Osmium doping of UAl 2. Department of Physics and Engineering Greenville, SC Department of Physics Gainesville, FL

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1 Osmium doping of UAl 2 T. D. Scott 1,2, D. J. Burntt 2, J. S. Kim 2, and G. R. Stwart 2 1 Bob Jons Univrsity Dpartmnt of Physics and Enginring Grnvill, SC Univrsity of Florida Dpartmnt of Physics Gainsvill, FL Abstract Sampls of UAl 2 and UAl 1.6 Os 0.4 wr mad and analyzd using x-ray diffraction and magntic suscptibility masurmnts. Th rsults of ths masurmnts indicat that th rplacmnt of aluminum with osmium dcrass th ffctiv mass of lctrons in this systm, suggsting that spin fluctuations ar bing supprssd.

2 Introduction In 1975, Trainor t al. prsntd rsarch indicating th prsnc of spin fluctuations in th systm UAl 2 [1]. Spin fluctuations in a substanc ar th rsult of magntic spins fluctuating in and out of alignmnt. Ths fluctuations intract with lctrons such that, in a sns, thy slow down th movmnt of lctrons rsulting in an ffctiv lctron mass (indicatd by m ) gratr than th normal bar (or non-intracting) lctron rst mass, m. This largr ffctiv mass influncs th proprtis of th substanc. Particularly, w ar intrstd in th spcific hat (spcific hat at T = 0 is givn by γ) and th magntic suscptibility (χ) of our sampls. Ths proprtis ar dirctly proportional to m, and thrfor to th spin fluctuation bhavior of th systm. Thrfor, w can draw conclusions prtaining to th spin fluctuations of th sampl basd on our obsrvations of χ and γ. Our projct involvd rplacing a small amount of aluminum in th UAl 2 systm with osmium. This procss, calld doping, changs th proprtis of th sampl. For this particular projct, w rplacd 20% of th aluminum to form UAl 1.6 Os 0.4. Our plan was to masur χ in th dopd sampl and compar it to th χ of our pur sampl. Ths masurmnts would indicat how th osmium in th compound was affcting th spin fluctuations of th systm. Masurmnts of th spcific hat γ ar plannd for th futur. Procdur W bgan by making th dsird sampls in th lab. W calculatd and wighd out th corrct masss of ach mtal using simpl stoichiomtry and an accurat balanc. Thn, using an Edmund Bühlr arc-mltr, w mltd ach compound svral tims in an 2

3 oxygn-fr, argon atmosphr. W mad ach sampl, succssfully kping th mass loss du to mlting blow 0.2%. Howvr, w did ncountr a fw of obstacls in th procss of making th sampls. Our first UAl 2 sampl xplodd on th scond mlt attmpt. Th xplosion was simply th sampl shattring into many pics upon bing hatd rapidly by th plasma arc in th arc-mltr. To countr this problm, w slowly warmd th bad by bringing th arc slowly toward th sampl until it mltd. Dspit mploying this tchniqu, our scond sampl xplodd again, this tim on th third mlt. Howvr, x-ray analysis of th sampl rvald it had formd th propr structur bfor xploding. In addition, during th first attmpt at doping th aluminum, w had difficulty gtting th osmium to mix in. Upon braking th sampl opn, w discovrd a small bad of osmium which had not mltd and mixd with th othr mtals. W trid svral tchniqus to gt th osmium to mlt bfor w wr succssful. Whn w mltd th sampl with high hat to mlt th osmium, w ncountrd high mass loss du to aluminum s high vapor prssur. Finally, w mltd just th uranium and osmium togthr svral tims at high hat to mix thm thoroughly. This was possibl bcaus both uranium and osmium hav rlativly low vapor prssurs. Thn, w mltd th aluminum into our UOs 0.4 mixtur. Aftr going through this procdur, th final product still had a vry small osmium bad in th cntr. Howvr, this unmixd osmium was only about 3mg in mass. Th loss of 3mg of osmium would not significantly affct our rsults. W put ach sampl through svral procdurs. First, aftr braking th sampl into pics, w took about mg of th sampl and ground it into a fin powdr using a mortar and pstl. W thn glud th powdr to a glass x-ray slid. Aftr th glu on ach slid drid, w took x-ray diffraction data at th Major Analytical and Instrumntation 3

4 Cntr (MAIC) at th Univrsity of Florida. W usd ths x-ray data to confirm w had mad a satisfactory sampl. If som of th lmnts had not mixd into th lattic proprly, th diffraction pattrn would hav indicatd a scond phas. Th data wr also usful to calculat th siz of th sampl s cubic lattic structur. Using th paks of th diffraction pattrn, our Unitcll computr program calculatd th lattic paramtr of ach sampl. Th lattic paramtrs allowd us to track what was happning to th siz of th structur of th sampl as w dopd it. Nxt, w put ach sampl in th Suprconducting Quantum Intrfrnc Dvic (SQUID) to masur magntic suscptibility. Ths χ data gav us an indication of how th magntism of th sampl racts to an applid magntic fild. Bcaus th suscptibility data ar rlatd to m, this stp was an intgral part of th projct. W masurd th suscptibility of both sampls ovr a tmpratur rang of 2-300K. W stord both sampls in th lab so thy ar availabl for spcific hat and rsistivity masurmnts in th futur. Rsults First, our x-ray diffraction data, shown in Fig. 1, indicat w producd sampls of adquat quality. Both pattrns ar clan and fr of too much background nois. Fig. 2 shows th lattic paramtrs of UAl 2, UAl 1.6 Os 0.4, and UOs 2. Th litratur-valu lattic paramtrs for UAl 2 and UOs 2 com from Lam t al. [2] and Hal and Williams [3], rspctivly. Th data show that th lattic structur of th osmium dopd sampl is smallr than that of UAl 2. This shrinking of th lattic is influntial in th supprssion of spin fluctuations, as th U-U intratomic spacing is movd away from th Hill limit. Th 4

5 Hill limit (3.5 Å) is th valu abov which f-f orbital ovrlap, without hybridization, cass. Our χ data in Fig. 3 clarly show th suscptibility of UAl 1.6 Os0.4 is lowr than that of pur UAl 2. Bcaus χ is dirctly proportional to m, th dcras in χ indicats th osmium doping has dcrasd Conclusion m by supprssing spin fluctuations. This rsarch indicats that th doping of UAl 2 with osmium supprsss th spin fluctuations of th systm. Bcaus of th proportionality of χ and γ to m, th lowr χ valus of UAl 1.6 Os 0.4 indicat a lowr m, and thrfor supprssd spin fluctuations. This conclusion could b rinforcd with spcific hat data, which w wr unabl to obtain du to th brif duration of th program. W would prdict th spcific hat valu γ (=142 mj/mol-k 2 in [1] pur UAl 2 ) to b lowr in th dopd sampl. Acknowldgmnts W would lik to thank th NSF and th Univrsity of Florida Physics Dpartmnt for sponsoring this program. T. D. Scott would also lik to thank Dr. G. R. Stwart for his patinc and dirctions during th projct. In addition, h would lik to acknowldg th hlp of Dr. J. S. Kim and D. J. Burntt in assisting and training th author in th laboratory. Finally, thanks to Dr. K. Ingrsnt and Kristin Nichola for arranging all th dtails of th REU program. 5

6 Rfrncs [1] R. J. Trainor, M. B. Brodsky, and H. B. Culbrt, Phys. Rv. Ltt. 34, 1019 (1975). [2] D. J. Lam, J. B. Darby, Jr., J. W. Downy, and L. J. Norton, J. Nucl. Matr. 22, 22 (1967). [3] T. J. Hal and G. I. Williams, Acta Crystallogr. 8, 494 (1955). Figurs Fig 1. Comparison of x-ray diffraction pattrns for UAl 2 and UAl 1.6 Os 0.4 6

7 Fig. 2 Lattic paramtrs of UAl 2-x Os x Fig. 3. Comparison of magntic suscptibility for UAl 2 and UAl 1.6 Os 0.4 7

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