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Register a new userNon-magnetic B-site Impurities Induce Ferromagnetic Tendencies in CE Manganites
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Using a two-orbital model and Monte Carlo simulations, we investigate the effect of nonmagnetic B-site substitution on half-doped CE-type manganites. The lattice defects induced by this substitution destabilize the CE phase, which transforms into (1) the ferromagnetic (FM) metallic competing state, or (2) a regime with short-range FM clusters, or (3) a spin-glass state, depending on couplings and on the valence of the B-site substitution. While a C-type antiferromagnetic state is usually associated with an average $e_{rm g}$ charge density less than 0.5, the nonmagnetic B-site substitution that lowers the $e_{rm g}$ charge density is still found to enhance the FM tendency in our simulations. The present calculations are in qualitative agreement with experiments and provide a rationalization for the complex role of nonmagnetic B-site substitution in modulating the phase transitions in manganites.
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Most previous investigations have shown that the surface of a ferromagnetic material may have antiferromagnetic tendencies. However, experimentally the opposite effect has been recently observed: ferromagnetism appears in some nano-sized manganites with a composition such that the antiferromagnetic charge-ordered CE state is observed in the bulk. A possible origin is the development of ferromagnetic correlations at the surface of these small systems. To clarify these puzzling experimental observations, we have studied the two-orbital double-exchange model near half-doping n=0.5, using open boundary conditions to simulate the surface of either bulk or nano-sized manganites. Considering the enhancement of surface charge density due to a possible AO termination (A = trivalent/divalent ion composite, O = oxygen), an unexpected surface phase-separated state emerges when the model is studied using Monte Carlo techniques on small clusters. This tendency suppresses the CE charge ordering and produces a weak ferromagnetic signal that could explain the experimental observations.
Ferromagnetic (FM) manganites, a group of likely half-metallic oxides, are of special interest not only because they are a testing ground of the classical doubleexchange interaction mechanism for the colossal magnetoresistance, but also because they exhibit an extraordinary arena of emergent phenomena. These emergent phenomena are related to the complexity associated with strong interplay between charge, spin, orbital, and lattice. In this review, we focus on the use of inelastic neutron scattering to study the spin dynamics, mainly the magnon excitations in this class of FM metallic materials. In particular, we discussed the unusual magnon softening and damping near the Brillouin zone boundary in relatively narrow band compounds with strong Jahn-Teller lattice distortion and charge/orbital correlations. The anomalous behaviors of magnons in these compounds indicate the likelihood of cooperative excitations involving spin, lattice, as well as orbital degrees of freedom.
We argue that in lightly hole doped perovskite-type Mn oxides the holes (Mn$^{4+}$ sites) are surrounded by nearest neighbor Mn$^{3+}$ sites in which the occupied $3d$ orbitals have their lobes directed towards the central hole (Mn$^{4+}$) site and with spins coupled ferromagnetically to the central spin. This composite object, which can be viewed as a combined orbital-spin-lattice polaron, is accompanied by the breathing type (Mn$^{4+}$) and Jahn-Teller type (Mn$^{3+}$) local lattice distortions. We present calculations which indicate that for certain doping levels these orbital polarons may crystallize into a charge and orbitally ordered ferromagnetic insulating state.
The charge order of CE phase in half-doped manganites is studied, based on an argument that the charge-ordering is caused by the Jahn-Teller distortions of MnO6 octahedra rather than Coulomb repulsion between electrons. The uantitative calculation on the ferromagnetic zigzag chain as the basic structure unit of CE phase within the framework of two-orbital double exchange model including Jahn-Teller effect is performed, and it is shown that the charge-disproportionation of Mn cations in the charge-ordered CE phase is less than 13%. In addition, we predict the negative charge-disproportionation once the Jahn-Teller effect is weak enough.
The heavy fermion ferromagnet CeRh$_6$Ge$_4$ is the first example of a clean stoichiometric system where the ferromagnetic transition can be continuously suppressed by hydrostatic pressure to a quantum critical point. In order to reveal the outcome when the magnetic lattice of CeRh$_6$Ge$_4$ is diluted with non-magnetic atoms, this study reports comprehensive measurements of the physical properties of both single crystal and polycrystalline samples of La$_x$Ce$_{1-x}$Rh$_6$Ge$_4$. With increasing $x$, the Curie temperature decreases, and no transition is observed for $x$ $>$ 0.25, while the system evolves from exhibiting coherent Kondo lattice behaviors at low $x$, to the Kondo impurity scenario at large $x$. Moreover, non-Fermi liquid behavior (NFL) is observed over a wide doping range, which agrees well with the disordered Kondo model for 0.52 $leq$ $x$ $leq$ 0.66, while strange metal behavior is revealed in the vicinity of $x_c$ = 0.26.
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