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[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

Four Cu(I) complexes were synthesized with a family of pyridylmethylamide ligands, HLR [HLR = N-(2-pyridylmethyl)acetamide, R = null; 2,2-dimethyl-N-(2-pyridylmethyl)propionamide, R = Me3; 2,2,2-triphenyl-N-(2-pyridylmethyl)acetamide, R = Ph3)]. Complexes 1–3 were synthesized from the respective ligand and [Cu(CH3CN)4]PF6 in a 2 : 1 molar ratio: [Cu(HL)2]PF6 (1), [Cu2(HLMe3)4](PF6)2 (2), [Cu(HLPh3)2]PF6 (3). Complex 4, [Cu(HL)(CH3CN)(PPh3)]PF6, was synthesized from the reaction of HL with [Cu(CH3CN)4]PF6 and PPh3 in a 1 : 1 : 1 molar ratio. X-Ray crystal structures reveal that complexes 1, 3 and 4 are mononuclear Cu(I) species, while complex 2 is a Cu(I) dimer. The copper ions are four-coordinate with geometries ranging from distorted tetrahedral to seesaw in 1, 2, and 4. Complexes 1 and 2 are very air sensitive and they display similar electrochemical properties. The coordination geometry of complex 3 is nearly linear, two-coordinate. Complex 3 is exceptionally stable with respect to oxidation in the air, and its cyclic voltammetry shows no oxidation wave in the range of 0–1.5 V. The unusual inertness of complex 3 towards oxidation is attributed to the protection from bulky triphenyl substituent of the HLPh3 ligand. A new geometric parameter for four-coordinate compounds, τ4, is proposed as an improved, simple metric for quantitatively evaluating the geometry of four-coordinate complexes and compounds.

Structural variation in copper(I) complexes with pyridylmethylamide ligands: structural analysis with a new four-coordinate geometry index, tau4.

Using the full-potential screened Korringa-Kohn-Rostoker method we study the full-Heusler alloys based on Co, Fe, Rh, and Ru. We show that many of these compounds show a half-metallic behavior; however, in contrast to the half-Heusler alloys the energy gap in the minority band is extremely small due to states localized only at the Co (Fe, Rh, or Ru) sites which are not present in the half-Heusler compounds. The full-Heusler alloys show a Slater-Pauling behavior and the total spin magnetic moment per unit cell ${(M}_{t})$ scales with the total number of valence electrons ${(Z}_{t})$ following the rule ${M}_{t}{=Z}_{t}\ensuremath{-}24.$ We explain why the spin-down band contains exactly 12 electrons using arguments based on group theory and show that this rule holds also for compounds with less than 24 valence electrons. Finally we discuss the deviations from this rule and the differences compared to the half-Heusler alloys.

/pdf/slater-pauling-behavior-and-origin-of-the-half-metallicity-1vlzvqk1h0.pdf

Slater-Pauling behavior and origin of the half-metallicity of the full-Heusler alloys

Giant elastocaloric effect in directionally solidified Ni-Mn-In magnetic shape memory alloy

The effect of heat treatments on Ni43Mn42Co4Sn11 meta-magnetic shape memory alloys for magnetic refrigeration

We report the results of a ${}^{27}\mathrm{Al}$ and ${}^{51}\mathrm{V}$ nuclear magnetic resonance study of ${\mathrm{Fe}}_{2}\mathrm{VAl}$ at temperatures between 4 and 550 K. This material has been a subject of current interest due to indications of possible heavy fermion behavior. The low-temperature NMR relaxation rate follows a Korringa law, indicating a small density of carriers at the Fermi level. At elevated temperatures, the shifts and relaxation rates go over to a thermally activated response, a semiconductorlike behavior, consistent with separate low-lying bands removed from the Fermi-level. These results are consistent with recent electronic structure calculations, and can explain both the reported activated resistivity as well as the Fermi cutoff exhibited in photoemission studies. While we observe nonstoichiometric samples of $({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{V}}_{x}{)}_{3}\mathrm{Al}$ to be magnetic, the $x=0.33$ composition is nonmagnetic, with narrow NMR linewidths.

Semimetallic behavior in Fe 2 VAl: NMR evidence

The hydrothermal syntheses, structures, and magnetic properties of the Cu(II), Co(II), and Mn(II) complexes with organophosphonic acid H2O3PCH2NH2+CH2PO3H- are reported. Compound Cu3[NH2(CH2PO3)2]2 (1) crystallizes in the orthorhombic space group Pbca, with unit cell parameters a = 9.4430(10) A, b = 9.3681(10) A, c = 16.0755(17) A, V = 1422.1(3) A3, and Z = 4. The X-ray crystallographic determination reveals copper ions in two different coordination environments that are bridged by one terminal oxygen atom to form a three-dimensional framework of corner-sharing layers. Antiferromagnetic coupling between the two types of Cu units in 1 was observed at 15 K, followed by ferromagnetic ordering at 9 K. Compound Co3[NH2(CH2PO3)2]2 (2) crystallizes in the orthorhombic space group P21212 with unit cell parameters a = 10.7346(8) A, b = 13.3704(10) A, c = 5.3699(4) A, V = 770.72(10) A3, and Z = 2. All Co(II) ions are in a tetrahedral environment with both ends of the ligand bound via O atoms to adjacent Co(II) ions...

Syntheses, Structure, and Magnetic Properties of New Types of Cu(II), Co(II), and Mn(II) Organophosphonate Materials: Three-Dimensional Frameworks and a One-Dimensional Chain Motif

The authors report NMR measurements of the $^{93}\mathrm{Nb}$ resonance in the charge-density-wave (CDW) conductor ${\mathrm{NbSe}}_{3}$ on aligned, multicrystalline samples. They have observed the resonance with and without current flow in the sample at temperatures above 59 K, where a single incommensurate CDW is present. Results include (1) a demonstration that the CDW is not discommensurate, (2) quantitative measurements of CDW displacements below threshold, and (3) evidence, including current-induced motional narrowing, of CDW motion throughout the entire sample, above the threshold.

/pdf/nmr-study-of-the-structure-and-motion-of-charge-density-38c5m1gcvm.pdf

Joseph H. Ross

Papers

Giant elastocaloric effect in directionally solidified Ni-Mn-In magnetic shape memory alloy

The effect of heat treatments on Ni43Mn42Co4Sn11 meta-magnetic shape memory alloys for magnetic refrigeration

Semimetallic behavior in Fe 2 VAl: NMR evidence

Syntheses, Structure, and Magnetic Properties of New Types of Cu(II), Co(II), and Mn(II) Organophosphonate Materials: Three-Dimensional Frameworks and a One-Dimensional Chain Motif

NMR study of the structure and motion of charge density waves in NbSe3