Exchange bias between magnetoelectric YMnO 3 and ferromagnetic SrRuO 3 epitaxial films

Orthorhombic YMnO3 YMO epitaxial thin films were deposited on SrTiO3 STO single-crystal substrates. We show that the out-of-plane texture of the YMO films can be tailored using STO substrates having 001 , 110 , or 111 orientations. We report on the magnetic properties of the YMO 010 films grown on STO 001 substrates. The dependence of the susceptibility on the temperature indicates that the films are antiferromagnetic below the Néel temperature around 35 K . Orthorhombic YMO 010 films were also deposited on an epitaxial buffer layer of ferromagnetic and metallic SrRuO3 SRO . The magnetic hysteresis loops of SRO show exchange bias at temperatures below the Néel temperature of YMO. These results confirm that the YMO films are antiferromagnetic and demonstrate that magnetoelectric YMO can be integrated in functional epitaxial architectures. © 2006 American Institute of Physics. DOI: 10.1063/1.2167333


INTRODUCTION
Multifunctional materials research is of fundamental importance for the next generation of devices of relevance in several areas of science and technology, including magnetoelectronics.Biferroic materials show the coexistence of ferroelectricity and magnetic order and are currently receiving much attention ͑see two recent reviews in Refs. 1 and 2͒.One of the most studied materials is the hexagonal phase of YMnO 3 ͑YMO͒, which is ferroelectric and antiferromagnetic ͑AF͒. 3The orthorhombic phase of YMO is not ferroelectric, but substantial changes of the dielectric constant close to the AF order temperature have been reported. 4he orthorhombic phase of YMO is metastable in bulk, and it forms only when high pressures are applied 4 or by soft chemistry techniques. 5,6YMO belongs to the family of RMnO 3 compounds, where R is a trivalent cation.These compounds can present either hexagonal or orthorhombic structure, depending on the size of the R 3+ cation. 7Specific volume is lower in the perovskite ͑orthorhombic͒ phase than in the hexagonal, and the orthorhombic phase can be stabilized in bulk samples under high pressures.][10] Whereas the phase stabilization is documented in these references, the physical properties of orthorhombic YMO films are not reported.
We report here on the growth and magnetic characterization of AF perosvkite YMO thin films.Epitaxial films were deposited on SrTiO 3 ͑STO͒ substrates having different orientations; we will show that this strategy allows selection of the YMO crystal out-of-plane orientation.This is of fundamental importance towards device design since it makes possible having films with precisely controlled magnetic, ferroelectric, or magnetoelectric axis orientation.We present magnetic measurements of the films grown on STO͑001͒, confirming their AF character below the Néel temperature, T N ϳ 35 K.Moreover, we report also on the growth and magnetic characterization of bilayers with YMO and SrRuO 3 ͑SRO͒, a ferromagnetic and metallic oxide below ϳ150 K.

EXPERIMENT
YMO thin films, about 150 nm in thickness, were grown on ͑001͒-, ͑110͒-, and ͑111͒-oriented STO substrates by pulsed laser deposition using a KrF excimer laser ͑248 nm wavelength and 34 ns pulse duration͒ with a repetition rate of 10 Hz.The laser beam was focused to a fluence around 2.5 J / cm 2 , on a stoichiometric target of YMO.The target was rotated during the ablation process to reduce nonuniform erosion.The substrate was placed parallel to the target at a distance of 5 cm.The films were deposited at substrate temperatures of 800 °C, under 0.15 mbar of oxygen.When the laser was switched off, the substrate was cooled down and 1 atm of oxygen was introduced at 500 °C.A YMO/SRO bilayer was fabricated on a STO͑001͒ substrate in a single-step process, SRO being the bottom layer.The crystal structure and lattice parameters were investigated by x-ray diffraction ͑XRD͒ using Cu K ␣ radiation.The magnetic properties of YMO films on STO͑001͒ and YMO/SRO bilayers on STO͑001͒ were measured by using a superconducting quantum interference device ͑SQUID͒ system, from 5 to 300 K and under field up to 5 T.

RESULTS AND DISCUSSION
Figure 1 shows the XRD /2 scans of YMO films on ͑a͒ STO͑111͒, ͑b͒ STO͑110͒, and ͑c͒ STO͑001͒ substrates.The diffraction peaks correspond to reflections from the substrates or the orthorhombic phase of YMO.There are no reflections corresponding to the hexagonal phase.Each film has a single out-of-plane orientation, which depends on the a͒ Electronic mail: fsanchez@icmab.esused substrate.Using the Pnma indexation ͑a = 5.84 Å, b = 7.36 Å, and c = 5.26 Å͒ of orthorhombic bulk YMO, the orientation relationships are YMO͑011͒ / STO͑111͒, YMO͑001͒ / STO͑110͒, and YMO͑010͒ / STO͑001͒.The full width at half maximum of the rocking curves ͑ scans͒ of the out-of-plane reflections was around 0.25°in all cases.scans ͑not shown͒ indicated that YMO films are epitaxial.From the position of the diffraction peaks in the /2 scans, the out-of-plane lattice parameters of the three films were determined.The out-of-plane parameters are 4.31, 5.27, and 7.45 Å, increasing with respect to the bulk phase by 0.5%, 0.1%, and 1.4% for films on STO͑111͒, STO͑110͒, and STO͑001͒, respectively.It is to be noted that the increase in the out-of plane parameter is consistent with an in-plane compressive stress.The stress is due to the epitaxial growth, which caused the stabilization of the orthorhombic phase.
We turn now to the magnetic properties of the YMO͑010͒ / STO͑001͒ samples.According to neutrondiffraction data 6 in the AF state of the orthorhombic YMO, the magnetic moments of Mn 3+ ions lie in the a-c crystal plane.Therefore, in our YMO͑010͒ / STO͑001͒ sample, with the b axis perpendicular to the film surface ͑as shown Fig. 1͒, the atomic spins are located in the film plane.The magnetic susceptibility was determined as a function of the temperature and with the magnetic field applied parallel to the plane of the sample ͑Fig.2͒.As expected, ͑T͒ rises when lowering the temperature; a peak in ͑T͒ signals the onset of AF ordering at the Néel temperature T N ϳ 35 K.We note that this value of T N is close to that reported ͑T N ϳ 42 K͒ for bulk samples. 4,6We have also analyzed the susceptibility data in the paramagnetic region.In Fig. 2 ͑inset͒, where we show the reciprocal of the susceptibility plotted as a function of the temperature, it can be appreciated that the susceptibility displays a Curie-Weiss behavior.From these data, we extract an extrapolated Curie of about −25 K and an effective paramagnetic moment eff Ϸ 5.14 B /f.u.We note that eff = 4.90 B is expected for a Mn 3+ ion, which is very close to the estimated value; the difference is likely due to some unavoidable inaccuracy on the evaluation of film thickness and substrate subtraction of the magnetic data.
We have also prepared bilayers composed by the AF YMO and the ferromagnetic SRO.SRO has a structure of distorted perovskite, with a pseudocubic lattice parameter very close to that of STO ͑lattice mismatch is ϳ0.6%͒ and it can be grown epitaxially on STO. 11XRD characterization ͑not shown here͒ indicates that YMO films grown on SRO/ STO͑001͒ are orthorhombic and ͑010͒ oriented, as they are when grown directly on the STO͑001͒ substrate.We measured the hysteresis loops with the magnetic field applied parallel to the plane of the sample.Spins in the AF layer lay in this plane, and an exchange bias field 12 ͑H EB ͒ in the loops could be observed below the Néel temperature of YMO.In Fig. 3͑a͒ ͑inset͒ we show a hysteresis loop measured at 10 K, after a field-cooling process from a temperature well above T N .Similar measurements were done at several temperatures; a zoom of the low-field region of the corresponding magnetization loops is included in Fig. 3͑b͒.It is clear from this figure the presence of an exchange bias that increases when reducing the temperature.In Fig. 3͑a͒ ͑main panel͒ we display the temperature dependence of the exchange bias field, H EB = ͑H c1 + H c2 ͒ / 2, where H c1 and H c2 are the crossing fields in both magnetization branches.Data in this figure indicate that the H EB vanishes at some temperature in the 30-50 K range and thus, in agreement with the measurements ͑Fig.2͒ performed on single YMO films, we infer that the antiferromagnetic transition occurs between 30 and 50 K.

SUMMARY
In summary, films of orthorhombic YMO oxide were epitaxially grown with an out-of-plane orientation that can be tuned by an appropriate choice of the substrate.The magnetic properties of YMO͑010͒ films were studied.The Néel temperature is determined to be around 35 K. YMO͑010͒ films were also grown on ferromagnetic SRO layers, and a well-defined exchange bias field is observed below the Néel temperature of YMO.The selection of texture of YMO films and the demonstration of the occurrence of an exchange bias between a ferromagnetic oxide and the magnetoelectric YMO could be also useful towards future applications.

FIG. 1 .FIG. 2 .
FIG.1.XRD /2 scans ͑intensity, in a.u., is plotted in a logarithmic scale͒ of YMO films on STO substrates: ͑a͒ ͑111͒, ͑b͒ ͑110͒, and ͑c͒ ͑001͒ oriented.Scans are vertically shifted for clarity.Pnma indexation is used for YMO peaks.The scans reveal that YMO films are orthorhombic, without traces of other phases, and with out-of-plane orientation that depends on the substrate orientation.