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Determination of cobalt behavior in TmCo2 and HoCo2 by means of polarized neutron diffraction
165 DETERMINATION OF COBALT BEHAVIOR IN TmCo2 AND HoCo2 BY MEANS OF POLARIZED NEUTRON DIFFRACTION D. G I G N O U X , F. G I V O R D Laboratoire de Magn~tisme, CNRS, 166X, 38042-Grenoble-Cedex, France
and W.C. K O E H L E R ORNL, Oak Ridge, Tennessee 37830, USA The determination, from polarized neutron diffraction measurements, of the Co moment in HoCo2 and TmCo2 at various temperatures shows that the Co paramagnetic susceptibility is of Pauli type. Below the ordering temperature a large increase of the Co susceptibility, related to the large value of the molecular field, is observed.
I. Introduction In the RCo2 compounds, a magnetic moment is observed on Co atoms only when the alloyed rare earth R is magnetic [I]. Thus LuCo2 and YCo2 are strongly enhanced Pauli paramagnets, whereas a moment on Co atoms (I #`B/Co in GdCo2) is created by the magnetic interactions. The measured thermal variation of the paramagnetic susceptibility can be interpreted with two different models assuming that the Co moment is either permanent [2] (localized moment model) or induced at each temperature by the total field acting on the 3d band [3] (collective electron model). A polarized neutron study on single crystals of TmCo2 and HoCo2 has been undertaken in order to enable us to separate the 3d-Co moment from ,
I
i
I
I
-0.8
/
TmCo 2 -0.6
?_
/
i
the strong 4f localized rare earth moment and from a diffuse density. The study has been done at various temperatures in order to determine the Co behavior in the ordered and paramagnetic states. The polarized neutron diffraction measurements were performed in Grenoble ( I L L and C E N G ) and in Oak Ridge (HFIR). Experimental conditions and result analysis are described elsewhere [4, 5]. The R and Co magnetic moments #`R and P-co thus deduced are shown on fig. 1 for TmCo2 and in table I for HoCo2. Table I Ho and Co moments under various conditions of field and temperature, and thermal variation of the Co susceptibility Xy in HoCo2 T (K)
H (kOe)
]J'Ho
(P-B)
77 95 108 120 120 128 250 250 300
16.9 11.6 11.6 57.2 10.0 16.9 57.2 15.0 11.6
6.8 (1) 1.35 (5) 0.80 (2) 2.31 (6) 0.425 (11) 0.68 (1) 0.663 (17) 0.181 (6) 0.135 (5)
P-Co(P-n)
X, (emu/Co at.g.)
-0.80 (4) -0.105 (6) -0.061 (6) -0.180 (5) -0.032 (2) -0.055 (5) -0.040 (2) -0.011 (1) -0.0085(15)
3.5 (2) 2.4(2) 2.4(3) 2.6(1) 2.5 (2) 2.7(3) 3.2(3) 3.2(5) 3.4(8)
2. Paramagnetic state
/
0
1 K
P,1, I
o
4.2K
,
,
,
,
I
I
I
I
2
IJT~ { Pa) Fig. 1. Co moment versus Tm moment, measured at various temperatures, in TmCo2.
Physica 86-88B (1977) 165-166 (~) North-Holland
In the molecular field model, #`Co can be expressed at each temperature as a function of #`R and the applied magnetic field H: #`Co = A [nRco#`R + n CoCo#`Co + H ]
=A'[naco#`a+H], with A ' -
A 1 - Ancoco'
where nRco and ncoco are the molecular field coefficients. Whether the cobalt moment is per-
166 manent or induced, the thermal variation of the Co susceptibility A ' is different. In the permanent moment model A ' observes a Curie-Weiss law: A ' = C / ( T - 0 ) , whereas if the moment is induced on the 3d band: A ' = ~(y [3], which is the enhanced Pauli susceptibility of Co, little temperature dependent. For TmCo2, points represented on fig. 1 have been measured for the same applied field H = 57.2 kOe. The Co moment changes sign between 125 and 250 K which yields to the value of nvmco: at point P, /.tco=0 and nv,,Co=-H/lzvm = - 28 -+ 3 emu/at.g. Moreover, points measured at T / > 2 5 K lie on a straight line showing that A'nTmco is temperature independent. The thermal variation of nvmco being generally weak, A ' is constant. The Co susceptibility cannot follow a Curie-Weiss law, but is in agreement with the enhanced paramagnetism model with ~(,. = (2.8 + 0.3) × 10 3 emu/Co at.g. For HoCo2, WHo and /~co values have been determined in different applied fields. It is then necessary to calculate H , = nHoco/~Ho+ H in order to get A'. nHoco cannot be evaluated directly from the polarized neutron measurements, because the Co moment is found to have the same sign at all temperatures. We have first used the value nHoco-- - 28 emu/at.g which is obtained in interpreting with both models the bulk susceptibility measurements [6]. In the paramagnetic state, the values of A ' thus obtained increase slightly with temperature, as the Y C o 2 and LuCo2 susceptibilities. As in TmCo2, this behavior agrees with the enhanced paramagnetism model. However, the mean value of A ' is higher than that of X~., which, associated with the preceding value of n HoCo,accounts for the thermal variation of the HoCo2 bulk susceptibility. In fact, the values of n Hoco and X~ are strongly correlated. The best agreement between the results of the polarized neutron study and the bulk susceptibility has been obtained, after refinement, for the values nHoco = - 3 4 emu/at.g. The corresponding values of A ' - ~ Xy are given in table I and are close to that found for TmCo2. However, these Co enhanced susceptibilities in TmCo2 and HoCo2 are higher than the corresponding YCo2 susceptibility (1.8 × 10-3 emu/Co at.g at 150 K). 3. Ordered state
Points measured in the ordered state, 4.2 K for
TmCo2 and 7 7 K for HoCo2, show a sudden increase of the Co susceptibility (fig. 1 and table I). This behavior is associated with the high values of the field Ht as illustrated by fig. 2. It has to be related to the high molecular field (> 500 kOe), due to R atoms, acting on the Co atoms. A similar variation has also been observed in YCo2: the susceptibility measured at 4.2 K increases of roughly 30% in an applied field of 380 kOe [7] (fig. 2). Such a field dependence of the cobalt susceptibility has been attributed [8] to the fact that the Fermi level lies in a dip of the density of states of the 3d band. In this case, it is then possible to observe a transition in high field called collective electron metamagnetism, which can explain the first order transitions observed in DyCo2, HoCo> ErCo2 [8]. It is a pleasure to thank R. Lemaire, who has proposed this subject, for his constant interest in this study.
/
08
06
Tm Co 2 ,
HoCo 2
% l
v
0,4
//i/ /
8 02
/
" ~ _ 0 0
/
_~-YC°2
.
o
g
~
~'
i
i
9~o
~2oo'
,soo
Ht =nRco ['JR * H (kC~)
Fig. 2. Co moment versus the field H,=nRc,,#.+H for R = Tin, Ho and Y.
References [1] [2] [3] 14] [5l [6] [7] [8]
R. Lemaire, Cobalt 33 (1966) 201. E. Burzo, Phys. Rev. B6 (1972) 2882. D. Bloch and R. Lemaire, Phys. Rev. B2 (1970) 2648. D. Gignoux, D. Givord, F. Givord, W.C. Koehler and R.M. Moon, Phys. Rev. BI4 (1976) 162. D. Gignoux, F. Givord and J. Schweizer, to be published. F. Chaiss6, Thesis, University of Grenoble, France (1971). C.J. Schinkel, private communication. D. Bloch, D.M. Edwards, M. Shimizu and J. Voiron, J. Phys. F5 (1975) 1217.
and W.C. K O E H L E R ORNL, Oak Ridge, Tennessee 37830, USA The determination, from polarized neutron diffraction measurements, of the Co moment in HoCo2 and TmCo2 at various temperatures shows that the Co paramagnetic susceptibility is of Pauli type. Below the ordering temperature a large increase of the Co susceptibility, related to the large value of the molecular field, is observed.
I. Introduction In the RCo2 compounds, a magnetic moment is observed on Co atoms only when the alloyed rare earth R is magnetic [I]. Thus LuCo2 and YCo2 are strongly enhanced Pauli paramagnets, whereas a moment on Co atoms (I #`B/Co in GdCo2) is created by the magnetic interactions. The measured thermal variation of the paramagnetic susceptibility can be interpreted with two different models assuming that the Co moment is either permanent [2] (localized moment model) or induced at each temperature by the total field acting on the 3d band [3] (collective electron model). A polarized neutron study on single crystals of TmCo2 and HoCo2 has been undertaken in order to enable us to separate the 3d-Co moment from ,
I
i
I
I
-0.8
/
TmCo 2 -0.6
?_
/
i
the strong 4f localized rare earth moment and from a diffuse density. The study has been done at various temperatures in order to determine the Co behavior in the ordered and paramagnetic states. The polarized neutron diffraction measurements were performed in Grenoble ( I L L and C E N G ) and in Oak Ridge (HFIR). Experimental conditions and result analysis are described elsewhere [4, 5]. The R and Co magnetic moments #`R and P-co thus deduced are shown on fig. 1 for TmCo2 and in table I for HoCo2. Table I Ho and Co moments under various conditions of field and temperature, and thermal variation of the Co susceptibility Xy in HoCo2 T (K)
H (kOe)
]J'Ho
(P-B)
77 95 108 120 120 128 250 250 300
16.9 11.6 11.6 57.2 10.0 16.9 57.2 15.0 11.6
6.8 (1) 1.35 (5) 0.80 (2) 2.31 (6) 0.425 (11) 0.68 (1) 0.663 (17) 0.181 (6) 0.135 (5)
P-Co(P-n)
X, (emu/Co at.g.)
-0.80 (4) -0.105 (6) -0.061 (6) -0.180 (5) -0.032 (2) -0.055 (5) -0.040 (2) -0.011 (1) -0.0085(15)
3.5 (2) 2.4(2) 2.4(3) 2.6(1) 2.5 (2) 2.7(3) 3.2(3) 3.2(5) 3.4(8)
2. Paramagnetic state
/
0
1 K
P,1, I
o
4.2K
,
,
,
,
I
I
I
I
2
IJT~ { Pa) Fig. 1. Co moment versus Tm moment, measured at various temperatures, in TmCo2.
Physica 86-88B (1977) 165-166 (~) North-Holland
In the molecular field model, #`Co can be expressed at each temperature as a function of #`R and the applied magnetic field H: #`Co = A [nRco#`R + n CoCo#`Co + H ]
=A'[naco#`a+H], with A ' -
A 1 - Ancoco'
where nRco and ncoco are the molecular field coefficients. Whether the cobalt moment is per-
166 manent or induced, the thermal variation of the Co susceptibility A ' is different. In the permanent moment model A ' observes a Curie-Weiss law: A ' = C / ( T - 0 ) , whereas if the moment is induced on the 3d band: A ' = ~(y [3], which is the enhanced Pauli susceptibility of Co, little temperature dependent. For TmCo2, points represented on fig. 1 have been measured for the same applied field H = 57.2 kOe. The Co moment changes sign between 125 and 250 K which yields to the value of nvmco: at point P, /.tco=0 and nv,,Co=-H/lzvm = - 28 -+ 3 emu/at.g. Moreover, points measured at T / > 2 5 K lie on a straight line showing that A'nTmco is temperature independent. The thermal variation of nvmco being generally weak, A ' is constant. The Co susceptibility cannot follow a Curie-Weiss law, but is in agreement with the enhanced paramagnetism model with ~(,. = (2.8 + 0.3) × 10 3 emu/Co at.g. For HoCo2, WHo and /~co values have been determined in different applied fields. It is then necessary to calculate H , = nHoco/~Ho+ H in order to get A'. nHoco cannot be evaluated directly from the polarized neutron measurements, because the Co moment is found to have the same sign at all temperatures. We have first used the value nHoco-- - 28 emu/at.g which is obtained in interpreting with both models the bulk susceptibility measurements [6]. In the paramagnetic state, the values of A ' thus obtained increase slightly with temperature, as the Y C o 2 and LuCo2 susceptibilities. As in TmCo2, this behavior agrees with the enhanced paramagnetism model. However, the mean value of A ' is higher than that of X~., which, associated with the preceding value of n HoCo,accounts for the thermal variation of the HoCo2 bulk susceptibility. In fact, the values of n Hoco and X~ are strongly correlated. The best agreement between the results of the polarized neutron study and the bulk susceptibility has been obtained, after refinement, for the values nHoco = - 3 4 emu/at.g. The corresponding values of A ' - ~ Xy are given in table I and are close to that found for TmCo2. However, these Co enhanced susceptibilities in TmCo2 and HoCo2 are higher than the corresponding YCo2 susceptibility (1.8 × 10-3 emu/Co at.g at 150 K). 3. Ordered state
Points measured in the ordered state, 4.2 K for
TmCo2 and 7 7 K for HoCo2, show a sudden increase of the Co susceptibility (fig. 1 and table I). This behavior is associated with the high values of the field Ht as illustrated by fig. 2. It has to be related to the high molecular field (> 500 kOe), due to R atoms, acting on the Co atoms. A similar variation has also been observed in YCo2: the susceptibility measured at 4.2 K increases of roughly 30% in an applied field of 380 kOe [7] (fig. 2). Such a field dependence of the cobalt susceptibility has been attributed [8] to the fact that the Fermi level lies in a dip of the density of states of the 3d band. In this case, it is then possible to observe a transition in high field called collective electron metamagnetism, which can explain the first order transitions observed in DyCo2, HoCo> ErCo2 [8]. It is a pleasure to thank R. Lemaire, who has proposed this subject, for his constant interest in this study.
/
08
06
Tm Co 2 ,
HoCo 2
% l
v
0,4
//i/ /
8 02
/
" ~ _ 0 0
/
_~-YC°2
.
o
g
~
~'
i
i
9~o
~2oo'
,soo
Ht =nRco ['JR * H (kC~)
Fig. 2. Co moment versus the field H,=nRc,,#.+H for R = Tin, Ho and Y.
References [1] [2] [3] 14] [5l [6] [7] [8]
R. Lemaire, Cobalt 33 (1966) 201. E. Burzo, Phys. Rev. B6 (1972) 2882. D. Bloch and R. Lemaire, Phys. Rev. B2 (1970) 2648. D. Gignoux, D. Givord, F. Givord, W.C. Koehler and R.M. Moon, Phys. Rev. BI4 (1976) 162. D. Gignoux, F. Givord and J. Schweizer, to be published. F. Chaiss6, Thesis, University of Grenoble, France (1971). C.J. Schinkel, private communication. D. Bloch, D.M. Edwards, M. Shimizu and J. Voiron, J. Phys. F5 (1975) 1217.