We report controlled ignition of magnetization reversal avalanches by surface acoustic waves in a single crystal of Mn 12 acetate. Our data show that the speed of the avalanche exhibits maxima on the magnetic field at the tunneling resonances of Mn 12 . Combined with the evidence of magnetic deflagration in Mn 12 acetate [Y. Suzuki et al., Phys. Rev. Lett. 95, 147201 (2005)], this suggests a novel physical phenomenon: deflagration assisted by quantum tunneling. DOI: 10.1103/PhysRevLett.95.217205 PACS numbers: 75.50.Xx, 45.70.Ht Magnetic properties of Mn 12 acetate have been intensively studied after the magnetic bi-stability of this molecular cluster below 3.5 K was demonstrated [1]. The bistability is caused by a large spin of the cluster, S 10, and by strong uniaxial magnetic anisotropy that provides a 65 K energy barrier between spin-up and spin-down states. At low temperature a magnetized Mn 12 crystal exhibits two modes of magnetic relaxation. The first mode is a slow one. It manifests itself in a staircase hysteresis curve which is due to thermally assisted quantum tunneling of the magnetization [2]. The second relaxation mode, exhibited by sufficiently large crystals, is a much more rapid magnetization reversal that typically lasts less than 1 ms. It was initially studied by Paulsen and Park [3] and attributed to a thermal runaway or avalanche [see also Ref. [4]]. In the avalanche, the initial relaxation of the magnetization towards the direction of the field results in the release of heat that further accelerates the magnetic relaxation. Recent local magnetic measurements of Mn 12 crystals [5] have demonstrated that during an avalanche the magnetization reversal occurs inside a narrow interface that propagates through a crystal at a constant speed of a few meters per second. It has been argued that this process is analogous to the propagation of a flame front (deflagration) through a flammable chemical substance. The conventional theory of deflagration, in the first approximation, yields the following expression for the velocity of the flame front [5-7]:Here U, 0 , and T f are the energy barrier, the attempt frequency, and the temperature of the ''flame'' in the expression 0 expU=k B T f for the ''chemical reaction'' time, and is thermal diffusivity. In the case of Mn 12 , 10 ÿ5 m 2 =s, 0 10 ÿ7 s, and the field dependence of the energy barrier, UH, is well known.In a flammable chemical substance the potential barrier is a constant determined by the nature of the chemical reaction that transforms a metastable chemical into a stable chemical (e.g., a mixture of hydrogen and oxygen transforms into water). On the contrary, in molecular magnets the energy barrier, as well as the released energy, can be controlled by the magnetic field. At certain values of the magnetic field the spin levels on the two sides of the energy barrier come to resonance and thermally assisted quantum spin tunneling under the barrier takes place; see Fig. 1. Therefore, the effect of the tunneling is roughly equivalent to cutting ...

This study examines the validity of a survey instrument on near-miss obstetric complications. Three groups of women--with severe complications, with mild complications, and with a normal delivery--were identified retrospectively in three hospitals in South Benin and interviewed at home. The concept of "near-miss" was used to identify women with severe episodes of morbidity. The questionnaire was able to detect, with some accuracy, eclamptic fits, abnormal bleeding in the third trimester for a recall period of at least three to four years, and all episodes of bleeding independent of timing within a period of two years. Questions concerning dystocia and infections of the genital tract generated disappointing results except when information on treatment was included. Overall, better results were achieved for antepartum and acute events. Severity made a positive difference only in the case of eclampsia, with an increase in sensitivity. The implications of the results for using women's recall of obstetric complications in surveys are discussed.

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The authors demonstrate a compact optical waveguide modulator based on a Mach-Zehnder interferometer driven by surface acoustic waves. The modulator was monolithically fabricated on GaAs with an active region length of approximately 15 m. It yields peak-to-peak modulation exceeding 90% of the average transmission and operation in the gigahertz frequency range. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2354411͔ Acousto-optic effects have been used for optical modulation for a long time, with the Bragg cells being probably the best known example. 1 The demand for fast and compact devices together with required phase matching, however, imposes several limitations on conventional acousto-optic devices in future generations of integrated photonics. Therefore, great attention has recently been devoted to alternative concepts for light modulation. A promising approach to increase the operation speed employs all-optical light control, which allows operation down to the subpicosecond time scales. 2 These devices are typically a few hundreds of microns long since they rely on optical nonlinearities that are usually small. An effective approach to reduce device dimensions employs photonic crystals ͑PhCs͒. Examples are thermo-optical switches based on a PhC Mach-Zehnder interferometer ͑MZI͒ with 12-m-long arms on AlGaAs/ GaAs system 3 as well as electro-optical switches based on carrier injection of 80-m-long silicon PhC-MZI. 4 In both cases the switching time is on the order of microseconds. Faster PhC-based all-optical switching devices have been realized in the ͑Al,Ga͒As system by taking advantage of the nonlinear properties of quantum dots embedded in the MZI arms. 5 PhC fabrication, however, requires a sophisticated technology with very strict tolerances.In this letter, we demonstrate a compact and monolithic modulator based on conventional ridge waveguides ͑WGs͒ on GaAs. The modulator consists of a MZI driven by a surface acoustic wave ͑SAW͒ in the gigahertz range, where the length of the interaction region between the acoustic and optical waves ͑the active region͒ is reduced to approximately 15 m. The design used, which is a modified version of the acousto-optic MZI proposed by Gorecki et al.,6 is based on the refractive index modulation of the interferometer arms by the wave fronts of a SAW propagating perpendicularly to the arms. The changes in refractive index are induced by the elasto-optic and electro-optic effects associated with the strain and piezoelectric fields, respectively. For photon energies away from electronic transitions-which is the case discussed here-the elasto-optical effect dominates. 7 The width of the WGs forming the arms is chosen to be much smaller than the acoustic wavelength ͑ SAW ͒ in order to ensure a constant modulation amplitude across the WG width. In our design, we introduce two fundamental modifications to increase the modulation efficiency and reduce the length of the active region. First, we enhance the modulation efficiency by modulating simultaneously both interferom...

Very fast magnetic avalanches in ͑La, Pr͒-based manganites are the signature of a phase transition from an insulating blocked charge-ordered antiferromagnetic state to a charge-delocalized ferromagnetic ͑CD-FM͒ state. We report here the experimental observation that this transition does not occur either simultaneously or randomly in the whole sample but there is instead a spatial propagation with a velocity of the order of tens of m/s. Our results show that avalanches originate from the inside of the sample, move to the outside, and occur at values of the applied magnetic field that depend on the CD-FM fraction in the sample. Moreover, upon application of surface acoustic waves at constant magnetic fields, we are able to trigger avalanches at very well-determined values of the temperature and magnetic field. Due to the interaction with the acoustic waves, the number of isolated ferromagnetic clusters in La 0.225 Pr 0.40 Ca 0.375 MnO 3 starts to grow across the entire sample in the same way as if it were a magnetic deflagration.

Mobile piezoelectric potentials are used to coherently transport electron spins in GaAs (110) quantum wells (QW) over distances exceeding 60 microm. We demonstrate that the dynamics of mobile spins under external magnetic fields depends on the direction of motion in the QW plane. This transport anisotropy is an intrinsic property of moving spins associated with the bulk inversion asymmetry of the underlying GaAs lattice.

Magneto-optic Kerr microscopy was employed to investigate the spin-orbit interactions of electrons traveling in semiconductor quantum wells using surface acoustic waves (SAWs). Two-dimensional images of the spin flow induced by SAWs exhibit anisotropic spin precession behaviors caused by the coexistence of different types of spin-orbit interactions. The dependence of spin-orbit effective magnetic fields on SAW intensity indicates the existence of acoustically controllable spin-orbit interactions resulting from the strain and Rashba contributions induced by the SAWs.

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