5f band metamagnetism in UCoAl: Effects of alloying and pressure

Journal of Magnetism and Magnetic Materials 169 (1997) 229-239

ELSEMER

5f band metamagnetism in UCoAl: Effects of alloying and pressure A.V. Andreeva>b, L. Havelac, V. Sechovskj+?*, M.I. Bartashevichb$d, T. Gotod, K. Kamishimad “Institute of Physics, Academy of Sciences, Na Slovance 2, 18040 Prague 8, The Czech Republic b Ural State University, 620053 Ekaterinburg, Russia ‘Department of Metal Physics, Charles University, Ke Kadovu 5, 12116 Prngue 2, The Czech Republic dISSP, University of Tokyo, Roppongi 7-22-1, Minato-ku, Tokyo 106, Japan

Received 15 November 1996

Abstract We report on the results of magnetisation

and susceptibility

studies of the effects induced in the 5f band metamagnet

UCoAl by doping with other elements and/or by application of external pressure. In majority of aspects UCoAl shows strong similarities to the 3d band metamagnets Co&, YCoz and LuCo2, which suggests an essentially same origin of field-induced phenomena in both the classes of materials. The main role of the observed alloying and pressure effects can

be attributed to the involvement of the Sf-ligand hybridisation in both the U 5f-moment formation and the 5f-5f exchange interactions. Keywords: Uranium

intermetallics;

Magnetic properties; Metamagnetism

1. Introduction Magnetic properties of uranium intermetallic compounds are strongly influenced by the hybridisation of uranium 5f electron states with valence electron states of ligands, which strongly affects the degree of delocalization of 5f electrons and the inter-uranium exchange interactions. These effects

*Corresponding author. Tel.: + 42-2-2191-1351;fax: + 422-2191-1351;e-mail: [email protected].

play a key role in the formation of the 5f magnetic moments and in magnetic ordering [l]. The evolution of magnetic properties across the isostructural groups of ternary UTX (T = transition metal, X = p-metal) compounds can be followed exhibiting a wide spectrum ranging from Pauli paramagnetism over different stages characterised by high-temperature Curie-Weiss paramagnetism (which indicates the existence of 5f magnetic moments) which may condense either to a nonmagnetic ground state with spin fluctuations or in ferromagnetic or antiferromagnetic ordering at low temperatures. This process is controlled

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predominantly by the hybridisation of the uranium 5f electron states with d electron states of transition metal atoms. This effect can be demonstrated by progressive development from Pauli paramagnetism of 5f itinerant electrons towards magnetic ordering of stable local moments, which we can trace within each series of UTX compounds with a fixed X element and varying transition metal component towards the right end of a particular d series [l]. The group of UTX compounds crystallising in the hexagonal ZrNiAl-type structure (schematic drawing of the structure can be seen in Fig. l), which comprises more than 20 compounds including UCoAl, seems to be very suitable for relevant studies. Also in the row UFeAl (Pauli paramagnet), UCoAl (paramagnetic ground state and 5f band metamagnetism), UNiAl (antiferromagnetic ordering of uranium moments) we can find a ‘monotonous’ trend of the development of 5f magnetism with increasing occupancy in transition metal 3d band [l, 21. Phenomenon of band metamagnetism, i.e. fieldinduced ferromagnetism in a system with paramagnetic ground state, is observed in a few compounds in which 3d-electrons are responsible for the magnetic properties (Co& [3] and RCo2 with nonmagnetic R = Y, Lu [4,5]). From the results of our recent studies of UCoAl, we may point to a number of similar features of this Sf-band metamagnet similar to the ‘conventional’ 3d-band metamagnets. In particular, we have shown on single crystals that the metamagnetism in UCoAl, similar to the 3d-band metamagnets, is very sensitive to external pressure [6]. The transition field linearly increases with pressure with dBJdp = 0.27 T/kbar and simultaneously the magnetisation jump at the transition is gradually reduced. A critical pressure of -24 kbar for suppression of metamagnetism in UCoAl can be estimated by the linear extrapolation from the O-10 kbar range to higher pressures. Another feature of 3d-band metamagnets is their high sensitivity to alloying. Therefore, we were motivated to check alloying effects on UCoAl. At first, we have observed that magnetic properties are considerably influenced by deviations of the composition from the exact 1 : 1 : 1 stoichiometry [7]. Our

U

co1

Fig. 1. Hexagonal crystal structure of UCoAl (the ZrNiAi-type).

‘alloying’ pilot study [S] revealed that the substitution of 10% Co by Fe or Ru yields ferromagnetism, while the same amount of Ni or Pd results in suppression of the metamagnetism. Since these results give clear evidence that the previously suggested correlations between the composition and magnetic behaviour may be violated when we enter the solid solutions, we have started detailed studies in this direction. In this context, it is worth mentioning that URuAl, besides UFeAl, has a paramagnetic ground state [9], UPdAl does not exist in this structure type, but all Pd members of the UTX family (UPdGa [lo] and UPdIn [ll]), like all Ni compounds, order magnetically with large U magnetic moments (1.5 PLgin UPdIn [ll]). In the pres. ent paper we have also studied in detail the low concentration range of solid solutions of Fe, Ni and Ru in UCoAl.

2. Experimental The alloys with nominal compositions UCol -,T,Al (T = Fe, Ni, Ru; 0 B x < 0.1) have been prepared by melting the corresponding

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amounts of the elemental components (uranium of 99.8%, other metals of 99.99% purity) in an arc furnace on a water-cooled copper bottom under a protective argon atmosphere. The ingots (of 3 g mass) were turned several times in order to avoid inhomogeneities, and afterwards they were wrapped in a Ta foil and annealedin vacuum at 750°C in a sealed quartz tube for one week. The magnetic properties were measured on both annealed and as-cast samples, no visible difference was found. The compound UO.sOLu~.~~COA~,where U is partially substituted by nonmagnetic Lu, was also prepared. The phase composition of the alloys and the lattice parameters of the compounds were determined by a standard X-ray diffractometry. The magnetisation at ambient pressure was measuredby a SQUID magnetometer in superconducting coil up to 5 T above T = 2 K. Then it was measured under pressure by an extraction-type magnetometer, with a high-pressure clamp cell made of Cu-Ti alloy, in superconducting coil up to 9 T at 4.2 K. The powders consisting of randomly oriented particles were fixed by a glue to avoid an alignment of the particles in high field. This corresponds to an ideal polycrystalline material. The Curie temperature was determined by AC susceptibility measurements. 3. Results and discussion 3.1. Ambient pressure

Despite the fact that the field and temperature dependencesof the magnetisation in the undoped UCoAl have been already studied in detail on a single crystal, we repeated thesemeasurementson polycrystals in order to get a proper reference for substituted compounds. Fig. 2 shows the magnetisation curves of an isotropic powder sample of UCoAl at different temperatures under ambient pressure. They generally coincide with the corresponding curves for single crystal along the c-axis, but the transition is obviously broader and the magnetisation gained across is by a factor of 2 smaller than that measured along the c-axis on the single crystal (since the metamagnetism takes place only in the c-axis, a distribution of applied

UCoAl 0.20 c’r”‘j 0.16

@ 0.12 3

E

0.08 0.04

0.00 0

1

2

3

4

5

6

B (T) Fig. 2. Magnetisation curves of isotropic powder of UCoAl at different temperatures under ambient pressure.

field projections on randomly oriented c-axes of polycrystal grains should be considered). One can see that the transition becomes less pronounced with increasing temperature and disappears at 18 K. At low temperatures, the sample exhibits a noticeable magnetic hysteresis (not shown in Fig. 2, where the results only for decreasing field are presented)at the metamagnetic transition indicating its first-order character. The width of hysteresis in polycrystal is larger than that in single crystals measured along the c-axis [6], but also rather small, and strongly decreaseswith increasing temperature (30 mT at 2 K, 7 mT at 4.2 K and becomes negligible at 6 K). Already the substitution of 1% Co by other d metals leads to strong changesin magnetic properties. This is illustrated in Fig. 3, where the magnetisation curves of UCO~.~~T~.~~AIsolid solutions at 4.2 K are presented. Whereas the presence of Ni yields generally a shift of the metamagnetic transition to higher fields, Ru and Fe lead to qualitative changes of magnetisation curves. The Rudoped compound becomes a ferromagnet. A large spontaneous moment is also exhibited by the Fe-doped compound, but at the same time there

232

AX

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Andveev et al. / Jowt~al of Magnetism and bfagnetic Materials I69 (1997) 229-239

T = Ni

T=

0.16

_

0.16

w

0.04

0.04 fixed randomly

oriented powder

o.no __

0

1

2

3

4

5

6

0.00 0

1

2

B CT)

Fig. 3. Magnetisation curves of isotropic powders of UC0 o.~~To.olAl solid solutions at 4.2 K under ambient pressure.

remains visible metamagnetic behaviour. Magnetisation jump at the transition is roughly equal to the spontaneous moment. (It is important to mention in the beginning of discussion that the lattice parameters do. not practically change upon substitutions. Only at the end of investigated range, x = 0.10, an observable difference appears [SJ.) Magnetisation isotherms of UCO~,~~RU~,~,AI at different temperatures shown in Fig. 4 are typical for a ferromagnet with Curie temperature Tc 20 K. This Tc value was also confirmed by AC susceptibility measurements. In the case of UCoO.,,Fe,,.O,A1 (Fig. 5), the superposition of ferromagnetic and metamagnetic behaviour exists in a wide temperature interval. A ratio between the ‘spontaneous’ and ‘metamagnetic’ parts of the magnetic moment increases with decreasing temperature and the ground state (T + 0 K) is almost completely ferromagnetic. T, = 20 K has been determined was found for the ferromagnetic part component from AC susceptibility. Further, Fedoping stabilises ferromagnetism. Magnetisation isotherms of UCo0.9sFeo.ozAl show no traces of metamagnetism.

3

4

5

6

B VI

Fig. 4. Magnetisation curves of isotropic powder of UCo0.99Ru0,0,Al at different temperatures under ambient pressure.

UCoo.99Feo.olA~ 0.20 r-----7

0

1

2

3

4

5

6

B 0’)

Fig. 5. Magnetisation curves of isotropic powder of UCoO,~eFe,,,,Al at different temperatures under ambient pressure.

A.Y. Andreev et al. 1 Journal of Magnetism and Magnetic Materials 169 (1997) 229-239

Fig. 6 presents the concentration dependence of Curie temperature and spontaneous magnetic moment M, for UCo, -,T,Al solid solutions (T = Ru, Fe), x < 0.1. (Note that the M, values presented in Fig. 6 correspond to single crystals along the c-axis, i.e. are twice larger than that obtained from the isotropic powder data). Dashed lines show extrapolations to zero content of the doping element. Such an extrapolation suggests a ferromagnetic ground state of the parent compound. Ferromagnetism in the UCo 1 _ .Fe,Al compounds has been reported by Trek et al. [12] for a wide concentration range also including UCoAl. Results of our detailed substitution studies in conjunction with other results on UCoAl single crystals (including neutron diffraction [13]), however, provide clear evidence why the conclusion of

501

1 ,

UCo,+&Al 1 , 1 ,

1 1 1 1

F

233

TroL et al. [12] on the UCoAl ferromagnetic ground state cannot be right. A lot of care should be taken for sample quality when studying the material which appears in such a delicate situation as UCoAI. We have shown, on the other hand, that as little as 1% Fe-doping is sufficient to induce ferromagnetism. Probably even a much lower amount of some impurities may produce a similar effect. The temperature development of magnetisation curves of UCo0.99Ni0,01A1 (Fig. 7) is qualitatively similar to that of UCoAl (Fig. 2). As mentioned above, Ni-doping does not induce ferromagnetism but, moreover, yields an increase of B, and a decrease of magnetisation gain across the metamagnetic transition. This is illustrated in Fig. 8, where the magnetisation curves of isotropic powders of UCol -.Ni,Al with different x are presented. The curve for x = 0.06 still has a slight S-shape. The compounds with x < 0.08 exhibit regular paramagnetism. This holds up to high Ni concentrations (0.80-0.85) where onset of the antiferromagnetic ordering occurs [14]. An increase of B, with Ni substitution is found also in the 3d-band metamagnets. In Y(Co1-xNix)2,

4

10 t

0

T=Ru

0

T=Fe

9 g E

0.04

0.12 0.08

0.06

T content x Fig. 6. Concentration dependence of Curie temperature Tc and spontaneous magnetic moment M, for slight UCo, -,T,Al solid solutions (T = Ru, Fe; x < 0.1). Dash lines show extrapolations to zero T content. Square symbol represents M, of weak ferromagnetic state in UCo0.99Fe0,01A1.

o.oq 0

1

2

3 B(T)

4

5

6 .

Fig. 7:‘Magtietisatioti curves of is’otropic powder of UCo,,,99Nio,01A1 at different tempe;‘aiures under ambient pressure.

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0.20

t

T = 4.2 K

0.16 t

0.00

0'

0

1

2

3

4

5

6

B 0') Fig. 8. Magnetisation curves of isotropic powders of UCo, -,Ni,Al solid solutions at 4.2 K under ambient pressure. The results on Uo.soLuo,,oCoAl, where metamagnetism is suppressed by dilution of U sublattice with nonmagnetic Lu, are also presented for comparison.

the metamagnetism is still observed at x = 0.03 with B, = 93 T (B, = 72 T in YCo2) [lS]. The value of dB,/dx = 7.2 T/% Ni is much larger than in UCoAl(O.4 T/“/o Ni), however, if we compare the effect related to B, of the parent compounds, the situation becomes reverse because of the extremely low critical field in UCoAl, the d(ln B,)/dx values are equal to 0.11% Ni and -OS/% Ni, respectively. On the other hand, the Fe substitutions reduce B, both in YCo, and UCoAl. We should note that very crude conclusions only may be derived from such comparisons, because the virtually similar effect of substitution may have different impact on electron structure. In particlar, we should recall that the transition metals are nonmagnetic in the UTX compounds. (The field-induced moment on the Co sites - 0.06 l.tg in the metamagnetic state is very small compared to the corresponding U moment (of 0.37 u.Lg,both in 5 T [13]).) On the other hand, the Co atoms do bear a magnetic moment in the YCo,-based compounds in the metamagnetic state.

0

I

I

I

20

40

60

T (W Fig. 9. Temperature dependence of magnetic susceptibility x in 0.2 T field for isotropic powders of UCo, -,NtAl solid solutions under ambient pressure. The results on U0.80L~0.20C~A1 are also presented.

Fig. 9 shows the temperature dependence of the magnetic susceptibility x for UCol-.Ni,Al in a 0.2 T. This field which is low enough to avoid any influence of the metamagnetic transition for all compounds presented (see Fig. 8). The susceptibility gradually decreases with increasing Ni content. All the compounds which exhibit the S-shape magnetisation curve at low temperatures, also display another feature of band metamagnets, the broad maximum in the x(T) dependence. The temperature of the susceptibility maximum T,,, is not sensitive to Ni-doping, its values fall in the range 16-17 K for all compounds from the Ni series except x = 0.06, the last compound which exhibits a nonmonotonous x(T) curve. In this sample, the wide plateau on X(T) prevents precise determination of T 0X7X* The compound with 8% Ni exhibits only a shoulder at low temperatures, as well as Uo.80Luo,zoCoAl with the diluted U sublattice (for clarity, the curve for x = 0.08 is not presented in Fig. 9 because it would cross several other curves; it differs from the curve for Uo,aoLuo,zoCoAl only by a factor of two higher absolute values). The

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of Magnetism

low sensitivity of T,,, to the composition is also found in the homogeneity range of UCoAl on single-crystalline samples measured in magnetic fields applied along the c-axis (15-18 K [7, 161). T max is slightly higher in the off-stoichiometric UCoAl-based single crystals showing a weak ferromagnetism compared to the purely metamagnetic crystals. One can see that T,,, in UCoAl, as well as in off-stoichiometric and Ni-substituted compounds, practically coincides with the temperature at which the magnetisation curve loses its S-shape. This differs from the situation in YCo, which has T max= 250 K while the metamagnetic character of the magnetisation curve vanishes already at T” = 100 K [15]. On the other hand, T,,, in the Y(Col -XAl,)z system decreases with increasing Al content and at x = 0.09 T,,, = 60 K becomes lower than T” (S-shape of the magnetisation curve is still visible at 100 K [17]). Thus, no clear correlation exists between T,,, and T” can be observed. A somewhat different story has been found when testing correlation between T,,, and B,. In the Y(Col-.A& system, the decrease of T,,, with increasing x is observed to follow the variation of B,,,, the transition field at T --f 0 K. In a wide range of T,,, and &o, a simple linear relation BJT,,, = 0.29 T/K holds 1151. However, this is not a case of UCoAl-based metamagnets.,For example, B, differs by a factor of 3 in UCoAl and UCo0.96Ni0.04A1 whereas T,,, keeps the same value. The transition field in 3d-band metamagnets depends on temperature as [15, 171: B,(T) = BcO + aT2.

(1)

The B, vs. T2 plots are presented in Fig. 10 for isotropic powders of UCol-,Ni,AI. The corresponding plot for UCoAl single crystal, measured in magnetic fields along the c-axis is also shown. For all the samples B,(T) dependence obeys the formula (1) with the coefficient a E 2 x lo- 3 T K-‘. Nearly the same a value (1.85 x 10m3 T Km2) is found for YCo2 [15] despite the two orders larger value of BcO in the latter case. In Y(CO~-.A~,)~, a tends to increase with Al-doping but it is still not so different (3 x 10e3 T Kw2 at x = 0.11 [17]). The positive a coefficient (i.e. increase of B, with temperature) is considered as an important feature of

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and Magnetic Materials 169 (1997) 229-239

UCo,-,N&Al

3

-m-

-C -a,9---

--.

---e, --6 -+-

T2 (K’) Fig..lO. Dependence of transition field B, of metamagnetic transition on squared temperature for isotropic powders of UCo,-,Ni,AI solid solutions under ambient pressure. The lowest plot corresponds to UCoAl single crystal, measured along the c-axis.

3d-band metamagnets. This suggests that the entropy of d-electron system is reduced by the metamagnetic transition. The entropy reduction originates from the suppression of spin fluctuations at the transition which leads to a decrease of the y coefficient of the electronic specific heat. The discontinuity in y can be estimated from the values of a and AM (magnetisation jump across the transition) as follows [15]: Ay = - 2aAM.

.

(2)

The reduction of y (from 35.5 mJmol-l Km2 in zero field to 25 mJmol-l K-2 in the fieldinduced ferromagnetic state) was confirmed in Lu(Co0.91Ga0.09)2r which has B, low enough to perform specific heat measurements in magnetic fields above the transition [18]. The value of Ay is consistent with that estimated by formula (2). With regard to UCoAl, Ay = - 6 mJ mol-’ Km2 if we take AM = 0.25 pg. In order to check this result properly, the specific heat should be measured on a single crystal in a magnetic field applied

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along the c-axis because of the huge magnetic anisotropy in UCoAl. Till now, only results on polycrystal are available [9]. The y-value changes from 70 mJ mol- 1 Km2 in zero field to 62 mJ mol-r Km2 at 5 T, which seems to be consistent with the above estimate. 3.2. External

0.24

presswe

The metamagnetic transition in UCoAl is very sensitive to external pressure. The value of B, increases with the rate dB,/dp = 0.27 T/kbar, whereas AM is reduced as shown on single crystal samples [6]. Now, we report the results obtained on doped polycrystals, Figs. 11-13 show magnetisation curves for isotropic powders of UCO~,~,T~.~~A~ (T = Ru, Fe, Ni) at 4.2 K under different pressures. In each figure, field dependences of the differential susceptibility dM/dB, from which the B, values have been determined, are also presented. UCO~.~~RU~.~~A~ at ambient pressure and at 2.6 kbar (Fig. 11) exhibits ferromagnetic behaviour with a considerably reduced magnetic moment. High-pressure (10 kbar) curve is typical for a metamagnet with a negligible spontaneous moment. The S-shape can be already seen at 7.6 kbar (a clear maximum in dM/dB). The pressure-dependence of B, for this compound is drawn tentatively in Fig. 14. Crossing of this line with the p-axis gives us a critical pressure pciz5 kbar needed to suppress ferromagnetism. Indeed, the magnetisation curve at this pressure exhibits a superposition of ferromagnetic and metamagnetic behaviour. In the case of Fe-doping (Fig. 12), the pressure affects the magnetisation process more effectively. The magnetisation curve has a metamagnetic character already at 2.6 kbar. The pc, value is very low, about 1 kbar (in compound with 1% Fe, the pc, value at 4.2 K is just equal to the ambient pressure, as seen from the similarity between the curve at pc, = 5 kbar in Fig. 11 and the ambient-pressure curve for UCoo.99Fe0,0,A1 in Fig. 3). From the development of magnetisation curves with increasing pressure, we can expect that not only ferromagnetism but also metamagnetism will be suppressed at a critical pressure pc,. In particular, this is seen well in UCo0,98Ni0.02A1, which is paramagnetic already at ambient pressure. Meta-

isotropic powder, T = 4.2 K I

0

2

4

6

8

10

B (T)

Fig. 11. Field dependenceof magnetisation M (top) and differential susceptibility dM/dB (bottom) for isotropic powder of UCO~,~~RU~,~,A~ at 4.2 K under different pressures.

magnetism looks almost completely suppressed at 10 kbar (Fig. 13). In the UCoAl-based single crystals, pc, was determined from pressure dependence of AM as 22-24 kbar only slightly dependent on the composition within the homogeneity range [6]. Results obtained on polycrystals do not give so accurate values of AM as in the case of single crystal, and we can only estimate pc,. It is natural to expect a decrease of pc, in UCo0.9sNi0.0,Al compared to UCoAl because both the doping and the external pressure work together to suppress the metamagnetism. pc, is estimated to be (18 + 4) kbar, which would seem somewhat smaller than in UCoAl. However, the same or even slightly lower pc, values were found also in the cases of Fe or Ru substitution despite the fact that such a doping enhances the magnetic coupling reaching finally ordering. Thus, pc, in UCoAl does not depend strongly on the composition. A hysteresis at the transition remains rather small with increasing pressure in all the compounds studied. We cannot compare these

A. V, Andreev et al. / Joumal of Magnetism and Magnetic Materials I69 (1997) 229-239

237

UC0 0.!#i0.02*1 I

0.24

3

I

I

1

0.24 - isotropic powder, T = 4.2 K 5 b t E

0.16

0.16

0.08

0.00

n.nn _.__ 8

0

2

I

I

4

6

4

I

8

0.06

t

p=o h

0

2

4

6

8

10

10 B U-1

B CO Fig. 12. Field dependence of magnetisation M (top) and differential susceptibility dM/dB (bottom) for isotropic powder of UCo0,9sFe0.02A1 at 4.2 K under different pressures.

features with 3d-band metamagnets because of lack of systematic results. In the compounds studied, the B,(p) dependence (Fig. 1) B,(p) = B(0) + k*p

Fig. 13. Field dependence of magnetisation M (top) and differential susceptibility dM/dB (bottom) for isotropic powder of UCoe.ssNi o.ozAl at 4.2 K under different pressures.

isotropic powder, 4.2 K 5

/( /

/

(3)

yields the coefficient k = dB,/dp = 0.29, 0.36 and 0.39 T/kbar in UCO,,~~T~.~~AI with T = Ru, Fe and Ni, respectively. This is somewhat larger than that found in the UCoAl-based single crystals (results for one of them are given in Fig. 14 for comparison). As regards the 3d-band metamagnets, magnetisation under pressure was studied in the Lu(CoO.asGa,,& [15]. This is a ferromagnet at ambient pressure but the ferromagnetism is lost under pressure and metamagnetism appears with very large pressure effect on the critical field. The behaviour is very similar to that of UCO~.~~T~.~~A~ with T = Ru, Fe (Figs. 11 and 12), but dB,/dp reaches the value 1.0 T/kbar. In this respect,

0

2

4

6

8

10

12

P @bar) Fig. 14. Pressure dependence of transition field B, of metamagnetic transition for isotropic powders of UCoa.asTa.02Al (T = Ru, Fe, Ni) solid solutions. The plot for Ul.lCoa.a~A1a.ss single crystal is also presented for comparison.

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metamagnetism in UCoAl-based compounds is less pressure-dependent. Nevertheless, both types of compounds can be classified as very sensitive to pressure. Another quantitative difference in magnetoelastic properties of UCoAl and RCo2 (R = rare-earth element) is the difference in volume magnetostriction. It is well known that the spontaneous [19-21-J or field-induced [22] ferromagnetism of Co sublattice in RCo2 is always accompanied by an increase of volume up to 5 x 10m3. Volume effect at the metamagnetic transition in UCoAl is also positive but much smaller, only 1 x 10m4 [23]. In Ref. [6] we determined the volume compressibility of UCoAl as k- = 0.89 x lop3 kbar-i which is in good agreement with other UTX [24]. Taking this Ic-value, the change of volume corresponding to the critical pressure pC, of suppression of metamagnetism may be estimated as AV/V = - 2%. One can see that the volume effect at metamagnetic transition is 200 times smaller than this estimate. This may reflect that ground state of UCoAl is much closer in energy scale to ferromagnetic than to a ‘normal’ paramagnetic. Extrapolation of B,(p) dependences for UCoAl and Ni-substituted compounds to the range of negative pressure gives pC, = - (4-5) kbar, which corresponds to a 0.5% lattice expansion (in volume) needed to stabilise the spontaneous ferromagnetism. It is impossible to find a substitute with larger atomic radius than components in UCoAl, which provides only the ‘chemical’ negative pressure and does not influence the electronic structure in another way. We can only mention in this respect that in the UCoAl,-,Sn, system the onset of ferromagnetism (x - 0.25 [25]), corresponds to a 3.5% volume expansion and ferromagnetic UCoGa has practically the same unit-cell volume as UCoAl (e.g. Ref. Cl]).

4. Conclusions In the course of the present investigation we have determined numerous features which are common to the 5f band metamagnet UCoAl and to the group of compounds containing YCo2, LuCoz and Co& in which the metamagnetic state is connected

with the magnetic-field-induced splitting of the majority and minority 3d subbands of Co. The strong sensitivity to stoichiometry variations, doping by other elements, and to changes of interatomic distances (induced by external pressure) is naturally expected in these materials with electron band states which are close to a magnetic instability. The present reported effects of substitutions of Co in UCoAl by some other transition metals seem to violate the previously determined semi-empirical rules relating to the development of magnetic ground-state in UTX compounds with the occupation of the d-states of the transition metal, which considerably affects the strength of the Sf-dhybridisation. In our earlier work, we considered almost exclusively that this hybridisation is responsible mainly for the delocalisation of the 5f states and consequently, for the washout of 5f magnetic moments. This oversimplified picture seemed to explain well the evolution from the ‘less-magnetic’ (no or small ordered 5f moments) towards the ‘more-magnetic’ (large ordered Sf-moments) compounds, which can be demonstrated, e.g. on the series UFeAl-UCoAl-UNiAl. The recent results reveal that the initial substitutions of Co by Fe or Ru in UCoAl lead to an almost immediate appearance of ferromagnetism and that the Ni-doping, on the other hand, causes suppression of metamagnetism (increase of B,). These phenomena also demonstrate the other aspects, namely, the Sf-ligand hybridisation which is involved not only in the delocalisation of 5f states but also mediates the 5f-5f exchange interactions between uranium ions. Since the Sf-ligand hybridisation is anisotropic depending on the geometry of surrounding of the U-atom by ligands this should be reflected to the anisotropy of exchange interactions. This feature is particularly pronounced in the ZrNiAl-type structure of the considered UTX compounds. This structure consists of U-T and T-X basal-plane layers stacking along the c-axis. Depending on which of the two nonequivalent transition metal sites is occupied preferentially by the substituent atoms various effects may be induced. Whereas the exchange coupling within the U-T planes is basically ferromagnetic, irrespective of the transition metal species, the coupling along the c-axis is very sensitive to substitution on the sites in the ,T-X plane,

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of Magnetism and Magnetic Materials I69 (1997) 229-239

which may yield different ground states (ferromagnetic, antiferromagnetic, nonmagnetic). Within this scenario, the ‘unexpected’results of Fe, Ni, Ru substitutions in UCoAl reflect the effect of substitution in the T-X plane on the exchange coupling along the c-axis. The change of volume effects(pressure experiments) may be interpreted only if we know the details about the anisotropy of linear compressibility. Acknowledgements

The authors thank L. Dobiasova for the kind help in the attestation of the samples. The stay of M.I.B. in ISSP was supported by the Ministry of Education, Scienceand Culture of Japan. The work was supported by the Grant Agency of the Academy of Sciences of the Czech Republic (grant #A1010614), the Grant Agency of the Czech Republic (Grant #202/96/0207) and the Ministry of Education of the Czech Republic (Grant #ES 011). References [l] [2] [3] [4] [S] [6]

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