Unveiling New Magnetic Phases of Undoped and Doped Manganites

Novel ground-state spin structures in undoped and lightly doped manganites are investigated based on the orbital-degenerate double-exchange model, via mean-field and numerical techniques. In undoped manganites, a new antiferromagnetic (AFM) state, called the $E$-type phase, is found adjacent in parameter space to the $A$-type AFM phase. Its structure is in agreement with recent experimental results. This insulating $E$-AFM state is also competing with a ferromagnetic metallic phase as well. For doped layered manganites, the phase diagram includes another new AFM phase of the $C_x{E}_{1-x}$ type. Experimental signatures of the new phases are discussed.

Figure 1

(a) Phase diagram obtained using the $4\times 4$ lattice. Open circles denote MC results and solid curves are obtained with MF calculations. (b) Spin correlation $S(\mathbf{q})$ vs $J_{\mathrm{A}\mathrm{F}}$ at $\lambda =1.5$. (c) Schematic view of the spin and orbital structure of the $E$-AFM phase. Solid and open symbols denote orbitals with up- and down-spins, respectively. Note the presence of zigzag chains for each spin orientation. (d) Density of states (DOS) obtained by MC simulations at $\lambda =0.25$. Solid and dashed curves denote, repsectively, the results for $E$ and FM phases. (e) Gap at the Fermi level for the FM phase vs $\lambda$. The magnitude is evaluated from the DOS obtained by MC simulations. (f) Total energy vs magnetic field for $E$-AFM and FM phases. (g) Phase diagram in the ($J_{\mathrm{A}\mathrm{F}},T$) plane at $x=0$ and $\lambda =1$, using a $4\times 4$ lattice. Estimations of the Curie and Néel temperatures are evaluated using MC simulations (see text).

Figure 2

(a) Energies of the FM, $A$-AFM, $E$-AFM, $C$-AFM, and $G$-AFM phases on $4\times 4\times 2$ bilayer lattices ($\lambda =1.5$). Solid circles indicate the results of optimizations, while lines denote the MF results. (b) Phase diagram for the $4\times 4\times 4$ cubic lattice. All solid lines emerge from MF calculations.

Figure 3

(a) Phase diagram in the ($x,J_{\mathrm{A}\mathrm{F}}$) plane for layered manganites at $\lambda =0$ obtained by analytic calculations. (b) Schematic view for spin structure of the $C_x{E}_{1-x}$-type phase at $x=1/6$. Hatched squares denote hole-rich $C$-type regions. (c) $S(\mathbf{q})$ and (d) the DOS in the $C_{1/6}{E}_{5/6}$-type phase.