M Hofmann

Structural evolution of ball-milled ZnFe2O4

Abstract Nanostructured zinc ferrite produced by milling in both low-energy and high-energy ball mills has been investigated by X-ray diffraction and Mossbauer effect spectroscopy. The lattice parameter of the milled products remains essentially unchanged from that of equilibrium ZnFe2O4 with the steady-state average particle size found to decrease to d=18(2) nm on low energy milling compared with d=8(1) nm on high energy milling. The room temperature Mossbauer spectra of the milled materials have been analysed using two doublets, one of which is considered to be associated primarily with the octahedral lattice sites. Spectral broadening is observed with decreasing particle size, particularly below d∼10 nm, for which the effects of magnetic hyperfine splitting become evident. The mean inversion parameter of nanostructured ZnFe2O4 is found to increase to c∼0.75 for particle sizes of d∼8 nm reflecting the systematic evolution of zinc ferrite from its normal spinel structure towards an inverse spinel structure on mechanical treatment as observed previously. The other factors which contribute to the Mossbauer spectra of nanostructured ZnFe2O4 (d∼8–70 nm) are discussed.

Magnetism of the nanostructured spinel zinc ferrite

Nanostructured zinc ferrite has been prepared by mechanical milling. The changes in the microstructure indicate a decrease in particle size and a simultaneous increase in the inversion parameter. Along with the structural changes, a magnetic transformation from the antiferromagnetic phase to a ferrimagnetic-like behaviour is observed by neutron diffraction.

Mechanochemical transformation of α-Fe2O3 to Fe3−xO4–microstructural investigation

Abstract The effects of wet-milling α-Fe 2 O 3 in vacuum for up to 144 h have been investigated by neutron diffraction measurements at room temperature and in situ at ∼950 K. Rietveld refinements show that the main product is iron-deficient magnetite of approximate stoichiometry ∼Fe 2.8 O 4 . Comparison of the phases derived from the neutron data with results of the Fe 2+ /Fe 3+ oxidation states as determined by chemical analysis reveals that a significant fraction of the unreacted α-Fe 2 O 3 occurs in an amorphous-like or disordered state. The wet-milled products are also found to contain ∼8% γ-Fe 2 O 3 . The transformation from the α-phase to the γ-phase occurs as a result of the shearing during the low-energy milling, with the further collisions and impacts leading to defect magnetite on extended milling. While other contributing effects take place, the transformation process from α-Fe 2 O 3 to Fe 3− x O 4 occurs mainly as a result of rupturing the oxide surface layers of α-Fe 2 O 3 and releasing oxygen with consequent reduction to Fe 3− x O 4 .

Antiferromagnetism of ternary lanthanide stannides RAuSn (R = Pr, Nd, Gd - Er)

Neutron diffraction and magnetometric data show that hexagonal (LiGaGe-type crystal structure) RAuSn compounds (R = Pr, Nd, Gd, Tb, Dy, Ho) order antiferromagnetically at low temperatures. Their magnetic structures are described by the wavevector ; the magnetic moments are normal to the hexagonal axis in TbAuSn and DyAuSn, parallel to it in PrAuSn and HoAuSn and make an angle of in NdAuSn. ErAuSn is cubic (space group ). At T = 1.6 K only a broad peak corresponding to short-range ordering is observed. The observed Neel points range from 2.8 K in PrAuSn to 24 K in GdAuSn. In all title compounds the magnitudes of ordered magnetic moments at 1.6 K derived from neutron diffraction experiments are smaller than the respective free ion values, indicating a strong effect of the crystalline electric field.

Competing magnetic interactions in La0.8Y0.2Mn2Si2-coexistence of canted ferromagnetism and antiferromagnetism

The magnetic structures of tetragonal La0.8 Y0.2 Mn2 Si2 have been investigated by neutron diffraction measurements over the temperature range ~2 - 513 K. This compound corresponds to the critical concentration xc ~ 0.2 in the La1 - x Yx Mn2 Si2 series for the transition from the antiferromagnetism of YMn2 Si2 to the predominant ferromagnetism of LaMn2 Si2 . The high temperature antiferromagnetic region of La0.8 Y0.2 Mn2 Si2 , ~ 300 K < T < T N 1 ~ 410 K, exhibits the planar antiferromagnetism common to the La-rich compounds in the La1 - x Yx Mn2 Si2 series, with c -axis canted antiferromagnetic and ferromagnetic structures coexisting in the region ~ 150 K < T < T N 2,C ~ 300 K. Below ~ 150 K, La0.8 Y0.2 Mn2 Si2 exhibits only the single phase canted antiferromagnetic structure. Rietveld refinements of the data demonstrate that the canted magnetic structures maintain the same angle of tilt with respect to the c -axis and the same total sublattice moment in the two phase region ~ 150 - 300 K, and that the two tetragonal phases (both with the ThCr2 Si2 structure) can be distinguished crystallographically with differences in their a lattice parameters and c lattice parameters of around a ~ 0.012 A and c ~ 0.007 A respectively. This simple description of these magnetic structures leads to a monotonic variation of the total Mn magnetic moment below T N 1 ~ 410 K with the onset of the c -axis component occurring below T N 2,C ~ 300 K. The canted antiferromagnetic ground state of La0.8 Y0.2 Mn2 Si2 can be described by the Mn moment values: µMn (2 K) = 2.25(4) µB , with the z -component of the moment µz (2 K) = 1.87(4) µB , corresponding to a tilt angle (2 K) = 33.7(9)°.

Magnetic structures and phase transitions in PrMn2-xFexGe2

The magnetic properties and magnetic structures of PrMn2−xFexGe2 compounds (space group I4/mmm) have been investigated using magnetic, F57e Mossbauer effect (x=1.0,1.3,1.6), and neutron diffraction measurements (x=0.4,0.6,0.8,1.3) over the temperature range of 3–410 K. This has enabled the existing magnetic phase diagram for PrMn2−xFexGe2 to be extended from Fe concentration x=0–1 to the full range x=0–2 in terms of concentration and dMn–Mn, the intralayer distance. Analysis of the Mossbauer spectra (4.5–300 K) using a model which takes nearest-neighbor environments into account confirms the nonmagnetic nature of Fe atoms in these compounds, and leads to hyperfine parameters which deviate around the magnetic transition temperatures derived from the magnetic and neutron investigations while also enabling the Debye temperatures of PrMn2−xFexGe2 (x=0.4–1.6) to be determined. The experimental values for TCinter are found to decrease rapidly with increasing Fe concentration in the range x=0.0–0.6 compared with...

Magnetic structure and magnetic phase transitions in TbPtGe2

Magnetic and neutron diffraction measurements have been performed on TbPtGe2 at low temperatures. The compound crystallizes in the orthorhombic YIrGe2-type structure (space group Immm); crystal structure parameters have been refined on the basis of the neutron diffraction pattern collected at T = 30.7 K (paramagnetic region). TbPtGe2 is antiferromagnetic below TN = 24.2 K. Below this temperature only one of the two Tb sublattices is ordered; the Tb magnetic moments localized at the 4(i) sites order with the simple propagation vector k = 0. Below Tt1 = 11.4 K the magnetic moments at the other Tb sites, 4(h), show an ordering with the propagation vector k1 = [0.2677(8),0.1312(24),0.6989(27)]. At Tt2 = 7 K a further phase transition to a new modulated phase described by the propagation vector k2 = [0.2584(5),0,0.5895(6)] is observed.

Neutron diffraction studies of the magnetic structures of the HoRhGe and ErRhGe compounds

HoRhGe and ErRhGe compounds crystallize in the orthorhombic TiNiSi-type of structure with the space group Pnma. Neutron diffraction measurements at T = 1.6 K indicate collinear magnetic structures with the propagation vector k = (1/2,0,1/2) for HoRhGe and k = (0,1/2,0) for ErRhGe. After increasing the temperature, the change to the incommensurate sine modulated structure is observed for ErRhGe at . The temperature dependence of the magnetic peak intensities gives the N?el temperature 4.6 K for HoRhGe and 9.5 K for ErRhGe.

Magnetic structures and magnetic phase transitions in TbMn0.33Ge2

Abstract A study of the magnetic structure of TbMn0.33Ge2 has been made using a neutron diffractometer of better resolution and new results have been obtained. The paramagnetic neutron diffraction data confirmed the CeNiSi2-type of crystal structure reported earlier for this compound. The magnetic moments are located on terbium. At 1.5 K the Tb magnetic moments have two components: a collinear and sine wave modulated one so the magnetic order at this temperature has a complex character. With increasing temperature, the change of the magnetic structure to the sine wave modulated one is observed near the Neel temperature TN=28 K.

Antiferromagnetism in YbMn2Ge2–Mn magnetic sublattice

Abstract The magnetic structures of YbMn 2 Ge 2 with the tetragonal ThCr 2 Si 2 type structure have been investigated by neutron diffraction measurements over the temperature range ∼10–526 K. Rietveld refinements demonstrate that YbMn 2 Ge 2 has a planar antiferromagnetic structure below T N1 ∼510 K with a canted antiferromagnetic structure below T N2 ∼185 K. The canted antiferromagnetic ground state of YbMn 2 Ge 2 has a Mn moment value of μ Mn (10 K)=3.03(5) μ B , with the z -component of the moment μ z (10 K)=1.72(5) μ B , corresponding to a canting angle relative to the c -axis of θ (10 K)=55.4(9)°. No evidence for magnetic ordering of the Yb lattice is obtained although an unusual variation of the a -lattice parameter with temperature is observed.