Preparation of magnetic tips for spin-polarized scanning tunneling microscopy onFe1+yTe

Abstract

The interplay of electronic nematic modulations, magnetic order, superconductivity and structural distortions in strongly correlated electron materials calls for methods which allow characterizing them simultaneously -to allow establishing directly the relationship between these different phenomena. Spin-polarized STM enables studying both, electronic excitations as well as magnetic structure in the same measurement at the atomic scale. Here we demonstrate preparation of magnetic tips, both ferromagnetic and antiferromagnetic, on single crystals of FeTe. This opens up preparation of spin-polarized tips without the need for sophisticated ultra-high vacuum preparation.PACS numbers: 74.55.+v, 74.70.Xa In many unconventional superconductors, the superconducting phase is reached from a magnetically ordered state by some external tuning parameter, such as doping, pressure or chemical substitution. Superconductivity emerges in close vicinity to a magnetically ordered phase [1]. This suggests an intimate relation between magnetism and superconductivity in these materials. Often, the phase diagrams exhibit even regimes of coexistence between the two, however the important question about whether the two coexist or compete at the microscopic level remains unresolved. One difficulty in probing their relation at the atomic scale is that most methods employed to characterize magnetic order, such as neutron scattering, probe a macroscopic sample volume, rendering statements about local phase separation difficult. A method which has been very successful to characterize both superconductivity and magnetism locally on an atomic scale is Scanning Tunneling Microscopy (STM). It has provided important information both about local variations in the superconducting properties and charge ordering in strongly correlated electron materials [2][3][4] and, using magnetic tips in spin-polarized STM, it has also been shown to allow for characterization of magnetism at the atomic scale in nanostructures [5,6]. Application of spin-polarized STM to strongly correlated materials has recently been demonstrated in the nonsuperconducting parent compound of the iron chalcogenide superconductors [7], providing real space images of the magnetic structure Fe 1+δ Te. Preparing and calibrating a magnetic tip for spin-polarized STM measurements has been an important obstacle towards its application to strongly correlated electron materials.In this work, we demonstrate preparation of spinpolarized tips and the characterization of their magnetic properties on Fe 1+y Te. Presence of small amounts of excess iron proves instrumental in the preparation of spin-polarized tips on this material. Specifically we show preparation of both ferromagnetic and antiferromagnetic clusters at the apex of the tip and the characterization of the magnetization of the tip-cluster as a function of field.

Charge ordering in stoichiometric FeTe: Scanning tunneling microscopy and spectroscopy

“…We have verified the absence of any indications of a magnetic STM tip, as reported in Ref. 25, in our measurements (see Supplementary Note 1 [31]). …”

“…It is noteworthy that excess Fe atoms could be picked up by the STM tip on a Fe 1+x Te surface, yielding a magnetic tip good for visualizing the ordering pattern of spins instead of charges [25]. Thus, the stoichiometry of FeTe samples is particularly critical to the STM study of charge dynamics in them [26][27][28][29][30].…”

Reorientation of the diagonal double-stripe spin structure at Fe1+yTe bulk and thin-film surfaces

“…STM data obtained for Fe 1+ y Te bulk and thin film samples have revealed atomic resolution of the Te-terminated surface and a superstructure on top of the atomic corrugation having a periodicity of λ =2 a Te 181920272829. The observation of this additional λ =2 a Te periodicity is mainly discussed in two contradictory models.…”

“…In contrast to a common representation of spin and charge modulations in correlated electron systems31, in this case the CDW would have the same periodicity as the SDW. Opposed to this interpretation, experiments performed with magnetically sensitive tips, which were prepared by attaching excess Fe atoms to the tip apex, give additional insight based on spin-polarized tunnelling2829. The 2 a Te superstructure can unambiguously be assigned to a direct imaging of the DDS order of the underlying Fe lattice.…”

Cryogenic STM in 3D vector magnetic fields realized through a rotatable insert

“…6,14,15 Our method of preparing spin polarized tips is to collect either excess Fe (FM tip) or FeTe clusters (AFM tip) from the surface of an FeTe reference sample 14 . This method is capable of producing soft magnetic tips that can be polarized by a small magnetic field (∼ 0.5 T), it can reproducibly produce spin-polarized tips of different (either FM or AFM) characters, and after tip preparation, characterization of the tip can be performed on the same sample.…”