TY  - JOUR
AU  - Lazo, C.
AU  - Caciuc, V.
AU  - Hölscher, H.
AU  - Heinze, S.
TI  - Role of tip size, orientation, and structural relaxations in first principles studies of magnetic exchange force microscopy and spin polarized scanning tunneling microscopy
JO  - Physical review / B
VL  - 78
IS  - 21
SN  - 1098-0121
CY  - College Park, Md.
PB  - APS
M1  - PreJuSER-2482
SP  - 214416
PY  - 2008
N1  - It is our pleasure to thank S. Blugel, Y. Mokrousov, P. Ferriani, A. Schwarz, U. Kaiser, R. Schmidt, and R. Wiesendanger for many insightful discussions. Computations were performed at the Hamburg University of Technology, the Norddeutscher Verbund fur Hoch-und Hochstleistungsrechnen (HLRN), and the Forschungszentrum Julich (JUMP). We acknowledge financial support from the DFG (Grants No. HO 2237/3-1 and No. HE 3292/4-1). S. H. thanks the Stifterverband fur die Deutsche Wissenschaft and the Interdisciplinary Nanoscience Center Hamburg for financial support.
AB  - Using first-principles calculations based on density-functional theory, we investigated the exchange interaction between a magnetic tip and a magnetic sample which is detected in magnetic exchange force microscopy (MExFM) and also occurs in spin-polarized scanning tunneling microscopy (SP-STM) experiments. As a model tip-sample system, we chose Fe tips and one monolayer Fe on W(001) which exhibits a checkerboard antiferromagnetic structure and has been previously studied with both SP-STM and MExFM. We calculated the exchange forces and energies as a function of tip-sample distance using different tip models ranging from single Fe atoms to Fe pyramids consisting of up to fourteen atoms. We find that modeling the tip by a single Fe atom leads to qualitatively different tip-sample interactions than using clusters consisting of several atoms. Increasing the cluster size changes the calculated forces, quantitatively enhancing the detectable exchange forces. Rotating the tip with respect to the surface unit cell has only a small influence on the tip-sample forces. Interestingly, the exchange forces on the tip atoms in the nearest and next-nearest layers from the apex atom are non-negligible and can be opposite to that on the apex atom for a small tip. In addition, the apex atom interacts not only with the surface atoms underneath but also with nearest neighbors in the surface. We find that structural relaxations of tip and sample due to their interaction depend sensitively on the magnetic alignment of the two systems. As a result the onset of significant exchange forces is shifted toward larger tip-sample separations which facilitates their measurement in MExFM. At small tip-sample separations, structural relaxations of tip apex and surface atoms can either enhance or reduce the magnetic contrast measured in SP-STM.
KW  - J (WoSType)
LB  - PUB:(DE-HGF)16
UR  - <Go to ISI:>//WOS:000262244400061
DO  - DOI:10.1103/PhysRevB.78.214416
UR  - https://juser.fz-juelich.de/record/2482
ER  -