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.     

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.     

Calculating the distribution of transferred hyperfine fields at the Sn site in tetragonal CeScSi-type RMgSn compounds

We present a simple model based on an isotropic transferred hyperfine field to calculate the transferred hyperfine field distribution at the Sn site of the antiferromagnetic CeScSi-type RMgSn compounds. The calculations correctly reproduce the main features observed in NdMgSn, TbMgSn, DyMgSn, HoMgSn, and ErMgSn. The transferred hyperfine field distributions are remarkably sensitive to the complex magnetic structures in the RMgSn compounds, and can be used to discriminate between competing magnetic structure models.     

Magnetic properties, Mössbauer effect and first principle calculations study of laves phase HfFe2

The magnetic properties and hyperfine interaction parameters for Laves phase HfFe₂ with C14 type structure are studied using SQUID magnetometer and M?ssbauer measurement. The saturation magnetization, remanent magnetization, coercive field, magnetic moment per unit formula and the hyperfine magnetic field at Fe site are reported. In addition, a detailed theoretical study of the electronic structure and hyperfine magnetic fields of the two possible HfFe₂ structures, C15 and C14, is presented. Using the full-potential linearized augmented plane wave (FP-LAPW) method as implemented in the WIEN 97 package, the equilibrium volume, bulk moduli, magnetic moments and hyperfine magnetic fields for the two structures are calculated. The obtained results are compared with the measured data.     

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.

     

基于超精细结构下的激光光泵铯磁力仪的理论研究

激光光泵碱金属磁力仪具有很高的灵敏度,测量范围可以从地球磁场到生物磁场。给出了铯（Cs）光泵磁力仪的理论分析和系统设计以及磁场梯度测量原理,铯原子能级在I—J耦合时形成超精细结构,在外磁场的作用下超精细结构进一步产生塞曼分裂形成塞曼子能级,利用激光泵浦和射频磁场能够使电子在超精细结构中进行能级跃迁,产生光磁双共振的结果,最终通过共振频率就能够达到精确测量外磁场的目的。     

Temperature dependence of hyperfine fields at impurities in metals

A model is developed to account for the spin-densities at dilute non- magnetic impurities in ferromagnetic metals which exhibits a non-linear dependence of the hyperfine field on the host magnetization. The theory is used to calculate the hyperfine field at an interstitial positive muon in gadolinium.     

Magnetism of ultrathin Cr layers and Fe/Cr multilayers studied by 119Sn probes

Cr/Sn and Fe/Cr/Sn/Cr multilayers, where monatomic Sn layers are embedded in Cr layers and Fe/Cr multilayers respectively, were prepared by means of ultrahigh-vacuum deposition technique, and the magnetic hyperfine field induced at the ¹¹⁹Sn nuclear sites was examined using conversion electron Mössbauer spectroscopy. The magnetic structures of the Cr layers are inferred from the size and direction of the magnetic hyperfine field transferred at the Sn sites.     

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.     

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.     

Anomalous temperature dependence of the magnetic hyperfine field of99Ru in gadolinium

The magnetic hyperfine field at dilute⁹⁹Ru impurities in ferromagnetic Gd has been investigated as a function of temperature by time differential perturbed angular correlation (TDPAC) measurements. The saturation field at 11 K is H_hf(Ru Gd)=51(2) KOe. This value fits well into the systematics of 4d impurity hyperfine fields in Gd. The magnetic hyperfine field of RuGd does not follow the magnetization of the host (T_c=290 K) but vanishes abruptly at about 70 K. A similar behaviour has previously been observed for the 5d impurity Os in Gd. From first TDPAC measurements of the hyperfine interactions in the intermetallic phases of the Ru-Sc system it can not be completely ruled out that the observed collapse of the hyperfine field at 70 K is due to the formation of the intermetallic compound RuGd₃.This work has been supported by the Minister für Wissenschaft und Forschung des Landes Nordrhein-Westfalen     

Study of Hyperfine Fields in CeIn3 by Electronic Structure Calculations

Electronic band structure calculations for the CeIn₃ compound utilizing the full potential linearized augmented plane waves method were performed with the aim to compute the hyperfine fields acting on Ce and In atoms. The latter are found to be in reasonable agreement with the values measured at low temperatures. The 4f orbital contribution dominates the magnetic hyperfine field at Ce ions while the contact field is negligible due to an almost complete cancellation of valence and core contributions. A non-zero magnetic hyperfine field appears at In sites due to the spin-polarization of the 5p sub-shells through the hybridization with the extended 6s, 6p and 5d Ce states which, in turn, are spin-polarized by the Ce 4f states. No net magnetic moment at In is observed since the sum of its 5p sub-shell spins is zero. The 5p shell of In is responsible for the presence of an electric field gradient at In nuclei. This revised version was published online in September 2006 with corrections to the Cover Date.     

Investigations of “soft-landed” Cd surface atoms via nuclear methods: hyperfine-field sign determination

Using refined preparation techniques, cadmium guest atoms have been positioned at different sites on the surfaces of nickel crystals. The magnetic hyperfine fields and the electric field gradients at the Cd nuclei were measured by time-dependent perturbed angular correlation (TDPAC) spectroscopy of the emitted gamma radiations. By measuring the combined interactions, electric field gradients and magnetic hyperfine fields can be unambiguously attributed to each surface site. The signs of the magnetic hyperfine fields are determined by applying an external magnetic field and choosing the appropriate γ-ray detector configuration. The measured fields correlate with the number of neighbouring host atoms. Band structure calculations confirm this finding and predict magnetic fields for various sp elements from the band structure of the s-like conduction electrons. The quadrupolar interactions are manifestations of the balance in the occupation of the guest p-sublevels. These results provide new information on the structure and formation of electronic configurations of sp elements in different local environments and will contribute to understanding electronic effects on surfaces.     

Anomalous behaviour of the magnetic hyperfine field in the microcrystalline YFeO3-Fe2O3 mixed phase system

The magnetic hyperfine field in 120 Å YFeO₃ microcrystals just below the superparamagnetic blocking temperature, as well as at 5 K are found to be surprisingly low. In addition, the hyperfine field fails to saturate even at the lowest recorded temperature. The observed features cannot be understood in terms of either collective magnetic citations or superferromagnetism. A negative equivalent pressure arising out of decreased particle size is proposed as a plausible explanation.     

Temperature Dependence of the Magnetic Hyperfine Field at 140Ce on Gd Sites in GdAg Compound

Perturbed gamma–gamma angular correlation technique was used to measure the magnetic hyperfine field at Gd sites in the intermetallic compound GdAg using the ¹⁴⁰La→¹⁴⁰Ce nuclear probe. A major and well-defined magnetic interaction is observed at ¹⁴⁰Ce substituting Gd sites in GdAg below 130 K, corresponding to a ferromagnetic ordering of Gd moments. The temperature dependence of magnetic hyperfine field, however, shows a sharp deviation from an expected Brillouin-like behavior for temperatures below 75 K. This additional magnetic interaction is believed to result from the polarization of Ce spin moments induced by the magnetic field from Gd atoms.     

Spin-canting and transverse relaxation in maghemite nanoparticles and in tin-doped maghemite

The magnetic properties of maghemite nanoparticles and tin-doped maghemite have been studied by ⁵⁷Fe and ¹¹⁹Sn Mössbauer spectroscopy at temperatures from 6 to 300 K with and without applied magnetic fields. The low-temperature ⁵⁷Fe spectra of both samples, obtained in a field of 4 T, can be described in terms of A-site and B-site components with perfect ferrimagnetic order and a strongly canted component, which seems to have its main contribution from B-site ions. At higher temperatures, the components with strong canting are influenced by transverse relaxation, which results in significant line broadening, a reduction of the magnetic hyperfine splitting and a reduction in the relative areas of lines 2 and 5. The ¹¹⁹Sn spectra show a very broad distribution of magnetic hyperfine fields at low temperatures. When the sample was exposed to applied magnetic fields the distribution became narrower. The spectra show that the direction of the hyperfine field of a large fraction of the tin ions in maghemite is antiparallel to the applied field, but a minor fraction of the tin ions have canted hyperfine fields.     

Search for spin canting in gadolinium iron garnet using the Mössbauer technique

We have studied the hyperfine interactions in GdIG in zero external magnetic field using⁵⁷Fe Mössbauer spectroscopy. Measurements were made at temperatures between 80 K and 310 K, whereby the region around the compensation point at 286 K was emphasized. The easy axis of magnetization was determined to be [111] at all the chosen temperatures. No indications of spin canting were found. The hyperfine parameters vary smoothly with temperature. The temperature dependences of the magnetization of the sublattices are deduced from the magnetic hyperfine fields.     

Investigation of the magnetic hyperfine field at197Au impurities in the heavy Rare Earth metals

The Mössbauer effect has been used to measure the magnetic hyperfine field at the site of¹⁹⁷Au impurities in the heavy Rare Earth metals Gd, Tb, Dy, Ho and Er at 4.2 K. The magnetic hyperfine field decreases in a non linear way with decreasing spin of the Rare Earth host. For¹⁹⁷Au this decrease is stronger than for any other impurity investigated up to now. Possible reasons for this behaviour are discussed.     

Resolved magnetic hyperfine spectra of Au-Fe alloys as a function of temperature

We have analyzed the Mössbauer hyperfine magnetic spectra of six compositions of Au-Fe from 10.5 to 33 at.% Fe at temperatures ranging from 4.2 K to the magnetic ordering temperature of each alloy. Only two elementary magnetic hyperfine spectra are required to fit the data, each of which has its own value of the magnetic hyperfine field, quadrupole coupling, isomer shift, and relative intensities. The intensity ratios of the two spectra for a given Fe concentration are essentially invariant with temperature. These results point to the existence of two chemically different Fe sites with all the Fe atoms magnetically coupled over the entire temperature regime below the upper magnetic ordering temperature. A relatively sharp increase in the local moment commences at both Fe sites, at ∼ 17 at.% Fe and continues to increase with Fe concentration.     

Hyperfine Interactions of Pm in Nd and Gd Hosts

Time differential perturbed angular correlation measurements were carried out using short lived isomeric 5/2⁺ state in ¹⁴⁷Pm to investigate the magnetic and electric hyperfine interactions in Nd and Gd hosts at different temperatures. At 10 K the magnetic hyperfine fields at ¹⁴⁷Pm in Nd and Gd hosts are 361(42) kG and 256(30) kG, respectively, and are very low as compared to the free-ion value while the electric field gradients are of comparable order. The magnetic hyperfine field in Gd is constant with temperature. The results are interpreted in terms of singlet ground state of Pm ion. This revised version was published online in September 2006 with corrections to the Cover Date.