Materials can be transformed from one crystalline phase to another by using an electric field to control ion transfer, in a process that can be harnessed in applications such as batteries, smart windows and fuel cells. Increasing the number of transferrable ion species and of accessible crystalline phases could in principle greatly enrich material functionality. However, studies have so far focused mainly on the evolution and control of single ionic species (for example, oxygen, hydrogen or lithium ions). Here we describe the reversible and non-volatile electric-field control of dual-ion (oxygen and hydrogen) phase transformations, with associated electrochromic and magnetoelectric effects. We show that controlling the insertion and extraction of oxygen and hydrogen ions independently of each other can direct reversible phase transformations among three different material phases: the perovskite SrCoO (ref. 12), the brownmillerite SrCoO (ref. 13), and a hitherto-unexplored phase, HSrCoO. By analysing the distinct optical absorption properties of these phases, we demonstrate selective manipulation of spectral transparency in the visible-light and infrared regions, revealing a dual-band electrochromic effect that could see application in smart windows. Moreover, the starkly different magnetic and electric properties of the three phases-HSrCoO is a weakly ferromagnetic insulator, SrCoO is a ferromagnetic metal, and SrCoO is an antiferromagnetic insulator-enable an unusual form of magnetoelectric coupling, allowing electric-field control of three different magnetic ground states. These findings open up opportunities for the electric-field control of multistate phase transformations with rich functionalities.

The complex interplay of spin, charge, orbital and lattice degrees of freedom provides a plethora of exotic phases and physical phenomena. In recent years, complex spin topologies have emerged as a consequence of the electronic band structure and the interplay between spin and spin-orbit coupling in materials. Here we produce complex topologies of electrical polarization--namely, nanometre-scale vortex-antivortex (that is, clockwise-anticlockwise) arrays that are reminiscent of rotational spin topologies--by making use of the competition between charge, orbital and lattice degrees of freedom in superlattices of alternating lead titanate and strontium titanate layers. Atomic-scale mapping of the polar atomic displacements by scanning transmission electron microscopy reveals the presence of long-range ordered vortex-antivortex arrays that exhibit nearly continuous polarization rotation. Phase-field modelling confirms that the vortex array is the low-energy state for a range of superlattice periods. Within this range, the large gradient energy from the vortex structure is counterbalanced by the corresponding large reduction in overall electrostatic energy (which would otherwise arise from polar discontinuities at the lead titanate/strontium titanate interfaces) and the elastic energy associated with epitaxial constraints and domain formation. These observations have implications for the creation of new states of matter (such as dipolar skyrmions, hedgehog states) and associated phenomena in ferroic materials, such as electrically controllable chirality.

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The bistability of ordered spin states in ferromagnets (FMs) provides the magnetic memory functionality. Traditionally, the macroscopic moment of ordered spins in FMs is utilized to write information on magnetic media by a weak external magnetic field, and the FM stray field is used for reading. However, the latest generation of magnetic random access memories demonstrates a new efficient approach in which magnetic fields are replaced by electrical means for reading and writing. This concept may eventually leave the sensitivity of FMs to magnetic fields as a mere weakness for retention and the FM stray fields as a mere obstacle for high-density memory integration. In this paper we report a room-temperature bistable antiferromagnetic (AFM) memory which produces negligible stray fields and is inert in strong magnetic fields. We use a resistor made of an FeRh AFM whose transition to a FM order 100 degrees above room-temperature, allows us to magnetically set different collective directions of Fe moments. Upon cooling to room-temperature, the AFM order sets in with the direction the AFM moments pre-determined by the field and moment direction in the high temperature FM state. For electrical reading, we use an antiferromagnetic analogue of the anisotropic magnetoresistance (AMR). We report microscopic theory modeling which confirms that this archetypical spintronic effect discovered more than 150 years ago in FMs, can be equally present in AFMs. Our work demonstrates the feasibility to realize room-temperature spintronic memories with AFMs which greatly expands the magnetic materials base for these devices and offers properties which are unparalleled in FMs

The critical size limit of electric polarization remains a fundamental question in nanoscale ferroelectric research 1 . As such, the viability of ultrathin ferroelectricity greatly impacts emerging low-power logic and nonvolatile memories 2 . Size effects in ferroelectrics have been thoroughly investigated for perovskite oxides -the archetypal ferroelectric system 3 . Perovskites, however, have so far proved unsuitable for thickness-scaling and integration with modern semiconductor processes 4 . Here, we report ultrathin ferroelectricity in doped-HfO2, a fluorite-structure oxide grown by atomic layer deposition on silicon. We demonstrate the persistence of inversion symmetry breaking and spontaneous, switchable polarization down to 1 nm. Our results indicate not only the absence of a ferroelectric critical thickness, but also enhanced polar distortions as film thickness is reduced, contradictory to perovskite ferroelectrics. This work shifts the focus on the fundamental limits of ferroelectricity to simpler transition metal oxide systems -from perovskite-derived complex oxides to fluoritestructure binary oxides -in which 'reverse' size effects counter-intuitively stabilize polar symmetry in the ultrathin regime.Ferroelectric materials exhibit stable states of collectively ordered electrical dipoles whose polarization can be reversed under an applied electric field 5 . Consequently, ultrathin ferroelectrics are of great technological interest for high-density electronics, particularly field-effect transistors and nonvolatile memories 2 . However, ferroelectricity is typically suppressed at the few nanometer scale in the ubiquitous perovskite oxides 6 . First-principles calculations predict six unit cells as the critical thickness in perovskite ferroelectrics 1 due to incomplete screening of depolarization fields 3 . Atomic-scale ferroelectricity in perovskites often fail to demonstrate polarization switching 7,8 , a crucial ingredient for application. Furthermore, attempts to synthesize ferroelectric perovskite films on silicon 9,10 are plagued by chemical incompatibility 4,11 and high temperatures required for epitaxial growth. Since the discovery of ferroelectricity in HfO2-based thin films in 2011 12 , fluorite-structure binary oxides (fluorites) have attracted considerable interest 13 as they enable lowtemperature synthesis and conformal growth in three-dimensional (3D) structures on silicon 14,15 , thereby overcoming many of the issues that restrict its perovskite counterparts in terms of complementary metal-oxide-semiconductor (CMOS) compatibility and thickness scaling 16 .