Tb Sites Research Articles

Magnetic properties of RPdGe2 (R=Y, Gd, Tb, Dy) compounds

Abstract Magnetic measurements have been performed on some orthorhombic RPdGe2 compounds (R=Y, Gd, Tb, Dy), as well as some non-stoichiometric RPdxGe2 compounds (R=Nd, Gd, Ho, x q → =0 ) propagation vector. Below 6.5 K the moments of the second Tb site become ordered with a different propagation vector q → =(0, 1/2, 1/2). Such a behaviour reveals a decoupling of both Tb crystallographic sites.

New magnetic phenomena in TbNi2

Structural investigations at room temperature revealed that TbNi2 does not crystallize in the Laves phase structure, but shows a superstructure of the Laves phase with the space group F-43m. Susceptibility, specific heat, magnetostriction and magnetoresistance measurements on polycrystalline specimens showed an additional magnetic phase transition at TR = 14 K below the Curie temperature of TC = 36±0.2 K. In order to clarify the nature of this magnetic phase transition at 14 K, elastic neutron diffraction below and above TR and TC was performed. The analysis of these data showed that this transition at TR is due to the rotation of the Tb moments on three of the total of eight non-equivalent Tb sites in the rhombohedrally distorted unit cell in the magnetic ordered state. This rotation of these Tb moments is out of the [111] direction into a plane perpendicular to the space diagonal. The cause for this magnetic instability is due to an interplay of the regularly arranged vacancies in the superstructure and the crystal field level position which has been studied by inelastic neutron scattering.

Spin-Reorientation and Magnetostriction in Highly Oriented (Er1-xTbx)2Fe14B

Magentostriction measurements in highly-oriented ribbons of (Er1-xTbx)2Fe14B with 0≤x≤0.45, fabricated by a rapid-quenching method have been performed in order to investigate the lattice deformation at the spin reorientation (SR) transitions. The c-axis of the tetragonal cell was found to be aligned perpendicular to the ribbon surface. The sum of the longitudinal and transverse magnetostriction λ//+λ⊥, with fields applied within the ribbon plane, took a maximum at successive SR temperatures. A model calculation of the volume change ΔV/V was made by assuming that only the crystalline electric field (CEF) potentials at Er and Tb sites affect the volume change. This calculation showed that ΔV/V exhibits a sharp peak at SR temperatures, which is in accordance with the experimental peaks of λ//+λ⊥.

Influence of thermal history on crystal structure, microstructure and magnetic properties of TbFe 6Ge 6 (II)

The relation between crystal structure, microstructure and magnetic structure in TbFe 6Ge 6 is derived on the basis of neutron powder diffraction for samples annealed at 925, 800 (part I) and 650°C. All samples display a density modulation of only the guest subsystem (Tb/Tb* and Ge5/Ge5*) described by the wave vector q=( 1 2 , 1 2 , 0) of the orthohexagonal YCo 6Ge 6 structure. The occupancy of the Tb site is 1 for the sample with T an=925°C and decreases to 0.830(6) for the sample with T an=650°C. At high temperatures (HT) T 1< T< T N ( T N≈450 K, T 1=9.5 K) the Fe moments order with the wave vector q 1=(0 1 0) and have an antiferromagnetic collinear arrangement (+−+−··) along the x-axis. The Tb magnetic order sets in at T 1=9.5 K and is associated with two wave vectors q 1=(0 1 0) (antiferromagnetic, AF) and q 2=(0 0 0) (ferromagnetic, F). The Tb F-component is parallel to x, while the Tb AF-component is confined to the ( x 0 z)-plane. However, their relative amount depends strongly on the thermal history and this in turn influences the Fe ordering. The largest Tb F-component is observed for the sample with T an=925°C and it induces a reorientation of the Fe moments below T 1 which at 1.4 K are tilted away from the x-axis by an angle φ xy =25.0(6)°. The 1.4 K ordered moment values are 2.22(4) μ B/Fe and 8.0(1) μ B/Tb. The coupling within the Tb[1 0 0]-layers stacked along x is F. The canted ferrimagnetic structure has moments in general directions and comprises four sublattices, two Fe AF and two Tb ferrimagnetic. For sample with T an=650°C the Tb AF-component is dominant, leading to a braking of the F coupling within the Tb[1 0 0]-layers. The Fe moments preserve the HT arrangement below T 1. The 1.4 K ordered moment values are 2.29(6) μ B/Fe, 9.5(2) μ B/Tb ( x=0) and 4.1(2) μ B/Tb ( x=0.5). The canted magnetic structure comprises six sublattices, two Fe AF and four Tb ferrimagnetic, all within the ( x 0 z) plane. The change of the Tb atoms distribution and coherency of the guest subsystem (peak broadening) are coupled in two ways with the magnetic ordering. They change the total magnetic structure and determine the correlation length of the long-range order of the Tb moments.

Mixed magnetic phases in tetragonal

A careful analysis of neutron diffraction experiments under magnetic field performed on a single crystal of tetragonal is presented. It shows that the zero-field-low-temperature magnetic phase is not the natural compensated antiphase structure proposed so far for the Q = (3/13, 0, 0) propagation vector, but a mixed phase where the moment of two of the 26 Tb sites of the unit cell vanishes. The magnetic structures of the first three field induced phases of the multistep metamagnetic process agree with previous analysis whereas the fourth field induced phase is not the non-mixed proposed one but a mixed one with two zero moments as in zero field. Contrary to previous interpretations, the transition at 4.7 K observed by specific heat and susceptibility corresponds to the change of the magnetic structure from the mixed low-temperature one to a slightly modulated non-mixed one. Numerical simulation allowed us to account for the main properties observed and emphasizes the crucial role of the crystal field.

Co-infection with Mycobacterium tuberculosis and HIV in high risk clinical care settings in Rhode Island

We assessed the extent to which co-infection with HIV and Mycobacterium tuberculosis (Mtb) was diagnosed at several high risk clinical care sites from 1992 to 1994 to determine whether surveillance for co-infection was performed. Information on PPD skin testing, HIV status, and HIV risk exposures was extracted from records at HIV clinics in Rhode Island and a large database (HIV sites) and from records at the state TB clinic and the Rhode Island Health Department (TB sites). At the HIV sites, 34 of 1,408 HIV infected subjects were newly diagnosed with Mtb infection in the study period. At the TB sites, 16 of 1,389 subjects with newly diagnosed Mtb infection or disease were identified as HIV infected. Eighty per cent of the records reviewed for this study were incomplete. Hispanic subjects were at higher risk of being identified as co-infected at the HIV sites. At the TB sites, US-born subjects were at higher risk of being identified as co-infected. Recommendations for high risk individuals include yearly tuberculosis skin testing. Adherence to these guidelines in selected high-risk clinical care sites in Rhode Island was substandard during the study period; the importance of Mtb screening was demonstrated in this study. Identification of groups that are at higher risk of having HIV and Mtb co-infection identified may enable health care providers to improve testing and prevention of tuberculosis at high risk clinical care settings.

Atomic disorder, magnetic order and phase transitions of TbFe 6Ge 6 studied by X-ray, neutron diffraction and magnetic measurements (I)

The crystal structure and the magnetic ordering of TbFe 6Ge 6 were studied by X-ray and neutron powder diffraction and by magnetic measurements on a single crystal. It has been found that TbFe 6Ge 6 forms a superstructure described by the wave vector q = ( 1 2, 1 2, 0 ) of the orthohexagonal YCo 6Ge 6 type of structure ( c, √ 3 a, a) h . High resolution neutron data of an as cast material annealed at 800°C indicate that the major factor leading to the superstructure formation is associated with a density modulation of the Tb/Ge ordering that can be described by a density modulation function with the wave vector q . The Fe ordering is antiferromagnetic between 1.4 and 293 K with the wave vector q 1 = (0 1 0) . The three Fe sites have a collinear moment arrangement along the x axis with the same moment value. The Tb sublattice, as a result of the density modulation, splits into two sites ( Tb(0, y, 1 4 )/ Tb∗( 1 2 , y, 1 4)) with (0.89(1)/0.11(1)) occupancy. Both Tb sites order magnetically below T 1 = 9 K. Their ordering can be described by two wave vectors q 1 = (0 1 0) and q 2 = (0 0 0) , the former giving rise to a collinear antiferromagnetic ordering along z (perpendicular to the Fe moments), the latter to ferromagnetic ordering along x (parallel with the Fe moments). At 1.4 K the Fe moment values are 2.02(4)μ B/Fe atom. Assuming that the two wave vectors correspond to a single magnetic phase, this would lead to the moment values μ Tb = 7.34(8)μ B , μ Tb∗ = 5.1(7)μ B with an angle of ±45° with the z direction in the plane (x 0 z) corresponding to a flopside structure. The nuclear and the magnetic patterns display an anisotropic peak broadening depending on the h k l direction and the annealing temperature. This effect is discussed in terms of microdomain structure associated with fluctuations of density modulation.

Antiferromagnetic ordering in the novel (two-step) and -type) compounds studied by neutron diffraction and magnetic measurements

The magnetic ordering of the novel binary compound with two Tb sites ( structure type, space group Cmcm) has been studied by neutron diffraction and magnetic measurements. orders antiferromagnetically in two steps. In a first step the sublattice orders below with a collinear moment arrangement along the shortest axis a which persists down to . Below a reorientation of the moments takes place. Simultaneously the moments order with a collinear arrangement along c. The ordering of the and sites is associated with the wavevector q = (010) and is described by the monoclinic magnetic space group . At 1.4 K the ordered moment values of and are and , respectively. The moment is tilted away from the x-direction by an angle of and by from the y-direction within the (0yz) plane. The resulting structure corresponds to a three-dimensional canting with six sublattices. The presence of diffuse scattering in the 16.5 - 53 K temperature interval is attributed to short-range ordered magnetic domains of the mean correlation length Å. Fourier analysis of the data reveals that the first- (up to 4.5 Å) and third- (6.5 - 7.5 Å) neighbour correlations are mainly ferromagnetic; the second (4.5 - 6.5 Å) and fourth (7.5 - 8.5 Å) ones are antiferromagnetic. The magnetic ordering of the coexisting novel phase with the structure type (space group P6/mmm) has been also studied. The Tb moments order antiferromagnetically below . This collinear moment arrangement is associated with the wavevector and is described by the magnetic space group . At and is confined to the (001) plane.

Dry deposition velocities of polycyclic aromatic hydrocarbons in the ambient air of traffic intersections

Abstract Dry deposition and air sampling were undertaken simultaneously at two traffic intersections (TA and TB sites) with several dry deposition plates and PS‐1 samplers from August to December 1995 in southern Taiwan. In addition, samples in a lane (L) and a residence (R) sites were also determined and compared. Twenty‐one individual polycyclic aromatic hydrocarbons (PAHs) were analyzed primarily with a gas chromatographyVmass spectrometer (GC/MS). At the traffic intersections, the dry deposition velocities of total‐PAHs averaged 0.38 and 0.23 cm sec‐1 in the day‐time and at night‐time, respectively. This result shows that, in the ambient of traffic intersections, significant fraction of PAH dry deposition are contributed by the road dust resuspension caused by traveling vehicles. In general, the lower molecular weight (LMW) PAHs, being primarily in the gas phase, have lower dry deposition velocities; while the higher molecular weight (HMW) PAHs, which are mainly associated with particulates, have higher dry deposition velocities. The mean dry deposition velocities for LMW PAHs ‐ Nap, AcPy and Acp ‐ are 0.011, 0.034 and 0.043 cm sec‐1, respectively. However, the dry deposition velocities of10 HMW PAHs, including BbF, BkF, BeP, BaP, PER, IND, DBA, BbC, BghiP and COR, are all at the same level but up to 2 orders of magnitude higher than those of LMW PAHs. Because of the “synergism”; of higher fraction in particle‐phase and higher deposition velocity, more than 94% of the HMW PAH mass in the dry deposition flux is contributed by the particle phase.

Crystal Structure of Escherichia coliPhosphoenolpyruvate Carboxykinase: A New Structural Family with the P-loop Nucleoside Triphosphate Hydrolase Fold

The crystal structure of ATP-dependent phosphoenolpyruvate carboxykin |dschyp|ase (ATP-oxaloacetate carboxy-lyase, (transphosphorylating), E.C. 4.1.1.49; PCK) from Escherichia colistrain K12 has been determined using a combination of multiple isomorphous replacement, density modification, and partial model phase combination, and refined to a conventional R-index of 0.204 ( R free=0.244) at 1.9 Å resolution. Each PCK molecule consists of a 275 residue N-terminal domain and 265 residue C-terminal or mononucleotide-binding domain, with the active site postulated to be within a cleft between the two domains. PCK is an open-faced, mixed α/β protein, with each domain having an α/β/α folding topology as found in several other mononucleoside-binding enzymes. The putative phosphate- binding site of ATP adopts the P-loop motif common to many ATP and GTP-binding proteins, and is similar in structure to that found within adenylate kinase. However, the β-sheet topology within the mononucleo |dschyp|tide-binding fold of PCK differs from all other families within the P-loop containing nucleoside triphosphate hydrolase superfamily, therefore suggesting it represents the first member in a new family of such proteins. The mononucleotide-binding domain is also different in structure compared to the classical mononucleotide-binding fold (CMBF) common to adenylate kinase, p21ras, and elongation factor-Tu. Several amino acid residues, including R65, K212, K213, H232, K254, D269, K288 and R333 appear to make up the active site of the enzyme, and are found to be absolutely conserved among known members of the ATP-dependent PCK family. A cysteine residue is located near the active-site, as has been suggested for other PCKs, although in the E. colienzyme C233 is buried and so is most likely not involved in substrate binding or catalysis. Two binding sites of the calcium-analog Tb 3+have been determined, one within the active site coordinating to the side-chain of D269, and the other within the C-terminal domain coordinating to the side-chains of E508 and E511.

The role of vacancies in the magnetic and magneto-optic properties of Tb-doped EuO and EuS films

Rocksalt structure solid solutions were prepared by coevaporating Eu2O3 and Tb metal. The films have the general formula Eu1−xTbxO1−y⧠y where y, the oxygen vacancy fraction, is given by y=(3x−1)/2. We have also prepared Tb-doped EuS solid solutions where y≂x so we can compare the effects of anion vacancies in both the oxide and sulfide systems. The coercivity, Hc, increases rapidly with Tb doping at about x=0.15 in the sulfide system and x=0.45 in the oxide system. The Hc of both systems falls on a universal curve, however, when plotted vs vacancies/RE (⧠/RE) with the rapid increase occurring at ⧠/RE=0.15. This value is close to 1/6 (0.166) which in octahedral coordination corresponds to one anion vacancy/RE. This suggests that the near neighbor vacancy is producing a strong crystal field at the Tb site which causes strong single ion anisotropy and thus a high coercivity. The MO spectra of the oxides show a strong dependence on x. At x=0.25, peaks are observed at 1.5 eV (−3°), 2.2 eV (5.5°), and 2.7 eV (−2°). At x=0.33, the spectra change completely. A narrow peak with a rotation θK=−20° appears at 2.5 eV, dominating the spectrum. The reflectivity, R, goes through a sharp minimum at the same energy (R&amp;lt;1%), possibly causing the enhancement in θK.

Canted antiferromagnetic ordering of Tb 2Si 3−δ

The crystal structure and magnetic properties of a compound of nominal composition TbSi 1.25 were studied by neutron diffraction. The crystal structure was found to be orthorhombic (Cmcm) with two formula units Tb 2Si 3 per unit cell, comprising two Tb and three Si sites. Refinement has shown that it is a stoichiometric compound contrary to isomorphic light rare earth compounds. This compound orders at T N = 22 K antiferromagnetically with a three dimensional canted magnetic moment arrangement comprising six Tb sublattices associated with a doubling of the a axis of the monoclinic primitive cell ( a, a 2 + b 2 , c). The corresponding magnetic space group is P a2 1 (Sh 10 4). There is a strong difference i n the orientations and magnitude of the Tb moments between the two Tb sites (7.26μ B/Tb 1 and 1.29μ B/Tb 2). The intersite interaction I AB is dominant and the two sites order at the same critical temperature.

On the mechanism of energy transfer to Tb3+ ions in proteins

Intraprotein energy transfer to terbium ions is widely used for probing distances of calcium sites in proteins. In this work we have performed a time-resolved study of the sensitized luminescence in elastase using a pulsed laser excitation at 265 nm. Terbium-sensitized luminescence was found to build-up within about 150 microseconds, which indicates that the protein transfers energy at a rate several orders of magnitude slower than expected for a singlet state donor. From the rise time of the signal and from its variation with the oxygen concentration, it can be deduced that 80% of the transfer originates from the first triplet excited state of one unique aromatic residue. From the comparison of protein fluorescence and sensitized terbium luminescence excitation spectra the sensitizer was identified as a tryptophan, presumably Trp141, which is situated only 0.7-0.9 nm away from the Tb site. The results are at variance with the usual assumption that energy is transferred from the first excited singlet state of aromatic residues according to a long-range dipole-dipole interaction and are more consistent with a short-distance exchange mechanism.

Carcinogenicity of bracken fern(Pteridium esculentum)in New Zealand

Bracken fern (Pteridium esculentum) was harvested from two sites LB and TB one of which (TB) was on a central North Island New Zealand farm where bovine enzootic haematuria (BEH) was known to occur. The fern was dried, ground and incorporated (25% w/w) into a pelleted diet and fed to female rats for a total of 162 days. Fifteen weeks later when the rats were autopsied it was found that numerous tumours, mainly of the ileum and urinary bladder were present in the animals fed the bracken fern from the TB site. Neoplasms were found in 85% of rats from the TB group compared with 11% in the LB group while only a single tumour (a haemangioma of the uterus) was observed in the controls. In all, there were neoplasms in 42, 5 and 1 organ/s from the TB, LB and control rat groups respectively (p<0.001). Analysis of the fern and pellets for ptaquiloside, the carcinogen in bracken fern, showed much higher levels present in the material from the TB site. There was 26 and 2270 microg ptaquiloside/g of dried fern and, for pellets from the same fern, 6.5 and 355 microg ptaquiloside/g of pellets, for one collection from the LB and TB sources respectively.

Sublattice magnetization and magnetization process in TbCu2

The TbCu2 compound exhibits antiferromagnetism and has two nonequivalent Tb sites below 54 K (=TN). The magnetization for the a-axis increases discontinuously at critical field and saturates at high fields. The magnetization process and magnetic moments have been analyzed in terms of the molecular field theory including the crystal-field interaction.

NMR of159Tb in TbFe2and TbFe3

The hyperfine parameters of 159Tb in TbFe2 and TbFe3 have been measured by nuclear magnetic resonance at liquid helium temperatures. The parameters presented remove previous uncertainties about the values in TbFe2. The new observations on TbFe3 show resolved spectra for the two crystallographically inequivalent Tb sites.

Magnetic overprinting in the Thanet gabbro complex, Ontario

Multi-component paleomagnetic remanences are reported for the Thanet gabbro complex of the Grenville Province in Ontario. Forty-five samples (10 sites) attain stable reversed (A) directions in AF's (alternating fields) of 150 Oe (1.2 × 104 A/m) or less. Blocking temperatures (TB's) are 300–350 °C (5 sites) and 525–560° (5 sites). The A1 paleopole of 158.4°E, 38.0°S (k = 593, α95 = 3.1°) for the low TB sites differs significantly at the 95% confidence level from the A2 pole of 172.0°E, 32.3°S (k = 151, α95 = 6.2°) for the high TB sites. Directions from 18 additional samples (at 8 sites) follow great circle paths towards the A direction upon AF demagnetization to 1000 Oe (8.0 × 104 A/m). TB's of the lower coercivity (B) component are 560–580 °C, above TB's of both A components. Upon thermal demagnetization, directions swing away from the A direction and endpoints reached above 560 °C define a normal B paleopole at 159.3°E, 20.3°N (k = 9.6, α95 = 11.7°).If the A and B components are both of thermal origin, B predates A, and was overprinted by A during a regional metamorphic event which reset components with TB's &lt;560 °C but left those unaffected with TB's &gt; 560°C. The difference in the A2 and A1 directions represents apparent polar wander during cooling from above 500 °C to less than 300 °C. If the B component, however, was generated as a low temperature (&lt;300 °C) chemical remanence, the relative age of A and B are reversed—B is younger than A. The applicaton of Thanet paleopoles to the problems of Grenville and Sveconorwegian tectonics is described.

Magnetic characteristics of TbCo 0·3Al 1·7

TbCo 0·3Al 1·7 crystallizes with the cubic MgCu 2 type structure. The material orders close to ferromagnetically in a field of 30 kOe but exhibits a Néel point of 35 K in a field of 500 Oe. At 1·6 K the magnetization increases in two discontinuous steps to an extrapolated saturation moment of 6·58 μ B . A neutron diffraction experiment at 4·2 K shows a magnetic moment of 6·98 μ B ordered ferromagnetically only at the Tb sites. The situation seems to be similar to ErCo 2−ErAl 2 alloys, where disordered antiferromagnetic components stabilize as a function of the free electron concentration.