Infrared Zeeman study of the ${\mathrm{Nd}}^{3+}\text{−}{\mathrm{Cu}}^{2+}$ anisotropic exchange interaction in ${\mathrm{Nd}}_2{\mathrm{CuO}}_4$

We have performed a Zeeman crystal-field infrared transmission study of ${\mathrm{Nd}}_2{\mathrm{CuO}}_4$. While a large discrepancy between the Zeeman calculations and the experimental data is observed below a magnetic field of 4 T applied either along the [100] or the [110] directions, the Zeeman calculations are in good agreement with the experimental data above that critical field. This indicates the suppression, above 4 T, of the ${\mathrm{Nd}}^{3+}\text{−}{\mathrm{Cu}}^{2+}$ anisotropic exchange interaction responsible for the nonzero Kramers doublet splittings observed in this compound. We show that the $\text{noncollinear}\to \text{collinear transition}$ of the magnetic structure is not responsible for this phenomena. This behavior is explained by a progressive spin reorientation of the ${\mathrm{Nd}}^{3+}$ spins from an antiferromagnetic to a paramagnetic configuration. For $\mathbf{H}\Vert \left[001\right]$, the exchange interaction is not significantly affected. Moreover, it is observed that the ${\mathrm{Nd}}^{3+}\text{−}{\mathrm{Nd}}^{3+}$ interactions, which modify only slightly the Kramers doublet splittings, is opposite to both the ${\mathrm{Nd}}^{3+}\text{−}{\mathrm{Cu}}^{2+}$ anisotropic exchange and the Zeeman interactions.

Figure 1

${\mathrm{Nd}}^{3+}{}^{4}I_{9∕2}\to {}^{4}I_{11∕2}$ low temperature spectra (8.5 K) of sample 1 for $\mathbf{H}\Vert \left[001\right]$. Italic and greek symbols correspond to assigned and unassigned ${\mathrm{Nd}}^{3+}$ regular site CF excitations, respectively.

Figure 2

Comparison between the Zeeman calculations and the experimental data in the ${\mathrm{Nd}}^{3+}{}^{4}I_{9∕2}\to {}^{4}I_{11∕2}$ spectral range for $\mathbf{H}\Vert \left[001\right]$. Solid and dashed lines correspond to calculated transitions from the lowest and highest energy component of the GSKD, respectively. Circles and squares are associated with assigned ${\mathrm{Nd}}^{3+}$ regular site experimental transitions at 8.5 K starting from the lowest and the highest energy component of the GSKD, respectively.

Figure 3

${\mathrm{Nd}}^{3+}{}^{4}I_{9∕2}\to {}^{4}I_{11∕2}$ low temperature spectra of (a) sample 2 for $\mathbf{H}\Vert \left[100\right]$; (b) sample 3 for $\mathbf{H}\Vert \left[110\right]$. Each spectrum with $\mathbf{H}<8\mathrm{T}$ and $\mathbf{H}>8\mathrm{T}$ has been obtained at 8.5 and 1.6 K, respectively. Italic and greek symbols correspond to assigned and unassigned ${\mathrm{Nd}}^{3+}$ regular site CF excitations, respectively, while asterisks are associated with CF transitions of ${\mathrm{Nd}}^{3+}$ in the vicinity of an apical oxygen.

Figure 4

Comparison between the Zeeman calculations and the experimental data in the ${\mathrm{Nd}}^{3+}{}^{4}I_{9∕2}\to {}^{4}I_{11∕2}$ spectral range for (a) $\mathbf{H}\Vert \left[100\right]$ or (b) $\mathbf{H}\Vert \left[110\right]$. Solid and dashed lines correspond to calculated transitions from the lowest and highest energy component of the GSKD, respectively. Circles (upward triangles) and squares (downward triangles) are associated with ${\mathrm{Nd}}^{3+}$ regular site assigned experimental transitions at 8.5 K (1.6 K) starting from the lowest and the highest energy component of the GSKD, respectively.

Figure 5

${\mathrm{Nd}}^{3+}{}^{4}I_{9∕2}\to {}^{4}I_{15∕2}$ low temperature spectra (8.5 K) of (a) sample 2 for $\mathbf{H}\Vert \left[100\right]$; (b) sample 3 for $\mathbf{H}\Vert \left[110\right]$. Italic and greek symbols correspond to assigned and unassigned ${\mathrm{Nd}}^{3+}$ regular site CF excitations, respectively, while asterisks are associated with CF transitions of ${\mathrm{Nd}}^{3+}$ in the vicinity of an apical oxygen. The inset of (a) compares the $5845–5900{\mathrm{cm}}^{-1}$ spectral range of the spectra recorded at 8.5 and 40 K under 7.5 T.

Figure 6

Comparison between the Zeeman calculations and the experimental data in the ${\mathrm{Nd}}^{3+}{}^{4}I_{9∕2}\to {}^{4}I_{15∕2}$ spectral range for (a) $\mathbf{H}\Vert \left[100\right]$ and (b) $\mathbf{H}\Vert \left[110\right]$. Solid and dashed lines correspond to calculated transitions from the lowest and highest energy component of the GSKD, respectively. Circles and white squares are associated with ${\mathrm{Nd}}^{3+}$ regular site assigned experimental transitions at 8.5 K starting from the lowest and the highest energy component of the GSKD, respectively. Black squares refer to assigned transitions detected at 40 K.

Figure 7

Experimental splittings (circles) measured in the ${}^{4}I_{9∕2}\to {}^{4}I_{15∕2}$ spectral range compared with the Zeeman calculations (lines) obtained for in-plane magnetic fields. The measured splittings correspond to the sum of the ground state doublet splitting and excited doublet splittings, as obtained from the subtraction of the transition wave numbers. (a) and (b), $s\text{−}p$; (c) and (d), $v\text{−}t$; (e) and (f), $y\text{−}x$; (g), $F\text{−}D$.

Figure 8

(Color online) Magnetic configuration of ${\mathrm{Nd}}_2{\mathrm{CuO}}_4$ in the collinear phase assuming either an antiferromagnetic alignment [(a) $\mathbf{H}\Vert \left[100\right]$; (b) $\mathbf{H}\Vert \left[110\right]$] or a ferromagnetic alignment [(c) $\mathbf{H}\Vert \left[100\right]$; (d) $\mathbf{H}\Vert \left[110\right]$] of the ${\mathrm{Nd}}^{3+}$ spins.