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Register a new userBound Magnetic Polarons in the 3d-electron Ferromagnetic Spinel Semiconductor CdCr$_2$Se$_4$
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Added by
Vyacheslav Storchak G.
Publication date
2010
fields
Physics
and research's language is
English
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Muon spin rotation/relaxation spectroscopy %(supported by magnetization measurements) has been employed to study electron localization around a donor center - the positive muon - in the 3d magnetic spinel semiconductor CdCr$_2$Se$_4$ at temperatures from 2 to 300 K in magnetic fields up to 7 T. A bound state of an electron around a positive muon - a magnetic polaron - is detected far above the ferromagnetic transition up to 300 K. Electron localization into a magnetic polaron occurs due to its strong exchange interaction with the magnetic 3d electrons of local Cr$^{3+}$ ions, which confines its wave function within Rapprox 0.3 nm, allowing significant overlap with both the nearest and next nearest shells of Cr ions.
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We report detailed dc and ac magnetic susceptibilities, specific heat, and thermal conductivity measurements on the frustrated magnet ZnCr$_2$Se$_4$. At low temperatures, with increasing magnetic field, this spinel material goes through a series of spin state transitions from the helix spin state to the spiral spin state and then to the fully polarized state. Our results indicate a direct quantum phase transition from the spiral spin state to the fully polarized state. As the system approaches the quantum criticality, we find strong quantum fluctuations of the spins with the behaviors such as an unconventional $T^2$-dependent specific heat and temperature independent mean free path for the thermal transport. We complete the full phase diagram of ZnCr$_2$Se$_4$ under the external magnetic field and propose the possibility of frustrated quantum criticality with extended densities of critical modes to account for the unusual low-energy excitations in the vicinity of the criticality. Our results reveal that ZnCr$_2$Se$_4$ is a rare example of 3D magnet exhibiting a field-driven quantum criticality with unconventional properties.
We demonstrate via a muon spin rotation experiment that the electronic ground state of the iridium spinel compound, CuIr$_2$S$_4$, is not the presumed spin-singlet state but a novel paramagnetic state, showing a quasistatic spin glass-like magnetism below ~100 K. Considering the earlier indication that IrS$_6$ octahedra exhibit dimerization associated with the metal-to-insulator transition below 230 K, the present result suggests that a strong spin-orbit interaction may be playing an important role in determining the ground state that accompanies magnetic frustration.
We report pressure-dependent reflection and transmission measurements on ZnCr$_2$Se$_4$, HgCr$_2$S$_4$, and CdCr$_2$O$_4$ single crystals at room temperature over a broad spectral range 200-24000 cm$^{-1}$. The pressure dependence of the phonon modes and the high-frequency electronic excitations indicates that all three compounds undergo a pressure-induced structural phase transition with the critical pressure 15 GPa, 12 GPa, and 10 GPa for CdCr$_2$O$_4$, HgCr$_2$S$_4$, and ZnCr$_2$Se$_4$, respectively. The eigenfrequencies of the electronic transitions are very close to the expected values for chromium crystal-field transitions. In the case of the chalcogenides pressure induces a red shift of the electronic excitation which indicates a strong hybridization of the Cr d-bands with the chalcogenide bands.
We determined the magnetic structure of CuCr$_2$O$_4$ using neutron diffraction and irreducible representation analysis. The measurements identified a new phase between 155 K and 125 K as nearly collinear magnetic ordering in the Cr pyrochlore lattice. Below 125 K, a Cu-Cr ferrimagnetic component develops the noncollinear order. Along with the simultaneously obtained O positions and the quantum effect of spin-orbit coupling, the magnetic structure is understood to involve spin-orbit ordering, accompanied by an appreciably deformed orbital of presumably spin-only Cu and Cr.
Using the Lanczos method in linear chains we study the double exchange model in the low concentration limit, including an antiferromagnetic super-exchange K. In the strong coupling limit we find that the ground state contains ferromagnetic polarons whose length is very sensitive to the value of K/t. We investigate the dispersion relation, the trapping by impurities, and the interaction between these polarons. As the overlap between polarons increases, by decreasing K/t, the effective interaction between them changes from antiferromagnetic to ferromagnetic. The scaling to the thermodynamic limit suggests an attractive interaction in the strong coupling regime (J_h > t) and no binding in the weak limit (J_h simeq t).
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