Antiferromagnetism in cobalt orthosilicate

J. Phys. Chem. Solids Pergamon

Press 1964. Vol. 25, pp. 901-905.

ANTIFERROMAGNETISM S. NOMURA,? Laboratory

IN COBALT

R. SANTORO,

for Insulation Research,

Printed in Great Britain.

J. FANG

Massachusetts

ORTHOSILICATE*

and R. NEWNHAh%

Institute of Technology,

Cambridge,

Mass.

(Received 12 March 1964)

Abstract-Cobalt orthosilicate, CosSiO4, is isomorphous with the mineral olivine and is antiferromagnetic below 49°K. The Curie-Weiss law coefficients for the paramagnetic region are peff = 5.09~~ and 0 = 65°K. A four-sublattice magnetic spin structure based on Co-O-Co superexchange interactions is proposed to explain the susceptibility data. Low-temperature neutron diffraction patterns substantiate the model and show that the spin direction is along b.

Cobalt orthosilicate was prepared by sintering an intimate mixture of CoCOs and finely divided SiO$Cab-0-Sil, Cabot Corp.) in air at 1200°C. Several regrinding operations plus firing times up to 130 hr gave a final product free from the constituent oxides. Identical results were achieved by a chemical method using sodium orthosilicate and cobalt nitrate. After the components were dissolved separately in distilled water, the orthosilicate

solution was slowly added to the nitrate solution, yielding a hydrated cobalt silicate precipitate, which was then filtered, washed, and fired at 1200°C for 24 hr to give violet-colored CosSiO4. Accurate lattice parameters were determined from X-ray diffractometer patterns using slow scanning speeds and FeKa radiation. High-angle data and least-squares refinement gave the orthorhombic cell dimensions d = 10*301+ O-005, b = 6.003 kO.002 and c = 4*782+0.002 A. The space group is Pnmd = Dfi, with four molecules per unit cell. Cobalt orthosilicate is isomorphous with olivine, (Mg, Fe)sSiOa (Fig. l(a)). The oxygens form a slightly distorted hexagonal close-packed array in which half the octahedral sites are filled by cobalt and one eighth of the tetrahedral interstices occupied by silicon. The divalent cobalt ions occupy sites of two different symmetries: Cot occupies a set of inversion centers at (0, 0, 0; 0, &, 0; 4, 0, a; 4, +, $), while the Corn sites at 5 (x, ;I, z; ++x,& 4-z) possess mirror symmetry. Si, 01, and 02 also lie on mirror-plane sites. 0s is located in general position at -t(x, y, z; 4+x, $-y, 4-z; x, B-y, z; 4+x, y, $--z). A recent refinement(s) of the olivine structure gave the coordinates listed in Table 1; these values proved satisfactory in calculating the Co&i04 neutron-diffraction intensities.

* Sponsored by the U.S. Air Force, Aeronautical Systems Division, under Contract AF 33 (616)--8353. t Present address: Physics Dept., Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo, Japan.

The magnetic susceptibility of crystalline CosSiO4 was measured from 20 to 300°K. The

INTRODUCTION

is a hexagonal close-packed analog to spinel. Both are AB2O4 compounds with tetrahedral A cations and octahedral B cations coordinated to a close-packed framework of oxygens. Olivine crystallizes in preference to spine1 for certain small A ions, such as Best, S?+, and P5+. In contrast to magnetic properties of the spinelferrite family, those of the transition-metal olivines have received little attention. We have undertaken an investigation of NisSi04, CozSiO4, FesSi04, MnsSi04, and CrzBeOe. LiMnP04 was studied by BOZORTH and KRAMER(~) and by MAYS.(~) All six compounds are antiferromagnetic at low temperatures. This paper describes the magnetic structure of cobalt orthosilicate, one of the simplest of the group. OLIVINE

SAMPLE

PREPARATION AND STRUCTURE

CRYSTAL

MAGNETIC

901

MEASUREMENTS

902

S.

NOMURA,

R.

SANTORO,

J.

FANG

.ooo

.723

:. . :

0.282

o ,230

bQ

‘574

Y,

0.723

m

NEWNHAM

/

Y

‘im,’

.:!A0

m

R.

+_r__+&-_-r--_ ,3\ \ \

l. . . . . . . . . . . . . . . . . . . . . . . ... . . ... ..

:.ooo :

and

fx

u

\/ m

Iy-4+---lx--*-

l .oicJ

,:

/I' ,/I'

0

I\

\

a

l: . . . . . . . . . . . . . . .

b . 0

.ssBsss.-wUnit cell

Cobalt Silicon

---Y---

Direct exchange

0 Oxygen - Cy--Superexchange (4

(b)

FIG. 1. (a) The cobalt orthosilicate crystal structure projected on (001). Inversion- and mirror-symmetry cobalt positions are denoted by i and m, respectively. (b) An idealization of the magnetic structure used in the Weiss field calculations, and verified by neutron diffraction.

Table 1. Atomic coordinates of olivine in cellfractions (after HANKE and I&MANN(~)) ~______ -__ -~ x Wg, Fe)< Wk, Fe)m Si

0 0.2775 0.0945

01

0.092

02

04495

03

0.163

z

Y 0 : i

0.040

0 -0.010

0.426 0.770 0.218 0.277

experiments were performed with a vibratingsample magnetometer using a field of 12 kG; no field dependence of the susceptibility was observed.

The reciprocal susceptibility goes through a minimum at 49 & 2°K (Fig. 2), indicating a paramagnetic-to-antiferromagnetic phase transition. The susceptibility data in the paramagnetic region conform closely to a Curie-Weiss law, with perf = 5.09 & 0.06~~ and 0 = 65 rt 2°K. GOODGAME and COTTON(~) report a Pen of 5 .Olpn and a 0 of 60°K. WEISS FIELD MODEL The low-temperature antiferromagnetic spin structure of cobalt orthosilicate was predicted from simple crystallographic considerations. If the small distortions from ideal close-packing are neglected, the magnetic ion arrangement is that shown in

ANTIFERROMAGNETISM

IN

COBALT

903

ORTHOSILICATE

I

I

I

1

I

I

I

I

I

30

60

90

120

150

180

210

240

270

Temperature FIG. 2. The reciprocal magnetic susceptibility

I

300

“K

of CosSiO4 plotted as a function of temperature.

Fig. l(b). Each mirror-symmetry cobalt (Co,) shares octahedral edges with two inversionsymmetry cobalts (Cog); it also shares corners with eight others, four Coe and four Ch. Every Cog ion shares edges with four other Co, two of each symmetry, and corners with four COG. The edgesharing cobalts subtend a Co-O-Co angle near 90” and are only about 3.0 A apart, conducive to direct exchange interaction. Those sharing a corner are somewhat more distant at 3.8 A, but the Co-O-Co angle of 132” is more favorable to superexchange. Longer-range interactions of the type proposed by MAY&?)were not included in our model. The directions in which direct and superexchange interactions may occur via shared comers and edges are given in Fig. l(b), where y and a denote the Co-Co and Co-O-Cointeractions, respectively. If, as in most transition metal oxides, an antiferromagnetic superexchange mechanism dominates, the geometry of the cobalt lattice leads directly to the four-sublattice model indicated by arrows. The spin directions were determined by neutron diffraction; only their relative orientations are predicted by the preceding argument. Using the Weiss field treatment, the ratio TxJt? was calculated and compared with experiment. The

sublattice magnetizations are given by T&.$

= c(H- 4aMm3 - 2yMq - 4aiWq)

TlWm$ = c(~-4~~+-~~~-2y~~+~ TMi,

(1)

= c(H-2yM,,-4,~~~-2y~~~)

TM& = c(H-4aMmr-2yMmS-2y.i&~) The Weiss field coefficients y and a are positive for antiferromagnetic direct and superexchsnge interactions. Simultaneous solution of equations (1) yields a magnetic susceptibility T3-+(y-6a)cT2+(8ay-6y2-8a2)cYf’ +(32a3+4y3)@ X

= 4c

~*+~c~a-4ca(l2as+

y2)T2-

16y3~?I’

+1~(16a~~~za2+~) (2) For temperatures well above TN, x reduces to a Curie-Weiss law,

c X

= - T+8

4c =

T+3c(2a+yj

(3)

Setting the denominator of equation (2) equal to zero, the Neel temperature is the largest positive

904

S.

NOMURA,

root of the fourth-order

R.

SANTORO,

J. FANG

equation

Table 2.

TL$+4cyT;-4c2(12&+y3)T;-16c3y3TN +16C4(16~8+4ysas+y4)

_____~ = 0.

(4)

hkl

and R. NEWNHAM Comparison of observed and calculated nuclear intensities 4-Q

I&C.

200 101 210 011 111 201

5.15 4.33 3.91 3.74 3.52 3.50

21 14 100 32 3 18 1

211 020

3.03 3 .oo

9 17 1

301 220 400

2.79 2.59 2.58

42 55 22 1

40

311 121

2.53 2.47

37 57

32 56

410 002 102 221 401

2.37 2.39 2.33 2.28 2.27

1 72 1 12 345 108 1

274+

0.6

112 202

2.17 2.17

51 17 1

51

0.5 0

411

2.12

8

The ratio TN/O, evaluated numerically from equations (3) and (4), is plotted as a function of y/a in Fig. 3. Agreement with the experimental result TN/O = 0.75 +0*06 is obtained if y/a = 0.45 kO.10. This would seem to justify the assumption that the superexchange interactions dominate. I.1

0.2

0.6

0.4

0.0

1.0

lobs.

22 15 100 34 22 26

75

72

545

5

Y/Q, FIG. 3. Ratio of the Nkel temperature to the asymptotic Curie temperature calculated as a function of the direct and superexchange interaction parameters.

DIFFRACTION

MEASUREMENTS

Neutron-diffraction patterns of Co&i04 taken at 4 and 300°K are shown in Fig. 4. The data were collected at 3-min angular intervals using a cylindrical aluminum sample holder of 2 cm dia. and 6 cm high. The observed integrated intensities presented in Tables 2 and 3 are on identical scales in which the most prominent nuclear peak, I,,,,.(210) = 100. The calculated nuclear intensities in Table 2 were computed using the olivine coordinates (Table 1) and the scattering lengths: Co 0.28, Si O-42, 0 0*577x lo-12 cm. Lorentz, multiplicity, and scale factors were included in the calculation, but no absorption or thermal vibration corrections were applied. The excellent agreement between the observed and calculated nuclear intensities confirm the CosSiO4 atomic positions. The Weiss field model of Fig. l(b) with spins parallel and antiparallel to b was used in calculating the magnetic intensities in Table 3. Identical

* Al(111) superposed.

Table 3. Comparison of observed and calculated magnetic intensities hkl

44

IC,lC.

100 110 001 300 310 120 221 401

10.30 5.18 4.78 3.43 2-98 2.88 2.28 2.27

334 54 362 107 35 10 92 68 I

112 202

2.17 2.17

29
lobs.

352 106 362 84 28 7 169 26

arrangements with spins along a and c gave poor agreement with experiment. Computations were carried out with the Cos+ form factor suggested by WATSON and FREEMAN(~) and an effective atomic moment of 3.3 pi. Following the analysis of B.&CON(~), the magnetic scattering length for

ANTIFERROMAGNETISM

600

-

500 400

IN

COBALT

905

ORTHOSILICATE

Co2SiO4 T = 300 OK x= 1.2 ii

-

300 -

2 2oor .z E 1 cn 2 iii

IOO01

600-

I

1 1221) tsso

CozSiO4

(401) v

200-

(II21 c

too-

0 0

J\

I 5

I IO

Counter angle FIG. 4. Neutron

I 25

I 20

I 15

I 30

I 35

20

diffraction patterns of polycrystalline CosSiOr taken at temperatures above and below the antiferromagnetic transition.

Co2+ was taken as pjf = + 0,538 Sx lo-12 cm, the plus or minus sign denoting the two collinear spin orientations, The close correspondence between lobs. and ICalc. in Table 3 establishes the magnetic order. Acknowledgements-We wish to express our gratitude to Prof. C. G. SHULL, A. WEDCSWOOD.and M. 5. RBDMAN for their assistance in the experimental work. The intensity calculations were carried out at the M.I.T. Computation Center.

REFERENCES *- BOZORTH R. M. and K~~lvne~V., Colloque Internal de Magne’tisme de Grenoble 329 (19.59). 2 MAYS J. M., Phys. Rev. 131, 38 (1963). 3: HANKE K. and ZEMANN J., Naturwismzscha&nSO, 91 (1963). 4. GOODGAME M. and COTTON F. A., J. Chem. Phys. 65,791 (1961). 5. WATSON R. E. and FREEMANA. J., Acta Cryst. 14, 27 (1961). 6. BACON G. E., Neutron Dzyraction, p. 149. O.U.P., London (1955).