Magnetic properties of NdMn6Sn6 and SmMn6Sn6 compounds from susceptibility measurements and neutron diffraction study

Journal of Alloys and Compounds 252 (1997) 41–49

L

Magnetic properties of NdMn 6 Sn 6 and SmMn 6 Sn 6 compounds from susceptibility measurements and neutron diffraction study a, a a b B. Malaman *, G. Venturini , B. Chafik El Idrissi , E. Ressouche a

´ , Universite´ Henri Poincare-Nancy ´ Laboratoire de Chimie du Solide Mineral I, associe´ au CNRS ( URA 158), B.P. 239, 54506 Vandoeuvre les Nancy Cedex, France b ´ ` Condensee ´ /SPSMS-MDN, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France CEA /Departement de Recherche Fondamentale sur la Matiere Received 26 August 1996

Abstract Investigations by susceptibility measurements and neutron diffraction experiments have been performed on the ternary stannides NdMn 6 Sn 6 and SmMn 6 Sn 6 of HoFe 6 Sn 6 - and HfFe 6 Ge6-type structure, respectively. In these structures, each of the rare earth and manganese atoms is successively distributed in alternate layers with the sequence Mn–R–Mn–Mn–R–Mn. Owing to the manganese atom coordination of the rare earth, they appear closely related to the CaCu 5 - and ThMn 12 -type structures. NdMn 6 Sn 6 and SmMn 6 Sn 6 are ferromagnetic below 357 and 405 K respectively. Two additional magnetic transitions related to the spin reorientation process are undergone in NdMn 6 Sn 6 at 135 and 35 K. A neutron diffraction study of NdMn 6 Sn 6 shows that both the rare earth and manganese sublattices order simultaneously above room temperature. The magnetic structure of NdMn 6 Sn 6 consists of ferromagnetically coupled Mn and Nd ferromagnetic (100) layers (i.e. positive Nd–Mn interaction). In the temperature range 135–35 K, the moment direction is along the a-axis whereas an easy plane occurs at 300 and 2 K with mNd 53.30(5) mB and mMn 52.39(4) mB . The results are discussed and compared with those previously obtained for the parent NdMn 6 Ge 6 and RMn 6 Sn 6 (R5Tb–Er, Lu, Y, Sc) compounds.

1. Introduction Most of the RMn 6 X 6 compounds (R5Sc, Y, rare earths; X5Ge, Sn) [1–3] crystallize in the Fe 3 Mn 4 Ge 6 -type structure (also denoted HfFe 6 Ge 6 ) [4,5] which is closely related to the CaCu 5 one [6]. Numerous neutron diffraction studies have shown the interesting magnetic properties of this class of materials [3,7–19]. In the heavy rare earth series, a classically antiferromagnetic R–Mn coupling was observed in all the studied compounds. On the contrary, it was shown that, in the light rare earth NdMn 6 Ge 6 compound [11], crystallizing in the closely related YCo 6 Ge 6 -type [20], a ferromagnetic Nd– Mn coupling occurs. It appeared therefore very interesting to check such magnetic exchange interaction in the corresponding stannide. From preliminary studies on the light rare earth stannide series [1], we have concluded the existence of the only SmMn 6 Sn 6 compound crystallizing in the disordered YCo 6 Ge 6 -type, at least above 973 K. Recently, crystallographic studies of the homologous RFe 6 Sn 6 series [3,21] *Corresponding author. 0925-8388 / 97 / $17.00  1997 Elsevier Science S.A. All rights reserved PII S0925-8388( 96 )02717-X

have shown that low temperature annealings yield long range ordering in these compounds previously reported of YCo 6 Ge 6 -type. It was then decided to reexamine the light rare earth manganese stannides at lower temperatures. SmMn 6 Sn 6 and NdMn 6 Sn 6 have been prepared in such a way [3]. During the preparation of this paper, Weitzer et al. [22] have published their results on the structural and magnetic properties of the RMn 6 Sn 6 (R5Pr, Nd, Sm) compounds, synthesized at 5458C. All three compounds were found to be isotypic with HoFe 6 Sn 6 [21]. According to these authors, the three compounds order ferromagnetically (or else in a more complex canted spin structure) slightly above room temperature with saturation moment values, at 5 K, of 12, 4.5 and 8 mB for Pr, Nd and Sm, respectively. ¨ However, in spite of a 119 Sn Mossbauer spectroscopy study, no realistic magnetic structure was proposed. The aim of this work was to determine the sign of the R–Mn exchange in the case of the light rare earth element RMn 6 Sn 6 stannides. Furthermore, the knowledge of the easy direction of the R and Mn moments was also important, since it has been observed that, in the RMn 6 X 6 series, the Ge,2.Sn substitution yields a change in the anisotropy direction of the R and Mn sublattices [3,7–19].

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B. Malaman et al. / Journal of Alloys and Compounds 252 (1997) 41 – 49

In this paper we report on the magnetic properties of NdMn 6 Sn 6 and SmMn 6 Sn 6 by the use of bulk magnetization measurements between 700 and 4.2 K and on the magnetic structure of NdMn 6 Sn 6 determined by neutron diffraction experiments.

2. Experimental procedures The compounds were prepared from commercially available high purity elements: Mn (powder, 99.9%), rare earth (R) elements (ingot, 99.9%) and tin (pieces, 99.999%). Pellets of stoichiometric mixture were compacted using a steel die and then introduced into silica tubes sealed under argon (100 mm Hg). The samples were annealed for 2 weeks at 973 K. After this, annealings of several weeks at different temperatures, between 723 and 923 K, were performed. The final products were checked by powder X-ray diffraction technique by using a curved position sensitive detector INEL CPS120 (CuKa). The magnetic measurements were carried out on a Faraday balance (above 300 K) and on a Manics magnetosusceptometer (between 4.2 and 300 K), in fields up to 1.6 T. Neutron experiments have been carried out at the Siloe ´ reactor of the Centre d’Etudes Nucleaires de Grenoble (CENG). Several patterns have been recorded in the temperature range 2–300 K with the DN5 multidetector ˚ ( l52.4965 A). Using the scattering lengths: b Sn 56.225 fm, b Mn 52

3.73 fm, b Nd 57.69 fm and the form factor for Nd 31 and Mn given in [23] and [24], respectively, the scaling factor, the atomic positions and the Nd and Mn magnetic moments were refined by the MiXeD crystallographic executive for diffraction (MXD) least-square fit procedure [25]. The MXD program allows the simultaneously fitting of the intensities of the nuclear and magnetic reflections. This program was also used for the refinements of the X-ray diffraction patterns.

3. Experimental results

3.1. Synthesis and crystal structure According to the particular atomic positions of the different atoms in the HfFe 6 Ge 6 -type [5], the structure factors of (hkl) with l52n11 lines become as an example: F(hk1) | A* [ f(R) 2 f(Sn)] and F(hk3) | A* [ f(R) 1 2 * f(Sn)] As the scattering factors ( f ) of the R elements are relatively close to that of Sn, the intensities of the (hk1) lines are largely weaker than those of the (hk3) lines. This behaviour is clearly illustrated by the characteristic diffraction pattern of the TbMn 6 Sn 6 compound (Fig. 1).

3.1.1. SmMn6 Sn6 X-ray diffraction patterns of SmMn 6 Sn 6 , annealed at 973 and 723 K, are shown in Fig. 1.

Fig. 1. X-ray diffraction patterns (CuKa) of TbMn 6 Sn 6 (HfFe 6 Ge 6 -type), NdMn 6 Sn 6 and SmMn 6 Sn 6 annealed at 723 K and ( * )SmMn 6 Sn 6 annealed at 923 K (see text).

B. Malaman et al. / Journal of Alloys and Compounds 252 (1997) 41 – 49 Table 1 Atomic coordinates and occupancy factors in SmMn 6 Sn 6 (HfFe 6 Ge 6 type, see text). HfFe 6 Ge 6

P6 / mmm Position

a55.552(2) ˚ A x y

c59.058(3) ˚ A z

Atom Mn Sn 1 Sn 91 Sn 2 Sn 3 Sm 1 Sm 91

mj

6(i) 2(e) 2(e) 2(c) 2(d) 1(a) 1(b)

1/2 0 0 1/3 1/3 0 0

|0.25 |0.33 |0.166 0 1/2 0 1/2

1 |0.75 |0.25 1 1 |0.75 |0.25

0 0 0 2/3 2/3 0 0

At 973 K, this study confirms the occurrence of the YCo 6 Ge 6 -type SmMn 6 Sn 6 compound [1] whereas in the 923–723 K annealing temperature range investigated, the diffraction patterns exhibit all the characteristic lines of the HfFe 6 Ge 6 -type structure. However, as displayed on Fig. 1, the intensities of the (hk3) reflections are clearly weaker than those expected for a fully ordered HfFe 6 Ge 6 -type. Refinements of this pattern yield the lattice constants, the atomic coordinates and the occupancy factors m j gathered in Table 1. According to these data, the crystal structure of SmMn 6 Sn 6 appears as

43

an intermediate state between the HfFe 6 Ge 6 and YCo 6 Ge 6 atomic arrangements (i.e. a partial HfFe 6 Ge 6 -type ordering of the YCo 6 Ge 6 structure). These results significantly differ from those of Weitzer et al. [22]. Nevertheless, we have shown in a previous paper [21] that the occurrence of each structural type could be very sensitive to the thermal treatment. In the same way, it is noteworthy that, up to now, we have not succeeded in preparing the PrMn 6 Sn 6 compound.

3.1.2. NdMn6 Sn6 In agreement with the results of Weitzer et al. [22], the existence of the NdMn 6 Sn 6 compound is revealed by long duration annealing performed below 800 K. Fig. 1 clearly shows that this compound crystallizes in the HoFe 6 Sn 6 type structure [21]. Table 2 gives the lattice constants and the mean atomic coordinates (i.e. approximate values according to the high number of adjustable parameters and the lack of experimental data) as deduced from the refinement of the X-ray diffractogram. 3.1.3. Crystal chemistry The HoFe 6 Sn 6 -type (Fig. 2), an intergrowth of ‘‘HfFe 6 Ge 6 blocks’’ and ‘‘ScFe 6 Ga 6 (ordered variant of

Table 2 Mean atomic coordinates in NdMn 6 Sn 6 (HoFe 6 Sn 6 -type [21], see text). HoFe 6 Sn 6 -type Atom

Immm Position

˚ a59.005(7) A x

˚ b528.76(2) A y

˚ c55.535(5) A z

Mn 1 Mn 2 Mn 3 Mn 4 Sn 1 Sn 2 Sn 3 Sn 4 Sn 5 Sn 6 Sn 7 Sn 8 Nd 1 Nd 2

4(e) 8(k) 8(n) 16(o) 4(g) 4(g) 4(g) 4(g) 4(g) 4(g) 4(e) 8(n) 4(g) 2(a)

0.251 1/4 0.251 0.251 0 1/2 0 1/2 0 1/2 0.336 0.336 0 0

0 1/4 1/3 1 / 12 1 / 18 1 / 18 1/9 1/9 2/9 2/9 0 1/6 0 1/6

1/2 1/2 1/2 1/2 1/2 1/2 0 0 0 0 0 1/2 1/2 0

Fig. 2. HoFe 6 Sn 6 -type structure [21].

B. Malaman et al. / Journal of Alloys and Compounds 252 (1997) 41 – 49

44

Fig. 3. Temperature dependence of magnetization in NdMn 6 Sn 6 and SmMn 6 Sn 6 compounds.

ThMn 12 ) slabs’’, has been largely described in [21]. It is worthwhile to note that, in both HfFe 6 Ge 6 - and HoFe 6 Sn 6 types, the R element has the same transition metal neighbouring (i.e. hexagonal dipyramid) while the transition metal builds the same Kagome´ net. Furthermore, in the HoFe 6 Sn 6 -type, rare earth and manganese atoms lie in (100) layers alternatively stacked along the a-axis yielding a similar ‘‘Mn–R–Mn–Mn–R–Mn sequence’’ to that observed in the HfFe 6 Ge 6 -type [(001) layers stacked along the six-fold axis]. A more detailed analysis of the crystal chemistry of the RT 6 X 6 (R5rare earth; T5Mn, Fe; X5Ge, Sn) compounds can be found in [21]..

3.2. Susceptibility measurements As previously claimed [14], NdMn 6 Sn 6 and SmMn 6 Sn 6 compounds order ferro(ferri)magnetically below T C 5357 and 405 K respectively (Fig. 3). The thermal variation of the magnetization of NdMn 6 Sn 6 (Happl. 5500 Oe) evidences two additional transitions at low temperature (Fig. 3): a sharp kink at T 1 5135 K and a smoother one at T 2 |30 K. This behaviour simply suggests the occurrence of a spin reorientation process at T 1 and T 2 as already observed in the

corresponding NdMn 6 Ge 6 germanide [11]. This hypothesis is confirmed by the neutron diffraction study (see below). In contrast, in the case of SmMn 6 Sn 6 , the magnetization remains almost constant in the whole temperature range below T C , except a light decrease below |16 K. It is noteworthy that these low temperature transitions were not observed by Weitzer et al.. This probably arises from the too high applied field used by these authors (|13 kOe) since, during this study, we did not detect any low temperature transition under applied fields greater than 10 kOe. In both cases, the coercive field remains very weak in the whole temperature range (4.2 K–T C ) investigated (Hc ,500 Oe) and the magnetization values measured at 4.2 K are 14.2 and 10.6 mB / mole for NdMn 6 Sn 6 and SmMn 6 Sn 6 , respectively (Table 3). In the paramagnetic state, the thermal variation of the inverse susceptibility well follows a Curie Weiss law and the shape of the thermomagnetic curve is characteristic of a ferromagnetic behaviour. This result and the high values of magnetization observed in both compounds rather suggest a ferromagnetic coupling between the R and Mn sublattices. Table 3, which compares our magnetic data with those published by Weitzer et al., underlines some strong discrepancies. Although these authors used applied fields up

Table 3 Main magnetic data of SmMn 6 Sn 6 and NdMn 6 Sn 6 compounds. TC (K) SmMn 6 Sn 6

405 [380] 357 [340]

NdMn 6 Sn 6 * Happl. 530 kOe;

1

Happl. 516 kOe)

T SR (K) 16 135, 30 -

up (K)

meff ( mB )

Hc (Oe)

M max / f.u. at 5 K ( mB )

Ref.

408 [380] 363 [274]

8.45 [7.70] 9.58 [8.50]

300 450 -

10.6 1 [8.0 * ] 14.2 1 [4.5 * ]

this work [14] this work [14]

B. Malaman et al. / Journal of Alloys and Compounds 252 (1997) 41 – 49

to 30 kOe, the measured magnetization values are largely weaker than those obtained in the present work. This is particularly true in the case of NdMn 6 Sn 6 (i.e. 4.5 mB against 14.2 mB ). In order to explain their results, these authors have concluded the occurrence of a ferromagnetic ordering or else a more complex canted spin structure for these compounds. This singular difference of magnetic data is ‘‘a priori’’ difficult to explain. However, as we will see, the neutron diffraction study of NdMn 6 Sn 6 unambiguously confirms our results.

3.3. Neutron diffraction study of NdMn6 Sn6 As shown in Section 3.1, NdMn 6 Sn 6 crystallizes in the orthorhombic (o) HoFe 6 Sn 6 -type structure (Immm) which is closely related to the hexagonal (h) HfFe 6 Ge 6 -type structure (P6 / mmm) adopted by the other members of the RMn 6 Sn 6 (R5Gd–Lu) series. A simple correspondence (amplitude and relative orientation) can be found for the lattice parameters (Table 2): ˚ b o 5 3a h ( | 28 A); ˚ c o 5 b h | (5.5 A) ˚ a o 5 c h ( | 9 A); As a result (h o →l h ) and according to the rather close values of the Fermi lengths of tin and neodymium, the intensities of the (1kl) lines will be largely weaker than the (3kl) ones. This implies a relatively simple neutron diffraction pattern despite the high values of the cell parameters. Finally, bearing these data in mind, it is important to stress that assuming a ferromagnetic ordering of the Mn sublattice yields (1kl) line intensities only correlated to the magnetic ordering of the Nd sublattice (i.e. magnitude and direction of the Nd moment).

3.3.1. Magnetic structure determination The temperature dependence of the neutron diffraction patterns recorded step by step from room temperature to 2 K shows only a variation in the intensity of the nuclear reflections, with no additional lines (Fig. 4). This observation implies a ferromagnetic ordering in the whole temperature range investigated, in agreement with the magnetometric measurements. A partial neutron thermogram is shown in Fig. 4a. One clearly observes that the intensity of the (200) line increases slowly down to |135 K, decreases abruptly below 135 K and increases again below |40 K. This result suggests the occurrence of a spin reorientation process and a careful analysis of the intensities of all lines (Fig. 4b) yields three temperature dependent magnetic states: 1. above 135 K, the moments lie in the (b,c) planes (the basal plane of the hexagonal subcell) i.e. the Mn and Nd layers, 2. between 135 and 40 K, they are along the a-axis (c-axis

45

of the hexagonal subcell) i.e. perpendicular to the Mn and Nd layers, 3. below 40 K, they are again in the (b,c) planes. These transition temperatures are in good accordance with those detected by magnetization measurements. Ac˚ the cording to the large value of the b parameter (|28 A), (110) plane is almost parallel to the (200) plane and its intensity follows the same thermal variation (Fig. 4 and Table 4). Above 135 K, its very weak intensity has to be related to a low value of the neodymium moment in this temperature range. Assuming a collinear ferromagnetic arrangement of both Mn and Nd sublattices, the refinements have been undertaken by refining the mean x Sn and x Mn abscissa, the Mn and Nd moment magnitudes and the angle u between the easy direction and the (100) planes. The thermal variation of u (Fig. 5) clearly displays the spin reorientation process, thus confirming our previous hypothesis. The u value is close to 08 at 2 and 250 K and has been fixed at this value in the final refinements (Table 4). The magnetic structures are displayed in Fig. 6. At 2 K, the moments are in the (100) plane and the relative intensities of the (110) and (130) lines show unambiguously that they are aligned along the b-axis. However, it was not possible to define precisely the orientation of the moments in this plane above 140 K. As the chemical cell is a superstructure built on an hexagonal subcell, the peaks corresponding to the Mn sublattice magnetic contributions have almost the same Bragg angles and any information concerning the Mn moment orientation in the (100) plane can be extracted. On the other hand, at high temperature, according to the observed data, the Nd moment value is not yet enough large to appreciably contribute to the intensities of the (110) and (130) lines. At 2 K, the Nd and Mn moments values are 3.30(5) and 2.39(4) mB , respectively yielding a total resulting moment of about 15 mB , in fair agreement with that obtained by magnetometric measurements. Their thermal variations (Fig. 7) suggest that the Nd and Mn sublattices probably order simultaneously at T C (5357 K). Table 4 gives the observed and calculated intensities together with the lattice constants and the various adjustable parameters at 260, 80 and 2 K. Remarks: It is noteworthy that from a X-ray diffraction study on oriented samples (Happl. 51.2 T), Weitzer et al. have concluded a net magnetization lying in the (b,c) plane at 4.2 K, in total accordance with the present neutron diffraction data. Nevertheless, in contrast with their work, this study unambiguously confirms our bulk magnetic measurements and shows that NdMn 6 Sn 6 behaves as a simple collinear ferromagnet. However, it remains very difficult to explain their largely weaker value of the saturation moment (Table 3). Obviously, a neutron diffraction study of their samples would be very interesting and

B. Malaman et al. / Journal of Alloys and Compounds 252 (1997) 41 – 49

46

Fig. 4. (a) Neutron thermogram of NdMn 6 Sn 6 recorded between 250 and 2 K. (b) Neutron diffraction patterns of NdMn 6 Sn 6 at 260, 80 and 2 K.

might show the prime importance of the synthesis procedure in the case of these materials crystallizing in large YCo 6 Ge 6 -type superstructures.

4. Discussion This study represents the first determination of the magnetic structure of a HoFe 6 Sn 6 -type representative. Although it is a more complex structural type, the analysis of the magnetic properties of the NdMn 6 Sn 6 compound nicely completes the conclusions about the

magnetic anisotropy and exchange interactions in the RMn 6 Sn 6 (R5rare earth, X5Ge, Sn) series. Since the magnetic behaviour of these compounds have already been largely detailed in [3,7–19], we will just discuss here some relevant features. According to the present neutron diffraction study, NdMn 6 Sn 6 is a collinear ferromagnet below 357 K. This implies that, as generally observed in rare-earth-3d intermetallics, the Nd–Mn couplings are positive. A plot of the ordering temperature observed all along the RMn 6 Sn 6 (R5Nd–Lu, Y, Sc) series versus the ionic size of the rare ´ earth element is drawn in Fig. 8. The variation of the Neel

B. Malaman et al. / Journal of Alloys and Compounds 252 (1997) 41 – 49

47

Table 4 NdMn 6 Sn 6 : calculated and observed intensities and adjustable parameters at 260, 80 and 2 K 260 K hkl 020 110 040 130 011 150 101 031 060 121 200 220 051 141 240 170 080 211 161 071 231 260 190 251 310 002 091 350 321 341 370 062 0120 202 291 400 ˚ a (A) ˚ b (A) ˚ c (A) z Sn z Mn mMn ( mB ) mNd ( mB ) u (8) R (%)

F

2 c

80 K F

0 2.7 0 0.5 0

2 o

F 0 3.5( 3) 0 0 0

F 0 3.2 0 2.5 0

270

264 ( 4)

1071 0 0 4.2 0 0.8 0 0 0.8 0

1075 ( 9) 0 0 0 0 0 0 0 0 0

864

979 (11)

1018

128

116 ( 6)

121

2958 134 261 260

2945 103 199 258

(19) ( 9) ( 9) (10)

1415 16.1 9.005(7) 28.76 (3) 5.535(5) 0.336(1) 0.251(3) 1.80 (2) 1.03 (7) 0 3.8

1500 (19) 19 (12)

2K

2 c

2 o

F c2 0 2.1( 5) 0 2.1( 5) 0

0 21.6 0 3.9 0

F o2 0 21.3( 4) 0 1.8( 5) 0

280

271 ( 4)

267

264 ( 3)

900 0 0 26.6 0 13.8 0 0 6.0 0

907 (10) 0 0 20 ( 4) 0 0 0 0 0 0

1139 0 0 34.6 0 4.9 0 0 5.7 0

1101 ( 8) 0

1073 (12)

1306

1309 (15)

108 ( 9)

148

136 ( 8)

2953 142 267 268

2929 142 283 262

2423 30.1 9.003 (8) 28.68 (4) 5.521 (6) 0.334 (2) 0.253 (4) 2.26 (8) 2.69 (19) 84 (4) 1.8

2423 (26) -

temperature of the diamagnetic R element compounds almost follows a linear variation and the equation: T N 5 6072304 r( R31 ) allows us to calculate the variation of the ordering temperature (DT ) corresponding to the insertion of a paramagnetic rare earth in the Mn–Sn sublattice. The DT values, correlated to the additional R–Mn interactions, are gathered in Table 5 together with the A values deduced from the variation of the threshold field in the R xY 12x Mn 6 Sn 6 solid solution [9]. Table 5 shows the close correlations between the two sets of values. The high values of T C observed for the NdMn 6 Sn 6 and SmMn 6 Sn 6 compounds have to be related to strong R–Mn interactions

(22) (12) (12) (13)

35 ( 5) 0 0 0 0 16 ( 5) 0

2950 152 300 295

2980 129 295 326

(25) (10) (12) (12)

2127 379 8.975(9) 28.65 (5) 5.513(8) 0.334(1) 0.253(1) 2.39 (4) 3.30 (5) 0 3.4

2170 (24) 263 (12)

occurring with the light rare earth elements. As already remarked by several authors, the Mn light rare earth magnetic interactions are larger than expected [26]. This effect has been attributed to the larger exchange interaction between the 4f and 5d electrons in the light rare earths resulting from the decrease in the spatial extent of the 4f shell with the rare earth atomic number. The simultaneous ordering of the Nd and Mn sublattice at T C reinforce the occurrence of strong interaction between the Nd and Mn moments in NdMn 6 Sn 6 . Finally, it is noteworthy that the R–Mn couplings are able to return and align the Mn moments, giving rise to the ferromagnetic

48

B. Malaman et al. / Journal of Alloys and Compounds 252 (1997) 41 – 49

Fig. 5. Temperature dependence of the angle u between the direction of the moments and the (100) plane.

Fig. 7. Temperature dependence of the Nd and Mn magnetic moments in NdMn 6 Sn 6 compound.

structure of the Nd and probably Sm compounds whereas an antiferromagnetic ordering of the Mn sublattice occurs in Y, Sc and LuMn 6 Sn 6 compounds. The second remark concerns the direction of the moments. In all the RMn 6 Sn 6 (R5Sc, Y, Gd–Lu) compounds, crystallizing in the HfFe 6 Ge 6 -type structure, it was shown that, at high temperature, the Mn moment lies in the Mn hexagonal planes. Furthermore, a spin rotation to the six-fold axis occurs when the rare earth (Tb–Ho) anisotropy acts at lower temperature [3,8,27]. According to the structural relationships previously described, one can conclude that, above 40 K, the same trends occur in NdMn 6 Sn 6 . The moments lie in the pseudohexagonal plane at high temperature ((100) o layers) and rotate to the perpendicular direction below |150 K. This is also in fair agreement with the same negative sign of the Steven’s factors of Nd and Tb–Ho. Bearing in mind these conclu-

sions and according to the positive sign of the Sm Steven’s factor, one can conclude that SmMn 6 Sn 6 , with a saturation moment value of about 11 mB , is an easy plane ferromagnet in the whole temperature range investigated. Finally, below 40 K, a new rotation occurs in NdMn 6 Sn 6 and the moments are again unambiguously aligned in the metal planes. This very surprising spin flip is probably related to the loss of the pseudohexagonal symmetry. In order to elucidate this last point, it will now be interesting to study the Y 12x Pr x Mn 6 Sn 6 solid solution [28], crystallizing in the less complicated HfFe 6 Ge 6 -type structure. This work is in progress.

5. Conclusion This study presents new information about the exchange interaction between light rare earth and Mn moments and on the anisotropy direction in the RMn 6 Sn 6 series as well

Fig. 6. Magnetic structures of NdMn 6 Sn 6 (a) at 260 and 2 K, and (b) between 140 and 40 K.

Fig. 8. Variation of (T C ,T N ) versus the R31 ionic radius (r) in the RMn 6 Sn 6 series (R5Sc, Y, Lu, Tm–Gd, Sm, Nd).

B. Malaman et al. / Journal of Alloys and Compounds 252 (1997) 41 – 49

49

Table 5 R–Mn interaction magnitudes according to threshold fields measurements (A) and to the variation of the ordering temperatures (DT ) R

Ce

Pr

Nd

Sm

Gd

Tb

Dy

Ho

Er

Tm

-A DT (8)

5.32

5.25

7.42 51

10.24 91

9.40 113

5.66 96

3.12 62

1.83 41

13

4

as the magnetic behaviour of a HoFe 6 Sn 6 -type structure compound. The magnetic structure of NdMn 6 Sn 6 clearly proves the occurrence of positive Nd–Mn coupling in the RMn 6 X 6 compounds. Its also confirms that the magnetocrystalline anisotropy arises from a competition between the magnetic contributions from the Nd and Mn sublattices. The change in the easy direction of the Mn moments upon substituting Sn by Ge, observed in the whole HfFe 6 Ge 6 -type RMn 6 X 6 (X5Ge, Sn) series, also occurs in the Nd compounds, in spite of different structural types. Then, NdMn 6 Sn 6 (HoFe 6 Sn 6 -type) has an easy axis (easy plane) anisotropy at low temperature (high temperature) while the NdMn 6 Ge 6 ferromagnet (YCo 6 Ge 6 -type) exhibits the inverse behaviour [11]. Finally, it is noteworthy that our results largely differ from those of Weitzer et al.. This suggests that the conditions of synthesis could be a critical parameter in the magnetic behaviour of such intergrowth compounds.

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