Article
Reference
Optical, electric and dielectric studies of cobalt-chlorine boracite, Co
3B
7O
13Cl
MENDOZA-ALVAREZ, Maria Eugenia, RIVERA, Jean-Pierre, SCHMID, Hans
Abstract
The principal spontaneous birefringence ΔnS(γ, α) of the trigonal ferroelec.-ferroelastic phase was measured at 5-468 K (λ = 489, 666 nm) and the dielec. const. ε33 of the same phase at 5-300 K (f = 100 kHz). The spontaneous polarization PS of the trigonal and monoclinic ferromagnetic phase was measured at 5-468 K. At the onset of the monoclinic phase at ∼11 K, PS undergoes a drastic change of slope which can be attributed to an order-parameter and magnetization-induced polarization. The spontaneous Faraday rotation in the ferromagnetic phase was measured vs. temp. at λ = 539 nm.
MENDOZA-ALVAREZ, Maria Eugenia, RIVERA, Jean-Pierre, SCHMID, Hans. Optical, electric and dielectric studies of cobalt-chlorine boracite, Co
3B
7O
13Cl. Japanese Journal of Applied Physics , 1985, vol. 24, Suppl. 24-2, p. 1057-1059
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Proceedings of the Sixth International Meeting on Ferroelectricity, Kobe 1985 Japanese Journal of Applied Physic11, Vol. 24 (1985) Supplement 24-2, pp. 1057-1059
Optical, Electric and Dielectric Studies of Cobalt-Chlorine Boracite, Co
3B
70
13CI
M.-E. MENDOZA-ALVAREZ, J.-P. RIVERA and H. ScHMID
Dfipartement de Chimie Minirale, analytique et appliquee, Universite de GenCve, CH-1211 Geneve 4, Switzerland
The principal spontaneous birefringence Lln,(y, a) of the trigonal ferroelectrk-ferroelastic phase has been measured between 5 and 468 K (..\.=489 nm, A. =666 nm) and the dielectric constant e33 of the same phase between 5 and 300 K
IJ= 100kHz). The spontaneous polarization P, of the trigonal and monoclinic ferromagnetic phase has been measured between 5 and 468 K. At the onset of the monoclinic phase at - II K P, undergoes a drastic change of slope which can be attributed to an order-parameter and magnetization-induced polarization. The spontaneous Faraday rotation in the fer- romagnetic phase has been measured versus temperature at A= 539 nm.
§1. Introduction
Cobalt chlorine boracite, Co,B,O"Cl (abbreviated
/""o. thereafter by Co-Cl}, is a member of the boracite struc- ture family and is known to show three struCtural phase transitions,l)43ml' ~623K mm21' -sJt::K ml' -46~K
3m!' and a paramagnetic-ferromagnetic one, 3ml'<-+m (20 K, '' 11.5 K"). With a view to improving the understan- ding of the latter transition and to finding an eventual coupling between the magnetic, electric and (or) dielec- tric properties, measurements of spontaneous Faraday rotation, spontaneous birefringence, dielectric constant and spontaneous polarization have been performed as a function of temperature.
§2. Experimental
2.1 Preparation of samples
Single crystals of Co-Cl have been synthesized by means of chemical vapour transport.'' Four types of thin platelets thereof have been cut and polished with dia- mond paste for different measurements:
i) (110)*-cuts (Thickness 16 and 21tJm), for spon-
~ taneous birefringence, as-grown/ as-cooled polydomain ' ' state, with some domains displaying the principal bire-
fringence Lln,(y, a), i.e. with P, in the plane of the platelet and extinction at 54°44' from the [001]-direc- tion.
ii) (112)-cut (thickness 351Jm) for Lln,(y, a), as-grown/
as-cooled polydomain state.
iii) (111)-cuts (thickness 16 and 2ltJm), for spon- taneous Faraday rotation, single domain state with P, .L (Ill), obtained by poling with a field of 500 k Vern - t in silicon oil at 150°C; transparent Au (on Cr)-elec- trodes.
iv) (II I)-cut (thickness 50 tJm, surface 0.917 mm2, for dielectric constant ell and spontaneous polarization P, as-grown/as-cooled trigonal single domain state with Au (on Cr)-electrode.
2.2 Optical measurements
2.2.1 Spontaneous birefringence. Measurements of the principal spontaneous birefringence Lln.(y, a) have been made in the trigonal paramagnetic phase 3ml' and the
*Throughout this paper pseudo-cubic indices are used.
pseudo-trigonal ferromagnetic phase m (down to 5 K) using a tilting compensator (Leitz type M) and compen- sating the path difference visually. A He-flow cryostat served for cooling. The chosen measuring wavelengths .<=489 nm and .<=666 nm correspond to minima of the absorption spectrum." The results are shown in Fig. I.
The magnitude and weak temperature dependence of Lln,(y, a) are comparable to those of other trigonal boracite phases (Zn-Cl, Fe-Cl1'). Within the accuracy of the tilting compensator no anomaly of Lin, was detec- table in the monoclinic ferromagnetic range m (T> - 11.5 K). With a view to measuring the expected anomaly of Ll n, in the ferromagnetic range with the more sensitive photoelastic modulator technique,'' (112)-cuts have been prepared on which certain Lln,(y, a) domains exempt of superposed Faraday rotation are possible, whereas in all domains on (110)-cuts Lin, and Faraday- rotation are superposed (Fig. 2 of reference 8). Unfor- tunately-inspite of several trials-no samples with suffi- ciently large domains have been found.
2.2.2 Faraday rotation. The effective spontaneous Fara- day rotation of the ferromagnetic phase m has been measured on a (111)-platelet with P, .L (Ill) (sample iii)) along a direction in a (112) plane containing the spon- taneous magnetization M, and spontaneous polarization P,. The incident ray formed an angle of 25 degrees with the optic axis; taking account of refraction with a mean
40,---.
~
'o
" ..,
c
; 20
c
0 100
o .l =489 nm ).=6156 nm
200 300 400
temperature K
500
Fig. 1. Spontaneous birefringence Llns()•, a) of the trigonal phase 3m I' and monoclinic phase m versus temperature [A =489 nm (C)), .1~666 nm (e)l.
1057
1058 M.~E. MENDOZA-ALVAREZ, J.-P. RIVERA and H. SCHI\.-t:D
refractive index of n
=
I. 76 (estimated by means of the Gladstone-Dale relationship'>, this corresponded to a pro- pagation direction 31 degrees off the optic axis in the in- teriour of the crystal and to an effective birefringence L1n=0.0013. During the measurements a magnetic bias field H~2000e was applied in the (111)-plane along [110]//M,. The measurement technique consisted in sear- ching the extinction angle by means of a polarizer, an analyser and a microphotometer. Figure 2(a) shows the measured rotation versus temperature. Close to the tran- sition temperature the data points scatter because of in- sufficient occuracy of the rotation values< 0_5 degree.With a view to resolving better the behaviour close to the phase transition, the effective Faraday rotation has been measured by means of the photoelastic modulator technique'> with an angle of incidence 10 degrees off the optic axis (Fig. 2(b)). This measurement complements that of Fig. 2(a) and shows clearly the second order character of the phase transition 3m I' +-+m, that might be questioned on the basis of magnetic susceptibility data.'>
Curve 2 (b) follows roughly a (T,-T)1121aw.
2.3 Dielectric measurements
2.3.1 The dielectric constant e" of the trigonal phase 1500y---,
E'
.11"'
"
'C
~1000 .!!
<i
2
i;' 500
'0
l'!
"
u..
- "
0
lol
·.
lo l
-
10 temperature [K]20
~ ~
....
"'~ ~~ ~
~ r::
0
,.,_
.. "'
..,_
u.... ..
~Fig. 2. Effective spontaneous Faraday rotation of the ferromagnetic phase m versus temperature (A= 539 nrn) a) measurement by means of a polarizer, an analyser and a photomultiplier; light propagation 31 deg. off the optic axis in a (1 10) plane containing M, and P,; b) measurement by means of the photoelastic modulator technique7l;
light propagation ll deg. off the optic axis, in same (110) plane.
-
r:: 0;; r::
00 10
0
E " "
"
'0
9+---~--~--~----~--~
0 100 2.00 300
temperature [K]
Fig. 3. Dielectric constant c33 of trigonal phase 3ml' and pseudo- trigonal phase m versus temperature (100kHz, 100 V rrm·cm-1).
3m!' and the pseudo-trigonal ferromagnetic phase m has been measured at I 00 kHz with 100 V ,m, ·em - I between 5 K and 300 K, using an impedance analyser (HP4192A).
The results are shown in Fig. 3. It is noteworthy that no anomaly of e" was detectable in the ferromagnetic range.
In the theoretical expression for " which can be derived from the free energy /1) terms linear and quadratic in Pap- pear. Since a drastic change of slope of P, occurs at the ferromagnetic Curie point (see 2.3.2), "" should be af- fected. However, the influence may be overshadowed by other contributions, so that no substantial effect is detec- table.
2.3.2 Spontaneous polarization P,. The temperature dependence of P, has been measured between 5 and 623 K upon heating, using an electrometer (Keithley 616) in the charge mode (Fig. 4). This curve shows the different structural phase transitions of Co-CI. The sample iv) was in a trigonal single domain state up to -474 K, where it transformed to monoclinic (ml ')and at -525 K to orthorhombic mm21' . Because the single domain_ of~
the rhombohedral phase remained stable up to -474 K, · the corresponding P, value represents the saturation polarization. The magnitude of P, compares well with that of certain orthorhombic boracites (Fe-!: 3.9 x 10-2 .C ·m-2; Cu-CI: 1.85 X 10-2 C ·m-2, Cu-Br: 3 X 10-2 C ·m-2
Y'.
but it is at variance with P, data on Co-Cl about one order of magnitude smaller derived from pyroelectric measurements of polydomain samples." The domain patterns of phases ml' and mm21' looked very4
"" .,
!.,
u
" ·~ 3 >m
'-"-"
1<>."'
"' iJm
2 mm>
1
\
"'
0 100 200 300 400 500 600 700
temperature [K]
Fig. 4. Spontaneous polarization P, of phase 3m I', and projection of P, of phases ml' and mm21' along [Ill] versus temperature.
2.4~---,
~
.,
(,) 2.31<>."' 2.2
[K]
Fig. 5. Detail of the spontaneous polarization P, of phases m and 3ml' between 5 and 50 K.
-..
Optical, Electric and Dielectric Studies of Cobalt-Chlorine Boracite, Co3B7013Cl 1059
intricate; the corresponding P, values (Fig. 4) are therefore probably somewhat smaller than the projec- tions of P, along [Ill].
Figure 5 shows detail of the P,-T dependence below 50 K with enlarged P, and temperature scales. One can see that the onset of magnetic ordering (T,= II K) is accom- panied by a drastic change of slope of P, versus temperature, similar to that found on the orthorhombic boracites Ni-Br101 and Co-Br111 whereas for Cu-Br12l and
Co-
I'"
the change of slope is of opposite sign. The con-tinuity of P, at the bent of P, of Co-Cl shows the second order character of the transition, at variance with a magnetic susceptibility-curve'' which mimics first order character, probably because of paucity of data points.
Recently a Landau theory has been elaborated with a view to explaining the magnetic phase transitions of boracites.'1 There the paramagnetic phase R3cl' of Co- Cl is considered as the parent phase of the ferromagnetic phase with Shubnikov group Cc, with a unit cell contain- - ing the same number of atoms as the trigonal cell.
Although the ferroelectric phase R3cl' remains struc- turally trigonal below T,,F.M>" this does not exclude that an anomalous temperature dependence of P, may be observed. Such anomalies are ruled by couplings between the order parameter on the one hand and non-spon- taneous components of P, on the other hand. By minimiz- ing the free energy (Table VI, Ref. 8) one obtains for P, at T < Tc(F.\1.):
P, =Po- X
/i(b,ql+
y,q,M,)where P,=spontaneous polarization, P0=spontaneous polarization in the paramagnetic phase,
x ii
=the electric susceptibility in the trigonal pa.rent phase, q, =compo- nent of order parameter, M,=spontaneous magnetiza- tion along the monoclinic b-axis and b1 and y1 coeffi- cients of the free energy. The second term represents the order-parameter and magnetization induced polariza- tion. This term should only modify the slope of the total ,-._polarization below TdF.M.> as is confirmed experimentally(Fig. 4).
§3. Conclusions
On the basis of the observation of the persistence of the trigonal ferroelectric domain pattern in the fer- romagnetic phase, the occurence of a change in slope of P, at the ferromagnetic Curie point and the second order type onset of ferromagnetism, the transition 3m!' --+m of Co-Cl can be considered as purely magnetic in character, a point of view already adopted in the Landau theoretical treatment. 'l
Acknowledgements
The authors are grateful for helpful discussions with Drs P. Tolectano and M. Clin, to R. Boutellier, E.
Burkhardt and R. Cros for technical assistance, and to the Fonds National Suisse de Ia Recherche scientifique for support (project No.2. 231-0.84). Thanks go to Mrs.
0. Hirth for typing.
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