Journal of Alloys and Compounds, 193 (1993) 303-305 303 JALCOM 2017

Neutron diffraction studies of metamagnetic phase transitions in TbRuzSi 2 and DyRu2Si2

S. Kawano Research Reactor Institute, Kyoto University, Kumatori, Sennan, Osaka 590-04 (Japan)

T. Shigeoka and N. Iwata Faculty of Science, Yamaguchi University, Yamaguchi 753 (Japan)

S. Mitani and Ridwan* Research Reactor Institute, Kyoto University, Kumatori, Sennan, Osaka 590-04 (Japan)

Abstract

Investigations of metamagnetic phase transitions in TbRu2Si 2 and DyRu2Si 2 using single-crystal neutron diffraction under magnetic fields up to 5 T along the c-axis are reported.

Without a magnetic field, TbRu2Si2 and DyRu2Si2 exhibit a one-dimensional magnetic order of a squared wave with a wavevector • = (0.23, 0, 0) and T = (0.222, 0, 0) respectively, with magnetic moments parallel or antiparallel to the c axis. The r =0.23 wave consists of 26 ferromagnetic (100) planes with a 5 + 4 - 4 + 5 - 4 + 4 - sequence along the a axis, where 5 + and 4 - denotes five and four consecutive ferromagnetic (100) planes with up and down spins respectively. The z = 0.222 structure can be expressed by a 5+5 4 + 4 - sequence. Applied fields induce a gradual change in this one-dimensionality up to a critical field Hot = 21 kOe; from this value up to He2 = 29 kOe, TbRu2Si 2 becomes a two-dimensionally modulated ferrimagnet with the wavevectors (0.23, 0, 0), (0.23, 0.23, 0) and a ferromagnetic component. Just below He1 the structure becomes ferrimagnetic with a 5 + 4 - 5 ~ 3 - 5 + 4 - sequence. Above Oc2 = 29 kOe all the moments are ferromagnetically aligned in the field direction. DyRu2Si2 shows a similar metamagnetic behaviour with Hc~ = 10 kOe and Hc2 = 17 kOe. The metamagnetic phase is two dimensional with wavevectors (0.222, 0, 0), (0.222, 0.222, 0) and a ferromagnetic component.

1. Introduction

The ternary compounds TbRu2Si2 and DyRu2Si2 crystallize in the tetragonal ThCr2Si2-type structure. Only rare earth ions bear a magnetic momen t and are simply arranged in a b.c.t, symmetry, as illustrated in Fig. 1. Their magnetic structures are one dimension- ally modula ted with a wavevector x = (z, 0, 0) below a N6el temperature T N and the magnetic moments are parallel or antiparallel to the c axis because o f a strong crystalline electric field effect [1, 2]. In addition, they show a metamagnet ic behaviour at 4.2 K, a l though details have not been clarified yet. Quite recently this metamagnet ic phase transit ion in a TbRu2Si2 single crystal has been extensively studied using neutron diffraction [3]. In the present paper we shall report the results o f neutron diffraction in external

*On leave of absence from National Atomic Energy Agency, Batan, Indonesia.

fields at 4.2 K on both TbRu2Si2 and DyRu2Sie single crystals.

2. Experimental details

Neut ron diffraction measurements were performed at 4.2 K on single crystals using a double-axis neutron diffractometer installed at the K y o t o University Reactor ( K U R - N D ) . The incident neutron wavelength is 1.006 ,~ and the second-order contaminat ion is less than 0.2%. The crystal was mounted in a cryostat such that the c axis was vertically oriented to collect (hkO)- type reflections. Magnet ic fields o f up to 50 kOe were applied parallel to the c axis with a superconduct ing magnet [4].

3. Results and discussion

Without an applied field at 4.2 K, ant iferromagnetic peaks with a propagat ion vector z = ( r , 0, 0) were

0925-8388/93/$6.00 (c~, 1993 - Elsevier Sequoia. All rights reserved

304 S. Kawano et al. / Neutron diffraction of phase transitions in TbRu2Si2 and DyRueSi2

C

O:Tb , Dy

, :Ru

p . . . . ~ a

Fig. 1. Crystal structure of RRuzSi2 (R - Tb, Dy).

observed for both crystals. Add]tional peaks of the third and fifth harmonics were seen besides main peaks. Figure 2(a) shows the temperature dependence of the main satellite and higher order harmonic components for magnetic reflections for TbRu2Si2. Although Fig. 2(b) has no harmonic components, the third-harmonic component was observed at 4.2 K in DyRuzSi 2. The magnetic structure, in consequence, at 4.2 K for these crystals can be expressed by a squared sinusoidal wave running along the a axis with the magnetic moments aligned along the c axis. With increasing temperature the structure becomes gradually sinusoidal and below ire it is perfectly sinusoidal. The wavevector is

= 0 . 2 3 ( = 3 ) for TbRu2Si2 and ~ =0 .222(=3) for 13 DyRu2Si2. The results are consistent with those reported for the powdered samples [1, 2]. The fact that the wavevector can be expressed by a fraction of an integer leads to a commensurate structure such that ferromagnetic (100) planes are one dimensionally arranged in a sequence 5 + 4 - 4 + 5 - 4 + 4 - along the [100] direction for T = 3 , where the ideogram "4" means a + + + + sequence with the magnetic moment parallel to the e axis, "4" is and so on. Similarly for DyRuzSi 2 with • = ~ the magnetic struc- ture can be represented by a sequence of 5 + 5 - 4 + 4 - . Bak and von Boehm [5] have called such structures in which ferromagnetic planes are one dimensionally

arranged in one direction a two-dimensional an- isotropic Ising system. It should be noted that, in these structures, local fields acting on spins at boundaries between up and down spins are small.

Figure 3(a) gives the field dependence of peak intensities for ferromagnetic (110), antiferromagnetic ( 2 - ~ , 0 , 0 ) , (1--T, 1--T, 0) and ( 1 - 2 T , 1,0) reflec- tions at 4.2 K for TbRu2Si 2. A similar field dependence was obtained for DyRu2Si2, as shown in Fig. 3(b). The application of magnetic fields causes a gradual develop- ment of an antiferromagnetic ( 1 - 2~, 1, 0) component as H is increased. At H = Hc] the (1 - 2 ~ , 1, 0) and (2 - T, 0, 0) reflections abruptly decrease, while a simul- taneous increase in ferromagnetic (1, 1, 0) and anti- ferromagnetic ( 1 - T, 1 - T, 0) reflections occurs. This indicates a clear metamagnetic phase transition with increasing H. At H = He2 the ferromagnetic components are nearly saturated and the antiferromagnetic compo- nents disappear. All the moments are ferromagnetically aligned in the field direction. The observed large hys- teresis indicates this transition to be of first order.

With increasing applied field from H = 0 to H = Hc~ the magnetic structure gradually changes from 5 + 4 - 4 + 5 - 4 + 4 - with a reduced magnetization (ratio of the magnetization to its saturation value) m = 0 to 5 + 4 - 5 + 3 - 5 + 4 - with m = 4 for TbRu2Si2 and from 5 + 5 - 4 + 4 - with m = 0 to 5 + 4 - 5 + 4 - with m = l~ for DyRu2 Si 2. Since local fields acting on down spins at boundaries are weak, at H = Hcl these down spins at boundaries are flipped to the field direction by the external field. This supposition is supported by magnetization measurements [3, 6]. All these structures are still one dimensional along the a axis. Beyond Hc~ the structures become two dimen- sional and ferrimagnetic with wavevectors of (T, 0, 0) and (z, z, 0) and a ferromagnetic component, where

= 0.23 for TbRuzSi2 and T =0.222 for DyRu2Si2. These two-dimensional phases persist up to H = H~2. Finally the moment reaches its full value, parallel to the field direction over H = Hc2. Table 1 summarizes the metamagnetic behaviour and structural properties.

5

0

"~ 4

o 3

+: 2 _c

I1.

0 (a)

TbRu~,Si= TbRu~,Si= o (1-.~.1,0) a (1- 5'r,1,0) o (1-5'{,1,0)

=0.23

20 4 0 6 0

Temperature (K)

0

4-

E

n.

(b)

oT 0

i

DyRu2Si2

(1,1-.r,0)

i i

10 2 0 5 0

T e m p e r a t u r e (K )

Fig. 2. Temperature dependences of main and higher harmonic reflections for (a) TbRuaSi 2 and (b) DyRu2Si2.

S. Kawano et al. / Neutron diffraction of phase transitions in TbRu2Si 2 and DyRu2Si2 305

20 1 2 1 " ' ' TbRu2Si2 - =- t DyRUzSi2/f'~//" " " " ~

o'~ ,0 T=4.2K ] [ /f , sl T=4.2K.~/ /f to~ (1,1,0) ~ . j . o~ i" J , ~ (11 O)

0 ~ I I

. 3 c2-,.o.o) = -=o __= ~ : 0 . 2 5 ~ .22

• . - - - ~ • •

• : ' ° o , ¢ - - - -

0 J o 1 2 3 0 ; 2

(a) H (x I0 kOe) (b) H(xlO kOe)

Fig. 3. Field dependences of some representative magnetic reflections at 4.2 K for (a) TbRu2Si 2 and (b) DyRu2Si~.

TABLE 1. Metamagnetic properties and magnetic structures at 4.2 K for TbRu2Si 2 and DyRu2Si 2

Properties TbRu 2 Si 2 DyRu2 Si2

Structure, H = 0 Squared wave//a axis Squared wave//a axis Moment direction lie axis //c axis • , H = 0 (0.23, 0, 0) (0.222, 0, 0) Sequence, H = 0 5 + 4 - 4 + 5 - 4 + 4 - 5 + 5 - 4 + 4 -

Structure, 0 < H < He, One dimensional One dimensional Sequence, just below He, 5 + 4 - 5 + 3 - 5 + 4 - 5 + 4 - 5 + 4 -

2 Reduced magnetization m rn = 75 m = T, 0 < H < H~ (0.23, 0, 0) (0.222, 0, 0) Hcl 21 kOe l0 kOe

Structure, Hc~ < H < He2 Two dimensional Two dimensional • , He, < H < He2 (0.23, 0, 0) (0.222, 0, 0)

(0.23, 0.23, 0) (0.222, 0.222, 0) Ferromagnetic component Ferromagnetic component

He2 29 kOe 17 kOe Structure, Hc2 < H Fully ferromagnetic Fully ferromagnetic

T h e t w o - d i m e n s i o n a l w a v e can be f o r m e d by a

c o m b i n a t i o n b e t w e e n m o l e c u l a r fields a c c o m p a n i e d

by the (r, 0 , 0 ) w a v e a n d a (0, r , 0) wave , wh ich is

e q u i v a l e n t c r y s t a l l o g r a p h i c a l l y to the (T, 0, 0) wave .

W h e n these t w o w a v e s are s u p e r i m p o s e d , loca l

fields at b o u n d a r i e s b e t w e e n up a n d d o w n spins are so

w e a k tha t an ex te rna l f ield eas i ly reverses the d o w n spins

at the b o u n d a r i e s . As a resul t , the free e n e r g y o f the

sys tem is dec reased . T h u s the p re sen t t w o - d i m e n s i o n a l

s t ruc tu res s h o u l d be rea l ized . D e t a i l e d w o r k on the

s tabi l i ty o f these s t ruc tu res is in p rogress , ba sed on a w a v e - l i k e m o l e c u l a r field m o d e l [7] a n d will be p u b l i s h e d

e l sewhere .

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4 S. Kawano, K. Takami, H. Yoshida, T. Shigeoka, N. Iwata, M. Machida, S. Fukui and I. Shibuya, Annu. Rep. Res. React. Inst. Kyoto Univ., 23 (1990) 144.

5 P. Bak and J. von Boehm, Phys. Rev. B, 21 (1980) 5279. 6 T. Shigeoka, Y. Fujiwara, N. Iwata, S. Kawano, S. Mitani, S.

Fukui and Ridwan, Abstr. Rep. Sci. Mtg. Res. React. Inst. Kyoto Univ., 25 (1991) 27.

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