Velocity measurements of the dendritic instability in YNi2B2C

I N ~ T I T ~ T E OF PI~YSICS PUBLISHTYG S U P E R C ~ N D U ~ R SCIENCE AND TLCHNI)LIIGY

Supercond. Sci Twhnol. I 8 (2005) 385-187 doi- 10.1088M953-2W8II RI1W2

Velocity measurements of the dendritic instability in YNi2B2C

R Biehler" B-U Rungel, S C ~ i r n b u s h * . ~ , B Holzapfe12 and P Leiderer "

I Depanmcnt of Physics, Univcnrly or Konstan~, Universitatssuafle 10.78464 Konstanz, Germany

? IFW Dresden. PO Box 2700 16.0 1 17 1 Dresrlen, Germany E-mail: Bjoem.Bichler@uni-konstanz.de

Rcccived 19 July

2004,

in final

form 8 November

2004

Published

1

February 2005

Online

at

stacks.iop.org/SUST/18/385

Abstract

Wc measured the

velocity

of

Ihc

flux front of an artificially

nucleated dendritic instability in

YNi2B2C.

The

required

t i m e resolution

In

the nanosecond

regime was

achieved

by our

magneto-optic pumpprobe

technique,

utilizing

a

f e m t o s e c o n d

laser

^system.

The

penetration vclocity of

the flux

front

is

on the order

of 360 km

s - ' .

(Some

figurcs in

this

article are in

colour

onIy

in the

electronic version)

1. Introduction

The dendritic ~ n s t a h ~ l i t y is a rapid redistr~bution of magnetic flux inside a type

E

supercunductor (e.g. see [I]) and was found in 1967 by Wertheimer er a /

[2];

ⁱⁿ YNi2B2C it was first observed by Wimbush e# a1 [ 3 ] . The instability can be tripgemd aflificially by disturbing the screening current distribution inside the wperconductor. Thls can be caused by sweeping the magnetic field 141. hy applying a transport current [S] or by heatrng a small ponion of thc s u ~ ~ ' c o n d u c t o l : The latter was used to determine the flux front velocity i n YBCO, which was found to be on the order of 160 km s-' [6].

Despite the efforts made to undcrstantl the details behind the instability mechanism they rcmain unknown. This is the reason why we want to put forward additional data

on

the dynamic behaviour of the dendritic instablli ty in YNi2B2C.

2. Sample preparation

Fur this work.

a

thln film of YNi2B2C was deposited in an ultrah~gh vaccuum (base presqure < 10-' mbar) pulsed laser deposition system, as descr~bed In [7]. A polycrystalline YNi2B2C target of stoichiometric cornpositron, prcpared by arc melt~ng, was lired upon at 30 Hz with an energy density a5 J cm-', leading to a depobition riite of around 2 nm s-' on the MgQ(001) substrate, polished on both

'

^{Present addrew Yational Institute} for Matenal~ Science. International

Center for Young Scientists. 1-1 Nam~hi. Tsukuba, lharak~ 305-0044. Japan

t

^YNlzBzC magneto-oplic ^{film wfth} layer ^camera

vanaMe delay line

Figure 1. For time resolved picrures we use a pumpprobe techn~quc The laser pulse is used to trigger the instability and some nanosecond^ later for illumination. For non-time-resolved images a cold light source can be used for illumination. For clarity the lenses of the pularizmg microscope (right part) arc not shown.

iides, held around 2 cm abovc the target. The deposition temperature was 750°C. and the film thickness was around 350 nm Structural analysis of the film was performed using x-ray 8-26! measurements followed by in-planc texture dctcrminatinn (pale figure measurement). 8-28 and pole figure x-ray characterization indicate c-axih oriented epitaxial f l m growth with the dornlnant orientation rotated by 45" in plane with respect to the substrate: YNiZB2C(OOi)[l 101

11

MgO(001)[100]. Thc svperconducting transition temperature, T,, was measured inductively using an alternating magnetic field shielding technique. yielding

a

Tc of

12

K with

a

transition width of 1.5 K.

0953-20jR/05/040385+03$30 00 O 2005 10P Publishing Ltd Printed In the UK 385 First publ. in: Superconductor Science and Technology 18 (2005), 4, pp. 385-387

Konstanzer Online-Publikations-System (KOPS) URL: http://www.ub.uni-konstanz.de/kops/volltexte/2007/2728/

URN: http://nbn-resolving.de/urn:nbn:de:bsz:352-opus-27281

R Biehler er a1

Figure 2. Magneto-opt~cal i~liages uC the flux d~stl.~butiortin YNi?B?C:: (a) initinl condii~on, (b) afrer 3 9 ns, and ( c ) -10 s (the final state).

T h e cxternnl field was B,, = 1.32 m T at 4 h K For clarity a plane was subtracted from the initinl image (a) and the contrast enhanced.

(h) and (c) were rercaled according to equsuon ( 1 ) The firs1 arrow ( 1 ) In part (b) marks the minimum penematton length and the second arrow (2) marks the maximum cornparible wfrh the measurement. The w h m line on the right of each picture indicates the sample edge.

Note that the front of the final state (c) 1s much clearer (ind~catcd by n m w ( 3 ) ) . The length scale remains the same in each panel.

3. Experimental set-up

A sketch of the experimental set-up 1s hewn in figure 1 We apply a p u m p p r o b e tcchiiique usrrlg ;i conimercial femtosecond laser system. It produces pulscs of VWHM =

150 fs at b = 800 nm. We split the beam in two parts; one is used to perturb thc current distr~butlon by Iwally heating the superconductor. Under appropriate cclnrlitionu [XI this w ~ l l triggel. the instability. T h e second pan of the pulse i s fed into a delay line which has been adjusted for time delays betwccn 1 aiid 6.5 ns. After passing through a polarizer t o ensure a well polarized beam, the light i~ used to take a rnagncto- optical image of the instantaneous flux distribution. This is done by an iron garnet film showing

a

large Faraday cffect, i.e. the polarization of light i s rotated proportionally to the local perpendicular mapnctic field compnnent. Thir gives

a

snapshot

of

the flux distribution just above the sample. We focused the pump beurn at the sample cdge. h r away from the comers. As

a

camera we usc a 12-bit slow scat1 CCD camera w h l c l ~ was coilled to -40 'C. Rcsults c ~ f a slrnilar experiment with YBCO thin films

are

published i n [6j.

4,

Results and discussion

A typical expcrirnent was conductcd as followh. The sarnpIc was zem-field cooled to a ternperatrtre of T = 4.6 K and an external magnetic field 1.3 mT

<

^R,,,

6

5 . 2 mT way

applied. Then the pump-prohe run was conducted and after

~ 1 0 s an additional picture of thc final qcare was taken. Wc measured the energy El,,,, of each lascr pulse and scaled

the intensity of the images by this factor. In addition we apphed a divisionldifference technique to improve the s~gnal- to-noiw ratio and reduce the effects of interference and uneven illumination. This was done by measuring the flux distribution:

twice before ( I I , 12) and once during the p u m p p r o b e run

(4).

The final images

as

seen in figures 2(b) and(c) were constructed by the fol!owing calcufat~on:

whcre El,,,,>, is the energy of thc h e r pulse with whlch the nth image was taken. The division of two pictures should be read

as

dtvtding the images pixel by pixel. Note that this will lead to nn image emphasis~ng the changes of the flux distribution. Note that this procedure cannot be applied for the initial condition (figure 2(a)), since the subtraction would result i n a uniformly black image.

Repeating thip experiment for different external fields and variuus delay times gives snapshots of the momentary flux disirihutinn. We measured the distance

from

the sample edge ro the Rux front; this is plotted in figure 3. The error bars lndicatc the maximum and minimum distance consistent with the magneto-optical images. The dotted line in figure 3 corresponds to a velocity of a360 km s-'. This is much faster than the velocities measured in YBCO. There we werc ablc fo distinguish two well separated velocities: an early strtge showing a dependence on the external field

(i.e, a

high field Ied to a high velocity of u p to 160

krn

s-I), and a decrease in velocity

to

18 km s-' at times larger than 10 ns, Largely independent of the

external

field and temperature.

Velocity measurements ofthe dendritic instability in Y N i z B z C

Figure 3. Flux penetmtron length as a runchon o f (a) lime and (b) exte~,nally appl~ed field. The doned tine is a guide to the eye corresponding to a velocity of -360 k111 s - ' Note the decrease in velocity f o r tlrneq 3 4 ns.

These wel! defined stages were not seen in YNi2B2C. A possible explanation may be that rhe low velocrty of the flux front In YBCO was observed as the dendrite penetrated into the Meissner phase region of the sample. In the case of YNi2B2C the Shubnikov phase extended over a region of z 1 . 2 mm, which 1s approx~~nately the total length of the

dendrites investigated. Therefore we would expect the veloc~ty to decrease as soon as the dendrite enters the Melssner region.

Acknowledgments

We are grateful for the sponsorship by the Gerrnnfi Isrneli Foun- dation. The work in Dresden was supported by the Deutschc Forschungsgemeinschaft as par? of SFTI 463 'Rare earth tran- sition metal compounds: stnlcture, magnetism and transpon'.

References

I l j Altshule~.Eand JohansenTH 2004Rev. Mod. Phys. 76471-87 [2J Wcrthcimer M R and Gilchrist J le G 1967 J. P h p . Chem.

Solidt 28 2509

[3] Wimbush S C, Holzepfel B and Jooss Ch 2004 J. Appl. Phys. 96 3589-91

[4] Jnhansen T H, Bar.rljevich M , Shantsev D V, Goa P E, Galperin Y M, Kang W N, Kim H J, Choi E M, Kim M-S and Lee S 12002 Europhys. Len 5 9 5 9 9 4 0 5 [ 5 ] Bobyl A V, Shantsev D V, Johansen T H, _Kang W N, Kim H J,

C h o ~ E M and Lee S I 2002 Rppl. Phys. Lett. 80 4588-90 [ 6 ] Bolz U, Schmidt D, Biehler B, Runge _B-tl and Leiderer P 2003

Eitrophy.~ Len 64 5 1 7-23

[ J l U'irnbush S C, Haise K. Schultz L and Holmpfel B 2001 J. P h p : Condens. Matrer 13 t35540

[ R I Bichlcr B, Runge B-U and Leiderer T 2004 J. Low Temp. Phys.

137 117-23