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Browsing by Author "Campbell, SJ"

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A comparative study of magnetic behaviors in TbNi2, TbMn2 and TbNi2Mn
(AIP Publishing LLC., 2014-01-01) Wang, JL; Din, MFM; Kennedy, SJ; Hong, F; Campbell, SJ; Studer, AJ; Wu, GH; Cheng, ZX; Dou, SX
All TbNi2, TbMn2, and TbNi2Mn compounds exhibit the cubic Laves phase with AB2-type structure in spite of the fact that the ratio of the Tb to transition-metal components in TbNi2Mn is 1:3. Rietveld refinement indicates that in TbNi2Mn the Mn atoms are distributed on both the A (8a) and B (16d) sites. The values of the lattice constants were measured to be a¼14.348A ° (space group F-43 m), 7.618A ° , and 7.158A ° (space group Fd-3 m) for TbNi2, TbMn2, and TbNi2Mn, respectively. The magnetic transition temperatures TC were found to be TC¼38K and TC¼148K for TbNi2 and TbNi2Mn, respectively, while two magnetic phase transitions are detected for TbMn2 at T1¼20K and T2¼49 K. Clear magnetic history effects in a low magnetic field are observed in TbMn2 and TbNi2Mn. The magnetic entropy changes have been obtained. © 2014 AIP Publishing LLC.
Critical behaviour of Ho2Fe17-xMnx - magnetisation and Mössbauer spectroscopy
(Springer, 2012-11-20) Wang, JL; Campbell, SJ; Kennedy, SJ; Dou, SX
The magnetic properties of Ho2Fe17 − xMnx compounds (x = 0–2) of ferromagnetic ordering temperatures up to TC ~344 K have been investigated by DC magnetization and Mössbauer effect measurements. The nature of the magnetic phase transitions and the critical behaviour around TC has been investigated by analysis of the magnetisation data and the critical exponents β, γ and δ determined. The critical exponents are found to be similar to the theoretical values of the mean-field model for which β = 0.5 and γ = 1.0, indicating the existence of a long-range ferromagnetic interactions. The isothermal entropy changes ΔS around TC have been determined as a function of temperature in different magnetic fields. © 2012, Springer
Critical magnetic transition in TbNi2Mn-magnetization and Mössbauer spectroscopy
(IOP Publishing LTD, 2011-06-01) Wang, JL; Campbell, SJ; Kennedy, SJ; Zeng, R; Dou, SX; Wu, GH
The structural and magnetic properties of the TbNi2Mnx series (0.9 ≤ x ≤ 1.10) have been investigated using x-ray diffraction, field- and temperature-dependent AC magnetic susceptibility, DC magnetization (5–340 K; 0–5 T) and 57Fe Mössbauer spectroscopy (5–300 K). TbNi2Mnx crystallizes in the MgCu2-type structure (space group Fd\bar {3}m ). The additional contributions to the magnetic energy terms from transition-metal–transition-metal interactions (T–T) and rare-earth–transition-metal interactions (R–T) in RNi2Mn compounds contribute to their increased magnetic ordering temperatures compared with RNi2 and RMn2. Both the lattice constant a and the Curie temperature TC exhibit maximal values at the x = 1 composition indicating strong magnetostructural coupling. Analyses of the AC magnetic susceptibility and DC magnetization data of TbNi2Mn around the Curie temperature TC = 147 K confirm that the magnetic transition is second order with critical exponents β = 0.77 ± 0.12, γ = 1.09 ± 0.07 and δ = 2.51 ± 0.06. These exponents establish that the magnetic interactions in TbNi2Mn are long range despite mixed occupancies of Tb and Mn atoms at the 8a site and vacancies. The magnetic entropy − ΔSM around TC is proportional to (μ0H/TC)2/3 in agreement with the critical magnetic analyses. The Mössbauer spectra above TC are fitted by two sub-spectra in agreement with refinement of the x-ray data while below TC three sub-spectra are required to represent the three inequivalent local magnetic environments.(c) 2011 IOP Publishing LTD
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.
Direct evidence of Ni magnetic moment in TbNi2Mn—X-ray magnetic circular dichroism
(Elsevier, 2014-12-01) Yu, DH; Huang, MJ; Su, HC; Lin, HJ; Chen, CT; Campbell, SJ; Wang, JL
We have investigated the individual magnetic moments of Ni, Mn and Tb atoms in the intermetallic compound TbNi2Mn in the Laves phase (magnetic phase transition temperature TC ~131 K) by X-ray magnetic circular dichroism (XMCD) studies at 300 K, 80 K and 20 K. Analyses of the experimental results reveal that Ni atoms at 20 K in an applied magnetic field of 1 T carry an intrinsic magnetic moment of spin and orbital magnetic moment contributions 0.53±0.01 μB and 0.05±0.01 μB, respectively. These moment values are similar to those of the maximum saturated moment of Ni element. A very small magnetic moment of order <0.1 μB has been measured for Mn. This suggests that Mn is antiferromagnetically ordered across the two nearly equally occupied sites of 16d and 8a. A magnetic moment of up to ~0.3 μB has been observed for the Tb atoms. Identification of a magnetic moment on the Ni atoms has provided further evidence for the mechanism of enhancement of the magnetic phase transition temperature in TbNi2Mn compared with TbNi2 (TC~37.5 K) and TbMn2 (TC~54 K) due to rare earth–transition metal (R–T) and transition metal–transition metal (T–T) interactions. The behaviour of the X-ray magnetic circular dichroism spectra of TbNi2Mn at 300 K, 80 K and 20 K – above and below the magnetic ordering temperature TC ~131 K – is discussed. © 2014 Elsevier
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.
The effect of Fe and Ni substitution in magnetocaloric MnCoGe
(Australian Institute of Physics, 2013-02-05) Ren, QY; Hutchinson, WD; Wang, JL; Kemp, W; Cobas, R; Cadogan, JM; Campbell, SJ
The MnCoGe family of compounds shows potential as a rare-earth free material for magnetocaloric applications around room temperature. We present initial findings on the effects of the substitution of Fe and Ni for Mn in a series of Mn1-xTxCoGe compounds (T = Fe, Ni; x = 0.04 - 0.10). Investigations include x-ray diffraction, differential scanning calorimetry(200 - 670 K) and magnetisation (5 - 350 K) measurements in magnetic fields up to 8 T. The influence of the Fe and Ni substitutions on the transformation temperature between the hexagonal and orthorhombic structures, the resultant phase fractions and their magnetic phase transitions are reported.
Enhancement of the refrigerant capacity in low level boron doped La0.8Gd0.2Fe11.4Si1.6
(Elsevier Science BV., 2013-06-01) Shamba, P; Zeng, R; Wang, JL; Campbell, SJ; Dou, SX
The effects of boron doping on the itinerant-electron metamagnetic (IEM) transition and the magnetocaloric effects (MCEs) in the cubic NaZn13-type La0.8Gd0.2Fe11.4Si1.6 compound have been investigated. The Curie temperature, TC, of La0.8Gd0.2Fe11.4Si1.6Bx compounds with x=0, 0.03, 0.06, 0.2 and 0.3 was found to increase from 200 K to 222 K with increase in boron doping, x. The maximum values of the isothermal magnetic entropy change, Delta S-M, (derived using the Maxwell relation for a field change ΔB=0–5 T) in La0.8Gd0.2Fe11.4Si1.6Bx with x=0, 0.03, 0.06, 0.2 and 0.3 are 14.8, 16, 15, 7.5 and 6.6 J kg−1 K−1 respectively, with corresponding values of the refrigerant capacity, RCP of 285, 361, 346, 222 and 245 J kg−1. The large Delta S-M values observed for the undoped sample, and the low level B doped La0.8Gd0.2Fe11.4Si1.6B0.03 and La0.8Gd0.2Fe11.4Si1.6B0.06 compounds are attributed to the first order nature of the IEM transition while the decrease of Delta S-M at x=0.2 and 0.3 is due to a change in the second order phase transition with increase in B doping. The nature of the magnetic phase transitions is also reflected by the magnetic hysteresis of 3.7, 9, 5.7, 0.4 and 0.3 J kg−1 for x=0.0, 0.03, 0.06, 0.2 and 0.30 respectively. The possibility of tuning the TC and the magnetocaloric properties at temperatures close to room temperature make this system interesting from the points of view of both fundamental aspects as well as applications. © 2013, Elsevier Ltd.
ESR studies of magnetocaloric PrMn2-xFexGe2
(Australian Institute of Physics, 2014-02-05) Ren, QY; Hutchison, WD; Campbell, SJ; Wang, JL
In a recent paper, we investigated the magnetic structures, phase transitions and magnetocaloric entropy of PrMn1.6Fe0.4Ge2 by a combination of bulk magnetometry, 57Fe Mössbauer spectroscopy and electron spin resonance (ESR) over the temperature range 5-300 K. This work followed on from a broader study of the PrMn2-xFexGe2 family of compounds, in which it was found that with decreasing temperature from the paramagnetic region, three magnetic phase transitions have been detected for PrMn1.6Fe0.4Ge2. The transition temperatures and related magnetic structures (using the notation of [3]) the magnetic structures are: (i) paramagnetism to intralayer antiferromagnetism (AFl) at TN intra=370 K; (ii) AFl to canted ferromagnetism (Fmc) at TC inter∼230 K, and (iii) a third transition around TC Pr∼30 K with ferromagnetic ordering of the Pr sublattice resulting in the combined magnetic region (Fmc+F(Pr)). Here the ESR, focusing on the Pr3+ 4f magnetic moment and undertaken in the vicinity of the lowest transition temperature, is the subject of further analysis in order to correlate the observed resonant line/s and changes in g-factors with the phases mentioned above. In particular an aim is to link the increase in g factor of the Pr3+ ion (from g = 0.85 in the region above TC Pr∼30 K to g ~ 2.5 at 8 K) with the bulk moments measured via DC magnetisation.
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.
Large magnetocaloric effect in re-entrant ferromagnet PrMn1.4Fe0.6Ge2
(Elsevier, 2011-02-17) Zeng, R; Dou, SX; Wang, JL; Campbell, SJ
Magnetocaloric effects (MCE) at multiple magnetic phase transition temperatures in PrMn1.4Fe0.6Ge2 were investigated by heat capacity and magnetization measurements. PrMn1.4Fe0.6Ge2 is of a re-entrant ferromagnet and performs multiple magnetic phase transitions in the temperature range from 5 to 340 K. A large magnetic entropy change (−ΔSM) 8.2 J/kg K and adiabatic temperature change (ΔTad) 4.8 K are observed for a field change of 0–1.5 T around 25.5 K, associated with the field-induced first order magnetic phase transition (FOMT) from the antiferromagnetic to the ferromagnetic state with an additional Pr magnetic contribution. These results suggest that a re-entrant ferromagnet is probably promising candidate as working material in the hydrogen and nature gas liquefaction temperature range magnetic refrigeration technology. © 2010, Elsevier Ltd.
Magnetic and structural properties of intermetallic NdMn2-xTixSi2 compounds
(Australian Optical Society, 2012-01-01) Din, MFM; Wang, JL; Zeng, R; Shamba, P; Hutchison, WD; Avdeev, M; Kennedy, SJ; Campbell, SJ
Giant magnetocalorie effects have been observed in NdMn2-xTixSi2 around the Curie Temperature Tc (with AB = 5-0 T.) The magnetic entropy charge decreases with increasing x from 27 J kg- K- for x=0 to 10 J kg- K- for x =0.3. Neutron investigations indicate that magnetostructural coupling contribution plays a critical role in the large value of magnetic entropy change.
Magnetic and structural transitions in magnetocaloric Mn(Co1-xNix)Ge alloys
(Australian Institute of Physics, 2017-02-01) Ren, QY; Hutchison, WD; Wang, JL; Studer, AJ; Cadogan, JM; Campbell, SJ
The magnetocaloric effect (MCE) - a significant temperature change due to the entropy change around magnetic transitions in materials driven by magnetisation or demagnetisation - has emerged as an increasingly important topic in condensed matter physics in the past two decades. A direct (positive) MCE occurs around a magnetic transition from ferromagnetism (FM) to paramagnetism (PM), while an inverse (negative) MCE is obtained around a magnetic transition from antiferromagnetism (AFM) to FM. If such magnetic transitions couple with a structural transition, a first-order magneto-structural transition can form and hence strengthen the MCE. In this work, the magnetic and structural transitions have been tuned by substitution of Ni for Co in MnCoGe. The Mn(Co1-xNix)Ge samples (x = 0.14 - 1.00) were studied by magnetisation, x-ray and neutron powder diffraction measurements over the temperature range 5 - 450 K. Mn(Co1-xNix)Ge alloys have an orthorhombic (Orth) TiNiSi-type structure (Pnma) at low temperature with transformation to a hexagonal (Hex) Ni2In-type structure (P63/mmc) at the martensitic transformation temperature TM. The increase of the Ni content changes the orthorhombic phase from FM (x < 0.55) to spiral-AFM (x ≥ 0.55). In addition, the transformation temperature TM for the reverse martensitic transformation - from orthorhombic to hexagonal - decreases with Ni content x when x < 0.55 and then increases when x ≥ 0.55. The adjustment of TM leads to the occurrences of first-order FM-Orth/PM-Hex magneto-structural transitions and large values of the direct MCE in the samples with ~0.20 < x < ~0.60. Moreover, the spiral-AFM/FM magnetic transitions in the orthorhombic phase for samples with ~0.55 < x < ~0.75 result in an inverse MCE.
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 order and structural properties of Tb2Fe2Si2C
(Elsevier, 2016-01-05) Susilo, RA; Cadogan, JM; Hutchison, WD; Avdeev, M; Cobas, R; Muñoz-Pérez, S; Campbell, SJ
The structural and magnetic properties of Tb2Fe2Si2C have been investigated by bulk measurements (magnetisation and specific heat), X-ray diffraction, neutron powder diffraction and 57Fe Mössbauer spectroscopy over the temperature range 3 K–300 K Tb2Fe2Si2C is antiferromagnetic with a Néel temperature TN of 44(2) K. The magnetic structure can be described with a propagation vector k = [0 0 ] with the Tb magnetic moments ordering along the b-axis. We also observed strong magnetoelastic effects in particular along the a- and c-axes associated with the antiferromagnetic transition. The 57Fe Mössbauer spectra show no evidence of magnetic splitting down to 10 K, indicating that the Fe atom is non-magnetic in Tb2Fe2Si2C. © 2015 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 ordering in Er2Fe2Si2C and Tm2Fe2Si2C
(Australian Institute of Physics, 2015-02) Susilo, RA; Cadogan, JM; Hutchison, WD; Campbell, SJ; Avdeev, M; Ryan, DH; Namiki, T
The magnetic ordering of two members of the R2Fe2Si2C (R = rare-earth) series of compounds (monoclinic Dy2Fe2Si2C-type structure with the C2/m space group), Er2Fe2Si2C and Tm2Fe2Si2C, have been studied by neutron powder diffraction and 166Er Mössbauer spectroscopy, complemented by magnetisation and specific heat measurements. In both cases, antiferromagnetic ordering of the R sublattice is observed, with Neel temperatures of 4.8(2) K and 2.6(3) K for Er2Fe2Si2C and Tm2Fe2Si2C, respectively. The magnetic structures of the Erand Tm-based compounds are quite different from those found for the other members of the R2Fe2Si2C series. Previous studies show that the common magnetic structure of the heavy- R2Fe2Si2C compounds involves ordering of the R sublattice along the b-axis with a propagation vector k = [0, 0, ½]. However, the antiferromagnetic structure of the Er sublattice in Er2Fe2Si2C is described by k = [½, ½, 0] with the Er magnetic moments lying close to the ac-plane. Tm2Fe2Si2C is found to exhibit a more complex magnetic structure that is characterised by a square-wave modulation of the Tm magnetic moments along the a-axis and a cell-doubling along the b-axis with k = [0.403(1), ½, 0]. The differences in the magnetic structures of these compounds are interpreted in terms of the RKKY exchange interaction, which depends on the R-R interatomic distances, and crystal field effects acting on the R3+ ions.
Magnetic ordering in Ho2Fe2Si2C
(American Institute of Physics, 2015-02-23) Susilo, RA; Cadogan, JM; Cobas, R; Hutchison, WD; Avdeev, M; Campbell, SJ
We have used neutron diffraction and 57Fe M€ossbauer spectroscopy, complemented by magnetisation and specific heat measurements, to examine the magnetic ordering of Ho2Fe2Si2C. We have established that Ho2Fe2Si2C orders antiferromagnetically below TN¼ 16(1) K with a magnetic structure involving ordering of the Ho sublattice along the b-axis with a propagation vector k ¼ ½0 0 12. 57Fe M€ossbauer spectra collected below TN show no evidence of a magnetic splitting, demonstrating the absence of long range magnetic ordering of the Fe sublattice. A small line broadening is observed in the 57Fe spectra below TN, which is due to a transferred hyperfine field—estimated to be around 0.3 T at 10 K—from the Ho sublattice. VC 2015 AIP Publishing LLC.
Magnetic phase transition and Mossbauer spectroscopy of ErNi2Mnx compounds
(American Institute of Physics, 2011-04-01) Wang, JL; Campbell, SJ; Zeng, R; Dou, SX; Kennedy, SJ
Investigation of ErNi(2)Mn(x) with 0.90 <= x <= 1.10 indicates that the MgCu(2)-type structure (with space group Fd-3m) is formed within x = 0.97-1.10. Curie temperature T(C) = 50 K of ErNi(2)Mn is significantly higher than those of the corresponding ErNi(2) (T(C) = 7 K) and ErMn(2) (T(C) = 15 K) compounds. Detailed analyses of DC magnetization data demonstrate that the magnetic phase transition at the Curie temperature is second order. The magnetic entropy change around T(C) has been found to be 4.8 J/kg K for a magnetic field change of 0 to 5 T with its relative cooling power similar to 283 J/kg. (57)Fe Mossbauer spectra below T(C) have been fitted with three sub-spectra representing three inequivalent local magnetic environments while the paramagnetic spectra are fitted using two sub-spectra for two inequivalent crystal sites (8a and 16d), which supports the conclusion based on Rietveld refinement that Mn atoms occupy at both 8a and 16d sites. (C) 2011 American Institute of Physics.
Magnetic phase transition and thermal expansion in LaFe13-x-yCoySiz
(Australian Institute of Physics, 2012-02-02) Wang, JL; Campbell, SJ; Kennedy, SJ; Shamba, P; Zeng, R; Dou, SX
The structural and magnetic properties of a series of LaFe13-x-yCoySix compounds have been investigated by X-ray diffraction, thermal expansion, magnetic and Mössbauer effect measurements. As is evident from the thermal expansion curves of Fig. 1(a), the Curie temperatures of LaFe13-xSix compounds increase with increasing Si content from TC~219 K for x=1.6 to TC~250 K for x=2.6. Further enhancement in the Curie temperature from TC~250 K to TC~281 K is obtained on substitution of Co for Fe in LaFe10.4Si2.6 to LaFe9.4CoSi2.6. A pronounced positive spontaneous volume magnetostriction has been observed below the Curie temperature TC (see Fig. 1(a)). The anomalous thermal expansion can be attributed to the volume dependence of the magnetic energy. Both the magnetization and Mössbauer spectroscopy studies (e.g. Fig. 1(b)) indicate that the type of the magnetic phase transition at TC changes from first order for LaFe11.4Si1.6 to second order for LaFe10.4Si2.6 and LaFe9.4CoSi2.6. The different natures of the magnetic transitions in LaFe13-x-yCoySix are discussed in terms of the classical model for itinerant ferromagnets and the volume dependence of the magnetic energy which is very sensitive to the distance between first-neighbor transition-metal atoms.

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