ANSTO :: Browsing by Author "Hofmann, M"

Browsing by Author "Hofmann, M"

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Crystal field excitations of YbMn2Si2
(Elsevier Science BV, 2013-12-01) Mole, RA; Hofmann, M; Adroja, DT; Moze, O; Campbell, SJ
The crystal field excitations of the rare earth intermetallic compound YbMn2Si2 have been measured by inelastic neutron scattering over the temperature range 2.5-50 K. The YbMn2Si2 spectra exhibit three low energy excitations (similar to 3-7 meV) in the antiferromagnetic AFil region above the magnetic phase transition at T-N2 = 30(5) K. The crystal field parameters have been determined for YbMn2Si2 in the antiferromagnetic AFil region. A further two inelastic excitations (similar to 9 meV, 17 meV) are observed below T-N2=30(5) K, the temperature at which the high temperature antiferromagnetic structure is reported to exhibit doubling of the magnetic cell. Energy level diagrams have been determined for Yb3+ ions in the different sites above (single site) and below the magnetic transition temperature (two sites). The excitation energies for both sites are shown to be temperature independent with the temperature dependences of the transition intensities for the two sites described well by a simple Boltzmann model. The spectra below T-N2 cannot be described fully in terms of molecular field models based on either a single Yb3+ site or two Yb3+ sites. This indicates that the magnetic behaviour of YbMn(2)Si2 is more complicated than previously considered. The inability to account fully for excitations below the magnetic phase transition may be due to an, as yet, unresolved structural transition associated with the magnetic transition. © 2013, Elsevier Ltd.
Driving magnetostructural transitions in layered intermetallic compounds
(American Physical Society, 2013-05-23) Wang, JL; Caron, L; Campbell, SJ; Kennedy, SJ; Hofmann, M; Cheng, ZX; Din, MFM; Studer, AJ; Brück, E; Dou, SX
We report the dramatic effect of applied pressure and magnetic field on the layered intermetallic compound Pr0.5Y0.5Mn2Ge2. In the absence of pressure or magnetic field this compound displays interplanar ferromagnetism at room temperature and undergoes an isostructural first order magnetic transition (FOMT) to an antiferromagnetic state below 158 K, followed by another FOMT at 50 K due to the reemergence of ferromagnetism as praseodymium orders (T-C(Pr)). The application of a magnetic field drives these two transitions towards each other, whereas the application of pressure drives them apart. Pressure also produces a giant magnetocaloric effect such that a threefold increase of the entropy change associated with the lower FOMT (at T-C(Pr)) is seen under a pressure of 7.5 kbar. First principles calculations, using density functional theory, show that this remarkable magnetic behavior derives from the strong magnetoelastic coupling of the manganese layers in this compound. © 2013, American Physical Society.
Inelastic neutron scattering of YbMn2Si2 – magnetic interplay of Mn and Yb sites
(Australian Institute of Nuclear Science and Engineering, 2016-11-29) Mole, RA; Yu, DH; Hofmann, M; Wang, JL; Campbell, SJ
The layered RT2X2 series of compounds (R = rare-earth, T = 3d, 4d transition metal, X = Si, Ge) of tetragonal body centred ThCr2Si2 –type structure (I4/mmm) is one of the most widely studied systems in condensed matter and materials science [e.g. 1]. Among the RT2X2 family, Yb- and Eu-based intermetallics continue to attract strong scientific interest, mainly as a result of their intermediate valence character and the related wide range of unusual physical and magnetic properties [e.g. 2]. Having delineated the spectral features of YbMn2Si2 in the region of the layered antiferromagnetism from TN1 = 526(4) K to TN2 = 30(5) K [3], we have investigated the crystal field splitting of YbMn2Si2 by inelastic neutron scattering using PELICAN over the temperature range 5-65 K. The high resolution - 800 µeV and dynamic range ~14 meV -svailable at λ = 2.375 Å has enabled the additional excitations observed below TN2 – the temperature below which the magnetic cell is doubled along the c-axis - to be investigated in detail. The results have been interpreted in terms of a crystal field model in which Yb3+ ions have a unique environment above TN2 with the doubled magnetic cell below TN2 leading to inequivalent sites for the Yb3+ ions. The calculated excitation spectra show good agreement with the observed spectra both above and below TN2. In particular the low temperature model describes a molecular field with components in the x, y and z directions with the x and y components related to the significant contraction of ~ 0.1 % of the c lattice parameter at TN2.
Magnetic interplay of Mn and Yb sites in YbMn2Si2 – crystal field and electronic structure studies
(Australian Institute of Physics, 2018-01-30) Mole, RA; Cortie, DL; Hofmann, M; Wang, JL; Yu, DH; Wang, X; Campbell, SJ
The layered RT2X2 series of compounds (R = rare-earth, T = 3d, 4d transition metal, X = Si, Ge) of bct ThCr2Si2–type structure (I4/mmm) is one of the most widely studied systems in condensed matter and materials science [e.g. 1]. Yb-based compounds are of particular interest with their intermediate valence character leading to a wide range of unusual physical and magnetic properties. Following investigation of the magnetic dynamics of YbT2Si2 for non-magnetic T = Ni, Co and Fe [2], we investigated YbMn2Si2 to explore the interaction between the crystal field excitations of Yb atoms and the magnetic ordering of Mn atoms by thermal inelastic neutron scattering [3]. Here we extend these studies of crystal field splittingof YbMn2Si2 to high resolution (800 μeV and a dynamic range of ~14 meV; PELICAN, OPAL) over the temperature range 5-65 K. The results have been analysed in terms of a crystal field model above and below TN2, the temperature at which the collinear antiferromagnetic structure AFil (TN1 = 526(4) K > T > TN2 = 32(2) K) transforms to the low temperature structure below TN2 in which the magnetic cell is doubled along the c-axis [e.g. 3]. Density functional theory with a Hubbard correction (DFT+U method) was used to model YbMn2Si2. A key finding is that the 4f electron Ueffparameter in this compound needs to be negative to stabilise the trivalent state of YbMn2Si2 with the strength of these interactions reflected in contraction of the lattice parameter.
Magnetic interplay of Mn and Yb sites in YbMn2Si2 – crystal field splitting
(Elsevier, 2020-12-10) Mole, RA; Cortie, DL; Hofmann, M; Wang, JL; Hutchison, WD; Yu, DH; Wang, XL; Campbell, SJ
The crystal field splitting of YbMn2Si2 has been investigated over the temperature range 5–65 K using inelastic neutron scattering at a wavelength of 2.345 Å (resolution 800 μeV; dynamic range ∼10 meV). The excitation spectra have been analysed using a crystal field model above and below TN2, the temperature at which the collinear antiferromagnetic structure AFil transforms to the low-temperature structure in which the magnetic cell is doubled along the c-axis (TN1 = 526(4) K > T > TN2 = 32(2) K). The calculated excitation spectra show good agreement with the observed spectra for the unique environment of Yb3+ ions in the collinear antiferromagnetic structure AFil above TN2 and for the inequivalent sites of Yb3+ ions below TN2. This agreement has been obtained with a model for the low-temperature region in which a molecular field with optimal components in the x, y and z directions of Bx = 13.5 T, By = 65 T, Bz = 21.3 T is included. The pronounced components in the x and y directions are discussed in relation to the significant contraction of ∼0.1% of the c lattice parameter below the TN2 magnetic transition. © 2020 Published by Elsevier B.V.
Magnetic order in YbMn2Si2 - neutron scattering investigation
(Springer, 2013-08-01) Campbell, SJ; Hofmann, M; Mole, RA; Prokes, K; Wallacher, D; Wang, JL
Several mechanisms have been proposed to account for the structural and magnetic behaviour of the rare earth intermetallic compound YbMn2Si2 below the transition that occurs around ∼30 K. We have carried out detailed neutron diffraction measurements over the temperature range ∼0.3–52 K and confirm both an antiferromagnetic structure with ferromagnetic Mn (001) planes coupled antiferromagnetically along the c-axis above ∼30 K and the absence of ordering of the Yb sublattice down to 0.3 K. The decrease in intensity of the (111) reflection around ∼30 K together with the appearance of satellite reflections of propagation vector k=0012 confirm that the magnetic unit cell doubles along the c axis below T N2. The resultant molecular field acting on half of the ions below ∼30 K removes the degeneracy of the Kramers doublets of the Yb3+ ions and accounts for the additional excitations observed below ∼30 K compared with the inelastic neutron scattering above ∼30 K. © 2013, The Korean Physical Society.
Magnetic properties of PrMn1.2Fe0.8Ge2
(Elsevier, 2006-11-15) Wang, JL; Studer, AJ; Campbell, SJ; Hofmann, M; Cadogan, JM
As part of our investigation of the PrMn2-xFexGe2 series we have investigated the magnetic structures of PrMn1.2Fe0.8Ge2 (space group I4/mmm) by powder neutron diffraction and magnetic measurements. The Mn moments in PrMn1.2Fe0.8Ge2 exhibit only antiferromagnetic ordering whereas for lower Fe concentrations, x≤0.6, the Mn-sublattice exhibits ferromagnetic ordering. Two magnetic phase transitions take place with decreasing temperature in PrMn1.2Fe0.8Ge2. The first transition from paramagnetism to intralayer antiferromagnetic order (AFl) within the (0 0 1) Mn layers occurs at TN intra∼242 K. The second transition occurs at TN inter∼154 K; this transition is related to the change from the intralayer antiferromagnetic state to the antiferromagnetic mixed commensurate magnetic structure (AFmc). The neutron data also reveal an anisotropic thermal expansion within the interplanar antiferromagnetic region (-0.39% in the a-axis and+0.25% in the c-axis from∼160 to 3 K). © 2006 Elsevier B.V.
Magnetic structures and phase transitions in PrMn2-xFexGe2
(American Institute of Physics, 2008-11-19) Wang, JL; Campbell, SJ; Studer, AJ; Avdeev, M; Hofmann, M; Hoelzel, M; Dou, SX
The magnetic properties and magnetic structures of PrMn2-xFexGe2 compounds (space group I4/mmm) have been investigated using magnetic, Fe-57 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 d(Mn-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 T-C(inter) are found to decrease rapidly with increasing Fe concentration in the range x=0.0-0.6 compared with calculated T-C(inter) values due to pressure (and therefore geometric) effects only. This behavior demonstrates that electronic effects and replacement of the magnetic Mn atoms with nonmagnetic Fe atoms contribute to the overall magnetic behavior of PrMn2-xFexGe2 compounds. Compared with intralayer Mn-Mn interactions, the interlayer Mn-Mn interactions play the major role in the anomalous thermal expansion observed at magnetic transition in these layered systems, with the interlayer Mn-Mn interactions governing the significant magnetovolume effects. © 2008, American Institute of Physics
Magnetic structures and valence states of YbMn2SixGe2-x
(Australian Institute of Physics, 2004-02-04) Grimm, D; Hofmann, M; Campbell, SJ; Edge, AVJ; Studer, AJ
Rare-earth intermetallic compounds containing ytterbium exhibit a wide range of interesting and unusual physical and magnetic properties. This occurs mainly as a result of their mixed valence states (II/III) or changes from one valence state to the other. Here we present a set of results for the YbMn2SixGe2-x series (8 samples from x = 0 to 2), focusing on the magnetic structure and valence state of these compounds as investigated by neutron powder diffraction (2-530 K). We have found that the valence state of the Yb ion changes from the trivalent state of YbMn2Si2 to a divalent-like behaviour for YbMn2Ge2 with increasing Ge concentration x, with significant changes in the magnetic structure taking place around 1.5
Magnetism and magnetic structures of PrMn2Ge2-xSix
(IOP Publishing, 2013-09-25) Wang, JL; Campbell, SJ; Hofmann, M; Kennedy, SJ; Zeng, R; Din, MFM; Dou, SX; Arulraj, A; Stusser, N
The structural and magnetic properties of seven PrMn2Ge2-xSix compounds with Si concentrations in the range x = 0.0-2.0 have been investigated by x-ray diffraction, magnetic (5-350 K), differential scanning calorimetry (300-500 K) and neutron diffraction (3-480 K) measurements. Replacement of Ge by Si leads to a contraction of the unit cell and significant modifications to the magnetic states-a crossover from ferromagnetism at room temperature for Ge-rich compounds to antiferromagnetism for Si-rich compounds. The compositional dependence of the room temperature lattice parameters exhibits non-linear behaviour around x = 1.2, reflecting the presence of magnetovolume effects. Re-entrant ferromagnetism has been observed in both PrMn2Ge1.0Si1.0 and PrMn2Ge0.8Si1.2 compounds with co-existence of canted ferromagnetism and canted antiferromagnetism detected, with both compounds exhibiting a larger unit cell volume in the canted Fmc state than in the canted AFmc. Combined with earlier studies of this system, the magnetic phase diagram has been constructed over the full range of PrMn2Ge2-xSix compositions (x = 0.0-2.0) and over the temperature range of interest (T = 3-480 K). In common with other systems in the RMn2X2 series, the overall magnetic behaviour of PrMn2Ge2-xSix compounds is governed by the strong dependence of the magnetic couplings on the Mn-Mn spacing within the ab-plane. Both total manganese moment mu(Mn)(tot) and in-plane manganese moment mu(Mn)(ab) at 5 K are found to decrease with increasing Si content, which can be ascribed to the reduction of Mn-Mn separation distance and stronger Si-Mn hybridization compared with Ge-Mn hybridization. Pr site ferromagnetic ordering occurs for x < 1.6 below T-C(Pr). © 2013, IOP Publishing Ltd.
Mechanosynthesis of nanocrystalline MgFe2O4—neutron diffraction and Mössbauer spectroscopy
(Springer Nature, 2010-12-22) Šepelák, V; Bergmann, I; Feldhoff, A; Litterst, FJ; Becker, KD; Cadogan, JM; Hofmann, M; Hoelzel, M; Wang, JL; Avdeev, M; Campbell, SJ
The evolution of nanocrystalline n-MgFe2O4 by high-energy milling a mixture of MgO and α-Fe2O3 for periods of between 0 h and 12 h has been investigated by neutron diffraction in addition to previous Mössbauer, XRD and HRTEM measurements. Complete transformation of the milled products to n-MgFe2O4 only occurs on milling to ∼8 h even though the average particle size decreases to < ∼10 nm after milling for 2 h. The applied field Mössbauer spectra of n-MgFe2O4 can be well described by two subspectra representing core and shell regions with different cation distributions and spin canting angles. The neutron pattern of nanocrystalline MgFe2O4 is described well by two components comprising nanoparticles of core and shell dimensions ∼7(1) nm and ∼0.7(1) nm, respectively, in support of the Mössbauer core-shell model. © 2021 Springer Nature Switzerland AG.
Neutron diffraction study of MnNiGa2—structural and magnetic behaviour
(AIP Publishing, 2014-01-07) Wang, JL; Ma, L; Hofmann, M; Avdeev, M; Kennedy, SJ; Campbell, SJ; Din, MFM; Hoelzel, M; Wu, GH; Dou, SX
MnNiGa2 crystallizes in the L21 (Heusler) structure and has a ferromagnetic ordering temperature TC ∼ 192 K. Rietveld refinement of the neutron diffraction patterns indicates that the Ga atoms occupy the equivalent 8c position, while Mn and Ni share the 4a (0, 0, 0) and 4b (0.5, 0.5, 0.5) sites with a mixed occupancy of Mn and Ni atoms. It is found that that ∼83% of Mn and ∼17% Ni are located at the 4a site while ∼83% of Ni and ∼17% Mn occupy the 4b site. There is no evidence of a magneto-volume effect around TC. In agreement with this finding, our detailed critical exponent analyses of isothermal magnetization curves and the related Arrott plots confirm that the magnetic phase transition at TC is second order. © 2014,AIP Publishing LLC.
Phase gap in pseudoternary R1-yRyMn2X2-xXx compounds
(American Physical Society, 2013-03-04) Wang, JL; Kennedy, SJ; Campbell, SJ; Hofmann, M; Dou, SX
Our neutron diffraction investigation of PrMn2Ge2-x Si-x reveals a clear separation into two magnetic phases, canted ferromagnetic (Fmc) and antiferromagnetic (AFmc), between x = 1.0 and 1.2 and a commensurate phase gap in the lattice, due to magnetostrictive distortion. This remarkable magnetoelastic phenomenon is driven by a nonuniform atomic distribution on the X site which in turn produces subtle variations in the local lattice and abrupt changes in the Mn-Mn magnetic exchange interaction. Our results show that coexistence of Fmc and AFmc phases depends on lattice parameter, chemical pressure from the rare-earth and metalloid sites, and local lattice strain distributions. We demonstrate that these magnetostructural correlations act across the entire family of R1-yRy' Mn2X2-xXx' compounds. © 2013, American Physical Society
Structural and magnetic phase separation in PrMn2Ge2-xSix compounds
(Australian Institute of Physics, 2011-02-03) Wang, JL; Kennedy, SJ; Campbell, SJ; Hofmann, M; Zeng, R; Dou, SX; Arulraj, A; Stusser, N
Ternary intermetallic compounds of RMn2X2 (where R = rare earth or Yttrium and X = Si or Ge) display a rich variety of magnetic structures due to sharp changes in magnetic exchange interactions between neighbouring manganese atoms, resulting from changes in chemical pressure. Observed variants in the magnetic structure include ferromagnetic (f), collinear & non-collinear antiferromagnetic (a/f), mixed axial f + planar a/f, and even incommensurate a/f structures. This remarkable behaviour is symptomatic of subtle changes in interatomic bond lengths, differentiated at sub-picometre length scales. Transformation between magnetic variants is often accompanied by structural distortions due to magnetoelastic coupling. Further to this, we find that some pseudoternaries, in which one or more sites has mixed occupancy (e.g. La & Y mixed on the R site or Si & Ge mixed on the X site), simultaneously display two structural variants with different axial magnetic order (f or a/f). Such behaviour is most clearly seen in PrMn2Ge2-xSix compounds where x ≈ 1. We report a neutron diffraction study on the PrMn2Ge2-xSix system, through which we gain new insights into the magnetic and structural origins of the curious behaviour of these compounds. In certain regions of the phase diagram we clearly see phase separation (both structural and magnetic), which leads us to propose a two-phase structural model driven by changes in the Mn-Mn magnetic exchange energy, and related to variations in local strain propagated by the shared crystallographic sites. This interpretation brings into question whether a random substitution could produce such remarkable magnetoelastic phenomena or whether local site-specific atomic order is prevalent in the family of mixed 122 compounds.
Substitution of Y for Pr in PrMn2Ge2-the magnetism of Pr0.8Y0.2Mn2Ge2
(American Institute of Physics, 2013-05-07) Wang, JL; Campbell, SJ; Hofmann, M; Kennedy, SJ; Avdeev, M; Din, MFM; Zeng, R; Cheng, ZX; Dou, SX
Pr0.8Y0.2Mn2Ge2 is found to exhibit four magnetic transitions on decreasing the temperature from the paramagnetic region: (i) paramagnetism to intralayer antiferromagnetism (AFl) at T-N(intra); (ii) AFl to canted ferromagnetism (Fmc) at T-C(inter); (iii) Fmc to conical magnetic ordering of the Mn sublattice (Fmi) at T-cc; and (iv) Fmi(Mn) to Fmi(Mn) + F(Pr) at T-C(Pr). These changes in magnetic structure are discussed in terms of changes in the Mn-Mn separation distances caused by the unit cell contraction and by electronic effects due to replacement of 20% of Pr with Y. © 2013 American Institute of Physics.

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