Polarized light studies of ferromagnetic/ferroelectric/ferroelastic domain patterns in NiCl and NiBr boracite

Article

Reference

Polarized light studies of ferromagnetic/ferroelectric/ferroelastic domain patterns in NiCl and NiBr boracite

BRUNSKILL, Ian Hedley, SCHMID, Hans

Abstract

Ferromagnetic stripe domains were observed in polarized light along the optic axes of the orthorhombic NiCl and NiBr boracites below ∼9K and ∼30K, respectively. The spontaneous magnetization vector is found to lie along the principal refractive index nγ. Measurements of spontaneous Faraday rotation and spontaneous birefringence down to 4.2K are presented.

BRUNSKILL, Ian Hedley, SCHMID, Hans. Polarized light studies of

ferromagnetic/ferroelectric/ferroelastic domain patterns in NiCl and NiBr boracite.

Ferroelectrics, 1981, vol. 36, no. 1, p. 395-398

DOI : 10.1080/00150198108218137

Available at:

http://archive-ouverte.unige.ch/unige:30824

Disclaimer: layout of this document may differ from the published version.

1 / 1

POLARIZED LIGHT STUDIES OF FERROMAGNETIC/FERROELECTRIC/FERROELASTIC DOMAIN PATTERNS IN NiCl and NiBr BORACITE

IAN H. BRUNSKILL and HANS SCHMID

Department of Mineral, Analytical and Applied Chemistry, University of Geneva, CH-1211 Geneva 4, Switzerland

Abstract - Ferromagnetic stripe domains have been observed in polarized light along the optic axes of the orthorhombic NiCl and NiBr boracites below ~9K

and ~30K, respectively. The spontaneous magnetization vector is found to lie along the principal refractive index ^~· Measurements of spontaneous Faraday rotation and spontaneous birefringence down to 4.2K are presented.

INTRODUCTION

The 3d-metal boraciteGM3B7

o

₁₃

x,

abbreviated MX, with the compos1t1ons M=Fe, Co, Ni, Cu and X=Cl, Br, I (except Cui) are known to be weakly ferromagnetic ferroelastic ferroelectricsl,2, being transparent in the visible. So far observations of ferromag- netic domains have been reported for Nii3,4,5 and CoC12 only. In this work the composi- tions remaining cristallographically orthorhombic down to 4.2K, MnCl,MnBr,Mni,NiCl,NiBr, CuCl and CuBr, have been examined with a view to make visib~ ferromagnetic domains. For NiCl and NiBr the search was successful; their respective spontaneous magnetizations Ms at 4.2K were found to be 0.72 emu/g (single crystal)6 and 2.15 emu/g (powder)l. Magneto- clectric7and magnetic torque6 measurements on NiCl yielded a magnetic Curie point of~lO~

SPONTANEOUS BIREFRINGENCE

In order to make visible ferromagnetic domains in strongly birefringent NiCl and

NiBr~

Jne has to observe along or close to an optic axis.Tbis isdue to strong interfererce of birefringent phase retardation with Faraday rotation9. Orthorhombic NiCl and NiBr are op!ically positive with ns//Ps//co.r.//<001>cub• na//bo.r.//<110>cub and ny//a0 .r.//

<110>cub with the optic axes lying in the (001)₀ .r. plane. With a view to locating the optic axes of NiCl and NiBr in the ferromagnetic range, we have extended the measure- ments of the principal birefringences of NiCl and NiBr from room temperature to 4.2K.

(see Figs. la and b). Using Mallard's approximation formulalO,sinVy=[(ns-na)/(ny-na)Jl/2, where Vy is half the acute optic angle and ns-ny, ny-na are princ1pal birefringences,

the opt1c angles 2Vy of ~79° rtnd ~71° are obtained for NiCl and NiBr at 4.2K, respecti- vely. This means that on (lOO)cub cut platelets with spontaneous polarization within that plane (Fig.2a) one of the two optic axes is coming out at ~5.5° and ~9.5° off the vertical for NiCl and NiBr, respectively. As shown below, these angles and the accom- panying weak birefringence ny-nB proved acceptable for observation of magnetic domains

(compare Figs 4 and 5). For NiCl .J.lso the birefringence section n~ -nB has been measured between 4.2 and 40K (Fig. 3) using a lock-in amplifier together with a photoelastic modulator11. Below lOK a contribution from the Faraday rotation is clearly observed.

FERROMAGNETIC DOMAINS AND ORIENTATION OF SPONTANEOUS MAGNETIZATION VECTOR

For observing ferromagnetic domains in the interiour of the ferroelectric ferro- elasticdomains,(100)cub-cut platelets of NiCl and NiBr were used, allowing in principle,

395

396 IAN H. BRUNSKILL and HANS SCHMID

the simultaneous observation of four different ferroelectric domains having Ps within the plane (see Table 2 of ref,4). At nearly crossed polars, close to extinction, ferro- rnagnetic domains have been observed in NiCl(Fig.4) and NiBr(Fig.5) in the interiour of such ferroelectric domains, In white light brown/bluish contrast was obtained between different states of magnetization. Minimization of magnetostatic energy leads to narrow stripe domains after cooling in the field free state. The stripes are separated by

walls following approximately (001) orth. rh. planes. The NiBr sample (Fig. 5) is particu- larly instructive because it shows ferroelectric domains with four different orienta- tions of Ps and all three types of ferroelectric/ferroelastic domain walls known for NiCl and NiBr 4, i.e. "180°¹¹

Q),

"head-head (tail-tail)"

(Y

and "head-tail"@ (Fig. 6) .

-

With a view to deducing the orientation of the spontaneous magnetization vector :rv.~

in NiCl and NiBr, stability tests of the stripe patterns in an external field H (~300G~;

permanent magnet rotated around the cryostat) were performed : (i) for rotation of H within the plane ns/na, the stripes are stable for any angular position of H; (ii) for rotation of H within the plane ns/na the stripes are stable for H parallel to ns only;

(iii) for H rotating within the plane nylna the stripes are. stable for H parallel to na only. From (i) to (iii) Ms is found to lie parallel to p. (a₀-a.xis of ref. 7)

in good agreement with results from magnetoelectric7and torque 6 measurements on NiCl.

From this result the schematic representation of the stripe domains follows (Fig.2b) and which - together with Fig.2a-explains the magnetic domain patterns of Figs.4 and 5.

The angles between Ms on either side of the ferroelectric walls @'®and G) (Fig. 2) are 90°, 60° and 60°, respectively. Whereas across the walls ~the magnetic domains seem to be joined both by magnetic head-head and head-tail junctions, the continuity of the stripes across the walls

(V

seems to indicate that in the latter case the magnetic stripes -while ¹¹bending¹¹ around 90° - are joined by head-head junctions.

FARADAY ROTATION

In Figure 6 the apparent Faraday rotation versus temperature of NiCl and NiBr is shown for observation perpendicularly to (100)cubcuts containing Ps within the plane

and characterized by the birefringence ny-lla· By 'apparent' we mean that the plotted ,~

rotation is the angle formed by the plane of the initial linearly polarized wave and the major axis of the elliptically polarized wave leaving the crystal. For measurement a conventional technique using a polarizer, an analyser and a microphotometer was used in conjunction with a photoelastic modulator acting as a high frequency polarization chopper.

CONCLUSIONS

The domain studies of NiCl and NiBr lead to the deduction of Aizu spec~es

43m1'Fm'm2' for both compositions at 4.2K, with Ms lying in both cases along ny.Further work is needed for separating the intrinsic Faraday rotation from birefringent

phase retardation.

ACKNOWLEDGMENTS

Warm thanks are extended toR. Boutellier, E. Burkhardt, R. Cros and Mrs. 0. Birth for drawings, photographs, samples preparation and typing, resp. and to the Fonds

National Suisse de la Recherche Scientifique for financial support.

C") 2 4 ··· · · .. · · · .. · · ... fY·r~ -~~-.. .

NiCI

~20

Q)

g16

~ ... --~~-n,

· ...

·· .. "

\

.£12

~ ... -~~~-~~~

ffi8

4

0o~-1~oo~2~o~o...~...::-3o~o~4~oo~s~oo~~soo~

Temperature [K]

20r---~

.. ... . '"!v-n.

C") 16~

... .

0 ...-;

Q)

g12 · .... · ...

ny-n₁₁

Q) . . .

0'1 ~

Ea~ c ... Q)

4

. . . . ~-n. ...

NiBr

. ...

'"-

..

··.

. ..

I I

50 100 150 200 250 300 350 400 Temperature [KJ

FIGURE Principal spontaneous birefringences versus temperature, (a) NiCl,(b) NiBr (A

=

546nm; tilting compensator)

a.--G--

^~

-G-d.

-~~-ny'

M's-G-

FIGURE 2 Schematic domains on a (100)cubcut

r~~) ferroelectric/ferroelastic single Jmain, (b) ferromagnetic stripe domains occuring in (a)

194 r

~193

Q)

~

^r

0'1 c

;£:192

Q)

m

L. r 191

·• • ....,. ... -... ·I •..

...

··· ...

_{... _}

NiCl

··-

0 5 10 15 20 25 30 35 40 Temperature [K)

FIGURE 3 Spontaneous birefringence ny-nS of NiCl versus temperature

(photoelastic modulator).

Sample thickness1 244~m

FIGURE 4 Ferromagnetic stripe domains of NiCl, (a) narrow stripes, as-cooled without magnetic field; T

=

4.2K, (b) large stripes after repeated switching; T

=

4.2K.

Sample thickness, 244~m.

398

IAN H. BRUNSKILL and HANS SCHMID

FIGURE 5 Ferromagnetic stripe domains of NiBr, (a) as-cooled without field, T = 12K, (b) ferroelectric/ferroelastic domains of Fig. 5-:t; G,) , (y , @ , are 180°,

head-head and head-tail domain walls, resp. (compare Fig. 6). Sample thickness) 77~m

180'- DOMAINS

it7i'

. .

tllfu~ [

0 g

HEAD- HEAD <TAIL-TAIL) DOMAINS

. ^. 0 ^{; -}

^{' . t i . ' i.~}

^{. ...}

HEAD- TAIL DOMAINS

FIGURE 6 Three types of ferroelectric/

ferroelastic domain wall occur~ng ~n

NiCl an NiBr4 REFERENCES

]140

""'

_2'120

.!2.

510o

1j 0 80

0:::

"0 ~60

~ 40

LL 0

. I .

• •• NiBr

.

lNiCl

2o- ~~., ... I .

oo~~~~~~~~~~~~

10 20 30 40

Temperature [K]

FIGURE 7 Apparent Faraday rotation of NiCl and NiBrJl(ny-ns);

A=

546mn; sample

thicknesses 244~m and 145~m, resp.

l. G. Quezel and H. Schmid, Solid State Comm. 6, 447 (1968) 2. H. Schmid, Int.J.Magnetism, 4, 337 (1973)

3. E. Ascher, H. Rieder, H. Schmid and H. Stossel, J.Appl.Phys. 37, 1404 (1966) 4. H. Schmid, Rost Krista1lov 7, 32 (1967) [Engl. : Growth of Crystals 7, 25 (1969)]

5. J.-P. Rivera and H. Schmid, Proc. of IMF-5, State College, Aug. 1981, Ferroelectrics 6. M. Haida, K. Kahn and H. Schmid, J.Phys.Soc.Jap. 37, 1463 (1974)

7. J.-P. Rivera, H. Schmid, J.M. Moret and H. Bill, Int.J.Magnetism 6, 211 (1974) 8. H. Schmid and H. Tippmann, Ferroelectrics 20, 21 (1978)

9. W.J. Tabor and F.S. Chen, J.App1.Phys. 40, 2760 (1969)

10. C. Burri, Das Po1arisationsmikroskop (Birkhauser, Easel, 1950), p. 48 11. J.C. Kemp, J.Opt.Soc.Amer. 59, 950 (1969)