The heterostructures of the ferromagnet (Cr2Te3) and topological insulator (Bi2Te3) have been grown by molecular beam epitaxy. The topological Hall effect as evidence of the existence of magnetic skyrmions has been observed in the samples in which Cr2Te3 was grown on top of Bi2Te3. Detailed structural characterizations have unambiguously revealed the presence of intercalated Bi bilayer nanosheets right at the interface of those samples. The atomistic spin-dynamics simulations have further confirmed the existence of magnetic skyrmions in such systems. The heterostructures of ferromagnet and topological insulator that host magnetic skyrmions may provide an important building block for next generation of spintronics devices.

We derive the effective single-band Hamiltonian in the flat NiO2 planes for nickelate compounds Nd1−xSrxNiO2. We first implement the first-principles calculation to study electronic structures of nickelates using the Heyd-Scuseria-Ernzerhof hybrid density functional and derive a three-band Hubbard model for Ni-O pdσ bands of Ni + 3d x 2 −y 2 and O 2− 2p x/y orbitals in the NiO2 planes. To obtain the effective one-band t-t -J model Hamiltonian, we perform the exact diagonalization of the three-band Hubbard model for the Ni5O16 cluster and map the low-energy spectra onto the effective one-band models. We find that the undoped NiO2 plane is a Hubbard Mott insulator, and the doped holes primarily locate on Ni sites. The physics of the NiO2 plane is a doped Mott insulator, described by the one-band t-t -J model with t = 265 meV, t = −21 meV and J = 28.6 meV. We also discuss the electronic structure for the "self-doping" effect and heavy fermion behavior of electron pockets of Nd 3+ 5d character in Nd1−xSrxNiO2. arXiv:1909.07427v2 [cond-mat.supr-con]

Magnetization reversal in magnetic soft/hard bilayer systems is studied analytically by means of a variational method for magnetic energies in a continuum model. The demagnetization curve is involved with nonlinear equations, and the solution is given implicitly in the form of Jacobi functions, which is valid for the total reversal process. Based on the non-trivial solutions, hysteresis loops, as well as the maximum energy product (BH)max versus thicknesses of soft/hard layers are obtained. With regard to (BH)max, improvement of the remanence competes with loss of coercive force. As a result, an optimum condition exists. For a given thickness of the hard layer, the optimum condition at which the largest (BH)max could be achieved is discussed, which is slightly different from previous works.