Control of the Ultrafast Photoinduced Magnetization across the Morin Transition in ${\mathrm{DyFeO}}_3$

Excitation of the collinear compensated antiferromagnet ${\mathrm{DyFeO}}_3$ with a single 60 fs laser pulse triggers a phase transition across the Morin point into a noncollinear spin state with a net magnetization. Time-resolved imaging of the magnetization dynamics of this process reveals that the pulse first excites the spin oscillations upon damping of which the noncollinear spin state emerges. The sign of the photoinduced magnetization is defined by the relative orientation of the pump polarization and the direction of the antiferromagnetic vector in the initial collinear spin state.

Figure 1

(a) Magnetooptical images of the two types of antiferromagnetic domains in the low-temperature ${\mathrm{\Gamma }}_1$ phase of the $z\text{−}{\mathrm{DyFeO}}_3$ sample. The bias temperature of the sample is 20 K. (b) Magnetooptical image of the magnetic domain pattern in the high-temperature ${\mathrm{\Gamma }}_4$ phase. The bias temperature of the sample is 50 K. (c) Images of the domains in $z\text{−}{\mathrm{DyFeO}}_3$ taken 100 ps after excitation with a single 60 fs pump pulse. Areas exposed to the pulsed excitation are marked by dashed circles. “shot 1” forms a black domain having $-M_z$ magnetization in the dark-grey area, corresponding to $\mathbf{L}-$. “shot 2” forms a white domain with the $+M_z$ magnetization in the light-grey area, $\mathbf{L}+$. “shot 3” affects both antiferromagnetic domains and induces a magnetization the sign of which is defined by the direction of $\mathbf{L}$.

Figure 2

(a) Time-resolved magnetooptical images of the photoinduced dynamics in $z\text{−}{\mathrm{DyFeO}}_3$. The images are obtained by taking the difference between the images at the given positive delay and those at negative delay. The bias sample temperature is set to 23 K. (b) The magnetization dynamics in a cross section of the laser-excited area for the different excitation fluences. The left vertical axis shows the distance from the center of the laser excited area. Each figure is defined by the fluence at the center of the pump spot. The color code represents the measured value of the magnetooptical signal and is the same for all pictures in the panel. (c) The distribution of the damping ratio of the spin oscillations and rise time of the photoinduced magnetization extracted in the cross section. Coordinates are replaced with values of the local fluence. Solid lines are the Gaussian fits of the dependencies with FWHM of the pump beam. (d) Amplitude of the pump-induced antiferromagnetic spin oscillations in the ${\mathrm{\Gamma }}_1$ phase for different pump fluences. The value of the amplitude was obtained by comparing the time-resolved magnetooptical signal with the Faraday rotation in the ${\mathrm{\Gamma }}_4$ phase. The solid line is a fit with a single exponent. The inset is a schematic representations of the spin oscillations.

Figure 3

(a) The geometry of the experiment with a single crystal cut perpendicular to the optical axis: the $o\text{−}{\mathrm{DyFeO}}_3$. The external magnetic field $H_x$ and applied stress ${\sigma }_{yz}$ are depicted on the picture. The pump is linearly polarized. (b) The total magnetization integrated over the spot as a function of the angle $\phi$ between the pump polarization and the $x$ axis for two antiferromagnetic domains with opposite signs of the $L_y$ component. The value is shown by a percentage of the value of the static Faraday rotation in the ${\mathrm{\Gamma }}_4$ phase. The solid lines are square wave fits. (c) The dynamics of the total magnetization for two antiferromagnetic domains obtained by applying magnetic field $H_x=\pm 1.7\mathrm{kOe}$. Each curve is obtained as a difference between two experiments performed for two polarizations of the pump $\phi =\pm 45$ degrees. The inset shows the amplitude of the spin oscillations as a function of the angle $\phi$.

Figure 4

(a) The square of the frequency $f_a$ of the photoinduced oscillations for different temperatures in the vicinity of the Morin point. Dashed lines are linear fits of frequencies extended to the intersection with the temperature axis. The intersection points $T_1$ and $T_2$ define the region where thermodynamically phases ${\mathrm{\Gamma }}_1$ and ${\mathrm{\Gamma }}_4$ may coexist. Insets show the magnetic anisotropy potential $W_a$ of the first-order Morin phase transition as a function of angle $\theta$, the spins form with the $y$ axis for two temperatures $T_1<T<T_M$ and $T_M<T<T_2$.