S. Lee

Unidirectional electric current flow, such as that found in a diode, is essential for modern electronics. It usually occurs at asymmetric interfaces such as p-n junctions or metal/semiconductor interfaces with Schottky barriers. We report on a diode effect associated with the direction of bulk electric polarization in BiFeO3: a ferroelectric with a small optical gap edge of approximately 2.2 electron volts. We found that bulk electric conduction in ferroelectric monodomain BiFeO3 crystals is highly nonlinear and unidirectional. This diode effect switches its direction when the electric polarization is flipped by an external voltage. A substantial visible-light photovoltaic effect is observed in BiFeO3 diode structures. These results should improve understanding of charge conduction mechanisms in leaky ferroelectrics and advance the design of switchable devices combining ferroelectric, electronic, and optical functionalities.

We report discovery of collinear-magnetism-driven ferroelectricity in the Ising chain magnet ${\mathrm{Ca}}_{3}{\mathrm{Co}}_{2\ensuremath{-}x}{\mathrm{Mn}}_{x}{\mathrm{O}}_{6}$ ($x\ensuremath{\approx}0.96$). Neutron diffraction shows that ${\mathrm{Co}}^{2+}$ and ${\mathrm{Mn}}^{4+}$ ions alternating along the chains exhibit an up-up-down-down ($\ensuremath{\uparrow}\ensuremath{\uparrow}\ensuremath{\downarrow}\ensuremath{\downarrow}$) magnetic order. The ferroelectricity results from the inversion symmetry breaking in the $\ensuremath{\uparrow}\ensuremath{\uparrow}\ensuremath{\downarrow}\ensuremath{\downarrow}$ spin chain with an alternating charge order. Unlike in spiral magnetoelectrics where antisymmetric exchange coupling is active, the symmetry breaking in ${\mathrm{Ca}}_{3}(\mathrm{Co},\mathrm{Mn}{)}_{2}{\mathrm{O}}_{6}$ occurs through exchange striction associated with symmetric superexchange.

In order to investigate spin-charge coupling in multiferroic oxides, we measured the optical properties of ${\text{BiFeO}}_{3}$. Although the direct 300 K charge gap is observed at 2.67 eV, absorption onset actually occurs at much lower energy with ${\text{Fe}}^{3+}$ excitations at 1.41 and 1.90 eV. Temperature and magnetic-field-induced spectral changes reveal complex interactions between on-site crystal-field and magnetic excitations in the form of magnon sidebands. We employ the sensitivity of these magnon sidebands to map out the magnetic-field-temperature phase diagram which demonstrates optical evidence for spin spiral quenching above 20 T and suggests a spin domain reorientation near 10 T.

We have studied quasi-two-dimensional multiferroic LuFe2O4 with strong charge-spin-lattice coupling, in which low-temperature coercivity approaches an extraordinary value of 9 T in single crystals. The enhancement of the coercivity is connected to the collective freezing of nanoscale pancakelike ferrimagnetic domains with large uniaxial magnetic anisotropy ("Ising pancakes"). Our results suggest that collective freezing in low-dimensional magnets with large uniaxial anisotropy provides an effective mechanism to achieve enhanced coercivity. This observation may help identify novel approaches for synthesis of magnets with enhanced properties.

We have used specific cDNAs to the rat vitamin D receptor (VDR) and to the mammalian vitamin D-dependent calcium-binding proteins (calbindin-D9k in intestine and calbindin-D28k in kidney) in order to obtain a better understanding of the regulation of the VDR gene and its relationship to calbindin gene expression. Hormonal regulation and development expression of the rat VDR gene were characterized by both Northern and slot blot analyses. Administration of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3; 25 ng/day for 7 days) to vitamin D-deficient rats resulted in an increase in calbindin mRNA in intestine and kidney but no change in VDR mRNA in these tissues. Vitamin D-deficient rats responded to dexamethasone treatment (100 micrograms/100 g of body weight/day for 4 days) with a 2.5-fold increase in intestinal VDR mRNA which was accompanied by a 4-fold decrease in intestinal calbindin-D9k mRNA. Developmental studies indicated a pronounced increase in renal VDR mRNA and calbindin-D28k mRNA between birth and 1 week of age. In the intestine, an induction of VDR and calbindin-D9k gene expression was observed at a later time, during the 3rd postnatal week (the period of increased duodenal active transport of calcium). Taken collectively, our data indicate that in the adult rat, target tissue response to hormone is not modified by a corresponding alteration in new receptor synthesis. However, developmental studies indicate that the induction of 1,25(OH)2D3 receptor mRNA is correlated with the induction of calbindin gene expression. Our results also demonstrate that glucocorticoid administration can result in an alteration in intestinal calbindin and VDR gene expression.