~ So~id State Communications, Vol.45,No.9, pp.795-797, 1983. 0038-1098/83/090795-03503.00/0 Printed in Great Britain. Pergamon Press Ltd.

STUDY OF THE MAGNETIC SUPERCONDUCTOR, Y9Co7, AT HIGH PRESSURE AND HIGH MAGNETIC FIELD

C.Y. Huang+ and C.E. Olsen +Center for Nonlinear Studies

Physics Division Los Alamos National Laboratory, Los Alamos, NM 87545

W.W. Fuller, J.H. Huang#, and S.A. Wolf Condensed Matter and Radiation Sciences Division Naval Research Laboratory, Washington, DC 20375

(Received 27 October 1982 by H. Suhl)

We have measured the electrical resistance and magnetic susceptibility of the magnetic superconductor, YQCo7, at pressures up to 20 kbar and in magnetic fields up to 6T. We h~ve found that pressure suppresses the magnetism resulting in a higher superconducting transition temperature and conclude that YQCo7 is an itinerant ferromagnet, not a spin-glass. Pressure also sharpens the superconducting transition and increases the cr i t ical magnetic f ie ld, signifying that the long range ferromagnetic and superconducting order parameters co-exist but vary spatially. For pressures greater than 6 kbar, the magnetoresistance is always positive, further indicating the s,ppresslon of magnetism by high pressure.

For the past several years the interplay between superconductivity and. magnetism has attracted considerable interest, t Most of the studies have been concentrated on the rare earth ternaries v/nose particular crystal structure give rise to weak magnetic interactions between the localized magnetic rare earth ions and the super- conducting d-electrons of the transition metal. Recently, a new intermetallic magnetic supercon- ductor, Y9Co7 , has been intensively invest i - gated. 2-I0 This compound becomes superconducting below T s ~ 3K, and is ferromagnetic below T m~ 6K,

- i.e. the magnetism precedes the superconductiv- i ty. .From the magnetic measurements, Sarkissian, et al ~ suggested that the magnetic state could be quasi-spin-glass freezing. However, the magneti-

zation study by Gratz, et al 3 and other experi-

mental results 9'I0 have concluded that the mag- netic state is itinerant electron band ferromagne- tism. In want of band structure calculations,

Sarkissian and Grover 8 have speculated that super- conductivity and magnetism in YoCo7 originate from i ts unusual Fermi surface co~si~ting of a par- t i a l l y exchange-split narrow 3d band of Co super- imposed on the 4d band of Y. They argued that the exchange f ie ld arising from the magnetic 3d electrons acting on superconducting paired 4d electrons is not enough to completely destroy the superconductivity yielding a complicated spacial variation of the order parameters, and they conjectured that at somewhat lower temperature, superconducting fluctuations are rapidly developed and dominate over the magnetic exchange coupling. All these effects are considered to originate from the unusual Fermi surface.

Present address: Massachusetts Institute of Technology, 416 Beacon St., Boston, MA 02115

In our previous studies, we have shown that unusual Fermi surfaces are very sensitive to

external pressure, 11 and that itinerant magnetism

could be easily suppressed by high pressure. 12 Accordingly, i t is expected that high pressure could raise T s by suppressing the 3d magnetic

correlations 13 in YgCo 7. In view of the fact that high pressure is a useful way to study the i~er- play between superconductivity and magnetism, we have measured the electr ical resistance and magnetic susceptibility as a function of hydro- static pressure (up to -20 kbar) and magnetic f ield (up to 6T).

The YnCo 7 was prepared from yttrium metal (99.9% pur~) and cobalt metal (99.999% pure) by arc melting appropriately weighed amounts of the materials in a gettered argon atmosphere using a tungsten electrode and a water-cooled copper hearth similar to the methods described in Ref. 8-10. The buttons were weighed after melting and the material losses from the melting were found to be less than 0.5%. The samples were heat treated at 530°C in a pure argon atmosphere, titanium gettered, for 20 days. The samples for the measurements were cut from the buttons using a diamond string saw. Metallographic examination of the buttons showed them to be homogeneous except for a few small islands of unreacted peritectlc material. X-ray examination showed the material to be single phase. The mean bulk density mea- surements on five different buttons prepared at

the same composition was 5.848 + 0.003 g/cm 3. Chemical analysis, within the limits of the atomic adsorption method used, show the material to have the composition YgCo 7. The resist iv i ty ratios

(R300/R4.2) of our samples are ~ 16, comparable to samples used by others. 8"I0

795

796 STUDY OF THtl MAGNETIC SUPERCONDUCTOR, Y9Co7

Simultaneous measurements of resistance, R, and ac magnetic susceptibil ity, x , of the samples were carried out in a self-clamped pressure cell as a function of temperature from ~I.2K to ~ 300K and at pressures up to ~20 kbar. The resistance was measured using a standard four-terminal ac technique at 33 Hz, while a 610 Hz susceptibllity signal was obtained from a secondary coil wrapped d i rec t l y around the sample. A lead manometer situated in dose proximity to the sample was used to determine the pressure.

Figure I shows the temperature dependence of the ac magnetic susceptibility at various pressure values. The increase o ~ e l o w ~SK in accordance with previous reports, v- v indicates the onset of ferromagnetism. Similar to other i t inerant

magnets, 12 x decreases wlth increasing pressure. This strong pressure dependence of x dearly indi- cates that Y9Co7 is not a spin glass because a

Vol. 45% No, 9

"!f .6L

0 o

2kbor ~ i B kbor ~ l

. . . . . . ,6 k,,o,//~.~:.-'.~

/ I / ~1- ~ I

I I / o 20 4o ,6o 80 ,ool / ! / T c,(I , I l l / , I , I , l J

2. 4 6 8 T(K)

Fig. 2 Resistance, R, (with respect to R at 6K) xvs T at ~3 kbar,~ kbar, and - 16 kbar. Inset shows R vs T at 11 kbar.

+4

+ 2 -

0 o x

- 2 - -

i

-'i - 6

• ,

2 4

T(K)

- - 0 kbot , ------~2 k l ~ Y9Co7 / ~ ~ kbOr

I , l 6 8

Fig. I Magnetic suscept ib i l i t y , x, vs T at various pressure values.

spin glass is rather insensitive to pressure. 15 Also shown in the figure is the increase of with increasing pressure, in agreement with t , observation that magnetism is suppressed by the applied pressure.

Figure 2 displays the temperature dependence of R at -11 kbars, which is consistent with

earlier reports. 8-I0 The resistance is independent of pressure (up to ~20 kbar) for T > ~ 8K . This pressure independence indicates that the magnetic order exists only for T < ~8K. The temperature dependence of R at various pressures is also demonstrated in Fig. 2. AS can be seen, the higher the pressure, the sharper the transition very simi lar to x(T,P) shown in Fig. I . This interesting observation suggests that, at ambient pressure, the long range ferromagnetic and super- conducting order parameters coexist around the superconducting transition but vary spatially In agreement with suggestions of Sarkissian and

Grover 8-I0. Thus the system is in a mixed state in which the ferromagnetic regions are separated from the superconducting regions, a state consid-

ered theoret ica l ly by Fulde and Ferrel116 and

others 17,18 However, as long as these order

parameters vary spa t ia l l y , our data could not preclude the possibil i ty that ferromagnetism and superconductivity overlap part ial ly in l~pace near T as discussed by Nakanishi, et a1. At high p~essure, the magnetism is suppressed and the sample becomes more "uniform" spatially, giving rise to a sharper superconducting t rans i t ion. Furthermore, the presence of the spatial distribu- tion of the order parameters point to the need of high purity Y and Co and the requirement of proper heat treatment in preparing the sample.

We have also measured the res is tance as a funct ion of an external magnetic f i e l d up to 6T. Figure 3 displays some of our data at ~15 kbar. The f i e l d dependencies of R at 1.251( and 2.5K are shown in Fig. lb. I t is s t r ik ing to note that the magnetoresistance for pressures > 6 kbars is always p o s i t i v e f o r T < 4.2K in c o n t r a d i c t i o n

with the ambient pressure resul t . 8 However, th is f i e l d dependence is consistent with an e a r l i e r conc lus ion tha t pressure suppresses i t i n e r a n t magnetism. In want of an appropriate theory for the c r i t l c a l f i e l d of a magnetic superconductor, we define the midpoint of the t rans i t i on as the c r i t i c a l f i e l d . Hc2 ( fo l lowing Ref. 14). Figure 4

shows Hc2 vs T at var ious values of pressure.

AS expected, the c r i t i c a l f i e l d increases with increasing pressure,

a)

8 1 3 K

6 - 4 2 K

2

O 0 , 2

HIT)

~ t5 kboI

I L 3 4

Fig. 3a Resistance, R, vs the magnetic f i e l d , H, at var ious temperatures at ~15 kbar.

@oI. 45, No. g STUDY OF THE PL~GNETIC SUPERCONDUCTOR, Y9Co7 797

b)

o

n r

2

o o

2 . 5 K _L- _u . . . .

1 2 5 K

/ / ,,C'/ - .... 6 kbo, ,' I / , '>" - . . . . . . . 15 kbar // ///,,;'/ .... 18 kbo,

~.,~ I I I I 2 3

H (T)

3b Resistance, R, vs H at various values of pressure and temperature.

1.0

A I - v

u 0 .5 I

\

q \

\

A ~0 kb~r

0 ~ 7 kbGr • ~ l l I(bor

• ~15 kbo r

• --18 kbor

T ( K )

I I I 0 i 2 3 4 5

In conclusion, we have found that in YgCo 7 (a) high pressure strongly suppresses magnetism; (b) the magnetism is not of a spin glass nature but is i t inerant ferromagnetism; (c) T s and H.o increase with pressure; (d) the superconductih~ transi t ion is sharper at higher pressures; (e) the magnetoresistance for P > ~8 kbar and T < 5K is always positive; and (f) the ferromagnetic and superconducting order parameters coexist around the superconducting t r ans i t i on but may vary strongly in space. In order to further understand the system, microscopic studies have to be made

Fig, 4 C r i t i ca l f i e l d , Hc2 , vs T at various values of pressure.

and we propose to invest igate NMR, neutron scattering, muon spin relaxation, and the M~ssbaue effect at high pressure.

Acknowledg~nts - We thank K. Machida (Ref. 18 and 19) and B.W. Sarklssian (Ref. 9 and 10) for pre- prints of their work, R.B. Roof, Jr. and A.W. Webb for the x-ray work, and N.C. Koon for the use of his superconducting magnet. Two of us (C.Y.H. and C.E.O.) are supported by the Department of Ener(~y.

References

I. Ternary Superconductors, ed by G.K. Shenoy, C.D. Dunlop, and F.Y. Fradin (North Holland, 1980).

2. A. Kolodziejczyk, B.V.D. Sarkissian, and B.R. Coles, J. Phys. FIO, L333 (1980).

3. E. Gratz, H.R. Kirchmayer, V. Sechovsky, and E.P. Wohlfarth, J. Ma9. Mat. 21, 191 (1980).

4. J. Sebek, J. Stehno, V. Sec~vsky, and E. Gratz, Solid State Commun. 40, 457 (1981).

5. E. Gratz, J.O. Strom-Olsen, and M.J. Zucker- mann, Solid State Commun. 40, 833 (198]).

6. £.V.D. Sarkissian, A.K. Grover, and E.R. Co]es, Physica B+C 109 and 110, 2041 (1982).

7. ~I. Cheng, G. Creuzet, P. Garoche, I.A. Campbell, and E. Gratz, J. Phys. FL2, 475 (1982).

~. B.V.D. Sarkissian and A.K. Grover, J. Phys. F.L2, LI07 (1982).

9. B.V.D. Sarkissian, to appear in proceedings of the IV Conference on Superconductivity in d- and f-Band Metals, Kernforschungszentrum Karlsruhe, 28-30 June 1982. B.V.D. Sarkissian, to appear in proceedings

of the 3rd Joint Intermag-Magnetism and Magnetic Materials Conference, Montreal, Canada, 20-23 July 1982.

10.

I I . D.W. Harrison, K.C. Lim, J.D. Thompson, C.Y. Huang, P.D. Hambourger, and H.L. Luo, Phys. Rev. Lett. 46, 280 (1981).

12. M.K. Wu, C.W. Chu, J.L. Smith, A.L. Giogi, C.Y. Huang, B.T. Matthias, and F.E. Wang, Solid State Cofmun. 34, 507 (1980).

13. Independently as c i ted in Ref. 9, J.J.M. Franse and P. Frings found that the ac suscept ibi l i ty at 6 kbar is smaller than at ambient pressure,

14. C.Y. Huang, D.W. Harrison, S.A. Wolf, W.W. Fuller, and H.L. Luo, Physica B+C 109, and 110, 1649 (1982).

15. M.K. Wu, R.G. Aitken, C.W. Chu, C.Y. Huang, and C.E. O]sen, J. Appl. Phys. 50, 7356 (1980).

16. P. Fulde and R.A. Ferrell, Phys. Rev. 135, A550 (1964).

17. A.I. Larkin and Y.N. Ovchinnikov, Soy. Phys. JETP 20, 762 (1965).

18. K. Machida, to appear in J. Phys. Soc. Japan.

19. H. Nakanishi, K. Machida, T. Matsubara, to appear in Solid State Commun.