Magnetic behavior in solid solution systems: I. (MnMg)Tb2S4

The solid solutions in the system Mn_xMg_1?xTb₂S₄ for 0 ≤ x ≤ 1 all have the orthorhombic MnY₂S₄ structure, space group Cmc2₁. In the temperature range 77–300 K the materials are paramagnetic and the Curie-Weiss law is obeyed. At low temperatures, ca. 20 K, there is a deviation from linearity of the curve of χ^?1 vs T. The curves of magnetization as a function of the magnetic field at 4.2 K are reminiscent of saturation curves, especially for low values of x. The magnetic interactions between the metal ions are discussed.     

Spin-chemical approach to photochemistry: reaction control by spin quantum operation

In this review, the contribution of spin chemistry (in particular, magnetic resonance-related chemistry) to the photochemical field is briefly introduced. First, the development of a time-resolved EPR method and its significant application to radical-related physical phenomena and chemical reactions are presented. Second, a reaction-control method by means of electron spin operations is introduced, and several reaction yield-detected magnetic resonance (RYDMR) methods are presented as applications of this concept. One of the most important physical conclusions is the introduction of the concept of “spin phase relaxation” termed singlet–triplet (S–T) and triplet–triplet (T–T) dephasing, instead of the traditional concepts of longitudinal (T₁) and transversal relaxations (T₂). The effects of strong microwave power on the RYDMR spectrum and time-domain data are analyzed according to this concept. Furthermore, a new detection method is introduced, termed “photoconductivity detected magnetic resonance” (PCDMR), which is applicable exclusively to the system of charge transfer reactions.     

Internal friction spectra of polymer materials in complex stressed states

A procedure for the quantitative characterization of complex stressed states and their effect on the relaxation properties of polymer materials has been proposed. Throughout the temperature interval where the thermal degradation of polymers can be ignored, three measurements runs for recording internal friction spectra Δ(T) are performed. As has been shown for cellulose diacetate and poly(vinylformals), a difference between attenuation decrements Δ₁(T)–Δ₂(T) for the initial samples and the same samples in the second measurement cycle reflects the presence of cryptoheterogeneity. This difference achieves its maximum values at temperatures close to the glass transition temperature T _α but becomes noticeable even at temperatures several tens of degrees lower than T _α.     

Novel Co complex with high transformation temperature of valence tautomerism

The preparation and characterization of [Co^II-HS(dpa)(3,5-DBSQ)₂] are presented first time. From the magnetic curve, the phase transition temperatures, T_c is 380 K. To our knowledge, the T_c value is the highest among the species exhibiting thermal valence tautomerism. After illumination with 532 nm light at 5 K, the magnetization values increase from μ_eff=1.73 to 2.0μ_B, showing valence tautomerism. The metastable state will recover to the original state with the increase of temperature. The alternate illumination with 532 and 830 nm light at 5 K can induce a reversible change in magnetization. The quick response to light means that this compound can be used for the development of optical–magnetic switch.     

Thermodynamic properties and phase transitions of Tutton salt (NH4)2Fe(SO4)2·6H2O from MAS NMR and single-crystal NMR

The thermodynamic properties and phase transitions of Tutton salt (NH₄)₂Fe(SO₄)₂·6H₂O were investigated using thermogravimetric analysis, differential scanning calorimetry, and nuclear magnetic resonance. The first mass loss occurs around 330 K (T _d), which is interpreted as the onset of partial thermal decomposition. Phase transitions were found at 387 K (=T _C1) and 500 K (=T _C2). The temperature dependences of the spin–lattice relaxation time in the rotating frame, T _1ρ, and that in the laboratory frame, T ₁, for the H nuclei change abruptly near T _C1. These changes are associated with changes in the geometry of the arrangement of octahedral water molecules and ammonium protons.     

Magnetic properties of ultrafine ferrihydrite clusters studied by Mossbauer spectroscopy and by thermodynamical analysis

Magnetic properties of ultrafine clusters of Fe₅HO₈·₄H₂O (ferrihydrite, FH), isolated in pores of polysorb, were studied by Mossbauer spectroscopy and by thermodynamical analysis. Thermodynamical analysis allowed the conclusion that magnetic properties of ultrafine clusters cannot be interpreted in terms of a secondorder magnetic phase transition or of superparamagnetic behavior alone but require the consideration of a jumplike first order magnetic phase transition (JMT). The critical radius R _cr below which the JMT is to be expected in clusters was derived from thermodynamic criteria. It was determined as R _cr = 2 α β η/(1 - T _cc/T ₀), where α, β and η are constants derived from surface energy, magnetostriction, compressibility and T _cc = 3/2 Nκ _B T _o ² ηβ ² (N is the number of iron atoms, κ _B is the Boltzmann constant, T _o is the Curie temperature of the clusters). For the smallest FH clusters isolated in pores of polysorb, the critical radius and the JMT temperature were estimated by Mossbauer spectroscopy to be R _cr ～ 1.5–2.0 nm and T _JMT ～ 4.2–6 K, respectively. Satisfactory agreement between the value R _cr, estimated from the experimental data and the one derived by thermodynamical analysis was achieved. Interfacial (cluster-surface) and intercluster interactions were found to destroy the JMT effect and to give rise to a second-order magnetic phase transition.     

Comparison of magnetocaloric properties of the Mn2−xFexP0.5As0.5 (x = 1.0 and 0.7) compounds

The Mn_2−xFe_xP_0.5As_0.5 compounds (x = 0.7 and 1.0) studied exhibit the magnetic phase transitions, which are accompanied by a magnetic entropy change. For x = 1 the PM–FM transition is of the first order one with a weak (2–3 K) thermal hysteresis in the vicinity of T_C = 275 K. The Mn_1.3Fe_0.7P_0.5As_0.5 compound possesses two magnetic transitions: the second-order PM–FM transition at T_C = 190 K, followed by the FM–AFM transition at T_N = 90 K, leading to normal and inverse magnetocaloric effects, respectively. The maximum values of magnetic entropy change are equal to 17 J kg⁻¹ K⁻¹ in MnFeP_0.5As_0.5 and 5 J kg⁻¹ K⁻¹ in Mn_1.3Fe_0.7P_0.5As_0.5 for a field change of 5 T. The magnetic entropy changes were calculated using both the isofield magnetization curves versus temperature and the isothermal magnetization curves versus applied magnetic field. The magnetocaloric effect in MnFeAs_0.5P_0.5 is discussed in the terms of both the thermodynamic Maxwell relation and the Clausius–Clapeyron equation.     

High performance heteroaromatic poly(azo-ester)s and their miscible blends: Thermomechanical and conductivity profile

An interesting type of thermally stable and processable poly(azo-ester)s (PAEs) have been synthesized through the polycondensation of a novel diol—(E)-1-(5-(3-hydroxypyridylazo)thiocarbamoyl-aminonaphthyl)-3-(3-hydroxypyridylazo)thiourea, and various diacid chlorides. The dihydroxy compound containing azo group (?N=N?) was prepared via multi-step procedure in which the coupling of bisthiourea compound with diazonium chloride (in alkali) yielded the desired monomer. The polymeric materials were characterized in terms of FTIR, ¹H NMR, solubility, solution viscosity, molecular weight, electrical conductivity, glass transition and thermal degradation temperatures. PAEs possessed high inherent viscosity 1.19–1.23 dL/g and molar mass (8.3–8.5) × 10⁴. The polymers were thermally stable in the range 531–541°C (10% gravimetric loss T ₁₀) having T _g′ = 258–266°C. PAEs were, thus, melt blended with polyaniline resulting in high performance materials that potentially combined the fine thermal properties and processability of poly(azo-ester)s with electrical characteristics of polyaniline. The miscible blends exhibited good heat stability (T ₁₀ = 525–527°C, T _g = 246–252°C) and mechanical strength (61.81–63.19 MPa) compared with several polyaniline-based blends. FESEM showed nano-level homogeneity of the microstructure liable for better electrical conductivity (3.2–4.2 S/cm).     

Synthesis and characterization of a new layered organic-inorganic hybrid nickel(II) 1,4:5,8-naphthalenediimide bis-phosphonate, exhibiting canted antiferromagnetism, with Tc∼21 K

A new Ni(II) layered hybrid organic-inorganic compound of formula Ni₂[(NDI-BP)(H₂O)₂]·2H₂O has been prepared in very mild conditions from N,N′-bis(2-phosphonoethyl)napthalene-1,4:5,8-tetracarboximide (NDI-BP ligand) and NiCl₂. The X-ray powder structure characterization of the title compound suggests a pillared layered organic-inorganic hybrid structure. The distance between the organic and inorganic layers has been found to be 17.8 Å. The inorganic layers consist of corner sharing [NiO₅(H₂O)] octahedra and they are pillared by the diphosphonate groups. DC and AC magnetic measurements as a function of temperature and field indicate the presence of 2D antiferromagnetic exchange interactions between the nearest-neighbor Ni(II) ions below 100 K. A long-range magnetic ordering at T_c∼21 K has been established and is attributed to the presence of spin canting. AC magnetic measurements as a function of temperature at different frequencies confirm the occurrence of the magnetic ordering temperature at T=21 K and the presence of a slight structural disorder in the title compound.     

Studies of fluorine-containing bismaleimide resins. Part I: Synthesis and characteristics of model compounds

A series of fluorine-containing bismaleimide (FBMI) monomers are synthesized by a 2-step reaction for using as the applications of low-k materials. The synthesized FBMI monomers are characterized by the ¹H, ¹³C, ¹⁹F nuclear magnetic resonance (NMR) spectroscopy and element analysis. These FBMI monomers react with free radical initiator or self-cure to prepare FBMI-polymers. All the self-curing FBMI resins have the glass transition temperatures T _g in the range of 130–141°C and show the 5% weight loss temperatures T _5% of 280–322°C in nitrogen atmosphere. The higher heat resistance of self-curing FBMI resin relative to FBMI-homopolymer is due to its higher cross-linking density. The FBMI resins exhibit improved dielectric properties as compared with commercial bismaleimide (BMI) resins with the dielectric constants ? lower than 2.44 which is related to the low polarizability of the C-F bond and the large free volume of CF₃ groups in the polymers. Besides, the flame retardancy of all these FBMI resins could be enhanced via the introduction of Br-atom.     

Neutron powder diffraction study of the magnetic and crystal structures of SrFe2(PO4)2

The crystal and magnetic structures of SrFe²⁺₂(PO₄)₂ have been determined by neutron powder diffraction data at low temperatures (space group P2₁/c (no. 14); Z=4; a=9.35417(13) Å, b=6.83808(10) Å, c=10.51899(15) Å, and β=109.5147(7)° at 15 K). Two magnetic phase transitions were found at T₁=7.4 K (first-order phase transition) and T₂=11.4 K (second-order phase transition). The transition at T₂ was hardly detectable by dc and ac magnetization measurements, and a small anomaly was observed by specific heat measurements. At T₁, strong anomalies were found by dc and ac magnetization and specific heat. The structure of SrFe₂(PO₄)₂ consists of linear four-spin cluster units, Fe2-Fe1-Fe1-Fe2. Below T₁, the propagation vector of the magnetic structure is k=[0,0,0]. The magnetic moments of the inner Fe1-Fe1 atoms of the four-spin cluster unit are ferromagnetically coupled. The magnetic moment of the outer Fe2 atom is also ferromagnetically coupled with that of the Fe1 atom but with spin canting. The four-spin cluster units form ferromagnetic layers parallel to the [−101] plane, while these layers are stacked antiferromagnetically in the [−101] direction. Spin canting of the outer Fe2 atoms provides a weak ferromagnetic moment of about 1 μ_B along the b-axis. The refined magnetic moments at 3.5 K are 4.09 μ_B for Fe1 and 4.07 μ_B for Fe2. Between T₁ and T₂, a few weak magnetic reflections were observed probably due to incommensurate magnetic order.     

Li ion dynamics in Li3xLa2/3–x□1/3–2xTiO3 studied by NMR

⁷Li nuclear magnetic resonance relaxation times, T₁ and T_1ρ, versus temperature are reported in the 150–900 K temperature range on the lithium lanthanum titanates, Li_3xLa_2/3–x□_1/3–2xTiO₃, which are fast ionic conductors. Two characteristic frequencies of Li⁺ motions are evidenced in these compounds: the first is in the range of the Larmor frequency when the second one is in the range of the radio-frequency field. These frequencies are respectively attributed to motion of the Li⁺ ion inside the cage formed by the oxygen ions and to jumps between the cages. The T₁ and T_1ρ studies on ⁶Li nuclei confirm the above results and show that the relaxation is not due to quadrupolar interaction at a variance, which is generally accepted.     

Synthesis and characterization of photocrosslinkable poly(l-lactide)s with a pendent cinnamate group

In this report, we describe the synthesis and characterization of photosensitive poly(l-lactide) with a pendent cinnamate group. α,ω-Dihydoxy terminated poly(l-lactide) (PLLA-diol) [molecular weights (MW); 2000, 4000 and 9000 g/mol ] was chain-extended with a diacyl chloride of 5-cinnamoyloxyisophthalic acid (ICA) to obtain high-molecular-weight photocrosslinkable poly(l-lactide)s (ICA/PLLA). The resulting polyesters were characterized by proton nuclear magnetic resonance spectroscopy, gel permeation chromatography, ultraviolet–visible spectroscopy, differential scanning calorimetry, thermomechanical analysis and thermogravimetry. The glass transition temperature (T_g), the melting temperature (T_m) and the degree of relative crystallinity (X_c) of ICA/PLLAs increased with the increasing MW of the PLLA-diols. The photosensitive ICA/PLLAs were irradiated with a 400 W high-pressure mercury lamp (λ > 280 nm) for various times to produce the PLLA gel films without a photoinitiator. The crosslinking rate monitored by a UV–vis spectrum decreased with the increasing MW of the PLLA-diols. The crosslinking of the ICA/PLLA ?4000 film enhanced the T_g slightly and the tensile strength and Young’s modulus significantly, while reduced the T_m and X_c. The enzymatic degradation was measured by the weight loss of the films in a phosphate buffer solution with proteinaze-k. The crosslinking of the films decreased markedly the degradation rate.     

Mesophase and magnetic behavior in cobalt(II) and iron(II) compounds

The metal complexes with long alkyl chains [Co(C16-terpy)₃](BF₄)₂ (1) and [Fe(C16-terpy)₂](BF₄)₂ (2) were synthesized and the physical properties of the complex were characterized by magnetic susceptibility, Mössbauer spectroscopy, polarizing optical microscopy, differential scanning calorimetry, and X-ray scattering, where C16-terpy is 4′-hexadecyloxy-2,2′:6′,2′′-terpyridine. Variable-temperature magnetic susceptibility measurements and/or Mössbauer studies revealed that the complex 1 exhibited unique spin transition (T_1/2↓ = 217 K and T_1/2↑ = 260 K) induced by structural phase transition, and the complex 2 was in the low-spin state in the temperature region of 5–400 K before the first mesophase transition. The cobalt(II) and iron(II) complexes exhibited liquid-crystal properties in the temperature range of 371–528 K and 466–556 K, respectively. After mesophase transition, the complex 1 exhibited only slight spin transition (T_1/2↓ = 266 K and T_1/2↑ = 279 K), and the complex 2 was in the low-spin state. The compounds with multifunction, i.e., magnetic property and liquid-crystal properties, are important in the development of molecular materials.     

Magnetic phase diagrams and helical magnetic phases inMxNi1−xBr2 (M =Fe,Mn): A neutron diffraction and magneto-optical study

BelowT_N = 44K, NiBr₂ orders as an easy-plane antiferromagnet, but atT_IC ～ 23K it undergoes a transition to a helimagnetic phase whose propagation vector, τ, increases smoothly to a limiting value [0.027,0.027,0] at 4.2 K. The magnetic order in NiBr₂ andM_xNi_1?xBr₂ (M =Fe, Mn) is investigated by single-crystal neutron diffraction. In Fe_xNi_1?xBr₂ τ changes from the [110] to [100] direction at 3.7(17)% Fe. Its magnitude, andT_IC, remain constant up to 9.9(14)% Fe, but above this concentration a collinear, easy-axis structure is stable at all temperatures belowT_N, which decreases towards the value of 11 K in FeBr₂. Doping 2.9% Mn²⁺ into NiBr₂ decreasesT_IC slightly, in the manner previously observed for diamagnetic dopants. Refinement of powder X-ray diffraction profiles show the lattice distortions in the doped materials to be small, while the structures of some Fe_xNi_1?xBr₂ samples are confirmed by refinement of the powder neutron diffraction profiles. The temperature and field dependence of hot and cold exciton-magnon combination bands in the optical absorption spectra of Fe²⁺- and Mn²⁺-doped crystals give additional information about magnetic order. The magnetic behaviour is interpreted in terms of perturbations of the exchange constantsJ_1,2,3, andJ′ and the anisotropy constantD.     

Formation of SrSnO3 shell-like inclusions in the Bi2Sr2CaCu2O8+x superconductor via chemical reaction

Superconducting composites Bi-2212–SrSnO₃ have been prepared by reacting between the Bi–Sr–Ca–Cu–O precursor and Sr₂SnO₄ or Ca₂SnO₄ at 800–950°C, followed by crystallisation of Bi-2212 from the partial melt at decreasing temperature. The samples have been characterised by powder X-ray diffraction, scanning electron microscopy and magnetic measurements. The materials consist of large Bi-2212 lamellae and complex-shaped fine inclusions of SrSnO₃. The composite obtained using Sr₂SnO₄ contains almost all the SrSnO₃ phase in the form of micron-sized spherical shells, which are partly included in Bi-2212 lamellae, partly agglomerated in-between. The shells are perforated, thus allowing the Bi-2212 crystals to grow through them. It has been found that the shell-like grains form at an early stage of the precursor thermal treatment between 800 and 850°C. A mechanism of the SrSnO₃ shell formation is proposed. The composites exhibit T_c in the range of 82–87 K and reveal up to five times better magnetic flux pinning at T≥30 K in comparison with the undoped Bi-2212 sample prepared using the same experimental procedure.     

Some applications of thermomagnetometry to the study of chromindur alloys

Thermomagnetic experiments are shown to be a useful and rapid technique for studying phase changes involving magnetic materials. Particular examples are chosen from Chromindur II (Fe28Cr10.5Co) alloys having different thermal histories. The Curie temperature of the single phase alloy is found to be ～650°C and the metastable spinodal decomposition boundary is at essentially the same temperature. The Cr-rich near equilibrium decomposition product is less magnetic and has a lower T_c, while the Cr-deficient product is more magnetic, with a T_c ? 720°C.Comparisons are made at different heating and cooling rates between temperatures measured using magnetic standards and those measured by a thermocouple in close proximity to the sample.     

Magnetic properties of the fullerene organic compounds in strong magnetic fields

Magnetisation study of the C₆₀·TMTSF·2CS₂ molecular complex in magnetic field up to 47 T for the temperature range 1.8–300 K and ESR spectroscopy of the molecular complex (ET)₂C₆₀ at T=1.8 K for the frequency range 60–90 GHz in magnetic field up to 32 T provide the experimental evidence that a paramagnetic centers with the reduced g-factor values g<1 control magnetic properties of these solids. Anomalous g-factor values may be caused by dynamic Jan-Teller effect on the negative C₆₀ ions, which appear as defects in crystalline structure with a weak charge transfer.     

An investigation of magnetocaloric effect and its implementation in critical behavior study of La1−xNdxMn2Si2 compounds

The magnetic properties of La_1−xNd_xMn₂Si₂ (x = 0.30–0.45) compounds are studied over the temperature range 5 K ≤ T ≤ 375 K. We report inverse and conventional magnetic entropy change values of La_1−xNd_xMn₂Si₂ (x = 0.35 and 0.40) compounds over the temperature range 5 ≤ T ≤ 375 K. In addition, we study critical behavior of La_1−xNd_xMn₂Si₂ (x = 0.35 and 0.40) compounds around their Curie temperatures. The field dependence of the magnetic entropy change is brought out and implemented to deduce the critical exponents. The critical behavior study shows that the magnetic interactions for the x = 0.35 sample have the same behavior below and above T_C. However, for the x = 0.40 sample has different behavior below and above T_C. Thus, the x = 0.40 sample behaves as a multiphase compound.     

A Uranium‐Based UO2+–Mn2+ Single‐Chain Magnet Assembled trough Cation–Cation Interactions

Single‐chain magnets (SCMs) are materials composed of magnetically isolated one‐dimensional (1D) units exhibiting slow relaxation of magnetization. The occurrence of SCM behavior requires the fulfillment of stringent conditions for exchange and anisotropy interactions. Herein, we report the synthesis, the structure, and the magnetic characterization of the first actinide‐containing SCM. The 5f–3d heterometallic 1D chains [{[UO₂(salen)(py)][M(py)₄](NO₃)}]_n, (M=Cd ( 1 ) and M=Mn ( 2 ); py=pyridine) are assembled trough cation–cation interaction from the reaction of the uranyl(V) complex [UO₂(salen)py][Cp*₂Co] (Cp*=pentamethylcyclopentadienyl) with Cd(NO₃)₂ or Mn(NO₃)₂ in pyridine. The infinite UMn chain displays a high relaxation barrier of 134±0.8 K (93±0.5 cm^?1), probably as a result of strong intra‐chain magnetic interactions combined with the high Ising anisotropy of the uranyl(V) dioxo group. It also exhibits an open magnetic hysteresis loop at T<6 K, with an impressive coercive field of 3.4 T at 2 K.