Transferred Hyperfine Interaction Research Articles

Hyperfine couplings as a probe of orbital anisotropy in heavy-fermion materials

The transferred hyperfine interaction between nuclear and electron spins in an heavy fermion material depends on the hybridization between the $f$-electron orbitals and those surrounding a distant nucleus. In CeMIn$_5$ (M=Rh, Ir, Co), both the hyperfine coupling to the two indium sites as well as the crystalline electric field at the Ce are strongly dependent on the transition metal. We measure a series of CeRh$_{1-x}$Ir$_x$In$_5$ crystals and find that the hyperfine coupling reflects the orbital anisotropy of the ground state Ce 4$f$ wavefunction. These findings provide direct proof that the localized to itinerant transition is dominated by hybridization out of the Ce-In plane in this system.

Spin echo studies on Fe3+ ions in GaN: Spin-phonon relaxation and ligand hyperfine interactions

The electron spin echo detected inversion recovery technique at the Q-band frequency was used to characterize spin diffusion effects in spin-lattice relaxation of compensating Fe3+ impurities in n-type doped GaN crystals. It was found that the selective saturation can be achieved in the GaN:Fe3+ system due to magnetization transfer based on the spin flip-flop cross-relaxation processes. The temperature dependence of 1/T1 can be explained by direct spin-phonon processes (∼T) below 25 K and by Raman two-phonon processes (∼T9) at higher temperatures. Spin diffusion in this system is characterized by an additional cross-relaxation rate which is weakly temperature-dependent below 25 K. The transferred hyperfine interactions of Fe3+ centers with gallium and nitrogen neighbor nuclei were resolved using pulsed-electron nuclear double resonance. A comparative analysis of quadrupole interactions indicates the essential increase in the electric field gradients on the nearest nitrogen and gallium shells.

ENDOR investigations of the Ce3+ ions in YAG: Transferred hyperfine interaction with nearest aluminum ions

The transferred hyperfine interactions of Ce3+ centers with 27Al neighbor nuclei in heavily cerium doped yttrium aluminum garnet (YAG) are resolved using Pulsed-Electron Nuclear Double Resonance (Pulsed-ENDOR). It is demonstrated that substitution of Ce3+ for Y3+ in YAG crystals leads to the strong reduction of the nuclear quadrupole coupling at the tetrahedral aluminum d-sites (e2qQ/h ≈ 4.67 MHz).

Structural, magnetic and hyperfine characterizations of nanocrystalline Zn-Cd doped nickel ferrites

In our present work, we have synthesized a series of Cd-Zn doped nickel ferrite ((Cd0.5−xZnx)Ni0.5Fe2O4; x=0, 0.1, 0.2, 0.3, 0.4 and 0.5) through standard chemical co-precipitation method to study the influence of diamagnetic ions (Cd, Zn) on the magnetic properties of ferrites. XRD and Raman spectroscopy were employed for the structural characterizations. The refinement of the X-ray diffractogram data augmented by the Williamson-Hall plots showed the presence of Cd2+ vacancies and a strained crystal structure. The vibrational spectroscopy indicated the presence of lower space-group symmetry and a distorted crystal structure. Magnetic measurements showed the samples possessed low magnetic anisotropy along with a canted spin structure. The Mössbauer measurements confirmed the cation distribution and gave evidence of super transferred hyperfine interactions arising due to canted spin structure of the system.

NMR Investigations in Li1.3Al0.3Ti1.7(PO4)3 Ceramics Part II: Lithium Dynamics, Experiments, and Model

Often the paramagnetic defects have important impact on the nuclear relaxation even in the “nominally” pure samples. These effects show themselves either by the nearest-neighbors only or by the dipolar long-range interaction. The former are addressed in this paper in which we built a model for calculation of T1, T1Q, T2, T1ρ when there is—or not—a residual quadrupolar interaction. This model shows that the maxima of R2 = 1/T2 and R1ρ = 1/T1ρ are shifted toward the low frequencies (low temperatures with regards to the one of R1 = 1/T1), the maximum of R1ρ is shifted toward the low frequencies with regards to the one of R2, and this model is general and can be applied to any system which relaxes by transferred hyperfine interaction. This model was applied to Li1.3Al0.3Ti1.7(PO4)3. It accounts for all the experimental results of T1 (6Li), T1 (7Li), T1Q (7Li), T2 (7Li), and T1ρ (7Li) which were used to highlight local dynamic properties of the lithium versus temperature. The analysis of these results allows u...

EPR, ENDOR and optical spectroscopy of Yb3+ ion in KZnF3 single crystals

The paramagnetic center of tetragonal symmetry formed by the Yb3+ ion in the KZnF3 crystal has been studied using methods of EPR, ENDOR and optical spectroscopy. The location of the impurity ion and the structural model of the complex differing from the model of the Yb3+ center in KMgF3 have been established. The empirical scheme of the energy levels of the Yb3+ ion has been found. The parameters of its interaction with the crystal electrostatic field and the hyperfine interaction with ligands of the nearest environment have been determined. The parameters of the crystal field were used for the analysis of the distortions of the crystal lattice in the vicinity of Yb3+. The parameters of the transferred hyperfine interaction have been calculated for the distances between Yb3+ and F− ions of the nearest environment obtained taking into account the found distortions. They are in good agreement with the experimental values.

Magnetic properties and hyperfine interactions in Cr₈, Cr₇Cd, and Cr₇Ni molecular rings from ¹⁹F-NMR.

A detailed experimental investigation of the (19)F nuclear magnetic resonance is made on single crystals of the homometallic Cr8 antiferromagnetic molecular ring and heterometallic Cr7Cd and Cr7Ni rings in the low temperature ground state. Since the F(-) ion is located midway between neighboring magnetic metal ions in the ring, the (19)F-NMR spectra yield information about the local electronic spin density and (19)F hyperfine interactions. In Cr8, where the ground state is a singlet with total spin S(T) = 0, the (19)F-NMR spectra at 1.7 K and low external magnetic field display a single narrow line, while when the magnetic field is increased towards the first level crossing field, satellite lines appear in the (19)F-NMR spectrum, indicating a progressive increase in the Boltzmann population of the first excited state S(T) = 1. In the heterometallic rings, Cr7Cd and Cr7Ni, whose ground state is magnetic with S(T) = 3/2 and S(T) = 1/2, respectively, the (19)F-NMR spectrum has a complicated structure which depends on the strength and orientation of the magnetic field, due to both isotropic and anisotropic transferred hyperfine interactions and classical dipolar interactions. From the (19)F-NMR spectra in single crystals we estimated the transferred hyperfine constants for both the F(-)-Ni(2+) and the F(-)-Cd(2+) bonds. The values of the hyperfine constants compare well to the ones known for F(-)-Ni(2+) in KNiF3 and NiF2 and for F(-)-Cr(3+) in K2NaCrF6. The results are discussed in terms of hybridization of the 2s, 2p orbitals of the F(-) ion and the d orbitals of the magnetic ion. Finally, we discuss the implications of our results for the electron-spin decoherence.

Angle-resolved NMR: Quantitative theory of75AsT1relaxation rate in BaFe2As2

While NMR measurements of nuclear energy spectra are routinely used to characterize the static properties of quantum magnets, the dynamical information locked in NMR 1/T1 relaxation rates remains notoriously difficult to interpret. The difficulty arises from the fact that information about all possible low-energy spin excitations of the electrons, and their coupling to the nuclear moments, is folded into a single number, 1/T1. Here we develop a quantitative theory of the NMR 1/T1 relaxation rate in a collinear antiferromagnet, focusing on the specific example of BaFe2As2. One of the most striking features of magnetism in BaFe2As2 is a strong dependence of 1/T1 on the orientation of the applied magnetic field. By careful analysis of the coupling between the nuclear and electronic moments, we show how this anisotropy arises from the "filtering" of spin fluctuations by the form-factor for transferred hyperfine interactions. This allows us to make convincing, quantitative, fits to experimental 1/T1 data for BaFe2As2, for different field orientations. We go on to show how a quantitative, angle-dependent theory for the relaxation rate leads to new ways of measuring the dynamical parameters of magnetic systems, in particular the spin wave velocities.

19F NMR study of LiTbF4 single crystals

The angular dependences of 19F NMR spectra have been measured in the external magnetic field of 0.5 T oriented in the basis plane of LiTbF4 at the room temperature. We have obtained the constants of transferred hyperfine interaction and the corrected set of crystal field parameters for the Tb3+ ions in LiTbF4. The results of simulations of the magnetization in high pulsed magnetic fields with taking into account magnetoelastic interactions agree satisfactorily with experimental data presented in the literature.

Transferred Hyperfine Interaction between a Tetrahedral Transition Metal and Tetrahedral Lithium: Li6CoO4

Li6CoO4 presents an antifluorite-type structure, with both the Co and Li ions in tetrahedral oxygen coordination. 7Li MAS NMR shows remarkably different shifts (+885 and −232 ppm) for the two different crystallographic types of Li. In order to assign the signals and to understand the mechanisms whereby the electron spins on the e orbitals of Co2+ ions (e4 t23 electronic configuration) are transferred toward the two different types of Li with opposite polarization, we have carried out GGA and GGA+U calculations of the electronic structure using the VASP code. Spin density maps in selected planes of the structure reveal (as expected) that lobes of the t2 orbitals point toward the faces of the CoO4 tetrahedra and can thus overlap with the neighboring Li(2) through empty square pyramidal sites. As concerns Li(1), a mechanism is evidenced where the (filled) e orbitals of Co2+ are polarized by the electron spins in the t2 ones. These polarized e orbitals overlap with Li(1) through the common edge of the tetrahedra. The relative magnitude of the experimental shifts for the two types of Li are however not fully reproduced by the calculations, and the influence of the U parameter as well as of the pseudopotential method used is discussed.

Transferred hyperfine interactions forYb3+ions inCsCaF3andCs2NaYF6single crystals: Experimental andab initiostudy

The results of an electron-nuclear double resonance study of the cubic paramagnetic ${\mathrm{Yb}}^{3+}$ center in ${\mathrm{Cs}}_{2}\mathrm{Na}\mathrm{Y}{\mathrm{F}}_{6}$ and $\mathrm{Cs}\mathrm{Ca}{\mathrm{F}}_{3}$ single crystals are presented. The values and signs of the transferred hyperfine interaction (THFI) parameters for several neighboring shells are determined. It is found that the relevant parameters for the two studied matrices differ, in spite of the fact that the nearest environment of the rare earth ion is nearly identical. A first-principles theoretical analysis is performed for the THFI parameters of the first coordination shell of ${\mathrm{F}}^{\ensuremath{-}}$ ions. Several mechanisms of metal ion-ligand coupling are considered and it is found that one of them, ligand polarization, explains the difference observed for the THFI parameters in ${\mathrm{Cs}}_{2}\mathrm{Na}\mathrm{Y}{\mathrm{F}}_{6}$ and $\mathrm{Cs}\mathrm{Ca}{\mathrm{F}}_{3}$.

P31-NMRstudy of hyperfine interactions and magnetic fluctuations in the neptunium-based filled skutteruditeNpFe4P12

$^{31}\text{P}\text{-NMR}$ measurements have been performed on a single crystal of the neptunium-based filled skutterudite ${\text{NpFe}}_{4}{\text{P}}_{12}$. The compound undergoes a ferromagnetic phase transition at ${T}_{C}=23\text{ }\text{K}$. From the field-orientation dependence of the $^{31}\text{P}\text{-NMR}$ line splitting, the angular dependence of the hyperfine interactions between $\text{Np}\text{ }5f$ spin and $^{31}\text{P}$ nuclear moments has been investigated. We have observed anisotropic transferred hyperfine interactions at the P sites, which lead to an estimate of the local spin density in the $\text{P}\text{ }3p$ orbitals. It is shown that a fraction of the $\text{Np}\text{ }5f$ spin moments is transferred mostly into the $\text{P}\text{ }3p$ orbitals extending toward the inside of a P cage. The weak hybridization between $\text{Np}\text{ }5f$ and $\text{P}\text{ }3p$ orbitals suggests a localized character for $\text{Np}\text{ }5f$ electrons in ${\text{NpFe}}_{4}{\text{P}}_{12}$. We have also measured the field and temperature dependences of the nuclear spin-lattice relaxation rate $(1/{T}_{1})$ in several magnetic fields between 18.5 and 78.0 kOe. The $1/{T}_{1}$ data reveal that the low-energy spin fluctuations of $\text{Np}\text{ }5f$ spin moments are strongly suppressed by applied fields over a relatively wide temperature range up to $4{T}_{C}$. In this compound, a large and negative magnetoresistance has been known to occur in the same temperature range. The present NMR results demonstrate that the negative magnetoresistance comes from a reduction in the magnetic scattering from $\text{Np}\text{ }5f$ spin moments by an applied field.

Anisotropic transferred hyperfine interactions inCs2CuCl4

We report 133Cs nuclear magnetic resonance (NMR) measurements of magnetic insulatorCs2CuCl4 in the paramagneticphase (T≥4.2 K) as a function of the orientation of an applied magnetic field with respect to the principalcrystalline axes. The magnetic shift tensor is determined. It is found that its principal axesdo not coincide with the principal axes of the crystal. The Cu–Cs dipolar interaction tensoris calculated as well. From these, we deduce the full transferred hyperfine tensor for the twoinequivalent Cs sites of the unit cell. We find that the tensors are anisotropic, containingnon-zero off-diagonal terms, and that the transferred hyperfine coupling betweenCu electronic spins and Cs nuclei dominates the NMR shift on both Cs sites.

Anisotropic Transferred Hyperfine Interaction in a Heavy-Fermion Compound: Paramagnetic NMR Line Shift in a CeCu6 Single Crystal

We report on single crystal nuclear magnetic resonance spectroscopy for 63Cu at the Cu(5) sites in the heavy-fermion paramagnet CeCu6 for constant external magnetic field strength (7 T) applied along the “hardest” magnetic direction b and varied temperature (5 − 100 K). The transferred field coupling constant α b (5) = + 3.40(31) kOe/μ B is derived from the van Vleck like paramagnetic line shift. These results establish the predominance of anisotropic spin and orbital polarization acting on the Cu sites neighbouring the Ce 4f local moment. Because magnetically induced electric quadrupolar interaction at this Cu site is not unusually enhanced, we conclude that the exchange polarization mechanism at the Ce site is the source of the observed anisotropy. Impact of the substitution of Cu(2) by Au on the Ce3+ single-ion ground state properties and magnetic characteristics of CeCu6-x Au x is discussed.

A study of the transferred hyperfine field and spin–lattice relaxation time of [formula omitted] single crystals

The molecular susceptibility and paramagnetic shift of [ N ( CH 3 ) 4 ] 2 CuCl 4 single crystals were measured, and from these experimental results we obtained the transferred hyperfine interaction, H hf , due to the spin transfer from Cu 2 + ions to [ N ( CH 3 ) 4 ] + ions. We conclude that the decrease in H hf with increasing temperature is not sufficient to allow the spin transfer between Cu 2 + ions and the nuclear spins of the nonmagnetic [ N ( CH 3 ) 4 ] + ions in the lattice. In addition, the proton spin–lattice relaxation time, T 1 , of [ N ( CH 3 ) 4 ] 2 CuCl 4 single crystals was determined as a function of temperature; the variation in T 1 with temperature exhibited discontinuities near 263 and 200 K, confirming previous reports of phase transitions at these temperatures. Comparison of the T 1 and H hf results revealed that T 1 decreases with increasing H hf , which is the opposite of the trend reported for [ N ( CH 3 ) 4 ] 2 CoCl 4 . This difference may be due to differences in the electronic structures of Co and Cu ions: Cu 2 + has one s electron outside closed d shells whereas Co 2 + has unpaired d electrons, which screen the nuclear charge from the motion of the outer electrons.

Nuclear spin lattice relaxation measurements in the heavy-fermion compoundsCeCu6andCeCu5Au: A quantitative approach

We derive the $^{63}\mathrm{Cu}$ and $^{65}\mathrm{Cu}$ nuclear spin lattice relaxation for the Cu(5) site of oriented powder prepared from the single-crystalline heavy-fermion compounds $\mathrm{Ce}{\mathrm{Cu}}_{6}$ and $\mathrm{Ce}{\mathrm{Cu}}_{5}\mathrm{Au}$. The quantitative description is based on single-crystal analysis of the anisotropic magnetic susceptibility, anisotropic transferred hyperfine interaction, and magnetically induced electric quadrupolar interaction. The temperature dependence of the correlation time of the $4f$ electron contribution is derived. An earlier discrepancy with neutron scattering results is removed.

Isoelectronically substituted heavy-fermion compoundsCeCu6−xAux, studied by local probe and electronic band structure calculation

For $\mathrm{Ce}{\mathrm{Cu}}_{6\ensuremath{-}x}{\mathrm{Au}}_{x}$ $(x=0,0.1,1)$, we measured the temperature dependence of $^{63,65}\mathrm{Cu}$ nuclear magnetic resonance spectra of oriented single-crystal powder at $7\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ in the $5--100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ range. ${\mathrm{Ce}}^{3+}$ crystal field and molecular field parameters are derived from single-crystal magnetic susceptibility data. Calculated electric field gradients are used to correctly assign nuclear quadrupole resonance frequencies to lattice sites $(n)$. Bare Knight shifts and transferred hyperfine interaction coupling constants $\ensuremath{\alpha}(n)$ as well as electric field gradient information are extracted for the various Cu sites $(n)$. Ab initio self-consistent energy-band calculations explain the electric field gradients and the alternation of the sign of $\ensuremath{\alpha}(n)$. The anisotropy of the hyperfine coupling parameter $\ensuremath{\alpha}$ is proved for the $a\text{\ensuremath{-}}c$ plane for the Cu(5) site.

Transferred hyperfine field of Rb 2CoCl 4 single crystals in the ferroelectric–incommensurate–normal phase by 87Rb NMR

The molecular susceptibility and paramagnetic shift of Rb 2CoCl 4 single crystals grown using the slow evaporation method were measured, and from these experimental results we obtained the transferred hyperfine interaction due to the transfer of spin density from Co 2+ ions to Rb + ions. The transferred hyperfine field was obtained for the ferroelectric, incommensurate, and normal phases. In the case of Rb(I), the transferred hyperfine interaction decreases with increasing temperature in the incommensurate phase, and increases with increasing temperature in the normal phase. The value of H hf in the incommensurate and normal phases increases abruptly with increasing temperature in the case of Rb(II). These results indicate that the effects due to the transfer of spin density from Co 2+ ions to the Rb(I) and Rb(II) ions are large above T i. In particular, the effect due to the transfer of spin density to Rb(II) ions in the normal phase is very large; the variations with temperature of the transferred hyperfine interactions of the Rb(I) and Rb(II) nuclei are more or less continuous in T c1 and T i, and are not affected by the ferroelectric–incommensurate–normal phase transitions.

The microscopic structure of the oxygen–aluminium hole center in natural and neutron irradiated blue topaz

The dominant O− hole center in natural and neutron irradiated blue topaz with general formula Al2SiO4(F,OH)2 has been investigated with electron paramagnetic resonance (EPR) and optical absorption. The analysis of the EPR spectra of the O− center yielded superhyperfine interactions with two equivalent Al neighbors in four magnetically inequivalent positions. From the g-factors it is found that the O− hole center is located in (8d) Wyckoff positions on fluoride sites that were substituted by hydroxyl molecules prior to the irradiation. The spin Hamiltonian parameters were successfully interpreted with the model of transferred hyperfine interaction by exchange polarization supporting the localization of O− in (F,OH)− sites. The O− center has the same thermal stability as the absorption band centered at 620 nm, which is responsible for the blue color. The intensity of the absorption band correlates linearly with the O− concentration. Its correlation with the blue color is discussed.

Nuclear magnetic resonance in [N(CH3)4]2CoCl4 single crystals: transferred hyperfine interaction and spin-lattice relaxation rate

The molecular susceptibility and paramagnetic shift of [N(CH3)4]2CoCl4 single crystals were measured, and from these experimental results we obtained the transferred hyperfine interaction, Hhf, due to the transfer of spin density from Co2+ ions to [N(CH3)4]+ ions. The transferred hyperfine interaction can be expressed as a linear equation, with Hhf increasing with increasing temperature. The remarkable change in Hhf near Tc5 (=192K) corresponds to a phase transition. The proton spin-lattice relaxation times of [N(CH3)4]2CoCl4 single crystals were also investigated, and it was found that the relaxation process can be described by a single exponential function. The variation of the relaxation time with temperature undergoes a remarkable change near Tc5, confirming the presence of a phase transition at that temperature. From the above results, we conclude that the increase in Hhf with increasing temperature is large enough to allow the transfer of spin density between Co2+ ions and the nuclear spins of the nonmagnetic [N(CH3)4]+ ions in the lattice, and thus the increase in the relaxation time with temperature is attributed to an increase in the transferred hyperfine field.