Mössbauer spectroscopic study of the thermal spin crossover in [Fe(II)(isoxazole)6](ClO4)2

The ⁵⁷Fe Mössbauer spectroscopy of mononuclear [Fe(II)(isoxazole)₆](ClO₄)₂ has been studied to reveal the thermal spin crossover of Fe(II) between low-spin (S=0) and high-spin (S=2) states. Temperature-dependent spin transition curves have been constructed with the least-square fitted data obtained from the Mössbauer spectra measured at various temperatures between 84 and 270 K during a cooling and heating cycle. This compound exhibits an unusual temperature-dependent spin transition behaviour with T_C(↓)=223 and T_C(↑)=213 K occurring in the reverse order in comparison to those observed in SQUID observation and many other spin transition compounds. The compound has three high-spin Fe(II) sites at the highest temperature of study of which two undergo spin transitions. The compound seems to undergo a structural phase transition around the spin transition temperature, which plays a significant role in the spin crossover behaviour as well as the magnetic properties of the compound at temperatures below T_C. The present study reveals an increase in high-spin fraction upon heating in the temperature range below T_C, and an explanation is provided.     

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.     

Electrical and magnetic characterization of a molecular material {[Ru(bpy)3][Fe(dca)3]2}n

Electrical and magnetic properties of {[Ru(bpy)₃][Fe(dca)₃]₂}_n (bpy=2,2′-bipyridine, dca=dicyanamide) have been studied. The compound is a non-extrinsic type of semiconductor and paramagnetic in nature. Mössbauer spectroscopy has established the presence of high spin Fe(II) as one major species in this compound, and no high spin-low spin transition of Fe(II) was detected down to 80 K under dark. The photo-response of electrical conductivity with time shows interesting behavior with repeated exposure.     

Quasi-two-dimensional magnetism in (CnH2n+1NH3)2CuCl4 studied by electron paramagnetic resonance

The quasi-two-dimensional magnetism in the layered transition metal compound (C_nH_2n+1NH₃)₂CuCl₄ (n=10, 14) was investigated by means of electron paramagnetic resonance (EPR) and superconducting quantum interference device measurements. As a result, the high temperature magnetic phase transitions were reflected in the EPR parameters in a sensitive manner.     

Triple quantum MQMAS spectroscopy of (I=7/2) in Na3Co(NO2)6 and trans-Co[(en2)(NO2)2]NO3 interplay between the quadrupole coupling and anisotropic shielding tensors

The purpose of this paper is to investigate the interplay between the chemical shielding anisotropy and quadrupole interaction in MQMAS spectra. in the compounds Na₃Co(NO₂)₆ and trans-Co[(en₂)(NO₂)₂]NO₃ provides model systems for such an investigation. Furthermore, only few results have been reported on the application of the MQMAS method to a spin I=7/2. The possibilities of the MQMAS spectroscopy for determining the relative orientation of the two tensors and its advantage over previous techniques are discussed. Reported experimental spectra at different spinning speeds of Na₃Co(NO₂)₆ are accurately reproduced by our theoretical simulations. The calculations are based on a recent approach, summarized in the present paper, which allows one to perform efficient simulations of MQMAS spectra including all interactions and their time-dependence throughout the experiment. This is necessary for calculating accurate MQMAS spectra including the spinning sideband pattern. In the case of trans-Co[(en₂)(NO₂)₂]NO₃ where the quadrupolar interaction and chemical shielding are stronger and their axes are non-coincident, the MQMAS spectrum is strongly distorted due to the unsufficient spinning speed and RF power. In this case, MAS at different spinning speeds is shown to provide valuable information.     

Ab initio study on phase transition and magnetism of BiFeO3 under pressure

In this paper the first-principles calculations within local spin density approximation (LSDA)+U show that BiFeO3 experiences a mixed phase state with P4mm structure being the intermediate phase before the pressure of phase transition is reached. The critical pressure for the insulator-metal transition (IMT) is found to be about 50 GPa. A pressure induced crossover of high-spin states and low-spin states is observed close to the IMT pressure in R3c structure. The LSDA+U calculations account well for the mechanism of the IMT and crossover of spin states predicted in recent experiment (Ref.[1]).     

EPR studies on exchange interactions in pairs of Jahn-Teller ions in Cu : Zn(pyO)6(ClO4)2

Semi-dilute 10% ⁶³Cu : Zn(pyO)₆(ClO₄)₂ [pyO = C₅H₅NO, pyridine-N-oxide] has been studied by EPR. Spectra of pairs of Cu(pyO)²⁺₆ ions are observed below the transition from dynamic to static Jahn-Teller distortion. Most of these pairs are antiferrodistortively oriented. They exhibit only the type of exchange which corresponds to the interchain interaction J′ in the pure Cu compound. From the Q-band AB type of spectrum it was found that |J′|/k = 0.041 K. An estimate of the zero- point spin reduction could be derived (～36%).     

Magnetic properties of the charge ordered Nd0.75Na0.25MnO3

Nd_0.75Na_0.25MnO₃ polycrystalline ceramic is prepared via sol-gel process and its magnetic properties and electron spin resonance (ESR) spectra have been investigated experimentally. As the compound is cooled from room temperature, a charge-ordered state first develops below 170 K. A high magnetic field melts the charge ordered state and stabilizes a ferromagnetic (FM) state below 170 K. A field induced transition, analogous to a spin flip transition, is observed between 40 and 170 K. The critical temperature for spin flip increases with increasing temperature. Below 130 K, the compound tends to be intrinsically inhomogeneous, i.e. FM clusters and paramagnetic domains coexist in this system at least, which is confirmed by ESR measurements. When the external magnetic field is zero, long range FM interaction is not developed in this system; however, a tendency of re-entrant FM transition is observed in this compound.     

Thermal expansion property of anomalous magnetic alloy Fe70Al30

D0₃-type Fe₇₀Al₃₀ shows a transition from ferromagnetism to spin glass with anomalous slow spin dynamics below 170 K. Furthermore, it shows a structural transition from D0₃ to body-centered cubic (BCC) at 823 K. In this article, the relationship between the magnetic properties, thermal expansion coefficient (TEC), and crystal structure transition of D0₃-type Fe₇₀Al₃₀ is discussed. Below 170 K, TEC decreases with temperature, accompanied by a decrease in the Fe moments. In the ferromagnetic state between 170 K and the Curie temperature (T_C), TEC increases gradually with temperature. Above T_C, TEC increases rapidly. These temperature variations of TEC could be connected to the high-spin/low-spin transition and thermal spin fluctuations. During transition from D0₃ to disordered BCC at 823 K, TEC shows discontinuous behavior, similar to a first-order transformation. With further increase in temperature, TEC becomes constant. This implies that the phase transition from D0₃ to disordered BCC is accompanied by a change in spin fluctuation.     

Magnetic resonance study of the two-dimensional spin diffusion in the Heisenberg paramaget (C18H37NH3)2MnCl4

¹H nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) techniques were employed to study the perovskite-type layered structure compound (C₁₈H₃₇NH₃)₂MnCl₄ undergoing structural phase transitions. The spin relaxation was found to sensitively reflect the two-dimensional electron spin diffusion.     

EPR Features of Spin-Crossover in Tris(<Emphasis Type="Italic">N</Emphasis>,<Emphasis Type="Italic">N</Emphasis>-Dialkyl-Dithiocarbamato)Iron(III)

The temperature dependence of electron paramagnetic resonance (EPR) spectra of a series of dithiocarbamates Fe(RR′dtc)₃ was studied in the temperature range from 5 to 300 K. A small part of solvated complexes serving as spin probes in the EPR-silent matrix enabled the observation of EPR of the Fe(III) ion in the whole temperature range. The spin transition was revealed in the reduction of the integral intensity of the signal from the high-spin complexes and in the non-monotonous change of the line width with temperature decrease due to the effect of the low-spin complexes with short spin–lattice relaxation times. Below ca. 60 K, the ferromagnetic ordering of the magnetic moments in low-spin particles (“domains”) arising at the spin transition was observed.     

Identification of the La6F37 cubooctahedral clusters in mixed crystals (BaF2)1 ? x(LaF3)x by the electron paramagnetic resonance method

The electron paramagnetic resonance (EPR) spectra of mixed crystals (BaF₂)_{1 − x} (LaF₃) _x (x = 0, 0.001, 0.002, 0.005, 0.010, 0.020) doped with Ce³⁺ ions (0.1%) are investigated at a frequency v ≈ 9.5 GHz in magnetic fields up to 1.45 T at temperatures T = 10 and 15 K. The EPR spectrum of “pure” barium fluoride BaF₂ (x = 0) is characterized by a single Ce³⁺-F⁻ center with tetragonal symmetry (i.e., the O center with g _‖ = 2.601 and g _⊥ = 1.555). For a lanthanum trifluoride concentration x ≠ 0, the spectrum exhibits new lines due to the presence of the clusters containing Ce³⁺ and La³⁺ ions. The intensity of EPR signals from the O centers decreases rapidly as the lanthanum trifluoride concentration x increases. The lines attributed to a paramagnetic center with tetragonal symmetry and strongly anisotropic g factors (i.e., the K center with g _‖ = 0.725 and g _⊥ = 2.52) are separated in the complex EPR spectrum with the use of the angular dependence of the EPR signal intensity measured for the samples with x ≥ 0.002. This center is identified as a cubooctahedral cluster of the La₆F₃₇ type in which one of the La³⁺ ions is replaced by the Ce³⁺ ion. Original Russian Text ? L.K. Aminov, I.N. Kurkin, S.P. Kurzin, I.A. Gromov, G.V. Mamin, R.M. Rakhmatullin, 2007, published in Fizika Tverdogo Tela, 2007, Vol. 49, No. 11, pp. 1990–1993.     

Proton irradiation defects in (fluoranthene)2PF6

Electron spin resonance and electron–proton double resonance (Overhauser shift method) are used for the comparison of proton radiation damaged and as-grown (fluoranthene)₂PF₆ single crystals. Chemical modification and various consequences of the nonuniform distribution of radiation induced defects in this quasi-one-dimensional organic conductor with defect dependent Peierls transition are worked out.     

Molecular material—{N(n-C4H9)4[Ni(II)0.5 Fe(II)0.5 Fe(III)(C2O4)3]}∞: Magnetic, Mössbauer and electrical conductivity studies

We have attempted to characterize the magnetic and electrical properties of a new mixed-metal molecular material {NBu₄[Ni(II)_0.5Fe(II)_0.5Fe(III)(ox)₃]}_N synthesized by the use of trioxalatoferrate as the building block. Mössbauer spectroscopy was utilized in order to understand local spin structures in this compound. The results indicate that the compound is a semiconducting ferrimagnet with T_N=30 K and room temperature conductivity of 6×10⁻¹⁵ Ω⁻¹ cm⁻¹ along with 1.8 eV activation energy under dark. The compound has no appreciable electrical response towards illumination.     

Fe（II）-NH(a1Δ)N4S2自旋翻转单分子磁体的电子输运性质

Spin-crossover (SCO) magnets can act as one of the most possible building blocks in molecular spintronics due to their magnetic bistability between the high-spin (HS) and low-spin (LS) states. Here, the electronic structures and transport properties through SCO magnet Fe(II)-N₄S₂ complexes sandwiched between gold electrodes are explored by performing extensive density functional theory calculations combined with non-equilibrium Green''s function formalism. The optimized Fe-N and Fe-S distances and predicted magnetic moment of the SCO magnet Fe(II)-N₄S₂ complexes agree well with the experimental results. The reversed spin transition between the HS and LS states can be realized by visible light irradiation according to the estimated SCO energy barriers. Based on the obtained transport results, we observe nearly perfect spin- filtering effect in this SCO magnet Fe(II)-N₄S₂ junction with the HS state, and the corresponding current under small bias voltage is mainly contributed by the spin-down electrons, which is obviously larger than that of the LS case. Clearly, these theoretical findings suggest that SCO magnet Fe(II)-N₄S₂ complexes hold potential applications in molecular spintronics.     

Physical properties of (TMTSF)2TaF6: Influence of the anion size

The physical properties of (TMTSF)₂TaF₆ are discussed through transport and magnetic measurements. A crossover between two different regimes is observed around 100K and a spin density wave ground state appears below 11 K. The phase diagram under magnetic field is determined. Finally, the properties of this compound are compared with that of the other members of the TMTSF series and the influence of the anion size is discussed.     

Effects of synthetic routes of Fe-urea complex on the synthesis of γ-Fe2O3 nanopowder

Maghemite (γ-Fe₂O₃) nanopowder was prepared by the thermal decomposition of Fe-urea complex ([Fe(CON₂H₄)₆](NO₃)₃) that was synthesized by various routes including wet and dry synthetic methods. Then the effects of synthetic routes of the [Fe(CON₂H₄)₆](NO₃)₃ on resulting iron oxide crystalline phases and their magnetic properties have been studied using X-ray powder diffraction (XRD) and magnetic measurements. The result of XRD shows that the iron oxide crystalline phases are strongly dependent on the synthetic routes of the [Fe(CON₂H₄)₆](NO₃)₃. When [Fe(CON₂H₄)₆](NO₃)₃ is synthesized in ethanol, thermal decomposition of the compound results in pure γ-Fe₂O₃. When [Fe(CON₂H₄)₆](NO₃)₃ is synthesized either by an aqueous synthetic method or by a dry synthetic method, however, thermal decomposition of the compound results in mixed phases of γ-Fe₂O₃ and α-Fe₂O₃. Magnetic measurements show that resulting iron oxide nanopowder exhibits a ferromagnetic characteristic with a maximum saturation magnetization (Ms) of 69.0 emu/g for the pure γ-Fe₂O₃ nanopowder.     

Uniaxial stress measurements on Rb2PbCu(NO2)6 and K2PbCu(NO2)6

EPR measurements on crystals of compressed tetragonal Rb₂PbCu(NO₂)₆ and K₂PbCu(NO₂)₆ subjected to uniaxial stress have been carried out at various temperatures. The results indicate that uniaxial stress can reorient the crystal axes in both compounds and that smaller stresses are required for Rb₂PbCu(NO₂)₆ than for K₂PbCu(NO₂)₆ at comparable temperatures. Larger stresses are required at lower temperatures.     

Fe2+ spin transition in [Fe(trz)(Htrz)2](BF4) as evidenced by a variable temperature EPR study

The EPR of Fe³⁺ ions has been used for the first time to evidence a low-spin (S=0) to high-spin (S=2) transition of Fe²⁺ ions in an octahedral ferrous complex [Fe(trz)(Htrz)₂](BF₄). The temperature dependence of the intensity of the Fe³⁺ EPR line atg=4.3 reveals a spin transition which occurs for the Fe²⁺ ions, with hysteresis. The transition temperatures areT _c↑=374 K in the warming mode andT _c↓=345 K in the cooling mode. The analysis of the EPR spectral data indicates the presence of a structural phase transition accompanying the spin transition.     

Magnetic order in Y6(57Fe:Mn)23H26

The ferrimagnetic compound Y₆Mn₂₃ and its hydride Y₆Mn₂₃H₂₆, both doped with 0.5%⁵⁷Fe, have been investigated using the ⁵⁷Fe Mössbauer resonance and dc field magnetization measurements. For the hydride a small ⁵⁷Fe magnetic hyperfine field is observed to increase abruptly below 110 K whereas the bulk magnetization results suggest antiferromagnetic ordering at T_N≈ 180 K.