EPR Spectroscopy of Impurity Thulium Ions in Yttrium Orthosilicate Single Crystals

The frequency-field and orientation dependences of the electron paramagnetic resonance (EPR) spectra are measured for impurity Tm³⁺ ions in yttrium orthosilicate (Y₂SiO₅) single crystals by stationary EPR spectroscopy in the frequency range of 50–100 GHz at 4.2 K. The position of the impurity ion in the crystal lattice and its magnetic characteristics are determined. The temperature dependences of the spin–lattice and phase relaxation times are measured by pulse EPR methods in the temperature range of 5–15 K and the high efficiency of the direct single-phonon mechanism of spin–lattice relaxation is established. This greatly shortens the spin–lattice relaxation time at low temperatures and makes impurity Tm³⁺ ions in Y₂SiO₅ a promising basis for the implementation of high-speed quantum memory based on rare-earth ions in dielectric crystals.     

Valence and magnetic states of impurity iron ions in the La<Subscript>0.75</Subscript>Sr<Subscript>0.25</Subscript>Co<Subscript>0.98</Subscript>Fe<Subscript>0.02</Subscript>O<Subscript>3</Subscript> perovskite

The local magnetic and valence states of impurity iron ions in the rhombohedral La_0.75Sr_0.25Co_0.98 ⁵⁷Fe_0.02O₃ perovskite were studied using Mössbauer spectroscopy in the temperature range 87–293 K. The Mössbauer spectra are described by a single doublet at 215–293 K. The spectra contained a paramagnetic and a ferromagnetic component at 180–212 K and only a broad ferromagnetic sextet at T < 180 K. The results of the studies showed that, over the temperature range 87–295 K, the iron ions are in a single (tetrahedral) state with a valence of +3. In the temperature range 180–212 K, two magnetic states of Fe³⁺ ions were observed, one of which is in magnetically ordered microregions and the other, in paramagnetic microregions; these states are due to atomic heterogeneity. In the magnetically ordered microregions in the temperature range 87–212 K, the magnetic state of the iron ions is described well by a single state with an average spin S = 1.4 ± 0.2 and a magnetic moment μ(Fe) = 2.6 ± 0.4μ _B .     

57Fe Mössbauer study of Eu1−x Sr x FeO3−y

The samples of Eu_1–x Sr _x FeO_3–y (x=0.0–1.0) were prepared by the solid state reaction method. Their X-ray diffraction patterns and⁵⁷Fe Mössbauer spectra at room temperature were measured. It is found that Sr ions incorporate in the lattice of EuFeO₃, the change of crystal structure is related to the dopant.⁵⁷Fe Mössbauer spectra consist of one magnetic, one doublet and one single paramagnetic components. The Fe ions in the cubic phase are in intermediate valence state between Fe(III) and Fe(IV) and may participate in electron hopping.     

Strangeness production in AA collisions at SIS18

Aluminium oxides doped with 1% ⁵⁷Fe were prepared by sol-gel method, and annealed for 3 hours at various temperatures between 550°C and 1100°C. Amorphous phases were obtained below 1000°C, and crystalline α–Al₂O₃ was formed at 1100°C. Although Al₂O₃ itself shows diamagnetism, the light doping of Fe ions into aluminium oxide induced a very weak ferromagnetism, but the ferromagnetism disappeared by longer annealing. M?ssbauer spectra were composed of paramagnetic Fe^2?+? and Fe^3?+? species for samples heated below 750°C, and of paramagnetic Fe^3?+? above 850°C, in addition to a magnetic sextet and relaxation peaks of Fe^3?+?. The magnetic and quadrupole interactions of the sextet and the relaxation peaks and the density functional calculations suggest that the lightly doped Fe^3?+? ions are substituted at Al sites in the Al₂O₃ lattice.     

Observation of magnetic Mn sites in cubic and hexagonal HoMn2: 55Mn NMR study

⁵⁵Mn nuclear magnetic resonance has been measured for both cubic C15 and hexagonal C14 HoMn₂. In the ordered state, we found a high-frequency signal, which can be assigned to magnetic Mn atoms, for both C15 and C14 phases together with a low-frequency signal from non-magnetic Mn atoms. The results of the spin-spin relaxation time T₂ in the ordered state and the NMR spectra in the paramagnetic state are also given to discuss the magnetic instability and the magnetic structure.     

Emission Mössbauer spectroscopy study of fluence dependence of paramagnetic relaxation in Mn/Fe implanted ZnO

Emission Mössbauer Spectroscopy following the implantation of radioactive precursor isotope ⁵⁷Mn⁺ (T _1/2= 1.5 min) into ZnO single crystals at ISOLDE/CERN shows that a large fraction of ⁵⁷Fe atoms produced in the ⁵⁷Mn beta decay is created as paramagnetic Fe³⁺ with relatively long spin-lattice relaxation times. Here we report on ZnO pre-implanted with ⁵⁶Fe to fluences of 2×10¹³, 5×10 ¹³ and 8 × 10¹³ ions/cm² in order to investigate the dependence of the paramagnetic relaxation rate of Fe³⁺ on fluence. The spectra are dominated by magnetic features displaying paramagnetic relaxation effects. The extracted spin-lattice relaxation rates show a slight increase with increasing ion fluence at corresponding temperatures and the area fraction of Fe³⁺ at room temperature reaches a maximum contribution of 80(3)% in the studied fluence range.     

Mössbauer study of cubic pyrochlores,Bi2−xYxFeSbO7

The solid solutions Bi_2?xY_xFeSbO₇ (x = 0,1,2) which are cubic pyrochlores of the type A₂B₂O₇ have been prepared for the first time and the lattice parameters determined. The ⁵⁷Fe Mössbauer spectra have been recorded both in the paramagnetic and magnetically ordered states. From the values of the Mössbauer parameters it is found that Fe ions are in the high spin trivalent state. The large value for the quadrupole splitting of these compounds is attributed to the highly distorted BO₆ octahedron in these compounds. The quadrupole splitting decreases with increase of Bi concentration and therefore, the BO₆ octahedron is found to have the lowest distortion for the composition Bi₂FeSbO₇. The magnetic transition temperature and the value of the magnetic hyperfine fields have been determined. The distribution of Fe³⁺ and Sb⁵⁺ ions in B sites gives rise to the distribution in magnetic hyperfine fields.     

Anomalously strong relaxation of the polarization of muons in the magnetically ordered and paramagnetic states of the TbMnO<Subscript>3</Subscript> multiferroic

An anomalously strong relaxation of the muon polarization in a magnetically ordered state in the TbMnO₃ multiferroic has been revealed by the method below the μSR Néel temperature (42 K). Such a relaxation is due to the muon channel of relaxation of the polarization and the interaction of the magnetic moment of the muon with inhomogeneities of the internal magnetic field of an ordered state in the form of a cycloid. Above the Néel temperature, beginning with temperatures depending on the applied magnetic field, a two-phase state has been revealed where one phase has an anomalously strong relaxation of the muon polarization for a paramagnetic state. These features of the paramagnetic state are due to short-range magnetic order domains that appear in strongly frustrated TbMnO₃. A true paramagnetic state has been observed only at T ≥ 150 K.     

EPR spectra of a GdMnO3 thin film on a SrTiO3 substrate

Electron paramagnetic resonance (EPR) spectra of a GdMnO₃/SrTiO₃ thin film in the X band have been measured in the temperature interval from 200 to 450 K. Signals from two types of paramagnetic centers have been observed in the spectra. The first paramagnetic center is a subsystem of Gd³⁺ ions, in the EPR spectrum of which the fine structure lines are resolved below 350 K. The second paramagnetic center is a system of manganese and gadolinium ions, in the EPR spectrum of which an exchange-narrowed line is observed with the width ΔH several times less than the width ΔH of an exchange-narrowed line observed in the GdMnO₃ single crystal. Unusual magnetic properties are due to the mismatch of the lattice parameters of the GdMnO₃ thin film and the SrTiO₃ substrate.     

Anomalous nuclear spin–lattice relaxation of <Superscript>3</Superscript>Не in contact with ordered Al<Subscript>2</Subscript>O<Subscript>3</Subscript> aerogel

Spin–lattice relaxation of ³Не in contact with the ordered Al₂O₃ fiber aerogel has been studied at the temperature of 1.6 K in fields of 0.1–0.5 T by the pulsed nuclear magnetic resonance (NMR) method. An additional mechanism of the relaxation of ³Не in aerogels is found and it is shown that this relaxation mechanism is not associated with the adsorbed layer. A hypothesis about the influence of intrinsic paramagnetic centers on the relaxation of gaseous ³Не is proposed.     

Magnetic properties of YMn2 and related compounds studied by μSR

Zero, longitudinal and transverse field SR was carried out between 3 and 250 K in YMn₂, Y_0.9Tb_0.1Mn₂, Y_0.98Sc_0.02Mn₂ and TbMn₂. The data had to be analyzed in the frame of electronic-nuclear double relaxation. The dynamics of Mn magnetic moments in the paramagnetic state is typical for an itinerant antiferromagnet approaching magnetic order. This behavior is suppressed abruptly when the first-order transition occurs. In all samples, a part enters a randomly ordered (spin glass like) magnetic state within a certain temperature range aboveT _N as an outcome of frustration. The temperature range of existence and the fraction of the sample taking part in this stale is enhanced by the presence of Tb³⁺ paramagnetic ions and also by the lattice pressure in the Sc doped compound. It is suspected that muon dynamics makes the extraction of information on magnetic relaxation questionable above 150–180 K.Part of this work was funded by the German Federal Minister for Research and Technology (BMFT) under Contract No. 03-KA2-TUM.     

Hyperfine interaction of161Dy imprities implated in iron and nickel

In this paper we present a full report on our Mössbauer measurements on implanted sources of¹⁶¹TbFe and¹⁶¹TbNi using the 26.7 keV Mössbauer transition in¹⁶¹Dy. From the measured spectra values were derived for the magnetic and quadrupole interaction strengths of rare earth ions in at least two non-equivalent lattice sites and information is obtained about the relaxation behaviour of these ions.Conclusions are drawn about the lattice location of the implanted ions. Information could also be obtained about the crystalline electric field acting upon the substitutionally implanted rare-earth ions.     

<Superscript>27</Superscript>Al NMR Study of the Structure and Dynamics of Natrolite

The temperature dependences of nuclear magnetic resonance and magic angle spinning nuclear magnetic resonance spectra of ²⁷Al nuclei in natrolite (Na₂Al₂Si₃O₁₀· 2H₂O) have been studied. The influence of water molecules and sodium ions mobility on the shape of the ²⁷Al NMR spectrum and framework dynamics have been discussed The temperature dependences of the spin–lattice relaxation times T₁ of ²⁷Al nuclei in natrolite have also been studied. It has been shown that the spin–lattice relaxation of the ²⁷Al is governed by the electric quadrupole interaction with the crystal electric field gradients modulated by translational motion of H₂O molecules in the natrolite pores. The dipolar interactions with paramagnetic impurities become significant as a relaxation mechanism of the ²⁷Al nuclei only at low temperatures (<270 K).     

Spin dynamics in LiCu<Subscript>2</Subscript>O<Subscript>2</Subscript> and NaCu<Subscript>2</Subscript>O<Subscript>2</Subscript> low-dimensional helical magnets

Comprehensive NMR investigation of low-frequency spin dynamics of LiCu₂O₂ (LCO) and NaCu₂O₂ (NCO) low-dimensional helical magnets in the paramagnetic state has been carried out for the first time. Temperature dependences of the spin–lattice relaxation rate and anisotropy on various LCO/NCO nuclei have been determined at various orientations of single crystals in an external magnetic field. The spatial asymmetry of spin fluctuations in LCO multiferroic has been discovered. The quantitative analysis of the anisotropy of spin–lattice relaxation in LCO/NCO has allowed estimating the contributions of individual neighboring Cu²⁺ ions to the transferred hyperfine field on Li⁺(Na⁺) ions.     

The magnetic phase transition in titanium oxide induced by proton irradiation

We have studied the magnetic properties of ⁵⁷Fe-doped TiO₂ compounds irradiated by proton with 0, 5 and 10 pC/μm². We have observed the enhancement of the magnetic moment, measured by superconducting quantum interference device magnetometer, with increasing proton irradiation ranging from 0 to 10 pC/μm². Mössbauer spectra of proton irradiated Ti_0.99⁵⁷Fe_0.01O₂ samples were taken at room temperature. Two sites of the wing (sextet) and the central (doublet) are shown in the spectra, which suggest the magnetically ordered phase and the paramagnetic phase, respectively. With increasing proton irradiation, the part of Fe³⁺ ions was converted to Fe²⁺ ions by compensation charge. This clearly suggests that the enhancement of magnetic moment after proton irradiation is contributed to the moment by the spin-orbit coupling of Fe²⁺ ions.     

Resonance Frequency and NMR Relaxation Times in Two Inequivalent <Superscript>133</Superscript>Cs in Cs<Subscript>2</Subscript>CuBr<Subscript>4</Subscript> and Cs<Subscript>2</Subscript>ZnBr<Subscript>4</Subscript> Single Crystals

The resonance frequencies and relaxation mechanisms of Cs₂CuBr₄ and Cs₂ZnBr₄ were examined by static nuclear magnetic resonance (NMR) method. Here, the two inequivalent Cs(1) and Cs(2) sites surrounded by Br ions were distinguished. The saturation recovery traces for ¹³³Cs nuclei in Cs₂CuBr₄ with the paramagnetic ions, and those in Cs₂ZnBr₄ without the paramagnetic ions were each fitted by four exponential functions. From these results, the spin–lattice relaxation times T₁ in the laboratory frame of ¹³³Cs nuclei in the two crystals were obtained, and Cs(1) surrounded by 11 bromide ions has a longer relaxation time than Cs(2) surrounded by 9 bromide ions.     

The charge states of iron in insulators implanted with57Co and57Fe

Single crystals of α-Al₂O₃ and LiNbO₃ were implanted with⁵⁷Co (dose: up to 2×10¹⁵ atoms/cm²) and with⁵⁷Fe (dose: 2×10¹⁵ atoms/cm²) ions. The Mössbauer spectra revealed the disordered atomic environment. Fe²⁺ and Fe³⁺ charge states were observed. The spectra were compared to the spectra of crystals doped with⁵⁷Co. It was remarkable that in the doped α-Al₂O₃ Fe³⁺ states with slow spin-spin relaxation have appeared. The CEMS study of the samples implanted with⁵⁷Fe resulted in Fe²⁺ ionic states indicating that a fraction of Co atoms can also be in Co²⁺ state.     

Mössbauer study of Zn x Ni5/3−x Fe1Sb1/3O4 spinel ferrite

The antimony substituted nickel ferrite Zn_xNi_5/3–xFe₁Sb_1/3O₄ with different values ofx are prepared, checked by x-ray and studied with⁵⁷Fe Mössbauer spectroscopy over a wide temperature range. Characteristic spectra of paramagnetic, magnetic and electronic relaxation types for the different compositions have been observed. The interpretation of the spectra allows the cation distribution of the compounds to be deduced. The Mössbauer effect parameters at different temperatures are calculated and their dependence on the substitution of non-magnetic Zn⁺² for the magnetic nickel ions are discussed. The temperature dependence of the hyperfine magnetic fields and the respective Néel temperature points are obtained.     

An investigation of the spin glass behaviour in iron antimonate by iron-57 and antimony-121 Mössbauer spectroscopy

Iron antimonate, which contains a superlattice composed of an ordered array of cations in the rutile-type structure, has been shown to undergo a spin glass transition atca. 20K which is induced by antisite atomic ordering. The iron-57 Mössbauer spectra recorded at 298 and 77K provide information on the relaxation times of short range magnetically ordered clusters identified by magnetic susceptibility measurements. The spectrum of 4K is tentatively interpreted in terms of the existence, at temperatures below the spin glass transition temperature, of a hyperfine magnetic field distribution at Fe³⁺ ions with a mean value of 487 kOe and which contains a supertransferred contribution from the nearest neighbour Fe³⁺ ions. The¹²¹Sb Mössbauer spectra are characteristic of Sb⁵⁺ and the increasing linewidths at lower temperatures are consistent with the presence of a supertransferred hyperfine magnetic field at the Sb⁵⁺ species ofca. 16^kOe.     

238U Mössbauer study on the magnetic properties of uranium-based heavy fermion superconductors

We have performed ²³⁸U Mössbauer spectroscopy of uranium-based heavy fermion superconductors, UPd₂Al₃ and URu₂Si₂, in order to investigate their physical properties, mainly their magnetic properties. The slow relaxation of magnetic hyperfine interaction in a paramagnetic state and the static hyperfine field has been observed in an antiferromagnetic ordered state for each compound. The line-widths have maximum at their characteristic temperatures where their magnetic susceptibilities have maximum values.