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Magnetic ordering of TmSe under pressure
M A G N E T I C O R D E R I N G O F TmSe UNDER P R E S S U R E C. V E T T I E R Institut Laue-Langevin, BP 156X, 38042 Grenoble, France
J. F L O U Q U E T , J. M. M I G N O T Centre de Recherches sur les Tres Basses Temperatures, CNRS, BP 166X, 38042 Grenoble-Cedex, France
and F. H O L T Z B E R G I B M T. J. Watson Research Center, PO Box 218, Yorktown Heights, N Y 10598, USA
We report the first neutron diffraction study of the magnetic ordering in stoiehiometrie TmSe under pressures up to 20 kbar. The most striking result is the persistence of the type-I antiferromagnetie structure. We have observed the pressure dependence of the magnetic moment and of the N~el temperature; the temperature dependence of the sublattice magnetization (in reduced units) is independent of pressure. Comparisons are made with abnormal cerium compounds.
TmSe has been the subject of intensive study because the thulium valence has been found to be intermediate between the magnetic T m 3+ and T m 2÷ states. It is now well established by neutron diffraction measurements that a "stoichiometric" sample of TmSe a 0 ~ 5.71 A, orders in a type-I antiferromagnetic structure below a characteristic temperature TN ~ 3.5 K with a magnitude of the magnetic m o m e n t / z equal to 1.7 /zn [1]. Far from stoichiometry, an A F type-II magnetic ordering has been reported for a sample of lattice parameter a 0 ~ 5.64 A. As the comparison between different samples has shown the main role of the T m vacancies, it is now fruitful to plan pressure experiments on a given sample. Previous magnetization experiments have been reported up to 10 kbar [2, 3]. We describe here neutron diffraction experiments performed under pressure up to 20 kbar on the DIA spectrometer of the ILL high flux reactor. The main results are: (i) the measurement of the compressibility; (ii) the persistence of the type-I antiferromagnetic structure up to 20 kbar; (iii) the correlated pressure variation of TN and #; (iv) the scaling of the data with a T / T N law. The measured sample has a lattice parameter at room temperature of 5.71 A. The pressure inside the experimental cell has been determined by reference to the measured lattice parameter of NaC1. Different hkl reflections have been measured in
order to observe the six magnetic domains. The lowest temperature reached is 1.7 K. Table 1 describes the pressure variation of T N, a0, /t, and the compressibility K at 4.2 K. The estimate of the percentage of 2 + valency, x, is given in the last two columns using successively linear interpolations of the lattice parameter from Vegard's law and from the high temperature Curie constant CM [3]. Fig. 1 represents at 20 kbar, the intensity of the 1,0,0 reflection as a function of the temperature T: in zero field the magnetic transition is a well defined second order type. It must be pointed out, as is reported in ref. [1], that the full width of the 1,0,0 lines is larger than the instrumental resolution. Rather good agreement is obtained by scaling the results with a T/TN(P) law. Contrary to the conclusion of Guertin et al. [2], the 3 + state can only be obtained far from p = 20 kbar; an extrapolation of the compressibility at high pressure shows that the compressibility of the pure T m 3+ state (0.95 x 10 -6 bar -1 [4]) is achieved for p > 30 kbar. This remark underlines the weakness of the valence definition based on Vegard's law (see table 1). The variation of T N is correlated with that of/~: an initial increase is observed up to 8 kbar, followed by an almost pressure independent behavior from 8-20 kbar. Varma has pointed out that double exchange may be responsible for coupling among T m ions [5]. With the Varma's assumption of a constant/~ under pressure plus the application of Vegard's law, TN(P) would be linear with x as
Journal of Magnetism and Magnetic Materials 15-18 (1980) 987-988 ©North Holland
987
988
C. Vettier et al./ Magnetic ordering of TmSe under pressure TABLE 1 Summary of the neutron diffraction results p (kbar)
TN (K)
a0 (A)
K ( lO-6bar- l )
/~ (/~B)
0 8 20
3.1 3.8 3.7
5.697 5.657 5.626
2.66
1.9 2.3 2.2
described by Vegard's law. Our measurement rules out such a situation. Further experiments must clarify the importance of the double exchange mechanism. We propose the following explanation. Under pressure the valence mixing remains initially constant [3] and consequently so does the electronic structure characterized by the Fermi wavelength k~-1, since the valence mixing gives the number of electronic carriers. The main pressure effect is the strong d e p e n d e n c e of an electronic correlation parameter called Tr analogous to observations on abnormal cerium compounds [6]. This parameter is a function of the local coupling exchange J between the localized moment and the Fermi sea. The first ~'l and second ~2 neighbour exchange couplings are related to J and k F by an RKKY-like law:
~1 ---- J2F(2kFrl),
h2 ---- JEF(2kFr2),
1.33
x(in %) Vegard
CM
25 11 0
50 50
distances. The pressure changes J and consequently Tr, hi and ~2' but the ratio ?h/?~2 is still constant (kvr ,~ cte). This last point implies the persistence of the type-I A F structure. The variation of TN(p) is the same as has been described for the Ce compounds. The true mechanism is more complicated than this description as a new insulating phase seems to occur at T N [7]. It will be particularly interesting to compare our neutron measurements with the corresponding resistivity resuits under pressure [8]. Finally it must be emphasized that there is no simple connection between stoichiometric and non-stoichiometric samples. Comparisons can be made regarding the deficiency in T m and the possible occurrence of vacancy planes. On this point it must be remarked that when the T m deficiency increases from stoichiometric concentration (a 0 decreases), TN decreases but increases again near the critical 5.64 A lattice parameter.
where r I and r 2 are the first and second neighbour
11.0.0
3k-(a u )
I
I
References
I
o o o o
o
20 k bar o o o o o o
o
I 2
I 3
°to 4
n
T(K) 5
Fig. 1. Intensity of the magnetic 1,0,0, reflection as a function of T for p = 20 kbar.
[1] S. H. Shapiro, H. B. MOiler, J. D. Axe, R. J. Birgeneau and E. Bucher, J. Appl. Phys. 49 (1978) 2101. [2] R. P. Guertin, S. Foner and F. P. Missell, Phys. Rev. Lett. 37 (1976) 529. [3] G. Chouteau, F. Holtzberg, O. Pefia, T. Penney and R. Tournier, J. de Phys. 40 (1979) C5-361. [4] B. Baflogg, H. R. Ott, E. Kaldis, W. Th~ni and P. Wachter, Phys. Rev. BI9 (1979) 217. [5] C. M. Varma, Solid State Commun. 30 (1979) 537. [6] A. Benoit, J. Flouquet and M. Ribault, J. de Phys. 40 (1979) C5-328. [7] P. Haen, F. Lapierre, J. M. Mignot, R. Tournier and F. Holtzberg, Phys. Rev. Lett. 43 (1979) 304. [8] J. Flouquet, P. Haen, F. Holtzberg, F. Lapierre, J. M. Mignot, M. Ribault and R. Toumier, Intern. Conf. SM79, MontpeUier, France, to be published in J. de Phys.
J. F L O U Q U E T , J. M. M I G N O T Centre de Recherches sur les Tres Basses Temperatures, CNRS, BP 166X, 38042 Grenoble-Cedex, France
and F. H O L T Z B E R G I B M T. J. Watson Research Center, PO Box 218, Yorktown Heights, N Y 10598, USA
We report the first neutron diffraction study of the magnetic ordering in stoiehiometrie TmSe under pressures up to 20 kbar. The most striking result is the persistence of the type-I antiferromagnetie structure. We have observed the pressure dependence of the magnetic moment and of the N~el temperature; the temperature dependence of the sublattice magnetization (in reduced units) is independent of pressure. Comparisons are made with abnormal cerium compounds.
TmSe has been the subject of intensive study because the thulium valence has been found to be intermediate between the magnetic T m 3+ and T m 2÷ states. It is now well established by neutron diffraction measurements that a "stoichiometric" sample of TmSe a 0 ~ 5.71 A, orders in a type-I antiferromagnetic structure below a characteristic temperature TN ~ 3.5 K with a magnitude of the magnetic m o m e n t / z equal to 1.7 /zn [1]. Far from stoichiometry, an A F type-II magnetic ordering has been reported for a sample of lattice parameter a 0 ~ 5.64 A. As the comparison between different samples has shown the main role of the T m vacancies, it is now fruitful to plan pressure experiments on a given sample. Previous magnetization experiments have been reported up to 10 kbar [2, 3]. We describe here neutron diffraction experiments performed under pressure up to 20 kbar on the DIA spectrometer of the ILL high flux reactor. The main results are: (i) the measurement of the compressibility; (ii) the persistence of the type-I antiferromagnetic structure up to 20 kbar; (iii) the correlated pressure variation of TN and #; (iv) the scaling of the data with a T / T N law. The measured sample has a lattice parameter at room temperature of 5.71 A. The pressure inside the experimental cell has been determined by reference to the measured lattice parameter of NaC1. Different hkl reflections have been measured in
order to observe the six magnetic domains. The lowest temperature reached is 1.7 K. Table 1 describes the pressure variation of T N, a0, /t, and the compressibility K at 4.2 K. The estimate of the percentage of 2 + valency, x, is given in the last two columns using successively linear interpolations of the lattice parameter from Vegard's law and from the high temperature Curie constant CM [3]. Fig. 1 represents at 20 kbar, the intensity of the 1,0,0 reflection as a function of the temperature T: in zero field the magnetic transition is a well defined second order type. It must be pointed out, as is reported in ref. [1], that the full width of the 1,0,0 lines is larger than the instrumental resolution. Rather good agreement is obtained by scaling the results with a T/TN(P) law. Contrary to the conclusion of Guertin et al. [2], the 3 + state can only be obtained far from p = 20 kbar; an extrapolation of the compressibility at high pressure shows that the compressibility of the pure T m 3+ state (0.95 x 10 -6 bar -1 [4]) is achieved for p > 30 kbar. This remark underlines the weakness of the valence definition based on Vegard's law (see table 1). The variation of T N is correlated with that of/~: an initial increase is observed up to 8 kbar, followed by an almost pressure independent behavior from 8-20 kbar. Varma has pointed out that double exchange may be responsible for coupling among T m ions [5]. With the Varma's assumption of a constant/~ under pressure plus the application of Vegard's law, TN(P) would be linear with x as
Journal of Magnetism and Magnetic Materials 15-18 (1980) 987-988 ©North Holland
987
988
C. Vettier et al./ Magnetic ordering of TmSe under pressure TABLE 1 Summary of the neutron diffraction results p (kbar)
TN (K)
a0 (A)
K ( lO-6bar- l )
/~ (/~B)
0 8 20
3.1 3.8 3.7
5.697 5.657 5.626
2.66
1.9 2.3 2.2
described by Vegard's law. Our measurement rules out such a situation. Further experiments must clarify the importance of the double exchange mechanism. We propose the following explanation. Under pressure the valence mixing remains initially constant [3] and consequently so does the electronic structure characterized by the Fermi wavelength k~-1, since the valence mixing gives the number of electronic carriers. The main pressure effect is the strong d e p e n d e n c e of an electronic correlation parameter called Tr analogous to observations on abnormal cerium compounds [6]. This parameter is a function of the local coupling exchange J between the localized moment and the Fermi sea. The first ~'l and second ~2 neighbour exchange couplings are related to J and k F by an RKKY-like law:
~1 ---- J2F(2kFrl),
h2 ---- JEF(2kFr2),
1.33
x(in %) Vegard
CM
25 11 0
50 50
distances. The pressure changes J and consequently Tr, hi and ~2' but the ratio ?h/?~2 is still constant (kvr ,~ cte). This last point implies the persistence of the type-I A F structure. The variation of TN(p) is the same as has been described for the Ce compounds. The true mechanism is more complicated than this description as a new insulating phase seems to occur at T N [7]. It will be particularly interesting to compare our neutron measurements with the corresponding resistivity resuits under pressure [8]. Finally it must be emphasized that there is no simple connection between stoichiometric and non-stoichiometric samples. Comparisons can be made regarding the deficiency in T m and the possible occurrence of vacancy planes. On this point it must be remarked that when the T m deficiency increases from stoichiometric concentration (a 0 decreases), TN decreases but increases again near the critical 5.64 A lattice parameter.
where r I and r 2 are the first and second neighbour
11.0.0
3k-(a u )
I
I
References
I
o o o o
o
20 k bar o o o o o o
o
I 2
I 3
°to 4
n
T(K) 5
Fig. 1. Intensity of the magnetic 1,0,0, reflection as a function of T for p = 20 kbar.
[1] S. H. Shapiro, H. B. MOiler, J. D. Axe, R. J. Birgeneau and E. Bucher, J. Appl. Phys. 49 (1978) 2101. [2] R. P. Guertin, S. Foner and F. P. Missell, Phys. Rev. Lett. 37 (1976) 529. [3] G. Chouteau, F. Holtzberg, O. Pefia, T. Penney and R. Tournier, J. de Phys. 40 (1979) C5-361. [4] B. Baflogg, H. R. Ott, E. Kaldis, W. Th~ni and P. Wachter, Phys. Rev. BI9 (1979) 217. [5] C. M. Varma, Solid State Commun. 30 (1979) 537. [6] A. Benoit, J. Flouquet and M. Ribault, J. de Phys. 40 (1979) C5-328. [7] P. Haen, F. Lapierre, J. M. Mignot, R. Tournier and F. Holtzberg, Phys. Rev. Lett. 43 (1979) 304. [8] J. Flouquet, P. Haen, F. Holtzberg, F. Lapierre, J. M. Mignot, M. Ribault and R. Toumier, Intern. Conf. SM79, MontpeUier, France, to be published in J. de Phys.