Metal frame as local protection of superconducting films from thermomagnetic avalanches

1.6 MB·

by  P. Mikheenko

Metal frame as local protection of superconducting films from thermomagnetic
avalanches
P. Mikheenko,1 J. I. Vestg˚arden,1 S. Chaudhuri,2 I. J. Maasilta,2 Y. M. Galperin,1,3 and T. H. Johansen1,4
1Department of Physics, University of Oslo, P.O. Box 1048 Blindern, 0316 Oslo, Norway
2Nanoscience Center, Department of Physics, P.O. Box 35,
University of Jyv¨aaskyla¨, FIN-40014 Jyva¨askyla¨, Finland
3Physico-Technical Institute RAS, 194021 St. Petersburg, Russian Federation
4Institute for Superconducting and Electronic Materials,
University of Wollongong, Northfields Avenue, Wollongong, NSW 2522, Australia
6 Thermomagnetic avalanches in superconducting films propagating extremely fast while forming
1 unpredictable patterns, represent a serious threat for the performance of devices based on such
0 materials. Itis shownhere thatanormal-metalframe surroundingaselectedregion insidethefilm
2 areacanprovideefficientprotectionfromtheavalanchesduringtheirpropagationstage. Protective
behaviorisconfirmedbymagneto-opticalimagingexperimentsonNbNfilmsequippedwithCuand
n
Al frames, and also by performing numerical simulations. Experimentally, it is found that while
a
J conventional flux creep is not affected by the frames, the dendritic avalanches are partially or fully
screenedbythem. Thelevelofscreeningdependsontheratioofthesheetconductanceofthemetal
9
andthesuperconductorintheresistivestate,andforratiosmuchlargerthanunitythescreeningis

very efficient.
]
n
o PACSnumbers: 74.25.fc,74.25.Ha,74.25.N-,74.25.Op
c
-
r
p The number of applications based on superconductors nomenon are severely limited by the lack of techniques
u increases, and with the important role these devices will allowing to investigate the flux dynamics on the relevant
s play in technology,[1–3] safe operation becomes crucially time scale of a few nanoseconds. This is the duration
.
t important. When loaded with a high electrical current, of a typical event,[16] and is also the time scale when
a
m or exposed to a strong magnetic field, the superconduc- damage is done to a device. In fact, an avalanche can
tors accumulate large amounts of energy. This energy permanentlydamagethesuperconductor,asshowninre-
-
d cansuddenlybereleasedthroughathermomagneticrun- cent work on YBa Cu O films.[17] Here, MOI revealed
2 3 x
n away, or avalanche, which in general is very harmful, that superconducting properties can be lost in parts of
o
e.g., to superconducting magnets and electronic devices. the avalanche path, and atomic force microscopy scans
c
[ Especially vulnerable are superconducting thin-film de- showed that the local heating can even cause complete
vices experiencing a perpendicular magnetic field, where disintegration of the material. Thus, from an applied
2
avalanche events may occur at fields as low as a few perspective is it essential to find means to prevent such
v
millitesla.[4, 5] events from happening, in particular, in regions of vital
5
2 importance for the functionality of a device.
The instability that triggers such avalanches is deeply
4
rooted in the nature of type-II superconductors, where Previous experiments demonstrated that a uniform
0
0 magneticfluxexistsintheformofquantizedvortices. Ifa metal layer deposited on the superconducting film can
. vortexmoves,itdissipatesenergycausingalocaltemper- suppress the avalanche activity.[18–22] It was also re-
1
ature rise in the material. This promotes motion of the ported that one may selectively prevent avalanche nucle-
0
6 neighboring vortices, and the positive feedback can cre- ation along the rim by coating parts of it with a normal
1 ate a massive thermomagnetic avalanche. In thin films, metal film.[23, 24] I In the present work, we show that
v: magneto-opticalimaging(MOI)hasrevealedthatthein- avalanches can be stopped even after they have nucle-
i stabilityleadstoabruptfluxmotionintheformoflarge, ated, i.e., during their fast and destructive propagation
X
often sample-spanning, dendritic structures, where each stage. This is done by adding a normal metal frame sur-
r
branch propagates at a speed up to 100 km/s.[6–8] The rounding a selected internal area of the superconducting
a
phenomenon has been observed in films of many super- film. Theefficiencyofthelocalprotectionisdocumented
conducting materials.[6, 9–14] by MOI observations and numerical simulations.
Although the avalanche behavior has already under- The sample configuration chosen in this work is a
gone extensive investigations,[15] one finds from a prac- squaresuperconductingfilm,seeFig.1,whereinthecen-
tical viewpoint that these events are largely out of con- tralpartanarrowsquaremetalframe(lightbrowncolor)
trol. Partly, this is due to (i) the unpredictability of is placed to protect the area it surrounds. The figure il-
when and where an avalanche starts, and (ii) the un- lustrates the result of a numerical simulation where such
predictable path that each branch of the avalanche will a sample is exposed to an increasing perpendicular mag-
follow. Furthermore, experimental studies of the phe- neticfield. Thesimulationswereperformedfollowingthe
2
FIG. 1. Numerical simulation of the flux distribution in a FIG. 2. Flux distribution in the upper right corner area of
superconductingfilm, wherethepropagationofanavalanche a NbN film, as the descending field reaches 9.2 mT. The
was blocked by a metal frame (light-brown). The brightness magneto-optical image was recorded using slightly uncrossed
ofthegreencolorrepresentsthemagnitudeofthefluxdensity. polarizers.
formalismdescribedinRefs.25and26,andusingtypical 1 µm thick Cu frame was then deposited on the sample,
parameters for low-resistive (in the normal state) super- with the same relative positioning as seen in Fig. 1. The
conductingfilms. Inthefigurethefilmedgeappearsasa squareframemeasures4mmexternally,andhasawidth
verybrightcontour,showingthepilingupoftheexternal of 0.5 mm. Visualization of magnetic flux distributions
fieldduetothediamagneticresponseofthesuperconduc- accross the sample area was done by MOI using a Fara-
tor. Thisparticularimagerepresentsthefluxdensityata day rotating ferrite garnet sensor plate [28, 29] placed
time65nsafteranavalanchestartedfromtheupperfilm directly on the sample.
edge. Evidently,thisavalanchepropagatedinabranched Shown in Fig. 2 is a magneto-optical image of the flux
fashion,andtheshowndendriticstructureisthefinalflux distributionformedbyanavalanchestartingfromtheup-
pattern after the avalanche came to rest. The spatially peredgeofanNbNfilm. Thesamplehadherefirstbeen
smooth penetration of flux from all the external edges zero-field-cooled to 4 K, and then exposed to a 12 mT
shows the regular behavior of a square superconducting perpendicular field, which caused full penetration of flux
film. without triggering any avalanche. The applied field was
From the flux distribution in Fig. 1 it is clear that the then reduced, and when reaching 9.2 mT the dendritic
avalanchewasstronglyinfluencedbythemetalframe. In- avalanche seen in the figure occurred.
deed, the frame fully preventedall the rapidly approach- It is evident that all the branches in this avalanche
ing flux branches from entering the enclosed central re- wereblockedbythemetalframe. Interestingly,theblock-
gion. Interestingly,thisprotectionoccursintwodifferent ing has to a large extent the character of reflection from
ways, namely (i) by reflecting incoming flux branches, the interface.[24] Another visible feature is that some of
and (ii) by damping the flux motion taking place under the avalanching flux, when hitting the frame, penetrates
the metal coating. Note also that in the upper horizon- smoothly into the metal coated area, where it finally
tal part of the metal frame, the flux branches hitting comes to rest in a critical-state-like distribution. The
theframeatapproximatelynormalincidencearecloseto framegivesherefullavalancheprotectionoftheenclosed
penetrate through the obstruction. Also the right verti- area, and does it with characertistics agreeing very well
cal part of the frame is activated, serving to guide some with the simulation results.
branches along the outer edge of the frame. When lowering the applied field further to 5.8 mT, a
To test in practice the avalanche-protection ability of secondavalancheoccured,startingfromadifferentpoint
a metal frame on superconducting films, pulsed laser de- on the upper sample edge, see Fig. 3. The new starting
position was used to grow films of NbN on MgO (001) point caused many of the branches in the avalanche to
single crystal substrates.[27] A film of thickness 170 nm hittheframeatnearperpendicularincidence. Evidently,
was shaped as a square with sides measuring 8 mm. A someofthesebranchestraversetheframe. Althoughthey
3
FIG.3. MagneticfluxdistributionasinFig.2whenthefield FIG.4. Simulatedpenetrationofadendritethroughthecon-
isfurtherreducedto5.8mT.Whenasecond,morepowerful, ductive frame. S =10.
avalanche occurs, the protection is incomplete.
ever,depositionofalayerwiththicknessmuchmorethan
leave only faint traces of their crossing, they form again onemicronisdifficultsincethickmetalfilmstendtopeel
a typical branched patterns when entering the uncoated off the sample. Also due to a finite skin depth in the
square. Thus, in this case the metal frame was not able metal at the ultra-short time scale typical for the den-
to provide full protection of the central area. drite propagation, only part of the thickness could con-
To verify the importance of the angle between the tributetoscreening. E.g.,theskindepthinAlis∼2µm
metal frame edge and the direction of the incoming for processes of duration of 1 ns, and 6 µm for processes
avalanche branches, simulations were performed using a with a characteristic time of 10 ns.
sample matrix causing the avalanche to start from a ge- A more reliable and flexible approach could be to use
ometrical location similar to what is seen in Fig. 3. The an external frame rather than one deposited on the film.
result of the numerical calculation is shown in Fig. 4. Totestthisconcept, aframewascutfroma13µmthick
The key features of the experimental and numerical re- Altechnicalfoil,andplacedonthesuperconductorusing
sults are again strikingly similar, in particular the fact the light pressure from the magneto-optical sensor plate
that several branches of the avalanche are now travers- for stable mounting.
ingthemetalframe. Moreover,thebranchesenteringthe Shown in Fig. 5 is the observed flux penetration pat-
area to be protected are seen to form further branching. tern inside the sample after it was initially zero-field
According to Ref. 30 the electromagnetic damping cooledto4K,andthensubjectedtoaperpendicularfield
of thermomagnetic avalanches by a normal-metal coat- of 2.9 mT. Panel a) displays the magneto-optical image
ing is governed by the dimensionless parameter, S = using color coding, where red color represents its max-
(ρ d )/(ρ d ). Here, ρ and ρ are resistivities of the imum brightness. The frame is indicated by the black
s m m s m s
metalandsuperconductor(intheresistivestate),respec- dashed lines showing a slight rotation relative to the su-
tively, while d and d are the thicknesses of the corre- perconductor square.
m s
sponding layers. In other words, S is the ratio of the In this figure, as in Fig. 3, one sees two regimes of
normal-state sheet resistance of the superconductor, R , flux penetration – one consisting of avalanche dendrites,
s
and the sheet resistance of the coating metal, R . For and one showing conventional penetration forming typ-
m
efficient screening the value of S should be much larger ical critical-state profiles. [31] The avalanching flux is
thanunity. Inthepresentcase,aftersubtractingthecon- again strongly influenced by the frame, which prevents
tact resistance, we measured at room temperature that nearly all the incoming branches from entering into the
R between points separated by 2.25 mm equals 7 Ohm, central uncoated area. Instead, they disperse into a
s
whileR betweenthesamepointsis0.5Ohm. Thisgives smooth flux distribution within the metal-coated part.
m
S ≈ 14 for the sample displayed in Figs. 2 and 3, quite Onlyonesmallfragmentofthefluxbranchesreachesthe
consistent with the fairly good avalanche protection. central frame area. Contrary, one sees that the flux hav-
An obvious way to increase the screening efficiency is ing penetrated the sample from the lower left side of the
toincreasethethicknessofthenormalmetallayer. How- square, was not at all influenced by the metal frame as
4
1.0 if a sensitive superconducting device were placed there,
a) it would not be affected by the avalanche event. Note
0.5
also that due to the finite skin depth, there is little to
gain by increasing the thickness of this frame any fur-
0.0
ther. However, improving the quality of the Al-foil and
itsmountingislikelytomakethescreeningmoreefficient.

Interestingly, one can see from panel b that the B-

profile across the frame is substantially steeper than the
Bean critical-state profile in the bare sample. Therefore,
coatingbya normalmetal improves thescreening offast

electromagnetic excitations in type-II superconductors.
- 1 mm Overall, with the deposition of a highly conductive nor-
mal metal layer and proper adjustment of the device ge-

ometry, onecanobtainsubstantialscreeningofdendritic
3.5
avalanches, the propagation of slowly moving magnetic
3.0 flux remaining essentially unperturbed.

2.5 In conclusion, it is demonstrated that a normal metal
b)
frame added on top of a superconducting film strongly
)Tm 2.0 impedes the propagation of thermomagnetic avalanches.
( B 1.5 Using such frames, it is possible to screen selected areas

of the film from the destructive avalanche events, while
1.0
keeping unaffected all slowly moving magnetic flux, e.g.,
0.5 aspartofthecommunicationwithsuperconductingelec-

0.0 tronicslocatedinsidetheareaenclosedbytheframe. The
0 1 2 3 4 5 experimentsalsorevealthatthepenetrationofavalanche
x (mm) branches into the framed region depends on the angle of

incidence. Avalanche branches hitting the frame at near
B
normal incidence are much less screened than those hit-
c) ting at smaller angles which are mostly reflected.

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