# Presentation: 547821

## 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). 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 study the spin 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 electron transport as a function of angle between the magnetic field of the tip and the magnetic field of the sample. The BEEM spectra show a dependence upon the azimuthal angle of the tip only when a ferromagnetic tip is utilized as shown in Fig. 3(a) & Fig. 3(b) due to spin dependent scattering in the Fe. The relative orientation of the two magnetic fields is determined by plotting the BEEM current or percent transmission at a constant energy vs. angle as shown in Fig. 4.  The maximum in the plot is taken as the zero angle, which corresponds to the parallel state of a GMR device. Spin dependent scattering observed here is due to the differences in the density of states between spin up and spin down electrons in ferromagnetic metals and is utilized in magneto-resistive devices such as GMR or TMR spin valves.  Our new technique provides a more complete picture of spin dependent transport due to the full 2π rotational capability and the nanoscale positioning of the STM tip.  Our future focus is on measuring the spin dependent attenuation lengths of the Fe film and the spin transport properties of this interface.

**Fig. 2**

**Spin Polarized BEEM**

**Fig. 4**

**Notes:**
BACKGROUND AND INTRODUCTION 
Devices that utilize the spin degree of freedom rely on transport of electron spin through materials and material interfaces. For example, tunneling magneto resistance (TMR) stacks used in magnetic random access memories utilize tunneling and spin dependent scattering to achieve a high magneto-resistive effect. Further knowledge of spin-polarized electron transport can aid in the development of these and other spintronic devices.  One method for studying hot electron transport is ballistic electron emission microscopy (BEEM). BEEM is a three terminal scanning tunneling microscopy (STM) based technique where electrons are injected from a STM tip into a grounded thin film metal base of a Schottky diode.  A small fraction of these electrons will travel ballistically through the metal film towards the metal-semiconductor interface.  Those electrons with enough energy to overcome the Schottky barrier will be collected as the BEEM current.  The atomic scale positioning capability of the STM tip gives BEEM nanometer resolution.  In addition, the electrons tunneling from the STM tip gives BEEM a high energy resolution (~0.02 eV)
BEEM has proven to be useful for investigating hot electron scattering in various thin metal films.  In these studies the hot electron attenuation lengths were determined by measuring the BEEM current as a function of metal thickness.  This technique assumes that as electrons traverse a material the scattering is linearly proportional to the distance traveled.  This results in a first order rate equation where the attenuation length is the proportionality constant.  These studies have given insight into ballistic electron transport in various metals.  Some recent work has been done to study spin polarized hot electron transport.  For example, BEEM has been used to study giant magneto resistance in Co/Cu/Co trilayers10.  When the Co layers were in ferromagnetic alignment the BEEM current was greater than when they were in anti-ferromagnetic alignment due to spin dependent scattering effects.  The spin dependent attenuation lengths were extracted by measuring this difference in BEEM current as a function of Co layer thickness.  Other studies on CoFe/Al2O3/CoFe/GaAs(001) magnetic tunnel junctions also measured spin dependent attenuation lengths by measuring current voltage spectra for both anti-ferromagnetic and ferromagnetic alignment and layer thickness. 
We invented a new spin polarized BEEM (SP-BEEM) technique, 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 spin polarized hot electron transport as a function of angle between the magnetic field of the tip and the magnetic field of the sample. The BEEM current shows a dependence upon the angle when a ferromagnetic tip is used and negligible dependence for a non-ferromagnetic tip.  This new technique provides a more complete picture of spin dependent transport due to the full 2-pi rotational capability and the nanoscale positioning of the STM tip.
EXPERIMENTAL 
The Schottky diodes were fabricated using commercially available phosphorous doped Si(001) wafers (Na= 3.6 x 1013).  Prior to any deposition indium solder is applied to the backside of the wafers in order to create an ohmic contact.  These wafers are then cleaned in a 49% HF solution in order to remove the native oxide and immediately introduced into a Varion 980 electron beam evaporator that was evacuated to a maximum pressure of 3 x 10-6 mbar.  An Fe film was then deposited with thickness of 6 nm.  A capping layer consisting of 5 nm of Au was then deposited to prevent the Fe from oxidizing when the sample was fixed to the holder ex situ.  The metal film thickness was monitored with a quartz microbalance.  After deposition, the samples were mounted to the holders by applying silver paste to the ohmic backside contact while a BeCu clip served as the front side contact.  Once the silver paste had finished drying, the entire sample was immersed in a uniform in-plane magnetic field (> 1T).  Once this was complete the sample was introduced into an ultra high vacuum chamber to perform BEEM. 
A modified low temperature (4-300 K) UHV STM system (Omicron) was utilized to perform the BEEM measurements.  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
Several normalized BEEM spectra taken at different tip angles for the Fe coated tip are shown in Fig. 3(a).  These spectra show an onset at the Schottky height along with a difference in the amount of BEEM current as a function of the angle.  In contrast, BEEM spectra obtained using an uncoated Au tip on a sample of similar thickness displayed negligible dependence upon angle as shown in Fig. 3(b). To further clarify this dependence the normalized BEEM current at 1.5 eV is plotted as a function of tip angle for both a coated and uncoated Au tip in Fig. 3. The data was normalized such that the minimum BEEM current was set to zero. A fit to a sinusoidal function was performed and the maximum of the fit was taken as the zero angle as shown by the dashed line in Fig. 4. These spectra demonstrates that spin dependent scattering of the hot electrons is occurring as they traverse the metal base layer due to the relative orientation of their polarization and the magnetic field direction of the ferromagnetic base layer. Spin dependent scattering arises from the differences in the density of states for spin up and spin down electrons in a ferromagnetic metal. Electron scattering in a metal is proportional to the density of states, thus in a ferromagnetic metal the scattering rates are different for the different spin orientations. As the angle is changed, the relative population of the hot electrons within these states changes, which changes the effective scattering rate of the total current of ballistic electrons.  This results in the dramatic change in the measured BEEM current for the Fe coated tip. 
This research is notable because it measures the spin dependent scattering on a local scale, and as a function of the azimuthal angle between the tip and the sample. 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 DIRECTION:
To measure the spin dependent attenuation lengths of the Fe overlayer as a function of electron energy and tip angle.  Extend the technique to other systems such as other ferromagnetic metals such as permalloy and semiconductors such as GaAs and exploit the imaging capabilities of the technique to spatially resolve spin dependent scattering on the atomic scale.
FIGURE CAPTIONS
Fig.1: (left) Wiring schematic of the BEEM experiment (right) Energy band diagram of the BEEM spectra
Fig 2: a) The angle dependent SP-BEEM experiment depicting coated STM tip, azimuthal angle between the 2 magnetic field vectors.  Circular insert shows a cross section of the STM tip apex with the thin coating of Fe (dark grey) on the Au tip.  In this inset the sample is not shown. 
Fig. 3: BEEM spectra take from a 5 nm Au / 6 nm Fe / Si(001) Schottky diode for different tip angles relative to the sample using (a):  an Fe coated Au tip and (b): an uncoated Au tip.  Magnetic tip BEEM spectra show dependence upon angle which indicates that spin dependent scattering is occurring in the Fe film.
Fig. 4:  Plot of the normalized BEEM current as a function of angle between the tip and the Schottky diode for electron energy of 1.5 eV.  Open squares Au tip. Filled squares: Fe coated Au tip. The error associated with each data point is 15%. 

## 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

_**Education and Outreach:**_  The PI is engaged in several educational outreach activities.  These include teaching first year graduate students quantum mechanics using an audience response system or clickers. Exposing these first year students to some laboratory research as shown in the top left picture.  His research group is composed of a diverse set of students from all over the world that includes 4 Ph.D. students and 2 undergraduates.  The PI is also involved in several outreach activities including judging at two high school and middle school science fairs this past spring.  In addition, he is tutoring a sophomore high school student in quantum mechanics.  The student, Lexa Vreslovic, is engaged in a special topics research program at her high school (The Academy of the Holy Names in Albany NY) where each student picks an interesting topic to study.  Lexa picked quantum mechanics after reading an interesting article in *Scientific American*!  This is an ongoing program that Lexa started in her freshman year and plans to continue until she is a senior.  Currently she is learning about the double-slit experiment and wave particle duality, next will be tunneling and some hand on experience taking STM images in the laboratory.  This study will lead to her competing in local and national level science fairs.

- PI LaBella works in the lab with first year grad students and CNSE V.P. Kaloyeros

- The spintronics research group at CNSE
**Standing Left to Right: **Chaffra Awo-Affoudda, John Garramone, Evan Spadafora, Andy Stollenwerk, Chris Penalver (undergrad)**Seated Left to Right:  **Qa'id Foulks (undergrad), Vincent LaBella (PI)

- PI LaBella tutors sophomore high school student Lexa Vreslovic in quantum mechanics

**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