On the pressure-temperature phase diagram of the Kondo compound CeAl 2

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On the pressure-temperature phase diagram of the Kondo compound CeAl 2

B. Barbara, J. Beille, B. Cheaito, J.M. Laurant, M.F. Rossignol, A. Waintal, S. Zemirli

To cite this version:

B. Barbara, J. Beille, B. Cheaito, J.M. Laurant, M.F. Rossignol, et al.. On the pressure-temperature phase diagram of the Kondo compound CeAl 2. Journal de Physique, 1987, 48 (4), pp.635-640.

�10.1051/jphys:01987004804063500�. �jpa-00210479�

On the pressure-temperature phase diagram

of the Kondo compound CeAl2

B. Barbara, ^J. Beille, ^B. Cheaito, J. M. Laurant (*), ^{M. F.} Rossignol, ^A. Waintal and S. Zemirli Laboratoire Louis-Néel, C.N.R.S., (*) C.R.T.B.T., C.N.R.S., 166X, ^{38042 Grenoble} Cedex, ^France

(Requ le 20 novembre 1985, révisé le 24 novembre 1986, accepté ^{le 28 novembre} 1986)

Résumé.

Des mesures de diffraction des rayons X ^sous pression, effectuées sur un monocristal de

CeAl2 ^{réduit en} poudre, ^{ne montrent aucune} anomalie de volume jusqu’a ^{150 kbar.} Cependant ^des expériences de résistance électrique ^{sous fortes} pressions effectuées sur un monocristal provenant ^{de la même} origine ^{montrent une} anomalie faible au voisinage ^{de 80} kbar, ^{à la} température ambiante, ^i.e. près ^{de la} pression pour laquelle ^{Croft et} Jayaraman [2] ^{ont observé une très} importante anomalie de volume.

L’anomalie de résistance a été suivie jusqu’à ² K, ^et conduit à une nouvelle ligne ^{dans le} diagramme

P-T de CeAl2. ^Cette ligne, interprétée ^{en termes d’un} changement ^{entre deux} régimes ^Kondo, coupe la ligne critique ^{où un ordre} magnétique apparaît. Sur la base de ces résultats obtenus sur CeAl2, ^nous proposons ^un

diagramme ^de phase détaillé pour les systèmes ^Kondo magnétiquement ^ordonnés.

Abstract.

Accurate high pressure X-ray diffraction experiments performed ^{on a} powdered single crystal ^of CeAl2 ^{do not} show any sizeable volume anomaly up ^to 150 kbar. However, high pressure electrical resistance

experiments performed ^{on a} single crystal ^{of the same} batch show a continuous anomaly ^{near 80 kbar at room} temperature, i.e. close to the pressure for which Croft and Jayaraman [2] ^{observed a} strong ^volume anomaly.

The resistance anomaly ^{has been} followed down to 2 K leading ^{to a new line} in the P-T diagram ^of CeAl2. ^This line, interpreted ^{in terms of a crossover between two} different Kondo regimes, ^crosses the critical line where magnetic order takes place. On the basis of our results in CeAl2, ^{a detailed} phase diagram ^for magnetically ordered Kondo compounds ^is proposed and discussed.

Classification

Physics ^Abstracts

72.15

1. Introduction.

The a - _y transition of Ce-metal has been discovered 60 _years ago [1] ^{and since,} it has been the object ^of

an increasing ^{interest on both} experimental ^and

theoretical sides. This transition is also observed on

several Ce-based intermetallics such as CeAl2 [2, 3], CexThl-x [4], ^CeNi [5] ^{and even on metallic} praseodymium [6] ^at extremely high pressures. The

a - y transition was first thought ^to correspond ^{to a}

f-d promotion [7]. ^Johansson suggested ^{that it was}

instead a Mott transition from localized f electrons

to an f-band [8]. Subsequently ^various experiments

have supported ^the idea that some sort of f delocali- zation is responsible ^{for the a - y} transition. More

recently ^this transition has been attributed to a

volume collapse ^{of a} compressible Kondo model [9, 10]. ^{This last} interpretation ^{is coherent} with the 7 %

softening ^{observed on the bulk modulus of} CeAl2 compared ^{to other} RAl2 compounds [11]. ^However,

it does not exclude the possibility ^of slight ^deviations

from an integer valency (say by ^a few per cent)

or/and of electrons having ^{a f} angular symmetry ^and

a sufficiently large radial extension so that they

contribute to the diffuse part of the form factor [12]

(these ^two aspects being ^{however not} relevant in

driving ^{the a - y} transition).

The apparition ^{of a} dense Kondo regime ^{can be}

associated either with a first ^order transition (a - y )

or with a simple regular ^variation of the volume (no transition), depending ^{on the} counterbalence be- tween’ Kondo and elastic volume energies. ^{Ce metal} [1, 13], CexTh1-x [4] ^{for x} 0.27, ^{Pr metal} [6] ^and

CeNi [5] ^{show a} first-order transition whereas

CexThl - x [4] ^for x > 0.27 shows a simple regular

variation of the volume. Concerning CeAl2, ^this question ^{was not} clear up ^{to now.} After X-ray experiments ^under pressure [4] ^{it has been} widely admitted that CeAl2 presents ^{a a - y} transition.

However, ^{the nature of this} transition has been

questioned after other lattice parameter experiments

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01987004804063500

636

performed ^{on the} alloys CeAl2 _ eCue ^with

0 -- E -- 0.05 [14]. ^On the other hand sample depend-

ences of the bulk modulus have been observed in different alloys ^of CeAl2 by ^{Bartholin et al.} [3].

In this paper ^we first describe accurate lattice parameter and electrical resistance experiments per- formed on a single crystal ^of CeAl2 (prepared ^{on a}

W crucible, ^{which has been shown to} provide ^best samples [11]) ^{under a} quasi-hydrostatic pressure

reaching ¹⁵⁰ kbar. Lattice parameter experiments performed ^{at room} temperature ^{do not} show any

sign of valence anomaly ; however, ^{one cannot} exclude the possibility ^of very smooth anomaly

between 50 and 100 kbar, considering ^{the error bars.}

Resistance experiments clearly ^{show a smooth ano-} maly ^{near 80 kbar at room} temperature. ^{This anoma-} ly evoluates rather strongly between 300 K and 2 K

leading ^{to a new} line in the P-T diagram ^of CeAl2. ^We interpret ^{this line as a crossover from a}

weak to a strong Kondo behaviour. Then we estab- lished that this line intercepts the critical line where

CeAl2 ^orders magnetically [15, 18]. ^The possible

influence of a crossover, from weak to strong ^Kondo effect, ^{on the} nature of the magnetic ^{order is}

discussed for the first time. This point ^{as well as}

other features of the P-T diagram ^of CeAl2 ^{can be}

understood on the simple ^{basis of a volume} depen-

dent Kondo temperature (see ^Ref. [10] ^{and refer-}

ences therein) already used in the study ^{of the} susceptibility ^of CeAl2 under pressure [12, 16, 19].

2. Pressure. Volume behaviour.

The set up [17] ^{used for} determining the pressure- volume behaviour of CeAl2 ^is built around a rotating molybdenum ^anode generator Marconi-Elliott type GX 21. The X-ray beam is made quasi-mono-

chromatic and focussed using ^{a set of two metallic}

mirrors. The diffraction pattern ^{is recorded} by ^a

curved gaseous detector INEL CPS 120 (with ^a

radius of curvature of 25 cm and an angular aperture of 120°), transmitted to and analysed by ^{a microcom-}

puter. ^The sample (carefully powdered single crystal)

was enclosed within a diamond anvil cell [20]. ^At

each pressure, ^at least three reflections have been

simultaneously ^{observed in} CeAl2. The pressure ^was determined with an accuracy of ± 2 kbar from the reflection lines (at ^least two) ^{of NaCI} powder ^mixed

with the CeAl2 ^one.

The pressure-volume behaviour obtained in our

sample ^of CeAl2 ^is given figure ^1. First of all the absence of _{any well} defined transition is clearly

observed in contradiction with the result of reference

[2]. ^A reasonable fit to the Birch-Murnagham equation [21], performed ^{up to} 140 kbar leads to values for the isothermal bulk modulus B

-dP/d(V/Vo) and its pressure derivative Bó ⁱⁿ

very good agreement with those previously ^obtained

at lower pressures (see Fig. 1). Therefore, ^if CeAl2

Fig. ^1.

Volume-pressure evolution of CeAl2 ^{at room} temperature ^{and fit to the} Birch-Mumagham [20]

equation. The values obtained for the bulk modulus

Bo

68.7 kbar and its first derivative Bo’

^{3.0 are in} good agreement with other determinations [3, 11].

exhibits an anomaly in its P-V behaviour below 150 kbar, ^this anomaly ^must be very smooth : the conventional a - y transition does not exist in our

sample CeAl2. This. result _{is very} important ^{since it}

was, up ^{to now,} very well admitted that the archetyp-

ical Kondo system CeAl2 presented ^a typical ^cerium-

like a - y transition. It has been carefully ^checked by doing ^{several runs} in pressure. On the other hand another _group performed ^{the same} kind of exper- iment on another sample ^of CeAl2 [22] ; ^their sample, ^as ours, does not show _any sharp ^{feature in}

the P-V behaviour.

3. High pressure resistivity.

Let us now consider our electrical resistance meas- urements of CeAl2 ^under pressure. They ^{have been}

undertaken on a small single crystal ^of CeAl2 (0.05 ^{x 0.2 x 1.6} mm3) ^{from the same batch that the}

sample ^{used for} X-ray experiments. ^A Bridgman

anvil technique [23] ^{was used in} attaining quasi- hydrostatic pressures P 140 kbar in a higher

pressure cell developed ^{in our} laboratory. ^{A Pb}

pressure ^{manometer was} sandwiched with our

CeAl2 crystal between steatite disks into a pyrophyl-

lite gasket. The pressure within the ^{cell was} deduced from a calibration of the superconductive ^transition

of Pb versus pressure [24]. ^{We also used the Pb} (I-II)

transformation fixed point ^{at 130 kbar.}

The pressure evolution ^of the resistance of our

single crystal ^of CeAl2 ^is given figure 2. First of all

we must mention that isobars all saturate to

-

35 mn, ^{but the curve A.} This lack of saturation is

just ^{due to the bad} quality of electrical contacts at

low pressure in this type of cell. In the curve B a

pressure of 16.2 kbar is large enough ^to compact ^the system and therefore to considerably improve ^electri-

cal contacts. General features of these curves are

Fig. ^2.

Electrical resistance-temperature plot ^{for a} CeAl2 single crystal ^{at different} hydrostatic pressures. The

high temperature portion ^{of the curve A shows an excess}

of resistance which is due to bad electrical contacts at low pressures. Results for LaAl2 ^at normal pressure ^are also

given.

similar to those of Nicolas-Francillon et al. [25] ^and

Probst and Wittig [26]. ^{However, one} cannot extract any well defined anomaly from these data due to

their limitation in a very restricted pressure range

(P -- 16 kbar) ^{in the first case and to an} important

pressure dependent background ^{in the second case} (experiments ^{have been} performed ^on powders ^and

therefore compacting ^effects modify ^the resistivity).

On the contrary, ^if plotted ^{in a} resistance-pressure

system ^of coordinates, ^{data of} figure ² clearly ^shows

a transition from a resistive to a less resistive state when the pressure increases (Fig. 3). When lattice contributions are subtracted (normalized ^resistance

of LaA12) it appears that the spin ^disorder resistivity

vanishes in the high pressure phase. ^{It is here}

assumed that lattice contributions are not affected

by ^a pressure ; this is generally ^{the case for} very stable materials such as CeAl2 ^or LaAl2. ^This

Fig. ^3.

Isotherms deduced from results of figure ^2.

Notice that, ^{in the} high pressure side, the resistance of

CeAl2 ^is nearly equal ^{to that of} LaAl2 (arrows) suggesting

a vanishing ^{of the} spin ^disorder resistivity.

hypothesis ^is furthermore corroborated by ^our X-ray experiments ^which clearly show that the MgCu2

structure of CeAl2 ^is preserved up ^to 150 kbar.

The continuous vanishing ^{of the} spin ^disorder resistivity ^of CeAl2 ^under pressure (beyond crystal

field effects [27] ^which always ^{lead to a finite} magnetic resistivity ^{at low} temperature) ^{must be} attributed, ^{in the} paramagnetic phase, ^to strong reductions in the time averaged ^moments (m2) T’

associated with a larger ^Kondo temperature ; ^{in the} high pressure/low temperature region, ^{the Kondo} temperature, TK ^{is larger than that,} TK, ^{of the low}

pressure/high temperature ^one. A continuous vol-

ume anomaly ^{could be} associated with this modifi- cation in the TK ^of CeAl2, ^as suggested by ^the

following arguments :

i) the evolution of the Neel temperature ^of CeAl2 ^with pressure has been in the past ^well interpreted ^{in terms of a volume} dependent ^Kondo temperature ^where TK (pJ) is the usual exponential

function and where the functional dependence pJ(P ) ^{has been} determined from resistivity exper- iment [16, 12] (nearly ^linear dependence).

ii) the bulk modulus of well characterized crystals

of CeAl2 ^is by ^{7 % softer than the one of the other} RA12 (an interpolation using ^the phenomenological

relation B - (valency/volume) ^{was used} [11]).

iii) ^The surprising coincidence of the pressures ^at which our resistance anomaly ^{and the} transition of Croft and Jayaraman [4] ^{occurs in another} sample ^of CeAl2,

iv) ^the alloys CexThl - x, ^{which are for} x > 0.27 similar to CeAl2 under pressure, show ^{a linear}

resistivity/volume relationship ^{when the} temperature

evoluates [4].

It is worth noticing ^{that in} CeAl2 ^{the resistance} anomaly ^is continuous at any temperature ^between 300 and 2 K, showing ^{that the} transition a - y does

not exist in CeAl2 ^{in this} temperature range, for P :!S: 140 kbar ; ^{instead we are in the} presence ^{of a} continuous passage betwen ^{two Kondo} regimes (weak ^to strong) ^{of the same Kondo} phase. ^{If the}

arguments given ^{above are} relevant, ^{then a smooth}

volume anomaly would be associated with this

crossover and the line of figure ^{4 would} represent the inflection point of isotherms above the critical

point (Pc7 Tc) given in the Kondo collapse ^model [9, 10]. ^In CeAl2 ^{it is not} clear whether the upper critical

point ^{lies at} very low temperature ^or both critical

points collapse ^{at finite} temperature. ^{This last} possi- bility seems more probable since, in the Gibbs energy, the volume dependent ^{term of Kondo} origin

is relatively small, compared ^{to the normal} one, ^as this can be estimated from the softening [11] ^{of 7 %}

of the bulk modulus. Accurate P- V experiments ^are

in progress which should allow ^to study ^{in more}

details the P- V diagram ^of CeAl2. ^{At the} present

638

time we shall only ^consider that, ^{at each} tempera- ture, there ^{is a} pressure ^{P above} which local

paramagnetic ^{moments vanish in} CeAl2 ^{due to a} large ^Kondo temperature. ^The corresponding ^P-T

line is shown figure ^{4. It has} been defined from the inflection point ^{of each isotherm of} figure ³ (another definition, in which the characteristic pressure is taken on each isotherm at Rmax/2, ^where Rmax ^{is the}

resistance maximum of crystal ^field origin, ^{leads to}

the same curve). ^{This curve} characterizes the cross- over from weak to strong ^Kondo regime ^{in which the}

time averaged paramagnetic ^moment vanishes prog-

ressively. ^{It is} very different from a a - y transition line. In particular ^{its curvature is} positive ^{instead of} negative. The existence of ^a continuous _passage from a magnetic ^{to a non} magnetic ^{state in} CeAl2 had been in fact suggested many years ago from a linear extrapolation ^of susceptibility ^measure-

ments performed ^{between 0 and 18 kbar at 1.5 K} [19], ^{i.e. in the} magnetically ^ordered phase. ^This point brings ^{us to the} question ^of possible ^connec-

tions between a crossover from weak to strong

Kondo behaviour occurring ^{in the} vicinity ^{of a} magnetically ^ordered phase.

Fig. ^4.

^{Line of crossover} between weak and strong Kondo regimes ⁱⁿ CeAl2. ^{Note the} positive ^curvature.

4. P-T Phase diagramme.

In CeAl2 ^{the crossover line} intercepts the critical line where the modulated magnetic order takes place ^at

about 2.5 K and 23 kbar (Fig. 5). ^{It is} interesting ^to

note that a phase transition from this modulated structure to a simple antiferromagnetic ^{structure of}

type ^{II has been observed near this} point [15]. ^More precisely, ^{below 15 kbar,} CeAl2 ^orders according ^to

Fig. ^5.

Temperature-pressure phase diagram ^of CeAl2

for T (K ) 10 and 10 =s= T (K ) ²⁰ (inset). Three diffe- rent magnetic phases ^are present : ^the paramagnetic phase (regions ^{II and} III), the modulated phase (regions IV), ^the type ^II phase (region VI). Magnetically ^ordered CeAl2

shows also a coexistence between modulated and type ^II phases, ⁱⁿ region ^V [15]. Expected ^lines separating ^re- gions ^{V with} regions ^{IV or VI are} indicated in dashed. In the paramagnetic region, ^{two different crossover lines are} given. One of them is not yet very well determined

(triangles ^and dashed). ^{It is} interpreted ^{as due to the}

crossover between thermal (II) ^{and weak} Kondo behaviour

(III). ^{The second one} determined from our pressure

experiments corresponds ^{to the crossover} between weak and strong Kondo behaviour (see ^{also Ref.} [32]).

a modulated structure and at lower temperature ^it shows a coexistence of the modulated and type ^II

structure. Between 15 and = 20 kbar these two types of magnetic ^{order coexist at} any temperature ^and

above = 23 kbar only ^the type ^{II structure} persists [31]. ^In particular ^the amplitude ^{of the} components of the propagation ^vector characterizing ^{the mod-}

ulated structure vanishes at 1.7 K and 22 kbar. It is remarkable that this point (0) ^falls exactly ^{on the} extrapolation, ⁱⁿ the ordered phase, ^{of the crossover}

like determined in the paramagnetic phase (this extrapolated portion ^{of line} coincides, ⁱⁿ fact, ^with

the curve determined, ^{in the ordered} phase, using

the same criterion). ^This suggests ^{that the} change

from the modulated to the type ^II antiferromagnetic

structure is a consequence of ^the increase of the Kondo temperature ^from TK ^to T H ^{It is} tempting ^to

consider that the crossover line, associated with the

progressive change from weak to strong ^Kondo behaviour in the paramagnetic region, ^becomes

critical in the magnetically ^ordered region (transition

between the modulated and type ^II structure). ^In

such a case another critical line, characterizing ^the

type ^{II to} paramagnetic transition in the high

pressure region, should be observed (region VI).

However, this ^line might ^{be rather} steep ^{due to a} strong pressure dependence ^{of the Neel} temperature, reducing the extention of the phase ^{VI and therefore} making ^{difficult to observe} it, unless very careful

investigations ^are performed between 20 and 40 kbar.

It might ^be interesting ^{to note} that the weak

(TK/T 1 ) ^or strong (TK/T > 1 ) ^Kondo regimes correspond ^{to the weak or} strong coupling ^{limits in a}

Kondo inpurity. ^The resistivity should vary - Ln T in the first case and - T2 (Fermi liquid) ^{in the second}

case. These different aspects will be discussed in

forthcoming papers.

Another aspect of the observation of a high

pressure magnetic ^{structure in} CeAl2 ^{concerns the}

nature of the magnetic ground ^{state of the f.c.c.}

Kondo lattice. In CeAl2, inhomogeneous ^moment

reductions (modulated ^structure [18]) ^{have been}

attributed to the competition ^{between « normal »} ferromagnetic interactions (PrAl2, ^{as well as most}

other rare earth-Al2 ^are ferromagnetic) ^and negative

interactions [18] resulting ^{from the Kondo effect in a} compound [12] (Kondo lattice). ^This interpretation

is still consistent with the conclusions of this paper,

since, ^{in the} high pressure region, negative ^interac-

tions of Kondo origin should become relevant lead-

ing ^{to a} simple antiferromagnetic structure. Within this interpretation ^the type ^II antiferromagnetic

structure would characterize the ground ^{state of the}

f.c.c. Kondo lattice. It would be interesting ^{to see}

whether such a conclusion could be obtained theoret-

ically ^{on the basis} of coherence effects in the f.c.c.

Kondo lattice. However, ^one must mention that

Jarlborg et al. [30] found that nesting properties ^of LaAl2 correspond reasonably ^{well to the} type ^II

antiferromagnetic ^{structure of} CeAl2. This indicates

that, ^even in the absence of Kondo lattice effects,

the magnetic ^{structure of} CeAl2 ^{would be} essentially unchanged.

Let us now consider some other features of t1¡e

P-T phase diagram ^of CeAl2 given ⁱⁿ figure ^{5. The} high temperature-low pressure region (weak ^Kondo coupling) ^{is divided in two} portions :

-

in region II, the Kondo temperature ^is negli- gible compared ^to the thermal energy kT and therefore the paramagnetic ^moment M2> T ^{is not}

reduced (in other words the Kondo enhanced quan-

tum fluctuation time _{TK -} hlkTK ^{is much larger}

than the characteristic time of thermal fluctuations TT - 1 /k7J . ^{The behaviour of} CeAl2 tends here ^to a

normal « thermal behaviour ».

-

in region III, TK ^: TT, and cerium paramagne- tic moments are reduced (paramagnetic scattering ^of CeAl2 ^{led to} J (M2) - 0.6 Jl-B ^{at 8 K). The be-}

haviour of CeAl2 ^is here ^{of weak Kondo} Type.

The hatched line separating ^{these two} regions

indicates the crossover from thermal to weak Kondo behaviour. This line can be located from _e.g. specific

heat experiments ^and/or resistivity experiments.

Specific heat results at P

0 [29] showed that the full entropy ^R In 2 of the crystal ^field ground ^{state of} CeAl2 is reached at 15 K ; ^this temperature ^{has been} interpreted ^as characteristic of the crossover above mentioned between Kondo and thermal fluctuations at P

0 [12]. ^{Two other data} points ^{can be obtained}

from the resistance experiments ^of figure ¹ where,

above the resistivity cusp of the Neel temperature, another cusp is clearly ^{observed which must be}

connected with the evolution of TK/T. ^{We believe}

that the cusp ^at the right ^{end of} the resistance

plateau ^of figure ¹ (curves ^{A and} B) ^{can be used to}

locate the crossover between thermal and Kondo behaviour at P $= 0 (Fig. ⁵ inset). ^{It is} interesting ^to

note that the resistance plateau (i.e. ^both cusps)

vanishes between 16.2 and 21.7 kbar : this is consis- tent with a progressive increasing ^{of the Kondo}

temperature with pressure which tends to suppress at the same time the magnetic order and the

crossover to a weak Kondo regime.

Finally ^{the recent work of} Schefzyk ^{et al.} [31]

shows the existence of a specific ^heat anomaly ^{at a} temperature T, slightly larger ^than TN, ^with

d Ln TcldP > 0. Our resistivity experiment ^under

pressure also shows ^an anomaly ^{in the} paramagnetic phase. ^{We have followed this} anomaly ^{between 5}

and 25 kbar ; ^{it can} very well be estrapolated ^when

P > 0 to the anomaly ^at Schefzyk et al. ^{In our}

interpretation ^{the line} Tc (P ) characterizes the oc- currence of a magnetic ^short range order in ^a Kondo system (magnetic coherence).

5. Conclusion.

To conclude, ^we have shown that our sample ^of CeAl2 ^{does not} present any sharp anomaly ^{in its}

pressure behaviour. Instead we evidenced a cros- sover line characterizing ^the passage from weak ^to strong ^Kondo regimes. ^The intersection of this line with the critical line along ^which magnetic ^order

takes place is located where the magnetic ^structure changes from modulated to type ^{II. This} suggests ^the

possibility ^{of a new} type of transition within the

magnetically ^ordered phase between different ordered phases of low and large TK. ^{Such a} phase

transition would be the analog, ^below TN, ^{of the}

weak Kondo coupling ^{to Fermi} liquid ^crossover

observed above TN. On the other hand a crossover

line between regions ^dominated by thermal fluctua-

tions or where single ion Kondo fluctuations start to

640

be observed has been identified. The existence of such crossover lines should be general ^{and their}

identification in CeAl2 ^should help ⁱⁿ understanding

anomalies observed in other Kondo compounds.

Acknowledgments.

It is a pleasure ^{to thank A.} Draperi for valuable technical assistance and C. Lacroix for interesting

discussions.

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[31] ^The study of different magnetic phases ^of CeAl2 under pressure is given in reference [15].

However, ^similar approaches ^{based on the effect}

of very small ^amounts of impurities ^{have been}

also performed, ^see e.g. BARBARA, B., BOUCHERLE, ^J. X., BUEVOZ, ^J. L., ROSSIGNOL,

M. F. and SCHWEIZER, J., ^J. Magn. Magn. ^Mat.

14 (1979) 221 ;

SCHEFZYK, R., LIEBE, W., STEGLICH, F., GOTO, ^T.

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[32] BARBARA, B., BEILLE, J., CHEAITO, B., LAURANT,

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