Spin solitons in molecular magnetic materials with the chiral structure

Quasiperiodic sequences of the maxima of microwave absorption with decreasing amplitudes have been observed in a temperature range of 4–50 K in the electron spin resonance spectra of ferrimagnetic chiral single crystals [Mn{(R/S)-pn}]₂[Mn{(R/S)-pn}₂(H₂O)][Cr(CN)₆]₂, as well as [Cr(CN)₆][Mn(S)-pnH-(H₂O)]H₂O. Theoretical estimates and previous experimental data indicate that the Dzyaloshinskii-Moriya interaction is the main factor determining the chirality of the spin density and the existence of soliton solutions for the spin dynamics in these crystals. The experimental dependences obtained for the distances between the microwave power absorption maxima on the constant component of the magnetic field of the spectrometer correspond to the theoretical predictions for spin solitons in three-dimensional magnetic materials and exhibit another behavior in crystals with quasi-two-dimensional magnetic ordering.     

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.     

Threshold effect of microwave power on ferromagnetic resonance in K<Subscript>0.4</Subscript>[Cr(CN)<Subscript>6</Subscript>][Mn(S)-pn](S)-pnH<Subscript>0.6</Subscript> single crystals

A stepwise change in the ferromagnetic resonance spectrum in a K_0.4[Cr(CN)₆][Mn(S)-pn](S)-pnH_0.6 chiral magnet has been observed in a critical microwave magnetic field of 1.8 Oe. The threshold changes in the ferromagnetic resonance spectra are caused by Suhl instability leading to the generation of defects of the magnetic structure, chiral spin solitons. The threshold effect is not observed in the same microwave range in similar chiral crystals [Cr(CN)₆][Mn(S)-pnH(H₂O)](H₂O) with a stronger Heisenberg exchange interaction.     

Nonlinear spin-wave phenomena in a [Mn{(R/S)-pn}2]2[Mn{(R/S)-pn}2H2O][Cr(CN)6] chiral molecular ferrimagnet

The instability of ferromagnetic resonance (FMR) spectra initiated by microwave power was observed in [Mn{(R/S)-pn}₂]₂[Mn{(R/S)-pn}₂H₂O][Cr(CN)₆] single crystals. It was established that the value of threshold microwave power P _th = 0.2−0.9 mW depends on the orientation of the easy magnetization axis relative to the sweeping magnetic field of the spectrometer. P > P _th spin-wave bistability occurs in the region of high microwave fields.     

Magnetic resonance in a [{Cr(CN)<Subscript>6</Subscript>} {Mn(S)-<Emphasis Type="Italic">pn</Emphasis>H-(H<Subscript>2</Subscript>O)}] · H<Subscript>2</Subscript>O single-crystal molecular ferrimagnet

The variations in the magnetic resonance spectra accompanying the transition from the paramagnetic to ferrimagnetic state in [{Cr(CN)₆} {Mn(S)-pnH-(H₂O) }] · H₂O orthorhombic chiral molecular crystals were studied. The dependence of the EPR linewidth on temperature in the proximity of the transition point T_C = 38 K argues for the two-dimensional character of spin ordering. The spin resonance line was found to undergo exchange narrowing at T > T_C. The ferrimagnetic phase has an easy magnetization axis coinciding with the a crystallographic axis.     

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 [Mn^II(HL-pn)(H₂O)][Mn^III(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.     

Effect of dehydration on the ferrimagnetic resonance in [Cr(CN)<Subscript>6</Subscript>][Mn(<Emphasis Type="Italic">S</Emphasis>)-<Emphasis Type="Italic">pn</Emphasis>H(H<Subscript>2</Subscript>O)](H<Subscript>2</Subscript>O) single crystals

The effect of dehydration-induced structural transitions on the ferrimagnetic resonance spectra of the [Cr(CN)₆][Mn(S)-pnH(H₂O)](H₂O) chiral molecular magnet has been studied for three crystalline modifications. The differences in the anisotropy field and exchange interaction constants due to the change in the dimensionality of the magnetic ordering upon the phase transitions have been established. In the dehydrated amorphous phase, apart from the ferrimagnetic resonance, additional isotropic magnetic resonance lines corresponding to the spin-glass state have been revealed.     

Spin dynamics and ferromagnetic resonance in an [Mn{(R/S)-pn}]2[Mn{(R/S)-pn}2(H2O)][Cr(CN)6]2 molecular magnetic

Ferrimagnetic resonance spectra of the [Mn{(R/S)-pn}]₂[Mn{(R/S)-pn}]₂(H₂O)][Cr(CN)₆]₂ molecular magnetics were examined. It was established that, within the high-temperature (paramagnetic) region (T > 53 K), the EPR spectrum features a single Lorentz-shaped peak, while, at temperatures below 53 K, in a three-dimensional ferrimagnetic state, this peak splits into several components, some of which correspond to different states of spins in the helical chains of atoms comprising the chiral crystalline structure.     

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).     

Effect of chirality on domain wall dynamics in molecular ferrimagnet [Mn<Superscript>II</Superscript>(HL-pn)(H<Subscript>2</Subscript>O)][Mn<Superscript>III</Superscript>(CN)<Subscript>6</Subscript>]·2H<Subscript>2</Subscript>O

In this paper we distinguish the contributions of switching, slide, creep and Debye relaxation modes of the domain wall dynamics to the low-frequency magnetic properties of chiral and racemic [Mn^II(HL-pn)(H₂O)][Mn^III(CN)₆]·2H₂O molecular ferrimagnets. We demonstrate that crystal and spin chirality affects the characteristic transition temperatures between different modes. In chiral crystals, transitions to the creep and Debye relaxation modes were observed at T = 7 K and 5 K, whereas in racemic crystals the same transitions occurred at higher temperatures T = 13 K and 9 K, respectively. Difference of the Peierls relief in chiral and racemic crystals is a possible reason of the chirality effect on the domain walls dynamics.     

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

Chiral and racemic molecular ferrimagnets [Mn^II(HL)(H₂O)][Mn^III(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 T _C = 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. Original Russian Text ? R.B. Morgunov, F.B. Mushenok, M.V. Kirman, 2008, published in Fizika Tverdogo Tela, 2008, Vol. 50, No. 7, pp. 1252–1256.     

High-spin metal-cyanide clusters: species incorporating [Mn(salen)] complexes as a source of anisotropy

The use of N,N′-ethylenebis(salycylideneiminato) (salen) complexes of Mn^III in assembling high-spin metal-cyanide coordination clusters with significant magnetic anisotropy is demonstrated. The reaction of [Mn(salen)(H₂O)₂]⁺with [Cr(CN)₆]³⁻ in aqueous solution generates {Cr[CNMn(salen)(H₂O)]₆}[Cr(CN)₆]·6H₂O (1), a previously reported compound featuring a heptanuclear cluster with a distorted octahedral geometry. A fit to the variable-temperature magnetic susceptibility data for 1 revealed the presence of weak antiferromagnetic coupling of within the cluster, giving rise to an S=21/2 ground state. In addition, variable-field magnetization data collected at low temperatures revealed the presence of magnetic anisotropy in the ground state, with the best fit yielding zero-field splitting parameters of D=+0.19 cm⁻¹ and A reaction intended to produce a direct analogue of 1 by employing [Fe(CN)₆]³⁻ in place of [Cr(CN)₆]³⁻ instead gave an unusually complex compound of formula {Fe(CN)₄[CNMn(salen)(MeOH)]₂}{[Mn(salen)(H₂O)]₂}[Mn(salen)(H₂O)(MeOH)]₂[Fe(CN)₆]·4H₂O (2). The crystal structure and magnetic properties of this compound are reported.     

Spin dynamics in (( BEDO-TTF )6[ M (CN)6](H3O,CH3CN)2 ( M = Fe, Cr) crystals

Layered single crystals of the (BEDO-TTF)₆[M(CN)₆](H₃O,CH₃CN)₂ (M = Fe, Cr) compounds with alternating conducting layers of BEDO-TTF and [M(CN)₆](H₃O,CH₃CN)₂ are studied. The contributions to the magnetic susceptibility from charge carriers in BEDO-TTF layers and from the subsystem of localized magnetic moments of iron (or chromium) transition metal complexes are separated for both compounds under investigation. It is revealed that the crystals with [Fe(CN))₆]³⁻ anions at a temperature of ∼80 K and the crystals with [Cr(CN))₆]³⁻ anions at ∼30 K undergo magnetic transitions which are accompanied by drastic changes in the parameters of the EPR lines associated with the BEDO-TTF layers and the subsystem of localized spins of transition metal complexes. It is established that the presence of the BEDO-TTF layers in the structure affects the magnetic properties of iron and chromium hexacyanide complexes. Original Russian Text ? R.B. Morgunov, E.V. Kurganova, T.G. Prokhorova, E.B. Yagubskiĭ, S.V. Simonov, R.P. Shibaeva, 2008, published in Fizika Tverdogo Tela, 2008, Vol. 50, No. 4, pp. 657–663.     

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).     

Structural and magnetic properties of MoS2 monolayer zigzag nanoribbon doped by Ti,V, Cr,and Mn

In order to find new functions of monolayer MoS₂ in nanoelectronics or spin electronic devices, using spin-polarized density functional theory (DFT) calculations with on-site coulomb interaction (U), we investigated substitutional doping of Mo atoms of monolayer zigzag MoS₂ nanoribbon (ZZ-MoS₂ NR) by transition metals (TM) (where TM = Ti, V, Cr, Mn) at the Mo-edge, S-edge, and the middle of the NRs. The results of this study indicate the NR widened irrespective of the doped TM position and type, and the Mo-edge was found as the easiest substitutional position. For ZZ-MoS₂ NR doped by Mn, Cr or V atoms, the preferred magnetic coupling state is the edge atoms of S at the S-edge, exhibiting the same spin polarization with TM (named the FM1 state), attributing the NR with metallic magnetism. For Ti-doped monolayer ZZ-MoS₂ NR, in addition to the FM1 state, other preferred magnetic coupling state was observed in which the edge atoms of S at the S-edge exhibit the opposite spin polarization with that of Ti (named the FM2 state). Thus, the NR doped by Ti atom possesses metallic (FM1 state) or half-metallic (FM2 state) magnetism. The total magnetic moments of the ZZ-MoS₂ NR doped by TM follows a linear relationship as a function of the TM dopants (Mn, Cr, V, and Ti). Under $>4\text{\%}$ applied strain, the NR doped by Ti atom only presents the characteristics of half-metallic magnetism as the initial one in the FM2 state, and its total magnetic moment always remained 0 μB, i.e., it was not affected by the width of the NR. This study provides a rational route of tuning the magnetic properties of ZZ-MoS₂ NRs for their promising applications in nanoelectronics and spin electronic devices.     

Magnetoacoustic resonance on nuclear spin waves in the cubic antiferromagnet RbMnF3

Magnetoacoustic resonance on nuclear spin waves is measured in the cubic antiferromagnet RbMnF₃. A resonance change with respect to a constant magnetic field H ₀ with maximum damping at H ₀≈4×10³ Oe is observed in the amplitude of an acoustic pulse passing through a sample owing to excitation of nuclear spin waves under nuclear magnetoacoustic resonance conditions. A study of the angular dependence of the damping revealed a 90° periodicity consistent with the fact that the [001] direction, around which the rotation takes place, is a four-fold axis of the crystal. An analysis of the dispersion law for nuclear spin waves shows that longitudinal ultrasound propagating along the [001] axis perpendicular to H ₀ excites a branch of nuclear spin waves whose frequency depends on the magnitude of the constant magnetic field. Fiz. Tverd. Tela (St. Petersburg) 41, 297–300 (February 1999)     

Molecular-based magnet with two magnetic transition temperature: A Prussian-blue analogue Mn3[Cr(CN)6]2·12H2O

A Prussian-blue analogue Mn₃[Cr(CN)₆]₂·12H₂O has been prepared and characterized by single crystal X-ray analysis and magnetic measurements. The complex has a disordered face-centered cubic lattice and shows two magnetic transition temperatures of 74 and 61 K.     

On the magnetic properties of Pr[Mn(CN)<Subscript>6</Subscript>]. 4H<Subscript>2</Subscript>O

A Prussian Blue type compound of the nominal composition Pr[Mn(CN)₆].4H₂O has been prepared. It is shown that the compound exhibits ferrimagnetism with a Curie temperature T_C=38.9 K. Observed magnetic relaxation displays a logarithmic behavior.     

The first detection of photomodulation by both DC and in-phase AC susceptibility for organic/inorganic hybrid materials containing cyano-bridged Gd–Cr complex and azobenzene

New organic/inorganic hybrid material containing a ferrimagnetic cyano-bridged binuclear complex, [Gd(DMF)₄(H₂O)₃Cr(CN)₆]·H₂O (DMF=N,N′-dimethylformamide), and photochromic azobenzene in poly(methylmethacrylate) (PMMA) cast films has been prepared. We characterized magnetic properties of bulk [Gd(DMF)₄(H₂O)₃Cr(CN)₆]·H₂O and confirmed that it was not changed by light illumination at all. For the first time, we could detect reversible changes of both DC and in-phase AC susceptibility for the hybrid materials accompanying with cis–trans photoisomerization of azobenzene after UV and visible light illumination for 3 min at 3 K. We also discuss the kind of detectable magnetic changes (e.g. DC, in-phase AC, and both DC and in-phase AC) and the nature of some photo-magnetic functional hybrid materials (cis–trans photoisomerization and increase/decrease of magnetization) known to estimate their mechanism and interactions.