Journal of Physics Conference Series

OPEN ACCESS

Fabrication of planar-type ferromagnetic tunneljunctions using electromigration method and itsmagnetoresistance propertiesTo cite this article Y Tomoda et al 2010 J Phys Conf Ser 200 062035

View the article online for updates and enhancements

You may also likeFundamentals of planar-type inductivelycoupled thermal plasmas on a substratefor large-area material processingMai Kai Suan Tial Hiromitsu Irie YujiMaruyama et al

-

A Method of Forming a Polycrystalline Siwith the Biomolecule FerritinJae Hwan Oh Eun Hyun Kim Dong HanKang et al

-

Preparation and investigations on thethermal structural and magnetic behaviorof Co-Ce substituted Ni nanoferritesSabih Qamar Saima Yasin NaveedRamzan et al

-

Recent citationsField-emission-induced electromigrationmethod for the integration of single-electron transistorsShunsuke Ueno et al

-

This content was downloaded from IP address 8324921313 on 14012022 at 0616

Fabrication of Planar-Type Ferromagnetic Tunnel Junctions Using Electromigration Method and Its Magnetoresistance Properties

Y Tomoda M Hanada W Kume S Itami T Watanabe and J Shirakashi

Department of Electrical and Electronic Engineering Tokyo University of Agriculture and Technology Koganei Tokyo 184-8588 Japan

E-mail shrakashcctuatacjp

Abstract The authors report electrical and magnetoresistance properties of planar-type NiVacuumNi ferromagnetic tunnel junctions fabricated by novel electromigration method This technique is based on the motion of atoms caused by field-emission-induced electromigration (ldquoactivationrdquo) The activation scheme is able to form nanogaps with separations of less than 10 nm which act as vacuum tunnel barriers We performed the activation technique for Ni initial nanogaps with separations of 20-50 nm The resistance of planar-type NiVacuumNi ferromagnetic tunnel junctions was changed by applying a magnetic field and MR ratio exhibited above 300 at 16 K The result suggests that the activation procedure is useful for the application to planar-type ferromagnetic tunnel junctions with vacuum tunnel barriers

1 Introduction Ferromagnetic single-electron transistors (FMSETs) are attractive nanoscale devices for the control of the interplay of spin and charge In RC-coupled FMSETs several metastable charge states within the Coulomb blockade regime cause hysteresis properties of tunnel magnetoresistance suggesting the multivalued functions of the devices [1] For the realization of FMSETs with higher operated temperature novel nanofabrication methods are required because of the nanometer-scale dimensions of the devices Recently electromigration (EM) scheme is emerging as a powerful technique for the fabrication of nanogaps [2-8] We have already reported planar-type tunnel junctions with ferromagnetic nanogap system fabricated by novel EM methods such as feedback-controlled EM (FCE) [9] and field-emission-induced EM (ldquoactivationrdquo) [10-13] Using these techniques one can easily fabricate the nanogaps with separations of less than 10 nm and precisely control the tunnel resistance of the nanogaps Therefore nanogap electrodes fabricated by these EM methods act as planar-type ferromagnetic tunnel junctions with vacuum tunnel barriers In this report electrical and magnetoresistive properties of planar-type NiVacuumNi ferromagnetic tunnel junctions are studied in detail

2 Experimental method

21 Fabrication of initial nanogap structure

International Conference on Magnetism (ICM 2009) IOP PublishingJournal of Physics Conference Series 200 (2010) 062035 doi1010881742-65962006062035

ccopy 2010 IOP Publishing Ltd 1

First metallic contact pads consisting of 5 nm Ti and 25 nm Au were defined on thermally oxidized silicon substrates using electron-beam (EB) lithography and lift-off process The total thickness and gap separation of the metallic contact pads were 30 nm and 500-800 nm respectively Then 25-30 nm thick Ni initial nanogaps with separations of 20-50 nm were also placed into the gap between the metallic contact pads by EB lithography and lift-off process Using the object-oriented micromagnetic framework (OOMMF) based on Landau-Lifshitz-Gilbert equation [14] we estimated the distribution of magnetization in Ni initial nanogap electrodes with an asymmetrical butterfly shape The geometry of the ferromagnetic electrodes should be designed appropriately so that their magnetization can be reliably controlled between anti-parallel and parallel configurations by magnetic shape anisotropy Finally activation procedure was applied to the initial nanogaps in a vacuum chamber with a pressure of 10-3 Pa

22 Field-emission-induced electromigration (ldquoactivationrdquo) Activation scheme is based on the motion of atoms induced by Fowler-Nordheim (F-N) field emission current at nanogaps [10-12] Figure 1 (a) shows the schematic of an initial nanogap By applying bias voltages a field emission current flows through the initial nanogap The metal atoms at the tip of the source electrode are activated by the field emission current as shown in Figure 1 (b) Then activated atoms move from source to drain electrode by electron wind force which is always caused along the direction of electron flow (figure 1 (c)) Finally hillock is formed by accumulated

Figure 1 Schematic of activation procedure (a) Initial nanogap before performing the activation (b) Field emission current passes through the initial nanogap by applying bias voltages (c) Metal atoms at the source electrode are activated by field emission current (d) Hillock is formed by accumulation of activated atoms at the tip of the drain electrode

Figure 2 AFM images of a Ni nanogap (a) before and (b) after performing the activation

Field Emission Current

(b)

(c)

lt10 nm

(d)

Source

Drain

A Few Tens nm

(a)

Metal Atoms Activated Atoms

Source

Drain

Source

Drain

Source

Drain

50 nm

Source

Drain

Ni SiO2

(a) (b)

50 nm

Source

Drain

NiSiO2

International Conference on Magnetism (ICM 2009) IOP PublishingJournal of Physics Conference Series 200 (2010) 062035 doi1010881742-65962006062035

2

atoms at the tip of drain electrode resulting in a decrease of the separation of the initial nanogap Consequently the tunnel resistance of the nanogaps after performing the activation becomes smaller than that before the activation

3 Results and Discussion Figure 2 (a) shows an atomic force microscope (AFM) image of a representative Ni initial nanogap The separation of the gap is approximately 40 nm which is completely defined by EB lithography The AFM image of the nanogap after performing the activation with the preset current of 65 microA is shown in Figure 2 (b) The inset also shows a scanning electron microscopy image after performing the activation The figures clearly indicate that the separation of the initial nanogap reduces from about 40 nm before the activation to less than 10 nm after the activation The AFM images before and after the activation imply that the initial gap separation is clearly reduced by performing the activation

Figure 3 (a) shows the relation between the resistances and the preset current Is during the activation procedure of the nanogap In this figure three kinds of resistances are shown such as tunnel resistance R of the nanogaps resistance Rs = VsIs when the bias voltage V is stopped at V = Vs in which the current reaches the Is in the activation and differential resistance dVsdIs during the activation The tunnel resistance of the nanogap decreases from 100 TΩ to 95 kΩ with increasing the preset current Is from 1 nA to 65 microA at room temperature Figure 3 (b) exhibits the current and derivative conductance as a function of the bias voltage at 16 K Since peak structures in the inelastic electron tunneling spectrum can be observed around plusmn20 mV which may be caused by the Ni phonon [15] the current is due to the electron tunneling between source and drain electrodes through the vacuum tunnel barrier The results suggest that the nanogap formed by the activation technique acts as

Figure 3 (a) Variation of resistances of a Ni nanogap during the activation procedure at room temperature (b) Current-voltage properties and inelastic electron tunneling spectrum of the nanogap at 16 K

Figure 4 (a) MR curve of a planar-type NiVacuumNi ferromagnetic tunnel junction (b) MR ratio as a function of bias voltage

(a) (b)

10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3100

102

104

106

108

1010

1012

1014

1016

R R

s d

Vsd

Is (Ω

)

Preset Current Is (A)

T = 300 K

R Rs dVsdIs

-400

-300

-200

-100

0

100

200

300

400

-01 -005 0 005 01

Dra

in C

urre

nt I

(nA

)

Bias Voltage V (V)

d2IdV

2

Ni 30 nmT = 16 K

Drain Current d2IdV2

0

50

100

-2000 -1000 0 1000 2000

MR

()

H (Oe)

Ni 30 nmT = 16 K

0

50

100

150

200

250

300

350

400

0 1 2 3 4 5 6 7

MR

()

Bias Voltage V (mV)

Ni 30 nmT = 16 K

(a) (b)

International Conference on Magnetism (ICM 2009) IOP PublishingJournal of Physics Conference Series 200 (2010) 062035 doi1010881742-65962006062035

3

a planar-type ferromagnetic tunnel junction with vacuum tunnel barrier Typically the effective barrier height and barrier thickness of the planar-type NiVacuumNi ferromagnetic tunnel junction formed by the activation were obtained [10] respectively as 4 eV and lt1 nm by the fitting of Simmons model [16]

Figure 4 (a) shows magnetoresistance (MR) properties of the planar-type ferromagnetic tunnel junction fabricated by the activation In order to eliminate the thermal excitation electrons we measured the MR characteristics at 16 K We obtained MR of 80 in the nanogap formed by the activation With decreasing the bias voltage MR increases from 80 to 300 at 16 K as shown in figure 4 (b) As seen in figure 2 (b) Ni island structure is formed in the nanogap Since the activation technique is based on the motion of atoms the Ni atoms tend to accumulate as Ni dots in the nanogap and act as multiple Ni islands Thus the high MR ratio may be due to the single-electron charging effects caused by ferromagnetic multiple tunnel junctions [17] The result implies that the activation technique is suitable for the fabrication of planar-type ferromagnetic tunnel junctions with vacuum tunnel barriers

4 Conclusion A novel electromigration method for the fabrication of nanogaps with less than 10 nm based on the motion of atoms induced by field emission current has been presented The nanogaps formed by activation technique act as planar-type tunnel junction devices with vacuum tunnel barriers Using the activation planar-type NiVacuumNi ferromagnetic tunnel junctions show MR of 300 at 16 K The MR increases from 80 to 300 with decreasing the bias voltage These results strongly suggest that planar-type NiVacuumNi ferromagnetic tunnel junctions show the possibilities for nanoscale magnetoresistive devices

Acknowledgment This study is partially supported by Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists

References [1] Shirakashi J and Takemura Y 2003 J Appl Phys 93 6873 [2] Park H Lim A K L Park J Alivisatos A P and McEuen P L 1999 Appl Phys Lett 75 301 [3] Khondaker S I and Yao Z 2002 Appl Phys Lett 81 4613 [4] Tsukagoshi K Watanabe E Yagi I and Aoyagi Y 2004 Microelectron Eng 73-74 686 [5] Bolotin K I Kuemmeth F Pasupathy A N and Ralph D C 2004 Appl Phys Lett 84 3154 [6] Strachan D R Smith D E Johnston D E Park T -H Therien M JBonnell D A and Johnson A T

2005 Appl Phys Lett 86 043109 [7] Esen G and Fuhrer M S 2005 Appl Phys Lett 87 263101 [8] Hoffmann R Weissenberger D Hawecker J and Stoumlffler D 2008 Appl Phys Lett 93 043118 [9] Takahashi K Tomoda Y Itami S and Shirakashi J 2009 J Vac Sci Technol B 27 805 [10] Kayashima S Takahashi K Motoyama M and Shirakashi J 2007 Japan J Appl Phys 46 L907 [11] Kayashima S Takahashi K Motoyama M and Shirakashi J 2008 J Phys Conf Ser 100 052022 [12] Tomoda Y Takahashi K Hanada M Kume W and Shirakashi J 2009 J Vac Sci Technol B 27

813 [13] Tomoda Y Takahashi K Hanada M Kume W Itami S Watanabe T and Shirakashi J 2009

IEEE Trans Mag in print [14] OOMMF is Object Oriented MicroMagnetic Framework a micromagnetic simulation code

available free from NIST at httpmath nistgovoommf [15] Tomoda Y Kayashima S Ogino T Motoyama M Takemura Y and Shirakashi J 2007 J Magn

Magn Mater 310 e641 [16] Simmons J G 1963 J Appl Phys 34 1793 [17] Shirakashi J and Takemura Y 2001 J Appl Phys 89 7365

International Conference on Magnetism (ICM 2009) IOP PublishingJournal of Physics Conference Series 200 (2010) 062035 doi1010881742-65962006062035

4

First metallic contact pads consisting of 5 nm Ti and 25 nm Au were defined on thermally oxidized silicon substrates using electron-beam (EB) lithography and lift-off process The total thickness and gap separation of the metallic contact pads were 30 nm and 500-800 nm respectively Then 25-30 nm thick Ni initial nanogaps with separations of 20-50 nm were also placed into the gap between the metallic contact pads by EB lithography and lift-off process Using the object-oriented micromagnetic framework (OOMMF) based on Landau-Lifshitz-Gilbert equation [14] we estimated the distribution of magnetization in Ni initial nanogap electrodes with an asymmetrical butterfly shape The geometry of the ferromagnetic electrodes should be designed appropriately so that their magnetization can be reliably controlled between anti-parallel and parallel configurations by magnetic shape anisotropy Finally activation procedure was applied to the initial nanogaps in a vacuum chamber with a pressure of 10-3 Pa

22 Field-emission-induced electromigration (ldquoactivationrdquo) Activation scheme is based on the motion of atoms induced by Fowler-Nordheim (F-N) field emission current at nanogaps [10-12] Figure 1 (a) shows the schematic of an initial nanogap By applying bias voltages a field emission current flows through the initial nanogap The metal atoms at the tip of the source electrode are activated by the field emission current as shown in Figure 1 (b) Then activated atoms move from source to drain electrode by electron wind force which is always caused along the direction of electron flow (figure 1 (c)) Finally hillock is formed by accumulated

Figure 1 Schematic of activation procedure (a) Initial nanogap before performing the activation (b) Field emission current passes through the initial nanogap by applying bias voltages (c) Metal atoms at the source electrode are activated by field emission current (d) Hillock is formed by accumulation of activated atoms at the tip of the drain electrode

Figure 2 AFM images of a Ni nanogap (a) before and (b) after performing the activation

Field Emission Current

(b)

(c)

lt10 nm

(d)

Source

Drain

A Few Tens nm

(a)

Metal Atoms Activated Atoms

Source

Drain

Source

Drain

Source

Drain

50 nm

Source

Drain

Ni SiO2

(a) (b)

50 nm

Source

Drain

NiSiO2

International Conference on Magnetism (ICM 2009) IOP PublishingJournal of Physics Conference Series 200 (2010) 062035 doi1010881742-65962006062035

2

atoms at the tip of drain electrode resulting in a decrease of the separation of the initial nanogap Consequently the tunnel resistance of the nanogaps after performing the activation becomes smaller than that before the activation

3 Results and Discussion Figure 2 (a) shows an atomic force microscope (AFM) image of a representative Ni initial nanogap The separation of the gap is approximately 40 nm which is completely defined by EB lithography The AFM image of the nanogap after performing the activation with the preset current of 65 microA is shown in Figure 2 (b) The inset also shows a scanning electron microscopy image after performing the activation The figures clearly indicate that the separation of the initial nanogap reduces from about 40 nm before the activation to less than 10 nm after the activation The AFM images before and after the activation imply that the initial gap separation is clearly reduced by performing the activation

Figure 3 (a) shows the relation between the resistances and the preset current Is during the activation procedure of the nanogap In this figure three kinds of resistances are shown such as tunnel resistance R of the nanogaps resistance Rs = VsIs when the bias voltage V is stopped at V = Vs in which the current reaches the Is in the activation and differential resistance dVsdIs during the activation The tunnel resistance of the nanogap decreases from 100 TΩ to 95 kΩ with increasing the preset current Is from 1 nA to 65 microA at room temperature Figure 3 (b) exhibits the current and derivative conductance as a function of the bias voltage at 16 K Since peak structures in the inelastic electron tunneling spectrum can be observed around plusmn20 mV which may be caused by the Ni phonon [15] the current is due to the electron tunneling between source and drain electrodes through the vacuum tunnel barrier The results suggest that the nanogap formed by the activation technique acts as

Figure 3 (a) Variation of resistances of a Ni nanogap during the activation procedure at room temperature (b) Current-voltage properties and inelastic electron tunneling spectrum of the nanogap at 16 K

Figure 4 (a) MR curve of a planar-type NiVacuumNi ferromagnetic tunnel junction (b) MR ratio as a function of bias voltage

(a) (b)

10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3100

102

104

106

108

1010

1012

1014

1016

R R

s d

Vsd

Is (Ω

)

Preset Current Is (A)

T = 300 K

R Rs dVsdIs

-400

-300

-200

-100

0

100

200

300

400

-01 -005 0 005 01

Dra

in C

urre

nt I

(nA

)

Bias Voltage V (V)

d2IdV

2

Ni 30 nmT = 16 K

Drain Current d2IdV2

0

50

100

-2000 -1000 0 1000 2000

MR

()

H (Oe)

Ni 30 nmT = 16 K

0

50

100

150

200

250

300

350

400

0 1 2 3 4 5 6 7

MR

()

Bias Voltage V (mV)

Ni 30 nmT = 16 K

(a) (b)

International Conference on Magnetism (ICM 2009) IOP PublishingJournal of Physics Conference Series 200 (2010) 062035 doi1010881742-65962006062035

3

a planar-type ferromagnetic tunnel junction with vacuum tunnel barrier Typically the effective barrier height and barrier thickness of the planar-type NiVacuumNi ferromagnetic tunnel junction formed by the activation were obtained [10] respectively as 4 eV and lt1 nm by the fitting of Simmons model [16]

Figure 4 (a) shows magnetoresistance (MR) properties of the planar-type ferromagnetic tunnel junction fabricated by the activation In order to eliminate the thermal excitation electrons we measured the MR characteristics at 16 K We obtained MR of 80 in the nanogap formed by the activation With decreasing the bias voltage MR increases from 80 to 300 at 16 K as shown in figure 4 (b) As seen in figure 2 (b) Ni island structure is formed in the nanogap Since the activation technique is based on the motion of atoms the Ni atoms tend to accumulate as Ni dots in the nanogap and act as multiple Ni islands Thus the high MR ratio may be due to the single-electron charging effects caused by ferromagnetic multiple tunnel junctions [17] The result implies that the activation technique is suitable for the fabrication of planar-type ferromagnetic tunnel junctions with vacuum tunnel barriers

4 Conclusion A novel electromigration method for the fabrication of nanogaps with less than 10 nm based on the motion of atoms induced by field emission current has been presented The nanogaps formed by activation technique act as planar-type tunnel junction devices with vacuum tunnel barriers Using the activation planar-type NiVacuumNi ferromagnetic tunnel junctions show MR of 300 at 16 K The MR increases from 80 to 300 with decreasing the bias voltage These results strongly suggest that planar-type NiVacuumNi ferromagnetic tunnel junctions show the possibilities for nanoscale magnetoresistive devices

Acknowledgment This study is partially supported by Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists

References [1] Shirakashi J and Takemura Y 2003 J Appl Phys 93 6873 [2] Park H Lim A K L Park J Alivisatos A P and McEuen P L 1999 Appl Phys Lett 75 301 [3] Khondaker S I and Yao Z 2002 Appl Phys Lett 81 4613 [4] Tsukagoshi K Watanabe E Yagi I and Aoyagi Y 2004 Microelectron Eng 73-74 686 [5] Bolotin K I Kuemmeth F Pasupathy A N and Ralph D C 2004 Appl Phys Lett 84 3154 [6] Strachan D R Smith D E Johnston D E Park T -H Therien M JBonnell D A and Johnson A T

2005 Appl Phys Lett 86 043109 [7] Esen G and Fuhrer M S 2005 Appl Phys Lett 87 263101 [8] Hoffmann R Weissenberger D Hawecker J and Stoumlffler D 2008 Appl Phys Lett 93 043118 [9] Takahashi K Tomoda Y Itami S and Shirakashi J 2009 J Vac Sci Technol B 27 805 [10] Kayashima S Takahashi K Motoyama M and Shirakashi J 2007 Japan J Appl Phys 46 L907 [11] Kayashima S Takahashi K Motoyama M and Shirakashi J 2008 J Phys Conf Ser 100 052022 [12] Tomoda Y Takahashi K Hanada M Kume W and Shirakashi J 2009 J Vac Sci Technol B 27

813 [13] Tomoda Y Takahashi K Hanada M Kume W Itami S Watanabe T and Shirakashi J 2009

IEEE Trans Mag in print [14] OOMMF is Object Oriented MicroMagnetic Framework a micromagnetic simulation code

available free from NIST at httpmath nistgovoommf [15] Tomoda Y Kayashima S Ogino T Motoyama M Takemura Y and Shirakashi J 2007 J Magn

Magn Mater 310 e641 [16] Simmons J G 1963 J Appl Phys 34 1793 [17] Shirakashi J and Takemura Y 2001 J Appl Phys 89 7365

International Conference on Magnetism (ICM 2009) IOP PublishingJournal of Physics Conference Series 200 (2010) 062035 doi1010881742-65962006062035

4

atoms at the tip of drain electrode resulting in a decrease of the separation of the initial nanogap Consequently the tunnel resistance of the nanogaps after performing the activation becomes smaller than that before the activation

3 Results and Discussion Figure 2 (a) shows an atomic force microscope (AFM) image of a representative Ni initial nanogap The separation of the gap is approximately 40 nm which is completely defined by EB lithography The AFM image of the nanogap after performing the activation with the preset current of 65 microA is shown in Figure 2 (b) The inset also shows a scanning electron microscopy image after performing the activation The figures clearly indicate that the separation of the initial nanogap reduces from about 40 nm before the activation to less than 10 nm after the activation The AFM images before and after the activation imply that the initial gap separation is clearly reduced by performing the activation

Figure 3 (a) shows the relation between the resistances and the preset current Is during the activation procedure of the nanogap In this figure three kinds of resistances are shown such as tunnel resistance R of the nanogaps resistance Rs = VsIs when the bias voltage V is stopped at V = Vs in which the current reaches the Is in the activation and differential resistance dVsdIs during the activation The tunnel resistance of the nanogap decreases from 100 TΩ to 95 kΩ with increasing the preset current Is from 1 nA to 65 microA at room temperature Figure 3 (b) exhibits the current and derivative conductance as a function of the bias voltage at 16 K Since peak structures in the inelastic electron tunneling spectrum can be observed around plusmn20 mV which may be caused by the Ni phonon [15] the current is due to the electron tunneling between source and drain electrodes through the vacuum tunnel barrier The results suggest that the nanogap formed by the activation technique acts as

Figure 3 (a) Variation of resistances of a Ni nanogap during the activation procedure at room temperature (b) Current-voltage properties and inelastic electron tunneling spectrum of the nanogap at 16 K

Figure 4 (a) MR curve of a planar-type NiVacuumNi ferromagnetic tunnel junction (b) MR ratio as a function of bias voltage

(a) (b)

10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3100

102

104

106

108

1010

1012

1014

1016

R R

s d

Vsd

Is (Ω

)

Preset Current Is (A)

T = 300 K

R Rs dVsdIs

-400

-300

-200

-100

0

100

200

300

400

-01 -005 0 005 01

Dra

in C

urre

nt I

(nA

)

Bias Voltage V (V)

d2IdV

2

Ni 30 nmT = 16 K

Drain Current d2IdV2

0

50

100

-2000 -1000 0 1000 2000

MR

()

H (Oe)

Ni 30 nmT = 16 K

0

50

100

150

200

250

300

350

400

0 1 2 3 4 5 6 7

MR

()

Bias Voltage V (mV)

Ni 30 nmT = 16 K

(a) (b)

International Conference on Magnetism (ICM 2009) IOP PublishingJournal of Physics Conference Series 200 (2010) 062035 doi1010881742-65962006062035

3

a planar-type ferromagnetic tunnel junction with vacuum tunnel barrier Typically the effective barrier height and barrier thickness of the planar-type NiVacuumNi ferromagnetic tunnel junction formed by the activation were obtained [10] respectively as 4 eV and lt1 nm by the fitting of Simmons model [16]

Figure 4 (a) shows magnetoresistance (MR) properties of the planar-type ferromagnetic tunnel junction fabricated by the activation In order to eliminate the thermal excitation electrons we measured the MR characteristics at 16 K We obtained MR of 80 in the nanogap formed by the activation With decreasing the bias voltage MR increases from 80 to 300 at 16 K as shown in figure 4 (b) As seen in figure 2 (b) Ni island structure is formed in the nanogap Since the activation technique is based on the motion of atoms the Ni atoms tend to accumulate as Ni dots in the nanogap and act as multiple Ni islands Thus the high MR ratio may be due to the single-electron charging effects caused by ferromagnetic multiple tunnel junctions [17] The result implies that the activation technique is suitable for the fabrication of planar-type ferromagnetic tunnel junctions with vacuum tunnel barriers

4 Conclusion A novel electromigration method for the fabrication of nanogaps with less than 10 nm based on the motion of atoms induced by field emission current has been presented The nanogaps formed by activation technique act as planar-type tunnel junction devices with vacuum tunnel barriers Using the activation planar-type NiVacuumNi ferromagnetic tunnel junctions show MR of 300 at 16 K The MR increases from 80 to 300 with decreasing the bias voltage These results strongly suggest that planar-type NiVacuumNi ferromagnetic tunnel junctions show the possibilities for nanoscale magnetoresistive devices

Acknowledgment This study is partially supported by Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists

References [1] Shirakashi J and Takemura Y 2003 J Appl Phys 93 6873 [2] Park H Lim A K L Park J Alivisatos A P and McEuen P L 1999 Appl Phys Lett 75 301 [3] Khondaker S I and Yao Z 2002 Appl Phys Lett 81 4613 [4] Tsukagoshi K Watanabe E Yagi I and Aoyagi Y 2004 Microelectron Eng 73-74 686 [5] Bolotin K I Kuemmeth F Pasupathy A N and Ralph D C 2004 Appl Phys Lett 84 3154 [6] Strachan D R Smith D E Johnston D E Park T -H Therien M JBonnell D A and Johnson A T

2005 Appl Phys Lett 86 043109 [7] Esen G and Fuhrer M S 2005 Appl Phys Lett 87 263101 [8] Hoffmann R Weissenberger D Hawecker J and Stoumlffler D 2008 Appl Phys Lett 93 043118 [9] Takahashi K Tomoda Y Itami S and Shirakashi J 2009 J Vac Sci Technol B 27 805 [10] Kayashima S Takahashi K Motoyama M and Shirakashi J 2007 Japan J Appl Phys 46 L907 [11] Kayashima S Takahashi K Motoyama M and Shirakashi J 2008 J Phys Conf Ser 100 052022 [12] Tomoda Y Takahashi K Hanada M Kume W and Shirakashi J 2009 J Vac Sci Technol B 27

813 [13] Tomoda Y Takahashi K Hanada M Kume W Itami S Watanabe T and Shirakashi J 2009

IEEE Trans Mag in print [14] OOMMF is Object Oriented MicroMagnetic Framework a micromagnetic simulation code

available free from NIST at httpmath nistgovoommf [15] Tomoda Y Kayashima S Ogino T Motoyama M Takemura Y and Shirakashi J 2007 J Magn

Magn Mater 310 e641 [16] Simmons J G 1963 J Appl Phys 34 1793 [17] Shirakashi J and Takemura Y 2001 J Appl Phys 89 7365

International Conference on Magnetism (ICM 2009) IOP PublishingJournal of Physics Conference Series 200 (2010) 062035 doi1010881742-65962006062035

4

a planar-type ferromagnetic tunnel junction with vacuum tunnel barrier Typically the effective barrier height and barrier thickness of the planar-type NiVacuumNi ferromagnetic tunnel junction formed by the activation were obtained [10] respectively as 4 eV and lt1 nm by the fitting of Simmons model [16]

Figure 4 (a) shows magnetoresistance (MR) properties of the planar-type ferromagnetic tunnel junction fabricated by the activation In order to eliminate the thermal excitation electrons we measured the MR characteristics at 16 K We obtained MR of 80 in the nanogap formed by the activation With decreasing the bias voltage MR increases from 80 to 300 at 16 K as shown in figure 4 (b) As seen in figure 2 (b) Ni island structure is formed in the nanogap Since the activation technique is based on the motion of atoms the Ni atoms tend to accumulate as Ni dots in the nanogap and act as multiple Ni islands Thus the high MR ratio may be due to the single-electron charging effects caused by ferromagnetic multiple tunnel junctions [17] The result implies that the activation technique is suitable for the fabrication of planar-type ferromagnetic tunnel junctions with vacuum tunnel barriers

4 Conclusion A novel electromigration method for the fabrication of nanogaps with less than 10 nm based on the motion of atoms induced by field emission current has been presented The nanogaps formed by activation technique act as planar-type tunnel junction devices with vacuum tunnel barriers Using the activation planar-type NiVacuumNi ferromagnetic tunnel junctions show MR of 300 at 16 K The MR increases from 80 to 300 with decreasing the bias voltage These results strongly suggest that planar-type NiVacuumNi ferromagnetic tunnel junctions show the possibilities for nanoscale magnetoresistive devices

Acknowledgment This study is partially supported by Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists

References [1] Shirakashi J and Takemura Y 2003 J Appl Phys 93 6873 [2] Park H Lim A K L Park J Alivisatos A P and McEuen P L 1999 Appl Phys Lett 75 301 [3] Khondaker S I and Yao Z 2002 Appl Phys Lett 81 4613 [4] Tsukagoshi K Watanabe E Yagi I and Aoyagi Y 2004 Microelectron Eng 73-74 686 [5] Bolotin K I Kuemmeth F Pasupathy A N and Ralph D C 2004 Appl Phys Lett 84 3154 [6] Strachan D R Smith D E Johnston D E Park T -H Therien M JBonnell D A and Johnson A T

2005 Appl Phys Lett 86 043109 [7] Esen G and Fuhrer M S 2005 Appl Phys Lett 87 263101 [8] Hoffmann R Weissenberger D Hawecker J and Stoumlffler D 2008 Appl Phys Lett 93 043118 [9] Takahashi K Tomoda Y Itami S and Shirakashi J 2009 J Vac Sci Technol B 27 805 [10] Kayashima S Takahashi K Motoyama M and Shirakashi J 2007 Japan J Appl Phys 46 L907 [11] Kayashima S Takahashi K Motoyama M and Shirakashi J 2008 J Phys Conf Ser 100 052022 [12] Tomoda Y Takahashi K Hanada M Kume W and Shirakashi J 2009 J Vac Sci Technol B 27

813 [13] Tomoda Y Takahashi K Hanada M Kume W Itami S Watanabe T and Shirakashi J 2009

IEEE Trans Mag in print [14] OOMMF is Object Oriented MicroMagnetic Framework a micromagnetic simulation code

available free from NIST at httpmath nistgovoommf [15] Tomoda Y Kayashima S Ogino T Motoyama M Takemura Y and Shirakashi J 2007 J Magn

Magn Mater 310 e641 [16] Simmons J G 1963 J Appl Phys 34 1793 [17] Shirakashi J and Takemura Y 2001 J Appl Phys 89 7365

International Conference on Magnetism (ICM 2009) IOP PublishingJournal of Physics Conference Series 200 (2010) 062035 doi1010881742-65962006062035

4