NEUTRON DIFFRACTION STUDY OF ORDERED Mn-ALLOYS

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NEUTRON DIFFRACTION STUDY OF ORDERED Mn-ALLOYS

E. Krén, G. Kádár, L. Pál, Eva Zsoldos, M. Barberon, R. Fruchart

To cite this version:

E. Krén, G. Kádár, L. Pál, Eva Zsoldos, M. Barberon, et al.. NEUTRON DIFFRACTION STUDY OF ORDERED Mn-ALLOYS. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-980-C1-981.

�10.1051/jphyscol:19711348�. �jpa-00214383�

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JOURNAL DE PHYSIQUE Colloque C 1, supplkment au no 2-3, Tome 32, Rvrier-Mars 1971, page C 1

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⁹⁸⁰

NEUTRON DIFFRACTION STUDY OF ORDERED Mn-ALLOYS

E. KREN, G.

KADAR,

^L.

PAL

and Eva ZSOLDOS Central Research Institute for Physics, Budapest, Hungary

and M. BARBERON and R. FRUCHART Centre &Etudes de Chimie Mktallurgique, Vitry, France

R&umk.

-

Dans le systkme Mn-Pd on a trouve que la surstructure de longue pkriode avec des domaines d'antiphase existe jusqu'a 36 at % Mn. En augmentant la concentration de Mn Ia penode croi! mais la configuration antiferromagnh- tique reste inchangk. Dans les khantillons refro~d~s lentement F e autre phase antiferromagnktique apparait, son point de Nee1 est plus klevk que celui de la structure des domaines d'antiphase dans le m6me khantillon.

Dans le syst&me Mn3PtNr les atomes d'azote sur les posltions lntersticielles du motif detypeCusAu dilatent la maille, done la tempkrature de la transformation triangulaire-colinkaire antiferromagnetique, observke antkrieurement dans Mn3Pt, dkcroit si la concentration de l'azote augmente. La valeur du moment magnetique sur les atom& Mn change dans la transformation triangulaire-colinkaire.

Abstract. - In the Mn-Pd system the long-period antiphase domain structure was found to exist up to 36 at % Mn- with increasing Mn concentrations the period increases while the antiferromagnetic configuration remains unchanged.

In slowly cooled specimens an additional antiferromagnetic phase appears with a Nee1 point considerable higher than TN of the antiphase domain structure in the same sample.

In the Mn3PtNz system the nitrogen atoms at the interstitial positions of the Cu3Au type cell cause the lattice to dilate, thus the temperature of the traingular-collinear antiferromagnetic transition, observed earlier in MnsPt, decreases with increasing N-concentration. A change in the value of the magnetic moment on the Mn atoms is observed in the triangular- collinear transformation.

A one-dimensional long-period superlattice based on the Cu3Au type order with periodic antiphase domains was observed in the Mn-Pd system at MnPd, composition [I]. This structure exists in the 23-30 at. %

Mn concentration range with a periodicity 2 M = 4, where M is the number of the Cu,Au type cells per domain [2, 31. The magnetic structure is colinear antiferromagnetic [I], the magnetic moments are parallel to the tetragonal axis below 25 at. % Mn and perpendicular to it above this concentration [3]. At the stoichiometric composition the dependence of the direction of magnetic moments on the atomic long- range order was observed [4].

In the present work the neutron diffraction investi- gation of the Mn-Pd system covered the 30-40 at. % Mn concentration range. The powdered samples were studied both on quenching from 800 DC and on slow cooling for a month from 800 OC to room temperature.

The compositions were checked by chemical analysis.

In both the quenched and the slowly cooled samples the subsistence of the long-period antiphase domain structure was observed up to 36 at. % Mn. However, above 30 % the periodicity 2 M increases with increasing Mn concentrations and has non-integer values, as shown in figure 1. The non-integer values of the periodicity are common in the antiphase domain structures and in these cases M represents an average over the domains with different integers for M. The sharpness of the diffraction lines can be explained without assuming a complete regularity in the arrangement of the different domains [5]. While the atomic order of the antiphase domain structure is complete at 30 at

% Mn, it decreases at higher Mn concentrations. The Mn atoms in excess over the MnPd, composition occupy preferentially one of the Pd sublattices produc- ing thus a CuAu type basic unit of increasing tetra- gonality with increasing Mn concentrations, as seen in figure 1.

The samples above 30 at. % Mn have the same collinear antiferromagnetic structure as that observed a t lower Mn concentrations, the magnetic moments of about 4 p,, like at 25-30 %, are perpendicular to the

Mn-Pd ALLOYS

m

\ O--O-O-~-- c / a

V - a

0.85

CONCENTRATION (at% Mn)

FIG. 1. - Temperature dependence of the periodicity 2M, the lattice parameters a and ^cof the basic unit, and the Nkel

temperature TN in the Mn-Pd system.

tetragonal axis. The magnetic moments of the excess Mn atoms on the Pd sites are aligned antiparallel t o that of the nearest neighbour Mn atoms on the regular Mn sites as in the CuAu type MnPd alloys [6]. The periodicity of the magnetic structure coincides with that of the crystal structure. The Nee1 temperature TN was obtained by measuring the temperature depen- dence of the magnetic reflections, its values are shown

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711348

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NEUTRON DIFFRACTION STUDY OF ORDERED Mn-ALLOYS C 1 - 9 8 1

in figure 1. For completeness, the values of TN mea- sured for the CuAu type structure occurring at higher Mn concentrations [6] are also included in the figure.

I n the slowly cooled samples a new additional phase appears in the 32-38 at. % Mn concentration range which as a single phase was observed only at 32 %. Though the exact crystal structure could not be determined from the powder neutron diffraction measurement, the numerous reflections which are both nuclear and magnetic in origin were indexed in an orthorhombic unit cell with a = 8,07

A,

b = 5.41 A

and c = 4.00 A. This phase is the same as the uni- dentified phase observed by Raub and Mahler [7], in the 33-36 at % Mn interval. The Niel temperature TN of the orthorhombic phase is remarkably higher than TN of the antiphase domain structure at the same composition, as shown in figure 1. Between 36-40 at. % Mn the Mn,Pd, phase [8] was also observed in slowly cooled samples. Apart from the orthorhombic phase, both the crystal and the magnetic structures show a continuous transition from MnPd, to MnPd.

In the Mn,Pt phase a first-order transformation between a triangular and a colinear antiferromagnetic structure (AF-AF transformation) was observed [9].

The transformation occurs a t a critical value of the lattice parameter thus its temperature T , decreases as the size of the lattice increases, as observed in the Mn,+,Pt,

-,

system at increasing Pt concentrations.

Owing to the limited stability range of the Mn,Pt phase, the value of T, remains very close to the Niel temperature TN even at the limit Pt concentration x = - 0.17 [9]. For the unambiguous determination of the colinear antiferromagnetic structure above T,, a further lowering of T, seemed necessary. The increas- ed temperature range of the colinear phase was expected at the same time to allow the study of a possible change in the value of the magnetic moment during the AF-AF transformation.

To obtain the desired slight increase in the lattice size, samples of the Mn,PtN, type were prepared.

This system was chosen because the perowskite type Mn,PtN with a ⁼3.968 A at 20 OC [lo] has an increas- ed lattice parameter, as compared with a = 3.833 A

of Mn,Pt at 20 OC, due to nitrogen atoms occupying the body-centred interstitial positions of the Cu,Au type unit cell. X-ray diffraction showed the samples with x = 0 - 0.07 to be of Cu,Au type single phase, the value of the lattice parameter increases with increasing x, as expected [I 11.

The temperature dependence of the magnetic susceptibility, the lattice parameter and the magnetic

neutron reflection (101) of the colinear phase have the same characteristics in the Mn,PtN, system as those observed for Mn,Pt [9]. An abrupt change with thermal hysteresis occurs at T,, while a change in the slope of the susceptibility and lattice parameter curves is observed at TN. The,magnetic phase diagram showing the concentration dependence of T, and TN is given in figure 2. The decrease in T, with increasing

FIG. 2. - Magnetic phase diagram of the Mn@tNz system.

5 0 0

nitrogen concentration supports the conclusions drawn for Mn,Pt [9]. The increase in TN is, however, incon- sistent with the phase diagram calculated in the molecular field approximation [9].

The neutron diffraction measurements confirmed the magnetic structures established for Mn,Pt and given in figure 2. I n the colinear antiferromagnetic structure, part of the Mn atoms have no magnetic moment while the others have antiparallel moments lying in the plane perpendicular to the axis in which the unit cell is doubled. According to the theoretical calculations [9] the magnetic moments in this plane may point to either [loo] or [I101 directions. Magnetic moment on the Pt atoms is not allowed by the sym- metry.

Accurate neutron diffraction measurements above and below T, showed a considerable change in the value of the magnetic moment during the AF-AF transformation. For x ⁼0.025 where T, = 265 OK, the values above and below T, were found to be p,, = 4.5 f 0.2 and 3.5

+

0.2 p,, respectively.

PARAMAGNETIC

$--9--9-- +-4-p-+-6-

---4-

References

-

^{4 0 0}

LL W

a ,'

[I] CABLE (W.), WOLLAN (E. O.), KOEHLER (W. C . ) and [7] RAUB (E.) and MAHLER (W.), Z. Metallkde, 1954, CHILD (H. R.), Phys. Rev., 1962, 12a, 2118. 45, 430.

[21 SATO (H.) and TOTH (R. S.), Phys. Rev., 1965, 139, ^[8]KRBN (E.), K.&D.~R (G.) and TARN~CZI (T.), Phys.

A1581. Letters, 1967, 25 A, 56.

[9] K R ~ N (E.), G D ~ R (G.), PAL (L.), SLOYOM (J.), 131 KRBN (E.) and KADAR (G.), Phys. Letters, 1969, S Z A B ~ (P.) and TARN~CZI (T.), Phys. Rev., 1968,

29A, 340. 171, 574.

[4] K R ~ N (E.), K ~ D ~ R (G.) and PAL (L.), J. Appl. Phys., [lo] IMADAR (R.), GILLE~ (J.1, ROUAULT (A.), BOUCHAUD

1970, 41, 941. (J. P.), FRUCHART (E.), LORTHIOIR (G.) and

[5] FUJIWARA (K.), J. Phys. Soc. Japan, 1957, 12, 7. FRUCHART (R.), C. R. Acad. Sci. Paris, 1967, 264, 308.

[6] PAL (L.), KREN (E.), KADAR (G.), S Z A B ~ (P.) and [I 11 BARBERON (M.), LORTHIOIR (G.), FRUCHART (R.), Bull.

TARN~CZI (T.), J. Appl. Phys., 1968, 39, 538. Soc. Chim. France 1969, 7 , 2329.