Theoretical study of the hyperfine fields of Co impurities in Mo hosts

The currently accepted measured value for the hyperfine field at a Co impurity in Mo hosts is -2.6 T. Contrary to this value, in a previous work we have calculated the Fermi contact contribution to the hyperfine field at an isolated Co impurity in Mo hosts using the RS-LMTO-ASA scheme and we have found the value -8.9 T. In order to investigate if this discrepancy is due to the formation of small Co clusters in the measured sample, here we study, theoretically, the behaviour of local magnetic moments and hyperfine fields in three systems, representing different local configurations of Co atoms in Mo hosts: Co sites at Co dimmers; Co with a nearest neighbour vacancy; and a 9 Co atoms cluster. In all cases we use the RS-LMTO-ASA scheme. Our results suggest that the hyperfine fields at Co atoms in Mo can be very sensitive to the homogeneity of the alloy. This revised version was published online in August 2006 with corrections to the Cover Date.     

Mössbauer studies of hyperfine fields in disordered Fe2CrAl

Heusler-like alloy Fe₂CrAl was prepared and studied. Structure determination was done by X-ray. The structure was found to conform to the B₂ type. Magnetic hyperfine fields in this sample were studied by the Mössbauer effect. The Mössbauer spectra were recorded over a range of temperature from 40 to 296 K. The Mössbauer spectra showed the co-existence of a paramagnetic part with a magnetic hyperfine portion at all recorded temperatures. Even with the distribution in the magnetic hyperfine field, the average hyperfine field follows the (T/T _c)^3/2 law. The paramagnetic part of the hyperfine field is explained in terms of the clustering of Cr atoms.     

The influence of small magnetic fields on the Mössbauer relaxation spectra of Au: Yb alloys

The influence of an external magnetic field on the hyperfine structure of the Γ₇ CEF ground state of dilute Yb impurities in Au is investigated through the Mössbauer effect. Strong changes in the shape of the hyperfine spectra are observed when small magnetic fields (?1 kG) are applied. The dependence of the hyperfine structure on applied magnetic fields is shown through a Breit-Rabi diagram. The electronic relaxation rate for this system is found to be independent of the fields applied. The nature of polarized radiation emitted by such sources is discussed.     

Optical pumping and population transfer of nuclear-spin states of caesium atoms in high magnetic fields

Nuclear-spin states of gaseous-state Cs atoms in the ground state are optically manipulated using a Ti:sapphire laser in a magnetic field of 1.516T, in which optical coupling of the nuclear-spin states is achieved through hyperfine interactions between electrons and nuclei. The steady-state population distribution in the hyperfine Zeeman sublevels of the ground state is detected by using a tunable diode laser. Furthermore, the state population transfer among the hyperfine Zeeman sublevels, which results from the collision-induced modification \delta a(\bm S \cdot \bm I) of the hyperfine interaction of Cs in the ground state due to stochastic collisions between Cs atoms and buffer-gas molecules, is studied at different buffer-gas pressures. The experimental results show that high-field optical pumping and the small change \delta a(\bm S \cdot \bm I) of the hyperfine interaction can strongly cause the state population transfer and spin-state interchange among the hyperfine Zeeman sublevels. The calculated results maybe explain the steady-state population in hyperfine Zeeman sublevels in terms of rates of optical-pumping, electron-spin flip, nuclear spin flip, and electron-nuclear spin flip-flop transitions among the hyperfine Zeeman sublevels of the ground state of Cs atoms. This method may be applied to the nuclear-spin-based solid-state quantum computation.     

Hyperfine fields in 3d impurities in metals

The dynamic spin polarization of conduction electrons by an impurity spin is shown to provide a significant positive contribution to the impurity hyperfine field, accounting for the fact that the hyperfine field per unit impurity spin is only about half as large for impurities in metals as in insulators. The anomalously small hyperfine fields of Fe impurities in the noble metals are found to result from an orbital contribution. Quantitative analysis of this orbital contribution suggests that a strong dynamic Jahn-Teller effect is present. Similar orbital contributions can yield a strongly anisotropic hyperfine field for Fe or Cr impurities in hexagonal host metals, depending on the type of crystal-field orbital ground state.     

Coordination-number dependence of magnetic hyperfine fields at (111)Cd on Ni surfaces

Ferromagnetic Ni surfaces were investigated on an atomic scale using the perturbed angular correlation spectroscopy probe (111)Cd. A comprehensive set of data for magnetic hyperfine fields (B(hf)) at various probe sites is presented. A field variation from -7 T in Ni bulk to the surprisingly large value of 16 T at the adatom position on Ni(111) is observed. A continuous nonlinear dependence is found, correlating the experimental B(hf) values with the number of their nearest Ni neighbors. The data are discussed on the basis of recent calculations on B(hf) values at sp-element impurities on ferromagnetic surfaces.     

Magnetic hyperfine fields in ultrathin Ni films on Cu(100)

The magnetic hyperfine field was measured at ¹¹¹In(¹¹¹Cd) probe atoms in ultrathin Ni films epitaxially grown on Cu(100) utilizing the perturbed -angular correlation (PAC) method. The behaviour of the hyperfine field as a function of temperature was studied for different film thicknesses ranging from 2 up to 10 monolayers. It was found that the strength of the hyperfine fields as well as the critical temperatures are strongly reduced for thin nickel films and approach the bulk value with increasing film thickness. The orientation of the hyperfine field is discussed.     

Moment compensation in NixRh1−x(Fe) from Moessbauer spectroscopy

The magnetic behaviour of very dilute ⁵⁷Fe(≈20 ppm) impurities in paramagnetic Ni_xRh_1?x (x = 0.42 and x = 0.55) alloys has been studied by Moessbauer spectroscopy in the temperature range between 11 and 0.05 K and in external fields up to 5.6 T. The magnetic moment associated with the Fe-impurity is determined via the dependence of the hyperfine magnetic field on applied magnetic field and temperature. Below 4.2 K deviations from a free spin behaviour are found. The saturation hyperfine field becomes dependent on the applied field, a behaviour which is typical for impurity spin compensation. This compensation decreases with Ni concentration.     

Hyperfine fields in iron-metalloid ferromagnetic metals

A new coordination parameter Z_eff is defined at each inequivalent iron site in the interstitial iron-metalloid ferromagnetic metals which uniquely determines the hyperfine field at that site with considerable accuracy. The dependence of Z_eff upon bonding lenghts is defined and can be directly used to generate hyperfine field distributions for amorphous aggregates. It thereby promises to provide a fairly quantitative bridge between computer models and hyperfine field probes in iron metglasses.     

Observation of atomic spin precessions of isolated fluorine ions on recoil through weak transverse magnetic fields

A new experimental method for studying the hyperfine interactions of multi-electron configurations in isolated ions is illustrated for fluorine using the long-lived 197 keV¹⁹F(5/2⁺) state as probe. The observed atomic precessions were found to vary linearly with the applied weak magnetic field integrated over the flight path of the recoiling ions. The data are well accounted for, assuming configurations from the coupling of 2s and 2p electrons in the relevant ionic charge states.     

Hyperfine fields at Ce in GdAl2 and DyAl2

The local fields at¹⁴⁰Ce in the cubic intermetallic compounds GdAl₂ and DyAl₂ have been measured with the DPAC method. At our lowest temperatures we obtainB _eff (30K)=54(2) T for GdAl₂ andB _eff(12.5K)=27(1) T for DyAl₂ which are considerably lower than the hyperfine field of the free Ce³⁺ ion (183 T). The (Ce)GdAl₂ field is quantitatively explained by a cubic crystal field splitting of 4f states but for DyAl₂ additional effects are discussed.     

The magnetic hyperfine fields of rare earth ions in metals

Data for the magnetic dipole hyperfine interaction of essentially single rare earth ions in metals, measured with different experimental methods, are collected and discussed. Depending on the host, the magnetic hyperfine field of these paramagnetic ions remains undisturbed by the environment, or it is enlarged, or weakened or can even become completely lost. If there are magnetic ions in the neighbourhood, the magnetic interaction can enlarge the hyperfine field of the single ion by a transferred hyperfine field. The reason of the demagnetization effect may be crystal field splitting and hybridization. The core polarization field of the free rare earth ions is redetermined from measurements of the hyperfine interaction in nonmagnetic metals at low magnetic ion concentration.     

On the origin of discontinuity of the hyperfine fields at Fe nuclei in bulk iron and aerosol Fe nanoparticles

Advancing the early work in which a discontinuity of hyperfine fields at ⁵⁷Fe nuclei in bulk iron and in aerosol Fe nanoparticles has been revealed by analyzing their Mössbauer spectra the present Letter evidences that the existence of several peaks in the hyperfine distribution (HFD) for bulk Fe is caused with the internal magnetic fields owing to its multidomain structure whereas aerosol Fe nanoparticles are single-domain and show only a unique peak in HFD. This argument has been corroborated by transformation of the HFD pattern for Fe foil after applying the external magnetic field of 0.03 T.     

Anomalous hyperfine fields detected with the differential angular correlation method

Time differential perturbed angular distributions of nanosecond-levels of some rare earth nuclei embedded in a ferromagnetic host lattice have been measured. Coulomb excitation with 4–10 MeV α-particles and recoil implantation technique have been used. Some evidences have been found for anomalous hyperfine fields acting on the nucleus in a time period which cannot be resolved. On a time scale starting at about 1 nsec where details can be resolved, the hypothesis of a static hyperfine field parallel to the external polarizing field seems to be justified.     

Hyperfine fields in dilute Cr-Rh systems

The hyperfine fields on rhodium nuclei in a chromium matrix have been investigated for rhodium concentrations ranging from 39 to 1000 at. p.p.m. For dilute alloys the hyperfine fields measured at the maximum of the spin density wave of chromium areH _o=28.9±0.4 kOe at 308 K just below the Néel temperature andH _o=45.2±1.0 kOe at 272.6 K. It is shown, in a simple model, that the hyperfine field is proportional to the local magnetization of chromium. Concentration effects have been found which prove the existence of long-range interactions between the impurities in the chromium matrix.     

Magnetic hyperfine fields of vacancy-associated119Sn in ion-implanted nickel

Magnetic hyperfine fields of¹¹⁹Sn impurity defects in nickel have been investigated by Mössbauer emission spectroscopy. Radioactive¹¹⁹Xe isotopes were implanted, annealing was performed after¹¹⁹Xe had decayed to¹¹⁹Sb. At least five different components with well-defined magnetic hyperfine fields, isomer shifts and Debye temperatures are identified in the rather complex spectra. One of these (B=2T) is known to be due to substitutional Sn. The hyperfine fields of the other components are pronouncedly larger (B=9T, B=15T, and B=17T, respectively, for single crystals). These defects are proposed to be Sn-multivacancy defects.     

Hyperfine fields and molecular fields in metals

Data on the temperature dependence of the hyperfine fields on Fe sites in Fe₃Si and FeRh (35 at.%) are presented. The sublattice magnetizations are practically independent of the number of magnetic neighbours at each site, suggesting that the molecular field approach is inapplicable to metallic systems.     

A study of the induced magnetic hyperfine fields in Fe/Ag multilayers

Near perpendicular magnetic hyperfine fields in ^110mAg have been observed in Fe/Ag multilayers. These fields are studied by low temperature nuclear orientation (LTNO) in multilayers [Ag(x ML)/Fe(y ML)]₂₀, with (x,y) monolayer (ML) values of (2,10), (3,9), (5,10) and (3,18). The ^110mAg γ-ray anisotropy was measured as a function of applied magnetic field parallel to the multilayer. The average induced hyperfine field of Ag is significantly influenced by the quality of the multilayer as measured by X-ray diffraction. This revised version was published online in August 2006 with corrections to the Cover Date.     

Changes in structure and magnetic hyperfine fields of Finemet alloys, induced by transition elements substitution

Amorphous as well as two-phase nanocrystalline Finemet-type soft magnetic alloys substituted by transition elements (Ni, Co, V) have been studied by means of Mössbauer spectroscopy. The main subject of interest was the distribution of magnetic hyperfine field (MHF) in Ni substituted alloys. The increase of mean MHF was stated as a result of crystallization. It was found that changes of transition metal concentration cause different kind of hyperfine field evolution, attributed both to the alteration in grain structure and other effects related to crystallization processes.     

Hyperfine fields in the half-metallic ferromagnet NiMnSb

Using nuclear spin-echo technique the distribution of the hyperfine fields in the half-metallic ferromagnet NiMnSb is measured. An asymmetry of the NMR spectra is observed. At liquid helium temperatures a resolution of the⁵⁵Mn and¹²¹Sb resonance lines is obtained in zero applied field.