C. Ferrater

Electric-field control of exchange bias in multiferroic epitaxial heterostructures.

The magnetic exchange between epitaxial thin films of the multiferroic (antiferromagnetic and ferroelectric) hexagonal YMnO3 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 YMnO3 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.

Emergence of ferromagnetism in antiferromagnetic TbMnO3 by epitaxial strain

We show that in oxide thin films of spiral antiferromagnetic orthorhombic TbMnO3, 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 YMnO3. Owing to the intimate relationship between magnetic order and ferroelectricity in TbMnO3 these results may provide a new route to induce magnetoelectric coupling and tailor their ferroelectric response.

Strain-driven noncollinear magnetic ordering in orthorhombic epitaxial YMnO3 thin films

We show that using epitaxial strain and chemical pressure in orthorhombic YMnO3 and Co-substituted (YMn0.95Co0.05O3) thin films, a ferromagnetic response can be gradually introduced and tuned. These results, together with the measured anisotropy of the magnetic response, indicate that the unexpected observation of ferromagnetism in orthorhombic o-RMnO3 (R=Y,Ho,Tb, etc.) films originates from strain-driven breaking of the fully compensated magnetic ordering by pushing magnetic moments away from the antiferromagnetic [010] axis. We show that the resulting canting angle and the subsequent ferromagnetic response, gradually increase (up to ∼1.2°) by compression of the unit cell. We will discuss the relevance of these findings, in connection to the magnetoelectric response of orthorhombic manganites.

Tunable epitaxial growth of magnetoresistive La2/3Sr1/3MnO3 thin films

We report on the growth of epitaxial La2/3Sr1/3MnO3 thin films on buffered Si(001) substrates. We show that a suitable choice of the buffer heterostructure allows one to obtain epitaxial (00h), (0hh), and (hhh) manganite thin films. The magnetotransport properties are investigated and we have found that the low-field magnetoresistance is directly related to the width of the normal-to-plane rocking curves, irrespective of the film orientation. The magnetic anisotropy of these films has also been determined.We report on the growth of epitaxial La2/3Sr1/3MnO3 thin films on buffered Si(001) substrates. We show that a suitable choice of the buffer heterostructure allows one to obtain epitaxial (00h), (0hh), and (hhh) manganite thin films. The magnetotransport properties are investigated and we have found that the low-field magnetoresistance is directly related to the width of the normal-to-plane rocking curves, irrespective of the film orientation. The magnetic anisotropy of these films has also been determined.

Magnetic studies of Fe-Y compositionally modulated thin films

Among other interesting problems superlattices or multilayers are very suitable systems to study magnetic properties of ultra thin films, surface effects, periodicity effects, etc. Due to this special properties a wide range of metallic multilayers have been studied with different techniques1,2,3,4,5,6. In our case we are mainly concerned with the iron-silicon system. Early studies on the magnetic properties of amorphous Fe1−xSix thin films were performed by Y. Shimada et al.7 and by D. Bloch et al.8, in this studies the range of composition in which a ferromagnetic ordering exist is available together with some of the structural features, as a function of x, of the FexSi1−x alloys. J.M. Alameda et al.9 have pointed out that the behavior of the saturation magnetization as a function of x between the values 0.66 < x ≤ 0.78 is greatly influenced by the particular conditions of the sample preparation. Clear differences are evident in the magnetic behavior of sputtered samples and evaporated samples pointing out the influence of the short range order effects.

SrRUO3/SrTiO3/SrRUO3 heterostructures for magnetic tunnel junctions

We report on the growth and characterization of SrRuO3 single layers and SrRuO3/SrTiO3/SrRuO3 heterostructures grown on SrTiO3(100) substrates. The thickness dependence of the coercivity was determined for these single layers. Heterostructures with barrier thickness tb=1, 2.5, and 4 nm were fabricated, with electrodes having thickness ranging from 10 to 100 nm. The hysteresis loops of heterostructures with tb=2.5u2009nm, 4 nm reveal uncoupled magnetic switching of the electrodes. Therefore, these heterostructures can be used for the fabrication of magnetic tunneling junctions.

Magnetic properties of multilayer ferromagnetic thin films of Fe/La

Abstract In this paper we have used SQUID magnetometry to measure the approach to magnetic saturation and thermal demagnetizing processes in several compositionally modulated Fe/La thin films obtained by electron beam evaporation. Different regions can be identified in the dependence of the magnetization. The analysis of the magnetization temperature dependence has been done in the framework of collective excitations.

Magnetic phase diagram of Fe-Y multilayers

Abstract In this work we have studied the existence of different magnetic states occuring in the Fe/Y system depending on the Fe layer thickness while keeping constant the Y layer thickness. The compositionally modulated samples are made up of alternating layers of Fe and Y by means of electron beam evaporation. A complete magnetic characterization at low and high field have been carried out by using SQUID magnetometry. In the low field regime the susceptibility shows an irreversible thermal behavior below a temperature T f which depends on the intensity of the applied field. To determine the region of irreversibility the zero-field cooling and field cooling magnetization process of the samples were studied. The starting point of the onset of irreversibility depends on both the applied field and the Fe layer thickness. The magnetization curves of the samples show different regimes depending on the temperature of the sample and a correlated spin glass state seems to be present below T f . Taking into account the behaviour of the M ( T ) and M ( H ) curves a tentative magnetic phase diagram of the Fe/Y compositionally modulated system could be proposed.

Kerr measurements on single-domain SrRuO3 thin films

We report on the magneto-optical measurements of an epitaxial SrRuO3 film grown on SrTiO3 (0 0 1), which previously was determined to be single domain orientated by x-ray diffraction and Raman spectroscopy techniques. Our experiments reveal a large Kerr rotation, which reaches a maximum value of about 0.5° at low temperature. By measuring magnetic hysteresis loops at different temperatures, we determined the temperature dependence of the Kerr rotation in the polar configuration. Values of the anisotropic magnetoresistance ∼20% have been measured. These values are remarkably higher than those of other metallic oxides such as manganites. This striking difference can be attributed to the strong spin-orbit interaction of the Ru 4d ion in the SrRuO3 compound.

Self-interference of charge carriers in ferromagnetic SrRuO3

We report a systematic study of the low-temperature electrical conductivity in a series of SrRuO3 epitaxial thin films. At relatively high temperature the films display the conventional metallic behavior. However, a well-defined resistivity minimum appears at low temperature. This temperature dependence can be well described in a weak localization scenario: the resistivity minimum arising from the competition of electronic self-interference effects and the normal metallic character. By appropriate selection of the film growth conditions, we have been able to modify the mean-free path of itinerant carriers and thus to tune the relative strength of the quantum effects. We show that data can be quantitatively described by available theoretical models.