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Ferromagnetic compounds of manganese with perovskite structure
P h y s i c a X V I , no 3
M a a r t 1950
FERROMAGNETIC COMPOUNDS OF MANGANESE WITH PEROVSKITE STRUCTURE by G. H. J O N K E R and J. H. VAN SANTEN Philips Research Laboratories, N.V. Philips' Gloeilampenfabrieken Eindhoven - Netherlands
Summary V a r i o u s m a n g a n i t e s of t h e g e n e r a l f o r m u l a L a 3 + M n 3÷ 0 2 - - M e 2 + M n 4 + O 3 2 h a v e b e e n p r e p a r e d in t h e f o r m of p o l y c r y s t a l l i n e p r o d u c t s . P e r o v s k i t e s t r u c t u r e s w e r e f o u n d , i.a. for all m i x e d c r y s t a l s L a M n O 3 - C a M n O 3, for L a M n O 3 - - S r M n O 3 c o n t a i n i n g up t o 70~/o SrMnO3, a n d for L a M n O 3 - B a M n O 3 c o n t a i n i n g less t h a n 50~/o B a M n O 3. T h e m i x e d c r y s t a l s w i t h pero v s k i t e s t r u c t u r e are f e r r o m a g n e t i c . C u r v e s for t h e Curie t e m p e r a t u r e versus c o m p o s i t i o n a n d s a t u r a t i o n v e r s u s c o m p o s i t i o n are g i v e n for L a M n O 3 - C a M n O 3, L a M n O 3 - S r M n O 3, a n d L a M n O 3 - B a M n O 3. B o t h t y p e s of c u r v e s s h o w r 0 a x i m a b e t w e e n 25 a n d 4 0 % Me2+Mn4+O2--; h e r e all 3d electrons available contribute with their spins to the saturation magnetiza t i o n . T h e f e r r o m a g n e t i c p r o p e r t i e s c a n be u n d e r s t o o d as t h e r e s u l t of a s t r o n g p o s i t i v e Mn 3+ - - M n 4+ e x c h a n g e i n t e r a c t i o n c o m b i n e d w i t h a weak Mn 3 + M n 3+ i n t e r a c t i o n a n d a n e g a t i v e M n 4 + Mn 4+ i n t e r a c t i o n . T h e M n 3+ - - M n 4+ i n t e r a c t i o n , p r e s u m a b l y of t h e i n d i r e c t e x c h a n g e t y p e , is t h o u g h t t o be t h e first clear e x a m p l e of p o s i t i v e e x c h a n g e i n t e r a c t i o n in oxidic s u b s t a n c e s .
1. Introduclion. During our investigations 1) into the occurrence of the perovskite structure we prepared i.a. compounds of the general formula A3+B3÷02- . One of these, LaMnO 3, showed ferromagnetic properties at liquid-air temperature, whereas LaCrO 3 and LaFeO 3 did not. It appeared, however, that LaMnO 3 was ferromagnetic at this temperature only when it contained some manganese of a valency higher than three ; by a suitable thermal treatment in an oxygen atmosphere, the substance took up more oxygen and Curie temperatures up to 210°K were found. A similar increase of valency of Mn was realized by preparing mixed crystals Laa+Mn3+02 - Me2+Mn4+02- (Me2+ = large divalent ion). An - -
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J O N K E R A N D J. H. VAN S A N T E N
investigation was therefore made with the binary systems LaMnO 3 CaMnOa, LaMnO a - - SrMnOa, LaMnO 3 - - BaMn03, LaMnO 3 - CdMnOa, LaMnO a - PbMnO a. In a number of experiments La was replaced by trivalent rare-earth metals. It appeared that in all these systems, at least over a certain range of compositions, ferromagnetic mixed crystals were formed. In this paper we shall discuss the preparation of these compounds, and their crystallographic and ferromagnetic properties *). In a later one their electric conductivity will be considered. For the sake of simplicity the compounds containing trivalent as well as those containing tetravalent manganese will be designated as " m a n ganites" **). -
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2. Chemicalpad. 2.1. P r e p a r a t i o n
of manganites. The manganites were prepared using standard ceramic technique. Oxides or carbonates of the metals were weighed in the desired proportions and milled for half an hour in water or ethanol to mix them thoroughly; afterwards the preparations were dried and pre-fired in air at about 1000°C. At this temperature an appreciable conversion already takes place. The product obtained was milled and dried a g a i n mixed with a binder and finally for~ned into discs or rods. The samples were fired again for some hours in an electric furnace at temperatures between 1350 and 1450°C. In most cases air was passed through the furnace but for certain compositions nitrogen or oxygen was used. It did not make any difference whether we prepared manganites starting from MnO 2 or from MnCOa. At the reaction temperature the valency of manganese adjusted itself to the composition of the sample and to the atmosphere. Some samples were prepared by a wet process. Solutions of nitrates of the metals were mixed in the desired proportions and from this mixture the metals were co-precipitated by adding a solution of ammonia, ammonium carbonate and hydrogen peroxide, or a solution of sodium carbonate. These preparations show a higher *) The authors wish to t h a n k Mr. M. B a n n i n k for his valuable e x p e r i m e n t a l assistance. **) Strictly speaking only the compounds Me2+Mn4+Oa c o n t a i n i n g t e t r a v a l e n t Mn should be called manganites 2), whereas the compound La3+Mn3+O~ c ont a i ni ng trivalent Mn might be called " h y p o m a n g a n i t e " . Furthermore one mi ght be averse to using the name man ganite since this would suggest t h a t these compounds were " s a l t s " r a t h e r th an double oxides.
F E R R O M A G N E T I C COMPOUNDS OF MANGANESE
339
reactivity, so that at lower firing temperatures the conversion is more complete. The properties of the samples prepared in this manner were not different from those obtained by the dry method. By the ceramic method just described polycrystalline products are obtained. We also tried to prepare single crystals from solutions in molten salts, but this has so far only been successful in the case of SrMnO a. Since, however, this compound does not crystallize in the perovskite lattice, the results will not be discussed here. 2.2. D e t e r m i n a t i o n of the valency of mang a n e s e. The physical properties of the samples are closely related to the valency of manganese. Therefore this quantity was determined for all samples, applying a method commonly used for manganese dioxide. About 50 mg of the substance to be examined is dissolved in hydrochloric acid under gradual heating. With the aid of a current of nitrogen the chlorine evolved is passed through a solution of potassium iodide and the iodine liberated is titrated with a solution of sodium thiosulfate. It was expected that LaMnO 3 would contain trivalent manganese but it appeared that this substance had a pronounced tendency to take up more oxygen, part of the manganese assuming a valency higher than three, presumably four. In table I the results of our titrations are given for a number of LaMnO a samples fired at different temperatures in air. By firing LaMnO a at 1400°C in an TABLE I atmosphere of nitrogen containing firing % tetravalent about 1% of oxygen a preparation temperature °C Mn containing only 3% Mn 4+ was ob1000 20.5 tained. The tendency to take up an 1150 25 excess of oxygen might be an indi1250 21 1350 14 cation of a region of mixed crystals extending from LaMnO a towards La2/3MnO3. The latter substance does not actually exist (at least not at high temperatures) but the corresponding titanate La2/3Ti 0 a is known and has perovskite structure. It could be expected that by replacing lanthanum by alkaline earth metals the valency of manganese could be appreciably increased even for samples that are fired at high temperatures. This proved in fact to be easy, e.g., for mixtures of LaMnO a and SrMnO a. When preparing samples containing more than 40% Mn 4÷ , however,
340
G. H. J O N K E R
A N D J . H. V A N S A N T E N
it was found necessary to fire in oxygen. Table n gives the Mn 4+ content of a n u m b e r of LaMnO 3 SrMnO 3 compositions fired in air at 1350°C. F r o m this it follows TABLE II t h a t for mixtures containing about LaMnO3 - - SrMnO3 % Mn *+ t7 35% Sr, firing in air is the right 100-0 90-10 19 w a y of preparation. In the case of 80-20 23 other mixtures a suitable atmos70-30 32 phere must be chosen. Bv firing in 60-40 38 42 pure oxygen it is possible to obtain 50-50 40-60 so a SrMnO 3 preparation containing 30-70 62 960/o tetravalent manganese. Simi0-100 85 lar results were obtained for the b i n a r y systems LaMnO 3 - - C a M n Q , LaMnO 3 BaMn03, LaMnO 3 CdMnO 3, and LaMnO 3 - - PbMnO 3. -
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3. Crystallographic considerations.
3. I. T h e
p e r o v s k i t e
structure of manganites. LaMnO 3 crystallizes in the perovskite structure, so called after the mineral perovskite, CaTiO 3. In fig. 1 the unit cell of a perovskite A B O 3 is given. Here A denotes a large ion like Ca 2+, Sr 2+, Ba 2÷, Cd 2+, Pb 2+, La3+, pr3+, Nd3+, Gd3+, y3+, while B stands f o r a small ion like A13+, Cr 3+, Mn 3+, Fe 3+, Ti 4+ , Mn 4+ . The A ions are situated at the corners of the unit cell, the B ions occupying the centre of the cube while the oxygen ions are placed at the centres of the faces. •A It was pointed out b y G 0 1 d s c h m i d t 3) 00 t h a t the perovskite structure is stable only if the Fig. 1. Unit cell of parameter t, defined b y t = ( r a + ro)/(rB+ro).V/2, perovskite. (ra, rB, r o denote respectively the radii of ions A, B, O) a p p r o x i m a t e l y equals unity. A purely cubic structure is found if it is equal to u n i t y (SrTi03). If t is slightly different from u n i t y the perovskite structure is slightly distorted (CaTi03, BaTi03) and for large deviations from u n i t y completely different crystal structures are found (e.g. ilmenite for t < 1, calcite and aragonite for t > 1). W h e n calculating the value of the p a r a m e t e r t we must know the radii of the ions for the special co-ordination t h e y possess in the perovskite structure 4). Since we considered only related compounds •
B
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341
COMPOUNDS OF MANGANESE
we simply used non-corrected G o 1 d s c h m i d t radii knowing that our values of t should be multiplied by a factor that is approximately equal for all compounds considered here. The ionic radii used are given in table III. TABLE III Ca z+ Sr"-+ Ba 2+ Cd 2+ P b "+
1.06 A. 1.27 1.43 1.03 1.32
La s+ Pr s+ Nd s+ Sm s+ Gd s+ Y~+
1.22 A 1.16 1.15 1.13 1.11 1.06
Mn ~+ Mn *+
0.70 A 0.52
02-
1.32A
The values of t calculated from these radii are given in table IV. T A B L E IV *LaMnO.~ t = 0.89 *(Pr, Nd)MnO~ 0.86 s SmMnO 3 0.86 *GdMnOs 0.85 YMnOs 0.83
*CaMnO s t = 0.915 SrMnO s 0.995 BaMnO s 1.055 CdMnOs 0.90~ PbMnO~ 1.015
Here the compounds showing perovskite structure are marked with an asterisk. The parameter t has also been calculated for mixed crystals taking for r A and r e the arithmetical ".0~ ~.~ tt /8. average of the radii of the ions occupying , / respectively the positions A and B. The results are represented in fig. 2, where t has been plotted for all compounds and binary systems investigated. Compounds and mixed crystals showing perovskite structure are indicated by O. Various series have been investigated in detail and perovskite structures were found to occur for values of t up to t = 0.965. For mixtures with t >0.965 products were obtainO4 ed containing a second phase with a different , ~n~o 4o 60 8o N o , ~ crystal structure. The lower t-limit of the Fig. 2. perovskite region was not determined. The Values of parameter t lowest value, t = 0.85, was obtained in the for various manganites. case of GdMnO 3. It is interesting to note that 0 indicates compounds YMnO a and SrMnO 3, which do not crystallize and mixed crystals with perovskite structure. in the perovskite structure, form mixed crystals, e.g. (Y0.4oSr0.6o) MnO 3, with perovskite structure.
rl--.
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G. H. J O N K E R AND J. H. VAN S A N T E N
In general the perovskites considered here are not perfectly cubic. Only some compositions, e.g. (Lao.ssBa0.4s) MnO3and (Lao.ssSro.4s)MnO 3 are probably cubic. In tbe TABLE V series LaMnO a - - SrMnO a average edge of unit cell % Mn ~+ mixed crystals containing A less than 40% or more LaMnOa I 3.90 s.9 than 50% Sr are not cubic. (Pr, Nd)MnOz 3.85 6.4 It is possible that for the GdMnO 3 3.82 CaMnOa 3.73 8o o perovskite co-ordination LaA1Oz. 3.78 the ionic radii have to be LaCrOa 3.88 corrected in such a way LaFeO a 3.89 that t is exactly equal to unity for mixed crystals showing cubic structures. In table V the average cell dimensions are given for a number of perovskites.
4. Magnetic measurements. The saturation magnetization has been measured for all samples as a function of temperature. From these data the saturation magnetization at absolute zero, i.e. the number of B o h r magnetons per molecule contributing to ferromagnetism, and the Curie temperature can be derived. 4.1. E x p e r i m e n t a l technique. For the determination of the saturation magnetization I s we used an apparatus constructed and described b y R a t h e n a u and S n o e k S ) . A weighed sample of the ferromagnetic material is placed in a pendulum moving horizontally between the pole shoes of an electromagnet and in a direction perpendicular to the line connecting them. The pole shoes are shaped in such a way that the value of the magnetic field strength perpendicular to the direction of motion of the pendulum is given b y H - - - - H 0 - ½ax2, where H 0 denotes the field strength at the centre between the pole shoes, x stands for the distance between the pendulum and the centre, and a is a constant. Provided the sample is magnetically saturated all the time the magnetic force acting on the pendulum is proportional to the distance of the sample from the equilibrium position; the same holds for the gravitational force. Consequently, b y inserting the sample, the harmonicity of the pendulum is not disturbed. The apparatus was calibrated for different field strengths with samples of pure nickel. Thus b y measuring the period of oscillation
FERROMAGNETIC
COMPOUNDS OF MANGANESE
343
both for the e m p t y pendulum and for the pendulum containing the sample, it was possible to calculate the saturation magnetization. The pole pieces were spaced far enough apart to allow of heating or cooling devices to be placed round the samples. The maximum field obtainable was about 6000 Orsted, amply sufficient to saturate the samples also at the lowest temperatures, as was verified by measuring B - - H curves between 90°K and 370°K. 4.2. E x p e r i m e n t a l results. Ir~ fig. 3 the measured values of the saturation magnetization are given as a function of temperature foi two g,~ preparations consisting of mixed crystals of LaMnO a = -''--2.,_ and SrMnO a. In general \ at 90°K the curves are so \ flat that extrapolation \ towards 0°K is easy. The corrections arising from the extrapolation appear ~o to be smaller than the ~T experimental error and the scattering between Fig. 3. Saturation magnetization I s as a samples of the same com- function of temperature T for mixed crystals (La0.90 Sr0.10)MnO3 (I) and position. Therefore the (Lao.70 Sr0.ao)MnOa (II). values of I~ were not extrapolated. For samples with low Curie temperatures, e.g. for LaMnO3, extrapolation becomes very uncertain; therefore here also extrapolation was omitted. A Curie temperature 0 was derived from the I , - - T curves by determining the intersection of the T-axis by the tangent in the inflexion point. For series of mixed crystals of manganites of trivalent and divalent metals it appears that in general the Curie temperature increases with the Me2+ content for Me2+ fractions up to 25-40% and decreases for higher ones. The increase of the Curie temperature is closely connected with the Mn 4+ concentration since the Curie temperature of a given sample can also be raised by increasing the oxygen content. This effect is very pronounced for LaMnO 3 and for mixed crystals with only small amounts of Mea+. For such compositions it was difficult to find the Curie temperature
344
G.H. JONKER AND J. H. VAN SANTEN
of "ideal" preparations showing the correct stoichiometric oxygen content. The Curie temperatures of the ideal preparations were determined b y interpolation or extrapolation, plotting for each composition the values of the ,x I Curie temperatures against the oxygen content. An example is given in fig. 4 for the composition (La0.9o Sr0.10) MnQ. In this w a y we determined the Curie temperatures for a number of ideal preparations of the systems L a M n Q - - C a M n Q I 5 fO ~J 30 35 30 40 (fig. 5), LaMnO 3 - - S r M n O 3 (fig. 7), LaMnQBaMnO 3 (fig. 9). Moreover Fig. 4. Determination of the in table VI the Curie temperatures of Curie temperature 0 and of the saturation magnetization some other mixed crystals are given; I s of the ideal preparation these values, however, have not been (Lao.90 Sr0A0)MnO3 • corrected for deviations from the stoichiometric oxygen content. In the same way as discussed for the Curie temperatures, the saturation magnetization at 90°K was corrected for non-stoichiometI
/!
~/,/n
4•
130
,o fl
/'xl
.j _ _ . I
\
~0
/
.c
!\i
o/
/
f~
./ 3~
°
8O 0
tO
20
3~
40
~%
50
t
60 ,tO
ean~os
Fig. 5. Curie temperature 0 of mixed crystals (La, Ca)MnO 3. Crosses indicate ideal preparations.
10
30
30
40
50
60
70
80
90 $00
Fig. 6. Saturation magnetization I s at 90°K of mixed crystals (La, Ca)MnO 3. Crosses 'indicate ideal preparations.
ric oxygen content. The results are given in figs. 6,8, I0 and in table VI. The same graphs give values of I s calculated on the assumption
FERROMAGNETIC
COMPOUNDS
OF MANGANESE
345
t h a t all 3d electrons present in the m a n g a n e s e ions c o n t r i b u t e with their spins to the s a t u r a t i o n magnetization. F o r all mixed crystals c o n t a i n i n g from 25 to 35°/o Mn 4+ it appears t h a t the s a t u r a t i o n reaches "K 38( 3~
/
34(
i"
/
30O
~0
,
11
.I
--
J 2z..
I
IN0
f~
[
,~ --
[
~
20 30 40 50 60 ,~ 80 90 ~0 ~
Fig. 7. Curie temperature 0 of mixed crystals (La, Sr)MnO 3. Crosses indicate ideal preparations.
% .~.M,.,o3
FAg. 8. Saturation magnetization I s at 90°K of mixed crystals (La, Sr)MnO 3. Crosses indicate ideal preparations.
"k' 3~ 34C 3,~ 30~
fO0 26C
a: ~
~,.~..
- -
f8~
f4(
20
[
20ha~cea
tX 0
tO 20
3040
.50
6O7O
60
tO
90
20
30
405O
60yo
80
gO fO0
• %ea~no3
Fig. 9. Curie temperature 0 of mixed crystals (La, Ba)MnO a. Crosses indicate ideal preparations.
F i g . 10. S a t u r a t i o n I s
at
(La, ideal
90°K Ba)
of
M n O 3.
preparations.
magnetization mixed
crystals
Crosses
indicate
346
G. H. J O N K E R A N D J. H. VAN S A N T E N T A B L E VI
Composition
% Mn,+
Is at 90°K
gauss/gram (La0.60Sr0.30Ba0.10) MnO3 ( La0.70 Pb0.30) MnO 3 (La0.60Pb0.40)MnO3 ( La0.70Cd0.30) MnO3 ((Pr, Nd)0.75Sr0.25)MnO 3 ((Pr, Nd)0.70Sr0.30)MnO 3 ((Pr, Nd)0.60Sr0.40)MnO 3 ( La0.45 (Pr, Nd)0" 15Ba0.40) MnO3 (La0.15(Pr, Nd)0.45Ba0.40)MnO3 ((Pr, Nd)0.60Ba0.40)IVlnO , (La0.30(Pr, Nd)0.30Sr0.40)MnO3 (La0.15(Pr, Nd)0.45Sr0.40)MnO3
m
21.4 45.0 29.8 29.5 32.0 44.0
39.0 41.5
85 61 76 81.5 94 91.5 81 73 80 78 91 87
magnetic Curie moment temperature fromCalculatedIs lab °K 3.46 2.88 3.68 3.40 3.92 3.77 3.26 3.17 3.50 3.42 3.61 3.50
358 361 337 326 201 263 315 275 215 188 341 327
this value For series of mixed crystals containing more than 40% Mn 4+ characteristic deviations occur. In the case of LaMnO 3 SrMnO 3 containing 60-700/0 SrMnO 3 abnormal I s - - T curves have
it" t
~ ~_
~0 ~20 ~
~
~0 ~ 0 2 ~ 2,~ ~ 0 280 ,~0*g ~T
F i g . 11. S a t u r a t i o n m a g n e t i z a t i o n I s a s a f u n c t i o n of t e m p e r a t u r e for m i x e d c r y s t a l s (Lao.40 S r 0 6 o ) M n O 3.
been measured (fig. 11): at 90°K low values of I s are found; the Curie temperatures are difficult to determine but they seem to be relatively high.
5. Discussion o/magnetic properlies. When discussing the magnetic properties we shall consider the manganites as heteropolar compounds containing besides ions with rare-gas configurations, Mn3+ and Mn4+ ions with respectively 3 # and 3d 3 configurations. From the theoretical point of view the manganites investigated are relatively easy to discuss, the manganese ions being situated at the points of a simple nearly-cubic lattice. Certain complications, however, arise from the fact that we never have to do with only Mn 3+ or only Mn4+ ions but always with mixtures of the two. Moreover the high electrical conductivity *) suggests that it might *) The electrical conductivity is of tile order of 10~.Q -I cm-X; details about the conductive properties will be given in a following paper
FERROMAGNETIC
COMPOUNDS
OF MANGANESE
347
be a poor approximation to speak in terms of Mn 3+ and Mn 4+ ions instead of using a zone picture. 5.1. T h e spin-only character of the saturat i o n m a g n e t i z a t i o n. As was stated already in section 4.2, for compositions containing from 25 to 35~/o Mn4+ the saturation magnetization can be calculated assuming that all 3d electrons contribute with their spins. The possibility of any appreciable crbit contribution to the saturation is ruled out by measurements of the gyromagnetic ratio. Mr. H. G. B e l i e r s of this laboratory carried out measurements on gyromagnetic resonance of (Lao.65Sr0.35) MnO 3 and found the g-factors to be 1.99 and 2.05 respectively, i.e. within the limits of experimental accuracy the spin-only values *). For other compounds containing trivalent or tetravalent manganese octahedrically surrounded by six oxygens, paramagnetic spin-only moments have been observed 8), while theoretically such moments have been explained by V a n V l e c k ~ ) and by S i e g e r t S ) . 5.2 T h e v a l u e s of the Curie temperature and the saturation magnetization. In view of the high values of the Curie temperatures the interaction between the manganese ions must be of the exchange type. S 1a t e r 9) has correlated positive exchange interaction, favouring situations with parallel spins, with large distance between the magnetic atoms, the interaction being negative for small interatomic distances. S 1 a t e r's ideas have been refined by S t o n e r 1 ° ) and b y N6e111). F o r a number of alloys N 6 e 1 showed that the variation of the exchange interaction with interatomic distance d could be represented by a single "Slater curve" if the interaction were plotted as a function of d - - ~ or, following S t o n e r lo), as a function of d/~, where denotes the diameter of the electronic orbitals responsible for the exchange interaction. Experimentally it has been found that such a general S 1 a t e r curve cannot be given for non-metallic compounds 12). Moreover, even for the same magnetic ion in different lattices exchange interactions are at variance with S 1 a t e r's curve is). The exchange interaction in non-metallic substances has been dis-
*) The m e t h o d a p p l i e d here b y Mr H. G. B e 1 j e r s was different from t h a t used e a r l i e r ~s) because of c o m p l i c a t i o n s a r i s i n g from the s k i n effect of these r e l a t i v e l y well c o n d u c t i n g s a m p l e s ; i t will be d e s c r i b e d in a p a p e r to be p u b l i s h e d s h o r t l y .
348
G. H. JONKER AND J. H. VAN SANTEN
cussed theoretically by K r a m e r s 14). It was pointed out that in addition to direct exchange interaction between magnetic ions also an indirect exchange through the medium of an interjacent diamagnetic ion or atom m a y play a part. It is obvious that this indirect exchange interaction will depend on the properties of the lattice. In the case of manganites, where the magnetic ions are separated from each other by oxygen ions situated just between them, a direct exchange would be difficult to understand. In the manganites three exchange interactions are to be distinguished: Mn 3+ - - M n 3+, Mn3+ - - M n 4+, Mn 4÷ - - M n 4+, each having their own characteristic dependency on the properties of the lattice, e.g. on interionic distance. I n v i e w of the similarity of the I s versus composition and 0 versus composition curves of the various binary systems L a 3 + M n 3 + 0 3 - Me2+Mn4+Q (figs 5-10), the three exchange interactions can be considered to a first approximation as being constant quantities. From the very low value of the Curie temperature of LaMnO 3 it can be concluded that the Mn3+ - - Mn 3+ interaction is approximately zero. The strong increase of the saturation and the Curie temperature for increasing concentrations of Mn 4 ~ must be ascribed to a strong positive Mn 3+ - - M n 4+ interaction; the sharp decrease of I s versus composition for higher Mn 4+ concentrations and the asymmetrical position of the maximum can be explained by assuming a negative Mn 4 + - Mn4+ interaction. Such a negative interaction is confirmed by the abnormal shape of the I s - - T curves in the systems (Lao.40 Sro.eo)MnO3 and (Lao.30 Sro.70) M n Q (Cf. fig. l 1), which most interestingly corresponds to a type predicted by N 6 e 1 in his 1948 paper ~2). As far as we know the case of the manganites investigated is the first example of a positive indirect exchange interaction. If we consider a fixed Mn3+/Mn4+ ratio all changes in the Curie temperature must be explained by changes of the three exchange interactions, thus by effects that have been ignored in our first approximation. In order to investigate these effects we prepared a number of mixed crystals all containing 40% Mn 4÷. If the situation were as simple as for the metals investigated by N 6 e 1 11) (see gection 5.2), the Curie temperature would be uniquely determined by the lattice constant alone. From fig. 12 it is easily seen, however, that such a simple relation does not exist: different samples with the same lattice constant have different Curie temperatures.
FERROMAGNETIC
COMPOUNDS
OF MANGANESE
349
The ferromagnetic properties of crystals "K 380 where different exchange interactions play a r61e can be treated by means of the molec- 340 ular field method applied by N 6 e 1 11)12) ~0 01' to ferromagnetic metals containing small ~00 amounts of other metal atoms at equivalent 38Q A/[ It1 lattice positions and to ferrites with spinel ~G structure containing magnetic ions distributed over non-equivalent lattice points. For certain composition ranges the ferromagnetic properties of manganites can be understood with the aid of N 6 e l ' s method; in the transition region between ferromagnetism and antiferromagnetism, however, difficul- F i g . 12. C u r i e t e m p e r a ties arise. Moreover, complications are caused t u r e 0 of v a r i o u s m i x e d by the fact that changes in the geometrical crystals containing 40% distribution of the Mn3+ and Mn 4+ ions,which tetravalent manganese. would affect I s and, particularly 0, can be easily brought about since only electron transfer is required. Eindhoven, January 24th, 1950. Received 3 0 - 1 - 5 0
REFERENCES 1) G . H .
Jonker
and J.H.
van
Santen,
Chem. W e e k b l a d 43, 672-679, 1947;
Science 109, 632-635, 1949. 2) P. P a s c a 1, Trait6 de chimie min6rale IX, Paris, 1933 pp. 591-593. 3) V. M. G o 1 d s c h m i d t, Geochemische Verteilungsgesetze der Elemente, V I I , V I I I , 1927-1928. 4) O. H a s s e 1, Kristallchemie, Dresden-Leipzig, 1934. 5) G . N . R a t h e n a u and J . L . S n o e k , P h i l i p s Res. Rep. l , 239, 1946. 6) S.S. B h a t n a g a r , P.I.. Kapur and B r a h m Prakash, CurrentSci. 8, 253-254, 1939. B . H . S c h u 1 t z, Thesis, Leiden, 1940, especially pp. 68-72. 7) J . H . V a n V l e c k , P h y s . Rev. 41, 2 0 8 - 2 1 5 , 1 9 3 2 . 8) A. S i e g e r t, P h y s i c a 3, 85-90, 1936; ibid. 4, 138-152, 1937. 9) J . C . S l a t e r , P h y s . Rev. 36, 57-64, 1930. 10) C. S t o n e r , Phil. Mag. 15, 1018-1034, 1932. 11) L. N 6 e 1, Ann. P h y s i q u e , 5, 232-279, 1936. 12) L. N 6 e 1, Ann. P h y s i q u e , 3, 137-198, 1948. 1 3 ) . C . F . S q u i r e, Phys. Rev. 56, 922-925, 1937. 14) H . A . K r a m e r s , Physica I, 182-192, 1933. 15) H . G . 13 e 1 j e r s, P h y s i e a 1 4 , 6 2 9 - 6 4 1 , 194~..
M a a r t 1950
FERROMAGNETIC COMPOUNDS OF MANGANESE WITH PEROVSKITE STRUCTURE by G. H. J O N K E R and J. H. VAN SANTEN Philips Research Laboratories, N.V. Philips' Gloeilampenfabrieken Eindhoven - Netherlands
Summary V a r i o u s m a n g a n i t e s of t h e g e n e r a l f o r m u l a L a 3 + M n 3÷ 0 2 - - M e 2 + M n 4 + O 3 2 h a v e b e e n p r e p a r e d in t h e f o r m of p o l y c r y s t a l l i n e p r o d u c t s . P e r o v s k i t e s t r u c t u r e s w e r e f o u n d , i.a. for all m i x e d c r y s t a l s L a M n O 3 - C a M n O 3, for L a M n O 3 - - S r M n O 3 c o n t a i n i n g up t o 70~/o SrMnO3, a n d for L a M n O 3 - B a M n O 3 c o n t a i n i n g less t h a n 50~/o B a M n O 3. T h e m i x e d c r y s t a l s w i t h pero v s k i t e s t r u c t u r e are f e r r o m a g n e t i c . C u r v e s for t h e Curie t e m p e r a t u r e versus c o m p o s i t i o n a n d s a t u r a t i o n v e r s u s c o m p o s i t i o n are g i v e n for L a M n O 3 - C a M n O 3, L a M n O 3 - S r M n O 3, a n d L a M n O 3 - B a M n O 3. B o t h t y p e s of c u r v e s s h o w r 0 a x i m a b e t w e e n 25 a n d 4 0 % Me2+Mn4+O2--; h e r e all 3d electrons available contribute with their spins to the saturation magnetiza t i o n . T h e f e r r o m a g n e t i c p r o p e r t i e s c a n be u n d e r s t o o d as t h e r e s u l t of a s t r o n g p o s i t i v e Mn 3+ - - M n 4+ e x c h a n g e i n t e r a c t i o n c o m b i n e d w i t h a weak Mn 3 + M n 3+ i n t e r a c t i o n a n d a n e g a t i v e M n 4 + Mn 4+ i n t e r a c t i o n . T h e M n 3+ - - M n 4+ i n t e r a c t i o n , p r e s u m a b l y of t h e i n d i r e c t e x c h a n g e t y p e , is t h o u g h t t o be t h e first clear e x a m p l e of p o s i t i v e e x c h a n g e i n t e r a c t i o n in oxidic s u b s t a n c e s .
1. Introduclion. During our investigations 1) into the occurrence of the perovskite structure we prepared i.a. compounds of the general formula A3+B3÷02- . One of these, LaMnO 3, showed ferromagnetic properties at liquid-air temperature, whereas LaCrO 3 and LaFeO 3 did not. It appeared, however, that LaMnO 3 was ferromagnetic at this temperature only when it contained some manganese of a valency higher than three ; by a suitable thermal treatment in an oxygen atmosphere, the substance took up more oxygen and Curie temperatures up to 210°K were found. A similar increase of valency of Mn was realized by preparing mixed crystals Laa+Mn3+02 - Me2+Mn4+02- (Me2+ = large divalent ion). An - -
Physica XVI
3 3 7
- -
22
338
G.H.
J O N K E R A N D J. H. VAN S A N T E N
investigation was therefore made with the binary systems LaMnO 3 CaMnOa, LaMnO a - - SrMnOa, LaMnO 3 - - BaMn03, LaMnO 3 - CdMnOa, LaMnO a - PbMnO a. In a number of experiments La was replaced by trivalent rare-earth metals. It appeared that in all these systems, at least over a certain range of compositions, ferromagnetic mixed crystals were formed. In this paper we shall discuss the preparation of these compounds, and their crystallographic and ferromagnetic properties *). In a later one their electric conductivity will be considered. For the sake of simplicity the compounds containing trivalent as well as those containing tetravalent manganese will be designated as " m a n ganites" **). -
-
2. Chemicalpad. 2.1. P r e p a r a t i o n
of manganites. The manganites were prepared using standard ceramic technique. Oxides or carbonates of the metals were weighed in the desired proportions and milled for half an hour in water or ethanol to mix them thoroughly; afterwards the preparations were dried and pre-fired in air at about 1000°C. At this temperature an appreciable conversion already takes place. The product obtained was milled and dried a g a i n mixed with a binder and finally for~ned into discs or rods. The samples were fired again for some hours in an electric furnace at temperatures between 1350 and 1450°C. In most cases air was passed through the furnace but for certain compositions nitrogen or oxygen was used. It did not make any difference whether we prepared manganites starting from MnO 2 or from MnCOa. At the reaction temperature the valency of manganese adjusted itself to the composition of the sample and to the atmosphere. Some samples were prepared by a wet process. Solutions of nitrates of the metals were mixed in the desired proportions and from this mixture the metals were co-precipitated by adding a solution of ammonia, ammonium carbonate and hydrogen peroxide, or a solution of sodium carbonate. These preparations show a higher *) The authors wish to t h a n k Mr. M. B a n n i n k for his valuable e x p e r i m e n t a l assistance. **) Strictly speaking only the compounds Me2+Mn4+Oa c o n t a i n i n g t e t r a v a l e n t Mn should be called manganites 2), whereas the compound La3+Mn3+O~ c ont a i ni ng trivalent Mn might be called " h y p o m a n g a n i t e " . Furthermore one mi ght be averse to using the name man ganite since this would suggest t h a t these compounds were " s a l t s " r a t h e r th an double oxides.
F E R R O M A G N E T I C COMPOUNDS OF MANGANESE
339
reactivity, so that at lower firing temperatures the conversion is more complete. The properties of the samples prepared in this manner were not different from those obtained by the dry method. By the ceramic method just described polycrystalline products are obtained. We also tried to prepare single crystals from solutions in molten salts, but this has so far only been successful in the case of SrMnO a. Since, however, this compound does not crystallize in the perovskite lattice, the results will not be discussed here. 2.2. D e t e r m i n a t i o n of the valency of mang a n e s e. The physical properties of the samples are closely related to the valency of manganese. Therefore this quantity was determined for all samples, applying a method commonly used for manganese dioxide. About 50 mg of the substance to be examined is dissolved in hydrochloric acid under gradual heating. With the aid of a current of nitrogen the chlorine evolved is passed through a solution of potassium iodide and the iodine liberated is titrated with a solution of sodium thiosulfate. It was expected that LaMnO 3 would contain trivalent manganese but it appeared that this substance had a pronounced tendency to take up more oxygen, part of the manganese assuming a valency higher than three, presumably four. In table I the results of our titrations are given for a number of LaMnO a samples fired at different temperatures in air. By firing LaMnO a at 1400°C in an TABLE I atmosphere of nitrogen containing firing % tetravalent about 1% of oxygen a preparation temperature °C Mn containing only 3% Mn 4+ was ob1000 20.5 tained. The tendency to take up an 1150 25 excess of oxygen might be an indi1250 21 1350 14 cation of a region of mixed crystals extending from LaMnO a towards La2/3MnO3. The latter substance does not actually exist (at least not at high temperatures) but the corresponding titanate La2/3Ti 0 a is known and has perovskite structure. It could be expected that by replacing lanthanum by alkaline earth metals the valency of manganese could be appreciably increased even for samples that are fired at high temperatures. This proved in fact to be easy, e.g., for mixtures of LaMnO a and SrMnO a. When preparing samples containing more than 40% Mn 4÷ , however,
340
G. H. J O N K E R
A N D J . H. V A N S A N T E N
it was found necessary to fire in oxygen. Table n gives the Mn 4+ content of a n u m b e r of LaMnO 3 SrMnO 3 compositions fired in air at 1350°C. F r o m this it follows TABLE II t h a t for mixtures containing about LaMnO3 - - SrMnO3 % Mn *+ t7 35% Sr, firing in air is the right 100-0 90-10 19 w a y of preparation. In the case of 80-20 23 other mixtures a suitable atmos70-30 32 phere must be chosen. Bv firing in 60-40 38 42 pure oxygen it is possible to obtain 50-50 40-60 so a SrMnO 3 preparation containing 30-70 62 960/o tetravalent manganese. Simi0-100 85 lar results were obtained for the b i n a r y systems LaMnO 3 - - C a M n Q , LaMnO 3 BaMn03, LaMnO 3 CdMnO 3, and LaMnO 3 - - PbMnO 3. -
-
-
-
-
-
3. Crystallographic considerations.
3. I. T h e
p e r o v s k i t e
structure of manganites. LaMnO 3 crystallizes in the perovskite structure, so called after the mineral perovskite, CaTiO 3. In fig. 1 the unit cell of a perovskite A B O 3 is given. Here A denotes a large ion like Ca 2+, Sr 2+, Ba 2÷, Cd 2+, Pb 2+, La3+, pr3+, Nd3+, Gd3+, y3+, while B stands f o r a small ion like A13+, Cr 3+, Mn 3+, Fe 3+, Ti 4+ , Mn 4+ . The A ions are situated at the corners of the unit cell, the B ions occupying the centre of the cube while the oxygen ions are placed at the centres of the faces. •A It was pointed out b y G 0 1 d s c h m i d t 3) 00 t h a t the perovskite structure is stable only if the Fig. 1. Unit cell of parameter t, defined b y t = ( r a + ro)/(rB+ro).V/2, perovskite. (ra, rB, r o denote respectively the radii of ions A, B, O) a p p r o x i m a t e l y equals unity. A purely cubic structure is found if it is equal to u n i t y (SrTi03). If t is slightly different from u n i t y the perovskite structure is slightly distorted (CaTi03, BaTi03) and for large deviations from u n i t y completely different crystal structures are found (e.g. ilmenite for t < 1, calcite and aragonite for t > 1). W h e n calculating the value of the p a r a m e t e r t we must know the radii of the ions for the special co-ordination t h e y possess in the perovskite structure 4). Since we considered only related compounds •
B
FERROMAGNETIC
341
COMPOUNDS OF MANGANESE
we simply used non-corrected G o 1 d s c h m i d t radii knowing that our values of t should be multiplied by a factor that is approximately equal for all compounds considered here. The ionic radii used are given in table III. TABLE III Ca z+ Sr"-+ Ba 2+ Cd 2+ P b "+
1.06 A. 1.27 1.43 1.03 1.32
La s+ Pr s+ Nd s+ Sm s+ Gd s+ Y~+
1.22 A 1.16 1.15 1.13 1.11 1.06
Mn ~+ Mn *+
0.70 A 0.52
02-
1.32A
The values of t calculated from these radii are given in table IV. T A B L E IV *LaMnO.~ t = 0.89 *(Pr, Nd)MnO~ 0.86 s SmMnO 3 0.86 *GdMnOs 0.85 YMnOs 0.83
*CaMnO s t = 0.915 SrMnO s 0.995 BaMnO s 1.055 CdMnOs 0.90~ PbMnO~ 1.015
Here the compounds showing perovskite structure are marked with an asterisk. The parameter t has also been calculated for mixed crystals taking for r A and r e the arithmetical ".0~ ~.~ tt /8. average of the radii of the ions occupying , / respectively the positions A and B. The results are represented in fig. 2, where t has been plotted for all compounds and binary systems investigated. Compounds and mixed crystals showing perovskite structure are indicated by O. Various series have been investigated in detail and perovskite structures were found to occur for values of t up to t = 0.965. For mixtures with t >0.965 products were obtainO4 ed containing a second phase with a different , ~n~o 4o 60 8o N o , ~ crystal structure. The lower t-limit of the Fig. 2. perovskite region was not determined. The Values of parameter t lowest value, t = 0.85, was obtained in the for various manganites. case of GdMnO 3. It is interesting to note that 0 indicates compounds YMnO a and SrMnO 3, which do not crystallize and mixed crystals with perovskite structure. in the perovskite structure, form mixed crystals, e.g. (Y0.4oSr0.6o) MnO 3, with perovskite structure.
rl--.
342
G. H. J O N K E R AND J. H. VAN S A N T E N
In general the perovskites considered here are not perfectly cubic. Only some compositions, e.g. (Lao.ssBa0.4s) MnO3and (Lao.ssSro.4s)MnO 3 are probably cubic. In tbe TABLE V series LaMnO a - - SrMnO a average edge of unit cell % Mn ~+ mixed crystals containing A less than 40% or more LaMnOa I 3.90 s.9 than 50% Sr are not cubic. (Pr, Nd)MnOz 3.85 6.4 It is possible that for the GdMnO 3 3.82 CaMnOa 3.73 8o o perovskite co-ordination LaA1Oz. 3.78 the ionic radii have to be LaCrOa 3.88 corrected in such a way LaFeO a 3.89 that t is exactly equal to unity for mixed crystals showing cubic structures. In table V the average cell dimensions are given for a number of perovskites.
4. Magnetic measurements. The saturation magnetization has been measured for all samples as a function of temperature. From these data the saturation magnetization at absolute zero, i.e. the number of B o h r magnetons per molecule contributing to ferromagnetism, and the Curie temperature can be derived. 4.1. E x p e r i m e n t a l technique. For the determination of the saturation magnetization I s we used an apparatus constructed and described b y R a t h e n a u and S n o e k S ) . A weighed sample of the ferromagnetic material is placed in a pendulum moving horizontally between the pole shoes of an electromagnet and in a direction perpendicular to the line connecting them. The pole shoes are shaped in such a way that the value of the magnetic field strength perpendicular to the direction of motion of the pendulum is given b y H - - - - H 0 - ½ax2, where H 0 denotes the field strength at the centre between the pole shoes, x stands for the distance between the pendulum and the centre, and a is a constant. Provided the sample is magnetically saturated all the time the magnetic force acting on the pendulum is proportional to the distance of the sample from the equilibrium position; the same holds for the gravitational force. Consequently, b y inserting the sample, the harmonicity of the pendulum is not disturbed. The apparatus was calibrated for different field strengths with samples of pure nickel. Thus b y measuring the period of oscillation
FERROMAGNETIC
COMPOUNDS OF MANGANESE
343
both for the e m p t y pendulum and for the pendulum containing the sample, it was possible to calculate the saturation magnetization. The pole pieces were spaced far enough apart to allow of heating or cooling devices to be placed round the samples. The maximum field obtainable was about 6000 Orsted, amply sufficient to saturate the samples also at the lowest temperatures, as was verified by measuring B - - H curves between 90°K and 370°K. 4.2. E x p e r i m e n t a l results. Ir~ fig. 3 the measured values of the saturation magnetization are given as a function of temperature foi two g,~ preparations consisting of mixed crystals of LaMnO a = -''--2.,_ and SrMnO a. In general \ at 90°K the curves are so \ flat that extrapolation \ towards 0°K is easy. The corrections arising from the extrapolation appear ~o to be smaller than the ~T experimental error and the scattering between Fig. 3. Saturation magnetization I s as a samples of the same com- function of temperature T for mixed crystals (La0.90 Sr0.10)MnO3 (I) and position. Therefore the (Lao.70 Sr0.ao)MnOa (II). values of I~ were not extrapolated. For samples with low Curie temperatures, e.g. for LaMnO3, extrapolation becomes very uncertain; therefore here also extrapolation was omitted. A Curie temperature 0 was derived from the I , - - T curves by determining the intersection of the T-axis by the tangent in the inflexion point. For series of mixed crystals of manganites of trivalent and divalent metals it appears that in general the Curie temperature increases with the Me2+ content for Me2+ fractions up to 25-40% and decreases for higher ones. The increase of the Curie temperature is closely connected with the Mn 4+ concentration since the Curie temperature of a given sample can also be raised by increasing the oxygen content. This effect is very pronounced for LaMnO 3 and for mixed crystals with only small amounts of Mea+. For such compositions it was difficult to find the Curie temperature
344
G.H. JONKER AND J. H. VAN SANTEN
of "ideal" preparations showing the correct stoichiometric oxygen content. The Curie temperatures of the ideal preparations were determined b y interpolation or extrapolation, plotting for each composition the values of the ,x I Curie temperatures against the oxygen content. An example is given in fig. 4 for the composition (La0.9o Sr0.10) MnQ. In this w a y we determined the Curie temperatures for a number of ideal preparations of the systems L a M n Q - - C a M n Q I 5 fO ~J 30 35 30 40 (fig. 5), LaMnO 3 - - S r M n O 3 (fig. 7), LaMnQBaMnO 3 (fig. 9). Moreover Fig. 4. Determination of the in table VI the Curie temperatures of Curie temperature 0 and of the saturation magnetization some other mixed crystals are given; I s of the ideal preparation these values, however, have not been (Lao.90 Sr0A0)MnO3 • corrected for deviations from the stoichiometric oxygen content. In the same way as discussed for the Curie temperatures, the saturation magnetization at 90°K was corrected for non-stoichiometI
/!
~/,/n
4•
130
,o fl
/'xl
.j _ _ . I
\
~0
/
.c
!\i
o/
/
f~
./ 3~
°
8O 0
tO
20
3~
40
~%
50
t
60 ,tO
ean~os
Fig. 5. Curie temperature 0 of mixed crystals (La, Ca)MnO 3. Crosses indicate ideal preparations.
10
30
30
40
50
60
70
80
90 $00
Fig. 6. Saturation magnetization I s at 90°K of mixed crystals (La, Ca)MnO 3. Crosses 'indicate ideal preparations.
ric oxygen content. The results are given in figs. 6,8, I0 and in table VI. The same graphs give values of I s calculated on the assumption
FERROMAGNETIC
COMPOUNDS
OF MANGANESE
345
t h a t all 3d electrons present in the m a n g a n e s e ions c o n t r i b u t e with their spins to the s a t u r a t i o n magnetization. F o r all mixed crystals c o n t a i n i n g from 25 to 35°/o Mn 4+ it appears t h a t the s a t u r a t i o n reaches "K 38( 3~
/
34(
i"
/
30O
~0
,
11
.I
--
J 2z..
I
IN0
f~
[
,~ --
[
~
20 30 40 50 60 ,~ 80 90 ~0 ~
Fig. 7. Curie temperature 0 of mixed crystals (La, Sr)MnO 3. Crosses indicate ideal preparations.
% .~.M,.,o3
FAg. 8. Saturation magnetization I s at 90°K of mixed crystals (La, Sr)MnO 3. Crosses indicate ideal preparations.
"k' 3~ 34C 3,~ 30~
fO0 26C
a: ~
~,.~..
- -
f8~
f4(
20
[
20ha~cea
tX 0
tO 20
3040
.50
6O7O
60
tO
90
20
30
405O
60yo
80
gO fO0
• %ea~no3
Fig. 9. Curie temperature 0 of mixed crystals (La, Ba)MnO a. Crosses indicate ideal preparations.
F i g . 10. S a t u r a t i o n I s
at
(La, ideal
90°K Ba)
of
M n O 3.
preparations.
magnetization mixed
crystals
Crosses
indicate
346
G. H. J O N K E R A N D J. H. VAN S A N T E N T A B L E VI
Composition
% Mn,+
Is at 90°K
gauss/gram (La0.60Sr0.30Ba0.10) MnO3 ( La0.70 Pb0.30) MnO 3 (La0.60Pb0.40)MnO3 ( La0.70Cd0.30) MnO3 ((Pr, Nd)0.75Sr0.25)MnO 3 ((Pr, Nd)0.70Sr0.30)MnO 3 ((Pr, Nd)0.60Sr0.40)MnO 3 ( La0.45 (Pr, Nd)0" 15Ba0.40) MnO3 (La0.15(Pr, Nd)0.45Ba0.40)MnO3 ((Pr, Nd)0.60Ba0.40)IVlnO , (La0.30(Pr, Nd)0.30Sr0.40)MnO3 (La0.15(Pr, Nd)0.45Sr0.40)MnO3
m
21.4 45.0 29.8 29.5 32.0 44.0
39.0 41.5
85 61 76 81.5 94 91.5 81 73 80 78 91 87
magnetic Curie moment temperature fromCalculatedIs lab °K 3.46 2.88 3.68 3.40 3.92 3.77 3.26 3.17 3.50 3.42 3.61 3.50
358 361 337 326 201 263 315 275 215 188 341 327
this value For series of mixed crystals containing more than 40% Mn 4+ characteristic deviations occur. In the case of LaMnO 3 SrMnO 3 containing 60-700/0 SrMnO 3 abnormal I s - - T curves have
it" t
~ ~_
~0 ~20 ~
~
~0 ~ 0 2 ~ 2,~ ~ 0 280 ,~0*g ~T
F i g . 11. S a t u r a t i o n m a g n e t i z a t i o n I s a s a f u n c t i o n of t e m p e r a t u r e for m i x e d c r y s t a l s (Lao.40 S r 0 6 o ) M n O 3.
been measured (fig. 11): at 90°K low values of I s are found; the Curie temperatures are difficult to determine but they seem to be relatively high.
5. Discussion o/magnetic properlies. When discussing the magnetic properties we shall consider the manganites as heteropolar compounds containing besides ions with rare-gas configurations, Mn3+ and Mn4+ ions with respectively 3 # and 3d 3 configurations. From the theoretical point of view the manganites investigated are relatively easy to discuss, the manganese ions being situated at the points of a simple nearly-cubic lattice. Certain complications, however, arise from the fact that we never have to do with only Mn 3+ or only Mn4+ ions but always with mixtures of the two. Moreover the high electrical conductivity *) suggests that it might *) The electrical conductivity is of tile order of 10~.Q -I cm-X; details about the conductive properties will be given in a following paper
FERROMAGNETIC
COMPOUNDS
OF MANGANESE
347
be a poor approximation to speak in terms of Mn 3+ and Mn 4+ ions instead of using a zone picture. 5.1. T h e spin-only character of the saturat i o n m a g n e t i z a t i o n. As was stated already in section 4.2, for compositions containing from 25 to 35~/o Mn4+ the saturation magnetization can be calculated assuming that all 3d electrons contribute with their spins. The possibility of any appreciable crbit contribution to the saturation is ruled out by measurements of the gyromagnetic ratio. Mr. H. G. B e l i e r s of this laboratory carried out measurements on gyromagnetic resonance of (Lao.65Sr0.35) MnO 3 and found the g-factors to be 1.99 and 2.05 respectively, i.e. within the limits of experimental accuracy the spin-only values *). For other compounds containing trivalent or tetravalent manganese octahedrically surrounded by six oxygens, paramagnetic spin-only moments have been observed 8), while theoretically such moments have been explained by V a n V l e c k ~ ) and by S i e g e r t S ) . 5.2 T h e v a l u e s of the Curie temperature and the saturation magnetization. In view of the high values of the Curie temperatures the interaction between the manganese ions must be of the exchange type. S 1a t e r 9) has correlated positive exchange interaction, favouring situations with parallel spins, with large distance between the magnetic atoms, the interaction being negative for small interatomic distances. S 1 a t e r's ideas have been refined by S t o n e r 1 ° ) and b y N6e111). F o r a number of alloys N 6 e 1 showed that the variation of the exchange interaction with interatomic distance d could be represented by a single "Slater curve" if the interaction were plotted as a function of d - - ~ or, following S t o n e r lo), as a function of d/~, where denotes the diameter of the electronic orbitals responsible for the exchange interaction. Experimentally it has been found that such a general S 1 a t e r curve cannot be given for non-metallic compounds 12). Moreover, even for the same magnetic ion in different lattices exchange interactions are at variance with S 1 a t e r's curve is). The exchange interaction in non-metallic substances has been dis-
*) The m e t h o d a p p l i e d here b y Mr H. G. B e 1 j e r s was different from t h a t used e a r l i e r ~s) because of c o m p l i c a t i o n s a r i s i n g from the s k i n effect of these r e l a t i v e l y well c o n d u c t i n g s a m p l e s ; i t will be d e s c r i b e d in a p a p e r to be p u b l i s h e d s h o r t l y .
348
G. H. JONKER AND J. H. VAN SANTEN
cussed theoretically by K r a m e r s 14). It was pointed out that in addition to direct exchange interaction between magnetic ions also an indirect exchange through the medium of an interjacent diamagnetic ion or atom m a y play a part. It is obvious that this indirect exchange interaction will depend on the properties of the lattice. In the case of manganites, where the magnetic ions are separated from each other by oxygen ions situated just between them, a direct exchange would be difficult to understand. In the manganites three exchange interactions are to be distinguished: Mn 3+ - - M n 3+, Mn3+ - - M n 4+, Mn 4÷ - - M n 4+, each having their own characteristic dependency on the properties of the lattice, e.g. on interionic distance. I n v i e w of the similarity of the I s versus composition and 0 versus composition curves of the various binary systems L a 3 + M n 3 + 0 3 - Me2+Mn4+Q (figs 5-10), the three exchange interactions can be considered to a first approximation as being constant quantities. From the very low value of the Curie temperature of LaMnO 3 it can be concluded that the Mn3+ - - Mn 3+ interaction is approximately zero. The strong increase of the saturation and the Curie temperature for increasing concentrations of Mn 4 ~ must be ascribed to a strong positive Mn 3+ - - M n 4+ interaction; the sharp decrease of I s versus composition for higher Mn 4+ concentrations and the asymmetrical position of the maximum can be explained by assuming a negative Mn 4 + - Mn4+ interaction. Such a negative interaction is confirmed by the abnormal shape of the I s - - T curves in the systems (Lao.40 Sro.eo)MnO3 and (Lao.30 Sro.70) M n Q (Cf. fig. l 1), which most interestingly corresponds to a type predicted by N 6 e 1 in his 1948 paper ~2). As far as we know the case of the manganites investigated is the first example of a positive indirect exchange interaction. If we consider a fixed Mn3+/Mn4+ ratio all changes in the Curie temperature must be explained by changes of the three exchange interactions, thus by effects that have been ignored in our first approximation. In order to investigate these effects we prepared a number of mixed crystals all containing 40% Mn 4÷. If the situation were as simple as for the metals investigated by N 6 e 1 11) (see gection 5.2), the Curie temperature would be uniquely determined by the lattice constant alone. From fig. 12 it is easily seen, however, that such a simple relation does not exist: different samples with the same lattice constant have different Curie temperatures.
FERROMAGNETIC
COMPOUNDS
OF MANGANESE
349
The ferromagnetic properties of crystals "K 380 where different exchange interactions play a r61e can be treated by means of the molec- 340 ular field method applied by N 6 e 1 11)12) ~0 01' to ferromagnetic metals containing small ~00 amounts of other metal atoms at equivalent 38Q A/[ It1 lattice positions and to ferrites with spinel ~G structure containing magnetic ions distributed over non-equivalent lattice points. For certain composition ranges the ferromagnetic properties of manganites can be understood with the aid of N 6 e l ' s method; in the transition region between ferromagnetism and antiferromagnetism, however, difficul- F i g . 12. C u r i e t e m p e r a ties arise. Moreover, complications are caused t u r e 0 of v a r i o u s m i x e d by the fact that changes in the geometrical crystals containing 40% distribution of the Mn3+ and Mn 4+ ions,which tetravalent manganese. would affect I s and, particularly 0, can be easily brought about since only electron transfer is required. Eindhoven, January 24th, 1950. Received 3 0 - 1 - 5 0
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