Now showing 1 - 10 of 19
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    Hard x-ray photoemission spectroscopy of GdNi and HoNi
    (14-10-2020)
    Chuang, C. W.
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    De Groot, F. M.F.
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    Liao, Y. F.
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    Chin, Y. Y.
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    Tsuei, K. D.
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    Malterre, D.
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    Chainani, A.
    We study the electronic structure of GdNi and HoNi, which are magnetic materials with a Curie temperature Tc=69 and Tc=36K, respectively. These materials are useful for magnetic refrigerator applications at low temperature as they exhibit a large magnetocaloric effect near Tc. We have used hard x-ray photoemission spectroscopy (HAXPES) to investigate the core-level and valence-band electronic states of GdNi and HoNi. HAXPES measurements of the Gd and Ho 3d, 4d, 4p, 5p, and 4s core-level spectra have been compared with atomic multiplet calculations of Gd3+ and Ho3+ ionic configurations. The good match between the experimental and calculated spectra clarify the important role of spin-orbit coupling, as well as Coulomb and exchange interactions in the intermediate-coupling scheme. The core-level spectra also show plasmons in addition to the atomic multiplets. The Gd and Ho 4s spectra show clear evidence of exchange splitting. The Ni 2p and 3s spectra of GdNi and HoNi show a correlation satellite at a binding energy of 7 eV above their main peaks. The Ni 2p and Ni 3s spectra could be reproduced using charge transfer multiplet calculations. Valence-band HAXPES of GdNi and HoNi shows that the Gd 4f and Ho 4f features are also consistent with atomic multiplets and occur at high binding energies away from the Fermi level. The Ni 3d density of states are spread from the Fermi level to about 3 eV binding energy. The results indicate a partially filled Ni 3d band and show that the charge transfer model is not valid for describing the electronic structure of GdNi and HoNi.
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    Preservation of large low temperature magnetocaloric effect in metamagnetic intermetallic compounds RCu2 (R = Gd, Tb, Dy, Ho and Er) upon rapid solidification
    (30-01-2020)
    Rajivgandhi, R.
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    Chelvane, J. Arout
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    Nigam, A. K.
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    Malik, S. K.
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    In this work, magnetic and magnetocaloric properties of melt-spun RCu2 (R = Gd, Tb, Dy, Ho and Er) compounds have been studied. The melt-spun samples have formed in the same orthorhombic crystal structure (space group Imma, no. 74) as that of the arc-melted samples but with a development of crystallographic texture and micron-size grains. The melt-spun ribbons of RCu2 (R = Gd, Tb, Dy, Ho and Er) order antiferromagnetically at 43 K, 51 K, 28 K, 10 K and 11 K (TN) respectively. The ordering temperatures are almost the same as that of the arc-melted samples. The RCu2 compounds show one or more field induced transitions below TN. Therefore, a normal magnetocaloric effect is observed in these compounds when the magnetic field change is larger than the metamagnetic critical field and inverse magnetocaloric effect is observed for smaller field changes. The microgranularity does not seem to smear out the metamagnetic transition. The maximum of isothermal magnetic entropy change, ΔSmmax, is about −2.1 Jkg−1K−1, -5.5 Jkg−1K−1, -5.2 Jkg−1K−1, -17.2 Jkg−1K−1 and -11.7 Jkg−1K−1 respectively for the melt-spun RCu2 (R = Gd, Tb, Dy, Ho and Er) samples for 50 kOe field change. The refrigeration capacity is found to be 40 Jkg-1, 160 Jkg-1, 166 Jkg-1, 230 Jkg-1 and 207 Jkg-1 respectively, for the above samples. These values are nearly the same as that of the arc-melted counterparts. Thus the large magnetocaloric effect in the temperature range of 10 K–70 K make these melt-spun RCu2 samples potential materials for low temperature refrigeration applications. The results also ascertain melt-spinning process as an alternate technique for the production of magnetocaloric alloys and intermetallics.
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    Magnetocaloric effect in mixed rare earth manganite Gd0.5Dy0.5MnO3
    (04-05-2020)
    Behera, P. Suchismita
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    Single phase, polycrystalline mixed rare earth manganite Gd0.5Dy0.5MnO3 (orthorhombic, Pbnm) has been prepared by solid state reaction. Magnetization vs temperature data measured in 100 Oe field shows a weak cusp and a tendency to order around 5 K. Paramagnetic susceptibility shows Curie-Weiss behaviour. Magnetocaloric effect (MCE) has been estimated in terms of isothermal magnetic entropy change (ΔSm). The maximum value of ΔSm is about -13.4 Jkg-1K-1 at 8 K for a field change of 70 kOe.
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    Large low field magnetocaloric effect in multicomponent Laves phase intermetallic compounds Gd<sub>0.33</sub>Dy<sub>0.33</sub>Ho<sub>0.33</sub>Al<sub>2</sub>, Tb<sub>0.33</sub>Ho<sub>0.33</sub>Er<sub>0.33</sub>Al<sub>2</sub> and Dy<sub>0.33</sub>Ho<sub>0.33</sub>Er<sub>0.33</sub>Al<sub>2</sub>
    (01-01-2023)
    Jesla, P. K.
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    Arout Chelvane, J.
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    Morozkin, A. V.
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    Multicomponent Laves phase intermetallic compounds Gd0.33Dy0.33Ho0.33Al2, Tb0.33Ho0.33Er0.33Al2 and Dy0.33Ho0.33Er0.33Al2 have been synthesized by arc-melting. The samples crystallize in cubic (MgCu2-type, Space group Fd-3m) structure. Temperature dependent magnetization measurements reveal ferromagnetic order in the Gd0.33Dy0.33Ho0.33Al2, Tb0.33Ho0.33Er0.33Al2 and Dy0.33Ho0.33Er0.33Al2 compounds at 84 K, 45 K and 33 K (TC) respectively. Magnetization vs magnetic field data at 5 K suggest soft ferromagnetism. Magnetocaloric effect is estimated in terms of isothermal magnetic entropy change and adiabatic temperature change in magnetic fields up to 15 kOe and it is quite large. Therefore, these multicomponent Laves phase compounds could be useful for realizing magnetic refrigeration-based hydrogen liquefaction.
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    Dynamic spin fluctuations in the frustrated A -site spinel CuAl2 O4
    (01-07-2020)
    Cho, Hwanbeom
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    Jeong, Jaehong
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    Baker, Peter J.
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    Takeda, Hikaru
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    Mera, Nobuyoshi
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    Blundell, Stephen J.
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    Takigawa, Masashi
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    Adroja, D. T.
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    Park, Je Geun
    We performed nuclear magnetic resonance (NMR) and muon spin relaxation (μSR) experiments to identify the magnetic ground state of the frustrated quantum A-site spinel, CuAl2O4. Our results verify that the ground state does not exhibit a long-range magnetic ordering, but a glasslike transition manifests at T∗=2.3K. However, the Gaussian shape and the weak longitudinal field dependence of μSR spectra below T∗ show that the ground state has dynamic spin fluctuations, distinct from those of conventional spin glasses.
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    Magnetic, magnetocaloric and electrical transport properties of electron-doped manganite Eu0.15Ca0.85MnO3
    (05-11-2020)
    Thangavel, Kavipriya
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    In the present work, magnetic and magnetocaloric properties of Eu0.15Ca0.85MnO3 oxide have been studied. The sample crystallizes in orthorhombic crystal structure (space group Pnma, no. 62) at 300 K and undergoes a paramagnetic to antiferromagnetic transition at ~115 K (TN). The magnetization value at 5 K in field of 70 kOe is only 0.11 μB/f.u. Thermal hysteresis in low field magnetization data around TN suggests a first order transition. Metamagnetic behaviour is observed at temperatures immediately below TN. This suggests the presence of competing magnetic interactions. Isothermal magnetic entropy change is calculated from magnetization-field data and its maximum value is ~ +10 Jkg-1K-1 at 107.5 K for a field change of 70 kOe. Inverse magnetocaloric effect and the magnetic moment value in the ordered state reveal the robust antiferromagnetism in this manganite. In addition, the electrical resistivity shows semiconducting behaviour and increases strongly at low temperatures.
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    Magnetism and magnetocaloric effect of melt-spun, nanostructured GdAl2
    (01-06-2020)
    Prusty, Mitali Madhusmita
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    Arout Chelvane, J.
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    Magnetocaloric effect (MCE) of melt-spun rare earth intermetallic compound GdAl2 (Cubic, MgCu2-type) has been studied. The sample becomes nanostructured upon melt-spinning and the crystallite size obtained from the powder x-ray diffraction data is about 48 nm. A sluggish paramagnetic to ferromagnetic transition occurs at a Curie temperature (TC) of about 136 K. This value is about 30 K lower than the ferromagnetic transition temperature of the arc-melted GdAl2. The maximum isothermal magnetic entropy change (ΔSm) is found to be ∼ -8.3 Jkg-1 K-1 at 133 K for 70 kOe field change around TC. This value is quite comparable to that of bulk sample prepared by arc-melting which is about -8.5 Jkg-1 K-1 at 168 K for the same field change. Thus melt-spinning process results in broadening of the peak in the isothermal magnetic entropy change versus temperature plot without compromising on the magnetocaloric effect.
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    Magnetocaloric effect in melt-spun rare earth intermetallic compound ErAl2
    (01-03-2022)
    Prusty, Mitali Madhusmita
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    Chelvane, J. Arout
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    Morozkin, A. V.
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    Gururaj, Karanam
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    Paulose, P. L.
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    Laves phase rare earth intermetallic compound ErAl2 (Cubic, MgCu2-type) has been prepared by melt-spinning process. Analysis of powder X-ray diffraction data of melt-spun ErAl2 yields crystallite size of about 54 nm. Transmission electron microscopy image reveals particles of size ∼70 nm. The melt-spun ErAl2 orders ferromagnetically ∼10 K (TC) whereas the ferromagnetic ordering temperature is ∼14 K for the arc-melted ErAl2 compound. From the magnetization vs field data, isothermal magnetic entropy change (ΔSm) has been computed as a function of temperature near TC. The maximum ΔSm value in the melt-spun ErAl2 is ∼-34 Jkg-1K-1 at 14 K for 70 kOe field change whereas the corresponding value in the arc-melted sample is ∼-42 Jkg-1K-1 at 16 K. Thus, rapid solidification results in crystalline, coarse grained ErAl2 with submicron sized particles leading to only minor changes in the magnetic and magnetocaloric properties.
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    Electronic structure investigation of GdNi using x-ray absorption, magnetic circular dichroism, and hard x-ray photoemission spectroscopy
    (15-03-2020)
    Chuang, C. W.
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    Lin, H. J.
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    De Groot, F. M.F.
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    Chang, F. H.
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    Chen, C. T.
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    Chin, Y. Y.
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    Liao, Y. F.
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    Tsuei, K. D.
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    Chelvane, J. Arout
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    Chainani, A.
    GdNi is a ferrimagnetic material with a Curie temperature TC=69 K which exhibits a large magnetocaloric effect, making it useful for magnetic refrigerator applications. We investigate the electronic structure of GdNi by carrying out x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) at T=25 K in the ferrimagnetic phase. We analyze the Gd M4,5-edge (3d-4f) and Ni L2,3-edge (2p-3d) spectra using atomic multiplet and cluster model calculations, respectively. The atomic multiplet calculation for Gd M4,5-edge XAS indicates that Gd is trivalent in GdNi, consistent with localized 4f states. On the other hand, a model cluster calculation for Ni L2,3-edge XAS shows that Ni is effectively divalent in GdNi and strongly hybridized with nearest-neighbor Gd states, resulting in a d-electron count of 8.57. The Gd M4,5-edge XMCD spectrum is consistent with a ground-state configuration of S=7/2 and L=0. The Ni L2,3-edge XMCD results indicate that the antiferromagnetically aligned Ni moments exhibit a small but finite total magnetic moment (mtot∼0.12μB) with the ratio mo/ms∼0.11. Valence band hard x-ray photoemission spectroscopy shows Ni 3d features at the Fermi level, confirming a partially filled 3d band, while the Gd 4f states are at high binding energies away from the Fermi level. The results indicate that the Ni 3d band is not fully occupied and contradicts the charge-transfer model for rare-earth based alloys. The obtained electronic parameters indicate that GdNi is a strongly correlated charge-transfer metal with the Ni on-site Coulomb energy being much larger than the effective charge-transfer energy between the Ni 3d and Gd 4f states.
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    Enhanced magnetocaloric effect in undercooled rare earth intermetallic compounds RNi (R = Gd, Ho and Er)
    (01-04-2020)
    Kurian, Jinu
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    Rahul, M. R.
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    Arout Chelvane, J.
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    Morozkin, A. V.
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    Nigam, A. K.
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    Equiatomic RNi (where R = Gd, Ho and Er) compounds have been prepared by undercooling. Magnetization data confirm ferromagnetic ordering of the samples at 69 K, 35 K and 10 K (TC) respectively. Magnetocaloric effect (MCE) has been estimated in terms of isothermal magnetic entropy change (ΔSm) near TC. The maximum ΔSm value (ΔSmmax) for 50 kOe field change is about −18 Jkg-1K−1, −20 Jkg-1K−1 and −30 Jkg-1K−1 respectively near TC for the undercooled RNi (R = Gd, Ho and Er) compounds. The ΔSmmax value is more than that obtained for the same compounds prepared by arc-melting and melt-spinning techniques. The observed enhancement in MCE of the undercooled samples could be due to the improved purity that results in faster change of magnetization around the magnetic transition. Thus undercooling rare earth intermetallics and alloys seems to be an attractive, alternative method to synthesize magnetocaloric materials.