Evolution of Spin-Wave Excitations in Ferromagnetic Metallic Manganites

Neutron scattering results are presented for spin-wave excitations of three ferromagnetic metallic $A_{1-x}{A}_x^{\prime }{\mathrm{MnO}}_3$ manganites (where $A$ and $A^{\prime }$ are rare- and alkaline-earth-metal ions), which when combined with previous work elucidate the systematics of the interactions as a function of carrier concentration $x$, on-site disorder, and strength of the lattice distortion. The long-wavelength spin dynamics show only a very weak dependence across the series. The ratio of fourth to first neighbor exchange ($J_4/J_1$) that controls the zone boundary magnon softening changes systematically with $x$, but does not depend on the other parameters. None of the prevailing models can account for these behaviors.

Figure 1

(a) Crystal structure of $A_{1-x}{A}_x^{\prime }{\mathrm{MnO}}_3$ with magnetic exchange coupling indicated. (b) Phase diagram of ${\mathrm{Pr}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ in the $T\mathrm{\text{−}}H$ plane [19]. (c) Phase diagram of ${\mathrm{Pr}}_{0.55}({\mathrm{Ca}}_{0.85}{\mathrm{Sr}}_{0.15}{)}_{0.45}{\mathrm{MnO}}_3$ in the $T\mathrm{\text{−}}H$ plane from transport measurements. Our neutron experimental conditions are marked as the (red) upper triangle and the (blue) square in the phase diagrams.

Figure 2

The $q$-dependent spin-wave excitations in LCMO25 (a), PCMO30 (b), and PCSMO45 (c) at low temperatures. The data at different $q$’s are incrementally shifted for clarity. The horizontal bars are the instrumental resolution and small shoulders around 25 meV in (c) are phonon scattering. $E$ versus $q^2$ are plotted in the insets to determine $D$.

Figure 3

(a) Spin-wave dispersion curves of various $A_{0.7}{A}_{0.3}^{\prime }{\mathrm{MnO}}_3$ manganites along the $[\xi ,0,0]$ direction. (b) Dispersion curves for a series of $A_{1-x}{A}_x^{\prime }{\mathrm{MnO}}_3$ as a function of $x$. The solid lines are least-square fits using the Heisenberg model with $J_1$ and $J_4$.

Figure 4

The on-site disorder dependence of (a) the spin-wave stiffness $D$ obtained from low-$q$ excitations, (b) the coupling $2S{J}_1$ and (c) the ratio of $J_4/J_1$. The average ionic radius dependence of (d) $D$, (e) $2S{J}_1$, and (f) $J_4/J_1$. Dashed lines are guides to the eye.

Figure 5

Doping dependence of (a) spin-wave stiffness $D$, (b) nearest-neighbor exchange coupling $J_1$, and (c) ratio of $J_4/J_1$. Dashed lines are guides to the eye. The solid line in (a) is the prediction of Ref. 3. The (green) solid, (blue) dash-dotted, and (red) dashed lines in (c) are calculations from different models in 15.