IACS Institutional Repository

Foreword

Ghosh, Sanjukta

Kanjilal, D

Som, T

Indian J. Phys. 81(1) 83-87(2007) 
Low energy ion irradiation induced modifications in 
Co/Pt bi-layers 
Sanjukta Ghosh1, D Kanjilal2 and T Som1* 
institute of Physics, Sachivalaya Marg, Bhubancswar-751 005, Onssa, India 
2Inter-University Accelerator Centre, Aruna Asaf All Marg, New Delhi-110 067, India 
E-mail tsom(« lopb.res in 
Abstract : We report on 150 keV Ar and 250 keV Kr ion-irradiation induced modification of Pt(8 nm)/Co(85 nm)/Si bilayers. 
X Ray diffraction, Rutherford backscattering spectrometry and magneto optical Kerr effect measurements have been performed 
to study the changes in structural and magnetic properties of the technologically important Co/Pt bi-layer system due to low 
energy ion irradiation The combined result from XRD and RBS measurements strongly indicate that ion beam induced mixing 
takes place across the Co/Pt interface leading to the formation of Co-Pt phase and simultaneous sputtering of the Pt atoms from 
the top most layer Magneto optical Kerr effect data reveal the corresponding changes in the magnetic properties due to the 
formation of Co-Pt alloy 
Keywords : Low energy ion irradiation, Pt/Co bilayers. XRD. RBS. MOKE. 
PACS Nos. : 61.80 Jh. 75.50 Ss. '75.70.-i, 82 80.Yc 
1. Introduction 
The fundamental understanding of ion beam induced interface modification of new 
materials is not yet sufficient and thus it is of recent interest among the researchers. 
It can be mentioned that ion beams play an important role not only in atomic mixing 
across the interfaces but also in materials modification and phase formation [1-4]. Low 
energy ((-keV/amu) ion irradiated atomic mixing in materials is an interesting phenomenon, 
which is predominantly governed by elastic collisions of ions and target nuclei with 
nuclear energy loss Sn. 
According to the investigations done on uniformly irradiated Co/Pt system [5-9], 
the structural properties of ultrathin magnetic films and layered structures can be 
purposefully modified by light ion irradiation in an energy range of 5-150 keV. The 
basic motivation for ion irradiation of Co/Pt system arises from the possibility of 
modifying the perpendicular magnetic anisotropy (PMA) through irradiation-induced 
intermixing at the interfaces [10,11]. Also, these investigations are of interest for future 
applications to ultra high-density magnetic recording media since this technique may be 
a way to produce magnetic nanostructures with strong PMA without affecting, the 
'Corresponding Author © 2007 IACS 
84 Sanjukta Ghosh, D Kanjilaland TSom 
surface topography. We should mention here that for Co/Pt bi/multi-layers, apart from 
the composition, crystallinity, and the grain sizes; the magnetic properties are often 
strongly dependent on surface and interface structures [12]. Based on the above facts, 
one can approach towards systematic tailoring of the magnetic properties of the Co/ 
Pt layered structures by low energy ion irradiation induced interface modification. 
In this paper, we present the correlation of changes in structural and magnetic 
properties of 150 keV Ar and 250 keV Kr irradiated Pt(8 nm)/Co(85 nm) bilayer 
samples grown on native oxide covered Si substrates. The samples were characterized 
by X-ray diffraction (XRD), Rutherford backscattering spectrometry (RBS) and magneto 
optical Kerr effect (MOKE) measurements. The results are attributed to the ion beam 
induced interface modification of Co/Pt bi-layers. 
2. Experimental 
[Pt (8 nm)/Co (85 nm)/Si] thin films were deposited using ion beam sputtering 
technique under a vacuum of -1CT6 mbar. The films were irradiated by 150 keV Ar and 
250 keV Kr ions obtained from an electron cyclotron resonance (ECR) ion source at 
Inter University Accelerator Centre. RBS was performed using 1.7 MeV He* ions at 
IOP. XRD was employed for phase analysis, while MOKE was used for studying 
magnetic properties of the Co/Pt system before and after ion irradiation. 
3. Results and discussion 
All irradiation induced significant changes are systematically shown in Figures 1 and 
2 (for XRD), Figures 3 and 4 (for RBS), and Figure 5 (for magnetic studies). 
Careful analyses of the XRD patterns (for both the ions)- clearly show that with 
irradiation some new peaks evolve, which match well with those corresponding to the 
CoPt3 phase. This strongly indicates the formation of Co-Pt phase as a result of ion 
2000 
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Figure 1. XRD patterns of Co/Pt bilayers 
irradiated with 250 keV Kr ions showing 
crystalline peaks of old and new phases with 
fluence* 
Figure 2. XRD patterns of Co/Pt bilayers 
irradiated with 150 keV Ar ions showing 
crystalline peaks of old and new phases with 
fluence. 
Low energy ion irradiation induced modifications in Co/Ptbi-layers 85 
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induced mixing of the Co and Pt atoms across the interface. From SRIM [13] 
simuiation it is seen that both the ions stop deep in the Si substrate penetrating both 
Pt and Co layers. The irradiation process, therefore, involves recoils of Pt and Co 
atoms, which are limited to one or two atomic distances leading to the substitution of 
Co-»Pt, Pt->Co [12]. Thus, if we consider this process as a "soft" irradiation process 
then the original crystallographic structure and the microstructure should remain nearly 
unaltered. From the XRD patterns, it is observed that although some new peaks have 
evolved due to mixing across the interface, the original peaks still remain unaltered. 
Therefore, it can be inferred that although short range motions are most likely 
due to irradiation, the appearance of the new XRD peaks confirm that long range 
motions also take place giving rise to new phase formation due to mixing. Essentially 
to have a better understanding of the mixing process, we need to consider the 
probability of chemical effect superimposed on the pure ballistic ion mixing. It has 
86 Sanjukta Ghosh, D Kanjllal and T Som 
already been reported [11] that Pt and Co atoms are being miscible, which favours 
atomic mixing across the interface. However, at this stage we are unable to provide the 
exact information whether any chemically ordered L10 or L12 phase has been formed 
or not. 
Let us now go on to discuss our RBS results (Figures 3 and 4) where we 
clearly notice that alloy formation is accompanied by the drastic loss of Pt atoms with 
increasing ion mass and fluence. There are also shifts in the Si and Co signals. All 
the observed changes in the RBS spectra are indicative of the fact that Pt atoms get 
sputtered. Now if we try to understand the ion irradiation effects on the Pt layer, we 
can consider the following two possibilities : (a) sputtering of Pt atoms leads to a 
situation so that it is present only in the form of Co-Pt alloy, (b) reduction in Pt 
thickness from the top layer accompanied by the formation of Co-Pt alloy. However, 
combining the results of XRD and RBS, we observe that the XRD patterns do show 
the presence of Pt, which essentially rules out the first possibility mentioned above. 
These observations match well with the SRIM simulation towards calculation of the 
sputtering yield of Pt due to Ar and Kr ion-irradiation. 
To correlate the structural properties with the change in magnetic properties of 
uniformly irradiated Co/Pt bi-layer films, we have employed MOKE measurements to 
study the hysteresis loop as a function of ion fluence. The dependence of coercivity 
on the ion fluence has been plotted in Figure 5. 
These plots show that there is a definite increase in both the values. The origin 
of the hysteresis loop is expected to be due to the strong magnetic nature of the Co 
atoms. It is also known that the value of coercivity saturates for Co layer of 40 nm 
or 50 nm [12]. Therefore, increase in coercivity, could result only due to the formation 
of the Co-Pt phase. Together increase in coercivity and in remanence may be 
interpreted as a direct consequence of interface mixing of Co atoms and Pt atoms. 
However, formation of chemically ordered phase is expected to give rise to a large 
increase in coercivity value, which is not observed in our case. This reveals that 
although Co-Pt alloy is formed, it is not in a chemically ordered state. 
4. Conclusions 
In conclusion, 150 keV Ar and 250 keV Kr ion-irradiation of Pt(8 nm)/Co(85 nrn) grown 
on Si substrates lead to the formation of Co-Pt alloy phase through ion beam mixing 
across the interface. This is followed by the increase in coercivity and remanence, 
which can also be attributed to the formation of Co-Pt alloy phase. Further studies are 
underway to understand the change in magnetic properties in the context of structural 
evolution in the Co/Pt bi-layer system with ion irradiation under varying parameters. 
Acknowledgments 
Authors gratefully acknowledge the help of Prof. Ajay Gupta (UGC-DAE CSR, Indore 
Centre) during this work. We would also like to thank the low energy ion beam facility 
group members at IUAC, New Delhi and the Pelletron operators at IOP, Bhubaneswar. 
Low energy ion irradiation induced modifications in ColPt bi-layers 87 
References 
[1] Y-T Cheng Mater. Sci. Rep. 5 45 (1990) 
[2] G S Was Prog. Surf. Sci. 32 211 (1990) 
[3] S Dhar, T Som, Y N Mohapatra and V N Kulkarni Appl. Phys. Lett. 671700 (1995) 
[4] S Dhar, T Som and V N Kulkarni J. Appl. Phys. 83 2363 (1998) 
[5] P F Carcia, SI Shah and W B Zeper Appl. Phys. Lett. 56 2345 (1990) 
[6] C Chappert, H Bernas, J Ferri, V Kottler, J P Jamet, Y Chen, E Cambril, T Dfvolder, F Rousseaux, V Mathet 
and H Launois Science 280 1919 (1998) 
[7] T Devolder, C Chappert, Y Chen, E Cambrill, H Bernas, J P Jamet and J Ferr6 Appl. Phys. Lett. 74 3383 
(1999) 
[8] T Devolder Phys. Rev. B62 5794 (2000) 
[9) C T Rettner, S Anders, J E E Baglin. T Thompson and B D Terris Appl. Phys. Lett. 80 279 (2002) 
[10] G S Chang, Y P Lee, J Y Rhee, J Lee, K Jeong and C N Whang Phys. Rev. Lett. 87 067208 (2001) 
[11] A Traverse, M G Le Boite and G Martin Europhys. Lett. 8 633 (1989) 
[12] J Fassbender, D Ravelsona and Y Samson J. Phys. D: Appl. Phys. 37 R179 (2004) 
[13] http://www.srim.org 


English

Low Energy Ion Irradiation Induced Modifications in Co/Pt Bi-Layers

Sarkar, S K

Kumar, Anil

Ahmed, Iqbal

Gupta, M M

Prasad, M V S N

De, Arun K

Das, J

Indian J. Phys. 81(7) 647-653(2007) 4p 
Rain attenuation at 13 GHz over the line of sight path 
situated between Panihati and Barrackpore in eastern India 
S K Sarkar*, Anil Kumar, Iqbal Ahmed, M M Gupta and M V S N Prasad 
Radio and Atmospheric Sciences Division, National Physical Laboratory, 
Dr K S Knshnan Road, New Delhi-110 012, India 
and 
Arun K De and J Das 
Electronics and Communication Sciences Unit, Indian Statistical Institute, 
203 B T Road, Kolkata-700 108, India 
E-mail sksarkar@mail nphndia ernet in 
akde^isical ac in 
Received 4 August 2006, accepted 19 June 2007 
Abstract : In this paper, results on attenuation measurements at 13 GHz under rainy conditions over another microwave link 
propagation path (Panihati-Barackpore) located over Kolkata have been presented The link suffers large attenuation due to heavy 
rain The communication link belongs to an operational agency The microwave amplitude variation was monitored and ram rate 
measurements were made on twenty four-hour basis by a fast response rain gauge in the monsoon months during 2005 The 
observed microwave radio signal was characterized with a steady signal level - -42 to -43 dBm with fade depth ~1 to 2 dB 
under non-precipitation condition The results on attenuation have been studied in the light of simultaneous rain rate observations 
The results on attenuation of radio wave under rainy condition are useful to design and installation of future communication links 
both for terrestrial and earth space paths in the regions in India where we have rain characteristics similar to Indian eastern coastal 
station The probability distribution of attenuation and rain rates have been deduced from measured observations 
Keywords : Microwave link, terrestrial path, rain rate, rain attenuation 
PACSNo. : 9410 -s 
1. Introduction 
In present days scenario of radio communication it has become necessary to provide 
more channels as per the demand of users. Using higher radio frequencies particularly 
above 10 GHz can meet such demand. In a tropical country, we have regions where 
heavy rainfall occurs with high rain intensity. The use of higher frequency in radio wave 
gives rise to problems like attenuation of radio wave due to rain [1-4] as well as 
performance deterioration of the links. Though efforts are on to generate more database 
'Corresponding Author © 2 0 0 7 , A C S 
648 S K Sarkar Ami Kumar Iqbal Ahmed, M M Gupta, MVSN Prasad, Arun K De and J Das 
of rain attenuation in microwave frequencies to develop model for estimation of 
attenuation due to rain over this part of the world But, still there is dearth of the 
measured results on rain attenuation of radio wave in the tropical regions of India. 
Recently, some systematic simultaneous monitoring of communication links (having 
different path length) belonging to different operational agencies at 13 GHz and 18 GHz 
and measurements of rain rate were undertaken in Indian eastern sector by using 
terrestrial communication links and some very useful results on attenuation were derived 
[5-7] Still more measurements on radio signal above 10 GHz are needed over a vast 
country like India where we have varied total rain fall over different places [8] for better 
planning of microwave communication links both for terrestrial and earth space paths 
[9] 
The communication link operating at 13 GHz situated between Panihati and 
Barackpore located in Kolkata (22 32° N, 88 27° E) region was monitored on twenty-four 
hours basis during July-August 2005 A rapid response rain gauge was also put on 
operation on continuous basis to measure the rain intensity It has been seen that the 
signal level is characterized with steady signal of the order —42/-43 dBm with a fade 
depth -1 -2 dB under non-prec.^itation condition The communication link used to 
exhibit heavy loss of signal around 20 to 25 dB during heavy rain Attenuation as high 
as 31 dB has been observed during torrential rain The analysis of the results on signal 
levels and rain rates has yielded the percentage of time and its association with the 
particular level of attenuation during monsoon months of July and August over Kolkata 
region These results are useful to design and install future links over the regions in 
Indian locations where we have this amount of rainfall 
2. Source of data 
A strip chart recorder was used to recora the radio signal level of the communication 
link situated between Panihati and Barackpore having path length of 8 km The link 
belongs to Kolkata Telephones of BSNL The amplitude variation measurements were 
made on twenty-four hour basis during July-August 2005 at Panihati The transmitter is 
situated at Barackpore and the receiver is situated at Panihati The rain rate 
measurements were carried out over Kolkata during July-August 2005 by a rapid 
response rain gauge having integration time -10 seconds The rain gauge was located 
near the receiver site The rapid response rain gauge monitors rain fall rate automatically. 
The program controls the sampling, storing and printing of the data of rain intensity. 
3. Results and discussions 
The probability distribution of the rain rate has been derived from the rain rate 
measurements of ali the rain events taken place during July-August 2005 over Kolkata. 
It would have been more useful and served the purpose better if we could have installed 
few more rain gauges along the path of the radio system But it is very difficult to do 
so because lot of infrastructure facilities is required for such purpose More over the 
long-term probability distribution of ram rate has been established. If long-term probability 
Rain attenuation at 13 GHz over the line of sight path etc 649 
distributions of rain rate are estimated from rain rate measurements over different 
locations in a region, we find that there is variation with in 5% in probability distribution 
of rain rate from one location to another location But, if case (rain event) by case (rain 
event) over different locations is taken then there exit's some differences 
The fast response rain gauge has been used to obtain rainfall rate and correlate 
them with variations in signal attenuation All the rain events occurred during the course 
of rain measurements over Kolkata during July-August 2005 were analyzed Different 
rain event was characterized with different variation of rain rate The minimum rain rate 
was recorded with fast response was ~4 mm/hr while the maximum rain rate was 
around -156 mm/hr The rain rate having low integration time is usually characterized 
with slow and fast variation The rain rate variation is similar to the amplitude variation 
of the radio signal Such amplitude variation of radio signal under rainy condition 
provides information of the fade dynamics of radio signal Fade dynamics are important 
for the determination of fade margins as well as eventually fhe performance of the link 
The cumulative distribution of rain rate, which has been obtained from rain events 
occurring during July-August 2005, is presented in Figurel It is seen that the rain rate 
-18 mm/hr exceeds for 50% of the time and rain rate -55 mm/hr exceeds for 5% 
of time The maximum ram rate obtained was -156 mm/hr during this period 
i i i ••" i ' i i i i i i ••"' i i ' ' 
0 01 01 2 5 10 30 50 70 90 95 98 100 
PERCENTAGE OF TIME ORDINATE EXCEEDED 
Figure 1. Probability distribution of rain rate during July-August 2005 over Kolkata 
The radio signal variation measured during different rainy conditions in July-August 
2005 over Panihati-Barackpore communication link is presented in Figures 2-5 It is 
seen that the radio signal exhibits slow and rapid amplitude variations The cumulative 
distribution of the signal level has also been obtained during ram events and just prior 
to start of the ram. It is seen from all such measurements that the signal level varies 
from —42 dBm to -73 dBm It is shown in Figures 2-5 that maximum attenuation 
during rain varies from - 6 dB to 31 dB. 
E 
E 
UJ 80 
< 
I " 
20 
650 S K Sarkar, Anil Kumar, Iqbal Ahmed, M M Gupta, MVSN Prasad, Arun K De and J Das 
•'fab: 
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1
 f 13 OH/ 
EYELU dBm 
•C 
i ; 
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Figure 2. (a, b) Typical amplitude variations over the microwave link under rainy condition where maximum 
attenuation is 10 dB (Figure a) and 6-8 dB (Figure b). 
Figure 3. (a, b) Typical amplitude variations over the microwave link under rainy condition where maximum 
attenuation is 18 dB (Figure a) and 15 dB (Figure b). 
It has been observed that rain occurrence can be categorized in four types such 
as drizzling, moderate rain, heavy rain and torrential rain. The variation of signals shown 
in Figures 2-5 is found to be associated with all types of rain. It has been found that 
the attenuation around -1 -5 dB are associated with low rain rate (from few mm/hr to 
20 mm/hr) while attenuation level -5 -10 dB are associated with moderate rain (30 
mm/hr to 60 mm/hr). The attenuation 15-20 dB is associated with heavy rainfall (from 
70 mm/hr to 120 mm/hr) while the attenuation above > 25 dB is associated with 
torrential rain (above 135 mm/hr). The results on attenuation have been derived by 
taking the difference of the normal signal level —42.5 dBm and the instantaneous 
signal level in dBm. If the observed signal level at a time is —57.5 dBm then the 
estimated attenuation is 15 dB. 
Rain attenuation at 13 GHz over the line of sight path etc. 651 
Figure 4. (a, b) Typical amplitude variations over the microwave link under rainy condition where maximum 
attenuation is 24 dB (Figure a) and 21 dB (Figure b). 
Figure 5. (a.b) Typical amplitude variations over the microwave link under rainy condition where maximum 
attenuation is 31 dB (Figure a) and 27 dB (Figure b). 
The probability distribution of the observed attenuation is presented in Figure 6. 
It is seen that under normal condition the measured radio signal level is -42.5 dBm. 
It is seen in Figure 6 that the attenuation increases with rain rate. The attenuation of 
-5.5 dB is found to be associated with rain rate -20 mm/hr while the attenuation -10 
dB corresponds with rain rate -36 mm/hr and attenuation of -31 dB correlates with 
rain rate -158 mm/hr. 
The estimated results on specific attenuation at different rain rate derived by 
using the ITU-R relation (earlier known as CCIR), A = 0.02515R1'164 [10] have been 
compared with the measured results. The specific attenuation in dB/km deduced by 
using ITU-R is presented in Table 1. The specific attenuation (dB/km) over the link path 
(measured total attenuation, dB is divided by the link path of 8 km length) is also 
652 S K Sarkar, Anil Kumar, Iqbal Ahmed, M M Gupta, MVSN Prasad, Arun KDe and J Das 
Panlhati - Barackpore (13GHz) 
Figure 6. Probability distribution of attenuation results at 13 GHz 
presented in Table 1 for comparison with ITU-R results. The specific attenuation 
deduced by using ITU-R and from the measured total attenuation over the path of 
length of 8 km shows good agreement upto rain rate 60 mm/h. 
Table 1. Specific attenuation in dB/km deduced by using ITU-R and from measured attenuation results 
Ram rate in 
10mm/hr 
20 mm/hr 
30 mm/hr 
60 mm/hr 
100 mm/hr 
130 mm/hr 
158 mm/hr 
ITU-R 
0 367 dB/km 
0.822 dB/km 
1.352 dB/km 
2.953 dB/km 
5.352 dB/km 
7.263 dB/km 
9 115 dB/km 
From measured 
attenuation results 
0 312 dB/km 
0.754 dB/km 
1.038 dB/km 
3.008 dB/km 
3.875 dB/km 
4.000 dB/km 
4.062 dB/km 
The specific attenuation deduced by ITU-R overestimates as compared to specific 
attenuation obtained from measured total attenuation. This difference is due to the fact 
that the path length of the link is of 8 km which is quite large. For higher rain intensity 
i.e. when the rain rate is from 60 mm/hr to 160 mm/hr, the effective rain path length 
is usually less and is of the order from 3.5 km to 5.5 km depending on rain intensity. 
If the effective rain path length of rain is taken around 3.5 km to 5.5 km for different 
Ram attenuation at 13 GHz over the line of sight path etc 653 
high rain intensity then specific attenuation derived from the measured total attenuation 
results also becomes comparable with the ITU-R results 
It is therefore very essential and important to carry out measurements of 
attenuation of radio wave at frequency above 10 GHz over communication links having 
varying path length whenever and wherever opportunities are available in different 
geographical regions over the Indian sub continent 
4. Conclusion 
The measured results on radio signal and rain rate indicate that during heavy rain, the 
signals are attenuated heavily as high as > 25 dB These measured results are useful 
to develop a rain attenuation model for the Indian tropics It is seen from this study 
as well as from earlier studies that the ITU-R model is not fully valid in India for 
different type of rainy condition. 
Acknowledgment 
This work has been carried out as a part of a project sponsored by the Department 
of Science and Technology (DST), Govt of India, New Delhi The authors are grateful 
to DST for sponsoring the project Facilities have been provided by the Bharat Sanchar 
Nigam Limited (Kolkata Telephones) for the monitoring work of their communication link. 
The authors are very thankful to BSNL particularly to the personnel at Panihati link site 
for their support, cooperation and help to carry out the field strength measurements 
during July-August 2005 The authors wish to express their gratitude to those of 
Kolkata Telephones (BSNL), National Physical Laboratory, New Delhi and Indian 
Statistical Institute, Kolkata who were involved directly or indirectly with this work 
References 
[1] H E Green IEEE Antennas and Propagation Magazine 46 31 (2004) 
[2] A D Panagopoulos P M Arapoglou and P G Cottis IEEE Communication Surveys and Tutonals (3rd Quarter) 
October (2004) 
[3] G Brussard and P A Watson Atmospheric Modelling and Millimetre Wave Propagation (UK Chapman 
and Hall) (1995) 
[4] K I Timothy, N C Mondal and S K Sarkar International Journal of Satellite Communication 16 53 (1998) 
[5] S K Sarkar, Rajesh Kumar and J Das Indian J Phys 77 271 (2003) 
[6] S K Sarkar, R Kumar, I Ahmad, M M Gupta, M V S N Prasad, J Das and A K De Indian J Radio Space Phys 
33 176(2004) 
[7] S K Sarkar, Iqbal Ahmad, M M Gupta, M V S N Prasad, J Das, A K De and Rajesh Indian J Phys 79117 
(2005) 
[8] Climatological Tables of Observatories in India India Meteoropgical Department (1988 ) 
[9] S K Sarkar Guest Editorial Indian J Radio Space Phys 34 297 (2J05) 
[10) ITU-R Recommendation (1994) 


Rain Attenuation at 13 GHz Over the Line of Sight Path Situated between Panihati and Barrackpore in Eastern India

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Indian J. Phys. 81 (7), 739-742 (2007) 
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Notes for Contributors 741 
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[e] U Fano and A R P Rao Atomic Collisions and Spactta (New Yoik Academic) Vol 1, Ch 2, Sec 4, p 25 
(1986) 
[7] T Atsumi, T Isihara, M Koyama and M Matsuzawa Phys Rev A42 6391 (1990) 
[ 111 T Le Brun, M Lavolled and P Morin X ray and Inner Shell Proc esses (AlP Conf Proc 215) (eds) T A 
Canson, M O Krause and S Manson (New York AIP) p 846 (1990) 
[14] S B Hansen, K B MacAdam and LG Gray 12th Int Conf on Atomic Physics (Ann Arbor) Abstracts px-
12(1990) 
[15] H Pauly Atomic and Molecular Beams Methods (eds) G Scoles, D Basst, U Buck and D Lame (London 
Oxford University Press) p 83 (1988) 
[19] W Fntsch (private communication) (1988) 
[21] K B MacAdam (to be published) (1991) 
[23] T Roy PhD Thesis (University of Calcutta, India) (1992) 
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Behaviour of Cosmic Ray Daily Variation and Solar Activity on Anomalous Days

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