Change in magnetic properties of MgB2 thin films after irradiation by 200 MeV Au ion beam

NIM B Beam Interactions with Materials & Atoms

Nuclear Instruments and Methods in Physics Research B 263 (2007) 414–418 www.elsevier.com/locate/nimb

Change in magnetic properties of MgB2 thin films after irradiation by 200 MeV Au ion beam Ravindra Kumar b

a,*

, H.M. Agrawal a, R.P.S. Kushwaha a, D. Kanjilal

b

a Department of Physics, G.B.P.U.A & T. Pantnagar, Uttaranchal 263145, India Inter University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110067, India

Received 21 August 2006; received in revised form 23 February 2007 Available online 5 July 2007

Abstract The SHI irradiation induced effects on magnetic properties of MgB2 thin films are reported. The films having thickness 300–400 nm, prepared by hybrid physical chemical vapor deposition (HPCVD) were irradiated by 200 MeV Au ion beam (Se  23 keV/nm) at the fluence 1 · 1012 ion/cm2. Interestingly, increase in the transition temperature Tc from 35.1 K to 36 K resulted after irradiation. Substantial enhancement of critical current density after irradiation was also observed because of the pinning provided by the defects created due to irradiation. The change in surface morphology due to irradiation is also studied.  2007 Elsevier B.V. All rights reserved. PACS: 74.78.Fk Keywords: Critical current density; Irradiation; Columnar defects

1. Introduction The newly discovered superconductor MgB2 [1] with transition temperature 39 K has initiated flurry of activities due to its unusual properties such as strongly coupled grain boundaries [2], time dependent electronic anisotropy [3] and multiple superconducting gap structure [4,5]. This material has already been proved to be promising candidate for technological applications. For further improvement of pinning characteristics envisioned for potential applications, however, effective pinning centres are necessary to be introduced in the material. Swift heavy ion (SHI) irradiation is a reproducible means of enhancing pinning properties by the introduction of defects due to interaction of ions with matter. Bugoslavsky et al. [6] showed that modest levels of atomic disorder *

Corresponding author. Present address: ICFAI University Dehradun, 248002, India. Tel.: +91 9412982072. E-mail address: ravindra_k_bhatt@rediffmail.com (R. Kumar). 0168-583X/$ - see front matter  2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.06.028

induced by proton irradiation enhance the pinning of vortices, thereby significantly increasing Jc at high field strength. They also showed degradation of Tc. Narayan et al. [7] reported that the defects created by 200 MeV Ag ions on polycrystalline MgB2 are metallic and provide optimum core pinning of flux vortices leading to Jc enhancement significantly. Chikumoto et al. [8] studied the effect of heavy ion irradiation (5.8 GeV Pb ion) on polycrystalline MgB2 samples and observed the enhancement of pinning but no appreciable change in Tc. Irradiation effects on MgB2 (bulk samples) were studied by Okayasu et al. [9]. They found the degradation of intergrain coupling by electron irradiation as a result of which the pinning properties were also degraded. However, they confirmed the presence of columnar defects which improved pinning in higher field regions when the same samples were subjected to heavy ion irradiation (3.54 GeV Xe ions). Recently, Narayan et al. [10] reported the severe degradation of superconducting properties in irradiated PLD films and absence of the same in EBE films. Interestingly, Shinde

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et al. [11] showed that formation of defect clusters are likely to be responsible for the observed improvement in Jc after 200 MeV Ag ion irradiation on MgB2 thin films. Present study reports the results of 200 MeV Au+15 ion irradiation (corresponding to electronic energy loss, Se = 23 keV/nm) of high quality MgB2 thin films at the fluence 1 · 1012 ions/cm2. The critical current density (Jc) increased due to the pinning caused by defects produced by the irradiation. Surprisingly, the irradiation also enhanced the critical temperature Tc at the fluence of 1 · 1012 ions/cm2.

pressor geometry. The total number of particles falling on sample was estimated by a combination of current integrator and pulse counter. Position of the samples during irradiation was continuously monitored from the control room by a CCD camera placed at one of the windows in the irradiation chamber and attached to a video port. The temperature dependence (10 K to T > Tc) of the diamagnetic moment was carried out at IIT Kanpur by a quantum design superconducting quantum interference device (SQUID; MPMS 5). Magnetization hysteresis loops were measured at 5 K and 10 K with an external field (B) up to 3 T along c axis (c axis is perpendicular to the film surface). The required size of films for magnetization studies was 3 mm · 3 mm and therefore, films were cut by a diamond cutter. Scanning electron microscopy (SEM) of the samples was carried out at SSPL, New Delhi by JSM 840 GEOL setup.

2. Experimental details For the present investigation, high quality MgB2 thin films were procured from Superconix Inc (USA), which were fabricated by hybrid physical chemical vapor deposition (HPCVD) [12,13] on Al2O3(0 0 0 1) substrate. The film thickness was 300–400 nm. To confirm the MgB2 phase, XRD was carried out by using Cu-Ka radiation. SRIM (the stopping and range of ions in matter) code was run to choose the ion energy such that no ion implantation takes place. For the present ion energy combination, the electronic energy loss (Se  23 keV/nm) was taken to be high enough to create the desired defects. Out of the same batch of samples, one was irradiated by 200 MeV 197Au beam from 14 MV tandem Pelletron accelerator at IUAC, New Delhi and one was kept pristine. The ion beam was made incident normally upon the samples of size 5 mm · 5 mm during the irradiation, scanning an area of about 7 mm · 7 mm, so that whole area of sample may be uniformly irradiated. The irradiation was carried out at fluence 1 · 1012 ions/cm2 and keeping the sample in a sup-

3. Results and discussion Fig. 1 shows the diamagnetic transition before and after irradiation in field cooled (FC) and zero field cooled (ZFC) conditions. For ZFC data, the samples are first cooled to the desired temperature below Tc. Subsequently, a field of 0.01 T is applied and diamagnetic moment as a function of slowly increasing temperature is recorded. To obtain FC data, the temperature of sample is reduced through Tc under fixed field of 0.01 T and measurements are carried out. The difference between ZFC and ZC signals arises due to trapped flux [14]. It is clear that superconducting transition temperature Tc before irradiation is 35.1 K. Surprisingly, after 200 MeV Au ion irradiation, Tc increased marginally to 36 K at the dose of 1 · 1012 ions/cm2, which

FC 0.0000 Unirradited 2

1x1012 ions/cm

-0.0002

M (emu)

ZFC

-0.0004

-0.0006

-0.0008 10

20

415

30

40

50

Temperature (K) Fig. 1. Moment versus temperature before and after irradiation.

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needs further confirmation. The difference between ZFC and FC signals for the irradiated sample with the fluence of 1 · 1012 ions/cm2 indicates that the weaker superconducting phase is providing flux trapping centres. In order to further confirm enhanced flux pinning after irradiation, M–H hysteresis loops were recorded at temperatures 5 K and 10 K. The observed data are shown in Fig. 2(a) and Fig. 2(b), respectively. For both the temperatures, magnetization hysteresis loops are almost closed

at about 3 T. The loops become larger with decrease in temperature. Fig. 3 shows the critical current density as a function of B at 5 K and 10 K before and after irradiation. The Jc values before and after irradiation were estimated by using Bean’s critical state model [15]. Jc is related to the width of magnetization loops by the formula J c ¼ 20  DM=Vað1  a=3bÞ;

0.0015

Unirradiated (5K) 12

2

20000

30000

1x10 ions/cm (5K)

0.0010

M (emu)

0.0005

0.0000

-0.0005

-0.0010

-0.0015 -30000

-20000

-10000

0

10000

B (Oe) Fig. 2(a). M–B loops at 5 K.

0.0015

Unirradiated (10K) 12

2

1x10 ions/cm (10K) 0.0010

M (emu)

0.0005

0.0000

-0.0005

-0.0010

-0.0015 -30000

-20000

-10000

0

10000

B (Oe) Fig. 2(b). M–B loops at 10 K.

20000

30000

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1x10

417

5

12

2

1x10 ions/cm (10K) Unirradiated (10K) Unirradiated (5K) 12

4

1x10

3

1x10

2

2

1x10 ions/cm (5K)

Jc(Amp/cm2)

1x10

0

5000

10000

15000

20000

25000

30000

B (Oe) Fig. 3. Variation of Jc with B before and after irradiation.

Fig. 4. SEM images before and after irradiation.

where V is superconducting volume and a and b are length and breadth of the film, respectively. After irradiation at a dose of 1 · 1012 ions/cm2 the value of Jc has increased from 1.24 · 104 amp/cm2 (before irradiation at 5 K) to 8.9 · 104 amp/cm2 at 5 K and from 0.4 · 104 amp/cm2 at 10 K to 8.5 · 104 amp/cm2 at 10 K. Swift heavy ion (SHI) irradiation results in crystalline disorders in the form of point defects and columnar defects (if the electronic stopping power is more than the material dependent threshold Sth). Such defects tend to depress the order parameter locally, thereby creating pinning sites for the vortices which enhance Jc [16]. The irradiation studies done so far have all used the ion energy combination where Se is not higher than what we have used. Chikumoto et al. [8] used 5.8 GeV Pb ion corresponding to Se  18–20 keV/ nm. Okayasu et al. [9] used 3.54 GeV Xe ions corresponding to Se  10 keV/nm. Shinde et al. [11] used 200 MeV Ag

ions corresponding to Se  16 keV/nm. In our case of 200 MeV Au ions, Se  23 keV/nm which is quite high in comparison to the studies done so far. Therefore, it is expected that columnar defects are formed which tend to depress the order parameter locally, thereby creating pinning sites for the vortices which enhance Jc [16]. In Fig. 4 , SEM micrographs are shown before and after irradiation. SEM micrograph before irradiation shows the granular structure having the grains of different size. The grains seem to be rectangular in shape. After irradiation, the change in grain size and shape can easily be seen in the figure. Acknowledgements Financial support by IUAC (UFUP-36307) is gratefully acknowledged. R.K. is extremely thankful to Dr. H. Kishan

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