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n-Si interfacial structures
Accepted Manuscript Effect of swift heavy ion irradiation on magnetic, surface morphology and electronic transport across CoFe/n-Si interfacial structures Arvind Kumar, P.C. Srivastava PII:
S0749-6036(16)30054-4
DOI:
10.1016/j.spmi.2016.02.010
Reference:
YSPMI 4188
To appear in:
Superlattices and Microstructures
Received Date: 9 January 2016 Accepted Date: 7 February 2016
Please cite this article as: A. Kumar, P.C. Srivastava, Effect of swift heavy ion irradiation on magnetic, surface morphology and electronic transport across CoFe/n-Si interfacial structures, Superlattices and Microstructures (2016), doi: 10.1016/j.spmi.2016.02.010. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Effect of swift heavy ion irradiation on magnetic, surface morphology and electronic transport across CoFe/n-Si interfacial structures Arvind Kumar and P. C. Srivastava*
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Department of Physics, Banaras Hindu University, Varanasi (U.P.) -221005, India
Abstract
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In this study, swift heavy ion induced modifications on magnetic, morphological and electronic transport properties of CoFe/n-Si bilayers was investigated. Structural
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investigations have revealed the interfacial intermixing across the interface upon irradiation to result in the formation of magnetic silicide phases with enhanced crystallite size as compared to unirradiated structure. On irradiation, surface topography (from atomic force microscopy) has revealed the columnar arrangement of grains with increased value of rms surface roughness which in turn also affects the magnetic behaviour. Magnetization
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measurements have shown the enhancement in saturation magnetization and coercivity value with increased magnetic signal strength after irradiation. Current-voltage measurement across the irradiated CoFe/n-Si interface has shown the enhancement in current data by two orders
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of magnitude as compared to unirradiated interface. The observed significant changes in magnetic and transport properties for the irradiated interface has been explained on the basis
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of disorder/defect creation and interfacial chemistry modifications in the structure due to swift heavy ions.
Keywords: CoFe; Swift heavy Ion; I-V characteristics, M-H characteristics *
Corresponding Author: Tel: +91 9839234757; Fax: +91 542 2368390 E-mail: [email protected] (Prof. P. C. Srivastava)
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1. Introduction Investigation of magnetic properties in magnetic metals/or and alloys interfaced with semiconductor, Si has been a topic of curiosity due to its potential applications in spintronic
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devices [1]. Due to complementary properties of both semiconductor and ferromagnetic system, there is a growing effort towards the studies of Ferromagnetic (FM)/Semiconductor (SC) interfaces. During the last decade, modification of structure and properties of materials
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by ion beam got significant interest from the both scientific and technological point of view. Heining et al. [2] studied the synthesis and modification of nanostructures by ion beams in
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detail. Irradiation of materials with ion beams can improve the optical, chemical, magnetic and electrical properties [3-6]. Among the various irradiation techniques, such as ion implantation, ion beam mixing and ion beam assisted deposition, swift heavy ion (SHI) irradiation is found to be a very effective tool for tailoring the material properties. Ion beam irradiation of magnetic bilayers and mutilayers has shown to modify the extrinsic magnetic
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properties, such as magnetic anisotropy, coercivity and magnetic exchange coupling [7-11]. Several studies [12-14] performed on Co/Pt multilayered structures, FePt and CoPt alloy have shown the modification in magnetic properties by ion beam which causes atomic
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displacements and hence permanent modifications of the structure result in magnetic modifications. Several studies are available on ion beam induced effects in metal/metal [15-
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16] and metal/semiconductor bilayers [17-20]. Significant findings on swift heavy ion irradiation induced modification on the properties of metal/semiconductor interfaces such as Fe/Si [21], Ni/Si[22], Mn/Si[23], NiFe/Si[24] and exchange biased interfaces[25-28] have also been reported by our group. Ion irradiation plays a significant role in modifications of material properties such as in morphology, magnetic and transport property. Our group has studied such modifications upon irradiation for metal/semiconductor structures [21,29-31]. Tripathi et al.[21-22] studied the transport behaviour of irradiated structures of Ni/Si and
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Fe/Si and found the increased value of current and magnetic field sensitivity (i.e., magnetoresistance) as compared to unirradiated ones for these structures. A large magneto-resistance, i.e., giant magneto-resistance of 2400% has been found for the irradiated Fe/c-Si strucrure
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[21]. Significant effect of irradiation on magnetic properties of metal/semiconductor interfaces have also been investigated using MFM and magnetization characteristics which shows increment in the value of magnetization and magnetic signal strength upon irradiation.
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In recent studies [26-28], effect of ion irradiation on magnetic, morphological and transport studies on exchanged coupled structures of Fe/NiO has also been investigated and significant
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findings are reported. In series, irradiation induced modifications on the properties of magnetic alloy of CoFe interfaced with Si has been studied. CoFe alloy has soft magnetic properties. It has saturation magnetisation (about 15% greater than Fe), high magnetic moment, low coercive force and high Curie temperature (~1500 K). In our previous reports [32-33], we have studied the CoFe/Si (on both p- and n-type) interfaces.
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So, motivation of the present study, to investigate the effects of swift heavy ion (SHI) irradiation on the structural, transport and magnetic properties of transition metal alloy (CoFe) interfaced with Si. Interfacial structures of CoFe/n-Si have been characterized from
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various techniques. Significant and interesting modifications have been observed after the swift heavy ion (SHI) irradiation in the magnetic properties and transport behaviour across
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CoFe/n-Si interfacial structures which could be understood in the realm of SHI induced modification.
2. Experimental Details
The procedure of realizing CoFe/n-Si interfacial structures and details of further characterizations have been discussed in detail in our previous reports [32-33].The above realized structures were then irradiated from ~100MeV Ni7+ with a fluence of 1×1012
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ions/cm2 at Inter University Accelerator Centre, New Delhi using 15UD Pelletron Facility. Detailed discussions of irradiation method were discussed in our previous study [33].The projected range of the 100MeV Ni7+ ions in CoFe/n-Si structure has been calculated from
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SRIM software [34] which is found to be ~ 9.70µm. Since, the junction depth in our system is much less than the projected range. So, energy deposited by the passage of Ni ion beam in CoFe/nSi structure is mainly dominated by electronic energy loss in the system. The
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estimated electronic energy loss (Se) and nuclear energy loss (Sn) are 1.572E+04 and 3.183E+01 keV/µm, respectively.
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3. Results and Discussion 3.1 XRD Study
XRD patterns have been collected for prior to and after irradiation for chemical phase identifications of CoFe/n-Si structures. Fig. 1(a, b) shows the XRD pattern for the
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unirradiated and irradiated CoFe/n-Si interfacial structures, respectively. XRD spectrum of unirradiated structure contains the signals due to metallic phases of CoFe and also some other phases of metallic silicides whereas after irradiation signals due to metallic phases are weak
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and more silicide phases are pronounced. Thus, it seems that after irradiation the interface is modified as a result of intermixing (across the interface of CoFe layer and Si) due to the large
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energy deposition of ion beams. Emergence of some new phases has also been observed on the irradiation which could be identified as different silicide phases related to Fe and Co. The observed peak positions are identified using standard JCPDS files. The reduction in the intensity of metallic CoFe peak after the irradiation and the appearance of other new peaks also support the role of interfacial modification as a result of interfacial intermixing across the interface. XRD data are also tabulated in table 1(a) and 1(b) for unirradiated and irradiated CoFe/n-Si interfacial structures, respectively. Crystallite size σ, has also been estimated using the Scherrer relation [35] and calculated crystallite size after irradiation is
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found to range from ~ 55-96 nm which is larger as compared to prior to irradiation of the structures. Such increase in crystallite size can be understood through swift heavy ion (SHI) irradiation induced interfacial intermixing. Several other groups have already reported SHI
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induced atomic mixing for metal-semiconductor interfaces [36]. SHI irradiation causes a thermal spike phenomena in which the the ion beam excites the electronic system at local site and electrons transfer their energy to phonon via electron-phonon coupling resulting in an
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increase in the local temperature which cause recrystallization and grain growth phenomenon in the irradiated structures [37]. From the SRIM calculations we have found that SHI loses its
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energy almost by electronic excitation and ionization of target atoms.
The full width and half maxima (FWHM) for metallic CoFe peak has been observed to decrease after the irradiation which can be due to either change in crystallite size or due to strain. Using the
single line analysis [38]
method, it is found that grain size ‘D’
corresponding to CoFe phase has increased from 43 nm to 57 nm after the irradiation. So, it
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looks that most of the decrease in FWHM corresponding to CoFe peak is related to the increase in crystallite size. The estimated strain has been found to be very small of the order of ∼10−3.
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3.2 AFM/MFM Study
Figs. 2 and 3(a-c) show the AFM images for unirradiated and irradiated structures of
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CoFe/nSi interface, respectively along with their line scan sectional profiles. All the images were scanned over a scan area of 2×2µm2. Line scan sectional analysis has also been done to estimate the grain size and root mean square (rms) surface roughness. Surface morphology of unirradiated CoFe/n-Si structure is shown in two and three-dimensional (2D and 3D) view (Fig. 2a, b) which shows the feature of clustered grains having size > 300nm. Clustering of grains can be understood due to clustering of smaller grains (with high surface area) to result in larger gains/cluster. The rms roughness has been estimated to be ~ 30 nm. Moreover, after
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irradiation (Fig. 3a, b), the surface shows the feature of isolated columnar shaped grains along a specific direction. The width and height of the grains has been found to vary from ~150 nm to ~ 250 nm and ~ 550 nm to ~ 800 nm, respectively. The rms surface roughness
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has found to be of ~ 50 nm. Such evolution of shape with increased rms surface roughness and grain size after irradiation can be understood due to irradiation induced recrystallization and intermixing phenomena at CoFe/n-Si interface. Thus, it is evident from the AFM images
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that the surface feature looks clearly different after irradiation. RMS surface roughness is the parameter in characterizing the morphology of a surface. The increased surface roughness
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after irradiation with fluence of 1×1012 ions/cm2 implies that the roughening process is predominant in our case which can be assumed due to evaporation of atoms from a hot surface heated by the ineleastic thermal spike. Similar observations have also been reported for Fe-Ni thin films [39].
Magnetic force microscopy (MFM) has been employed as a tool to analyze the
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domain pattern in the structures before and after irradiation. MFM images have been scanned over a scan area of 2×2µm2 in a tapping mode with a lift height of 90 nm to avoid any topographical interference due to van der Waal force or AFM signals. Since, it is well known
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that van der Waal forces are active only up to ~ 10nm [40]. A magnetized cantilever tip coated with 50 nm thin film of CoCr has been used to measure the dipole-dipole interaction
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between the magnetic tip and stray field of the sample. Figs.4 and 5 (a, b) shows the MFM images along with their line scan sectional
analysis for unirradiated and irradiated structures, respectively. 2D MFM image for unirradaited structure is shown in fig. 4(a) depicting the clear bright and dark contrast which quantifies the domain formation. Domain size (estimated from line scan profile, Fig. 4b) is found to be ~ 30 nm with a magnetic signal strength ~ 0.23˚. Moreover, the MFM image of irradiated structure shows the formation of fine domain structure ~ 10nm with an increased
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magnetic signal strength ~0.29˚.The observed decrease in domain size and increased magnetic signal strength after irradiation can be attributed due to the formation of more magnetic silicide phases as a result of irradiation induced intermixing.
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3.3 Magnetization characteristics
Magnetization measurement of the structures, i.e., M-H characteristics have been recorded for in plane configuration, i.e., magnetic field was applied along the plane of the
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interface by sweeping the applied external magnetic field from -15kOe to 15kOe and back. The diamagnetic contributions arising due to Si substrate and the glass sample holder were
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also subtracted before analysis. Several magnetic parameters (extracted from M-H data) were tabulated in table 2 for unirradiated and irradiated structures. Fig. 6 shows the M-H characteristics of unirradiated and irradiated CoFe/n-Si structures.
Magnetization, coercivity and remanence values were found to increase after the. It is
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significant to observe that irradiation causes the change in magnetization curve from antiferromagnetic coupled phase (in case of unirradiated structure) to ferromagnetic phase (in case of irradiated structure) with increased value of magnetization which could be understood
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due to irradiation induced modifications. It has been suggested that after irradiation the surface state pinning of domains are released via surface modifications and the saturation
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magnetization is increased [41]. The increased coercivity and saturation magnetization in the ion irradiation induced is attributed to the domain wall pinning due to irradiation induced effects and large collision energy transfer causing roughening of surface which provides pinning sites for inhibiting domain wall motion [39, 42]. The increased coercivity can also be understood due to changes in magnetic structure (e.g., domain size) [43]. Similar observation has also been found by Thomas et al.[39] in SHI irradiated Fe-Ni amorphous thin films where the increased coercivity on the irradiation has been related to the increased roughness on the irradiation [39].
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3.4 Electronic Transport Study Fig. 7 shows the I-V characteristics across unirradiated and irradiated CoFe/nSi interface. It is interesting to observe that for the irradiated structure, the magnitude of current
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has increased by two orders of magnitude for both the polarity of applied bias voltage as compared to unirradiated ones. Moreover, the I-V characteristics of unirradiated structure show the rectifying behaviour of typical Metal/Semiconductor (M-S) interface. However,
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after irradiation the nature of the I-V curve seems to be bit symmetrical. The significant increase of the current value after the irradiation is a significant and curious observation
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because usually in the irradiated interfaces with a non magnetic metal layer, the current is generally observed to decrease [24] due to irradiation induced disorders and defects leading to electron traps. In our case, such significant increase in current after irradiation by two orders of magnitude for CoFe/n-Si interfacial structure seems to be due to the intermixing at the interfaces which causes the formation of intimate interface and/or decrease in interface
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resistance which provides the easy path for the current flow across the interface. Our XRD results have shown the formation of silicide phases which can be in support to the observation. It seems that the defects created due to ion irradiation (causing decrease in
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interface resistance) or formed magnetic silicide phases are somehow favouring the paths for easy orientation of spins to cause less scattering and thus increase in current value. The
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observation seems interesting to us because usually the interface resistance increases heavily after the irradiation. Thus, it looks that irradiation induced interfacial intermixing causes decrease in the interface resistance to make the interface more intimate and thus increase in the current flow across the interface.
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4. Conclusions Effect of swift heavy ion irradiation on the magnetic, morphological and transport properties of CoFe thin films with nSi substrate has been studied.Structural study has
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revealed the formation of more silicide phases as a result of irradiation induced enhanced intermixing. The crystallite size was found to increase after the irradiation. Shape evolution from clustered grains to isolated columnar shaped grains has been observed after irradiation
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with increased rms roughness and grain size. Strong magnetic signal strength with fine domain structure has been found which significantly affects the magnetic behaviour.
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Magnetization characteristics have shown a change from antiferromagnetic coupled phase to ferromagnetic phase with significant increase in magnetization value and coercivity after the irradiation. Enhancement in current value has been observed for the irradiated structure by two orders of magnitude as compared to unirradiated ones. The observed changes in magnetic, morphological and transport characteristics have been understood in the realm of
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irradiation induced defect creation and/or interfacial intermixing across the interface to make the interface more intimate.
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Acknowledgements
We would like to thank the IUAC, New Delhi personnel (Dr. D Kanjilal, Dr. Indira
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Sulania) for their co-operation during the Irradiation and AFM/MFM measuremnets. One of the authors Arvind Kumar also acknowledges the financial support received from the University Grants Commission, New Delhi, India in the form of senior research fellowship (UGC-SRF).
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[12] Chappert, C.; Bernas, H.; Ferre´, J.; Kottler, V.; Jamet, J.P.; Chen, Y.; Cambril, E.; Devolder, T.; Rousseaux, F.; Mathet, V.; Launois, H.; Science 280 (1998) 1919. [13] Devolder, T.; Chappert, C.; Chen, Y.; Cambril, E.; Bernas, H. Jamet, J.P.; Ferre,J.; Appl. Phys. Lett. 74 (2000) 236. [14] Ravelosona, D.; Chappert, C.; Mathet, V.; Bernas, H.; Appl. Phys. Lett. 76 (2000) 236.
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[31]Srivastava, P.C.; Srivastava, M.K.; Pandey, P.S.; Appl. Sur. Sci. 254 (2008) 5116–5119 [32] Kumar, A.; Srivastava, P.C.; J. Exp. Nano. 10 (2015) 803–818 [33] Kumar, A.; Srivastava, P.C.; Radiation Effects & Defects in Solids 170 (2015)(6)461476. [34]SRIM [15] http://www.srim.org/. [35]Eberhart, J.P.; Analyse structurale et chimiques des materiaux [Structural and chemical analysis of materials. Dunod: Paris; 1989
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Figure Captions: Figure 1. XRD pattern of CoFe/n-Si interfacial structures of (a) unirradiated ones, (b)
region.)
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irradiated from a fluence of 1 × 1012 ions/cm2 (Insets show the expanded view of the selected
Figure 2. AFM image of the CoFe/n-Si structure (a) 2-dimensional view, (b) 3-dimensional
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view and (b) sectional analysis of the unirradiated structure.
Figure 3. AFM image of the CoFe/n-Si structure (a) 2-dimensional view, (b) 3-dimensional
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view and (b) sectional analysis of the irradiated structure.
Figure 4. MFM image of the CoFe/n-Si structure (a) 2-dimensional view and (b) sectional analysis of the unirradiated structure.
Figure 5. MFM image of the CoFe/n-Si structure (a) 2-dimensional view and (b) sectional
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analysis of the unirradiated structure
Figure 6. In-plane magnetization (M–H) characteristics of the CoFe/n-Si interfacial structure;
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before and after the irradiation from a fluence of 1 × 1012 ions/cm2 Figure 9. Current–voltage (I–V) characteristics of the CoFe/n-Si interfacial structure; before
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and after the irradiation from a fluence of 1 × 1012 ions/cm2.
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Table Captions:
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Table 1(a). XRD data of the unirradiated CoFe/p-Si interfacial structure. Table 1(a). XRD data of the irradiated CoFe/p-Si interfacial structure from a fluence of 1×1012ions/cm2.
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Table 2. Magnetization data of CoFe/n-Si interfacial structure before and after irradiation
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from a fluence of 1×1012ions/cm2.
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Table 1(a) Peak Intensity (counts) 42 19 16 25 67 2391
32.255 44.460 53.895 55.665 61.025 65.240 65.790 68.470
1.4185 1.3692
0.150 0.150
475 59487
Identified Peak
β-FeSi2 <022> CoFe <110> Fe3Si <311> CoFe <111> β-FeSi2 <602> CoFe <200>/β-FeSi2 <424> ε-FeSi <310> Si <400>
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Grain Size (nm) 55.5 43.1 59.7 60.2 61.8 63.3
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2.7731 2.0361 1.6998 1.6499 1.5172 1.4290
Peak width (2θ) 0.150 0.200 0.150 0.150 0.150 0.150
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d-value (Å)
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Angle (2θ)
63.3 -----
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Table 1(b)
Peak Intensity (counts) 380
Fe5Si3<111>,Co2Si<111>
Grain Size (nm) 15.4
44.465
2.0359
0.150
26
CoFe<110>
16.0
61.310
1.5108
0.150
85
Fe5Si3<302>,Co2Si<022>,βFeSi2<602>
0.100
9
68.745
1.3644
0.100
53963
68.995
1.3601
0.100
21638
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1.4794
Fe5Si3<400>,Co2Si<202>,βFeSi2<621>
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61.8 93.3
Fe5Si3<222>,Co2Si<222>,βFeSi2<711>
96.5
Si<400>
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62.755
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Peak width (2θ) 0.150
Identified Peaks
32.605
dvalue (Å) 2.7441
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Angle (2θ)
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Table 2 Before Irradiation
After Irradiation (1×1012ions/cm2)
Coercivity (Hc)
47Oe
89Oe
Magnetization at Hmax
2.61×10-4 emu
1.98×10-3 emu
Remanence (Mr)
0.57×10-5 emu
4.67×10-4 emu
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Magnetic Parameter
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Research Highlights
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1. CoFe/n-Si interfaces have been irradiated from SHI. 2. Significant increment has been observed in value of saturation magnetization of SHI irradiated CoFe/n-Si structure. 3. Current across the interface gets increased for SHI irradiated structures. 4. The observed results could be understood in the realm of SHI induced modifications.
S0749-6036(16)30054-4
DOI:
10.1016/j.spmi.2016.02.010
Reference:
YSPMI 4188
To appear in:
Superlattices and Microstructures
Received Date: 9 January 2016 Accepted Date: 7 February 2016
Please cite this article as: A. Kumar, P.C. Srivastava, Effect of swift heavy ion irradiation on magnetic, surface morphology and electronic transport across CoFe/n-Si interfacial structures, Superlattices and Microstructures (2016), doi: 10.1016/j.spmi.2016.02.010. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Effect of swift heavy ion irradiation on magnetic, surface morphology and electronic transport across CoFe/n-Si interfacial structures Arvind Kumar and P. C. Srivastava*
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Department of Physics, Banaras Hindu University, Varanasi (U.P.) -221005, India
Abstract
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In this study, swift heavy ion induced modifications on magnetic, morphological and electronic transport properties of CoFe/n-Si bilayers was investigated. Structural
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investigations have revealed the interfacial intermixing across the interface upon irradiation to result in the formation of magnetic silicide phases with enhanced crystallite size as compared to unirradiated structure. On irradiation, surface topography (from atomic force microscopy) has revealed the columnar arrangement of grains with increased value of rms surface roughness which in turn also affects the magnetic behaviour. Magnetization
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measurements have shown the enhancement in saturation magnetization and coercivity value with increased magnetic signal strength after irradiation. Current-voltage measurement across the irradiated CoFe/n-Si interface has shown the enhancement in current data by two orders
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of magnitude as compared to unirradiated interface. The observed significant changes in magnetic and transport properties for the irradiated interface has been explained on the basis
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of disorder/defect creation and interfacial chemistry modifications in the structure due to swift heavy ions.
Keywords: CoFe; Swift heavy Ion; I-V characteristics, M-H characteristics *
Corresponding Author: Tel: +91 9839234757; Fax: +91 542 2368390 E-mail: [email protected] (Prof. P. C. Srivastava)
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1. Introduction Investigation of magnetic properties in magnetic metals/or and alloys interfaced with semiconductor, Si has been a topic of curiosity due to its potential applications in spintronic
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devices [1]. Due to complementary properties of both semiconductor and ferromagnetic system, there is a growing effort towards the studies of Ferromagnetic (FM)/Semiconductor (SC) interfaces. During the last decade, modification of structure and properties of materials
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by ion beam got significant interest from the both scientific and technological point of view. Heining et al. [2] studied the synthesis and modification of nanostructures by ion beams in
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detail. Irradiation of materials with ion beams can improve the optical, chemical, magnetic and electrical properties [3-6]. Among the various irradiation techniques, such as ion implantation, ion beam mixing and ion beam assisted deposition, swift heavy ion (SHI) irradiation is found to be a very effective tool for tailoring the material properties. Ion beam irradiation of magnetic bilayers and mutilayers has shown to modify the extrinsic magnetic
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properties, such as magnetic anisotropy, coercivity and magnetic exchange coupling [7-11]. Several studies [12-14] performed on Co/Pt multilayered structures, FePt and CoPt alloy have shown the modification in magnetic properties by ion beam which causes atomic
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displacements and hence permanent modifications of the structure result in magnetic modifications. Several studies are available on ion beam induced effects in metal/metal [15-
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16] and metal/semiconductor bilayers [17-20]. Significant findings on swift heavy ion irradiation induced modification on the properties of metal/semiconductor interfaces such as Fe/Si [21], Ni/Si[22], Mn/Si[23], NiFe/Si[24] and exchange biased interfaces[25-28] have also been reported by our group. Ion irradiation plays a significant role in modifications of material properties such as in morphology, magnetic and transport property. Our group has studied such modifications upon irradiation for metal/semiconductor structures [21,29-31]. Tripathi et al.[21-22] studied the transport behaviour of irradiated structures of Ni/Si and
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Fe/Si and found the increased value of current and magnetic field sensitivity (i.e., magnetoresistance) as compared to unirradiated ones for these structures. A large magneto-resistance, i.e., giant magneto-resistance of 2400% has been found for the irradiated Fe/c-Si strucrure
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[21]. Significant effect of irradiation on magnetic properties of metal/semiconductor interfaces have also been investigated using MFM and magnetization characteristics which shows increment in the value of magnetization and magnetic signal strength upon irradiation.
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In recent studies [26-28], effect of ion irradiation on magnetic, morphological and transport studies on exchanged coupled structures of Fe/NiO has also been investigated and significant
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findings are reported. In series, irradiation induced modifications on the properties of magnetic alloy of CoFe interfaced with Si has been studied. CoFe alloy has soft magnetic properties. It has saturation magnetisation (about 15% greater than Fe), high magnetic moment, low coercive force and high Curie temperature (~1500 K). In our previous reports [32-33], we have studied the CoFe/Si (on both p- and n-type) interfaces.
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So, motivation of the present study, to investigate the effects of swift heavy ion (SHI) irradiation on the structural, transport and magnetic properties of transition metal alloy (CoFe) interfaced with Si. Interfacial structures of CoFe/n-Si have been characterized from
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various techniques. Significant and interesting modifications have been observed after the swift heavy ion (SHI) irradiation in the magnetic properties and transport behaviour across
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CoFe/n-Si interfacial structures which could be understood in the realm of SHI induced modification.
2. Experimental Details
The procedure of realizing CoFe/n-Si interfacial structures and details of further characterizations have been discussed in detail in our previous reports [32-33].The above realized structures were then irradiated from ~100MeV Ni7+ with a fluence of 1×1012
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ions/cm2 at Inter University Accelerator Centre, New Delhi using 15UD Pelletron Facility. Detailed discussions of irradiation method were discussed in our previous study [33].The projected range of the 100MeV Ni7+ ions in CoFe/n-Si structure has been calculated from
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SRIM software [34] which is found to be ~ 9.70µm. Since, the junction depth in our system is much less than the projected range. So, energy deposited by the passage of Ni ion beam in CoFe/nSi structure is mainly dominated by electronic energy loss in the system. The
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estimated electronic energy loss (Se) and nuclear energy loss (Sn) are 1.572E+04 and 3.183E+01 keV/µm, respectively.
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3. Results and Discussion 3.1 XRD Study
XRD patterns have been collected for prior to and after irradiation for chemical phase identifications of CoFe/n-Si structures. Fig. 1(a, b) shows the XRD pattern for the
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unirradiated and irradiated CoFe/n-Si interfacial structures, respectively. XRD spectrum of unirradiated structure contains the signals due to metallic phases of CoFe and also some other phases of metallic silicides whereas after irradiation signals due to metallic phases are weak
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and more silicide phases are pronounced. Thus, it seems that after irradiation the interface is modified as a result of intermixing (across the interface of CoFe layer and Si) due to the large
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energy deposition of ion beams. Emergence of some new phases has also been observed on the irradiation which could be identified as different silicide phases related to Fe and Co. The observed peak positions are identified using standard JCPDS files. The reduction in the intensity of metallic CoFe peak after the irradiation and the appearance of other new peaks also support the role of interfacial modification as a result of interfacial intermixing across the interface. XRD data are also tabulated in table 1(a) and 1(b) for unirradiated and irradiated CoFe/n-Si interfacial structures, respectively. Crystallite size σ, has also been estimated using the Scherrer relation [35] and calculated crystallite size after irradiation is
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found to range from ~ 55-96 nm which is larger as compared to prior to irradiation of the structures. Such increase in crystallite size can be understood through swift heavy ion (SHI) irradiation induced interfacial intermixing. Several other groups have already reported SHI
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induced atomic mixing for metal-semiconductor interfaces [36]. SHI irradiation causes a thermal spike phenomena in which the the ion beam excites the electronic system at local site and electrons transfer their energy to phonon via electron-phonon coupling resulting in an
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increase in the local temperature which cause recrystallization and grain growth phenomenon in the irradiated structures [37]. From the SRIM calculations we have found that SHI loses its
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energy almost by electronic excitation and ionization of target atoms.
The full width and half maxima (FWHM) for metallic CoFe peak has been observed to decrease after the irradiation which can be due to either change in crystallite size or due to strain. Using the
single line analysis [38]
method, it is found that grain size ‘D’
corresponding to CoFe phase has increased from 43 nm to 57 nm after the irradiation. So, it
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looks that most of the decrease in FWHM corresponding to CoFe peak is related to the increase in crystallite size. The estimated strain has been found to be very small of the order of ∼10−3.
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3.2 AFM/MFM Study
Figs. 2 and 3(a-c) show the AFM images for unirradiated and irradiated structures of
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CoFe/nSi interface, respectively along with their line scan sectional profiles. All the images were scanned over a scan area of 2×2µm2. Line scan sectional analysis has also been done to estimate the grain size and root mean square (rms) surface roughness. Surface morphology of unirradiated CoFe/n-Si structure is shown in two and three-dimensional (2D and 3D) view (Fig. 2a, b) which shows the feature of clustered grains having size > 300nm. Clustering of grains can be understood due to clustering of smaller grains (with high surface area) to result in larger gains/cluster. The rms roughness has been estimated to be ~ 30 nm. Moreover, after
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irradiation (Fig. 3a, b), the surface shows the feature of isolated columnar shaped grains along a specific direction. The width and height of the grains has been found to vary from ~150 nm to ~ 250 nm and ~ 550 nm to ~ 800 nm, respectively. The rms surface roughness
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has found to be of ~ 50 nm. Such evolution of shape with increased rms surface roughness and grain size after irradiation can be understood due to irradiation induced recrystallization and intermixing phenomena at CoFe/n-Si interface. Thus, it is evident from the AFM images
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that the surface feature looks clearly different after irradiation. RMS surface roughness is the parameter in characterizing the morphology of a surface. The increased surface roughness
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after irradiation with fluence of 1×1012 ions/cm2 implies that the roughening process is predominant in our case which can be assumed due to evaporation of atoms from a hot surface heated by the ineleastic thermal spike. Similar observations have also been reported for Fe-Ni thin films [39].
Magnetic force microscopy (MFM) has been employed as a tool to analyze the
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domain pattern in the structures before and after irradiation. MFM images have been scanned over a scan area of 2×2µm2 in a tapping mode with a lift height of 90 nm to avoid any topographical interference due to van der Waal force or AFM signals. Since, it is well known
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that van der Waal forces are active only up to ~ 10nm [40]. A magnetized cantilever tip coated with 50 nm thin film of CoCr has been used to measure the dipole-dipole interaction
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between the magnetic tip and stray field of the sample. Figs.4 and 5 (a, b) shows the MFM images along with their line scan sectional
analysis for unirradiated and irradiated structures, respectively. 2D MFM image for unirradaited structure is shown in fig. 4(a) depicting the clear bright and dark contrast which quantifies the domain formation. Domain size (estimated from line scan profile, Fig. 4b) is found to be ~ 30 nm with a magnetic signal strength ~ 0.23˚. Moreover, the MFM image of irradiated structure shows the formation of fine domain structure ~ 10nm with an increased
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magnetic signal strength ~0.29˚.The observed decrease in domain size and increased magnetic signal strength after irradiation can be attributed due to the formation of more magnetic silicide phases as a result of irradiation induced intermixing.
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3.3 Magnetization characteristics
Magnetization measurement of the structures, i.e., M-H characteristics have been recorded for in plane configuration, i.e., magnetic field was applied along the plane of the
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interface by sweeping the applied external magnetic field from -15kOe to 15kOe and back. The diamagnetic contributions arising due to Si substrate and the glass sample holder were
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also subtracted before analysis. Several magnetic parameters (extracted from M-H data) were tabulated in table 2 for unirradiated and irradiated structures. Fig. 6 shows the M-H characteristics of unirradiated and irradiated CoFe/n-Si structures.
Magnetization, coercivity and remanence values were found to increase after the. It is
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significant to observe that irradiation causes the change in magnetization curve from antiferromagnetic coupled phase (in case of unirradiated structure) to ferromagnetic phase (in case of irradiated structure) with increased value of magnetization which could be understood
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due to irradiation induced modifications. It has been suggested that after irradiation the surface state pinning of domains are released via surface modifications and the saturation
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magnetization is increased [41]. The increased coercivity and saturation magnetization in the ion irradiation induced is attributed to the domain wall pinning due to irradiation induced effects and large collision energy transfer causing roughening of surface which provides pinning sites for inhibiting domain wall motion [39, 42]. The increased coercivity can also be understood due to changes in magnetic structure (e.g., domain size) [43]. Similar observation has also been found by Thomas et al.[39] in SHI irradiated Fe-Ni amorphous thin films where the increased coercivity on the irradiation has been related to the increased roughness on the irradiation [39].
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3.4 Electronic Transport Study Fig. 7 shows the I-V characteristics across unirradiated and irradiated CoFe/nSi interface. It is interesting to observe that for the irradiated structure, the magnitude of current
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has increased by two orders of magnitude for both the polarity of applied bias voltage as compared to unirradiated ones. Moreover, the I-V characteristics of unirradiated structure show the rectifying behaviour of typical Metal/Semiconductor (M-S) interface. However,
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after irradiation the nature of the I-V curve seems to be bit symmetrical. The significant increase of the current value after the irradiation is a significant and curious observation
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because usually in the irradiated interfaces with a non magnetic metal layer, the current is generally observed to decrease [24] due to irradiation induced disorders and defects leading to electron traps. In our case, such significant increase in current after irradiation by two orders of magnitude for CoFe/n-Si interfacial structure seems to be due to the intermixing at the interfaces which causes the formation of intimate interface and/or decrease in interface
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resistance which provides the easy path for the current flow across the interface. Our XRD results have shown the formation of silicide phases which can be in support to the observation. It seems that the defects created due to ion irradiation (causing decrease in
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interface resistance) or formed magnetic silicide phases are somehow favouring the paths for easy orientation of spins to cause less scattering and thus increase in current value. The
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observation seems interesting to us because usually the interface resistance increases heavily after the irradiation. Thus, it looks that irradiation induced interfacial intermixing causes decrease in the interface resistance to make the interface more intimate and thus increase in the current flow across the interface.
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4. Conclusions Effect of swift heavy ion irradiation on the magnetic, morphological and transport properties of CoFe thin films with nSi substrate has been studied.Structural study has
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revealed the formation of more silicide phases as a result of irradiation induced enhanced intermixing. The crystallite size was found to increase after the irradiation. Shape evolution from clustered grains to isolated columnar shaped grains has been observed after irradiation
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with increased rms roughness and grain size. Strong magnetic signal strength with fine domain structure has been found which significantly affects the magnetic behaviour.
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Magnetization characteristics have shown a change from antiferromagnetic coupled phase to ferromagnetic phase with significant increase in magnetization value and coercivity after the irradiation. Enhancement in current value has been observed for the irradiated structure by two orders of magnitude as compared to unirradiated ones. The observed changes in magnetic, morphological and transport characteristics have been understood in the realm of
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irradiation induced defect creation and/or interfacial intermixing across the interface to make the interface more intimate.
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Acknowledgements
We would like to thank the IUAC, New Delhi personnel (Dr. D Kanjilal, Dr. Indira
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Sulania) for their co-operation during the Irradiation and AFM/MFM measuremnets. One of the authors Arvind Kumar also acknowledges the financial support received from the University Grants Commission, New Delhi, India in the form of senior research fellowship (UGC-SRF).
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Figure Captions: Figure 1. XRD pattern of CoFe/n-Si interfacial structures of (a) unirradiated ones, (b)
region.)
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irradiated from a fluence of 1 × 1012 ions/cm2 (Insets show the expanded view of the selected
Figure 2. AFM image of the CoFe/n-Si structure (a) 2-dimensional view, (b) 3-dimensional
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view and (b) sectional analysis of the unirradiated structure.
Figure 3. AFM image of the CoFe/n-Si structure (a) 2-dimensional view, (b) 3-dimensional
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view and (b) sectional analysis of the irradiated structure.
Figure 4. MFM image of the CoFe/n-Si structure (a) 2-dimensional view and (b) sectional analysis of the unirradiated structure.
Figure 5. MFM image of the CoFe/n-Si structure (a) 2-dimensional view and (b) sectional
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analysis of the unirradiated structure
Figure 6. In-plane magnetization (M–H) characteristics of the CoFe/n-Si interfacial structure;
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before and after the irradiation from a fluence of 1 × 1012 ions/cm2 Figure 9. Current–voltage (I–V) characteristics of the CoFe/n-Si interfacial structure; before
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and after the irradiation from a fluence of 1 × 1012 ions/cm2.
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Table Captions:
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Table 1(a). XRD data of the unirradiated CoFe/p-Si interfacial structure. Table 1(a). XRD data of the irradiated CoFe/p-Si interfacial structure from a fluence of 1×1012ions/cm2.
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Table 2. Magnetization data of CoFe/n-Si interfacial structure before and after irradiation
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from a fluence of 1×1012ions/cm2.
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Table 1(a) Peak Intensity (counts) 42 19 16 25 67 2391
32.255 44.460 53.895 55.665 61.025 65.240 65.790 68.470
1.4185 1.3692
0.150 0.150
475 59487
Identified Peak
β-FeSi2 <022> CoFe <110> Fe3Si <311> CoFe <111> β-FeSi2 <602> CoFe <200>/β-FeSi2 <424> ε-FeSi <310> Si <400>
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Grain Size (nm) 55.5 43.1 59.7 60.2 61.8 63.3
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2.7731 2.0361 1.6998 1.6499 1.5172 1.4290
Peak width (2θ) 0.150 0.200 0.150 0.150 0.150 0.150
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d-value (Å)
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Angle (2θ)
63.3 -----
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Table 1(b)
Peak Intensity (counts) 380
Fe5Si3<111>,Co2Si<111>
Grain Size (nm) 15.4
44.465
2.0359
0.150
26
CoFe<110>
16.0
61.310
1.5108
0.150
85
Fe5Si3<302>,Co2Si<022>,βFeSi2<602>
0.100
9
68.745
1.3644
0.100
53963
68.995
1.3601
0.100
21638
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1.4794
Fe5Si3<400>,Co2Si<202>,βFeSi2<621>
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61.8 93.3
Fe5Si3<222>,Co2Si<222>,βFeSi2<711>
96.5
Si<400>
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62.755
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Peak width (2θ) 0.150
Identified Peaks
32.605
dvalue (Å) 2.7441
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Angle (2θ)
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Table 2 Before Irradiation
After Irradiation (1×1012ions/cm2)
Coercivity (Hc)
47Oe
89Oe
Magnetization at Hmax
2.61×10-4 emu
1.98×10-3 emu
Remanence (Mr)
0.57×10-5 emu
4.67×10-4 emu
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Magnetic Parameter
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Research Highlights
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1. CoFe/n-Si interfaces have been irradiated from SHI. 2. Significant increment has been observed in value of saturation magnetization of SHI irradiated CoFe/n-Si structure. 3. Current across the interface gets increased for SHI irradiated structures. 4. The observed results could be understood in the realm of SHI induced modifications.