NEUTRON SCATTERING STUDIES OF SOME SPINELS

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NEUTRON SCATTERING STUDIES OF SOME SPINELS

N. Murthy, L. Rao, R. Begum, M. Natera, S. Youssef

To cite this version:

N. Murthy, L. Rao, R. Begum, M. Natera, S. Youssef. NEUTRON SCATTERING STUD- IES OF SOME SPINELS. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-318-C1-319.

�10.1051/jphyscol:19711104�. �jpa-00213919�

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NEUTRON SCATTERING STUDIES OF SOME SPINELS

N. S. S. MURTHY, L. M. RAO, R. J. BEGUM, M. G. NATERA and S. I. YOUSSEF

Nuclear Physics Division, Bhabha Atomic Research Centre, Trombay, Bombay, 85, India

Rksumk.

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Par diffraction de neutrons polarisks on a ttudie les structures magnktiques de nombreux cristaux de spinelles ferrites (mono et mixte). On a demontre que cette mkthode est applicable meme aux structures colinkaires. On a employe l'expression du champ moleculaire pour expliquer la courbe experimentale d'aimantation de MgFe204 et en tirer les valeurs des interactions dominantes. On trouve J A B = - 22 OK et J A A = - 4 OK.

Par diffusion de neutrons froids, on a pu determiner les valeurs des interactions dominantes dans ZnFe204 et ZnCrzO4.

JBB (Fe3+-Fe3+) = 1,04 OK et JBB (Cr3+-Cr3+) = 2,38 ^{O K .}Dans MnA1204 (inversion de 9 %), J A A (Mn2+-MnZf) ⁼- 0,21 OKet JAB (Mn2+-Mn2+) ⁼- 16 OK.

Abstract.

-

The magnetic structures of a number of single and mixed spinel ferrites have been determined using the polarised neutron diffraction technique. It is shown that this method ^ISapplicable even when the structure is non- collinear. The experimental magnetization curves in MgFe2O4 have been fitted to a molecular field expression to derive the dominant exchange interactions as JAB =

-

22 OK and J A A = - 4 OK.

From paramagnetic neutron Scattering, the only dominant exchange interactions in ZnFezO4 and ZnCr204 have been determined as ^JBB(Fe3+-Fe3+) ⁼1.04 OK and ^JBB(Cr3+-Cr3+) ⁼2.38 OK. In MnA1204 (9 % inverted)

J A A (Mn2+-Mn2+) =

-

0.21 OK, JAB (Mnz+-Mn2+) ⁼- 16 OK.

I. Diffraction studies. - In the study of the che- mical and magnetic structures of spinel ferrites, the number of parameters to be determined is usually large, especially in mixed systems. In the absence of single crystals, the only way in which the data can be increased is to use polarised neutron diffraction intensities I+ and I- along with the unpolarised neu- tron data, I, [I]. A programme, PUPILS, has been developed on a CDC-3600 computer to refine simultaneously the various crystal and magnetic structure parameters 121. The increased data enables a larger number of parameters to be simultaneously refined and works even when there is sizeable depolarisation due to the sample thickness. I t has been shown to work even for non-collinear structures ; it has, however, t o be used with care for samples with high ani- sotropy.

NiFe204 has been shown unambiguou.sly to be completely inverted (within 1

%).

Ni2" has an extra- polated moment of 2.3 pB due to orbital contribution.

A sample of CoFe204 was found to be 88

%

inverted.

Cobalt is divalent on both the sites and the room temperature moment values of 2.97 pB (~0:') and 2.41 pB (CO;+) indicate small orbital contributions.

The cation distribution in MgFe204 indicated that this ferrite is 86

%

inverted and this remained unchanged in the temperature range of 90 OK-700 OK.

The temperature dependence of several magnetic intensities were studied to yield the individual sublattice magnetization curves. These were fitted to molecular field expressions involving the significant exchange integrals JAB, JA4 and JBB between Fe3" ions.

The values obtained were, JAB =

-

22 OK and J A A = - 4 OK using JBB ⁼I OK. Actually, the solution of the molecular field expressions yielded unrealis- tically high values for JBB as predicted by Smart [3]

and so JBB was fixed at 1 O K as determined by neutron scattering (see Section 11).

Previous neutron diffraction [4] and Mossbauer effect [5] studies have asserted that MnFe204 is

81

%

normal. The present studies have shown our sample to be 93

%

normal. Further, contrary to previous suggestions, all Mn ions are in the divalent state and the extrapolated magnetic moment at 0 OK is 4.85 p,. There is no evidence of any canting of the moments as suggested recently [6]. The A and B sublattice magnetizations have been studied and can be fitted with exchange integrals which are essen- tially in agreement with those obtained by Sawatzky et al. [6].

In the system Mg,Mn,-,Fe204 it has been shown that the amount of Mn going to the A sites is highly irregular, being 38.7

%,

77.4

%

and 40

%

respectively for x = 0.25, 0.50 and 0.75. This is in direct conflict with the observations of Nathans et al. 171, but corro- borates the findings of Yamzin and coworkers [8]

who obtained an anomalous distribution of Mn and Fe ions over the two sites in Mn,Fe,-,O, system.

Earlier unpolarised neutron diffraction work [9]

on the system Zn,Ni,-,Fe204 showed that Zn,,Ni.,Fe2O4 has a non-collinear Yafet-Kittel (YK) type of magnetic structure at room temperature.

Polarised neutron intensity measurements using a field of 6 kOe led to a Yafet-Kittel angle, a,, of 13O, identical to the value obtained with unpolarised neutrons at room temperature. This excellent agreement proves that the polarised neutrons d o lead to correct results even in the case of canted moments if there is complete saturation of the moments in the field direction.

The analysis of the data on Co2Ti0, at 300 OK showed it to be completed inverted and paramagnetic. But the neutron diffraction pattern at 4.2 OK showed a broad peak near the (1 11) position which is purely magnetic in origin. This suggests the exis- tence of a complicated spin structure, possibly due to competing distant neighbour interactions among the Co2+ ions.

11. Paramagnetic studies.

-

An experimental knowledge of the exchange integrals operating bet-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711104

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NEUTRON SCATTERING STUDIES O F SOME SPINELS C 1

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³¹⁹ ween pairs of magnetic ions on the A and B sites in

spinels is bound to be extremely useful in their phe- nomenology. Temperature dependence of sublattice magnetization as well as paramagnetic susceptibility data are often insensitive to some of the exchange interactions and their analysis takes recourse to molecular field theory. Direct energy measurement using neutron scattering is the most appropriate method to determine these quantities. Ideally, spin wave dis- persion relations will yield the best values of exchange integrals. But, paramagnetic neutron scattering [lo]

can also provide reliable values when only one dominant exchange integral is operative.

We have chosen the spinels ZnFe20,, ZnCr204 and MnA1204 with a view to determining the exchange interactions between Fe3 +-Fe3

+,

Cr3+-Cr3+ and Mn2+-Mn2+ ions. The technique of cold neutron inelastic scattering in the paramagnetic region was used and the exchange integrals were extracted by the de Gennes method of moments. The data on these spinels were collected at 85 OK. Figure 1 shows the typical time-of-flight distribution of 4.1

A

neutrons scattered by ZnFe204. ZnFe204 and ZnCr204 were found to be completely normal from room temperature diffraction patterns and hence the only exchange integrals operating are those between the B site ions.

The experiments yield the dominant interactions in the two cases as JBB ( ~ e ~ + - ~ e " ) ⁼1.04 ^O^K and JBB (Cr3 -Cr3 +) = 2.38 ^{O K .}

In the case of MnA1204, the sample was found to have a small inversion of 9

%

and hence there is, in addition to JAA, a significant JAB interaction between the Mn2+ ions. Using therefore, a molecular field expression for TN (= 18 OK) along with the second moment of the scattered neutron energy distribution, it was found that JAA (Mn2 ⁺-Mn2 +) ⁼

-

0.21 OK and JAB =

-

16 ^OK.

It is well known that both ZnFe204 and ZnCr20, [I13 have complicated types of ordering at low tempe- ratures and these have been argued as due to the influence of distant neighbour B-B interactions.

However, it is also known [I I, 121 that these interactions are at least an order of magnitude smaller than the dominant, nearest neighbour B-B superexchange.

In the paramagnetic scattering experiment, the energy exchange with the neutron can be reasonably assumed

FIG. I. - At the top is shown the <<raw D time-of-flight spec- trum of 4.1 A neutrons scattered from ZnFe201 powder at 85 OK and at scattering angle of 44 ^O. The final corrected intensity plotted on a semi-logarithmic scale against the square of the energy transfer is shown at the bottom. The Gaussian nature of the energy distribution is clearly seen. The inverse of the

slope of the straight line yields the second moment.

1200

1000

to arise mainly from this dominant interaction (especially as the second moment involves the squares of the J's).

In the case of MnA1204, Friedman and Goland [13]

also using a neutron scattering method derived JAA (Mn2+-Mn2+) as 2.2 ^OK, ignoring the small inversion. Although the present results have used a molecular field expression for TN the widely differ- ing values of JAA underline the importance of taking into account inversion in view of the dominant nature of JAB.

-

ELASTIC PEAK

-

s %*

PARAMAGNETIC

***

SCATTERING

-

e ^*' ^*a²^.

References [I] YOUSSEF (S. I.), NATERA (M. G.), BEGUM (R. J.),

SRINIVASAN (B. S.) and SATYA MURTHY (N. S.), J. Phys. Chem. Solids, 1969, 30, 194.

[2] BEGUM (R. J.), RADHAKRISHNAN (N. K.) and SATYA MURTHY (N. S.), B. A. R. C. Report (to be published).

[3] SMART (J. S.), Magnetism, Vol. 111, edited by G . T. Rado and H. Suhl. Academic Press. New York. 1963.

HASTINGS (J. s.) and CORLISS (L: M.), phys: Rev., 1956, 104, 328.

SAWATZKY (6. A.), VAN ^DERWOUDE and MORRISH (A. H.), Physics Letters, 1967, 25A, 147.

SAWATZKY (G. A.), VAN ^DER WOUDE and MOR-

RISH (A. H.), Phys. Rev., 1969, 187, 747.

NATHANS (R.), PICKART (S. J.), HARRISON (S. E.) and KRIESSMAN (C. J.), Proc. IEE, 1957, B 104,

m."

[8] YAMZIN (I. I.), BELOV (N. V.) and NOZIK (Yu. 2.) J. Phys. Soc. Japan, 1962, 17, B 111, 55.

- .

(S. I.), BEGUM (R. 'J.) and SR~VASTAVA (C. M.), Phys. Rev., 1969, 181, 969.

[lo] SATYA MURTHY (N. S.) and MADHAV RAO (L.), Proc. Nucl. Phys. Solid State Phys. Syrnp. (DAE), 1968, 1 , 167 and references therein.

[11] PLUMIER (R.), D. &S SC. Thesis, University of Paris, 1968.

[I21 DWIGHT (K.) and MENYUK (N.), Phys. Rev., 1967, 163, 435.

[13] FRIEDMAN (E. A.) and GOLAND ( A . W.), Phys. Rev., 1966, ^147,457.