S. Dong

Charge-order breaking and ferromagnetism in La0.4Ca0.6MnO3 nanoparticles

La(0.4)Ca(0.6)MnO(3) nanoparticles of grain size as small as similar to 20 nm are prepared and their magnetic behaviors are investigated in order to understand the size effect of the charge ordering in manganites. The highly stable charge-ordered state can be significantly suppressed upon reduction of the grain size down to nanometer scale, while the ferromagnetism is enhanced. The magnetic phase separation due to the competition between ferromagnetic state and charge-ordered state as well as the surface spin disordering is responsible for the spin-glass-like state at low temperature

Nonlinear excited state absorption in cadmium texaphyrin solution

Nonlinear absorption at 532 nm in a cadmium texaphyrin solution has been studied using 8 ns and 23 ps laser pulses. The experiments show that reverse saturable absorption occurs in the nanosecond case. For picosecond pulses, reverse saturable absorption occurs only at low fluences, and the transmission increases with increasing incident fluence at high fluences. A six‐level model is presented to explain these nonlinear absorption effects. Several photophysical parameters for cadmium texaphyrin, such as absorption cross sections at 532 nm and lifetimes, have been evaluated by theoretical simulations of the experimental results.

Specific heat anomalies and possible Griffiths-like phase in La0.4Ca0.6MnO3 nanoparticles

The specific heat of La(0.4)Ca(0.6)MnO(3) in bulk and nanoparticle (similar to 60 nm in grain size) forms was investigated. It is found that the charge-ordered state highly stabilized in the bulk samples can be significantly suppressed in the nanoparticle. The low temperature specific heat data reveal a Schottky-like anomaly at similar to 5 K for the bulk sample, while a large electronic linear term (gamma=17.8 mJ/mole K(2)) was identified for the nanoparticle samples. The magnetic measurements unveil the small magnetic entropy as low as similar to 0.255 J/kg K and the possible existence of a Griffiths-like phase in the nanoparticle samples. We argue that the physics underlying the size effect is associated with the dimension-dependent interactions based on which the ferromagnetic/charge-ordering transition occurs. (c) 2008 American Institute of Physics.

Charge order suppression and weak ferromagnetism in La1∕3Sr2∕3FeO3 nanoparticles

Perovskite-type polycrystalline La1/3Sr2/3FeO3 particles with different sizes (80-2000 nm) were prepared using a simple sol-gel technique. In samples of nanoparticles with a diameter of less than 300 nm, weak ferromagnetism was observed at room temperature, which was attributed to the lattice distortion. The magnetic and specific heat measurements suggest that the charge ordering state was largely suppressed due to the lowering of the particle size, but the charge ordering temperature remained unaffected

Ferromagnetic metal to cluster-glass insulator transition induced by A-site disorder in manganites

The magnetotransport behaviors of a series of rare earth manganites with the same A-site cational mean radius and different A-site ionic radii variance A-site disorder are investigated. It is found that the system’s ground state transforms from ferromagnetic metal to cluster-glass insulator with increasing A-site disorder. In the cluster-glass state, the magnetization shows the steplike behavior, indicating the existence of short-range magnetically ordered clusters. The significant effect of the A-site disorder on the electronic phase separation is revealed by detecting the cluster-glass ground state at low temperature.

NONLINEAR ABSORPTION IN METALLO-PORPHYRIN-LIKE

Abstract Nonlinear absorption at 532 nm in a series of metallo-porphyrin-like compounds using 8 ns and 23 ps laser pulses is reported. The experiments show that for picosecond pulses, reverse saturable absorption occurs only at low fluences, and saturable absorption occurs at high fluences, for nanosecond pulses, only reverse saturable absorption occurs. The density matrix equations of motion for a six-level model are deduced to explain these nonlinear absorption effects, and numerical results calculated by the model agree well with the experimental results.

Enhancement of ferroelectricity in Cr-doped Ho2Ti2O7

A series of polycrystalline pyrochlore rare-earth titanate Ho(2-x)Cr(x)Ti(2)O(7) are synthesized in order to enhance the ferroelectricity of pyrochlore Ho(2)Ti(2)O(7). A giant enhancement of polarization P from 0.54 mu C/m(2) at x=0 up to similar to 660 mu C/m(2) at x=0.4 is obtained, accompanied with an increment of ferroelectric transition point T(c) from T(c)similar to 60 K up to T(c)similar to 140 K. A magnetic anomaly at T(c)similar to 140 K together with the polarization response to magnetic field is identified, implying the multiferroic effect in Ho(2-x)Cr(x)Ti(2)O(7)

Fluence dependence of nonlinear optical response of cadmium texaphyrin

Third‐order nonlinear optical response of a cadmium texaphyrin solution at 532 nm was measured using time‐resolved degenerate four‐wave mixing with picosecond pulses. The molecules of cadmium texaphyrin exhibit a large optical nonlinearity, which arises from the excited‐state population. The dynamics of the nonlinear optical response show an evident fluence dependence. The temporal response is characterized by three components, a slower, a faster, and a fastest, which are attributed to populations of the first triplet excited state, the first singlet excited state, and the higher singlet excited state, respectively.

Multiferroic phase competitions in perovskite manganite thin films

Based on the Mochizuki-Furukawa model, the cycloidal spin structures of orthorhombic RMnO3 manganite thin films on various magnetic substrates are simulated using Monte Carlo method. It is revealed that the long range cycloidal spin order can be modulated by the film thickness and substrate spin structure. In particular, the ferromagnetic and antiferromagnetic spin orders of the substrate in different orientations have different pinning effects on the cycloidal spin order of the thin film. The simulated results are discussed in terms of the competition between the single-ion anisotropy and spin-orbit coupling.

Spin frustration destruction and ferroelectricity modulation in Ca3CoMnO6: Effects of Mn deficiency

We present careful experiments on the spin order and multiferroicity of Mn-deficient Ca3CoMn1−xO6 with respect to the Co/Mn compensated Ca3Co1+yMn1−yO6. It is revealed that a slight destruction of the Co/Mn ionic order does not damage but significantly enhances the ferroelectricity in terms of the transition point and polarization magnitude, due to the Mn-deficiency enhanced degree of Co/Mn spin ordering in comparison with Ca3Co1+yMn1−yO6. The delicate competition between the ionic (charge) order and spin frustration is the core physics for improving the ferroelectric performance.