N. D. Mathur

Multiferroic and magnetoelectric materials

A ferroelectric crystal exhibits a stable and switchable electrical polarization that is manifested in the form of cooperative atomic displacements. A ferromagnetic crystal exhibits a stable and switchable magnetization that arises through the quantum mechanical phenomenon of exchange. There are very few ‘multiferroic’ materials that exhibit both of these properties, but the ‘magnetoelectric’ coupling of magnetic and electrical properties is a more general and widespread phenomenon. Although work in this area can be traced back to pioneering research in the 1950s and 1960s, there has been a recent resurgence of interest driven by long-term technological aspirations.

Nanotechnology: The third way

The emergence of nanoscale features — such as magnetic and electronic patterns — in materials that are otherwise homogeneous provides a potential alternative to conventional top-down and bottom-up fabrication techniques. The way these features arise in manganite crystals is contentious, but could be explained using elasticity theory.

Giant Electrocaloric Effect in Thin-Film PbZr0.95Ti0.05O3

An applied electric field can reversibly change the temperature of an electrocaloric material under adiabatic conditions, and the effect is strongest near phase transitions. We demonstrate a giant electrocaloric effect (0.48 kelvin per volt) in 350-nanometer PbZr0.95Ti0.05O3 films near the ferroelectric Curie temperature of 222°C. A large electrocaloric effect may find application in electrical refrigeration.

Magnetically mediated superconductivity in heavy fermion compounds

In a conventional superconductor, the binding of electrons into the paired states that collectively carry the supercurrent is mediated by phonons — vibrations of the crystal lattice. Here we argue that, in the case of the heavy fermion superconductors CePd2Si2 and CeIn3, the charge carriers are bound together in pairs by magnetic spin–spin interactions. The existence of magnetically mediated superconductivity in these compounds could help shed light on the question of whether magnetic interactions are relevant for describing the superconducting and normal-state properties of other strongly correlated electron systems, perhaps including the high-temperature copper oxide superconductors.

Giant sharp and persistent converse magnetoelectric effects in multiferroic epitaxial heterostructures

Magnetoelectric coupling permits a magnetic order parameter to be addressed electrically or vice versa, and could find use in data storage, field sensors and actuators. Coupling constants for single phase materials such as chromium dioxide, boracites and manganites are typically as low as 10^{-12} - 10^{-9} s/m, e.g. because the polarisations and magnetisations are small. Two phase multiferroics with strain mediated coupling, such as laminates, composites and epitaxial nanostructures, are more promising because each phase may be independently optimised. The resulting magnetoelectric switching can be larger, e.g. 10^{-8} s/m, but it is not sharp because clean coupling is precluded by the complexity of the microstructures and concomitant strain fields. Here we report a giant sharp magnetoelectric effect at a single epitaxial interface between a 40 nm ferromagnetic stress-sensitive La_{0.67}Sr_{0.33}MnO_3 film, and a 0.5 mm BaTiO_3 substrate that is ferroelectric, piezoelectric and ferroelastic. By applying a small electric field (4-10 kV/cm) across the entire structure, we achieve persistent changes in film magnetisation of up to 65% near the BaTiO_3 structural phase transition at around 200 K. This represents a giant magnetoelectric coupling (2.3*10-7 s/m) that arises from strain fields due to ferroelastic non-180 degree domains whose presence we confirm using x-ray diffraction. The coupling persists over a wide range of temperatures including room temperature, and could therefore inspire a range of sensor and memory applications.

Giant tunnel electroresistance for non-destructive readout of ferroelectric states

Ferroelectrics possess a polarization that is spontaneous, stable and electrically switchable, and submicrometre-thick ferroelectric films are currently used as non-volatile memory elements with destructive capacitive readout. Memories based on tunnel junctions with ultrathin ferroelectric barriers would enable non-destructive resistive readout. However, the achievement of room-temperature polarization stability and switching at very low thickness is challenging. Here we use piezoresponse force microscopy at room temperature to show robust ferroelectricity down to 1 nm in highly strained BaTiO3 films; we also use room-temperature conductive-tip atomic force microscopy to demonstrate resistive readout of the polarization state through its influence on the tunnel current. The resulting electroresistance effect scales exponentially with ferroelectric film thickness, reaching ∼75,000% at 3 nm. Our approach exploits the otherwise undesirable leakage current—dominated by tunnelling at these very low thicknesses—to read the polarization state without destroying it. We demonstrate scalability down to 70 nm, corresponding to potential densities of >16 Gbit inch-2. These results pave the way towards ferroelectric memories with simplified architectures, higher densities and faster operation, and should inspire further exploration of the interplay between quantum tunnelling and ferroelectricity at the nanoscale.

Caloric materials near ferroic phase transitions

A magnetically, electrically or mechanically responsive material can undergo significant thermal changes near a ferroic phase transition when its order parameter is modified by the conjugate applied field. The resulting magnetocaloric, electrocaloric and mechanocaloric (elastocaloric or barocaloric) effects are compared here in terms of history, experimental method, performance and prospective cooling applications.

Ferroelectric Control of Spin Polarization

Spin into Control Spintronics—the use of the spin direction of subatomic particles to control on and off states, instead of electric charge—has the potential to create low-power electronics, because less energy is needed to flip spin states than to flip switches to create voltage barriers. Theoretical work hints that spin-polarized electrons from a ferromagnetic electrode can be controlled by a change in polarization created in a ferroelectric thin film. Garcia et al. (p. 1106, published online 14 January) fabricated an iron-barium titanate junction on a lanthanum strontium manganate substrate that acts as a spin detector. Local control of spin polarization was observed in the ferroelectric layer, which retained its polarization without any applied power. Ferroelectric tunnel junctions control the spin polarization of electrons emitted from iron electrodes. A current drawback of spintronics is the large power that is usually required for magnetic writing, in contrast with nanoelectronics, which relies on “zero-current,” gate-controlled operations. Efforts have been made to control the spin-relaxation rate, the Curie temperature, or the magnetic anisotropy with a gate voltage, but these effects are usually small and volatile. We used ferroelectric tunnel junctions with ferromagnetic electrodes to demonstrate local, large, and nonvolatile control of carrier spin polarization by electrically switching ferroelectric polarization. Our results represent a giant type of interfacial magnetoelectric coupling and suggest a low-power approach for spin-based information control.

A ferroelectric memristor

Memristors are continuously tunable resistors that emulate biological synapses. Conceptualized in the 1970s, they traditionally operate by voltage-induced displacements of matter, although the details of the mechanism remain under debate. Purely electronic memristors based on well-established physical phenomena with albeit modest resistance changes have also emerged. Here we demonstrate that voltage-controlled domain configurations in ferroelectric tunnel barriers 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, 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.

Giant electrocaloric effect in the thin film relaxor ferroelectric 0.9 PbMg1/3Nb2/3O3-0.1 PbTiO3 near room temperature

The authors have recently observed a giant electrocaloric effect (12K in 25V) in 350nm sol-gel PbZr0.95Ti0.05O3 films near the ferroelectric Curie temperature of 242°C. Here the authors demonstrate a giant electrocaloric effect (5K in 25V) in 260nm sol-gel films of the relaxor ferroelectric 0.9PbMg1∕3Nb2∕3O3–0.1PbTiO3 near the Curie temperature of 60°C. This reduction in operating temperature widens the potential for applications in cooling systems.