Peculiarities of hyperfine field formation in systems with a complicated magnetic order

First principles calculations of the local magnetic moments and parameters of hyperfine interactions were performed for the systems Fe₅SiC and Fe _x Al_{1 ? x} , and for multilayer Fe9/Cr5. The efficiency of using the first principles calculations in combination with Mössbauer spectroscopy experiments is demonstrated.     

On the hyperfine magnetic field distribution at118Sn in spinel ferrites

The distributionP(H) of the hyperfine magnetic fieldsH at the¹¹⁹Sn nucleus in spinel ferrites can be reliably obtained by means of the decomposition method [10] adapted for this case. To interpretP(H), a polynomial model is proposed which accounts for the contribution inH of the magnetic ions in both octahedral (B) and tetrahedral (A) sites.     

The temperature dependence of the57Fe hyperfine magnetic field distribution in Fe-Ni

The temperature dependence of the⁵⁷Fe hyperfine magnetic field (hmf) in Fe-Ni is stronger than the temperature dependence of the⁵⁷Fe hmf in pure Fe. By analyzing the shape of the⁵⁷Fe hmf distribution, and with the help of experiments with Si in Fe-Ni, we deduce that this anomalous temperature dependence originates from a large thermal sensitivity of the magnetic moments at those Fe atoms with more Ni nearest neighbors. A strong temperature dependence of the recoilfree fraction was also observed in Fe-Ni alloys. We suggest that a large mean square thermal displacement of Fe atoms in Fe-Ni is the cause of the anomalous temperature dependence.     

The supertransferred magnetic hyperfine field in chromium-iron tellurates

The broadening of the¹²⁵Te Mössbauer spectral absorption with decreasing temperature in compounds of the type Cr_2?x Fe _x Te _x TeO₆ (x=0.4, 0.8, 1.0, 1.5) is attributed to the presence of a supertransferred magnetic hyperfine field. The origin of the supertransferred magnetic hyperfine field, which is not observed in antiferromagnetically ordered Cr₂TeO₆ or Fe₂TeO₆, is associated with an imbalance in the magnetic exchange interaction at tellurium which results from the replacement of chromium by iron.     

The magnetic hyperfine field at140Ce in nickel

The hyperfine interaction of¹⁴⁰Ce in nickel has been investigated by the time-differential perturbed-angular-correlation technique (TDPAC). The probe was produced by isotope separator implantation of the fission product¹⁴⁰Xe, the ^- decay chain of which finally populates excited states of¹⁴⁰Ce.Different spin rotation spectra were observed before and after an 8 h annealing at 415°C. The analysis of the spectra led to the conclusion that the Ce ions were in the diamagnetic 4⁺ state. The dominant contributions to the hyperfine interaction are two different magnetic hyperfine fields: |H _hf ¹|=385±7 kOe and |H _hf ²|=276±12 kOe.H _hf ¹ disappears after annealing. The fraction of nuclei which observeH _hf ² is increased by the annealing procedure from 16% to 75%. It is assumed thatH _hf ¹ is the hyperfine field of CeNi in an unperturbed substitutional site andH _hf ² is attributed to Ce ions which have trapped a single vacancy.     

The hyperfine magnetic field of172Yb in Fe and Ni

Time-differential perturbed angular correlation measurements have been performed on the 91–1095 keV -ray cascade emitted by¹⁷²Yb nuclei in the capture decay of¹⁷²Lu, using implanted sources of¹⁷²Lu in Fe and Ni. From these measurements hyperfine field valuesB(YbFe)=–1253±83 kG andB(YbNi)=–143±12 kG follow at room temperature. From the modulation amplitude of the spectra it follows that only about 20% of the ytterbium nuclei participate in the precession.     

External field influence on the magnetic hyperfine fields in alpha-manganese

We report a study on the magnetic nature of the two hyperfine fields observed in α-Mn by PAC. Both of the field sites behave completely differently under the influence of an external field ranging from zero to 4 Tesla. The so-called high field originates from the local moment antiferromagnetism of the matrix, while the low field should be associated with an itinerant magnetism.     

The magnetic hyperfine field at Hg impurities in ferromagnetic Gd

The magnetic hyperfine field at dilute Hg impurities in Gd has been investigated by the conversion electron (e ^–)--time differential perturbed angular correlation (TDPAC) technique. The radioactivities^197m Hg and¹⁹⁹Tl were implanted into Gd foils by means of an isotope separator. TDPAC measurements were performed with the 165 keV-L-conversion electron—134 keV--cascade of¹⁹⁷Hg at different temperatures and with the 334 keV--158 keV-K-conversion electron cascade of¹⁹⁹Hg at 200 K.The regular site occupation probabilities were found to be 15(3)% for an annealed^197m HgGd sample and 29(5)% in unannealed¹⁹⁹TlGd samples.From the magnetic hyperfine interaction frequencies measured for the regular sites at 200 K the magnetic hyperfine fields |H _hf(¹⁹⁷HgGd; 200 K)|=256(13) kG and |H _hf(¹⁹⁹HgGd; 200 K)|=267(7) kG were deduced.On leave from the University of Lisboa, Portugal     

First-principles studies of the magnetic hyperfine field in Fe multilayers

A detailed theoretical study of the magnetic moments and magnetic hyperfine fields in several Fe multilayers (Fe fcc(001)/5X fcc(001), X=Cu and Ag, and Fe bcc(001)/5X fcc(001), X=Ag and Au) as well as in bulk Fe is presented. The calculations have been performed using the spin-polarized, relativistic linear muffin-tin orbital (SPR-LMTO) method of band structure calculation. Therefore, not only the contribution to the hyperfine fields due to the conventional Fermi contact interaction but also due to the spin dipolar and orbital contributions induced by the crystal field and by spin-orbit coupling are accounted for. To decompose the hyperfine field of non-s-electrons into these contributions it has been assumed that they are proportional to the corresponding so-called magnetic dipole moment and the orbital magnetic moment, respectively. In contrast to previous results for pure metals and alloys not only the orbital but also the spin dipolar hyperfine field was found to be non-negligible. The anisotropy of the hyperfine field determined by calculations for in-plane and perpendicular orientation of the magnetisation was found to be very pronounced and closely connected with the corresponding anisotropy of the magnetic dipole moment and the orbital moment.     

On the effective magnetic hyperfine field in naturally occuring goethite

The asymmetric broadening of the Mössbauer absorption lines frequently observed in goethite is shown to be due to relaxation effects in the surface layers rather than being an intrinsic property of this material. The particle size determines entirely the shape of the observed M-spectrum at room temperature which can vary from a magnetically ordered spectrum to a pure paramagnetic one.     

Local magnetic moment and hyperfine field in hydrogenated iron and iron-vanadium alloy

The local magnetic moment and hyperfine fieldB _hf at Fe and V sites in hydrogenated iron and iron-vanadium were calculated using the discrete variational method. The variations in andB _hf with H occupation of the octahedral (O) site were considered. It was found that when H occupies the O site neighbouring an Fe atom, both local moment and hyperfine field at this atom decrease linearly with increasing number of H atoms. The rate of decrease is larger for Fe in iron as compared to iron in vanadium. On the other hand, when H resides at an O site next neighbouring an Fe atom, whether in iron metal or in iron-vanadium, the Fe magnetic moment increases slowly, while the hyperfine field remains almost constant. The V moment in iron, which is negative (–0.83 _B ), becomes less negative (–0.30 _B) as H occupies the neighboring O sites, whereas slight changes occur (–0.88 _B) when H is at the next neighbouring O site. The net effect of H on Fe in iron is to decrease the average magnetic moment at a rate of 1.2 _B per H/Fe for low H content. On the other hand, the average Fe moment in an iron-vanadium alloy increases if H resides at O sites which are immediate neigbours of V and next neighbours of Fe. This may explain the development of a magnetic state on hydrogenation of Fe-V alloys, which is exhibited by the specific heat and susceptibility measurements. The changes in the isomer shift were found to agree with experimental trends.     

Inhomogenous hyperfine field distribution versus relaxation broadening in Fe-Ni invar

Using an alloy sample of Fe-33.6at%Ni at 300 K we show experimentally that the shape of the Mössbauer spectra is determined by an inhomogenous hyperfine field distribution and not by relaxation broadening. Following Vince /1/ redundant information has been removed from the spectra. The Wegener-Ritter relaxation theory /2,3/ was used to find arguments for the choice of temperature.     

The hyperfine magnetic field in Fe-Hf amorphous alloys at low temperatures

The time-differential perturbed angular correlation (TDPAC) technique was applied to study the hyperfine magnetic field in amorphous Fe-Hf alloys with different Hf contents at low temperatures. The Hf content was changed from 20 to 40 at% andthe TDPAC measurements were carried out in the temperature range from room temperature down to 18 K. Very strong perturbations were observed for the specimen of low Hf content at low temperatures due to a strong hyperfine magnetic field. The characteristic feature observed by the present study is that the temperature dependence of the perturbation for the Fe-30 at% Hf source is quite different from that for other sources, indicating a maximum around 80 K. Discussions have been given on the explanation of the experimental results in the light of magnetic properties of these alloys.     

Relation between the critical magnetic field and energy gap of a superconductor

A dynamical model is used to derive a relationship between the slope of the reduced critical magnetic field for T = T_c and the energy gap Δ_o of a superconductor. Good agreement with the relation |dh/dt|_t=1 = Δ_o/ k T_c proposed by Toxen is found up to Δ_o/k T_c = 4.     

海洋与大气光学传递函数与自然光辐射场的关系

本文根据海洋辐射传递的变换模型确定海水光学传递函数与海中自然光场辐亮度分布的关系,分析了海中辐射场角频率谱变化。提出一种新的估算海中辐射场分布和测定海水光学传递函数的方法。这种方法完全适用于大气。     

A nonfrustrated magnetoelectric with incommensurate magnetic order in magnetic field

We discuss a model nonfrustrated magnetoelectric in which a sufficiently strong magnetoelectric coupling produces an incommensurate magnetic order leading to ferroelectricity. Properties of the magnetoelectric in the magnetic field directed perpendicular to the wave vector describing the spin helix are considered in detail. Analysis of the classical energy shows that in contrast to the naive expectation, the onset of ferroelectricity occurs at a field H _c1 that is lower than the saturation field H _c2. We have H _c1 = H _c2 at large enough magnetoelectric coupling. We show that at H = 0, ferroelectricity occurs at T = T _FE < T _N. A qualitative discussion of the phase diagram in the H-T plane is presented within the mean-field approach. The text was submitted by the author in English.     

Temperature dependence of the hyperfine field distribution in an amorphous ferromagnet

The rapid temperature decrease in magnetization characteristic of amorphous alloys is often attributed to short-range exchange interactions. The temperature dependence of the hyperfine field distribution in an amorphous Fe₈₀B₂₀ (METGLAS 2605) measured by Mössbauer technique disagrees with such an explanation. It is shown that for this alloy each magnetic moment follows the same magnetization curve, i.e. the temperature dependence of the magnetization is determined by long-range correlations.