Many metal–insulator–metal systems show electrically induced resistive switching effects and have therefore been proposed as the basis for future non-volatile memories. They combine the advantages of Flash and DRAM (dynamic random access memories) while avoiding their drawbacks, and they might be highly scalable. Here we propose a coarse-grained classification into primarily thermal, electrical or ion-migration-induced switching mechanisms. The ion-migration effects are coupled to redox processes which cause the change in resistance. They are subdivided into cation-migration cells, based on the electrochemical growth and dissolution of metallic filaments, and anion-migration cells, typically realized with transition metal oxides as the insulator, in which electronically conducting paths of sub-oxides are formed and removed by local redox processes. From this insight, we take a brief look into molecular switching systems. Finally, we discuss chip architecture and scaling issues.

http://chialvo.org/Curso/UBACurso/DIA7/Papers/Aono_NatMat_07_Ionic%20Switching%20Memory.pdf

Nanoionics-based resistive switching memories

Redox‐Based Resistive Switching Memories – Nanoionic Mechanisms, Prospects, and Challenges

Recent research activities on the linear magnetoelectric (ME) effect?induction of magnetization by an electric field or of polarization by a magnetic field?are reviewed. Beginning with a brief summary of the history of the ME effect since its prediction in 1894, the paper focuses on the present revival of the effect. Two major sources for 'large' ME effects are identified. (i) In composite materials the ME effect is generated as a product property of a magnetostrictive and a piezoelectric compound. A linear ME polarization is induced by a weak ac magnetic field oscillating in the presence of a strong dc bias field. The ME effect is large if the ME coefficient coupling the magnetic and electric fields is large. Experiments on sintered granular composites and on laminated layers of the constituents as well as theories on the interaction between the constituents are described. In the vicinity of electromechanical resonances a ME voltage coefficient of up to 90?V?cm?1?Oe?1 is achieved, which exceeds the ME response of single-phase compounds by 3?5 orders of magnitude. Microwave devices, sensors, transducers and heterogeneous read/write devices are among the suggested technical implementations of the composite ME effect. (ii) In multiferroics the internal magnetic and/or electric fields are enhanced by the presence of multiple long-range ordering. The ME effect is strong enough to trigger magnetic or electrical phase transitions. ME effects in multiferroics are thus 'large' if the corresponding contribution to the free energy is large. Clamped ME switching of electrical and magnetic domains, ferroelectric reorientation induced by applied magnetic fields and induction of ferromagnetic ordering in applied electric fields were observed. Mechanisms favouring multiferroicity are summarized, and multiferroics in reduced dimensions are discussed. In addition to composites and multiferroics, novel and exotic manifestations of ME behaviour are investigated. This includes (i) optical second harmonic generation as a tool to study magnetic, electrical and ME properties in one setup and with access to domain structures; (ii) ME effects in colossal magnetoresistive manganites, superconductors and phosphates of the LiMPO4 type; (iii) the concept of the toroidal moment as manifestation of a ME dipole moment; (iv) pronounced ME effects in photonic crystals with a possibility of electromagnetic unidirectionality. The review concludes with a summary and an outlook to the future development of magnetoelectrics research.

https://iopscience.iop.org/article/10.1088/0022-3727/38/8/R01/pdf

Revival of the Magnetoelectric Effect

Multiferroic magnetoelectrics are materials that are both ferromagnetic and ferroelectric in the same phase. As a result, they have a spontaneous magnetization that can be switched by an applied magnetic field, a spontaneous polarization that can be switched by an applied electric field, and often some coupling between the two. Very few exist in nature or have been synthesized in the laboratory. In this paper, we explore the fundamental physics behind the scarcity of ferromagnetic ferroelectric coexistence. In addition, we examine the properties of some known magnetically ordered ferroelectric materials. We find that, in general, the transition metal d electrons, which are essential for magnetism, reduce the tendency for off-center ferroelectric distortion. Consequently, an additional electronic or structural driving force must be present for ferromagnetism and ferroelectricity to occur simultaneously.

Why Are There so Few Magnetic Ferroelectrics

The recent literature concerning the magnetocaloric effect (MCE) has been reviewed. The MCE properties have been compiled and correlations have been made comparing the behaviours of the different families of magnetic materials which exhibit large or unusual MCE values. These families include: the lanthanide (R) Laves phases (RM2, where M = Al, Co and Ni), Gd5(Si1−xGex)4 ,M n(As1−xSbx), MnFe(P1−xAsx), La(Fe13−xSix) and their hydrides and the manganites (R1−xMxMnO3, where R = lanthanide and M = Ca, Sr and Ba). The potential for use of these materials in magnetic refrigeration is discussed, including a comparison with Gd as a near room temperature active magnetic regenerator material. (Some figures in this article are in colour only in the electronic version)

https://iopscience.iop.org/article/10.1088/0034-4885/68/6/R04/pdf

Recent developments in magnetocaloric materials

Magnetoresistive devices (based on, for example, magnetic multilayers1) exhibit large changes in electrical resistance in response to a magnetic field, which has led to dramatic improvements in the data density and reading speed of magnetic recording systems Manganese oxides having a perovskite structure (the so-called manganites) can exhibit a magnetoresistive response that is many orders of magnitude larger than that found for other materials, and there is therefore hope that these compounds might similarly be exploited for recording applications2,3,4,5,6,7,8,9,10,11 Here we show that the switching of resistive states in the manganites can be achieved not only by a magnetic field, but also by an electric field For manganites of the form Pr1−xCaxMnO3, we find that an electrical current (and by implication a static electric field) triggers the collapse of the low-temperature, electrically insulating charge-ordered state to a metallic ferromagnetic state We suggest that such a phenomenon could be exploited to pattern conducting ferromagnetic domains within an insulating antiferromagnetic matrix, and so provide a route for fabricating micrometre- or nanometre-scale electromagnets

Current switching of resistive states in magnetoresistive manganites

An electronic (metal-to-insulator) phase transition of the first order, which can be caused by an external magnetic field, was discovered in Nd 1/2 Sr 1/2 MnO 3 . A clear hysteresis was observed during the increase and decrease of an external magnetic field at a fixed temperature. The hysteretic field region was observed to depend critically on temperature and to drastically expand with a decrease of temperature, perhaps as a result of suppression of the effect of thermal fluctuations on the first-order phase transition. Although it has seldom been observed, this is thought to be a generic feature of the first-order phase transition at low temperatures near 0 kelvin.

https://science.sciencemag.org/content/sci/270/5238/961.full.pdf

A First-Order Phase Transition Induced by a Magnetic Field

Field-induced insulator-to-metal transitions have been found in ${\mathrm{Pr}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}(0.3l~xl~0.5)$ single crystals, which are accompanied by a melting of the insulating charge-ordered (i.e., ${\mathrm{Mn}}^{3+}$/${\mathrm{Mn}}^{4+}$ ordered) state. The transition is of the first order with a hysteresis and is even irreversible at low temperatures. The $x$-dependent metal-insulator phase diagrams in the $H\ensuremath{-}T$ plane indicate that a deviation of $x$ from 0.5 modifies the robustness of the charge-ordering state, which is argued in terms of the effect of discommensuration of the charge concentration on the charge-ordered state.

Magnetic-field-induced metal-insulator phenomena in Pr1-xCaxMnO3 with controlled charge-ordering instability

The perovskite-type manganese oxide, ${\mathrm{Pr}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ $(x\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.5)$, shows a first-order phase transition from a ferromagnetic (FM) metal to antiferromagnetic (AFM) nonmetal at 140 K, which accompanies the transition to the charge-ordered state (generalized Wigner lattice). The nonmetal-to-metal transition temperature can be decreased down to near 0 K with a magnetic field up to 70 kOe. The temperature-field phase diagram for the charge-ordered state is presented together with a discussion on the mechanism of this field-induced phase transition.

Collapse of a charge-ordered state under a magnetic field in pr1/2sr1/2mno3

We have observed an insulator-to-metal (I-M) transition triggered by the photocarrier injection into the charge-ordered (CO) state of a perovskite manganite crystal, ${\mathrm{Pr}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_{3}$. The photocurrent is a highly nonlinear function of applied electric field and of light intensity; both show a threshold behavior for the I-M transition. The dependence of the anomalous photocurrent on the excitation photon energy and the temperature excludes the laser heating as the cause of the effect, suggesting the photocarrier-mediated collapse of the CO state.

Y. Tomioka

Papers

Current switching of resistive states in magnetoresistive manganites

A First-Order Phase Transition Induced by a Magnetic Field

Magnetic-field-induced metal-insulator phenomena in Pr1-xCaxMnO3 with controlled charge-ordering instability

Collapse of a charge-ordered state under a magnetic field in pr1/2sr1/2mno3

Photoinduced Insulator-to-Metal Transition in a Perovskite Manganite