Sound propagation in the two dimensional magnetic system K2NiF4

We have measured the classic constants C₁₁, C₃₃, C₅₅ and C₆₆ in K₂NiF₄ as a function of temperature between 4.2 and 200 K. A small velocity and attenuation anomaly was observed in the C₁₁ mode at T_N = 97.2 K.     

Study of alkyl chain length dependent characteristics of imidazolium based ionic liquids [CnMIM]+[TFSA]− by Brillouin and dielectric loss spectroscopy

This study examined the acoustic phonon mode of ionic liquids consisting of 1-alkyl-3-methyl-imidazolium family (C_nMIM) cations with n values ranging from 2 to 10 and bis(trifluoromethylsulfonyl)amide (TFSA) anion in the temperature range from 300 K to 100 K. [C_nMIM]⁺[TFSA]^? showed depolarized (VH) components of Brillouin peaks at temperatures below the glass transition temperature when n is larger than 4. On the other hand, in the case of ionic liquids with different anions, such as [C₄MIM]⁺[BF₄]^?, [C₄MIM]⁺[PF₆]^? and [C₈MIM]⁺[BF₄]^?, the VH component of Brillouin peaks was not observed in the temperature range investigated. The dielectric loss spectra showed that the temperature dependence of alkyl chain domain relaxation of all ionic liquids followed the Arrhenius law and showed an increase in activation energy at the temperature where the VH component of Brillouin peak appeared. These results suggest that the observed depolarized component of Brillouin peak might originate from uniquely induced polarization in the 2nd domain composed of head groups of cations and anions.     

Magnetic and 57Fe Mössbauer studies of collinear spin rotation in Ho2Fe14B

Magnetic properties of Ho₂Fe₁₄B compounds have been studied by the ⁵⁷Fe Mössbauer effect and magnetization measurements. The axes of easy and hard magnetizations lie along the [001] and the [100] directions in the tetragonal structure, respectively, above T_sc = 58 K. From the comparison of the Mössbauer results with the magnetization measurements, it became clear that the Fe and the Ho moments tilt collinearly from the c-axis to the [110] direction throughout the temperature range of 4.2–58 K, and the canting angle reaches to 22° at 4.2 K. The Mössbauer spectra are consistently resolved with six subspectra above T_sc and with twelve below T_sc, together with reasonable site-assignments. We have estimated the mean Ho moment at 10.0μ_B, using the mean Fe moment of 2.3μ_B derived from the average hyperfine field or using the magnetization of Y₂Fe₁₄B as the Fe-sublattice magnetization of Ho₂Fe₁₄B.     

Stiffness matrix and debye temperature of ReO3 from ultrasonic measurements

We have measured the propagation velocities of bulk acoustic waves in the simple cubic transition-metal oxide ReO₃ by ultrasonic pulse propagation. The elastic stiffness constants at 300 K are: C₁₁ = (47.9 ± 1.4) × 10¹¹ dyne/cm²; C₄₄ = (6.1 ± 0.2) × 10¹¹ dyne/cm²; C₁₂ = (?0.7 ± 2.8) × 10¹¹ dyne/cm². These elastic constants indicate a crystal with highly anisotropic shear propagation. The Debye temperature of the compound from these measurements is 528 K. This value is somewhat higher than previous results from specific heat and resistivity determinations.     

Crystal structure of martensite and intermediate phases in Ni<Subscript>2</Subscript>MnGa studied by neutron diffraction

We have investigated crystal structures of martensite and intermediate phases in stoichiometric Ni₂MnGa. The neutron diffraction profile of the martensite phase measured at T = 4.2 K exhibits four satellites between [2 0 0]_P^* and [0 2 0]_P^* reflections (P stands for the parent phase) at incommensurate positions of [h 2-h 0]_P^* with h = 0.428, 0.863, 1.136 and 1.572. The profile of the intermediate phase measured at T = 210K exhibits two satellites between [2 0 0]_P^* and [0 2 0]_P^* reflections at incommensurate positions of [h 2-h 0]_P^* with h = 0.343 and 1.657. Although each satellite of the martensite phase moves toward its nearest fundamental reflection as temperature increases, that of the intermediate phase does not move significantly. On the contrary, the intensity of each satellite decreases significantly in the intermediate phase as temperature increases while not in the martensite phase. A synchrotron X-ray diffraction and a Rietveld analysis of the result reveal that, for both the phases, the displacement of atoms from the parent phase are represented by a sine wave whose propagation vector is parallel to [1 1 0]_P.     

Magneto-acoustic attenuation in AuSb2

Quantum oscillations in the ultrasonic attenuation in AuSb₂ were studied as a function of temperature, magnetic field and crystal orientation. The effective masses of the carriers associated with the F₅ and F₆ oscillations were measured in a (110) plane. For the F₅ oscillations, the Dingle temperature and apparent magnetic breakdown field appear to depend strongly upon orientation. For the F₆ oscillations, however, there were no signs of magnetic breakdown up to the highest magnetic fields available (70 kOe) and the Dingle temperature was roughly independent of orientation. From the acoustic velocities, the elastic constants were determined at 77 K: C₁₁ = (14·7 ± 0·9) × 10¹¹ dyne/cm², C₁₂ = (6·0 ± 0·9) × 10¹¹ dyne/cm², and C₄₄ = (2·59 ± 0·06) × 10¹¹ dyne/cm². These elastic constants give an adiabatic compressibility K_s = (1·13 ± 0·12) × 10^?12 cm²/dyne and a Debye temperature ?_D = (203 ± 15) K.     

On the nature of the valence transition in Eu(Pd1−xAux)2Si2

The intermediate valent compound series Eu(Pd_1−xAu_x)₂Si₂ has been studied by Mössbauer effect measurements on ¹⁵¹Eu(T = 4.2−300K) and ¹⁹⁷Au (4.2K) and by X-ray diffraction (10K, 300K). The temperature induced valence transition Eu²⁺ → Eu³⁺ for x < 0.175 is not of first order type, as suggested in a previous phase diagram [1]. The valence change of the Eu-ion is reflected also in the isomer shift of the ¹⁹⁷Au Mössbauer-resonance.     

Attenuation of longitudinal ultrasonic wave near the diffuse phase transition in CdF2

The attenuation of a longitudinal ultrasonic wave propagating in the [111] direction in CdF₂ is studied as function of temperature from 300K to 1030K. An ultrasonic attenuation peak has been observed for the first time near 983K. This peak is used to define the diffuse transition temperature (T_c = 983K) in CdF₂ which is well below its melting temperature of 1372K. The Arrhenius activation energy of anion motion above T_c was obtained from the temperature dependence of the attenuation and the theory of the dynamics of the coupled crystalline-cage-charged-liquid fluctuations. The elastic constant, $\text{C}{}_{11}\text{+2}\text{C}{}_{12}\text{+4}\text{C}{}_{44}\text{)}\text{3}$, measured simulataneously with the ultrasonic attenuation displays a large decrease near 983K in addition to the nearly linear decrease in the elastic constant with temperature.     

Low-temperature magnetic properties of the 2D molecular ferrimagnet N(n-C5H11)4 [FeIIFeIII(C2O4)3]

⁵⁷Fe Mössbauer and magnetometric studies of the molecular ferrimagnet N(n-C₅H₁₁)₄ [ Fe^IIFe^III(C₂O₄)₃] are indicative of a 2D magnetic character with strong uniaxial anisotropy in the basal plane of the crystal. It is established that the change in the sign of the net magnetization of this compound is related to a compensation between Fe^III and Fe^II sublattice magnetizations at T _comp=31.2 K. The form and parameters of the magnetic Hamiltonian describing the temperature dependence of the Fe^III sublattice and the net magnetizations are determined.     

Magnetic characteristics of PrCd single crystal

Magnetic measurements have been made along the [100], [110] and [111]-axes of PrCd single crystal in magnetic fields up to 70 kOe in the temperature range from 4.2 to 300 K. PrCd shows metamagnetism below T_t = 25 K in which two critical fields are observed in the magnetization processes along the [110] and [111]-axes and a critical field along the [100]-axis. The metamagnetic behaviour is interpreted by a noncollinear antiferromagnetic model with the Pr moments of ～ 2μ_B directed to the 〈111〉-axes.     

Faraday rotation in single crystal erbium iron garnet

Measurements of the Faraday rotation of ErIG, Er₃Fe₅O₁₂, have been performed in the 4.2–300 K temperature range in magnetic field up to 20 kOe applied along the [111] direction and at 1.15 μm wavelength. The results are analysed under the assumption that the contribution of the Fe³⁺ ions to the total Faraday rotation is the same as that of YIG, Y₃Fe₅O₁₂. The temperature and field dependences of the contribution of the Er³⁺ ions are deduced. Both magnetic and electric dipole contributions of the Er³⁺ ions are calculated; the electric dipole coefficient C_e is found to present a linear temperature dependence between 30–300 K. The temperature dependence of the Faraday rotation susceptibility differs strongly from that of the magnetic susceptibility.     

Magnetization and Mössbauer studies of 57Fe doped SrCoO3

⁵⁷Fe (1%) doped SrCoO₃ obtained by high-pressure method, has been investigated by magnetization and Mössbauer spectroscopy studies (MS) in the temperature range 4.2 K to 300 K. The ferromagnetic ordering temperature T _C obtained is 272(2) K. Isothermal magnetization curves have been measured at various temperatures, from which the saturation moments (M _sat) have been deduced. The ⁵⁷Fe MS spectra display standard six-line patterns with an isomer shift typical of Fe^3?+? and a very small quadrupole splitting (QS = 0.14(1) mm/s above T _C). The magnetic hyperfine field at 4.2 K is 276(1) kOe. The temperature dependencies of the iron hyperfine field and M _sat (1.83 µ _B at 5 K) are almost identical. This shows that the Fe^3?+? is replacing Co^4?+?, both of the same electronic configuration. They also interact similarly, namely the Fe–Co exchange is almost identical to the Co–Co exchange.     

Iron sulfur clusters in benzoyl–CoA reductase

The enzyme benzoyl-CoA reductase (BCR) has been studied by Mössbauer spectroscopy in fields up to 7 T and at temperature down to 4.2 K. It has been shown that the oxidized BCR contains three diamagnetic [4Fe-4S]²⁺ centers. Treatment of BCR with dithionite or deazaflavin reduces only one of the three centers from [4Fe-4S]²⁺ to [4Fe-4S]¹⁺. The latter exhibits Mössbauer spectra at 4.2 K in applied fields characteristic for Fe^2.5+Fe^2.5+ (S=9/2) and Fe²⁺Fe²⁺ (S=4) pairs, both coupled antiferromagnetically to total spin S=1/2.     

Zero-field elastic constants of uvite

The theories of Kyame [1] and Koga et al. [2] on stress wave propagation in piezoelectric media are critically studied and compared with each other. Solutions of the modified Christoffel's equations predicted by each theory for piezoelectric crystals in class 3m are derived. The velocities of ultrasonic waves propagated through a gem quality uvite specimen in particular directions were measured at 293 K using the pulse-echo overlap method. All of the independent zero-field elastic moduli were calculated from the measured wave velocities and compared with the available data in the literature. The values of the elastic constants in 10¹²$\text{dyn}\text{cm}{}^2$ were found to be C₁₁ = 3.016 ± 0.2% C₆₆ = 1.028 ± 0.3% C₃₃ = 1.698 ± 0.2% C₁₃ = 0.47 ± 3% C₄₄ = 0.655 ± 0.3% c₁₄ = ?0.089 ± 6%. The calculated bulk modulus, shear modulus and Poisson's ratio, using the Voigt-Reuss-Hill (VRH) approximation, are K_H = 1.226 × 10¹²$\text{dyn}\text{cm}{}^2$, G_H = 0.826 × 10¹²$\text{dyn}\text{cm}{}^2$ and σ_H = 0.224. The Debye temperature is 764 K.     

Mössbauer study of the intermetallic compound Nd2Fe14B. II. Temperature dependence and spin reorientation

The ⁵⁷Fe Mössbauer effects of Nd₂Fe₁₄B were measured in a temperature range of 4.2−300 K. Below the spin reorientation transition temperature T_sc = 148 K, the spectra were satisfactorily analyzed with twelve Zeeman sextuplets due to splitting of six crystallographic Fe-sites into twelve non-equivalent sites. It was shown that the magnetic moments of the Fe and the Nd atoms are non-collinearly coupled in the magnetic structure with canted moments below T_sc. The directions of the moments at 4.2 K are inclined at 27° for Fe and at 58° for Nd from the c-axis to the [110] direction. The average moments are 2.27μ_B for Fe and 3.3μ_B for Nd at 4.2 K. The increase of the average hyperfine field with decreasing temperature is suppressed below T_sc, and its value at 4.2 K is reduced by 1% from the value of 337 kOe which is observed in Y₂Fe₁₄B and also estimated for Nd₂Fe₁₄B by extrapolating the values above T_sc. On the other hand, the Nd moment increases abruptly around T_sc as the temperature decreases. The directions of the principal axes of electric field gradients on the six distinct Fe-sites were also obtained. The anomalous temperature dependence of quadrupole splittings and isomer shifts was observed around T_sc. They were discussed in a framework of the changes in the band structure and the lattice parameters incidental to the spin reorientation transition.     

Magnetic properties of tetragonal uranium compounds I. the U2N2Z-ternaries

In the present work we report the magnetic behaviour of the tetragonal ternaries of the U₂N₂Z-type (Z=Sb, Bi and Te). Magnetic susceptibility measurements performed in a wide temperature range (4.2–1000 K) have shown, that they are ferromagnetically ordered with Curie temperatures 166, 154 and 71 K for U₂N₂Sb, U₂N₂Bi and U₂N₂Te, respectively. For all the investigated compounds the κ^-1_M(T) function in the temperature range above T_C can be expressed as (A/T + B)^-1 - λ.Magnetization measurements were carried out up to magnetic field strengths of 140 kOe in the temperature range from 4.2 to the respective T_C. It follows from these measurements that σ(T)_H and σ(H)_T for U₂N₂Sb and U₂ N₂Bi are typical as for normal ferromagnets. On the other hand, U₂N₂Te exhibits, unexpectedly, two distinct maxima on the σ(T) curves up to fields of 4.5 kOe; one at 45 K and the other one at 71 K. Previous neutron diffraction studies of this compound have shown that the magnetic moments of uranium atoms at 4.2 K are titled by about 20° from the basal plane.The results obtained are interpreted in terms of the crystal-field interaction of the 5f² electrons of the U⁴ ion. In consequence a γ_5t doublet is expected to be the ground-state crystal field level in the U₂N₂Z-type as well as in many other ternary uranium tetragonal compounds. However, in the case U₂N₂Te, the singlet-singlet-doublet “band” as a ground system is postulated.     

Lattice instability induced by 3<Emphasis Type="Italic">d</Emphasis> impurities in a zinc selenide crystal

It was experimentally shown that, in a 3d-impurity-doped ZnSe crystal with the zinc blende structure, nanosized ionic shear displacements of the trigonal (ZnSe : Ni, ZnSe : V) and tetragonal (ZnSe : Cr) types arise at temperatures of 300 and 120 K. As the temperature decreases in the range 100.0–4.2 K, the shear elastic moduli C ₄₄ (ZnSe : Ni) and (C ₁₁–C ₁₂)/2 (ZnSe : Cr) are softened owing to the 3d impurities. A new peak at a frequency of 90 cm^?1 appears in the Raman scattering spectrum of ZnSe : Ni at 5 and 20 K.     

On the mixed valence of the chromium ions in the noncollinear ferrimagnetic spinels Mn1−xCuxCr2S4 (Where x = 0.85, 0.90, 0.95)

For the polycrystalline samples of Mn_1?xCu_xCr₂S₄ (x = 0.85, 0.90, 0.95) the magnetization was measured in the temperature range between 77 K and the Curie temperature, T_C, using a magnetic balance (Faraday's method) and pulsed magnetic fields up to 2.0 T. The magnetic susceptibility was measured between T_C and about 600 K. The Curie temperatures were obtained using the kink point method.In the temperature range between 4.2 and 77 K the magnetization was measured in stationary magnetic fields up to 14 T. The data indicate a noncollinear ferrimagnetic structure. The compounds under investigation can be treated as CuCr₂S₄ slightly doped with Mn, with a valence distribution Mn²⁺_1?xCu¹⁺_xCr³⁺_2?xCr⁴⁺_xS^2?₄.     

Low temperature heat capacity and magnetic excitation of HoAl2 in a magnetic field

The specific heat of single crystalline HoAl₂ in magnetic fields up to 7.5 T has been measured for the temperature range 1.5–16 K. In addition the energy of a magnetic excitation in a magnetic field of 5 T at 4.2 K has been determined by inelastic neutron scattering. The results have been interpreted with a cubic crystalline electric field and an exchange interaction using the same parameter set B₄=-0.85×10^-4 meV, B₆=+0.71× 10^-6 meV and T_C=31.5 K previously obtained by magnetization measurements.