Spin and temperature dependence of the magnetic hyperfine field of Cd in the rare earth-aluminum Lavesphase compounds RAl2

Perturbed angular correlation spectroscopy has been used to investigate the combined magnetic and electric hyperfine interaction of the probe nucleus ¹¹¹Cd in ferromagnetically ordered rare earth (R)-dialuminides RAl₂ as a function of temperature for the rare earth constituents R=Pr, Nd, Sm, Eu, Tb, Dy, Ho and Er. In compounds with two magnetically non-equivalent Al sites (R=Sm, Tb, Ho, Er), the magnetic hyperfine field was found to be strongly anisotropic. This anisotropy is much greater than the anisotropic dipolar fields, suggesting a contribution of the anisotropic 4f-electron density to magnetic hyperfine field at the closed-shell probe nucleus. The spin dependence of the magnetic hyperfine field reflects a decrease of the effective exchange parameter of the indirect coupling with increasing R atomic number. For the compounds with the R constituents R=Pr, Nd, Tb, Dy and Ho the parameters B₄, B₆ of the interaction of the crystal field interaction have been determined from the temperature dependence of the magnetic hyperfine field. The ¹¹¹Cd PAC spectrum of EuAl₂ at 9 K confirms the antiferromagnetic structure of this compound.     

The magnetic hyperfine field at Cd nuclei in the rare earth ferromagnets Gd,Tb, Dy,Ho, Er and Tm

The time differential perturbed angular correlation technique has been used to study the combined magnetic and electric hyperfine interactions at the site of a¹¹¹Cd impurity in the rare earth ferromagnets Gd, Tb, Dy, Ho, Er, and Tm at 4.2 °K. The following magnetic hyperfine fields at the site of¹¹¹Cd have been found: ¦H _hf ¦=340(7) kG in Gd, 275 (5) kG in Tb, 221 (4) kG in Dy, 116 (3) kG in Er and 60 (6) kG in Tm. In Ho two magnetically different sites were observed with magnetic fields of 159 (3) and 139 (3) kG. Both sites are equally populated. The coupling constantJ _5f of the conduction electron-4f interaction has been calculated for the different rare earth metals from the measured hyperfine fields by means of the RKKY theory.     

Study of combined magnetic and electric hyperfine interactions for cadmium in Dysprosium by time differential perturbed angular correlations

The combined magnetic and electric hyperfine interaction at the site of a¹¹¹Cd impurity in magnetically ordered Dysprosium has been investigated as a function of temperature by time differential perturbed angular correlation measurements. Three different phases have been found in metallic Dy with transition temperatures of 85 and 179 °K in agreement with the results of bulk material measurements. In the paramagnetic phase above 179 °K a pure electric quadrupole interaction has been observed. The various contributions to the electric fieldgradient are analyzed and it is shown, that the dominant contribution comes from the conduction electrons. In the ferromagnetic phase which extends from 0 to 85 °K the magnetic hyperfine field at the site of¹¹¹Cd has the same temperature dependence as the spontaneous magnetization. The value of the hyperfine field at 4.2 °K is ¦H _eff¦=(221 ± 4) kG. At 85 °K a transition to the antiferromagnetic phase of Dy occurs, which shows a hysteresis of the transition temperature. In the antiferromagnetic phase the temperature dependence of the hyperfine field deviates considerably from the magnetization curve. It is suggested that this deviation might be due to a temperature dependence of thes-f exchange interaction.     

Nuclear magnetic resonance of175Hf oriented in iron

Nuclear magnetic resonance of¹⁷⁵Hf oriented at low temperature in iron has been observed with a sample prepared by ion implantation. The centre frequency of the broad resonance line isv _L (B _ext = 0)=138.53(36)MHz. Possible origins of the large inhomogeneous line width of FWHM=11.0(1.1) MHz are discussed. A comparison with model calculations for combined magnetic and electric hyperfine interaction indicates that the centre frequency may be interpreted as the magnetic interaction frequency for¹⁷⁵Hf in unperturbed substitutional sites of the host iron. With theg-factor of¹⁷⁵Hf from literature the magnetic hyperfine field of Hf in Fe is derived asB _hf=?64.9(9.3) T fitting well into systematics.     

Level crossing in hyperfine interactions studied by perturbed γ-γ angular correlations

The technique of differential γ-γ angular correlation measurements has been applied to an investigation of the hyperfine interactions in the 482 keV level of¹⁸¹Ta. The activity was embedded in the lattice of a hafnium single crystal. The investigation of the quadrupole interaction gave for the electric interaction frequencyω ₀=(313±4) MHz. The electric field gradient was found to be axially symmetric, the asymmetry parameter beingη<0.1. Furthermore the combined collinear magnetic dipole and electric quadrupole interaction was studied. The angular correlation was investigated as a function of the strength of the external magnetic field by integral as well as time differential measurements. The integral anisotropy as function of the magnetic field has the shape of a resonance curve. The maximum was observed at a magnetic field ofB _res=(24.2±0.5)kG.     

Hyperfine interactions in intermetallic rare earth-gallium compounds studied by 111 Cd PAC

Magnetic and electric hyperfine interaction of the nuclear probe ¹¹¹In/¹¹¹Cd in intermetallic compounds of the rare earth-gallium system have been investigated by perturbed angular correlation (PAC) spectroscopy. The PAC measurements, supported by X-ray diffraction, provide evidence for a marked phase preference of ¹¹¹In for hexagonal RGa₂ over orthorhombic RGa and of RGa₃ with the L12 structure over RGa₂. In the case of SmGa₂, the magnetic hyperfine field B_hf, the electric quadrupole interaction and the angle β between B_hf and the symmetry axis of the electric field gradient have been determined as a function of temperature. The angle β?=?0 is consistent with the results of previous magnetization studies. Up to T?≤?17 K the magnetic hyperfine field has a constant value of B_hf?=?3.0(2) T. The rapid decrease at higher T gives the impression of a first-order transition with an order temperature of T_N?=?19.5 K. In the RKKY model of indirect 4f interaction the ratio T_C/B_hf(0) is a measure of the coupling constant. For ¹¹¹Cd:SmGa₂ (T_C/B_hf(0)~6.5 K/T) this ratio is significantly smaller than for the same probe in other R intermetallics (SmAl₂ ~9.5 K/T, Sm₂In ~13.5 K/T).     

Magnetic hyperfine fields at Gd and in sites in GdPdIn compound

Perturbed gamma-gamma angular correlation (PAC) technique was used to measure the hyperfine interactions in the intermetallic compound GdPdIn using ¹¹¹In→ ¹¹¹Cd and ¹⁴⁰La→ ¹⁴⁰Ce probe nuclei at the In and Gd sites, respectively. The PAC results for ¹¹¹Cd show two well-defined electric quadrupole frequencies above T _C assigned to probes occupying Gd and In sites, with ~50% of site occupation each. The fraction corresponding to In sites increases with temperature reaching 95% around 500 K. Below T _C the measurements for ¹¹¹Cd probe showed combined electric quadrupole plus magnetic dipole interaction with sharp increase in the magnetic field below around 80 K. A pure magnetic interaction with lower hyperfine field values was observed at the Gd sites occupied by ¹⁴⁰Ce below 100 K.     

119Sn Mössbauer study of the CePtSn Kondo compound

We have measured¹¹⁹Sn Mössbauer spectra of the CePtSn and (Ce_0.9La_0.1)NiSn compounds in the range from 1.5 to 293 K. In CePtSn, the spectra observed above 8 K are well explained by an electric quadrupole interaction. The spectral shape changes below 8 K due to the presence of a magnetic hyperfine field produced by the ordering of the Ce magnetic moments. We have analyzed these spectra assuming an incommensurate magnetic structure. The temperature dependence of the magnetic hyperfine field matches with anS=1/2 mean field curve with a step at 5 K. In (Ce_0.9La_0.1)NiSn, no magnetic order exists down to 1.5 K.     

Hyperfine Fields of 181Ta in UFe4Al8

The γ–γ Perturbed Angular Correlation technique was used to study the hyperfine interaction of ¹⁸¹Ta at the Hf site(s) in UFe₄Al₈ at room temperature and 12 K. The data at room temperature are well described by two electric field gradients, while at low temperature two combined hyperfine interactions have to be considered, one with the magnetic hyperfine field collinear with the c-axis and another with the magnetic hyperfine field in the basal plane. The results are compared with previous Mössbauer and neutron diffraction experiments and the lattice site of Hf is discussed.

     

Mössbauer study of the magnetic and electric hyperfine interaction of dilute57Fe impurities in rare earth metal hosts

The hyperfine interaction of dilute⁵⁷Fe in the rare earth (RE)metals Gd to Lu was investigated by Mössbauer measurements with⁵⁷Co doped RE sources. In all hosts well split, 2-lines spectra were observed at room temperature, with slight asymmetries of the line intensities in some cases. The quadrupole splitting eQV_zz/2 increases from 0.29 mm/sec for Gd to 0.50 mm/sec for Tb, and decreases by less than 10 % between Tb and Lu. Only about 10 % of the corresponding electric fieldgradient (EFG) can be accounted for by the ionic EFG on a substitutional RE site. The temperature dependence of the EFG was measured in the case of Tb. No variation within 3 percent was found between 300 K and 700 K. Measurements of the magnetic hyperfine interaction at low temperatures were carried out in Tb. The saturation field of⁵⁷Fe in this host is H_hf(FeTb;4.2 K)=25(2) KOe. The temperature dependence of the magnetic hyperfine field does not follow the host magnetization (T_c=220K) but vanishes at about 80 K. Similar anomalies of H_hf(T) have previously been observed for other transition element impurities in the RE ferromagnets.     

The temperature dependence of the magnetic hyperfine field of tantalum in dysprosium

The temperature dependence of the magnetic and electric hyperfine interactions at the site of ¹⁸¹Ta impurities in polycrystalline Dy has been measured between 4.2 and 178 K using the time differential perturbed angular correlation technique. The value of the magnetic hyperfine field at 4.2 K is: |H_hf(TaDy)| = 212(9) kG The temperature dependence of the magnetic hyperfine field follows closely the prediction of the molecular field model.     

Möβbauereffekt am61Ni

With the Möβbauer technique the hyperfine splittings of the 67.4 keV transition of⁶¹Ni in compounds and alloys have been studied. A magnetic momentΜ _61,4=+ (0,477 ±0.031)Μ _n of the first excited state has been determined. From isomer shifts an order of magnitude estimate of the change in radius ofδ〈r ²〉/〈r ²〉=?6· 10^?4 can be inferred. The electric quadrupole interaction of the first excited level has been observed in Ni-J-boracite. In addition the hyperfine fields at the nickel sites were determined in the alloy series Ni _x Fe_1-x (0≦x≦1). In one case an effect of polarization in a high external field is measured. The fields in some rare earth (RE) intermetallic compounds of the form RENi₂ were deduced from linewidth to be less than 15 kOe.     

Nuclear hyperfine effects in Dy(OH)3

A crystal field analysis of the experimental data on magnetic, optical and thermal properties of Dy(OH)₃ single crystals have been published The nuclear hyperfine properties of Dy³⁺ in Dy(OH)₃ were studied using a crystal field thus obtained. The hyperfine spectra were computed from 4–20 K with a minimum number of approximations. Under a weak crystal field, the lowest electronic level is a Kramers' doublet For this highly anisotropic crystal, the magnetic hyperfine and the quadrupole interactions are both prominent The quadrupole interaction energy is temperature dependent The value of the magnetic Sternheimer factor R_hf/R is determined to be 0 14 The observed specific heat $\text{C}{}_{\mathrm{hf}}\text{R}$ arising from hyperfine interactions have been explained satisfactorily A maxima is expected at 21 mK.     

Magnetic hyperfine field changes at different Fe-sites below spin reorientation temperature in polycrystalline untextured Nd2Fe14B

In order to estimate the magnetic hyperfine fields at different sites, an analysis of the Mössbauer spectra of polycrystalline untextured Nd₂Fe₁₄B, recorded at temperatures above and below the spin reorientation temperature T_s(≈150 K), has been made. Here the signs of the electric field gradients (efg) and the principal axes of efg components have been constrained according to the available crystal structure information below and above T_s. It has been observed that the magnetic hyperfine field changes are similar to the recently reported magnetic moment changes at different sites in an isostructural alloy Tm₂Fe₁₄B.     

The temperature dependence of the hyperfine interaction of the transition metal osmium in a gadolinium host matrix

Integral perturbed angular correlations of the 931-155keVγγ-cascade of¹⁸⁸Os in Gd have been measured. With this technique the combined magnetic and electric hyperfine interaction of the 155 keV level of¹⁸⁸Os as an impurity in a Gd host has been studied as a function of temperature. The result for the electric field gradient of Os in Gd at 300 K is: $$\left| {V_{zz} \left( {Os:\underline {Gd} } \right)} \right| = \left( {12.8_{ - 1.9}^{ + 3.1} } \right) \cdot 10^{17} {V \mathord{\left/ {\vphantom {V {cm^2 }}} \right. \kern-\nulldelimiterspace} {cm^2 }}.$$ For the magnetic hyperfine field at 4.2 K the value $$H_{hf} \left( {Os:\underline {Gd} } \right) = - 134\left( {26} \right)kG$$ was obtained. Sign and magnitude of the magnetic hyperfine field suggest the existence of a localized moment of about ?0.4 µ _B at the site of Os in Gd. With increasing temperature the magnetic hyperfine field decreases much stronger than the magnetization of the host. Possible explanations for this anomalous temperature dependence are discussed.     

Crystal field effects on the magnetic and hyperfine properties of Yb3+ in ytterbium ethylsulfate

The magnetic and hyperfine properties of Yb³⁺ in Yb(C₂H₅SO₄)₃·.9H₂O have been studied using the crystal field (CF) obtained from an analysis of the observed absorption spectra of the crystal. The principal magnetic susceptibilities are in reasonable agreement with those observed both at liquid oxygen and liquid helium temperatures. The observed reversal of magnetic anisotropy at 17.7K is also corroborated. The Schottky heat capacity shows a peak at around 40K. The magnetic hyperfine field due to the 4f electrons at the nucleus is found to be 1.79MG. The hyperfine heat capacity C_N has a Schottky anomaly at about 25 mK. Above 1K, in the liquid helium range, C_N follows a simple T^?2 law as observed by Cooke et al. It is concluded that in the rare earth ethylsulfates a single suitable CF gives a good account of the various properties from room temperature down to liquid He temperatures.     

Angular anisotropy of the alpha emission from241Am nuclei oriented by magnetic hyperfine interaction

Angular anisotropy of the alpha emission from²⁴¹Am nuclei oriented by magnetic hyperfine interaction at low temperatures in ZrFe₂-host have been observed. The sign of the anisotropy indicates unambiguously the enhanced alpha emission from the poles of²⁴¹Am nucleus. The value of magnetic hyperfine field on²⁴¹Am nuclei have been estimated from the temperature dependence of the anisotropy.     

Mössbauer study of the hyperfine and magnetic properties of organo-di-iron electron reservoirs containing a polyaromatic bridging ligand

Organo di-iron electron reservoirs Fe(CP^*)₂(Ar) ⁿ⁺ withn=2, 1, 0, where Cp^* is C₅(CH₃)₅ and where Ar are the following bridges: biphenyl, dihydrophenanthrene, triphenylene, have been studied by Mössbauer spectroscopy in the solid state. Complexes withn=2, with 36e^? in the coordination spheres of the metals, exhibit the usual diamagnetic behaviour of 18e^?, Fe^II mono-iron systems. Complexes withn=1, 37e^?, are delocalized mixed valence (Fe^IIFe^I) with a spin 1/2; the magnetic hyperfine interaction, measured under an external field, shows equal delocalization of the 37^th e^? on the two iron centers and the two bridging carbon atoms of the biphenylene. Complexes withn=0, formally with 38e^?, have a practically temperature-independent quadrupole splitting, and isomer shift values which constrast with the expected behaviour of independent Fe^I, 19e^? centers. This indicates that the 37^th and 38^th electrons are mostly located on the polyaromatic bridge. Spectra obtained in an external field show a negligible magnetic hyperfine interaction and support this conclusion. In the case of biphenyl and dihydrophenanthrene bridges, this electron localization can be related to a strong intramolecular chemical coupling, evidenced by other spectroscopic and X-ray data [1].     

Der Einfluß eines elektrischen Feldes auf Level-Crossings des 6d 2 D 3/2-Zustands im TlI-Spektrum

The influence of an electric field on the energy levels of the 6d²D_3/2-state in the Tl I-spectrum was studied by measuring the shifts of level crossing signals relative to their magnetic field positions. The following values of the magnetic hyperfine constantA and the Stark parameterβ were deduced: ¦A¦=42(2) Mc/sec · g_J/0.8, ¦β¦=0.12(1) Mc/sec/(kV/cm)² · g_J/0.8 and A/β>0. Assuming that the main part of the energy shifts are caused by admixtures of the 7p²P-states the sign of the Stark parameterβ and —from the measured ratio A/β>0 —the sign of theA-factor should be negative. For electric field strength E?30 kV/cm the energy shifts of the 6d²D_3/2state are considerably greater than the hyperfine structure splitting. Therefore the case of decoupled hyperfine structure is considered.     

Mixed hyperfine interactions and defect structure in magnesiowüstite Fe−Mg−O

An analysis of the 4.2 K spectra of Fe_xO (x∼0.91) and (Fe_0.4Mg_0.6)_xO is presented in which it is considered that because of the large electric field gradient at Fe²⁺ defect sites, the spectra cannot be described by Lorentzian sextets. It is assumed that the magnetic hyperfine field vector is oriented at random in the coordinate system defined by the EFG main axis, and that the EFG coordinate system is also distributed randomly. The simplifying assumption of the asymmetry parameter η=0 allows an analytical formula to be used to describe the complex spectra. Distributions of both magnetic hyperfine field and quadrupole interaction were progressively refined resulting in reasonable fits to the spectra with the main features being well reproduced. The magnetic hyperfine field distribution is rather broad with several features present while distinct values were obtained in the distribution of quadrupole interactions. These latter values are considered to correspond to the defect configurations around the Fe²⁺ sites. The distribution of hyperfine fields is considered to reflect the varying strengths of superexchange due to the high defect concentration as well as the effects of magnetic relaxation.