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Magnetic behaviour of the heusler alloy Ru2FeSi
Journal of Magnetism and Magnetic Materials 51 (1985) 359-364 North-Holland, Amsterdam
359
MAGNETIC BEHAVIOUR OF THE HEUSLER ALLOY Ru2 FeSi S.N. M I S H R A , D. R A M B A B U , A . K . G R O V E R a n d P.N. T A N D O N
Tata Institute of FundamentalResearch, Bombay-400 005, India Received 15 March 1985
Magnetization measurements and 57Fe Mrssbauer effect studies in a new Heusler alloy Ru2FeSi indicate that the moments of Fe in this alloy have antiferromagnetic orientation. The values obtained at 4.2 K for/~/f.u, and the average value of the hyperfine field at Fe are 4× 10-2~tB (in a field of 8 kOe) and 225 kOe, respectively. The alloy exhibits a complex magnetic behaviour due to the presence of inherent aIomic disorder in the sample depicting two ordering temperatures as seen from Xac(T) at 280 and 35 K. The latter temperature is conjectured to be the temperature at which the reorientation of Fe moments occur. The Ru substitution of Fe in Fe3Si thus brings about the transformation from a ferromagnetic ordering of Fe3Si to an antiferromagnetic ordering in Ru 2FeSi.
localized o n iron has been seen to u n d e r g o antiferr o m a g n e t i c ordering.
1. Introduction I n c o n t i n u a t i o n with o u r earlier investigations to s t u d y the m a g n e t i c p r o p e r t i e s of the H e u s l e r alloys [1,2], in p a r t i c u l a r those c o n t a i n i n g Ru, F e a n d s2p 2 element, we r e p o r t here o n the m a g n e t i c b e h a v i o u r o f R u 2 F e S i . I n c o n t r a s t to its o t h e r isoelectronic c o u n t e r p a r t s , n a m e l y , R u z F e S n a n d R u 2 F e G e b o t h of which are collinear f e r r o m a g nets, the p r e s e n t M 6 s s b a u e r a n d m a g n e t i c m e a s u r e m e n t s i n d i c a t e that F e m o m e n t s in R u 2 F e S i have a n t i f e r r o m a g n e t i c orientation. T o our k n o w l edge, no o t h e r H e u s l e r alloy c o n t a i n i n g m o m e n t s
Table 1 Comparison of the observed and calculated (assuming perfect atomic ordering, see text) X-ray line intensities from various planes. The measured intensities are normalized to the (220) plane Planes
Measured intensity (~)
Calculated intensity (~)
lll 200 220 222 400 420 422
0.5 (1) 22 (1) 100 7 (1) 19 (1) 10 (1) 36 (1)
3 24 100 6 16 7 33
2. Experimental T h e alloy R u z F e S i was p r e p a r e d b y m e l t i n g together the s t o i c h i o m e t r i c quantities o f high p u r i t y e l e m e n t s in a r g o n a t m o s p h e r e in i n d u c t i o n a n d arc furnaces. T h e alloy b u t t o n s were a n n e a l e d at 800°C for one week, followed b y slow cooling to r o o m t e m p e r a t u r e . T h e X - r a y d i f f r a c t o g r a m of p o w d e r e d s a m p l e s i n d i c a t e d it to be essentially in single p h a s e with cubic L21 H e u s l e r structure ( a = 5.87 ,~). T h e m e a s u r e d value o f p h y s i c a l density, o f 9.21 g / c m 3, agrees within 2% with that calcul a t e d using the m e a s u r e d a value; this indicates the n e a r a b s e n c e of vacancies in the matrix. T h e degree of a t o m i c o r d e r i n g in the s p e c i m e n can b e e s t i m a t e d b y c o m p a r i n g the e x p e r i m e n t a l values of relative intensities of even (h + k + 1 = 4n + 2) a n d o d d (h, k, l = all o d d ) s u p e r s t r u c t u r e lines of L21 structure with those expected f r o m ideal a t o m i c o r d e r i n g with Ru, F e a n d Si o c c u p y i n g the (A,C), B a n d D sites [3]. T h e o b s e r v e d values of IF(200) 12/I F(220) 12 a n d IF(l11) IZ/IF(220)I 2 i n d i c a t e little (A, C) ---, B / D d i s o r d e r b u t considerable, -- (30 + 10)%, B ---, D d i s o r d e r (see table 1).
0 3 0 4 - 8 8 5 3 / 8 5 / $ 0 3 . 3 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics P u b l i s h i n g Division)
360
S.N. Mishra et al. / Heusler alloy Ru2FeSi
Ru2Fe Si MvsH
80
'~O
77K
4"2K
6O
298K
A
E 339
(5 40
K
j455K f
20
0
~
I
I
2
I
I
4
I
I
6
I
|
I
8
MAG. FIELD
IO (kOe)
I
I
I
12
14
i
I
16
Fig. 1. M vs. H plots at various temperatures for Ru2FeSi.
The magnetic measurements made on the specimen include the low field ac susceptibility measured from 4.2-300 K and the dc magnetization data recorded in field values from 120 Oe to 15.6 kOe in the temperature range 4.2-570 K. Fig. 1 shows M vs. H plots at different temperatures and fig. 2 depicts Xac(T) and M vs. T plots in 120, 1500 and 8000 Oe. The temperature variation of inverse susceptibility corresponding to data obtained in 8 kOe is also shown in fig. 2. Fig. 3 shows the 57Fe Mrssbauer absorption spectra recorded at different temperatures from 4.2 to 500 K. The average hyperfine field values at Fe at different temperatures are obtained by fitting the spectra to a distribution in hyperfine field following the procedure given by Window [4]. The variation with temperature of the average hyperfine field, normalized to its value at 4.2 K, and the relative changes in isomer shift at 57Fe with respect to the 57Co(Rh) source are shown in fig. 4.
3. Results and discussions
The MOssbauer spectra (fig. 3) change gradually from an unsplit single line at 300 K to a split six finger pattern below 145 K. At still lower temperatures the spectra are well split and are indicative of a small distribution in the hyperfine field at Fe. The average hyperfine field value at 4.2 K is obtained to be (225 +_ 3) kOe. Two important facts which emerge from the Mrssbauer data are: (i) On the time scale of = 10 -8 s, the magnetic freezing starts to set in just below the room temperature (line broadening of the M6ssbauer line). The temperature variation of the hyperfine field shows that the slowing down of the spin dynamics is a (gradual) slow process. The isomer shift also changes in a very smooth manner as a function of temperature. (ii) Even though there is a distribution in hyperfine field at Fe, which presumably is due to
S.N. Mishra et al. / Heusler alloy Ru2FeSi
361
Oe 0.7 1500 Oe
~xlO 00
0.6 O 0 B 0 Q 0
-
O
0.,5
I
:50
D
A O Q
E
A O O
E
f
Z O 0.4 IN I-IM Z (.9 0-3 <
t
- 200
t
-8 O O
,2oo, ,oo
\
~e -
150
100
0"2
5O
0.1
I
I00
200
300 TEMR
400
400
6OO
(K)
Fig. 2. M vs. T plots at various magnetic fields for Ru2FeSi. The I / x ( T ) and Xac(T) are also shown.
the distribution of Fe a t o m s on multiple sites ([B] --- 70% and [D] -- 30%), the observed width of the field distribution (65 kOe of total width at half maximum) indicates that almost all the Fe atoms possess local moments in the range 1.3 to 1.7# B (assuming that a moment of 2.2#B on Fe gives rise to a hyperfine field value of
330 kOe, like Fe [B] in Fe3Si ). Table 2 lists the hyperfine field and the moment values in isoelectronic Ru2FeSn and Ru2FeGe for comparison [1,2]. There is no sharp signature of the onset of magnetic ordering in the bulk magnetic data, see figs. 1 and 2. The ac susceptibility (fig. 2) starts to
S.H. Mishra et al. / Heusler alloy Ru2FeSi
362 RuzFe Si
TN ~ 2 9 0 K
0 ........................
\
.W -~"
~-
Ru 2 Fe Si
--~- . . . . . . . . . .
+0-15
300K 5.0-
"~.
o: . - - - - .
ft t t tt
E E
li
. . .
tt
_,,,---~-----'--
+0-05
tt tt
0 1.0' o-e
"1o
.,
&
o.61
.
-I-
o
03 ,'n <
°
0.4
"'":.:'-....
2
0.2
N I00
200
:300
TEMR (K) Fig. 4. Average hyperfine field values HF(Fe), normalized to 4.2 K, and isomer shift values w.r.t. 57Co(Rh) source for Ru2FeSi at various temperatures.
i I
1
I
1
1
+6
+3
0
-3
-6
VELOCITY
(mm/s)
Fig. 3. Mrssbauer spectra for Ru2FeSi at various temperatures taken with a 57Co(Rh) source. The solid lines are fit to the data incorporating a field distribution.
build up just below room temperature, the first point of inflexion in the cooling down cycle being at = 280 K. There are two maxima in the X,c(T) plot centred around 200 and 35 K. A broad h u m p centred around 250 K is also observed in dc magnetization recorded in an applied field of 120 Oe (fig. 2). The hump gets broadened and diffused in higher values of the applied dc field. The mag-
netization recorded in ~000 Oe shows a monotonic increase across the hump temperature region and down to = 1 5 K. The M vs. H plots, fig. 1, are completely linear for T > 300 K. The non-linear behaviour is observed below ~ 250 K but only at lower values of field, H ~< 2 kOe. In the region 0 ~< H ~< 2 kOe the M vs. H curves have curvature usually seen in metamagnetic systems, the typical behaviour is most clearly evident in M vs. H plots Table 2 Comparison of lattice constant a, ordering temperature, Tin, moment values, g / f . u , and the hyperfine field values at Fe, HF(Fe), in Ru based Heusler alloys [1,2] Alloy
a (,&)
Tm (K)
/z/f.u. (/%)
Hf(Fe) (kOe)
Ru2FeSn Ru2 FeGe Ru 2FeSi
6.20 5.98 5.87
595(F) 610(F) 280(AF)
3.30 1.92 0.04
314 317 225
S.H. Mishra et al. / Heusler alloy RueFeSi
at 77 and 4 K (fig. 1). At 77 K upto 15.6 kOe there is no indication of impending saturation expected at higher field values in metamagnetic systems. The magnetization value in 8 kOe at 4.2 K corresponds to /~/f.u. of 4 × 10-2/~B. If we fit the inverse susceptibility in the region 350 K 4 T ~ 570 K to a Curie-Weiss behaviour (X - l = 3 k ( T + 8p)//~eff(/leff + 1)), we obtain a value of 3.2/~B for /zetJf.u. of RuzFeSi with 0p = +85 K. Such a value of/~eff is more in accordance with t h e / ~ / F e atom expected from the hyperfine field data as compared to that from the magnetization data. One can view Ru2FeSi as being obtainable from the system Fe3Si by progressive replacement of Fe(A,C) atoms by Ru. Our recent studies of the series Fe3_xRuxSi show that in the region of 0 ~< x ~< 1.35, the alloys behave like collinear ferromagnets, and as x increases, 1.5 ~< x ~ 1.8, the Fe moments gradually become progressively non-collinear [5]. The observed very low magnetization value of Ru2FeSi, 4 × 10-2/xB, can be reconciled with the large average hyperfine field at Fe only if Fe moments have some kind of antiferromagnetic orientations resulting in vanishing net magnetization. We conjecture that Fe moments in Ru2FeSi have orientations similar to that of Mn moments in the antiferromagnetic Pd 2Mnln system [6]. In a well ordered specimen of Pd2Mnln, Mn moments occupying only the B sites are seen to form a rhombohedral antiferromagnetic lattice with a magnetic unit cell twice that of the L21 chemical unit cell. However, in disordered Pd2Mnln specimens, Mn moments on adjacent [B] and [D] sites form an antiferromagnetic lattice with the magnetic unit cell equal to the L21 unit cell. Our present specimen of Ru2FeSi containing = 70% Fe(B) and = 30 Fe(D) moments may, therefore, be viewed as an admixture of two antiferromagnetic lattices. The observed switching in magnetic order from ferro- to antiferromagnetic, from Fe 3Si to Ru 2FeSi, has an interesting parallel in the transformation seen ealier in the series Fe3_xMnxSi [7]. The Mn moments in the atomically well ordered Fe2MnSi alloy are expected to form a rhombohedral antiferromagnetic lattice. However, in specimens made at stoichiometry FezMnSi, there exists about 10% Fe-Mn(A,C)--, (B) disorder. Fe(A,C) atoms with
363
4Mn(B) and 4Si(D) in their l n n shell are non-magnetic, the moment carrying atoms in the system are = 90% Mn(B), = 10% Mn(A,C) and a small fraction of Fe(B) atoms. Fe2MnSi displays two transitions around = 220 and = 70 K in the bulk magnetization data [8]. Neutron diffraction studies [8] show that the higher temperature transition corresponds to freezing of moments with the magnetic unit cell equal to the L21 unit cell; the moments continue to reorient as the temperature is lowered further and the = 70 K transition corresponds to the appearance of a rhombohedral antiferromagnetic lattice with a magnetic cell twice that of the L21 unit cell. The = 220 K transition is caused by the presence of a small fraction of Fe and Mn moments on their disordered B and (A, C) sites, it would have been absent in atomically well ordered Fe2 MnSi. The presence of two maxima centred around 200 and 35 K in the X a c ( T ) plot (see fig. 2) of our specimen of Ru2FeSi hints towards the existence of two possible magnetic transitions in it. In view of earlier results in different specimens made at stoichiometrics P d z M n l n and Fe2MnSi, we believe that in Ru2FeSi, the = 280 K transition corresponds to the appearance of an antiferromagnetic lattice with a L21 a value and = 35 K transition to the antiferromagnetic lattice with 2a value. Neutron diffraction studies are desired to confirm the magnetic structure of RuzFeSi.
Acknowledgements The authors thank V.S. Patil and R.G. Pillay for their help in the initial stages of this work, to C.R.K. Murthy for his help in ×a~ measurements. They also thank R. Vijayaraghavan and H.G. Devare for their encouragement.
References [1] A.K. Grover, R.G. Pillay, V. Nagarajan and P.N. Tandon, Phys. Stat. Sol. (b) 98 (1980) 495. [2] V.S. Patil, R.G. Pillay, P.N. Tandon and H.G. Devare, Phys. Stat. Sol. (b) 118 (1983) 57. [3] V.A. Niculescu, K. Raj, J.I. Budnick, T.J. Butch, W.A. Hines and A.M. Menotti, Phys. Rev. B14 (1976) 41~i0.
364
S.N. Mishra et a L / Heusler alloy Ru2fe$i
[4] B. Window, J. Phys. E 4 (1971) 401. [5] S.N. Mishra, D. Rambabu, A.K. Grover, R.G. Pillay, P.N. Tandon, H.G. Devare and R. Vijayaraghavan, Solid State Commun. 53 (1985) 321; J. Appl. Phys. (in press).
[6] P.J. Webster and R.S. Tebble, Phil. Mag. 16 (1967) 347. [7] V.A. Niculescu, T.J. Burch and J.l. Budnick, J. Magn. Magn. Mat. 39 (1983) 223, and refs. therein. [8] S. Yoon and J.G~ Booth, J. Phys. F 7 (1977) 1079.
359
MAGNETIC BEHAVIOUR OF THE HEUSLER ALLOY Ru2 FeSi S.N. M I S H R A , D. R A M B A B U , A . K . G R O V E R a n d P.N. T A N D O N
Tata Institute of FundamentalResearch, Bombay-400 005, India Received 15 March 1985
Magnetization measurements and 57Fe Mrssbauer effect studies in a new Heusler alloy Ru2FeSi indicate that the moments of Fe in this alloy have antiferromagnetic orientation. The values obtained at 4.2 K for/~/f.u, and the average value of the hyperfine field at Fe are 4× 10-2~tB (in a field of 8 kOe) and 225 kOe, respectively. The alloy exhibits a complex magnetic behaviour due to the presence of inherent aIomic disorder in the sample depicting two ordering temperatures as seen from Xac(T) at 280 and 35 K. The latter temperature is conjectured to be the temperature at which the reorientation of Fe moments occur. The Ru substitution of Fe in Fe3Si thus brings about the transformation from a ferromagnetic ordering of Fe3Si to an antiferromagnetic ordering in Ru 2FeSi.
localized o n iron has been seen to u n d e r g o antiferr o m a g n e t i c ordering.
1. Introduction I n c o n t i n u a t i o n with o u r earlier investigations to s t u d y the m a g n e t i c p r o p e r t i e s of the H e u s l e r alloys [1,2], in p a r t i c u l a r those c o n t a i n i n g Ru, F e a n d s2p 2 element, we r e p o r t here o n the m a g n e t i c b e h a v i o u r o f R u 2 F e S i . I n c o n t r a s t to its o t h e r isoelectronic c o u n t e r p a r t s , n a m e l y , R u z F e S n a n d R u 2 F e G e b o t h of which are collinear f e r r o m a g nets, the p r e s e n t M 6 s s b a u e r a n d m a g n e t i c m e a s u r e m e n t s i n d i c a t e that F e m o m e n t s in R u 2 F e S i have a n t i f e r r o m a g n e t i c orientation. T o our k n o w l edge, no o t h e r H e u s l e r alloy c o n t a i n i n g m o m e n t s
Table 1 Comparison of the observed and calculated (assuming perfect atomic ordering, see text) X-ray line intensities from various planes. The measured intensities are normalized to the (220) plane Planes
Measured intensity (~)
Calculated intensity (~)
lll 200 220 222 400 420 422
0.5 (1) 22 (1) 100 7 (1) 19 (1) 10 (1) 36 (1)
3 24 100 6 16 7 33
2. Experimental T h e alloy R u z F e S i was p r e p a r e d b y m e l t i n g together the s t o i c h i o m e t r i c quantities o f high p u r i t y e l e m e n t s in a r g o n a t m o s p h e r e in i n d u c t i o n a n d arc furnaces. T h e alloy b u t t o n s were a n n e a l e d at 800°C for one week, followed b y slow cooling to r o o m t e m p e r a t u r e . T h e X - r a y d i f f r a c t o g r a m of p o w d e r e d s a m p l e s i n d i c a t e d it to be essentially in single p h a s e with cubic L21 H e u s l e r structure ( a = 5.87 ,~). T h e m e a s u r e d value o f p h y s i c a l density, o f 9.21 g / c m 3, agrees within 2% with that calcul a t e d using the m e a s u r e d a value; this indicates the n e a r a b s e n c e of vacancies in the matrix. T h e degree of a t o m i c o r d e r i n g in the s p e c i m e n can b e e s t i m a t e d b y c o m p a r i n g the e x p e r i m e n t a l values of relative intensities of even (h + k + 1 = 4n + 2) a n d o d d (h, k, l = all o d d ) s u p e r s t r u c t u r e lines of L21 structure with those expected f r o m ideal a t o m i c o r d e r i n g with Ru, F e a n d Si o c c u p y i n g the (A,C), B a n d D sites [3]. T h e o b s e r v e d values of IF(200) 12/I F(220) 12 a n d IF(l11) IZ/IF(220)I 2 i n d i c a t e little (A, C) ---, B / D d i s o r d e r b u t considerable, -- (30 + 10)%, B ---, D d i s o r d e r (see table 1).
0 3 0 4 - 8 8 5 3 / 8 5 / $ 0 3 . 3 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics P u b l i s h i n g Division)
360
S.N. Mishra et al. / Heusler alloy Ru2FeSi
Ru2Fe Si MvsH
80
'~O
77K
4"2K
6O
298K
A
E 339
(5 40
K
j455K f
20
0
~
I
I
2
I
I
4
I
I
6
I
|
I
8
MAG. FIELD
IO (kOe)
I
I
I
12
14
i
I
16
Fig. 1. M vs. H plots at various temperatures for Ru2FeSi.
The magnetic measurements made on the specimen include the low field ac susceptibility measured from 4.2-300 K and the dc magnetization data recorded in field values from 120 Oe to 15.6 kOe in the temperature range 4.2-570 K. Fig. 1 shows M vs. H plots at different temperatures and fig. 2 depicts Xac(T) and M vs. T plots in 120, 1500 and 8000 Oe. The temperature variation of inverse susceptibility corresponding to data obtained in 8 kOe is also shown in fig. 2. Fig. 3 shows the 57Fe Mrssbauer absorption spectra recorded at different temperatures from 4.2 to 500 K. The average hyperfine field values at Fe at different temperatures are obtained by fitting the spectra to a distribution in hyperfine field following the procedure given by Window [4]. The variation with temperature of the average hyperfine field, normalized to its value at 4.2 K, and the relative changes in isomer shift at 57Fe with respect to the 57Co(Rh) source are shown in fig. 4.
3. Results and discussions
The MOssbauer spectra (fig. 3) change gradually from an unsplit single line at 300 K to a split six finger pattern below 145 K. At still lower temperatures the spectra are well split and are indicative of a small distribution in the hyperfine field at Fe. The average hyperfine field value at 4.2 K is obtained to be (225 +_ 3) kOe. Two important facts which emerge from the Mrssbauer data are: (i) On the time scale of = 10 -8 s, the magnetic freezing starts to set in just below the room temperature (line broadening of the M6ssbauer line). The temperature variation of the hyperfine field shows that the slowing down of the spin dynamics is a (gradual) slow process. The isomer shift also changes in a very smooth manner as a function of temperature. (ii) Even though there is a distribution in hyperfine field at Fe, which presumably is due to
S.N. Mishra et al. / Heusler alloy Ru2FeSi
361
Oe 0.7 1500 Oe
~xlO 00
0.6 O 0 B 0 Q 0
-
O
0.,5
I
:50
D
A O Q
E
A O O
E
f
Z O 0.4 IN I-IM Z (.9 0-3 <
t
- 200
t
-8 O O
,2oo, ,oo
\
~e -
150
100
0"2
5O
0.1
I
I00
200
300 TEMR
400
400
6OO
(K)
Fig. 2. M vs. T plots at various magnetic fields for Ru2FeSi. The I / x ( T ) and Xac(T) are also shown.
the distribution of Fe a t o m s on multiple sites ([B] --- 70% and [D] -- 30%), the observed width of the field distribution (65 kOe of total width at half maximum) indicates that almost all the Fe atoms possess local moments in the range 1.3 to 1.7# B (assuming that a moment of 2.2#B on Fe gives rise to a hyperfine field value of
330 kOe, like Fe [B] in Fe3Si ). Table 2 lists the hyperfine field and the moment values in isoelectronic Ru2FeSn and Ru2FeGe for comparison [1,2]. There is no sharp signature of the onset of magnetic ordering in the bulk magnetic data, see figs. 1 and 2. The ac susceptibility (fig. 2) starts to
S.H. Mishra et al. / Heusler alloy Ru2FeSi
362 RuzFe Si
TN ~ 2 9 0 K
0 ........................
\
.W -~"
~-
Ru 2 Fe Si
--~- . . . . . . . . . .
+0-15
300K 5.0-
"~.
o: . - - - - .
ft t t tt
E E
li
. . .
tt
_,,,---~-----'--
+0-05
tt tt
0 1.0' o-e
"1o
.,
&
o.61
.
-I-
o
03 ,'n <
°
0.4
"'":.:'-....
2
0.2
N I00
200
:300
TEMR (K) Fig. 4. Average hyperfine field values HF(Fe), normalized to 4.2 K, and isomer shift values w.r.t. 57Co(Rh) source for Ru2FeSi at various temperatures.
i I
1
I
1
1
+6
+3
0
-3
-6
VELOCITY
(mm/s)
Fig. 3. Mrssbauer spectra for Ru2FeSi at various temperatures taken with a 57Co(Rh) source. The solid lines are fit to the data incorporating a field distribution.
build up just below room temperature, the first point of inflexion in the cooling down cycle being at = 280 K. There are two maxima in the X,c(T) plot centred around 200 and 35 K. A broad h u m p centred around 250 K is also observed in dc magnetization recorded in an applied field of 120 Oe (fig. 2). The hump gets broadened and diffused in higher values of the applied dc field. The mag-
netization recorded in ~000 Oe shows a monotonic increase across the hump temperature region and down to = 1 5 K. The M vs. H plots, fig. 1, are completely linear for T > 300 K. The non-linear behaviour is observed below ~ 250 K but only at lower values of field, H ~< 2 kOe. In the region 0 ~< H ~< 2 kOe the M vs. H curves have curvature usually seen in metamagnetic systems, the typical behaviour is most clearly evident in M vs. H plots Table 2 Comparison of lattice constant a, ordering temperature, Tin, moment values, g / f . u , and the hyperfine field values at Fe, HF(Fe), in Ru based Heusler alloys [1,2] Alloy
a (,&)
Tm (K)
/z/f.u. (/%)
Hf(Fe) (kOe)
Ru2FeSn Ru2 FeGe Ru 2FeSi
6.20 5.98 5.87
595(F) 610(F) 280(AF)
3.30 1.92 0.04
314 317 225
S.H. Mishra et al. / Heusler alloy RueFeSi
at 77 and 4 K (fig. 1). At 77 K upto 15.6 kOe there is no indication of impending saturation expected at higher field values in metamagnetic systems. The magnetization value in 8 kOe at 4.2 K corresponds to /~/f.u. of 4 × 10-2/~B. If we fit the inverse susceptibility in the region 350 K 4 T ~ 570 K to a Curie-Weiss behaviour (X - l = 3 k ( T + 8p)//~eff(/leff + 1)), we obtain a value of 3.2/~B for /zetJf.u. of RuzFeSi with 0p = +85 K. Such a value of/~eff is more in accordance with t h e / ~ / F e atom expected from the hyperfine field data as compared to that from the magnetization data. One can view Ru2FeSi as being obtainable from the system Fe3Si by progressive replacement of Fe(A,C) atoms by Ru. Our recent studies of the series Fe3_xRuxSi show that in the region of 0 ~< x ~< 1.35, the alloys behave like collinear ferromagnets, and as x increases, 1.5 ~< x ~ 1.8, the Fe moments gradually become progressively non-collinear [5]. The observed very low magnetization value of Ru2FeSi, 4 × 10-2/xB, can be reconciled with the large average hyperfine field at Fe only if Fe moments have some kind of antiferromagnetic orientations resulting in vanishing net magnetization. We conjecture that Fe moments in Ru2FeSi have orientations similar to that of Mn moments in the antiferromagnetic Pd 2Mnln system [6]. In a well ordered specimen of Pd2Mnln, Mn moments occupying only the B sites are seen to form a rhombohedral antiferromagnetic lattice with a magnetic unit cell twice that of the L21 chemical unit cell. However, in disordered Pd2Mnln specimens, Mn moments on adjacent [B] and [D] sites form an antiferromagnetic lattice with the magnetic unit cell equal to the L21 unit cell. Our present specimen of Ru2FeSi containing = 70% Fe(B) and = 30 Fe(D) moments may, therefore, be viewed as an admixture of two antiferromagnetic lattices. The observed switching in magnetic order from ferro- to antiferromagnetic, from Fe 3Si to Ru 2FeSi, has an interesting parallel in the transformation seen ealier in the series Fe3_xMnxSi [7]. The Mn moments in the atomically well ordered Fe2MnSi alloy are expected to form a rhombohedral antiferromagnetic lattice. However, in specimens made at stoichiometry FezMnSi, there exists about 10% Fe-Mn(A,C)--, (B) disorder. Fe(A,C) atoms with
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4Mn(B) and 4Si(D) in their l n n shell are non-magnetic, the moment carrying atoms in the system are = 90% Mn(B), = 10% Mn(A,C) and a small fraction of Fe(B) atoms. Fe2MnSi displays two transitions around = 220 and = 70 K in the bulk magnetization data [8]. Neutron diffraction studies [8] show that the higher temperature transition corresponds to freezing of moments with the magnetic unit cell equal to the L21 unit cell; the moments continue to reorient as the temperature is lowered further and the = 70 K transition corresponds to the appearance of a rhombohedral antiferromagnetic lattice with a magnetic cell twice that of the L21 unit cell. The = 220 K transition is caused by the presence of a small fraction of Fe and Mn moments on their disordered B and (A, C) sites, it would have been absent in atomically well ordered Fe2 MnSi. The presence of two maxima centred around 200 and 35 K in the X a c ( T ) plot (see fig. 2) of our specimen of Ru2FeSi hints towards the existence of two possible magnetic transitions in it. In view of earlier results in different specimens made at stoichiometrics P d z M n l n and Fe2MnSi, we believe that in Ru2FeSi, the = 280 K transition corresponds to the appearance of an antiferromagnetic lattice with a L21 a value and = 35 K transition to the antiferromagnetic lattice with 2a value. Neutron diffraction studies are desired to confirm the magnetic structure of RuzFeSi.
Acknowledgements The authors thank V.S. Patil and R.G. Pillay for their help in the initial stages of this work, to C.R.K. Murthy for his help in ×a~ measurements. They also thank R. Vijayaraghavan and H.G. Devare for their encouragement.
References [1] A.K. Grover, R.G. Pillay, V. Nagarajan and P.N. Tandon, Phys. Stat. Sol. (b) 98 (1980) 495. [2] V.S. Patil, R.G. Pillay, P.N. Tandon and H.G. Devare, Phys. Stat. Sol. (b) 118 (1983) 57. [3] V.A. Niculescu, K. Raj, J.I. Budnick, T.J. Butch, W.A. Hines and A.M. Menotti, Phys. Rev. B14 (1976) 41~i0.
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S.N. Mishra et a L / Heusler alloy Ru2fe$i
[4] B. Window, J. Phys. E 4 (1971) 401. [5] S.N. Mishra, D. Rambabu, A.K. Grover, R.G. Pillay, P.N. Tandon, H.G. Devare and R. Vijayaraghavan, Solid State Commun. 53 (1985) 321; J. Appl. Phys. (in press).
[6] P.J. Webster and R.S. Tebble, Phil. Mag. 16 (1967) 347. [7] V.A. Niculescu, T.J. Burch and J.l. Budnick, J. Magn. Magn. Mat. 39 (1983) 223, and refs. therein. [8] S. Yoon and J.G~ Booth, J. Phys. F 7 (1977) 1079.