ThMn-TYPE COMPOUNDS OF RARE EARTHS AND ACT1NIDES 157 M atoms enter the sites which have a minimal contact with the R-sublattice 2(a) sites (Zarechnyuk and Kripyakevieh 1963). This is confirmed by the occupation of the 8(f) and 8(j) and not 8(i) positions by the Si atoms in GdFel0Si2 (Dirken et al. 1989), in SmFel0Si2 (Buschow 1988a) and in UFel0Si2 (Berlureau et al. 1991); Si enters the 8(f) sites in UCol0Si2 (Berlureau et al. 1991) and the 8(j) sites in UNia0Si2 (Suski et al. 1993a).
Finally, in YNil0Si2 (Moze et al. 1991) the Si atoms preferentially oceupy the 8(f) sites, with some small occupancy also for both the 8(i) and 8(j) positions. However, in SmFellTi the Ti atoms enter both the 8(i) and 8(j) sites (Ohashi et al. 1988b).
The statement about the rare-earth atoms exclusively occupying the 2(a) position is not true exactly: Bodak and coworkers reported some admixture of the lanthanide atom in other positions in the following systems: Ce-Co-Mn (Bodak et al. 1981), alloys of Fe and W with Ce, Pr, Nd and Sm (Bodak and Berezyuk 1983), alloys of Fe and Mo with the same lanthanides (Bodak and Berezyuk 1985), and the Dy-Fe-Re system (Sokolovskaya et al. 1985). Moreover, the Ce-Fe-Mo phase exists over a broad composition range of Fe and Mo, and the fraction of Ce entering other than 2(a) positions amounts to 0.025 (Bodak and Berezyuk 1985). This fraction is typical for other lanthanides and does not reach a value higher than 0.05. Unfortunately, the location of these excess lanthanide atoms has not been determined.
From the above discussion one ean see that the ThMn12-type strueture is complex, and it is impossible to formulare any clear-cut general rules of formation for the respective compounds. For the sake of simplieity pseudoternaries are omitted in the above considerations; some attention to their structure will be devoted in the description of their magnetic properties, for which the location of the individual components is of paramount importance.
158 W. SUSKI
that the moment values of the 8(i) iron exceed those of the 8(j) and 8(f) iron atoms only in compounds in which R = Nd, Tb, Er or Y, but the iron moments at the 8(i) sites are sub- stantially lower than those in the remaining sites in the eompounds where R = Ho and Dy.
The authors have no explanation for this nonsystematic variation of the iron moments.
3.1. (R, An)M12-type compounds 3.1.1. Compounds with M = Mn
Magnetic properties of binary M = M n compounds am listed in table 4. They exist for heavier rare earths ineluding Y and for Th. Preparation is difficult due to the high vapor pressure of Mn. For this reason the starting eomposition is usually RMn12.5-13.0 and therefore the resultant alloys always contain some amount of the neighboring phases as impurities. After prolonged annealing these impurities are not observed by XRD.
Unfortunately, we could not find any magnetie data coneeming ThMnl2. Attempts to obtain UMn12 turned out to be unsuccessful. The parameters presented in table 4 prove that AF-type magnetic ordering exists in the Mn sublattice below 87 K (Er) and
Table 4
Binary compounds of the ThMnlz-type with Mn
Compound Ps T~ c T~ TN p«f O Easy Remarks Ref.
(#B) (K) (K) (K) (#B) (K) direct.
GdMn12 4.2 a 5.0 3.6 160 7.92 - 8 0 55Mn NMR, peculiar magn. str. 1
5.5 9.4(5) b magn. struet. 4
spin echo 3
~120 electr, resist, vs. tempemture 5
TbMn~2 4.4 4.7 3.0 108 10.26 - 7 S»Mn NMR 1
6.0 8.3(3) b spin echo 3
I c magn. str.: Mn at 2 diff. 4 magn. sites, Tb moment in basal plane
~120 electr, resist, vs. temperature 5
DyMn12 5.05 2.2 100 10.25 -11 5»Mn NMR 1
_l_c 4
HoMn12 6.4 1.7 90 10.4 - 8 ~SMn NMR 1
IIc 4
ErMn12 6.5 1.9 87 9.8 II c 4
YMn~2 0.42 c 120 a = 0.859 nm, c = 0.479 nm; 2
ND: magn. str. fig. 8
G d , f(x) 7 Gdl_xYxMnl2 3
• Variable composition; see Remarks column, b Arrott plot.
a Magnetic moment canted from the a-axis by an c Po.
angle of 56.6 ° .
References
(1) Okamoto et al. (1987) (3) Amako et al. (1992) (2) Deportes and Givord (1976) (4) Deportes and Givord (1977)
(5) Amako«al. (1993)
ThMn-TYPE COMPOUNDS OF RARE EARTHS AND ACTINIDES 159
ùZ I" ~" i. T i" ~ 1
•
" . ~,'.i ,¥- / ""
¥" T ,~,m , "~" "r
-(?~ / - ~ \ -(
~ - J ~ . , «
«j)- ~t~ "Nr- "t,t 4~
)
Fig. 8. Magnetic structure of YMn12 determined using neutron diffraction by Deportes and Givord (1976). Y atoms at the 2(a) sites and Mn atoms at the 80) and 8(j) sites lie in the planes z = 0 (full circles) and z = 1/2 (dashed cireles). Mn atoms on the 8(f) sites are located at z = 1/4 and z = 3/4.
160 K (Gd) as the lowest and highest TN temperature, whereas for the eompounds containing magnetic lanthanides, ferromagnetic ordering is observed in the sublattiee of these elements below 10 K. This low temperature obviously stems from the strong dilution of the lanthanide atoms (1:12). The various values of TN for the Mn sublattiee in other eompounds than YMn12 indieate a strong influenee of the lanthanide atoms on magnetie properties, most probably through the band strueture, i.e. the transfer of the rare-earth eonduetion eleetrons to the 3d band.
The magnetic strueture of the Mn sublattiee determined by neutron diffraetion in YMn12 by Deportes and Givord (1976) is presented in fig. 8. The Shubnikov group of the magnetic structure is Ip4/mtmm. As the mirror planes contain atoms of the magnetie group, a given magnetie moment must transform into itself through the assoeiated mirror plane. Although the magnetie strueture is not eollinear, the arrangements of magnetie moments between first neighboring atoms are nearly eollinear. The magnetic strueture of GdMnl2 has been proposed on the basis of the magnetie and NMR measurements (see sect. 1). This strueture is presented sehematieally in fig. 9 (Okamoto et al. 1987); it has been described in sect. 1 and shows that the eoupling between the lanthanide and transition metals is not necessarily parallel. AdditionaUy, we should note that the Gd moments eant from the a-axis by an angle of 56.6 ° in the a - c plane.
In a discussion of the influence of the magnetic lanthanide atom on eleetronie strueture of the ThMn]2-type Amako et al. 1992 drew an interesting eonelusion from the coneentration dependence of the saturation magnet-ic moment in the Gdl-xYxMnl2 system. They showed that the magnetic moment of Gd gets eloser to the free-ion value (7ttB) when the Gd is more diluted. At the same time the Néel point deereases from 160 K to 120K for the Gd and Y compounds, respeetively. This differenee suggests that the Gd atom induees an additional positive magnetie interactions in the Mn sublattiee whereas the other lanthanide atoms seem to induce negative interaetion resulting in lower TN.
160 W. SUSKI
°°°l~
©
Fig. 9. Magnetic strueture of GdMn12 proposed by Okamoto et al.
(1987) on the basis of magnetic and »SMn NMR examination.
Atoms loeated at 8(f) have negligible magnetic moment and are not shown.
The eleetric resistivity o f YMnl2, GdMnl2 and TbMnl2 has been measured by Amako et al. (1993). The tempera~tre dependenee of the electrie resistivity exhibits a Néel point about 120 K for all compounds with a remarkable anomaly below this temperature. It has been suggested that the anomaly is due to an AF band gap.
3.1.2. Compounds with M = Fe
Only the SmFel2 binary compound has been synthesized (Hegde et al. 1991) as an oriented sputtered thin film with a slight iron deficiency. The crystallographic and magnetic data are listed in table 5. This film is strongly (002) textured and the anisotropy field is estimated to be about 13 T. Navarathna et al. (1992) have shown that the texturing can be switehed from (002) to (200) and the SmFe12 (222)-textured films could be synthesized with higher coereivities than (002)-textured films. For (222)-textured films the crystallite c-axes make an angle of 51 ° with respect to the film plane, versus 90 ° for (002)-textured films. Consequently, their hysteresis loop is less square for this texture than for an ideal (002)-textured films. The He values neeessary to switch the texture direction, perpendicular and in-plane for the (222) sample, were 2.5 and 3.2 kOe, respectively.
GdFel2 does not exist as a stable compound but electronic structure calculations using the LMTO-ASA method by Trygg et al. (1992) have been performed. As was observed in the experiment on the RMn12 systems there is a pronounced influence of localized 4fmagnetism on the conduction-band magnetism (transition-metal sublattice) which gives noticeable changes in the local moment of the iron (transition element). The presenee of the 4f spin moment is found to induce a redistribution of the spin moment between the rare-earth and iron sites, while the total conduetion-eleetron moment remains constant. It
seems that these conclusions have also some importanee for the temary materials.
3.1.3. Compounds with M = Zn
The lattice parameters and magnetic properties of compounds with M = Zn are listed in table 5. According to Iandelli and Palenzona (1967) only the compounds of the heavy lanthanides exist. Since the Zn sublattice is nonmagnetic the magnetic properties of these alloys are related exclusively to the lanthanide atoms. The lattice parameters reported by Iandelli and Palenzona (1967) follow the lanthanide contraction, however, the data of
ThMn-TYPE COMPOUNDS OF RARE EARTHS AND ACTINIDES Table 5
Binary eompounds of the ThMn~2-type with Fe and Zn
161
Compound a c T~ a TE a Pcfr 69 Remarks Ref.
(nm) (nm) (K) (K) (gB) (K)
YFe12
StuFen2 0.8438(5) 0.4805(5)
GdFe12
YZn12 0.8861 0.5205
SmZnl2 0 . 8 9 2 7 0.5215 15 14 0.74 -27.8 GdZnl~ 0 . 8 8 9 8 0.5210 16 15.25 8.40 -58 TbZn~2 0 . 8 8 5 6 0.5199
0.8884 0.5200 14 13 10.14 -38
DyZn12 0 . 8 8 7 2 0.5204 0.8877 0.5198 HoZnlz 0 . 8 8 6 8 0.5195 ErZn12 0.8850 0.5195
0.8863 0.5195 2.8 2.76 9.67 +3 TmZnl2 0 . 8 8 6 3 0.5190 0 0
YbZnl2 0 0
LuZn12 0 . 8 8 4 8 0.5186 0 0
»TFe ME, H hf =37.7T 1 crystalline thin film, 2 Ps = 1.8pB/Fe atom,
H¢ =0.6Th; c-axis
perpendicular to the film plane does not exist as a stable 3 compound;
ab initio calculation of magnetism
4 5,6 c 5,6 c 4 5,6 ¢ 4 6 6 4 5,6 ~ 5,6 ~ 5 5,6 c T~ determined in
b For the onset of ferromagnetic order.
c Lattice parameters.
References
(1) Denissen et al. (1990) (2) Hegde et al. (1991) (3) Trygg et al. (1992)
magnetometric measurements; Tr~ determined in resistivity measurements.
(4) Kuz'ma et al. (1965) (5) Stewart and Coles (1974) (6) Iandelli and Palenzona (1967)
K u z ' m a et al. (1965) exhibit some irregularity for Dy and Er compounds. Contrary to the M n compounds, magnetic order in the lanthanide sublattiee has antiferromagnetie eharacter. The Néel points determined in magnetic measurements are close to that obtained from examination o f the resistivity by Stewart and Coles (1974). The magnetie suseeptibility o f these materials follows a Curie-Weiss law above 1 0 0 K and effeetive magnetic m o m e n t values are close to that expeeted for the trivalent lanthanide free ions.
Because the A F ordering temperatures o f the RZnl2 eompounds are low, the p h o n o n resistivity is small eompared to the spin-disorder resistivity, Ps, even at TN. Therefore, the disorder contribution ean be determined with good aeeuraey and the dependence o f ps
162 W. SUSKI
A
E
Er F Sm Tb Ga
j j l
s .
ùe..-"
, s / /
ù t |
5
I I
10 15
(g-1)2j (J*l)
Fig. 10. Total spin disorder resistivity, B, versus (g-1)2j(J+l); p~ has been corrected by a small phonon contribution at T u (Stewart and Coles 1974).
on temperature T is Ps ~ T1'38 for GdZnl2, Ps ~ T 1'45 for TbZnl2, Ps ~ T 5'9 for SmZnl2, and Ps ~ T6"5 for ErZn12 (Stewart and Coles 1974). These results cannot be related to any existing theories o f electron-magnon seattering. In fig. 10 the total spin-disorder resistivity, Ps, normalized to that of GdZnl2 after correcting for the small phonon contribution at TN, is presented versus ( g - 1)2 J ( J + 1), where g is the Landé factor and J is the angular momentum of the rare earth. The ps values of Tb and Er compounds are smaller than expected from the linear plot. This might result from a non-S ground state. Specific-heat measurements (Stewart 1973) demonstrate that in ErZnl2 probably only one CEF doublet (the ground state) is involved in the magnetic transition. In turn, Ps for SmZn]2 is slightly larger than expected, suggesting a contribution other than exchange scattering.
3.2. (R, An)TxM12-x-type compounds 3.2.1. Aluminides
The magnetic and some related properties of the ThMnl2-type aluminides will be presented below. The authors usually distinguish three types within these materials:
(R, An)TxAll2_x, with x = 4 , 5 and 6. However, this is only a simplifieation for easier presentation of the results, and in reality solid solutions with 3 < x ~<6 or sometimes even higher x were reported (see e.g. Kamimori et al. 1988, Zeleny et al. 1991). Nevertheless, we present in principle the results according to stoichiometry with x = 4, 5 and 6. At the end of this subseetion we will describe the systems which are explicitly determined as a solid solutions.
X-ray and ME investigations show that for the materials with x = 4 the transition metal oeeupies mostly the 8(f) crystallographie position, and with increasing x the 8(j) site is populated, sometimes exclusively as reported for the UCu4+xAls_x system (Krimmel et al.
1992) or with a strong preferenee over the 8(i) position. It might be that in the case of T = Fe this diserimination of the 8(i) position is a reason for the relatively low Curie point, below about 300 K in these materials.
ThMn-TYPE COMPOUNDS OF RARE EARTHS AND ACTINIDES 163 3.2.1.1. (R, An)M4Als-type compounds. These compounds form a kind of superstructure because the transition element enters exclusively the 8(f) position as has been shown in numerous ND and ME experiments. The results of different examinations are collected in tables 6 - 12 below.
3.2.1.1.1. M = Cr compounds. As follows from table 6 these compounds exhibit AF ordering with the exception of La, Ce, Nd, Yb, Lu, Y, Th and U compounds.
The ordering exists only in the lanthanide sublattice, and that is the reason for the relatively low Néel points (below 20 K). Among the actinides, the magnetic ordering in NpCr4A18_x is observed in the Np as weil as Cr sublattices but the ordering temperatures are quite close (~55K) and cannot be distinquished (Gal et al. 1987). The angle between hyperfine field and c-axis observed in the 55Gd Mössbauer experiment in GdCr4A18 by Feiner and Nowik (1979) suggests a possibility that the AF structure in these materials is not necessarily collinear. Also the temperature dependence of magnetization reported by Feiner and Nowik (1979) is not typical for AF. In most of the compounds a sharp rise of the magnetization is clearly seen at low temperature.
On the one hand the magnetic moments determined at 1.1 K under an applied field of 1.7T are low compared to their free-ion value. Only the temperature dependence of magnetization of GdCr4A18 is typical for an antiferromagnet. On the other hand, the paramagnetic Curie temperatures 69 are negative for all magnetic RCr4A18 type compounds, hence Feiner and Nowik (1979) concluded that all these compounds order antiferromagnetically. The lack of magnetic order in the compounds of La, Ce, Lu, Y, Th and U is clear evidence that the Cr ions do not carry any localized magnetic moment. However, the enhanced Curie constants determined for magnetic compounds indicate that magnetic R ions induce small magnetic moment (about 0.1#B) in the Cr sublattice. For SmCr4AI8 there is evidence of a contribution of the ionie excited states into the 6H5/2 ground state, and thus it is not justified to compare the results for this compound to the free-ion value for Peff- The positive Curie-Weiss temperature as weil as the value of the magnetic moment for YbCr4A18 may suggest a mixed-valence state. ME provides evidence of ~10% admixture of Yb 3+ in the predominantly Yb 2+
(Feiner and Nowik 1979). The mixed-valence state is also confirmed by unit cell volume results (Feiner and Nowik 1979) and an X-ray spectroscopy experiment by Shcherba et al. (1992).
At present we do not know why UCr4A18 does not follow the Curie-Weiss law (Baran et al. 1987). Magnetic order as the cause of this behavior is excluded by a neutron diffraction study (Bourée-Vigneron et al. 1990). Therefore, the cause could be a partial delocalization of the uranium 5f electrons.
3.2.1.1.2. M=Mn compounds. The results concerning magnetic and related properties of the (R, An)Mn4A18-type compounds are collected in table 7. In these compounds only the lanthanide sublattice seems to order magnetically (AF) except for the compounds of La, Ce, Yb and Lu. The compounds of Y, Th and U are also nonmagnetic. This observation is strange because in the ThCr2Si2-type compounds the exchanges in the Mn sublattice are strong and magnetic order is observed above room temperature (see Szytuta 1991). It is, however, possible that in the compounds containing a magnetic
164 W. S U S K I
t,
.<
,v
CD
t~
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~ ~ ~ li i i i i i i • 7
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O O O O O O
O O O O O O O O O O O O O O O O
o'~ O t ~ I'~ O ~ ~dD O t ~- ~ " t,~ t ~ ~ ~ ~ -
ThMn-TYPE COMPOUNDS OF RARE EARTHS AND ACT1NIDES 165
~J
~o
o o
o +
F~ 8
166 W. SUSKI
~ <
<
ù~
o o
r~
ù ~
~ E ~ ~
+ +
< < < < <
,
T h M n - T Y P E C O M P O U N D S OF R A R E E A R T H S A N D A C T I N I D E S 167
ù ~
Z
o / ) t ~
<
' , o ¢-,I ~ ~ t"q ',,o t ' q o o ~ t'-,I ¢'q ~ ¢ q '~" ¢-,I ~ ~ O~ ~
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r'z.
¢ q t",-
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• n- ,n- , 4 ~
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õ
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..,q
t'-,I ù'¢;I
168 w. SUSKI
lanthanide atom, the lanthanide element induces magnetic order in the Mn sublattice and the R and Mn sublattices order antiferromagnetically.
Coldea et al. (1994a,b) claim that the oceurrenee of a magnetie moment, at least in YMn4AI8 and GdMn4A18 is strongly eorrelated with the eritieal value of the M n - Mn distance d ~ 0.26 nm, below whieh the Mn moment is not stable. This value had been postulated for the first time for the Laves phases with manganese by Wada et al.
(1987) and has recently been confirmed by Kim-Ngan et al. (1994) who gave a more precise value of 0.267 nm. However, as mentioned above, the magnetie properties of the ThCr2Si2 phases as weil as the UMnxAl12-x alloys suggest that the band structure and geometry o f the close environment of the Mn atom also have substantial influence.
Moreover, the partial structural disorder in UMn4A18 resulting from the permutation of the Mn and A1 atoms between 8(f) and 8(j) sites reported by Bourée-Vigneron et al. (1990) eould be the cause of the disappearance of the ordered magnetic moment. Although the Mn-Mn distance is just below the critical value (d = 0.255 nm), likely as in YMn4A18 (d=0.256nm) (Coldea et al. 1994b), the geometrical condition can prevent formation of a magnetic order. These authors claim that an increase in Mn eoneentration results in appearanee of localized moment in the Mn sublattice. Their additional statement that the degree of localization changes with temperature is based on rather speculative arguments. On the contrary, an increase in x in the uranium alloys does not cause any remarkable change of charaeter of magnetic behavior, although for UMn3A19 there is an indieation ofmagnetic ordering below 50 K (Suski et al. 1995). The 57Fe Mössbauer probe studies of rare-earth compounds indicate that at 4.1 K the Mn sublattice is magnetically ordered. The low magnetic moment at 4.1 K and in 1.7T (see table 7) and the shape of the magnetization eurves show that both the R and the Mn subsystem transform to antiferromagnetie stare.
The Mössbauer experiments carried out by Felner and Nowik (1979) On the lanthanide nuclei (X55Gd, 166Er, 161Dy and 17°yb) show that the hyperfine field is directed at an angle to the c-axis, and this observation suggests that the AF structure is not simple, collinear.
Inelastie neutron diffraction experiments performed by Moze et al. (1990a) revealed a substantial influence of CEF on the properties of the Tb and Ho compounds. Elastic neutron examination of several lanthanide compounds shows that the 8(f) site is the majority site (93%) for the Mn atoms but 7% is oceupied by A1 atoms. The remaining 7% of the Mn atoms is not distributed statistically over the 8(j) and 8(i) sites but 5%
of them enter into 8(j) sites while only 2% enter into 8(i) sites (Moze et al. 1990b).
The unit cell volume of the RMn4A18 compounds suggests that the eompounds of Ce, Eu and Yb could be in a mixed-valence state (Feiner and Nowik 1979). For the first compound this conelusion is confirmed by X-ray spectroscopy, where v = 3.18, see table 3 (Shcherba et al. 1992). The experimental observation of 151Eu ME and the susceptibility curve above TN indicate a mixed-valence stare in EuMnnA18 (Feiner and Nowik 1979). In YbMn4A18 Yb is predominantly divalent (Feiner and Nowik 1979). The suseeptibility of ThMn4A18 follows a modified Curie-Weiss law resulting from pronounced contribution of the Pauli paramagnetism of the Mn sublattiee. The susceptibility of UMn4A18 does not follow a Curie-Weiss law and this behavior most probably is due to the competition of
ThMn-TYPE COMPOUNDS OF RARE EARTHS AND ACTINIDES 169 the itinerant conduction electrons of the U and Mn sublattices, and partly loealized and partly hybridized 5f and 3d electrons, respectively. Elastic neutron diffraction excluded magnetic order in the uranium compound above 1.5 K (Bourée-Vigneron et al. 1990) as mentioned above.
3.2.1.1.3. M=Fe compounds. The compounds of the (R, An)Fe4A18-type have been investigated most frequently of all (R, An)M4Als-type compounds. The results are collected in tables 8 and 9. One should note that this is the group of compounds in which several examples of single crystals are available (GdFe4A18, Fujiwara et al. 1987;
YFe4AI8, Chetkowski et al. 1991, Drzazga et al. 1994; DyFe4AI8 and HoFe4A18, Drzazga et al. 1994; UFe4AIs, St~piefi-Damm et al. 1984, Gonqalves et al. 1992). Some of the (R, An)Fe4A18 compounds offer the unique possibility to perform ME examination on both the 57Fe and the R or Np nuclei. These compounds can be divided into two distinct groups: In the first group, where R = La, Ce, Y, Lu and An = Th, only the Fe atoms carry a magnetic moment. According to a preliminary examination of these compounds the more or less sharp maxima in the temperature dependence of the magnetic susceptibility are interpreted as being due to AF ordering with the Fe moment ofiented along the c- axis (La, Ce, Buschow and van der Kraan 1978). In the seeond group (R = Eu, Gd, Yb), however, an angle is observed between the hyperfine field direction and c-axis (Feiner and Nowik 1978). The conclusion about simple AF order for the first group is not valid here because the magnetic susceptibility of these alloys below TN is markedly field dependent (Buschow and van der Kraan 1978), and even a tiny remanence at 4.2K was observed for the Th compound (Suski 1989). In the paramagnetic region the susceptibility curves obey a modified Curie-Weiss law.
The compounds of the second group have two independent magnetic sublattices:
one has Fe atoms ordered AF with magnetic moment oriented, according to 57Fe ME (Buschow and van der Kraan 1978), along the c-axis at 4.2K (Nd, Gd and Tb), and the second has R or An atoms where the R atoms order at a TN which does not exceed 20K (Feiner and Nowik 1978). The low magnetic moments at 4.2K and 1.7T (see table 8) in all the RFe4A18 eompounds indicate that both the Fe and R sublattices are initially AF coupled. In the majority of these compounds the temperature dependence of magnetization gives rise to a maximum at low temperature, with magnitude and location strongly dependent on the applied field strength. However, these maxima are absent if the samples are cooled down in the presence of a magnetic field prior to the magnetization measurements. In the high-temperature regime the susceptibility can be described by a Curie-Weiss law with the 69 and Peff values collected in table 8. Single-crystal studies of the Dy, Ho and Y compounds (Drzazga et al. 1994) have revealed that at RT all investigated samples exhibit a small but nonnegligible torque of the sin20-type, which is strongly influenced by the external field. It follows that a slight preference of easy magnetization direction (observed below Tc) occurs in the temperature range treated as paramagnetic (above Tc). This phenomenon may be associated with partial disorder of Fe and A1 atoms. For R = Gd, Tb, Dy, Ho and Er there is another region below To where the X -1 (T) plot is linear (but To is higher than the temperature of magnetic order), although the (9 and Peff values are different from those found above To. The effective moments