Room‐Temperature Insulating Ferromagnetic (Ni,Co)_1+2xTi_1−xO₃ Thin Films

Abstract

Insulating uniaxial room-temperature ferromagnets are a prerequisite for commonplace spin wave-based devices, the obstacle in contemporary ferromagnets being the coupling of ferromagnetism with large conductivity. It is shown that the uniaxial A 1 + 2x Ti 4+ 1 − x O 3 (ATO), A = Ni 2+ ,Co 2+ , and 0.6 < x ≤ 1, thin films are electrically insulating ferromagnets already at room temperature. The octahedra network of the ATO and the corundum and ilmenite structures are the same yet different octahedra-filling proved to be a route to switch from the antiferromagnetic to ferromagnetic regime. Octahedra can continuously be filled up to x = 1, or vacated (−0.24 < x < 0) in the ATO structure. TiO-layers, which separate the ferromagnetic (Ni,Co)O-layers and intermediate the antiferromagnetic coupling between the ferromagnetic layers in the NiTiO 3 and CoTiO 3 ilmenites, can continuously be replaced by (Ni,Co)O-layers to convert the ATO-films to ferromagnetic insulator with abundant direct cation interactions.

Permanently Magnetized Insulating Thin‐Film Devices by Reduction

“…We note that no pure antiferromagnetic phase was observed in the NCT solid solution, though a ferrimagnetic ordering was observed in the (Ni,Co) 1þ2x Ti 1-x O 3 samples with x % 0, see sample J and Figure 4 in ref. [11]. The ordering was revealed by a cusp at 78 K. The susceptibility peak on top of the antiferromagnetic response at around 30 K (see Figure 3f ) is similar to the anomalies reported in ref.…”

“…Sample 1, Stages A and B: (Ni 0.4 Co 0.6 ) 3 O 3 film was grown on C-plane-oriented (0001 plane) Al 2 O 3 substrate by pulsed laser deposition [11,13,18,19] by shooting 2500 pulses on the NiCoO 4 target. Substrate temperature was 783 K during the first growth, laser beam fluence of 3.0 J cm À2 , and the pulse repetition rate 1 Hz.…”

“…[15] NCT films are uniaxial; Ti-rich compounds are rhombohedral and (Ni,Co)-rich compounds are hexagonal. [10,11] Metallic compounds can be adjusted by Co and Ti alloying to stabilize to the hexagonal close-packing structure. Ti alloying affects the reduction rate: (Ni 0.4 Co 0.6 ) 3 O 3 films can be readily reduced at 300 C, whereas Ti-alloyed samples require higher processing temperatures.…”

“…Here we report a manufacturing route of a permanently and strongly magnetized building block with high-quality interfaces, based on uniaxial (Ni,Co) 1þ2x Ti 1Àx O 3 (NCT)-oriented thin films [10,11] on top of a (Ni,Co)-film obtained through a complete reduction of the NCT layer. We demonstrate the route for the end composition, x ¼ 1.…”

“…The characteristic features of the NCT compounds are the short and direct overlap interactions, discussed in ref. [11] and [13], between cation orbitals both in the hexagonal and in the basal plane directions. As shown, the overlap interaction facilitates the coupling between conducting and insulating layers.…”