Muon Knight shift in palladium

The Knight shift at positive muons implanted in pure palladium has been measured as a function of temperature from 19.8 to 883 K. The Knight shift variation is strictly proportional to the Pd magnetic susceptibility with ΔK^μ/Δ_x=-(0.43±0.02) mole/emu=-(2.39±0.11)kG/μ_B. A temperature independent term K^μ(x=0)=+45±10 ppm is found. The results are discussed in terms of the electronic structure of H in Pd.     

205Tl NMR in the Cuprate Superconductor Tl2Ba2CaCu2O8−δ: Evidence for a pseudo spin gap

²⁰⁵Tl-NMR in Tl₂Ba₂Ca_n-1Cu_nO_8?δ for differentn and δ is reviewed. Forn=2,3 the²⁰⁵Tl spectra consist of two lines. The main line arises from Tl(1) in the TlO-layer whereas the defect line belongs to Tl(2) in the Ca-layers between the CuO₂-layers. The temperature dependent Knight shift of the Tl(2) (defect) line is caused by the hyperfine interaction with the CuO₂-layer and is strongly related to the oxygen content of the sample. We find quasipartticle behavior for the T1(2) line similar to the⁸⁹Y Knight shift in the YBa₂Cu₃O_6+δ system. In contrast the T1(1) line shows strong antiferromagnetic correlations with indication of pseudo spin gap opening above the superconducting transition temperatureT _c.     

Mössbauer effect study of hyperfine interactions in145Nd

The magnetic hyperfine splitting of the 72.5 keV γ rays of¹⁴⁵Nd was investigated in intermetallic compounds of Nd and in the paramagnetic salts Nd _x Y _1-x Cl₃·6H₂O (withx=0.02 andx=0.05) at 4.2 K. With the magnetic hyperfine tensorA of Nd_0.01Y_0.99Cl₃·6H₂O known from EPR spectroscopy, the analysis of the unresolved magnetic hyperfine spectra yieldsI _e =5/2 for the spin of the 72.5 keV state, in contradiction to a previous result. The multipolarity of the 72.5 keV γ transition was found to be essentiallyM1 with δ²=0.010±0.014, and the magnetic moment of the 72.5 keV state was determined as μ(5/2)=?0.319±0.004 nm. For various divalent and trivalent Nd compounds as well as for metallic Nd the isomer shift IS of the 72.5 keV γ line was measured. A value for the change of the mean square nuclear charge radius during the 72.5 keV γ transition of Δ〈r ²〉=+(1.9±0.9)·10^?3fm² was deduced using electron density differences from free-ion Hartree-Fock calculations.     

EPR study of the ferroelastic phase transition in CsLiSO4

EPR spectra of SO^-₃ ion-radical in X-ray irradiated CsLiSO₄ single crystals were used for the study of the ferroelastic phase transition at T_c = 203.0 K. The splitting (ΔH) of the SO₃ line in the low-temperature ferroelastic phase has been interpreted as proportional to the square of the order parameter. The splitting shows the temperature dependence ΔH ∞ (T_c ? T) 1.01^± 0.01     

EPR studies of phase transitions in perchlorates [M 2+(ClO4)2 · 6H2O] at high pressures

The EPR spectra of the Mn²⁺ ion in crystals of the perchlorate hexahydrates Zn(ClO₄)₂ · 6H₂O, Mg(ClO₄)₂ · 6H₂O, and Cd(ClO₄)₂ · 6H₂O were studied in the temperature range 77–320 K under hydrostatic pressure. It is shown that the octahedron of six molecules H₂O surrounding this paramagnetic ion is contracted along the c axis and that pressure decreases this distortion. The second-order phase transition that occurs near 200 K in the perchlorates and in other crystal hydrates is shown to be associated with changes in the bonds in the nearest ligand environment. As the pressure is increased, the phase-transition temperatures shift and the perchlorate crystals tend to a single-phase state. The low-temperature phase is assumed to disappear as the pressure increases, and this phase exists in a closed T-P region in the phase diagram. As the pressure increases, the character of the high-temperature transition in the Cd(ClO₄)₂ · 6H₂O changes: the jumplike transition at T ₁ with a 1-K hysteresis changes into a smooth transition and then disappears as the pressure increases further.     

Susceptibility — Knight shift correlation and the band structure of VO2

The magnetic susceptibility and⁵¹V Knight shift have been measured in powdered VO₂ from the semiconductor-metal transition temperatureT _t=341°K to 478°K. The fact that both depend linearly on temperature enables the orbital and spin contributions to both quantities to be evaluated. The orbital and spin susceptibilities at 341°K are found to be 1.30±0.07 and 7.49±0.07μemu g^?1, respectively, and the corresponding shifts +0.61±0.04% and ?1.00±0.04%. The existence and magnitude of the orbital term are consistent with the currently accepted two band model of VO₂ aboveT _t.     

EPR of cadmium ferric voltaite above the ferrimagnetic transition

A single isotropic EPR line of Fe³⁺ in synthetic cadmium ferric voltaite, (NH₄)₂Cd₅Fe₃Al(SO₄)₁₂ · 18H₂O, was observed in a wide temperature range from 295 to 1.57°K. The ferrimagnetic transition temperature of CdFe voltaite was determined to be ～ 0.7°K using the temperature dependence of the g-factor and the line width. The cubic crystal field parameter, a, for Fe³⁺ in CdFe voltaite is extracted from the EPR line width measurement using the exchange-narrowed line width model of Anderson and Weiss. The parameter a for Fe³⁺ in CdFe voltaite at 4.2°K is 157 × 10^-4cm^-1 which is consistent with the corresponding values for Fe³⁺ in other cubic structures.     

Jahn-Teller effect in low-spin Ni3+ in the LaAlO3 ceramic

This paper reports on an EPR study of LaAl_1?x Ni_xO₃ solid solutions with x≤0.12 made in the 4.2-to 300-K temperature range. In the X range, the broadening of the single EPR line with g _eff=2.148 was observed at temperatures below ～40 K. In the Q range, a slightly anisotropic EPR line with g _⊥ ^′ =2.145±0.002 and g _‖ ^′ =2.154±0.002 transforms to a rhombic-symmetry spectrum with g ₁=2.183±0.002, g ₂=2.143±0.002, and g ₃=2.118±0.002. It is shown that the observed low-temperature effects are due to the lowered symmetry of the complex under the combined action of the tetragonal Jahn-Teller distortions and of the trigonal component of the crystal field.     

Angular dependence of the Knight shift, electric field gradient, and spin-lattice relaxation time of 9Be NMR in beryllium metal

Angular dependences of ⁹Be NMR (±1/2) and (±1/2 ? ±3/20 transition frequencies are measured in a single-crystal beryllium metal plate in a field of 7.04T. The isotropic K _iso and anisotropic K _aniso components of the Knight shift are determined. The measured values of K _iso and T ₁ are considered in terms of the contact, polarization, and orbital contributions.     

Crystal electric field next to a hydrogen-like interstitial— µ+ in PrNi5

Muon spin rotation measurements of the temperature dependence and the anisotropy of the μ⁺ Knight shift in single crystals of the crystal electric field singlet ground state system PrNi₅ reveal pronounced deviations from a linear scaling of the Knight shift with the bulk magnetic susceptibility atT≤50 K. They are explained by a μ⁺ induced modification of the atomic susceptibility of neighboring Pr³⁺ ions. From the Knight shift anisotropy atT> 50 K it is determined that the implanted μ⁺ occupy a single intersitial site, namely the 6i site (0.5, 0, 0.21±0.02). Using this site information, good model fits to the measured data are obtained assuming a μ⁺ induced perturbation of the crystal electric field at the Pr³⁺ ions next to the μ⁺. Apparently, the presence of the μ⁺ leads to a lowering of the local symmetry, causing a lifting of the degeneracy and a pronounced rearrangement of the low lying crystal electric field levels for these ions.     

NMR detected by Mössbauer-effect,applied to the system57CoFe

The ratio of the Larmor frequencies of¹¹¹Cd and²H has been measured in an aqueous solution of CdSO₄ with high accuracy.¹¹¹Cd chemical shifts have been investigated in aqueous solutions of CdCl₂, Cd(NO₃)₂, CdSO₄, and Cd(ClO₄)₂ as a function of concentration. Using these, the ratio of the Larmor frequencies of the¹¹¹Cd nuclei for infinite dilution relative to²H in pure D₂O is given. From this ag _I -factor for¹¹¹Cd has been derived and has been compared with theg _I -factor of an optical pumping experiment. The resulting shielding constant is σ^*(hydrated¹¹¹Cd versus¹¹¹Cd atom)=?1.106(4) · 10^?3. This yields an atomic reference scale for all measured NMR line shifts in the liquid and solid state and the possibility of comparing experimental and theoretical shielding constants. As a consequence, the amount of the Knight shift of metallic cadmium becomes 20% larger.     

Effect of hydrostatic pressure and temperature on the EPR spectrum of the Mn2+ ion in Zn(BF4)2 · 6H2O

A study of the effect of hydrostatic pressure and temperature on the EPR spectrum of the Mn²⁺ ion in Zn(BF₄)₂ · 6H₂O is reported. The break in the temperature dependence of the b ₂ ⁰ parameter at 196 K is evidence of the existence of a phase transition accompanied by a change in the thermal expansion coefficient. It is shown that pressure considerably affects the spectral parameters by reducing the axial parameter b ₂ ⁰ and increasing the cubic one, b ₄ ⁰ . At 9 kbar, the b ₂ ⁰ parameter is temperature independent. A comparison of the pressure dependences of the spectra of Zn(BF₄)₂ · 6H₂O, ZnSiF₆ · 6H₂O, ZnTiF₆ · 6H₂O, and MgSiF₂ · 6H₂O crystals suggests equal hydrogen-bond lengths in these compounds. A ligand hyperfine structure has been detected, which originates from the Zeeman interaction with the proton nuclei surrounding Mn²⁺ and manifests itself in the formation of satellites at each EPR line, their separation being proportional to the magnetic field. The nonlinear pressure dependence of the linewidth is related to the structural features of the crystal under study.     

A Low Temperature (10 K) High-Frequency (208 GHz) EPR Study of the Non-Kramers Ion Mn3+ in a MnMo6Se8 Single Crystal

A high-frequency (208 GHz) electron paramagnetic resonance (EPR) study on Mn³⁺ (3d⁴, S = 2) ions embedded in a MnMo₆Se₈ single crystal has been performed at 10 K. The experimental spectra reveal the presence of only one set of EPR lines from Mn³⁺ ions, whose magnetic axes are oriented along the crystal axes. The spin-Hamiltonian parameters are evaluated by the method of least-squares, fitting all the observed line positions simultaneously, for the three orthogonal orientations of the external magnetic field. The symmetry of the spin Hamiltonian at the site of the Mn³⁺ ions has been deduced from the EPR spectra.     

EPR and Optical Absorption Study of Cr3+-Doped Dipotassium Tetrachloropalladate

X-band electron paramagnetic resonance (EPR) study of Cr³⁺-doped dipotassium tetrachloropalladate single crystal is done at liquid nitrogen temperature. EPR spectrum shows two sites. The spin-Hamiltonian parameters have been evaluated by employing hyperfine resonance lines observed in EPR spectra for different orientations of crystal in externally applied magnetic field. The values of spin-Hamiltonian and zero-field splitting (ZFS) parameters of Cr³⁺ ion-doped DTP for site I are: g _x  = 2.096 ± 0.002, g _y  = 2.167 ± 0.002, g _z  = 2.220 ± 0.002, D = (89 ± 2) × 10^?4 cm^?1, E = (16 ± 2) × 10^?4 cm^?1. EPR study indicates that Cr³⁺ ion enters the host lattice substitutionally replacing K⁺ ion and local site symmetry reduces to orthorhombic. Optical absorption spectra are recorded at room temperature. From the optical absorption study, the Racah parameters (B = 521 cm^?1, C = 2,861 cm^?1), cubic crystal field splitting parameter (Dq = 1,851 cm^?1) and nephelauxetic parameters (h = 2.06, k = 0.21) are determined. These parameters together with EPR data are used to discuss the nature of bonding in the crystal.     

Controversy about the reference compound for correct chemical shift determination of 195Pt nuclei

Several groups exploring the ¹⁹⁵Pt NMR in solids, including metallic and magnetic materials, use different standards for chemical shift (Knight shift) determination. Commonly applied H₂PtCl₆ and Na₂PtCl₆ (IUPAC δ scale) lead to considerable underestimation of the shifts since H₂PtCl₆ has considerable own ¹⁹⁵Pt NMR shift due to its Van Vleck paramagnetism. In this Letter new results on ¹⁹⁵Pt NMR in heavy fermion system CeInPt₄ are presented and rationalized scale for the Knight shift determination is discussed.     

NMR and magnetic susceptibility of the intermetallic pseudo-binary compounds Ce1−xRxAl3 with R=Gd,Tb and Er

The NMR and magnetic susceptibility of the intermetallic pseudo-binary compounds Ce_1?xGd_xAl₃ (x=0·01; 0·03; 0·05; 0·07), Ce_0·95Tb_0·05Al₃ and Ce_0·975Er_0·025Al₃ were investigated. The susceptibility is given by the sum of a temperature independent term and a Curie-Weiss one. The last one results from the contribution of the localized free-ion magnetic moments of all the rare-earth ions which are also responsible for the strong temperature dependent Knight shifts of the NMR lines of ²⁷Al nuclei via the exchange polarization of the conduction electrons. The ²⁷Al NMR spectra exhibit besides the line due to Al sites surrounded in the first coordination sphere as in CeAl₃ a second peak with a lower Knight shift due to Al nuclei positioned in the vicinity of a Gd, Tb or Er ion.     

Temperature dependence of the positive-muon knight shift in MnSi

The temperature dependence of the μ⁺ Knight shift in weakly-helimagnetic MnSi has been measured in the temperature range between 28 and 300 K. The observed shift is found to be directly proportional to the host susceptibility in the paramagnetic state with a hyperfine coupling constant of —4.8 kOe/_μB.     

London penetration depth and coherence peak in ammonia-intercalated fulleride superconductors

The compound (NH₃)_xNaRb₂C₆₀ (x ～ 1.6) has been studied by¹H nuclear magnetic resonance. The proton line shows no shift at any temperature, while lineshape analysis of the low-temperature (30 K) spectra reveals the presence of a Knight shift interaction of pure anisotropic nature, whose amplitude has been estimated. Relaxation measurements show a single-exponential recovery of the magnetization at all investigated temperatures and fields. A Korringa behavior is observed below 65 K and a clear Hebel-Slichter coherence peak is detected just below T_c. The London penetration depth can be estimated from the line broadening observed below T_c, which yields λ ≈ 825 nm.     

The effect of clustering of VO2+ ions in sub- and supercritical water. An in situ EPR study

Temperature variations of the EPR spectra of VO²⁺ ions in sub- and supercritical water under isothermal and temperature gradient conditions are investigated using an in situ EPR. Broadening of the hyperfine structure at increasing temperature and the appearance of an unresolved broad low-intensity line (ΔH _pp ≈ 300 Oe) in the supercritical state are observed in the absence of temperature gradients, indicating an increase of exchange interaction between VO²⁺ ions in supercritical water. An exchange-narrowed anisotropic absorption line is observed under the temperature-gradient conditions in the subcritical water near the transition to a supercritical state. The shape of this line is close to that observed in the solid salt VOSO₄ · 3H₂O. It is shown that in situ EPR allows us to investigate the effects of changing the local environment of paramagnetic ions, which precedes the well-known process of clustering and formation of amorphous oxide particles in sub- and supercritical conditions.     

The potential barrier height for a Jahn-Teller center

The EPR spectrum of the Cu²⁺ ion in a ZnSiF₆·6H₂O crystal is studied in the temperature range T=5?300K. It is shown that the EPR spectrum can be represented in the form of a superposition of three contributions with essentially different properties. The first contribution is characterized by the maximal intensity at low temperatures and is described by a spin Hamiltonian with a large anisotropy of parameters. The second contribution has the maximal intensity at high temperatures and is described by a spin Hamiltonian with a low anisotropy of parameters. The third contribution cannot be described by a spin Hamiltonian and has the form of a partly orientationally averaged EPR spectrum. The reason for the emergence of these contributions is substantiated along with the form of the temperature dependence of their intensities on the basis of variation of the populations of vibronic states upon a change in temperature. The height (E₀=4±1cm_?1) of the potential barrier separating three equivalent Jahn-Teller potential wells of the Cu²⁺ ion is determined from analysis of the temperature dependence of the integrated intensity of the EPR spectrum. The obtained value of the barrier height substantially differs from the estimate (100 cm^?1) obtained earlier [2, 3] for the Cu²⁺ ion in ZnSiF₆·6H₂O on the basis of the tunneling model. It is shown that the forms of the temperature dependences of the linewidth of the low-and high-temperature EPR spectra are essentially different. This difference indicates that the contributions of the low-and high-temperature EPR spectra are associated with quantum-mechanical transitions between these states. It is proposed that the low-and high temperature contributions to the EPR spectrum are associated with the filling of under-the-barrier and above-the-barrier vibronic states, respectively.