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The ferromagnetism of CeRu2Ge2
Physica B 171 (1991) 350-352 North-Holland
The ferromagnetism
of CeRu,Ge,
M.J. Besnus”, A. Essaihi”, N. Hamdaoui”, J. Pierre’, P. Haenc and P. Lejay’
G. Fischera,
“IPCMS-GEMME, Znstitut de Physique, 67084 Strasbourg, bLaboratoire de Magnttisme, 38042 Grenoble, France ‘C. R. T. B. T., 166X, 38042 Grenoble, France
France
J.P. Kappler”,
A. Meyer”,
Results of neutron diffraction, magnetization, specific heat and resistivity are reported for CeRu,Ge, which orders ferromagnetically at 7.9 K with a spontaneous magnetization of 1.96 k 0.02~~ along the tetragonal c axis. The anomalous behaviour of polycrystalline samples in the vicinity of the Curie temperature is not observed in single crystals.
1. Introduction
CeRu,Ge,, which has the tetragonal ThCr,Si, structure, has been previously described as ferromagnetic below 8 K [ 1, 21. Polycrystalline samples were found to saturate in fields of some kOe with a moment of 1.12& at 4.2K [l]. The magnetization decrease at the Curie point appeared to show a two step decrease, which was also detected by specific-heat measurements [2]. They show further a Schottky type anomaly centred at 220 K which is fitted by a crystal-field level scheme of three doublets at 0, 500 and 750 K. Previous inelastic neutron-scattering experiments [3] show only a linear T dependence of the magnetic relaxation rate, failing to evidence any quasi-elastic contribution. To get further insight into the properties of this compound, we studied its magnetic structure by neutron diffraction and reinvestigated its magnetic properties and specific heat on a single crystal grown in a tri-arc furnace by the Czochralski pulling method.
2. Results and discussion
Neutron-diffraction experiments were performed at the Institut Laue-Langevin in Grenoble using the DlB diffractometer (A = 2.525 A), 0378-4371/91/$03,50 0
on a powdered polycrystalline sample obtained by arc melting without subsequent heat treatment. Patterns were recorded at 1.3 and 18K. The 18 K pattern is consistent with the crystallographic structure (space group 14/m mm). Lattice parameters at 18 K are obtained as 4.268 A, c = 10.07 A. The difference pattern (1.3 - 18 K) only shows additional intensity which coincides with the allowed nuclear reflections, indicating pure ferromagnetism. From the intensity of the magnetic reflections and particularly the absence of the (002) reflection, the ferromagnetic moment is aligned along the c direction of the tetragonal cell. The moment calculated for CeRu,Ge, at 1.3 K is (1.9 -+ 0.15)~~. Magnetization measurements in low fields detect the spontaneous magnetization along the c direction. At the Curie temperature, the magnetization measured along the c axis decreases reversibly, with a slope -dMldT being largest at 7.9 K. Above this temperature the decrease becomes hyperbolic with small irregularities; however, strongly reduced if compared to those seen on the polycrystal. Above -9 K, the inverse susceptibility is linear, extrapolating to zero near 8 K. The slopes s = Ax-‘/AT, determined in a small temperature range above 9 K along the two reflect a considerable principal directions, crystal-field anisotropy through their ratio s, / s,, = 34 + 4, which results from the ground state
1991 - Elsevier Science Publishers B.V. (North-Holland)
M.J.
Besnus et al. I The ferromagnetism
Fig. 1. Magnetization at 4.2 K of a single crystal of CeRu,Si, along the tetragonal c axis. The dashed line denotes linear extrapolation to H = 0 yielding M,, = 1.96&mol.
20 ,
I
k +
CeRu,Ge, polycrystal
;+
OO
5
10
T (K)
351
of CeRu,Ge,
doublet of the system as the population of the higher doublets is yet negligible in the concerned temperature range. Magnetization data taken along the c axis in fields up to 20 T at 4.2 K (S.N.C.I., Grenoble), extrapolated linearly backwards to zero field, in the 5-20T range (fig. l), lead to the so defined spontaneous magnetization M,, = (1.96 4 0.02) pB/mol. The same procedure applied along an a axis yields M, = (0.26 + O.O2)&mol instead of zero as expected from the presently available neutron-scattering results. Specific-heat measurements were done on both a polycrystal and the single crystal in zero magnetic field (fig. 2a and 2b). The polycrystal shows, similarly to previous results also obtained on unannealed samples [2], an anomalous specific-heat jump with here a double peaked maximum at 7.51 and 7.90 K and a further rate change at 8.0 K, where the magnetization also shows a weak anomaly. No such features are seen on the single-crystal results. Here, the reversible C(T) variation draws a typical secondorder phase transition curve. C(T) is at maximum at 7.91 K with a value of 34.1 J/mol K, to be compared to the value of SR = 12.5 J/mol K calculated for a S = i classical two level system. As the specific heat witnesses -J dM*/dT, where J is an exchange constant, one is here
15
CeRu,Ge, single crwtal
-
01
0
0
0
5
IO
Fig. 2. (a) Specific heat of a CeRu,Ge, Specific heat of a CeRu,Ge, single crystal.
T (K) polycrystal.
I5 (b)
I
100
200
T (K)
3
Fig. 3. Temperature dependence of the magnetic contribution (obtained by subtracting the lattice contribution taken as the LaRu,Ge, data) to the resistivity of polycrystalhne CeRu,Ge,.
352
M.J.
Besnus et al. i The ferromagnetism
faced with a very steep homogeneous demagnetization process, which in the polycrystal, in contrast, appears to proceed in two main discontinuous steps. We note further that the entropy gains are the same for the polycrystal and the single crystal, beginning to saturate at 8 K where S/ R In 2 = 0.85. At low temperature (T < 4 K), our data are for both the single and polycrystal fully consistent with those of Bohm et al. [4]: an enhanced electronic term of 20 mJ/mol K2 and a ferromagnetic magnon spectrum with an energy gap of here 11 K. In addition, we report resistivity measurements on a polycrystal (fig. 3). There is a continuous increasing spin disorder contribution of the magnetic component up to 8 K, followed by a progressive approach towards saturation at higher temperatures; these features are clearly in
of CeRu,Ge,
line with the overall magnetic behaviour and the known crystal-field splittings. As a conclusion we find it worth to mention that, at hand of the presently available results, ferromagnetic CeRu,Ge, seems not to carry valence instability features: the only possible indice being the slightly enhanced low-temperature electronic specific heat of 20 mJ/mol K2. References [l] P. Lehmann, Thesis, Strasbourg (1987). [2] H. Rietschel, B. Renker, R. Felten, F. Steglich and G. Weber, J. Magn. & Magn. Mater. 76-77 (1988) 105. [3] A. Loidl, G. Knopp, H. Spille, F. Steglich and A.P. Murani, Physica B 156-157 (1989) 794. [4] A. Bohm, R. Caspary, U. Habel, L. Pawlak, A. Zuber, F. Steglich and A. Loidl, J. Magn. & Magn. Mater. 76-77 (1988) 150.
The ferromagnetism
of CeRu,Ge,
M.J. Besnus”, A. Essaihi”, N. Hamdaoui”, J. Pierre’, P. Haenc and P. Lejay’
G. Fischera,
“IPCMS-GEMME, Znstitut de Physique, 67084 Strasbourg, bLaboratoire de Magnttisme, 38042 Grenoble, France ‘C. R. T. B. T., 166X, 38042 Grenoble, France
France
J.P. Kappler”,
A. Meyer”,
Results of neutron diffraction, magnetization, specific heat and resistivity are reported for CeRu,Ge, which orders ferromagnetically at 7.9 K with a spontaneous magnetization of 1.96 k 0.02~~ along the tetragonal c axis. The anomalous behaviour of polycrystalline samples in the vicinity of the Curie temperature is not observed in single crystals.
1. Introduction
CeRu,Ge,, which has the tetragonal ThCr,Si, structure, has been previously described as ferromagnetic below 8 K [ 1, 21. Polycrystalline samples were found to saturate in fields of some kOe with a moment of 1.12& at 4.2K [l]. The magnetization decrease at the Curie point appeared to show a two step decrease, which was also detected by specific-heat measurements [2]. They show further a Schottky type anomaly centred at 220 K which is fitted by a crystal-field level scheme of three doublets at 0, 500 and 750 K. Previous inelastic neutron-scattering experiments [3] show only a linear T dependence of the magnetic relaxation rate, failing to evidence any quasi-elastic contribution. To get further insight into the properties of this compound, we studied its magnetic structure by neutron diffraction and reinvestigated its magnetic properties and specific heat on a single crystal grown in a tri-arc furnace by the Czochralski pulling method.
2. Results and discussion
Neutron-diffraction experiments were performed at the Institut Laue-Langevin in Grenoble using the DlB diffractometer (A = 2.525 A), 0378-4371/91/$03,50 0
on a powdered polycrystalline sample obtained by arc melting without subsequent heat treatment. Patterns were recorded at 1.3 and 18K. The 18 K pattern is consistent with the crystallographic structure (space group 14/m mm). Lattice parameters at 18 K are obtained as 4.268 A, c = 10.07 A. The difference pattern (1.3 - 18 K) only shows additional intensity which coincides with the allowed nuclear reflections, indicating pure ferromagnetism. From the intensity of the magnetic reflections and particularly the absence of the (002) reflection, the ferromagnetic moment is aligned along the c direction of the tetragonal cell. The moment calculated for CeRu,Ge, at 1.3 K is (1.9 -+ 0.15)~~. Magnetization measurements in low fields detect the spontaneous magnetization along the c direction. At the Curie temperature, the magnetization measured along the c axis decreases reversibly, with a slope -dMldT being largest at 7.9 K. Above this temperature the decrease becomes hyperbolic with small irregularities; however, strongly reduced if compared to those seen on the polycrystal. Above -9 K, the inverse susceptibility is linear, extrapolating to zero near 8 K. The slopes s = Ax-‘/AT, determined in a small temperature range above 9 K along the two reflect a considerable principal directions, crystal-field anisotropy through their ratio s, / s,, = 34 + 4, which results from the ground state
1991 - Elsevier Science Publishers B.V. (North-Holland)
M.J.
Besnus et al. I The ferromagnetism
Fig. 1. Magnetization at 4.2 K of a single crystal of CeRu,Si, along the tetragonal c axis. The dashed line denotes linear extrapolation to H = 0 yielding M,, = 1.96&mol.
20 ,
I
k +
CeRu,Ge, polycrystal
;+
OO
5
10
T (K)
351
of CeRu,Ge,
doublet of the system as the population of the higher doublets is yet negligible in the concerned temperature range. Magnetization data taken along the c axis in fields up to 20 T at 4.2 K (S.N.C.I., Grenoble), extrapolated linearly backwards to zero field, in the 5-20T range (fig. l), lead to the so defined spontaneous magnetization M,, = (1.96 4 0.02) pB/mol. The same procedure applied along an a axis yields M, = (0.26 + O.O2)&mol instead of zero as expected from the presently available neutron-scattering results. Specific-heat measurements were done on both a polycrystal and the single crystal in zero magnetic field (fig. 2a and 2b). The polycrystal shows, similarly to previous results also obtained on unannealed samples [2], an anomalous specific-heat jump with here a double peaked maximum at 7.51 and 7.90 K and a further rate change at 8.0 K, where the magnetization also shows a weak anomaly. No such features are seen on the single-crystal results. Here, the reversible C(T) variation draws a typical secondorder phase transition curve. C(T) is at maximum at 7.91 K with a value of 34.1 J/mol K, to be compared to the value of SR = 12.5 J/mol K calculated for a S = i classical two level system. As the specific heat witnesses -J dM*/dT, where J is an exchange constant, one is here
15
CeRu,Ge, single crwtal
-
01
0
0
0
5
IO
Fig. 2. (a) Specific heat of a CeRu,Ge, Specific heat of a CeRu,Ge, single crystal.
T (K) polycrystal.
I5 (b)
I
100
200
T (K)
3
Fig. 3. Temperature dependence of the magnetic contribution (obtained by subtracting the lattice contribution taken as the LaRu,Ge, data) to the resistivity of polycrystalhne CeRu,Ge,.
352
M.J.
Besnus et al. i The ferromagnetism
faced with a very steep homogeneous demagnetization process, which in the polycrystal, in contrast, appears to proceed in two main discontinuous steps. We note further that the entropy gains are the same for the polycrystal and the single crystal, beginning to saturate at 8 K where S/ R In 2 = 0.85. At low temperature (T < 4 K), our data are for both the single and polycrystal fully consistent with those of Bohm et al. [4]: an enhanced electronic term of 20 mJ/mol K2 and a ferromagnetic magnon spectrum with an energy gap of here 11 K. In addition, we report resistivity measurements on a polycrystal (fig. 3). There is a continuous increasing spin disorder contribution of the magnetic component up to 8 K, followed by a progressive approach towards saturation at higher temperatures; these features are clearly in
of CeRu,Ge,
line with the overall magnetic behaviour and the known crystal-field splittings. As a conclusion we find it worth to mention that, at hand of the presently available results, ferromagnetic CeRu,Ge, seems not to carry valence instability features: the only possible indice being the slightly enhanced low-temperature electronic specific heat of 20 mJ/mol K2. References [l] P. Lehmann, Thesis, Strasbourg (1987). [2] H. Rietschel, B. Renker, R. Felten, F. Steglich and G. Weber, J. Magn. & Magn. Mater. 76-77 (1988) 105. [3] A. Loidl, G. Knopp, H. Spille, F. Steglich and A.P. Murani, Physica B 156-157 (1989) 794. [4] A. Bohm, R. Caspary, U. Habel, L. Pawlak, A. Zuber, F. Steglich and A. Loidl, J. Magn. & Magn. Mater. 76-77 (1988) 150.