A magnetic eigenvalue bound in the disk

We consider the magnetic Schr\"odinger operator in the unit disk with constant magnetic field of strength $b>0$ and magnetic Neumann boundary condition. If $\lambda_1(b)$ denotes its lowest eigenvalue, then we prove that $\lambda_1(b) < \Theta_0 b$ for all $b>0$, where $\Theta_0$ is the de Gennes constant. The proof has two parts, both based on Rayleigh's principle. For large $b$, we use a trial state built from the de Gennes ground state. For the remaining bounded range of $b$, we divide the interval into finitely many overlapping subintervals and, on each of them, choose a trial state from a finite-dimensional space. This reduces the proof to finitely many inequalities between rational numbers.