The two magnetic structures of Ho5Ge4. A neutron diffraction study

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The two magnetic structures of Ho5Ge4. A neutron diffraction study

P. Schobinger-Papamantellos, A. Niggli

To cite this version:

P. Schobinger-Papamantellos, A. Niggli. The two magnetic structures of Ho5Ge4. A neu- tron diffraction study. Journal de Physique Colloques, 1979, 40 (C5), pp.C5-156-C5-158.

�10.1051/jphyscol:1979559�. �jpa-00218976�

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JOURNAL DE PHYSIQUE

Colloque C5, supplément au n° 5, Tome 40, Mai 1979, page C5-156

The two magnetic structures of Ho

⁵

Ge

⁴

. A neutron diffraction study

P. Schobinger-Papamantellos and A. Niggli

Institut fiir Kristallographie und Petrographie ETH, Zurich, Switzerland

Résumé. — La structure magnétique de Ho

5

Ge

4

groupe d'espace 'Prima est étudiée par diffraction neutronique.

Entre 21 et 18 K la structure affinée non collinéaire est décrite dans le groupe magnétique Pn'm'a' (r

^{5 u}

et k = 0).

Les moments des trois sites Ho sont 6,9, 5,2, 5,8 ju

^B

. Une transition du second ordre fait apparaître au-dessous de 18 K une deuxième phase P

^a

( r

^u

et k = |00). D'après raffinement simultané les deux structures semblent être en proportions égales et avoir leurs moments voisins des plans (010).

Abstract. — The magnetic structure of Ho5

Ge

4

, space group Prima is studied by neutron diffraction. Between 21 and 18 K the refined three dimensional canted magnetic structure is described in the Pn'm'a' (T

^5u

and k = 0) space group. The magnetic moment values of the three Ho positions are 6.9, 5.2, 5.8 /*

B

. Below 18 K through a second order transition an additional phase P

^a

(r

^lk

and k = |00) appears. The two structures refined simulta- neously at 6 K seem to be in equal amounts and have their moments close to the (010) plane.

1. Introduction. — Ho5

Ge

4

[1-3] crystallizes in the Pnma space group with four formula units per unit cell a = 7.565, b = 14.582, c = 7.635 A. The Ho atoms occupy two general 8(d) positions and one special 4(c), the Ge atoms two 4(c) and one 8(d) positions. Similar to all 5 : 4 compounds Ho

⁵

Ge

⁴

is antiferromagnetic T

^N

= 21 K, with positive para- magnetic Curie temperature 8 = 16 K indicating ferromagnetic interactions.

2. Neutron diffraction. — Measurements were car- ried on two powder samples in the EL-3 reactor in Saclay (X 1.14 A) as well as in Wiirenlingen (X 2.312 A) for the three temperatures 293, 6 and 18 K. The calcu- lated nuclear profile intensities [6] at 293 K based on the same parameters as for Tb

^s

Ge

⁴

are shown in figure 1 the reliability factor R

^N

= 0.10. The results were used to scale the magnetic intensities. The 6 K neutron data of both samples show two groups of magnetic reflections. One indexed in the same cell (k = 0) as at 293 K but with strong additional reflec- tions like (010). The second requests a a-doubling (k = |00) of the cell called in the following P

B

. The temperature dependence measurements show that the phase with k = 0 orders at 7*

N

= 21 K in agreement with [1], this transition is of first order.

The P

^fl

phase orders at 18 K with a second order transition figure 2.

2.1 THE MAGNETIC STRUCTURE WITH k — 0.—

The 18 K neutron data figure 1 contain only reflections of the group with k = 0. The strong (010), (030) intensities indicate that (OyO) is the main plane of antiferromagnetism therefore the mirror plane m at (0^0) [3] containing the 4(c) atoms becomes an anti-

mirror plane m'. Among the 8 magnetic space groups associated with k = 0 only Fnm'a and Pn'm'a' are having a plane m' and are antiferromagnetic [5, 3].

Ho

5

Ge

4

orders in Vn'm'a' and Tb

5

Ge

4

in Vnm'a, with moments respectively near x and z. Both have a 3-dimensional canted antiferromagnetic arrangement with 10 sublattices. The interaction in the characteristic

S layers parallel to (OyO) [2] figure 3 is mainly ferro- magnetic. The x and z modes are interchanged in the two space groups. Both structures are compensated through operation 1'. The Pn'm'a'(r

^s

J modes are for 8(o?) atoms R

^x

+{ + + ) or

C

^x

C

^x

,Q

^y

+(+- + -) or Gfi

^y

,L

^z

+ (+--+) or A

^Z

A

^Z

and for 4(c) atoms A

^x

(-\ (-) and G

^z

(-\ h—). The refined moment values at 18 K (table I) for the 3 Ho positions, 6.9, 5.2, 5.8 n

^B

are lower than the free ion values. The low R-factor values show that the model chosen is probably correct.

2.2 THE MAGNETIC STRUCTURE WITH k = ^00. —

Below 18 K H o

5

G e

4

undergoes a second order tran- sition (rise of the \ 10) reflection figure 2 to the magne- tic phase P

^B

with k = ^00, both phases coexist in a temperature-dependent ratio down to 4.2 K. Once the magnetic modes fitting the strongest lines of the P

^a

structure were found at 6 K, we carried an iterative simultaneous refinement of the two magnetic struc- tures using two transformation matrices for the magnetic modes, one for each magnetic phase. How- ever the number of parameters to observations ratio didn't allow refinement of the positional and lattice parameters (table I) of the two phases separately.

The P

a

magnetic structure belongs to a two dimen-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979559

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THE TWO MAGNETIC STRUCTURES OF Ho5Ge4. A NEUTRON DIFFRACTION STUDY C5-157

Xn 10"3 H o ^ G e . I 2 9 3 K w ^ 5 4 row *^

U2.312A) " c i O ON

6 - I o " "

£<N I^Nc> ""

| S ° W « ™ _^

O^OCM „ J l IT" J JJ „ 5

Or- T-T- ^ ^ N D W ^ M ^ O Hfr- K§i| 2 5m _ WT- O r t i n o r o o n n n I JTitl" ~ S

* ~ o o *-'"" " " o o p i i - c t r O N *(t i t ' l l '- « i l l » 1 1 * loWinfN - 11 >i i i nil i i mi WU f i ? " r " i

A • WHriCA o-l , 1 1 W f

T

W^,

0 10 2 0 3 0 4 0 5 0 6 0 « 7 0 29<°)

Yn-In M^I||VWvWl

I „ . 1 0 - 3 In.10"4 2 .

J ° 18 K

14-

12-

2 -

1 0 -

- o o o a- o S » ; 2 ^ S

5 o j g S >-" 2 ; o - o m " "

i) I 2 0 4 0 6 0 29CI

In.10-3 I n , 1 0 -4 _ . ' 4 - ~

o 6 K 14-

1 2 -

2" o

1 0 - I

8- fi °"^" «

g 1 T T *

«S SS

6 - S n I " I ' S . I O

,-o « ° " " E l "

O«M ™m | n P>0 J J i o l

*- 5 "i 57. l i p 55

( 1' J "• ' I ' l 0" °ii t' I r iflfl

⁵

' N H *" nil ffl Jifl 11' s i r

•iiBitt p i : ,—

Fig. 1. — Neutron diffraction patterns y„ (points) of Ho⁵Ge⁴ : at 293 K and 18 K (Pn'm'a' k = 0) indexed in the a, b, c, cell. At 6 K the mixture of Pn'm'a' and P„ (& = 4<W antiferromagnetic phases with only P„ reflections in the cell 2 a, 6, c. The full curves are the calculated profiles /„ T61- Y„ — I„ the difference diagrams.

° ^ ^ v VoOi

0 1

°

X \ Z I

c

- \ ?T

4- \l

N

2 J

o-j , , , -r—

0 10 2 0 3 0 T ( K ) Fig. 2. — The temperature dependence measurements of the strongest magnetic reflections of Ho5Ge4 (010) Vn'm'a' and (^ 10) P„.

\ / \ ⁸ ^ \ ,A

W \ 4" V \

<tf ^ /

^v

\ '/

^a

^ /

S LAYER Y=0.11

Fig. 3. — The magnetic structure of the characteristic S layer of Ho5Ge4 at 18 K (Pn'm'a' space group). 1, 8 and V, 8' are for the two Ho 8(d) positions. 1", 4" with z = { the 4(c) atoms projected onS.

8 * ' \ 1 6 ^ '^1/4 \

S LAYER Y=0.11

Fig. 4. — The magnetic structure P„ (k = £00) at 6 K of the S layer. The projected 4(c) atoms have moment values almost zero.

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C5-158 P. SCHOBINGER-PAPAMANTELLOS AND A. NIGGLI

Table I.

-

ReJinedparameters from neutron intensities of Ho,Ge, at 18 K and 6 K . The estimated standard deviations are given in parenthesis and correspond to the last digit. p is the ordered moment of H o and

pX,,=

its x, y,

2,

components

;

a, 6, c, the lattice constants R,, R,, R,, the agreement values for nuclear, magnetic and weighted profile intensities [6].

Ho at 8(d)

-

x 0.123(6) 18 K y 0.125(1) z 0.376(5) Pn'm'a'

P 6.9(3)

x 0.049(6) 6 K y 0.133(3) z 0.344(9) Pn 'm'a'

CL 7.0(9)

r,

5.5(6) fly -1.5(1.2) pz 4.20.0) in [r,l

CL 8.4(8) P, ^P, 7.5(7) ,uy 1.5(1.7)

rZ

- 3.7(8)

Ho' at 8 ( d )

-

0.984(8) 0.092(2) 0.823(6) 5.2(2) 4.0(3) - 0.3(6) -3.3(2)

a R"

Ho" bin[A] R, at 4(c) c

-

- ^RW,-

0.311(9) 7.578 0.05 0.25 14.557 0.05 0.965(3) 7.586 0.13 5.8(2) Overall temp.

5.8(2) factor 0.76

[A2]

0. Scale factor 0.3(5) 0.26

sional real representation l',,

[5],

the modes fitting our dataare G,,I?,,T(+ - + - + -

+-),

and

for the 8(d) atoms and C , ~ ~ T ( + +

^-^-)

^and

F T Z + ( + + +

⁺⁾

for 4(c) atoms. We show the Po magnetic structure of an S layer in figure 4. The main axis of antiferrornagnetism is

x

like in the Pn'm'a' structure of Ho,Ge,.

Acknowledgments. - One of us (P. S.) thanks Dr. P . Meriel, Dr. R. Plumier and Dr. M. Sougi for their help during a stay in Saclay.

References 4.9(5) Overall temp.

4.6(4) factor 0.76

[A2]

0. Scale factor 1.9(7) 0.96(1)

[I] HOLTZBERG, F. et al., J. Phys. Chem. Solids 28 (1967) 2283.

[2] SMITH, G. S. et al., Acta Cryst. 22 (1967) 269.

[3] SCHOBINGER, P., J. Phys. Chem. Solids 39 (1978) 197.

[4] BERTAUT, E. F., Treatise of Magnetism (Academic Press New York) 1963 Vol. 111, p. 186.

151 BBRTAUT, E. F., Acta Cryst. A24 (1968) 217.

[6] RIETVELD, H. M., RCN Rep. 104, Petten, The Netherlands, 1969.