We have combined neutron scattering and piezoresponse force microscopy to study the relation between the exchange bias observed in CoFeB/BiFeO3 heterostructures and the multiferroic domain structure of the BiFeO3 films. We show that the exchange field scales with the inverse of the ferroelectric and antiferromagnetic domain size, as expected from Malozemoff's model of exchange bias extended to multiferroics. Accordingly, polarized neutron reflectometry reveals the presence of uncompensated spins in the BiFeO3 film at the interface with the CoFeB. In view of these results we discuss possible strategies to switch the magnetization of a ferromagnet by an electric field using BiFeO3.PACS numbers: 75.50. Ee, 75.70.Cn, 75.70.Kw The renaissance of multiferroics [1, 2], i.e. materials in which at least two ferroic or antiferroic orders coexist, is motivated by fundamental aspects as well as their possible application in spintronics [3]. Such compounds are rare and the very few that possess simultaneously a finite magnetization and polarization usually order below about 100K [4,5,6]. Ferroelectric antiferromagnets (FEAF) are less scarce, and some exhibit a coupling between their two order parameters. This magnetoelectric (ME) coupling allows the reversal of the ferroelectric (FE) polarization by a magnetic field [7] or the control of the magnetic order parameter by an electric field [8].The practical interest of conventional antiferromagnets (AF) is mainly for exchange bias in spin-valve structures. The phenomenon of exchange bias (EB) [9] manifests itself by a shift in the hysteresis loop of a ferromagnet (FM) in contact with an AF and arises from the exchange coupling at the FM/AF interface [10,11]. Combining this effect with the ME coupling in a FEAF/FM bilayer can allow the reversal of the FM magnetization via the application of an electric field through the FEAF, as reported recently at 2K in YMnO 3 /NiFe structures [12].To exploit these functionalities in devices one needs to resort to FEAF materials with high transition temperatures. BiFeO 3 (BFO) is a FE perovskite with a Curie temperature of 1043K [13] that orders antiferromagnetically below T N =643K (T N : Néel temperature) [14]. BFO thin films have a very low magnetization (∼0.01 µ B /Fe) compatible with an AF order [15,16], and remarkable FE properties with polarization values up to 100 µC.cm −2 range [17]. Recently, we reported that BFO films can be used to induce an EB on adjacent CoFeB layers at room temperature [18]. This observation together with the demonstration of a coupling between the AF and FE domains [8] paves the way towards the room-temperature electrical control of magnetization with BFO. However, several questions remain before this can be achieved. Key issues concern the precise magnetic structure of BFO thin films, and the mechanisms of EB in BFO-based heterostructures.In this Letter, we report on the determination of the magnetic structure of BFO films by means of neutron diffraction (ND), and the analysis of the EB effect in CoFeB/BFO ...

High‐quality monodisperse metallic cobalt nanorods are obtained by the reduction of carboxylate salts of CoII in 1,2‐butanediol using a rapid, simple, and solid‐template‐free procedure. In this polyol process, particle shape can be controlled via the growth rate, which depends on three parameters: i) the nature of the cobalt carboxylate, ii) the temperature ramp, and iii) the basicity of the medium. Cobalt in the hexagonally close‐packed phase favored the growth of anisotropic particles. Magnetic measurements of the cobalt nanorods indicate they are ferromagnetic at room temperature. They have a very high coercivity of 9.0 kOe at 140 K, much higher than that observed for wires prepared with solid templates. This can be attributed to their small mean diameter and high crystallinity.