Photoinduced melting of magnetic order in the correlated electron insulator NdNiO${}_3$

Using ultrafast resonant soft x-ray diffraction, we demonstrate photoinduced melting of antiferromagnetic order in the correlated electron insulator NdNiO${}_3$. Time-dependent analysis of the resonant diffraction spectra allows us to follow the temporal evolution of the charge imbalance between adjacent Ni sites. A direct correlation between the melting of magnetic order and charge rebalancing is found. Furthermore, we demonstrate that the magnetic ordering on the Ni and Nd sites, which are locked together in equilibrium, become decoupled during this nonthermal process.

Figure 1

Multiple orders in the low-temperature insulating phase of NdNiO${}_3$. (a) Monoclinic crystal structure as measured by high-resolution x-ray diffraction (Ref. 21). In this configuration Ni${}_1$ (Ni${}_2$) atoms are surrounded by a large (small) oxygen octahedron. (b) Magnetic structure of NdNiO${}_3$ consistent with resonant soft x-ray diffraction data. Both Nd and Ni magnetic moments are ordered. (c) Energy dependence of the magnetic reflection at the Ni $L_{2,3}$ absorption edge measured at $T$ $=$ 15 K. Inset: Angular dependence (θ-2θ scan) of the magnetic reflection measured at the maximum intensity of the Ni $L_3$ edge. (d) Energy dependence of the magnetic reflection at the Nd $M_{4,5}$ absorption edge measured at $T$ $=$ 15 K. Inset: Angular dependence (θ-2θ scan) of the magnetic reflection measured at the maximum intensity of the Nd $M_4$ edge.

Figure 2

Ultrafast charge rebalancing in NdNiO${}_3$. (a) Evolution of the energy-resolved intensity of the (1/4 1/4 1/4) magnetic reflection around the Ni $L_3$ edge after ultrafast laser excitation (100 fs, 800 nm, 1.5 mJ/cm${}^2$). (b) Example of best fits of two Lorentzian peaks to individual spectra measured at different time delays, which allows the determination of the energy-integrated scattering intensity of the individual peaks (SW${}_1$, SW${}_2$). (c) Scatter points: Evolution of the ratio of the energy-integrated scattering intensity of the two Lorentzian peaks after photoexcitation. Solid line: Guide to the eye. (d) Scatter points: Evolution of the normalized peak intensity $I$/$I_0$ after photoexcitation. Solid line: Exponential fit to the data.

Figure 3

Photomagnetism in NdNiO${}_3$ for low excitation fluence. Scatter points: Evolution of the intensity $I$ of the (a) Ni and (b) Nd magnetic diffraction peak, with respect to the equilibrium intensity $I_0$, observed for different near-infrared laser fluences. Solid lines: Best fits to double exponential functions.

Figure 4

Photomagnetism in NdNiO${}_3$ for high excitation fluence. Scatter points: Evolution of the intensity $I$ of the (a) Ni and (b) Nd magnetic diffraction peak, with respect to the equilibrium intensity $I_0$, observed for different near-infrared laser fluences. Solid lines: Best fits to exponential functions. (a) Inset, scatter points: Slow and fast exponential recovery times of Ni (τ${}_1$,τ${}_2$) as a function of excitation.