Surface phase transitions in BiFeO3 below room temperature

We combine a wide variety of experimental techniques to analyze two heretofore mysterious phase transitions in multiferroic bismuth ferrite at low temperature. Raman spectroscopy, resonant ultrasound spectroscopy, EPR, X-ray lattice constant measurements, conductivity and dielectric response, specific heat and pyroelectric data have been collected for two different types of samples: single crystals and, in order to maximize surface/volume ratio to enhance surface phase transition effects, BiFeO3 nanotubes were also studied. The transition at T=140.3K is shown to be a surface phase transition, with an associated sharp change in lattice parameter and charge density at the surface. Meanwhile, the 201K anomaly appears to signal the onset of glassy behaviour

Sub 15 fs X ray pump and X ray probe experiment for the study of ultrafast magnetization dynamics in ferromagnetic alloys

In this paper, we present a new setup for the measurement of element specific ultrafast magnetization dynamics in ferromagnetic thin films with a sub 15 fs time resolution. Our experiment relies on a split and delay approach which allows us to fully exploit the shortest X rays pulses delivered by X ray Free Electrons Lasers close to the attosecond range , in an X ray pump X ray probe geometry. The setup performance is demonstrated by measuring the ultrafast elemental response of Ni and Fe during demagnetization of ferromagnetic Ni and Ni80Fe20 Permalloy samples upon resonant excitation at the corresponding absorption edges. The transient demagnetization process is measured in both reflection and transmission geometry using, respectively, the transverse magneto optical Kerr effect T MOKE and the Faraday effect as probing mechanism

Toward ultrafast magnetic depth profiling using time resolved x ray resonant magnetic reflectivity

During the last two decades, a variety of models have been developed to explain the ultrafast quenching of magnetization following femtosecond optical excitation. These models can be classified into two broad categories, relying either on a local or a non local transfer of angular momentum. The acquisition of the magnetic depth profiles with femtosecond resolution, using time resolved x ray resonant magnetic reflectivity, can distinguish local and non local effects. Here, we demonstrate the feasibility of this technique in a pump probe geometry using a custom built reflectometer at the FLASH2 free electron laser FEL . Although FLASH2 is limited to the production of photons with a fundamental wavelength of 4 amp; 8201;nm amp; 8771;310 amp; 8201;eV , we were able to probe close to the Fe L3 edge 706.8 amp; 8201;eV of a magnetic thin film employing the third harmonic of the FEL. Our approach allows us to extract structural and magnetic asymmetry signals revealing two dynamics on different time scales which underpin a non homogeneous loss of magnetization and a significant dilation of 2 amp; 8201; of the layer thickness followed by oscillations. Future analysis of the data will pave the way to a full quantitative description of the transient magnetic depth profile combining femtosecond with nanometer resolution, which will provide further insight into the microscopic mechanisms underlying ultrafast demagnetizatio

A tool for predicting the thermal performance of a diesel engine

This paper presents a thermal network model for the simulation of the transient response of diesel engines. The model was adjusted by using experimental data from a completely instrumented engine run under steady-state and transient conditions. Comparisons between measured and predicted material temperatures over a wide range of engine running conditions show a mean error of 7◦C. The model was then used to predict the thermal behavior of a different engine. Model results were checked against oil and coolant temperatures measured during engine warm-up at constant speed and load, and on a New European Driving Cycle. Results show that the model predicts these temperatures with a maximum error of 3◦C.Torregrosa, AJ.; Olmeda González, PC.; Martín Díaz, J.; Romero Piedrahita, CA. (2011). A tool for predicting the thermal performance of a diesel engine. Heat Transfer Engineering. 32(10):891-904. doi:10.1080/01457632.2011.548639S891904321

Nonmagnetic Fe-site doping of BiFeO3 multiferroic ceramics

International audienceIn this paper, we show that a pure single phase by doping Fe-site of BiFeO3 (BFO) using tetravalent Zr4+ ions can be achieved by introducing cation (Bi3+) vacancies. The structural analysis reveals that the ferroelectric nature of BFO should be weakly affected by 10% of Zr4+ doping as the c/a ratio and the Curie temperature T-C remain roughly unchanged compared to that of pure BFO. In contrast, the magnetic properties are affected as a weak ferromagnetism and a change of Neel temperature T-N are observed. Beyond the double-exchange interactions arising from the creation of Fe2+, we propose another simple model inducing a local ferromagnetic coupling rather than an antiferromagnetic which considers the replacement of the magnetically active Fe3+, time to time, by a nonactive Zr4+

High-temperature ferroic phase transitions and paraelectric cubic phase in multiferroic Bi0.951dFe0.9Zr0.1O3

International audienceThe temperature-dependent phase transitions of Bi0.95þdFe0.9Zr0.1O3 have been studied using hightemperature x-ray powder diffraction together with differential scanning calorimetry measurements. The results show that Bi0.95þdFe0.9Zr0.1O3 undergoes two phase transitions at 815 C and 905 C before decomposition at 920 C. It appears that Zr-doping seems to stabilize the high temperature phases in such compound. Both the sharp contraction in the unit-cell volume and enthalpy thermal hysteresis demonstrate that the ferroelectric phase transition (a/!b) at 815 C is of first order nature. In contrast, the highest b/!c phase transition at 905 C appears to be a second-order-like one. The analysis of the diffraction pattern in addition to Rietveld refinement strongly suggests a cubic symmetry for c-phase

Interface Magnetoelectric Coupling in Co/Pb(Zr,Ti)O 3

International audienceMagnetoelectric coupling at multiferroic interfaces is a promising route toward the nonvolatile electric-field control of magnetization. Here, we use optical measurements to study the static and dynamic variations of the interface magnetization induced by an electric field in Co/PbZr0.2Ti0.8O3 (Co/PZT) bilayers at room temperature. The measurements allow us to identify different coupling mechanisms. We further investigate the local electronic and magnetic structure of the interface by means of transmission electron microscopy, soft X-ray magnetic circular dichroism, and density functional theory to corroborate the coupling mechanism. The measurements demonstrate a mixed linear and quadratic optical response to the electric field, which results from a magneto-electro-optical effect. We propose a decomposition method of the optical signal to discriminate between different components involved in the electric field-induced polarization rotation of the reflected light. This allows us to extract a signal that we can ascribe to interface magnetoelectric coupling. The associated surface magnetization exhibits a clear hysteretic variation of odd symmetry with respect to the electric field and nonzero remanence. The interface coupling is remarkably stable over a wide frequency range (1–50 kHz), and the application of a bias magnetic field is not necessary for the coupling to occur. These results show the potential of exploiting interface coupling with the prospect of optimizing the performance of magnetoelectric memory devices in terms of stability, as well as fast and dissipationless operation

Surface phase transitions in BiFeO3 below room temperature

We combine a wide variety of experimental techniques to analyze two heretofore mysterious phase transitions in multiferroic bismuth ferrite at low temperature. Raman spectroscopy, resonant ultrasound spectroscopy, EPR, X-ray lattice constant measurements, conductivity and dielectric response, specific heat and pyroelectric data have been collected for two different types of samples: single crystals and, in order to maximize surface/volume ratio to enhance surface phase transition effects, BiFeO3 nanotubes were also studied. The transition at T=140.3K is shown to be a surface phase transition, with an associated sharp change in lattice parameter and charge density at the surface. Meanwhile, the 201K anomaly appears to signal the onset of glassy behaviour