- Email: [email protected]
Magnetic field-induced magnetic transition in Gd2Al compound
Journal of Magnetism and Magnetic Materials 205 (1999) 307}310
Magnetic "eld-induced magnetic transition in Gd Al compound Xingguo Li* Department of Materials Science and Engineering, Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japan Received 9 February 1999
Abstract Magnetic properties of Gd Al compound with C23 structure were investigated from 4.5 to 300 K in magnetic "elds up to 50 kOe. The compound is paramagnetic above 50 K without dependence on the magnetic "eld. However, below 50 K it is antiferromagnetic in low magnetic "eld and is ferromagnetic in high magnetic "eld. The "eld-induced magnetic transition occurs around 25 kOe. 1999 Elsevier Science B.V. All rights reserved. Keywords: Rare earth compounds; Magnetization curve; Field-induced magnetic transition
1. Introduction Magnetic properties of rare earth metals and alloys have been widely investigated and as a result, many interesting phenomena have been reported. The research focuses especially on compounds consisting of rare earth metals and 3d transition metals because of their huge economical potential [1}4]. However, the complicated magnetism is di$cult to be understood since both 3d transition metals and 4f rare earth atoms possess magnetic moments. For this reason, magnetic behaviors of Gd Al are inves tigated in this study. In Gd Al only Gd atom has magnetic moments originating simply from spin angular momentum, and therefore magnetic properties of Gd Al are dominated by Gd atom. In most cases, magnetism in matters is classi"ed into
* Tel.: #81-19-621-6350; fax: #81-19-621-6373. E-mail address: [email protected] (X. Li)
ferromagnetism, antiferromagnetism, paramagnets and diamagnetism at a given temperature, which is magnetic "eld independent. However, our recent measurement showed that Gd Al has several di!erent magnetic states at low temperatures and alignment of atomic moment varies on altering the applied magnetic "eld. This paper reports on the magnetic properties of Gd Al compound and dis cusses its unusual behaviors.
2. Experimental The intermetallic compound Gd Al was pre pared from 99.9% pure Gd and 99.99% pure Al by arc melting in an argon gas atmosphere. Arcmelted ingot was #ipped over and was remelted thrice. Weight loss was negligible during arcmelting. To obtain a single phase, the ingot was homogenized at 973 K, for 605 ks in an evacuated quartz tube. The ingot was pulverized into powder under 100 mesh as experimental sample. The
0304-8853/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 9 ) 0 0 4 7 0 - 9
308
X. Li / Journal of Magnetism and Magnetic Materials 205 (1999) 307}310
structure of the sample was identi"ed by powder X-ray di!raction (XRD) using monochromated Cu K radiation. XRD shows that Gd Al crystallizes in a C23 structure with lattice constants of a"0.6612, b"0.5127 and c"0.9543 nm. The magnetic properties were measured at temperatures from 4.5 to 300 K by a superconducting quantum interference device (SQUID) magnetometry with applied "eld up to 50 kOe and was calibrated with Pd.
3. Results and discussion Fig. 1 shows magnetization curves of Gd Al at a series of temperatures. At 4.5 K, Gd Al exhibits di!erent magnetic behavior below and above 25 kOe. Below 25 kOe the spontaneous magnetization increases linearly with increasing magnetic "eld, but above 25 kOe it increases rapidly and then approaches a saturation value from 40 kOe. The staircase behavior on magnetization curves expresses the occurrence of a "eld-induced magnetic transition. The spontaneous magnetization in 50 kOe is 203 emu/g, i.e. 6.02 Bohr moments/ Gd atom. Gadolinium is normally considered an S state ion with seven unpaired spins. Atomic spin moments in Gd Al in 50 kOe are close to localized moment of 7 Bohr magnetons on the Gd atoms, indicating spins in Gd Al aligned in the same direc tion. This fact leads us to believe that the saturation part on the magnetization curve results from a ferromagnetic contribution. In contrast, the linear part on the magnetization curve below 25 kOe can be explained by a paramagnetic or an antiferromagnetic contribution. It is known that the "eld dependence of magnetization for a paramagnet can be expressed by a Langevin function. According to this model, it is impossible to transform into ferromagnetism from a paramagnet by a sudden increase in magnetization. Thus, it is concluded that the linear part is attributed to antiferromagnetism. A transition from antiferromagnetism to ferromagnetism in Gd Al is induced by the magnetic "eld and the critical magnetic "eld for the transition is 25 kOe. The magnetization curve at 4.5 K for the decreasing "eld is shown in Fig. 2. A large hysteresis loop is observed in magnetic "eld between 25 and
Fig. 1. Magnetization curves of Gd Al at a series of temper atures.
Fig. 2. Magnetization curves of Gd Al in the increasing and decreasing "elds at 4.5 K.
35 kOe, which is probably caused by inversion of the spin orientation. A slight curvature is also noticeable in "elds lower than 5 kOe, which is another "eld-induced transition and illustrates the presence of a small remnant moment. Buschow [5] and Oesterreicher [6] studied Gd Al in the paramagnetic region and gave results of k "8.02 l and ¹ "150. Sill and . Biggers investigated magnetic behavior of Gd Al
X. Li / Journal of Magnetism and Magnetic Materials 205 (1999) 307}310
with applied "eld up to 26 kOe. They noticed a "eld-induced magnetic transition around 5 kOe and reported Gd Al was antiferromagnetic with ¹ "44 K [7]. Our result given below is close to , the latter. However, because Gd Al was not investi gated at low temperatures and in higher "elds in their experiments, the ferromagnetic region and the "eld-induced magnetic transition were not found. Up to 40 K, the saturation part is visible, but the saturation magnetization decreases remarkably and the critical magnetic "eld for the transition increases a little with increasing temperature. Conversely, change in the linear part is small. The energy for the critical magnetic "eld to act with a Bohr magneton in Gd Al is about 2.32;10\ J, which corresponds to a mere energy of 1.7 k . Thus, the temperature range where the magnetic-induced transition occurs is quite wide in consideration of the magnetic action energy. The Arrott plot of magnetization obtained in high magnetic "elds gives the Curie temperature ¹ "38 K. ! At 50 K, the "eld dependence of the magnetization is a straight line up to the highest available "eld of 50 kOe without the saturation e!ect as shown below 50 K. If the Curie temperature is "eld-dependent, the transition perhaps shifts to a magnetic "eld higher than 50 kOe, which needs to be con"rmed further. Between 50 and 300 K, the spontaneous magnetization shows a linear dependence on the magnetic "eld and slope of line decreases gradually with increasing temperature, which is the typical behavior of a paramagnetic matter. Fig. 3 shows the temperature dependences of magnetization and the inverse magnetization in 10 and 50 kOe. In 50 kOe, it keeps a constant below 40 K, begins an abrupt decrease from 40 K and then decreases gradually above 60 K with increasing temperature, being a temperature dependent of ferromagnetism. However, in 10 kOe a small peak is observed around 50 K, which is characteristic of antiferromagnetism and demonstrates the antiferromagnetic state of Gd Al at low temperature. This result supports the discussion mentioned above. The peak's temperature of 50 K can be considered the NeH el point. The inverse susceptibility in 10 kOe linearly increases with increasing temperature except for a small positive deviation from linearity
309
Fig. 3. Temperature dependence of the magnetizations and the reverse magnetization in 10 and 50 kOe for Gd Al.
appearing above 250 K. Sill and Biggers also reported this deviation, but did not give an explanation for it. Although the nature of the deviation is not con"rmed in this study, it can be suggested to be caused by a rapid drop of the small remnant moment. The temperature dependence for a magnetic substance is normally characterized by a Curie temperature or a NeH el point. However, for a substance showing the magnetic "eld-induced transition, the Curie temperature and the NeH el point can be observed simultaneously. For Gd Al, the NeH el temperature is a little higher than the Curie temperature. The susceptibility was obtained from slope of magnetization curves, which is given in Fig. 4 together with the inverse susceptibility. The temperature dependence of susceptibility is di!erent from that of magnetization both in 10 and 50 kOe. The di!erence between them is due to the "eld-induced magnetic transitions. From the linear part of the reverse susceptibility, the paramagnetic Curie temperature ¹ and the e!ective magnetic moment are derived to be 42 K and 7.2 l /Gd atom, respectively. The e!ective magnetic moment is identical with that of a free Gd atom. In rare earth intermetallic compounds the exchange mechanism is usually attributed to interactions with conduction electrons (RKKY). These interactions are normally of long range [8]. Gd Al compound has the orthorhombic Ni Si-type space
310
X. Li / Journal of Magnetism and Magnetic Materials 205 (1999) 307}310
magnetic moment 7.2 l /Gd atom above 50 K. At the lower temperature, however, it is antiferromagnetic below a critical magnetic "eld, but ferromagnetic in the higher "eld. The Curie temperature and NeH el point are 38 and 50 K, respectively. The critical magnetic "eld for the "eld-induced magnetic transition is 25 kOe at 4.5 K and increases a little with increasing temperature. The saturation magnetization of Gd Al in the ferromagnetic state is 6.02 Bohr moments/Gd atom.
Fig. 4. Temperature dependence of the susceptibility and the inverse susceptibility.
group Pnma. The unit cell consists of four formula units. There are two nonequivalent rare earth sites with four Gd atoms each. The antiferromagnetism can be suggested to be due to the negative (i.e. antiferromagnetic) exchange interaction of these two sites. Since anisotropy usually plays an important role in rare earth intermetallic compounds, more information can be obtained if a single crystal Gd Al is studied in the higher magnetic "eld even though distribution of 4f electrons in Gd atom is spherical symmetry. In addition, interesting behaviors in other properties such as speci"c heat, resistivity and magnetostriction can be expected to occur accompanied with the "eld-induced magnetic transition. A detailed study on this "eld-induced magnetic transition is in progress.
4. Conclusions Gd Al is paramagnetic with the paramagnetic Curie temperature ¹ "42 K and the e!ective
Acknowledgements The author would like to acknowledge Prof. S. Takahashi of Department of Materials Science and Technology, Iwate University for helpful correspondence and encouragement during the preparation of the manuscript.
References [1] K.A. Gschneidner, J. Eying (Eds.), Handbook of Physics and Chemistry of Rare Earths, Vol. 4, North-Holland, Amsterdam, 1979. [2] K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, Vol. 6, North-Holland, Amsterdam, 1991. [3] I.R. Harris, J. Less-Common Met. 131 (1987) 245. [4] H. Sun, J.M.D. Coey, Y. Otani, D.P.F. Hurley, J. Phys.: Condens. Matter 2 (1990) 6465. [5] K.H.J. Buschow, J. Less-Common Met. 43 (1975) 55. [6] H. Oesterreicher, Phys. Stat. Sol. (a) 39 (1977) K91. [7] L.R. Sill, R.R. Biggers, J. Appl. Phys. 49 (1978) 1500. [8] M.A. Continentino, N. Rivier, Physica B 86}88 (1977) 793.
Magnetic "eld-induced magnetic transition in Gd Al compound Xingguo Li* Department of Materials Science and Engineering, Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japan Received 9 February 1999
Abstract Magnetic properties of Gd Al compound with C23 structure were investigated from 4.5 to 300 K in magnetic "elds up to 50 kOe. The compound is paramagnetic above 50 K without dependence on the magnetic "eld. However, below 50 K it is antiferromagnetic in low magnetic "eld and is ferromagnetic in high magnetic "eld. The "eld-induced magnetic transition occurs around 25 kOe. 1999 Elsevier Science B.V. All rights reserved. Keywords: Rare earth compounds; Magnetization curve; Field-induced magnetic transition
1. Introduction Magnetic properties of rare earth metals and alloys have been widely investigated and as a result, many interesting phenomena have been reported. The research focuses especially on compounds consisting of rare earth metals and 3d transition metals because of their huge economical potential [1}4]. However, the complicated magnetism is di$cult to be understood since both 3d transition metals and 4f rare earth atoms possess magnetic moments. For this reason, magnetic behaviors of Gd Al are inves tigated in this study. In Gd Al only Gd atom has magnetic moments originating simply from spin angular momentum, and therefore magnetic properties of Gd Al are dominated by Gd atom. In most cases, magnetism in matters is classi"ed into
* Tel.: #81-19-621-6350; fax: #81-19-621-6373. E-mail address: [email protected] (X. Li)
ferromagnetism, antiferromagnetism, paramagnets and diamagnetism at a given temperature, which is magnetic "eld independent. However, our recent measurement showed that Gd Al has several di!erent magnetic states at low temperatures and alignment of atomic moment varies on altering the applied magnetic "eld. This paper reports on the magnetic properties of Gd Al compound and dis cusses its unusual behaviors.
2. Experimental The intermetallic compound Gd Al was pre pared from 99.9% pure Gd and 99.99% pure Al by arc melting in an argon gas atmosphere. Arcmelted ingot was #ipped over and was remelted thrice. Weight loss was negligible during arcmelting. To obtain a single phase, the ingot was homogenized at 973 K, for 605 ks in an evacuated quartz tube. The ingot was pulverized into powder under 100 mesh as experimental sample. The
0304-8853/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 9 ) 0 0 4 7 0 - 9
308
X. Li / Journal of Magnetism and Magnetic Materials 205 (1999) 307}310
structure of the sample was identi"ed by powder X-ray di!raction (XRD) using monochromated Cu K radiation. XRD shows that Gd Al crystallizes in a C23 structure with lattice constants of a"0.6612, b"0.5127 and c"0.9543 nm. The magnetic properties were measured at temperatures from 4.5 to 300 K by a superconducting quantum interference device (SQUID) magnetometry with applied "eld up to 50 kOe and was calibrated with Pd.
3. Results and discussion Fig. 1 shows magnetization curves of Gd Al at a series of temperatures. At 4.5 K, Gd Al exhibits di!erent magnetic behavior below and above 25 kOe. Below 25 kOe the spontaneous magnetization increases linearly with increasing magnetic "eld, but above 25 kOe it increases rapidly and then approaches a saturation value from 40 kOe. The staircase behavior on magnetization curves expresses the occurrence of a "eld-induced magnetic transition. The spontaneous magnetization in 50 kOe is 203 emu/g, i.e. 6.02 Bohr moments/ Gd atom. Gadolinium is normally considered an S state ion with seven unpaired spins. Atomic spin moments in Gd Al in 50 kOe are close to localized moment of 7 Bohr magnetons on the Gd atoms, indicating spins in Gd Al aligned in the same direc tion. This fact leads us to believe that the saturation part on the magnetization curve results from a ferromagnetic contribution. In contrast, the linear part on the magnetization curve below 25 kOe can be explained by a paramagnetic or an antiferromagnetic contribution. It is known that the "eld dependence of magnetization for a paramagnet can be expressed by a Langevin function. According to this model, it is impossible to transform into ferromagnetism from a paramagnet by a sudden increase in magnetization. Thus, it is concluded that the linear part is attributed to antiferromagnetism. A transition from antiferromagnetism to ferromagnetism in Gd Al is induced by the magnetic "eld and the critical magnetic "eld for the transition is 25 kOe. The magnetization curve at 4.5 K for the decreasing "eld is shown in Fig. 2. A large hysteresis loop is observed in magnetic "eld between 25 and
Fig. 1. Magnetization curves of Gd Al at a series of temper atures.
Fig. 2. Magnetization curves of Gd Al in the increasing and decreasing "elds at 4.5 K.
35 kOe, which is probably caused by inversion of the spin orientation. A slight curvature is also noticeable in "elds lower than 5 kOe, which is another "eld-induced transition and illustrates the presence of a small remnant moment. Buschow [5] and Oesterreicher [6] studied Gd Al in the paramagnetic region and gave results of k "8.02 l and ¹ "150. Sill and . Biggers investigated magnetic behavior of Gd Al
X. Li / Journal of Magnetism and Magnetic Materials 205 (1999) 307}310
with applied "eld up to 26 kOe. They noticed a "eld-induced magnetic transition around 5 kOe and reported Gd Al was antiferromagnetic with ¹ "44 K [7]. Our result given below is close to , the latter. However, because Gd Al was not investi gated at low temperatures and in higher "elds in their experiments, the ferromagnetic region and the "eld-induced magnetic transition were not found. Up to 40 K, the saturation part is visible, but the saturation magnetization decreases remarkably and the critical magnetic "eld for the transition increases a little with increasing temperature. Conversely, change in the linear part is small. The energy for the critical magnetic "eld to act with a Bohr magneton in Gd Al is about 2.32;10\ J, which corresponds to a mere energy of 1.7 k . Thus, the temperature range where the magnetic-induced transition occurs is quite wide in consideration of the magnetic action energy. The Arrott plot of magnetization obtained in high magnetic "elds gives the Curie temperature ¹ "38 K. ! At 50 K, the "eld dependence of the magnetization is a straight line up to the highest available "eld of 50 kOe without the saturation e!ect as shown below 50 K. If the Curie temperature is "eld-dependent, the transition perhaps shifts to a magnetic "eld higher than 50 kOe, which needs to be con"rmed further. Between 50 and 300 K, the spontaneous magnetization shows a linear dependence on the magnetic "eld and slope of line decreases gradually with increasing temperature, which is the typical behavior of a paramagnetic matter. Fig. 3 shows the temperature dependences of magnetization and the inverse magnetization in 10 and 50 kOe. In 50 kOe, it keeps a constant below 40 K, begins an abrupt decrease from 40 K and then decreases gradually above 60 K with increasing temperature, being a temperature dependent of ferromagnetism. However, in 10 kOe a small peak is observed around 50 K, which is characteristic of antiferromagnetism and demonstrates the antiferromagnetic state of Gd Al at low temperature. This result supports the discussion mentioned above. The peak's temperature of 50 K can be considered the NeH el point. The inverse susceptibility in 10 kOe linearly increases with increasing temperature except for a small positive deviation from linearity
309
Fig. 3. Temperature dependence of the magnetizations and the reverse magnetization in 10 and 50 kOe for Gd Al.
appearing above 250 K. Sill and Biggers also reported this deviation, but did not give an explanation for it. Although the nature of the deviation is not con"rmed in this study, it can be suggested to be caused by a rapid drop of the small remnant moment. The temperature dependence for a magnetic substance is normally characterized by a Curie temperature or a NeH el point. However, for a substance showing the magnetic "eld-induced transition, the Curie temperature and the NeH el point can be observed simultaneously. For Gd Al, the NeH el temperature is a little higher than the Curie temperature. The susceptibility was obtained from slope of magnetization curves, which is given in Fig. 4 together with the inverse susceptibility. The temperature dependence of susceptibility is di!erent from that of magnetization both in 10 and 50 kOe. The di!erence between them is due to the "eld-induced magnetic transitions. From the linear part of the reverse susceptibility, the paramagnetic Curie temperature ¹ and the e!ective magnetic moment are derived to be 42 K and 7.2 l /Gd atom, respectively. The e!ective magnetic moment is identical with that of a free Gd atom. In rare earth intermetallic compounds the exchange mechanism is usually attributed to interactions with conduction electrons (RKKY). These interactions are normally of long range [8]. Gd Al compound has the orthorhombic Ni Si-type space
310
X. Li / Journal of Magnetism and Magnetic Materials 205 (1999) 307}310
magnetic moment 7.2 l /Gd atom above 50 K. At the lower temperature, however, it is antiferromagnetic below a critical magnetic "eld, but ferromagnetic in the higher "eld. The Curie temperature and NeH el point are 38 and 50 K, respectively. The critical magnetic "eld for the "eld-induced magnetic transition is 25 kOe at 4.5 K and increases a little with increasing temperature. The saturation magnetization of Gd Al in the ferromagnetic state is 6.02 Bohr moments/Gd atom.
Fig. 4. Temperature dependence of the susceptibility and the inverse susceptibility.
group Pnma. The unit cell consists of four formula units. There are two nonequivalent rare earth sites with four Gd atoms each. The antiferromagnetism can be suggested to be due to the negative (i.e. antiferromagnetic) exchange interaction of these two sites. Since anisotropy usually plays an important role in rare earth intermetallic compounds, more information can be obtained if a single crystal Gd Al is studied in the higher magnetic "eld even though distribution of 4f electrons in Gd atom is spherical symmetry. In addition, interesting behaviors in other properties such as speci"c heat, resistivity and magnetostriction can be expected to occur accompanied with the "eld-induced magnetic transition. A detailed study on this "eld-induced magnetic transition is in progress.
4. Conclusions Gd Al is paramagnetic with the paramagnetic Curie temperature ¹ "42 K and the e!ective
Acknowledgements The author would like to acknowledge Prof. S. Takahashi of Department of Materials Science and Technology, Iwate University for helpful correspondence and encouragement during the preparation of the manuscript.
References [1] K.A. Gschneidner, J. Eying (Eds.), Handbook of Physics and Chemistry of Rare Earths, Vol. 4, North-Holland, Amsterdam, 1979. [2] K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, Vol. 6, North-Holland, Amsterdam, 1991. [3] I.R. Harris, J. Less-Common Met. 131 (1987) 245. [4] H. Sun, J.M.D. Coey, Y. Otani, D.P.F. Hurley, J. Phys.: Condens. Matter 2 (1990) 6465. [5] K.H.J. Buschow, J. Less-Common Met. 43 (1975) 55. [6] H. Oesterreicher, Phys. Stat. Sol. (a) 39 (1977) K91. [7] L.R. Sill, R.R. Biggers, J. Appl. Phys. 49 (1978) 1500. [8] M.A. Continentino, N. Rivier, Physica B 86}88 (1977) 793.