M. V. Kirman

Spin solitons and waves in chiral molecular ferrimagnets

Resonance modes corresponding to a spin-soliton resonance have been found in the electron spin resonance spectra of [Cr(CN)6][Mn(S)-pnH-(H2O)]H2O two-dimensional (2D) chiral single crystals and [Mn{(R/S)-pn}]2[Mn{(R/S)-pn}2(H2O)][Cr(CN)6]2 chiral single crystals with a 3D magnetic order. It is also established that the chiral crystals of both types exhibit a spin-wave resonance analogous to the excitation of standing spin waves in thin magnetic films. At the same time, racemic crystals of the first type do not exhibit spin-soliton resonance. The entire body of experimental data indicates that the chirality of crystals influences the spin excitations (standing spin waves and solitons) in these media.

Stochastic jumps of magnetization in [Mn{(R/S)-pn}]2[Mn{(R/S)-pn}2(H2O)][Cr(CN)6]2 molecular magnet

Series of stochastic jumps of the magnetic moment (up to five individual jumps) have been observed at the demagnetization of single crystals of [Mn{(R/S)-pn}]2[Mn{(R/S)-pn}2(H2O)][Cr(CN)6]2 molecular magnet in a narrow range of magnetic fields near the coercive force (Hc = 7.5 Oe). The magnetic field at which jumps of the magnetization arise decreases with an increase in the temperature.

Ordered nanowires of photochromic compounds based on spiropyrane and transition metal complexes

Ordered nanowires of photochromic antiferromagnet SpFeMn(C2O4)3 (Sp is 1-{1′,3′,3′-trime-thyl-6-nitro-5′-chlorspiro[2H-1-benzopyran-2,2′-indolin]-8-yl)methyl}pyridinium) are synthesized in anodized aluminum oxide (AAO) pores 20 nm and 200 nm in diameter. A change in the magnetic properties of the material due to its nanostructuring is observed, resulting in the suppression of canted spin ordering (spincanted phase) in favor of the uniaxial antiferromagnet phase.

Influence of chirality on the electron spin resonance in molecular magnets [MnII(HL)(H2O)][MnIII(CN)6] · 2H2O with chiral ligands L

Chiral and racemic molecular ferrimagnets [MnII(HL)(H2O)][MnIII(CN)6] · 2H2O, where L = S-or R-1,2-diaminopropane for chiral samples (S-pn, R-pn) and L = rac-pn for racemic samples, are investigated by the electron spin resonance technique. It is revealed that the electron spin resonance spectra of the chiral and racemic samples differ from each other at temperatures below the Curie temperature TC = 21 K. The maximum in the temperature dependence of the integrated magnetic susceptibility χ(T) calculated by the double integration of the line centered at a negative field of −250 Oe for the racemic samples is broadened as compared to the maxima in the corresponding dependences for the enantiomers of the chiral samples with “right” (R) and “left” (S) symmetry. The new compounds under investigation differ from the previously synthesized crystals by the strong spin-orbit interaction between Mn3+ ions, which leads to a dependence of their magnetic properties on the chirality of the structure.

Low-temperature phase transition in α′-(BEDT-TTF)2IBr2 single crystals detected by the ESR method

A hysteresis of electron spin resonance spectra, which indicates a first-order phase transition, has been revealed in α′-(BEDT-TTF)2IBr2 single crystals in the temperature range 15–30 K. Two lines corresponding to the high-temperature and low-temperature phases have been observed in the ESR spectra.

Transitions between quantum states of the spin-soliton structure in molecular magnets [Mn{(R/S)-pn}]2[Mn{(R/S)-pn}2(H2O)][Cr(CN)6]2

A series of jumps of the magnetic moment (up to five jumps) with an amplitude of 1–10% of the saturation magnetization has been observed upon demagnetizing the molecular magnet [Mn{(R/S)-pn}]2[Mn{(R/S)-pn}2(H2O)][Cr(CN)6]2 in a narrow region of magnetic fields close to the coercive force. A decrease in the temperature leads to an increase in the critical magnetic field, which corresponds to the onset of the series of demagnetization jumps. The obtained experimental data agree with theoretical predictions on jump-like transitions between the magnetization curves upon attaining critical magnetic fields caused by the energy quantization of spin solitons.

Competition of two mechanisms of retardation of domain walls in the molecular ferrimagnet [Mn{(R/S)-pn}]2[Mn{(R/S-pn}2(H2O)][Cr(CN)6]2

The temperature dependence of the magnetization reversal dynamics of the chiral molecular ferrimagnet [Mn{(R/S)-pn}]2[Mn{(R/S)-pn}2(H2O)][Cr(CN)6]2 has been studied at low frequencies of 1–1400 Hz, which are characteristic of the domain wall motion. It has been found from the Cole-Cole plots that domain walls undergo relaxation (at temperatures T > 10 K) and creep (at T < 10 K), and the main parameters determining these modes and the transition between them have been determined. It has been shown that the quantitative regularities of the transition between the modes of the domain wall motion correspond to the concepts of the competition between the contributions of two mechanisms to the domain wall retardation (the periodic Peierls relief and random structural defects).

Influence of dehydration on the electron spin resonance in the Cu3[W(CN)8]2(Pyrimidine)2 · 8H2O molecular magnet

The influence of dehydration on the electron spin resonance spectra in the Cu3[W(CN)8]2(pyrimidine)2 · 8H2O molecular magnet has been found. It has been established that two ferromagnetic phases with the Curie temperatures of ∼12–17 and ∼50 K, which correspond to two different lines of the ferromagnetic resonance, are present in crystals. Dehydration leads to increasing the Curie temperature of the low-temperature phase from 12 to 17 K and to varying the temperature dependences of the spectrum parameters of the high-temperature phase (the resonant field and the width line).

Effect of chirality on the dynamics of domain walls in the molecular ferrimagnet [MnII(HL-pn)(H2O)][MnIII(CN)6] · 2H2O

The contributions from modes of switching, sliding, creep, and Debye relaxation of pinned domain walls to the low-frequency magnetic properties of the chiral and racemic molecular ferrimagnets [MnII(HL-pn)(H2O)][MnIII(CN)6] · 2H2O have been separated. It has been found that the chirality of the atomic and spin structures affects the temperatures of the transitions from the sliding mode to the creep mode and from the creep mode to the mode of Debye relaxation. In the chiral crystals, transitions to the creep and Debye relaxation modes have been observed at temperatures T = 7 and 5 K, respectively. In the racemic crystals, these transitions have been observed at temperatures T = 13 and 9 K, respectively, all other factors being equal.

FMR determination of the period of an incommensurate magnetic structure in chiral organometallic crystals

A sequence of maxima of microwave absorption has been found in the ferromagnetic resonance (FMR) spectra of the chiral molecular ferrimagnet [Mn{(R/S)-pn}]2[Mn{(R/S)-pn}2(H2O)][Cr(CN)6]2, which, as is shown, corresponds to the spin-soliton resonance. It has been established that this sequence corresponds to an incommensurate magnetic structure induced by the competition between the symmetric and antisymmetric exchange interactions. On the basis of the FMR spectra and their dependence on the temperature, the parameters of the modulated magnetic structure have been estimated.