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Register a new userElectric-field-induced spin-flop in BiFeO3 single crystals at room-temperature
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Bismuth ferrite, BiFeO3, is the only known room-temperature multiferroic material. We demonstrate here, using neutron scattering measurements in high quality single crystals, that the antiferromagnetic and ferroelectric orders are intimately coupled. Initially in a single ferroelectric state, our crystals have a canted antiferromagnetic structure describing a unique cycloid. Under electrical poling, polarisation re-orientation induces a spin flop. We argue here that the coupling between the two orders may be stronger in the bulk than that observed in thin films where the cycloid is absent.
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Electric polarization loops are measured at room temperature on highly pure BiFeO3 single crystals synthesized by a flux growth method. Because the crystals have a high electrical resistivity, the resulting low leakage currents allow us to measure a large spontaneous polarization reaching 100 microC.cm^{-2}, a value never reported in the bulk. During electric cycling, the slow degradation of the material leads to an evolution of the hysteresis curves eventually preventing full saturation of the crystals.
We find the realization of large converse magnetoelectric (ME) effects at room temperature in a multiferroic hexaferrite Ba$_{0.52}$Sr$_{2.48}$Co$_{2}$Fe$_{24}$O$_{41}$ single crystal, in which rapid change of electric polarization in low magnetic fields (about 5 mT) is coined to a large ME susceptibility of 3200 ps/m. The modulation of magnetization then reaches up to 0.62 $mu$$_{B}$/f.u. in an electric field of 1.14 MV/m. We find further that four ME states induced by different ME poling exhibit unique, nonvolatile magnetization versus electric field curves, which can be approximately described by an effective free energy with a distinct set of ME coefficients.
First-principals calculations show that up-spin and down-spin carriers are accumulating adjacent to opposite surfaces of BiFeO3(BFO) film with applying external bias. The spin carriers are equal in magnitude and opposite in direction, and down-spin carriers move to the direction opposing the external electric field while up-spin ones along the field direction. This novel spin transfer properties make BFO film an intriguing candidate for application in spin capacitor and BFO-based multiferroic field-effect device.
The search for oxide-based room-temperature ferromagnetism has been one of the holy grails in condensed matter physics. Room-temperature ferromagnetism observed in Nb-doped SrTiO3 single crystals is reported in this Rapid Communication. The ferromagnetism can be eliminated by air annealing (making the samples predominantly diamagnetic) and can be recovered by subsequent vacuum annealing. The temperature dependence of magnetic moment resembles the temperature dependence of carrier density, indicating that the magnetism is closely related to the free carriers. Our results suggest that the ferromagnetism is induced by oxygen vacancies. In addition, hysteretic magnetoresistance was observed for magnetic field parallel to current, indicating that the magnetic moments are in the plane of the samples. The x-ray photoemission spectroscopy, the static time-of-flight and the dynamic secondary ion mass spectroscopy and proton induced x-ray emission measurements were performed to examine magnetic impurities, showing that the observed ferromagnetism is unlikely due to any magnetic contaminant.
We demonstrate spin-accumulation signals controlled by the gate voltage in a metal-oxide-semiconductor field effect transistor structure with a Si channel and a CoFe/$n^{+}$-Si contact at room temperature. Under the application of a back-gate voltage, we clearly observe the three-terminal Hanle-effect signal, i.e., spin-accumulation signal. The magnitude of the spin-accumulation signals can be reduced with increasing the gate voltage. We consider that the gate controlled spin signals are attributed to the change in the carrier density in the Si channel beneath the CoFe/$n^{+}$-Si contact. This study is not only a technological jump for Si-based spintronic applications with gate structures but also reliable evidence for the spin injection into the semiconducting Si channel at room temperature.
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