- Email: [email protected]
CoO Thin Films
Available online at www.sciencedirect.com
ScienceDirect Physics Procedia 54 (2014) 87 – 89
International Conference on Magnetic Materials and Applications, MagMA 2013
Magnetic Studies of ion beam irradiated Co/CoO thin films A.S.Bhattacharyyaa*, D.Kabirajb, S.M.Yusufc, B.N.Devd a Centre for Nanotechnology,Central University of Jharkhand,Ranchi 835 205, India Inter University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110 067, India Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400 085, India d Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata 700 032, India b
c
Abstract Ion beam irradiation was performed on Co/CoO films deposited on Si (111) substrates by magnetron sputtering. Ion-beaminduced dewetting was observed. Dewetting is due to larger surface free energy of Co compared to CoO. Magnetic studies performed by SQUID and PPMS, showed exchange bias at the Co/CoO interface which forms a FM/AFM system. © © 2014 2014 The The Authors. Authors.Published Publishedby byElsevier ElsevierB.V. B.V.This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer-review under responsibility of Department of Physics, Indian Institute of Technology Guwahati. Peer-review under responsibility of Department of Physics, Indian Institute of Technology Guwahati Keywords: Ion beam; Co/CoO thin films; dewetting; exchange bias
1. Introduction Ion beam irradiation is an effective means of studying the morphological changes occurring in thin films. Surface energy changes as well as displacement of atoms due to localized heating can bring about interesting phenomena which can lead to possible patterning and device fabrication in the future. Film thickness and temperature affect the Co nanoparticles size and uniformity formed by dewetting. Co films have been found to undergo dewetting on annealing. Higher is temperature higher is the uniformity of the dewetted surface. It has been reported that a phase transition from mixed hcp and fcc to twinned fcc crystals with weak magnetic anisotropy occurs by dewetting. These Magnetic metal nanoparticle arrays are used in magnetic recording and carbon or semiconductor nanotubes. Other means of obtaining patterned nanoparticle arrays is by lithography, spin-coating, droplet drying, dip-coating or Langmuir–Blodgett methods. These are given in details by Lian et al. (2006) and Oh et al. (2009).
* Corresponding author. Tel.:+91-7870674251 E-mail address: [email protected]
1875-3892 © 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of Department of Physics, Indian Institute of Technology Guwahati doi:10.1016/j.phpro.2014.10.041
88
A.S. Bhattacharyya et al. / Physics Procedia 54 (2014) 87 – 89 2. Experiment Co/CoO thin films were deposited by magnetron sputtering. Thickness of each film was 25 nm. The films were then irradiated with Si and Ar ions at different fluences and energies. Scanning electron microscopy (SEM) and magnetic force microscopy (MFM) measurements were performed to study the changes in morphology and magnetic structure that occurred with the irradiation while SQUID and PPMS measurements were performed to study temperature and magnetic field dependence of dc magnetization. . 3. Results and discussions Due to Si ion irradiation at 50 keV on the Co/CoO sample, the phenomenon of ion-beam-induced dewetting occurred. Due to larger surface free energy of Co compared to CoO, globule-like structures, as shown in the SEM image [Fig-1(a)], have formed. This phenomenon resulted in the loss of connectivity between the different regions and as a result the ferromagnetic property got lost and the Co clusters act as separate entities with no interaction between each other as also shown by Lian et al. (2006). The phenomenon known as ion-beam induced dewetting was also observed in MFM image [Fig-1(b)].
Fig1: (a) SEM and (b) MFM images showing ion-beam-induced dewetting in Co/CoO films. (c) Observation of superparamagnetism with blocking temperature of around 300K (cooling field and measuring field in the FC case was 100 Oe). (d) Increase in coercivity and observation of exchange bias with reduction in temperature (the sample was first cooled under 100 Oe field from 300K to the measured temperature).
A.S. Bhattacharyya et al. / Physics Procedia 54 (2014) 87 – 89 Superparamagnetism was observed for Co/CoO films, which were irradiated with 200 keV Ar ions at a fluence of 1015 ions/cm2. Blocking temperature around 300 K was obtained from the measured ZFC-FC magnetization curves shown in Fig. 1(c). Increase in coercivity was evident from an increase in area of the hysteresis curve and the presence of an exchange bias was evident from the shift in the hysteresis curve as also observed by Oh et al. (2009). Increase in coercivity and exchange bias with lowering of temperature was found. The ratio |-Hc| / |Hc| changes from less than unity to higher than unity with decrease in temperature [Fig-1(d)]. This indicates that exchange bias varies from positive to negative with lowering of temperature. Similar results have also been reported by Manna et al. (2010, 2011). Table 1.Exchange bias in Ar ion irradiated Co/CoO thin films. Temperature (K)
|-HC| (Oe)
HC (Oe)
Ratio |-Hc| / |Hc|
300
236.2
260.6
0.91
100
326.8
351.5
0.93
50
439.9
362.8
1.21
4. Conclusions Irradiation with Si ions caused dewetting of the surface of Co/CoO films deposited by magnetron sputtering. Exchange bias and increase in coercivity with decrease in temperature was observed in this FM/AFM system. Evidence of Superparamagnetism was also found with Ar ion irradiation. Acknowledgements The authors acknowledge Prof D.P. Mahapatra, IOP Bhubaneswar, for help with Si ion beam irradiation. The work is supported by the IBIQuS project. References Lian, J., Grove, A., 2006. Patterning Metallic Nanostructures by Ion-Beam-Induced Dewetting and Rayleigh Instability. Nano Letters 6, 1047 -1052. Oh, Y-J., Ross, C.A., Jung, Y.S., Yang Wang, Thompson, C.V., 2009. Cobalt Nanoparticle Arrays made by Templated Solid-State Dewetting, Small 5, 860–865 and references within. Manna, P.K., Yusuf, S.M., Shukla, R., Tyagi, A.K., 2011. Exchange bias in BiFe0.8Mn0.2O3 nanoparticles with an antiferromagnetic core and a diluted antiferromagnetic shell. Physical Review B 83, 1844121-5. Manna, P.K., Yusuf, S.M., Shukla, R., Tyagi, A.K., 2010. Coexistence of sign reversal of both magnetization and exchange bias field in the core-shell type La0.2Ce0.8CrO3 nanoparticles. Applied Physics Letters 96, 2425081-3.
89
ScienceDirect Physics Procedia 54 (2014) 87 – 89
International Conference on Magnetic Materials and Applications, MagMA 2013
Magnetic Studies of ion beam irradiated Co/CoO thin films A.S.Bhattacharyyaa*, D.Kabirajb, S.M.Yusufc, B.N.Devd a Centre for Nanotechnology,Central University of Jharkhand,Ranchi 835 205, India Inter University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110 067, India Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400 085, India d Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata 700 032, India b
c
Abstract Ion beam irradiation was performed on Co/CoO films deposited on Si (111) substrates by magnetron sputtering. Ion-beaminduced dewetting was observed. Dewetting is due to larger surface free energy of Co compared to CoO. Magnetic studies performed by SQUID and PPMS, showed exchange bias at the Co/CoO interface which forms a FM/AFM system. © © 2014 2014 The The Authors. Authors.Published Publishedby byElsevier ElsevierB.V. B.V.This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer-review under responsibility of Department of Physics, Indian Institute of Technology Guwahati. Peer-review under responsibility of Department of Physics, Indian Institute of Technology Guwahati Keywords: Ion beam; Co/CoO thin films; dewetting; exchange bias
1. Introduction Ion beam irradiation is an effective means of studying the morphological changes occurring in thin films. Surface energy changes as well as displacement of atoms due to localized heating can bring about interesting phenomena which can lead to possible patterning and device fabrication in the future. Film thickness and temperature affect the Co nanoparticles size and uniformity formed by dewetting. Co films have been found to undergo dewetting on annealing. Higher is temperature higher is the uniformity of the dewetted surface. It has been reported that a phase transition from mixed hcp and fcc to twinned fcc crystals with weak magnetic anisotropy occurs by dewetting. These Magnetic metal nanoparticle arrays are used in magnetic recording and carbon or semiconductor nanotubes. Other means of obtaining patterned nanoparticle arrays is by lithography, spin-coating, droplet drying, dip-coating or Langmuir–Blodgett methods. These are given in details by Lian et al. (2006) and Oh et al. (2009).
* Corresponding author. Tel.:+91-7870674251 E-mail address: [email protected]
1875-3892 © 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of Department of Physics, Indian Institute of Technology Guwahati doi:10.1016/j.phpro.2014.10.041
88
A.S. Bhattacharyya et al. / Physics Procedia 54 (2014) 87 – 89 2. Experiment Co/CoO thin films were deposited by magnetron sputtering. Thickness of each film was 25 nm. The films were then irradiated with Si and Ar ions at different fluences and energies. Scanning electron microscopy (SEM) and magnetic force microscopy (MFM) measurements were performed to study the changes in morphology and magnetic structure that occurred with the irradiation while SQUID and PPMS measurements were performed to study temperature and magnetic field dependence of dc magnetization. . 3. Results and discussions Due to Si ion irradiation at 50 keV on the Co/CoO sample, the phenomenon of ion-beam-induced dewetting occurred. Due to larger surface free energy of Co compared to CoO, globule-like structures, as shown in the SEM image [Fig-1(a)], have formed. This phenomenon resulted in the loss of connectivity between the different regions and as a result the ferromagnetic property got lost and the Co clusters act as separate entities with no interaction between each other as also shown by Lian et al. (2006). The phenomenon known as ion-beam induced dewetting was also observed in MFM image [Fig-1(b)].
Fig1: (a) SEM and (b) MFM images showing ion-beam-induced dewetting in Co/CoO films. (c) Observation of superparamagnetism with blocking temperature of around 300K (cooling field and measuring field in the FC case was 100 Oe). (d) Increase in coercivity and observation of exchange bias with reduction in temperature (the sample was first cooled under 100 Oe field from 300K to the measured temperature).
A.S. Bhattacharyya et al. / Physics Procedia 54 (2014) 87 – 89 Superparamagnetism was observed for Co/CoO films, which were irradiated with 200 keV Ar ions at a fluence of 1015 ions/cm2. Blocking temperature around 300 K was obtained from the measured ZFC-FC magnetization curves shown in Fig. 1(c). Increase in coercivity was evident from an increase in area of the hysteresis curve and the presence of an exchange bias was evident from the shift in the hysteresis curve as also observed by Oh et al. (2009). Increase in coercivity and exchange bias with lowering of temperature was found. The ratio |-Hc| / |Hc| changes from less than unity to higher than unity with decrease in temperature [Fig-1(d)]. This indicates that exchange bias varies from positive to negative with lowering of temperature. Similar results have also been reported by Manna et al. (2010, 2011). Table 1.Exchange bias in Ar ion irradiated Co/CoO thin films. Temperature (K)
|-HC| (Oe)
HC (Oe)
Ratio |-Hc| / |Hc|
300
236.2
260.6
0.91
100
326.8
351.5
0.93
50
439.9
362.8
1.21
4. Conclusions Irradiation with Si ions caused dewetting of the surface of Co/CoO films deposited by magnetron sputtering. Exchange bias and increase in coercivity with decrease in temperature was observed in this FM/AFM system. Evidence of Superparamagnetism was also found with Ar ion irradiation. Acknowledgements The authors acknowledge Prof D.P. Mahapatra, IOP Bhubaneswar, for help with Si ion beam irradiation. The work is supported by the IBIQuS project. References Lian, J., Grove, A., 2006. Patterning Metallic Nanostructures by Ion-Beam-Induced Dewetting and Rayleigh Instability. Nano Letters 6, 1047 -1052. Oh, Y-J., Ross, C.A., Jung, Y.S., Yang Wang, Thompson, C.V., 2009. Cobalt Nanoparticle Arrays made by Templated Solid-State Dewetting, Small 5, 860–865 and references within. Manna, P.K., Yusuf, S.M., Shukla, R., Tyagi, A.K., 2011. Exchange bias in BiFe0.8Mn0.2O3 nanoparticles with an antiferromagnetic core and a diluted antiferromagnetic shell. Physical Review B 83, 1844121-5. Manna, P.K., Yusuf, S.M., Shukla, R., Tyagi, A.K., 2010. Coexistence of sign reversal of both magnetization and exchange bias field in the core-shell type La0.2Ce0.8CrO3 nanoparticles. Applied Physics Letters 96, 2425081-3.
89