Magnetic resonance study of bulk and thin film EuTiO3.

Magnetic resonance spectra of EuTiO3 in both bulk and thin film form were taken at temperatures from 3-350 K and microwave frequencies from 9.2-9.8 and 34 GHz. In the paramagnetic phase, magnetic resonance spectra are determined by magnetic dipole and exchange interactions between Eu2+ spins. In the film, a large contribution arises from the demagnetization field. From detailed analysis of the linewidth and its temperature dependence, the parameters of spin-spin interactions were determined: the exchange frequency is 10.5 GHz and the estimated critical exponent of the spin correlation length is ≈0.4. In the bulk samples, the spectra exhibited a distinct minimum in the linewidth at the Néel temperature, T N ≈ 5.5 K, while the resonance field practically does not change even on cooling below T N. This is indicative of a small magnetic anisotropy ~320 G in the antiferromagnetic phase. In the film, the magnetic resonance spectrum is split below T N into several components due to excitation of the magnetostatic modes, corresponding to a non-uniform precession of magnetization. Moreover, the film was observed to degrade over two years. This was manifested by an increase of defects and a change in the domain structure. The saturated magnetization in the film, estimated from the magnetic resonance spectrum, was about 900 emu cm-3 or 5.5 µ B/unit cell at T = 3.5 K.

Transversal spin freezing and re-entrant spin glass phases in chemically disordered Fe-containing perovskite multiferroics.

We propose experimental verification and theoretical explanation of magnetic anomalies in the complex Fe-containing perovskite multiferroics like PbFe1/2Nb1/2O3 and PbFe1/2Ta1/2O3. The theoretical part is based on our model of coexistence of the long-range magnetic order and spin glass in the above compounds. In our model, the exchange interaction is anisotropic, coupling antiferromagnetically z spin components of Fe(3+) ions. At the same time, the xy components are coupled by much weaker exchange interaction of ferromagnetic sign. In the system with spatial disorder (half of the corresponding lattice sites are occupied by spinless Nb(5+) ions) such frustrating interaction results in the fact that the antiferromagnetic order is formed by the z projection of the spins, while their xy components contribute to spin glass behaviour. Our theoretical findings are supported by the experimental evidence of such a coexistence of antiferromagnetic and spin glass phases in chemically disordered Fe-containing complex perovskite multiferroics.

Size effects in a relaxor: further insights into PMN.

Dielectric measurements of PbMg1/3Nb2/3O3 (PMN) powder and dense ceramics with grain sizes between 15 nm and two microns were carried out in a broad frequency range (20 Hz-1 GHz). Clear grain size dependence of relaxor behavior was evidenced. A progressive transformation from Vogel-Fulcher behavior towards the Arrhenius process in the PMN with reduction of grain size in both ceramics and powder was observed. In the case of ceramics we were able to extract deeper information from the distributions of relaxation times and an analysis using the Vogel-Fulcher law, revealing two main contributions: a fast part of distribution of relaxation times with a maximum close to 10(-11) s, which is almost grain-size independent and has a non-polar origin; whereas, a process with long relaxation times (in the time range of 10(-8) to 10(-5) s) is associated with the dynamics of the polar nanoregions and is strongly suppressed with reduction of grain size. The results of dielectric investigations are confirmed by Nuclear Magnetic Resonance experiments.

Domains within domains and walls within walls: evidence for polar domains in cryogenic SrTiO3.

Resonant piezoelectric spectroscopy shows polar resonances in paraelectric SrTiO3 at temperatures below 80 K. These resonances become strong at T<40 K. The resonances are induced by weak electric fields and lead to standing mechanical waves in the sample. This piezoelectric response does not exist in paraelastic SrTiO3 nor at temperatures just below the ferroelastic phase transition. The interpretation of the resonances is related to ferroelastic twin walls which become polar at low temperatures in close analogy with the known behavior of CaTiO3. SrTiO3 is different from CaTiO3, however, because the wall polarity is thermally induced; i.e., there exists a small temperature range well below the ferroelastic transition point at 105 K where polarity appears on cooling. As the walls are atomistically thin, this transition has the hallmarks of a two-dimensional phase transition restrained to the twin boundaries rather than a classic bulk phase transition.

7Li NMR investigation of Li-Li pair ordering in the paraelectric phase of weakly substitutionally disordered K(1-x)Li(x)TaO3.

Breaking of the average cubic symmetry in Li-doped potassium tantalate was observed with quadrupole-perturbed 7Li NMR at temperatures (150-400 K) far above the nominal glass transition temperature (≈50 K for Li concentration x=0.03). The observed spectrum consists of contributions from both isolated Li ions (i.e., with no nearest-neighbor Li) and from Li-Li pairs. The isolated Li ions move among six equivalent off-center sites in a potential having cubic symmetry. These have zero average electric field gradient and, hence, exhibit no quadrupole splitting. In addition, very low intensity, but well resolved, quadrupole satellites having a temperature-dependent splitting were observed. This splitting indicates that the various Li-Li pair configurations are not all equally probable. These are the first direct observations of biased Li ion ordering that persists in the paraelectric phase at temperatures high above the glass phase.

Terahertz emission from tubular pB(Zr,Ti)O3 nanostructures.

We report intense terahertz emission from lead zirconate titanate (PZT) tubular nanostructures, which have a wall thickness around 40 nm and protrude on n-type Si substrates. Such emission is totally absent in flat PZT films or bulk; hence the effect is attributed to the nanoscale geometry of the tubes. The terahertz radiation is emitted within 0.2 ps, and the spectrum exhibits a broad peak from 2 to 8 THz. This is a gap in the frequency spectrum of conventional semiconductor terahertz devices, such as ZnTe, and an order of magnitude higher frequency peak than that in the well-studied p-InAs, due to the abnormally large carrier concentration gradient in the nanostructured PZT. The inferred mechanism is optical rectification within a surface accumulation layer, rather than the Dember effect. The terahertz emission is optically pumped, but since the tubes exhibit ferroelectric switching, electrically driven emission may also be possible. EPR reveals 02 molecules adsorbed onto the nanotubes, which may play some role in the emission.

Field cooled and zero field cooled 207Pb NMR of the relaxor ferroelectric PMN.

A 2.8 kV/cm electric field has been applied parallel to the external magnetic field along the [111] direction of a PMN single crystal and the 207Pb NMR spectra were measured at 9.1 T. Whereas the zero field cooled (ZFC) spectrum exhibits a Gaussian-like line shape, the FC spectrum clearly shows a two peak structure. One of the two peaks coincides with the ZFC spectrum. The other peak is shifted by about 100 kHz towards lower frequencies with respect to the ZFC peak and seems to be characteristic for the ferroelectric state. The ferroelectric shift agrees with the predictions of the spherical random bond-random field model.