Evidence for the magnetoelectric effect in nickel-bromine boracite, Ni₃B₇O₁₃Br

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Evidence for the magnetoelectric effect in nickel-bromine boracite, Ni₃ B₇O₁₃Br

RIVERA, Jean-Pierre, SCHMID, Hans

Abstract

A ferromagnetic/ferroelec./ferroelastic phase, consistent with Point Group m'm2' and TC(f.m.)

= 17 K, was identified by measurements of the magnetoelec. coeff. α32, peak value at TC (7 ps/m), and the ferromagnetic coercive field, Hc, vs. temp. A magnetoelec. effect persists above 17 K up to ∼30 K, indicating a phase of still unknown nature. The dielec. const. ε33 is insensitive to magnetic ordering, but a striking anomaly of spontaneous polarization, Ps, occurs in the magnetic ordering range and is attributed to a spontaneous magnetoelec.

interaction. Further, Ps is (290 nC/cm2) at TC(f.e.).

RIVERA, Jean-Pierre, SCHMID, Hans. Evidence for the magnetoelectric effect in nickel-bromine boracite, Ni₃B₇O₁₃Br. Ferroelectrics, 1984, vol. 55, no. 1, p. 295-298

DOI : 10.1080/00150198408015393

Available at:

http://archive-ouverte.unige.ch/unige:32135

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F•'"•••·l··• trt« '· IQK.&_ \'111 ~~- rr .:!4~--=~H 1Mll~"ll4_1 ~ ~<UIII~'I<~IK ~~~II

'~· I'IH4 G,,,..J,,n JnJ Br\.'Jr..:h. S\.t\!nu: Puf'llhh.;:r ... ln ...

Pnnh:J tn !h.: t·nth:J Sl..tr...~ .. 11f Ank.·rh."J

EVIDE~CE FOR THE ~GNETOELECTRIC EFFECT IN NICKEL-BRO~Il~E

BORACITE, Ni 3B

7o 13Br

JEAN-PIERRE RI\'ERA and HA~S SCID-liD

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

Abstract A ferromagnetic/ferroelectric/ferroelastic phase, consistent with point group m'm2' and TC(f.m.)• 17K, has been identified by measurements of the magnetoelectric coefficient a_{12 ,} peak value at Tc(7 ps/m), and the ferro- magnetic coercive field, He:• versus temperature. A magneto- electric: effect persists above 17K up to~ )OK indicating a phase of still unknown nature. The dielectric constant ~ll

is insensitive to magnetic ordering, but a striking anomaly of spontaneous polarization, Ps• occurs in the magnetic -+

orderin~ range and is attributed to a spontaneous magneto-₊ electric: interaction. Furthermore, P has its maximum value

(290 nC/cm²⁾ at TC(f.e.) s

I ~TRODt:CTI 0:-l

The 3d-metal boracites M₃B,(\,X, abbreviated~. with~ • Fe, Co, Ni, Cu and X • Cl, Br, I have weakly ferromagnetic/ferroelectric/

ferroelastic phases 1,2,3,4 except for Cui, for which no data are as yet available. This paper reports magnetoelectric and dielectric data on SiBr which - on the basis of Faraday effect and do~ain

switching properties - was believed to have Shubnikov point group m'm2' at low temperature 4.

S~~PLE PREPARATIO~

For the measurement of a12(T), Hc(T), £33(T) and P (T)

..

the same ferroelectric: single domain sample, prepared ~t room te=pe- rature from an unpoled virgin crystal, was used .. : (IOO)c-platelet, thickness t • 8~~m, surfaceS • 1.795mm², with Ps ~surface, hence orthorhombic indicatrix principal section ny/na in the plane of the platelet, electroded with transparent gold.

~GSETOELECTRIC ~ASL'RE~!ESTS (~[E)H

Linear ma~netoelectric coefficient : a 32 . In this paper the same coordinate system as that described in detail for the magne:~­

electric measurements of SiCl 5 is adopted, because the sace corre-

• J

lation bet~een the orientations of P_{5 ,}

Ms

and the optical indicatrix

2%•1'~~>~1 1.-P RIVER.-\ A~D H SCH\110

vas deduced for ~iBr 4 as found for NiCl 4,S : P₃ ^s a₃₂Hz (P3// Psll n

8, Hz// Msll ny). For the supposed point group m'mZ' of NiBr, two coefficients of the linear magnetoelectric effect,

labelled a₃₂ and a₂₃ in the adopted system, are allowed. So far it vas only possible to isolate a single domain sample (see above) permitting the measurement of a_{32 •} The measurement technique adop- ted was similar to that described earlier S. A He-flow cryostat served for cooling the sample down to 1.5K, a Varian 12" magnet allowed to increase/decrease the magnetic field linearly in time.

An electrometer (Keithley 642) served for charge measurements. The results are given in figure 1. The temperature dependence of a₃₂ is characterised by a sign reversal at about 9K and a peak value of 7 ps/m, reached at ~ t6.5K. Above that temperature the magneto- electric signal drops sharply to about 36% of the peak value and decreases smoothly to reach zero at about 30K. The peak of a₃₂ of NiBr resembles the peak of a₃₂ of NiCl 5 at 9K, the temperature at which ~ 6 and the Faraday effect 4 of NiCl disappear.

s

Ferromagnetic coercive field, He• Figure 2 shows the ferromagnetic coercive field versus temperature, which has been measured by the

"butterfly" loop technique 7,5. Figure 3 gives some examples of loop. We note that on crossing the temperature of sign reversal of

a32 (~ 9K) the loop turns upside down ! The remarkable feature of He is that it tends sharply to zero at ~ 16.8K, practically the same temperature at which a₃₂ reaches its maximum. From this coincidence it is concluded that 17K marks the Curie point of a ferromagnetic phase.

DIELECTRIC ~ASCRE~NTS

The dielectric constant c;₃ of the ferroelectric orthorhombic phase of :-v iBr has been measured at 300kHz and 36Vrms/cm from 1. 5 to 440K using an impedance analyzer (HP4192A) (Fig.4). Its value is very small (about 7 at 1.5K) and no anomaly whatsoever is seen in the temperature range of magnetic ordering(at variance with the

case of£ for CuBr 8,9), nor any anomaly due to defects as found for Nil 10,11 •

The change of spontaneous polarization with temperature has been measured between 1.5 and 400K upon heating, using an electrometer

(Keithley 616) in the charge mode (Fig. 5 and Fig. 6). Leaving aside the unusual temperature dependance ~f + Ps , which is not yet understood, a strong anomalous change of P₅ is found below 28K.

It is noteworthy that the Ps(T) curve is marked by kinks at

~ 17K, the temperature of Tc(f.m.) and at 28K, where a₃₂ tends to zero. The peak values of ~s and p (pyroelectric coefficient), 290 nC/cm2 and~ 5.0 nC/(K.cm2), respectively, are obtained at Tc(f.e)• in agreement with P+ ₅ (T) of MgCl 12 b~t disagree with pre- viously calculated temperature dependence of Ps for NiBr 13.

MAGNETOELECTRIC EFFECT IN NICKEL-BROMINE BORACITE

.. -·

T l .!.

: ·4 -I

• ^a ~

..

II -lw

•

..

• • ft _, •IY • •

~ y

..

"

•

..

I!

..

I i •I

I

•

FIGURE I ME-coefficient a32 of NiBr (if// ny) vs.temperature

:zooo

'! ^'

. _. .

;;1500

_...

^~

.... ^, ..

_•

_.

ii: • •

..

~1l00

...

~

~sao ^•

00 _{5 Ill 15 20}

TEMPERATURE [K)

:25 lO

FIGURE 2 Ferromagnetic coercive field ^He of NiBr vs.temperature deduced from butterfly loops.

~ ^uo

.---""'""!...,

'.

::.

§

c

! ..

uo

2 ..

i

..

mm21' !/

: m'm2'

I

uogL-....&..--I0~--1.--!Lo--..L.---'JO

TPif'fUTUitf ~

F!GL~E 6 Detail of the sponta- neous polarization • ^P₅ of NiBr vs.temperature in the region of

the magnetic phase transitions.

Arrows show sens of ^(~)H signal.

at !H·~ fer

!HI>!Hcl·

l

_•

.: "

T.S_OK

---..;:====-~""'----~

[L --~----~J ..---;

-====-rc----..:-

... ,.

....

^:

T.H.I.K

FIGURE 3 Some examples of magne- toelectric butterfly loop1 of NiBr .

·.S

•2Q ~

..

z

8 ' w 0:

• :0: IC

i ~---

0 100 200 JOO oOO

TE .. PERATuRE [KJ

FIGURE 4 Dielectric constant

£]3 of NiBr vs.temperature

o;-

y

:-J.D

•

,.· .. r

"'"'''"

..

^/

U~---~

I . . ,_ :MID ...a

I [ .... (UIUII( [aJ

FIGVR~ 5 Spontaneous polariza- tion P₅ of NiBr vs.temperature

("- 4K/rnn)

COt-:CLL'S IONS

It is concluded that a ferromagnetic phase exists below

Tc(f.m.)"' 17K, its properties being consistent with Shubnikov group m'm2'. Above 17K up to about 30K, another magnetically ordered

phase exists. A similar behaviour above Tc(f.m) (9K) of NiCl was interpreted as an antiferromagnetic phase mm2 ~. Such an interpre- tation for NiBr would, however, be at variance with the observation of a Faraday effect above 17K 4. There seems little doubt that the ....

strong anomaly of Ps found for NiBr below 28K is caused by a kind of spontaneous magnetoelectric coupling.

ACK~Qt,'L EDGE:-tENTS

thanks go to E. Burkhardt and R. Cros for the painstaking prepara- tion of samples, to R. Boutellier for technical assistance and to Mrs 0. Hirth for typing the manuscript. This work was supported by the Fonds National Suisse de la Recherche Scientifique.

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!·

211 (1974)

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V.A. Koptsik and V.K. Sovik, Kristallografiva, 20, 854 (1975), (Sov.Phys.Crvstallogr. 20,526 (1976)] . -

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12•

475 ( 1982)] - - - -

10. J.-P. Rivera and H. Schmid, Ferroelectrics, 42, 35 (1982) 11. J.-P. Rivera and H. Schmid, this conference,E:01F-5 (1983) 12. L.P. Torre, S.C. Abrahams and R.L. Barns, Ferroelectrics, ~.

291 (1972)

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