Zili Chu

Charge disproportionation and ordering in La1/3Sr2/3FeO3-δ

The perovskite La1/3Sr2/3FeO3−δ was investigated by neutron diffraction, magnetic and Mossbauer spectroscopy measurements. La1/3Sr2/3FeO3−δ undergoes magnetic ordering at T = 190–200 K accompanied by charge disproportionation. Magnetic peaks due to charge ordering are observed below 200 K. The charge ordering is gradually developed below 200 K along with a charge disproportionation, 2Fe4+ Fe3+ + Fe5+. La1/3Sr2/3FeO3−δ shows an antiferromagnetic structure at low temperature. Magnetic moments of about 3 and 1.3 μB were obtained from the neutron diffraction data refinement for Fe3+ and Fe5+ at 15 K, respectively, which suggest that both Fe ions are in a low spin state. These values are significantly lower than those reported by Battle et al for La1/3Sr2/3FeO2.98. Mossbauer spectra indicate that full charge ordering might be reached below 20 K with no Fe4+.

Observation of novel disordered rhombohedral R2Fe17 (R=rare earth) based compounds

A recent study of (SmGd)Fe14Si3 showed an anomalous x-ray diffraction pattern [P. C. Ezekwenna, G. K. Marasinghe, J.-H. Nam, W. J. James, W. B. Yelon, M. Ellouze, and Ph. I’Heritier, J. Appl. Phys. 87, 6716 (2000)]. Although all observed peaks could be indexed to the rhombohedral 2:17 phase (R-3m) many lines were strongly reduced in intensity. Subsequently, the same phenomenon was seen in neutron diffraction patterns from Nd2−xDyxFe17−ySiy alloys and conventional refinement using the rhombohedral phase left significant residuals. Fourier mapping revealed additional density along the c-axis chains of Fe dumbbells and rare earth atoms. The perfect a-b-c stacking of the CaCu5 layers with regular 1/3 replacement of the rare earth atoms by Fe dumbbells appears to be broken. Detailed analysis shows that excess Fe dumbbells are incorporated into the structure and some reverse substitution is seen. In addition, the weak intensities are associated with a near equivalence of the 9d and 18h Fe sites, as in the paren...

Magnetic properties of iron-rich Nd2−yDyyFe17−xSix mixed rare-earth system

A series of Nd2−yDyyFe17−xSix solid solutions with x=0, 1, 2, and 3 and y=0, 0.5, 1.0, 1.5, and 2.0 were prepared by induction melting stoichiometric amounts of high-purity elements. The x-ray diffraction data confirmed that the postannealed samples are 2:17 intermetallics of the R3m space group. The lattice parameters and unit-cell volumes were calculated using FULLPROF to analyze the neutron diffraction (ND) data. It was observed that for a particular y, the unit-cell volume decreased almost linearly with increasing silicon content (x⩽3), as is observed for single rare-earth 2:17 intermetallic compounds. For a given x, the cell volume decreased almost linearly with the increasing replacement of neodymium with dysprosium. The ND studies revealed additional density along the c-axis chains of Fe dumbells and rare-earth atoms. The perfect a-b-c stacking of the CaCu5 layers with a regular one third replacement of the rare-earth atoms by Fe dumbells appear to be broken, resulting in a disordered rhombohedral...

Neutron diffraction analysis of melt spun 2:14:1 type (NdPr)–Fe–B compounds with Ti and Zr additions

There has been continued technological and research interest in Nd 2 Fe 14 B -based compounds due to their excellent intrinsic magnetic properties [(BH) max * ∼64 MGOe ,H a ∼73 kOe ]. It is found that Ti and Zr substitution in Nd 2 Fe 14 B leaves its magnetic properties largely unaffected. We have carried out neutron diffraction studies on Zr and Ti substituted 2:14:1 compounds. Three specimens of melt spun alloys of compositions ( Nd 0.75 Pr 0.25 ) 12 Fe 80 B 8 , ( Nd 0.75 Pr 0.25 ) 8.4 Fe 79.7 Ti 4.7 B 7.2 and ( Nd 0.75 Pr 0.25 ) 9.2 Fe 79.2 Ti 2 Zr 2.2 B 7.4 were studied. Rietveld refinements reveal that the 4f site is equally occupied by Nd and Pr while the 4g site is occupied only by Nd. In the Ti-containing sample, the lattice expands and Ti is found on the j 1 , j 2 sites normally occupied by Fe, (4% and 8%, respectively). This accounts for about 30% of the added Ti. In the third sample, the lattice contracts, which could occur only if Zr replaces the larger rare-earth atoms. However, due to the possibility of four different species occupying the 4f site (i.e., Nd, Pr, Ti, and Zr), it is not possible to determine their relative concentrations from neutron diffraction data alone. The balance of Ti and Zr in this sample must be locked in the grain boundaries. Despite peak broadening associated with the fine grain structure, robust refinements were achieved.

Magnetization and neutron diffraction studies on Dy5Si2Ge2

The compound Dy5Si2Ge2 crystallizes in an orthorhombic structure (Sm5Ge4 type, space group Pnma). Magnetization measurements performed in the temperature range of 2–300 K in applied fields up to 7 T reveal that this compound orders antiferromagnetically at 56 K (TN) but with a positive paramagnetic Curie temperature θP. Magnetization-field isotherms, obtained at 5 K and 20 K, display a field-induced antiferromagnetic to ferromagnetic transition. The magnetization approaches saturation in a field of 6 T with a moment value of ∼8μB∕Dy3+. Neutron diffraction measurements, carried out at 9.2 K, suggest that Dy moments arrange spirally along the a axis giving rise to a canted antiferromagnetic structure. The analysis of neutron diffraction data yields an ordered state magnetic moment of 7.63μBperDy3+ ion.

Effect of oxygen vacancies on the magnetic structure of the La0.6Sr0.4FeO3−δ perovskite: A neutron diffraction study

Magnetic interactions in perovskite compounds of the type La1−xSrxMO3−δ (M=3d transition such as Mn and Fe) are presumed to arise through a super exchange between 3d electrons of the magnetic ions via oxygen orbitals. The magnetic structure of La0.6Sr0.4FeO3−δ has been studied with neutron diffraction. Oxygen vacancies were created by annealing samples under various gases including N2, air and mixtures of CO/CO2. All La0.6Sr0.4FeO3−δ compounds maintain the rhombohedral structure (space group R3c). The air- or oxygen-annealed samples have almost no oxygen vacancies while those made in the reducing atmosphere show 7%–11% oxygen vacancies. The rhombohedral distortion decreases in the reduced samples. All the samples exhibit antiferromagnetic ordering at room temperature, although a small ferromagnetic moment may also be present. The samples with little or no oxygen vacancies show a room temperature magnetic moment of ∼1.4 μB at the Fe site while those having >7% oxygen vacancies show a moment of ∼4.0 μB. Ma...

Magnetic properties of Er2Mn3Si5

Abstract Magnetization measurements have been carried out on the Er 2 Mn 3 Si 5 compound (tetragonal Sc 2 Fe 3 Si 5 -type, space group P 4/ mnc ) in the temperature range 1.8–300 K. There is a crossover from a room temperature paramagnetic phase to a ferromagnetic phase at 75 K ( T C1 ). A second magnetic transition is observed at 11.5 K ( T C2 ). The magnetic susceptibility in the paramagnetic region, above 100 K, follows simple Curie–Weiss type behavior. The magnetization does not saturate at 2 K even in applied fields of 7 T. Neutron diffraction data collected at 300 and 9.2 K confirm that both the rare earth and the Mn moments in this compound carry magnetic moments. There are two Mn sublattices (Mn1 and Mn2) coupled orthogonal to each other. Both the rare earth and the Mn1 sublattice order along the positive a axis but the Mn2 moments order along the positive c axis, thus leading to a competition of magnetic interactions at low temperatures.

Magnetic and neutron diffraction studies on PrMnSb2

Magnetic and neutron diffraction studies have been carried out on PrMnSb2. Magnetization data in the temperature range of 5–400 K show two magnetic transitions: one at ∼175 K attributed to the antiferromagnetic ordering of the Mn moments, and the other at ∼35 K possibly due to the antiferromagnetic ordering of the Pr moments. The magnetization-field isotherms at various temperatures are consistent with the above. Neutron diffraction data obtained at various temperatures can be fitted with a nearly antiferromagnetically coupled Mn moment of ∼3μB at 70 K. At 10 K, moments both on Mn (∼3.5μB) and Pr (∼0.94μB) are ordered antiferromagnetically with small canting angles.

Site occupancy of Sm in (Nd1−xSmx)5(Fe1−yTiy)17

The ferromagnetic compound Nd5Fe17 has a large average Fe moment (2.1 μB), a reasonably high Curie temperature (503 K), but basal plane spin orientation. Partial replacement of Nd by Sm leads to a uniaxial spin arrangement with substantial anisotropy field. Natural Sm has a large neutron absorption cross section (5650 b) which is usually thought to prevent neutron diffraction measurements unless Sm is replaced by a separated isotope with low absorption. However, the unique geometry of the MURR neutron powder diffractometer allows measurements on thin samples and data have been successfully collected on samples with up to about 50% Sm (Nd2.5Sm2.5Fe17). Samples prepared without Ti contain the 2:17 phase as well as the 5:17 phase, while those prepared with Ti have a very high concentration of the 5:17 phase which forms more readily (with minor amounts of TiFe2). The locations of Sm atoms in these compounds have been determined. These results may provide important information about the crystal fields at diffe...