Structural and magnetic properties of Y6(Fe1-xMnx)23

(1)

HAL Id: jpa-00218994

https://hal.archives-ouvertes.fr/jpa-00218994

Submitted on 1 Jan 1979

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Structural and magnetic properties of Y6(Fe1-xMnx)23

W. James, K. Hardman, W. Yelon, B. Kebe

To cite this version:

W. James, K. Hardman, W. Yelon, B. Kebe. Structural and magnetic properties of Y6(Fe1-xMnx)23.

Journal de Physique Colloques, 1979, 40 (C5), pp.C5-206-C5-208. �10.1051/jphyscol:1979577�. �jpa-

00218994�

(2)

Structural and magnetic properties of Y

6

(Fe

1

_

x

Mn

A:

)

2

3 W. J. James and K. Hardman

Graduate Center for Materials Research, University of Missouri-Rolla, Rolla. MO 65401. U.S.A

W. Yelon

University of Missouri Research Reactor, University of Missouri-Columbia, Columbia. MO 65201. U.S.A.

B. Kebe

Laboratoire Louis-Neel. CNRS, 166X.

38042 Grenoble Cedex. France

Abstract. — The ternary system Y6(Fe1_xMn;e)23 exhibits unusual magnetic behavior. Although both Y6Mn2 3

and YgF e2 3 are magnetically ordered, these ternaries show a striking reduction in both Curie temperature and magnetization. In the compositional range of JC = 0.5 to 0.75, there exists no magnetic ordering at liquid helium temperatures.

The Mn and Fe atoms are found to exhibit strong site preference in the Y6(Fe0 2 5M n0 7 5)23 unannealed,

Y6(Fe0 2 5Mn0 7 5)2 3 annealed and Y6(Fe0 5Mn0 5)2 3 unannealed samples. The magnetization of these com-

pounds is field-dependent and suggests possible spin glass behavior.

1. Introduction. — The R ^ F e j ^ M n J ; ^ system exhibits unusual magnetic behavior relative to the end members, R

⁶

M n

^{2 3}

and R

⁶

Fe

^{2 3}

. In Y ^ F e ^ ^ M n ^ j there is no magnetic ordering in the compositional range, x = 0.5 to 0.75. Considerable controversy exists concerning the origin of the magnetic anomalies in these compounds. Itinerate and localized models have been proposed for the Y

6

(Mn, Fe)

2 3

system [1].

The localized model assumes antiferromagnetic cou- pling of Fe and Mn moments which leads to a decrease of the magnetic moment and also to a decrease in the Curie temperature. The band model assumes a common band for Fe and Mn. Both of these models are inadequate in explaining the experimental data of the central compositional range. The electronic nature of these compounds is expected to be important in understanding the magnetic interactions of the atoms.

Manganese is known to have many different valence

states. This would affect the electronic nature of these compounds. Although Fe-Mn interactions are expected to be antiferromagnetic, this does not account for the paramagnetic nature of the Y

6

(Fe

1

_

;c

Mn

x

)

2

3 system. In the YMn

1 2

structure, there are three transition metal sites with an occu- pancy of four atoms per site. In YMn

4

Al

8

, Al occupies two sites completely. The paramagnetic ternaries also show significant preferred ordering in the crys- tallographic sites of this system. This could lead to a breakdown in long range magnetic ordering.

Neutron diffraction and magnetic studies offer the most straight-forward approach in determining ordering effects and magnetic character of these compounds. Since the nuclear scattering lengths of Mn and Fe are very different, b

Ma

= — 0.37 and b

Fe

= 0.95 x 1 0 "

1 2

cm, the nuclear data indicate very readily the degree of preferred ordering on each of the crystallographic sites.

JOURNAL DE PHYSIQUE Colloque C 5 , supplément au n° 5, Tome 40, Mai 1979, page C5-206

Résumé. — Le système ternaire Y g C F e ^ ^ M n ^ j montre un comportement magnétique extraordinaire. Bien que Y6M n2 3 et Y6F e2 3 soient ordonnés du point de vue magnétique, ces alliages ternaires montrent une réduc- tion marquée de la température de Curie et de l'aimantation. Dans la gamme de compositions x = 0,5 à 0,75, l'ordre magnétique n'existe pas à la température de l'hélium liquide.

Les atomes Mn et Fe montrent une forte préférence pour certains sites dans un échantillon coulé de Y6(Fe0>25Mn0i75)23 et Y6(Fe0 5Mn0 5)2 3 et dans un échantillon recuit de Y6(Fe0 2 5M n0 7 5)2 3. L'aimantation de ces composés dépend du champ magnétique, ce qui suggère un comportement possible de type verre de spin.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979577

(3)

STRUCTURAL AND MAGNETIC PROPERTIES OF Y ,(Fel - ,Mn,),

,

^C5-207

2. Experimental.

-

Ingots of Y6(Feo.25Mno.75)23 and Y6(Feo.5Mno,s)23 were prepared by induction melting of the elements in the appropriate stoichio- metric amounts in a water-cooled Cu boat under Ar atmosphere. The Y6(Feo,25Mno.75)23 sample was annealed at 920 OC for four days under a vacuum.

A small portion of the Y6(Feo.5Mno.5)2, was annealed at 950 OC for three days. X-ray diffraction photo- graphs of both annealed and unannealed samples showed no evidence of other phases. Neutron dif- fraction data were collected on Y6(Feo,2sMno.75)23 unannealed, 6 ( 0 2 5 0 7 s ) 2 3 annealed and Y6(Feo.5Mno~5)23 unannealed. The two-axis neutron diffractometer at the University of Missouri Research Reactor was used with A

=

1.103 A. Scans were made a t room temperature with data collected from a 2 0 of 5.00 to 50.00.

Refinement of the data were carried out using the profile technique developed by Rietveld [2]. The profile program compares the data with a proposed model and calculates the best fit. By varying different nuclear parameters, better fits can be obtained. This method of refinement is particularly powerful in that it accounts for peak overlap.

The magnetic measurements were completed at the Service National des Champs Intenses. CNRS, Grenoble, France. Magnetic fields up to 150 kOe were used a t 4.2 K.

3. Results and discussion.

-

The four crystallo- graphic sites of this system are b (1 atom). d (6 atoms), f, with x

=

0.378 (8 atoms) and f2 with x

=

0.178 (8 atoms). While the magnetic moments of the transition atoms are coupled ferromagnetically in Y6Fez3, Y,Mnz3 is ferrimagnetically coupled with the b and d sites opposite the f sites. The computer refined stoichiometry for these compounds is Y,(Fe0.2, ,Mn0.7,,)2

3

for Y6(Fe0.25Mn0.

7512 3

,man-

fK=aled Y6(Feo.225MnO.,75),3 for Y,(Feo.2sMno.,5),3 annealed and Y6(Feo,5,6Mno,,,,)23 for

unannealed. See table I below.

The lattice parameter change suggests different valence states of Mn. For Y6(Feo.,sMno.75)23 an- nealed, a , is 12.337 A. The lattice parameter for Y6(Feo~,Mno.s)23 is 12.229 A. Definite site prefe- rence can be seen for Mn and Fe in this region. If a random distribution of Mn and Fe atoms in each site is assumed, the R-factor is greater than 50 %.

When Mn and Fe atoms are allowed to atomically order in each site, the R-factor decreases to 5 %.

The Mn atoms prefer the b and f, sites while Fe atoms prefer the f2 site. The f,-f, interatomic dis- tance is approximately 2.6 A. This would suggest possible antiferromagnetic coupling between Mn and Fe.

Table I.

-

Occupation of atoms in crvstallographic sites

Fig. 1 .-Magnetization and Curie temperature of Y,(Fel -,Mn,),,.

Y6(Fe0.2SMn0.7 5)23U Y6(Fe0.25Mn0.7S)23A

No. of atoms (%) No. of atoms (%) Y6(Fe0.5Mn0.5)23 No. of atoms (%)

-

0.654 (65) 0.346 (35) 2.396 (40) 3.605 (60) 6.025 (75) 1.978 (25) 2.049 (26) 5.954 (74) 11.124 (48.4) 11.883 (51.6)

5.0 %

58.5 %

(4)

C5-208 W. J. JAMES, K. HARDMAN, W. YELON A N D B. KEBE

The drastic decrease in both Curie temperature

^YS( F ~ I - ~ M ~ X ) ~ ~

and ordered magnetic moment can be seen in

figure 1 [3]. The Curie temperature is represented

3 -

by +

^.

Magnetization results obtained by Kirchmayr and Steiner are indicated by circles and by Bechman

et al. are squares. Bechman et al. [3] concluded that

the paramagnetic region consisted of microscopic

^O

regions of antiferromagnetically coupled atoms along 3

with microscopic regions of ferromagnetic coupling.

=i o + x = a 5 X=0.75

Hilscher et al. [I] has suggested magnetic clustering

I

-

^AX=O.S annealed

9 X.0.75 onnealed

effects at the critical concentration leading to spin glass behavior in the paramagnetic region. Oester-

reicher et al. [4] proposed a complex disordered

I I I

magnetic state originating from fluctuations of the

50 100 150

magnetic exchange from site to site and anisotropy

H ( k 0 e )

interactions.

Fig. 2. ^-Magnetization of Y,(Fe,-,Mn,),,.

A high field magnetic study of these compounds is given in figure 2. Magnetic clustering or spin glass

behavior is suggested by these results. Experiments Acknowledgment. - The authors thank the Natio- in small angle neutron scattering are in progress on nal Science Foundation for support of this study Y ,(Fe ,.,, ^Mn,, ,

^,),

, unannealed. under Grant NSF INT 76-02665.

References

[l] HILSCHER, G., BUIS, N. and FRANSE, J. J. M., Physica 91B (1977) 170.

[2] RIETVELD, H. M., J . Appl. Cryst. 2 (1969) 65.

[3] BECHMAN, C. A., NARASIMHAN, K. S . U. L., WALLACE, W. E., CRAIG, R. S. and BUTERA, R. A., J. Phys. Chem. Solid 37 (1976) 245.

[4] OESTERREICHW, H., BITTNER, H. F. and PARKER, F. T., Magne- tism Lett. 1 (1978) 89.