We report an unconventional and promising route to self-assemble distributed
superconductor-ferromagnet-superconductor (S-F-S) Josephson Junctions on single
crystal [100] MgO. These structures consist of [110] epitaxial nano-plaquettes
of Fe covered with superconducting NbN films of varying thickness. The S-F-S
structures are characterized by strong magnetoresistance (MR) anisotropy for
the in-plane and out-of-plane magnetic fields. The stronger in-plane MR
suggests decoherence of S-F-S junctions whose critical current follows a
(1-T/Tc) and (1-T/Tc)1/2 dependence for T Tc and T<<Tc respectively, in
accordance with the theory of supercurrent transport in such junctions.Comment: 9 pages, 4 figure

Buzdin A. I.

R. C. Budhani

S. K. Bose

English

arXiv

We report an unconventional and promising route to self-assemble distributed superconductor-ferromagnet-superconductor (S-F-S) Josephson junctions on single crystal [100] MgO. These structures consist of [110] epitaxial nanoplaquettes of Fe covered with superconducting NbN films of varying thickness. The S-F-S structures are characterized by a strong magnetoresistance (MR) anisotropy for the in-plane and out-of-plane magnetic fields. The stronger in-plane MR suggests decoherence of S-F-S junctions whose critical current follows a (1-T/Tc) and (1-T/Tc)1/2 dependence for T ≈ Tc and T  Tc, respectively, in accordance with the theory of supercurrent transport in such junctions

Bose, S. K.

Budhani, R. C.

Publications of the IAS Fellows

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Robust coupling of superconducting order parameter in a
mesoscale NbN-Fe-NbN epitaxial structure
S. K. Bose and R. C. Budhani∗
Condensed Matter - Low Dimensional Systems Laboratory, Department of Physics,
Indian Institute of Technology Kanpur, Kanpur - 208016, India
(Dated: July 9, 2009)
Abstract
We report an unconventional and promising route to self-assemble distributed superconductor-
ferromagnet-superconductor (S-F-S) Josephson Junctions on single crystal [100] MgO. These struc-
tures consist of [110] epitaxial nano-plaquettes of Fe covered with superconducting NbN films of
varying thickness. The S-F-S structures are characterized by a strong magnetoresistance (MR)
anisotropy for the in-plane and out-of-plane magnetic fields. The stronger in-plane MR suggests
decoherence of S-F-S junctions whose critical current follows a (1 − T/Tc) and (1 − T/Tc)1/2 de-
pendence for T ≈ Tc and T ≪ Tc respectively, in accordance with the theory of supercurrent
transport in such junctions.
PACS numbers: 74.81.Cp, 75.75.+a, 74.50.+r
∗Electronic address: rcb@iitk.ac.in
1
The tunneling of Cooper pair order parameter through a thin barrier separating two bulk
superconductors, as predicted by Josephson[1] and seen subsequently in thin film junctions[2]
has had a far reaching impact on physics and technology. The applications of Josephson
Junctions (JJs) range from sensors for ultralow magnetic fields and weak electromagnetic
radiation[3], millimeter wave resonators[4], programmable voltage standards[5], supercon-
ducting flux qubits[6], etc. The physics of JJs changes remarkably when the barrier material
is a ferromagnet[7, 8]. Interesting effects with rich underlying physics and many promis-
ing applications are expected in superconductor-ferromagnet (S-F) hybrids of nanoscale
dimensions[9]. In recent years nanoscale S-F structures have been synthesized using the
conventional approaches of nano-lithography such as electron beam patterning, atomic force
microscopy, and focused ion beam milling.
In this letter we report observation of a giant anisotropic magnetoresistance (MR)
in self-assembled nanometer-scale-distributed junctions of Fe and superconducting NbN.
Our methodology of synthesis utilizes stress-tuned Volmer-Weber (VW) type[10] plaquette
growth of Fe on [100] MgO, whose electrical connectivity is tuned by NbN layers of different
thickness (dNbN) deposited on top of the VW template. A KrF excimer laser (λ =248 nm)
based pulsed laser ablation technique was used to deposit the nanostructured Fe and epi-
taxial NbN thin films as described in our earlier works[11, 12]. In brief, the growth and the
post-growth annealing temperature for Fe plaquettes was ∼700 ◦C whereas the NbN layer
was deposited at 200 ◦C to inhibit the formation of iron nitride at NbN-Fe interfaces. The
nominal thickness of the Fe base layer is 40 nm, whereas dNbN =10, 20, and 30 nm have
been used.
The scanning electron micrographs (SEM) of the samples as shown in Fig. 1(a & b)
reveal that the Fe template consists of nearly perfect square plaquettes of ≈100 × 100 nm2
2
FIG. 1: (a & b) Scanning electron micrographs of the Fe(40 nm) / NbN(30 nm) sample. The top
of nearly perfect square Fe plaquettes and the inter-plaquette space is covered with epitaxial NbN.
(c) A typical atomic-force-microscope line scan showing the height of Fe plaquettes. (d) A sketch
showing the two distinct parallel paths for the flow of supercurrent through the S-F-S hybrid. Path
♯1 is for the NbN-Fe-NbN junctions and ♯2 for current flow through the percolating backbone of
NbN.
area, separated by ≈20 nm gaps and are aligned along MgO [110] direction. The NbN grows
epitaxially on the plaquettes and the inter-plaquette gaps. This has been confirmed with x-
ray diffraction and x-ray fluorescence mapping of niobium. A typical atomic force microscope
(AFM) line scan shown in Fig. 1(c) reveals that the requirement of mass conservation makes
the Fe nano-plaquettes thicker than the programmed thickness of ≈40 nm. Due to this
unique structure, the flow of supercurrent in this S-F hybrid occurs through two parallel
channels as shown in Fig. 1(d). One of these paths (i.e. ♯1) is the double S-F-S junction, in
which supercurrent goes vertically up through thin Fe layers into the intra-plaquette NbN
and then comes down, again through the Fe. The other route (♯2) is the thin percolating
backbone of NbN in the inter-plaquette spaces. As will be shown later, the path ♯1 involving
3
4 8 12 16
0
4
8
12
0.0
0.2
0.4
20 nm R
 
(kΩ
)
30 nm
30 nm NbN
R
 
(kΩ
)
T (K)
10 nm
 
MgO (001) 
Fe plaquettes (40 nm)  
NbN (10 / 20 / 30 nm) 
FIG. 2: (Color online) The temperature dependence of resistance of the 10, 20 , 30 nm NbN covered
Fe plaquettes (left Y-scale) and 30 nm thick pure NbN film (right Y-scale). The onset temperature
of superconductivity (Tonset) for 30 nm hybrid is the same as of plane 30 nm NbN film, whereas
the Tc and ∆Tc are affected significantly when the NbN cover layer thickness is reduced.
double S-F-S junctions is of greater significance for supercurrent transport in these hybrids.
Fig. 2 shows the superconducting (SC) transition measured across bridges of ≈75 × 1300
µm2 area, created by Ar+ ion milling of the S-F hybrids. For comparison the SC-transition
of a plane 30 nm thick NbN film is also shown in Fig. 2. A significant drop in transition
temperature (Tc) and increase in the width of the transition (∆Tc) along with a gain in
the normal state resistance (Rn) is seen as dNbN is reduced from 30 to 10 nm. In fact, the
Tc drops nearly twice as fast for the hybrid (≈49%) in comparison to the drop seen in a
plane NbN film (≈24%)[13] as the film thickness dNbN is reduced from 30 to 10 nm. The Rn
increases with decrease in temperature, but remains lower than the quantum resistance for
Cooper pairs (RQ = h/4e
2 ∼6.4 kΩ/), above which a superconductor-insulator transition
is seen in granular films[14]. We also notice that although the Rn of the hybrid with 30 nm
4
0.2 0.4 0.6 0.8 1.0
0.0
0.5
1.0
1.5
0.2 0.4 0.6
0.21
0.28
0.35
30 nm  ∆(0) ~2.5 meV
 
I C 
(m
A
)
t (=T/T
c
)
 
Ιc α (1-t)1/2 
20 nm  ∆(0) ~1.9 meV
Ιc α (1-t) 
I c(
m
A
) 20 nm
 
 
 
 
t
 
FIG. 3: (Color online) Critical current (Ic) variation with temperature for the 30 and 20 nm NbN
covered Fe plaquettes. Two distinct regimes of temperature dependence can be seen in the figure.
At t > 0.7 it goes as ∼ (1-t) and for t < 0.6 the dependence is of the type (1-t)1/2. Inset shows
the (1-t)1/2 dependence of the 20 nm NbN sample over an expanded scale.
thick NbN is three orders of magnitude higher compared to the Rn of pure NbN film, the
onset temperature of superconductivity (Tonset ≈14.7 K) remains nearly the same in the
two cases. The ∆Tc (≈2 K) of this hybrid, however is seven times higher than of pure NbN
(≈0.3 K). This observation suggests that superconductivity in inter-plaquette epitaxial NbN
sets in at ≈14.7 K, but the realization of the zero-resistance state depends on the strength
and phase factor of the supercurrent through the double S-F junctions and the narrow
constrictions of the epitaxial NbN backbone as sketched in Fig. 1(d).
In Fig. 3 we show the critical current (Ic) of two films (dNbN ≈20 & 30 nm) in the
t(=T/Tc) range of 0.2 to 1.0. The Ic of a proximity coupled S-N-S junction depends on the
number of Andreev bound states (ABS) in the normal metal (N) spacer. If the spacer is a
5
ferromagnet, then the spectrum of ABS is affected by the extent of exchange splitting of its
conduction band[15]. It has been shown that as long as the ferromagnet thickness dF ≪ ξF ,
the coherance length in F layer, defined as ξF =
√
~D/I, where D is the diffusion coefficient
and I the exchange splitting of the ferromagnet, the maximum current (Ic) through the
junctions is given as[7];
Ic(T ) =
32
√
2(∆/e)
RN
F (∆/T ) y exp (−y) sin (y+π/4) (1)
where,
y =
dF
ξF
(
2I
πTc
)1/2
Here, F(∆/T) has limiting values of pi
128
(
∆/Tc
)
for T ≈ Tc and 0.071 for T ≪ Tc. For a
given dF and a BCS temperature dependence of the gap parameter ∆(T) ≃ 3.2kBTc
√
1− t,
we get Ic(T ) ∼ (1 − t) for T ≈ Tc and ∼
√
1− t for T ≪ Tc. This temperature depen-
dence of Ic is similar to that predicted by Ambegaokar and Baratoff for weakly coupled
granular superconductors provided the suppression of gap parameter by supercurrent is not
significant[16, 17]. The data in Fig. 3 have been fitted with the (1− t) and √1− t depen-
dence for 1.0 & t & 0.7 and 0.6 & t & 0.2, respectively. This dependence of Ic (t) fits well
over the entire temperature range and leads to ∆(0) values of 2.5 meV for 30 nm and 1.9
meV for 20 nm case respectively, which matches well with the BCS energy gap [2∆(0) =
3.5kBTc] values of 2.2 meV (30 nm) and 1.93 meV (20 nm). To further point out the quality
of fit, the inset shows the magnified version of
√
1− t dependence of Ic for the 20 nm case.
In order to address the magnetic field (H) dependence of the coupling between the NbN
layers, we investigate the state of the magnetization (M) of the Fe plaquettes. In Fig. 4 we
show the M-H plots taken at 5 K, with H applied along in-plane (H‖) and out-of-plane (H⊥)
direction. The out-of-plane M-H shows that the moment of Fe plaquettes is in the plane of
6
-4 -2 0 2 4
-10
0
10
H⊥ 
 
 
M
 
(10
-
4 
e
m
u
)
B (kG)
Hc ~60 Oe
H|| 
30 nm
0
50
100
 
 
H|| 
B (kG)
R
-
R
0/ 
R
0 
 
(%
)
 
-2 -1 0 1 2
0
25
50H⊥ 
 
 
 
 
1.6 K
 
FIG. 4: (Color online) Magnetization of the 30 nm NbN covered Fe plaquettes measured at 5 K
with in-plane (H ‖) and out-of-plane (H⊥) fields. Inset shows the magnetoresistance measured at
1.6 K. The positive MR is twice as large for the H ‖, which is distinctly different from the behavior
of a plane superconducting film where one sees a much stronger effect of H⊥.
the film in agreement with the earlier studies[18].
The in-plane anisotropy of magnetization as shown in Fig. 4 affects Josephson coupling
in the S-F-S junctions significantly as seen through measurements of MR, carried out at
currents exceeding the critical current (Ic). While details of the angular dependence of MR
will be presented elsewhere, in the inset of Fig. 4 we show the MR measured at 1.6 K
as a function of applied field strength. It is clear from the figure that the positive MR
at a peak field of 1500 Oe is higher by a factor of two when the field is in the plane of
the film. This is a very striking result in view of the fact that for thin superconducting
films it is the out-of-plane field which contributes significantly to MR, due to a copious
motion of Abrikosov vortices, which is unlikely to be arrested by a possible weak pinning
7
by the inhomogeneous magnetization of Fe plaquettes. A stronger response to the H‖ seen
here is consistent with the fact that the phase of the tunneling order parameter is affected
significantly when the field is in the plane of the junctions. We also note that the MR for
H‖ shows two distinct cusps at ±113 Oe, whose position is higher than the coercive field
(Hc ∼60 Oe) as measured by SQUID at 5 K. This observation can be understood in the
context of number of magnetic entities actually taking part in the magnetization reversal
process. Magnetization measurement reflects the total average response of all the magnetic
plaquettes, whereas in MR the transport current samples only a fraction of the magnetic
plaquettes which fall on its path[11].
In summary, we have provided a unique approach for fabrication of distributed S-F
Josephson-Junctions of nanometer length scale. Our self-assembled NbN-Fe-NbN hybrids
on [100] MgO shows ≈100% MR for H‖ which is higher by a factor of two as compared to the
MR for H⊥. This large in-plane response suggests breaking of phase coherence in S-F-S junc-
tions by the planar field. Temperature dependence of supercurrent in these self-assembled
structures is consistent with the theory of supercurrent transport in S-F-S junctions.
This research has been supported by a grant from the Department of Science & Tech-
nology under its Nanoscience & Technology Initiative and by the Board for Research in
Nuclear Science. S. K. Bose acknowledges financial support from the Council for Scientific
and Industrial Research, Government of India. Technical help of P. C. Joshi, H. Pandey and
R. Sharma are acknowledged.
[1] B. D. Josephson, Phys. Lett. 1, 251 (1962).
[2] P. W. Anderson, and J. M. Rowell, Phys. Rev. Lett. 10, 230 (1963).
8
[3] S. Krey, O. Bru¨gmann, and M. Schilling, Appl. Phys. Lett. 74, 293 (1999).
[4] A. M. Klushin, M. He, S. L. Yan, and N. Klein, Appl. Phys. Lett. 89, 232505 (2006).
[5] H. Schulze, R. Behr, F. Mu¨eller, and J. Niemeyer, Appl. Phys. Lett. 73, 996 (1998).
[6] I. Chiorescu, Y. Nakamura, C. J. P. M. Harmans, and J. E. Mooij, Science 299, 1869 (2003).
[7] A. I. Buzdin, and M. Yu. Kupriyanov, Pis’ma Zh. Eksp. Teor. Fiz. 53, 308 (1991) [JETP Lett.
53, 321 (1991)].
[8] V. V. Ryazanov, V. A. Oboznov, A. Yu. Rusanov, A. V. Veretennikov, A. A. Golubov, and J.
Aarts, Phys. Rev. Lett. 86, 2427 (2001).
[9] Yuriy Makhlin, Gerd Sco¨hn, and Alexander Shnirman, Nature 398, 305 (1999).
[10] S. M. Jordan, R. Schad, A. M. Keen, M. Bischoff, D. S. Schmool, and H. van Kempen, Phys.
Rev. B 59, 7350 (1999).
[11] S. K. Bose, R. Sharma, and R. C. Budhani, Phys. Rev. B 78, 115403 (2008).
[12] K. Senapati, N. K. Pandey, Rupali Nagar, and R. C. Budhani, Phys. Rev. B 74, 104514
(2006).
[13] S. K. Bose, and R. C. Budhani, Unpublished results.
[14] A. F. Hebard and M. A. Paalanen, Phys. Rev. Lett. 65, 927 (1990); Ali Yazdani and Aharon
Kapitulnik, Phys. Rev. Lett. 74, 3037 (1995).
[15] T. Kontos, M. Aprili, J. Lesueur, F. Geneˆt, B. Stephanidis, and R. Boursier, Phys. Rev. Lett.
89, 137007 (2002).
[16] John R. Clem, B. Bumble, S. I. Raider, W. J. Gallagher, and Y. C. Shih, Phys. Rev. B 35,
6637 (1987).
[17] Vinay Ambegaokar and Alexis Baratoff, Phys. Rev. Lett. 10, 486 (1963).
[18] Yongsup Park, S. Adenwalla, G. P. Felcher, and S. D. Bader, Phys. Rev. B 52, 12779 (1995).
9


Robust coupling of superconducting order parameter in a mesoscale NbN-Fe-NbN epitaxial structure

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Robust coupling of superconducting order parameter in a mesoscale NbN–Fe–NbN epitaxial structure

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Robust coupling of superconducting order parameter in a mesoscale NbN-Fe-NbN epitaxial structure

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