Temperature dependence of the hyperfine fields in NiFe2O4 and CuFe2O4 oxides

We report the temperature dependence of the magnetic properties of (Ni, Cu)Fe₂O₄ spinel oxides. Mössbauer spectra for NiFe₂O₄ at various temperatures (79 ≤?T?≤ 900 K) are well fitted by two sextets associated with ⁵⁷Fe nuclei at tetrahedral (A) and octahedral (B) sites. The Curie point T _C was deduced by zero velocity Mössbauer technique to be 873 ± 3 K. The hyperfine fields are observed to vary with temperature according to the equation $B_{\rm hf} (T)=B_{\rm hf} (0)[{1-(T/T_{\rm C})^n}]^{\beta_n}$ where n?=?1 (based on the Landau–Ginzburg theory) and n?=?2 (based on the Stoner theory). A systematic decrease of the Mössbauer spectrum shift with increasing temperature is observed.     

Production of spin polarized 12N through heavy ion reactions

The structural and magnetic properties of Ho substituted BiFeO₃ (BHFO) have been investigated using ⁵⁷Fe Mössbauer spectroscopy and X-Ray diffraction (XRD) as a function of temperature. The Mössbauer spectrum obtained at room temperature for the as-synthesized BHFO sample exhibits broadened features due to the hyperfine field distributions related to the local variation of the neighbourhood of Fe and the magnetic hyperfine splitting patterns are indicative of magnetic ordering, mostly probably screwed or slightly antiferromagnetic. The spectrum was fitted with two superimposed asymmetric sextets, with similar hyperfine magnetic fields of B_hf1 = 48.0(1) T and B_hf2 = 49.0(1) T, corresponding to rhombohedral BFO. The hyperfine fields of the magnetic components decreased systematically with increasing temperature to a ‘field distribution’ just below the Néel temperature, T_N ~ 600 K. At temperatures above 600 K, the spectral line associated with the Bi₂₅FeO₄₀ impurity phase dominates the spectra. This phase is confirmed by XRD measurements. From the temperature dependence of the site populations of the spectral components an average Debye temperature of θ _D = 240(80) K has been estimated.     

Hyperfine interactions for181Ta nuclei in the laves phases LuFe2 and GdFe2

The hyperfine magnetic fields for¹⁸¹Ta in the cubic (C 15) Laves phases LuFe₂ and GdFe₂ have been measured by the TDPAC method. At 300 K, B_hf=−20.5(4) T for LuFe₂ and +19.0(5) T and +10.2(4) T for the samples of GdFe₂, prepared at normal and high (7.7 GPa) pressure, have been obtained. Temperature dependence of these fields in the range 77–900 K has been also measured.     

Mössbauer spectroscopic study of FeAl1−x Co x

We report the results of a Mössbauer study of the alloy sytem FeAl_1?x Co _x forx ≥ 0.3 at temperatures down to 83 K. Magnetic splitting is observed forx ≥ 0.35 at all temperatures. However, forx=0.3, no splitting is observed at room temperature, and superparamagnetic behavior occurs at LN₂ temperature. The magnetically split spectra are fitted each with a distribution of hyperfine fields and the average hyperfine field \(\bar B_{hf} \) as a function of temperature is obtained. The variation of \(\bar B_{hf} \) withT is explained using the model of magnetic clusters with collective magnetic excitations from which the saturation hyperfine field and the magnetic anisotropy energy for these clusters are obtained. Also, the results are discussed using the model of random atomic distributions, and the agreement between the calculated and the experimentally obtained distributions of hyperfine fields is found improve asx increases.     

Radio-frequency Mössbauer spectra of the “easy”-plane type magnetic system (FeBO3)

The theory of formation of the RF Mössbauer spectra for the “easy”-plane type magnetics (FeBO₃) and for various types of RF field polarization is presented. Experiments using both linearly and circularly polarized external RF fields were carried out at different temperatures. At room temperature the experimental spectra for both cases are well described by switching hyperfine (hf) field model. At temperatures close to the Neel temperature (335 K), the spectra in the oscillating and rotating RF field were obtained and their forms are described by models of switching and rotating hf field, respectively.     

CEMS-study of the interface in Fe/Cr multilayer systems

The magnetic hyperfine (hf.) fields at the Fe/Cr interface were analyzed in epitaxial Fe/Cr thin film structures of (100)- and (110)-orientation with monolayer resolution by means of in-situ⁵⁷Fe Conversion Electron Mössbauer Spectroscopy (CEMS). The hf. field (300 K) in the 1st Fe-monolayer (ML) at the interface has been found to be strongly reduced to 22.0/20.9 T for (110)-/(100)-orientation, whereas the 2nd and 3rd ML reveal a slightly increased hf. field of 33.7 T as compared with the Fe-bulk value of 33.4 T. The temperature dependence of the hf. field at the interface shows aT ^3/2 spin wave law. The spin wave parameters are enlarged with respect to the bulk value indicating a reduced exchange interaction. A discontinuity in theT ^3/2-dependence is interpreted by the onset of magnetic order (Néel-temperature) of the Cr layers adjoining the⁵⁷Fe probe layer.     

High pressure-high field Mössbauer spectroscopy on pure and magnetically diluted europium-monochalcogenides

By simultaneously applying pressures up to 19.6 kbar and magnetic fields up to 13 T, the magnetic hyperfine (hf) fields of the Eu-monochalcogenides Eu X (X = S, Se, Te) and of the magnetic dilution system Eu_xSr_1-xS were studied using the ¹⁵¹Eu Mössbauer effect. Expected relations between the magnetic exchange interactions and the transferred hyperfine (thf) fields have been confirmed. The pressure dependence of the total hf field in the ferromagnetically saturated state of EuS, EuSe and EuTe has been shown to be due mainly to the pressure response of the thf field of the nearest Eu-neighbours. Except with EuO the thf fields of the next nearest Eu-neighbours and the Eu-core polarization field are nearly insensitive to pressure. The results are discussed in terms of the exchange model proposed by T. Kasuya.     

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.     

Effect of Mg2+ on the Magnetic Compensation of Lithium–Chromium Ferrite

PAC measurements on ¹¹¹In(¹¹¹Cd) implanted and thermally treated α-Fe have shown an indication for a cubic defect with the ¹¹¹Cd probe in the centre of it. The measured room temperature (R. T.) magnetic hyperfine fields are B _hf1 = −38.4(8) T for substitution and B _hf2 = +11.5(3) T for the cubic defect. Additionally, probes with pure quadrupole frequency distributions were observed, which are incorporated in surface contaminations.     

Phase Relations and EOS of ZrO 2 and HfO 2 Under High-temperature and High-pressure

The phase relations and equations of state of ZrO 2 and HfO 2 high-pressure polymorphs have been investigated by means of in situ observation using multi-anvil type high-pressure devices and synchrotron radiation. Baddeleyite (monoclinic ZrO 2 ) transforms to two distorted fluorite (CaF 2 )-type phases at 3-4 GPa depending on temperature: an orthorhombic phase, orthoI, below 600 °C and a tetragonal phase, which is one of the high-temperature forms of ZrO 2 , above 600 °C. Both orthoI and tetragonal phases then transform into another orthorhombic phase, orthoII, with a cotunnite (PbCl 2 )-type structure above 12.5 GPa and the phase boundary is almost independent of temperature. OrthoII is stable up to 1800 °C and 24 GPa. In case of HfO 2 , orthoI is stable from 4 to 14.5 GPa below 1250-1400 °C and transforms to the tetragonal phase above these temperatures. OrthoII of HfO 2 appears above 14.5 GPa and is stable up to 1800 °C at 21 GPa. The unit cell parameters and the volumes of these high-pressure phases have been determined as functions of pressure and temperature. The orthoI/tetragonal-to-orthoII transition of both ZrO 2 and HfO 2 is accompanied by about 9% volume decrease. The bulk moduli of orthoII calculated using Birch-Murnaghan's equations of state are 296 GPa and 312 GPa for ZrO 2 and HfO 2 , respectively. Since orthoII of both ZrO 2 and HfO 2 are quenchable to ambient conditions, these are candidates for super-hard materials.     

Field-induced suppression of the heavy-fermion state in YbRh2Si2

We report dc-magnetization measurements on YbRh2Si2 at temperatures down to 0.04 K, magnetic fields B< or =11.5 T, and under hydrostatic pressure P< or =1.3 GPa. At ambient pressure a kink at B* =9.9 T indicates a new type of field-induced transition from an itinerant to a localized 4f state. This transition is different from the metamagnetic transition observed in other heavy-fermion compounds, as here ferromagnetic rather than antiferromagnetic correlations dominate below B*. Hydrostatic pressure experiments reveal a clear correspondence of B* to the characteristic spin fluctuation temperature determined from specific heat.     

Collapse of 5 f -Electron Ferromagnetism in UPtAl Under High Pressures

UPtAl exhibits a ferromagnetic ordering of U magnetic moments at temperatures below T C =42.5 K. The magnetic-ordering transition is accompanied by an anomaly in the temperature dependence of electrical resistivity. This allows us to determine the value of Curie temperature from 𝜌 vs. T data that can be measured at very high pressures, at which reliable magnetization measurements are difficult. We report on resistivity measurements performed on an UPtAl single crystal under hydrostatic pressures p h 8 GPa. It was observed that the initial increase of T C with p becomes gradually reduced for p >2 GPa until the maximum T C value of , 48 K is reached between 4 and 6 GPa that is followed by a progressively developing downturn of the T C ( p ) curve. The latter result is attributed to the approaching collapse of the U 5 f -moment ferromagnetism. Low-temperature resistivity data point to a rapidly reduced magnetic anisotropy at highest pressures.     

A57Fe Mössbauer study of the high pressure hydride phases of Fe and Co

Hydrides of iron and cobalt prepared at pressures between 4.0 and 9.5 GPa were studied by⁵⁷Fe Mössbauer spectroscopy at 4.2 K. Iron hydride was found to be nearly stoichiometric FeH. The two iron sites in its dhep lattice have hyperfine fields of 33.8 and 28.8 T. Practically the same results were found for the deuteride. In hep ε-CoH_x, the hyperfine fields decrease with hydrogen content by about 6% betweenx=0 andx=0.5. In all studied hydrides the electron densities at the⁵⁷Fe nuclei are smaller than in the pure metals.     

57Fe Mössbauer study of SrFeO3 under ultra-high pressure

⁵⁷Fe Mössbauer absorption spectra under ultra-high pressure up to 53 GPa have been measured using a diamond anvil cell for SrFeO_2.97 which is one of the typical Fe⁴⁺ oxides having a cubic perovskite structure. External high pressure up to 53 GPa makes no indication of structural transformation and does not show any change in valence state of iron, however the Néel temperature of 131 K at 0 GP increases to 300 K and the⁵⁷Fe magnetic hyperfine field decreases from 32.9 T at 0 GPa and 6.5 K to 23.3 T at 53 GPa and 300 K.     

Observation of a second order magnetic phase transition in CsFeS2

Mössbauer experiments performed on CsFeS₂ at temperatures between 4.2 K and 300 K show that the orthorhombic high temperature phase undergoes a second order magnetic phase transition near 69 K, when the previously reported first order magnetic and structural transition to a triclinic modification near 75 K is suppressed by lattice defects or internal stresses. The saturation values of the hyperfine fields are 19.1 T for the triclinic and 15.5 and 14.1 T for the orthorhombic phase.     

Pressure evolution of the EFG and magnetic hyperfine field in the layered antiferromagnet FeI2

When FeI₂ is subjected to pressures of up to 20 GPa, a change of approximately 20% occurs in the unit cell volume.⁵⁷Fe Mössbauer spectroscopy (MS) in a diamondanvil cell has been used to monitor the pressure evolution of the hyperfine interaction parameters of this layered antiferromagnetic insulator. The pressure dependence of the quadrupole splittingQS at 296 K exhibits a maximum at 12 GPa and the saturation magnetic hyperfine fieldH ₀ increases from 7.4 T at ambient pressure to 12 T at 18 GPa. A qualitative analysis identifies the pressure evolution ofQS with changes in the trigonal component of the crystal field splitting. The pressure variation ofH ₀ is attributed to an increase in the average value of the 3d charge density distribution.     

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.     

Mössbauer Effect Study of Eu0.88Fe4Sb12 Skutterudite

The partly filled skutterudites Eu_0.88Fe₄Sb₁₂ were investigated by ⁵⁷Fe Mössbauer spectroscopy in the temperature range 4.2 K ≤ T ≤ 295 K and in external fields up to 13.5 T. The results favour a statistical distribution of Eu and voids in the Fe near neighbour shell. Below 82 ± 1 K magnetic order is present. Debye temperatures Θ _D = 460 ± 20 K and 165 ± 30 K were obtained for Fe with completely occupied Eu sites and for Fe with vacancies in the R-neighbour shell, respectively. The temperature dependence of the quadrupole splitting reflects the thermal expansion of the lattice. The induced hyperfine fields at 4.2 K are negative and differ by roughly a factor of two for the two Fe surroundings.     

Diamond anvil cell for measurements in high magnetic fields

Abstract

We constructed a diamond anvil cell for pressures up to 100 GPa in magnetic fields up to 12 T. The cell can be operated at any temperature between 10 and 350 K. Loading by condensation of gases is possible.     

Electrical resistivity and critical temperature of Bi-based High-TC superconductors to 1 GPa

Abstract

We have measured the electrical resistance R of a sintered, two-phase, high-T_C superconductor with the nominal composition BiSrCaCu₂O_x, as a function of T and p. We find d(lnR)/dp ? -0.06 GPa^?1 at 295 K, while dT_C/dp is 2.5 K/GPa for the phase with T_c ? 76 and 2 K/GPa for that with T_C ? 106 K.