Investigation of 4f-magnetism in CeNi2P2, EuNi2P2 and YbNi2P2 by susceptibility and NMR studies

The results of ³¹P NMR Knight shift (KS) and spin-lattice relaxation time (T₁) as well as magnetic susceptibility (_ξ) measurements in the temperature interval 4.2–300K are reported for the compounds RENi₂P₂(RE = Ce, EuandYb) in order to understand the nature of the 41 magnetism in these compounds. The KS results conclusively establish that all these compounds exhibit non-magnetic ground states. There is a low-temperature upturn in _ξ for CeNi₂P₂ and YbNi₂P₂ presumably due to magnetic impurities. The estimated spin fluctuation temperatures (T_sf) for EuNi₂P₂ above 50K agree qualitatively with those obtained earlier from Mössbauer and _ξ data employing ionic interconfigurational fluctuation model, but seem to disagree at low temperatures. Implications of this result are discussed.     

Temperature dependence of the contribution to the transport coefficients of nearly ferromagnetic metals from electron-paramagnon scattering

We present calculations of the temperature dependence of the contribution from electron-paramagnon scattering to the electrical and thermal resistivity of a simple model of a nearly ferromagnetic metal. The purpose of the work is to explore the behavior of these quantities when the temperature is the order of or greater than the spin fluctuation temperature T_sf. As the temperature T is raised from zero through T_sf, the electrical resistivity varies more slowly with temperature than the T² law characteristic of the regime T?T_sf. When T?T_sf, the electrical resistivity becomes proportional to T, although this asymptotic behavior is approached very slowly. The Lorenz number rises monotonically with temperature, and appears to approach the ideal Sommerfeld value when T?T_sf, although this limit is also approached slowly.     

Change in the size dependences of the surface properties of nanodiamond upon variation in the isotope composition

The isothermal-isomorphic dependences of the specific surface energy (σ), its isochoric temperature derivative (σ′), and surface pressure (P _sf) on the size of nanodiamonds ¹²C and ¹³C at 100 and 4300 K are calculated. It is shown that the ratio σ(¹²C)/σ(¹³C) decreases with decreasing size along both isotherms, and σ′(¹²C)/σ′(¹³C) increases with decreasing size, and this effect is higher at lower temperatures. The ratio P _sf(¹²C)/P _sf(¹³C) increases with decreasing size. The effect is stronger at higher temperatures.     

Possible manifestation of spin fluctuations in the temperature behavior of the resistivity of Sm<Subscript>1.85</Subscript>Ce<Subscript>0.15</Subscript>CuO<Subscript>4</Subscript> thin films

A pronounced step-like (kink) behavior in the temperature dependence of resistivity ρ(T) is observed in the optimally doped Sm_1.85Ce_0.15CuO₄ thin films around T _sf = 87 K and attributed to the manifestation of strong-spin fluctuations induced by Sm³⁺ moments with the energy ħω_sf = k _B T _sf ≃ 7 meV. The experimental data are found to be well fitted by the residual (zero-temperature) ρ_res, electron-phonon ρ_e-ph(T) = AT, and electron-electron ρ_e-e(T) = BT ² contributions in addition to the fluctuation-induced contribution ρ_sf(T) due to thermal broadening effects (of the width ω_sf). According to the best fit, the plasmon frequency, impurity scattering rate, electron-phonon coupling constant, and Fermi energy are estimated as ω_p = 2.1 meV, τ ₀ ⁻¹ = 9.5 × 10⁻¹⁴ s⁻¹, λ = 1.2, and E _F = 0.2 eV, respectively. The text was submitted by the authors in English.     

Specific heat and magnetic susceptibility of single-crystalline ZnCr2−xAlxSe4 (x=0.15, 0.23)

A correlation between the second critical field H_c2 of the helix to paramagnetic transition and the magnetic specific heat C-peak was found in ZnCr_2−xAl_xSe₄ spinel single crystals with x=0.15, 0.23. The specific heat peak is anomalously sharp for all finite magnetic fields used here and this points to a first order magneto-structural transition (from cubic to tetragonal symmetry). The C(T)-peak is increasingly suppressed as the external field increases. Approaching the Neel temperature T_N, a broad ac-magnetic susceptibility peak is observed for zero dc-magnetic field. That peak does not show an energy loss and thus points towards a return to a second order type of transition. The magnetic contribution to the specific heat displays a sharp peak at T_N and is maximal at the spin fluctuation temperature T_sf=34 K. T_sf is related to the maximum of the magnetic susceptibility at T_m=40 K (at 50 kOe) in the spin fluctuation region, as evidenced by the entropy exceeding 90% of the entropy calculated classically for the complete alignment of the Cr spins, (2−x)R ln(2S+1). The X-ray photoelectron spectroscopy (XPS) data indicate that Al-substitution does not affect Cr³⁺ 3d³ electronic configuration.     

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.     

An ESR study of phase transitions in crystals of M(BF4)2·6H2O (M = Fe,Co, Ni,Zn)

From a temperature dependent ESR study of Mn²⁺-doped crystals of M(BF₄)₂·6H₂O, M Zn, Co and Ni, new structural phase transitions have been detected and studied. First order structural phase transitions occur in Co(BF₄)₂·6H₂O at T₁ ~ 281K, T₂~189 K and T₃~172K (during cooling), in Zn(BF₄)₂·6H₂O at T₁ ~ 286 K and in Ni(BF₄)₂·6H₂O at T₁ ~ 301 K. A continuous phase transition occurs in Co(BF₄)₂·6H₂O at T_p ~ 257 K, in Zn(BF₄)₂·6H₂O at T_p ~ 277 K and in Ni(BF₄)₂·6H₂O at T_p ~ 294 K. The ESR spectral characteristics suggest similarities in the structures of these fluoroborate compounds in the phase above T₁ with the room temperature structure of Mg(ClO₄)₂·6H₂O. All these compounds are found to have a tendency to crystallise in a triply-twinned pseudo-hexagonal form, although the unit cell above T₁ is found to be orthorhombic. The structural changes related to the water octahedron around the metal at T₁ were found to be very small and basically the same for these three compounds. Although the unit cell structure of Fe(BF₄)₂·6H₂O above the first order phase transition temperature T₁ was found to be similar to that of the other fluoroborate compounds, the structural changes occurring at T₁ appeared to be quite different. The low temperature thermal behaviour differs considerably in the Co, Fe and Zn compounds.     

A Mössbauer study of the bipyramidal lattice site (2b) in BaFe12O19

The hyperfine fieldsH _f(T), center shifts δ(T) and relative recoilless fractionsf _a(T) at different lattice sites of barium ferrite were determined by Mössbauer spectroscopy of⁵⁷Fe over a temperature range from 12 to 723 K. For ferric iron in the crystalline site (2b), the recoilless fractionf _a(T) decreases most with increasing temperature in comparison with that of other sites, corresponding to a characteristic temperature {ie219-1}. These results indicate that the ferric iron in the site (2b) oscillates between two equivalent positions (4e) on either side of the symmetry plane normal to thec-axis.     

Sb Magnetic Resonance as a Local Probe for the Gap Formation in the Correlated Semimetal FeSb2

We report on a comparative study of the narrow-band semimetals FeSb₂ and its structural homologue RuSb₂ by means of ^121,123Sb nuclear quadrupole (NQR) and nuclear magnetic resonance (NMR) spectroscopy. From NQR for both compounds two temperature regimes could be identified by use of ¹²³(1/T ₁) measurements. Above 40 K a conventional activated behavior (with Δ/k _B ? 400 K for FeSb₂) dominates in ¹²³(1/T ₁), whereas below 40 K in both systems an unconventional ¹²³(1/T ₁) behavior with a smooth maximum at around 10 K is observed. To analyze this behavior, we propose the presence of T-dependent in-gap states forming a narrow energy level of localized spins with S = ½ near the bottom of the conduction band. These states might have originated from an inherent Sb-deficiency in both compounds. This model enables us to fit the ¹²³(1/T ₁) data in the entire investigated temperature range (2–200 K) for FeSb₂. Ab initio band structure calculations reveal more than a factor of two larger Δ value for RuSb₂ as compared with FeSb₂. This results in dissimilar behavior of ¹²³(1/T ₁) in FeSb₂ and RuSb₂ above 40 K evidencing the inefficiency of thermal activation of electrons over the large energy gap at T ≤ 300 K in RuSb₂ and dominating of quadrupole relaxation channel in RuSb₂ in this temperature range caused by phonon relaxation involving two-phonon (Raman) scattering. In addition, extra wide range field-sweep NMR measurements are performed at various temperatures on FeSb₂ and RuSb₂. The complex broad spectra could be modeled and from the shift of the ¹²¹Sb central transition the 3d component of the shift K _3d (T) could be extracted.     

Der Zerfall des125m Xe,des127m Xe und des127g Xe

The decay of^125m Xe produced by the reaction¹²²Te(α, n)^125m Xe using a target enriched in¹²²Te (95.4%) and the decay of^127m Xe produced by the reaction¹²⁷J(d, 2n)^127m Xe have been investigated: ^125m Xe decays with a half-life ofT _1/2=(56±3) sec by ayy- cascade withE _γ1=(140.4 ±0.5) keV andE _γ2=(110.5 ±0.5) keV. The experimental conversion coefficients yield multipolarities ofE3 for the 140.4 keV isomeric transition and predominantlyM1 for the 110.5 keV-transition. ^127m Xe decays with a half-life ofT _1/2=(71±2) sec. The decay also proceeds by aγγ-cascade with an isomeric E3 transition ofE _γ1=(172.5±0.3) keV and a predominantlyM1 transition ofE _γ2=(124.6±0.3) keV. In the decay of^127g Xe an additional branching of the electron capture to a level at (618.1±0.3) keV was observed. The relative probability forK-captureP _K618/P_K375=0.40 ±0.07 yields a total transition energyQ _EC=(664 ±4)keV. A spin of 1/2⁺ was assigned to the ground state.     

Magnetic behaviour of U(MnxAl1−x)2 compounds

The results of magnetic measurements performed on U(Mn_xAl_1−x)₂ compounds in the temperature range 4.2K < T < 800K are reported. In the low temperature range (T < 200K), UMn₂ shows a Pauli-type paramagnetism. Above 420K a Curie-Weiss behaviour is evidenced. The magnetic properties of U(Mn_xAl_1−x)₂ compounds were analysed assuming a superposition of a temperature dependent term on a Pauli-type contribution, χ_O. The effective moments as well as the χ_O values were determined both in the low (T < 200K) and high (T > 420K) temperature range. The experimental data were discussed considering changes in the band structure and/or quenching of spin fluctuations.     

Electron spin-lattice relaxation of Yb3+ and Gd3+ ions in glasses

The electron spin-lattice relaxation rate (T ₁ ^?1) was measured in two glass samples: (i) a phosphate glass doped with 1 wt% Yb₂O₃ and (ii) a Li₂Si₄O₉ glass sample doped with 0.2 wt% Gd₂O₃. In the Yb³⁺-doped glass sample,T ₁ was measured by an electron-spin-echo technique from 4.2 to 6 K, by the modulation method from 10 to 26 K and by the EPR linewidth from 30 to 100 K. It was found thatT ₁ ^?1 αT ⁿ withn=9 in the range 4.2–6 K.n decreased gradually as the temperature was increased and tended towards 2 above 40 K. Over the entire temperature range 4.2–100 K,T ₁ ^?1 was fitted toAT+BT ⁹ J ₈ (Θ _D/T) (whereA andB are two temperature-independent constants,J ₈ is the well-known Van Vleck integral andΘ _D is the Debye temperature). The value ofΘ _D (=46.3±0.9 K) so determined is in good agreement with that of Stevens and Stapleton from theirT ₁ measurements in the range 1.5 to 7 K. In the Gd³⁺-doped glass, it was observed thatT ₁ ^?1 αT over the range 50–150 K. The data for Ye³⁺-doped glass sample were accounted for by assuming that the phonon modulation of the ligand field is the dominant mechanism, associated with a low Debye temperature in accordance with the published data obtained by using other techniques to study lattice dynamics. On the other hand, the data on the Gd³⁺-doped glass sample were explained to be predominantly due to a mechanism involving Two-Level-Systems (TLS)     

Relaxation phenomena and nuclear magnetic resonance of116In(T 1/2=14s) produced by capture of polarized neutrons in the indium III–V compounds

Polarized¹¹⁶In nuclei have been produced by capture of polarized thermal neutrons in several In compounds. At temperaturesT below 77 °K and magnetic field strengthsH ₀ of several kOe, asymmetries of a few percent of the β^? decay of the¹¹⁶In ground state could be observed in polycrystalline InP, InAs and InSb, thus indicating the nuclear polarization. Nuclear magnetic resonance signals have been measured with the result for the magnetic moment μ _i (¹¹⁶In)=2.7723 (10) nm (uncorrected). β^? decay asymmetry and spin lattice relaxation timeT ₁ have been studied as a function ofH ₀ andT. The effect ofH ₀ is to decouple the hyperfine interaction caused by the capture-γ recoil process. However,H ₀ has no influence uponT ₁, which demonstrates the absence of nuclear relaxation due to paramagnetic impurities.T ₁ is determined by quadrupolar relaxation. A quadrupole momentQ(¹¹⁶In)=0.09 (2) b was calculated by comparison of the¹¹⁶In relaxation rates with those of the stable¹¹⁵In isotope in the same compounds. Above 30 °K the temperature dependence of 1/T ₁ agrees with a recent theoretical investigation. Below 30 °K the relaxation rate shows an anomalous behaviour, which can be explained by resonance modes due to recoil lattice defects.     

On the surface properties and baric fragmentation of iron

The dependence of the specific surface energy (??) on the normalized volume (V/V ₀) and temperature (T) for the body-centered cubic (BCC) lattice of iron has been studied on the basis of the Mie-Lennard-Jones potential of interatomic interaction. It is shown that below the definite value of the normalized volume (V/V ₀)_fr the specific surface energy of the BCC-Fe lattice passes to the negative range of values: ??(V/V ₀)_fr = 0, and the (V/V ₀)_fr value increases almost linearly with temperature. In the case of compression, when (V/V ₀) < (V/V ₀)_fr, the exothermal process of crystal fragmentation into dendritic domains with the maximum possible specific intercrystalline surface takes place. In the case of nanofragmentation, surface pressure (P _sf) emerges, leading to self-packing of the formed nanocrystals. The dependence of the ??(V/V ₀) and P _sf (V/V ₀) functions on the size and shape of the BCC-Fe nanocrystals has been studied at different temperature values T = 1500?C3500 K. It has been shown that the P _sf function increases with a decrease in the size; this occurs more strongly, the more the nanocrystal deviates from the most thermodynamically stable cubic shape. The dimensional compression of the lattice parameter of BCC-Fe nanocrystals has been studied. The specific (per volume unit) amount of heat released during fragmentation of the BCC-Fe lattice at high pressures and temperatures has been estimated.     

Optical pumping of short lived Β-radioactive isotopes and the magnetic moment of37K

²¹Na (T _1/2=23 sec) and³⁷K(T _1/2=1.25 sec) have been produced in gas targets by (d, n) reactions and polarized by means of optical pumping or spin exchange scattering with optically pumped⁸⁷Rb. An asymmetry up to 3% in theΒ-decay of the polarized nuclei was found, which served to detect rf transitions between hfs levels of the atomic ground states of²¹Na and³⁷K.δF=0 andδF=1 resonances have been recorded, yielding spin, hfs separation and magnetic moment, especiallyI(³⁷K)=3/2,δW(³⁷K)=240.266 (3) Mc/sec andΜ _I(³⁷K)=0.02033(6) nm (diamagnetically corrected). For²¹Na earlier rf-spectroscopic data have been confirmed.     

Electron capture intensities in the decay of133Ba

The ground state decay of¹³³Ba was investigated using NaI(Tl) scintillation crystals.P _L/P_K electron capture ratios were measured for the transitions to the 437 keV and to the 384 keV level of¹³³Cs. The results are: (P _L/P_K)₄₃₇=0.371±0.007 and (PP _L/P_K)₃₈₄=0.221±0.005. From these data a mass difference between the ground states of the¹³³Ba- and the¹³³Cs-atom of (515±3) keV was derived. Besides, these capture ratios yield an exchange correction of 1.03±0.03. Furthermore, the intensity of the transition to the 161 keV level was determined to be smaller than 0.5% of all decays.     

Thermodynamic Study of Formamidinium Lead Iodide (CH5N2PbI3) from 5 to 357 K

In the present study, the molar heat capacity of solid formamidinium lead iodide (CH₅N₂PbI₃) was measured over the temperature range from 5 to 357 K using a precise automated adiabatic calorimeter. In the above temperature interval, three distinct phase transitions were found in ranges from 49 to 56 K, from 110 to 178 K, and from 264 to 277 K. The standard thermodynamic functions of the studied perovskite, namely the heat capacity C°_p(T), enthalpy [H⁰(T) − H⁰(0)], entropy S⁰(T), and [G°(T) − H°(0)]/T, were calculated for the temperature range from 0 to 345 K based on the experimental data. Herein, the results are discussed and compared with those available in the literature as measured by nonclassical methods.     

Crystal electric field excitations in ferromagnetic CeTX compounds

A ferromagnetic ground state was identified for the compounds CeCuGe (T_C=10 K), CeCuSi (T_C=15 K) [F. Yang, et al., J. Appl. Phys. 69 (1991) 4705] and CeAuGe (T_C=10 K) [R. Pottgen, J. Magn. Magn. Mater. 152 (1996) 196]. The observed saturation magnetic moment values at low temperatures for all three compounds are considerably less than the theoretically expected value g_JJ=2.14μ_B for the free Ce³⁺ ion involving the entire six-fold J=5/2 multiplet, and thus provide a first indication of partial lifting of the f-electron level degeneracy in these compounds. Specific heat data yield crystal electric field (CEF) excitation energies (Δ_Sch) equivalent to 140 K for CeCuGe, 110 K for CeCuSi and 280 K for CeAuGe. To confirm the presence of CEF excitations directly, we have carried out inelastic neutron scattering (INS) measurements on all three compounds, using the HET spectrometer at ISIS Facility. Here, we present a detailed analysis of the INS spectra of CeCuSi on the basis of a CEF model and the detailed analysis of the INS of the other two compounds will be reported elsewhere.     

Optical and time-resolved electron paramagnetic resonance studies of the excited states of para-methylcinnamic acid and para-methylcinnamate anion

Time-resolved electron paramagnetic resonance (EPR), fluorescence, and phosphorescence spectra have been observed for the excited states oftrans-p-methylcinnamic acid (p-MeCA) andtrans-p-methylcinnamate anion (p-MeCA^?) in rigid organic glasses at 77 K. With a stretched-polymer-film technique, we assigned the resonance fields in the time-resolved EPR spectra of the lowest excited triplet (T₁) states ofp-MeCA andp-MeCA^?. From the analysis of these spectra we concluded that the deviation from planar structure in the T₁ state is small inp-MeCA andp-MeCA^? and the direction of C=O iss-cis to the ethylenic C=C bond inp-MeCA. The deprotonation appears to have little effect on the zero-field splitting (ZFS) parameters. The ZFS parameters were calculated theoretically using semiempirical molecular orbitals. The sublevel preferentially populated by intersystem crossing (ISC) is T _y in bothp-MeCA andp-MeCA^?. However, following the deprotonation ofp-MeCA, (P _y?P _z)/(P _x?P _z) changes from 5.7 inp-MeCA to 2.1 inp-MeCA^? (P _i are relative populating rates; thex andy axes are close to the long and short in-plane molecular axes, respectively, andz-axis is perpendicular to the molecular plane). The fluorescence lifetimes indicate that the deprotonation has little effect on the sum of the three ISC rate constants for the three T₁ sublevels. A decrease in acidity ofp-MeCA upon excitation has been observed.     

151Eu Mössbauer effect study of T′- and T*-phase superconductors

The (La,Eu(₂ CuO ₄ system shows superconductivity by dopingCe orSr. Carriers inT′-phase doped withCe are electrons and those inT ^*-phase doped withSr are holes. In this work,¹⁵¹ Eu Mössbauer analysis is applied for theT′-phase(La _1?x Eu _x)_2?y Ce _y CuO ₄ and theT ^*-phase(La _1?x Eu _x)_2?y Sr _y CuO ₄ in order to compare the electronic state and the lattice vibration ofEu in these superconductors. In addition, correlations betweenT _c and Mössbauer parameters are examined. The isomer shift of¹⁵¹ Eu is 0.784–0.840 mm/s in theT ^*-phase and 0.689–0.733 mm/s in theT′-phase, which shows that the lanthanide in these superconductors is tri-valent. The Debye temperature of¹⁵¹ Eu is 180–208 K in theT ^*-phase and 160–192 K in theT′-phase. The difference of isomer shift between these two phases is explained by theEu?O distance. For(La _1?x Eu _x)_2?y Ce _y CuO ₄, a light correlation betweenT _c and the Debye temperature is observed, which means the importance of the lattice vibration in high-Tc superconductivity.