NEWS & VIEWS nature materials | VOL 7 | JUNE 2008 | www.nature.com/naturematerials 425 Manuel Bibes and Agnès Barthélémy are at the Unité Mixte de Physique CNRS/Thales, Route départementale 128, 91767 Palaiseau, France. e-mail: manuel.bibes@thalesgroup.com M ultiferroics represent an appealing class of multifunctional materials that simultaneously exhibit several ferroic orders such as ferroelectricity and antiferromagnetism 1 . he coexistence of several order parameters brings about novel physical phenomena and ofers possibilities for new device functions. Of particular interest is the existence of a cross-coupling between the magnetic and electric orders, termed magnetoelectric coupling. his coupling enables the control of the ferroelectric polarization by a magnetic ield and, conversely, the manipulation of magnetization by an electric ield. Although the former efect has been demonstrated in several materials such as TbMnO 3 and TbMn 2 O 5 (refs 2,3), the electrical control of magnetism in magnetoelectric multiferroics has rarely been reported, even though it is much more attractive for device applications. On page 478 of this issue, Ying-Hao Chu and colleagues address this central question, reporting the magnetoelectric manipulation of magnetization at room temperature 4 . With multiferroics, the coexistence of several order parameters and the magnetoelectric coupling can both be exploited in novel types of memory elements. As ferroelectric polarization and magnetization are used to encode binary information in FeRAMs (ferroelectric random access memories) and MRAMs (magnetic random access memories), respectively, the coexistence of magnetization and polarization in a multiferroic material allow the realization of four-state logic in a single device 5 . More complex schemes have even been proposed in order to store up to eight logic states 6 . Beyond the combination of ferroic properties in a single device, the electrical control of magnetization via the magnetoelectric coupling ofers the opportunity of combining the respective advantages of FeRAMs and MRAMs in the form of non-volatile magnetic storage bits that are switched by an electrical ield. Indeed, although the characteristics of MRAMs equal or surpass those of alternative non-volatile memory technologies in terms of access time and endurance, they have a large handicap in their high writing energy. A possible solution for reducing the writing energy uses a spin-polarized current to reverse the magnetization of the storage layer by spin-transfer 7 rather than magnetic ields. Spin-transfer MRAMs are currently being developed by several companies and a 2 Mb memory was recently demonstrated 8 . An alternative solution that could drastically reduce the writing energy of MRAMs is the use of a write scheme based on the application of a voltage rather than large currents. he magnetoelectric coupling in multiferroics provides such an opportunity. he basic operation of such magnetoelectric random access memories (MERAMs) combines the magnetoelectric coupling with the interfacial exchange coupling between a multiferroic and a ferromagnet to switch the magnetization of the ferromagnetic layer by using a voltage (Fig. 1). In MERAMs, the magnetoelectric coupling enables an electric ield to control the exchange coupling at the interface of the multiferroic with the ferromagnet. he exchange coupling across the interface then controls the magnetization of the ferromagnetic layer, so that ultimately this magnetization can be switched by the electric polarization of the multiferroic. Driven by The room-temperature manipulation of magnetization by an electric field using the multiferroic BiFeO 3 represents an essential step towards the magnetoelectric control of spintronics devices. MULTIFERROICS Towards a magnetoelectric memory Electrode P Electrode P Voltage Resistance V– R p R ap FE – AFM FE – AFM V+ Figure 1 Sketch of a possible MERAM element. The binary information is stored by the magnetization direction of the bottom ferromagnetic layer (blue), read by the resistance of the magnetic trilayer (R p when the magnetizations of the two ferromagnetic layers are parallel), and written by applying a voltage across the multiferroic ferroelectric– antiferromagnetic layer (FE-AFM; green). If the magnetization of the bottom ferromagnetic layer is coupled to the spins in the multiferroic (small white arrows) and if the magnetoelectric coupling is strong enough, reversing the ferroelectric polarization P in the multiferroic changes the magnetic configuration in the trilayer from parallel to antiparallel, and the resistance from R p to antiparallel (R ap ). A hysteretic dependence of the device resistance with voltage is achieved (blue curve). © 2008 Nature Publishing Group