• Journal of the Korean Magnetics Society
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• v.15 no.3
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• pp.167-171
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• 2005

Detailed aspects of the charge disproportionation (CD) transition for a polycrystalline $La_{1/3}Sr_{2/3}FeO_{2.96}$ were studied with the X-ray diffraction, $M\ddot{o}ssbauer$ spectroscopy, and SQUID magnetometer. The crystal structure was found to be rhombohedral with a space group R/3c. The lattice parameters were $a_R=5.4874\;\AA,\;and\;a_R=60.07^{\circ}$, respectively. $M\ddot{o}ssbauer$ spectra were taken within a wide range of temperature from 4.2 K up to room temperature. In the low temperature region, the spectra were comprised of two superimposed sextets which originated from $Fe^{3+}\;and\;Fe^{5+}$, respectively. This was the antiferromagnetic mixed valence state produced by the charges disproportionated into two different species. In the high temperature region, however, only a singlet from $Fe^{3.6+}$ was observed, indicating that it was a paramagnetic averaged valence state. The CD transition occurred in the temperature range from 175 K to 200 K, in which the two phases coexisted. The origin for the CD transition was explained by the thermally generated fast hopping of electrons. Hysteresis loop showed that there existed a strong antiferromagnetic interaction among magnetic ions. As the temperature increased thru the CD transition temperature, it was very likely that the interaction between $Fe^{3+}\;and\;Fe^{5+}$ was replaced by a more stronger one.

• Journal of the Korean Magnetics Society
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• v.15 no.2
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• pp.81-84
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• 2005

The origin for the charge disproportionation (CD) transition in polycrystalline $La_{1/3}Sr_{2/3}FeO_{2.96}$ was examined using X-ray diffraction and the external field $M\ddot{o}ssbauer$ssbauer spectroscopy. In order to see how the external magnetic field affects the CD state above its transition temperature, an external magnetic field of up to 6 T was applied either parallel or perpendicular to the $\gamma-ray$ direction with the sample temperature fixed at 225 K, which was above the CD transition temperature. Without an external magnetic field, a completely paramagnetic singlet was obtained in the temperature range of the averaged valence state above the transition temperature, which was interpreted as coming from the average valence $Fe^{3.6+}$. In the longitudinal geometry, a magnetic Zeeman with its intensity ratio 3:0:1:1:0:3 is superimposed to the central singlet. In the transverse geometry, however, the central singlet disappears and only a magnetic component with its intensity ratio 3:4:1:1:4:3 emerges. The existence of a singlet is understood as an evidence of the fast electron-transfer among Fe ions. Since the singlet still exists under the magnetic field, the application of an external field has little effect on the conduction mechanism of hopping electrons.

• Current Applied Physics
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• v.18 no.12
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• pp.1577-1582
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• 2018

While controlling the cation contents in perovskite rare-earth nickelate thin films, a metal-to-insulator phase transition is reported. Systematic control of cation stoichiometry has been achieved by manipulating the irradiation of excimer laser in pulsed laser deposition. Two rare-earth nickelate bilayer thin-film heterostructures with the controlled cation stoichiometry (i.e. stoichiometric and Ni-excessive) have been fabricated. It is found that the Ni-excessive nickelate film is structurally less dense than the stoichiometric film, albeit both of them are epitaxial and coherent with respect to the underlying substrate. More interestingly, as a temperature decreases, a metal-to-insulator transition is only observed in the Ni-excessive nickelate films, which can be associated with the enhanced disproportionation of the Ni charge valence. Based on our theoretical results, possible origins (e.g. anti-site defects) of the low-temperature insulating state are discussed with the need of future work for deeper understanding. Our work can be utilized to realize unusual physical phenomena (e.g. metal-to-insulator phase transitions) in complex oxide films by manipulating the chemical stoichiometry in pulsed laser deposition.

• Proceedings of the Korean Vacuum Society Conference
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• 2014.02a
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• pp.92-92
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• 2014

Recently, many state-of-art spectroscopy techniques are used to unravel the mysteries of condensed matters. And numerous heterostructures have provided a new avenue to search for new emergent phenomena. Especially, near the interface, various forms of symmetry-breaking can appear, which induces many novel phenomena. Although these intriguing phenomena can be emerged at the interface, by using conventional measurement techniques, the experimental investigations have been limited due to the buried nature of interface. One of the ways to overcome this limitation is in situ investigation of the layer-by-layer evolution of the electronic structure with increasing of the thickness. Namely, with very thin layer, we can measure the electronic structure strongly affected by the interface effect, but with thick layer, the bulk property becomes strong. Angle-resolved photoemission spectroscopy (ARPES) is powerful tool to directly obtain electronic structure, and it is very surface sensitive. Thus, the layer-by-layer evolution of the electronic structure in oxide heterostructure can be investigated by using in situ ARPES. LaNiO3 (LNO) heterostructures have recently attracted much attention due to theoretical predictions for many intriguing quantum phenomena. The theories suggest that, by tuning external parameters such as misfit strain and dimensionality in LNO heterostructure, the latent orders, which is absent in bulk, including charge disproportionation, spin-density-wave order and Mott insulator, could be emerged in LNO heterostructure. Here, we performed in situ ARPES studies on LNO films with varying the misfit strain and thickness. (1) By using LaAlO3 (-1.3%), NdGaO3 (+0.3%), and SrTiO3 (+1.7%) substrates, we could obtain LNO films under compressive strain, nearly strain-free, and tensile strain, respectively. As strain state changes from compressive to tensile, the Ni eg bands are rearranged and cross the Fermi level, which induces a change of Fermi surface (FS) topology. Additionally, two different FS superstructures are observed depending on strain states, which are attributed to signatures of latent charge and spin orderings in LNO films. (2) We also deposited LNO ultrathin films under tensile strain with thickness between 1 and 10 unit-cells. We found that the Fermi surface nesting effect becomes strong in two-dimensions and significantly enhances spin-density-wave order. The further details are discussed more in presentation. This work was collaborated with Hyang Keun Yoo, Seung Ill Hyun, Eli Rotenberg, Ji Hoon Shim, Young Jun Chang and Hyeong-Do Kim.