The magnetic exchange between epitaxial thin films of the multiferroic (antiferromagnetic and ferroelectric) hexagonal YMnO 3 oxide and a soft ferromagnetic (FM) layer is used to couple the magnetic response of the FM layer to the magnetic state of the antiferromagnetic one. We will show that biasing the ferroelectric YMnO 3 layer by an electric field allows control of the magnetic exchange bias and subsequently the magnetotransport properties of the FM layer. This finding may contribute to paving the way towards a new generation of electric-field controlled spintronic devices. DOI: 10.1103/PhysRevLett.97.227201 PACS numbers: 75.70.Cn, 85.80.Jm Multiferroic materials have been proposed for building a new generation of devices in spintronics, eventually allowing us to overcoming critical limitations in technology [1]. Much effort has been directed to searching for materials displaying the elusive coexistence of ferroelectricity (FE) and ferromagnetism (FM) [2,3], which is thought to be essential for progress in this direction. In contrast, materials displaying coupled FE and antiferromagnetic (AF) behavior have received much less attention. To exploit the multiferroic character of a material, it is essential that the ferroic properties (magnetic and electric, in the present context) are coupled. Hexagonal YMnO 3 (YMO), in bulk form, is ferroelectric up to 900 K and exhibits an antiferromagnetic character at low temperature (T N 90 K). It has been shown that in YMO single crystals, both order parameters are coupled [4], and this observation has triggered a renewed attention to this oxide [5,6]. The electric polarization axis of YMO is along the c axis; the Mn atomic spins lie in a perpendicular plane, forming a two dimensional, frustrated antiferromagnetic, triangular network [7,8]. Hence, in principle, one could use AF YMO to pin the magnetic state of a FM material and subsequently exploit its ferroelectric character and the coupling between FE and AF order parameters to tailor the properties of the FM layer. As a first step, it has been recently shown that indeed it is possible to exchange-bias NiFe (Permalloy-Py) with AF epitaxial (0001) YMO films which display a remanent electric polarization [5].Attempts towards electric-field control of exchange bias have been recently reported by Borisov et al. using magnetoelectric, but not multiferroic (AF) Cr 2 O 3 single crystals as pinning layers [9]. Here, we will show that it is possible to grow heterostructures that, exploiting the AF and FE character of YMO, allow us to control the magnetic state of a FM layer by an electric field. For that purpose, an epitaxial layer of YMO has been sandwiched between metallic electrodes (Pt and Py), and the exchange bias between YMO and Py has been monitored as a function of a biasing electric field applied across the YMO layer [ Fig. 1(b)].When a magnetic field is applied parallel to the interface between FM and AF materials, the magnetization of the FM layer does not follow (neglecting the anisotropy of the FM layer) the ex...

The search for novel functional materials is an extremely active field and to this purpose transition metal oxides have been under scrutiny for more than twenty years now. Their properties mainly arise from the interaction between the transition metal and oxygen ions and their sensitivity to bond length and angles.[1] Consequently, a large number of materials showing different physical effects can be found within one structural family. Perovskites are the most popular, as this crystal structure is shared by high superconducting temperature (T C ) superconductors, ferroelectrics, half-metallic ferromagnets, etc. This structural compatibility allows one to combine thin layers of very different materials to design multifunctional epitaxial heterostructures. Furthermore, the strong sensitivity of the physical properties to structural modifications often reveals unexpected behavior in strained thin films [2,3] or at the interface between two adjacent layers.[4]Besides perovskites, spinel oxides are also very attractive as they can be half-metallic (such as Fe 3 O 4 [5] ), ferrimagnetic insulators (like most spinel ferrites [6] ), transparent conductors (such as Cd 2 SnO 4[7] ), superconductors (LiTi 2 O 4[8]), or heavyfermion compounds (LiV 2 O 4 [9] ). Furthermore, spinel is a complex crystal structure, with many degrees of freedom available to engineer physical properties. Yet, their study in thin-film form has not been so intensive, [10] and much remains to be learnt concerning their properties in low dimensions and their potential in heteroepitaxial architectures.In this paper, we report on the growth and properties of ferrimagnetic NiFe 2 O 4 thin (3-12 nm) films onto perovskite SrTiO 3 (STO) substrates, layers of the ferromagnetic metallic oxide La 2/3 Sr 1/3 MnO 3 (LSMO), or LSMO/STO bilayers. These NiFe 2 O 4 nanometer-sized films have a considerably enhanced magnetic moment compared to the bulk, and their electronic properties can be tuned from conductive to insulating by changing the growth conditions. We have integrated these layers into epitaxial heterostructures to probe their potential for spintronics. We find that both types of NiFe 2 O 4 produce spin-polarized currents, but via two different mechanisms that might be combined to fabricate novel spintronics devices.NiFe 2 O 4 (NFO) films have been grown by target-facing-target radio frequency (RF) sputtering onto either STO(001) substrates or onto LSMO/STO or LSMO templates (see Experimental). The growth atmosphere consisted either of pure Ar or of a Ar/O 2 mixture. After growth, the films were cooled to room temperature in the growth atmosphere. Reflection high-energy electron diffraction (RHEED) patterns recorded before and after film deposition (Fig. 1a) indicate epitaxial growth of the spinel material onto the perovskite for both Ar and Ar/O 2 conditions. The pattern consists of organized spots for films grown in pure Ar and of streaks for Ar/O 2 . This indicates that the growth mode is modified from three-dimensional to two-dimensional when O 2 is i...

Bulk NiFe2O4 is an insulating ferrimagnet. Here, we report on the epitaxial growth of spinel NiFe2O4 ultrathin films onto SrTiO3 single-crystals. We will show that -under appropriate growth conditions -epitaxial stabilization leads to the formation of a spinel phase with magnetic and electrical properties that radically differ from those of the bulk material : an enhanced magnetic moment (MS) -about 250% larger -and a metallic character. A systematic study of the thickness dependence of MS allows to conclude that its enhanced value is due to an anomalous distribution of the Fe and Ni cations among the A and B sites of the spinel structure resulting from the offequilibrium growth conditions and to interface effects. The relevance of these findings for spinel-and, more generally, oxide-based heterostructures is discussed. We will argue that this novel material could be an alternative ferromagetic-metallic electrode in magnetic tunnel junctions.