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Magnetization of a Gd3Ni single crystal
Journal of Alloys and Compounds 334 (2002) 40–44
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Magnetization of a Gd 3 Ni single crystal a, a,b a c a N.V. Tristan *, S.A. Nikitin , T. Palewski , K. Skokov , J. Warchulska a
International Laboratory of High Magnetic Fields and Low Temperatures, 95 Gajowicka str., 53 – 421, Wroclaw, Poland b Faculty of Physics, Moscow State University, Vorobievy Gory, Moscow, 119899, Russia c Faculty of Physics, Tver State University, 33 Geljabova str., Tver, 170002, Russia Received 13 May 2001; accepted 16 July 2001
Abstract Magnetization and dc-susceptibility measurements of a Gd 3 Ni single crystal and polycrystalline material were performed. The Gd 3 Ni compound crystallizes in the orthorhombic Fe 3 C-type structure. Below the ordering temperature, T N 5100 K, the field dependence of magnetization, measured along three basic symmetry axes, exhibit one or two field induced transitions. This behavior is discussed in terms of a model that includes the influence of the magnetic field on the exchange and anisotropy energies. We also discussed the anomalous behavior of the magnetization in the paramagnetic temperature range appearing as an inflection of the field dependence of the magnetization. 2002 Elsevier Science B.V. All rights reserved. Keywords: Rare earth compounds; Magnetically ordered materials; Phase transition; Magnetic measurements
1. Introduction The rare earth–transition metal compounds of stoichiometry R 3 M, where R is a rare earth atom and M is a transition metal atom, are the compounds richest in the rare earth component of all known compounds formed with nickel or cobalt. All the R 3 Ni intermetallic compounds crystallize in the orthorhombic Fe 3 C-type structure belonging to space group Pnma [1,2]. Except for Y 3 Ni and Sm 3 Ni exhibiting only a weak paramagnetism fairly independent of the temperature, all the other compounds (R5Pr–Er) follow the Curie–Weiss law at high tempera´ tures and show a metamagnetic transition below their Neel temperatures [3,4]. Studies of the magnetic structure of Er 3 Ni [5] and Tb 3 Ni [6] by neutron diffraction showed that only the rare earth atoms carry magnetic moments. There are two nonequivalent positions of rare earth atoms in the structure and their magnetic moments in the ordered state are noncollinear which results in an antiferromagnetic character of the R 3 Ni compounds in low magnetic fields. The investigation of the magnetic properties of polycrystalline R 3 Ni compounds in pulsed magnetic fields up to 16 T [7], and of single crystals of Tb 3 Ni [6] and Dy 3 Ni [8] in static
*Corresponding author. E-mail address: [email protected] (N.V. Tristan).
fields up to 15 T pointed to the existence of complex metamagnetic transitions in the compounds. Because of the complicated magnetic properties of the R 3 Ni compounds, investigations performed on single crystals of other R 3 Ni compounds are very necessary. The purpose of the present work was to investigate the magnetic properties of a single crystal of the Gd 3 Ni compound. The latter compound was chosen from a number of R 3 Ni compounds because of its favorable features which may facilitate the analysis of magnetic properties. The 4f electron shell of the Gd atom has no orbital moment (L50) and, therefore, crystal field effects can be neglected. The value of the Gd magnetic moment in intermetallic compounds is close to that of the Gd 31 ion. Moreover, the magnetic transition temperature of the parent compound Gd 3 Ni is the highest (T N 5100 K) among all other R 3 Ni compounds [9] allowing us to study magnetic properties of the compound in the magnetic ordered state as well as in the paramagnetic one in a broad range of temperatures.
2. Experimental part The samples used in the present work were obtained by high-frequency melting of the constituent metals in a purified argon atmosphere under a pressure of 1.5 atm and then cooled with rate of cooling |1–2 K s 21 . The purity of the gadolinium and nickel was 99.9 and 99.98%, respec-
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 01 )01767-4
N.V. Tristan et al. / Journal of Alloys and Compounds 334 (2002) 40 – 44
tively. The X-ray diffraction measurements on the powdered samples showed that the diffraction pattern exhibit only the lines characteristic of the Fe 3 C-type structure which confirmed that the samples were a single-phase compound. Single crystal samples weighing 3–15 mg were extracted from the solidified ingots and then were oriented by the conventional Laue back reflection method. The lattice parameters are a50.6949 nm, b50.9695 nm, c50.6355 nm. Magnetization measurements were carried out using a capacitance sensor magnetometer in a superconducting solenoid in magnetic fields up to 14 T. The measurements of the temperature dependence of the dc magnetic susceptibility were performed using a Faraday balance method in the temperature range 4.2–290 K in a static magnetic field of m0 H50.43 T.
3. Results and discussion The temperature dependence of the magnetic susceptibility of the polycrystalline Gd 3 Ni is shown in Fig. 1. ´ One can see a maximum on the curve at the Neel temperature, T N 5100 K, corresponding to the transition from the paramagnetic to the antiferromagnetic state at a temperature which is in good agreement with earlier data [9,10]. Unlike ordinary antiferromagnets, an anomalous increase of the magnetic susceptibility is observed in the temperature range below T N , particularly at T580 K. This is possibly due to some crystalline and / or magnetic structure transformation [11]. Investigations of the temperature dependence of the electrical resistivity also detected an anomalous behavior around T580 K [12]. The inverse susceptibility is a linear function of temperature and the susceptibility obeys the Curie–Weiss law above T5115 K as is shown in Fig. 1. The value of the
Fig. 1. The temperature dependence of the magnetic susceptibility (squares, left scale) and inverse magnetic susceptibility (circles, right scale) for the Gd 3 Ni compound. The solid line shows the linear fitting for the inverse susceptibility.
41
effective magnetic moment calculated from the Curie– Weiss constant is meff 521.6 mB / f.u. According to Ref. [2] the nickel atoms do not carry noticeable magnetic moment. One can calculate the effective moment of the rare-earth atom as m Gd eff 57.2 mB , which is slightly lower than the Gd effective moment m eff 57.94 mB for the gadolinium atom in pure metallic Gd. The paramagnetic Curie temperature of the compound is positive and equals Qp 557.9 K. The existence of the maximum on the dc x (T ) curve at T5T N and the positive value of Qp are rather unusual for classical antiferromagnetics. These experimental data give some support that in Gd 3 Ni both positive as well as negative exchange interactions exist. In the sandwich like structure of Gd 3 Ni it is possible to discern layers of Gd atoms where the exchange interaction is positive inside the layers and is negative between the layers. The isotherms of magnetization of the single crystal Gd 3 Ni were measured in the temperature range 4.2–150 K along the main crystallographic directions. At T54.2 K the saturation of magnetization is achieved for all crystallographic directions at m0 H .9 T and its value is mS 523.9 mB / f.u. Assuming again that the nickel atoms do not carry magnetic moment one can calculate the saturation magnetic moment per gadolinium atom, m SGd 58.0 mB , which is slightly higher than magnetic moment m Gd S 57.55 mB per gadolinium atom in pure metallic Gd at T →0 K. Some of the selected isotherms s (H ) and curves of differential susceptibility ds / dH are shown in Figs. 2 and 3, respectively. One can see, that if magnetic field is applied along the crystallographic directions b or c the magnetization curve exhibits two magnetic transitions at the fields Hcr1 and Hcr2 while if it is applied along a only the high field magnetic transition is observed. The low field magnetic transition along b or c is accompanied by a sharp jump of the magnetization. The differential susceptibility at H5Hcr1 has a symmetric narrow maximum, i.e. the rate of the derivative increase does not depend on which side we are approaching the critical field. The combination of these features is evidence for a first-order magnetic transition, accompanied by a discontinuous change of the magnetic structure. The behavior of the magnetization and its derivative near the high field magnetic transition at Hcr2 differs from the above described one. In this case the differential susceptibility has the character of a l-type transition, i.e. a smooth increase at H ,Hcr2 , reaching a maximum at H5Hcr2 and a sharp drop at H .Hcr2 . The combination of such behavior of the differential susceptibility with a lack of any pronounced jump of the magnetization s (H ) is evidence of the second-order phase transition. We also observed that Hcr2 depends on the magnetic field direction and hence is anisotropic. The temperature dependencies of the critical field of the induced magnetic transitions are shown in Fig. 4. The magnitude of the magnetization jump at Hcr1 decreases as
42
N.V. Tristan et al. / Journal of Alloys and Compounds 334 (2002) 40 – 44
Fig. 2. The magnetisation of Gd 3 Ni single crystal measured along main crystallographic directions at different temperatures.
Fig. 4. The temperature dependence of the critical fields associated with magnetic transitions. Solid circles show critical fields Hcr when H / /a, open and closed triangles show Hcr1 and Hcr2 when H / /b, open and closed squares represent Hcr1 and Hcr2 when H / /c.
Fig. 3. The field dependence of the differential magnetic susceptibility dm / dH for Gd 3 Ni single crystal at T54.2 K (a) and 100 K (b). The peaks show the fields of the magnetic transitions.
the temperature increases and the transition disappears when the temperature reaches T N . The critical field Hcr2 also decreases with temperature increase and for T $T N the two magnetic transitions at Hcr1 and Hcr2 transforms into a single inflection point Hinf . It is well known that magnetic structure of the R 3 Ni compounds is complicated. Besides the crystallographic and magnetic unit cells are different. In spite of the complicated character of the magnetic structure of the
N.V. Tristan et al. / Journal of Alloys and Compounds 334 (2002) 40 – 44
Er 3 Ni [5] and Tb 3 Ni [6] compounds, their magnetic structures have two common features: (a) There are two positions for the rare earth atoms R I and R II , and each of these is characterized by its own local direction of the magnetic moment. (b) The existence of periodic magnetic structures which can be viewed as superposition of spin waves whose spin vectors are parallel to one of the main crystallographic directions. Evidently, this common feature remains for all R 3 Ni compounds including Gd 3 Ni. It is well known that two main contributions to the total energy should be considered: the exchange energy and the anisotropy energy [13]. In Fig. 5, a scheme of the magnetic structure (Fig. 5a) and two extreme cases showing the influence of the internal field are given (Fig. 5b and c). If the internal magnetic field is parallel to the y axis, the apex angle and the magnitude of the magnetic moment projections on the plane perpendicular to the y axis continuously decrease. The angles between the magnetic moment projections remain constant. In this situation the action of the internal magnetic field is against the anisotropy energy and no change of the exchange energy occurs. In that case a second-order phase transition takes place (Fig. 5b.). If the internal magnetic field is perpendicular to the y axis then neither the apex angle nor the magnitudes of the magnetic moments but the angles between the magnetic moment projections change. The action of the internal magnetic field is against the exchange energy and no change in anisotropy energy occurs. The first-kind phase transition occurs (Fig. 5c). In the case of Gd 3 Ni the wave vector is, probably, oriented along a and, if the magnetic field is applied along a, no jump of magnetization at Hcr1 is observed. The field induced transition at Hcr2 are caused by rotation of magnetic moments into the direction of the applied magnetic field. If the internal magnetic field is applied along b or c, the projection of the wave vector onto the direction of internal magnetic field is distinct from zero and the jump of the magnetization at H5Hcr1 is caused by a discontinu-
43
ous reorientation of the magnetic moments into the directions corresponding to local minimums of the magnetic anisotropy and exchange energy. An anomalous character of the magnetization is observed in the paramagnetic temperature range, T .T N . The magnetization does not increase linearly with increasing magnetic field and shows an inflection point at Hinf . At H ,Hinf the magnetization curve has positive curvature and at H .Hinf its curvature becomes negative. The differential susceptibility at T $T N has the character of a l-type transition and the maximum of ds / dH, corresponding to the inflection field Hinf , moves into the direction of higher magnetic fields as the temperature increases. No maximum of the differential susceptibility is observed for T $150 K in the field range m0 H50–14 T. We suppose that the Gd 3 Ni compound is not a classical paramagnet at T .T N because of the presence of two electron subsystems: a subsystem of the localized 4f electrons of gadolinium and a subsystem of the collective 3d electrons of nickel. The influence of the internal magnetic field causes ordering of the Gd magnetic moments. When the field reaches the critical value Hinf , non-zero localized magnetic moments of the Ni atoms, probably, appear. Since the magnetic moments of the heavy rare earth atoms and the transition metals atoms are oriented antiparallel in a magnetically ordered state, this should lead to a decrease of susceptibility at H .Hinf , which was observed experimentally.
Acknowledgements Authors are grateful to Dr. J. Stepien-Damm (W. Trzebiatowski Institute of Low Temperatures and Structure Research, Wroclaw, Poland) for single crystal orientation, Prof. R. Horyn, for X-ray measurements of powder samples and Prof. V.I. Nizhankovski for useful consultations during magnetization measurements. The part of the work performed by S.A. Nikitin was supported by RFBR Grant [ 99-02-17821.
References
Fig. 5. The scheme showing a magnetic structure (a) and two cases of the internal field influence: H parallel to the wave vector of the magnetic structure (b) and H perpendicular to the wave vector (c).
[1] R. Lemaire, D. Paccard, Bull. Soc. Franc. Mineral. Cryst. 90 (1967) 311. [2] E. Parthe, J.M. Moreau, J. Less-Common Metals 53 (1977) 1. [3] J.L. Feron, R. Lemaire, D. Pacckard, R. Pauthenet, Compt. Rend. Acad. Sci. Paris 267 (1968) 371. [4] C.S. Garde, J. Ray, J. Phys: Condens. Matter 9 (1997) 7419. [5] C. Becle, R. Lemaire, D. Paccard, J. Appl. Phys. 41 (1976) 855. [6] D. Gignoux, J.C. Gomez-Sal, D. Pacckard, Solid State Commun. 44 (1982) 695. [7] G.J. Primavesi, K.N.R. Taylor, J. Phys. F: Metal Phys. 2 (1971) 761. [8] E. Talik, T. Mydlarz, A. Gilewski, J. Alloys Comp. 233 (1996) 136.
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N.V. Tristan et al. / Journal of Alloys and Compounds 334 (2002) 40 – 44
¨ [9] E. Burzo, H.R. Kirchmayer, in: H.P.J. Wijn (Ed.), Landolt-Bornstein, New Series III / 19d2, Springer, Berlin, 1990, p. 468 [10] E. Talik, Physica B 193 (1994) 213. ¨ [11] J. Kusz, H. Bohm, E. Talik, J. Appl. Cryst. 33 (2000) 213.
[12] S.A. Nikitin, N.V. Tristan, T. Palewski, Yu.V. Skourski, K. Nenkov, V.N. Verbetsky, A.A. Salamova, J. Alloys Comp. 313 (2001) 22. [13] S.A. Nikitin (Ed.), Magnetic Properties of Rare Earth Metals and Their Alloys, Moscow University Publishing House, 1989, p. 248.
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www.elsevier.com / locate / jallcom
Magnetization of a Gd 3 Ni single crystal a, a,b a c a N.V. Tristan *, S.A. Nikitin , T. Palewski , K. Skokov , J. Warchulska a
International Laboratory of High Magnetic Fields and Low Temperatures, 95 Gajowicka str., 53 – 421, Wroclaw, Poland b Faculty of Physics, Moscow State University, Vorobievy Gory, Moscow, 119899, Russia c Faculty of Physics, Tver State University, 33 Geljabova str., Tver, 170002, Russia Received 13 May 2001; accepted 16 July 2001
Abstract Magnetization and dc-susceptibility measurements of a Gd 3 Ni single crystal and polycrystalline material were performed. The Gd 3 Ni compound crystallizes in the orthorhombic Fe 3 C-type structure. Below the ordering temperature, T N 5100 K, the field dependence of magnetization, measured along three basic symmetry axes, exhibit one or two field induced transitions. This behavior is discussed in terms of a model that includes the influence of the magnetic field on the exchange and anisotropy energies. We also discussed the anomalous behavior of the magnetization in the paramagnetic temperature range appearing as an inflection of the field dependence of the magnetization. 2002 Elsevier Science B.V. All rights reserved. Keywords: Rare earth compounds; Magnetically ordered materials; Phase transition; Magnetic measurements
1. Introduction The rare earth–transition metal compounds of stoichiometry R 3 M, where R is a rare earth atom and M is a transition metal atom, are the compounds richest in the rare earth component of all known compounds formed with nickel or cobalt. All the R 3 Ni intermetallic compounds crystallize in the orthorhombic Fe 3 C-type structure belonging to space group Pnma [1,2]. Except for Y 3 Ni and Sm 3 Ni exhibiting only a weak paramagnetism fairly independent of the temperature, all the other compounds (R5Pr–Er) follow the Curie–Weiss law at high tempera´ tures and show a metamagnetic transition below their Neel temperatures [3,4]. Studies of the magnetic structure of Er 3 Ni [5] and Tb 3 Ni [6] by neutron diffraction showed that only the rare earth atoms carry magnetic moments. There are two nonequivalent positions of rare earth atoms in the structure and their magnetic moments in the ordered state are noncollinear which results in an antiferromagnetic character of the R 3 Ni compounds in low magnetic fields. The investigation of the magnetic properties of polycrystalline R 3 Ni compounds in pulsed magnetic fields up to 16 T [7], and of single crystals of Tb 3 Ni [6] and Dy 3 Ni [8] in static
*Corresponding author. E-mail address: [email protected] (N.V. Tristan).
fields up to 15 T pointed to the existence of complex metamagnetic transitions in the compounds. Because of the complicated magnetic properties of the R 3 Ni compounds, investigations performed on single crystals of other R 3 Ni compounds are very necessary. The purpose of the present work was to investigate the magnetic properties of a single crystal of the Gd 3 Ni compound. The latter compound was chosen from a number of R 3 Ni compounds because of its favorable features which may facilitate the analysis of magnetic properties. The 4f electron shell of the Gd atom has no orbital moment (L50) and, therefore, crystal field effects can be neglected. The value of the Gd magnetic moment in intermetallic compounds is close to that of the Gd 31 ion. Moreover, the magnetic transition temperature of the parent compound Gd 3 Ni is the highest (T N 5100 K) among all other R 3 Ni compounds [9] allowing us to study magnetic properties of the compound in the magnetic ordered state as well as in the paramagnetic one in a broad range of temperatures.
2. Experimental part The samples used in the present work were obtained by high-frequency melting of the constituent metals in a purified argon atmosphere under a pressure of 1.5 atm and then cooled with rate of cooling |1–2 K s 21 . The purity of the gadolinium and nickel was 99.9 and 99.98%, respec-
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 01 )01767-4
N.V. Tristan et al. / Journal of Alloys and Compounds 334 (2002) 40 – 44
tively. The X-ray diffraction measurements on the powdered samples showed that the diffraction pattern exhibit only the lines characteristic of the Fe 3 C-type structure which confirmed that the samples were a single-phase compound. Single crystal samples weighing 3–15 mg were extracted from the solidified ingots and then were oriented by the conventional Laue back reflection method. The lattice parameters are a50.6949 nm, b50.9695 nm, c50.6355 nm. Magnetization measurements were carried out using a capacitance sensor magnetometer in a superconducting solenoid in magnetic fields up to 14 T. The measurements of the temperature dependence of the dc magnetic susceptibility were performed using a Faraday balance method in the temperature range 4.2–290 K in a static magnetic field of m0 H50.43 T.
3. Results and discussion The temperature dependence of the magnetic susceptibility of the polycrystalline Gd 3 Ni is shown in Fig. 1. ´ One can see a maximum on the curve at the Neel temperature, T N 5100 K, corresponding to the transition from the paramagnetic to the antiferromagnetic state at a temperature which is in good agreement with earlier data [9,10]. Unlike ordinary antiferromagnets, an anomalous increase of the magnetic susceptibility is observed in the temperature range below T N , particularly at T580 K. This is possibly due to some crystalline and / or magnetic structure transformation [11]. Investigations of the temperature dependence of the electrical resistivity also detected an anomalous behavior around T580 K [12]. The inverse susceptibility is a linear function of temperature and the susceptibility obeys the Curie–Weiss law above T5115 K as is shown in Fig. 1. The value of the
Fig. 1. The temperature dependence of the magnetic susceptibility (squares, left scale) and inverse magnetic susceptibility (circles, right scale) for the Gd 3 Ni compound. The solid line shows the linear fitting for the inverse susceptibility.
41
effective magnetic moment calculated from the Curie– Weiss constant is meff 521.6 mB / f.u. According to Ref. [2] the nickel atoms do not carry noticeable magnetic moment. One can calculate the effective moment of the rare-earth atom as m Gd eff 57.2 mB , which is slightly lower than the Gd effective moment m eff 57.94 mB for the gadolinium atom in pure metallic Gd. The paramagnetic Curie temperature of the compound is positive and equals Qp 557.9 K. The existence of the maximum on the dc x (T ) curve at T5T N and the positive value of Qp are rather unusual for classical antiferromagnetics. These experimental data give some support that in Gd 3 Ni both positive as well as negative exchange interactions exist. In the sandwich like structure of Gd 3 Ni it is possible to discern layers of Gd atoms where the exchange interaction is positive inside the layers and is negative between the layers. The isotherms of magnetization of the single crystal Gd 3 Ni were measured in the temperature range 4.2–150 K along the main crystallographic directions. At T54.2 K the saturation of magnetization is achieved for all crystallographic directions at m0 H .9 T and its value is mS 523.9 mB / f.u. Assuming again that the nickel atoms do not carry magnetic moment one can calculate the saturation magnetic moment per gadolinium atom, m SGd 58.0 mB , which is slightly higher than magnetic moment m Gd S 57.55 mB per gadolinium atom in pure metallic Gd at T →0 K. Some of the selected isotherms s (H ) and curves of differential susceptibility ds / dH are shown in Figs. 2 and 3, respectively. One can see, that if magnetic field is applied along the crystallographic directions b or c the magnetization curve exhibits two magnetic transitions at the fields Hcr1 and Hcr2 while if it is applied along a only the high field magnetic transition is observed. The low field magnetic transition along b or c is accompanied by a sharp jump of the magnetization. The differential susceptibility at H5Hcr1 has a symmetric narrow maximum, i.e. the rate of the derivative increase does not depend on which side we are approaching the critical field. The combination of these features is evidence for a first-order magnetic transition, accompanied by a discontinuous change of the magnetic structure. The behavior of the magnetization and its derivative near the high field magnetic transition at Hcr2 differs from the above described one. In this case the differential susceptibility has the character of a l-type transition, i.e. a smooth increase at H ,Hcr2 , reaching a maximum at H5Hcr2 and a sharp drop at H .Hcr2 . The combination of such behavior of the differential susceptibility with a lack of any pronounced jump of the magnetization s (H ) is evidence of the second-order phase transition. We also observed that Hcr2 depends on the magnetic field direction and hence is anisotropic. The temperature dependencies of the critical field of the induced magnetic transitions are shown in Fig. 4. The magnitude of the magnetization jump at Hcr1 decreases as
42
N.V. Tristan et al. / Journal of Alloys and Compounds 334 (2002) 40 – 44
Fig. 2. The magnetisation of Gd 3 Ni single crystal measured along main crystallographic directions at different temperatures.
Fig. 4. The temperature dependence of the critical fields associated with magnetic transitions. Solid circles show critical fields Hcr when H / /a, open and closed triangles show Hcr1 and Hcr2 when H / /b, open and closed squares represent Hcr1 and Hcr2 when H / /c.
Fig. 3. The field dependence of the differential magnetic susceptibility dm / dH for Gd 3 Ni single crystal at T54.2 K (a) and 100 K (b). The peaks show the fields of the magnetic transitions.
the temperature increases and the transition disappears when the temperature reaches T N . The critical field Hcr2 also decreases with temperature increase and for T $T N the two magnetic transitions at Hcr1 and Hcr2 transforms into a single inflection point Hinf . It is well known that magnetic structure of the R 3 Ni compounds is complicated. Besides the crystallographic and magnetic unit cells are different. In spite of the complicated character of the magnetic structure of the
N.V. Tristan et al. / Journal of Alloys and Compounds 334 (2002) 40 – 44
Er 3 Ni [5] and Tb 3 Ni [6] compounds, their magnetic structures have two common features: (a) There are two positions for the rare earth atoms R I and R II , and each of these is characterized by its own local direction of the magnetic moment. (b) The existence of periodic magnetic structures which can be viewed as superposition of spin waves whose spin vectors are parallel to one of the main crystallographic directions. Evidently, this common feature remains for all R 3 Ni compounds including Gd 3 Ni. It is well known that two main contributions to the total energy should be considered: the exchange energy and the anisotropy energy [13]. In Fig. 5, a scheme of the magnetic structure (Fig. 5a) and two extreme cases showing the influence of the internal field are given (Fig. 5b and c). If the internal magnetic field is parallel to the y axis, the apex angle and the magnitude of the magnetic moment projections on the plane perpendicular to the y axis continuously decrease. The angles between the magnetic moment projections remain constant. In this situation the action of the internal magnetic field is against the anisotropy energy and no change of the exchange energy occurs. In that case a second-order phase transition takes place (Fig. 5b.). If the internal magnetic field is perpendicular to the y axis then neither the apex angle nor the magnitudes of the magnetic moments but the angles between the magnetic moment projections change. The action of the internal magnetic field is against the exchange energy and no change in anisotropy energy occurs. The first-kind phase transition occurs (Fig. 5c). In the case of Gd 3 Ni the wave vector is, probably, oriented along a and, if the magnetic field is applied along a, no jump of magnetization at Hcr1 is observed. The field induced transition at Hcr2 are caused by rotation of magnetic moments into the direction of the applied magnetic field. If the internal magnetic field is applied along b or c, the projection of the wave vector onto the direction of internal magnetic field is distinct from zero and the jump of the magnetization at H5Hcr1 is caused by a discontinu-
43
ous reorientation of the magnetic moments into the directions corresponding to local minimums of the magnetic anisotropy and exchange energy. An anomalous character of the magnetization is observed in the paramagnetic temperature range, T .T N . The magnetization does not increase linearly with increasing magnetic field and shows an inflection point at Hinf . At H ,Hinf the magnetization curve has positive curvature and at H .Hinf its curvature becomes negative. The differential susceptibility at T $T N has the character of a l-type transition and the maximum of ds / dH, corresponding to the inflection field Hinf , moves into the direction of higher magnetic fields as the temperature increases. No maximum of the differential susceptibility is observed for T $150 K in the field range m0 H50–14 T. We suppose that the Gd 3 Ni compound is not a classical paramagnet at T .T N because of the presence of two electron subsystems: a subsystem of the localized 4f electrons of gadolinium and a subsystem of the collective 3d electrons of nickel. The influence of the internal magnetic field causes ordering of the Gd magnetic moments. When the field reaches the critical value Hinf , non-zero localized magnetic moments of the Ni atoms, probably, appear. Since the magnetic moments of the heavy rare earth atoms and the transition metals atoms are oriented antiparallel in a magnetically ordered state, this should lead to a decrease of susceptibility at H .Hinf , which was observed experimentally.
Acknowledgements Authors are grateful to Dr. J. Stepien-Damm (W. Trzebiatowski Institute of Low Temperatures and Structure Research, Wroclaw, Poland) for single crystal orientation, Prof. R. Horyn, for X-ray measurements of powder samples and Prof. V.I. Nizhankovski for useful consultations during magnetization measurements. The part of the work performed by S.A. Nikitin was supported by RFBR Grant [ 99-02-17821.
References
Fig. 5. The scheme showing a magnetic structure (a) and two cases of the internal field influence: H parallel to the wave vector of the magnetic structure (b) and H perpendicular to the wave vector (c).
[1] R. Lemaire, D. Paccard, Bull. Soc. Franc. Mineral. Cryst. 90 (1967) 311. [2] E. Parthe, J.M. Moreau, J. Less-Common Metals 53 (1977) 1. [3] J.L. Feron, R. Lemaire, D. Pacckard, R. Pauthenet, Compt. Rend. Acad. Sci. Paris 267 (1968) 371. [4] C.S. Garde, J. Ray, J. Phys: Condens. Matter 9 (1997) 7419. [5] C. Becle, R. Lemaire, D. Paccard, J. Appl. Phys. 41 (1976) 855. [6] D. Gignoux, J.C. Gomez-Sal, D. Pacckard, Solid State Commun. 44 (1982) 695. [7] G.J. Primavesi, K.N.R. Taylor, J. Phys. F: Metal Phys. 2 (1971) 761. [8] E. Talik, T. Mydlarz, A. Gilewski, J. Alloys Comp. 233 (1996) 136.
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N.V. Tristan et al. / Journal of Alloys and Compounds 334 (2002) 40 – 44
¨ [9] E. Burzo, H.R. Kirchmayer, in: H.P.J. Wijn (Ed.), Landolt-Bornstein, New Series III / 19d2, Springer, Berlin, 1990, p. 468 [10] E. Talik, Physica B 193 (1994) 213. ¨ [11] J. Kusz, H. Bohm, E. Talik, J. Appl. Cryst. 33 (2000) 213.
[12] S.A. Nikitin, N.V. Tristan, T. Palewski, Yu.V. Skourski, K. Nenkov, V.N. Verbetsky, A.A. Salamova, J. Alloys Comp. 313 (2001) 22. [13] S.A. Nikitin (Ed.), Magnetic Properties of Rare Earth Metals and Their Alloys, Moscow University Publishing House, 1989, p. 248.