Exchange bias between magnetoelectric YMnO3 and ferromagnetic SrRuO3 epitaxial films

Abstract

Orthorhombic YMnO 3 ͑YMO͒ epitaxial thin films were deposited on SrTiO 3 ͑STO͒ single-crystal substrates. We show that the out-of-plane texture of the YMO films can be tailored using STO substrates having ͑001͒, ͑110͒, or ͑111͒ orientations. We report on the magnetic properties of the YMO͑010͒ films grown on STO͑001͒ substrates. The dependence of the susceptibility on the temperature indicates that the films are antiferromagnetic below the Néel temperature ͑around 35 K͒. Orthorhombic YMO͑010͒ films were also deposited on an epitaxial buffer layer of ferromagnetic and metallic SrRuO 3 ͑SRO͒. 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: 10.1063/1.2167333͔ INTRODUCTIONMultifunctional 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. 3 The orthorhombic phase of YMO is not ferroelectric, but substantial changes of the dielectric constant close to the AF order temperature have been reported. 4 The orthorhombic phase of YMO is metastable in bulk, and it forms only when high pressures are applied 4 or by soft chemistry techniques. 5,6 YMO 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. 7 Specific 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. This phase can be also stabilized in YMO thin films under compressive epitaxial stress. [8][9][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....

Dielectric anomaly and magnetic response of epitaxial orthorhombic YMnO3 thin films

“…Thus, within the experimental resolution, the film is purely orthorhombic, and it is (00l) textured; we used the Pbnm setting (a ‫ס‬ 5.26 Å, b ‫ס‬ 5.85 Å, c ‫ס‬ 7.36 Å in bulk YMO). This is in agreement with the results of previous studies [7][8][9] that have reported the stabilization of this phase in films epitaxially grown on STO substrates. The out-of-plane lattice parameter of the film has been determined from the position of the YMO(004) peak, after a calibration by using the STO(002) reflection.…”

“…Although under normal synthesis conditions, the hexagonal phase is the stable one, the metastable orthorhombic o-YMnO 3 phase can be obtained using either high pressures (bulk materials) 6 or appropriate epitaxial strain engineering (thin films). [7][8][9] It has been reported that polycrystalline o-YMnO 3 displays a pronounced dielectric constant (⑀) anomaly at T⑀ < T N , 10 which has been taken as a signature of magnetoelectric coupling. The magnetic structure of o-YMnO 3 has been studied in great detail.…”

Emerging Multiferroic Memories

“…This report found that, indeed, the multiferroic layer could be used as an antiferromagnetic pinning layer that gives rise to exchange bias and enhanced coercivity, but suggested that YMnO 3 would likely be an inappropriate choice for continued study as these values varied greatly with crystal orientation and rendered actual device generation unlikely. Soon after this initial result, Marti et al [258] reported the observation of exchange bias in all-oxide heterostructure of the ferromagnet SRO and the antiferromagnetic, multiferroic YMnO 3 . In both of these studies, the exchange bias existed only at very low temperatures due to the low magnetic ordering temperature of the YMnO 3 .…”

Magnetic ordering anisotropy in epitaxial orthorhombic multiferroic YMnO3 films

“…13 The discrepancies were ascribed to the unavailability of single crystalline samples and additional ways of preparing orientation-specific orthorhombic RMnO 3 films have received extensive attention recently. [18][19][20][21][22] Among them, Y +3 ͑1.06 Å͒ has the critical ionic size right at the boundary of structure variations. As a result, it is expected that YMnO 3 can form either the orthorhombic ͑o-YMO͒ or the hexagonal ͑h-YMO͒ phase with just a slight variation in preparation conditions.…”