Browsing by Author "Wang, JL"
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- ItemA 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, SXAll 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.
- ItemCritical behaviour of Ho2Fe17-xMnx - magnetisation and Mössbauer spectroscopy(Springer, 2012-11-20) Wang, JL; Campbell, SJ; Kennedy, SJ; Dou, SXThe 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
- ItemCritical 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, GHThe 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
- ItemCritical phenomena and estimation of the spontaneous magnetization by a magnetic entropy analysis in Mn(0.96)Nb(0.04)CoGe alloy(American Institute of Physics., 2013-06-21) Debnath, JC; Strydom, AM; Shamba, P; Wang, JL; Dou, SXMagnetic and magnetocaloric properties of the alloy Mn0.96Nb0.04CoGe have been investigated. According to the mean-field theory prediction, the relationship between Delta S-M proportional to (H/T-C)(2/3) has been confirmed in the temperature region near T-C for that system. To investigate the nature of the magnetic phase transition, a detailed critical exponent study has been performed. The critical components, gamma, beta, and delta determined using the Kouvel-Fisher method, the modified Arrott plot, as well as the critical isotherm analysis agree well. Moreover, these critical exponents are confirmed by the Widom scaling law and the validity of the calculated critical exponents was also confirmed by the scaling theory. The values deduced for the critical exponents are close to the theoretical prediction of the mean-field model values, thus indicating that long range interactions dominate the critical behavior in the Mn0.96Nb0.04CoGe system. It is also speculated that the competition between the localized Mn-Mn magnetic interactions should be responsible for the critical behavior in this system. Moreover, an excellent agreement is found between the spontaneous magnetization determined from the entropy change (-Delta S-M vs. M-2) and the classical extrapolation from the Arrott curves (H/M vs. M-2), thus confirming that the magnetic entropy change is a valid approach to estimate the spontaneous magnetization in this system. © 2013, American Institute of Physics.
- ItemDirect 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, JLWe 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
- ItemDriving 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, SXWe 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.
- ItemThe 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, SJThe 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.
- ItemEffects of Cr substitution on structural and magnetic properties in La0.7Pr0.3Fe11.4Si1.6 compound(AIP Scitation, 2014-04-03) Din, MFM; Wang, JL; Studer, AJ; Gu, QF; Zeng, R; Debnath, JC; Shamba, P; Kennedy, SJ; Dou, SXIn an effort to explore the effect of substitution Fe by Cr in NaZn13-type La0.7Pr0.3Fe11.4−xCrxSi1.6 (x = 0, 0.06, 0.12, 0.26, and 0.34) compounds, the structure and magnetic properties have been investigated by high intensity of x-ray and neutron diffraction, scanning electron microscopy, specific heat, and magnetization measurement. It has been found that a substitution of Cr for Fe in this compounds leads to decrease in the lattice parameter a at room temperature but variation on Curie temperature (TC). While the first order nature of magnetic phase transition around TC does not change with increasing Cr content up to x = 0.34. High intensity x-ray and neutron diffraction study at variable temperatures for highest Cr concentration x = 0.34 confirmed the presence of strong magneto-volume effect around TC and indicated the direct evident of coexistence across magnetic transition as characteristic of first order nature. The values of −ΔSM around TC decrease from 17 J kg−1 K−1 for x = 0 to 12 J kg−1 K−1 for x = 0.06 and then increases with further increasing Cr content up to 17.5 J kg−1 K−1 for x = 0.34 under a change of 0–5 T magnetic field. Similar behavior on relative cooling power which is decrease from 390 J kg−1 for x = 0 to 365 J kg−1 for x = 0.06 at the beginning and then increases up to 400 J kg−1 for x = 0.34. From the point of this view with the highest Cr concentration (x = 0.34) exhibits favourable material candidate for magnetic refrigerator application therefore should inspire further study concerning on higher Cr concentration in this compound. © 2014, AIP Publishing LLC.
- ItemEffects of Cu substitution on structural and magnetic properties of La0.7Pr0.3Fe11.4Si1.6 compounds(Elsevier Ltd., 2013-05-01) Din, MFM; Wang, JL; Zeng, R; Shamba, P; Debnath, JC; Dou, SXThe structure and magnetic properties of the La0.7Pr0.3Fe11.4−xCuxSi1.6 compounds have been investigated by X-ray diffraction, scanning electron microscopy and magnetisation measurements. Cu substitution for Fe in La0.7Pr0.3Fe11.4−xCuxSi1.6 (x = 0, 0.06, 0.12, 0.23, 0.34) leads to a reduction in hysteresis loss, a decrease in magnetic entropy change but an increase in Curie temperature (TC). The influences of annealing processes at different temperatures on TC, magnetic hysteresis, and the magnetocaloric effect (MCE) of La0.7Pr0.3Fe11.4Si1.6 are investigated in detail. It has been found that a short-time and high temperature annealing process has benefits for the formation of the NaZn13 types as phase compared to a long-time and low temperature annealing process. © 2013, Elsevier Ltd.
- ItemEnhancement 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, SXThe 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.
- ItemESR studies of magnetocaloric PrMn2-xFexGe2(Australian Institute of Physics, 2014-02-05) Ren, QY; Hutchison, WD; Campbell, SJ; Wang, JLIn 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.
- ItemInelastic 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, SJThe 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.
- ItemInvestigation of the critical behavior in Mn[sub 0.94]Nb[sub 0.06]CoGe alloy by using the field dependence of magnetic entropy change(American Institute of Physics., 2013-03-07) Debnath, JC; Shamba, P; Strydom, AM; Wang, JL; Dou, SXThe critical behaviour of Mn0.94Nb0.06CoGe alloy around the paramagnetic-ferromagnetic phase transition was studied based on the field dependence on magnetic entropy change. By using the obtained exponents, the modified Arrott plot is consistent with that by using conventional method. These critical exponents are confirmed by the Widom scaling relation. Based on these critical exponents, the magnetization, field and temperature data around Tc collapse into two curves obeying the single scaling equation M(H, epsilon) = epsilon(beta)f+/-(H/epsilon(beta+gamma)). The calculated critical exponents not only obey the scaling theory but also anastomose the deduced results from the Kouvel-Fisher method [J. S. Kouvel and M. E. Fisher, Phys. Rev. 136, A1626 (1964)]. The values deduced for the critical exponents in the Mn0.94Nb0.06CoGe alloy are close to the theoretical prediction of the mean-field model, indicating that the magnetic interactions are long range. This method eliminates the drawback due to utilization of multistep nonlinear fitting in a conventional manner. So it provides an alternative method to investigate the critical behaviour. © 2013 American Institute of Physics.
- ItemLarge magnetocaloric effect in re-entrant ferromagnet PrMn1.4Fe0.6Ge2(Elsevier, 2011-02-17) Zeng, R; Dou, SX; Wang, JL; Campbell, SJMagnetocaloric 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.
- ItemLarge magnetoelectric coupling in magnetically short-range ordered Bi5Ti3FeO15 film(Nature Research, 2014-06-11) Zhao, HY; Kimura, H; Cheng, ZX; Osada, M; Wang, JL; Wang, XL; Dou, SX; Liu, Y; Yu, JD; Matsumoto, T; Tohei, T; Shibata, N; Ikuhara, YMultiferroic materials, which offer the possibility of manipulating the magnetic state by an electric field or vice versa, are of great current interest. However, single-phase materials with such cross-coupling properties at room temperature exist rarely in nature; new design of nano-engineered thin films with a strong magneto-electric coupling is a fundamental challenge. Here we demonstrate a robust room-temperature magneto-electric coupling in a bismuth-layer-structured ferroelectric Bi5Ti3FeO15 with high ferroelectric Curie temperature of ~1000 K. Bi5Ti3FeO15 thin films grown by pulsed laser deposition are single-phase layered perovskit with nearly (00l)-orientation. Room-temperature multiferroic behavior is demonstrated by a large modulation in magneto-polarization and magneto-dielectric responses. Local structural characterizations by transmission electron microscopy and Mössbauer spectroscopy reveal the existence of Fe-rich nanodomains, which cause a short-range magnetic ordering at ~620 K. In Bi5Ti3FeO15 with a stable ferroelectric order, the spin canting of magnetic-ion-based nanodomains via the Dzyaloshinskii-Moriya interaction might yield a robust magneto-electric coupling of ~400 mV/Oe·cm even at room temperature. © 2020 Springer Nature Limited
- ItemMagnetic 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, SJGiant 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.
- ItemMagnetic 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, SJThe 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.
- ItemMagnetic and structural transitions tuned through valence electron concentration in magnetocaloric Mn(Co1–xNix)Ge(American Chemical Society, 2018-02) Ren, QY; Hutchison, WD; Wang, JL; Studer, AJ; Campbell, SJThe structural and magnetic properties of magnetocaloric Mn(Co1-xNix)Ge compounds have been studied. Two responses to the increase of valence electron concentration on substitution of Ni (3d84s2) for Co (3d74s2) in the orthorhombic phase (Pnma) are proposed: expansion of unit-cell volume and redistribution of valence electrons. We present experimental evidence for electronic redistribution associated with the competition between magnetism and bonding. This competition in turn leads to complex dependences of the reverse martensitic transformation temperature TM (orthorhombic to hexagonal (P63/mmc)) and the magnetic structures on the Ni concentration. Magnetic transitions from ferromagnetic structures below x = 0.50 to noncollinear spiral antiferromagnetic structures above x = 0.55 at low temperature (e.g., 5 K) are induced by modification of the density of states at the Fermi surface due to the redistribution of valence electrons. TM is found to decrease initially with increasing Ni content and then increase. Both direct and inverse magnetocaloric effects are observed. © 2018 American Chemical Society.
- ItemMagnetic 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, SJThe 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.
- ItemMagnetic 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, SJThe 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.