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Register a new userTuning the Magnetic Ground State by Charge Transfer Energy in SrCoO2.5 via Strain Engineering
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SrCoO2.5 (SCO) is a charge transfer insulator with 3d6 ground state configuration leading to antiferromagnetic nature. It is observed that substrate induced strain engineering modifies the ground state of SCO thin film with 3d7L (L:O-2p hole) configuration causing negative charge transfer energy.The consequent strong hybridization between O-2p and Co-3d bands causes a hole in O-2p band leading to hole mediated unconventional ferromagnetic ordering in SrCoO2.5 thin film. This opens up a new avenue to tune the electronic structure vis a vis magnetic property via strain engineering.
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The anisotropic triangular lattice of the crednerite system Cu(Mn1-xCux)O2 is used as a basic model for studying the influence of spin disorder on the ground state properties of a two-dimensional frustrated antiferromagnet. Neutron diffraction measurements show that the undoped phase (x=0) undergoes a transition to antiferromagnetic long-range order that is stabilized by a frustration-relieving structural distortion. Small deviation from the stoichiometric composition alters the magnetoelastic characteristics and reduces the effective dimensionality of the magnetic lattice. Upon increasing the doping level, the interlayer coupling changes from antiferromagnetic to ferromagnetic. As the structural distortion is suppressed, the long-range magnetic order is gradually transformed into a two-dimensional order.
Electron spin resonance and magnetization data in magnetic fields up to 55 T of a novel multicenter paramagnetic molecular complex [L_2Ni_4(N_3)(O_2C Ada)_4](Cl O_4) are reported. In this compound, four Ni centers each having a spin S = 1 are coupled in a single molecule via bridging ligands (including a mu_4-azide) which provide paths for magnetic exchange. Analysis of the frequency and temperature dependence of the ESR signals yields the relevant parameters of the spin Hamiltonian, in particular the single ion anisotropy gap and the g factor, which enables the calculation of the complex energy spectrum of the spin states in a magnetic field. The experimental results give compelling evidence for tuning the ground state of the molecule by magnetic field from a nonmagnetic state at small fields to a magnetic one in strong fields owing to the spin level crossing at a field of ~25 T.
In the high spin-orbit coupled Sr2IrO4, the high sensitivity of the ground state to the details of the local lattice structure shows a large potential for the manipulation of the functional properties by inducing local lattice distortions. We use epitaxial strain to modify the Ir-O bond geometry in Sr2IrO4 and perform momentum-dependent Resonant Inelastic X-ray Scattering (RIXS) at the metal and at the ligand sites to unveil the response of the low energy elementary excitations. We observe that the pseudospin-wave dispersion for tensile-strained Sr2IrO4 films displays large softening along the [h,0] direction, while along the [h,h] direction it shows hardening. This evolution reveals a renormalization of the magnetic interactions caused by a strain-driven crossover from anisotropic to isotropic interactions between the magnetic moments. Moreover, we detect dispersive electron-hole pair excitations which shift to lower (higher) energies upon compressive (tensile) strain, manifesting a reduction (increase) in the size of the charge gap. This behavior shows an intimate coupling between charge excitations and lattice distortions in Sr2IrO4, originating from the modified hopping elements between the t2g orbitals. Our work highlights the central role played by the lattice degrees of freedom in determining both the pseudospin and charge excitations of Sr2IrO4 and provides valuable information towards the control of the ground state of complex oxides in the presence of high spin-orbit coupling.
The metal-insulator transitions and the intriguing physical properties of rare-earth perovskite nickelates have attracted considerable attention in recent years. Nonetheless, a complete understanding of these materials remains elusive. Here, taking a NdNiO3 thin film as a representative example, we utilize a combination of x-ray absorption and resonant inelastic x-ray scattering (RIXS) spectroscopies to resolve important aspects of the complex electronic structure of the rare-earth nickelates. The unusual coexistence of bound and continuum excitations observed in the RIXS spectra provides strong evidence for the abundance of oxygen 2p holes in the ground state of these materials. Using cluster calculations and Anderson impurity model interpretation, we show that these distinct spectral signatures arise from a Ni 3d8 configuration along with holes in the oxygen 2p valence band, confirming suggestions that these materials do not obey a conventional positive charge-transfer picture, but instead exhibit a negative charge-transfer energy, in line with recent models interpreting the metal to insulator transition in terms of bond disproportionation.
The effect of Ir substitution for Os in CeOs2Al10, with an unusually high Neel temperature of T~28.5K, has been studied by high-resolution neutron diffraction and magnetization measurements. A small amount of Ir (~ 8%) results in a pronounced change of the magnetic structure of the Ce-sublattice. The new magnetic ground state is controlled by the single ion anisotropy and implies antiferromagnetic arrangement of the Ce-moments along the a-axis, as expected from the anisotropy of the paramagnetic susceptibility. The value of the ordered moments, 0.92(1) mu_B, is substantially bigger than in the undoped compound, whereas the transition temperature is reduced down to 21K. A comparison of the observed phenomena with the recently studied CeRu1.9Rh0.1Al10 system, exhibiting similar behaviour [A. Kondo et al., J. Phys. Soc. Jpn. 82, 054709 (2013)], strongly suggests the electron doping as the main origin of the ground state changes. This provides a new way of exploring the anomalous magnetic properties of the Ce(Ru/Os)2Al10 compounds.
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