Magnetic anisotropy of UFe6Al6

A comparative study of magnetization in UFe₆Al₆ and LuFe₆Al₆ single crystals gives clear evidence for the magnetic state of uranium in UFe₆Al₆. Both compounds exhibit the easy-plane type of magnetic anisotropy, however, the anisotropy energy in UFe₆Al₆, characterized by the anisotropy constants K₁=−7.3 MJ m⁻³ and K₂=−1.25 MJ m⁻³ at 2 K, exceeds by an order of magnitude that in LuFe₆Al₆. Anisotropy within the easy plane in the ferromagnetic state and anisotropy in the paramagnetic range, both absent in LuFe₆Al₆, are observed in UFe₆Al₆.     

Ferromagnetic-to-antiferromagnetic transition in (Hf1−xTix)Fe2 intermetallic compounds induced by geometrical frustration of the Fe(2a) sites

The ferromagnetic-to-antiferromagnetic transition in the hexagonal (Hf_1−xTi_x)Fe₂ (0?x?1) intermetallic compounds has been investigated by ⁵⁷Fe Mössbauer spectroscopy. At 10 K, the transition occurs within rather narrow concentration limits, around x=0.55–0.65. We found that the key factor governing the unexpected quick change of the magnetic structure is the magnetic frustration of the Fe(2a) sites. The magnetic frustration is caused by the noncollinearity of the Fe(6h) magnetic sublattice. The noncollinearity arises from the rotation of the magnetic moments due to the competition between the ferromagnetic exchange interactions and the antiferromagnetic Fe(6h)–Ti–Fe(6h) interaction. In the compounds with x=0.4–0.6, the temperature transitions to the antiferromagnetic state are observed. As an example, the Hf_0.4Ti_0.6Fe₂ compound is completely antiferromagnetic above 200 K.     

Magnetic order and hyperfine interactions in RFe6Al6 (R = rare earth)

The systems RFe₆Al₆(R = Y, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb) crystallize in the tetragonal body centered I4/mmm structure. In striking contrast to the magnetic behaviour of RFe₄Al₈ (weakly coupled R and Fe sublattices, complicated magnetic structure, low T_c ~ 130 K), in the RFe₆Al₆ systems all magnetic sublattices order simultaneously at a relatively high temperature. The magnetization curves start with low values at low temperatures and rise to very high values at T_max ~ 230 K and then drop to 0 at T_c ~ 330 K. All samples show strong hysteresis effects at temperatures just below T_max. Mossbauer studies of ⁵⁷Fe in the (f) and (j) sites and ¹⁵¹Eu, ¹⁵⁵Gd, ¹⁶¹Dy, ¹⁶⁶Er and ¹⁷⁰Yb in the (a) site yield all hyperfine interaction parameters and temperature dependence of the local magnetic moments. All Mossbauer and magnetization experimental results can be explained in a self consistent way with a simple molecular field model. The Fe in the (j) site plays the dominant role in its strong intrasublattice ferromagnetic exchange and its strong antiferromagnetic exchange with the rare earth site. The Fe in the (f) site have an antiferromagnetic intrasublattice exchange, they have a canted strcuture with the ferromagnetic component parallel to the (j) sublattice magnetization.     

Investigations of Sm6(Mn1-xFex)23 system

The new Sm₆(Mn_1-xFe_x)₂₃(0?x?1.0) system hasbeen synthesized and investigated in a wide temperature range by the X-ray, magnetometric and Mössbauer effect methods. The X-ray studies show that the system forms solid solutions which are isostructural with the Th₆Mn₂₃ type crystal structure throughout the entire compositional range. Both Fe-rich and Mn-rich regions of the system are magnetically ordered and are separated from each other by the non-magnetically ordered 0.22?x?0.33 region. The substitution of Fe atoms for Mn atoms in the Mn-rivh region and similarly of Mn atoms for Fe atoms in the Fe-rich region decreases both the Curie temperature and the value of the magnetic moment per molecule. The temperature dependence of the reciprocal susceptibility obeys the Néel law. The Mössbauer absorption spectra reflect wide distributions of the ⁵⁷Fe hyperfine interaction parameters, and disappearance of long range magnetic coupling of Fe atoms in the magnetically ordered x=0 to 0.22 composition range.     

Li mobility in Li0.5 − xNaxLa0.5TiO3 perovskites (0 ≤ x ≤ 0.5): Influence of structural and compositional parameters

The dependence of Li mobility on structure and composition of Li_0.5 − xNa_xLa_0.5TiO₃ perovskites (0 ≤ x ≤  0.5) has been investigated by means of neutron diffraction, nuclear magnetic resonance and impedance spectroscopy. At 300 K, all samples display a rhombohedral superstructure (R-3c S.G.), where octahedra are out of phase tilted along [111] direction of the ideal cubic cell. The elimination of the octahedral tilting is responsible for the rhombohedral–cubic transformation, detected near 1000 K. In these perovskites, La and Na cations are randomly distributed in A sites, but Li ions are fourfold coordinated at unit cell faces of the cubic perovskite. Lithium conductivity, σ_300 K, decreases with the sodium content, decreasing from values typical of fast ionic conductors, 10^− 3 S/cm, to those of good insulators, 10^− 10 S/cm, when the interconnectivity between vacant A sites is lost (x > 0.3). In samples with x < 0.3, dc conductivity displays a non-Arrhenius behaviour, decreasing activation energy from ~ 0.37 to 0.25 eV when the sample is heated between 77 and 500 K. The temperature dependence of B_Li factors shows the existence of two regimes for Li motion. Below 373 K, Li ions remain partially located near square oxygen windows that connect contiguous A sites, but above 400 K, extended Li motions become dominant. The additional decrease of activation energy from 0.25 to 0.16 eV (low-temperature ⁷Li NMR value), should require the full elimination of octahedral tilting which is only produced above 1000 °C.     

A 57Fe and 119Sn Mössbauer study of BaFe4Sn2O11

The Mössbauer spectrum of BaFe₄Sn₂O₁₁ has been recorded for both ⁵⁷Fe and ¹¹⁹Sn isotopes at a variety of temperatures. In the paramagnetic state the ⁵⁷Fe spectra are interpreted in terms of three iron environments. Magnetic ordering begins at 77 K and is virtually complete by 4.2 K to give an average magnetic hyperfine field of 504 kG. The ¹¹⁹Sn spectra also reflect the magnetic ordering and a magnetic hyperfine field of 45 kG is transferred to the tin nuclei.     

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.     

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.     

High temperature ferromagnetism of the vacuum-annealed (In1−xFex)2O3 powders

(In_1−xFe_x)₂O₃ (x = 0.02, 0.05, 0.2) powders were prepared by a solid state reaction method and a vacuum annealing process. A systematic study was done on the structural and magnetic properties of (In_1−xFe_x)₂O₃ powders as a function of Fe concentration and annealing temperature. The X-ray diffraction and high-resolution transmission electron microscopy results confirmed that there were not any Fe or Fe oxide secondary phases in vacuum-annealed (In_1−xFe_x)₂O₃ samples and the Fe element was incorporated into the indium oxide lattice by substituting the position of indium atoms. The X-ray photoelectron spectroscopy revealed that both Fe²⁺ and Fe³⁺ ions existed in the samples. Magnetic measurements indicated that all samples were ferromagnetic with the magnetic moment of 0.49-1.73 μ_B/Fe and the Curie temperature around 783 K. The appearance of ferromagnetism was attributed to the ferromagnetic coupling of Fe²⁺ and Fe³⁺ ions via an electron trapped in a bridging oxygen vacancy.     

Nanocrystalline FeAl intermetallics obtained in mechanically alloyed Fe50Al40Ni10 powder

B2-Fe₄₇Al₅₃ intermetallics has been produced by mechanical alloying in a planetary ball mill, using elemental Fe, Al and Ni powder mixture. The microstructural and magnetic properties of the mechanically alloyed Fe₅₀Al₄₀Ni₁₀ powdered samples were investigated by X-ray diffraction and ⁵⁷Fe Mössbauer spectrometry at 300 and 77 K. As resulted from the X-ray diffraction studies, the ordered B2 structure was formed in the Fe₅₀Al₄₀Ni₁₀ powder, together with the bcc α_i-Fe(Al, Ni) (i = 1, 2) solid solutions. Further milling led to a partial disordering of B2-Fe₄₇Al₅₃; it has undergone an order–disorder transition which is characterized by an expansion of the volume Δa₀ (lattice disorder) and a magnetic transition from the paramagnetic to ferromagnetic state which is characterized by strong ferromagnetic interactions in the alloy. The nanocrystalline bcc α_i-Fe(Al, Ni) solid solution was ferromagnetic with a mean crystallite size of 6 nm.     

Evidence for low-temperature internal dynamics in Cu12As4S13 according to copper NQR and nuclear relaxation

^63,65Cu nuclear quadrupole resonance (NQR) was applied to study the natural mineral Cu₁₂As₄S₁₃ (tennantite) in the temperature range 4.2–210 K. The obtained results point to the presence of field fluctuations caused by internal motions in tennantite. Consistently with the crystal structure, the experimental data can be described by an occurrence of a magnetic phase transition, which takes place near 65 K. The low-temperature phase is characterized by Cu(II) electron magnetic moments freezing in the form of a spin-glass-like constitution.     

Magnetic anisotropy in (, .44, .55, .67, and .78) alloy film grown on and MgO(001) by molecular beam epitaxy

Fe_xPd_1−x (x=.30, .44, .55, .67, and .78) films were directly grown on SrTiO₃(001) and MgO(001) by molecular beam epitaxy at 500 C. The thickness of all films is 50 nm. X-ray diffraction shows epitaxial quality and face-center- tetragonal (00l), (002), and (003) peaks indicating an FePd L1₀ order state for films of x=.30, .44 and .55. X-ray diffraction only shows (002) peaks with a relatively weak intensity for the film of x=.67 and no (00l) peak is observed, but a broad body-center-cubic Fe(002) is identified for the film of x=.78. Magnetic hysteresis curves are carried out by a vibrating sample magnetometer (VSM) with an applied field within 12 kOe. Magnetization of both in-plane and perpendicular-to- the-plane measurements show a linear increase of the magnetization saturation from 560 emu/cm³ to 1250 emu/cm³ as x increasing from .30 to .78. For the results of the in-plane measurements, remanence (M_r), however, shows a minimum while the anisotropy field (H_k) shows a maximum for the film with x=.44 indicating the optimal content ratio of Fe/Pd for perpendicular anisotropy in the present alloy films. Further, negative remanence is observed in the hysteresis curves where the field is perpendicular to the film of x=.78 This may indicate that the L1₀order state still affects the magnetic anisotropy for high Fe content films even though the film has a body-center-cubic structure.     

Iron and molybdenum valences in double-perovskite (Sr,Nd)2FeMoO6: electron-doping effect

Double perovskite, (Sr_1−xNd_x)₂FeMoO₆, was doped with electrons through partial substitution of divalent Sr by trivalent Nd (0≤x≤0.2). The Fe valence and the degree of B-site order were probed by ⁵⁷Fe Mössbauer spectroscopy. Replacing Sr by Nd increased the fraction of Fe and Mo atoms occupying wrong sites, i.e. antisite disorder. It had very little effect on the Fe valence: a small but visible increase in the isomer shift was seen for the mixed-valent Fe^II/III atoms occupying the right site indicating a slight movement towards divalency of these atoms, which was more than counterbalanced by the increase in the fraction of antisite Fe atoms with III valence state. It is therefore argued that the bulk of the electron doping is received by antisite Mo atoms, which - being surrounded by six Mo^V/VI atoms-prefer the lower IV/V valence state. Thus under Nd substitution, the charge-neutrality requirement inflicts a lattice disorder such that low - valent Mo^IV/V can exist.     

Magnetization and heat capacity studies of double perovskite compounds Ba2SmRuO6 and Ba2DyRuO6

Here we report the magnetic and heat capacity studies of the double perovskite compounds Ba₂SmRuO₆ and Ba₂DyRuO₆. Antiferromagnetic transitions are inferred at 54 and 47 K in Ba₂SmRuO₆ and Ba₂DyRuO₆, respectively, in the magnetization measurements. Heat capacity measurements show large jumps at the corresponding temperatures and confirm the bulk magnetic ordering. Both the measurements provide clear indication of the ordering of the rare earth moments also along with the Ruthenium moments. However, the heat capacity results suggest that the ordering of rare earth magnetic moments is spread over a large temperature range. An anomaly observed in the magnetization measurements at 42 K (below the magnetic ordering) in Ba₂SmRuO₆ is discerned as a reorientation of Sm³⁺ moments.     

Action d'un champ magnetique sur le taux de polarisation de la raie interdite 2655,6 Å (61S0-63P0) du mercure

The forbidden radiation 2655.6 Å (6¹S₀-6³P₀) emitted by the odd isotopes of mercury is polarized in a magnetic field, even if the atoms are isotropically excited. The degree of polarization has been calculated for the case of ¹⁹⁹Hg. The experimental results agree with the theoretical predictions.     

Magnetic phases in lanthanum-strontium manganite-cobaltite La1.25Sr0.75MnCoO6

Two methods—the solid-phase high-temperature (1300 °C) and the liquid-phase low-temperature (750 °C) routes—were used to synthesize the complex oxide La_1.25Sr_0.75MnCoO₆, which has the structure of rhombohedral perovskite and is characterized by a disordered distribution of Mn and Co in structural sites. It was found by means of X-ray absorption near edge spectroscopy (XANES) at the K-edge that mixed valence states of Co²⁺/Co³⁺ and Mn³⁺/Mn⁴⁺, exist in both phases. Measurements of dc magnetization and real (χ′) and imaginary (χ″) parts of the ac susceptibility showed that the magnetic properties of these oxides are determined by a ferromagnetic transition at T_C=217 K and a frequency-dependent transition at T_g<100 K. The high frequency dependence of T_g is indicative of the cluster-glass behavior of La_1.25Sr_0.75MnCoO₆ (7 5 0) at T<T_C within the ferromagnetic state.     

Analysis of FTIR spectra of CH2BrCl; the ν4 and ν5 fundamentals and their hot-bands ν4+ν6−ν6 and ν5+ν6−ν6

The infrared spectrum of isotopically pure CH₂⁷⁹BrCl has been recorded at a resolution of 0.0023 cm⁻¹ (FWHM) in the range 550-800 cm⁻¹ with a Bruker IFS 120 HR Fourier transform spectrometer in Wuppertal. Here we report the full rotational analysis of the ν₄ and ν₅ fundamentals and of the hot-bands ν₄+ν₆−ν₆ and ν₅+ν₆−ν₆. Ground state combination differences were constructed for all bands, yielding improved ground state constants, up to quartic terms, as well as reliable rotational constants for the ν₄, ν₅, and ν₆ states.     

Effect of Fe substitution on magnetocaloric effect in La0.7Sr0.3Mn1−xFexO3 (0.05≤x≤0.20)

We have studied the effect of Fe substitution on magnetic and magnetocaloric properties in La_0.7Sr_0.3Mn_1−xFe_xO₃ (x=0.05, 0.07, 0.10, 0.15, and 0.20) over a wide temperature range (T=10-400 K). It is shown that substitution by Fe gradually decreases the ferromagnetic Curie temperature (T_C) and saturation magnetization up to x=0.15 but a dramatic change occurs for x=0.2. The x=0.2 sample can be considered as a phase separated compound in which both short-range ordered ferromagnetic and antiferromagnetic phases coexist. The magnetic entropy change (−ΔS_m) was estimated from isothermal magnetization curves and it decreases with increase of Fe content from 4.4 J kg⁻¹ K⁻¹ at 343 K (x=0.05) to 1.3 J kg⁻¹ K⁻¹ at 105 K (x=0.2), under ΔH=5 T. The La_0.7Sr_0.3Mn_0.93Fe_0.07O₃ sample shows negligible hysteresis loss, operating temperature range over 60 K around room temperature with refrigerant capacity of 225 J kg⁻¹, and magnetic entropy of 4 J kg⁻¹ K⁻¹ which will be an interesting compound for application in room temperature refrigeration.     

Vacancy-induced spin-glass behavior of Prussian blue analogue Fe1.1Crx[Cr (CN)6]0.6−x·nH2O nanowires

Prussian blue analogue Fe^II_1.1Cr^II_x[Cr^III(CN)₆]_0.6−x·nH₂O nanowires were synthesized by electrodeposition. The magnetic properties investigation indicates that the nanowires exhibit cluster spin-glass behavior, which undergoes a magnetic transition to a frozen state below about 62 K. Spin disorder arising from reduced coordination and broken exchange bonds between spin centers due to the structural defects may be the reason that causes the spin-glass freezing behavior. The negative magnetization observed at temperature lower than the compensation temperature (T_comp∼43 K) at a field of 10 Oe may be due to the different temperature dependences of the ferromagnetic site Fe-Cr and antiferromagnetic site Cr-Cr.     

Antiferromagnetic phase transition in garnet-type AgCa2Co2V3O12 and AgCa2Ni2V3O12

Antiferromagnetic phase transition in two vanadium garnets AgCa₂Co₂V₃O₁₂ and AgCa₂Ni₂V₃O₁₂ has been found and investigated extensively. The heat capacity exhibits sharp peak due to the antiferromagnetic order with the Néel temperature T_N=6.39 K for AgCa₂Co₂V₃O₁₂ and 7.21 K for AgCa₂Ni₂V₃O₁₂, respectively. The magnetic susceptibilities exhibit broad maximum, and these T_N correspond to the inflection points of the magnetic susceptibility χ a little lower than T(χ_max). The magnetic entropy changes from zero to 20 K per mol Co²⁺ and Ni²⁺ ions are 5.31 J K⁻¹ mol-Co²⁺-ion⁻¹ and 6.85 J K⁻¹ mol-Ni²⁺-ion⁻¹, indicating S=1/2 for Co²⁺ ion and S=1 for Ni²⁺ ion. The magnetic susceptibility of AgCa₂Ni₂V₃O₁₂ shows the Curie-Weiss behavior between 20 and 350 K with the effective magnetic moment μ_eff=3.23 μ_B Ni²⁺-ion⁻¹ and the Weiss constant θ=−16.4 K (antiferromagnetic sign). Nevertheless, the simple Curie-Weiss law cannot be applicable for AgCa₂Co₂V₃O₁₂. The complex temperature dependence of magnetic susceptibility has been interpreted within the framework of Tanabe-Sugano energy diagram, which is analyzed on the basis of crystalline electric field. The ground state is the spin doublet state ²E(t₂⁶e) and the first excited state is spin quartet state ⁴T₁(t₂⁵e²) which locates extremely close to the ground state. The low spin state S=1/2 for Co²⁺ ion is verified experimentally at least below 20 K which is in agreement with the result of the heat capacity.