Ultrafast Magnetism of a Ferrimagnet across the Spin-Flop Transition in High Magnetic Fields

We show that applying magnetic fields up to 30 T has a dramatic effect on the ultrafast spin dynamics in ferrimagnetic GdFeCo. Upon increasing the field beyond a critical value, the dynamics induced by a femtosecond laser excitation strongly increases in amplitude and slows down significantly. Such a change in spin response is explained by different dynamics of the Gd and FeCo magnetic sublattices following a spin-flop phase transition from a collinear to a noncollinear spin state.

Figure 1

(a) Schematic representation of the spin-flop behavior at $T<T_M$ of the rare earth and transition metal magnetic sublattices ($M_R$ and $M_T$, respectively) upon increasing the external magnetic field (adapted from [15]). (b) Static MOKE data of the GdFeCo sample measured at 630 nm wavelength at different temperatures from 230.0 to 321.0 K. A paramagnetic background was subtracted from the measurements. The magnetization compensation temperature $T_M$ lies at around 283 K. MOKE and VSM measurement are shown for 230 K. (c) Coercive field $H_c$ and spin-flop field $H_{\mathrm{sf}}$ extracted from the static MOKE data as a function of temperature. The error lies within the symbol size.

Figure 2

(a) and (b) show the magnetization dynamics measured by TR MOKE in GdFeCo at different applied magnetic fields below and above $T_M$, respectively. As an example, we show the exponential fit (red dashed line) used to quantify the dynamics in the first picoseconds after pump arrival for both temperatures at 5 T. Also at 5 T and in the first panel for fields $<H_{\mathrm{sf}}$, the position of the rare earth (green) and transition metal (red) magnetic sublattice is depicted schematically for two cases, probe delay $<0$ and $>0$. In the measurements at 1.5 T and 230 K the signal slightly decreases in the range between 40–50 ps. Such a slight decrease is most likely an artifact due to instabilities of the sample temperature and laser intensity. Note that ferrimagnets are especially sensitive to such instabilities in the vicinity of the compensation temperature and spin-flop transitions.

Figure 3

Absolute value of the amplitude $|A|$ (red circles) and decay time $\tau$ (black squares) of the single-exponential fits to the data in Fig. 2 are shown for the measurement below (a) and above $T_M$ (c). (b) and (d) show the extracted frequencies from the same oscillations as a function of applied magnetic field. The lines (red) are linear fits of the data according to Eq. (2).