MAGNETIC STRUCTURE DEPENDENCE OF 57Fe HYPERFINE FIELD IN Fea-xMnxAs (a = 2.0, 2.1)

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MAGNETIC STRUCTURE DEPENDENCE OF 57Fe

HYPERFINE FIELD IN Fea-xMnxAs (a = 2.0, 2.1)

T. Goto

To cite this version:

JOURNAL DE PHYSIQUE

Colloque C8, Supplkment au no 12, Tome 49, dkembre 1988

MAGNETIC STRUCTURE DEPENDENCE

OF

5 7 ~ e

HYPERFINE FIELD IN

Fe,

--

,Mn=As (a = 2.0, 2.1)

Faculty of Technology, Tokoku Galcuin University, Tagajo 985, Japan

Abstract. - The Mossbauer effect measurement of 5 7 ~ e has been performed for Fe,-,Mn,As system (a = 2.0, 2.1).

This system has three several magnetic phases, which are FezAs type and MnzAs type antiferromagnetic phases and a ferrimagnetic phase. The magnetic hyperfine field depends strongly on magnetic structure.

The intermetallic compounds FezAs and MnzAs crystallize in the tetragonal CuzSb type structure and two magnetic sites I and I1 exist in this structure. Both compounds are antiferromagnetic and the mag- netic moments on site-I and I1 are in parallel and an- tiparallel coupling each other for FezAs and MnzAs, respectively [l, 2, 31. On the other hand, in the sys- tem Fe,-,Mn,As (1.95

I

a

5

2.35) a ferrimagnetism occurs in a narrow composition range around x = 1.3 and a first order phase transition from ferrimagnetic to antiferromagnetic state occurs with increasing tem- perature [4, 5, 61. A characteristic feature of this phase transition is that the magnetic hyperfine field a t 5 7 ~ e changes drastically through the transition

M.

This result suggests that the hyperfine field of this system is very sensitive to the magnetic structure. In this study, the Mossbauer effect measurements of 5 7 ~ e in Fe,-,Mn, As (a = 2.0, 2.1) are made over a wide range of Mn concentration s a n d the magnetic phase diagram in this system is discussed on the basis of the variation of the magnetic hyperfine field with composition and

T.78 K -. - - . . . 295 K FQ.sM~o.& . FeMnPs

,

I I I -4 -2 0 2 4 -2 0 2

Velocity (rnrnlsec) Velocity (mrnlsec)

temperature.

wi+J7$

--b+

The samples were prepared in the same manner as Veloc~ty (rnmlsec) Veloc~ty (mm~sec) in the previous study 161, but enriched with small

amounts of 5 7 ~ e . The observed Mijssbauer spectra are considered to be classified into four types with Mn con- centration. Four types of spectra are shown in figure 1 (1). In the range 0

5

x

<

1, where Fe atoms oc- cupy necessarily both site-I and 11, the spectrum can be fitted by two sites model with intensity ratio (site- I/site-11) of 1

/

(1 - x)

.

The hyperfine field at site-I1 is about 160 kOe, and nearly independent of x. While, the spectrum of site-I is very broad and the hyperfine field decreases rapidly with increasing x. ((2 In the range 1

<

x

5

1.3, where Fe atoms occupy preferen- tially site-I, the hyperfine field is small, and nearly in- dependent of x. (3) In the range 1.3

5

x

<

1.4, the hy- perfine field in the low temperature ferrimagnetic state is much larger than in the high temperature antiferro- magnetic state. (4) In the range x

2

1.4, where the compound is antiferromagnetic over all temperatures below the NBel point, the hyperfine field is obviously

Fig. 1. - MGssbauer spectra for Fe,-,Mn,As. (a) Fel.5Mno.5As. Solid curve is the combined spectrum of site-I and I1 spectra (dotted curves). (b) FeMnAs. (c) Feo.75Mnl.35As. (d) Feo.5Mnl.sAs. Solid curves of (b), (c) and (d) are the spectra calculated using Hesse-Rubartsch method.

large as compared with the field in the antiferromag- netic state of 1

<

x

5

1.4.

The temperature dependence of the hypefine field is shown in figure 2 for some samples. The field a t each temperature has been obtained from the hyperfine field distribution calculated using Hesse-Rubartsch method [8]. The hyperfine fields of the samples with x = 1.3 and 1.35 change abruptly at the ferrimagnetic to an- tiferromagnetic transition temperature. The concen- tration dependence of the magnetic hypefine field a t 78 K and 295 K is shown in figure 3. It seems that the

C8 - 166 JOURNAL DE PHYSIQUE

I I I I I

0 200 300 400

loo TEMPERATURE ( K )

Fig. 2. - Temperature dependence of the magnetic hyper- fine field Fe2.1-,Mn,As.

Fig. 3. - Concentration dependence of the magnetic hyper- fine field of Fe,-,Mn,As ( a = 2.0, 2.1) at 78 K and 295 K. The results by Grandjean et al. [9] and Yuzuri et al. 21 are also plotted with the present results.

150- m

2

Y 0 -I 9 0 0 - LL. W f

composition range of ferrimagnetism slightly narrower in a = 2.0 system than a = 2.1 system. It is ap- parent from the figure that the hyperfine field in the antiferromagnetic state changes discontinuously near z = 1.45. This change may be due to the phase change from FezAs type t o Mn2As type spin structure with increasing Mn concentration. The phase change be-

0 - - -

----_-_.---

1.. l .

'.

T.78 K I,

,

! I , I ' \ r, I

;\:*

,,,-o---.* t , ! ' , =

tween two antiferromagnetic states may be caused by a change in exchange interaction between the site-I and I1 moment. The ferrimagnetic phase occurs near

,

! $,' ,,*.O--.-

r : r,,

the boundary between two antiferromagnetic phases. Thus, the occurence of ferrimagnetism may be due to the coexistence of ferromagnetic and antiferromagnetic interaction. It can be concluded from the behaviour of hyperfine field that the iron from the behaviour of hyperfine field that the iron moment of this system de- pends on spin structure. Therefore, itinerant electron model would be suitable for understanding the phase change in this system. According to the theory by Moriya and Usami [lo], it is possible that the coexis- tence of ferromagnetic and antiferromagnetic compo- nents of spin fluctuation modes and the temperature variation of spin fluctuation amplitude induce the first order magnetic phase transition.

[I] Katsuraki, H. and Achiwa, N., J. Phys. Soc. Jpn

21 (1966) 2238.

[2] Yuzuri, M. and Yamada, M., J. Phys. Soc. Jpn

15 (1960) 1845.

[3] Austin, A. E., Adelson, E. and Cloud, W. H., J. Appl. Phys. 33 (1962) 1356.

[4] Rosenberg, R. M., Cloud, W. H., Darnell, F. J.

and Flippen, R. B., Phys. Lett. A 25 (1967) 723. [5] Rosenberg, R. M., Cloud, W. H., Darnell, F. J., Flippen, R. B. and Butter, S. R., J. Appl. Phys.

40 (1969) 1361.

[6] Kanomata, T., Goto, T. and Ido, H., J. Pfiys.

Soc. Jpn 43 (1977) 1178.

[7] Goto, T., J. Magn. Magn. Mater. 54-57 (1986) 931.

[8] Hesse, J. and Rubartsch, A., J. Phys. E 4 (1971) 401.

[9] Grandjean, F. and Gerard, A., J. Magn. Magn. Mater. 1 (1975) 64.