MAGNETIC BEHAVIOUR OF Ru IN BiCaVIG CRYSTALS

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MAGNETIC BEHAVIOUR OF Ru IN BiCaVIG

CRYSTALS

R. Krishnan

To cite this version:

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JOURNAL DE PHYSIQUE Colloque C1, suppliment au no 4, Tome 38, At;ril 1977, page Cl-183

MAGNETIC BEHAVIOUR OF Ru IN

BiCaVIG

CRYSTALS

R. KRISHNAN

Laboratoire de Magnetisme, C. N. R. S. 1, Place Aristide-Briand 92190 Bellevue, France

Rbumk. - Nous avons Ctudie I'anisotropie magnetique dans les grenats

Bio,3Ca2,7Fe3,65V1,35012

dopes au ruthenium. Le K I reste negatif et sa valeur augmente avec la teneur en Ru et lorsque la temperature de mesure diminue. Le K2 est relativement faible et positif, quoique on puisse dkeler une contribution negative 2i T < 100 K. Le K1 < 0 et K2 > 0 sont attribues 2i I'ion Ru4' dans le site octaedrique dont la distorsion trigonale est consider& comme Btant positive. Enfin, la contribution negative a K2 pourrait venir de l'ion Ru3+ dans le site tetraedrique.

Abstract. - The magnetic anisotropy of B ~ o . 3Ca2.7Fe3.6gV1.35012 : RU crystals is invesitgated by ferromagnetic resonance techniques in the X-band. The K I is negative in all the samples and its value increases with Ru content and as the temperature decreases. The K2 is much smaller than KI and is positive. However below 100 K, a negative contribution to K2 is appearing. The above results are discussed in the light of data available today in the litterature and are explained interms of Ru4+ in the octahedral site with a positive trigonal distortion. The negative contribution to K Z

at low temperatures could be attributed to Ru3+ in the tetrahedral site.

1. Introduction. - The metal ions of the 4d and 5d transition series are quite interesting both from the fundamental and application points of view as they can show very anisotropic magnetic behaviour due to their large spin orbit coupling.

Investigations of the magnetic properties of 4d and 5d ions in ferrimagnetic compounds have started recently. The magnetic spinels and garnets are quite suitable for studying magnetic ions like RU,',

Ru4+(4dn) and Ir4+(5dS). For instance strong contributions from Ru and Ir to magnetic anisotropy (K, K2) in Y,FesO12(YIG) and NiFe20, have been experimentally shown and also theoretically discussed [I-51. Considering the lowest levels of octahedral Ru3+ and Ir4+ which are in the low spin state, Hansen successfully explained the observed K, and K2 in YIG. However when substituted in NiFe204 the contribution from Ru to K, changes sign with respect to what is observed in YIG. This would indicate the possibility of contributions to K, from other sources like Ru4+ and or distributions in the octahedral and tetrahedral sites. Theoretical calculations in such cases are rather complex and could be accomplished only if one can by chemical or magnetic methods arrive at the valence states and the site distribution of the ions involved. From the experimental point of view one could expect some solutions by conceiving divers compositions of host crystals chosen judiciously to investigate the magnetic behaviour of these ions in them. We have gone in this direction and have been able to confirm the presence of Ru4+ in the octahedral sites and demonstrate a strong positive contribution to K, from Ru4+ in YIG [6]. Also we showed a coherent behaviour of Ir4+ in both YIG

and NiFe204 [7]. Pursuing these lines we thought it could be informative to investigate the magnetic behaviour of Ru in the garnet { Bi3-2,Ca2, ) (Fe,) [Fe, - ,V,]O12 (designated hereafter as BiCaVIG). The choice of this host crystal with t = 1.35 is based on our earlier studies where we have noted some singular behaviour of CoZ+ and Tb3+ [8]. The purpose of this paper is to present the results on K, K2 of BiCaVIG : Ru and to discuss them in the light of recently available experimental data on R u [9].

2. Experimental methods. - The single crystals were prepared by the flux method. The melt composition was typically Pbo 22.2 ; Bi20, 8.3 ; F e 2 0 3 29.4 ;

V,O, 6.6 ; CaCo, 33.2 and RuO, (0-l), all expressed in mole

%.

The starting materials were of purity better than 99.99 "/,. The oxides were put into solution at 1 200 OC and the melt program cooled at the rate of 0.5-1.0 "C/h to 1 040 OC when the flux was poured out and the crystals were separated in the usual way. The crystals were then subjected to slow cooling from 1 000-600 OC at 3-5 "C/h. The crystals were analyzed to get their actual composition.

Spherical specimens of about 0.6 mm diameter (polished to 10 pm) were oriented such that the static magnetic field could be swept in the (110) plane. Ferromagnetic resonance was observed on these samples at 9 GHZ and in the range 4.2 - 300 K.

The lowest temperature that one could go was deter- mined by the K, value : K, and K2 were computed from the fields for resonance. Three specimens were measured for each composition. The magnetisation of the samples were measured with a vibrating sample magnetometer.

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Cl-184 R. KRISHNAN

3. Results and discussion. - The crystals obtained were incompletely formed with linear dimensions upto 15 mm and with well defined (1 10) faces. The flux inclusion was generally very low. The composition of the host crystal was

Bi,,3Ca2.,Fe3.65V1.35012

to within 10

%.

This variation in composition does not entail any significant variation in K, and K2 of the host crystal. The Ru content was analyzed to be 0, 0.65 and 1.1. wt

%.

The general formula of the samples could be expressed as

where we analyze x

+

y of Ru. We have in the above formula assumed that Ru can enter as both Ru3+ and Ru4+. From anisotropy considerations we may conclude that Ru is essentially present as R U ~ +

hence in the first approximation y = 0 and x in the two samples studied turns out to be as 0.04 and 0.07 respectively. Further the higher oxidation state Ru4+ is also to be expected as in our experience with the growth of BiCaVIG type garnets, it has not been possible to introduce Fez+ into the crystal even if one tries to do so by intentionally counterdoping the melt with Si4+. It is to be remembered here that in YIG growth on the contrary even traces of Si4+ in the melt readily induces Fe2+ in the crystal. This property we attribute to the oxidising nature of the flux system Pbo-Bi203-V,05. Based on work on rare earth rothenates one could expect Ru4+ to enter the octahedral sites and to be in the low spin state [lo]. Our results on K , are coherent with these assumptions as we shall see in what follows.

The quantity K , / M is negative for the two samples

and its temperature dependence is shown in figure 1.

For x = 0.07, the measurements were not possible below 200 K due to high anisotropy. The strong negative contribution to K, from Ru is clear. Figure 2 shows the temperature dependence of this contribution AK,, where AK, = K, (BiCaVlG : Ru)-K, (BiCaVIG). Also AK, f (x) is seen to be linear. While the contribution to K, is somewhat clear, that to K, is seen to be rather complex in the sense that for x = 0.04. K2 is positive and it goes through a broad maximum near 150 K and

1

K2

I

tends to decrease at lower temperatures (Fig. 3). K2 is much smaller than K, and unfortunately K2 measurement is complete only for one con~position making difficult any definite conclusion. Nevertheless this result on K2 would indicate that a strong negative K2 is appearing at lower temperatures suggesting more than one type of Ru ions present in the crystal though one particular type is dominant.

FIG. 2. - Temperature dependence of AK1 for x(Ru) = 0.04

and 0.07 ; AK1 = Kl(BiCaV1G : Ru)-Kl(BiCaV1G)

I I I I

0 100 z o o 3 0 0

T K

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MAGNETIC BEHAVIOUR O F Ru IN BiCaVIG CRYSTALS

Ground states and anisotropic energy contributions from low spin Ru3+ Ru4'

in the octalredral and tetrahedral sites

Octahedral site symmetry axis Tetrahedral site symmetry axis Ground [1111 Ground [loo]

Ion Configuration state AKI AK2 state AKl AK2

-

2

T2, large large 2T2 large large

positive negative negative negative

3 ~ 1 g medium medium ~ A I 0 0

positive negative

At this stage of this work we can explain our results by referring to table 1 where the respective contributions to K, and K2 from Ru3+ and Ru4+ are consider- ed in relation to their site occupancy. The symmetry axis of the each site is also indicated. It is clear that Ru3+ cannot explain our K,

<

0 K2

>

0 which are also not large but medium. So in the first approximation one can eliminate R U ~ + and consider Ru4+. These ions in the tetrahedral sites are in the 'A, diamagnetic state and hence cannot influence the anisotropy. Hence we are left with Ru4+ in the octahedral sites. But table I shows that for a negative trigonal field that is with [ I l l ] as the symmetry axis, AK, is

>

0 and AK2

<

0 and have medium values as against our results where AK, is < 0 and AK2

>

0. It is here that we are led to make the assumption that the symmetry axis in BiCaVIG crystal could be as well [loo] or in other words the trigonal field is positive. In fact certain garnets of the type NaCaCo2V3012 d o lend some support to this assumption where optical studies have indeed shown that the trigonal distortion is positive unlike in YIG [ll]. Of course it would be desirable to do independent measurements to verify this assumption. Unfortunately the obtention of diamagnetic counterpart of BiCaVIG has so far defied our techniques and as such any studies on them are not to be hoped in the near future. Spin echo studies on these crystals were unsuccessful due to very broad linewidths encountered. We have thus assumed in the above conclusion that K, is

>

0. If there is also a negative contribution to K2 then this could be understood in terms of Ru3+ in the tetrahedral sites. For as shown in table I one

can then expect a negative contribution from Ru3+ to both K, and K2. Naturally then for K, values this will add up to that from Ru4+ whereas it will diminish K2 as it has an opposite sign to that from Ru4+. Finally the one ion contributions to K, at 4 K from Ru3+ and Ru4+ in different host crystals is shown in table 11. The relatively lower contributions measured in BiCaVIG would again indicate that Ru4+ might be involved.

Experimental data on the one ion contribution to AK, from Ru3+ and Ru4+

Host crystal Ion - A K ~ c m - l Ref. N 4 ~

-

Y 3 F e 5 0 ~ z Ru3 +

+

28 [I]

Gd3Fe501z Ru3+

+

25 [I21 Y3Fe4.,A1,.,012 Ru3+

+

8.7

Ru4 + - 2.8

[I31

BiCaVIG Present

work

In conclusion, we have shown that Ru contribution to K, in BiCaVIG is negative and of medium values. The results can be explained in terms of Ru4+ in

the octahedral sites with a positive trigonal field. More work is needed particularly in the smaller concentration range to get more insight into the problem.

Experimental help in connection with magnetic resonance from Dr. Cagan is gratefully acknowledged.

References

HANSEN, P., Philips Tech. Rev. Suppl. No 7 (1970) ; Phys.

Rev. BC 862 (1971).

KRISHNAN, R., Phys. Stat. Sol. (a). K 17 (1970) 1. HANSEN, P., SCHULDT, J. and TOLKSDORF, W., Phys. Rev.

B 8 (1973) 4274.

KRISHNAN R., Phys. Stat. Sol. (a) K 177 (1971) 4. HANSEN, P., SCHULDT, J., HOLKSTRA, B. and DAMEN, J. P.

M., Phys. Stat. Sol. (a) 30 (1975) 289.

KRISHNAN, R., OUDBT, X., PORTE, M. and MARAIS, A., Int. Conf. Magnetism, Paper 7c3 (1976).

KRISHNAN, R., A. I. P. Conf. Proc. 29 (1975) 678.

[8] KRISHNAN, R., Z. Angew. Phys. 32 (1971) S 222.

[9] HANSEN, P . and KRISHNAN, R., International Conference on Ferrites 2, J. Physique Colloq. 38 (1977) C-1.

[ l o ] ALEONARD, R. BERTAUT, E. F., MONTMORY M. C. and

PAUTHENET, R. J. Appl. Phys. SuppI. to V 33 (1962) 1205.

[ l l ] SOKOLOV, V . I., SZYMCZAK, H. and WARDZINSKI, W., Phys.

Stat. Sol. (b) 55 (1973) 781.

1121 HANSEN, P., Phys. Rev. B 5 (1972) 3737.