MAGNETIC STRUCTURE AND EXCHANGE INTEGRAL MEASUREMENT IN TlMnF3

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MAGNETIC STRUCTURE AND EXCHANGE INTEGRAL MEASUREMENT IN TlMnF3

L. Madhav Rao, N. Satya Murthy

To cite this version:

L. Madhav Rao, N. Satya Murthy. MAGNETIC STRUCTURE AND EXCHANGE INTEGRAL MEASUREMENT IN TlMnF3. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-617-C1-618.

�10.1051/jphyscol:19711209�. �jpa-00214034�

JOURNAL DE PHYSIQUE Colloque C I , supplkment au no 2-3, Tome 32, Fe'orier-Mars 1971, page C 1 - 617

MAGNETIC STRUCTURE AND EXCHANGE INTEGRAL MEASUREMENT IN TlMnF,

L. MADHAV RAO and N. S. SATYA MURTHY

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

Rbum6. - Une etude par diffraction neutronique dans le c ompod TIMnF3 polycristallin de 300 OK B 4,2 OK indique qu'il a une structure cubique du type perovskite (a0 ⁼ 4,248 A). I1 subit une transition antiferromagnetique au-dessous de 76,5 OK a une structure du type G sans aucune deformation cristallographique.

La maille magnktique TIMnF3 est cubique simple. Chaque ion magnttique (Mnzt) est lit5 A ses proches voisins par une interaction d'echange du type a, qui est l'interaction dominante dans ce cristal. En se basant sur cette hypoth6seon a Bvaluk cette integrale d'kchange en phase paramagnttique en utilisant la technique de temps-de-vol de neutrons et en se servant de la theorie de de Gennes. On trouve J &gal a 3,O f 0,3 OK. Ceci est en bon accord avec les calculs thbriques.

Abstract. - Neutron diffraction patterns of polycrystalline TIMnF3 from 300 OK to 4.2 OK indicate that it has a cubic perovskite structure (a0 - 4.248 A) which undergoes an antiferromagnetic transition below 76.5 OK to a structure of the G-type without any crystallographic distortion. The magnetic moment of the Mnzt ion in this compound is 4.90 p~

at 4.2 OK.

The dominant exchange interaction among the magnetic ions in TIMnF3 which form a simple cubic lattice is a a-type superexchange among the nearest neighbours. Under this assumption, the exchange integral has been evaluated by para- magnetic neutron scattering using the moments method of de Gennes. J is found to be 3O & 0.3 OK which is in very good agreement with theoretical calculations.

I. Introduction. - TlMnF, belongs to a class of double fluorides with the perovskite structure which has been reported [I] to have the cubic antiferroma- gnetic structure. As part of our systematic investiga- tion of the exchange interactions in transition metal halides [2] it was thought interesting to study the substance in its ordered and paramagnetic phases by neutron scattering. The magnetic structure was inves- tigated to establish the NBel temperature and whether the antiferromagnetic ordering was preceded by any crystallographic distortion. Paramagnetic neutron scattering was studied at room temperature to extract the 2xchange integral using the de Gennes formalism [3]

as the nature of the simple cubic magnetic lattice lends itself to interpretation on the basis of adominant nearest neighbour superexchange interaction.

11. Magnetic structure. - A room temperature neu- tron diffraction pattern of pure TIMnF, powder was taken on the diffractometer at the CIRUS reactor, Trombay. An X-ray diffractometer tracing using CuK, radiation was also taken, at room temperature.

All the reflections of the combined data could be indexed assuming cubic symmetry and a least squares analysis yielded a, = 4.248

,

A diffraction pattern was then taken at 4.20K.

Figure 1 shows the diffraction patterns at 300 OK and 4.2 ^{O K .} The low temperature pattern is characterised by the appearance of three distinct superlattice reflec- tions indicating that TIMnF, is antiferromagnetic. All the reflections (magnetic and nuclear) could be indexed on the basis of an enlarged cubic magnetic unit cell with its cube edge twice that of the chemical unit cell.

Clearly, TIMnF, possesses the G-type magnetic structure. A least squares analysis of the intensities of the three superlattice reflections and four nuclear

10 15 20 25 30 3 5 40 4 5 50 J ANGLE 28

FIG. 1. -The neutron diffraction patterns in powder TlMnF3 at ³⁰⁰ OK and 4.2 OK. The appearance of three distinct super-

lattice reflections at 4.2OK is clearly seen.

reflections was carried out on the premise that there is no departure from cubic symmetry upon ordering.

There is no evidence of any such distortion in the diffraction patterns as the temperature is lowered from 300 OK. Table I shows the comparative values of ~ , 2 b = and F ~ , , , ~ , , , ~ . The magnetic moment of Mn2+

ion at 4.2 OK was established to be 4.90 p,.

A temperature dependence study of (1 11) super- lattice reflection yielded a value of 76.5 OK for TN. This is in agreement with the value quoted by Eastman and Shafer [I]. The temperature dependence of the super- lattice intensity is also explained well by the theore- tical Brillouin function with ti' = 512. Thus amongst the double fluoride perovskites, only RbMnF, and

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711209

C 1 - 6 1 8 L. MADHAV RAO AND N . S. SATYA MURTHY

Neutron Diffraction Analysis of TlMnF, a t 4.2 OK

Reflection F2 Fz

observed calculated

- -

lllrnag 54.57 53.80 Temperature

200 31.35 31.61 factor

311ma~ 30.65 29.61 B4.2% =

222 535.68 537.54 0.167 x 10-lGcmz

420 39.95 31.03 u of Mn2' ion

TlMnF, preserve their cibic symmetry even in the ordered phase. The near identity of the ionic radii of TI+ and Rb+ is perhaps significant.

111. Paramagnetic scattering. - TlMnF, powder was packed in a disc-shaped aluminium cassette and mounted on the Trombay rotating crystal spectro- meter which furnishes an incident monoenergetic neutron beam of 4.1 A. The time-of-flight-spectrum of the scattered neutrons was measured at room temperature and at a scattering angle of ^{Q, =} 45O (which satisfies the de Gennes criterion of large momentum transfers) using a 256 channel 16 ps channel-width, time analyser.

The ^cc raw ⁾⁾ time-of-flight spectrum is shown in the inset to figure 2. The time-of-flight spectrum was

FIG. 2. - The inset shows the ((raw >> time-of-flight spectrum in powder TIMnF3 at room temperature and at scattering angle 4S0. The corrected intensity is plotted on a semi-loga- rithrnic scale against the square of the energy transfer. The Gaussian nature of the energy distribution is clearly seen.

- f

2 ^{8 ;}

(I

i

I

Referc T

( Mn F3 (300°K) fi - g ^{= 4 5 O}

60 100 I40 180 220

l o r C H A N N E L N U M B E R

5

: i

; .

-

I I I I I I I 1

0 4 8 12 16 20 2 4 28 32

EASTMAN 0. E.) and SHAFER (M. W.), J. Appl. Phys., 1967, 38, 1274.

SATYA MURTHY (N. S.) and MADHAV RAO (L.), Proc.

Nuclear Physics and Solid State Physics Symposium (DAE) Vol. I , pp. 177 (1968) and references quoted therein.

DE GENNES (P. G . ) , J . Phys. Chem. Solids, 1958,4,223.

SQUARE OF ENERGY TRANSFER IN ( r n a ~ ) '

converted into an energy spectrum after correcting for absorption, magnetic form factor, air scattering, detailed balance factor and detector efficiency. Figure 2 shows the corrected intensities plotted against the square of the energy transfer on a semi-logarithmic scale. The Gaussian character of the energy distribu- tion is clearly seen. From the inverse of the slope the second moment < ^0' > was measured. Using the relation

where S = 512 and z = 6, J the dominant exchange integral was evaluated as J = 3.0 OK. A second mea- surement at @ = 330 yielded J ⁼ 3.3 ^{O K .} This diffe- rence is of the order of the experimental error.

IV. Discussion. - Eastman and Shafer [I] as also Kizhaev et al. [4] have measured the NCel temperature and susceptibility both at TN and in the paramagnetic phase. From their data, J was evaluated in the mole- cular field approximation (MF), BPW theory and the Green's function (G-F) formalism of Oguchi and Honma [S]. The series expansion method of Danielian and Stevens [6] (D-S) was also applied to the para- magnetic susceptibility data of Kizhaev et al. to obtain J. All these results are assembled in Table 11.

Exchange Integral J in TIMnF, in OK

Scattering Paramagnetic Models

angle Neutron

Scattering G-F BPW D-S M. F.

- - - -

45O 3.0 2.7 ( b )

3.3 ^{( 0 )} 3.5 (a)

33" 3.3 3.4 (C) 2.2 ( 8 )

( a ) From Nee1 Temperature TN (Ref. 151).

f b ) From TN.

ic) From ,y at T H ( ~ e f . 111).

From paramagnetic susccptibility (Ref. [4]).

(e) From TN.

It is seen that the present value of J agrees better with the values predicted by BPW, G-F, and D-S models while the M F value is decidedly on the lower side. This is quite in consonance with the fact that these models (except MF) are based on the nearest neighbour Heisenberg interaction.

[4] KIZHAEV (S. A.), TUTOV (A. .G.) and B o ~ o v ^(V. A.), Soviet Physics, 1966, 7 , 2325.

[ 5 ] OGUCHI (T.) and HONMA (A.), J . Appl. Phys., 1963, 34, 1153.

[6] DANIELCAN (A.) and STEVENS (K. W. H.), Proc. Phys.

Soc. (London) 1957, B70, 326 and 1961, B77, 116.