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...

Epitaxial and fully strained SrRuO 3 thin films have been grown on SrTiO 3 (100). At initial stages the growth mode is three-dimensional-͑3D-͒like, leading to a finger-shaped structure aligned with the substrate steps and that eventually evolves into a 2D step-flow growth. We study the impact that the defect structure associated with this unique growth mode transition has on the electronic properties of the films. Detailed analysis of the transport properties of nanometric films reveals that microstructural disorder promotes a shortening of the carrier mean free path. Remarkably enough, at low temperatures, this results in a reinforcement of quantum corrections to the conductivity as predicted by recent models of disordered, strongly correlated electronic systems. This finding may provide a simple explanation for the commonly observed-in conducting oxidesresistivity minima at low temperature. Simultaneously, the ferromagnetic transition occurring at about 140 K, becomes broader as film thickness decreases down to nanometric range. The relevance of these results for the understanding of the electronic properties of disordered electronic systems and for the technological applications of SrRuO 3 -and other ferromagnetic and metallic oxides-is stressed.

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We show that in oxide thin films of spiral antiferromagnetic orthorhombic TbMnO 3 , ferromagnetism emerges resulting from epitaxially induced strain. The unit cell volume can be tuned ͑contracting up to a 2%͒ by varying thickness and deposition conditions; it is found that the ferromagnetic response correlates with the unit cell deformation. Such effect of strain on the magnetic properties turns out to be similar to that occurring in collinear orthorhombic antiferromagnets such as YMnO 3 . Owing to the intimate relationship between magnetic order and ferroelectricity in TbMnO 3 these results may provide a new route to induce magnetoelectric coupling and tailor their ferroelectric response. © 2010 American Institute of Physics. ͓doi:10.1063/1.3443714͔The coupling of ferroelectricity and ferromagnetism in a single material or heterostructure attracts much attention due to its promising applications in sensor and data storage technologies. The scarcity of materials which individually display this feature 1 triggered the development of varied strategies where, for instance, the coupling is mediated by elastic 2 or exchange-bias 3 effects. Particularly relevant is the discovery that ferroelectricity can be obtained subsequent to magnetic ordering. Antiferromagnetic orthorhombic rare earth manganites ͑RMnO 3 ͒ probably constitute the best known example of such materials. Either if the magnetic structure is collinear ͑E-type as in YMnO 3 ͑YMO͒ ͑Ref. 4͒ or ͑HoMnO 3 ͒ ͑Ref. 5͒ or noncollinear ͓spiral as in TbMnO 3 ͑TMO͔͒, 6 magnetic interactions give rise to atomic displacements destroying the center of symmetry and thus allowing an electrical polarization.7 Due to this intimate link between magnetic structure and ferroelectricity, changes in the electrical polarization upon applied magnetic fields are to be expected. Indeed, it was the early experimental observation of such effects in bulk TMO ͑Ref. 8͒ that triggered intensive theoretical research and exploration of epitaxial thin films. However, investigation of magnetic properties on RMnO 3 films ͑Refs. 9-17͒ revealed the unexpected existence of a ferromagnetic response in otherwise bulk antiferromagnetic materials. Whereas in the case of E-type spin-ordered structures, its origin has been attributed to strain-induced deformation of the unit cell and subsequent unbalancing of magnetic interactions, [9][10][11][12]17 it also has been proposed domain walls boundaries to be relevant in the case of the spiral TbMnO 3 . 16 However, it turns out that the signatures of ferromagnetism reported 9-17 are in all cases intriguingly similar. Whether the origin of the ferromagnetism in both compounds is the same irrespective of their bulk magnetic structure is a critical open issue that we aim to disentangle in this paper.We report here on the magnetic properties of epitaxial TMO thin films. Data shows that the ferromagnetic response can be selected at wish by tuning the unit cell volume ͑V C ͒ through the film thickness and deposition conditions. Of relevance here is that the obta...

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