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Date: 21-2-2021
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Date: 10-2-2021
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Date: 7-10-2020
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Molecular clusters
Molecular magnets consist of several transition metal ions surrounded by organic and inorganic ligands. Their dimensions are a few nanometres, The best-known example is Mn12 acetate ([Mn12O12(CH3COO)16(H2O)4] which is a cluster of twelve manganese ions. Eight are Mn3+ ions and four are Mn4+ ions. The interaction between Mn3+ and Mn4+ is strongly antiferromagnetic, so the net spin of the molecular cluster is S = [8 × 2 − 4 × (3/2)] = 10, giving a moment of 20 μB per cluster. The overall symmetry of the molecule is tetragonal, and there is strong uniaxial anisotropy due to the crystal field
which produces an overall crystal field splitting of about 300 K. The ±|10〉 doublet is the ground state, and if the material is magnetized at low temperature (∼1 K), it remains in the −|10〉 state because relaxation to the +|10〉 state is extremely slow. In a reverse field, however, it is possible for the molecule to tunnel into an excited state, and a square, staircase hysteresis loop results. Similar magnetization dynamics are observed for other molecular magnets, and even for isolated rare earth ions with large J , the ultimate atomic nanomagnets.
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تفوقت في الاختبار على الجميع.. فاكهة "خارقة" في عالم التغذية
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أمين عام أوبك: النفط الخام والغاز الطبيعي "هبة من الله"
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قسم شؤون المعارف ينظم دورة عن آليات عمل الفهارس الفنية للموسوعات والكتب لملاكاته
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