Spontaneous birefringence and spontaneous Faraday effect in cobalt bromine boracite Co₃B₇O₁₃Br

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Spontaneous birefringence and spontaneous Faraday effect in cobalt bromine boracite Co₃B₇O₁₃Br

MENDOZA-ALVAREZ, Maria Eugenia, SCHMID, Hans, RIVERA, Jean-Pierre

Abstract

Measurements of the spontaneous birefringence of the ferroelec. phase (4 K to TC(f.e.) = 466 K), of its dispersion in the visible (300 K) and of the spontaneous Faraday rotation in the ferromagnetic phase (4 K to TC(f.m.) = 16 K) are reported. The rotation is 3450 deg.cm-1 at 4 K and λ = 643 nm along an optic axis.

MENDOZA-ALVAREZ, Maria Eugenia, SCHMID, Hans, RIVERA, Jean-Pierre. Spontaneous birefringence and spontaneous Faraday effect in cobalt bromine boracite Co₃B₇O₁₃Br.

Ferroelectrics, 1984, vol. 55, p. 227-230

Available at:

http://archive-ouverte.unige.ch/unige:32130

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1 / 1

Ft,_lrcrrics. 1984, Vol. SS. pp. 227-230 OOU.0193184/SSO J.Q227S 18.SOIO

~ 1984 Gordon and B~<:ach, Science Publishers, Inc.

Prinr.cd in the Unir.cd States of America

SPONTANEOUS BIREFRINGENCE AND SPONTANEOUS FARADAY EFFECT IN COBALT BROMINE BORACITE Co3B₇0₁₃Br

MARIA EUGENIA MENDOZA ALVAREZ*, HANS SCHMID and JEAN-PIERRE RIVERA

Laboratory of Applied Chemistry, University of Geneva, CH-1211 Geneva 4, Switzerland

Abstract Measurements of the spontaneous birefringence of the ferroelectric phase (4K to Tc(f.e.) • 466K), of its dispersion in the visible (300K) and of the spontaneous Faraday rotation in the ferromagnetic phase (4K to

Tc(f.m.) :16K) are reported. The rotation amounts to 3450 deg.cm-¹ at 4K and

A

^z 643nm along an optic axis.

I NTRODU CTI ON

Cobalt bromine boracite (abbreviated CoBr) undergo~s an improper phase transition at Tc(f.e.); 466K from a cubic (43m1') to an orthorhombic ferroelectric/ferroelastic phase (mm21') 1, which becomes ferromagnetic at low temperature 2. This work intended to determine the magnetic point symmetry and to measure the sponta- neous Faraday rotation of the ferromagnetic phase on single

domains. To date only little magnetic data on CoBr - obtained from powders - have been available : spontaneous magnetisation

11s(4.2K) "'9.5 emu.g-¹ ( : 0.0482 T), coercive field Hc(4.2K) =0.3T, and effective magnetic moments corresponding to high spin Co²+

-... ns 2.

SPO~TANEOUS BIREFRINGENCE

Orthoscopic observation on (100)** and (110) platelets

(thickness t ^z 3Q-80~) and conoscopic studies on (100)-cuts

(t

=

170pm) confirmed earlier studies that~the optical indicatrix of CoBr resembles that of NiBr 1 with ns// Ps// [001] and the acute bisectrix parallel tony If [ 110], where lla• ns, ny are the principal refractive indices and ~s the spontaneous polarisation. The ferro- electric domain structure is similar· to that of other orthorhombic boracites 3. The ferroma~netic domains have been evinced at

T < TC(f.m.) • 16K owing to high and strongly dispersive Faraday rotation, even along highly birefringent directions whereas

*

supported with a scholarship from CONACYT, Mexico

**

pseudo cubic indices are used throughout this paper

(89SJI227

228/(896) M. E. MENDOZA-ALVAREZ. H. SCHMID AND J.-P. RIVERA

in NiCl 4 and NiBr 4 magnetic domains are seen along and close to the optic axes only. For the measurement of the Faraday rotation it is advantageous to direct the light beam along an optic axis.

With a view to finding the orientation of the optical axes by means of Mallard's approximation 4 the three principal birefringences of CoBr have been measured from 16K to 466K (Fig. 1), yielding the optic axial angle 2V ~ 71° at 16K. There exist two strong absorp-

tion bands centred at ~ 17500 cm-¹ and ~ 22000 cm- 1 connected with the ~T

1

^{g+ ~T}

1

^~(P) transition of high spin Co²+ 5 which causes strong

"anomalous" d1spersion of the birefringence (Fig. 2) and hence also of the optic axial angle 2V. In addition it causes the crystal to act as a filter for red and blue at t ~150~. Owing to i) the necessity to use very thin platelets (30-SO~m) due to absorption and ii) because of extreme sensitivity to slight misorientation due to the strong anisotropic dispersion (Fig. 2), the measurements of birefringence were delicate.

FERROMAGNETIC DOMAINS

On cooling ferroelectric domains with ~ns

=

ns-na ((110-cut) in zero magnetic field through Tc(f.m.) • 16K, ferromagnetic stripe domains - sometimes elongated "bubbles" - form with vivid contrast, e.g. red/green, if viewed with nearly crossed polars. At crossed polars the domain wall traces appear dark (Fig. 3a). They are

!ssentially directed along~ [001]ffns. By rotating a field

H ; 0.022T ~round Oa•n8 and ny, th~ stripe pattern was found to be stable for H .1. ny only, sugges!ing Ms to lie along

ny.

Just below Tc saturation was achieved with

u· =

0.022Tffny. After field removal the energetically more favourable stripe pattern reappeared. Rapid cooling from T • 16K (or above) in

H:

0.022Tff ny leads to the pattern of figure 3b, with the traces of the walls aligned along

~ [112]. On field removal this pattern relaxes to the one of

figure 3a. Explanation of this phenomenon requires further study of the parameters of the material , such as magnetocrystalline energy, coercive field, etc. The black areas of figure 3a represent ferro- electric domains with ~ns = ny-n8 and

Ms//

[110)//ny in extinction position (No Faraday effect

!).

FARADAY ROTATION

The measurement of the rotation angle, 0o.a.• for light propagating along an optic axis was realised by means of a (100) platelet

(t '"' 170~m) on a domain with Ps (// ns) within (100). In that case the +

optic axis lies close to the vertical. After preorientation- based on 2V derived from ~n~ata, see above - the accurate orientation of the incidence angle (n) in the ny/na - plane for light

(A =

643nm) propagation along the optic axis, was realised by using the Faraday effect itself as a detector i.e. by searching the maximal rotation

BIREFRINGENCE AND FARADAY EFFECT OF Co,lhO,,Br (897)1229

•r---~

l:!ll • • • • • • • .. 1\p•n.

~

.

^-~-·

.. ; .

~ ^I

f

...

i '

n 0

.

. . . .

^..

.

.

. . . . .... _...

-· . . ... _{. . . . . . .}

.

. ...

..

"""""':

. ...

-:

lOO ]OJ 400

T~RATURE (K\

.IGURE 1 Spontaneous birefrin- gences versus temperature.

l

16,---,

~12

~ z w

0 8

z ~

...

~ iii I.

h298 K

n,-n,

.--.

.

^{. ...}

^__ .,.-....,..._

0~--~----~----~----~

30 25 20 15 10

-ENERGY (.1o•cm-' I

FIGURE 2 Dispersion of

spontaneous birefringences ~n

the visible.

....

-

0 0

....

f

(b)

FIGURE 3 Ferromagnetic domains, ns/n~-cut, (a) stripe domains 1

=

8 K, polars crossed, (b) walls along~ [112)at T

=

^{4.5 K}

after quenching from T = 16 K, polars decrossed.

'0.&.1

~~---~---~ _{'E lul4l own}

r - ..

..:!!.

• l1ooo

i i'-

_c

• ~

-

''

.. ... ../'''* .... "' / '

·--·l

tL---~----~---~L---~

a : Tilt-. ...,. e1 noot -'•••••t .n n. ..,,,.. •

. . . . . . . . tc ...

• ' • q

Tt:MI'UAMil (a)

FIGURE 4 Experimental arrange- FIGl~ 5 Spontaneous Faraday ment for the measurement of the rotation versus temperature.

Faraday rotation along an optic axis.

•

230tC898J M. E. MENDOZA-ALVAREZ, H. SCHMID AND J.-P. RIVERA

angle below Tc(f.m.) as a function of the angle or rotation of the crystal around ns· Small misalignments decrease the effective rota- tion due to birefringent phase retardation. This represents a direct method for determining 2V in a ferromagnetic phase (Fig. 4). The

ferroelectric single domain, used for the measurement of 8₀ .a .• has been magnetically poled by cooling through Tc(f.m.) in a field

H • 0.045T parallel to

ny.

To determine the extinction angle, a polariser, analyser and microphotometer were employed. Figure 5 shows the measured Faraday rotation, 8₀ .a •• along an optic axis, and the intrinsic, calculated one, 8i, along nyflMs at

A

^{c 643nm.}

These rotations are related by 8i • 8₀ .a./cos V. Half the acute optic axial angle was found to be 35.8°. The temperature dependence of 8i is unusual (Fig. 5) and does not seem to follow that of

M

as in NiCl 4. ⁶

CONCLUSIONS

Spontaneous birefringence versus temperature and the immobility of the ferroelectric domain p~ttern between 4 and 466K (Tc(f.e.)) show that the phase transition 43m1~~21' at 466K is the only structu- ral one of CoBr. Below Tc(f.m.) = 16K a ferromagnetic phase -most probably with Aizu species 43m1Fm'm2' - is evinced with a sponta- neous Faraday rotation along an optic axis of ~ 3450 deg.cm-¹ at 4K and A • 643nm. More detailed work is necessary to describe and to explain the optical and magneto-optical properties of CoBr, e.g.

the potential reciprocal rotatory power - neglected in the present study - will have to be separated from birefringence and Faraday rotation.

ACKNOWLEDGEMENTS

The authors thank R. Boutellier, E. Burkhardt and R. Cros for tech- nical assistance, Mrs 0. Hirth for typing and the Fonds National Suisse de la Recherche Scientifique for financial support.

REFERENCES

1. H. Schmid, H. Tippmann, Ferroelectrics, 20, 21 (1978) 2. G. Quezel and H. Schmid, Solid State Co~, 6, 447 (1968) 3. H. Schmid, Rost Kristallov, 7, 32 (1967) [Engl. : Growth of

Crystals

z,

25 (1969)]

4. I.H. Brunskill, H. Schmid, Ferroelectrics, 36, 395 (1981)

5. R.V. Pisarev, V.V. Druzhinin, S.D. Prochoro;;, N.N. Nesterova, G.T. Andreeva, phys.stat.sol., 35, 395 (1969)