The compound CeIrAl crystallizes in the orthorhombic -TiNiSi-type structure in which the cerium ions are in a mixed-valent state as inferred from its weakly temperature dependent magnetic susceptibility. This compound absorbs hydrogen at ambient temperature to form CeIrAlH 2 . A structural transformation from orthorhombic to hexagonal structure takes place on hydrogen absorption. Further, the magnetic susceptibility of the hydride exhibits Curie-Weiss behavior, typical of local moment on Ce 3ϩ ions. Thus hydrogen absorption brings about dramatic changes in the structure and the valence state of cerium in CeIrAl.
The hydriding behavior of the heavy fermion compound UNiAl has been investigated. A range of hydride compositions with orthorhombic structure ͑space group Pnma͒ has been stabilized. As a result, a different nomenclature for the UNiAlH y hydride phases is proposed. X-ray diffraction and magnetic studies are reported on three compositions in this range of hydrides, viz., yϭ0.06, 0.14, and 0.58. All these compositions do not show magnetic ordering down to 2 K, though weak ferromagnetic correlations are apparent in their magnetic studies. Low effective moment values on uranium atoms in these cases, appreciably below the value for itinerant antiferromagnetic UNiAl, indicate increased itinerant character of U 5 f electrons. We also report magnetization studies on hexagonal UNiAlH y (yϭ0.7). It shows ferromagnetic ordering below 87 K. This means that in the hexagonal hydride phases, the nature of magnetic ordering changes from antiferromagnetic (yϭ0) to ferromagnetic (yϭ0.7) and again back to antiferromagnetic for yϭ2.3. It is shown that the moment on the uranium atoms in the hexagonal hydride phases ͑yХ0.7 and 2.0-2.5͒ is close to the free-ion local moment value.
Heavy fermion itinerant antiferromagnetic UNiAl is one of the very few U-containing compounds which absorbs H2/D2 without disproportionation. The present neutron diffraction studies on UNiAlDy (y=2.2) are directed towards resolving controversies with regard to the occupancy of Ni atoms and the associated interstitial sites for (H/D) atoms, as well as the nature of magnetic ordering in the higher hydride phase with y⩾2. The fit to the neutron diffraction data is found to improve considerably if the Ni atoms originally lying in the U-atoms’ plane in UNiAl get shifted to the Ni–Al atoms’ plane in the deuteride. This is in agreement with an earlier neutron diffraction report on a deuteride sample of similar composition [T. Yamamoto et al., J. Alloys Compd. 269, 162 (1998)] and our x-ray structural studies on UNiAlH2.3 [P. Raj et al., Phys. Rev. B 63, 94414 (2001)], but differs from those of Bordallo et al., [H. N. Bordallo et al., Physica B 276–278, 706 (2000)] and of Kolomiets et al. [A. V. Kolomiets et al., J. Appl. Phys. 87, 6815 (2000)]. Our values of the structural parameters including the D-site occupancies are broadly in agreement with the results of Yamamoto et al. The magnetization studies on UNiAlD2.2 show a single antiferromagnetic transition with Néel temperature, TN=95 K.
We have observed x-ray magnetic circular dichroism ͑XMCD͒ in the helicity modulation mode at Ir L 2,3 edges in the ferromagnet IrMnAl ͑Curie temperature 379 K͒ at room temperature, 100 K and 30 K. This observation proves that Ir has a 5d magnetic moment in IrMnAl. Using the magneto-optic sum rules, which relate the integrated intensity of the XMCD and x-ray absorption spectra to the expectation values of spin and orbital angular momenta of Ir, the orbital moment to spin moment ratio, the spin moment and the orbital moment of Ir at 30 K have been deduced to be Ϫ0.17(2), 0.018͑1͒ B , and Ϫ0.0031(8) B , respectively. dc magnetization measurements yielded a total moment of 0.123 B /atom showing that the magnetic moment of Mn is also strongly reduced. We suggest that the extremely small moments of Mn and Ir in IrMnAl are associated with the suppression of local moments and delocalization of 3d and 5d electrons due to their strong hybridization with the sp electrons of Al atoms.
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