- ------------------~

R 579 r , Philips Res. Repts 21, 49-62, 1966

FERROELECTRIC PEROVSKITES CONTAININGMANGANESE IONS

by E. E. HAVINGA

Abstract

The investigations described relate to dielectric and magnetic propertiesof perovskites containing Mn3+ ions. Properties measured on ceramicsamples of compounds in the systems PbTi03-LaMn03 and PbTi03-PbMno.5Nbo.503 are given in detail. Other systems investigated arereferred to briefly. In all systems the substitution of more than 25 to35% MnH ions disturbs the occurrence of ferroelectricity. This is attrib-uted to the influence of local Jahn-Teller distortions. Other transition-metal ions may be substituted without meeting this difficulty. However,only Mn3+ ions give rise to ferromagnetic interactions. The object ofthe investigations was to find a compound showing both ferroelectricand ferromagnetic properties. The best results in this respect are obtain-ed with a mixed composition, 0·67PbTi03.0·25LaMn03.0'08LaNi03.Its ferroelectric transition point lies at 60 OK. The maximum valuereached by the dielectric constant is 350. The spontaneous polarizationamounts to about 0·5 !LC/cm2; however, no saturation is obtained.The extrapolated paramagnetic Curie point is 50 OK. Below this tem-perature no coherent ferromagnetism is observed, more complexbehaviour being found due to cluster formation in this very diluted spinlattice. No mutual interaction is detected between the ferroelectric andthe magnetic properties. .

1. Introduction

The investigations described in this paper were carried out with the aim ofarriving at ferroelectric ferromagnetic compounds. The existence of substances,combining special dielectric and specialmagnetic properties has been the subjectof several papers.Materials with a composition Ba6-2zGd2zNb9-zFe1+z030 were reported by

Fang and Roth 1) to be simultaneously ferriniagnetic and ferroelectric. Ourattempts to reproduce their results yielded only paramagnetic materials. How-ever, in most samples also traces of a second phase of barium hexaferriteBaFe12019were present. This contamination cannot easily be detected in X-raydiagrams because of a coincidence of peaks, but it reveals itself upon micro-scopic examination of polished samples. Very small amounts « 1%) ofBaFe12019 are already appropriate to explain the seemingly ferrimagneticbehaviour of the samples of Fang and Roth.Russian workers 2,3)have prepared numerous ferroelectric perovskites con-

taining Fe, Co or Ni ions, which compounds show antiferromagnetic inter-actions 4,5). Especially PbFeo.5Nbo.503 and BiFe03 have been studied ex-tensively, also in solid solutions with other perovskites 6-10). In a few com-positions a faint parasitic ferromagnetism was observed, and in BiFe03 slight

50 E. E. HA VINGA

anomalies in the lattice constants and possibly also in the dielectric constantare present at the Néel point 11).Other antiferromagnetic ferroelectrics are the hexagonal compounds YMnOa

and YbMnOa 12) and some compounds with the boracite structure. In the lattera parasitic ferromagnetism is observed showing interaction with the dielectricproperties la). However, as yet no example of a really ferroelectric ferromagneticcompound has been found.We have prepared numerous ferroelectric perovskites containing manganese

ions ..The perovskite structure was chosen because abundant experimental andtheoretical evidence exists that this structure is especially favourable for theoccurrence of ferroelectricity. Moreover transition-metal ions with magneticmoments can easily be substituted at the octahedral sites of the structure.Manganese ions were substituted because among the interactions betweenidentical transition-metal ions in perovskites only that between a pair ofMn3+ ions is found to be generally positive 14). Because most ferroelectricperovskites do not contain trivalent ions, often other ions had to be substitutedsimultaneously to reestablish the charge balance. To substitute Mn3+ ions inBaTiOa, for example, we can replace 2Ti4+ by Mn3++ Nb5+, or 3Ti4+ by2Mn3+ +W6+, or Ba2+ + Ti4+ by La3++ Mn3+, respectively. This corre-sponds to the formation of mixed crystals in the pseudo-binary systems ofBaTiOa with BaMno.5Mbo.50a, BaMno.67Wo.3303or LaMnOa, respectively.Several such systems have been investigated.The method of preparing the samples and its influence on their properties

are described in sec. 2. At the end of that section the methods used to measurethe dielectric and magnetic properties are discussed. Section 3 gives the resultsobtained on two systems, PbTiOa-PbMno.5Nbo.503 and PbTi03-LaMn03,respectively. In these systems ferroelectric compounds occur with ferromagneticinteractions between the manganese ions; however, ferroelectricity alreadydisappears at about 30% Mn3+. Therefore the ferroelectric compounds arevery diluted ferromagnetics. Attempts to arrive at better results in other systemswere unsuccessful; they' are reviewed briefly in sec. 4. The addition of othertransition-metal ions to ferroelectric perovskites containing the maximummanganese content results in compounds with magnetic properties only a littlemore favourable. They are discussed in sec. 5. The conclusions are summarized

. in sec. 6:2. Experimental

The compounds were prepared by conventional ceramic techniques. Appro-priate weights of oxides or carbonates of p.a. qualities were wet-milled' in agateball mills for 15minutes under absolute alcohol, filtered off, dried, compressedto small bricks and prefired at 850 oe in air for 14 hours. The prefired sampleswere wet-milled 'again Ct hr), formed to disks of 18mm diameter and 2 to 3 mm

•FERROELECTRIC PEROVSKITES CONTAINING MANGANESE IONS 51

thickness, pressed isostatically at 104 atm and wrapped in platinum foil. Thefinal firing was in a C02 atmosphere at 1100-1250 °C and 1 atm, the exact,firing temperature being adjusted to arrive at minimal loss tangents. After14 hours of firing the samples were shifted quickly to a cool part of the furnace.With this procedure the loss of volatile oxides (PbO, Bi203) was always lessthan 1% and the porosity of the samples was of the order of 5%.

The choice of the final firing temperature is rather critical, since a change ofabout 10°C gives already rise to a significant increase of the dielectric losstangent. At too low temperatures the samples are oxidized and contain Mn4+ions, whereas at too high temperatures the samples are spoiled by reductionprocesses, resulting in the formation of Mn2+ ions, mostly present in a secondphase. Moreover volatilization of PbO or Bi203 then begins to become notice-able. The Mn4+ ions formed in too oxidizing circumstances manifest themselvesnot only in a larger conductivity or an increase of the loss tangent, but alsoin a slight increase of the paramagnetic Curie temperature (in accordancewith ref. 7) and a decrease of the ferroelectric transition point. The lattereffect is illustrated in fig. 1, where for samples with nominal composition

T(OK) (1-x) Pb f"e().5 Nb(J.50a• X Pb Mn()'5Nb(J.f,oai4~~------~----~------~------~--~---1

().80 «o-xFig. 1. Temperatures Tm (where e reaches its maximum value in the s vs T curve) of sampleswith nominal composition (1-x)PbFeo.5Nbo.50s.xPbMno.5Nbo.50s, prepared under differentfiring conditions. The upper curve, 1180 °C C02, corresponds to stoichiometric samples, theother conditions are oxidizing. Dots indicate that at Tm a ferroelectric transition takes place.

(I-x)PbFeo.sNbo.s03.xPbMno.sNbo.s03 the temperatures (Tm) are plotted atwhich the dielectric constant e reaches a maximum. At this place only the influ-ence ofthe firing conditions on Tm will be discussed; the systemis treated furtherin sec. 4. The firing at 1180 °C in C02 atmosphere results in samples withoutMn4+; the two other conditions are much too oxidizing. Only at low Mn con-centrations (x< 0·40)does Tm coincide with a ferroelectric transition point, which

/.

52 E. E. HAVINGA

clearly is lowered upon oxidation. At high Mn concentrations (x > 0·40)feiroelectric hysteresis loops could no longer be detected; the samples fired inoxygen did not even show a maximum in the e vs T curve. This disappearanceof ferroe1ectricity at higher Mn concentrations was found generally and will beexplained in the next section.The dielectric constant of the samples was determined by measuring the

parallel capacitance of silvered disks with the aid of a Schering bridge (Rohdeand Schwarz, type VKB BN3520) at 250 kHz. Ferroe1ectric hysteresis loops .were traced with a simple Sawyer-Tower circuit at 50 Hz. In cases of high lossesuse was made of a circuit in which provision was made to compensate for losstangent and norma11inear dielectric constant (the principle of such a circuit wasdescribed in ref. 15). Polarizations ofthe order of 10-2 [J.Cjcm2 could easily be

I

detected. For alldie1ectric measurements sample holders covering the temper-ature range 4-800 OKwere available.The paramagnetic susceptibility X was measured, generally at 9000 Oe, by a

Faraday method in the temperature range 77-1000 OK; below 77 OK it wasmeasured with the aid of a null-coil pendulum magnetometer (described inref. 16). This magnetometer was also used for measurements of magnetizationas a function of field strength and of temperature. To check the absence ofsecond phases and to determine the lattice constants of the perovskite phaseCuKa X-ray diagrams were taken.

3. The systems PbTi03-LaMn03 and PbTi03-PbMno.sNbo.S03Results obtained. on high-ohmic samples with compositions

(l-x)PbTiOa.xLaMnOa and (1-2x) PbTiOa.2xPbMno.5Nbo.50a are plotted infigs 2 to 5 as a function of x, the concentration of Mn3+ ions. Figure 2 gives theextrapolated paramagnetic Curie temperatures 8 as determined from X-1 vs Tcurves in the temperature region 300-800 "K, and the temperatures Tm at whiche reaches its maximum value em. The values of em for the LaMnOa-substitutedcompositions are shown in fig. 3; the behaviour for PbMno.5Nbo.50a-sub-stituted compositions is similar. The dimensions of the unit cells as determinedfrom X-ray diagrams are plotted in figs 4 and 5, respectively.The unit-cell volume increases upon PbMno.5Nbo.50a substitution, whereas

it remains aböut constant upon LaMnOa substitution. Because the strength ofthe ferromagnetic interaction between an Mn3+ pair separated by an oxygenion decreases with increasing ionic separation 17), the extrapolated para-magnetic Curie temperature 8 is higher in the LaMnOa-substituted composi-tion at equal concentrations x (fig. 1..). Measurements at low temperatures showthat no real ferromagnetic Curie points are present at small Mnê+ concentra-tions (x < 0'50). This is due to the absence of a coherent lattice of spin-bearing ions and is discussed further in sec. 5.

At low Mns+ concentrations the temperatures Tm (fig. 2) coincide with ferro-

-,

FERROELECTRIC PEROVSKITES CONTAINING MANGANESE IONS

~ r=T,:-m"_e:::r--com-p-O-'S~:-:-:tion"--S---.L.-I-----,

700 \"" 0 • (1-X)PbTiO~.xLaMn0.3 1-Tt \ <,~ ·1~-2!9PbTio.,.2XPb_"",O,

\Î\ ~'"5oor----+-~--+_---~-~I---~---~

\ ~r--_~~----_4------~,+-----~~-----+--~--~

JIOOI----t---tI\------\::----l----J.-l2001---+----t----------:\~

\~

\\\\\oolr- 4- -+ ~-J-'\~-+_---~

~r::====:=O_L------=__::.___"..~L.-------_____=__!=L.-------~:_:_::~~~:~~~~o ().10 0·20 DUO (J.4() oeo

-xFig. 2. Temperatures Tm and paramagnetic Curie temperatures 8 as a function of x, theconcentration of Mn3+ ions in the systems PbTiOa-LaMnOa and PbTiOa-PbMno.5Nbo.50a.

electric transition points. The ferroelectricity of the low-temperature phase isalready suggested by the tetragonal distortion of the structure (figs 4 and 5)and was proved by the observation of D-E hysteresis loops. The values foundfor the spontaneous polarization are only of the order of"l fLCfcm2; however,as the coercive fields are very large saturation was certainly not obtained.At higher Mn3+ concentrations, x > 0'28 in the system with LaMn03 and

x > 0·37 in the system with PbMno.5Nbo.503, no tetragonal distortion of the. structure is found and no D-E hysteresis loops could be detected. Thereforethese compounds are no more ferroelectric. Nevertheless the e vs T curves stillshow a distinct maximum, though the temperature Tm at which these maxima

53

54 E. E. HA VINGA

0·20-x

Fig. 3. Maximum values em reached by the dielectric constants of samples(1-x)PbTi03.xLaMn03.

icm

\ I I(1-x) Pb Ti Da-X LaMnDJ

1\ ,

e»

\()fY

\,t:IY 1\

~-

r--o--o0

(oc-90':- 1O'i

occur are somewhat higher than 'was to be expected from extrapolation of thecurves through the ferroelectric transition points (see fig. 2).Another indication that ferroelectric transitions are absent at high x values

is the behaviour upon substitution of Sr for Pb. In a ferroelectric perovskite thisgenerally lowers the ferroelectric transition point because the polarizability ofSr2+ ions is much smaller than that of Pb2+ ions. Such a decrease is in-

7

5

2

o 0010 ouo

4~j~------r------.-------.----~.-------,4-10"" '"'" ~ (Lx) Pb TiOJ.LaMn DJ-t-------;

a,c a~ o300oKr: ~ ·100 -« -------1--------;'-....r--.o

I_:..o

Fig. 4. Lattice constants and cja ratio of samples (1-x)PbTi03.xLaMn03.

FERROELECTRIC PEROVSKlTES CONTAINING MANGANESE IONS 55

deed found for the compound O·4PbTi03.0·6PbMno.5Nbo.503 (fig. 6) inaccordance with the other arguments for ferroelectricity. In the compoundPbMno.5Nbo.503, on the other hand, no change in Tm is observed upon Srsubstitution, the only effect being a flattening of the e vs T curve.The disappearance of ferroelectricity at high MnH concentrations, which is

also found in other systems (sec. 4) can be explained by the influence of a Jahn-Teller effect. The interactions between the outer-electron configuration 3d4 ofthe Mn3+ ions and the ligand field of an oxygen octahedron induce a tetragonal

4·15c,c(~)4.f

14'05

M)O

C----"~ 2x) Pb Ti D,r2X Pb Mn().5Nb(J.f,oJ0 -...,

~ -~cr

ç::::;;;-

L----- ~5---- '.SJ..---

6 --~4~ i',.2 " I

040 0·20 000 ().4()-x

100cia

i1'01-0

1-00o

Fig. 5. Lattice constant and cja ratio of samples (l-2x)PbTiOa.2xPbMno.sNbo.sOa.

2

60? 1\ • ((}4-X) Pb TiDj' 05Pb Mn.o.5VbofJi~SrTiDJ40 \ -C(1-X) Pb Mn(J.sNbolJJ •X Sr Mnod'lbo{JJ

20 .,00 -,80 -,60 <,

<; -20 r-;001

o 005 040 0./5-x

Fig. 6. Decrease in Tm by Sr substitution for Pb in two compounds of the system PbTiOa-PbMno.sNbo.sOa.

56 E. E. HA VlNGA

distortion of this octahedron 18). In the first instance this is only an electron-ordering effect, but sometimes the induced elastic stresses can be released bycooperative distortion of all octahedra. For example the transition inLaMn03at 600 oe is due to such a cooperative Jahn-Teller effect. The cooperativedistortion can be removed by substitution of other ions; the room-temperaturedeformation of LaMn03 disappears upon substitution of about 10% BaTi0314)or 50% LaGa03 19). The considerable difference in the amount of foreign ionsneeded can to a great extent be ascribed to a difference in volume; the Jahn-Teller effect is the stronger the smaller the volume, since the ligand fields(crystalline field and/or covalent-bond formation) then become larger.In our compounds with a rather low Mn3+ content no cooperative Jahn-

Teller effect is present, but no doubt in Mn-rich clusters the Jahn-Teller effectslocally induce distortions of the regular lattice. Beyond a certain Mn3+ con-centration these may very well prevent the onset of ferroelectricity because therather long-range ferroelectric interactions are weakened considerably by latticedistortions. In the system PbTi03-LaMn03 the critical Mn3+ concentrationwill be reaclied earlier than in the system PbTi03-PbMno.5Nbo.503 because inthe former system on the one hand the unit-cell colume is smaller, which favoursthe Jahn-Teller effect and on the other hand there is less tendency to becomeferroelectric which follows from the lower transition temperatures. The maximain the S vs T curves at higher Mn3+ concentrations can be explained as follows.On cooling the compound has a tendency to become ferroelectric and thedielectric constant increases, but before the transition point is reached localJahn-Teller lattice distortions gradually set in, disrupting the ferroelectriccoupling, and the dielectric constant decreases again. Substitution of othertransition-metal ions in ferroelectric perovskites already containing Mn3+ ionsdoes not disturb the ferroelectricity (see sec. 5). This proves that the presenceof Mn3+ ions beyond a certain critical concentration indeed prevents theoccurrence of ferroelectricity in the compounds discussed, in accordance withthe explanation offered.The maximum value reached by the dielectric constant, Srn, is lowered strongly

with increasing Mn3+ content (see fig. 3). This can partly be attributed to thenon-ideal quality of our samples (a little porous and fine crystalline) and mustpartly be ascribed to local fluctuations in the composition of the samples.In these mixed crystals we have at best a random mixture of ions favourable

to the occurrence of ferroelectricity (pb, Nb, Ti) and ions markedly unfavour-able (Mn3+, as we saw above). Therefore clusters of favourable ions will havea higher transition temperature than clusters rich in Mn3+ ions. A distributionof transition temperatures is the result, which broadens and lowers the maxi-mum in the S vs T curves.

Summarizing we find in the' systems PbTi03-LaMn03 and PbTi03-PbMno.5Nbo.503 ferroelectric phases only at low Mn3+ concentrations. The spin

FERROELECTRIC PEROVSKITES CONTAINING MANGANESE IONS

system in these ferroelectric compounds is so diluted that no coherent ferro-magnetism is present, although ferromagnetic interactions are found. More-over, the ferroelectric transition temperatures are much higher than the magne-

. tically interesting temperatures, making the conditions for observing possiblemutual interactions unfavourable. Therefore other systems containing Mn3+ions were investigated.

4. Other ferroelectric perovskites withMnH ions

Itwas already mentioned that the substitution of Sr for Pb in the ferroelectricperovskites lowers their transition points. Such a substitution has little or noinfluence on the unit-cell volume but because the tendency to become ferro-electric is smaller in the substituted compounds we 'may expect that the localJahn-Teller effects will be able to prevent ferroelectricity already at lower Mn3+concentrations. This shift of the critical Mn3+ concentration has been verifiedexperimentally and is illustrated in fig. 7. Substitution of Sr for Pb in the com-

Ol ~~----~=-----~----~0- 040 0·20' OUO ().4()-x

Fig. 7. Decrease in Tm by Sr substitution for Pb in two compounds of the system PbTiOa-LaMnOa.

pound 0·25LaMn03.0·75PbTi03 gives a linear decrease of the ferroelectrictransition temperature with the strontium content, whereas in the compound0·27LaMn03.0·73PbTi03 Gust at the border line of the ferroelectric regionwhich ends at 28% Mn) only up to 10% SrTi03 ferroelectricity occurs. Beyondthis Sr concentration we could not obtain D-E hysteresis loops, and moreoverthere the temperatures Tm are higher than would be expected from extrapolationof the curve through the ferroelectric transition points. So the criticalMnê+concentration in the system PbTi03-LaMnÛ3 is decreased (from 28% Mn3+ to

57

58 E. E. HAVINGA

27% Mn3+) by the substitution of 10% SrTi03 for PbTi03. The magneticproperties are not changed upon this substitution., Substitution of Ba for Pb also lowers the ferroelectric transition points. Sucha substitution involves an increase in unit-cell volume, which will weaken theJahn-Teller effects. Therefore no shift in the critical Mn3+ concentration wasobserved for this substitution. Unfortunately the strength of the ferromagnetic .interactions decreases upon Ba substitution, just because of this increase involume.

Substituting Bi for part of the La, or taking W as the ion with compensatingvalency results already at rather low concentrations of these ions (R:J 10%) inantiferroelectric transitions. When instead of PbTi03 another ferroelectricperovskite was taken as the starting point for the substitutions of Mn3+ ions,the results were even less satisfying than those already obtained.A special case is the substitution of Mn3+ ions in ferroelectric perovskites

containing large numbers of other transition-metal ions. The dielectric behaviourofthe compounds (l-x) PbFeo.5Nbo.503.xPbMno.5Nbo.503 was already discuss-ed in sec. 2 (fig. I). Beyond x = 0·40 (20% Mn3+) local Jahn-Teller distortionsalready prevent the occurrence of ferroelectricity. The behaviour of the cor-responding system with Ta instead of Nb is similar, all values of Tm being about120°C lower. The study of themagnetic properties of these systems was greatlyhampered by a contamination oftraces ofmagnetoplumbite ~PbFeI2019). Onlyby choosing compositions which had deliberately been given an excess of PbO,forming a diamagnetic second phase (e.g. Pb5Nb40I5) this difficulty could beovercome. The compounds PbFeo.5Nbo.503 and PbFeo.5Tao.503 have nearlyidentical Xmolvs T curves in the temperature range 77-1000 "K. This illustratesthat the presence of ferroelectric transition points (at 370°C and 190 "K,respectively) has no marked influence on the magnetic behaviour. The X;;;~l vs Tcurves are identical with that published for PbFeo.5Nbo.503 in ref. 5; the high-temperature part shows the presence of antiferromagnetic interactions and atabout 400 "K a change in slope is found that is due to the dilution of the spinsystem. The extrapolated Curie temperature (determined from the high-temperature part of the X-I vs T curves) shifts towards positive values whenMn3+ ions are substituted, because the Mn3+-O-Mn3+ interactions are ferro-magnetic. However, all ferroelectric compounds show the behaviour of dilutedantiferromagnetics.

5. Compositions O.25LaMn03·(O·75-x)PbTi03·xLaX03In the preceding section we found that perovskites with a magnetically favour-

able lattice parameter and containing more than about 25% Mn3+ ions cannotbecome ferroelectric at low temperatures. Because no coherent ferromagnetismis present at such low concentrations of magnetic ions, other magnetic ions weresubstituted in the best system PbTi03-LaMn03. As valency-compensating ion

FERROELECTRIC PEROVSKITES CONTAINING MANGANESE IONS

La3+ ions were chosen in order to get only small changes in unit-cell volume.The results obtained for four different ions, Fe3+, C03+, Ni3+ and Cr3+, areplotted in fig. 8. The temperature Tm is lowered upon substitution and below

g~~~----7~~S~----~O------~O~~~S------~~Wx- -x

Fig. 8. Temperatures Tm and paramagnetic Curie temperatures 8 of samples with composition(O·75-x)PbTi03.0·25LaMn03.xLaX03.

Tm the samples are ferroelectric, as was verified by the observation of D-Ehysteresis loops. In sec. 3 this was already mentioned as an argument that thepresence of Mn3+ ions beyond a critical concentration is indeed responsible forthe disappearance of ferroelectricity.The substitution of Fe3+ or Cr3+ lowers the extrapolated paramagnetic Curie

point; it remains about constant upon substitution of Ni3+ and increases uponC03+ substitution. This was to be expected from the "results quoted in theliterature for the substitution of these ions in LaMn03 18); only in the case ofCr3+ substitution is the observed decrease in Curie point unexpectedly large.For Fe3+-, Ni3+- and Cr3+- substituted compounds the slopes of the X-I vs T

59

60 E. E. HA VINGA

curves lead to magnetic moments in reasonable agreement with the spin-onlyvalues. In the case of C03+ the observations are in agreement with the assump-tion that part of the C03+ ions are in the low-spin state. However, the presenceof an increasing amount of non-magnetic COllI ions with decreasing temper- /ature probably is the reason for the rather disappointing magnetic behaviourof Co-substituted samples at low temperatures. No improvement at all couldbe found in comparison with the samples containing only Mn3+ ions. Onlywith Ni3+ substitution a small improvement was noticed. As an example of themagnetic behaviour of these compounds we therefore will discuss the propertiesof the compound 0·25LaMn03.0·67 PbTi03.0·08LaNi03. In fig. 9, e and X-I

12X!1Ö*cr: I I ,I

0067Pb TiOJ• 0·25 La MnOa' ()'08 La Ni 0a

/.1I

~

'----- !//.: 1

27KOe / c:;>1~

8 400ê(250kHz)

JJOI6

200

002

1ro 200

Fig. 9. Dielectric constant e and reciprocal paramagnetic susceptibility x-1 at differentmagnetie-field strengths as a function of T for a ceramic sample with compositionO·6.7Pb~i03.0·25LaMn03.0·08LaNi03.

are plotted versus T. It is seen that the maximum of the e vs T curve is ratherlow and flat; nevertheless the compound is ferroelectric below Tm = 60 "K ,(D-E hysteresis with Ps Rj 0·5 (.LCfcm2). The x-values at low temperatures aremarkedly dependent on the magnetie-field strength H. The sharp rise in X(drop in x-1 in fig. 9) at about 100 "K measured at 450 Oe indicates that atthis temperature there is already some formation of ferromagnetic clusters ofions having 5 or 6 nearest-neighbour magnetic ions. The number of clustersand their sizes are small at first, as can be seen from the field dependence of X,

FERROELECTRIC PEROVSKITES CONTAINING MANGANESE IONS

but both will increase gradually upon lowering of the temperature. However,because of the great dilution (only about 1/3 of the B ions are magnetic ions)an appreciable number of magnetic ions, viz. those that have no or only a fewmagnetic neighbours, are not incorporated in these clusters. Therefore at theextrapolated paramagnetic Curie point X is not infinite, but decreases ratherrapidly with increasing field strength. The (J vs T curves (fig. 10) illustrate this

10 '}j) 30-H(kOe)

Fig. 10. Magnetization o at different temperatures as a function of field strength H for aceramic sample with composition Q·67PbTiOa.0·25LaMnOa.0·08LaNiOa.

behaviour. At 4·2 "K and 30 kOe a magnetization of only about 9 gauss cm3/gwas reached, which is about half the calculated saturation magnetization.Clearly, then, the magnetic lattice is too diluted to give coherent ferromagnetism.At the ferroelectric transition point no irregularities in the magnetic.behaviourcould be detected, showing that there is no appreciable interaction between thetwo properties.

6. Conclusions

The investigations presented show clearly that it is indeed possible to preparecompounds that exhibit both ferroelectric and ferromagnetic properties. How-ever, the ferroelectric properties are not very pronounced and moreover theferromagnetic properties suffer from the appreciable dilution of magnetic ions,leading to a lack of coherence between the ferromagnetic clusters. Largeramounts of ferromagnetically coupled spins could not be incorporàted in theperovskites without disturbing the ferroelectricity because oflocal Jahn-Teller

61

62 E. E. HAVINGA

distortions induced around the Mn3+ ions. The lack of coherence between theferromagnetic clusters is, in our opinion, the reason for the failure to detectinteraction between ferroelectric and magnetic properties. .The situation may be more favourable in antiferromagnetic compourids with

a parasitic ferromagnetism, because then at least a coupling throughout thewhole lattice is present. .

AcknowledgementI

The author wishes to thank Mr A. van der Grijp for designing and construct-ing the apparatus for measuring D-E hysteresis loops with compensation forlosses and normal capacitance, and Mr J. F. Fast for making the low-temper-ature magnetic measurements. .

Eindhoven, October 1965

REFERENCES1) P. H. Fang and R. S. Roth, J. appl. Phys. 31, 278S, 1960.2) A. I.Agranovskaya, Bull. Acad. Sci. S.S.S.R. 24, 1271-1277, 1960.3) V. A. Bokov and I. E. Myl 'nikova, SOy.Phys. solid State 2, 2428-2433,,1961.4) G. A. Smolenskii and V. A. Bokov, J. appl. Phys. 35, 915-918, 1964.5) V. A. Bokov, I.E. Myl'nikova and G. A. Smolenskii, SOY.Phys. JETP 42, 447-449,

1962.6) Yu. E. Roginskaya, Yu. N. Venevtsev, S.A. Fedulov and G. S. Zhdanov, SOY.

Phys. Crystallography 8, 490-494, 1964.7) Yu. Ya. Tomashpol'skii and Yu. N. Venevtzev, SOy. Phys, solid State 6,2388-2392,

1965.8) V. A. Bokov, S. A. Kizhaev, E. I.Myl'nikova and A. G. Tutov, SOy. Phys. solid

State 6, 2419-2424, 1965.9) S. A. Fedulov, P. B. Ladyzhinskii, I. L. Pyatigorskaya and Yu. N. Venevtsev,

SOy.Phys, solid State 6, 375-378, 1964.10) G. A. Smolenskii and V. M. Yudiz, SOy.Phys, solid State 6, 2936-2942, 1965.11) Yu. Ya. Tomashpol'skii, Yu. N. Venevtsev and G. S. Zhdanov, SOy.Phys, JETP

19, 1294-1295, 1964.12) F. Ber t aut, F. Forr at and P. H. Fang, C.R. Acad. Sei. Paris 256, 1958-1961, 1963.13) D. Tar, Helv. phys, Acta 37, 165-172, 1964.

E. Ascher, private communication.14) G. H. Jonker, Physica 22, 707-722, 1956.15) H. Diamant, K. Drenck and R. Pepinsky, Rev. sci. Instr. 28,30-33, 1957.16) U. Enz, J. F. Fast, S. van Houten and J. Smit, Philips Res. Repts 17, 451-463, 1962.17) E. E. Havinga, to be published.18) J. B. Goodenough in Landolt-Börnstein, Zahlenwerte und Funktionen, Springer

Verlag, Berlin-Göttingen-Heidelberg, 1964, 6th Ed., Vol. 11/9,2-187-2-221.19) J. B. Goodenough, A. Wold, R. J. Arnott and N. Me nyn k, Phys. Rev. 124, 373-384,

1961.