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Magnetic structure of the cubic U2Te3 with a non-stoichiometric Th3P4 type of structure
Solid State Communications, Vol. 43, No. 7, pp. 5 8 7 - 5 8 9 , 1982. Printed in Great Britain.
0038-t098/82/310587-03503.00/0 Pergamon Press Ltd
MAGNETIC STRUCTURE OF THE CUBIC U2Te3 WITH A NON-STOICHIOMETRIC Th3P4 TYPE OF STRUCTURE P. Burlet, J. Rossat-Mignod and W. Suski* Laboratoire de Diffraction Neutronique, D0partement de Recherche Fondamentale, Centre d'Etudes Nucl6aires, 85 X, 38041 Grenoble COdex, France and B. Janus Institute for Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 937, 50-950 Wroclaw 2, Poland
(Received 5 March 1982 by E. F. Bertaut) The neutron diffraction patterns of the cubic U2T% have been obtained at 15 and 150 K. There are no extrapeaks of magnetic origin below the Curie point T¢ = 70 -+ 5 K. Thus the ordering corresponds to a collinear ferromagnetic structure. The value of the magnetic moment is 1.78 ~B which is close to those values determined for a few cubic uranium pnictides and chalcogenides.
MAGNETIC PROPERTIES of the uranium pnictides and chalcogenides with the Th3P4-type of structure are investigated since years. The pnictides appeared to be ferromagnetic (for review see [ 1 ] ), whereas chalcogenides seem to have more complicated properties [2, 3]. However, the magnetic structure of the pnictides has been predicted to be more complicated than collinear ferromagnetic [ 4 - 6 ] . This prediction was recently con. firmed by neutron diffraction experiment [7] which revealed that the magnetic ordering is a non-collinear three axial structure in which magnetic moments are tilted from the ( 111 ) axis by an angle of about twenty degrees within (11 O) planes. As concerns the chalcogenides the magnetic structure of U3Se4 has been investigated above 77 K only and this experiment did not show any magnetic ordering [8]. Also preliminary neutron diffraction experiments on the uranium telluride have failed to detect the existence of a magnetic structure [9]. Moreover, the tellurides are reported to exist over a range of homogeneity with U3Te4 and U2T% as the terminal compositions. The cubic form is a high temperature metastabie modification, while the stable one is orthorhombic [3]. The temperature dependence of the magnetic susceptibility of U2Te3 exhibits a maximum at about 6 0 100 K depending on magnetic field strength and the dependence of the magnetization on the magnetic field
*On leave from Institute for Low Temperature and Structure Research, Wroclaw, Poland
shows a ferromagnetic character below about 60 K [3]. Because the preliminary neutron diffraction experiment [9], as mentioned above has failed to reveal any magnetic ordering we have decided to reexamine this material using the facility of the Siloe reactor of the CEN-Grenoble with the multidetector spectrometer. 1. SAMPLE PREPARATION The sample has been obtained via simple synthesis of stoichiometric amounts of powdered uranium and tellurium in vacuum at 900 ° C [31. This procedure was followed by arc-melting during a few minutes and fast cooling down to room temperature. It has been proved to be nonadequate because the peaks belonging to the orthorhombic form are still present in the neutron diffraction pattern. Moreover, it had the disadvantage of introducing a preferential orientation of the crystals as was deduced later from the neutron diffraction intensities. However, at present we are not able to get a better specimen. The sample used for experiments was composed of several ingots with a total weight of about 10 g. 2. EXPERIMENTAL Neutron diffraction patterns were recorded using the multidetector set up at the Siloe reactor at the CEN-Grenoble. The sample was introduced into a vanadium container in a glove-box filled with hellium in order to prevent any oxydation. The neutron wavelength was 2.4 A filtered by pyrolitic graphite. The temperature of examination was kept constant at T = 15 587
~!
MAGNETIC STRUCTURE OF THE CUBIC U,.T%
588
and 150 K during 2 0 h for each spectrum. The temperature dependence of the magnetic contribution of the (211 ) peak, for which the ratio of the magnetic to the nuclear contribution was the largest, has been obtained using the two axes spectrometer DN3.
% 3400
3. RESULTS
°c 3200 S
The neutron diffraction spectra have been obtained as mentioned above, at T = 15 and 150 K. In these patterns in addition of the peaks belonging to the Thai'atype of structure there are parasitic peaks which we were able to index as belonging to the orthorhombic form of U2Te 3. At high Bragg angles extrapeaks appear which come from some parts of the cryostat. The comparison o f the spectra obtained below and above the Curie point shows clearly that there are no additional peaks of magnetic origin; therefore the ordering corresponds to a ferromagnetic structure. Unfortunately at present, because of the lack of single crystals, we are not able to decide if that structure is a simple collinear or a multiaxial one as it has been discovered recently for U3P4 and U 3 A s 4 [ 7 ] . The thermal variation of the (2 1 1 ) magnetic contribution reported in Fig. 1 gives an ordering temperature T e = 70 + 5 K. As preferential orientations have been detected we are obliged to normalize the magnetic contribution of each peak by considering individual nuclear intensities. Normalized magnetic intensities are taken as:
:~ 3~00
3500 F
Vol. 43, No. 7
u~r~
i
3300 [
Tc=70-+5 K
o
3000 ......... 20 40
50 80 Temperature [K]
°
t 100
Fig. I. Thermal variation of the [211] peak intensity.
Table 1: Comparison o f observed and calculated magnetic' in tensities hkl
Io observed
I0 calculated
211
6.0 3.5 1.63 3.8 1.5
6.21 3.66 1.51 3.71 1.16
310 321 332 422
-+0.3 +0.3 +0.17 +-0.2 +0.3
where I(15 K) and I(150 K) are the observed intensities at 15 and 150 K respectively and Fnuel is the nuclear structure factor calculated using the Fermi length b u = 0.85 and ba,~ = 0.58. Then:
difficult to compare the Curie point obtained in the present experiment and in magnetic measurements because the results of the last are clearly dependent on the magnetic field strength [3]. Moreover the maxima observed in the temperature dependence of the magnetization suggest the importance of a domain structure and magnetocrystalline anisotropy [3], It is worthwhile to mention that electrical resistivity of U2Te3 exhibits a minimum at about 50 K [ 11 ]. Similar minimum has been discovered in other ferromagnetic compound UAsSe at temperatures which also do not correspond to
Io = (0.27)~f 2 p2 sin2e~F=m,
Tc [12].
lo(hkl) = I(15 K) - - I ( 1 5 0 K) F~uei(hkl), z(150 K)
where f i s the U '~+ form factor, sin2~ = 2/3, ,u is the uranium magnetic moment and Fm is the magnetic structure factor. As indicated in Table 1 good agreement is obtained between observed and calculated intensities for a moment o f 1.78 #B per uranium ion. This value is very close to those obtained for the ferromagnetic component in U3P4 and U3As4 [7] and is very common in ferromagnetic uranium compounds (see, e.g. [ 1 ] ). The value of the magnetic moment obtained by magnetic measurements in a magnetic field o f 50 kOe is i .2 ~B only [3], so one can expect that we are still far from saturation due to a high magnetic anisotropy, or that this difference is due to the influence of a negative polarization of conduction electrons as it has been observed in the uranium monochalcogenides [10]. It is
The present results, obtained on polycrystalline samples, have still a preliminary character. We hope to get more conclusive data from a single crystal as soon as it will be available. REFERENCES 1. 2. 3. 4. 5.
W. Trzebiatowski, Ferromagnetic Materials (Edited by E.P. Wohlfarth), VoI. 1, p. 115. North-Holland. Amsterdam (1980). W. Suski, T. Mydlarz & V.U.S. Rao, Phys. Status Solidi(a) 14, M137 (1972). W. Suski & B. Janus, Bull. Acad. Polon. SoL, Sdr. Sci. Chirn. 28, 199 (1980). C.F. Buhrer, J. Phys. Chem. Solids 30, 1273 (1969). J. Przystawa & J. Steslicki, Lectures on the 6th Winter School o f Theoretical Physics, p. 113. Karpacz (I 969); J. Przystawa, J. Phys. Chem. SolMs 31,2158 (1970); L. Dobrzynski & J. Przystawa,
Vol. 43, No. 7
6. 7. 8.
MAGNETIC STRUCTURE OF THE CUBIC U2T%
Phys. Status Solidi (b) 42, K 15 (1970); J. Przystawa & E. Praweczki, J. Phys. Chem. Solids 33, 1943 (1972). R. Tro& J. Mulak & W. Suski, Phys. Status. Solidi (b) 43,(1971). P. Burlet, J. Rossat-Mignod, R. Trod & Z. Henkie, Solid State Commun. 39,745 (1981). A. Szytula & W. Suski, Acta Phys. Polon. A43, 631 (1973).
9. 10, 11, 12.
589
J. Leciejewicz (Unpublished results). W. Eib, M. Erbudak, F. Greuter & B. Heiht, Phys. Lett. 68A, 391 (1978). A. Blaise, B. Janus & W. Suski, Solid State Commun, 37,417 (1981). A. Wojakovski, Z. Henkie & Z. Kletovski, Phys. Status Solidi (a) 14, 517 (1972).
0038-t098/82/310587-03503.00/0 Pergamon Press Ltd
MAGNETIC STRUCTURE OF THE CUBIC U2Te3 WITH A NON-STOICHIOMETRIC Th3P4 TYPE OF STRUCTURE P. Burlet, J. Rossat-Mignod and W. Suski* Laboratoire de Diffraction Neutronique, D0partement de Recherche Fondamentale, Centre d'Etudes Nucl6aires, 85 X, 38041 Grenoble COdex, France and B. Janus Institute for Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 937, 50-950 Wroclaw 2, Poland
(Received 5 March 1982 by E. F. Bertaut) The neutron diffraction patterns of the cubic U2T% have been obtained at 15 and 150 K. There are no extrapeaks of magnetic origin below the Curie point T¢ = 70 -+ 5 K. Thus the ordering corresponds to a collinear ferromagnetic structure. The value of the magnetic moment is 1.78 ~B which is close to those values determined for a few cubic uranium pnictides and chalcogenides.
MAGNETIC PROPERTIES of the uranium pnictides and chalcogenides with the Th3P4-type of structure are investigated since years. The pnictides appeared to be ferromagnetic (for review see [ 1 ] ), whereas chalcogenides seem to have more complicated properties [2, 3]. However, the magnetic structure of the pnictides has been predicted to be more complicated than collinear ferromagnetic [ 4 - 6 ] . This prediction was recently con. firmed by neutron diffraction experiment [7] which revealed that the magnetic ordering is a non-collinear three axial structure in which magnetic moments are tilted from the ( 111 ) axis by an angle of about twenty degrees within (11 O) planes. As concerns the chalcogenides the magnetic structure of U3Se4 has been investigated above 77 K only and this experiment did not show any magnetic ordering [8]. Also preliminary neutron diffraction experiments on the uranium telluride have failed to detect the existence of a magnetic structure [9]. Moreover, the tellurides are reported to exist over a range of homogeneity with U3Te4 and U2T% as the terminal compositions. The cubic form is a high temperature metastabie modification, while the stable one is orthorhombic [3]. The temperature dependence of the magnetic susceptibility of U2Te3 exhibits a maximum at about 6 0 100 K depending on magnetic field strength and the dependence of the magnetization on the magnetic field
*On leave from Institute for Low Temperature and Structure Research, Wroclaw, Poland
shows a ferromagnetic character below about 60 K [3]. Because the preliminary neutron diffraction experiment [9], as mentioned above has failed to reveal any magnetic ordering we have decided to reexamine this material using the facility of the Siloe reactor of the CEN-Grenoble with the multidetector spectrometer. 1. SAMPLE PREPARATION The sample has been obtained via simple synthesis of stoichiometric amounts of powdered uranium and tellurium in vacuum at 900 ° C [31. This procedure was followed by arc-melting during a few minutes and fast cooling down to room temperature. It has been proved to be nonadequate because the peaks belonging to the orthorhombic form are still present in the neutron diffraction pattern. Moreover, it had the disadvantage of introducing a preferential orientation of the crystals as was deduced later from the neutron diffraction intensities. However, at present we are not able to get a better specimen. The sample used for experiments was composed of several ingots with a total weight of about 10 g. 2. EXPERIMENTAL Neutron diffraction patterns were recorded using the multidetector set up at the Siloe reactor at the CEN-Grenoble. The sample was introduced into a vanadium container in a glove-box filled with hellium in order to prevent any oxydation. The neutron wavelength was 2.4 A filtered by pyrolitic graphite. The temperature of examination was kept constant at T = 15 587
~!
MAGNETIC STRUCTURE OF THE CUBIC U,.T%
588
and 150 K during 2 0 h for each spectrum. The temperature dependence of the magnetic contribution of the (211 ) peak, for which the ratio of the magnetic to the nuclear contribution was the largest, has been obtained using the two axes spectrometer DN3.
% 3400
3. RESULTS
°c 3200 S
The neutron diffraction spectra have been obtained as mentioned above, at T = 15 and 150 K. In these patterns in addition of the peaks belonging to the Thai'atype of structure there are parasitic peaks which we were able to index as belonging to the orthorhombic form of U2Te 3. At high Bragg angles extrapeaks appear which come from some parts of the cryostat. The comparison o f the spectra obtained below and above the Curie point shows clearly that there are no additional peaks of magnetic origin; therefore the ordering corresponds to a ferromagnetic structure. Unfortunately at present, because of the lack of single crystals, we are not able to decide if that structure is a simple collinear or a multiaxial one as it has been discovered recently for U3P4 and U 3 A s 4 [ 7 ] . The thermal variation of the (2 1 1 ) magnetic contribution reported in Fig. 1 gives an ordering temperature T e = 70 + 5 K. As preferential orientations have been detected we are obliged to normalize the magnetic contribution of each peak by considering individual nuclear intensities. Normalized magnetic intensities are taken as:
:~ 3~00
3500 F
Vol. 43, No. 7
u~r~
i
3300 [
Tc=70-+5 K
o
3000 ......... 20 40
50 80 Temperature [K]
°
t 100
Fig. I. Thermal variation of the [211] peak intensity.
Table 1: Comparison o f observed and calculated magnetic' in tensities hkl
Io observed
I0 calculated
211
6.0 3.5 1.63 3.8 1.5
6.21 3.66 1.51 3.71 1.16
310 321 332 422
-+0.3 +0.3 +0.17 +-0.2 +0.3
where I(15 K) and I(150 K) are the observed intensities at 15 and 150 K respectively and Fnuel is the nuclear structure factor calculated using the Fermi length b u = 0.85 and ba,~ = 0.58. Then:
difficult to compare the Curie point obtained in the present experiment and in magnetic measurements because the results of the last are clearly dependent on the magnetic field strength [3]. Moreover the maxima observed in the temperature dependence of the magnetization suggest the importance of a domain structure and magnetocrystalline anisotropy [3], It is worthwhile to mention that electrical resistivity of U2Te3 exhibits a minimum at about 50 K [ 11 ]. Similar minimum has been discovered in other ferromagnetic compound UAsSe at temperatures which also do not correspond to
Io = (0.27)~f 2 p2 sin2e~F=m,
Tc [12].
lo(hkl) = I(15 K) - - I ( 1 5 0 K) F~uei(hkl), z(150 K)
where f i s the U '~+ form factor, sin2~ = 2/3, ,u is the uranium magnetic moment and Fm is the magnetic structure factor. As indicated in Table 1 good agreement is obtained between observed and calculated intensities for a moment o f 1.78 #B per uranium ion. This value is very close to those obtained for the ferromagnetic component in U3P4 and U3As4 [7] and is very common in ferromagnetic uranium compounds (see, e.g. [ 1 ] ). The value of the magnetic moment obtained by magnetic measurements in a magnetic field o f 50 kOe is i .2 ~B only [3], so one can expect that we are still far from saturation due to a high magnetic anisotropy, or that this difference is due to the influence of a negative polarization of conduction electrons as it has been observed in the uranium monochalcogenides [10]. It is
The present results, obtained on polycrystalline samples, have still a preliminary character. We hope to get more conclusive data from a single crystal as soon as it will be available. REFERENCES 1. 2. 3. 4. 5.
W. Trzebiatowski, Ferromagnetic Materials (Edited by E.P. Wohlfarth), VoI. 1, p. 115. North-Holland. Amsterdam (1980). W. Suski, T. Mydlarz & V.U.S. Rao, Phys. Status Solidi(a) 14, M137 (1972). W. Suski & B. Janus, Bull. Acad. Polon. SoL, Sdr. Sci. Chirn. 28, 199 (1980). C.F. Buhrer, J. Phys. Chem. Solids 30, 1273 (1969). J. Przystawa & J. Steslicki, Lectures on the 6th Winter School o f Theoretical Physics, p. 113. Karpacz (I 969); J. Przystawa, J. Phys. Chem. SolMs 31,2158 (1970); L. Dobrzynski & J. Przystawa,
Vol. 43, No. 7
6. 7. 8.
MAGNETIC STRUCTURE OF THE CUBIC U2T%
Phys. Status Solidi (b) 42, K 15 (1970); J. Przystawa & E. Praweczki, J. Phys. Chem. Solids 33, 1943 (1972). R. Tro& J. Mulak & W. Suski, Phys. Status. Solidi (b) 43,(1971). P. Burlet, J. Rossat-Mignod, R. Trod & Z. Henkie, Solid State Commun. 39,745 (1981). A. Szytula & W. Suski, Acta Phys. Polon. A43, 631 (1973).
9. 10, 11, 12.
589
J. Leciejewicz (Unpublished results). W. Eib, M. Erbudak, F. Greuter & B. Heiht, Phys. Lett. 68A, 391 (1978). A. Blaise, B. Janus & W. Suski, Solid State Commun, 37,417 (1981). A. Wojakovski, Z. Henkie & Z. Kletovski, Phys. Status Solidi (a) 14, 517 (1972).