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We discover the experimental case of a quantum valence criticality leading to the breakdown of the Fermi liquid state., A valence critical end point existing near the absolute zero provides a unique case for the study of a quantum version of the strong density fluctuation at the Widom line in the supercritical fluids. Although singular charge and orbital dynamics are suggested theoretically to alter the electronic structure significantly, breaking down the standard quasi-particle picture, this has never been confirmed experimentally to date. We provide the first empirical evidence that the proximity to quantum valence criticality leads to a clear breakdown of Fermi liquid behavior. Our detailed study of the mixed valence compound α-YbAlB4 reveals that a small chemical substitution induces a sharp valence crossover, accompanied by a pronounced non–Fermi liquid behavior characterized by a divergent effective mass and unusual T/B scaling in the magnetization.

The magnetic properties of high-quality ∼25 at.% Fe doped α-YbAlB4 with an orthorhombic Pbam structure were investigated by using magnetization measurements and 57Fe Mossbauer spectroscopy. The temperature dependence of the magnetization along the a-axis showed anomalies at TN2 ∼ 7.7 K and TN1 ∼ 10.4 K. Broadenings of the peaks in the Mossbauer spectrum were observed at temperatures below 6 K, which were due to the magnetic hyperfine fields transferred from the Yb magnetic moments. The evaluated transferred magnetic hyperfine field almost disappears around TN2. The anomalous temperature dependence of the transferred magnetic hyperfine field reveals that the magnetic structure and the ordered magnetic moments of the Yb ions show complex temperature dependences at temperatures below TN1.

A (001)-epitaxial-BiFeO3/CoFe2O4 bilayer was grown by self-assembly on SrTiO3 (100) substrates by just coating a mixture precursor solution. The thickness ratio of the bilayer could be controlled by adjusting the composition ratio. For example, a BiFeOx:CoFe2Ox = 4:1 (namely Bi4CoFe6Ox) mixture solution could make a total thickness of 110nm divided into 85-nm-thick BiFeO3 and 25-nm-thick CoFe2O4. Self-assembly of the bilayer occurred because the perovskite BiFeO3 better matched the lattice constant (misfit approximately 1%) and crystal symmetry of the perovskite SrTiO3 than the spinel CoFe2O4 (misfit approximately 7%). The magnetic domains of the hard magnet CoFe2O4 were switched by the polarization change of BiFeO3 due to an applied vertical voltage and the switched magnetic domain size was approximately 100nm in diameter. These results suggest that self-assembled BiFeO3/CoFe2O4 bilayers are interesting in voltage driven nonvolatile memory with a low manufacturing cost.

BiFeO3-CoFe2O4 composite thin films were formed on Pt/Ti/SiO2/Si(100) substrates by chemical solution deposition from a mixed precursor solution. X-ray diffraction and transmission electron microscopy analyses confirmed that CoFe2O4 nanoparticles less than 10 nm were uniformly distributed in the BiFeO3 matrix. The BiFeO3-CoFe2O4 composite films exhibited the same ferroelectric switching charge as BiFeO3 thin films, although a larger applied electric field was necessary. However, the magnetic properties were significantly improved by incorporation of CoFe2O4 nanoparticles into BiFeO3; a saturated magnetization of 80 emu/cm3 and a magnetic coercive field of 450 Oe were attained at 300 K. Furthermore, the composite films did not show superparamagnetic behavior in zero-field-cooling and field-cooling measurements, which suggest that the thermal fluctuation of CoFe2O4 nanoparticles was suppressed by exchange coupling with BiFeO3.

Mn- and Co-BiFeO3 polycrystalline films were prepared by a chemical solution deposition (CSD) method and their structures were investigated by grazing incidence X-ray diffraction (GIXRD) and θ–2θ XRD in conjunction with transmission electron microscopy (TEM). GIXRD measurement revealed that the Mn-BiFeO3 film contains the Bi2Fe4O9 pyrochlore phase, which might be attributed to the low magnetization of the Mn-BiFeO3 film. For the Co-BiFeO3 film, secondary phases were not observed by structural analyses and the composition was almost homogeneous. From the analytic aspect of the structure, the magnetization enhancement in the Co-BiFeO3 films is considered to be associated with Co substitution for Fe in BiFeO3.

BiFeO3 (BFO) films were formed on (001), (110), and (111) La-doped SrTiO3 single-crystal substrates. All the films were epitaxially grown and had mainly rhombohedral structure. The BFO film formed on the (001) substrate had only a (001) component whereas the BFO films on (110) and (111) substrates had (110) and (111) components, respectively. The (001), (110), and (111) epitaxial BFO films showed the remanent polarizations of 63, 84, and 106 µC/cm2, respectively. It seems that the (110) and (111) components were changed to (110) and (111) ones, respectively, by applying an external voltage. The (001) epitaxial BFO film showed marked asymmetry in its electrical properties. It was found that the (001) epitaxial BFO film had a thin tetragonal layer with spontaneous polarization fixed in the downward direction near the substrate. Finally, we concluded that this tetragonal layer caused the accumulation of space charges at the interface, causing a downward built-in field to be generated. The downward build-in field facilitated the switching of upward polarization and caused asymmetric relaxation.