Structure, magnetic properties and the 57Fe Mössbauer effect in the LuFe11−xAlxTi system

Physica B 100

(1903) 126-130

North-Holland

Structure, magnetic properties and the “Fe MCssbauer effect in the LuFe 1 1 _ M,Al,Ti system F.G. Vagizov”* YA.V. Andreev”‘,

W. Suski”, I-L Drulis” and T. Goto”

“Polish Aca&m_v of Sciences, W. Trtebicltowski Institute of Low Tempcmtwe and Structure Rcssem-ch, P.O. Box 9.37, .50-9%) Wroclrw 2, Polnncl “institute for Solid Stutc Physics, The University of Tokyo, Roppon,qi, Minato- Ku, Tokyo 1016,Jupm Kcceived

29 January

The structure, temperature parameters

1993

magnetic

propcrtics

and the “Fc

Miissbauer

follow Vegard’s

law. It has been found that these alloys arc fcrromagnctic

moments decreasing with an increase in Al concentration, X(i)

positions and a diminution

neighboring

cffcct of LuFe ,, ,AI,Ti

alloys were

range of 4 .2-500 K. The system exists as a single philsc for the s < 4 concentration

of hyperfine

interaction

with Curie

investigated

in the

range.

lattice

The

points and magnetic

x. The Fc atoms arc subslitutcd by the Al atoms mostly in the results from a decrease in the average

number

of the nearest

Fc atoms.

1. Introduction The f-electron compounds of the ThMn,,-type tetragonal structure with iron as the other components exist in a form of ternaries with various elements as stabilizing additives, only. These compounds arc promising magnetic materials (see e.g. ref. [l]). As far as we know, rare earth pseudoternaries in which the Fe atoms are substituted by the Al atoms have not been investigated whereas such studies have been carried out for the UFe ,,,_.tAl.,Si, system [2] in which a change in the mr;gnetocrystalline anisotropy type has been observed. UFe,,,Si, is a uniaxial ferromagnet [3] and with increasing Al content. the anisotropy becomes of

the basal plane type. This has been interpreted as a result of competition among the different Fe sublattices assuming an unchanged ground state of the U atoms. This last conclusion seems to be invalidated by an increase in the U magnetic moment from O.S~,, to 1.4~~~with increasing Al content obscrvcd in the “Fe M(issbauer expcriment [4]. One should remember that in the ThMn,, type of strucbure, there are four inequivalcnt crystallogl aphic positions: the 2(a) sites occupied by the f-electron elements (rare earths or actinidcs), whereas the iron, silicon/ titanium and aluminum atoms are distributed over the (Y(f), S(i) and 8(j) positions. For the 1JFc ,,,_ ,Al,Si, system, MGssbauer examination [4] sho ved that the Al atoms mainly occupy the S(i) positions which arc the positions of the Fe atoms having the strongest infiucncc on the ferromagnetic behavior of the compound. 1-0 Ic:trn more about the contribution of the iron atoms to the magnetism of We,,,_ ,Al.,Si,, WChave invcstigatcd the LuFc,,,_ ,Al,Si, system c ur;mium is substituted by nonmagnetic lutetium. The lack of a cc>nccntration spin oricnt:ttion obscrvcd in the last system strongly

F.G. Vagizov et al. I On the LuFe,~ ,AI, Ti system

suggests that the origin of this phenomenon in uranium compounds [2] does not result from competition of tl,c magnetism of various iron sublattices but should be attributed to the change of electronic state of uranium in agreement with the M6ssbauer results. [4]. Recently, the structure and the magnetic properties of a single crystal of LuFe~Ti have been investigated by Andreev et al. [6]. They showed that this compound is ferromagnetic below about 490 K and the c-axis is an easy axis of magnetization. This compound was investigated as a standard with the nonmagnetic rare earth (Lu) sublattice for RFe~Ti compounds considered as the most promising magnetic materials among the compounds with the ThMnte-type of structure (for a review see rcf. I71). In the present paper, we report on the crystallographic and magnetic properties of LuFe~_,AI.,Ti alloys. The influence of A! substitution for Fe on the magnetic properties and M6ssbauer hyperfine parameters are presented.

127

3. Results and discussion

XRD studies revealed a single phase state with tetragonal ThMn ~2-type structure for x < 4. Only in the LuFet~Ti sample did the STFe M6ssbauer spectra reveal a small contribution (--3%) of sextet corresponding to a free et-Fe admixture. To obtain the best fit in the central part of the M6ssbauer spectrum for LuFe7AI4Ti (fig. 4), we were forced to add a line corresponding to iron atoms with spin-glass behavior [8]. Similar features as the interactions of the Fe atoms have been observed in the UFem_,AI,Si 2 system for x > 2.5 [41. Figure 1 demonstrates the lattice parameters a and c, and the c/a ratio versus A! concentration, x. Vcrgard's law is roughly fulfilled for this concentration range and an increase in both Lu F e l l . x A I x T i 860

f E

_~ 850

11"

f

° I

2. Experimental details

The LuFe~_,AI,Ti alloys were obtained by melting of the components in stoichiometric proportion (Lu, 99.9% purity; Fe, A! and Ti, 99.9% purity) in an arc furnace under a protective argon atmosphere. The samples were then annealed at 900°C during 2 weeks. The X-ray (XRD) patterns were used to determine the phase composition. Magnetometric measurements were carried out at 4.2 K using a vibrating sample magnetometer with a field strength of 2 T and a pulsefield installation in a magnetic field of 5 T. The Curie temperatures were determined by AC susceptibility measurements. The S7Fe M6ssbauer measurements wcrc performed using a conventional constant-acceleration spectrometer with a .~7Co in Cr matrix source. The velocity scale was calibrated using an a-Fc absorber at room temperature.

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.

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.

.

.

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clo

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i

o.s7 U

E

Aluminium

content

x

Fig. 1. Lattice parameters a (upper panel) and c (middle panel) and c;a ratio (lower panel) for LuFe~l A I T i alloys. versus aluminum concentration x.

F.G. Vagizov et al. I On the

128

parameters corresponds to substitution of the smaller Fe atoms by larger AI atoms. The LuFett_,Al.~Ti alloys are ferromagnetic with field and the temperature dependences of magnetization similar to those observed for single crystalline LuFe~tTi (see figs. 1 and 2 in ref. [6]). The Curie points versus AI concentration, x , are presented in fig. 2 and Tc for LuFettTi is close to that for a single crystal sample [6]. One can see that Tc diminishes with increasing x almost linearly very much like the magnetization versus x, presented in fig. 3. The uniaxial magnetic anisotropy also sharply decreases; the compound with x = 4 is nearly isotropic. However, there is no change in anisotropy type within the homogoneity range. The behavior is the same as in LuFe~0_,AlxSi 2 [5] and differs from that in UFe~o_xAlxSi: [2]. Thus, one can conclude that the substitution of the Fe atoms by the A! atoms corresponds to simple magnetic dilution. The ~TFe M6ssbauer spectra for LuFe~_~

AI,Ti solid solutions with x = 0 , . . . , 4 at 2 0 K are shown in fig. 4. The spectra are approximated by three sextets resulting from the three nonequivalent positions of the Fe atoms (8(f), 8(i) and 8(j)). The sextet exhibiting the largest hyperfine field is related to the Fe atoms in the 8(i) positions having the largest number of the nearest neighbor (nn) Fe atoms and the largest average Fe-Fe separation, dFc_v¢. The sextet with the smallest hyperfine field is attributed to the Fe atoms in the 8(f) site. The assumed ratio of the hyperfine fields Hht> Hihr>Hfhf corresponds to earlier results obtained for RFe1,_.,.MTi compounds [9,10]. The distribution of hyperfine fields connected with various numbers of A! and Ti atoms in the nearest neighborhood of the Mfssbauer nuclei has been taken into account by ascribing a different line width for the external and internal lines of the Zeeman sextets. The best fit is shown in fig. 4 by solid

i,,i,,i,,i,,i,,i,,i,, 100 ~

600

_

"

LuFe,, ,AI, Ti system

0

_

I

1

__~i_.

. . . . . . . . . . . . .

1,,t,,i, .

J

2 3 Aluminium content

x

4

Fig. 2.

The Curie points, T c, of LuFe,,_.,AI,Ti alloys versus AI concentration x. °I

20 ~

E

M,,,

(/3 O

~ 1 0 ~-

~-

E

100 1

0

1-

2

A[uminium

content

3 x

Fig. 3. Magnetization M (in p.r,/f.u.) of the LuFe,~ ,AI,Ti system determined in magnetometdc ( 0 ) and " F c Mossbauer experiments (©1 versus aluminum concentration if.

-6

.

0

3

ve J-c~clty, mm/s

6

Fig. 4 The Fc Mosshauer spectra of LuFe,~ ,AI,Ti alloys for different .~ - 4.() at 20 K.

129

F.G. Vagizov et ai. / On the LuFea~ ,AI, Ti system

~_~ (D

0

v~O06'

6

E 0

&

o

1

2

~

o

~

2

~

o

1

Composition ( x ) Fig. 5. The occupation of various t:rystailographic positions by iron for the LuFe~,

.-(

,,

,

,

i

,

,

,

,

,

. . . .

i

. . . .

,AI,Ti

4

system versus Ai concentration

x.

effect, is presented in fig. 5. The M6ssbauer and X R D data allow us to calculate the average number of nearest neighbor Fe atoms (fig. 6(a)) and the average Fe-Fe distance (fig. 6(b)). Because the average d~.:_F~ is constant over AI concentration, investigated (fig. 7), a diminution of the hyperfine field on the M6ssbauer nuclei, apparently, has to result from diminution of the number of nearest neighboring Fe atoms (see fig. 6(a)). The magnetic moments of the Fe atoms calculate(] from the hypcrfine field using the

lines and it is apparently a very good approximation except for the above-mentioned alloy with x = 4. The substitution of the iron by the aluminum results in the change in intensity of sextets corresponding to the various Fe sublattices. The intensity of sextets resulting from the 8(i) sites strongly decreases, whereas the line intensities of the 8(0 and 8(j) sublattices are practically constant. The occupation of various crystallographic positions as a function of AI concentration x, concluded from the M6ssbauer

12!

3

2

0.275

I

~ , ~ I l l l i l l i l i l l

l l i 7

E

E 0.270

11-:

Lu F e 1 ~_~AI xTi

29

O Q) 8 b_

0.255

I,

I

7 7" Z

L.L

1

0

i

i

n 8j-site ,,, 8 f - s i t e

> 0.240 0 0.235

l

,

~

1

i

I

i

2

Composition

i

i

3

,

(X)

i

O /x

~0.245~_,.3I

I

O

0.250

o 8J-site

1

o

~ 0.265 o .~ 0.260 "o

1o-

il,:l

o

d

,

l

4

! (b)

0

....

, ............. 1 2

Composition

3

(X)

4

Fig. 6. (a) The average number of nearest neighbor Fe atoms for the Fe atoms located in various crystallographic positions versus A! concentration x. (b) The average Fe-Fe distance for various crystallographic positions versus A1 concentration x.

F.G. Vagizov et al. / On the LuFe~ ,AI, Ti system

130

magnetic interactions follows the decrease in the average number of the nearest neighbor Fe atoms. This result confirms once again the importance of the Fe atoms located in the 80) positions for magnetic properties of ThMn~2-type compounds.

30 Hhf

" 1-~ 2 0

100

•

I

1

1 ~

I,

2 3 Atuminiurn content x

I,

Fig. 7. The hyperfine magnetic feld, Hht, at 20K for LuFett ,AI,Ti alloys versus AI concentration x.

coefficient 15.6 T//z• [10,11] are presented in fig. 3 and exhibited a close similarity to the magnetometric results.

4. Conclusions The LuFel~_,AlxTi alloys are single phase samples over their limited AI concentration range (x <4), similar to other ThMn~2 systems with admixtures of this element [2,4,5]. "i~'he lattice parameters follow Vegard's law in the above-mentioned concentration range. These solid solutions are ferromagnets with rather low Curie points (below 490 K) which decrease with increasing x, similar to the magnetic moment and hyperfine field. According to the M6ssbauer data, this diminution results from substitution of the Fe atoms by the AI atoms, mostly in the 8(i) positions and therefore the decrease in the

References !1] B.D. de Mooij and K.I-I.J. Buschow, Philips J. Rcs. 42 (1987) 246. [2i A.V. Andreev and W. Suski, J. Alloys Compounds 187 (1992) 381. 131 w. Suski, A. Baran and T. Mydlarz, Phys. Lett. A 136 (1989) 89. [4! F.G. Vagizov, H. Drulis, W. Suski and A.V. Andreev, J, Alloys Compounds 191 (1993) 213. [5 ! A.V. Andreev, Ye. V. Shcherbakova, T. Goto and W. Suski, J. Alloys Compounds, in press. [61 A.V. Andreev, V. Sechovsky, N.V. Kudrevatykh, S.S. Sigaev and E.N. Tarasov, J. Less-Common Metals 144 (1988) 121. [7] K.H.J. Buschow, J. Magn. Magn. Mater. 100 (1991) 79; J.M.D. Coey, Phys. Scripta T39 (1991) 21. [81 J. Gal, i. Yaar, D. Regev, S. Fredo, G. Shani, E, Arbaboff, W. Petzel, K. Aggarwal, J.A. Pereda, G.M, Kalvius, F.J. Litterst, W. Sch~ifer and G. Will, Phys. Rev. B 42 (1990) 8507. [91 Yingchang Yang, Linshu Kong, Mingjun Yu and Xiaotong Wang, J. Less-Common Metals 167 (1991) 213. [101 B.-P. Hu, H.-S. Li, J.P. Gavigan and J.M.D. Coey, J. Phys.: Condcns. Matter 1 (1989) 755. Illl B.-P. Hu, H.-S. Li and J.M.D-Cocy, Hypcrfine Interactions 45 (1989) 233.