Physica B 194-196 (1994) 333-334 North-Holland PflYSICA

Spin echo and Knight shift, in the intermetallic c'onq)OUll(l AuIn2 T h o m a s Wagner , Siegfried GStz. Naoto Masuhara and (',eorg Eska

Physikalisches lnst i tut , Universil~it Bayreuth, D-8580 Bayreuth, Ge rmany

We have investigated two bulk samples of Auln2 from the same batch by pulsed NMR on USIn. The first sample (I) was as received front rf-melting. The second sample (Ill was annealed. The magnetic field was varied between 0.06 and 0.35 T and the nuclem" spin temperature ranged fl'om 0A mK to 20 inK. For sample I, a temperature independent T2=32(5) t~s was fom,d in spin echo experiments. Temperature and field ,lependences of Knight shift and Korringa constant were insignificant, with 1(=+0.00811(5) and ~=0.110(5) sI(, respectively. However, the annealed sample I1 even did not follow (hn'io's law over the whole t.emperature and tield ranges. The NMR line could only be seen in fields above 90 mT and showed at higher fields an intensity loss and a line broadening by roughly factors of 3 and 2 , respectively, wil.h respect l.o sample 1. No spin echo could be detected. The l(night shift, depended on magnetic history. At 270 m f w e g i l l 1~=:-1-0.007,R!){5). Some Sl)eculalions on the influence of magnetic impurities and/or SUl)erc~m(luctinta inclusions on the N,MR line of this c~mq)oun,I wilt be presented.

1. I N T R O D U C T I O N

Cubic s y m m e t r y is seen by In a toms in the i,> termetallic compound Auln2. This fact makes Auln2 useful for applications where t.he large nuclear magnet ic moments of In are of interest and, at the same time. interaction due to the large quadrupole moments has to be a\,oided. As AuIn2 possesses a small l(orringa constant and good thermal conductivity, applica.tiolls may range f rom nuclear susceptibility tttenno~uet.ry [1] to nuclear refl'igeration, as first pointed out by Andres el; al. [2]. Also, spin-spin interaction is s t rong in AuIn2 [2], which is of interest in the field of nuclear magnet ism. Unexpected results were reported recently [3,4].

In order to get more information about tlle interactions on In in bulk Auln2 we perfornmd pulsed N M R experiments on tlSln. From spin- echo and Knight shift measuretu,qlts infornmtion about internal fields act ing on the lu sit~ • can t)~, obtained.

2. E X P E R I M E N T A L

Our samples were prepared fi'om originally 5N pure Auln2 by rf-melting yielding sa,nples of residual resistivity ratios R R R ~30. One sa, nple (I) was directly soldered with In to a Cu-screw whiclr made the contact to the nuclear cooling stage. Sample II was mmealed in 10 - s Torr of air for 8 hours at 450 °C and thell glued with Ag- epoxy into a Cu-screw which was tiglLtly sq~,~,-'zed

around the Auln~ rod ( technique adapted from ref. 4 ). The annealing has l~ot improved RRR. The samples were investiga.ted by X-ray pow- der diffractornetry and by n,agnetic susceptibil- ity Ineasurelnellts [5]. Both samples consisted of a pure cubic phase of Au ln : . In the Kelvin tem- perature range and at 5 T we found a 2 tirnes larger l)aranmgtmtic signal in the ar~Ti.ealed sam- ple II than ill sa,,,ple I wllMl itself showed a 10 t.imes higher pararnagnetic contarni~mtion than a ultra-l)ure Smul)le ( R R R ~500 ) [-1].

'Fhe static field for the N M R was produced by the same magnet which was used for nuclear de- magnetizat ion. An additional magnet ic field gra- dient of 2.5 m T / c m could be applied for echo experiments, Many of tile N M R spect ra were taken with 3 ps wide rf-pulses and an effective rf- field s t rength around 0A roT. This caused s t rong eddy current heating and co,,sequeutly time de- f )e l H t e l i1, ( 'Ol i d t lCt i~ ~ll e l e c t l ' O l l t e l i l p e r a i l l r e s 1 nuch

i~l er t lmll Jim:lear spi~l t.elill)eratAires [(5]. To get. the l¢nigllt shift 1( fi'om the known K

of ( 'u and the In resonance t'reqtMmies, we deter- mined the external field at the Auln2 rods (3 m m clia.) by simultaneously moni tor ing 6a'sSC'u lines fronl the rf-coil ( 30 p m wire ) which was tightly wound aronnd the sample [1].

3. R E S U L T S a n d D I S C U S S I O N

Tyl)ical NMR spectra obtaitLed at 5 mK and (3.27 T are shown in fig. l. Two features of the In

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334

line are obvious: a) the line width o{' saluple 11 is broader by a factor of two and b) tile-, line inten- sity for sample II is only one • third of tlje intensity of the un-annealed sample (I), Theref'ore, in tile mmealed sample less nuclei are affect~ed by rf and those who are affected see a wider field distribu- tion than the In nuclei of the un-mmealed sample. This broadening seems not to be purely ' inhomo- geneous ' because we could not see any indication of a spin echo for smnple II ( T2 <7tLs ), whereas we found echoes in smnple [, yielding a tempera- ture and field independent T2=32(5) lZS ( in ac- cordance with measured line width and results of ref. 2 ).

The most striking observation was disap- pearence of the N M R signal in sample I1 for fields below 90 roT. Also, line width told l(tdght sldl't of sample II depended on the field and its llistory. None of those holds for sample I, for which we ob- tained mean values for the Knight shill and Ko- rr inga constant , K=+0 .00811(5) and ~=0.110(5) sK , respectively ( in reasonable agreement with ref. 7; measurements at high tenlperatures ). Spin-lattice relaxation time was also nleasured for smnple II yielding similar, but less acmlrate, val- ues to sample 1.

The Knight shift in saml)le I1 was SJl~aller. Af- ter cycling the field up to 350 m T and back to 270 naT we got for the mean value of K=+0.0078.~1(5), indicating a change in the Fermi contact interac- tion probably due to changes in conduction elec- t ron polarization.

This change may be caused by field and tem- pera ture dependent scat ter ing of conduction elec- t rons on impur i ty al~olns. This scat, tering would also influence the line width. As the effect is in- duced by mmealing, oxygen interstitials nly be re- sponsible. Another scenario could be ml anneal- ing induced change of the Juaterial towards k','pe II superconductor in the surface layer. Super- conduct ivi ty would reduce the Knight shift. At low fields, no N M R signal would be observable due to Meissner effect and at higher fields only nuclei located in regions of t rapped flux would contr ibute to a s t rongly broadened signal. Beside these speculations there is one fact: annealing of AuIn2 changes the N M R sensitive properties of the material.

45o I

400;

350

'~ 300

• ~ 250

200

150 <~

100

5o

0 , , , I , J I

tk 2000 2500 3000

f r e q u e n c y [kHz] 75

× x

"~ 5o

0 ~ - , , l l

2000 2500

o~ 25

3000

f r e q u e n c y [kHz]

Figure 1, Fourier Spectra of tl~e un-annealed (top) and annealed (bottom) sample. 115In line at left: at right the lines of the two C:u isotopes

R E F E R E N C E S

1. U. Angerer and G. Eska, Cryogenics 5 1 , 515 iv)s2)

2. K. Andres and Bavarian Millimill©,J. de Physique C__66, 769 (1978)

3. 1(. Glooms, R. l(&nig, P. Smeibidl and F. Pobell, Furophys. Lett.., 12. 661 (1990)

-1. see: T. tlerrmamlsd6rfer, 13. Schr6der-Smeibidl, P. Snleibidt and F. Pobell. this volume

5, the help of W. Widder and T. Herrmmmsd6rfer is gratefully acknowledged

6. G. Eska, J. Peters, E.N. Smith and E. Syskakis, Phys. Letters A 136, 509 (1989)

7. V~7.W. Warren, IR.W. Shaw, A. Menth, F.J. DiS- alvo, A.IR. Storm and J.H. Wernick, Phys. Rev. B 7, 1247 (1973)