Dewhurst, C.D.

Kes, P.H.

Marchevsky, M.

McK. Paul, D.

Nieuwenhuys, G.J.

Saha, N.

Surdeanu, R.

Wijngaarden, R.J.

DSpace at VU

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PHYSICAL REVIEW B, VOLUME 63, 020502~R!Magnetic twin boundaries and flux pinning in the antiferromagnetic superconductor ErNi2B2C
N. Saha,1 R. Surdeanu,2 M. Marchevsky,3 G. J. Nieuwenhuys,1 C. D. Dewhurst,4 R. J. Wijngaarden,2
D. McK. Paul,5 and P. H. Kes1
1Kamerlingh Onnes Laboratory, Leiden University, POB 9504 Leiden, The Netherlands
2Division of Physics and Astronomy, Vrije Universiteit, De Boelelaan 1081, 1081HV Amsterdam, The Netherlands
3NEC Research Institute, 4 Independence Way, Princeton, New Jersey 08540
4Institut Laue-Langevin, 6 rue Jules Horowitz, BP 156-38042 Grenoble, France
5Department of Physics, University of Warwick, Coventry CV4 7AL, United Kingdom
~Received 20 September 2000; published 20 December 2000!
We show that in the antiferromagnetic superconductor ErNi2B2C, the increase in flux pinning below TN
coincides with the appearance of twin boundaries between antiferromagnetic domains. The pinning mechanism
is provided by a ferromagnetic spin component localized at the twin boundaries parallel to the crystallographic
c axis.
DOI: 10.1103/PhysRevB.63.020502 PACS number~s!: 74.70.Dd, 74.60.Ge, 74.60.JgThe study of the mixed state of antiferromagnetic super-
conductors has made a remarkable revival since the discov-
ery of the (R)Ni2B2C, where R5rare earth ~RNBC!.1,2 Co-
existence of antiferromagnetism and superconductivity was
known to exist in (R)Rh4B4 and (R)Mo6(S/Se)8 with the
exception of ErRh4B4 and HoMo6S8 , but the value of the
Ne´el temperature (TN) is typically 2 K or less3 and the su-
perconducting transition temperatures (Tc) are much larger.
However, RNBC compounds have a higher TN and a Tc
which is comparable, and this provides the opportunity to
study the influence of long-range antiferromagnetic ~AF! or-
der on superconductivity. Hall-probe-array measurements on
HoNi2B2C ~Ref. 4! show that bulk pinning is weak both in
the c-axis spiral AF state (T.6 K! and the low-temperature
commensurate AF phase (T,5 K! but that the pinning is
much stronger in the intermediate (5 K,T,6 K! incom-
mensurate AF phase. Similarly, in ErNi2B2C,5 bulk pinning
is weak in the paramagnetic phase between TN (56 K! and
Tc (510.5 K! and much stronger below TN where the ma-
terial magnetically orders into an incommensurate AF phase
with a transversely polarized spin-density wave along the a
axis with wave vector ~0.553,0,0!,6 and the magnetic Er31
moments parallel to ~0,1,0!. A weak ferromagnetically or-
dered component appears in this compound below Tw f m
52.5 K ~Refs. 7 and 8! and is accompanied by a further
increase of the bulk pinning.9 In Refs. 4 and 5 it is suggested
that the enhanced flux pinning is related to the incommensu-
rability of the magnetic order and in Ref. 9 the further in-
crease below Tw f m is speculated to be due to strong local-
pair breaking by the ferromagnetism at the domain walls
between differently oriented AF domains. Such AF twinning
is quite common in R compounds.10 In this paper we will
further address the nature of flux pinning in the AF phase of
ErNi2B2C.
In order to explore the underlying mechanism of flux pin-
ning in these magnetic superconductors in general and in
ErNi2B2C in particular, we have performed Bitter decora-
tion, magneto-optical and scanning Hall-probe experiments
on single crystals of ErNi2B2C in both the paramagnetic and0163-1829/2000/63~2!/020502~4!/$15.00 63 0205AF phases. These experiments provide clear evidence for the
formation of domain walls and the interaction of these walls
with the vortex lattice ~VL!. The features of the VL observed
in the AF phase are very similar to those observed in
strongly twinned single crystals of YBa2Cu3O7 ~YBCO!.12,13
In addition, a field modulation across the domain wall is
detected which clearly reveals weak local ferromagnetism.
The weak ferromagnetism sets in immediately below TN and
therefore we think that this is the mechanism for the en-
hancement of flux pinning below the AF transition. The fur-
ther increase below Tw f m as observed by Gammel et al.9
requires an additional mechanism which has not yet been
explored.
The experiments which are described here were per-
formed on single crystals prepared using a high-temperature
flux method. The crystals were platelets with the c axis nor-
mal to the platelet surface and typical sample sizes are 700
by 600 mm and 80 mm thick along the c axis. The orienta-
tion of the crystal was determined using Laue diffraction and
the published c/a ratio of ErNi2B2C.6 The sample was char-
acterized using electron probe microanalysis ~EPMA! and
atomic force microscopy. It is chemically uniform and me-
chanically smooth. All experiments were done on the same
single crystal. The surface was cleaned between successive
decorations using rf-plasma etching. The ErNi2B2C crystal
has a Tc510.5 K. Typical values of low-temperature pen-
etration depth l are 700 Å and coherence length j’150 Å .11
Three types of experiments were carried out on these
samples. Bitter decoration experiments were performed after
field cooling ~FC! below and above TN , scanning Hall-probe
experiments after FC at 5.2 K and magneto-optic experi-
ments both FC and zero-field cooled ~ZFC! above and below
TN . Due to radiation heating the actual decoration tempera-
ture is slightly ~,1.5 K! higher than the preset temperature.
In this paper the temperature referred to is the estimated
actual temperature of the sample during decoration.
A typical decoration picture on ErNi2B2C ~5 K! is shown
in Fig. 1 for an applied field of 6 mT. Clearly visible are
stripes at right angles to each other directed along @110# and©2000 The American Physical Society02-1
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N. SAHA et al. PHYSICAL REVIEW B 63 020502~R!@11¯0#. These stripes continue without deviation across large
~a few micron! surface steps. This indicates that the origin of
these linelike patterns is a bulk and not a surface effect. The
distances between the stripes in all decorations below 6 K
ranges between 3 and 10 mm. An enlargement of the
marked area is shown in @Fig. 2~a!#. It is seen that the stripes
consist of hexagonal vortex arrangements with one of the
close-packed directions along @110# or @11¯0# . Square vortex
arrangements are observed in the regions where two stripes
meet at right angles. These observations are very reminiscent
to the vortex configurations in twinned YBCO ~Ref. 13!
where vortices line up along the twin boundaries. The dis-
tance between the vortices inside the stripes is governed by
the applied field. A study of the autocorrelation function of
the VL in wide stripes, Fig. 2~c! shows that the translational
correlation length is about 3 to 4 lattice spacings in all direc-
FIG. 1. A decoration picture of ErNi2B2C ~field cooled in 6 mT
to 5 K!. The separation between the stripes is approximately 3–10
mm along @110# and @11¯0# . They continue without deviation
across surface steps.
FIG. 2. ~a! Enlargement of the vortex lattice in the marked area
in Fig. 1 after field cooling in 6 mT to 5 K. ~b! Structure factor
shows that the vortex lattice is hexagonal with orientation along
@110# and @11¯0# . ~c! The two-dimensional autocorrelation function
shows that translational correlation of the lattice exists over 3–4
lattice spacing.02050tions. In the areas between the stripes occasionally faint im-
ages of the VL are visible. For different FC experiments the
line structure at the same sample site can change from @110#
to @11¯0# or vice versa. There were no essential differences
between FC experiments at 3 or 5 K. Our experimental setup
does not allow us to go below Tw f m’2.5 K. The decoration
experiments done above 6 K do not show the stripes, in
contrast a conventional hexagonal VL was observed with a
correlation length of 3–4 lattice spacing and with the princi-
pal axis along @110# or @11¯0# . Such a VL orientation is in
agreement with the predictions of Kogan et al.14 The fact
that the stripes disappear above 6 K implies that their exis-
tence is related to the AF transition.
The observation of line patterns reminiscent to the twin
boundaries in YBCO points to the formation of domains co-
incident with the antiferromagnetic phase transition. As has
been shown by Detlefs et al.15 the crystal lattice of
ErNi2B2C undergoes a tetragonal (T.6 K! to orthorhombic
(T,6 K! distortion at the onset of long-range antiferromag-
netic order due to magneto-elastic coupling. In order to mini-
mize strains the crystal lattice breaks up into domains with
domain walls parallel to the c axis and @110# or @11¯0# , on
either side of which the direction of the a and b axes are
interchanged.
In order to further confirm that these domain walls are
related to the linelike patterns and to explore their effect on
the pinning of the VL, magneto-optical experiments were
carried out. The sample was ZFC to T,Tc and the field was
cycled from zero to 40 mT and back to zero. The resulting
images were recorded. The experiments were repeated at 2,
4.2, 5.5, 6, 7, and 7.5 K. Distinctly different features were
only observed below and above 6 K. There was no notice-
able difference in flux penetration behavior at 2 K and above.
Figure 3 shows the nature of flux penetration into
ErNi2B2C at 2 K. Such images are typical for the behavior
below TN . It is seen that the flux penetrates very inhomoge-
FIG. 3. Magneto-optical observation of flux penetration into
ErNi2B2C at 2 K. The sample was zero-field cooled to 2 K, the field
was cycled to 45 mT and thereafter back to zero. The above picture
was taken at 20 mT in increasing field.2-2
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MAGNETIC TWIN BOUNDARIES AND FLUX PINNING . . . PHYSICAL REVIEW B 63 020502~R!neously in the form of bubbles and finger shaped flux fronts.
These observations are globally similar to the flux penetra-
tion in 2H-NbSe2 ,16 but there is a distinct difference in that
there are two preferential directions of flux penetration cor-
responding to the directions of the stripe structures seen in
the Bitter decoration. The directions of flux penetrations are
indicated by the black arrows in Fig. 3. It means that flux
lines easily penetrate along the domain walls and that in
addition there are strong screening currents along the domain
walls which drive the vortices into the areas in between. This
kind of flux penetration behavior is very similar to that seen
in twinned single crystals of YBCO,17,18 indicating that the
domain walls act as planar pinning centers which promote
vortex channeling along them. In contrast, above 6 K, the
flux penetrates uniformly yielding a flux profile which is
dome shaped. A typical example at 7 K is shown in Fig. 4.
The dome shaped flux profile is caused by strong screening
currents at the edges of the sample due to its platelet geom-
etry in combination with absence of bulk pinning.19 An esti-
mate of the edge current density Je is obtained from the
measured value of the first penetration field Hp ~12 mT at 4.2
K! and the expressions19 Hp5Hc1Ad/W and Je52Hc1 /d ,
where Hc1 is the lower critical field, d is the sample thick-
ness, and W is the sample width. The result is Je59
3108 A/m2. Information about the strength of the bulk pin-
ning is deduced from the slope of the profile at the bubble
front. The result at 4.2 K is Jc’73108 A/m2. Similar ob-
servations have been made with Hall-probe arrays by Dew-
hurst et al.5 who concluded in agreement with our findings
that bulk pinning is comparable to the geometrical barrier in
the AF phase while in the paramagnetic phase bulk pinning
is negligibly small.
We now turn to the questions why the line pattern is ob-
served below 6 K and what the actual pinning mechanism in
the AF phase is. The fact that we do not observe vortices in
the regions between the stripes or occasionally only a faint
image, means preferential deposition of the nickel particles
takes place at and around the location of the domain walls. In
FIG. 4. The dome shaped flux profile deduced from magneto-
optics at 7 K showing the absence of bulk pinning in the paramag-
netic phase. Sample edges are at x’0 mm and 700 mm.02050order to explore this further we undertook scanning Hall
probe experiments after cooling down the sample in different
fields to a temperature of 5.2 K as shown in Fig. 5.
In analyzing the images it should be known that the size
of the scanning Hall probe ~SHP! is 1 by 1 mm, the distance
to the sample surface is around 0.5 mm, the field calibration
is 900 V/T at 5.2 K, and that only the field modulation is
shown after subtraction of the applied background field. Fig-
ure 5 shows that across a domain wall the field variation is
around 1 mT. The actual field variation at the domain wall is
expected to be larger, but it cannot be precisely detected due
to the finite size of the SHP. It is further seen that the am-
plitude of the modulation is independent of the applied field
and that upon changing the polarity of the applied field the
overall image reverses sign.
This is convincing evidence for local ferromagnetism at
the domain walls with an internal field component parallel to
the c axis. The resulting magnetic field above the sample will
thus have strong gradients at the domain walls which attract
the nickel particles during decoration. The stripes in Fig. 1
therefore show the positions of the domain walls. On the
other hand, SHP probes the field modulation and this has
twice the periodicity of the domain walls. Thus the distance
between the brighter areas in Fig. 5 should approximately be
twice as large as the distance between the stripes in Fig. 1,
which is indeed the case.
The origin of the local ferromagnetism can be explained
as follows. At the domain walls both the crystallographic
orientation and the orientation of the Er moments change by
90°. This gives rise to a region of magnetic frustration which
we believe leads to local canting of the Er moments away
from the basal (a-b) plane. Depending on the amount of
FIG. 5. Scanning Hall probe microscope images across a meet-
ing point of two perpendicular stripes at 5.2 K and various applied
fields. The size of the scanned region is approximately 40 by 40
mm. The background field has been subtracted. The field calibra-
tion is 900 V/T yielding a typical field modulation averaged over
the size of the SHP of 1 mT. Note the black-white reversal upon
zero-field crossing.2-3
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N. SAHA et al. PHYSICAL REVIEW B 63 020502~R!canting this results in a strong local magnetic field. We esti-
mate that this local field can be 300 mT if we assume a
canting of the Er31 moments by an angle of 10° out of the
ab plane and a width of the domain wall of a few crystal
lattice parameters. We further note that Mossbauer
spectroscopy20 has demonstrated the existence of a net c-axis
component of the Er31 moments in the AF phase. Since the
experiments in Ref. 20 were done on polycrystalline
samples, these results were not decisive as to the location of
the canted moments. Our work shows that they coincide with
the boundaries between the AF domains.
It is obvious that strong local internal fields at the domain
walls will severely suppress superconductivity. In addition, it
may also be the reason for the drop in Hc2 below TN in the
phase diagram of ErNi2B2C. The domain walls will there-
fore act as strong planar pinning centers, very similar to the
twin boundaries in YBCO,21 except that the pinning mecha-
nism will be dTc pinning rather than the dk pinning sug-
gested by Gammel et al.9 We believe that the domain-wall
related ferromagnetism provides a plausible explanation for
the sudden appearance of flux pinning below TN . Since the
onset of bulk pinning coincides with the appearance of a
transverse spin-wave modulation of Er moments, one might
suspect that coupling of the VL to the spin-wave structure02050could be a source of flux pinning. However, the modulation
will be smeared out over the relevant length scales of the VL
which will make this pinning mechanism less effective. We
finally note that it would be interesting to study the interplay
of internal magnetism and the VL when the formation of
domain walls could be suppressed.
In conclusion, we have performed Bitter decoration,
magneto-optical and scanning Hall-probe measurements in
the mixed state of the antiferromagnetic superconductor
ErNi2B2C. The enhancement of flux pinning in the antifer-
romagnetic phase is caused by the formation of magnetic
twin boundaries at which a local variation of the internal
magnetic field is related to the ferromagnetic component of
the Er31 moments along the crystallographic c axis.
The authors wish to thank Professor Aronson from the
University of Michigan and Professor Ramakrishnan from
Tata Institute, Bombay for helpful discussions. Technical as-
sistance from Marcel Hesselberth, Ton Gortenmulder, and
Ruud Hendrikx was of great help. This work was part of the
research program of FOM, the ‘‘Stichting voor Fundamental
Onderzoek der Materie,’’ which was financially supported
by ‘‘NWO.’’1 C. Mazumdar et al., Solid State Commun. 87, 413 ~1993!.
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21 P. H. Kes et al., Cryogenics 29, 228 ~1989!.2-4


Magnetic twin boundaries and flux pinning in the antiferromagnetic superconductor ErNi_{2}B_{2}C

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VU Research PortalMagnetic twin boundaries and flux pinning in the antiferromagnetic superconductorErNi_{2}B_{2}CDewhurst, C.D.; Kes, P.H.; Marchevsky, M.; McK. Paul, D.; Nieuwenhuys, G.J.; Saha, N.;Surdeanu, R.; Wijngaarden, R.J.published inPhysical Review B. Condensed Matter and Materials Physics2000document versionPublisher's PDF, also known as Version of recordLink to publication in VU Research Portalcitation for published version (APA)Dewhurst, C. D., Kes, P. H., Marchevsky, M., McK. Paul, D., Nieuwenhuys, G. J., Saha, N., Surdeanu, R., &Wijngaarden, R. J. (2000). Magnetic twin boundaries and flux pinning in the antiferromagnetic superconductorErNi_{2}B_{2}C. Physical Review B. Condensed Matter and Materials Physics, 63(2), [020502].General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright ownersand it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.            • Users may download and print one copy of any publication from the public portal for the purpose of private study or research.            • You may not further distribute the material or use it for any profit-making activity or commercial gain            • You may freely distribute the URL identifying the publication in the public portal ?Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.E-mail address:vuresearchportal.ub@vu.nlDownload date: 22. May. 2021RAPID COMMUNICATIONSPHYSICAL REVIEW B, VOLUME 63, 020502~R!Magnetic twin boundaries and flux pinning in the antiferromagnetic superconductor ErNi2B2CN. Saha,1 R. Surdeanu,2 M. Marchevsky,3 G. J. Nieuwenhuys,1 C. D. Dewhurst,4 R. J. Wijngaarden,2D. McK. Paul,5 and P. H. Kes11Kamerlingh Onnes Laboratory, Leiden University, POB 9504 Leiden, The Netherlands2Division of Physics and Astronomy, Vrije Universiteit, De Boelelaan 1081, 1081HV Amsterdam, The Netherlands3NEC Research Institute, 4 Independence Way, Princeton, New Jersey 085404Institut Laue-Langevin, 6 rue Jules Horowitz, BP 156-38042 Grenoble, France5Department of Physics, University of Warwick, Coventry CV4 7AL, United Kingdom~Received 20 September 2000; published 20 December 2000!We show that in the antiferromagnetic superconductor ErNi2B2C, the increase in flux pinning belowTNcoincides with the appearance of twin boundaries between antiferromagnetic domains. The pinning mechanismis provided by a ferromagnetic spin component localized at the twin boundaries parallel to the crystallographicc axis.DOI: 10.1103/PhysRevB.63.020502 PACS number~s!: 74.70.Dd, 74.60.Ge, 74.60.Jgecoatonaserdsn-l-llngoin-da-ndr thellsdinism.n-r-eener-ture700nd-e-mesiven-eseftereri-ra-e.tedeThe study of the mixed state of antiferromagnetic supconductors has made a remarkable revival since the disery of the (R)Ni2B2C, whereR5rare earth~RNBC!.1,2 Co-existence of antiferromagnetism and superconductivity wknown to exist in (R)Rh4B4 and (R)Mo6(S/Se)8 with theexception of ErRh4B4 and HoMo6S8 , but the value of theNéel temperature (TN) is typically 2 K or less3 and the su-perconducting transition temperatures (Tc) are much larger.However, RNBC compounds have a higherTN and a Tcwhich is comparable, and this provides the opportunitystudy the influence of long-range antiferromagnetic~AF! or-der on superconductivity. Hall-probe-array measurementsHoNi2B2C ~Ref. 4! show that bulk pinning is weak both ithe c-axis spiral AF state (T.6 K! and the low-temperaturecommensurate AF phase (T,5 K! but that the pinning ismuch stronger in the intermediate (5 K,T,6 K! incom-mensurate AF phase. Similarly, in ErNi2B2C,5 bulk pinningis weak in the paramagnetic phase betweenTN (56 K! andTc (510.5 K! and much stronger belowTN where the ma-terial magnetically orders into an incommensurate AF phwith a transversely polarized spin-density wave along thaaxis with wave vector~0.553,0,0!,6 and the magnetic Er31moments parallel to~0,1,0!. A weak ferromagnetically or-dered component appears in this compound belowTw f m52.5 K ~Refs. 7 and 8! and is accompanied by a furtheincrease of the bulk pinning.9 In Refs. 4 and 5 it is suggestethat the enhanced flux pinning is related to the incommenrability of the magnetic order and in Ref. 9 the further icrease belowTw f m is speculated to be due to strong locapair breaking by the ferromagnetism at the domain wabetween differently oriented AF domains. Such AF twinniis quite common inR compounds.10 In this paper we willfurther address the nature of flux pinning in the AF phaseErNi2B2C.In order to explore the underlying mechanism of flux pning in these magnetic superconductors in general anErNi2B2C in particular, we have performed Bitter decortion, magneto-optical and scanning Hall-probe experimeon single crystals of ErNi2B2C in both the paramagnetic an0163-1829/2000/63~2!/020502~4!/$15.00 63 0205r-v-soneu-sfintsAF phases. These experiments provide clear evidence foformation of domain walls and the interaction of these wawith the vortex lattice~VL !. The features of the VL observein the AF phase are very similar to those observedstrongly twinned single crystals of YBa2Cu3O7 ~YBCO!.12,13In addition, a field modulation across the domain walldetected which clearly reveals weak local ferromagnetisThe weak ferromagnetism sets in immediately belowTN andtherefore we think that this is the mechanism for the ehancement of flux pinning below the AF transition. The futher increase belowTw f m as observed by Gammelt al.9requires an additional mechanism which has not yet bexplored.The experiments which are described here were pformed on single crystals prepared using a high-temperaflux method. The crystals were platelets with thec axis nor-mal to the platelet surface and typical sample sizes areby 600mm and 80mm thick along thec axis. The orienta-tion of the crystal was determined using Laue diffraction athe publishedc/a ratio of ErNi2B2C.6 The sample was characterized using electron probe microanalysis~EPMA! andatomic force microscopy. It is chemically uniform and mchanically smooth. All experiments were done on the sasingle crystal. The surface was cleaned between succesdecorations using rf-plasma etching. The ErNi2B2C crystalhas aTc510.5 K. Typical values of low-temperature peetration depthl are 700 Å and coherence lengthj'150 Å .11Three types of experiments were carried out on thsamples. Bitter decoration experiments were performed afield cooling~FC! below and aboveTN , scanning Hall-probeexperiments after FC at 5.2 K and magneto-optic expments both FC and zero-field cooled~ZFC! above and belowTN . Due to radiation heating the actual decoration tempeture is slightly~,1.5 K! higher than the preset temperaturIn this paper the temperature referred to is the estimaactual temperature of the sample during decoration.A typical decoration picture on ErNi2B2C ~5 K! is shownin Fig. 1 for an applied field of 6 mT. Clearly visible arstripes at right angles to each other directed along@110# and©2000 The American Physical Society02-1rgoTh6sthripceisdolecim-heestuph aci-ino-asg-ini-ithreonreingat 2,rece-ve.toiore-–1eann–toldtureRAPID COMMUNICATIONSN. SAHA et al. PHYSICAL REVIEW B 63 020502~R!@11̄0#. These stripes continue without deviation across la~a few micron! surface steps. This indicates that the originthese linelike patterns is a bulk and not a surface effect.distances between the stripes in all decorations belowranges between 3 and 10mm. An enlargement of themarked area is shown in@Fig. 2~a!#. It is seen that the stripeconsist of hexagonal vortex arrangements with one ofclose-packed directions along@110# or @11̄0#. Square vortexarrangements are observed in the regions where two stmeet at right angles. These observations are very reministo the vortex configurations in twinned YBCO~Ref. 13!where vortices line up along the twin boundaries. The dtance between the vortices inside the stripes is governethe applied field. A study of the autocorrelation functionthe VL in wide stripes, Fig. 2~c! shows that the translationacorrelation length is about 3 to 4 lattice spacings in all dirFIG. 1. A decoration picture of ErNi2B2C ~field cooled in 6 mTto 5 K!. The separation between the stripes is approximately 3mm along @110# and @11̄0#. They continue without deviationacross surface steps.FIG. 2. ~a! Enlargement of the vortex lattice in the marked arin Fig. 1 after field cooling in 6 mT to 5 K.~b! Structure factorshows that the vortex lattice is hexagonal with orientation alo@110# and@11̄0#. ~c! The two-dimensional autocorrelation functioshows that translational correlation of the lattice exists over 3lattice spacing.02050efeKeesnt-byf-tions. In the areas between the stripes occasionally faintages of the VL are visible. For different FC experiments tline structure at the same sample site can change from@110#to @11̄0# or vice versa. There were no essential differencbetween FC experiments at 3 or 5 K. Our experimental sedoes not allow us to go belowTw f m'2.5 K. The decorationexperiments done above 6 K do not show the stripes, incontrast a conventional hexagonal VL was observed witcorrelation length of 3–4 lattice spacing and with the prinpal axis along@110# or @11̄0#. Such a VL orientation is inagreement with the predictions of Koganet al.14 The factthat the stripes disappear above 6 K implies that their exis-tence is related to the AF transition.The observation of line patterns reminiscent to the twboundaries in YBCO points to the formation of domains cincident with the antiferromagnetic phase transition. As hbeen shown by Detlefset al.15 the crystal lattice ofErNi2B2C undergoes a tetragonal (T.6 K! to orthorhombic(T,6 K! distortion at the onset of long-range antiferromanetic order due to magneto-elastic coupling. In order to mmize strains the crystal lattice breaks up into domains wdomain walls parallel to thec axis and@110# or @11̄0#, oneither side of which the direction of thea and b axes areinterchanged.In order to further confirm that these domain walls arelated to the linelike patterns and to explore their effectthe pinning of the VL, magneto-optical experiments wecarried out. The sample was ZFC toT,Tc and the field wascycled from zero to 40 mT and back to zero. The resultimages were recorded. The experiments were repeated4.2, 5.5, 6, 7, and 7.5 K. Distinctly different features weonly observed below and above 6 K. There was no notiable difference in flux penetration behavior at 2 K and aboFigure 3 shows the nature of flux penetration inErNi2B2C at 2 K. Such images are typical for the behavbelowTN . It is seen that the flux penetrates very inhomog0g4FIG. 3. Magneto-optical observation of flux penetration inErNi2B2C at 2 K. The sample was zero-field cooled to 2 K, the fiewas cycled to 45 mT and thereafter back to zero. The above picwas taken at 20 mT in increasing field.2-2tstratornaruxtahenotthisnio-blewsiinbinsaile.allentizeisig-isl isduem-ldtheattoillract. 1thehascebe. 1,edicbychayftogeet-ied40bra-ronRAPID COMMUNICATIONSMAGNETIC TWIN BOUNDARIES AND FLUX PINNING . . . PHYSICAL REVIEW B63 020502~R!neously in the form of bubbles and finger shaped flux fronThese observations are globally similar to the flux penetion in 2H-NbSe2 ,16 but there is a distinct difference in thathere are two preferential directions of flux penetration cresponding to the directions of the stripe structures seethe Bitter decoration. The directions of flux penetrationsindicated by the black arrows in Fig. 3. It means that fllines easily penetrate along the domain walls and thaaddition there are strong screening currents along the domwalls which drive the vortices into the areas in between. Tkind of flux penetration behavior is very similar to that sein twinned single crystals of YBCO,17,18 indicating that thedomain walls act as planar pinning centers which promvortex channeling along them. In contrast, above 6 K,flux penetrates uniformly yielding a flux profile whichdome shaped. A typical example at 7 K is shown in Fig. 4.The dome shaped flux profile is caused by strong screecurrents at the edges of the sample due to its platelet geetry in combination with absence of bulk pinning.19 An esti-mate of the edge current densityJe is obtained from themeasured value of the first penetration fieldHp ~12 mT at 4.2K! and the expressions19 Hp5Hc1Ad/W and Je52Hc1 /d,whereHc1 is the lower critical field,d is the sample thick-ness, andW is the sample width. The result isJe593108 A/m2. Information about the strength of the bulk pinning is deduced from the slope of the profile at the bubfront. The result at 4.2 K isJc'73108 A/m2. Similar ob-servations have been made with Hall-probe arrays by Dhurst et al.5 who concluded in agreement with our findingthat bulk pinning is comparable to the geometrical barrierthe AF phase while in the paramagnetic phase bulk pinnis negligibly small.We now turn to the questions why the line pattern is oserved below 6 K and what the actual pinning mechanismthe AF phase is. The fact that we do not observe vorticethe regions between the stripes or occasionally only a fimage, means preferential deposition of the nickel partictakes place at and around the location of the domain wallsFIG. 4. The dome shaped flux profile deduced from magneoptics at 7 K showing the absence of bulk pinning in the paramanetic phase. Sample edges are atx'0 mm and 700mm.02050.--ineininiseengm-e-ng-inntsInorder to explore this further we undertook scanning Hprobe experiments after cooling down the sample in differfields to a temperature of 5.2 K as shown in Fig. 5.In analyzing the images it should be known that the sof the scanning Hall probe~SHP! is 1 by 1mm, the distanceto the sample surface is around 0.5mm, the field calibrationis 900 V/T at 5.2 K, and that only the field modulationshown after subtraction of the applied background field. Fure 5 shows that across a domain wall the field variationaround 1 mT. The actual field variation at the domain walexpected to be larger, but it cannot be precisely detectedto the finite size of the SHP. It is further seen that the aplitude of the modulation is independent of the applied fieand that upon changing the polarity of the applied fieldoverall image reverses sign.This is convincing evidence for local ferromagnetismthe domain walls with an internal field component parallelthec axis. The resulting magnetic field above the sample wthus have strong gradients at the domain walls which attthe nickel particles during decoration. The stripes in Figtherefore show the positions of the domain walls. Onother hand, SHP probes the field modulation and thistwice the periodicity of the domain walls. Thus the distanbetween the brighter areas in Fig. 5 should approximatelytwice as large as the distance between the stripes in Figwhich is indeed the case.The origin of the local ferromagnetism can be explainas follows. At the domain walls both the crystallographorientation and the orientation of the Er moments change90°. This gives rise to a region of magnetic frustration whiwe believe leads to local canting of the Er moments awfrom the basal (a-b) plane. Depending on the amount o--FIG. 5. Scanning Hall probe microscope images across a ming point of two perpendicular stripes at 5.2 K and various applfields. The size of the scanned region is approximately 40 bymm. The background field has been subtracted. The field calition is 900V/T yielding a typical field modulation averaged ovethe size of the SHP of 1 mT. Note the black-white reversal upzero-field crossing.2-3stealueeneniin, ith-llfofiguronVLeayofn,s intorr-eticalt ofthemas-andhetaledRAPID COMMUNICATIONSN. SAHA et al. PHYSICAL REVIEW B 63 020502~R!canting this results in a strong local magnetic field. We emate that this local field can be 300 mT if we assumecanting of the Er31 moments by an angle of 10° out of thab plane and a width of the domain wall of a few crystlattice parameters. We further note that Mossbaspectroscopy20 has demonstrated the existence of a netc-axiscomponent of the Er31 moments in the AF phase. Since thexperiments in Ref. 20 were done on polycrystallisamples, these results were not decisive as to the locatiothe canted moments. Our work shows that they coincide wthe boundaries between the AF domains.It is obvious that strong local internal fields at the domawalls will severely suppress superconductivity. In additionmay also be the reason for the drop in Hc2 below TN in thephase diagram of ErNi2B2C. The domain walls will there-fore act as strong planar pinning centers, very similar totwin boundaries in YBCO,21 except that the pinning mechanism will be dTc pinning rather than thedk pinning sug-gested by Gammelet al.9 We believe that the domain-warelated ferromagnetism provides a plausible explanationthe sudden appearance of flux pinning belowTN . Since theonset of bulk pinning coincides with the appearance otransverse spin-wave modulation of Er moments, one msuspect that coupling of the VL to the spin-wave structs,sov.ys02050i-arofthterahtecould be a source of flux pinning. However, the modulatiwill be smeared out over the relevant length scales of thewhich will make this pinning mechanism less effective. Wfinally note that it would be interesting to study the interplof internal magnetism and the VL when the formationdomain walls could be suppressed.In conclusion, we have performed Bitter decoratiomagneto-optical and scanning Hall-probe measurementthe mixed state of the antiferromagnetic superconducErNi2B2C. The enhancement of flux pinning in the antiferomagnetic phase is caused by the formation of magntwin boundaries at which a local variation of the internmagnetic field is related to the ferromagnetic componenthe Er31 moments along the crystallographicc axis.The authors wish to thank Professor Aronson fromUniversity of Michigan and Professor Ramakrishnan froTata Institute, Bombay for helpful discussions. Technicalsistance from Marcel Hesselberth, Ton Gortenmulder,Ruud Hendrikx was of great help. This work was part of tresearch program of FOM, the ‘‘Stichting voor FundamenOnderzoek der Materie,’’ which was financially supportby ‘‘NWO.’’F.s.ok,.1C. Mazumdaret al., Solid State Commun.87, 413 ~1993!.2R. Nagarajanet al., Phys. Rev. Lett.72, 274 ~1994!.3D. C. Johnstonet al., Superconductivity in Ternary CompoundVol. II, edited by M. B. Maple and O. Fisher~Springer, Berlin,1982!, p. 11.4C. D. Dewhurst, R. A. Doyle, E. Zeldov, and D. McK. Paul, PhyRev. Lett.82, 827 ~1999!.5C. D. Dewhurst, S. S. 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Magnetic twin boundaries and flux pinning in the antiferromagnetic superconductor ErNi_{2}B_{2}C

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