Strong Enhancement of the Critical Current at the Antiferromagnetic Transition in ErNi2B2C Single Crystals

0.81 MB·English

by  M. Weigand

Strong Enhancement of the Critical Current at the Antiferromagnetic Transition
in ErNi B C Single Crystals
2 2
M. Weigand,1 L. Civale,1 F.J. Baca,1 Jeehoon Kim,1 S.L. Bud’ko,2 P.C. Canfield,2 and B. Maiorov1
1Materials Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
2Ames Laboratory, US DOE and Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA
(Dated: January 31, 2013)
We report on transport and magnetization measurements of the critical current density Jc in
3 ErNi2B2C single crystals that show strongly enhanced vortex pinning at the N´eel temperature TN
1
andlow applied fields. Theheight of theobserved Jc peak decreases with increasing magnetic field
0
in clear contrast with that of the peak effect found at the upper critical field. We also performed
2
the first angular transport measurements of Jc ever conducted on this compound. They reveal the

n correlated nature of this pinning enhancement, which we attribute to the formation of antiphase
a boundaries at TN.
J

0 PACSnumbers: 74.25.F-,74.25.Sv,74.25.Wx, 74.70.Dd
3

] Single quanta of magnetic flux enter a type II super- slightly suppressedjust below T =TN for Hkc and has
n conductor in the form of vortices when it is exposed to aninflectionpoint for Hkab [17]. This is a consequence
o
a magnetic field H. This is a big setback for applica- of the local moments ordering in the antiferromagnetic
c
- tions, because when an electrical current J is applied, phase,aswasshownexperimentally[17,18]andtheoret-
r
p vortices move and energy is dissipated. This movement ically [19, 20].
u can be arrestedby non-superconducting defects that pin Inadditiontotheoccurrenceofantiferromagnetism,at
s vortices by lowering the system energy. The interaction T = T a tetragonal to orthorhombic crystal structure
. N
t between vortices and pinning centers has been studied transition takes place [21]. The resulting twin bound-
a
m extensively for decades, focused mainly on the so-called aries were shown by Bitter decoration, magneto-optical,
vortex core pinning (caused by the local suppression of and scanning Hall-probe experiments to act as pinning
-
d the superconducting order parameter) [1, 2]. Less ex- centers at lower temperatures (T < T ) [22, 23]. The
N
n plored is the interplay between vortices and magnetic pinningmechanismwasproposedtobecausedbyaferro-
o
media, whichcouldoffer pinning forcessuperiorto those magneticspincomponentparalleltothecrystallographic
c
[ fromcorepinning[3–6]. Acommondifficultyofstudying c axis localized at the twin boundaries [22].
magnetic pinning is the inability of separating the mag- Although borocarbides (and particularly the com-
1
netic and core contributions. It is, therefore, of great pound with RE = Er) have been extensively studied,
v
6 advantage to study systems where the magnetic transi- few criticalcurrentmeasurementsbyelectricaltransport
4 tiontakesplaceinside thesuperconductingphase,allow- have been performed, with the exception of the seminal
1 ing one to compare the behavior with and without the work by Gammel et al. [24], exploring the weak ferro-
7 magnetically ordered phase. magnetism below T = 2.3 K [25–28]. This was shown
.
1 Thereareaconsiderablenumber ofmaterialsinwhich to lead to an enhanced critical current density J , which
c
0 to study the coexistence between superconductivity and was ascribed to an increase in pinning force due to pair
3 orderedmagnetic phases, suchas the Chevrel-phases[7], breaking by the ferromagnetism[24]. Canfield et al. [29]
1
CeCoIn [8],andrecentlyironpnictides[9–11]. However, pointed out that the data in Ref. [24] extrapolated to
: 5
v the rare earth-nickel-borocarbide family (RENi B C, J =0 at T , indicating a clear linkage between pinning
2 2 c N
i
X whereREisarareearthelement)[12–14]hasseveralad- and the antiferromagnetic order. However, this extrapo-
vantages,namelyarelativelyhighsuperconductingtran- lation could not be confirmed because transport experi-
r
a sition temperature T and the tunability of the ratio be- ments were only done for T < 4.2 K to take advantage
c
tween its magnetic and superconducting ordering tem- of the cooling power of liquid helium [24].
peratures, which can be changed by using different rare The lack of transport experiments is to a great extent
earthelements[15,16]. Thereadilyavailablehigh-purity due to heating problems caused by the applied current
single crystals allow one to study the effects of magnetic being very high because of the large cross section of sin-
pinning without any significant defect (non-magnetic) gle crystals. To the best of our knowledge, the present
contribution [15]. work is the first report both of transport measurements
AmongtheborocarbidefamilythecompoundwithRE on ErNi2B2C single crystals at temperatures around TN
= Er has a Tc of 10.5 K and a N´eel temperature TN of and of angular Jc measurements performed on this com-
6.0K[17]. Theoccurrenceofantiferromagnetismdirectly pound.
influences the superconducting properties, as seen in the In this Letter, we present the results of transport
uppercriticalfieldvstemperaturecurve,whereH (T)is and magnetization measurements on ErNi B C single
c2 2 2
2
crystals, which revealed a local maximum in J (T) at value just below and above it. These are the key results
c
T = T . We study this large increase in J as a func- of this Letter. Finally, at a temperature just above 7 K
N c
tionof fieldstrengthandorientation,andwe findthat it we observe the well-documented peak-effect at H .
c2
occurs only for fields oriented along the crystallographic

c axis. Unlike the peak effect near H , the height of
c2
5 6 7 8
thenewlydiscoveredmaximumdecreaseswithincreasing
12
magnetic field. We rule out pinning from twin bound- (a)
aries as we are able to determine its angular fingerprint 10 = 0(cid:176)
at lowertemperatures, which differs fromthe increase in H||c
J observed at T . We attribute the findings to vortex 2 ) 8
c N - m
pinning due to antiphase boundaries between antiferro- A
magnetic domains. 5 6
0
1
groLwanrgeu,sihnogmtohgeenNeio2uBs flsiunxglegrcorwytshtaltsecohfnEiqruNei2[B302C]. wTehree J (c 4 = 30(cid:176)
samples used for transport measurements were polished 2
mechanically down to a thickness of 40−60 µm parallel (cid:181)0H = 0.5 T T N
to the c axis before being cut to pieces ∼250 µm wide 0
6 -2
and ∼1.5 mm long. Sputtered Au contacts allowedus to 0.0 1.25•10 A m (b)
481.1.102.32.26.2.6303.0573.56340.980E4.307E5.7013E5.13+0E5.55+7E6.06+3E06.48+00E7.80+7E074.21+30E74.63+0E085.05+7E085.56+3E095.98+0E095.30+7E015.71+3E015.13+0E015.55+7E015.06+3E015.08+0E015.00+7E015.14+3E05.18+0E05.22+2E0526+3E050+5E055+7E05+8E05+0E05+E05+E05+06+06+06060666
apply currents along the ab-planes while measuring the 1.0 H||c
voltage across the sample. We observed a slight reduc-
Hc2
tion of T (∼0.5 K) due to sample preparation, which
c
)
is consistent with the previously reported increase in T T 0.8
after annealing [31]. The small decrease in Tc does noct H (0 TN
(cid:181)
affectthe overallresultsofthe presentstudy; itleftboth
T and the shape of H (T) unchanged. 0.6
N c2
Transport measurements were performed in three sys-
tems characterized by different cooling methods: (i) a
Quantum Design, Inc. PPMS with a semi-closedHe sys- 0.4
5 6 7 8
tem, (ii) a variable temperature insert with flowing He,
and (iii) with the sample immersed in liquid He. In all T (K)
systems we used high-precision rotators and the maxi- FIG. 1. (Color online) (a) Transport measurements of Jc for
mum Lorentz force configuration (J ⊥ H). The criti- Hkc and θ =30◦. A new peak in Jc is observed at T =TN
for H k c. The sketch gives the measurement geometry. (b)
cal currentdensity was obtained from IV-curves using a
1 µV criterion. Measurements of R(T) and R(H) gave ContourplotofJc asafunctionoftemperatureandmagnetic
fieldHkcshowingthefieldrangewherethepeakatT =TN
the upper critical field vs temperature, using a value of
is present.
R corresponding to 90% of the normal conducting re-
sistance. The comparison of the J results obtained in
c
ThemaximumatT =T occursoverarangeoffields,
the three systems allowed us to assess the magnitude of N
as seen in the contour plot in Fig. 1(b), obtained from
heating effects.
multiple J (T) measurements at different H k c, where
Magnetization measurements were carried out in a c
differentcolorsrepresentdifferentvaluesofJ . Notethat
Quantum Design, Inc. MPMS Superconducting Quan- c
the height of the J peak at T shows the opposite field
tumInterferenceDevice(SQUID).J wasobtainedusing c N
c dependence than that of the one observed near H , in-
the Bean critical state model with J = |mi| · 4 , c2
c abc b(1−b/3a) dicating a different origin of the two maxima. Whereas
wherea≥b,miistheirreversiblemagneticmoment,and the peak near Hc2 becomes more pronouncedas H is in-
a, b, and c are the sample length, width, and thickness, creased,thepeakatT decreaseswithincreasingH until
N
respectively [32]. ithasessentiallyvanishedatµ H =0.7T. Thisleadsus
0
InFig.1(a)wepresentaJ transportmeasurementvs to speculate that the peak at T has a magnetic origin,
c N
temperature at µ H = 0.5 T for H k c. Starting at low as magnetic pinning tends to be attenuated as the mag-
0
temperature,J (T)decreasesrapidlyasT increases,and netic field modulation of the vortices becomes smaller.
c
aroughextrapolationofJ (T)leadsto 0atT =T ,the Our observations are consistent with the dip found in
c N
samebehaviorfoundinRef.[24]. Ournewmeasurements the dynamic magnetic susceptibility of ErNi B C single
2 2
presented here, however, show that J instead changes crystals at T =T , measured by Prozorovet al. [33] us-
c N
abruptly at T . The slope of J (T) is much lower at ing a tunnel-diode resonator technique. They explained
N c
T > T than below, and a peak in J (T) is observed at theirresultsbyapinning enhancementdueto theoccur-
N c
T ,correspondingtoatwo-foldincreasecomparedtothe rence of antiferromagnetic order, accompanied by large
N
3
magnetic fluctuations. The decrease in the peak height inFig.2(b). Itcanbe seenthatJ isenhancedonlyover
c
with increasing H also rules out the drastic change in a narrow temperature and angular range near T = 6 K
H (T) at T as the origin for the maximum in J (T), and θ =0◦.
c2 N c
since the peak disappears far below Hc2(TN). The ab-
sence of a J (T) peak at θ = 30◦, shown in Fig. 1(a), is
c 0 30 60 90
further proof that this peak is not associated with the 2.0
2.0
sudden change in H (T) at T , since for θ = 30◦ this (a) T = 4.2 K
c2 N
feature in Hc2(T) is still visible (not shown). 1.5
30I◦ttihsactleathrifsroemffetchteisJcst(rTo)ngmlyeaasnugrelem-deenptsenadteθn=t. 0T◦haunsd, -2 A m) 1.5 1.0
further insight can be gained from the dependence of Jc 6 0 1.0 -30 0 30
otwnefieenldthoeriseunpteartcioonn,dsuicntciengitaanlldowthseomneagtonedtiisctianngiusoisthrobpey-. (1Jc TT == 56..7050 KK
T = 6.25 K
FigT.h2e(ao)b.taTinheedy raenvgeuallarricJhcacnudrvceosmaprleexrfeepartoudruesc.edDien- 0.5 (cid:181)0H = 0.4 T
spite the small change of the absolute value of Hc2 with H||c H||ab
fieldorientation,wefoundthatJchasapronouncedmax- 0.80 361.1.301.62.03.2.3620.6033.0373.0360E3.603E4.0037E4.36+0E4.60+3E5.03+7E05.36+0E05.60+3E064.03+70E64.36+0E065.60+3E075.03+7E075.36+00E75.60+3E085.03+7E085.36+00E85.60+3E095.03+7E095.36+00E95.60+30E15.03+7E05.36+00E5.60+3E0503+7E056+0E050+30E5+7E05+0E05+E05+0E5+ 05+05+05050556
imum for H k ab at all temperatures investigated. This (b) H||c 0 1•106 A m-2 H||ab
indicates strong pinning for H k ab, in contrast to what
(cid:181)0H = 0.6 T
would be expected from the anisotropy of H , where
c2 7
Hkc >Hkab. Italsodiffers fromwhat hasbeen observed
c2 c2
in non-magnetic YNi B C thin films, where both J and )
2 2 c K
Hc2 show the same anisotropy [34, 35]. The maximum T ( 6
we find in J (θ) around H k ab can be explained by
c
magnetic pinning, and it is consistent with the angular
dependence ofthe magnetic susceptibility ofthe Erions, 5
namely χ ∼0 for H kc, while for H kab the full Curie-
Weiss form is observed [16]. We also find small maxima
at θ = ±22◦ both above and below T , which are not 0 30 60 90
N
related to the sample shape and will require further in- (deg)
vestigation.
FIG. 2. (Color online) (a) Angular transport Jc measure-
At T = TN a wide peak centered at H k c is visible, ments at µ0H = 0.4 T reveal the narrow temperature range
whereas at temperatures only 0.25 K above and below at TN over which the peak at Hkc occurs. (b) The contour
a plateau is observed. The peak in the angular depen- plot of Jc as a function of magnetic field orientation θ and T
dence of J indicates that the nature of this pinning is shows the location of the peak at TN and θ =0◦ in the T–θ
c
space.
correlated. Naturallyoneistemptedtoascribethismaxi-
mumto the alreadyreportedpinning bya ferromagnetic
spin component localized at magnetic twin boundaries The correlated nature of the pinning and its location
[22]; however we have measured a much narrower J (θ) aroundT lead us to conclude that it is due to dynamic
c N
peak at lower temperatures [T = 4.2 K, see the inset of magneticdomainsthatarecreatedatT andannihilated
N
Fig. 2(a)], which is clearly due to the presence of twin as the temperature decreases further. Domain walls are
boundaries, as has been observed in Bitter decoration knowntoproduceplanarpinning potentialsinferromag-
and scanning Hall-probe experiments [22, 23]. Its differ- netic/superconducting hybrids [36, 37]. This poses the
ent shape indicates that the J peak at T is not caused interestingquestionwhythepinningenhancementoccurs
c N
by twin boundaries. The peak due to twin boundaries for H k c, while the easy axis of the magnetic moment
appears for T <T and exhibits a monotonic growth as for ErNi B C is the b axis [38, 39]. This could be due
N 2 2
the temperature decreases. The height of this peak can to the magnetic moment being flipped to the c axis; it
be roughly extracted when comparing J (T) at θ = 0◦ has to be noted, however,that this is highly unlikely be-
c
and 30◦ shown in Fig. 1(a). The disappearance of this cause of the high crystalline electric field anisotropy of
peak just below T is also confirmed by the angular de- the local moment sublattice. Another explanation could
N
pendenceatT =5.75K[seeFig.2(a)]. Thetemperature be vortices bending into the ab planes at the magnetic
evolution of J (θ) indicates that the peak at T has a domain boundaries in order to gain the Zeeman energy.
c N
different origin than that at low temperatures, although Asthevorticesmoveawayfromthecaxistheenergygain
both are correlated along the c axis. gets gradually smaller, until at the ab planes there is no
The dependence of the T peak on temperature and gain since vortices have zero net gain. A phenomeno-
N
fieldorientationbecomesevenclearerinthecontourplot logical model indicates that J ∝ cos3(θ) [40]. This
c
4
gives a broader maximum than the one observed experi-
14
mentally,neverthelessitcanexplainqualitativelytheen- -2 m)9
hancement observed. The presented results and the pro- 12 A
posedmodelmotivatemeasurementsinothercompounds 7 0
18
csuacxhisas[4T1m].NIit2Bis2Cw,owrthhicnhohtiansgittsheaatsyasasxoicsiaptaerdallwelitthotthhee -2 m) 10 J (c
sptetirntarnaiingnogonca[c4lu2rt]so. toWhraththeoctrohhueolrdmtabhliiscsotibrsaeninrseditseipeoodnnstinhibeElecrfaNosire2Becn2ohCualdn[2c1be]de, 7 J (10 A c 68 TN 75.5 T6 (.0K) 6.5
4
answered by investigating a borocarbide which does not
undergoastructuraltransitionatT =TN,namelyagain 2 (cid:181)0H = 0.02 T
TmNi B C.
2 2 H||c
0
In order to further explore the effects at lower fields,
4 5 6 7 8 9 10
which are not accessible to transport measurements due
T (K)
to heating, different samples were taken from the same
FIG. 3. The critical current density vs temperature at
batchandinvestigatedintheSQUID.Thisalsoservesas
verification of our transport measurement results. After µ0H = 0.02 T (with H k c), obtained from magnetization
creating a Bean profile [32] by applying a field H k c measurements, exhibits a local maximum at T =TN.
largerthan2H∗ (thefieldabovewhichscreeningcurrents
flow in the entire sample volume) at T = 4.5 K, the
knowninEr,HoandTmborocarbides. Inaddition,new
magnetic moment m was measured vs temperature at
developmentscanbe expectedforother superconductors
µ H = 0.02 T. In order to capture the increase in J
0 c withcoexistenceofmagneticphases,suchasunderdoped
around T the Bean profile was generated at different
N copper- and iron-based superconductors [43, 44].
temperatures near T before setting µ H = 0.02 T and
N 0
The authors are grateful to S. Lin, C.D. Batista,
measuring m(T).
L.N. Bulaevskii, and R. Prozorov for useful discussions.
We then compiled an m(T) curve from the different
This publication was made possible by funding from the
branches, selecting the branch with the highest (abso-
Los Alamos LDRD Program, Project No. 20110138ER.
lute) value of m for each temperature. The obtained
The transport measurements were performed in part at
curverepresentsanenvelopeofall m(T)branches. With
the Center for Integrated Nanotechnologies and at the
the Bean model the irreversible magnetic moment then
National High Magnetic Field Laboratory, both at Los
gives the critical current density [32].
AlamosNationalLaboratory. WorkatAmes Laboratory
As can be seen in Fig. 3, the obtained J (T) shows a
c (PCC and SLB) was supported by the U.S. Department
peak exactly at T = T , similar to the local maximum
N ofEnergy,OfficeofBasicEnergyScience,DivisionofMa-
observed in the transport measurements [see Fig. 1(a)].
terialsSciencesandEngineering. AmesLaboratoryisop-
Although the shape of J (T) at this field resembles that
c erated for the U.S. Department of Energy by Iowa State
of H (T), it is not directly governed by H (T), as the
c2 c2 University under Contract No. DE-AC02-07CH11358.
heightoftheJ peakobtainedfrommagnetizationcurves
c
decreaseswithincreasingmagneticfield,vanishingfarbe-
low H (T ), as in transport measurements. The peak
c2 N
in J at T and its field dependence are seen in mag-
c N
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