Physica B 163 (1990) 117-120

North-Holland

OBSERVATION OF LONG-RANGE FERROMAGNETIC ORDER IN THE HEAVY FERMION

COMPOUND URu,~,Re,$3i, BY NEUTRON SCATTERING

M.S. TORIKACHVILI”.‘, L. REBELSKY”, K. MOTOYA”.“, S.M. SHAPIRO”, Y. DALICHAOUCHh and M.B. MAPLEb “Physics Department, Brookhaven National Laboratory, Upton, NY 11973, USA ‘Institute for Pure and Applied Physical Sciences, University of California, San Diego, San Diego, CA 92093. USA

We have performed a neutron scattering study of the heavy fermion compound URu, ,Re, ,Si,, in order to verify the

occurrence of long range ferromagnetic order. This study consisted of measurements of elastic. inelastic, and polarized

neutron scattering, measurements of low angle critical scattering, and the determination of the temperature dependence of

the order parameter. We found (1) a sharp peak in the critical scattering at =30 K for the wave vector Q = 0.0X A-‘, and

(2) an enhancement of the intensity at the position of the (10 1) nuclear Bragg reflection below the Curie temperature.

These measurements suggest the occurrence of long range magnetic order. The value of the ordered magnetic moment is estimated to be ~(0.49 2 0.05)~~ at 10 K, in good agreement with the 0.46~~ value obtained from magnetization

measurements.

Several cerium and uranium compounds with gener- al composition CeT,Si, and UT,.%,, where T is a 3d-, 4d-, or Sd-transition metal, form with the body cen- tered tetragonal ThCr,Si,-type crystal structure. While some of these compounds exhibit Pauli paramagnetism, many exhibit antiferromagnetism (CeT,S& with T = Rh, Pd, Ag, Pt, and Au; UT,%, with T = Ni, Rh, Pd, Ir, and Au), and a few exhibit ferromagnetism (UT,Si, with T= Mn, Co, and Cu) [l-5]. The compounds with partially substituted tran- sition metal U(Ru,_,ReX)Si, (0 <x s 1.0) (61, U(Ru,_lTcX)Si, (0 s: x < 1.0) [6], U(Cu,_,MnX)Si, (0 <X G 0.3) [7], as well as U(Ru,_,MnX)Si, [S] also display ferromagnetic interactions. Palstra et al. [2] pointed out that the magnetic behavior in these com- pounds evolves from magnetic ordering towards Pauli paramagnetism within each series of transition metals. Anomalous Kondo-like behavior can be observed in the compounds located in the region connecting these two extremes, as in CeCu,Si, for example, which is a heavy fermion superconductor [9], CeRu,Si, [lo] and URu$, [ll-131. The compound URu,Si, is an espe- cially intriguing member of this family, since it dis- plays two ground states at low temperatures. At T, = 17 K, URu,Si, undergoes a phase transition to a weak antiferromagnetic state, and it becomes superconduct-

’ Permanent address: Department of Physics, San Diego

State University. San Diego, CA 92182, USA. ’ Permanent address: Department of Physics, Faculty of Sci-

ences, Saitama University, Urawa 338, Japan.

0921-4526/90/$03.50 0 Elsevier Science Publishers B.V.

(North-Holland)

ing at -1.5 K. Elastic neutron scattering data by Broholm et al. [14] suggest that the order parameter (OP) develops gradually from above the 17 K onset of antiferromagnetism, and that the ordered moment reaches a maximum value of -0.03~, at 21 K. Specific heat data below the ordering temperature by Maple et al. reveal an exponential behavior below To [13], which suggests that the ground state associated with this phase could be a spin density wave, with the formation of a gap of -11 meV over a portion of the Fermi surface. These latter data are in reasonable agreement with the observation of gap like excitations in inelastic neutron scattering measurements [14, 151.

Dalichaouch et al. [5] found from measurements of magnetic susceptibility versus temperature, as well as from magnetization curves at various temperatures that the URu,_,Re,Si2, and URu,_,TcSi, com- pounds displayed ferromagnetic interactions. Previ- ously, the heavy fermion compounds CeSi,_XCuX, and CeSi,_X have also been found to exhibit ferromag- netism [16]. In this work we selected URu, 2Re,,, Si, and performed a detailed neutron scattering inves- tigation in order to verify the occurrence of long range magnetic order, and determine the characteristics of this state. This particular compound has the largest saturation moment of =0.46&U-atom amongst the

URu,_, Re,Si, compounds studied in ref. [6]. The value of the effective moment in the paramagnetic state is pefr = 1.59&U-atom; the Curie temperature yielded by Arrott plots is T, = 38 K; and the elec- tronic specific heat coefficient y obtained from the

118 M.S. Tnrikachvili et al. I Long range ferromagnetic order in URu I 2Rr,, hSi_’

extrapolation to 0 K of the C/T versus T’ data at low temperatures is about 117 mJ/mol K2 [6]. The specific heat was measured [6] only below T,., and it did not reveal any ferromagnetic spin-wave contribution

C, J: T”’ (61. The 15 g sample utilized in this study was prepared

by arc melting. The sample was finely ground and mounted in an aluminum cylinder of 12 mm in diam- eter, which was sealed inside an aluminum can filled with He exchange gas. The sample can was mounted on the cold finger of a Displex closed cycle re- frigerator. The unpolarized neutron scattering experi- ments were performed on the triple axis spectrometers H4M and H4S at the High Flux Beam Reactor, Brookhaven National Laboratory. The (0 0 2) reflec- tion of pyrolytic graphite (PG) crystals was used for the monochromator and analyzer. The elastic scatter- ing measurements reported here were performed utilizing either 5.0 meV neutrons, in which case a Be filter was used to reduce higher order contamination. or 14.8 meV neutrons, and PG filters. For the inelastic scattering experiments the final neutron energy was fixed at 5 meV, and the energy resolution was 0.060 meV. Measurements using polarized neutrons with polarization analysis of the diffracted beam were performed on triple axis spectrometer HS, with verti- cally magentized Heusler { 1 1 l} transmission crystals used as monochromator, and analyzer. The incident energy was either 14.7meV or 41 meV.

An elastic powder diffraction 0-20 scan with neu- tron incident energy of 14.8 meV revealed all the Bragg reflections with wave vector magnitude Q be- tween 1.2 and 3.6&l, corresponding to crystallo-

graphic planes with spacings from 1.75 to 5.24 A. The section of the scan for 1.2Am’<Q<2.6A is dis- played in fig. 1. All reflections could be indexed either to URu, ,Re,, $i, or to the sample holder material (aluminum). A powder diffraction intensity calculation suggested that the reflections from the { 1 0 1). and { 12 1 } planes would be the most sensitive to the onset of ferromagnetic order commensurate with the U- sublattice, and as such they would provide the largest additions to the nuclear Bragg reflections from these planes upon the onset of long range ferromagnetic order. Scans of the Bragg reflection centered at { 10 1) taken at temperatures above and below T, indicate an enhancement of this reflection below T,.. A plot of intensity versus Q centered at the (1 0 1) reflection, for the temperatures of 10 K and 40 K is displayed in the inset of fig. 1. and it shows clearly the enhance- ment of this reflection due to the onset of magnetic order. The value of the magnetic moment at 10 K was

Fig. I A-20 powder diffraction scan of URu, LRe,, HSi, for

wave vector Q between 1.2 and 2.6 A I at T = 10 K. The

inset displays the enhancement of the { 10 1) reflection at

10 K due to the onset of long range magnetic ordering.

estimated by comparing the additional intensity of the { 10 1) reflection below T,. to the intensity of this reflection at 50 K. yielding p = (0.49 + 0.04)pt,.

The temperature dependence of the OP can be inferred from the increase in intensity of the { 10 1) reflection below T, Since the position of the max- imum of the { 1 0 I} Bragg reflection (Q = 1.665 A -‘), as well as the intensity of the nuclear reflections did not change significantly as the temperature was varied between 10 and 50 K, we monitored the peak intensity at several temperatures in this interval, and then subtracted the intensity at Q = 1.63 A-‘. in order to take into account any changes in background scatter- ing with temperature. These data are shown in fig. 2.

The large dynamic fluctuations (in time and posi- tion) of the short range magnetically ordered regions expected to take place near a second order phase transition from a paramagnetic to a ferromagnetic state can be the source of critical scattering at low angles, because the relaxation time of these fluctua- tions near T,. can become much larger than the amount of time spent by a neutron traveling through a correlated region [ 171. We found that for URu, :-

Re,, xSiZ the scattered intensity for Q = 0.08 A- ’ (IT,,>‘ = 14.8 meV) clearly yields a critical temperature

T, -30 K. as indicated in fig. 3. Combined with the enhancement of the {I 0 I} reflection at low tempera- tures, the peak in critical scattering at 30 K suggests the occurrence of long range magnetic order.

In conclusion, our neutron scattering measurements indicate clearly the occurrence of long range ferromag- netic order in URu, ?Re,, ,Si,. with T,. = 30 K. How-

M.S. Torikachvili et al. I Long range ferromagnetic order in URu, 2Re, ,Si, 119

lo j

E = 14.7 meV

i

1 i

3 1 d 1 81,! A’

0 10 20 30 40 50 Gh 70

Temperature ( K )

Fig 2. Intensity of the (101) reflection (Q = 1.665A’)

minus background (Q == 1.63 A-‘) versus temperature. This

curve yields the temperature dependence of the order param-

eter in URu, ?Re, *Siz between 10 and 50 K.

ever, we would like to point out some interesting characteristics of the ordered state in this compound. First, it is important to note that the OP starts to develop at temperatures above T,, in a manner quite similar to the behavior in URu,Si, [14]. Secondly. our measurements (not shown) of elastic scattering using polarized neutrons reveal only a partial depolarization of the beam [18], which suggest that this material may be either a weak ferromagnet, or that the domains are very small [19].

L

URU,,R~,~SI~

H4--S

0 10 -20 30 40 50 60

Temperature (K)

Fig. 3. Low angle magnetic critical scattering intensity as a

function of temperature in URu, zRe, RSi,. These data were

taken with wave vector Q = 0.08 A-’ and incident neutron

beam energy E = 14.8 meV

Acknowledgements

The research at Brookhaven National Laboratory was supported by the Division of Materials Science, US Department of Energy, under Contract No. DE- AC02-76CH00016. The research at the University of California, San Diego was supported by the US De- partment of Energy under Contract DE-FG03- 86ER45230, and the National Science Foundation under Contract No. DMR-84-11839.

References

[II

PI

[31

[41

[51

VI

[71

PI

[91

[lOI

[Ill

1121

[I31

(141

[151

[161

L. CheImicki, J. Leciejewicz and A. Zygmunt, J. Phys.

Chem. Solids 46 (1985) 529.

T.T.M. Palstra, A.A. Menovsky, G.J. Nieuwenhuys

and J.A. Mydosh, J. Magn. Magn. Mat. 54-57 (1986)

435.

K.H.J. Buschow and D.B. de Mooij, Philips J. Res. 41

(1986) 55.

M. Saran and S.P. McAlister, J. Magn. Magn. Mat. 75

(1988) 345.

A. Szytula, S. Siek, J. Leciejewicz, A. Zygmunt and Z.

Ban, J. Phys. Chem. Solids 49 (1988) 1113.

Y. Dalichaouch, M.B. Maple, M.S. Torikachvili and

A.L. Giorgi, Phys. Rev. B 39 (1989) 2423.

A.L. Giorgi, A.C. Lawson, J.A. Goldstone, K.J. Volin

and J.D. Jorgensen, J. Appl. Phys. 63 (1988) 3604.

Y. Dalichaouch, M.B. Maple and M.S. Torikachvili,

A.L. Giorgi, M.V. Kuric, R.P. Guertin, to be published.

F. Steglich, J. Aarts, C.D. Bred], W. Liecke, D. Mes-

chede, W. Franz and J. Schafer, Phys. Rev. Lett. 43

(1979) 1892.

M.J. Besnus, T.P. Kappler, P. Lehman and A. Meyer,

Solid State Commun. 55 (1985) 779.

W. Schlabitz, J. Bauman, B. Pollit, U. Rauchshwalbe,

H.M. Mayer, U. Alheim and C.D. Bredl, Z. Phys. B 62

(1986) 171.

T.T.M. Palstra, A.A. Menovsky, J. van den Berg, A.J.

Dirkmaat, P.H. Kes, G.J. Nieuwenhuys and J.A. Myd-

osh, Phys. Rev. Lett. 55 (1985) 2727.

M.B. Maple, J.W. Chen, Y. Dalichaouch, T. Kohara, C.

Rossel, M.S. Torikachvili, M.W. McElfresh and J.D.

Thompson, Phys. Rev. Lett. 56 (1986) 185.

C. Broholm, J.K. Kjems, W.J.L. Buyers, P. Matthews,

T.T.M. Palstra, A.A. Menovsky and J.A. Mydosh,

Phys. Rev. Lett. 58 (1987) 1467.

U. Walter, C.-K. Loong, M. Loewenhaupt and W.

Schlabitz, Phys. Rev. B 33 (1986) 7875.

E. Holland-Moritz, W. Schlabitz. M. Loewenhaupt, U.

Walter and C.-K. Loong, J. Magn. Magn. Mat. 63 & 64 (1987) 187.

H. Yashima, H. Mori, T. Satoh and K. Kohn, Solid

State Commun. 43 (1982) 193.

120 M.S. Torikachvili et al. / Long raqr ferromagnetic order in URu, ,Re,, $i_?

P. Boni. G. Shirane. Y. Nakazawa, M. Ishikawa and S.

Tomiyoshi. J. Phya. Sot. Jpn. 56 (1987) 3801.

M. Khogi, F. Hippert. L.T. Regnault. J. Rossat-M&not,

B. Hennion, P. Satoh. F.L. Chui, T. Miura and H.

Takei. to be published. [ 171 G.E. Bacon, Neutron Diffration. 3rd Edn. (Clarendon

Press. Oxford, 1975).

1181 M.S. Torikachvili. L. Rebelsky, K. Motoya, S.M. Sha-

piro. Y. Dalichaouch and M.B. Maple, to be published.

[ 191 0. Halpern and T. Holstein. Phys. Rev. SY (1941) Y60.

M. Burgy, D.J. Hughes, J.R. Wallace. R.B. Heller and

W.E. Woolf. Phys. Rev. 80 (1950) 953.