Quaternary borocarbides: the new and exciting magnetic superconductors

Journal of

ELSEVIER

Journal o[ Alloys and Comlw~unds 2~2-2fi3 (1tl97) 22-27

Quaternary borocarbides: the new and exciting magnetic superconductors L.C. G u p t a Tara ht~tit.te of Fnmlamenta/ Re,~earch. ttomi Bhahha Road, Mumhai 400 (~5, hulia

Abstr~tct

Ouatcrnary borocarbide superconductors RENi:B:C (RE ~ Y, Dy. Ho, Er. Tm and Lu) have ~veral features which make them ideal candidates to study one of the most exciting and intriguing phenomena of conden~d matter, viz., the interplay of ~u~rconductivily and long range magnetic order, No~ only their su~rconducting transition tem~ratures are the highest ~mong the intcrmetallic su~r¢onductors, their magnetic ordering temperatures are al~ the highest among all the known m~gneli¢ su~rconductors, DyNi: B:C prc~nts a very interesting situation: su~rconductivity ,~ts in an already magnetically ordered h~llicc, Other ex~mple~of thi,~interplay in title borocarbide family will be presented. Absence of superconductivity in YbNi~B~C i~ rather ~mom~!loo~;it poin[~ to the 4[ocondtletion electron hybridization, ~c~1q97 Elsevier Soil,nee S,A, h~c~w-~d~ ~ Boro¢~rhidc~; Quaternary; Hi~h°'~ ~u!~:rconductivity; Magn¢|ism: Interplay

I, Int~$uclton Di~overy of su~rconductivity in the multiphase quaternary ~rocarbide :,ystcm Y=Ni=B-C [I,2] i~; ;~, major event in condensed matter. It t ~ k place nearly 7 years after the ~dnorz=Muller di~.:overy of High='~ su~r¢onductivity and injected fresh life into su~r° conduclivity re.arch. One of the most exciting tealures of borocarbide su~rconductors is the co=ex= istence/interplay of superconductivity and magnetism excurring at temperatures much higher than observed ever ~forle.

This discoveD~came abou[ as a resull of our intcro c~t ao:d efforts, spanning over the last 2 decades, in the phenomena of valence fluctuations. Among a ratio ely of mt~ed va|encc com~mnds, we investigated in several Niocontaining materials, Members of the se° t'ies of ternary |~3rides RENi4B (RE ~ rare earth) w~ere sludied as part of this program. We observed (~e Fig. 1). in samples of nominal com~sition of ~ 1~7 Ehcvi,ci Scici~cc S.A. All tighls reserved Pf# SO'~25-S3~81 ~7)!t0323o X

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YNi4B [I]. a weak but reproducible signal of super= conductivity at 7: as high as 12 K. This was an exciting result as no binary or ternary superconducting phases of the elements Y, Ni and B are known with such high ~.. We, thus, had a new superconducting pha~ with T,. as high as 12 K. In our efforts 1o investigate the origin of this weak superconducting signal, we prepared alloys YNi~BC, in which carbon had ~ e n introduced. A dramatic enhancement of the SUlrerconducting properties took place in these alloys. Several samples of the composi= lions YNi, B,C., with varying relative proportions of Ni, B. and C, were studied. Su~rconducting sample (resistivity and diamagnetism shown in Fig. 2) YNi~ B~C,~ ~ exhibited a sizeable specific heat anomaly at -~ 13 K, thereby establishing bulk superconductivity in the quaternary ~stem Y=Ni-B-C [2]. This pioneer= ing and ~minal discovery [I,2] laid down the foundation of the new and exciting field of quaternary borecarbide superconductors.

L.C Gupta / goumal of AIIoys and Compomuls 262-263 (1997) 22-27

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Subsequently, T,, as high as 23 K [3] and two superconducting transitions with T,. = 22 K and 10 K [4] were reported in multiphase samples of the Pdcontaining quaternary system Y-Pd-B-C. Some of our samples of the composition YNi2B.,C showed, in microwave absorption studies [5], a feature suggesting the existence of a superconducting phase with T,. = 23 K. Subsequently, possibility of a superconducting transition at Tc ~ 24 K was indicated [6] in samples of the quaternary system Y-Ni-B-C. These samples were synthesized using powder metallurgy under high temperature ~ high pressure conditions. 2. Structure of the REN| 2 B2C'phase

Fig. 3 shows a unit cell of the structure [7] of the superc,mducting single phase LuNi:B2C ( ~ ~ 16.5 K [8]) belonging to the quaternary system REoNi°B-C.

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This is a filled-in version of the well known tetragonal ThCr.,Si.,-structure (space group 14/mmm). It was confirmed [9] that YNi:B:C and several other members of the series RENi~B:C (RE = Ho, Er and Tin) have the same structure. Highly anisotropic vibrations of C-atoms were inferred from the X°ray diffraction [9]. The structure is essentially a stack of infinite REoC sheets and Ni:B: ('B-Ni-B sandwiches') layers arranged in an alternating sequence along the c-axis. The short B=C bonds provide contact between RE°C planes and Ni:B:olayers: they are also responsible for the conduction path along the c-axis. It is to be recalled that in the ternary superconduco tors such as RERh4B4 and REMo,,S~, to.existence of superconductivity and magnetism and relatively high T,. have been regarded [10a,b] to arise because their respective structure contains clusters of transition metal atoms. Borocarbides have no such clusters but they exhibit high T, values and high TN values (see below). Therefore, clusters are not a crucial and eSo sential ingredient for high T, and the coexistenc~ phenomena in intermetallics.

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Borocarbides are particularly important as they have rather high T, values and TN values, see Table 1. 111 fact, the co.existence occurs at the highest ever reported temperature. Further, RENi:B:C series is

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unique as its members have all the conceivable combinations of 71 and T.+, namely. 11 > T,,. "/: -,- T~ and 71 < T~. In this paper, we focus on some of the phenomena arising due to the interplay of superconductivity and magnetism in these materials. 3,1, D v N i : B : C : IT, + + hi. T~ .. I I K: 7: < T~

From the magnetic susceptibility [I 1.12] and speck fie heat [12] measurements, it was shown that ~ N i : B : C undergo¢~ an anliferromagnetic ordering ~t T,~ ~ I1 K0 Superconductivity at a fairly high 1: t + ~ K) wa~ reported bofll in si~gl¢ ¢lT~tal [13.14] as well a~ polyery~tal m+mple~ of DyNi:B:C [12.151. Fig. ~l [15! ~how~ ~¢o~u~¢¢plibi!ity of a polyc~T~lallin¢ mmio pie of |)yNi:B:C wll~re one clearly ~ee~ the occur° r¢~!c¢ o!' mt~gnetic trml~ithm at - II K and supcr¢ono duglhl~ transition ~l - ~, K. A~ E i~ les~ than T,., one ob~e~,'e~ here superconductivity in an already magnet° ically ordered lattice. Such materials are rather rare:

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only two other stoichiometric compounds (Er.,Fe3Sis: 7: = 1 K and Tx = 2.9 K[16]; and Tb_,Mo3Si4: T, = 1.5 K and T,, = 19 K [17]) are known. DyNi:B:C has the highest 7:. among them. Only a few materials with T, < Tx are known inspite of the fact that there is no fundamental reason why many more such materials should not exist. However, in view of the fact that non-magnetic impurities give rise to pair-breaking - - and therefore depression of T , in anti-ferromagnetic superconductors [18a-d], it is conceivable that there are many anti-ferromagnetic superconductors; their T, is. however, suppressed beyond observation by non-magnetic impurities which, presumably, may always be present in small concentrations and are hard to detect. It must be pointed out that non-magnetic impurities do not appreciably affect ~ in conventional isotropic superconductors (Anderson's theorem [19]). The following two observations are very instructive in this regard: (i) superconductivity in DyNi:B,C was not observed down to 2 K in the earlier studies [1 !]. Perhaps. some extrinsic/intrinsic disorder caused the depression of 71 in these studies (it) 7~ in Dym++,Lu,Ni_,B:C [20] geis depressed strongly by non-magnetic Lu-ions. One possible approach, as it emerges from what has been said above, to identify magnetic supercon' to examine mc:t i~: °~ ¢ +i o.dy S' ductors with 7] < T,+ ~s such anti°ferromagnets whose non-magnetic analogues exhibil superconductivity. One can easily find sc era! such examples from literature [21]. These materials +s to minimize the eft~¢ls of ~h~m!d he ps~p, °+ :t+~ ~.1 s~ a, m~ll.niag+~elic impurities. The basic message is: observation of superconductivity m D yNi:B~C at i~, less than T~+ is a clear assurance of the existence of many more of such materials and that the two earlier known materials with 7: < T,~.(see al~3ve) are not a freak of nature. As has already been stressed [21], such materials are signiticant for investigating phenomena related to coexistence of superconductivity and magnetism, such as gapless su~rconduclivity in anti°ferromagnetic superconductors, aton|ic+level Meissner shielding, nature of pairing in the anti-femmmgnetic state and so on. Magnetic structure of DyNI:B:C, as determined from neutron diffraction studies [.,+a,b: ~'~ ' 23a,bl is quite simple. Ferromagnetic ~heets of Dy-moments iconlined in the a4~ plane witll no component along |lie c-axis) are stacked anti°ferromagneticaHy along the c+axis+ ~"~Dy+Mossbauer absorption studies in DyNi:B~C show [2,t] that Dy+spins undergo a first order magnetic transition a t - - I 1 K and that the Dy-C layers are insulating, The Ni, B+4avrrs are then the carriers of supercurrent in RENi:B:C superconductors.

L C Gupta /,Ioumal oj'Allo?; and Comt~nmds 262-2(~3 ~i997~22-27

3.2. HoNi_,B,_C (T,. ,.-8 K', Tx~,8.5 K;" 7",.-,, Tx) A remarkable feature of HoNi,B,C is that it exhibits double re-entrant superconducting transition (DRST) [11,25]: it is the first material in which such a behavior has been observed in zero applied field. Coming from high temperature, HoNi,,B_,C orders magnetically (c-axis spiral [25]) at = 8.5 K and goes into the superconducting state (Srstate) almost at the same temperature (T,. --- 8 K). It re-enters the normal state at = 5 K, and regains superconductivity (S,.-state) as the material is further cooled below 5 K. There is a modification in the nature of the magnetic ordering as the material passes from the S~-state to S.,-state. Magnetic ordering is incommensurate in the St-state whereas it is commensurate in the S~-state [22a,b; 25]. Drastic reduction of the superconducting fluid density p,,,p(T) [26] in the normal state, sandwiched between the two superconducting states St and S.,, is consistent with DRST in HoNi,B,C. Double re-entrant superconducting transition is very sensitive to the chemical composition (sample dependent) and the heat treatment [27a,b; 28a,b]. Fig. 5 shows ac-x(T) of two samples i and !! of HoNi_,B:C having nearly the same nominal composition. Sample !I undergoes a transition at T,~-- 8 K whereas sample I exhibits DRST. We believe that non-magnetic impurities, which are hard to detect and are likely to be present, are responsible for this sample-tlependent behavior of HoNi, B,C.

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in ErNi.,B,C was observed through s~cific heat and resistivity studies [291. With its T,~> T,~, this material provides a complementa~ combination of T,. and Tx with respect to that in DyNi,B2C. Neutron diffraction studies, both in polyc~stal as well as in single crystal samples, show that Er-magnetic moments undergo an incommensurate anti-ferromagnetic ordering [30,31]. The Er-moments are ordered in a transversely polar.~zed planar sinusoidal structure propagating along the a(b)-axis with Er-moments parallel to the b(a)-axis. No c-component was observed in these measurements. Though speculations [32a,b] have been made on the possibility of ErNi,B.,C undergoing a transition to a weakly ferromagnetic state below 2.3 K, the existing data [32a,b] do not conclusively prove it. Implications of such a state with respect to a spontaneous vortex phase in a magnetic superconductor have been pointed out [33]. From the Mossbauer studies of ""Er in ErNi B2C [34] it was shown that Er-spins undergo a first order magnetic transition at /~,~=- 6 K into an incommensurate magnetic phase and the sinusoidal modulation gets fully squared up at low temperatures (T = 1.4 K). Er-spin relaxation rate exhibits an anomaly at T¢ = 10.6 K, suggesting that the band electrons are exchange-coupled with the 4f-moments and are involved in the formation of the superconducting state. In fact, high Tx values in borocarbides also imply that the RKKY-interaction is the dominant exchange interaction between the RE.. and the conduction electron spins. In this respect, borocarbides are very different from the ternary superconductors RERh~B~ REMo, S,. wherein it is usually assumed that the superconducting electrons (derived from Rh° and Mo-metal atoms) do not have appreciable interaction with the REospins.

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Temperature

20

P.5

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Fig. 5. Diamagnetic response of two polycrystalline samples of ItoNi~B:C° Sample ii shows a superconducting transition at 8 K whereas sample I clearly shows the double re.entrant superconducting transition (Z. Hossain. Chandan Mazumdar. R. Nagarajan and L. C. Gupta. unpublished results).

TmNi:B,C has the smallest ratio Ts/T, ~ 1.5/11 among all the magnetic superconducting quaternary borocarbides. The magnetic structure of this material is quite different from that of the three borocarbide magnetic superconductors, namely, magnetic moments are oriented along the coaxis [22a,b] whereas they are perpendicular to the c.axis in RENi:B:C (RE ~ Dy, Ho and Er). H,.:(T) is highly anisotropic below ~ 6 K, H~:( ± c) ~- 2 H,,(llc) [35]. This is consistent with the anisotropy of magnetic susceptibility in the normal state, namely, Xt~,~.> ~'HL~'" These observations suggest that the magnetic p,ur-I teakuig is responsible for the overall anisotropy of H~:. Specific heat measurements [36] have been interpreted to suggest that the Tm-C sheets order ferromagnetically (strong ferromagnetic interaction within the Tm-C sheet) and that these ferro-

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magnettc sheets are weakly coupled anti-ferromagneticatly~ Two-dimensional-ferromagnetic magnons, arising due to the ferromagnetically aligned sheets of Tin.moments, contribute a large gamma term to the low temperature specific heat [36]. Mu-SR measurements have been carried out on TmNi:B,C [37] down to 1~ mK to learn about the ~+namies of Tin- and Ni-moments. The Tin-spins are correlated, at the Mu-SR time scale, at temperatures as high as T > 10 T'~'. The rapid 2D-correlations in the normal phase are slowed down by the onset of su~rconductMty. Anti-ferromagnetic fluctuations associated with Ni-moments have been studied in YNi:B:C by NMR techniques with divergent conclusions drawn with respect to their existence [3.8] and the ab+~nce [39]. We believe that magnetic fluctuations of Ni-spins in borocarbide superconductors, non-magnetic as well as magnetic, is an open and important question considering that Ni-electrons form Cooper pairs. Anti-ferromagnetic fluctuations may play a role in the mechanism of superconductivity. 3.5~ WoNi : Bo:C ~ a non.magnetic normal heat~, fermion

Yb-ions are quite close to being trivalent in YbNi: B:C [1530]. From the pattern of variation of in RENi:B:C [21], YbNi:B:C should superconduct at T~ ~ 12 K. Remarkably, YbNi:B:C does not super¢('~nduct down to 2 K [15,4!1] nor does it order magnet° icatly. Single crystal studies have also confirmed this behavior of YbNi:B:C [41]. Specific heal studies show that a heavy fermion state evolves at low tempera° turc~ [4tt,41] suggesting au enhanced hybridization of the k~alized Ybo4f and the itinerant conduction clot. irons which may bc responsible for the anomalous depression of 7~ in YbNi:B:C. 4. Summaff To summarize, we have demribed here briefly the events that led tO the dimove~ of superconductivity in muhiphase samples of the ttuatcrnary~ • ~ system Y-NiBoC. Tetragonal structure of RENi,B~C does not !rove clusters which used to be considered e,,enttal'ss" ', for high 7:, in intermetallics. We commented briefly upon ~me of the features of bor~arbides, for exam. pie. highest 7:,~ in the superconducting state, all possi= hie combinations of T and T~' 7~ > T. ~ ~ 7~, a.d 71 < 7~ and anomalous suppression of 7~ due to the hybridization of the 4f° and conduction electrons. Mog~bauer studies ErNi~B~C suggest that the con. duction electrons which °are exchangeocoupled with REospins are also involved in the formation of the supmrconduc|ing state. This raises some basic queslions with ~ s ~ c t to the very survival of su~rconductivity under such circumstances, and one is motivated

to take a fresh look at the co-existence phenomena. These materials provide us with an opportunity to investigate new and yet unexplored phenomena [21,42] associated with interplay of superconductivity and magnetism. Borocarbides have great potential to generate new physics; they have led to the pathway to identify new superconducting materials, possibly with even more exciting properties.

Acknowledgements Several of our studies described here are a part of our collaborative program with laboratories within and outside India. Work in borocarbides at TIFR is being carried out in collaboration with R. Nagarajan, S.K. Dhar and Z. Hossain.

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L.C Gulna ~Journal o]'Alloys aml Coml~mnds 202-263 U997) 22-27

[151 [161

[171 [18a] [iSb]

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12,Sbl E. Tominez, E. Alleno, C. Godarl, J. AIh~ysComp. ~2-263 199"/) 4 6 2 - ~ ~this issue),

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