The creation of an atomically thin ferromagnetic and conducting electron system has been a long-standing goal in science. If realized, it will combine the advantages of two-dimensional electron systems with those of magnetic materials, i.e., state control by electric and magnetic fields. Atomically thin transition metal films can remain ferromagnetic, but these electron systems are only stable in vacuum, limiting their impact. Transition metal oxide heterostructures circumvent this issue. Most magnetic and conducting transition metal oxide materials, however, lose their functional properties well before the single-unit-cell layer thickness is reached; typically a non-conducting and non-magnetic dead layer is present. SrRuO3 is one of the oxide materials with the highest conductivity and it is chemically inert. In addition, it is an itinerant ferromagnet with a saturation moment of 1.6 μB/Ru and a Curie temperature TC of 160 K. As SrRuO3 has low intrinsic disorder and its epitaxial growth is well understood, it is a good candidate for realizing a two-dimensional spin-polarized electron system.
Several studies have investigated the behavior of ultrathin SrRuO3 films and SrRuO3 superlattices. In most studies, however, an insulating state is observed when the SrRuO3 thickness is less than three unit cells. Moreover, this insulating state has been proposed to be antiferromagnetic. Several theoretical studies agree with the antiferromagnetic and insulating ground state in ultrathin SrRuO. Nonetheless, one theoretical study concludes that ferromagnetism remains down to two-unit-cell-thick layers. Indeed, it has been proposed that a one-unit-cell-thick SrRuO3 layer, i.e., a single RuO2 plane, remains metallic and is fully minority spin-polarized if embedded in a SrTiO3 lattice . To test whether one-unit-cell-thick SrRuO3 is indeed magnetic and conducting if embedded with SrTiO3 in a heterostructure, we fabricated high-quality (SrRuO3)1–(SrTiO3)5 superlattices as illustrated in Fig. 1 . These layers exhibit conductivity and ferromagnetism with a TC of 100 K , in support of the proposal that a ferromagnetic groundstate can be stabilized for this atomically thin electron system.
The (SrRuO3)1–(SrTiO3)5 superlattices were grown at Cornell University by reactive molecular-beam epitaxy (MBE) on (001) SrTiO3 substrates . In independent deposition runs, we fabricated two samples A and B, both of which have twenty repetitions of the building blocks. The sample structure is depicted in Fig. 2, along with scanning transmission electron microscopy STEM images of sample A, showing excellent ordering in the superlattice with no measurable interdiffusion between the SrRuO3 and SrTiO3 layers and no significant population of two-unit-cell-thick SrRuO3 islands.
The temperature dependence of the resistivity ρ of the two samples is shown in Fig. 3 together with literature data. In the temperature range between 2 and 300 K the samples have a resistivity of ≈1000 μΩcm. This is higher than either the bulk or the thick-film resistivity, but significantly lower than the resistivities of the two-unit-cell-thick films and the (SrRuO3)1,2–(ABO3)n superlattices of the previous studies. The decreased resistivity is due to the superlattice structure and to the high structural quality of our samples. The samples show a minimum of the resistivity at 120 K (sample A) and 80 K (sample B). Below these temperatures, dρ/dT is negative, possibly owing to localization of the charge carriers.
We now turn to the magnetic properties of the superlattices. The magnetocrystalline anisotropy of SrRuO3 favors a predominantly out-of-plane magnetic moment for thin films. Therefore magnetic domain formation is expected to occur, resulting in a reduction of the global possible magnetic moment compared to that of a monodomain sample. As the anisotropy field is very large, it is difficult to saturate the moment, which makes conventional magnetization measurements challenging. We therefore used scanning superconducting-quantum-interference-device (SQUID) microscopy, which clearly proved the samples to be ferromagnetic with a TC of ≈100 K .
In conclusion, we have shown atomically thin SrRuO3 to be ferromagnetic and conducting if embedded in SrTiO3. The Curie temperature is close to the phase-transition temperature of SrTiO3, supporting the prediction that the ferromagnetic state is stabilized by the SrTiO3 lattice. In (SrRuO3)1–(SrTiO3)5 superlattices the electron system comprises only single RuO2 planes. These superlattices are a rare example of two-dimensional ferromagnetism and may therefore serve as a model system for further studies.