CRYSTALLOGRAPHIC AND MAGNETIC STUDIES OF Nd2Co14B AND Y2Co14B

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CRYSTALLOGRAPHIC AND MAGNETIC STUDIES OF Nd2Co14B AND Y2Co14B

D. Le Roux, H. Vincent, P. l’Héritier, R. Fruchart

To cite this version:

D. Le Roux, H. Vincent, P. l’Héritier, R. Fruchart. CRYSTALLOGRAPHIC AND MAGNETIC

STUDIES OF Nd2Co14B AND Y2Co14B. Journal de Physique Colloques, 1985, 46 (C6), pp.C6-243-

C6-247. �10.1051/jphyscol:1985642�. �jpa-00224895�

CRYSTALLOGRAPHIC AND MAGNETIC STUDIES, OF Nd

Co

B AND Y

Co

B

. . + + D. Le Roux, H. Vincent, P. L'Heritier and R. Fruchart

Laboratoire de Crystallographie du C.N.R.S, associe a I'U.S.M.G., 166 X, 38042 Grenoble Cedex, France

+

U.A. N°1109 du C.N.R.S, Institut National Polytechnique de Grenoble, 38402 Saint Martin d'Heres, France

Résumé : Nc^ComB est isotype de Nd2Feii,.B /!/. Il présente une rotation de l'aimantation spontanée à basse température dans le plan (110). A 300 K, l'axe c est de facile aimantation. Une seconde réorientation a lieu à 543 K vers le plan (001). Elle est déterminée par 1'anisotropie planaire de Co, comme le montre l'étude de YjCo-^B.

Abstract : Crystal structure of Nd2Coi4B, isotype to Nd2Feiij.B 11/ is present- ed. Nd2Con4.B exhibits a spin rotation in the (110) plane at low temperature.

The c axis is of easy magnetization at 300 K. Another reorientation occurs at 543 K toward the (001) plane. This plane is favoured by Co anisotropy, as shown in Y2C01H.B study.

I - CRISTALLOGRAPHIC STUDY Preparation of samples

Constituents were melted by means of induction heating in an evacuated so called cold crucible, at about 1500 K. Crushed ingots were then annealed for several hours at 1300 K. Single crystals suitable for structure determination and magnetostatic measurements could then be extracted from the bulk.

Structure determination of Nd„Co,. B

^ — j _ q —

A spherically ground fragment of 0.007 cm radius (pR = 1.4) was used to measure 5706 reflexions with Z > 0 on an ENRAF-NONIUS CAD4 diffractometer at Ag radiation,

o

up to 8 = 25°. Accurate cell parameters were found to be : a = 8.646(2) A,

c = 11.864(5) A and no h0£, h+2 •= 2n + 1 reflexion was observed. Averaging of equi- valent reflexions in 4/mmm Laue class led to 781 independent observations. We then performed a least square refinement of atomic parameters in P42/mnm space group, using the 654 reflexions with I > 3o(I), to R = 0.011 and R„ = 0.013. The weighting scheme was w = l/a

(F), with cr(F) = ^[aod) + (0.01I)

/ I]-5 where <J

(I) is the stan- dard deviation on observed intensities. Extinction was taken into account by means of SDP 121 correction (g = 0.98 x 10"

).

Final positional and thermal parameters are listed in table I and interatomic distan- ces in table II. An ORTEP view of the structure near the mirror plane at z = h is shown on figure 1. We noticed systematic differences between two sets of unaveraged equivalent reflexions, namely hk£, hk£, hkS., hkZ on one hand, and kh£, RhJZ, kFU, RFU on the other. Moreover, hk« reflexions with |h| > |k| were systematically about 10 % stronger than those with |h| < |k|. This was observed on several samples. However, attempts to describe the structure in orthorhombic subgroups of P42/mnm, with all or half the reflexions averaged in mini class didn't give significantly better results.

Further investigations by means of electron diffraction,and imaging are'in progress.

It will be seen further that magnetization curve of Nd

lio

|

,B shows a peak at T = 543 K.

In order to know wether the differences observed between equivalent reflexions at

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985642

C6-244 JOURNAL DE PHYSIQUE

room t e m p e r a t u r e h o l d o v e r T = 543 K , some hkP and e q u i v a l e n t r e f l e x i o n s were measured up t o 773 K. The same c r y s t a l a s f o r s t r u c t u r e d e t e r m i n a t i o n w a s mount- ed on a P h i l i p s 4 - c i r c l e d i f f r a c t o m e t e r , w i t h t e m p e r a t u r e c o n t r o l a c h i e v e d by means of a h e a t e d argon f l o w . Simultane- o u s l y , a c c u r a t e c e l l p a r a m e t e r s were de- t e r m i n a t e d by 8 and -8 c e n t e r i n g o f a b o u t 20 r e f l e x i o n s a t each t e m p e r a t u r e . The same d i f f e r e n c e s o f i n t e n s i t y s t i l l oc- c u r , b u t c e l l p a r a m e t e r s v a r i a t i o n d e v i - a t e from r e g u l a r i n c r e a s i n g w i t h T a t 543 K ( f i g u r e 2 ) .

F i g . 1 - ORTEP view of Ndl and B s i t e s . P l a n e of t h e f i g u r e p a r a l l e l t o ( 1 1 0 ) .

Ndl 4f .l4414(4)

0 .0062 Ull -0059 -.0018 0 0 Nd2 &g .72471(4) 5 ⁰ .0062 Ull .0052 -.0014 0 0

CO.

0 5 0 .0077 ,0088 -0066 -.0001 0 0

CO2 16k .7237514) .06925(4) .37357( 3) .0048 .0069 .0072 -.0011 -.0019 -.0001 CO, 16k .46300(4) .13985(4) .32192(3) .0054 .0051 .0059 .0007 -.0006 -0010 CO, 8j .18165(8)

.25336(5) .0064 Ull .0041 .DO09 -.0003 U13 CO5 8j .40123(8)

.29495(5) .0059 Ull .0063 -0011 .0010 U13 Co,4e 0 0 .38420(6) .0057 U,, -0064 .0026 0 0 B 4f .37665(96)

0 -0103 Ull .0074 -0018 0 0

T a b l e I - P o s i t i o n a l and t h e r m a l p a r a m e t e r s . (AUij 0.0005 i2 f o r Nd and C O , AUij < 0.004 i2 f o r B).

Fig. 2 - Temperature dependence o f c e l l p a r a m e t e r s i n Nd CO

2 1 4 ~ .

Co4 3.0407(4) 2 Cog 2.4461(4)

Co3 3.2256(3)

4 Co6 2.4656(3)

Co5 3.2377(5)

4 Co3 2.5002(4)

Col 3.3196(1)

2 Col 2.5201(3)

Ndl 3.5249(4)

2.5310(5)

Nd2 3.7996(4)

2 Co5 2.5332(6)

Co4 2.6531(6) Co4 2.7219(6) Ndl 2.9773(3) Nd2 2.9846(3) Co3 CO 2.3441(6)

CO;

2.4146(4)

Nd2 Cog 2.9731(4)

2 Co2 2.4461(4)

Co2 2.9846(3) X 4 Col 2.4552(3)

Co3 3.0409(3)

4 Co3 2.4578(5)

2

Cog 3.0719(3)

2 Co2 2.5002(4)

Col 3 . 0 7 2 4 ( 2 ) '

2 Cog 2.5015(5)

B 3.1343(62)

2 Co4 2.5903(6)

Co5 3.2522(4)

2 Co4 2.5987(5)

Ndl 3.7996(4)

2 Nd2 3.0490(3)

Ndl 3.2256(3) Cob Cog 2.5903(6)

2

Co5 2.5923(7)

2 Co3 2.5987(5)

2

Co2 2.6531(6)

2 B Co6 2.0402(34)

2

Co6 2.7097(5) Co2 2.0533(42)

4

Col Cog 2.4552(3)

4 Co2 2.7219(6)

2 Ndl 2.8430(46)

Co2 2.5201(3)

4 Co5 2.7299(5) B 3.0164(91)

Nd2 3.0724(2) X 2 Nd2 2.9731(4) Nd2 3.1343(62)

2

Ndl 3.3196(1)

2 Ndl 3.0407(4) Col 3.4267(62)

2

0

T a b l e I1 - I n t e r a t o m i c d i s t a n c e s i n Nd2Co14B (A).

I1 - MAGNETIC STUDY E x p e r i m e n t a l s

O r d e r i n g t e m p e r a t u r e s were d e t e r m i n e d on p o l y c r y s t a l l i n e samples by means of a h i g h t e m p e r a t u r e t o r q u e b a l a n c e . Small s i n g l e c r y s t a l s of good c r y s t a l l o g r a p h i c q u a l i t y (m = 0.077(1) mg f o r Y2Co14B, m = 0.228(1) mg f o r Nd2Co14B) were o r i e n t e d on a p r e - c e s s i o n chamber and used f o r m a g n e t o s t a t i c measurements a l o n g d i f f e r e n t d i r e c t i o n s . T h i s was done on a s u p e r c o n d u c t i n g c o i l VSM up t o 5 T , from l i q u i d helium t o room t e m p e r a t u r e .

R e s u l t s

F i g . 3 - M a g n e t i z a t i o n f o r Nd2Co14B and Y2Co14

v s t e m p e r a t u r e c u r v e s B. H a p p l i e d

300 Oe.

F i g . 4 - F i e l d dependence of magnet-

i z a t i o n i n Y2Col4B.

C6-246 JOURNAL DE PHYSIQUE

2C014B

Curie temperature is 1003 K, as shown on figure 3. Field dependence of single crys- tal magnetization at 5 K (figure 4) shows that the easy magnetization direction is located in the basal plane (001). Within the precision of our experiments, no aniso- tropy was found in this plane. Spontaneous magnetization is 19.8 pB/FU (1.4 pB/Co).

Anisotropy energy deduced from the area between magnetization curves when H is ap- plied along Cl001 or COO11 is - K = 1.2

107 erg/cm3, leading to an anisotropy field Ha = 28 kOe. Both spontaneous magnetization and anisotropy energy remain cons- tant up to room temperature (figure 6).

Curie temperature is 993 K. Temperature de- pendence of the magnetization shows a cusp- 5K like behaviour at 543 K (figure 3). Single ''OK

crystal magnetization curves along COO11 and 200K [l101 directions at different temperatures 300K show that, at low temperature, spontaneous magnetization has a component in the basal plane (figure 5). Magnetization along diffe- 5K rent directions in this plane (figure 7) pre- 200K sent a maximum along C1101 so that spontane- 1 0 0 ~ ous magnetization lies in the (110) plane.

From low field variation of magnetization along C1101 and C0011, the angle between spon- taneous magnetization and 75 axis is estimated

I

H to about 15O at 5 K. Spontaneous magnetization O 10 20 30 40 50 *koe deduced from these curves is 27 !JB/FU at 5 K

and its temperature dependence is shown on Fig. 5 - Field dependence of figure 6. When temperature increases, sponta;

magnetization in Nd CO

2 1 4 ~ - neous magnetization reorientates toward the c axis which is of easy magnetization at 300 K.

Anisotropy energy deduced from magnetization curves along [0011 and Cl101 at this temperature is K' = 2.2 x 107 erg/cm 3

- 1 . (Ha = 52 kOe). It is much

larger at lower temperature, but lack of saturation did not allow us to estimate it.

S .

Fig. 6 - Temperature denendence of spontaneous magnetization in Nd2Co14B and Y CO B.

2 14

Fig. 7 - Magnetization al-ong different directj.ons of the (003.) plane j.n Nd2Col4B.

A d i f f e r e n c e of 7.2 VB i s o b s e r v e d a t 5 K between Nd2Co14B and Y2Co14B s p o n t a n e o u s m a g n e t i z a t i o n s . T h i s i n d i c a t e s a n induced moment on CO a t o m s , a s i n Nd2Fe14B / 4 / . A t low t e m p e r a t u r e , t h e r a r e e a r t h a n i s o t r o p y d e t e r m i n e a c a n t e h s t r u c t u r e w l t h r e s p e c t t o t h e 3 a x i s . C r y s t a l f i e l d c a l c u l a t i o n s were made i n a p o i n t c h a r g e model, u s i n g c r i s t a l l o g r a p h i c p a r a m e t e r s r e f i n e d i n s t r u c t u r e d e t e r m i n a t i o n . A c h a r g e + 3 was p u t on each Nd, no c h a r g e on B and - 6/14 on each Co. Second o r d e r p a r a m e t e r s /5/ a r e l i s t e d i n t a b l e 111.-The v a l u e s o b t a i n e d a r e s i m i l a r t o t h o s e i n Nd2Fe14B /4/ w i t h o p p o s i t e s i g n s f o r B2 t e r m s on Ndl and Md2 s i t e s . However, a s CO a n l s o t r o p y f a v o u r s t h e b a s a l p l a n e , on t h e c o n t r a r y t o F e , one would have e x p e c t e d a l a r g e r v a l u e t h a n t h e 15O o b s e r v e d (30° i n Nd2Fe14B).

T a b l e 111 - C r y s t a l f i e l d p a r a m e t e r s ono

~ d i n Nd2Co14B. C a l c u l a t i o n up t o 40 ~ + A.

A t 300 K , Nd2Col4B and Y2Col4B have n e a r l y t h e same s p o n t a n e o u s m a g n e t i z a t i o n . T h i s is s u r p r i s i n g , a s it would need an a l m o s t a n t i f e r r o m a g n e t i c c o u p l i n g of Nd atoms.

Neutron powder d i f f r a c t i o n p a t t e r n s have been r e c o r d e d a t t e m p e r a t u r e s between 10 K and 300 K on Nd2Co14B and Y2Co14B and w i l l a l l o w f u r t h e r i n v e s t i g a t i o n of t h e s e f e a t u r e s .

A s a t 300 K s p o n t a n e o u s m a g n e t i z a t i o n i n Nd2Co14B i s c o l i n e a r w i t h t h e a x i s , one e x p e c t s a t h i g h e r t e m p e r a t u r e a s p i n r e o r i e n t a t i o n toward t h e b a s a l p l a n e , when CO a n i s o t r o p y becomes predominent. T h i s happens a t 543 K and e x p l a i n s t h e peak on f i - g u r e 3. T h i s b e h a v i o u r i s s i m i l a r w i t h t h e one o b s e r v e d i n Er2Fe14B a t 316 K / 3 / , where r a r e e a r t h and t r a n s i t i o n m e t a l a n i s o t r o p y f e v o u r perpendicular d i r e c t i o n s ( [ l 0 0 1 f o r E r and C O O 1 1 f o r F e ) . M a g n e t o s t a t i c measurements on a n o r i e n t e d c r y s t a l of Nd?Co14B up t o 800 K a r e b e i n g c a r r i e d o u t , and w i l l a l l o w a p r e c i s e d e s c r i p t i o n of t h l s r e o r i e n t a t i o n .

Aknowlegments : m a g n e t o s t a t i c measurements were performed a t t h e L a b o r a t o i r e d t E t u d e d e s P r o p r i e t e ' s E l e c t r o n i q u e s d e s S o l i d e s of t h e C.N.R.S., Grenoble. The a u t h o r s would l i k e t o thank R. Buder and J. Marcus f o r t h e i r c o l l a b o r a t i o n and t e c h n i c a l a s s i s t a n c e .

REFERENCES

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/2/ S t r u c t u r e D e t e r m i n a t i o n Package, Enraf-Nonius, D e l f t , H o l l a n d . /3/ Hirosawa, S. and Sagawa, M - , S o l i d S t a t e Comm. 54 (1985) 335.

/4/ G i v o r d , D . , P e r r i e r de l a B a t h i e , R . , S o l i d S t a t e Comm. 51 (1984) 857.

/5/ H u t c h i n g s , M.T., S o l i d S t a t e P h y s i c s 16 (1964) 227.