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Electron paramagnetic resonance of impurities in uranium trichalcogenides
0038-1098/83 $3.00 + .00 Pergamon Press Ltd.
Solid State Communications, Vol. 48, No. 6, pp. 569-571, 1983. Printed in Great Britain.
ELECTRON PARAMAGNETIC RESONANCE OF IMPURITIES IN URANIUM TRICHALCOGENIDES* M. Baran and H. Szymczak Institute of Physics, Polish Academy of Sciences, al. Lotnik6w 32/46, 02-668 Warszawa, Poland and B. Janus and W. Suski Institute for Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 937, 50-950 Wrod'aw, Poland
(Received 12 December 1982; in revised form 4 July 1983 by E.F. Bertaut) Uranium trichalcogenides USa and UTea were examined by EPR. The measurements were performed at the X-band over temperature range 4.2-300 K and with the magnetic field applied perpendicular and parallel to the (0 01) plane. The EPR spectra consist of two lines with g-factors about 2 and 4 respectively. Results are discussed in terms of interactions of the U 4+ ion with impurities. A magnetic ordering in USa below 50 K is suggested. RECENTLY THERE HAS BEEN great interest in the study of the interaction of magnetic impurities in some Van Vleck paramagnets examined by electron paramagnetic resonance (EPR). However, up to now all the Van Vleck paramagnets studied by this technique have been systems containing non-Kramers rare earth ions with a singlet as a crystal-field-ground-state. There is some evidence (see e.g. [1 ]) that Van Vleck paramagnetism occurs also in actinide compounds. In the present work we report the first observation of EPR of magnetic impurities in the Van Vleck paramagnetic actinide system. The uranium trichalcogenides UTea and USa have been chosen for such an experiment because of the low symmetry of these compounds. The suggestion that uranium trichalcogenides are Van Vleck paramagnets due to the U 4÷ ion in a low symmetry crystal field, has been formulated in [2, 3] on the basis of magnetic investigations. Previous studies [2, 3] have shown maxima in the temperature dependence of magnetic susceptibility or magnetization at 11 and 50 K for UTea and USa, respectively. Preliminary neutron diffraction studies on UTea failed to show any magnetic ordering in zero magnetic field. The uranium trichalcogenide monocrystals used in this study were prepared by the chemical transport method using bromide as a transport agent [4]. Thin platelets corresponding to the (0 0 1) plane were obtained. The EPR measurements were performed at the
X-band over temperature range 4.2-300 K. The magnetic fields were varied in the (0 0 1) plane and in planes perpendicular to the (0 0 1) plane. The EPR spectra consist of two lines: the first with a g-factor of about 2 (at higher temperatures) and the second with g = 4. The g-factors for both lines are almost isotropic. The anisotropy of the g-factors has been estimated to be smaller than 10%. The observed resonance lines are weak and cannot be interpreted as a resonance of the host U 4÷ ions. There is also an additional reason for such an interpretation. The U 4÷ ion is a non-Kramers ion placed in a low-symmetry crystal-field which completely removes the (2J + 1)-fold degeneracy of the ground state. In such a case EPR is observed if the ground state is an accidental doublet. The spin Hamiltonian for an accidental doublet has the following form [5]:
~fs = gll/3HzSz + AxSx + AySy,
(1)
where S = 1/2,/~ is the Bohr magneton. The first term in equation (1) is the Zeeman term (H is the external magnetic field) and the remaining terms describe the zero-field splitting. The ions with the ground state described by equation (1) have an extremely anisotropic EPR spectrum:
H,~,(O) =
x/(hvy-
Ax~ - Ay=
gll/3 cos/9
,
(2)
where v is the resonance frequency, 0 the angle between applied field and crystal field axis. The EPR spectrum for UTe3 and USa has a rather isotropic character contrary to the characteristic angular variation (2) expected for the U 4+ ion. The EPR spectra for UTe3 and USa should be
* The authors devote this work to the memory of the late Professor W. Trzebiatowsld. 569
570
EPR OF IMPURITIES IN URANIUM TRICHALCOGENIDES
interpreted as a resonance of impurities which were not intentionally introduced into single crystals. The line with g ~ 4 may be assigned to Fe 3+ ions in a strong (compared to Zeeman interactions at the X-band) crystal field of definite synrmetry giving rise to the EPR value g = 30/7 (= 4.28) at one of the Kramers doublets [6-8]. The spin Hamiltonian for the Fe 3+ ion has the form:
~ s = gI3(H'S) + DISZe --(1/3)S(S + I )] + E(S2~ -- Sy)'2 + (a/6)[S 4 + S 4Y + $ 2 - - ( 1 / 3 ) S ( S + 1)(3S 2 + 3 S -
1)l.
(3) As shown theoretically [ 6 - 8 ] , the EPR transitions with the isotropic g-factor g = 30/7 can be obtained in the strong crystal field approximation for two cases: (1) for the lowest Kramers doublet in the axial field [6], (2) for the middle Kramers doublet for orthorhombic symmetry i r e = 1/3D [7, 8].
Vol. 48, No. 6
existence of axial crystal field in tow synrmetry crystals seems to be not well-founded, one should assign the EPR line with g ~ 2 to an unpaired electron. The disappearance of both lines for USa below 50 K and the decrease of EPR lines intensity in the case of UTe3 can be explained only by antiferromagnetic ordering of both compounds at low temperature. The most significant feature of the line with g v 2 is the large g-shift observed for UTe3 (g-shift has not been detected for US3). The observed g-shift can be interpreted in terms o f the isotropic exchange interaction between the spin of impurity "~'imp ( Us+ or localized electron) and the U 4+ ions (ff)u,~+ of the form: ~ox~h -
Y/~',qm.'Y
~;>
K
E = O, D = - 2 / 3 a
In the case of the line with g ~ 2 it seems that the explanations (a) and (b) given below are equally reliable: (a) this line can be assigned to U s+ ions existing for some nonstoichiometry present. The ground state of the U s+ ion is 2Fs/2, therefore one can expect the appearance of isotropic EPR line under the same conditions as given above for Fe a+. The only difference is a value of the g-factor which can be easily calculated to equal g = 90/49 ( = 1.85), (b) this line can be assigned to unpaired, localized electrons arising from bond fission near to different types of crystal defects. Such a suggestion was given in [9] for EPR of some uranium chelates. From the lines intensity measurements it is well seen that the intensities decrease with decreasing temperature (for USa both lines disappear below 50 K). These results suggest that the observed EPR signal of Fe a+ ions could be assigned to transitions inside the middle Kramers doublet. It means that Fe a+ ions are located in an orthorhombic crystal field. For the U s+ ions one can expect that crystal field splitting o f J = 5/2 state should be considerably greater than in the case of Fe a+ ions. Therefore the observations of EPR line with g ~- 2 in UTe3 in low temperature region suggest that either the signal comes from the lowest Kramers doublet or the distance between the lowest doublets is (accidentally) small. Other explanations of the temperature dependence of line intensity can be based on the assumption of antiferromagnetic ordering at low temperatures (for USa below 50 K, and for UTea below 4 K) since usually the EPR lines intensities decrease to zero in the critical region. If this suggestion is valid the observed EPR signal for U s+ ions should be assigned to transitions inside the lowest Kramers doublet. Since the
K
where ga is the Land~ factor for U 4+ ga = 4/5,/K is an
isotropic exchange interaction coefficient between impurity and its kth host uranium neighbour, Ju~÷ is the total angular nronrentum of U 4+ ions. The interaction (4) induces a magnetic field on the impurity site which leads (in molecular field approximation) to the following value of theg-shift [10]: Ag--
ga__--1 XCF/[ ga 1 7 - / [ 1 - - 2 j ' \
(gd--1'] 2 XCF I ga
/
~T-],
(5)
where j is the total exchange interaction,/' is the total exchange interaction between the host U a ions. XCF, the crystal-field-only magnetic susceptibility of the host has the form: Xc~ = (A/2g,~/32 a2)[ 1/th (A/2kT)],
(6)
where A is the energy distance between the first excited level and the ground state and a is the matrix element of Jz between the ground state and the excited state. It has to be noticed that the above calculations are performed within the two-level approximation. It means that only one excited level is taken into account and therefore A plays the role of an "effective" crystal field splitting parameter. Finally Ag -1 = A + B cth (A/2kT).
(7)
The experimentally observed g-shift is described satisfactorily (Fig. 1) by the expressions (5) and (7) with the antiferromagnetic exchange interaction between the impurity and the U 4+ ions ( / < 0) and with A = (360 -+ 40) K. For US3 crystal the g-shift of both lines appears near the region of line disappearance and practically could not be measured. In UTe3 g-shift for Fe 3+ ions
EPR OF IMPURITIES IN URANIUM TRICHALCOGENIDES
Vol. 48, No. 6
571
practically independent on the angle 0. As mentioned above, in contrary to the present result the paramagnetic resonance for the U'~ ion has to exhibit the characteristic angular variation according to equation (2). Finally, it should be mentioned that the anomaly in the temperature dependence of the magnetic susceptibility of USa [3] has not been confirmed by low temperature specific heat measurements [ 11]. Thus, the possibility of magnetic ordering in US3 has to be checked by other independent experiments.
~.--4.0
60
u. 20 O
z 2.0
REFERENCES 1.
1.0
0
"
20
' 4~
" 60
' 80
' 1~
'
1~'
TEMPERATURE (K)
Fig. 1. Temperature dependence of g-value for UTe3. Solid line: theoretical prediction (7); +: experimental values. was not observed probably because of the small value of]. The EPR spectra discussed in this paper are very similar to those obtained in [9] for some uranium chelates, but the explanation, especially concerning the lines with g ~ 4, is completely different. A suggestion given in [9] about the possibility of observation of the paramagnetic resonance for the U4÷ ions is inconsistent with the fact that the line with g ~ 4 observed by us is
W. Suski, T. Gibiflski, A. Wojakowski & A. Czopnik, Phys. Status Solidi (a) 9,653 (1972). 2. W. Suski, Bull. Acad. Polon. Sci., Ser. Sci. Chim. 24, 75 (1976). 3. B. Janus, W. Suski & A. Blaise, Proc. IVInt. Conf. on Crystalline Electric FieM and Structural Effects in f-electron Systems, Wroc~a.w1981 (Edited by R.P. Guertin, W. Suski & Z. Zofnierek), p. 539. Plenum Press, New York and London (1982). 4. A. Plaise, B. Janus & W. Suski, Solid State Commun. 37, 417 (1981). 5. A. Abragram & B. Bleaney, Electron Paramagnetic Resonance of Transition lons, Clarendon Press, Oxford (1970). 6. J.V. Chepeleva, Dokl. Akad. NaukSSSR 202, 1042 (1972). 7. H.H. Wickman, M.P. Klein & D.A. Shirley, J. Chem. Phys. 42, 2113 (1965). 8. R. Aasa, J. Chem. Phys. 52, 3919 (1970). 9. T. Yoshimura, C. Miyake & S. Imoto, J. Inorg. Nucl. Chem. 37,739 (1975). 10. K. Sugawara, C.Y. Huang & B.R. Cooper, Phys. Rev. B l l , 4455 (1975). 11. F. Gr~nvold & E.F. Westrum Jr., Z Inorg. Nucl. Chem. 30, 2127 (1968).
Solid State Communications, Vol. 48, No. 6, pp. 569-571, 1983. Printed in Great Britain.
ELECTRON PARAMAGNETIC RESONANCE OF IMPURITIES IN URANIUM TRICHALCOGENIDES* M. Baran and H. Szymczak Institute of Physics, Polish Academy of Sciences, al. Lotnik6w 32/46, 02-668 Warszawa, Poland and B. Janus and W. Suski Institute for Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 937, 50-950 Wrod'aw, Poland
(Received 12 December 1982; in revised form 4 July 1983 by E.F. Bertaut) Uranium trichalcogenides USa and UTea were examined by EPR. The measurements were performed at the X-band over temperature range 4.2-300 K and with the magnetic field applied perpendicular and parallel to the (0 01) plane. The EPR spectra consist of two lines with g-factors about 2 and 4 respectively. Results are discussed in terms of interactions of the U 4+ ion with impurities. A magnetic ordering in USa below 50 K is suggested. RECENTLY THERE HAS BEEN great interest in the study of the interaction of magnetic impurities in some Van Vleck paramagnets examined by electron paramagnetic resonance (EPR). However, up to now all the Van Vleck paramagnets studied by this technique have been systems containing non-Kramers rare earth ions with a singlet as a crystal-field-ground-state. There is some evidence (see e.g. [1 ]) that Van Vleck paramagnetism occurs also in actinide compounds. In the present work we report the first observation of EPR of magnetic impurities in the Van Vleck paramagnetic actinide system. The uranium trichalcogenides UTea and USa have been chosen for such an experiment because of the low symmetry of these compounds. The suggestion that uranium trichalcogenides are Van Vleck paramagnets due to the U 4÷ ion in a low symmetry crystal field, has been formulated in [2, 3] on the basis of magnetic investigations. Previous studies [2, 3] have shown maxima in the temperature dependence of magnetic susceptibility or magnetization at 11 and 50 K for UTea and USa, respectively. Preliminary neutron diffraction studies on UTea failed to show any magnetic ordering in zero magnetic field. The uranium trichalcogenide monocrystals used in this study were prepared by the chemical transport method using bromide as a transport agent [4]. Thin platelets corresponding to the (0 0 1) plane were obtained. The EPR measurements were performed at the
X-band over temperature range 4.2-300 K. The magnetic fields were varied in the (0 0 1) plane and in planes perpendicular to the (0 0 1) plane. The EPR spectra consist of two lines: the first with a g-factor of about 2 (at higher temperatures) and the second with g = 4. The g-factors for both lines are almost isotropic. The anisotropy of the g-factors has been estimated to be smaller than 10%. The observed resonance lines are weak and cannot be interpreted as a resonance of the host U 4÷ ions. There is also an additional reason for such an interpretation. The U 4÷ ion is a non-Kramers ion placed in a low-symmetry crystal-field which completely removes the (2J + 1)-fold degeneracy of the ground state. In such a case EPR is observed if the ground state is an accidental doublet. The spin Hamiltonian for an accidental doublet has the following form [5]:
~fs = gll/3HzSz + AxSx + AySy,
(1)
where S = 1/2,/~ is the Bohr magneton. The first term in equation (1) is the Zeeman term (H is the external magnetic field) and the remaining terms describe the zero-field splitting. The ions with the ground state described by equation (1) have an extremely anisotropic EPR spectrum:
H,~,(O) =
x/(hvy-
Ax~ - Ay=
gll/3 cos/9
,
(2)
where v is the resonance frequency, 0 the angle between applied field and crystal field axis. The EPR spectrum for UTe3 and USa has a rather isotropic character contrary to the characteristic angular variation (2) expected for the U 4+ ion. The EPR spectra for UTe3 and USa should be
* The authors devote this work to the memory of the late Professor W. Trzebiatowsld. 569
570
EPR OF IMPURITIES IN URANIUM TRICHALCOGENIDES
interpreted as a resonance of impurities which were not intentionally introduced into single crystals. The line with g ~ 4 may be assigned to Fe 3+ ions in a strong (compared to Zeeman interactions at the X-band) crystal field of definite synrmetry giving rise to the EPR value g = 30/7 (= 4.28) at one of the Kramers doublets [6-8]. The spin Hamiltonian for the Fe 3+ ion has the form:
~ s = gI3(H'S) + DISZe --(1/3)S(S + I )] + E(S2~ -- Sy)'2 + (a/6)[S 4 + S 4Y + $ 2 - - ( 1 / 3 ) S ( S + 1)(3S 2 + 3 S -
1)l.
(3) As shown theoretically [ 6 - 8 ] , the EPR transitions with the isotropic g-factor g = 30/7 can be obtained in the strong crystal field approximation for two cases: (1) for the lowest Kramers doublet in the axial field [6], (2) for the middle Kramers doublet for orthorhombic symmetry i r e = 1/3D [7, 8].
Vol. 48, No. 6
existence of axial crystal field in tow synrmetry crystals seems to be not well-founded, one should assign the EPR line with g ~ 2 to an unpaired electron. The disappearance of both lines for USa below 50 K and the decrease of EPR lines intensity in the case of UTe3 can be explained only by antiferromagnetic ordering of both compounds at low temperature. The most significant feature of the line with g v 2 is the large g-shift observed for UTe3 (g-shift has not been detected for US3). The observed g-shift can be interpreted in terms o f the isotropic exchange interaction between the spin of impurity "~'imp ( Us+ or localized electron) and the U 4+ ions (ff)u,~+ of the form: ~ox~h -
Y/~',qm.'Y
~;>
K
E = O, D = - 2 / 3 a
In the case of the line with g ~ 2 it seems that the explanations (a) and (b) given below are equally reliable: (a) this line can be assigned to U s+ ions existing for some nonstoichiometry present. The ground state of the U s+ ion is 2Fs/2, therefore one can expect the appearance of isotropic EPR line under the same conditions as given above for Fe a+. The only difference is a value of the g-factor which can be easily calculated to equal g = 90/49 ( = 1.85), (b) this line can be assigned to unpaired, localized electrons arising from bond fission near to different types of crystal defects. Such a suggestion was given in [9] for EPR of some uranium chelates. From the lines intensity measurements it is well seen that the intensities decrease with decreasing temperature (for USa both lines disappear below 50 K). These results suggest that the observed EPR signal of Fe a+ ions could be assigned to transitions inside the middle Kramers doublet. It means that Fe a+ ions are located in an orthorhombic crystal field. For the U s+ ions one can expect that crystal field splitting o f J = 5/2 state should be considerably greater than in the case of Fe a+ ions. Therefore the observations of EPR line with g ~- 2 in UTe3 in low temperature region suggest that either the signal comes from the lowest Kramers doublet or the distance between the lowest doublets is (accidentally) small. Other explanations of the temperature dependence of line intensity can be based on the assumption of antiferromagnetic ordering at low temperatures (for USa below 50 K, and for UTea below 4 K) since usually the EPR lines intensities decrease to zero in the critical region. If this suggestion is valid the observed EPR signal for U s+ ions should be assigned to transitions inside the lowest Kramers doublet. Since the
K
where ga is the Land~ factor for U 4+ ga = 4/5,/K is an
isotropic exchange interaction coefficient between impurity and its kth host uranium neighbour, Ju~÷ is the total angular nronrentum of U 4+ ions. The interaction (4) induces a magnetic field on the impurity site which leads (in molecular field approximation) to the following value of theg-shift [10]: Ag--
ga__--1 XCF/[ ga 1 7 - / [ 1 - - 2 j ' \
(gd--1'] 2 XCF I ga
/
~T-],
(5)
where j is the total exchange interaction,/' is the total exchange interaction between the host U a ions. XCF, the crystal-field-only magnetic susceptibility of the host has the form: Xc~ = (A/2g,~/32 a2)[ 1/th (A/2kT)],
(6)
where A is the energy distance between the first excited level and the ground state and a is the matrix element of Jz between the ground state and the excited state. It has to be noticed that the above calculations are performed within the two-level approximation. It means that only one excited level is taken into account and therefore A plays the role of an "effective" crystal field splitting parameter. Finally Ag -1 = A + B cth (A/2kT).
(7)
The experimentally observed g-shift is described satisfactorily (Fig. 1) by the expressions (5) and (7) with the antiferromagnetic exchange interaction between the impurity and the U 4+ ions ( / < 0) and with A = (360 -+ 40) K. For US3 crystal the g-shift of both lines appears near the region of line disappearance and practically could not be measured. In UTe3 g-shift for Fe 3+ ions
EPR OF IMPURITIES IN URANIUM TRICHALCOGENIDES
Vol. 48, No. 6
571
practically independent on the angle 0. As mentioned above, in contrary to the present result the paramagnetic resonance for the U'~ ion has to exhibit the characteristic angular variation according to equation (2). Finally, it should be mentioned that the anomaly in the temperature dependence of the magnetic susceptibility of USa [3] has not been confirmed by low temperature specific heat measurements [ 11]. Thus, the possibility of magnetic ordering in US3 has to be checked by other independent experiments.
~.--4.0
60
u. 20 O
z 2.0
REFERENCES 1.
1.0
0
"
20
' 4~
" 60
' 80
' 1~
'
1~'
TEMPERATURE (K)
Fig. 1. Temperature dependence of g-value for UTe3. Solid line: theoretical prediction (7); +: experimental values. was not observed probably because of the small value of]. The EPR spectra discussed in this paper are very similar to those obtained in [9] for some uranium chelates, but the explanation, especially concerning the lines with g ~ 4, is completely different. A suggestion given in [9] about the possibility of observation of the paramagnetic resonance for the U4÷ ions is inconsistent with the fact that the line with g ~ 4 observed by us is
W. Suski, T. Gibiflski, A. Wojakowski & A. Czopnik, Phys. Status Solidi (a) 9,653 (1972). 2. W. Suski, Bull. Acad. Polon. Sci., Ser. Sci. Chim. 24, 75 (1976). 3. B. Janus, W. Suski & A. Blaise, Proc. IVInt. Conf. on Crystalline Electric FieM and Structural Effects in f-electron Systems, Wroc~a.w1981 (Edited by R.P. Guertin, W. Suski & Z. Zofnierek), p. 539. Plenum Press, New York and London (1982). 4. A. Plaise, B. Janus & W. Suski, Solid State Commun. 37, 417 (1981). 5. A. Abragram & B. Bleaney, Electron Paramagnetic Resonance of Transition lons, Clarendon Press, Oxford (1970). 6. J.V. Chepeleva, Dokl. Akad. NaukSSSR 202, 1042 (1972). 7. H.H. Wickman, M.P. Klein & D.A. Shirley, J. Chem. Phys. 42, 2113 (1965). 8. R. Aasa, J. Chem. Phys. 52, 3919 (1970). 9. T. Yoshimura, C. Miyake & S. Imoto, J. Inorg. Nucl. Chem. 37,739 (1975). 10. K. Sugawara, C.Y. Huang & B.R. Cooper, Phys. Rev. B l l , 4455 (1975). 11. F. Gr~nvold & E.F. Westrum Jr., Z Inorg. Nucl. Chem. 30, 2127 (1968).