Hyperfine-enhanced nuclear polarization inNdGaO3

Abstract

The three-dimensional magnetic ordering of the Nd' ions in NdGa03 has been studied by means of neutron powder diffraction. The transition to the antiferromagnetic state takes place at T&=1 K.Below this temperature, the Nd magnetic moments are oriented along the c axis in a c, mode. The value of the saturated magnetic moment ( = 1.1p& ) is drastically reduced below the free ion value (3.27p& ) due to crystal-field effects. The main contribution to the Nd-Nd magnetic interaction appears to be superexchange, the classical dipole-dipole interaction being too small to account for the observed Neel temperature. Below 300 mK, we observe an additional enhancement of the magnetic reAections. The analysis of the Q dependence of the neutron powder-diff'raction intensities allowed us to interpret this increase as due to the polarization of the nuclear moments of the ' 'Nd and ' 'Nd isotopes in the hyperfine field created by the Nd + electronic moments. The particular conditions which lead to the observation of this phenomenon in NdGa03 are briefly discussed. I. INTRQDUCTIQNIt is well known that the hyperfine field created by the electronic magnetic moments can be used to orient the nuclear spins. ' Although several experimental methods can be used for the study of this phenomenon, the scattering of thermal neutrons is particularly appealing since its cross section depends on the relative orientation of the spins of the neutrons and the nuclei.Because of its high scattering power for this process, the ' Nd and ' Nd isotopes are ideal candidates for the study of nuclear polarization effects. In fact, in the recent literature, they have been already observed in several Nd-based compounds as NdPd3, Nd2Cu04, or NdNi03. The quantitative analysis of this phenomenon in the former cases was however rather complicated. In NdPd3, the reason was the incommensurate magnetic structure displayed by the Nd magnetic moments. For Nd2CuO4 and NdNi03, the presence of a second magnetic ion (Cu and Ni +, respectively) introduced also additional difficulties: in NdNi03, for example, a nuclear polarization of 60% and an increase of the electronic magnetic moment of Nd + from 2p~t o = 3pz produce Rietveld refinements with identical reliability factors. To avoid these difficulties, we decided to study the perovskite NdGa03, with only one type of magnetic ion (Nd +). The crystallographic structure of NdGa03 is nowadays well known. In a previous paper dealing with the rare-earth gallates RGa03 (R=La,pr, Nd), we reported several neutron powder-diffraction experiments showing that the three compounds are orthorhombically distorted perovskites between room temperature and 12 K. In the case of LaGaO3 and PrGa03, the collective tilts of the Ga06 octahedra correspond to those of the GdFe03-type structure, which is described by the centrosymmetric Pbnm space group. On the other hand, NdGa03 was described by the noncentrosymmetric Pbn 2, space group, following the single-crystal x-ray diffraction study of Brusset. Recent higher-resolutionx-ray diffraction exp...

Reanalysis of crystal field parameter datasets for rare-earth ions at low symmetry sites: Nd3+ in NdGaO3 and Pr3+ in PrGaO3

“…The new interest in NdGaO 3 has been driven by its applications as a substrate in making a thin film of high-T c superconductors as well as semiconductors; see, e.g., reference [6]. There exists, however, a controversy concerning the crystal field parameter (CFP) datasets [1][2][3], since they are highly disparate and incompatible. Two CFP datasets [1,2] are non-standard, whereas one CFP dataset [3] is standard.…”

“…However, other compatibility problems have emerged, which require more general analysis [9]. Firstly, which axis systems were actually assumed in the CF analysis [1][2][3]? No clear definitions of the axis systems adopted could be found in [1][2][3].…”

“…To enable meaningful comparison we have first applied the standardization idea [7,8] to express the CFP datasets [1][2][3] in the same axis system corresponding to the standard range [7,8]. However, other compatibility problems have emerged, which require more general analysis [9].…”

“…Two CFP datasets [1,2] are non-standard, whereas one CFP dataset [3] is standard. The authors [1][2][3] are apparently unaware of the standardization idea [7,8], which enables comparison and analysis of the CFP datasets expressed in different axis systems for orthorhombic, monoclinic, and triclinic symmetry cases.…”

“…To study the energy levels [1][2][3] and crystal structure [4,5] of Nd 3+ ion in the neodymium gallate NdGaO 3 and Pr 3+ ion in PrGaO 3 due to the crystal field (CF) effects the inelastic neutron scattering (INS) technique and Raman spectroscopy have often been employed. The new interest in NdGaO 3 has been driven by its applications as a substrate in making a thin film of high-T c superconductors as well as semiconductors; see, e.g., reference [6].…”