Volodymyr Pashchenko

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.

Exchange broadening of EPR line in ZnO:Co

We study the X-band EPR spectra of Co2+ in single-crystalline Zn1−xCoxO (x=0.001–0.075) thin films grown by plasma-assisted molecular beam epitaxy. By analyzing the EPR linewidth behavior we argue that the exchange-narrowing model, usually applied to Mn-based II-VI DMS, fails here and that a combined effect of exchange and dipolar broadening can explain the linewidth variation with Co content and temperature.

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.

Electron Spin Resonance in the Spin-1/2 Quasi-One-Dimensional Antiferromagnet with Dzyaloshinskii-Moriya Interaction BaCu~2Ge~2O~7

We have investigated the electron spin resonance (ESR) on single crystals of BaCu2Ge2O7 at temperatures between 300 and 2 K and in a large frequency band, 9.6-134 GHz, in order to test the predictions of a recent theory, proposed by Oshikawa and Affleck (OA) [Phys. Rev. Lett. 82, 5136 (1999)]], which describes the ESR in a spin-1/2 Heisenberg chain with the Dzyaloshinskii-Moriya interaction. We find, in particular, that the ESR linewidth, Delta H, displays a rich temperature behavior. As the temperature decreases from T(max)/2 approximately 170 to 50 K, Delta H shows a rapid and linear decrease, Delta H approximately T. At low temperatures, below 50 K, Delta H acquires a strong dependence on the magnetic field orientation and for H axially c it shows a (h/T)(2) behavior which is due to an induced staggered field h, according to OAs prediction.

Magneto-structural correlations in a new oxalato-bridged Cu(II) alternating-exchange spin-chain compound

A new oxalato-bridged copper(II) compound, Cu(ox)(pyOH)H2O (ox = oxalate, pyOH = 3-hydroxypyridine), has been synthesized and characterized by x-ray diffraction and magnetic susceptibility measurements. The Cu(II) ions are bridged by oxalate molecules with two different arrangements alternating along a chain parallel to the b-axis. To the best of our knowledge, this is the first example of a metal–oxalate chain with such a combination of oxalate bridges. Due to the specific structural properties, the magnetic susceptibility was analysed in the framework of an alternating-exchange spin-chain. From a least-squares fit, an antiferromagnetic coupling constant J1 = (442 ± 5) K and an alternation parameter α = 0.13 ± 0.06 were derived, which classify Cu(ox)(pyOH)H2O as a strongly dimerized spin-chain compound. It is argued that the strength of the magnetic coupling is mainly determined by the displacement of the Cu(II) ions out of the basal plane of the local Cu environment.

Magnetic properties of a metal-organic antiferromagnet on a distorted honeycomb lattice

For temperatures T well above the ordering temperature T*=3.0+-0.2K the magnetic properties of the metal-organic material Mn[C10H6(OH)(COO)]2x2H20 built from Mn^2+ ions and 3-hydroxy-2-naphthoic anions can be described by a S=5/2 quantum antiferromagnet on a distorted honeycomb lattice with two different nearest neighbor exchange couplings J2 \approx 2J1 \approx 1.8K. Measurements of the magnetization M(H,T) as a function of a uniform external field H and of the uniform zero field susceptibility \chi(T) are explained within the framework of a modified spin-wave approach which takes into account the absence of a spontaneous staggered magnetization at finite temperatures.