- Email: [email protected]
A neutron diffraction study of the atomic ordering in Er(Fe1 − xCox)3
Journal of the fess-Common
363
Metals, 149 (1989) 363 - 369
A NEUTRON DIFFRACTION IN Er(Fe, _ XCo,)3 *
STUDY OF THE ATOMIC ORDERING
DWAYNE E. THARP, 0. A. PRINGLE, W. J. JAMES, G. K. MARASINGHE GARY J. LONG
and
Departments of Chemistry and Physics and the Graduate Center for Materials Research, University of Missouri-Rolla, Rolla, MO 65401 (U.S.A.) DECAI XIE and W. B. YELON University of Missouri Research Reactor, Columbia, MO 65211
(U.S.A.)
(Received June 15,1988)
Summary Neutron diffraction studies of the intermetallic compounds ErFe,_,sCO0.72, ErFe1.6sCo1.32, and ErFel.116C01.s4, above the magnetic ordering temperatures, indicate that iron atoms preferentially occupy the 6c site, while cobalt atoms show a strong preference for the 3b site and a slight preference for the 18h site. The room-~mperat~e pattern for ErFe,_,,CO 1.84, below the Curie temperature, indicates a collinear arrangement of the magnetic moments along the c axis. A more complex, non-collinear, array of magnetic moments is indicated for ErFez.2sCoo.,2.
1. Introduction Intermetallic compounds of the type ErM3, where M is Fe, Co, and Ni, crystallize in the rhombohedral R&n space group isotypic with PuNiJ. The erbium atoms occupy two non~uiv~ent sites, 3a and 6c, whereas the transition metals occupy three non-equiv~ent sites, 3b, 6c, and 18h. The magnetic behavior of these intermetallic compounds has been studied by several investigators, and a variety of magnetic transitions in ErM, have been reported [l - 81. In ErCo3, all magnetic moments are collinear, parallel to the c axis, with the erbium moments antiparallel to the cobalt moments. No evidence of spin reorientation has been observed from 4.2 to 300 K. In ErFe3, three distinct magnetic structures have been observed [7]. Spin reorientations are found at 42 K (TR1) and at 210 K the iron and erbium moments are collinear and parallel C&2). Below GI, to c. Above TRI , a non-collinear magnetic structure is observed. The erbium *Paper presented at the 18th Rare Earth Research Conference, September 12 - 16,1988. 0022-5088/891$3.50
Lake Geneva, WI,
@ Elsevier Sequoia/Printed in The Netherlands
364
3a moments make an angle of 38” with the c axis, and the erbium 6c and iron moments cant 57” from the c axis, with the iron moments antiparallel to the erbium moments. This magnetic structure is due to the competition of local magnetic anisotropies between the moments at the two erbium sites and those at the 3b and 6c sites of iron [6, 71. As the temperature is increased above TR2, a collinear magnetic structure is re-established with all moments perpendicular to the c axis. We have previously reported the atomic and magnetic structure of Er(Fei _ ,Ni,)3 solid solutions, where the effect of a non-magnetic transition metal was used to elucidate the magnetic structure as a function of both temperature and nickel substitution [8]. In this paper we focus on the preferential site occupancy of the iron and cobalt atoms, and report some preliminary results on the magnetic structures.
2. Experimental details Samples of Er(Fei _ xCo,)3, where 3c is 0.24, 0.44, and 0.61, were prepared by induction melting of the elements in a water-cooled copper cold boat under an argon atmosphere. The samples were examined by X-ray powder diffraction and no lines of other phases were detected, thus confirming that complete solid solubility is achieved. Neutron diffraction data were collected by using the two-axis diffractometer at the University of Missouri Research Reactor, with neutrons of wavelength 1.2892 A. The nuclear scattering lengths used were 0.803, 0.954, and 0.253 X lo-i2 cm for erbium, iron, and cobalt, respectively. Refinements of the resulting patterns were performed by using a modified Rietveld profile method [9]. In these refinements, no constraint was used for the iron to cobalt ratio, but a constraint was imposed to insure that the total atomic population would produce full occupancy on each of the transition metal sites. The erbium sites were constrained to their respective populations dictated by the crystallographic space group.
3. Results and discussion 3.1. Preferred site occupancy of iron and cobalt atoms The ordering temperatures for ErFes and ErCos are 555 and 401 K respectively [l, lo], and hence we expected that their solid solutions would exhibit Curie temperatures below about 600 K. However, our magnetization curves measured on the Er( Fe1 _ xCo,)3 samples show higher Curie temperatures, as indicated in Fig. 1. The Curie temperatures obtained are 647, 668, and 641 K for ErFe2.28C00.,2, ErFe,.,sCo1.32, and ErFe,.&01.s4 respectively. Neutron diffraction data were collected above these magnetic ordering temperatures in order to eliminate the presence of magnetic neutron scattering. This allowed us to determine precisely the extent and
365
Co CONCENTRATION
(%f
Fig. 1. Curie temperatures of Er(Fe+.Co,)a
as a function of cobalt concentration.
nature of the atomic ordering. The patterns and the resulting best fits are shown in Fig. 2. The iron and cobalt population parameters obtained from the best fits by using Rietveld analysis are shown in Table 1, and are also represented as the per cent deviations from a random dis~ibution in Fig. 3. These results indicate p~ferenti~ site occupancies in all three compounds and for all the transition metal sites. Cobalt atoms favor the 3b site and show only a slight preference for the 18h site, whereas iron atoms show a strong preference for the 6c site.
TABLE 1 Atomic occupations
for the crystallographic sitesa Number of atoms
Fe( 3b) Fe(&) Fe( 18h)
ErFe
~rFe2.zaCo0.72
ErFe1.dh.32
1.98(3) 5.08(4) 13.48(6)
1.25(3) 4.46(4) 9.39(6)
0.74(3) 3.48(4) 6.22(5) 2.26(10) 2.52(14) 11.78(20)
1.14m.84
Co(3b) Co(6c) Co(l8h)
1.02(12) 0.92(16) 4.52(22)
1.75(12) 1.54(16) 8.61(23)
&I
7.56
7.37
6.57
x
1.57
1.58
1.32
aFrom neutron diffraction data obtained above the Curie temperature. Standard deviations of the final digits are enclosed in parentheses.
366 12.5 1 9
6.
7 . 2.
4.8
2.4
6
10
20
30
40
2-THETA
50
60
70
80
90
(DEGREES)
Fig. 2. Neutron diffraction patterns and fits for (top) ErFez i28’ CO,-JJ~; (middle) ErFel.w Qq32, and (bottom) ErFe1~16C~t.~ obtained above the Curie temperature.
3.2. Magnetic structures of the Er(Fe, _J?o,)~ compounds Neutron diffraction patterns of ErFe1.16C01S84obtained at room temperature and 8 K are shown in Fig. 4. All the lines are characteristic of the PuNi3-type structure. The intensity of the (003) reflection, which has at most a very weak nuclear contribution, is useful in the determination of the magnetic structure of this compound. The absence of the (003) magnetic reflection requires that all magnetic moments be collinear with
367
4 40
20 Co
CONCENTRATION
Fig. 3. The per cent departure versus the cobalt concentration. site.
80
60
100
(%)
of the cobalt occupancy from a random distribution A positive per cent deviation represents a cobalt-rich
the hexagonal c axis. Hence, the absence of the (003) reflection at room temperature indicates that all moments lie along the c axis in ErFe I. &o~.~~. Table 2 summarizes the room-temperature magnetic results. The intensity of the (003) magnetic reflection is determined by the magnitude of the basal plane component of the magnetic moment. The data at 8 K also appear to favor a collinear array of moments along the c axis; however, a small cant of the moments away from the c axis may also be possible. Thus, in ErFe&Zo1.s4, it appears that the local anisotropies of the erbium 3a and cobalt atoms, which favor c axis alignment, dominate TABLE 2 The room-temperature
K,
magnetic moments on the crystallographic sites of ErFe1.r6C01.s4 &B)
K,
(PB)
M (FBI
fJW> cP
Er(c)
0.00 0.00
4.15 3.90
4.15 3.90
180 180
Fe co
0.00 0.00
1.76 1.38
1.76 1.38
0 0
Er(a)
Rn ElII
6.05 9.29
X
1.32
YqM, c) is the angle between the c axis and the magnetic moment direction,
RT
8K
0
10
20
30
40
2-THETA
50
60
TO
80
90
Lf
(DEGREES)
Fig. 4. Neutron diffraction patterns for EYF~~.J~CO~.~ obtained at, room temperature and 8 K, The 8 K pattern shows only the data.
over those of the erbium 6c and iron atoms, which favor basal plane alignment 171. It is evident from the neutron diffraction and Massbauer effect data for ErFe2.28C00.72that complex magnetic transitions involving non-collinear structures, as observed in ErFe3, are present. Detailed magnetic structures of all compounds will be reported at a later date, Acknowledgments The authors acknowledge support from the University of Missouri under a Weldon Spring Grant, and from the Army Research Office under contract DAAG 29-83-K.0159. References 1 K. Ii. J. Buschow and A. S. Van der Goat, Phys. Status Solidi, 35 (1969) 515. 2 A. N. Van der Kraan, P. C. N. Gubbens and K, ii. J. Buschow, Phys. Status Solidi A, 31 (1915) 495.
369 3 G. J. Bowden and R. K. Day,J. Phys. F, 7 (1977) 181. 4 R. L. Davis, R. K. Day and J. B. Dunlop,J. Phys. F, 7 (1977) 1885. 5 B. Kebe, W. J. James, J. Deportes, R. Lemaire, W. Yelon and R. K. Day, J. Appl. Phys., 52 (1981) 2052. 6 R. Ballou, J. Deportes, B. Kebe and R. Lemaire, J. Magn. Magn. Mater., 54 57 (1986) 494. 7 B. Kebe, Thesis, Grenoble, France, 1983. 8 D. E. Tharp, Y. C. Yang, W. J. James, W. B. Yelon, D. Xie and J. Yang, J. Appl. Phys., 61 (1987) 4249. 9 H. M. Rietveld, J. Appl. Crystollogr., 2 (1969) 65. 10 R. Lemaire, Cobalt, 32 (1966) 132.
363
Metals, 149 (1989) 363 - 369
A NEUTRON DIFFRACTION IN Er(Fe, _ XCo,)3 *
STUDY OF THE ATOMIC ORDERING
DWAYNE E. THARP, 0. A. PRINGLE, W. J. JAMES, G. K. MARASINGHE GARY J. LONG
and
Departments of Chemistry and Physics and the Graduate Center for Materials Research, University of Missouri-Rolla, Rolla, MO 65401 (U.S.A.) DECAI XIE and W. B. YELON University of Missouri Research Reactor, Columbia, MO 65211
(U.S.A.)
(Received June 15,1988)
Summary Neutron diffraction studies of the intermetallic compounds ErFe,_,sCO0.72, ErFe1.6sCo1.32, and ErFel.116C01.s4, above the magnetic ordering temperatures, indicate that iron atoms preferentially occupy the 6c site, while cobalt atoms show a strong preference for the 3b site and a slight preference for the 18h site. The room-~mperat~e pattern for ErFe,_,,CO 1.84, below the Curie temperature, indicates a collinear arrangement of the magnetic moments along the c axis. A more complex, non-collinear, array of magnetic moments is indicated for ErFez.2sCoo.,2.
1. Introduction Intermetallic compounds of the type ErM3, where M is Fe, Co, and Ni, crystallize in the rhombohedral R&n space group isotypic with PuNiJ. The erbium atoms occupy two non~uiv~ent sites, 3a and 6c, whereas the transition metals occupy three non-equiv~ent sites, 3b, 6c, and 18h. The magnetic behavior of these intermetallic compounds has been studied by several investigators, and a variety of magnetic transitions in ErM, have been reported [l - 81. In ErCo3, all magnetic moments are collinear, parallel to the c axis, with the erbium moments antiparallel to the cobalt moments. No evidence of spin reorientation has been observed from 4.2 to 300 K. In ErFe3, three distinct magnetic structures have been observed [7]. Spin reorientations are found at 42 K (TR1) and at 210 K the iron and erbium moments are collinear and parallel C&2). Below GI, to c. Above TRI , a non-collinear magnetic structure is observed. The erbium *Paper presented at the 18th Rare Earth Research Conference, September 12 - 16,1988. 0022-5088/891$3.50
Lake Geneva, WI,
@ Elsevier Sequoia/Printed in The Netherlands
364
3a moments make an angle of 38” with the c axis, and the erbium 6c and iron moments cant 57” from the c axis, with the iron moments antiparallel to the erbium moments. This magnetic structure is due to the competition of local magnetic anisotropies between the moments at the two erbium sites and those at the 3b and 6c sites of iron [6, 71. As the temperature is increased above TR2, a collinear magnetic structure is re-established with all moments perpendicular to the c axis. We have previously reported the atomic and magnetic structure of Er(Fei _ ,Ni,)3 solid solutions, where the effect of a non-magnetic transition metal was used to elucidate the magnetic structure as a function of both temperature and nickel substitution [8]. In this paper we focus on the preferential site occupancy of the iron and cobalt atoms, and report some preliminary results on the magnetic structures.
2. Experimental details Samples of Er(Fei _ xCo,)3, where 3c is 0.24, 0.44, and 0.61, were prepared by induction melting of the elements in a water-cooled copper cold boat under an argon atmosphere. The samples were examined by X-ray powder diffraction and no lines of other phases were detected, thus confirming that complete solid solubility is achieved. Neutron diffraction data were collected by using the two-axis diffractometer at the University of Missouri Research Reactor, with neutrons of wavelength 1.2892 A. The nuclear scattering lengths used were 0.803, 0.954, and 0.253 X lo-i2 cm for erbium, iron, and cobalt, respectively. Refinements of the resulting patterns were performed by using a modified Rietveld profile method [9]. In these refinements, no constraint was used for the iron to cobalt ratio, but a constraint was imposed to insure that the total atomic population would produce full occupancy on each of the transition metal sites. The erbium sites were constrained to their respective populations dictated by the crystallographic space group.
3. Results and discussion 3.1. Preferred site occupancy of iron and cobalt atoms The ordering temperatures for ErFes and ErCos are 555 and 401 K respectively [l, lo], and hence we expected that their solid solutions would exhibit Curie temperatures below about 600 K. However, our magnetization curves measured on the Er( Fe1 _ xCo,)3 samples show higher Curie temperatures, as indicated in Fig. 1. The Curie temperatures obtained are 647, 668, and 641 K for ErFe2.28C00.,2, ErFe,.,sCo1.32, and ErFe,.&01.s4 respectively. Neutron diffraction data were collected above these magnetic ordering temperatures in order to eliminate the presence of magnetic neutron scattering. This allowed us to determine precisely the extent and
365
Co CONCENTRATION
(%f
Fig. 1. Curie temperatures of Er(Fe+.Co,)a
as a function of cobalt concentration.
nature of the atomic ordering. The patterns and the resulting best fits are shown in Fig. 2. The iron and cobalt population parameters obtained from the best fits by using Rietveld analysis are shown in Table 1, and are also represented as the per cent deviations from a random dis~ibution in Fig. 3. These results indicate p~ferenti~ site occupancies in all three compounds and for all the transition metal sites. Cobalt atoms favor the 3b site and show only a slight preference for the 18h site, whereas iron atoms show a strong preference for the 6c site.
TABLE 1 Atomic occupations
for the crystallographic sitesa Number of atoms
Fe( 3b) Fe(&) Fe( 18h)
ErFe
~rFe2.zaCo0.72
ErFe1.dh.32
1.98(3) 5.08(4) 13.48(6)
1.25(3) 4.46(4) 9.39(6)
0.74(3) 3.48(4) 6.22(5) 2.26(10) 2.52(14) 11.78(20)
1.14m.84
Co(3b) Co(6c) Co(l8h)
1.02(12) 0.92(16) 4.52(22)
1.75(12) 1.54(16) 8.61(23)
&I
7.56
7.37
6.57
x
1.57
1.58
1.32
aFrom neutron diffraction data obtained above the Curie temperature. Standard deviations of the final digits are enclosed in parentheses.
366 12.5 1 9
6.
7 . 2.
4.8
2.4
6
10
20
30
40
2-THETA
50
60
70
80
90
(DEGREES)
Fig. 2. Neutron diffraction patterns and fits for (top) ErFez i28’ CO,-JJ~; (middle) ErFel.w Qq32, and (bottom) ErFe1~16C~t.~ obtained above the Curie temperature.
3.2. Magnetic structures of the Er(Fe, _J?o,)~ compounds Neutron diffraction patterns of ErFe1.16C01S84obtained at room temperature and 8 K are shown in Fig. 4. All the lines are characteristic of the PuNi3-type structure. The intensity of the (003) reflection, which has at most a very weak nuclear contribution, is useful in the determination of the magnetic structure of this compound. The absence of the (003) magnetic reflection requires that all magnetic moments be collinear with
367
4 40
20 Co
CONCENTRATION
Fig. 3. The per cent departure versus the cobalt concentration. site.
80
60
100
(%)
of the cobalt occupancy from a random distribution A positive per cent deviation represents a cobalt-rich
the hexagonal c axis. Hence, the absence of the (003) reflection at room temperature indicates that all moments lie along the c axis in ErFe I. &o~.~~. Table 2 summarizes the room-temperature magnetic results. The intensity of the (003) magnetic reflection is determined by the magnitude of the basal plane component of the magnetic moment. The data at 8 K also appear to favor a collinear array of moments along the c axis; however, a small cant of the moments away from the c axis may also be possible. Thus, in ErFe&Zo1.s4, it appears that the local anisotropies of the erbium 3a and cobalt atoms, which favor c axis alignment, dominate TABLE 2 The room-temperature
K,
magnetic moments on the crystallographic sites of ErFe1.r6C01.s4 &B)
K,
(PB)
M (FBI
fJW> cP
Er(c)
0.00 0.00
4.15 3.90
4.15 3.90
180 180
Fe co
0.00 0.00
1.76 1.38
1.76 1.38
0 0
Er(a)
Rn ElII
6.05 9.29
X
1.32
YqM, c) is the angle between the c axis and the magnetic moment direction,
RT
8K
0
10
20
30
40
2-THETA
50
60
TO
80
90
Lf
(DEGREES)
Fig. 4. Neutron diffraction patterns for EYF~~.J~CO~.~ obtained at, room temperature and 8 K, The 8 K pattern shows only the data.
over those of the erbium 6c and iron atoms, which favor basal plane alignment 171. It is evident from the neutron diffraction and Massbauer effect data for ErFe2.28C00.72that complex magnetic transitions involving non-collinear structures, as observed in ErFe3, are present. Detailed magnetic structures of all compounds will be reported at a later date, Acknowledgments The authors acknowledge support from the University of Missouri under a Weldon Spring Grant, and from the Army Research Office under contract DAAG 29-83-K.0159. References 1 K. Ii. J. Buschow and A. S. Van der Goat, Phys. Status Solidi, 35 (1969) 515. 2 A. N. Van der Kraan, P. C. N. Gubbens and K, ii. J. Buschow, Phys. Status Solidi A, 31 (1915) 495.
369 3 G. J. Bowden and R. K. Day,J. Phys. F, 7 (1977) 181. 4 R. L. Davis, R. K. Day and J. B. Dunlop,J. Phys. F, 7 (1977) 1885. 5 B. Kebe, W. J. James, J. Deportes, R. Lemaire, W. Yelon and R. K. Day, J. Appl. Phys., 52 (1981) 2052. 6 R. Ballou, J. Deportes, B. Kebe and R. Lemaire, J. Magn. Magn. Mater., 54 57 (1986) 494. 7 B. Kebe, Thesis, Grenoble, France, 1983. 8 D. E. Tharp, Y. C. Yang, W. J. James, W. B. Yelon, D. Xie and J. Yang, J. Appl. Phys., 61 (1987) 4249. 9 H. M. Rietveld, J. Appl. Crystollogr., 2 (1969) 65. 10 R. Lemaire, Cobalt, 32 (1966) 132.