Crystalline electric field splitting in YbNi₄P₂ measured by inelastic neutron scattering

Abstract

The heavy fermion (HF) compound YbNi 4 P 2 is a promising new system for the study of ferromagnetic quantum criticality. Its particular crystal structure in which the Yb ions form chains along the c-axis might cause quasi-1D magnetic interactions. In thermodynamic and transport measurements, highly anisotropic behaviour has been observed. To clarify the role of the crystalline electric field (CEF) on this anisotropy, we have measured its level scheme by inelastic neutron scattering. We observed a broad magnetic excitation feature between 7 and 13 meV that is composed of two or three CEF transitions. We discuss the probability of an additional CEF level at either very high or very low energy.

Analysis of the crystal electric field parameters of YbNi4P2

“…All fit parameters, the energies E 1 , E 2 , E 3 , and Γ 0 and Γ 1 , can in principle be obtained by neutron scattering measurements, Γ 0 and Γ 1 being the half-width at half maximum of the ground state and the first excited state, respectively. From references [8] and [13], we get E 1 = 8.5 meV, E 2 = 12.5 meV, Γ 0 = 0.6 meV and Γ 1 = 2.5 meV, leaving only E 3 as a free fit parameter. This fit is shown by the blue line in figure 2(a) and yields E 3 = 29.4 meV.…”

“…Thus, in an INS experiment three transitions from the ground state should appear. However, only two transitions were detected in the range up to 60 meV, namely at 8.5 and 12.5 meV [8,13], see also figure 2(b). The third transition seems to have a vanishing transition matrix element for neutron scattering.…”

“…INS was measured at IN4 at ILL, Grenoble, on powder samples of YbNi 4 P 2 , as published previously [8]. Further data at lower energy transfer were obtained at ToFToF, FRM2 [13].…”

“…There is only one crystallographic P position, but as in the case of Yb, the local symmetry of the P site is orthorhombic, with half of the P sites rotated by 90°around the c-axis. The basal plane main local axes are along [110] and [1][2][3][4][5][6][7][8][9][10], and thus parallel to the basal plane local main axes of Yb. As a result, one expects an basal plane anisotropic hyperfine field on the P site which is connected to the basal plane anisotropic 4f susceptibility at the Yb site.…”

“…The CEF excitations of YbNi 4 P 2 have been studied by inelastic neutron scattering (INS), identifying two excitations at 8.5 and 12.5 meV [8]. In the present study, we combine these data with results from heat capacity, susceptibility and NMR measurements to perform a comprehensive analysis of the CEF scheme.…”