></b:msub><b:mo>}</b:mo></b:math> are characterized by a 14-site unit cell with <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mrow><c:mi>S</c:mi><c:mo>=</c:mo><c:mn>2</c:mn></c:mrow></c:math> spin sites arranged in a circular geometry. Experimentally, these systems exhibit a magnetic ground state with a notably low total spin <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:mrow><d:msub><d:mi>S</d:mi><d:mtext>tot</d:mtext></d:msub><d:mo>=</d:mo><d:mn>5</d:mn><d:mo>–</d:mo><d:mn>7</d:mn></d:mrow></d:math>. Up to now, this low-spin ground state has been up difficult to describe theoretically due to the complexity of the quantum Heisenberg model for such a large system. In this paper, we fill this gap and demonstrate that the ground state of {<e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:msub><e:mi>Mn</e:mi><e:mn>70</e:mn></e:msub></e:math>} and {<f:math xmlns:f="http://www.w3.org/1998/Math/MathML"><f:msub><f:mi>Mn</f:mi><f:mn>84</f:mn></f:msub></f:math>} is in fact governed by a small, finite <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"><g:msub><g:mi>S</g:mi><g:mtext>tot</g:mtext></g:msub></g:math> in quantitative agreement with the experiment. We employ accurate, large-scale SU(2)-symmetric density-matrix renormalization group calculations for a quantum Heisenberg model with previously published exchange parameters obtained by density-functional theory. We do not find a low-spin state for the same parameters and <h:math xmlns:h="http://www.w3.org/1998/Math/MathML"><h:mrow><h:mi>S</h:mi><h:mo>=</h:mo><h:mn>1</h:mn></h:mrow></h:math> and thus propose that frustrated systems with <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"><i:mrow><i:mi>S</i:mi><i:mo>≥</i:mo><i:mn>2</i:mn></i:mrow></i:math> are inherently prone to weak ferromagnetic interactions. This could account for the prevalence of similar low-spin Mn-based single-molecule magnets. Finally, we compute the full magnetization curve and find wide plateaus at 10/14, 11/14, 12/14, and 13/14 of the saturation, which can be traced back to nearly independent three-site clusters with broken intercluster bonds. Ground state ; Spin (aerodynamics) ; Magnet ; Physics ; Atomic physics ; Thermodynamics ; Quantum mechanics Magnetism in coordination complexes ; Advanced NMR Techniques and Applications ; Organic and Molecular Conductors Research