Measurement of the angle dependence of magnetostriction in pulsed magnetic fields using a piezoelectric strain gauge.

We present a high resolution method for measuring magnetostriction in millisecond pulsed magnetic fields at cryogenic temperatures with a sensitivity of 1.11×10-11/Hz . The sample is bonded to a thin piezoelectric plate such that when the sample's length changes, it strains the piezoelectric and induces a voltage change. This method is more sensitive than a fiber-Bragg grating method. It measures two axes simultaneously instead of one. The gauge is small and versatile, functioning in DC and millisecond pulsed magnetic fields. We demonstrate its use by measuring the magnetostriction of Ca3Co1.03Mn0.97O6 single crystals in pulsed magnetic fields. By comparing our data to new and previously published results from a fiber-Bragg grating magnetostriction setup, we confirm that this method detects magnetostriction effects. We also demonstrate the small size and versatility of this technique by measuring angle dependence with respect to the applied magnetic field in a rotator probe in 65 T millisecond pulsed magnetic fields.

Mimicking Synaptic Plasticity and Neural Network Using Memtranstors.

Artificial synaptic devices that mimic the functions of biological synapses have drawn enormous interest because of their potential in developing brain-inspired computing. Current studies are focusing on memristive devices in which the change of the conductance state is used to emulate synaptic behaviors. Here, a new type of artificial synaptic devices based on the memtranstor is demonstrated, which is a fundamental circuit memelement in addition to the memristor, memcapacitor, and meminductor. The state of transtance (presented by the magnetoelectric voltage) in memtranstors acting as the synaptic weight can be tuned continuously with a large number of nonvolatile levels by engineering the applied voltage pulses. Synaptic behaviors including the long-term potentiation, long-term depression, and spiking-time-dependent plasticity are implemented in memtranstors made of Ni/0.7Pb(Mg1/3 Nb2/3 )O3 -0.3PbTiO3 /Ni multiferroic heterostructures. Simulations reveal the capability of pattern learning in a memtranstor network. The work elucidates the promise of memtranstors as artificial synaptic devices with low energy consumption.

Electrochemical-reaction-induced synaptic plasticity in MoOx-based solid state electrochemical cells.

Solid state electrochemical cells with synaptic functions have important applications in building smart-terminal networks. Here, the essential synaptic functions including potentiation and depression of synaptic weight, transition from short- to long-term plasticity, spike-rate-dependent plasticity, and spike-timing-dependent plasticity behavior were successfully realized in an Ag/MoOx/fluorine-doped tin oxide (FTO) cell with continual resistance switching. The synaptic plasticity underlying these functions was controlled by tuning the excitatory post-synaptic current (EPSC) decay, which is determined by the applied voltage pulse number, width, frequency, and intervals between the pre- and post-spikes. The physical mechanism of the artificial synapse operation is attributed to the interfacial electrochemical reaction processes of the MoOx films with the adsorbed water, where protons generated by water decomposition under an electric field diffused into the MoOx films and intercalated into the lattice, leading to the short- and long-term retention of cell resistance, respectively. These results indicate the possibility of achieving advanced artificial synapses with solid state electrochemical cells and will contribute to the development of smart-terminal networking systems.

Uniaxial ferroelectric quantum criticality in multiferroic hexaferrites BaFe12O19 and SrFe12O19.

BaFe12O19 is a popular M-type hexaferrite with a Néel temperature of 720 K and is of enormous commercial value ($3 billion/year). It is an incipient ferroelectric with an expected ferroelectric phase transition extrapolated to lie at 6 K but suppressed due to quantum fluctuations. The theory of quantum criticality for such uniaxial ferroelectrics predicts that the temperature dependence of the electric susceptibility χ diverges as 1/T(3), in contrast to the 1/T(2) dependence found in pseudo-cubic materials such as SrTiO3 or KTaO3. In this paper we present evidence of the susceptibility varying as 1/T(3), i.e. with a critical exponent γ = 3. In general γ = (d + z - 2)/z, where the dynamical exponent for a ferroelectric z = 1 and the dimension is increased by 1 from deff = 3 + z to deff = 4 + z due to the effect of long-range dipole interactions in uniaxial as opposed to multiaxial ferroelectrics. The electric susceptibility of the incipient ferroelectric SrFe12O19, which is slightly further from the quantum phase transition is also found to vary as 1/T(3).

Moisture effects on the electrochemical reaction and resistance switching at Ag/molybdenum oxide interfaces.

An important potential application of solid state electrochemical reactions is in redox-based resistive switching memory devices. Based on the fundamental switching mechanisms, the memory has been classified into two modes, electrochemical metallization memory (ECM) and valence change memory (VCM). In this work, we have investigated a solid state electrochemical cell with a simple Ag/MoO3-x/fluorine-doped tin oxide (FTO) sandwich structure, which shows a normal ECM switching mode after an electroforming process. While in the lower voltage sweep range, the switching behavior changes to VCM-like mode with the opposite switching polarity to the ECM mode. By current-voltage measurements under different ambient atmospheres and X-ray photoemission spectroscopy analysis, electrochemical anodic passivation of the Ag electrode and valence change of molybdenum ions during resistance switching have been demonstrated. The crucial role of moisture adsorption in the switching mode transition has been clarified based on the Pourbaix diagram for the Ag-H2O system for the first time. These results provide a fundamental insight into the resistance switching mechanism model in solid state electrochemical cells.

Quantum electric-dipole liquid on a triangular lattice.

Geometric frustration and quantum fluctuations may prohibit the formation of long-range ordering even at the lowest temperature, and therefore liquid-like ground states could be expected. A good example is the quantum spin liquid in frustrated magnets. Geometric frustration and quantum fluctuations can happen beyond magnetic systems. Here we propose that quantum electric-dipole liquids, analogues of quantum spin liquids, could emerge in frustrated dielectrics where antiferroelectrically coupled electric dipoles reside on a triangular lattice. The quantum paraelectric hexaferrite BaFe12O19 with geometric frustration represents a promising candidate for the proposed electric-dipole liquid. We present a series of experimental lines of evidence, including dielectric permittivity, heat capacity and thermal conductivity measured down to 66 mK, to reveal the existence of an unusual liquid-like quantum phase in BaFe12O19, characterized by itinerant low-energy excitations with a small gap. The possible quantum liquids of electric dipoles in frustrated dielectrics open up a fresh playground for fundamental physics.

Electrical control of large magnetization reversal in a helimagnet.

Reversal of magnetization M by an electrical field E has been a long-sought phenomenon in materials science because of its potential for applications such as memory devices. However, the phenomenon has rarely been achieved and remains a considerable challenge. Here we report the large M reversal by E in a multiferroic Ba0.5Sr1.5Zn2(Fe0.92Al0.08)12O22 crystal without any external magnetic field. Upon sweeping E through the range of ±2 MV m(-1), M varied quasi-linearly in the range of ±2 µB per f.u., resulting in the M reversal. Strong electrical modulation of M at zero magnetic field were observable up to ~\n150 K. Nuclear magnetic resonance measurements provided microscopic evidence that the electric field and the magnetic field play equivalent roles in modulating the volume of magnetic domains. Our results suggest that the soft ferrimagnetism and the associated transverse conical state are key ingredients to achieve the large magnetization reversal at fairly high temperatures.

Effects of Al substitution and thermal annealing on magnetoelectric Ba0.5Sr1.5Zn2Fe12O22 investigated by the enhancement factor of 57Fe nuclear magnetic resonance.

The magnetoelectric properties of hexaferrite Ba0.5Sr1.5Zn2Fe12O22 are significantly improved by Al substitution and thermal annealing. Measuring the enhancement factor of 57Fe NMR, we found direct microscopic evidence that the magnetic moments of the L and S blocks are rotated by a magnetic field in such a way as to increase the net magnetic moment of a magnetic unit, even after the field is removed. Al substitution makes magnetoelectric property arise easily by suppressing the easy-plane anisotropy. The effect of thermal annealing is to stabilize the multiferroic state by reducing the number of pinning sites and the electron spin fluctuation. The transverse conic structure gradually changes to the alternating longitudinal conic structure where spins fluctuate more severely.

Electric field control of nonvolatile four-state magnetization at room temperature.

We find the realization of large converse magnetoelectric (ME) effects at room temperature in a magnetoelectric hexaferrite Ba0.52Sr2.48Co2Fe24O41 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 0.62µ(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.

Realization of giant magnetoelectricity in helimagnets.

We show that low field magnetoelectric (ME) properties of helimagnets Ba0.5Sr1.5Zn2(Fe1-xAlx)12O22 can be efficiently tailored by the Al-substitution level. As x increases, the critical magnetic field for switching electric polarization is systematically reduced from approximately 1 T down to approximately 1 mT, and the ME susceptibility is greatly enhanced to reach a giant value of 2.0x10{4} ps/m at an optimum x=0.08. We find that control of the nontrivial orbital moment in the octahedral Fe sites through the Al substitution is crucial for fine-tuning the magnetic anisotropy and obtaining the conspicuously improved ME characteristics.