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Magnetic and magnetoelastic properties of the UGa2 intermetallic compound
A. Andreev, K. Belov, A. Deriagin, R. Levitin, A. Me_ovský
To cite this version:
A. Andreev, K. Belov, A. Deriagin, R. Levitin, A. Me_ovský. Magnetic and magnetoelastic properties of the UGa2 intermetallic compound. Journal de Physique Colloques, 1979, 40 (C4), pp.C4-82-C4-83.
�10.1051/jphyscol:1979427�. �jpa-00218823�
JOURNAL DE PHYSIQUE Colloque C4, supplément au n" 4, Tome 40, avril 1979, page C4-82
Magnetic and magnetoelastic properties of the UGa
2intermetallic compound
A . V . A n d r e e v (*), K . P . Belov (**), A . V. Deriagin (*), R. Z . Levitin (**) a n d A . Meriovsky (***) (*) Ural State University, Sverdlovsk, U.S.S.R.
(**) Moscow State University, Moscow, U.S.S.R.
(***) Charles University, Prague, C.S.S.R.
Résumé. — Dans l'état ferromagnétique (Tc = 125 K) on a observé une distorsion orthorhombique de la maille du réseau cristallin hexagonal due à la grande magnétostriction (A yl = - 4 x 10 3 à 4,2 K) et aussi à la grande anisotropie magnétocristalline {K = - 2 x 107 erg/g à 4,2 K). Le moment magnétique de U diffère de celui de l'ion libre qui a une valence différente, probablement à cause de la délocalisation des électrons 5f.
Abstract. — In the ferromagnetic state of the UGa2 (Tc = 125 K) the rhombic distortions of the hexagonal lattice due to the great magnetostriction (Ar'2 = - 4 x 10~* at 4.2 K) as well as the large magnetocrystalline anisotropy (K = - 2 x 107 erg/g at 4.2 K) were founded. The magnetic moment per U-atom (/AU = 2.71 /xB) is different from one for a free ion of different valency, that may be caused by partial derealization of the 5f-electrons.
T h e c o m p o u n d U G a2 h a s a f e r r o m a g n e t i c ordering at low t e m p e r a t u r e s . B u t m o s t of r e s e a r c h e s of m a g n e t i c p r o p e r t i e s of U G a2 w e r e m a d e using poly- crystals and s o m e a s u r e m e n t s of s u c h i m p o r t a n t c h a r a c t e r i s t i c s as t h e s p o n t a n e o u s m a g n e t i c m o - m e n t , m a g n e t i c a n i s o t r o p y , m a g n e t o s t r i c t i o n h a v e a n e s t i m a t e f e a t u r e . I n o u r w o r k t h e t e m p e r a t u r e d e p e n d e n c e s of t h e s e characteristics a s well as of lattice c o n s t a n t s a r e investigated using b y t h e single c r y s t a l s of U G a2. T h e single crystals ( m a s s u p t o 300 mg) w e r e p r e p a r e d by recrystallization anneal- ing of t h e ingot m e l t e d in a r c f u r n a c e in helium a t m o s p h e r e .
X - r a y s analysis s h o w e d t h a t in p a r a m a g n e t i c t e m - p e r a t u r e s region ( T > Tc) U G a2 h a d a hexagonal s t r u c t u r e of AlB2-type (space g r o u p P 6 / m m m ) and lattice p a r a m e t e r s w h i c h d e c r e a s e linearly with tem- p e r a t u r e lowering. I n t h e ( T < Tc) region t h e r h o m - b i c distortions of lattice ( s p a c e g r o u p C m m m ) a p - p e a r a n d their v a l u e A = —•= — a i n c r e a s e with
V 3
t e m p e r a t u r e lowering (Fig. 1). Figure 2 s h o w s t h e m a g n e t i z a t i o n c u r v e s of single crystal U G a2 along
Fig. 1. —Temperature dependences of the lattice parameters.
Fig. 2. — Magnetization in magnetic field along [100]-O, [120]-A, [001]-« axes at 4.2 K.
t h e different crystallographic a x e s at 4.2 K . It is s e e n t h a t [001]-axis is t h e h a r d magnetization axis a n d e a s y m a g n e t i z a t i o n directions lie in t h e b a s a l p l a n e along ( 1 0 0 )-type a x e s . It is followed f r o m figure 2 t h a t t h e U G a2 h a s a great m a g n e t o c r y s t a l l i n e a n i s o t r o p y . T h e effective c o n s t a n t of uniaxial aniso- t r o p y K* = K , + 2 K2 is equal t o - 2 x 107 e r g / g , a n d t h e b a s a l plane a n i s o t r o p y c o n s t a n t K3 is equal t o - 0.6 x 106 e r g / g a t 4.2 K .
T h e t e m p e r a t u r e d e p e n d e n c e of s p o n t a n e o u s m a - gnetization o b t a i n e d f r o m m e a s u r e m e n t s along
Fig. 3. — Temperature dependences of the spontaneous magneti- zation <r, and reversible paramagnetic susceptibility x~'-
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979427
MAGNETIC A N D MAGNETOELASTIC PROPERTIES C4-83 easy direction is typical for ferromagnetics (Fig. 3).
The Curie temperature Tc determined from a 2 = f(H/u) dependence is equal to 125 K that is close to data [I]. The obtained value of magnetic moment per U-atom 2.71 p B is essentially more than pu = 2.28 p B determined by neutrons dif- fraction [2]. Possibly it is connected with that in [2] was calculated using the neutrons diffraction forrnfactor obtained assuming that magnetic mo- ment of U has spin only nature. Another possible reason is the presence of the delocalization spin density indefinable by neutrons diffraction method [3].
The temperature dependences of reversible para- magnetic susceptibility along [001]-axis (xi') and in basal plane (x;') are shown on figure 3 too. Both dependences are linear. However use of Curie- Weiss law in its usual form
x
= C / ( T-
8,) for description of these dependences givesand 8,1= 127K, p i f = 3 . 0 p , i.e. different values pef in different crystallographic directions. It is possible that the vagueness in value pef is caused by inapplication of the Curie-Weiss law in usual form for description of experimental depen- dences x(T). For its solution we supposed a presen- ce of temperature-independent contribution X, into ,y due to polarization of conduction electrons. Indeed in this case
x
II (T) and x , ( T ) may be described as a sum of a contribution conforming Curie-Weiss law with O! = - 64 K, 0;f; = 134 K and same value of we, = 280 p B and temperature-independent contribu- tionx,
= 2.8 xFigure 4 shows the temperature dependence of
Fig. 4. -Temperature dependences of the magnetostriction coefficient A
'.'
obtained from measurements of : magnetostric- tion along [100]-0, [120]-A, polycrystals-x, lattice parame- ters-..magnetostriction constant A
'.'
which characterizes the anisotropic magnetostriction in basal plane and was obtained from measurements in magnetic field as well as from data of lattice parameters. It is seen that there is satisfactory agreement between values of A"'
obtained by different methods. Magnetostric- tion of UGa, is great (A"'
= - 4 x at 4.2 K) and compared with one of rare-earth metals and f-d compounds RFe2-type [4].At present they usually consider that Sf-electrons in pure uranium are collectivized. In uranium compounds with nonmagnetic partners the degree of localization of the Sf-electrons depends on U-U distance. Probably, in UGa, Sf-electrons are partly collectivized and so p, is not equal to value of pU calculated using Russel-Sounders scheme for diff e- rent (3-6) valency of U-ions. Of course more precise analysis of p, needs to take into consideration possibility of influence of crystalline field on the magnetic state of U-ions. We consider, that the great magnetic anisotropy and magnetostrictibn of UGa, are due to interaction of Sf-electronic shell of U-ions with crystalline field of lattice.
References
[I] STERNBERK, J . , HREB~K, I., MENOVSK?, A., SMETANA, Z., J.
Physique Colloq. 32 (1971) C1-744.
[2] SECHOVSKY, V., SMETANA, Z., MENOVSKY, A., Phys. Status Solidi (a) 28 (1975) K-37.
[3] ALDRED, A., DUNLAP, B . , LANDER, G . , Phys. Rev. B 14 (1976) 1276.
[4] CLARK, A. E . , AIP Conf. Proc. 18 (1974) 1015.
