W. Echtenkamp

Increasing the Néel temperature of magnetoelectric chromia for voltage-controlled spintronics

Boron doped chromia (Cr2O3) thin films with substitutional doping levels between zero and 3% are grown using pulsed laser deposition in borane background gases. Magnetometry reveals a tunable increase in the Neel temperature of the (0001) textured Cr2BxO3−x thin films at a rate of about 10% with 1% oxygen site substitution preserving a net boundary magnetization. Spin resolved inverse photoemission measured after magnetoelectric annealing in subsequently reversed electric fields evidences voltage-controlled reversal of boundary magnetization and thus magnetoelectricity of Cr2BxO3−x. Conservation of magnetoelectricity far above room temperature makes ultra-low power voltage-controlled spintronic devices feasible.

Spin polarization asymmetry at the surface of chromia

We demonstrate boundary spin polarization at the surface of a Cr2O3 single crystal using spin-polarized low-energy electron microscopy (SPLEEM), complementing prior spin polarized photoemission, spin polarized inverse photoemission, and x-ray magnetic circular dichroism photoemission electron microscopy measurements. This work shows that placing a Cr2O3 single crystal into a single domain state will result in net Cr2O3 spin polarization at the boundary, even in the presence of a gold overlayer. There are indications that the SPLEEM contrast for the two polarization states may be different, consistent with scanning tunneling microscopy spectroscopy results obtained from ultrathin films of Cr2O3.

Scaling of the Magnetoelectric Effect in Chromia Thin Films

Scaling behavior of chromia (0001)-oriented films is investigated for film thicknesses ranging from the nanoscale up to the mesoscopic regime. Unusually prominent finite-size scaling of the critical point shift is observed which is accompanied by finite-size effects of magnetic response functions. We investigate the scaling of the antiferromagnetic ordering temperature by reviewing data from the literature and complementing them with our findings. Particular attention is paid to the hitherto unexplored regime of mesoscopic film thicknesses where conventionally no finite-size effects are expected. Results of previously unreported direct measurements of the magnetic susceptibility of chromia thin films grown by molecular beam epitaxy are presented. The magnetometry data reveal qualitative changes in the temperature dependence and peak value of the susceptibility in comparison with the susceptibility of a bulk single crystal. These non-trivial finite-size effects may originate from oxygen defects which accompany reactive sputtering and evaporation growth processes. Significant implications for the scaling behavior of the magnetoelectric effect and consequences for potential spintronic applications are discussed.

Low temperature growth of cobalt on Cr2O3(0 0 0 1)

The thickness and temperature dependence of in situ grown cobalt thin films on Cr2O3(0u20090u20090u20091) single crystalline substrate has been studied by low energy electron microscopy (LEEM). The LEEM images indicate that growth of thin Co films (⩽5 monolayers) on chromia at 100u2009K tends to be continuous and flat with suppressed island growth compared to films grown on chromia at room temperature and above (to ~440u2009K). Low energy electron diffraction indicates that disorder builds and crystallinity of the cobalt thin film decreases with increased film thickness. Compared with cobalt thin films on Al2O3(0u20090u20090u20091) single crystalline substrate, cobalt thin films on Cr2O3(0u20090u20090u20091) show larger magnetic contrast in magnetic force microscopy indicating enhancement of perpendicular anisotropy induced by Cr2O3.

The surface stability of Cr2O3 (0 0 0 1)

The surface of chromia (Cr2O3) has a surface electronic structure distinct from the bulk and a packing density distinct from the bulk. More than a demarcation between the solid and the vacuum, the surface differs from the bulk of chromia, not just because of a partial occupancy of chromium sites, but also because of an increased number of unoccupied surface oxygen sites (vacancy sites), evident in angle-resolved core level photoemission. In spite of the structural differences that exist at the surface, there is, as yet, no evidence that these complications affect the surface Debye temperature beyond the most simple of assumptions regarding the lower coordination of the surface. Using low-energy electron diffraction (LEED), the effective surface Debye temperature (∼490xa0K) is found to be lower than the bulk (∼645xa0K) Debye temperature of Cr2O3(0u20090u20090u20091). This surface effective Debye temperature, indicative of vibrations along the surface normal, uncorrected for anharmonic effects, has a value reduced from the effective bulk Debye temperature yet close to the value √2 expected from a simple mean field argument.

Dielectric breakdown along c-axis boundaries in magnetoelectric O2O3 for spintronic devices

Voltage controlled boundary magnetism is crucial for spintronic devices with reduced power consumption. Magnetoelectric and antiferromagnetic Cr2O3 is an ideal material due to its electric field switching of nonvolatile boundary magnetism [1]. The boundary magnetization can switch an adjacent soft ferromagnetic layer via voltage-controlled exchange bias. Bulk Cr2O3 has a high dielectric breakdown field of 1000 KVmm and a large bandgap of 3.4 eV. However, even with a small electrode size of 0.04 mm and thick Cr2O3 film of 0.5 μm, the highest reported dielectric breakdown field of Cr2O3 films is only 200 KVmm [2]. The breakdown field drops rapidly to 8 KVmm if electrode size increases to 35 mm and film thickness decreases to 250 nm [2], making electric field induced switching of Cr2O3 based heterostructures very difficult. Here, we combine aberration corrected STEM characterization and spin polarized density functional theory (DFT) calculations to elucidate the structure, electronic properties, and magnetic properties of a new type of interface-stabilized planar crystallographic defect in Cr2O3 thin films that explains the structural origin of dielectric breakdown.