Bias-Voltage Dependence in Atomic-Scale Spin Polarized Scanning Tunneling Microscopy of Mn 3 N 2 (010)
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Bias-Voltage Dependence in Atomic-Scale Spin Polarized Scanning Tunneling Microscopy of Mn3N2 (010) Arthur R. Smith, Rong Yang, and Haiqiang Yang Department of Physics and Astronomy, Condensed Matter and Surface Science Program and Nanoscience and Quantum Phenomena Institute, Ohio University, Athens, OH 45701 ABSTRACT Atomic-scale spin-polarized scanning tunneling microscopy results on the manganese nitride Mn3N2 (010) surface are presented. The images show the row-wise antiferromagnetic structure of the surface. It is shown that the bias voltage between tip and sample affects both the magnetic and non-magnetic components of the height profile. In particular, a reversal of the magnetic contrast is shown to occur at a certain bias voltage. INTRODUCTION Spin-polarized scanning tunneling microscopy (SP-STM) is a very promising technique for the study of surface magnetism since detailed magnetic contrast can be obtained down to the atomic scale [1,2,3]. Previous work done by Heinze et al. showed magnetic contrast for a Mn monolayer on W (110) [1]. Such a capability to study the fine detail of the spin structure is very important considering the growing interest in nanoscale magnetism as well as spintronics. Unfortunately, not enough is known about this technique. For example, little is known how the magnetic contrast varies with tunneling parameters, such as bias voltage. The surface magnetic structure of Mn3N2 (010) – a model row-wise antiferromagnetic surface – makes an ideal testing ground for the technique of atomic-scale SP-STM. In recent work, we have demonstrated both magnetic and non-magnetic contrast obtained simultaneously on this surface [3]. Here, we present results showing bias-dependence of the magnetic contrast amplitude, and also that the magnetic contrast reverses at a certain bias. The explanation for this behavior is discussed. EXPERIMENTAL DETAILS The experiments are performed in a custom molecular beam epitaxy/scanning tunneling microscopy (MBE/STM) ultra-high vacuum system. The sample of Mn3N2 (010) is prepared by MBE using a Mn effusion cell and a radio frequency (rf) plasma N source. The substrate used is MgO(001). The grown sample is transferred directly into the adjoining STM chamber without air exposure for the SP-STM study. The tips for SP-STM are prepared in various ways, typically by coating outgassed W tips with a 5-10 monolayer Mn or Fe film. In the case of Fe coatings, a small magnetic field is also applied normal to the tip axis following deposition. The SP-STM experiments are performed at room temperature in constant current mode. DISCUSSION The principle of SP-STM is based not on the magnetic force between the tip and the sample, but rather on the existence of a spin-polarized local density of states (LDOS) for the tip and also for the sample. Shown in Figure 1 is a schematic diagram illustrating the basic concept of the
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atomic-scale SP-STM method. In this method, a magnetic-coated tip scans a magnetic surface, here depicted as an antiferromagnetic surface in which
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