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Influence of low-temperature annealing on magnetic properties of (Nd0.625Ni0.375)85Al15 metallic glass
Journal of Alloys and Compounds 463 (2008) 226–229
Influence of low-temperature annealing on magnetic properties of (Nd0.625Ni0.375)85Al15 metallic glass Feng Xu a,∗ , Zhiming Wang b , Guang Chen a , Jianzhong Jiang c , Youwei Du d a
Department of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China b School of Mechanical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China c Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China d Physics Department and National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China Received 7 May 2007; received in revised form 17 September 2007; accepted 18 September 2007 Available online 25 September 2007
Abstract After a review of the selection process of (Nd0.625 Ni0.375 )85 Al15 as a metallic glass with a relatively high glass-forming ability, we investigate the influences of its phase transitions by duplicating the heating process of the isochronal thermal analysis with low-temperature annealings. The structure, thermal stability and magnetic properties are characterized. And the influences on magnetic properties are particularly discussed with emphasis. Both the annealing processes, to the glass-transition temperature and to the onset temperature of crystallization, bring about a higher coercivity of the sample and a higher freezing temperature of the spin-glass-state. For the sample annealed to the onset temperature of crystallization, the influence is quite obvious and is ascribed to the formation of ferrimagnetic Nd7 Ni3 phase, as detected by XRD. For the sample annealed to the glass-transition temperature, the indistinct influence is further identified with the analysis of the frequency dependence of the spin-glass-state, and it is mainly attributed to the change of the short-range order in the amorphous matrix. © 2007 Elsevier B.V. All rights reserved. Keywords: Metallic glasses; Spin glasses; Magnetic measurements
1. Introduction Nd–Fe–Al metallic glasses have attracted lots of attention because of their possible applications as both structure materials and hard magnetic materials [1,2]. Based on this ternary glassy alloy system, other similar Nd-based metallic glasses have been investigated, with Fe substituted by different transitional metals [3]. In our previous studies, we reported a melt-spun (Nd0.625 Ni0.375 )85 Al15 metallic glass [4], and studied primarily the glass-forming ability, the magnetic properties and the isothermal crystallization mechanism performed in the supercooled liquid region [5]. Similar as most bulk metallic glasses, the glass-transition and crystallization were observed and their influences on the structure has been primarily investigated. However, the change of magnetic properties, which could reflect the subtler influences of phase transitions, has not been studied yet.
∗
Corresponding author. Tel.: +86-25-84315159; fax: +86-25-84315797. E-mail addresses: xufeng [email protected] (F. Xu).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.09.069
In the present work, we duplicate the heating processes of phase transitions during the isochronal differential scanning calorimeter (DSC) measurement with low-temperature annealings, and investigate the influences of phase transitions. The influences on magnetic properties are particularly discussed with emphasis. 2. Experimental Master alloys were prepared by arc-melting from elemental Nd, Ni and Al with a purity of 99.9% in an argon atmosphere. The ingots were remelted several times to ensure the homogeneity. Thin ribbons with the cross section of 0.03 mm × 2 mm were prepared by a single-roller melt spinner at a wheel speed of 41 m/s in the purified argon atmosphere. The structures of the melt-spun ribbons were characterized by a conventional X-ray diffractometer (XRD) using a Philips PW 1820 diffractometer with Cu K␣ radiation. The thermal analyses were performed with a Pyris Diamond power compensation differential scanning calorimeter (DSC) under a flow of purified argon (99.998%) at a heating rate of 20 K/min. In order to realize a precise control of the temperature and repeat the heating process of phase transitions, DSC was also used for the low-temperature annealings. The low-temperature annealings were controlled to follow a twostep procedure: first, to heat the sample from 323 K to the set temperatures at 20 K/min, and then to let it cool down with the furnace without any isothermal
F. Xu et al. / Journal of Alloys and Compounds 463 (2008) 226–229
227
Fig. 1. XRD patterns of melt-spun Ndx Ni100−x binary alloys (x = 55, 60, 62.5, 65) and XRD patterns of S1, S2 and S3, which represent asquenched (Nd0.625 Ni0.375 )85 Al15 metallic glass (S1), and the annealed (Nd0.625 Ni0.375 )85 Al15 metallic glasses after heated to Tg and Tx (S2, S3).
Fig. 2. Parts of the isochronal DSC measurement curves of S1, S2 and S3, performed from 323 K to 600 K at 20 K/min. The inset is the enlarged presentation of the curve of S1, showing the supercooled liquid region, Tg and Tx .
processes. Magnetic properties measurements were carried out in a commercial superconducting quantum interference device (SQUID) magnetometer.
In order to investigate the phase transitions at Tg and Tx , two as-quenched (Nd0.625 Ni0.375 )85 Al15 samples were annealed with DSC to Tg and Tx , respectively. The sample heated to Tg is named S2 and the other heated to Tx is named S3. The annealing processes were performed following the procedures of low-temperature annealing, as explained in Section 2. After annealing, the structure was examined with XRD, and the XRD patterns are shown in Fig. 1. No differences between S2 and S1 can be detected by XRD. However, two observable crystalline peaks can be found in S3, indicating the formation of Nd7 Ni3 phase. Since the samples are still primarily amorphous, their thermal stabilities were examined by isochronal DSC (shown in Fig. 2). A slight shift of the crystallization peak to a lower temperature is observed in S2. This indicates the limited thermal stability of the alloy under the cycling thermal treatment. The DSC curve of S3, however, indicates a phase separation which should be brought by the crystallization of Nd7 Ni3 crystallites. Fig. 3 presents the M(T) relations of all three samples (S1, S2 and S3). Both the zero-field-cooled (ZFC) and field-cooled (FC) M(T) branches were measured at 100 Oe on warming after initially cooled from 100 K. And the applied magnetic field during the field-cooling is 1 T. The M(T) curves of S1 and S2 are quite close, and almost in superposition, indicating a relative stability at a temperature lower than Tg . The magnetizations exhibit the onset of the irreversibility between the ZFC and FC curves at around 10 K, which corresponds to the development of a collective frozen magnetic state after ZFC with the randomly oriented and distributed magnetic moments. Above the bifurcation temperature (freezing temperature: Tf ), the ZFC and FC curves coincide; whereas below this temperature, they bifur-
3. Results and discussions The XRD patterns of melt-spun Ndx Ni100−x binary alloys (x = 55, 60, 62.5, 65) are presented in Fig. 1, as a review of the selection process of (Nd0.625 Ni0.375 )85 Al15 . Here the four compositions are chosen from a series of Ndx Ni100−x (x = 55, 60, 62.5, 65, 70, 75, 80, 85, 90) alloys, and their XRD patterns were referred as the selection criterion for the potential development as metallic glasses. Among them, the composition of Nd62.5 Ni37.5 was selected and then Al was incorporated to form the (Nd0.625 Ni0.375 )1−y Aly (y = 5, 10, 15, 20) ternary alloys. The XRD patterns of these as-quenched ternary alloys show typically amorphous characteristics (the XRD pattern of as-quenched (Nd0.625 Ni0.375 )85 Al15 (named S1) is shown in Fig. 1 as an example). The glass-forming abilities are evaluated by the isochronal DSC measurements at 20 K/min, and the crystallization peak of S1 is selectively shown in Fig. 2 as an example. From the DSC curves, some transition temperatures and deduced parameters are listed into Table 1. The parameters include glass transition temperatures (Tg ), the onset temperatures of the primary crystallization (Tx ), the supercooled liquid region T = Tx − Tg , the melting temperatures (Tm ), the liquidus temperatures (Tl ) and the reduced glass transition temperatures Trg = Tg /Tm , T’rg = Tg /Tl . As clearly indicated, both (Nd0.625 Ni0.375 )85 Al15 and (Nd0.625 Ni0.375 )80 Al20 metallic glasses have relatively higher glass-forming abilities. In this work, (Nd0.625 Ni0.375 )85 Al15 is selected for the further study. Table 1 Parameters obtained from the isochronal DSC measurements measured at 20 K/min Composition
Tg (K)
Tx (K)
T (K)
Tm (K)
Tl (K)
Trg
T’rg
(Nd0.625 Ni0.375 )95 Al5 (Nd0.625 Ni0.375 )90 Al10 (Nd0.625 Ni0.375 )85 Al15 (Nd0.625 Ni0.375 )80 Al20
501.5 512.5 513.6 529.6
518.4 536.2 548.7 565.5
16.9 23.7 35.1 35.9
820.0 825.2 802.3 832.6
826.0 830.7 809.6 841.0
0.612 0.621 0.640 0.636
0.607 0.617 0.634 0.630
228
F. Xu et al. / Journal of Alloys and Compounds 463 (2008) 226–229
Fig. 3. DC ZFC-FC M–T curves of all the three samples (S1, S2 and S3). The applied magnetic field is 100 Oe.
cate, and the ZFC branch decreases rapidly while the FC branch decreases slightly to a plateau. This is a typical characteristic of canonical spin glass. The comparison of the M(T) curves of S1 and S2, shows that S2 has a higher magnetization after FC and an indistinct increase of the freezing temperature (Tf ). As for S3, an obvious difference of M(T) relations is clearly shown. Its FC magnetization branch keeps increasing with decreasing temperature; and the FC and ZFC branches keep bifurcating when the temperature is higher than Tf . This phenomenon indicates a coexistence of spin-glass state and another new magnetic phase with stronger exchange interaction. As detected by XRD, the appearance of new magnetic phase should be caused by the growth of Nd7 Ni3 nuclei or nanocrystals, the magnetism of which changes from ferrimagnetism to antiferromagnetism at a Curie temperature of about 14 K [6]. Fig. 4 presents the enlarged sections of magnetic hysteresis loops of all three samples measured at 5 K after ZFC. The coercivities of the samples are 290 ± 10 Oe for S1, 334 ± 5 Oe for S2, 570 ± 10 Oe for S3, respectively. Relative to S3, the lowtemperature annealing to Tg only brings slight influences to the magnetic properties of S2. Corresponding to the change of M(T) relations, the high coercivity of S3 should be also ascribed to the appearance of Nd7 Ni3 phase after annealing. For all samples, the magnetization does not show saturation in fields up to
Fig. 4. Magnetic hysteresis loops for all three samples measured at 5 K, and the inset indicating the shape of loops at high fields.
Fig. 5. The real and imaginary (as the inset) components of ac susceptibility of S2 at the frequency range 0.02 ≤ ω ≤ 1488 Hz measured between 5 K and 35 K.
7 T (as shown in the inset), and this is also a characteristic of spin-glass. To further distinguish the influence of low-temperature annealing to Tg , and to further characterize the spin-glass behavior of the sample, we performed frequency-dependent susceptibility measurements of S1 and S2. As an example, Fig. 5 selectively presents the temperature dependence of the real and imaginary components of the ac susceptibility of S2 between 5 K and 35 K at the frequency range 0.02 ≤ ω ≤ 1488 Hz. The real exhibit pronounced maximums with amplitude components χac and position [Tf (ω)] depending on the frequency of the applied magnetic field. As ω increases, Tf increases from 10.71 K at ω = 0.7 Hz to 11.56 K at ω = 1488 Hz for S1, and increases
Fig. 6. The frequency dependencies of the dynamic spin freezing temperature Tf , plotted as ln(1013 /f) vs. Tf and 1000/ln(f /f0 ) vs. Tf , shown in (a) and (b), respectively.
F. Xu et al. / Journal of Alloys and Compounds 463 (2008) 226–229
from 10.89 K at ω = 0.02 Hz to 11.96 K at ω = 1488 Hz for S2. This is a typical characteristic suggesting the formation of the spin-glass state in the sample. The initial frequency shift of Tf defined as ␦Tf = Tf /(Tf log ω) is determined to be about ␦Tf = 0.02 comparable to the typical values (from a few thousandths to a few hundredths) for most spin glasses [7]. In order to estimate the dynamical parameters characterizing the spin-glass state of the sample, the obtained Tf (ω) data are fitted to the standard expression τ max = τ 0 [(Tf − Ts )/Ts ]−zv (the critical slowing down) [8] and to the Vogel–Fulcher law ω = ω0 exp[−Ea /kB (Tf − T0 )], respectively [9]. Following Tholence [10], f0 = ω0 /2π = 1/τ 0 = 1013 Hz was kept fixed, the fitting using the former equation [Fig. 6(a)] yields the static freezing temperature Ts = 9.1 ± 0.3 K and the critical (dynamical) exponent zv = 18 ± 2 for S1; Ts = 10.1 ± 0.1 K and zv = 13.2 ± 0.2 for S2. And the fitting using the latter one (Fig. 6(b)) yields the activation energy Ea = 8.33kB Ts and the Vogel–Fulcher temperature T0 = 8.2 K for S1; Ea = 7.23kB Ts and T0 = 8.7 K for S2. These fitting parameters clearly indicate the increases of the characteristic temperatures (including both Ts and T0 ) after annealing to Tg . These increases infer stronger exchange interactions among the magnetic moments in S2 than those in S1. Different from S3, only an endothermic behavior was detected during the heating process of S2. Although the quenched-in nuclei with net magnetic moments were identified in S1 [5], as widely observed in rare-earth-based materials [11,12], the growth of the nuclei and the nucleation of the amorphous matrix are thought to be negligible in S2, since at a relatively lowtemperature about Tg , it always takes much longer time to crystallize during the isothermal process. For S2, since the amorphous structure keeps maintained, the spin-glass state is primarily influenced by the atomic and site disorders, which bring about a competition of the randomly distributed exchange interactions between Nd–Nd, Nd–Ni and Ni–Ni bondings. Similar spin-glass behavior is also observed in Pr-based bulk metallic glasses [11]. In this case, the enhancement of exchange interactions in S2 should be mainly ascribed to the change of the short-range order, which is caused by the rearrangement of atoms during the glass transition.
229
4. Conclusion Through the low-temperature annealings to Tg and Tx , we duplicate the heating processes in DSC measurement and investigate the influences of phase transitions. For the sample annealed to Tg (S2), slight influences of low-temperature annealing can be detected through the measurement of magnetic properties, as compared with S1, although no obvious structural changes can be detected through XRD analysis. Both the slight increases of coercivity and characteristic temperatures of S2, are ascribed to the change of the short-range order in the amorphous matrix during the glass-transition. For the sample annealed to Tx (S3), both the coercivity and freezing temperature increase obviously, and a new magnetic phase is found coexistent with the spin-glass-state. These influences are ascribed to the formation of ferrimagnetic Nd7 Ni3 phase, which is also indicated by XRD. References [1] A. Inoue, T. Zhang, W. Zhang, A. Takeuchi, Mater. Trans. JIM 37 (1996) 99. [2] R. Sato Turtelli, D. Triyono, R. Grossinger, H. Michor, J.H. Espina, J.P. Sinnecker, H. Sassik, J. Eckert, G. Kumar, Z.G. Sun, G.J. Fan, Phys. Rev. B 66 (2002) 054441. [3] Z.G. Sun, G. Kumar, W. L¨oser, J. Eckert, K.-H. M¨uller, L. Schultz, Mater. Sci. Eng. A 375–377 (2004) 403. [4] F. Xu, X.S. Wu, Y.W. Du, C. Cui, G. Chen, J. Appl. Phys. 99 (2006) 08B524. [5] F. Xu, Z.M. Wang, Z.R. Yuan, G. Chen, J.Z. Jiang, J. Alloys Compd. 458 (2008) 261. [6] T. Tsutaoka, H. Fukuda, T. Tokunaga, H. Kadomatsu, Y. Ito, J. Magn. Magn. Mater. 167 (1997) 249. [7] J.A. Mydosh, Spin Glass: An Experimental Introduction, Taylor & Francis, London, 1993. [8] K. Gunnarsson, P. Svedlindh, P. Nordblad, L. Lundgren, H. Aruga, A. Ito, Phys. Rev. Lett. 61 (1988) 754. [9] D.X. Li, T. Yamamura, S. Nimori, K. Yubuta, Y. Shiokawa, Appl. Phys. Lett. 87 (2005) 142505. [10] J.L. Tholence, Solid State Commun. 35 (1980) 113. [11] Y.T. Wang, H.Y. Bai, M.X. Pan, D.Q. Zhao, W.H. Wang, Phys. Rev. B 74 (2006) 064422. [12] Y.T. Wang, W.H. Wang, J. Non-Cryst. Solids 352 (2006) 444; G.J. Fan, W. L¨oser, J. Eckert, L. Schultz, Appl. Phys. Lett. 75 (1999) 2948.
Influence of low-temperature annealing on magnetic properties of (Nd0.625Ni0.375)85Al15 metallic glass Feng Xu a,∗ , Zhiming Wang b , Guang Chen a , Jianzhong Jiang c , Youwei Du d a
Department of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China b School of Mechanical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China c Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China d Physics Department and National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China Received 7 May 2007; received in revised form 17 September 2007; accepted 18 September 2007 Available online 25 September 2007
Abstract After a review of the selection process of (Nd0.625 Ni0.375 )85 Al15 as a metallic glass with a relatively high glass-forming ability, we investigate the influences of its phase transitions by duplicating the heating process of the isochronal thermal analysis with low-temperature annealings. The structure, thermal stability and magnetic properties are characterized. And the influences on magnetic properties are particularly discussed with emphasis. Both the annealing processes, to the glass-transition temperature and to the onset temperature of crystallization, bring about a higher coercivity of the sample and a higher freezing temperature of the spin-glass-state. For the sample annealed to the onset temperature of crystallization, the influence is quite obvious and is ascribed to the formation of ferrimagnetic Nd7 Ni3 phase, as detected by XRD. For the sample annealed to the glass-transition temperature, the indistinct influence is further identified with the analysis of the frequency dependence of the spin-glass-state, and it is mainly attributed to the change of the short-range order in the amorphous matrix. © 2007 Elsevier B.V. All rights reserved. Keywords: Metallic glasses; Spin glasses; Magnetic measurements
1. Introduction Nd–Fe–Al metallic glasses have attracted lots of attention because of their possible applications as both structure materials and hard magnetic materials [1,2]. Based on this ternary glassy alloy system, other similar Nd-based metallic glasses have been investigated, with Fe substituted by different transitional metals [3]. In our previous studies, we reported a melt-spun (Nd0.625 Ni0.375 )85 Al15 metallic glass [4], and studied primarily the glass-forming ability, the magnetic properties and the isothermal crystallization mechanism performed in the supercooled liquid region [5]. Similar as most bulk metallic glasses, the glass-transition and crystallization were observed and their influences on the structure has been primarily investigated. However, the change of magnetic properties, which could reflect the subtler influences of phase transitions, has not been studied yet.
∗
Corresponding author. Tel.: +86-25-84315159; fax: +86-25-84315797. E-mail addresses: xufeng [email protected] (F. Xu).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.09.069
In the present work, we duplicate the heating processes of phase transitions during the isochronal differential scanning calorimeter (DSC) measurement with low-temperature annealings, and investigate the influences of phase transitions. The influences on magnetic properties are particularly discussed with emphasis. 2. Experimental Master alloys were prepared by arc-melting from elemental Nd, Ni and Al with a purity of 99.9% in an argon atmosphere. The ingots were remelted several times to ensure the homogeneity. Thin ribbons with the cross section of 0.03 mm × 2 mm were prepared by a single-roller melt spinner at a wheel speed of 41 m/s in the purified argon atmosphere. The structures of the melt-spun ribbons were characterized by a conventional X-ray diffractometer (XRD) using a Philips PW 1820 diffractometer with Cu K␣ radiation. The thermal analyses were performed with a Pyris Diamond power compensation differential scanning calorimeter (DSC) under a flow of purified argon (99.998%) at a heating rate of 20 K/min. In order to realize a precise control of the temperature and repeat the heating process of phase transitions, DSC was also used for the low-temperature annealings. The low-temperature annealings were controlled to follow a twostep procedure: first, to heat the sample from 323 K to the set temperatures at 20 K/min, and then to let it cool down with the furnace without any isothermal
F. Xu et al. / Journal of Alloys and Compounds 463 (2008) 226–229
227
Fig. 1. XRD patterns of melt-spun Ndx Ni100−x binary alloys (x = 55, 60, 62.5, 65) and XRD patterns of S1, S2 and S3, which represent asquenched (Nd0.625 Ni0.375 )85 Al15 metallic glass (S1), and the annealed (Nd0.625 Ni0.375 )85 Al15 metallic glasses after heated to Tg and Tx (S2, S3).
Fig. 2. Parts of the isochronal DSC measurement curves of S1, S2 and S3, performed from 323 K to 600 K at 20 K/min. The inset is the enlarged presentation of the curve of S1, showing the supercooled liquid region, Tg and Tx .
processes. Magnetic properties measurements were carried out in a commercial superconducting quantum interference device (SQUID) magnetometer.
In order to investigate the phase transitions at Tg and Tx , two as-quenched (Nd0.625 Ni0.375 )85 Al15 samples were annealed with DSC to Tg and Tx , respectively. The sample heated to Tg is named S2 and the other heated to Tx is named S3. The annealing processes were performed following the procedures of low-temperature annealing, as explained in Section 2. After annealing, the structure was examined with XRD, and the XRD patterns are shown in Fig. 1. No differences between S2 and S1 can be detected by XRD. However, two observable crystalline peaks can be found in S3, indicating the formation of Nd7 Ni3 phase. Since the samples are still primarily amorphous, their thermal stabilities were examined by isochronal DSC (shown in Fig. 2). A slight shift of the crystallization peak to a lower temperature is observed in S2. This indicates the limited thermal stability of the alloy under the cycling thermal treatment. The DSC curve of S3, however, indicates a phase separation which should be brought by the crystallization of Nd7 Ni3 crystallites. Fig. 3 presents the M(T) relations of all three samples (S1, S2 and S3). Both the zero-field-cooled (ZFC) and field-cooled (FC) M(T) branches were measured at 100 Oe on warming after initially cooled from 100 K. And the applied magnetic field during the field-cooling is 1 T. The M(T) curves of S1 and S2 are quite close, and almost in superposition, indicating a relative stability at a temperature lower than Tg . The magnetizations exhibit the onset of the irreversibility between the ZFC and FC curves at around 10 K, which corresponds to the development of a collective frozen magnetic state after ZFC with the randomly oriented and distributed magnetic moments. Above the bifurcation temperature (freezing temperature: Tf ), the ZFC and FC curves coincide; whereas below this temperature, they bifur-
3. Results and discussions The XRD patterns of melt-spun Ndx Ni100−x binary alloys (x = 55, 60, 62.5, 65) are presented in Fig. 1, as a review of the selection process of (Nd0.625 Ni0.375 )85 Al15 . Here the four compositions are chosen from a series of Ndx Ni100−x (x = 55, 60, 62.5, 65, 70, 75, 80, 85, 90) alloys, and their XRD patterns were referred as the selection criterion for the potential development as metallic glasses. Among them, the composition of Nd62.5 Ni37.5 was selected and then Al was incorporated to form the (Nd0.625 Ni0.375 )1−y Aly (y = 5, 10, 15, 20) ternary alloys. The XRD patterns of these as-quenched ternary alloys show typically amorphous characteristics (the XRD pattern of as-quenched (Nd0.625 Ni0.375 )85 Al15 (named S1) is shown in Fig. 1 as an example). The glass-forming abilities are evaluated by the isochronal DSC measurements at 20 K/min, and the crystallization peak of S1 is selectively shown in Fig. 2 as an example. From the DSC curves, some transition temperatures and deduced parameters are listed into Table 1. The parameters include glass transition temperatures (Tg ), the onset temperatures of the primary crystallization (Tx ), the supercooled liquid region T = Tx − Tg , the melting temperatures (Tm ), the liquidus temperatures (Tl ) and the reduced glass transition temperatures Trg = Tg /Tm , T’rg = Tg /Tl . As clearly indicated, both (Nd0.625 Ni0.375 )85 Al15 and (Nd0.625 Ni0.375 )80 Al20 metallic glasses have relatively higher glass-forming abilities. In this work, (Nd0.625 Ni0.375 )85 Al15 is selected for the further study. Table 1 Parameters obtained from the isochronal DSC measurements measured at 20 K/min Composition
Tg (K)
Tx (K)
T (K)
Tm (K)
Tl (K)
Trg
T’rg
(Nd0.625 Ni0.375 )95 Al5 (Nd0.625 Ni0.375 )90 Al10 (Nd0.625 Ni0.375 )85 Al15 (Nd0.625 Ni0.375 )80 Al20
501.5 512.5 513.6 529.6
518.4 536.2 548.7 565.5
16.9 23.7 35.1 35.9
820.0 825.2 802.3 832.6
826.0 830.7 809.6 841.0
0.612 0.621 0.640 0.636
0.607 0.617 0.634 0.630
228
F. Xu et al. / Journal of Alloys and Compounds 463 (2008) 226–229
Fig. 3. DC ZFC-FC M–T curves of all the three samples (S1, S2 and S3). The applied magnetic field is 100 Oe.
cate, and the ZFC branch decreases rapidly while the FC branch decreases slightly to a plateau. This is a typical characteristic of canonical spin glass. The comparison of the M(T) curves of S1 and S2, shows that S2 has a higher magnetization after FC and an indistinct increase of the freezing temperature (Tf ). As for S3, an obvious difference of M(T) relations is clearly shown. Its FC magnetization branch keeps increasing with decreasing temperature; and the FC and ZFC branches keep bifurcating when the temperature is higher than Tf . This phenomenon indicates a coexistence of spin-glass state and another new magnetic phase with stronger exchange interaction. As detected by XRD, the appearance of new magnetic phase should be caused by the growth of Nd7 Ni3 nuclei or nanocrystals, the magnetism of which changes from ferrimagnetism to antiferromagnetism at a Curie temperature of about 14 K [6]. Fig. 4 presents the enlarged sections of magnetic hysteresis loops of all three samples measured at 5 K after ZFC. The coercivities of the samples are 290 ± 10 Oe for S1, 334 ± 5 Oe for S2, 570 ± 10 Oe for S3, respectively. Relative to S3, the lowtemperature annealing to Tg only brings slight influences to the magnetic properties of S2. Corresponding to the change of M(T) relations, the high coercivity of S3 should be also ascribed to the appearance of Nd7 Ni3 phase after annealing. For all samples, the magnetization does not show saturation in fields up to
Fig. 4. Magnetic hysteresis loops for all three samples measured at 5 K, and the inset indicating the shape of loops at high fields.
Fig. 5. The real and imaginary (as the inset) components of ac susceptibility of S2 at the frequency range 0.02 ≤ ω ≤ 1488 Hz measured between 5 K and 35 K.
7 T (as shown in the inset), and this is also a characteristic of spin-glass. To further distinguish the influence of low-temperature annealing to Tg , and to further characterize the spin-glass behavior of the sample, we performed frequency-dependent susceptibility measurements of S1 and S2. As an example, Fig. 5 selectively presents the temperature dependence of the real and imaginary components of the ac susceptibility of S2 between 5 K and 35 K at the frequency range 0.02 ≤ ω ≤ 1488 Hz. The real exhibit pronounced maximums with amplitude components χac and position [Tf (ω)] depending on the frequency of the applied magnetic field. As ω increases, Tf increases from 10.71 K at ω = 0.7 Hz to 11.56 K at ω = 1488 Hz for S1, and increases
Fig. 6. The frequency dependencies of the dynamic spin freezing temperature Tf , plotted as ln(1013 /f) vs. Tf and 1000/ln(f /f0 ) vs. Tf , shown in (a) and (b), respectively.
F. Xu et al. / Journal of Alloys and Compounds 463 (2008) 226–229
from 10.89 K at ω = 0.02 Hz to 11.96 K at ω = 1488 Hz for S2. This is a typical characteristic suggesting the formation of the spin-glass state in the sample. The initial frequency shift of Tf defined as ␦Tf = Tf /(Tf log ω) is determined to be about ␦Tf = 0.02 comparable to the typical values (from a few thousandths to a few hundredths) for most spin glasses [7]. In order to estimate the dynamical parameters characterizing the spin-glass state of the sample, the obtained Tf (ω) data are fitted to the standard expression τ max = τ 0 [(Tf − Ts )/Ts ]−zv (the critical slowing down) [8] and to the Vogel–Fulcher law ω = ω0 exp[−Ea /kB (Tf − T0 )], respectively [9]. Following Tholence [10], f0 = ω0 /2π = 1/τ 0 = 1013 Hz was kept fixed, the fitting using the former equation [Fig. 6(a)] yields the static freezing temperature Ts = 9.1 ± 0.3 K and the critical (dynamical) exponent zv = 18 ± 2 for S1; Ts = 10.1 ± 0.1 K and zv = 13.2 ± 0.2 for S2. And the fitting using the latter one (Fig. 6(b)) yields the activation energy Ea = 8.33kB Ts and the Vogel–Fulcher temperature T0 = 8.2 K for S1; Ea = 7.23kB Ts and T0 = 8.7 K for S2. These fitting parameters clearly indicate the increases of the characteristic temperatures (including both Ts and T0 ) after annealing to Tg . These increases infer stronger exchange interactions among the magnetic moments in S2 than those in S1. Different from S3, only an endothermic behavior was detected during the heating process of S2. Although the quenched-in nuclei with net magnetic moments were identified in S1 [5], as widely observed in rare-earth-based materials [11,12], the growth of the nuclei and the nucleation of the amorphous matrix are thought to be negligible in S2, since at a relatively lowtemperature about Tg , it always takes much longer time to crystallize during the isothermal process. For S2, since the amorphous structure keeps maintained, the spin-glass state is primarily influenced by the atomic and site disorders, which bring about a competition of the randomly distributed exchange interactions between Nd–Nd, Nd–Ni and Ni–Ni bondings. Similar spin-glass behavior is also observed in Pr-based bulk metallic glasses [11]. In this case, the enhancement of exchange interactions in S2 should be mainly ascribed to the change of the short-range order, which is caused by the rearrangement of atoms during the glass transition.
229
4. Conclusion Through the low-temperature annealings to Tg and Tx , we duplicate the heating processes in DSC measurement and investigate the influences of phase transitions. For the sample annealed to Tg (S2), slight influences of low-temperature annealing can be detected through the measurement of magnetic properties, as compared with S1, although no obvious structural changes can be detected through XRD analysis. Both the slight increases of coercivity and characteristic temperatures of S2, are ascribed to the change of the short-range order in the amorphous matrix during the glass-transition. For the sample annealed to Tx (S3), both the coercivity and freezing temperature increase obviously, and a new magnetic phase is found coexistent with the spin-glass-state. These influences are ascribed to the formation of ferrimagnetic Nd7 Ni3 phase, which is also indicated by XRD. References [1] A. Inoue, T. Zhang, W. Zhang, A. Takeuchi, Mater. Trans. JIM 37 (1996) 99. [2] R. Sato Turtelli, D. Triyono, R. Grossinger, H. Michor, J.H. Espina, J.P. Sinnecker, H. Sassik, J. Eckert, G. Kumar, Z.G. Sun, G.J. Fan, Phys. Rev. B 66 (2002) 054441. [3] Z.G. Sun, G. Kumar, W. L¨oser, J. Eckert, K.-H. M¨uller, L. Schultz, Mater. Sci. Eng. A 375–377 (2004) 403. [4] F. Xu, X.S. Wu, Y.W. Du, C. Cui, G. Chen, J. Appl. Phys. 99 (2006) 08B524. [5] F. Xu, Z.M. Wang, Z.R. Yuan, G. Chen, J.Z. Jiang, J. Alloys Compd. 458 (2008) 261. [6] T. Tsutaoka, H. Fukuda, T. Tokunaga, H. Kadomatsu, Y. Ito, J. Magn. Magn. Mater. 167 (1997) 249. [7] J.A. Mydosh, Spin Glass: An Experimental Introduction, Taylor & Francis, London, 1993. [8] K. Gunnarsson, P. Svedlindh, P. Nordblad, L. Lundgren, H. Aruga, A. Ito, Phys. Rev. Lett. 61 (1988) 754. [9] D.X. Li, T. Yamamura, S. Nimori, K. Yubuta, Y. Shiokawa, Appl. Phys. Lett. 87 (2005) 142505. [10] J.L. Tholence, Solid State Commun. 35 (1980) 113. [11] Y.T. Wang, H.Y. Bai, M.X. Pan, D.Q. Zhao, W.H. Wang, Phys. Rev. B 74 (2006) 064422. [12] Y.T. Wang, W.H. Wang, J. Non-Cryst. Solids 352 (2006) 444; G.J. Fan, W. L¨oser, J. Eckert, L. Schultz, Appl. Phys. Lett. 75 (1999) 2948.