- Email: [email protected]
Spin–glass behavior of mechanically milled TbCu2
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 310 (2007) e506–e508 www.elsevier.com/locate/jmmm
Spin–glass behavior of mechanically milled TbCu2 D.P. Rojas, J.I. Espeso, L. Ferna´ndez Barquı´ n, J. Rodrı´ guez Ferna´ndez, J.C. Go´mez Sal DCITIMAC, Facultad de Ciencias, Universidad de Cantabria, Santander 39005, Spain Available online 14 November 2006
Abstract We report changes on the structural and magnetic properties of a TbCu2 crystalline alloy when submitted to high-energy ball milling. The samples were characterized by means of X-ray diffraction, DC and AC susceptibility. The starting compound was found to crystallize in the orthorhombic CeCu2 structure and orders antiferromagnetically at T N ¼ 50 K. After 120 h of milling time, the atomic structure is basically amorphous and the antiferromagnetic phase disappears giving rise to a spin–glass (SG)-like magnetic arrangement below T f ¼ 14 K. r 2006 Elsevier B.V. All rights reserved. PACS: 75.50.Kj; 75.50.Lk; 81.20.wk Keywords: Antiferromagnetic; Ball milling; Spin–glass; AC and DC susceptibility; Random anisotropy
In the last years the study of mechanically milled rare earth alloys have revealed changes on the magnetic properties with the introduction of disorder [1]. Some results claim to have encountered SG-like features on the basis of DC magnetization irreversibility and frequencydependent AC susceptibility behavior due to the presence of random magnetic interactions [1,2]. It is worth noticing that most of the studies have been constrained to materials with dominant ferromagnetic interactions. These systems are related to the well-known random anisotropy magnets in which the magnetic ground state is determined by the competition between the average local anisotropy D and the exchange, J, which in fact results in the existence of frustrated spin correlations [3]. So far, information about the disorder effects on rare earth antiferromagnetic (AFM) intermetallic systems upon mechanical milling is scarce, although such a production technique appears as a straightforward route to study heterogeneous magnetic systems [1,2]. Thus, in this work we will present the changes observed on the magnetic properties of antiferromagnetic TbCu2 bulk intermetallic upon mechanical milling monitored by AC and DC susceptibility measurements. Corresponding author. Tel.: +34 942 20 1512; fax: +34 942 20 1402.
E-mail address: [email protected] (D.P. Rojas). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.628
The starting policrystalline TbCu2 was prepared in an arc furnace. It shows an orthorhombic CeCu2 structure with unit cell parameters a ¼ 4:3176ð3Þ, b ¼ 6:8454ð9Þ, and ˚ as deduced from Rietveld refinement. The c ¼ 7:3318ð9Þ A, latter are in close agreement with those already reported for this intermetallic [4]. In a further step, this sample was milled for 120 h in a planetary high-energy ball system in Ar-sealed WC containers at a rotation speed of 200 rpm. The results of the X-ray diffraction patterns for unmilled and 120 h milled samples are presented in the Fig. 1. It can be seen that for 120 h of grinding time the peaks practically disappear and the X-ray diffraction pattern characteristic of amorphous material with a very broad halo centered around 38 is obtained. Also, broad peaks from the highest intensity reflections stemming from terbium oxide ðTb2 O3 ) emerge. Regarding the magnetic properties, ZFC (zero field cooled) and FC (field cooled) DC-magnetization measurements on the unmilled sample (not shown) presents the typical AFM cusp at T N ¼ 50 K, in agreement with previous data, and no irreversibility. By contrast, for the 120 h milled sample (presented in the Fig. 2), an irreversibility is clearly observed for the magnetic field of 500 Oe, and the maximum of the ZFC curve defines the freezing temperature ðT f Þ which reaches a value of 13.8 K
ARTICLE IN PRESS D.P. Rojas et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e506–e508
e507
χ´
120 h
*
χAC(emu/g.10-3)
I (au)
4
*
*
* *
χA C(emu/g.10- 3)
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3
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TbCu2 unmilled
0.3
χ´´
0.0 30
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70
T(K)
10 Hz
2
100 Hz 5 kHz 10 kHz
1
χ´´
0h 0 0
20
30
40
50
60
70
80
90
Fig. 1. X-ray diffraction patterns for unmilled and 120 h milled samples. The peaks associated to the Tb2 O3 are indicated by the markers.
14 FC
Tf (K)
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χ(emu/mol)
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12 11
1.2
10 0.8
0
100 Oe
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)
2k Oe 0.4
5k Oe 0
40
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20
30
40
50
T(K)
2θ°
2.0
10
60
T (K) Fig. 2. ZFC–FC (H ¼ 500 Oe) and ZFC curves for different magnetic fields for the 120 h milled sample. The field dependence of T f is depicted in the inset.
at H ¼ 100 Oe. It is worthy of note that the deviation of the ZFC and FC curves starts a few degrees above T f , different to canonical SG where the curves coincide at the peak. This irregularity in the ZFC–FC curves could stem from short-range correlations of magnetic clusters at temperatures higher than T f [5]. Despite this fact, the magnetic field dependence of the ZFC maximum (T f ) follows a H 2=3 law (depicted in the inset) usually taken as the irreversibility line according to the predictions of an Ising mean-field model derived for canonical SG systems by Almeida–Thouless [6]. However, information about the process of the freezing of the spins can be better furnished by the AC-susceptibility. Thus, the frequency dependence of the in-phase (w0 ) and out-phase (w00 ) components of the AC susceptibility for the 120 h milled sample is presented in the Fig. 3. The shift
Fig. 3. Real and imaginary parts (w0 and w00 ) of the AC susceptibility at several frequencies for the 120 h milled sample. The inset presents the AC susceptibility result for the Antiferromagnetic unmilled TbCu2 .
of the peak of w0 to higher temperatures and the decrease (increase) of w0 (w00 ) with the increase of the frequency is also characteristic of SG-like systems, ruling out the existence of an AFM phase transition. The temperature T f ¼ 14:3 K can be defined at the maximum of the lowest measured frequency ðv ¼ 10 HzÞ, which is in fact very close to the static DC-susceptibility and obviously this value agrees with that observed above from such a result. Furthermore, the value of the relative shift per frequency decade DT f =T f D log n ¼ 0:034 is larger than those of canonical SG, such as CuMn and AuMn, but comparable to the values reported for non-magnetic atom disorder(NMAD) SG of the type Ce2 TX3 (where T ¼ transition metal and X ¼ In,Ge) [7]. The inset shows the w0 (T) and w00 (T) curves for the bulk TbCu2 intermetallic. A hump near 50 K in the w0 and the absence of peak in w00 confirm an AFM behavior for this material. It is clear that despite the macroscopic results for the amorphous alloy (milled for 120 h) resemble those of bulk canonical and NMAD SG systems, here the microscopic origin of the magnetic phenomena is somewhat different. In rare earth amorphous materials the structural disorder induces random exchange and anisotropy and the frustration of the interactions leads to such features [3]. The relative large shift of the frequency at T f is a hint for the existence of a spin–clustered state. This heterogenous behavior has also been found in other amorphous alloys as magnetically concentrated Fe–Zr–B, in which the use of the non-linear susceptibility (which should diverge at the freezing temperature) allows to establish the existence of correlated spin–clusters [8]. Summarizing, the results have shown a crossover from bulk AFM in crystalline TbCu2 to SG-like behavior in a disordered amorphous alloy obtained by mechanical milling. It would be interesting to extend this study by preparing compounds at intermediate milling times which allow to establish a tendency for the evolution of the
ARTICLE IN PRESS e508
D.P. Rojas et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e506–e508
atomic structure and its relationship with the development of a frustrated magnetic behavior found in the 120 h amorphous TbCu2 sample. This work has been supported by the Direction of the Universities of the Ministry of Science and Education of Spain under Contracts MAT 2003-06815 and SB20030102.
References [1] [2] [3] [4] [5] [6] [7] [8]
M.A. Morales, et al., Phys. Rev. B 70 (2004) 184407. P.M. Shand, et al., J. Appl. Phys. 97 (2005) 10J505. R.I. Bewley, R. Cywinski, Phys. Rev. B 54 (1996) 15251. K. Sugiyama, et al., J. Magn. Magn. Mater. 177 (1998) 361. N. Marcano, et al., Phys. Rev B 71 (2005) 134401. J.R.L.D. Almeida, D.J. Thouless, J. Phys. A 11 (5) (1978) 983. D.P. Rojas, et al., Physica B 378 (2006) 847. L. F. Barquin, et al., Eur. Phys. J. B 35 (1) (2003) 3.
Journal of Magnetism and Magnetic Materials 310 (2007) e506–e508 www.elsevier.com/locate/jmmm
Spin–glass behavior of mechanically milled TbCu2 D.P. Rojas, J.I. Espeso, L. Ferna´ndez Barquı´ n, J. Rodrı´ guez Ferna´ndez, J.C. Go´mez Sal DCITIMAC, Facultad de Ciencias, Universidad de Cantabria, Santander 39005, Spain Available online 14 November 2006
Abstract We report changes on the structural and magnetic properties of a TbCu2 crystalline alloy when submitted to high-energy ball milling. The samples were characterized by means of X-ray diffraction, DC and AC susceptibility. The starting compound was found to crystallize in the orthorhombic CeCu2 structure and orders antiferromagnetically at T N ¼ 50 K. After 120 h of milling time, the atomic structure is basically amorphous and the antiferromagnetic phase disappears giving rise to a spin–glass (SG)-like magnetic arrangement below T f ¼ 14 K. r 2006 Elsevier B.V. All rights reserved. PACS: 75.50.Kj; 75.50.Lk; 81.20.wk Keywords: Antiferromagnetic; Ball milling; Spin–glass; AC and DC susceptibility; Random anisotropy
In the last years the study of mechanically milled rare earth alloys have revealed changes on the magnetic properties with the introduction of disorder [1]. Some results claim to have encountered SG-like features on the basis of DC magnetization irreversibility and frequencydependent AC susceptibility behavior due to the presence of random magnetic interactions [1,2]. It is worth noticing that most of the studies have been constrained to materials with dominant ferromagnetic interactions. These systems are related to the well-known random anisotropy magnets in which the magnetic ground state is determined by the competition between the average local anisotropy D and the exchange, J, which in fact results in the existence of frustrated spin correlations [3]. So far, information about the disorder effects on rare earth antiferromagnetic (AFM) intermetallic systems upon mechanical milling is scarce, although such a production technique appears as a straightforward route to study heterogeneous magnetic systems [1,2]. Thus, in this work we will present the changes observed on the magnetic properties of antiferromagnetic TbCu2 bulk intermetallic upon mechanical milling monitored by AC and DC susceptibility measurements. Corresponding author. Tel.: +34 942 20 1512; fax: +34 942 20 1402.
E-mail address: [email protected] (D.P. Rojas). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.628
The starting policrystalline TbCu2 was prepared in an arc furnace. It shows an orthorhombic CeCu2 structure with unit cell parameters a ¼ 4:3176ð3Þ, b ¼ 6:8454ð9Þ, and ˚ as deduced from Rietveld refinement. The c ¼ 7:3318ð9Þ A, latter are in close agreement with those already reported for this intermetallic [4]. In a further step, this sample was milled for 120 h in a planetary high-energy ball system in Ar-sealed WC containers at a rotation speed of 200 rpm. The results of the X-ray diffraction patterns for unmilled and 120 h milled samples are presented in the Fig. 1. It can be seen that for 120 h of grinding time the peaks practically disappear and the X-ray diffraction pattern characteristic of amorphous material with a very broad halo centered around 38 is obtained. Also, broad peaks from the highest intensity reflections stemming from terbium oxide ðTb2 O3 ) emerge. Regarding the magnetic properties, ZFC (zero field cooled) and FC (field cooled) DC-magnetization measurements on the unmilled sample (not shown) presents the typical AFM cusp at T N ¼ 50 K, in agreement with previous data, and no irreversibility. By contrast, for the 120 h milled sample (presented in the Fig. 2), an irreversibility is clearly observed for the magnetic field of 500 Oe, and the maximum of the ZFC curve defines the freezing temperature ðT f Þ which reaches a value of 13.8 K
ARTICLE IN PRESS D.P. Rojas et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e506–e508
e507
χ´
120 h
*
χAC(emu/g.10-3)
I (au)
4
*
*
* *
χA C(emu/g.10- 3)
5
3
χ´
1.0 0.8 0.5
TbCu2 unmilled
0.3
χ´´
0.0 30
40
50
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70
T(K)
10 Hz
2
100 Hz 5 kHz 10 kHz
1
χ´´
0h 0 0
20
30
40
50
60
70
80
90
Fig. 1. X-ray diffraction patterns for unmilled and 120 h milled samples. The peaks associated to the Tb2 O3 are indicated by the markers.
14 FC
Tf (K)
13
χ(emu/mol)
1.6
12 11
1.2
10 0.8
0
100 Oe
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500 Oe
150
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250
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)
2k Oe 0.4
5k Oe 0
40
20
20
30
40
50
T(K)
2θ°
2.0
10
60
T (K) Fig. 2. ZFC–FC (H ¼ 500 Oe) and ZFC curves for different magnetic fields for the 120 h milled sample. The field dependence of T f is depicted in the inset.
at H ¼ 100 Oe. It is worthy of note that the deviation of the ZFC and FC curves starts a few degrees above T f , different to canonical SG where the curves coincide at the peak. This irregularity in the ZFC–FC curves could stem from short-range correlations of magnetic clusters at temperatures higher than T f [5]. Despite this fact, the magnetic field dependence of the ZFC maximum (T f ) follows a H 2=3 law (depicted in the inset) usually taken as the irreversibility line according to the predictions of an Ising mean-field model derived for canonical SG systems by Almeida–Thouless [6]. However, information about the process of the freezing of the spins can be better furnished by the AC-susceptibility. Thus, the frequency dependence of the in-phase (w0 ) and out-phase (w00 ) components of the AC susceptibility for the 120 h milled sample is presented in the Fig. 3. The shift
Fig. 3. Real and imaginary parts (w0 and w00 ) of the AC susceptibility at several frequencies for the 120 h milled sample. The inset presents the AC susceptibility result for the Antiferromagnetic unmilled TbCu2 .
of the peak of w0 to higher temperatures and the decrease (increase) of w0 (w00 ) with the increase of the frequency is also characteristic of SG-like systems, ruling out the existence of an AFM phase transition. The temperature T f ¼ 14:3 K can be defined at the maximum of the lowest measured frequency ðv ¼ 10 HzÞ, which is in fact very close to the static DC-susceptibility and obviously this value agrees with that observed above from such a result. Furthermore, the value of the relative shift per frequency decade DT f =T f D log n ¼ 0:034 is larger than those of canonical SG, such as CuMn and AuMn, but comparable to the values reported for non-magnetic atom disorder(NMAD) SG of the type Ce2 TX3 (where T ¼ transition metal and X ¼ In,Ge) [7]. The inset shows the w0 (T) and w00 (T) curves for the bulk TbCu2 intermetallic. A hump near 50 K in the w0 and the absence of peak in w00 confirm an AFM behavior for this material. It is clear that despite the macroscopic results for the amorphous alloy (milled for 120 h) resemble those of bulk canonical and NMAD SG systems, here the microscopic origin of the magnetic phenomena is somewhat different. In rare earth amorphous materials the structural disorder induces random exchange and anisotropy and the frustration of the interactions leads to such features [3]. The relative large shift of the frequency at T f is a hint for the existence of a spin–clustered state. This heterogenous behavior has also been found in other amorphous alloys as magnetically concentrated Fe–Zr–B, in which the use of the non-linear susceptibility (which should diverge at the freezing temperature) allows to establish the existence of correlated spin–clusters [8]. Summarizing, the results have shown a crossover from bulk AFM in crystalline TbCu2 to SG-like behavior in a disordered amorphous alloy obtained by mechanical milling. It would be interesting to extend this study by preparing compounds at intermediate milling times which allow to establish a tendency for the evolution of the
ARTICLE IN PRESS e508
D.P. Rojas et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e506–e508
atomic structure and its relationship with the development of a frustrated magnetic behavior found in the 120 h amorphous TbCu2 sample. This work has been supported by the Direction of the Universities of the Ministry of Science and Education of Spain under Contracts MAT 2003-06815 and SB20030102.
References [1] [2] [3] [4] [5] [6] [7] [8]
M.A. Morales, et al., Phys. Rev. B 70 (2004) 184407. P.M. Shand, et al., J. Appl. Phys. 97 (2005) 10J505. R.I. Bewley, R. Cywinski, Phys. Rev. B 54 (1996) 15251. K. Sugiyama, et al., J. Magn. Magn. Mater. 177 (1998) 361. N. Marcano, et al., Phys. Rev B 71 (2005) 134401. J.R.L.D. Almeida, D.J. Thouless, J. Phys. A 11 (5) (1978) 983. D.P. Rojas, et al., Physica B 378 (2006) 847. L. F. Barquin, et al., Eur. Phys. J. B 35 (1) (2003) 3.