Dependence of the Electric Field Gradient of Ir in Fe on the Direction of Magnetization with respect to the Crystallographic Axes

Abstract

The electric quadrupole interaction of 188 Ir ͑I p 1 2 ; T 1͞2 41.5 h͒ in a Fe single crystal was measured for magnetization parallel to the crystallographic [100], [110], and [111] axes. Contrary to all previous experiments, a strong dependence of the electric field gradient on the direction of the magnetization with respect to the crystallographic axes was observed for the first time.[ S0031-9007(97) PACS numbers: 76.60. Jx, 75.50.Bb, 76.60.Gv, 76.80.+ y The electric field gradient (EFG) at the site of impurity nuclei in cubic ferromagnetic hosts such as Fe, Ni, and fcc-Co has been the subject of many investigations, both experimental [1-9] and theoretical [5,10,11]. It was first observed by nuclear magnetic resonance (NMR) of where eq V zz (henceforth denoted as EFG) is the principal component of the traceless EFG tensor and eQ is the nuclear spectroscopic quadrupole moment. If the EQI is collinear with the magnetic hyperfine interaction, the magnetic resonance frequency is split into a set of 2I equidistant subresonances, where I is the nuclear spin. The subresonance separation is given byThe satellite resonances were considerably broader than the central line. Mainly, the following two explanations were discussed: (i) The additional broadening is due to lattice inhomogeneities. (ii) The EFG depends on the direction of the magnetization with respect to the crystal axes. To explore whether the second possibility was the origin for this broadening, Aiga and Itoh prepared single crystal alloys of Fe-0.3% Ir by the stress annealing method. The NMR measurements yielded exactly the same spectra as observed for the polycrystal samples [5]. Thus Aiga and Itoh concluded that the quadrupole interaction is almost independent of the direction of the magnetization, and even if there is some anisotropy, it is of the order of, or less than, a few hundred kHz. Johnston and Stone performed NMR on oriented nuclei (NMR-ON) measurements on 192

Spin-Orbit Electric Field Gradient in Cubic Ferromagnets: Strong Magnetization-Direction Dependence of the Field Gradient Distribution

“…The latter were found to be 0.38 (3) Fig. 3, together with the EFG's for Re, Os, Ir, and Au in Fe [11,12]. The width of the EFG distribution is 0.058(12), 0.037(4), and 0.021͑2͒ 3 10 17 V͞cm 2 for the [100], [110], and [111] directions, respectively; i.e., the [100] width is by a factor of 2.8(6) larger than the [111] width.…”

“…In the general framework of their model, the EFG may, depending on the relative sizes of spin-orbit coupling, the crystal field, and the exchange splitting, depend on the direction of the magnetization with respect to the crystal axes, or not. Led by the experiments suggesting the EFG to be invariant, they concluded that the effects of the crystal field can be neglected, which leads to an invariant EFG.Recent NMR-ON and modulated adiabatic fast passage on oriented nuclei (MAPON) measurements on Z 77 188 Ir in an Fe single crystal showed a strong dependence of the EFG on the direction of magnetization with respect to the crystallographic axes [11]. In these experiments no special attention was directed to the widths of the EFG distributions, as there was no theoretical hint that the EFG width could be directional dependent.…”

“…Recent NMR-ON and modulated adiabatic fast passage on oriented nuclei (MAPON) measurements on Z 77 188 Ir in an Fe single crystal showed a strong dependence of the EFG on the direction of magnetization with respect to the crystallographic axes [11]. In these experiments no special attention was directed to the widths of the EFG distributions, as there was no theoretical hint that the EFG width could be directional dependent.…”

“…There have been many investigations for the understanding of this EFG, both experimental [1][2][3][4][5][6][7][8][9][10][11][12] and theoretical [5,[13][14][15][16]. It was first observed by Aiga and Itoh with nuclear magnetic resonance (NMR) measurements on 191 Ir and 193 Ir in polycrystalline Fe and Ni [1,5].…”

Measurements of the Electric Quadrupole Moment of Nb and Zr Isotopes with Modulated Adiabatic Fast Passage after Recoil Implantation into hcp Co

“…In addition, it is inhomogeneously broadened, the broadening being of the same order of magnitude as the total splitting. This is due to the fact that, as detected recently [6], the electric field gradient (EFG) at the impurity site in Fe depends strongly on the direction of magnetization with respect to the crystallographic axes. Therefore single crystal hcp Co as host lattice was supposed to be superior: The EFG originating from the noncubic lattice symmetry is well defined and, in addition, much larger than the EFG in Fe.…”