Magnetic properties of some amorphous silicates

Journal of Magnetism lind Magnetic Materials 90 & 91 (1990) 337-339 North-Holland

337

Magnetic properties of some amorphous silicates N. Miyata, Y. Kimishima, N. Akutsu and I. Oguro

a

Department of Physics, Faculty of Engineering, Yokohama National University. Yokohama 240, Japan a The Institute for Solid State Physics. The University of Tokyo. Tokyo 106, Japan The amorphous transition metal silicat es were prepared by the wet process using MCI 2'xH 20 and Na2SiOJ' where M = Ni, Co, Fe and Mn. Sleep change of magnetization was ob served on the M = Ni and Co systems at about 20 and 5 K. respectively. The magnetization curve at 4.2 K did not saturate even under the magnetic field of 8 T.

The amorphous transition metal silicates were prepared by wet method using MCI 2 • xH 20 and Na 2SiOJ• where M = Ni, Co, Fe and Mn. The precipitates were washed several times with distilled water and dried at about 120°C. The X-ray fluorescence analysis, which can detect heavier elements than Na, showed only M and Si as main elements. The chemical analysis for four silicates gave the wt% of both elements lying between 35-38 and 15-16, respectively. Other possible main elements are and H. Referring to the electrical neutrality, the mean structural unit is written as (MO)(Si02)t_l/m + nH 20 or MmSim_IOJm_2 + (IIH 20)m' Here the M-ion is assumed to be divalent and it is not clear whether H 20 is crystal water or not. Numbers of 111, II and m/(m - 1) for each silicate are given in table

Table 1 Chemical cornpo sruon of samples in the expression of (MO)(SiOZ)l_l/m +(nH 20) or MmSim_IOJm_2 +(nH 20)m M

111

n

m/(I/I-l)

Ni Co Fe Mn

5.24 12.50 10.52 5.46

1.67 1.89 1.99 1.31

1.24 1.09 1.10 1.22

°

small angle region. It indicates the packing of clusters with a size around 10-30 A depending on the model used for the 'analysis [1,2J. As shown in the last column in the table 1, the prepared silicates have the ratio of atomic concentration of M to Si lying between those of MSiO J and M 2SiO.j'

1.

Although no Bragg peak was observed in the X-ray diffraction pattem, a broad scattering appeared in the

•

I

I

!

i

-O-M +O-Si-O +M: I I I 0 I

!

I

I

-0 -M to -Sj-O tM1 1

a I I 9 ~! t !

I

-O-M+O-Si-OfM- 0 - M

:

!

0 - Si - 0 I

0

i i I

!j

I I

!

M-

I

iI

!I

-O-MtO-Si-O tM-

I 9 I l a, !i i

-O-M+O-Si-O"'M-0 -M + 0 -Si-O f M : I : ! 0 !

!

I

I

!

I

!

- 0 - M t 0 - Si - 0 t M -

(0 )

I 9 I

-O-MfO-Si-O-i-M-

1... .9... ..1 M

["·········0··········1 i

I

I 9

I

:

j

1 -O-M+O-Si-O+M-

1...

I

...9

1

M

r········ f·····..] -0 -M +0 -Si-O +M-

I 9 !

-O-M+O-Si-O'i'M-

!

I

( b)

;

M

:-·_·······1 ··--·····; 0 i

-

I

,

-O-M +O-Si-OtM-

L.. .. i . . . . !

,..·.. . 9.·.· . ·1 M

I

-O-MtO-Si-OtM-

!

,.._-----...;

:

;

I

-O-M tO-Si-O tM-

j

-O-M -'-0 -Si-O +M-

2 1

1.

r-........

9.. . . . 1

M

-O-M ';'O-Si-O+Mi

•

!

~- . .......9......... j M

r-..··..··9....·..·..1 -O-M fO-Si-O+M-

a

~············1···········:i M ..........•...........;

(c)

Fig. 1. Two -dimentional scheme of divalent met al silicate. (a) MSiO J : (b) MmSim_lOJm_2; (c) M 2Si04 • 0304-8853/90/S03.50 iD 1990 - Elsevier Science Publishers B.V. (North-Holland) and Yamada Science Foundation

N. Miyata et al. / Properties ofsome amorphous silicates

338

Considering that the Na rnetasilicate, which is one of the raw materials, has -Si-O- chains in its structure (3), the local atomic arrangement of the present amorphous silicates is supposed to be intermediate between those of the pyroxene type inosilicate MSiOJ and the olivine type nesosilicate M 2Si04 • The two-dimensional scheme of the continuous structural change between them is shown in fig. 1. The character of the structure is as fol1ows. A long -Si-O- main chain is divided into short ones through replacing Si by M. The M2+·ions surrounded by non-bridging oxygen ions construct a three-dimensional network which encloses the short -Si-O- chain in each cell. The mean length of repetition along the chain is m times the distance between neighboring two Si atoms, about 2.62 A (4). It is noticeable that this length is comparable to the cluster size found in the smal1 angle scattering of X-ray mentioned above. The results of magnetic studies are as follows. 1) The paramagnetic susceptibility X was measured in the temperature range 77-300 K. The diamagnetic contribution was subtracted. The results are shown in fig. 2. The X of the M = Ni and Co systems obeyed the

Table 2 Effective magnetic moment !'elr and paramagnetic Curie temperature 8 p • Concerning the data for M = Fe or Mn silicate. refer to the lex! M

Ni Co Fe Mn

l1.rr

8p

(IlD)

(K)

3.23 5.34 4.09 4.62

+37 +8 +12 -33

C :l

...

.ri

o

IIX (mol-Mion/emu) 200r---------------~

Ni

o

40

T{ K)

Fig. 3. Temperature dependence of magnetization of M = Ni and Co silicates in a magnetic field of 0.2 T. The ordinate is arbitrary scale.

150

o

20

tOO

200

300

T{K) Fig. 2. Reciprocal susceptibility vs, temperature (or four kinds of silicate.

o

2

4 H(r)

6

8

Fig. 4. Magnetization curve of M = Ni and Co silicates at 4.2 K. The ordinate expresses the mean magnetic moment per M-ion .

N. Miyata et 01. / Properties ofsome amorphous silicates

Curie-Weiss law X = C/(T - 8 p ) ' Values of !letr per M-ion and 8 p are given in table 2. The former is consistent with the divalent M-ion and the interaction is ferromagnetic. On the other hand, the X of M = Fe and Mn systems showed the deviation from the law below 200 K. The l/X vs. T curves were concave upward for the former and convex for the latter, respectively. !letr and 8 p obtained from the linear approximation of the data above 200 K are also given in table 2. The values of !lerr are much smaller than those from M Z+ or M H usually known in many oxides. 2) The rapid increase of the magnetization with decreasing temperature, which suggested the magnetic transition, was observed on the M = Ni and Co systems at about 20 and 5 K, respectively (fig. 3). 3) The magnetization curves were measured at 4.2 K for M = Ni and Co systems in the magnetic field up to 8 T (fig. 4). The magnetization still continued increasing even in the highest field. This behavior suggests the possibility of the spin-glass-like state. Almost all transition metal silicates in the previous reports had negative 8 p (5-8). CoSiOj was one of the

339

exceptions (7). But it showed the anti ferromagnetic transition at 50 K in spite of the positive 8 p (10 K). The present amorphous Co-system. which had nearly the same composition and 8 p , showed the transition different from the anti ferromagnetic one. The synthesis of the NiSiOJ crystalline powder was unsuccessful (7), so that the comparison between the crystalline and amorphous states is not possible for the Ni-systern at present. References (11 L.H. Lund and G.H. Vineyard, J. Appl. Phys. 20 (1949) 593. (2) O. Kratby and G . Porod, 1. Colloid Sci. 4 (1949) 35. (3) P.A. Grund and M.M. Pizy, Acta Cryst. 5 (1952) 837. (41 RE. Warren and J. Briscoe, Z. Krist. 80 (1931) 391. [5] H. Kondo and S. Miyahara. J. Phys, Soc. Ipn. 21 (1966) 2193. (6) R.P. Santoro, R.E. Newnham and S. Nomura. J. Phys. Chern. Solids 27 (1966) 655. [7) A. Sawaoka, S. Miyahara and S. Akimoto, J. Phys. Soc. Jpn. 26 (1968) 1253. [8] R.A. Verhelsr, R.W. Kline, A.M. de Graaf and B .O. Hooper, Phys. Rev. B 11 (1975) 4427.