Studies of structural, magnetic, electrical and photoconducting properties of Bi1−x_{1-x}Cax_{x}MnO3_{3} epitaxial thin films

The dynamics of the charge ordered (CO) state under non-equilibrium conditions created by strong dc-electric field (~106 V/cm) and photo-illumination with short (~ 6 ns) laser pulses is investigated in Bi1-xCaxMnO3 (x > 0.5) epitaxial films. A pulsed laser deposition method was used to synthesize films on (100) LaAlO3 (LAO) and (100) SrTiO3 (STO) substrates. The crystallographic structure, temperature dependence of electrical resistivity and magnetization of the samples of different composition prepared under different oxygen partial pressure (pO2) and deposition temperature (TD) are studied. For the x = 0.6 sample grown on LAO, a clear signature of charge ordering at ~275 K is seen in the magnetization and at ~ 260 K in the resistivity data. The same sample grown on STO revealed a complex behavior, which entails charge ordering at ~300 K, a Neel order at ~150 K and finally a weak ferromagnetic phase below 50 K. A strong correlation between charge ordering temperature (TCO) and the c-axis lattice parameter (c) of the type (dTCO/dc ~-350 K/A) imerges from measurements on films deposited under different growth conditions. Since the out of plane lattice parameter (c) increases with in plane compressive strain, this effect directly show a compressive strain induced suppression of the TCO. The current (I)- voltage (V) characteristics of the samples at T < TCO show hysteresis due to a compound effect of Joule heating and collapse of the CO state. Transient changes in conductivity of lifetime ranging from nano to microseconds are seen at T < TCO on illumination with pulsed UV (355 nm) radiation. These observations are explained on the basis of the topological and electronic changes in the charge ordered phase.Comment: 19 figures, 34 page

Structural ordering driven anisotropic magnetoresistance, anomalous Hall resistance and its topological overtones in full-Heusler Co2MnSi thin films

We report the evolution of crystallographic structure, magnetic ordering and electronic transport in thin films of full-Heusler alloy Co$_2$MnSi deposited on (001) MgO with annealing temperatures ($T_A$). By increasing the $T_A$ from 300$^\circ$C to 600$^\circ$C, the film goes from a disordered nanocrystalline phase to $B2$ ordered and finally to the $L2_1$ ordered alloy. The saturation magnetic moment improves with structural ordering and approaches the Slater-Pauling value of $\approx 5.0 \mu_B$ per formula unit for $T_A$ = 600$^\circ$C. At this stage the films are soft magnets with coercive and saturation fields as low as $\approx$ 7 mT and 350 mT, respectively. Remarkable effects of improved structural order are also seen in longitudinal resistivity ($\rho_{xx}$) and residual resistivity ratio. A model based upon electronic transparency of grain boundaries illucidates the transition from a state of negative $d\rho/dT$ to positive $d\rho/dT$ with improved structural order. The Hall resistivity ($\rho_{xy}$) derives contribution from the normal scattering of charge carriers in external magnetic field, the anomalous effect originating from built-in magnetization and a small but distinct topological Hall effect in the disordered phase. The carrier concentration ($n$) and mobility ($\mu$) have been extracted from the high field $\rho_{xy}$ data. The highly ordered films are characterized by $n$ and $\mu$ of 1.19$\times$ 10$^{29}$ m$^{-3}$ and 0.4 cm$^2V^{-1}s^{-1}$ at room temperature. The dependence of $\rho_{xy}$ on $\rho_{xx}$ indicates the dominance of skew scattering in our films, which shows a monotonic drop on raising the $T_A$. The topological Hall effect is analyzed for the films annealed at 300$^\circ$C. ......Comment: 10 pages, 9 figures, 1 tabl

Crossover from magnetostatic to exchange coupling in La0.67Ca0.33MnO3/YBa2Cu3O7/La0.67Ca0.33MnO3 heterostructures

The influence of YBa2Cu3O4 (YBCO) superconductor layer (S-layer) with varying thickness d-YBCO = 20 to 50 nm on the magnetic coupling between two La0.67Ca0.33MnO3 (LCMO) ferromagnet layers (F-layer, thickness d-LCMO = 50 nm) in F/S/F heterostructures (HSs) was investigated by measuring global magnetization (M) in a temperature (T) range = 2 - 300 K and magnetic field (H) range = 0 - 10 kOe. All the HSs were superconducting with critical temperature (Tc) decreasing from = 78 to 36 K with decrease in d-YBCO, whereas the ferromagnetic ordering temperature Tm = 250 K did not change much. Systematically measured M-H loops of all HSs at both T > Tc and T < Tc show three main results- (a) the two step magnetic reversal above Tc converts into a four step reversal below Tc in HSs with d-YBCO >= 30 nm, (b) the magnetic field corresponding to the additional two switching steps and their magnitude show characteristic evolution with T and d-YBCO and (c) the HS with d-YBCO = 20 nm shows radically different behaviour, where the two step magnetic reversal above Tc continues to persist below Tc and converts into a single step reversal at T << Tc. The first two results indicate magnetostatic coupling between the magnetic domains and the vortices across the two F/S interfaces resulting in reversal dynamics different from that deep within the LCMO layers. Whereas, the result c reveals indirect exchange coupling between LCMO layers through the superconducting YBCO layer, which is a clear experimental evidence of coexistence of ferromagnetism and superconductivity in nm scale F/S/F HSs expected theoretically by C.A.R. Sa de Melo (Physica C 387, 17-25 (2003)).Comment: 23 pages, 7 figures, accepted in JPC

Long-Term Neuropsychological Effects in Survivors of Pediatric Low-Grade Gliomas

Statement of the problem: Survival rates of low-grade gliomas (diagnosed and treated during childhood) have improved resulting in a population of long-term survivors, albeit with limited knowledge of their neurocognitive function, quality of life, and adaptive function. Methods: Patients treated at The Childrens Hospital at Westmead, Sydney, Australia during childhood (ages 6-18), and a minimum of five years post diagnosis (n = 20) participated in a neuropsychological evaluation. Demographic and tumour related variables were independently analyzed. Results: The majority of the participants demonstrated average ability on most cognitive tasks, although we found some variability resulting in three subgroups ranging from mild to severe functioning. The lower cognitive functioning subgroup demonstrated challenges on tasks of memory, processing speed, and executive function. In addition, they self-reported sub-clinical to clinical ranges in internalizing and externalizing symptoms as well as difficulties in physical health, and social and emotional well-being. Medication consumption and less education were moderators of biological risk. None of the tumour related factors were identified as moderators, in part due to the small sample size. Positive associations between challenges in adaptive function (internalizing and externalizing symptoms) and difficulties in perceived cognitive abilities and health related quality of life were identified in the sample. Conclusions: This study highlighted the variability in long-term outcomes of low-grade gliomas and the necessity for routine follow-up care over the course of recovery and survivorship

Complementarity of perturbations driving insulator-to-metal transition in a charge ordered manganite

Modulation of charge carrier dynamics and hence electrical conductivity of solids by photoexcitation has been a rich field of research with numerous applications. Similarly, electric and magnetic field assisted enhancement of conductivity are of fundamental importance and technological use. Hole doped manganites of the type (A$_{1-x}$B$_{x})$MnO$_{3}$, where A and B are rare and alkaline earth metals respectively have the distinction of showing all three effects. Here we establish the complementarity of the electric, magnetic and photon fields in driving an insulator-metal transition in epitaxial thin films of La$_{0.175}$Pr$_{0.45}$Ca$_{0.375}$MnO$_{3}$ whose electrical ground state is insulating. Both pulsed and CW lasers cause a giant photon flux dependent enhancement of conductivity. It is further observed that electric and magnetic fields trigger the persistent enhancement of conductivity whose magnitude can be accentuated by application of these fields in parallel.Comment: 17 pages 6 figure

Formation of double ring patterns on Co2MnSi Heusler alloy thin film by anodic oxidation under scanning probe microscope

Double ring formation on Co2MnSi (CMS) films is observed at electrical breakdown voltage during local anodic oxidation (LAO) using atomic force microscope (AFM). Corona effect and segregation of cobalt in the vicinity of the rings is studied using magnetic force microscopy and energy dispersive spectroscopy. Double ring forma-tion is attributed to the interaction of ablated material with the induced magnetic field during LAO. Steepness of forward bias transport characteristics from the unperturbed region of the CMS film suggest a non equilibrium spin contribution. Such mesoscopic textures in magnetic films by AFM tip can be potentially used for memory storage applications.Comment: 7 pages, 5 figure