Ferrimagnetism and disorder in the RCr6Ge6 compounds (RDy, Ho, Er, Y): A neutron study

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ALL@~ AHD CO~POL!~DS ELSEVIER

Journal of Alloys and Compounds 256 (1997) 92-96

Ferrimagnetism and disorder in the RCr6Ge 6 compounds (R=Dy, Ho, Er, Y): A neutron study •

•

E S c h o b i n g e r - P a p a m a n t e l l o s ~'*, J. R o d r t g u e z - C a r v a j a l

b

, K.H.J. Buschow ~

~Laboratorimn fiir Kristalh)graphie. ETHZ. CH-8092 Ziirich. Switzerland ~l~borandre L~on Brilhmin (CEA-CNRS). Centre d'Etudes de Satiny. 91191. Gif-sur-Yvette. France "Van der Waals-Zeeman hlstitute. University (~,[ Amsterdam. 1018 XE Amsterdam. Tile Netherlands

Received 2 December 1996

Abstract High-resolution neutron powder diffraction has been used to determine the crystal structure of the hexagonal compounds RCr~Ge,, (P6/mmm). It is shown that most oftbe compounds investigated (R=Y, Dy and Er) can be regarded as partly disordered derivatives of the HfFe~Ge~-type structure where a comparatively small fraction of the R atoms occupy intermediate lattice positions which are shifted from the normal R positions by c/2 along the c axis. The fully ordered HfFe~Ge~-type structure was observed only for HoCr~,Ge6. The observed atomic disorder is shown to strongly control the R-R interactions and the corresponding magnetic ordering temperatures. The compounds DyCr~Ge~ and ErCr6Ge 6 order below Tc=3.0 K and Tc=2.5 K, respectively. Both compounds exhibit canted ferrimagnetic moment arrangements, where the moments of the ferromagnetic R and Cr sublattices have different orientations. No magnetic order above 1.5 K was observed in YCr~Ge~ and HoCr~Ge~. © 1997 Elsevier Science S.A. Keywords: Ran~earth chromium gel'manides;Neutron diffraction- Crystal structure; Magnetic structure

1. Introduction In a recent investigation the crystal and the magnetic structures of the TbCr6Ge 6 compound have been reported on the basis of neutron diffraction data [1]. According to magnetic measurements [2,3] this compound was known to order ferromagnetically below 12 K while all the compounds RCre,Ge 6 (R=Dy, Ho, El', and Y) did not show any magnetic order down to 5 K. From measurements of magnetic isotherms of YCr6Ge 6 at 1.5 K and 5 K it was derived that a very weak moment of 0.2 P,B may be present at the Cr atoms. The crystal structure of the TbCr6G % compound was found [!] to have a slightly different P and Ge 3 dis.tribution, as known for the antiferromagnetic RMn6Ge 6 compounds (HfFe6G%-type structure, P6/mmm space group) [4,5]. The Mn compounds have very high ordering temperatures, well above room temperature (400-500 K). These high ordering temperatures are associated with a dominant antiferromagnetic M n - M n interaction while the R - R interaction sets in at temperatures below 100 K. Our results obtained on the TbCr6Ge 6 compound suggested *Corresponding author. 0925-83881971517.00 © 1997 ElsevierScience S.A. All rights reserved P l l S0925-8388(96)03109-X

that the ordering is governed by the R - R interaction along the c-axis. Although this interaction was found to be ferromagnetLc for TbCr6Ge 6, it is a different one from that observed in ThMn6Ge6 because in the former compound there is a small percentage (18%) of Th sites not present in the former occurring at distances of e / 2 along e. Apparently the observed partial atomic disorder associated with the appearance of this additional Th site strongly modifies the magnetic properties. For this reason it is of considerable interest to extend the neutron study~of magnetic ordering to s,weral other compounds of the RCr6Ge 6 series. In the present paper we will report on neutron diffraction results obtained on RCr6Ge 6 compounds formed with R = D y , Ho, Er and Y.

2. Experimental and results The RCr6Ge 6 samples (R=Dy, Ho, Er and Y) were prepared by arc melting starting materials of at least 99% purity. The samples were wrapped in Ta foil and annealed at 800 °C for 4 weeks in an evacuated quartz tube. The annealed samples were investigated by X-ray diffraction

P. Scimbinger-Papamm*tellos et ai. / Jour::a! of Alloys and Compounds 256 (1997) 92-96

and were found to be single phase ( H f F % G % structure type) [4-6].

60

ErCr6Ge6 $K

t

1

.=-40

3. N e u t r o n d i f f r a c t i o n Neutron diffraction experiments were carded out at the facilities o f the Grph6e reactor (LLB-Saclay). Data were collected between 1.5 and 20 K on the G4.1 diffractometer (800 cells multideteetor). The step increment in 20 was 0.1 °. The data were analyzed with the program Fullprof

"~ 20 .d 0

t~x

[71. 3.1. The nuclear structure

The neutron patterns obtained in the paramagnetic regime were indexed with the hexagonal cell o f the HfFe6Ge 6 type structure; see Fig. I Fig. 2 Fig. 3 (top part). However, similar to our findings in the TbCr6G % case, the patterns o f D y C [ 6 G % aeA ErCr6Ge 6 show a minor intensity reduction o f some specific reflection lines, such as, for instance, (103), superimposed on an enhanced background. Furthermore, the patterns contain weak additional lines belonging to a not yet identified foreign phase. These additional lines, labelled (i) in the corresponding figures,

HoCr, Ge, t.s K

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Fig. 2. Observed, calculated and difference neutron diagram of ErCre,Ge~ measured in the paramagnetic state at 4.95 K (top pan) and in the magneti~ ordered state at 1.5 K (bottom pa~).

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Fig. '1. Observed, c~culated and difference neutron diagram of HoCr,,Ge~ measured in the paramagnetic state at 1.5 K showing only nuclear peaks of HfFe6Ge~ (top part). Difference diagrams between different temperarares of the HoCrt,G%, 0.5-5 K) and YCr~,Ge~,(I.5-10 K) confirming the absence of magnetic order down to 1.5 K.

20

40

20(deg.)

60

80

Fig. 3. Observed. calculated and difference neutron diagram of DyCr~,Ge,~ measured in the paramagnetic state at 10 K (top part) and in the magnetic ordered state at 1.5 K (bottom pan).

94

P. &'hobinger-Papamantrlh~set aL i J.!mtal of Alloys and Compound~ 256 (1997) 92-96

Table I Refined parametersof RCr~Ge~ (R=Dy.Ho.E,j compounds: (a) in ~he magneticallyordered state at 1.5 K (LT), and (b) the paramagneticstate for T>T c Compound. P6/mmm

DyCr~Ge,, 1.5 K

oca,: ~,, oca,: _~,:, ZG,., at 2(e). (0,O.z)I oct,...... ocG..... ,,:, zc, at 6(i). ( I/2.0.zl, /a~ [/aB], d~a(deg.) /.%, [/%] ~bc,{deg.) a (nm) c (rim) R.(%). Rm(%) R,n(%). R~,r('Tc)

0.916~ I ) O.Og4(I ) 0.3596 0.924( I ) 0.076( i ) 0.2518(8) 5.0(I) 44(3) 0.48(12) 149(6) 0.51586(4) 0.82771(7) 3.9. 8.2 12.3.2

10 K 0.916( ! ~ 0.08411 ) 0.3596(9~ 0.924( I ) 0.076( I ~ 0.2517(14) 0.51588(6) 0.82773(9) 4.4 13.9, 2.0

ErCr,,Ge~ 1.5 K 0.958( I ) 0.042( I ) 0.3469(9) 0.960( 1) 0.040( I ) 0.2491 7.4(I) 14(2) 0.46(12) 110(4) 0.51495(5) 0.82639(7) 3.4. 6.5 14.1. 3.0

4.95 K

HoCr,,Ge~ 1.5 K

0.958( I ) 0.042( I ) 0.3470(6) 0.960( I ) 0.040( I ) 0.2492(6)

1.0 0 0.346(I) 1.0 0 0.259( I -

0.51474(7) 0.82617(9) 4.4.11.4. 2

0.5149( I ) 0.8262(2) 5.0.11.2. 2

R°. Rmare the reliability factors for the integrated nuclear and magnetic intensities respectively. The R and Ge~ atoms occup~ partly two sites each R:l(a) (000) and I(a) (0OI/2), and Ge~: 2(e) (0,0,z) and (O,O,z+I/2). The Ge, and Ge: atoms are at 2(d). (1/3,2/3,1/2) and 2(c), (1/3,2/3.0) si~es,respectively. p is the moment value Oc its angle with the c axis. B,,t=0.0018(5) nm:. have been excluded from the refinement. The refined parameters giw~n in Table 1 show that the Dy and Er compounds have a modest disorder at the R and Ge3 sites, existing for 9% at the former and for 4% at the latter while no disorder w~s found in the HoCr6Ge 6 compound. This means that a small percentage of R and Ge3 atoms occupy new sites at a distance of l / 2 c from the l(a) site of R at (000) and the 2(c) site of Ge3 at (00z) with z =0.35. This is also obvious from the results given in Table l. 3.2. Magnetic ordering With the exception of the Y and Ho compounds, the neutron diffraction patterns of the magnetically ordered RCr6Ge 6 phases display the same peak topology. All magnetic reflections appea; at reciprocal lattice positions of the chemical cell (q=O) indicating ferromagnetic order. 3.2. I. The compounds YCr~Ge 6 and HoCr6Ge ~ The difference diagrams shown in Fig. l were obtained by substraction of the high-temperature data (lO K for YCr6Ge 6 and 5 K for HoCr6Ge 6) from the 1.5 K data. Both difference diagrams indicate the absence of longrange magnetic: order down to 1.5 K. Apparently, the very small moment of 0.2/An derived for the Cr atoms from the magnetic mea~urements at low temperatures has to be regarded as a moment induced by the applied field. 3.2.2 The compounds ErCr6Ge ~ and DyCr6Ge 6 The neutron diffraction patterns of the ErCrbGe o and DyCrbGe 6 compounds are shown in the top parts of Figs. 2 and 3. The relative intensities of all observed magnetic reflections in the neutron patterns of both compounds remain unchanged over the entire magnetically ordered regime. This is indicative of a ferromagnetic moment arrangement of both sublattices. The (001) reflection,

associated with contributions of only the R sublattice, has a very small intensity. From this result we can derive that the R moments are oriented in a direction close to the c direction. The temperature dependence of the intensities of several magnetic reflections shown in the bottom part of Fig. 4 Fig. 5 indicates that the R and Cr sublattices order at the same temperature. In a way similar to the TbCr6Ge 6 compound [ l ] the saturation value of the Cr sublattice (see Figs. 4 and 5, top part) does not exceed 0.55/za. The refinement of the magnetic intensities shows that the Er moments make an angle of 14(2)o with the e axis while the moments of the ferromagnetic Cr sublattice make an angle of 110(4)o with the c axis. The Dy moments make an angle of 44(3)o with the c axis and here the Cr moments are oriented at an angle of 149(6) o relative to the c axis. The saturation moment values of the rare earth atoms Dy ( 5 . 0 / z a) and Er (7.4 P a ) are inferior to the free ion R 3+ values gjJ (/z a) of lO (P-a) and 9 (P-a), respectively. This is most likely related to crystal field effects.

4. Concluding remarks Apparently the ordering of the RCr6G % compounds is governed by the ferromagnetic R - R interaction. The ordering temperatures, as derived from the data shown in Figs. 4 and 5, decrease in the sequence Tb (Tc=10.3 K), Dy ( T c = 3 . 0 K), Er (Tc=2.5 K) and Ho ( T c < l . 5 K). This sequence is different from that expected on the basis of the de Gennes function. Apparently there is still another mechanism that controls the ferromagnetic ordering of the rare earth sublattice. We mentioned already that the observed partial atomic disorder associated with the appearance of the additional R site may strongly modify the magnetic properties. If the introduction of this new site (at

95

P. Schobinger-Papamantellos et al. I Journal of Alloys and Compounds 256 (1997) 92-96

ErCr6Ge6

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I

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Dy

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sublattices in DyCr~Ger,. Also shown is the thermal variation of the integrated intensity of several magnetic reflections (bottom part).

e l 2 ) is largely responsible for the ferromagnetism of the rare earth sublattice one would expect that the magnetic ordering temperature depends on the number of the new sites introduced, in other words one would expect a correlation between the Curie temperature and the degree of disorder, the magnetic ordering temperatures being stronger the larger the degree of R - R disorder. This degree of disorder in the RCr6Ge 6 compounds decreases in the sequence Tb (18%). Dy (9%). Er (4%) and Ho (0%), which is the same sequence as that of the magnetic ordering temperatures. The R--Cr interaction is antiferromagnetic and is probably the reason for the presence of the Cr moments. From

T(K)

3

Fig. 5. Temperature dependence of file moment value of the Er and Cr sublattices in ErCr~Ge~ (top part). Also shown is the thermal variation of the integrated intensity of .several magnetic reflections (bottom part).

the fact that no Cr moments were observed in the compounds where an ordered R moment is absent (YCr6Ge 6 and HoCr6Ge~), it can be derived that the C r - C r interaction is too small for 3d moment formation. Evidently, the Cr moment formation requires the polarising influence of the molecular field of the ordered R moments. The presence of canted structures rather than co-linear ferrimagnetic structures in ErCr6Ge 6 and DyCr6Ge 6 is probably due to the fact that the R and the Cr sublattices have different preferred moment directions, the Cr moment directions showing a preference for the hexagonal plane. As far as we know, the present study is file first experimental observation of the antiferromagnetic nature of the magnetic coupling between ordered Cr moments and

96

P Schobinger-Papama,~tellos et al. I Joun~al of AIh,ys and Compm.~ds 2~6 (1997) 92-96

o r d e r e d rare earth spins. It s h o w s that C r b e h a v e s in a similar w a y as o t h e r late 3d transition metals.

References [I] E Schobingerr-Papamantellos, J. Rodrfguez-Carvajal and K.HJ. Buschow. J. AIh,ys Comp., in press. [21 J.H.V.J. Braberrs,V.H.M Daijn, F.R. de Boer and K.H.J. Buschow. J. AIh,ys Comp.~ 198 (1993) 127.

131 J.H.V.J. Br,.aber~,., K.H.J. Buschow, F.R, de Boer, J. AIhLvs Comp..

205 (1994) 77. [4] G. Venturini. R. Welter and B. Malaman. J. Alloys Comp.. 185 (1992) 99. [5] R.R. Olentich, L.G. Akselrud and Ya P. Yarmoliuk, Dopov. Akad. Nauk. Ukr. RSR Ser.. A(2) (1981) 84. 161 E. Parth4 and 13. Chabot. in K.A. Gschneidner. Jr. and L. Eyring (eds.), Handbook on the Physics and Chemisto" of Rare Earths, Vol. 6, North-Holland, Amsterdam, 1984. p, 113. 171 J. Rodriguez-Carvajal. Physica B. 192 (1993) 55.