Spin-sensitive shape asymmetry of adatoms on noncollinear magnetic substrates

Abstract

The spin-resolved density of states of Co atoms on a noncollinear magnetic support displays a distinct shape contrast, which is superimposed on the regular height contrast in spin-polarized scanning tunneling microscopy. The apparent atom height follows the well-known cosine dependence on the angle formed by the tip and adatom local magnetization directions, whereas the shape contrast exhibits a sine dependence. We explain this effect in terms of a noncollinear spin density induced by the substrate, which in our case is the spin spiral of the Mn monolayer on W(110). The two independent contrast channels, apparent height and shape, are identified with the Co magnetization projections onto two orthogonal axes. As a result, all components of the overall atom magnetic moment vector can be determined with a single spin-sensitive tip in the absence of an external magnetic field. This result should be general for any atom deposited on noncollinear magnetic layers. DOI: 10.1103/PhysRevB.93.125424The control of the spin degree of freedom down to the single-atom limit is nowadays a central research issue [1][2][3][4][5][6][7], fostered by the increasing need for miniaturization and lowering power consumption in communication technologies. The most suitable technique to address the problem is spinpolarized scanning tunneling microscopy (SP-STM) [8], which has proven to be an excellent tool to perform single-atom magnetometry [1,6]. The design of magnetic structures with a given functionality calls for atomic-scale engineering by means of atomic manipulation or self-assembly of lowdimensional structures. Both of them can be combined with SP-STM [6,7,9,10]. For that reason, getting insight into the underlying physics of SP-STM and extending its range of applications is of crucial relevance. The SP-STM principle is the magneto-conductance effect in magnetic tunnel junctions [8,[11][12][13][14], and relies on probing the imbalance between majority and minority spins in the local density of states (LDOS) with respect to a particular magnetic quantization axis. Following the same notation as in Ref. [15], the spinpolarized tunneling current at sample bias V readswhere ρ T and P T are the tip's density of states and spin polarization,ρ s andm s the integrated sample total density of states and magnetization at r (tip position), and θ the angle formed between the tip and local sample magnetization. As a consequence, a SP-STM tip is only sensitive to the projection onto its magnetization direction, m s cos θ , and not to all other components ofm s . Here we show that the situation is radically different if the substrate's magnetic ground state is noncollinear within the spatial extent of an atomic wave function. Owing to the breaking of translational symmetry in spin space associated with the noncollinearity, the atoms' apparent shapes become distorted in spin-resolved STM images. We find that the strength of such distortion provides a quantitative measurement of the total spin component orthogonal to the default sensitivity direct...

Tunneling anisotropic magnetoresistance of Pb and Bi adatoms and dimers on Mn/W(110): A first-principles study

“…Similar behavior has been reported previously by Serrate et al for different d-states of a Co adatom adsorbed on Mn/W(110) 34 . Hence, we can conclude that in an STM experiment with a magnetic tip it will be possible to identify the spin direction of the Pb adatom by means of the respective orbitals dominating near E F yielding similar effects observed in spin-polarized STM [34][35][36] .…”

“…A monolayer Mn grown on W(110) surface (Mn/W(110)) is a prominent example which exhibits a noncollinear magnetic structure with a cycloidal 173 • spin-spiral ground state along the [110] direction 33 that is driven by the Dzyaloshinskii-Moriya interaction. Using this magnetic surface with a noncollinear spin structure, it is possible to control the spin direction of adsorbed Co adatoms due to local exchange coupling which has been demonstrated in recent experiments using scanning tunneling microscopy (STM) by Serrate et al 34,35 . The noncollinear spin state of the Mn monolayer is reflected due to hybridization even in the orbitals of the adsorbed Co adatom 36 .…”

Spin-spiral state of a Mn monolayer on W(110) studied by soft x-ray absorption spectroscopy at variable temperatures

“…Later, additional field-dependent STM measurements provided further evidence in favor of this [6]. Recently the Mn/W(110) system was exploited as a substrate inducing noncollinear magnetism in adatoms [7][8][9].…”

Electronic and magnetic properties of 3d transition-metal adatoms on Mn/W(110)

“…Using SP-STM, Serrate et al have imaged the spin direction of individual Co adatoms adsorbed on Mn/W(110) 22,23 . These experiments show that by means of SP-STM, the spin direction of the Co adatom can be controlled while maintaining the magnetic sensitivity by laterally moving the adatom along the Mn/W(110) surface.…”

“…The other aspect of the experiment of Co adatoms on Mn/W(110) is the indirect evidence of tunneling anisotropic magnetoresistance (TAMR) which has been observed in the STM line profiles 22,23 . The TAMR is very appealing for spintronic applications as it can be observed with only one ferromagnetic electrode and it does not need any coherent spin-dependent transport 27,28 .…”