$\beta$-detected NMR spin relaxation in a thin film heterostructure of ferromagnetic EuO

We present $\beta$-detected NMR measurements of the spin-lattice relaxation of ${}^8\mathrm{Li}^{+}$ implanted into an epitaxial heterostructure based on a 100 nm thick film of ferromagnetic (FM) EuO as a function of temperature through its FM transition. In the FM state, the spin-lattice relaxation rate follows the same temperature dependence, determined by magnon scattering mechanisms, observed in the bulk by ${}^{153}\mathrm{Eu}$ NMR, but above 40 K, the signal is wiped out. We also find that ${}^8\mathrm{Li}^{+}$ stopped in material adjacent to the magnetic layer exhibits spin relaxation related to the critical slowing of the Eu spins. A particularly strong relaxation in the Au overlayer suggests an unusual strong nonlocal coupling mechanism to ${}^8\mathrm{Li}$ in the metal.

Figure 1

The spin relaxation at low temperatures. The pronounced kink at 4 s corresponds to the end of the beam pulse. The fit curves shown convolve the spin relaxation function with the beam pulse that starts at $t=0$. At 5 K, in the ferromagnetic phase, we see the full asymmetry. On warming towards $T_C$, $1/T_1$ increases rapidly, wiping out the signal from ${}^8\mathrm{Li}$ in the EuO layer.

Figure 2

Relaxation of the remaining asymmetry, due to ${}^8\mathrm{Li}^{+}$ stopping in layers adjacent to the EuO. Note factor of 2 in the vertical scale compared to Fig. 1, with the fits discussed in the text. Inset: The implantation profile of ${}^8\mathrm{Li}^{+}$ at 18 keV into the nominal thicknesses of the sample heterostructure from srim2013 [30].

Figure 3

The relaxation rates at high temperature from fits described in the text. The signal is attributed to ${}^8\mathrm{Li}^{+}$ in the nonmagnetic layers adjacent to EuO. The curves, which differ only by a scale factor, are guides to the eye. The dashed gray curve shows the Korringa relaxation in pure Au foil modulated by the ${}^8\mathrm{Li}^{+}$ site change at $\sim 170$ K [31].

Figure 4

The relaxation rates at low temperature from the fits described in the text. The same curves as in Fig. 3 show the behavior of relaxation in the proximal layers. At the lowest temperatures, it becomes difficult to distinguish the relaxation in the EuO from the fast proximal relaxation below 10 K. The dashed green curve is the same as in Fig. 5

Figure 5

The relaxation rates in EuO (green points) at low temperature and $3.33$ T (single exponential fits, open diamonds; slow component, filled diamonds) in comparison with the NMR ${}^{153}\mathrm{Eu}$ rates in zero field [8] (black points) and at $2.9$ T [7] (red curve). The dashed green curve is obtained from the red curve by scaling by $0.28$.

Figure 6

Schematic of coupling of ${}^8\mathrm{Li}^{+}$ to spin fluctuations in the EuO layer. Dipolar fields from a fluctuation of wavelength $2\pi /q$ fall off as $exp(-qd)$, strongly filtering this coupling to zone center ($q=0$) modes. The substantial relaxation rate in the Au layer suggests another nonlocal coupling to the Eu spins.