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A magnetic phase transition in UCu2Ge2
So'lid State Communications, Vol. 41, No. 2, pp. 167-168, 1982. Printed in Great Britain.
0038-1098/82/020167-02502.00/0 Pergamon Press Ltd.
A MAGNETIC PHASE TRANSITION IN UCu2Ge 2 J. Leciejewicz and L. Chetmicki Institute of Nuclear Research, Swierk Research Establishment, 05 400 Otwock, Poland and A. Zygmunt Institute of Low Temperatures and Structural Research, Polish Academy of Sciences, 50-950 Wrodaw
(Received 31 August 1981 by E.F. Bertaut) Neutron-diffraction study of polycrystalline UCu2Ge 2 carried out at low temperatures shows that collinear antiferromagnetic order of + + - - - - type present at 4.2 K transforms between 25 and 40 K into collinear ferromagnetism. The Curie point is at 100-+ 3 K. The magnitude of magnetic moment on U ion is 1.61 -+ 0.10 Bohr magnetons at 4.2 K and 1.30 -+ 0.05 Bohr magnetons at 90 K. IN THE COURSE of neutron-diffraction studies of magnetic properties of uranium intermetallic systems with the crystal structure of CeA12Si2 type we found that UCu2Ge2 exhibits particularly interesting magnetic behaviour. A polycrystalline sample of UCu2Ge2 was synthesized by melting stoichiometric amounts of high purity component elements in an induction furnace under argon atmosphere. X-ray and neutron diffraction patterns have shown that the sample contains small amounts of an unidentified second phase. The lattice constants of UCu2Ge2 were determined from X-ray diffractograms to be: a = 4.061 + 0.002 A and c = 10.244 + 0.005 A. UCu2Ge2 belongs to the CeAI2Si2 type of crystal structure. Space group is I4/mmm, and atomic sites are as follows: 2U in 2(a)
O, O, 0
4Cu in 4(d)
0,½,¼
½,0,1
4Ge in 4(e)
O, O, z
O, O,
+ body
- centering translation.
The free atomic parameter z was determined from neutron intensities measured at 4.2 K to be 0.3835 + 0.0010. Neutron scattering lengths were taken after [1] to be (in 10 -14 m): b o : 0.85, bcu = 0.76, bGe = 0.82. No temperature factor was allowed for. Neutron-diffraction measurements were performed at 4.2, 90 and 300 K on a 15 g sample stored in thinwalled aluminium container closed under helium atmosphere to prevent possible oxidation. DN-500 powder diffractometer at the reactor EWA was used. The neutron wavelength was 1.326 -+ 0.001 A. 167
Two well-separated diffraction peaks of magnetic origin were found on the diffraction pattern obtained at 4.2 K. Besides that a small increase in the intensity of (110) nuclear peak was observed. Single peaks were indexable as (3/1 0 ½) and (M 1 0 3), while those overlapping with the (110) nuclear peak as (M 1 0 2 ~) and (M 1 1 ½). The indices indicate that the magnetic unitcell is twice the chemical one in the direction of the c-axis. The absence of magnetic peaks with the indices (M 0 0 1) was taken as an indication that the magnetic moments are aligned along the tetragonal axis. The above experimental data are consistent with a magnetic structure in which the magnetic moments on U ions are coupled ferromagnetically within the (001) planes, but the planes are stacked in the direction of the c-axis with the sequence + - - - - + . The same type of magnetic structure was earlier found in NdFe2Si 2 [2]. The magnitude of magnetic moment on U ion at 4.2 K was determined from the intensities of two single magnetic peaks. It is 1.61 -+ 0.10Bohr magnetons. Uranium form factor for the 5f z electron configuration was adopted [3]. Temperature dependence of the (M 1 0 ½) superlattice peak height shows that antiferromagnetism disappears between 25 K and 40 K. In the same temperature range magnetic contribution to the intensities of (1 0 1) and (1 1 0) nuclear peaks clearly rises indicating the appearance of ferromagnetic order. This follows from Fig. 1. The observed fairly large magnetic contribution to the (1 1 O) reflection implies that the magnetic moments are also pointing in the direction of the c-axis. This conclusion is additionally supported by the absence of any magnetic contribution to the intensity of (0 0 2) reflection. At 90 K the value of magnetic moment on U
168
A MAGNETIC PHASE TRANSITION IN UCu2G%
100+3K
,~ ~oL ~
diffraction experiments performed on isostructural UPd2Si2, UPd2G% and URh2Si2 [5]. We intend to study whether RKKY model is also applicable for the UM2X2 compounds (M = Co, Ni, Cu;X = Si and Ge). It seems worthy to mention that the 4.2 K UNi2Ge2 orders antiferromagnetically but its magnetic structure is of simple collinear +-- type [6]. Full account of these studies will be reported at a later date.
T
r,4 l o l
j
0
50
t00
Vol. 41,No. 2
~
T(K
Fig. 1. Temperature dependence of the height of antiferromagnetic peak (M 1 0 ½) and the height of(1 0 1) nuclear peak containing fairly large magnetic contribution. ion is 1.30 +- 0.05 Bohr magnetons. The Curie point was determined to be (100 -+ 3)K. Up to now only one case of ferromagnetic order was found among CeA12Si2 type intermetallics: NpCu2Si2 [4]. RKKY model of magnetic coupling via conduction electrons was recently used to explain the stability of different magnetic structures observed in neutron-
REFERENCES 1. 2. 3. 4. 5. 6.
Neutron Diffraction Comm. IU Cryst., Acta CrysL A28,357 (1972). F.A. Wedgewood,J. Phys. C: Solid State Phys. 5, 2477 (1972). H. Pinto & H. Shaked, Phys. Rev. B7, 3261 (1973). C.H. de Novion, J. Gal & J.L. Buevoz, J. Magn. Magn. Mat. 21, 85 (1980). H. Ptasiewicz-B~k, J. Leciejewicz & A. Zygmunt, J. Phys. F." MetalPhys. 11, 1225 (1981). L. Chetmicki, J. Leciejewicz & A. Zygmunt (to be published).
0038-1098/82/020167-02502.00/0 Pergamon Press Ltd.
A MAGNETIC PHASE TRANSITION IN UCu2Ge 2 J. Leciejewicz and L. Chetmicki Institute of Nuclear Research, Swierk Research Establishment, 05 400 Otwock, Poland and A. Zygmunt Institute of Low Temperatures and Structural Research, Polish Academy of Sciences, 50-950 Wrodaw
(Received 31 August 1981 by E.F. Bertaut) Neutron-diffraction study of polycrystalline UCu2Ge 2 carried out at low temperatures shows that collinear antiferromagnetic order of + + - - - - type present at 4.2 K transforms between 25 and 40 K into collinear ferromagnetism. The Curie point is at 100-+ 3 K. The magnitude of magnetic moment on U ion is 1.61 -+ 0.10 Bohr magnetons at 4.2 K and 1.30 -+ 0.05 Bohr magnetons at 90 K. IN THE COURSE of neutron-diffraction studies of magnetic properties of uranium intermetallic systems with the crystal structure of CeA12Si2 type we found that UCu2Ge2 exhibits particularly interesting magnetic behaviour. A polycrystalline sample of UCu2Ge2 was synthesized by melting stoichiometric amounts of high purity component elements in an induction furnace under argon atmosphere. X-ray and neutron diffraction patterns have shown that the sample contains small amounts of an unidentified second phase. The lattice constants of UCu2Ge2 were determined from X-ray diffractograms to be: a = 4.061 + 0.002 A and c = 10.244 + 0.005 A. UCu2Ge2 belongs to the CeAI2Si2 type of crystal structure. Space group is I4/mmm, and atomic sites are as follows: 2U in 2(a)
O, O, 0
4Cu in 4(d)
0,½,¼
½,0,1
4Ge in 4(e)
O, O, z
O, O,
+ body
- centering translation.
The free atomic parameter z was determined from neutron intensities measured at 4.2 K to be 0.3835 + 0.0010. Neutron scattering lengths were taken after [1] to be (in 10 -14 m): b o : 0.85, bcu = 0.76, bGe = 0.82. No temperature factor was allowed for. Neutron-diffraction measurements were performed at 4.2, 90 and 300 K on a 15 g sample stored in thinwalled aluminium container closed under helium atmosphere to prevent possible oxidation. DN-500 powder diffractometer at the reactor EWA was used. The neutron wavelength was 1.326 -+ 0.001 A. 167
Two well-separated diffraction peaks of magnetic origin were found on the diffraction pattern obtained at 4.2 K. Besides that a small increase in the intensity of (110) nuclear peak was observed. Single peaks were indexable as (3/1 0 ½) and (M 1 0 3), while those overlapping with the (110) nuclear peak as (M 1 0 2 ~) and (M 1 1 ½). The indices indicate that the magnetic unitcell is twice the chemical one in the direction of the c-axis. The absence of magnetic peaks with the indices (M 0 0 1) was taken as an indication that the magnetic moments are aligned along the tetragonal axis. The above experimental data are consistent with a magnetic structure in which the magnetic moments on U ions are coupled ferromagnetically within the (001) planes, but the planes are stacked in the direction of the c-axis with the sequence + - - - - + . The same type of magnetic structure was earlier found in NdFe2Si 2 [2]. The magnitude of magnetic moment on U ion at 4.2 K was determined from the intensities of two single magnetic peaks. It is 1.61 -+ 0.10Bohr magnetons. Uranium form factor for the 5f z electron configuration was adopted [3]. Temperature dependence of the (M 1 0 ½) superlattice peak height shows that antiferromagnetism disappears between 25 K and 40 K. In the same temperature range magnetic contribution to the intensities of (1 0 1) and (1 1 0) nuclear peaks clearly rises indicating the appearance of ferromagnetic order. This follows from Fig. 1. The observed fairly large magnetic contribution to the (1 1 O) reflection implies that the magnetic moments are also pointing in the direction of the c-axis. This conclusion is additionally supported by the absence of any magnetic contribution to the intensity of (0 0 2) reflection. At 90 K the value of magnetic moment on U
168
A MAGNETIC PHASE TRANSITION IN UCu2G%
100+3K
,~ ~oL ~
diffraction experiments performed on isostructural UPd2Si2, UPd2G% and URh2Si2 [5]. We intend to study whether RKKY model is also applicable for the UM2X2 compounds (M = Co, Ni, Cu;X = Si and Ge). It seems worthy to mention that the 4.2 K UNi2Ge2 orders antiferromagnetically but its magnetic structure is of simple collinear +-- type [6]. Full account of these studies will be reported at a later date.
T
r,4 l o l
j
0
50
t00
Vol. 41,No. 2
~
T(K
Fig. 1. Temperature dependence of the height of antiferromagnetic peak (M 1 0 ½) and the height of(1 0 1) nuclear peak containing fairly large magnetic contribution. ion is 1.30 +- 0.05 Bohr magnetons. The Curie point was determined to be (100 -+ 3)K. Up to now only one case of ferromagnetic order was found among CeA12Si2 type intermetallics: NpCu2Si2 [4]. RKKY model of magnetic coupling via conduction electrons was recently used to explain the stability of different magnetic structures observed in neutron-
REFERENCES 1. 2. 3. 4. 5. 6.
Neutron Diffraction Comm. IU Cryst., Acta CrysL A28,357 (1972). F.A. Wedgewood,J. Phys. C: Solid State Phys. 5, 2477 (1972). H. Pinto & H. Shaked, Phys. Rev. B7, 3261 (1973). C.H. de Novion, J. Gal & J.L. Buevoz, J. Magn. Magn. Mat. 21, 85 (1980). H. Ptasiewicz-B~k, J. Leciejewicz & A. Zygmunt, J. Phys. F." MetalPhys. 11, 1225 (1981). L. Chetmicki, J. Leciejewicz & A. Zygmunt (to be published).