ELSEVIER

Journal of Magnetism and Magnetic Materials 140-144 (1995) 903-904 journal of magnetism and magnetic materials

Field-induced magnetic phase transitions in DyNi2Si 2

Yuzo Hashimoto a,., Toru Shigeoka b Nobuo Iwata b Hideki Yoshizawa c Yasuaki Oohara c, Masakazu Nishi c

" Department of Physics, Fukuoka Uniuersity of Education, Munakata, Fukuoka 811-41, Japan b Department of Physics, Yamaguchi Uniuersity, Yamaguchi 753, Japan

Institute for Solid State Physics, Uniuersity of Tokyo, Roppongi, Tokyo 106, Japan

Abstract The magnetization process in the low-temperature phase of DyNi2Si 2 was studied by means of neutron diffraction

experiments on a single crystal. The magnetic structure changes to another one with a spin-flip process when the field is applied along the c-axis. These results are consistent with three-step metamagnetic transition of the magnetization curve.

The magnetic properties of the ternary compounds RNizSi 2 series have been investigated on polycrystalline samples [1] and some on single crystals of PrNizSi 2 [2], TbNi2Si 2 [3,4] and DyNizSi 2 [5] compounds. Almost all of them indicate metamagnetic transitions and many show various spin configurations described as incommensurate antiferromagnetism. Susceptibility and specific heat mea- surements on a single crystal of DyNi2Si 2 [5] have indi- cated an order-order phase transition at T t = 3.7 K below the N6el temperature at 5.6 K. We had determined the magnetic structure at zero magnetic field by neutron diffraction experiment on a single crystal [6]. The magnetic structure below T t is a square-wave-type structure with the magnetic moment parallel to the c-axis. The spin modula- tion is described by the commensurate wave vector Q1 =

1 (½ - ~'l, 5 + ~1, 0), ~'1 = 0.125. Fig. 1 shows the schematic spin arrangement in the c-plane, where the equal moments of Dy align along the c-axis, which direction is designated by open (up) and closed (down) circles. The magnetic unit cell is eight times of the chemical unit cell, as described in Fig. 1, with a rectangular form. When the temperature is increased above T t the magnetic structure transforms into an amplitude-modulated sine-wave-type, with the spin di- rection along the c-axis, through the first-order phase transition. The propagation vector changes from Q1 = (0.375, 0.625, 0) to an incommensurate Q2 = ( ½ - "rz,

I -}- T 2 , 0 ) T 2 = 0.132, through the phase t r a n s i t i o n .

The isothermal magnetization curve in the c-direction at 2.3 K is shown in Fig. 2. The three-step metamagnetic transition was observed below T t. These three critical

* Corresponding author. Fax: +81 940 35 1712; emaih hasi moto@fueipc.fukuoka-edu.ac.jp.

c-plane

.......... +......¢ .,........~ .......... L ' i ' i ' ~ i ' i ° i ~ i ' i ~ / • - . . . © . t - . . . . . . - c ' . - . . . . . . . * . . . . . . . . - 6 - . . . . . C , . . . . . . 6 . - . . . ~

L [" ! ' ~ i ' i i . i . ~ i . | ......... 6........+......6. • ....*........+........,.... • .~.. ~.

. . . . . I _ . . . . . .+ . , . . . . . + .........

1 . . . . . . . . . . . ........ 6-. . . . . . . .* . -....-4 . . . . . . . . . + . . . . . . . . * ........ 3 ~ i • c i ~ i •

. ~., a i ~ ~ ~ . . . . aP- [ 1 0 0 ]

Fig. 1. Schematic magnetic structure below 3.7 K projected onto the c-plane. Open and closed circles represent up- and down-spins along the c-axis. The positions of the atoms in the center of lattice

1 1] translation of the atom on the lattice. are obtained with [½, y, o The 16 atoms closed by the rectangle represent the magnetic unit cell.

fields were determined from the d M / d H versus H curves to be 5.5, 7 and 13.6 kOe. When the magnetic field strength is in 5.5 kOe < H < 7 kOe, the magnitude of

DyNi2Si 2 c-axis 10

"?. 8 ../

"® ) , / / at 2.3 K ~L 4 . / f

2

0 ................ i , ,

S 10 15 20 2S 30

H ( kOe )

Fig. 2. Isothermal magnetization curve along the c-axis at 2.3 K. 1 1 The arrows indicate z M~ and ~ M~ values.

0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 3 0 4 - 8 8 5 3 ( 9 4 ) 0 1 0 2 8 - 5

904 Y. Hashimoto et al /Journal of Magnetism and Magnetic Materials 140-144 (1995) 903-904

magnetization corresponds to about 1 / 4 of the saturation value Ms, then about 1 / 2 M s at 7 kOe < H < 13.6 kOe, and then approaches M S ( g J = 10/x B) at high magnetic fields.

In this work neutron diffraction measurements on a single crystal of DyNi2Si 2 were made at 1.5 K under magnetic fields up to 20 kOe. The neutron diffraction experiments were performed with the ISSP triple-axis spectrometer (4G) installed at the JRR-3M reactor of JAERI (Tokai). Magnetic fields were applied along the c-axis, perpendicular to the beam direction, with a super- conducting magnet.

When the field strength is less than 5.5 kOe magnetic reflections were indexed as Q1 = (0.375, 0.625, 0), which structure is described in Fig. 1. Then in the field range 5.5 kOe < H < 7 kOe, magnetic reflections were indexed as Q1 and Q3 = (0.5, 0.5, 0) and a ferromagnetic one. Fig. 3 shows the field dependence of the peak intensity of the (0.5, 0.5, 0) reflection, where no traces of thermal hystere- sis were recognized at either phase boundary. In the field range 7 kOe < H < 13.5 kOe magnetic reflections with the propagation vector Q3 disappeared, then reflections with Q1 and a ferromagnetic one were observed. Under a high magnetic field above 13.5 kOe, only ferromagnetic reflec- tions were observed.

These results can be explained by the wave-like molec- ular field model in which a long-range oscillatory interac- tion involving a two-dimensional wave-like mean field affects each magnetic atom, as has been described for PrCo2Si 2 [7]. In the case of DyNi2Si 2 the mean field distributes in a wave form described by the propagation vector Q1, which stabilizes the spin configuration in Fig. 1. The magnitude of the molecular field is small at the boundary positions, corresponding to the area of neighbor- ing open and closed circles shown in Fig. 1. So, the spins

(C) 7 kOe < H < 13.6 kOe

4b • ~ 2 ,

(b) 5.5 kOe < H < 7 kOe

(a) H < 5,5 kOe

Fig. 4. Magnetic unit cell of each magnetic field range. The atoms enclosed within the rectangles are equivalent to those in Fig. 1. In the field range 5.5 kOe, < H < 7 kOe ¼ values of total moments appear in the c-direction (b), then ~, values in the range 7 kOe < H < 13.6 kOe (c).

at these points are easily affected by an external magnetic field. When the external magnetic field is applied along the c-direction (up), the spins at these depositions return to the field direction. Consequently, this behavior reduces the total free energy of the spin system. The spin system of DyNi2Si e can be described as follows. As a magnetic field of 5.5 kOe < H < 7 kOe is applied along the c-axis (up), two down-spins (closed circles) return to the up-direction, as indicated in Fig. 4(b). Then the magnitude of the

1 magnetization indicates ~ M s along the c-axis. When the field is increased to 7 kOe < H < 13.6 kOe two more spins also return to the up-direction, as shown in Fig. 4(c), where the magnetization value of ½ M s appears along the c-axis. The magnitude of the magnetization at each field range is consistent with that obtained from the magnetiza- tion curve in Fig. 2. These results indicate that the mag- netic phase transition induced by an external magnetic field in DyNi2Si 2 below T t can be understood as the spin-flip of the Ising-like spin system.

(0,5, 0.5, 0)

2500 n i i i i

2,00 ¢:p

4~ 2300 ''%

2 2 0 0 °

2100

v 2000 .;--.. """ " "I'~ ". ;" " % %°, o" % • " ~" ," • ," o•° ,,

H 1900 ° • ° • •~" ;

1800 I I I I I

5 6 7 8 9 I0

H (kOe)

Fig. 3. Temperature dependence of the peak intensity of the (0.5, 0.5, 0) reflection. This reflection is observed only in the field range 5.5 kOe < H < 7 kOe. There is no hysteresis at either transition field.

References

[1] J.M. Barandiaran, D. Gignoux, D. Schmitt, J.C. Gomez-Sal and R. Fernandez, J. Magn. Magn. Mater. 69 (1987) 61.

[2] J.M. Barandiaran, D. Gignoux, D. Schmitt and J.C. Gomez-Sal, Solid State Commun. 57 (1986) 941.

[3] J.A. Blanco, D. Gignoux, D. Schmitt and C. Vettier, J. Magn. Magn. Mater. 97 (1991) 4.

[4] T. Shigeoka, H. Fujii, M. Nishi, T. Uwatoko, T. Takabatake, I. Oguro, I. Motoya, N. Iwata and Y. Ito, J. Phys. Soc. Jpn. 61 (1992) 4559.

[5] M. Ito, H. Deguchi, K. Takeda and Y. Hashimoto, J. Phys. Soc. Jpn. 9 (1994) 3019.

[6] Y. Hashimoto, T. Shigeoka, N. lwata, H. Yoshizawa, Y. Ohara, M. Nishi, A. Murata, M. Ohashi, H. Onodera and Y. Yamaguchi, Jpn. J. Appl. Phys. 32 (1993) 338.

[7] N. lwata, J. Magn. Magn. Mater. 86 (1990) 225.