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We present the evidence of surface magnetism detection using highly charged ions as a probe, and without any external magnetic field application [1]. Based on x-ray spectroscopy, our investigation puts an end to a longstanding controversy from contradictory studies on ion–magnetic surface interaction based on Auger spectroscopy [2,3]. We measured the $n=2 \to 1$ transition of excited argon ions (hollow atoms) produced in grazing incidence collisions of an Ar$^{17+}$ ion beam (E$_\text{kin}$ = 170 keV) with a monocrystalline (110) nickel sample [4]. When increasing the sample temperature, the $n=2 \to 1$ x-ray unresolved transitions show an asymmetric broadening to lower energies, indicating a larger electron population of the $n=2$ level. This agrees with the expected behaviour of a reduction of the Pauli exclusion effect in the $n=2$ level (like a reduction of the Pauli shielding effect [5]) as a result of a spin alignment loss of the captured electrons caused by the change of magnetic phase (from a ferromagnetic phase at low temperature to a paramagnetic one at higher temperature). A similar behaviour is observed in $n=3 \to 2$ x-ray transition for lower charge incoming argon ions. Because of the selective electron capture of the highly charged ions already above the surface, these findings open new interesting perspectives for the magnetic order surface characterization of the very first atomic layer of samples and of 2D magnetic materials.
[1] P. Dergham, C. Prigent, C.V. Ahmad et al., in preparation for Phys. Rev. Lett.
[2] M. Unipan, A. Robin, R. Morgenstern and R. Hoekstra, Phys. Rev. Lett. 96 177601 (2006)
[3] M. Busch, S. Wethekam and H. Winter, Phys. Rev. A 78 010901 (2008)
[4] P. Dergham, F. Aumayr, E. Lamour et al., Atoms 10, 151 (2022)
[5] I. Madesis, A. Laoutaris, T.J.M. Zouros et al., Phys. Rev. Lett. 124 113401 (2020)
