Antiferromagnetism of YMn2 intermetallic compound

Antiferromagnetism of YMn2 intermetallic compound

Physica 120B (1983) 212-215 North-Holland Publishing Company A N T I F E R R O M A G N E T I S M OF YMn2 I N T E R M E T A L L I C C O M P O U N D Y...

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Physica 120B (1983) 212-215 North-Holland Publishing Company

A N T I F E R R O M A G N E T I S M OF YMn2 I N T E R M E T A L L I C C O M P O U N D Y. N A K A M U R A a n d M. S H I G A Department of Metal Science and Technology, Kyoto University, Kyoto 606, Japan S. K A W A N O Research Reactor Institute, Kyoto University, Kumatori, Osaka 590-04, Japan We have made neutron diffraction measurements on powdered YMn2 and determined its spin structure to be a collinear antiferromagnet with a Mn moment of 2.7p~B. By analyzing both the intensity of magnetic peaks and the NMR spin-echo spectrum at 4.2 K, the direction of the spin axis was found to be parallel to the [lll] direction. The N6el temperature TN of YMn2 has been determined to be about 100 K by means of magnetic susceptibility and thermal expansion measurements. These measurements reveal that the magnetic transition is of first order and shows a thermal hysteresis around TN.

1. Introduction A 3d t r a n s i t i o n m e t a l T in Laves phase intermetallic c o m p o u n d s A T 2 exhibits a wide variety of m a g n e t i c properties, d e p e n d i n g u p o n the e l e m e n t A. In s o m e cases t r a n s i t i o n m e t a l a t o m s h a v e stable localized m o m e n t s a n d they align f e r r o m a g n e t i c a l l y as in YFe2. O n the o t h e r h a n d , YCo2 is a strongly e x c h a n g e - e n h a n c e d Pauli p a r a m a g n e t a n d the C o m a g n e t i c m o m e n t is i n d u c e d by the m o l e c u l a r field from rare earth a t o m s in R C o 2 (R = rare earth metal). T h e m a g n e t i c p r o p e r t i e s of c o m p o u n d s cont a i n i n g M n , AMn2, have not b e e n extensively s t u d i e d so far. A m o n g t h e m YMn2 was b e l i e v e d to be a Pauli p a r a m a g n e t b e c a u s e its susceptibility was f o u n d to decrease m o n o t o n i c a l l y with i n c r e a s i n g t e m p e r a t u r e [1]. W e have f o u n d , h o w e v e r , a distinct t h e r m a l e x p a n s i o n a n o m a l y a r o u n d 100 K, which suggests a phase t r a n s i t i o n [2, 3]. In this p a p e r we r e p o r t the results of n e u t r o n diffraction a n d N M R m e a s u r e m e n t s as well as the susceptibility of a sample of YMn2, which was carefully p r e p a r e d to avoid f o r m a t i o n of f e r r o m a g n e t i c i m p u r i t i e s such as Y6Mnz3.

2. Experiments T h e YMn2 sample was p r e p a r e d by m e l t i n g

99.9% p u r e m e t a l s of Y a n d M n in the p r o p o r tion of 1.06 to 2. T h e ingot was a n n e a l e d at 800°C for o n e week. No o t h e r phase than C15 was d e t e c t e d by X-ray diffraction. P o w d e r pattern spectra of n e u t r o n diffraction were m e a s u r e d by a K U R diffractometer at r o o m t e m p e r a t u r e a n d at 77 a n d 4.2 K. A spin-echo N M R s p e c t r u m was t a k e n at 4.2 K.

3. Results and discussion In the Y - M n b i n a r y system several kinds of i n t e r m e t a l l i c c o m p o u n d s are f o r m e d , such as YMn2, Y6Mn23 a n d YMnl2. A m o n g them, YMn2 was b e l i e v e d to be a Pauli p a r a m a g n e t . H o w e v e r , mixing of a small a m o u n t of a 2nd phase, which is f e r r o m a g n e t i c Y6Mn23 in the p r e s e n t case, m a k e s it very difficult to m e a s u r e the intrinsic susceptibility of YMn2. W e p r e p a r e d the sample very carefully to avoid f o r m a t i o n of the Y6Mn23 phase. T h e results of the susceptibility m e a s u r e m e n t s are shown in fig. 1. A d i s c o n t i n u i t y of the susceptibility was o b s e r v e d in both cooling a n d h e a t i n g curves a r o u n d 100 K. T h e t e m p e r a t u r e s of the d i s c o n t i n u i t y differ for the two directions by a b o u t 2 0 K . In t h e r m a l e x p a n s i o n m e a s u r e m e n t s we also f o u n d disc o n t i n u i t i e s at nearly the same t e m p e r a t u r e s [2, 3].

0378-4363/83/0000-0000/$03.00 © 1983 N o r t h - H o l l a n d a n d Y a m a d a Science F o u n d a t i o n

Y. Nakamura et al. / Antiferromagnetism of YMn2

peaks are assigned to simple Miller indices due to magnetic Bragg reflections. Before discussing the magnetic structure, we analyze nuclear peaks to estimate the factors determining their intensity. The expected intensities Ica~ are calculated for the C15 structure using bMn = --0.36 × 10-12 cm and by = 0.8 × 10 1=cm. The Iobs/Ica~ VS. 0 plot is almost constant with no systematic deviation, implying that corrections due to the Debye-Waller factor and the extinction effect are negligible. The reliability factor R, which is defined as

2 x

-1 4

T 0

[

I

|

I00

200

300

213

T(K)

]lobs- La, I

~,

Fig. 1. Temperature dependence of susceptibility of YMn2 for cooling and heating processes.

R-

/obs These facts strongly indicate that a first order phase transition takes place around 100 K. In order to determine the origin of the phase transition, we carried out powder pattern neutron diffraction measurements. The results at room temperature and at 4.2 K are shown in fig. 2. All peaks in the room temperature spectrum are assigned to nuclear Bragg peaks for the C15 structure and reflections from an AI capsule• At 4 . 2 K extra peaks appear whose position and intensity are given in table I. These additional

is 4%, which is small enough for the present statistics of neutron counts. Analyses of magnetic peaks were done as follows. First, noting only the absence of magnetic reflections for particular indices, we determined the combination of up and down spin atoms of Mn for a C15 unit cell. A possible arrangement is shown in fig. 3. Although alternative arrangements also explain the systematics of magnetic peaks, all of them are reducible to the structure given in

(m) N

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500

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Fig. 2. Neutron diffraction spectra of YMn2 at room temperature and at 4.2 K.

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Y. N a k a m u r a et al. / Antiferromagnetism of YMn=

214

Table I Line positions and intensities of magnetic peaks h k 1

20

(counts)

l*bs

spin axis (110)

spin axis (111)

1 2 2 3 3

10.51 16.82 18.43 23.87 27.27

1823 2050 784 414 373

0.889 1.00 /).382 0.204 0.182

1t.876 1.00 0.320 11.178 0.168

0.818 1.00 0.384 0.166 0.204

1 1 1 1 2

0 0 1 11 0

*Normalized intensity.

\

-

20 Fig. 3. A p r o p o s e d m a g n e t i c s t r u c t u r e of YMn2. O n l y M n sites

are shown. Open and closed circles represent Mn atoms with up and down spins, respectively. fig. 3 by a p p r o p r i a t e s y m m e t r y o p e r a t i o n s . This s t r u c t u r e m u s t b r e a k t h e cubic s y m m e t r y of t h e crystal a n d h e n c e t h e i n t e n s i t y of m a g n e t i c reflections d e p e n d u p o n the spin d i r e c t i o n . A s s u m i n g c o l l i n e a r a n t i f e r r o m a g n e t i s m , we calc u l a t e d t h e i n t e n s i t i e s of m a g n e t i c p e a k s for t h r e e cases of t h e spin axis a l o n g m a i n crystallog r a p h i c d i r e c t i o n , (1001, (1101 a n d (1111. H e r e , we e m p l o y e d t h e M n ++ f o r m factor, which gives a b e t t e r fit to t h e o b s e r v e d results t h a n t h e M n + o r M n ÷++ f o r m factor. T h e fitting was s a t i s f a c t o r y for b o t h [110] a n d [111] d i r e c t i o n s . W e calc u l a t e d t h e R f a c t o r by c h a n g i n g the spin axis in a (110) p l a n e as s h o w n in fig. 4. A s seen in t h e figure, it is difficult to d e t e r m i n e t h e d i r e c t i o n u n i q u e l y o n l y f r o m this analysis. T h e n we n o t e d t h e i n t e n s i t y r a t i o of I(110)/I(211) which is m o s t sensitive to the spin d i r e c t i o n . A s seen in fig. 4, t h e o b s e r v e d v a l u e lies on t h e c a l c u l a t e d line n e a r t h e [111] d i r e c t i o n . T h u s w e d e t e r m i n e d t h e spin s t r u c t u r e of YMn2 as s h o w n in fig. 3, with t h e spin d i r e c t i o n p a r a l l e l to [111]. By c o m p a r i n g t h e i n t e n s i t i e s of m a g n e t i c p e a k s with t h o s e of n u c l e a r p e a k s , we d e t e r m i n e d the m a g n i t u d e of M n m o m e n t as /ZMn = 2.71/~B.

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v ¢)-

)0

I 110

a

I 111

a

1 112

i

spin axis Fig. 4. Open circles: Reliability factor of magnetic peaks for different spin directions within a (110) plane. Crosses: Calculated intensity ratio of (ll0)M to (210)M peaks. An open square indicates the observed value. In a d d i t i o n to n e u t r o n diffraction analysis we h a v e m e a s u r e d s p i n - e c h o N M R at 4.2 K. A z e r o field s p i n - e c h o s p e c t r u m is s h o w n in fig. 5. T w o a b s o r p t i o n p e a k s w e r e d e t e c t e d at 118 a n d 132 M H z , w h o s e intensity r a t i o is a p p r o x i m a t e l y 3 : 1 . Such a splitting of N M R s p e c t r a is o f t e n o b s e r v e d in cubic L a v e s p h a s e c o m p o u n d s such as RCo2, which is e x p l a i n e d by t h e e x i s t e n c e of different sites with r e s p e c t to the m a g n e t i c local s y m m e t r y a n d a n i s o t r o p i c h y p e r f i n e fields [4]. F o r t h e [111] spin d i r e c t i o n in a cubic L a v e s p h a s e s t r u c t u r e , w e h a v e two p e a k s with an i n t e n s i t y r a t i o of 3 : 1, while we m i g h t e x p e c t two

Y. Nakamura et al. / Antiferromagnetism of YMn2

_,=

2 c

I

80

I

100

120 requency ( M Hz )

140

Fig. 5. Zero-field NMR spin-echo spectrum of YMn2 at 4.2 K. peaks with equal intensities for the [110] spin direction. T h e r e f o r e , the present N M R result strongly supports the neutron diffraction analysis. Finally we discuss the magnetic character of this material. Since it b e c o m e s evident that YMn2 is antiferromagnetic at 4 . 2 K , the discontinuities o b s e r v e d in the susceptibility vs. t e m p e r a t u r e curve, as well as in the thermal expansion curve, m a y c o r r e s p o n d to a first o r d e r m a g n e t i c transition at the N6el t e m p e r a t u r e . A b o v e the N6el t e m p e r a t u r e the susceptibility increases with increasing t e m p e r a t u r e in contrast to the previous r e p o r t [1]. This suggests that the a n t i f e r r o m a g n e t i s m of YMn2 is of an itinerant electron type. O n e of the surprising results of the n e u t r o n diffraction analysis is that the lattice p a r a m e t e r at 4.2 K is m u c h larger than that at r o o m t e m p e r a t u r e . By measuring the thermal expansion we f o u n d a significant v o l u m e shrinkage a b o v e TN of a b o u t 6%. R e c e n t b a n d calculations predict a large v o l u m e shrinkage of the

215

same o r d e r if the local m o m e n t disappears a b o v e the magnetic transition t e m p e r a t u r e [5]. T h e present result m a y be explained as the result of collapse of Mn local m o m e n t s . T h e m a g n i t u d e of the v o l u m e c h a n g e is reasonable for the complete collapse of a m o m e n t of 2.7txB [5]. A s far as we know, the value o b t a i n e d here is the largest v o l u m e c h a n g e for any material of purely magnetic origin. W e believe that YMn2 is an itinerant electron antiferromagnet, and provides the possibility for studying itinerant electron antiferr o m a g n e t i s m in a system having large magnetic moment.

Acknowledgements T h e authors would like to thank Professor I. Shibuya of R e s e a r c h R e a c t o r Institute of K y o t o University for n e u t r o n diffraction experiments. T h e y are m u c h i n d e b t e d to Mr. M. S u z u m u r a and Mr. K. Y o s h i m u r a for sample p r e p a r a t i o n and N M R m e a s u r e m e n t s .

References [1] S.A. Marei, R.S. Craig, W.E. Wallace and T. Tsuchida, J. Less-Common Met. 13 (1967) 391. [2] Y. Nakamura, Proc. Intern. Conf. on Magnetism, Kyoto (1982), to appear in J. Magn. Magn. Mat. [3] M. Shiga, H. Wada and Y. Nakamura, Proc. Intern. Conf. on Magnetism, Kyoto (1982), to appear in J. Magn. Magn. Mat. [4] S. Hirosawa and Y. Nakamura, J. Magn. Magn. Mat. 25 (1982) 284. [5] J.F. Janak and A.R. Williams, Phys. Rev. B14 (1976) 4199.