On the interesting magnetic behaviour of UFe2, CeFe2 and their pseudobinary compounds

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On the interesting magnetic behaviour of UFe2, CeFe2 and their pseudobinary compounds

S.B. Roy, G. Williams, B.R. Coles

To cite this version:

S.B. Roy, G. Williams, B.R. Coles. On the interesting magnetic behaviour of UFe2, CeFe2 and their pseudobinary compounds. Journal de Physique, 1989, 50 (18), pp.2773-2779.

�10.1051/jphys:0198900500180277300�. �jpa-00211101�

On the interesting magnetic behaviour of UFe2, CeFe2 ^and

their pseudobinary compounds

S. B. Roy, G. Williams

(*)

^{and B. R. Coles}

Blackett Laboratory, Imperial College, London SW7 2BZ, G.B.

(Reçu ^{le 6 mars 1989,} accepté ^{le 29 mars} 1989)

Résumé. ^- De nouvelles études des propriétés magnétiques ^{de U} (Fe, ^Al )2, (U, ^Ce )Fe2 ^{et des}

effets de substitution du Ce par l’U ^sur le comportement anormal de certains alliages Ce (Fe, X)2 (où ^{X = Co ou} Al) permettent de comparer les rôles respectifs ^{de l’U et du Ce dans}

les phases de Laves C15 avec le Fe. On en conclut que, à la fois, l’hybridation des états f avec les états étendus d et les effets de polarisation ^ou de fluctuation de spin ^sur les sites de l’élément f sont

importants. Toutefois, ^{à cause de leur} plus grande ^{extension et leur} plus ^forte hybridation, ^les

états 5f de l’U donnent de plus faibles anomalies que les états 4f.

Abstract. ^- New results for the magnetic properties ^{of U} (Fe, ^Al )2, (U, ^Ce )Fe2, and for the effects of U substitution for Ce on the anomalous behaviour of some Ce (Fe, X)2 alloys (where

X ⁼ Co or Al) ^make possible ^a comparison of the roles of U and Ce in their C15 Laves phases

with Fe. It is concluded that in both, hybridization of f-states with d-band states and polarization

or spin fluctuations on the f-element sites are important, but that the U 5f states, presumably by being ^more extended and more strongly hybridized, ^{are a weaker source} of anomalies than the 4f states.

Classification

Physics ^Abstracts

75.30C - 75.50C

Introduction.

In their intermetallic compounds ^{Ce and U show} interesting relationships

(and

^{contrasts with}

other 4f and 5f

systems)

that have been emphasized ^{in a recent} survey by ^Friedel

[1].

^{Of the}

various rare earth and actinide based C15 Laves phase compounds, UFe2 ^and CeFe2 ^have

drawn much theoretical and experimental attention in recent years

[2-5].

Though ^it belongs ^to

the rare earth group,

CeFe2

has much in common with UFe2 ; the ordered Fe moment is much smaller than those of the other RFe2 compounds ^{and the} particularly small value of

7c

(235 K)

^is comparable with that of UFe2. Moreover, ^as pointed by Rastogi et ^al.

[3],

^{in the}

behaviour of its specific heat, thermopower and electrical resistivity CeFe2 ^ressembles UFe2. ^{Also the lattice constants of} CeT2

(T

⁼ Fe, Co,

Ni)

compounds ^are smaller in

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:0198900500180277300

2774

comparison with those of other

RT2

compounds and, ^{as in the} UT2 series, ^{show an anomalous}

minimum at Co.

The various

properties

^of

UFe2

^{can be}

interpreted

^{within a} theoretical framework of itinerant electron magnetism ^which takes into account 5f-3d hybridization ^and its influence on

bonding ^and magnetism

[4].

The theoretical results suggest ^{that the} magnetism ⁱⁿ UFe2 has contributions from large spin ^{moments on both U} and Fe sites which are coupled antiparallel. Also there is a large orbital contribution on the U site, ^{which is} antiparallel ^to

and almost cancels the spin contribution. Recent polarized neutron measurements

[6]

^indeed

showed that the orbital and spin magnetic ^{moments which} individually ^{have a value of}

0.23 9B

^almost completely cancel in the U sublattice. A related itinerant electron model

[5]

with a contribution of 4f band magnetism ^has recently ^been suggested ^for

CeFe2

^to explain

various aspects of its anomalous behaviour. So far it has been assumed that Ce is tetravalent in CeFe2, transferring ^the single 4f electron to the conduction band

[7]

^{but a} very ^recent

polarized ^neutron measurement

[8]

indicates that the Ce site is

carrying

^{a moment in}

CeFe2.

^{Also recent} experimental ^{work on} alloying effects in

CeFe2 [3, 9-14]

strongly suggest ^a crucial role for Ce in the magnetic properties ^of

CeFe2

^{and its} pseudobinaries. This idea is

supported by ^a Lm X-ray absorption study

[15]

^on CeFe2 which indicates a nonintegral ^Ce

effective valence of 3.29. Finally the idea that Ce sites might ^be carrying ^{a small moment is}

reinforced by ^{the recent} discovery of small 4d moments at Y and Zr sites of YFe2 ^and ZrFe2 respectively

[16].

We have seen that a small substitution of Fe atoms by Al, Co, Ru, Re, ^{Os and Ir can} disrupt

the ferromagnetic ordering ^of CeFe2

giving

^{rise to an} antiferromagnetic ^{or canted structure}

([14]

and references

therein).

^{With the} striking similarity ^between UFe2 ^and CeFe2 ^{in mind, in}

this paper ^we would like to address the

interesting questions : (1)

what is the effect of small substitutions on the Fe sites of

UFe2 ?

^{We shall} present ^our careful studies of alloying ^{with Al}

and discuss these results along ^with existing ^ones for other elements ;

(2)

how does the

magnetic ^behaviour change ^{if one} goes from the UFe2 ^{end to the} CeFe2 ^{end in}

(Ul -.,Ce.,)Fe2 ?

^{To our} knowledge ^{this has not} previously ^been studied ;

(3)

what is the effect of U substitution at the Ce site on the anomalous magnetic behaviour of various

pseudobinaries ^of

CeFe2,

^for example ^Ce

(FeI - xCOx)2 [10]

^{or Ce}

(FeI - xAlx)2 [ll].

In the following ^{sections we} shall deal with each of the above points, drawing ^{on our} X-ray diffraction, metallographic ^and ac-susceptibility results, ^{and we} shall seek to

interpret

^these

within the framework of recent theoretical models

[4, 5].

Experimental.

The alloys ^were prepared by argon ^arc melting from metals of at least nominal 99.99 % purity

and suction chill casting ^into copper moulds ^to produce square cross-section rods. The

U (Fe, Al )2

and uranium rich

(U, Ce )Fe2

alloys ^were homogenised ^{at 800 °C for one to two}

weeks while all other alloys ^{were annealed at 600 °C for two} days ^followed by ^{700 °C for five} days. ^The samples ^were subjected ^to metallographic analysis ^to investigate ^the possible

presence of second phase. ^This study ^indicates that the solid solubility ^{limit for}

U (FeI - xAlx)2

is probably ^{around x =} 0.15, ^{while a solid} solution is possible ^{across the} whole range of

(Ul - xCex)Fe2-

X-ray diffraction studies using ^a Phillips diffractometer have been made on

the alloys ^{to check the nature of the} constituent

phases.

The results

of

^our X-ray study ^on

(Ul - xCex)Fe2

^are presented ⁱⁿ figure ^{1. In this} series the C15 Laves phase ^{structure is found}

across the whole concentration range while the lattice ^constant

(correct

^to the second decimal

place)

^vs. concentration plot ^{shows a} deviation from Vegard’s ^law

(Fig. 1).

^{Such a deviation}

from linearity has also been observed in the series

(Ce, Y)Ni2 [17].

^{In the case of}

U (FeI - xAlx)2

^the single phase ^{Laves structure is observed} only up ^{to x =} 0.12. The

Fig. 1. - Plots of lattice constant (a) ^vs. concentration of Ce (x) for the series of (U 1 - xCex )Fe2.

diffraction patterns ^{for x =} 0.15 and x ⁼ 0.2 reveal a number of lines of unknown origin along

with those for C15 structure, supporting ^our metallographic analysis ^{that a} single phase

structure does not exist in these alloys.

ac

Susceptibility

measurements were performed ^{with a} driving

frequency

of 320 Hz and a

driving field of 0.7 Oe parallel ^to the axis of the samples.

Results.

1. ac SUSCEPTIBILITY OF

U (Fe1-xAlx)2;

^{0 _ x _ 0.125. - ac} Susceptibility

(X)

^{data for}

these alloys ^{are shown in} figure 2. We deduce the Curie temperature

(Te)

from the inflexion

point ^{of the} sharp ^{rise in} susceptibility

( X )

^{as a} function of temperature

(T).

^The 7c for pure UFe2 turns out to be around 170 K, which is in good agreement with the earlier work

[18].

^{There is a} shoulder in the X ^{vs. T} plot ^of UFe2 around 65 K. The origin ^{of this} shoulder, ^{which was} also observed by Grossinger et ^al.

[18]

^{is not} very clear ^at the moment ; it may be related ^to the interesting magnetostrictive properties ^{of the} system

[18].

^{With the}

substitution of Al for Fe, T, ^decreases rapidly

(T,

⁼ 140 K for x ⁼ 0.05 and T, ^{= 50 K for}

x ⁼

0.075)

and goes ^{to zero at} around x ⁼ 0.08. For x ⁼ 0.085, 0.1 and 0.152 we observed ^a small and almost temperature independent susceptibility. ^Previous magnetization ^{and Arrott}

Fig. ^{2. - Plots of ac} susceptibility (,y ) ^vs. temperature

x ⁼ 0.05 and (9) x ^{= 0.075.}

2776

plot ^studies

[19]

indicated the concentration for the

disappearance

^of ferromagnetism ⁱⁿ

U

(FeI - xAlx)2

^{to be x = 0.12,} which is somewhat different from our result.

2. ac SUSCEPTIBILITY OF

(Ul - xCex)Fe2 ;

0.1, x ,1. - The susceptibility results for this system ^are presented ⁱⁿ figure 3, ^{and it} should be noted that the magnitude ^{of the} susceptibility ^around Te ^rises steadily ^{with Ce} concentration, while that at

liquid

^helium

temperature ^increases by ^{an even} greater ^amount as one crosses the system.

(It

^{should be}

mentioned here that samples of identical shape

(with

typical dimensions of 10 ^x 1.5 ^x 1.5

mm 3)

have been used in the present measurements although ^no correction has been made for the demagnetization

factor).

^{At no} composition ^however is there the abrupt

loss of

ferromagnetic

response ^{at a} well defined temperature that is found in

CeFe2

alloys ^with

substitutions of Co, Ru, Ir, ^{Os or Al on} the Fe sites

[14].

Fig. ^{3. - Plots of ac} susceptibility (,y ) ^vs. temperature

Fig. ^{4. -} Plots of Curie temperature (Tc) ^vs. Ce concentration (x ) for the series (Ul - xCex)Fe2-

The behaviour of the Curie temperature

(see

Fig.

4)

^{is also} interesting. ^{It rises} fairly rapidly

with initial

substitutions

of Ce, ^{so that} by x ⁼ 0.3 it has arrived at about the value

(235 K)

^for

pure

CeFe2 ;

^{but at} higher concentrations it varies very little, ^{with a} very shallow maximum ^at about x ⁼ 0.5. This is in contrast to the behaviour of

(U, Zr)Fe2 [20]

^and

(Ce, Y)

Fe2

[21] ;

^these systems show rather straight forward variation of Tc with concentration.

We have not attempted ^to study the critical character of the magnetic phase transitions in this system, partly because of the difficulty ^of ensuring ^{that one} has ideal random solid

solubility ^{on the A} sites of the AB2 compound ^and perfect stoichiometry ^with respect ^{to the} distribution of atoms of the f-elements and iron atoms on the two types of site. Furthermore the presence of ^a crystallographic distortion at the Curie temperature of pure UFe2

[18]

^means

that the magnetic transition must be, ^{even if} only weakly, first order. We do not yet ^{know how} far into the system this behaviour persists ^on the substitution of Ce, ^and hope ^to explore ^this shortly.

Although only UFe2 ^{has a} spontaneous

magnetostrictive

distortion at its Curie temperature, CeFe2 ^{also shows some} magnetostrictive ^anomalies which underlie the rhombohedral distortion and onset of antiferromagnetism ^{when some Co or Ru is} substituted for Fe

[22, 23].

3. ac SUSCEPTIBILITY OF

(CeO.8SUO.1s)(Feo.9CoO.1)2

^and

(Ceo.85UO.15)(Feo.95AI0.05)2-

^{- We}

mentioned in the introduction that various substitutions for Fe in CeFe2 gave rise ^to

interesting magnetic properties. ^For instance 10 % Co substitution for Fe apart ^from decreasing T,, ^also gives ^{rise to a ferro to} antiferromagnetic transition at lower temperature

[10, 22].

Similar but less straight ^{forward behaviour} has been observed with Al substitutions

[9, 23].

^{Our ac} susceptibility ^results

(Fig. 5)

show that 15 % U substitution for Ce in Ce

(FeO.9CoO.1)2

^{and Ce}

(Feo.95Alo.o5 )2

affects the magnetic ^character quite appreciably. ^{The low} temperature ^{loss of}

ferromagnetic

^character disappears for the former and is reduced to a

canting type transition in the latter, while there are increases in Tc ^{in both the} systems, ⁱⁿ spite

of the almost negligible ^{effect on}

Tc

^{of U} substitutions _{in pure}

CeFe2.

Fig. ^{5. - Plots of ac} susceptibility (X ) against temperature ( T ) ^for (Ceo.8sUo.1s)(Feo.9sAlo.os)2 (V), (Ceo.85UO.15)(Feo.gCoo.1)2 () The result of Ce(Fe,.95AI0.05)2 (V) is shown for comparison. ^{For similar} comparison ^with Ce(Feo.,Coo.1)2 ^{see reference} [10].

Discussion.

All these results, along with those of earlier work on UFe2 ^and CeFe2

(as

discussed in the

introduction),

highlight interesting features of these two compounds. ^{It is} quite apparent ^now that, ^{much more} than Zr and Y in their Laves phase ^with Fe, ^U and Ce have a very important

role in determining ^the magnetic character of the parent

compounds

^{as well as their}

pseudobinaries.

^The similarity ^{of these} compounds probably lies in the existence of f like

character, ^{in both U and} Ce, ^{and f-d} hybridization ^as suggested ^{in the recent} theoretical work

[4, 5].

^These suggestions draw further strength ^{from our Curie} temperature

(Tc)

^vs.

2778

concentration

(x )

plot

(Fig. 4)

for the series of

(U 1 - xCex )Fe2

alloys. On the other hand the nature of the f moment, whether it is spin only ^{or some} combination of both spin ^{and orbital}

contribution and also the extent of f-d

hybridization

^are likely ^to introduce subtle differences between

CeFe2

^and UFe2. In fact there is a difference in the effects of alloying ^{in the Fe}

sublattice of CeFe2 ^and UFe2. Small substitutions

(2-4 %)

^of Al, Ru, Re, Ir, Os and Co for Fe in

CeFe2

gave rise ^to

interesting

spin-canting ^and antiferromagnetic ^{behaviour at low} temperatures

[14],

^{which on} further increase in dopant concentrations ultimately ^{led to a total}

breakdown of ferromagnetic

ordering (except

^{in the case of Co}

[10]).

^Similar dramatic effects of alloying ^{have not} yet been observed in UFe2. For instance we have seen that with Al substitutions for Fe in UFe2 although ferromagnetic ^{order was lost} quite rapidly ^{there is no}

trace of the onset of any other type ^of magnetic ^{order at low} temperatures. ^{Even the} concentration for the loss of long range ferromagnetic order is somewhat higher ^for

U (Fe1-xAlx)2 (x,

⁼

0.08 )

than that of Ce

(Fe1-xAlx)2 (xc

⁼

0.055 ) [9, 23],

although ^the Tc

(170 K)

^of UFe2 is lower than the Tc

(235 K)

^of CeFe2. Similar behaviour has been observed for

U(Fe1-xRex)2 [24].

^{Also the} magnetic phase diagram ^for

U(Fe1-xCox)2 [25]

^is quite straight forward in

comparison

^{to that of}

Ce (Fe, - xCOx)2 [10].

^Further indications of the difference in the role of U and Ce come from the fact that 15 % U substitution for Ce in Ce

(Feo.95Alo.o5 )2

alters the magnetic ^character

quite

appreciably ^{and erases the low} temperature anomaly totally ^{in Ce}

(Feo 9000.1)2.

The latter effect is similar to the substitution of Y, which also totally ^{erases the low} temperature antiferromagnetic ^or spin-canting phase ^of

the CeFe2 pseudobinaries. On the basis of the effects of U substitution for Ce in the

CeFe2 alloys in which anomalous low temperature behaviour has been produced by ^small

substitutions for Fe, and of the comparison of these with the effects of yttrium, ^{it seems} justifiable ^to say that the ^more delocalized character of the 5f electrons

(and

probably ^their greater hybridization with the conduction

band)

ⁱⁿ UFe2 make the effects of U somewhat similar to those of Y. It does however maintain enough f character for low temperature anomalies to persist ^{in an} attenuated form in

Ce (Fe, Al )2

^after partial substitution of Ce by

U.

One of the most interesting features of this family ^of compounds is that while

UC02 ^{is not} superconducting ^a large substitution of Fe is

required

^to produce ferromagnetism,

while in CeCo2, ^{which is a} superconductor, ferromagnetism is induced very rapidly by ^Fe

substitution for Co, ^as it is in YCo2-YFe2. ^The subject ^is complicated by ^{the fact} that in both

CeCo2

^and YC02

(if

the results for YFe2 ^are

relevant)

exchange enhancement effects and spin

fluctuations are probably operative in the 4d and 5d electron bands as well as in the 3d band of Co. The admixture of 5f character on the U sites of UC02 would appear ^to have little drastic effect

(EPR

studies of Gd on the U sites suggest ^less magnetic U character than in

UAl2

[26]) ;

^{but the} temptation ^is strong ^to believe that the superconductivity ^of CeCo2, like that of UPt3 is associated with the right balance of f-hybridization and f-like

spin

fluctuations.

Acknowledgements.

We would like to thank Drs. Brown and Kennedy ^for permission ^{to refer to their} unpublished

neutron scattering ^results and Dr. H. E. N. Stone for preparing ^the alloys, various other

metallurgical help and discussions.

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