# Presentation: 471952

## Towards an Atomic Scale Understanding of Spin Polarized Electron TransportVincent P. LaBella, College of Nanoscale Science and Engineering, University at Albany / SUNY, DMR-0349108

**Towards an Atomic Scale Understanding of Spin Polarized Electron Transport****Vincent P. LaBella, *****College of Nanoscale Science and Engineering, ******University at Albany / SUNY*****, DMR-0349108**

_**The objective of this research**_ is to study spin polarized electron transport through materials and material interfaces on the atomic scale. Ferromagnetic-metal/semiconductor contacts are one component of high efficiency spin injectors such as magnetic tunnel junctions (MTJ) and magnetic tunnel transistors (MTT). One method for studying the electron transport properties of these interfaces is ballistic electron emission microscopy (BEEM). This three terminal scanning tunneling microscopy (STM) technique injects electrons from a STM tip into a grounded metal base of a Schottky diode as shown in Fig. 1. A small fraction of these electrons will travel ballistically through the metal to the interface and those electrons with sufficient energy to surmount the Schottky barrier will be detected by a backside contact as BEEM current.
_**To measure the spin dependent transport properties**_ of these interfaces we developed a new technique called spin polarized BEEM (SP-BEEM) where a ferromagnetic STM tip is utilized to inject spin-polarized electrons into a ferromagnetic metal semiconductor Schottky diode as shown in Fig. 2. This technique allows measurement of hot electron transport as a function of angle between the magnetic field of the tip and the magnetic field of the sample. By measuring this dependence as a function of the thickness of the Fe in the Schottky diode the spin dependent attenuation lengths of the Fe can be extracted as shown in Fig. 3. These spin dependent attenuation lengths quantify the spin dependent scattering in this material which is the underlying physical mechanism that is utilized in magneto-resistive devices. In addition, these attenuation lengths were measured as a function of tip bias above the Schottky height as displayed in the inset of Fig. 3 and show significant variation with bias. We attribute this variation to changes in the amount of allowed transverse momentum for different bands in the semiconductor. Our future focus is on investigating the effects of the interface band structure upon the measured attenuation lengths of the metal as these results suggest that the attenuation length of a metal is not solely a property of the metal but a property of the interface as well.

**Fig. 1**

**Fig. 2**

**Spin Polarized BEEM**

**BEEM**

**Fig. 3**

***- *= 1.8 ± 0.2 nm
***+ *= 2.5 ± 0.3 nm

**A. Stollenwerk *****et al. Phys. Rev. B***** (in press) (2007)**

**Notes:**
BACKGROUND AND INTRODUCTION 
Transport of electron spin through materials and material interfaces has attracted a lot of interest recently due to the desire to manufacture devices that harness the spin of the electron. For example, tunneling through metal-insulator-metal barriers and spin dependent scattering can produce a large and technologically useful magneto-resistive effect. This effect arises in part due to the difference in scattering rates between the minority and majority carriers in a ferromagnetic metal due to the differences in the density of states of the d bands near the Fermi level 
Measuring spin dependent scattering rates or attenuation lengths of electrons in ferromagnetic (FM) metals is challenging. However, a few methods have been developed, such as spin polarized photoemission, which measured spin dependent mean free paths in Fe for electron energies ranging from 14–44 eV. Recently, ballistic electron magnetic microscopy (BEMM), a scanning tunneling microscopy (STM) based technique, has been used to study transport properties of the interface between a FM metal and a normal metal in Co/Cu/Co layers on a Au/Si diode. The spin dependent attenuation lengths for Co were measured as a function of tip bias providing insight into the scattering at the metal-metal interfaces. Current-voltage studies on CoFe/AlO/CoFe/GaAs(001) and NiFe/AlO/NiFe/GaAs(001) magnetic tunnel junction devices measured attenuation lengths of these alloys as a function of applied bias. It was determined that the ratio of the mean free paths decreased monotonically with energy showing no dependence upon the band structure at the metal-semiconductor interface. These results are significant for understanding spin transport. However, measuring the effects of the interface band structure between a ferromagnetic metal and a semiconductor on spin dependent hot electron transport is even more challenging. This is due in part to the lack of suitable techniques with sufficient spatial and energetic resolution
In our work the spin polarized ballistic electron attenuation lengths measured in a FM-metal-semiconductor system are found to have an energetic dependence upon the interface band structure. This was accomplished by measuring spin polarized hot electron transport through Fe/Si(001) Schottky diodes using spin polarized ballistic electron emission microscopy (SP-BEEM).
EXPERIMENTAL 
A modified low temperature (4-300 K) UHV STM system (Omicron) was utilized to perform the BEEM measurements on Au/Fe/Si(001) Schottky diodes. The STM stage was in contact with a cryostat that was cooled with liquid nitrogen resulting in a tip and sample temperature of approximately 80 K as measured by a Si diode mounted on the imaging stage. The setup for this experiment is shown in Fig. 2. BEEM spectroscopy was performed using a Au tip fabricated by cutting 10 mil wire at a sharp angle. In order to perform the spin polarized BEEM the Au tip was made ferromagnetic by depositing 5 ML of Fe onto the tip in situ using an electron beam evaporator. Spectroscopy was performed at approximately 100 locations over an area of several square microns. These spectra where then averaged together in order to reduce the affects of surface roughness on the BEEM current. Once this was complete the tip was removed from the STM and rotated in situ. The angle was measured by observing a tick mark on the tip holder through a top-down viewport and placed back onto the STM. After each rotation BEEM spectra were acquired and averaged together as described above. This same method was then repeated with an uncoated Au tip. 
For a bulk ferromagnetic STM tip the magnetization is on axis due to shape anisotropy. This is perpendicular to the magnetization of the thin film sample, which would result in negligible spin dependent scattering of the hot electrons. However, it has been demonstrated that coating a tip with 3-10 ML of Fe results in a magnetization perpendicular to the axis of the tip and in plane with the sample. The deposition creates a thin Fe film on the relatively flat microscopic tunneling region of a macroscopically curved tip resulting in an in plane magnetization. SEM images of the tips used in these experiments showed that the tip was microscopically dull at the apex similar to the zoomed in schematic shown in Fig. 2.
RESULTS AND DISCUSSION
The maximum and minimum in the normalized BEEM current obtained as the tip was rotated are plotted as a function of the thickness of the Fe in the Schottky diode for a tip bias of 1.5 eV in Fig. 3. The line through each set of data points is a least squares fit to the equation under Fig. 3, where C+(-) is the majority (minority) collection efficiency at the metal-semiconductor interface, d is the thickness of the metal, λ+(-) is the majority (minority) hot electron attenuation length, E is the electron energy, T is the temperature, and P0 is the initial polarization of the tunneling current in the plane of the sample. The initial polarization is assumed to be 44% for the maximum current and -44% for the minimum current. To extract the attenuation lengths, C+ and C- are assumed to be equal for both spin orientations. From this fit, the spin dependent attenuation lengths at 1.5 eV were found to be 1.8 ± 0.2 nm and 2.5 ± 0.3 nm for the minority and majority electrons, respectively. 
The spin dependent attenuation lengths were determined using the same assumptions described above for tip biases from 1.0 eV to 1.7 eV and displayed in the inset of Fig. 3 The majority spin attenuation length was found to decrease with increasing energy until it reaches a tip bias of 1.25 eV, after which it begins to increase. A similar behavior was observed for the minority spin attenuation length. This increase indicates that the spin dependent transport properties of the diode are changing. This effect has been observed for unpolarized BEEM studies on Fe/Si(001) diodes and was attributed to the nature of the Fe/Si(001) interface band structure. More specifically, the second threshold voltage occurring at 1.29 eV, which exists due to an additional conduction band minimum (CBM) at the interface not present in pure Si. Theoretical calculations of the Fe/Si interface predict a change in the interface band structure of Si due to the poor coordination of dangling d bonds in the Fe with Si. The increase in attenuation length at energies above this band can be attributed to a larger amount of available parallel momentum states with greater parallel momentum when compared to the energy band associated with the Schottky barrier. As the tip bias is increased through the second threshold, electrons with greater parallel momentum will be allowed into the semiconductor. This makes the BEEM current less sensitive to the increased amount of elastic scattering that occurs in the thicker metal overlayers thus increasing the attenuation length. 
This research is notable because it can quantify the spin dependent attenuation lengths of electrons in a ferromagnetic metal. It can also measure the spin dependent scattering on a local scale, and as a function of the azimuthal angle between the tip and the sample. In addition, it demonstrates the effect of the interface band structure upon the measured attenuation lengths. This indicates that the (spin dependent) attenuation length of a metal in a Schottky diode is not solely a property of the metal but also the semiconductor. This new knowledge will impact the development of Schotky contacts to be utilized as spin injectors into semiconductors. This capability combined with the ability to control the temperature of the STM has the potential to provide new insight into spin dependent scattering and spin transport through materials and material interfaces. 
FUTURE DIRECTIONS
Quantify the effect of the semiconductor band structure upon the spin dependent and non spin dependent attenuation lengths in metals. 
Improve the SP-BEEM technique by designing a new tip that will allow the field direction to be rotated in the plane while tunneling, which will eliminate the need to physically rotate the tip.

FIGURE CAPTIONS
Fig.1: Wiring schematic of the BEEM experiment
Fig 2: Experimental setup for the SP-BEEM experiment. The directions of the magnetization of the sample and tip are indicated by MSample and MTip, respectively. The angle θ between the magnetization directions is measured by a tick mark on the back of the tip holder. Circular insert shows the thin coating of Fe on the Au tip. 
Fig. 3: Semi-log plot of the percent transmission of the ballistic electrons for FM and AFM alignment of the magnetization of the tip and the sample as a function of the Fe thickness of the Schottky diode. The solid lines represent the least squares fit to the equation under the figure giving attenuation lengths of 2.5 ± 0.3 nm for the majority spin attenuation length and 1.8 ± 0.2 nm for the minority spin attenuation length. (inset) Spin dependent hot attenuation lengths as a function of tip bias. 

## Towards an Atomic Scale Understanding of Spin Polarized Electron TransportVincent P. LaBella, College of Nanoscale Science and Engineering, University at Albany / SUNY, DMR-0349108

- The spintronics research group at the College of Nanoscale Science and Engineering (Left to Right)** **Joseph Abel, Ilona Sitnitski, John Garramone, Lexa Vreslovick, Evan Spadafora, Vincent LaBella

- High school student Lexa Vreslovic (right) and Grad Students Ilona Sitnitski (center) and Joeseph Abel (left) work on the ambient STM/BEEM instrument

**Towards an Atomic Scale Understanding of Spin Polarized Electron Transport****Vincent P. LaBella, *****College of Nanoscale Science and Engineering, ******University at Albany / SUNY*****, DMR-0349108**

- Second year graduate student John Garramone works on UHV STM/BEEM instrument