Badiur Rahaman

Microscopic modeling of a spin crossover transition

In spin crossover materials, an abrupt phase transition between a low-spin state and a high-spin (HS) state can be driven by temperature, pressure or by light irradiation. Of special relevance are Fe(II) based coordination polymers where, in contrast to molecular systems, the phase transition between a spin S=0 and 2 state shows a pronounced hysteresis which is desirable for technical applications. A satisfactory microscopic explanation of this large cooperative phenomenon has been sought for a long time. The lack of x-ray data has been one of the reasons for the absence of microscopic studies. In this work, we present an efficient route to prepare reliable model structures and within an ab initio density functional theory analysis and effective model considerations we show that in polymeric spin crossover compounds magnetic exchange between HS Fe(II) centers is as important as elastic couplings for understanding the phase transition. We discuss the relevance of these interactions for the cooperative behavior in these materials.

Crystal structure, physical properties, and electronic and magnetic structure of the spin S = 5/2 zigzag chain compound Bi2Fe(SeO3)2OCl3.

We report the synthesis and characterization of the new bismuth iron selenite oxochloride Bi2Fe(SeO3)2OCl3. The main feature of its crystal structure is the presence of a reasonably isolated set of spin S = 5/2 zigzag chains of corner-sharing FeO6 octahedra decorated with BiO4Cl3, BiO3Cl3, and SeO3 groups. When the temperature is lowered, the magnetization passes through a broad maximum at Tmax ≈ 130 K, which indicates the formation of a magnetic short-range correlation regime. The same behavior is demonstrated by the integral electron spin resonance intensity. The absorption is characterized by the isotropic effective factor g ≈ 2 typical for high-spin Fe(3+) ions. The broadening of ESR absorption lines at low temperatures with the critical exponent β = 7/4 is consistent with the divergence of the temperature-dependent correlation length expected for the quasi-one-dimensional antiferromagnetic spin chain upon approaching the long-range ordering transition from above. At TN = 13 K, Bi2Fe(SeO3)2OCl3 exhibits a transition into an antiferromagnetically ordered state, evidenced in the magnetization, specific heat, and Mössbauer spectra. At T < TN, the (57)Fe Mössbauer spectra reveal a low saturated value of the hyperfine field Hhf ≈ 44 T, which indicates a quantum spin reduction of spin-only magnetic moment ΔS/S ≈ 20%. The determination of exchange interaction parameters using first-principles calculations validates the quasi-one-dimensional nature of magnetism in this compound.

Effects of Two Energy Scales in Weakly Dimerized Antiferromagnetic Quantum Spin Chains

By means of thermal expansion and specific heat measurements on the high-pressure phase of (VO)(2)P(2)O(7), the effects of two energy scales of the weakly dimerized antiferromagnetic S=1/2 Heisenberg chain are explored. The low-energy scale, given by the spin gap Delta, is found to manifest itself in a pronounced thermal expansion anomaly. A quantitative analysis, employing the density-matrix renormalization-group approach for transfer matrices calculations, shows that this feature originates from changes in the magnetic entropy with respect to Delta, partial differentialS(m)/partial differentialDelta. This term, inaccessible by specific heat, is visible only in the weak-dimerization limit, where it reflects peculiarities of the excitation spectrum and its sensitivity to variations in Delta.

Vehement Competition of Multiple Superexchange Interactions and Peculiar Magnetically Disordered State in Cu(OH)F

The mixed anion copper compound Cu(OH)F was studied in measurements of dc- and ac-magnetic susceptibility, static and pulsed field magnetization, specific heat, X-band electron magnetic resonance and muon-spin spectroscopy. In variance with its layered structure, the magnetic behavior shows no evidence of low-dimensionality. Cu(OH)F reaches short range static antiferromagnetic order at TN = 9.5–11.5 K and experiences the spin-flop transition at B ∼ 3.5 T. This behavior is in a sharp contrast with physical properties of earlier reported isostructural compound Cu(OH)Cl. The first principle calculations reveal highly competitive nature of ferromagnetic and antiferromagnetic superexchange interactions, the details being rather sensitive to choice of magnetic structure employed in the extraction of magnetic interaction. Rather broad anomaly in Cp(T) dependence at phase transition and smeared magnetization curve M(B) at low temperatures suggest static disorder at low temperatures however no frequency dependence...

Color properties of model spin chain materials: VOHPO412H2O and (VO)2P2O7

We report the optical properties of two prototypical S=1/2 magnetic materials: vanadyl (IV) hydrogen-phosphate hemihydrate VOHPO{sub 4} {center_dot} H{sub 2}O and its derivative vanadyl pyrophosphate (VO){sub 2}P{sub 2}O{sub 7}. Local density approximation electronic structure calculations are used to identify and evaluate correct structures, assign the observed excitations, and quantify bonding and hybridization effects in both materials. VOHPO{sub 4} {center_dot} H{sub 2}O displays a strong color band that is derived from V{sup 4+} d {yields} d excitations. It is sensitive to both the local vanadium environment and the enhanced low-temperature hydrogen bonding between layers. In contrast, (VO){sub 2}P{sub 2}O{sub 7} displays a diffuse and gradually rising near infrared absorption in all directions. The O p {yields} V d charge transfer gaps in both materials are similar. We predict that the on-site excitations of the transition metal centers may be sensitive to a magnetic state via a magnetic field control of p-d hybridization.

Microscopic model for the frustrated Cu II-spin tetrahedron-based Cu 4 Te 5 O 12 X 4 (X=Cl, Br) systems

We present a microscopic study of the electronic and magnetic properties of the recently synthesized spin tetrahedron system

Comparative investigation of the coupled-tetrahedra quantum spin systems Cu2Te2O5X2, X = Cl, Br and Cu4Te5O12Cl4

{\mathrm{Cu}}_{4}{\mathrm{Te}}_{5}{\mathrm{O}}_{12}{\mathrm{Cl}}_{4}

Microscopic model for the frustratedCuII-spin tetrahedron-basedCu4Te5O12X4(X=Cl, Br) systems

based on density functional calculations and on ab initio-derived effective models. In view of these results, we discuss the origin of the observed differences in behavior between this system and the structurally similar

Microscopic model for the frustrated Cu II-spin tetrahedron-based Cu4Te5O12X4 (X=Cl, Br) systems

{\mathrm{Cu}}_{2}{\mathrm{Te}}_{2}{\mathrm{O}}_{5}{\mathrm{Cl}}_{2}

Cu-based metalorganic systems: an ab initio study of the electronic structure

. Since the Br analog of the title compound has not been synthesized yet, we derive the crystal structure of