Jia-Mian Hu

Recent Progress in Multiferroic Magnetoelectric Composites: from Bulk to Thin Films

Multiferroic magnetoelectric composite systems such as ferromagnetic-ferroelectric heterostructures have recently attracted an ever-increasing interest and provoked a great number of research activities, driven by profound physics from coupling between ferroelectric and magnetic orders, as well as potential applications in novel multifunctional devices, such as sensors, transducers, memories, and spintronics. In this Review, we try to summarize what remarkable progress in multiferroic magnetoelectric composite systems has been achieved in most recent few years, with emphasis on thin films; and to describe unsolved issues and new device applications which can be controlled both electrically and magnetically.

High-density magnetoresistive random access memory operating at ultralow voltage at room temperature

The main bottlenecks limiting the practical applications of current magnetoresistive random access memory (MRAM) technology are its low storage density and high writing energy consumption. Although a number of proposals have been reported for voltage-controlled memory device in recent years, none of them simultaneously satisfy the important device attributes: high storage capacity, low power consumption and room temperature operation. Here we present, using phase-field simulations, a simple and new pathway towards high-performance MRAMs that display significant improvements over existing MRAM technologies or proposed concepts. The proposed nanoscale MRAM device simultaneously exhibits ultrahigh storage capacity of up to 88 Gb inch−2, ultralow power dissipation as low as 0.16 fJ per bit and room temperature high-speed operation below 10 ns.

Multiferroic Heterostructures Integrating Ferroelectric and Magnetic Materials

Multiferroic heterostructures can be synthesized by integrating monolithic ferroelectric and magnetic materials, with interfacial coupling between electric polarization and magnetization, through the exchange of elastic, electric, and magnetic energy. Although the nature of the interfaces remains to be unraveled, such cross coupling can be utilized to manipulate the magnetization (or polarization) with an electric (or magnetic) field, known as a converse (or direct) magnetoelectric effect. It can be exploited to significantly improve the performance of or/and add new functionalities to many existing or emerging devices such as memory devices, tunable microwave devices, sensors, etc. The exciting technological potential, along with the rich physical phenomena at the interface, has sparked intensive research on multiferroic heterostructures for more than a decade. Here, we summarize the most recent progresses in the fundamental principles and potential applications of the interface-based magnetoelectric effect in multiferroic heterostructures, and present our perspectives on some key issues that require further study in order to realize their practical device applications.

Electric-field control of strain-mediated magnetoelectric random access memory

A strain-mediated magnetoelectric random access memory with electric-field-writing is presented, which consists of a magnetic tunnel junction (MTJ) in intimate contact with a ferroelectric (FE) layer. The calculations show that the magnetization vector in the free layer of the MTJ unit can switch in-plane by 90° upon applying an appropriate electric field to the FE layer, as compared to the common 180° reversal induced by magnetic field or spin-current. A perfect interface between the FE layer and the MTJ is assumed. The free layers used for illustration include either (001)-oriented or polycrystalline magnetic films of Fe–Co alloy, CoFe2O4 (CFO), Ni, and Fe3O4. Among them, the (001)-oriented FeCo and CFO films with positive magnetocrystalline anisotropy constant (i.e., K1>0) show an abrupt magnetization switching, while a gradual magnetization switching takes place in the (001)-oriented Ni and Fe3O4 films with K1<0 as well as the polycrystalline films. Such electric-field-induced in-plane magnetization s...

Design of a Voltage‐Controlled Magnetic Random Access Memory Based on Anisotropic Magnetoresistance in a Single Magnetic Layer

A simple and fully gate-voltage-controlled magnetic random access memory is designed based on anisotropic magnetoresistance. This multiferroic memory device consists of just a single magnetic film grown on a ferroelectric layer with bistable in-plane anisotropic ferroelastic or piezo strains induced by out-of-plane voltages. It can simultaneously achieve ultrahigh storage density, ultralow energy consumption, and GHz high-speed operation at room temperature.

Size-dependent electric voltage controlled magnetic anisotropy in multiferroic heterostructures: Interface-charge and strain comediated magnetoelectric coupling

(Received 21 November 2010; revised manuscript received 10 January 2011; published 8 April 2011)We present a phenomenological scheme to study the size-dependent electric voltage-controlled magneticanisotropy in ferromagnetic (FM) and ferroelectric (FE) heterostructures. The FM layers are either metallic[Fe(001), Ni(001), Co(0001)] or half-metallic [(La, Sr)MnO

Purely Electric-Field-Driven Perpendicular Magnetization Reversal

If achieved, magnetization reversal purely with an electric field has the potential to revolutionize the spintronic devices that currently utilize power-dissipating currents. However, all existing proposals involve the use of a magnetic field. Here we use phase-field simulations to study the piezoelectric and magnetoelectric responses in a three-dimensional multiferroic nanostructure consisting of a perpendicularly magnetized nanomagnet with an in-plane long axis and a juxtaposed ferroelectric nanoisland. For the first time, we demonstrate a full reversal of perpendicular magnetization via successive precession and damping, driven purely by a perpendicular electric-field pulse of certain pulse duration across the nanoferroelectric. We discuss the materials selection and size dependence of both nanoferroelctrics and nanomagnets for experimental verification. These results offer new inspiration to the design of spintronic devices that simultaneously possess high density, high thermal stability, and high reliability.

Magnetoelectric quasi-(0-3) nanocomposite heterostructures

Magnetoelectric composite thin films hold substantial promise for applications in novel multifunctional devices. However, there are presently shortcomings for both the extensively studied bilayer epitaxial (2-2) and vertically architectured nanocomposite (1-3) film systems, restricting their applications. Here we design a novel growth strategy to fabricate an architectured nanocomposite heterostructure with magnetic quasiparticles (0) embedded in a ferroelectric film matrix (3) by alternately growing (2-2) and (1-3) layers within the film. The new heteroepitaxial films not only overcome the clamping effect from substrate, but also significantly suppress the leakage current paths through the ferromagnetic phase. We demonstrate, by focusing on switching characteristics of the piezoresponse, that the heterostructure shows magnetic field dependence of piezoelectricity due to the improved coupling enabled by good connectivity amongst the piezoelectric and magnetostrictive phases. This new architectured magnetoelectric heterostructures may open a new avenue for applications of magnetoelectric films in micro-devices.

Full 180° Magnetization Reversal with Electric Fields

Achieving 180° magnetization reversal with an electric field rather than a current or magnetic field is a fundamental challenge and represents a technological breakthrough towards new memory cell designs. Here we propose a mesoscale morphological engineering approach to accomplishing full 180° magnetization reversals with electric fields by utilizing both the in-plane piezostrains and magnetic shape anisotropy of a multiferroic heterostructure. Using phase-field simulations, we examined a patterned single-domain nanomagnet with four-fold magnetic axis on a ferroelectric layer with electric-field-induced uniaxial strains. We demonstrated that the uniaxial piezostrains, if non-collinear to the magnetic easy axis of the nanomagnet at certain angles, induce two successive, deterministic 90° magnetization rotations, thereby leading to full 180° magnetization reversals.

Phase-field simulation of strain-induced domain switching in magnetic thin films

The strain-induced magnetic domain switching in epitaxial CoFe2O4 (CFO) thin films was studied using phase-field method. In particular, we investigated the domain switching from an initial in-plane direction to out-of-plane under the action of in-plane elastic strains. An abrupt switching feature is observed for a single-domain film while the switching of a multidomain CFO thin film is gradual. Typical magnetic domain structures as a result of the biaxial isotropic in-plane strains are presented.