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Magnetic and electronic properties of CrxIr1−x Alloys
Journal of Magnetism and Magnetic Materials 54-57 (1986) 1071-1072
MAGNETIC AND ELECTRONIC PROPERTIES
1071
O F Cr~Ir t _ ~ A L L O Y S
R. J E S S E R , R. K U E N T Z L E R L.M.S.E.S., 3, rue de l'Universitb, 67084 Strasbourg, France and R.M. WATERSTRAT Health Foundation, Gaithersburg, MD 20899, USA
The magnetic and electronic properties of Cr~Ir 1-x alloys with x = 0.04 to 0.28 are reported. The Ir-based solid solutions Crxlq x (x = 0.04 to 0.15) are paramagnetic with weak susceptibilities (X ~<1.2 p.emu g-~Oe ~) and their specific heat coefficient y increases with x. The Crxlrl_ x alloys with x = 0.21 to 0.28 are ferrimagnetic in the Cu3Au-type ordered structure. These ferrimagnets show relatively weak absolute moments (/~ ~<0.164/~Bat-l) and nearly constant y values (y = 4.4 mJat 1K-2).
1. Introduction The properties and stability study of the ordered alloys is now a research subject of increasing interest. We have reached some conclusions concerning nonmagnetic (ordered) alloys [1] for which the theories are most advanced. When magnetic interactions occur, they may compete with the chemical interactions and the situation becomes rather complex [2]. The main parameters which we use, are (Nd, AN, x ) and ( n ( E v ) , Tform), where Nd is the mean number of d electrons, A N the difference in valence between the two constituents, x the alloy composition, n ( E v ) is the density of states at the Fermi level E v and Tform is the temperature of formation of the alloy [1]. In the case of binary alloys of transition metals which possess the ordered Cu3Au-type structure, a certain number of general trends can be seen [3], and the magnetic alloys can be distinguished from the non-magnetic ones in the (F/d vs. A N ) and (y vs. Tform ) plots. But Ir3Cr is peculiar: its A N value is typical of a magnetic alloy while its Na value and its assumed Tform value are typical of a non-magnetic alloy. Very little information exists about the properties of the I r - C r alloys (see ref. [4] and references therein), and a revised constitution diagram has been made available [5] only relatively recently for this system. We report here the magnetic and electronic properties of the Crxlr l_x alloys in the composition range x = 0.04 to 0.28.
2. Elements of the constitution diagram The composition range considered here (0-28 at% Cr) represents the solubility range of Cr in fcc Ir. The temperatures of the order-disorder transformation are not well established but they appear to be strongly composition dependent. It was found that rapid cooling after arc-melting did not suppress the ordering reaction in alloys with x = 0.23 to 0.28, but that in Ir0.84Cr0.16 0304-8853/86/$03.50
atomic ordering could be produced only this powdered alloy for several days at between 600 and 700°C. All Crxlrl_ x been studied here after high-temperature annealing.
by annealing temperatures samples have (T>~ 1900 K)
3. Experimental results First, we summarize briefly the main results of our magnetic study. The CrxIrl_ x alloys with x = 0.04 to 0.16 are only paramagnets, even down to 4.2 K. Their weak susceptibility (at 4.2 K, X ~< 1.2 ~emu g - l O e -1) is consistent with the band-paramagnetism of I r - C r solid solutions. But the alloys with x = 0.21 to-0.28, which have the Cu3Au-like ordered structure (OS), are found to order magnetically below T~ = 385 to 425 K. This magnetic order may be characterized by the following features. (i) The magnetization o ( H ) at 4.2 K, cycled in the field range H = - 1 9 to + 1 9 kOe ( H being the f i e l d corrected for demagnetization) showed classical hysteresis loops with moderate coercive forces ( H c = 0.4 to 0.5 kOe). (ii) The paramagnetic susceptibility could be fitted to a classical Curie-Weiss law x ( T ) = X' + C ( T - 0p) -1 in the high-temperature range 450 K ~< T ~ 920 K with 0p slightly higher than T~ and X'-~0.17 to 0.38 ~emu g - l O e - a . The paramagnetic moment /~' (in #B per mean-atom of alloy) decreases from 0.84/1B for x = 0.21 to 0.78# B for x = 0.28. (iii) The high-field (H~< 150 kOe) magnetization of these alloys at 4.2 K (fig. 1) obeys the classical law of approach-to-saturation o ( H ) = HXH + Os(1 - b / H 2) with XH ~ 0.2 to 1.4 ~emu g - l O e -1. The absolute saturation magnetization Oo(X ) (fig. 2), taken here as o0 = o s (4.2 K), leads to an absolute moment /~ ranging from 0.148 to 0.164/~ B per mean-atom of alloy. (iv) The remanent magnetization of these alloys shows a T-dependence typical of ferrimagnetism. In view of all these results, we conclude that the C r x l r l - x alloys with x = 0.21 to 0.28 are ferrimagnetic
© E l s e v i e r S c i e n c e P u b l i s h e r s B.V.
1072
R. Jesser et aL / Properties of Cr~I0
~ alloys
/
4l
4.81 4.6
4.2
0
~ ~-~¢-~k,._y :
, . . . . . . . . . . . . . . . . 200 4.00
600
800
i
Y
0
.I
3 0.250
/
•
/
/%
i6
• /
~
_ ~ .2
~
~ 5 .3
X
~
1000 1200 1400 1600 H-2
,
Fig. 1. High-field magnetization of ferrimagnetic Cr~Ir~ alloys at 4.2 K: o ' = o - - H X H (emu g l) vs. H 2 (MCIe 2). Linearity holds over: H >i 23 kOe for x = 0.21 and 0.23; H i> 42 kOe for x = 0.25 and H >~ 48 kOe for x = 0.28.
in the C u 3 A u - t y p e OS. The choice /~tr = -0.27p, B / I r a t o m in the stoichiometric Ir3Cr alloy (resembling/~et = --0.27/~B/Pt atom in the Pt3Cr alloy [6] also ferrimagnetic in the C u 3 A u - t y p e OS) allows a direct comparison of the Cr m o m e n t /~Cr = + l'47/~B/Cr in Ir3Cr with that k n o w n [6] # c r = + 2 . 3 3 / ~ B / C r in Pt~Cr. The specific heat capacity C ( T ) at T = 1.4 to 20 K, analysed by means of the usual relation C / T = 2/+ B T 2 (where 2/ is the electronic coefficient related to n ( E v ) a n d 13 the lattice coefficient related to the Debye temp e r a t u r e 0D) is affected by atomic ordering [7] a n d by magnetic ordering [2]. The plot C / T vs. T 2 is a straight line below T = 8 K for all the investigated samples. M o r e o v e r the absence of any u p t u r n at low t e m p e r a t u r e in the C / T vs. T 2 plot seems to rule out the presence of magnetic clusters in these alloys [2]. The variations of 2/ (fig. 2) a n d 0 D with Cr content x, call for the following comments. (i) T h e increase of 2/ in the x range 0.04 < x ~< 0.15 seems consistent with the fact that E v lies in a valley of the density of states (DOS) of Ir. The Or> t e m p e r a t u r e is roughly c o n s t a n t (0 D ~ 430 K) for x = 0.04 to 0.15. (ii) In the x range 0.15 < x ~< 0.28, in which atomic order and ferrimagnetism appear, 2/ is nearly c o n s t a n t ( 2 / = 4 . 4 2 mJ a t - l K 2 for Ir3Cr annealed at 2173 K d u r i n g 5 h) with a maximal value at x = 0.15, while 0 o increases up to 505 K (for x = 0.23) a n d then, decreases. (iii) The c o m p o s i t i o n range x = 0.15 where atomic ordering a n d ferrimagnetism disappear in the I r - C r system, is yet not clearly characterized. Nevertheless, we assume that the magnetic o r d e r - d i s o r d e r transition in this system, is h o m o g e n e o u s in view of the high quality of metallurgical homogeneity of the investigated samples.
Fig. 2. Electronic specific heat coefficient y in mJ at IK-2 (O and scale left) and absolute magnetization % in emu g l (e and scale right) of Cr~Irj ~ against Cr content x.
4. Concluding remarks A comprehensive view of the electronic a n d magnetic properties of the Crxlr t -x alloys investigated here, can yet only be given for the two composition ranges 0.04 < x~<0.15 a n d 0.21~
MAGNETIC AND ELECTRONIC PROPERTIES
1071
O F Cr~Ir t _ ~ A L L O Y S
R. J E S S E R , R. K U E N T Z L E R L.M.S.E.S., 3, rue de l'Universitb, 67084 Strasbourg, France and R.M. WATERSTRAT Health Foundation, Gaithersburg, MD 20899, USA
The magnetic and electronic properties of Cr~Ir 1-x alloys with x = 0.04 to 0.28 are reported. The Ir-based solid solutions Crxlq x (x = 0.04 to 0.15) are paramagnetic with weak susceptibilities (X ~<1.2 p.emu g-~Oe ~) and their specific heat coefficient y increases with x. The Crxlrl_ x alloys with x = 0.21 to 0.28 are ferrimagnetic in the Cu3Au-type ordered structure. These ferrimagnets show relatively weak absolute moments (/~ ~<0.164/~Bat-l) and nearly constant y values (y = 4.4 mJat 1K-2).
1. Introduction The properties and stability study of the ordered alloys is now a research subject of increasing interest. We have reached some conclusions concerning nonmagnetic (ordered) alloys [1] for which the theories are most advanced. When magnetic interactions occur, they may compete with the chemical interactions and the situation becomes rather complex [2]. The main parameters which we use, are (Nd, AN, x ) and ( n ( E v ) , Tform), where Nd is the mean number of d electrons, A N the difference in valence between the two constituents, x the alloy composition, n ( E v ) is the density of states at the Fermi level E v and Tform is the temperature of formation of the alloy [1]. In the case of binary alloys of transition metals which possess the ordered Cu3Au-type structure, a certain number of general trends can be seen [3], and the magnetic alloys can be distinguished from the non-magnetic ones in the (F/d vs. A N ) and (y vs. Tform ) plots. But Ir3Cr is peculiar: its A N value is typical of a magnetic alloy while its Na value and its assumed Tform value are typical of a non-magnetic alloy. Very little information exists about the properties of the I r - C r alloys (see ref. [4] and references therein), and a revised constitution diagram has been made available [5] only relatively recently for this system. We report here the magnetic and electronic properties of the Crxlr l_x alloys in the composition range x = 0.04 to 0.28.
2. Elements of the constitution diagram The composition range considered here (0-28 at% Cr) represents the solubility range of Cr in fcc Ir. The temperatures of the order-disorder transformation are not well established but they appear to be strongly composition dependent. It was found that rapid cooling after arc-melting did not suppress the ordering reaction in alloys with x = 0.23 to 0.28, but that in Ir0.84Cr0.16 0304-8853/86/$03.50
atomic ordering could be produced only this powdered alloy for several days at between 600 and 700°C. All Crxlrl_ x been studied here after high-temperature annealing.
by annealing temperatures samples have (T>~ 1900 K)
3. Experimental results First, we summarize briefly the main results of our magnetic study. The CrxIrl_ x alloys with x = 0.04 to 0.16 are only paramagnets, even down to 4.2 K. Their weak susceptibility (at 4.2 K, X ~< 1.2 ~emu g - l O e -1) is consistent with the band-paramagnetism of I r - C r solid solutions. But the alloys with x = 0.21 to-0.28, which have the Cu3Au-like ordered structure (OS), are found to order magnetically below T~ = 385 to 425 K. This magnetic order may be characterized by the following features. (i) The magnetization o ( H ) at 4.2 K, cycled in the field range H = - 1 9 to + 1 9 kOe ( H being the f i e l d corrected for demagnetization) showed classical hysteresis loops with moderate coercive forces ( H c = 0.4 to 0.5 kOe). (ii) The paramagnetic susceptibility could be fitted to a classical Curie-Weiss law x ( T ) = X' + C ( T - 0p) -1 in the high-temperature range 450 K ~< T ~ 920 K with 0p slightly higher than T~ and X'-~0.17 to 0.38 ~emu g - l O e - a . The paramagnetic moment /~' (in #B per mean-atom of alloy) decreases from 0.84/1B for x = 0.21 to 0.78# B for x = 0.28. (iii) The high-field (H~< 150 kOe) magnetization of these alloys at 4.2 K (fig. 1) obeys the classical law of approach-to-saturation o ( H ) = HXH + Os(1 - b / H 2) with XH ~ 0.2 to 1.4 ~emu g - l O e -1. The absolute saturation magnetization Oo(X ) (fig. 2), taken here as o0 = o s (4.2 K), leads to an absolute moment /~ ranging from 0.148 to 0.164/~ B per mean-atom of alloy. (iv) The remanent magnetization of these alloys shows a T-dependence typical of ferrimagnetism. In view of all these results, we conclude that the C r x l r l - x alloys with x = 0.21 to 0.28 are ferrimagnetic
© E l s e v i e r S c i e n c e P u b l i s h e r s B.V.
1072
R. Jesser et aL / Properties of Cr~I0
~ alloys
/
4l
4.81 4.6
4.2
0
~ ~-~¢-~k,._y :
, . . . . . . . . . . . . . . . . 200 4.00
600
800
i
Y
0
.I
3 0.250
/
•
/
/%
i6
• /
~
_ ~ .2
~
~ 5 .3
X
~
1000 1200 1400 1600 H-2
,
Fig. 1. High-field magnetization of ferrimagnetic Cr~Ir~ alloys at 4.2 K: o ' = o - - H X H (emu g l) vs. H 2 (MCIe 2). Linearity holds over: H >i 23 kOe for x = 0.21 and 0.23; H i> 42 kOe for x = 0.25 and H >~ 48 kOe for x = 0.28.
in the C u 3 A u - t y p e OS. The choice /~tr = -0.27p, B / I r a t o m in the stoichiometric Ir3Cr alloy (resembling/~et = --0.27/~B/Pt atom in the Pt3Cr alloy [6] also ferrimagnetic in the C u 3 A u - t y p e OS) allows a direct comparison of the Cr m o m e n t /~Cr = + l'47/~B/Cr in Ir3Cr with that k n o w n [6] # c r = + 2 . 3 3 / ~ B / C r in Pt~Cr. The specific heat capacity C ( T ) at T = 1.4 to 20 K, analysed by means of the usual relation C / T = 2/+ B T 2 (where 2/ is the electronic coefficient related to n ( E v ) a n d 13 the lattice coefficient related to the Debye temp e r a t u r e 0D) is affected by atomic ordering [7] a n d by magnetic ordering [2]. The plot C / T vs. T 2 is a straight line below T = 8 K for all the investigated samples. M o r e o v e r the absence of any u p t u r n at low t e m p e r a t u r e in the C / T vs. T 2 plot seems to rule out the presence of magnetic clusters in these alloys [2]. The variations of 2/ (fig. 2) a n d 0 D with Cr content x, call for the following comments. (i) T h e increase of 2/ in the x range 0.04 < x ~< 0.15 seems consistent with the fact that E v lies in a valley of the density of states (DOS) of Ir. The Or> t e m p e r a t u r e is roughly c o n s t a n t (0 D ~ 430 K) for x = 0.04 to 0.15. (ii) In the x range 0.15 < x ~< 0.28, in which atomic order and ferrimagnetism appear, 2/ is nearly c o n s t a n t ( 2 / = 4 . 4 2 mJ a t - l K 2 for Ir3Cr annealed at 2173 K d u r i n g 5 h) with a maximal value at x = 0.15, while 0 o increases up to 505 K (for x = 0.23) a n d then, decreases. (iii) The c o m p o s i t i o n range x = 0.15 where atomic ordering a n d ferrimagnetism disappear in the I r - C r system, is yet not clearly characterized. Nevertheless, we assume that the magnetic o r d e r - d i s o r d e r transition in this system, is h o m o g e n e o u s in view of the high quality of metallurgical homogeneity of the investigated samples.
Fig. 2. Electronic specific heat coefficient y in mJ at IK-2 (O and scale left) and absolute magnetization % in emu g l (e and scale right) of Cr~Irj ~ against Cr content x.
4. Concluding remarks A comprehensive view of the electronic a n d magnetic properties of the Crxlr t -x alloys investigated here, can yet only be given for the two composition ranges 0.04 < x~<0.15 a n d 0.21~