Spin noise fluctuations from paramagnetic molecular adsorbates on surfaces

The measurement of spin noise in nuclei was demonstrated on bulk samples more than two decades ago. An ensemble of spins can produce a coherent signal at the Larmor frequency of a static magnetic field, known as spin noise, an effect due to the statistical polarization of small ensembles. The difficulty of these measurements is that the signal is extremely small—even if electron spins are detected. Although the statistical polarization of $N$ spins dominates the Boltzmann statistics if $N$ approaches unity, a more sensitive tool is requested to measure the polarization of the magnetic moment of a single spin. In this paper we report on the verification of recent results on the detection of spin noise from paramagnetic molecules of $\alpha ,\gamma$ -bisdiphenylene-$\beta$ -phenylallyl (BDPA) by Durkan and Welland [Appl. Phys. Lett. 80, 458 (2002)]. We also present results on a second paramagnetic specie 1,1-diphenyl-2-picrylhydrazyl (DPPH), deposited on Au(111) surfaces. Electron spin resonance spectra from ultrathin films of DPPH and BDPA grown on Au(111) are reported. We prove that the paramagnetic molecules preserve their magnetism on the surface. These data and a thorough analysis of the signal recovery apparatus help us to understand the low statistical recurrence of the spin noise in the data set. A detailed description of the experimental apparatus together with an analysis of the parameters that determine the sensitivity are also presented.