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Topology created by quasi-continuous spatial variations of a local polarization direction represents an exotic state of matter, but field-driven manipulation has been hitherto limited to creation and destruction. Here we report that relatively small electric or mechanical fields can drive the non-volatile rotation of polar spirals in discretized microregions of the relaxor ferroelectric polymer poly(vinylidene fluoride- ran -trifluoroethylene). These polar spirals arise from the asymmetric Coulomb interaction between vertically aligned helical polymer chains, and can be rotated in-plane through various angles with robust retention. Given also that our manipulation of topological order can be detected via infrared absorption, our work suggests a new direction for the application of complex materials. Polar spirals induced in a relaxor ferroelectric can be quasi-continuously rotated by applying electric/mechanical fields, due to an asymmetric Coulomb interaction. The rotations are non-volatile with robust retention, and can be optically read out.

Current interest in barocaloric effects has been stimulated by the discovery that these pressure-driven thermal changes can be giant near ferroic phase transitions in materials that display magnetic or electrical order. Here we demonstrate giant inverse barocaloric effects in the solid electrolyte AgI, near its superionic phase transition at ~420 K. Over a wide range of temperatures, hydrostatic pressure changes of 2.5 kbar yield large and reversible barocaloric effects, resulting in large values of refrigerant capacity. Moreover, the peak values of isothermal entropy change (60 J K −1  kg −1 or 0.34 J K −1  cm −3 ) and adiabatic temperature changes (18 K), which we identify for a starting temperature of 390 K, exceed all values previously recorded for barocaloric materials. Our work should therefore inspire the study of barocaloric effects in a wide range of solid electrolytes, as well as the parallel development of cooling devices. Barocaloric materials offer promise in solid-state cooling devices, but few materials have been show to display giant barocaloric effects near room temperature. Here, the authors demonstrate that solid electrolyte AgI displays giant inverse barocaloric effects near its superionic phase transition at ~420 K.

Multicaloric materials show thermal changes that can be driven simultaneously or sequentially by more than one type of external field. The use of more than one driving field can induce larger thermal changes, with smaller field magnitudes, over wider ranges of operating temperature, and can also eliminate hysteresis in one control parameter by transferring it to another. The thermodynamics behind multicaloric effects is well established, but only a small number of multicaloric materials have been experimentally studied to date. Here, we describe the fundamentals of multicaloric effects and discuss the performance of representative multicaloric materials. Exploiting multicaloric effects could aid the future development of cooling devices, where key challenges include energy efficiency and the span of the operating temperature.

The elastic degree of freedom is widely exploited to mediate magnetoelectric coupling between ferromagnetic films and ferroelectric substrates. For epitaxial Fe films grown on clean BaTiO 3 substrates, shear strain can determine the underlying magnetoelastic coupling. Here, we use PhotoEmission Electron Microscopy of ferroic Fe and BaTiO 3 domains, combined with micromagnetic simulations, to directly reveal an inverted interfacial magnetoelastic coupling in the low-dimensional limit. We show that the magnetocrystalline anisotropy competes with the epitaxial shear strain to align the local magnetization of ultrathin Fe films close to the local polarization direction of the ferroelectric BaTiO 3 in-plane domains. Poling the BaTiO 3 substrate creates c -domains with no shear strain contribution with the local magnetization rotated by ~45°. Tuning shear strain magnetoelastic contributions suggests new routes for designing magnetoelectric devices. The magnetoelastic coupling at a ferroelectric-ferromagnetic interface is shown to be dominated by shear-strain effects. Using polarised x-ray microscopy to simultaneously image the ferroic domain structures, the authors demonstrate an anomalous coupling in the ultrathin film limit.

For more than a century, humankind has achieved refrigeration by exploiting volatile gases that harm the environment when released to the atmosphere. More recently, the observation of electrocaloric effects in commercial multilayer capacitors has inspired the possibility of environmentally friendly cooling. In this article, we describe electrocaloric effects in multilayer capacitors for cooling applications, compare the electrocaloric performance of existing multilayer capacitors, and discuss the improvements required for practical cooling devices.

Magnetocaloric and electrocaloric effects are driven by doing work, but this work has barely been explored, even though these caloric effects are being exploited in a growing number of prototype cooling devices.

Bulk PbMg0.5W0.5O3 (PMW) is an antiferroelectric in which an electric field of 12 V μm−1 is sufficient to initiate a nominally reversible transition to a dipole-aligned (ferroelectric) phase if operating just below the Néel temperature TN, near room temperature (Li et al 2021 Adv. Funct. Mater.31 2101176). Here we describe multilayer capacitors (MLCs) of PMW that permit 27 V µm−1 to be applied without breakdown. Below TN, nominally reversible driving of the partial (full) antiferroelectric–ferroelectric (AF–FE) transition over a wide (narrow) range of temperatures yields large inverse electrocaloric (EC) effects that peak at ΔTj ∼ –2.6 K when applying 25 V μm−1 at 293 K (ΔTj denotes directly measured temperature jumps). Above TN, nominally reversible driving of the partial (full) paraelectric–ferroelectric (PE–FE) transition yields large conventional EC effects that peak at ΔTj ∼ +5.2 K when applying 25 V μm−1 at 302 K. This good EC performance near room temperature implies that MLCs of PMW could be exploited in prototype EC coolers.

Memristors are devices whose dynamic properties are of interest because they can mimic the operation of biological synapses. The demonstration that ferroelectric domains in tunnel junctions behave like memristors suggests new approaches for designing neuromorphic circuits. Memristors are continuously tunable resistors that emulate biological synapses 1 , 2 . Conceptualized in the 1970s, they traditionally operate by voltage-induced displacements of matter, although the details of the mechanism remain under debate 3 , 4 , 5 . Purely electronic memristors based on well-established physical phenomena with albeit modest resistance changes have also emerged 6 , 7 . Here we demonstrate that voltage-controlled domain configurations in ferroelectric tunnel barriers 8 , 9 , 10 yield memristive behaviour with resistance variations exceeding two orders of magnitude and a 10 ns operation speed. Using models of ferroelectric-domain nucleation and growth 11 , 12 , we explain the quasi-continuous resistance variations and derive a simple analytical expression for the memristive effect. Our results suggest new opportunities for ferroelectrics as the hardware basis of future neuromorphic computational architectures.

Ferroic-order parameters 1 are useful as state variables in non-volatile information storage media because they show a hysteretic dependence on their electric or magnetic field. Coupling ferroics with quantum-mechanical tunnelling allows a simple and fast readout of the stored information through the influence of ferroic orders on the tunnel current. For example, data in magnetic random-access memories 2 are stored in the relative alignment of two ferromagnetic electrodes separated by a non-magnetic tunnel barrier, and data readout is accomplished by a tunnel current measurement. However, such devices based on tunnel magnetoresistance 3 typically exhibit OFF/ON ratios of less than 4, and require high powers for write operations (>1 × 10 6  A cm −2 ). Here, we report non-volatile memories with OFF/ON ratios as high as 100 and write powers as low as ∼1 × 10 4  A cm −2 at room temperature by storing data in the electric polarization direction of a ferroelectric tunnel barrier. The junctions show large, stable, reproducible and reliable tunnel electroresistance, with resistance switching occurring at the coercive voltage of ferroelectric switching. These ferroelectric devices emerge as an alternative to other resistive memories 4 , and have the advantage of not being based on voltage-induced migration of matter at the nanoscale 5 , 6 , but on a purely electronic mechanism 7 . A tunnel junction that consists of a ferroelectric barrier layer sandwiched between two electrodes can operate as a fast, low-power and non-volatile nanoscale solid-state memory.