Magnetic properties of Sm-based bulk metallic glasses

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Magnetic properties of Sm-based bulk metallic glasses

C.L. Lu

^a

, H.M. Liu

^a,b

, K.F. Wang

^a,b

, S. Dong

^a,b

, J.–M. Liu

^a,b,ⁿ

, Q. Wang

^c

, C. Dong

^c

aLaboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China

bChina International Center for Materials Physics, Chinese Academy of Sciences, Shenyang, China

cLaboratory of Materials Modiﬁcation, Dalian University of Technology, Dalian 116024, China

a r t i c l e i n f o

Article history:

Received 5 December 2008 Received in revised form 12 April 2010

Available online 24 April 2010 Keywords:

Metallic glass Cluster spin glass Exchange interaction

a b s t r a c t

We perform detailed investigation on the magnetization and speciﬁc heat of Sm-based ternary bulk metallic glasses with different Co contents at low temperature. A low temperature cluster spin-glass phase below Tf25 K is evidenced for all samples. This cluster spin-glass behavior is ascribed to competition among the multi-fold magnetic interactions and the intrinsic structural inhomogeneity.

The magnetic relaxation behavior of the spin-glass phase can be well described using the stretched exponential dynamics. The magnetic hysteresis under ﬁeld-cooling condition and spin dynamics further demonstrate the low temperature cluster spin-glass behavior. It is revealed that the Co atoms play an important role in modulating the physical properties of the present Sm-based ternary metallic glasses.

1. Introduction

Bulk metallic glasses (BMGs) exhibit high thermal stability, unique physical, and mechanical properties that can be exploited for possible engineering and functional applications[1–7]. Glass forming ability (GFA), considered as a measure of the competition between cooling rate and crystallization kinetics, is one crucial ingredient of physics for BMGs. A high GFA is a pre-requisite for practical applications of BMGs. In the past decades, it has been reported that the rare-earth (RE) elements are very effective for improving the GFA, and a number of RE-based BMGs with extremely large GFA have been successfully developed [8]. In addition to the intensive research activities on thermal and mechanical properties, RE-based BMGs are interested for their speciﬁc magnetic properties[9]. In particular, quite a number of BMGs are excellent hard/soft magnetic materials [10–12] and demonstrate the interesting spin-glass (SG) behavior[13].

So far, a series of magnetic RE-based BMGs have been synthesized since the ﬁrst work on RE–Fe–Al (RE¼Nd, Pr) of Inoue et al.[14]. The Nd-based BMGs possess high coercivity and are promising candidates as permanent magnets[9]. The physics underlying this high coercivity is associated with the large magnetic anisotropy given the nanophase comparable with the magnetic correlation length on one hand and strong domain wall pining effects on the other hand. The magnetic behaviors of Pr-based magnetic BMGs seem to be diverse. For instance, high

coercivity was observed in Pr40Fe30Co15Al10B5 [11], while the as-cast Pr60Cu20Ni10Al10 shows paramagnetic (PM) behavior. If element Cu in Pr60Cu20Ni10Al10is partially replaced by Fe, one can observe the ferromagnetic (FM) behavior at room temperature (T) and diamagnetic response at lowT[15]. The mechanism underlying these diverse magnetic properties in Pr-based BMGs is still controversial. Furthermore, a multiple spin-glass (SG) like behavior was observed in some Pr-based magnetic BMGs, and ascribed to the intrinsic structural inhomogeneity, spontaneous nano-conﬁnement and competing magnetic interactions [13], bringing additional interest in rare-earth based BMGs from the point of view of fundamental researches.

The term ‘‘spin-glass’’ is derived from dilute magnetic alloys, in which the magnetic moments with enough randomness and frustration due to the structure disorder preclude the long-range spin order. Usually, the Ruderman–Kittel–Kasaya–Yosida (RKKY) interaction dominates in SG system [16]. Nevertheless, the SG behavior was often observed in ordered crystal structures, such as Ni3Sn-type Mn3.1Sn0.9 [17], pyrochlore molybdate Y2Mo2O7 [18], and phase separated manganites Nd0.55(Ca0.76Ba0.24)0.45MnO3[19].

Recently, it was revealed that some RE-based magnetic BMGs also exhibit the SG behavior due to their unique microstructures[20].

Nevertheless, it should be mentioned that the magnetic property of RE-based BMGs is not well understood and a successful prediction/design of RE-based BMGs for particular magnetic applications is not yet possible. For example, the hard magnetic performance in Nd(Pr)-based BMGs was suppressed upon a substitution by heavier RE element Sm[21], while it is known that the Sm–Co-based alloys exhibit excellent permanent- magnetic properties. Unfortunately, no reason for this suppression has been identiﬁed. On the other hand, the Sm-based BMGs Contents lists available atScienceDirect

journal homepage:www.elsevier.com/locate/jmmm

Journal of Magnetism and Magnetic Materials

doi:10.1016/j.jmmm.2010.04.040

nCorresponding author at: Nanjing University, Laboratory of Solid State Microstructures, Hankou Road. No.22, Nanjing 210093, China.

E-mail address:[email protected] (J.–M. Liu).

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as an important class of BMG materials have been paid less attention. The ﬁrst Sm-based BMG is Sm60Fe10Al10Co15Cu5 alloy [21], and the measured magnetic coercivity is low and the hard magnetic performance no longer exists. Subsequently, a quatern- ary Sm–Al–Ni–Cu BMG [22], and recently several ternary Sm–Al–TM (TM represents transition metals) BMGs were prepared[23,24]. However, subsequent researches mainly focused on the thermodynamic aspect of the materials synthesis and less attention to their magnetic property was paid.

We address in this article the signiﬁcance of understanding the magnetic performance of Sm-based magnetic BMGs. A prelimin- ary study has been performed on this sample series, and in the current work, a systematic investigation on this type of Sm-based magnetic BMGs were carried out as a complementary to our earlier work[25]. It is well known that BMG alloys possess highly disordered structure which determines the special physical property. Nanocrystalline agglomerates widely exist in magnetic BMGs, and previous works mainly focused on the effects of nanocrystalline agglomerates on the magnetic properties in BMGs [12]. Therefore, it also would be of interest to study the

‘‘homogeneous’’ BMG without nanocrystalline regions. In this work, we perform careful investigation on the magnetism and spin dynamics, and speciﬁc heat of ternary Sm–Al–Co BMG alloys with three different Co compositions at low T. These recently invented ternary Sm–Al–Co magnetic BMGs do exhibit ‘‘fully amorphous’’ structure without evident nanocrystalline region, as conﬁrmed using the X-ray diffractometry (XRD) and high-resolution transmission electron microscopy (HRTEM).

This ‘‘fully amorphous’’ disorder structure can be obtained only within a fairly narrow range[24]. Our results demonstrate the interesting cluster SG transitions at Tf25 K and its spin dynamics.

The remaining part of this article is organized as follow: Section 2 is devoted to the sample preparation and characterization, while details of the measured data are presented in Section 3, where we discuss every aspect of the SG behaviors in this class of BMG systems. A short summary is given in Section 4.

2. Experimental details

Ingots of ternary BMG series, Sm54Al23Co23, Sm52Al24Co24, and Sm50Al25Co25, were prepared by arc-melting the mixture of high purity constituent elements under argon atmosphere. A small excess of Sm was added to compensate for mass loss due to the evaporation during the alloy melting. All cast ingots were prepared by means of copper mould suction casting. Detail of the composition design was given elsewhere [23,24]. The Co content of these samples is denoted by y¼23, 24, and 25, respectively. The as-prepared samples are rods of 3 mm in diameter, and the amorphous structure was identiﬁed by XRD and HRTEM[24].

Because of the high Sm and Co contents, fascinating magnetic behavior at lowTis expected. We performed extensive measurements on the magnetic property using a quantum design superconducting quantum interference device (SQUID). The specific-heat data were collected using thermal relaxation method with a quantum design physical property measurement system (PPMS). A latest improved measurement puck for specific heat from quantum design, in which the field dependence of thermometer sensor on chip and the thermal conductance of thermal linking wires are negligible, was used for the specific heat measurements.

3. Results and discussion 3.1. A. dc magnetization

Fig. 1shows the measured dc magnetizationMas a function of Tunder zero-field cooled (ZFC) and field-cooling (FC) conditions with a measuring field of 100 Oe. At a first glance, the measured M–Tcurves of different samples exhibit similar behaviors: a cusp- like peak in the ZFC case atTf¼25 K, large magnetic irreversibility betweenM_FCandM_ZFC, andM_FCfinally reaches a plateau. These phenomena immediately allow us to argue a SG-like transition occurring at freezing pointTf[17–19]. Additional evidence will be presented in next section.

Besides these similar behaviors, the M–T curves also show some dependence on y, the Co content. For sample y¼23, MFC

shows a distinct kink at39 K, indicating the formation of some FM clusters. However, this kink becomes ambiguous for sample y¼24, and cannot be observed for sample y¼25. Furthermore, the measured MFC plateaus for y¼23 and y¼24 at low T are comparable with each other, but about ten times larger than that for y¼25. These suggest that the spin disordered degree is continuously increased withy, and fory¼25 the spin alignment is seriously frustrated. One observes a remarkable suppression of the magnetization in the FC case by only 1% change of Co content, demonstrating the signiﬁcant enhancement of magnetic frustration.

To further understand the path-dependent magnetization of these BMG alloys, we choose sample Sm52Al24Co24 (y¼24) for

Fig. 1.MeasuredMas a function ofTunder the ZFC and FC cases atH¼100 Oe for (a) Sm54Al23Co23. The SG transition temperatureTfindicated by solid arrow, and a distinct kink at T39 K indicated by dash arrow. (b) Sm52Al24Co24. The SG transition temperature Tf indicated by solid arrow, and the kink at T39 K indicated by dash arrow becomes ambiguous. (c) Sm50Al25Co25. The SG transition temperatureTfindicated by solid arrow, and no kink can be observed.

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more detailed investigation. TheT-dependences ofMin both FC and ZFC cases under different measuring fieldHmare shown in Fig. 2. No qualitative difference between the high-field data and low-field data is identified. It is revealed that the freezing pointTf

and bifurcation point Tb both shift to the low T side with increasingH_m. The measuredT_fas a function ofH_mis shown in the inset of Fig. 2(b), noting the logarithm Hm axis. While no dependence of Tf on Hm is shown at Hmo1.0 T, a rapid suppression of Tf withHm is observed at Hm41.0 T. It is also theoretically predicted and experimentally evidenced that the ﬁeld dependent transition of an Ising SG follows a relationship of TfHm2/3

in the low field region, while a deviation from such a relation would be observed when the Ising SG is transformed to a Heisenberg type one, caused by the sufficiently large field[26,27].

For the present case, an interesting linearTf–Hm2/3relationship can also be obtained forH_mo1.0 T, but asH_m41.0 T a clear deviation is identified. This feature allows us to assume that the crossover point atHm1.0 T is to possibly originate from the transition of an Ising SG system into a Heisenberg type SG system, due to the field suppression of the random anisotropy from infinite to a finite value. In addition, the measuredTfatHm3.0 T remains as high as 18 K, not much lower thanTf25 K atHm100 Oe, which also demonstrates the high stability of spin configuration in the present BMGs.

3.2. B. ac magnetic susceptibility

0210⁷sec reﬂect the relatively strong inter-cluster interaction.

t

0 is much longer than the microscopic single-spin ﬂipping time (10¹³s), indicating that the spin conﬁguration is not the same as the atomic-scale spin glass.

The dynamic critical exponent product (zv¼3.4) is also smaller than the value (zv8.0) for the atomic-scale SG, but very close to the value for cluster SG state (zv3.5)[18,29]. Therefore, one may argue that the present BMG system is more like a cluster SG rather than ordinary (atomic-scale) SG system. Here what should be mentioned is that a quantitative comparison in these parameters between the Vogel–Fulcher law and the critical slowing down law makes no sense.

3.3. C. M–H hysteresis

For a SG state, theM–Hhysteresis loop is usually very thin and no magnetization saturation can be observed until the freezing state is melt under a high magnetic ﬁeld. This is different from a long-range ordered FM state, in which the FM domains can be easily switched, causing a saturation ofMunder a relatively small magnetic ﬁeld. Such a difference provides a possible avenue to Fig. 2.(a) MeasuredMas a function ofTunder the ZFC and FC cases at high

measuring fieldH¼1, 2, 3 T for Sm52Al24Co24, (b) the difference between the FC and ZFC conditions. The inset in (b) shows the field dependence of Tf, the measuring fieldH axis is in a logarithm scale and the middle shadow region indicates the crossover behavior.

Fig. 3.Measured ac susceptibility real (w⁰ac) and imaginary (w’’ac) components as a function ofTato¼1, 10, 100, and 1000 Hz for Sm52Al24Co24shown in the main panel and the inset, respectively.

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identify a SG state from theM–H hysteresis loop. In Fig. 5 we present the measured hysteresis loops at 300 and 4 K for the ZFC case. At 300 K, the M–H hysteresis is almost linear although a small loop with coercivity of60 Oe can be identiﬁed, as shown

in the inset of Fig. 5(b), indicating the possible existence of magnetic Co clusters at 300 K. AtT¼4 K, the hysteresis (Fig. 5(a)) is also thin and no saturated magnetization can be observed so long asHreaches up to 3.0 T[28,31,32]. Note that a weak loop can always be observed at all fields atT¼4 K, indicating the very rigid SG state, in agreement with theM–Tresults. In addition, for spin frustrated systems, either a step-likeMas a function ofHor a multiple-peakeddM/dH–Hdependence should be available[33], as shown in the inset ofFig. 5(a) wheredM/dHas a function ofH does show three distinct peaks. This multi-peaked behavior reflects the infinite equilibrium states of the cluster SG systems[34].

The M–H hysteresis loops at T¼4 K for the sample upon different cooling ﬁelds (HFC) were measured, shown in Fig. 6.

Remarkable positive shift of theM–Hloops along theMaxis can be observed asHFCincreases. The coercivityHCalso increases with HFC.Fig. 6(b) shows theHFCdependence of vertical coordinate of the loop centerMEandHC. BothMEandHCmonotonically increase withH_FC, confirming that the SG state is far from broken. To understand this effect, one notes that in the cluster SG state there are two competing energy terms: the Zeeman energy and the exchange energy arising from the interface of magnetic clusters[35]. For the field cooling case, the FM clusters would be frozen along the field orientation as the sample was cooled across the Tf, causing the vertical shift of M. In the present case, a magnetic field of 3 T is still too low and the induced Zeeman energy cannot overcome the magnetic interactions within the system, so giving rise to a monotonically increase inME. On the other hand, it is well known that the SG state is facilitated with multiple equilibrium states, as confirmed by the multi-peak behavior indM/dHas a function ofHat 4 K. When the sample was cooled down acrossTfunder a relatively high field, the SG phase fell into some higher energy state associated with the vertical shift inM[35,36]. The increase inHCis coherent with the increase inM_E, and should be ascribed to reduction of the average random anisotropy, caused by the preferential direction along the cooling Fig. 4.Best fit to (a) the Vogel–Fulcher lawt¼t0exp [Ea/kB(TfT0)] and (b) the

power lawt/t0¼(Tf/Tg1)^zv.

Fig. 5.MeasuredMas a function ofHmeasured at 4 K (a) and 300 K (b) for sample Sm52Al24Co24. The inset in (a) showsdM/dHas a function ofHat 4 K.

Fig. 6.(a) MeasuredMas a function ofHmeasured at 4 K under different cooling ﬁelds, (b) the cooling ﬁeld dependence of vertical shifts and coercivity of the magnetization hysteresis loop at 4 K for sample Sm52Al24Co24.

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field[36]. This remarkable vertical shift of Malso confirms the strong random anisotropy of the cluster SG state, which is different from the prototype SG state where the shift along the field axis would be observed[16].

3.4. D. Spin dynamics

For a SG system, the spin dynamics would exhibit speciﬁc relaxation features. We investigate the magnetic relaxation by measuring the time-dependent isothermal remnant magnetization (MIRM)[37].Fig. 7shows the normalizedMIRMas a function of timetat four different temperatures (T¼5, 10, 20, 24 K) belowTf. An evident and slow relaxation is observed. The data were collected by the following procedure: cooling the sample from 300 K down to a given T under zero ﬁeld, and then applying H¼1000 Oe for 20 min beforeMIRM(t) is measured.

The simple logarithmic law[38]or power law[13]does not work well to ﬁt the data. Instead, a stretched exponential law:

M(t)¼M0+Mrexp[(t/

t

)¹ⁿ], is chosen here to ﬁt the data, where M0relates to an intrinsic FM component which is lower at higher T,M_rrelates to the glassy component which is mainly attributed to the relaxation, parameters

t

andnare the characteristic time and critical exponent of the relaxation [37–40]. Obviously, an exponential relaxation is obtained as n¼0, while there is no relaxation at all ifn¼1 (or the relaxation is so rapid that the time for relaxation is zero). The ﬁtted parameters using this equation are listed inTable 1, and it is distinct that all the four parameters show signiﬁcantT-dependence. The increasingnwith increasingT indicates that the weaker the cluster interaction is when the higher temperatureTis, a quite reasonable statement.

3.5. E. Speciﬁc heat

The cluster SG state at low T means no long-range spin ordering in the low-T range, which can be reflected by specific heat data in the language of phase transition[28]. The measured specific heatCpas a function ofTunderH¼0 and 7 T for sample Sm52Al24Co24is shown inFig. 8. No obvious anomaly can be seen from theCp–Tcurve no matter under zero field or 7 T. No obvious difference between the two curves can be seen too, indicating no phase transition with an entropy change sufficient for the long-range spin ordering. The frozen SG state is very rigid, in agreement with magnetic measurements shown above. We also presentDCp¼Cp(H¼0)Cp(H¼7 T) as a function ofTinFig. 8(b).

A peak at27 K, slightly higher thanT_f¼25 K, can be identiﬁed, probably due to the paramagnetic-SG phase transition, noting that the peak value is only30

m

J/g-K, too small for a long-range spin ordering.

The cluster SG state in Sm52Al24Co24BMG seems to contribute little to the specific heat for its intrinsic short-rang ordering, thus Cpdoes not respond much to magnetic field. This enables us to fit the specific heat data in the lowTrange from 2 to 10 K without including the magnetic contribution. The fitting is done using Cp¼

g

T+bT³[41], and fitted parameters are listed inTable 2. The similar specific heat measurements and fittings were done for samples Sm54Al23Co23and Sm50Al25Co25, as shown inFig. 9, and the fitted parameters are listed in Table 2 too. All the three samples show a large electron specific heat term which increases with increasingy, the Co content. The third-order term, however, Fig. 7.Normalized IRM,MIRM(t)/MIRM(0), as a function of time at 5, 10, 20, 24 K for

sample Sm52Al24Co24. Fitting curves (the solid lines) using the stretched exponential function.

Table 1

Fitting parameters of isothermal remnant magnetization (IRM)MIRM(t) using the stretched exponential function at different temperatures.

T(K) M0(emu/g) Mr(emu/g) t(s) n

5 0.00005 0.00049 659 0.4373

10 0.00036 0.00205 867 0.4367

20 0.00258 0.00825 875 0.4503

24 0.019 0.00288 619 0.5347

Fig. 8.Measured specific heatCpas a function of temperature for Sm52Al24Co24. (a) Under zero field overTrange from 2 to 50 K (black solid circle), and under a field ofH¼7 T overTrange from 2 to 30 K (red solid circle); (b) temperature dependence ofCp(0 T)–Cp(7 T) up to 30 K. Clear specific heat anomalous can be seen neatTf(0 T) andTf(7 T) indicated by black arrows; (c) lowTspecific heat up to 10 K fitted usingCp¼gT+bT³. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Table 2

Fitting parameters of the speciﬁc heat using:Cp¼gT+bT³.

g(mJ/g-K²) b(mJ/g-K⁴)

Sm54Al23Co23 0.30 7.28

Sm52Al24Co24 0.32 7.12

Sm50Al25Co25 0.34 6.99

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is relatively small at several

m

J/g-K⁴, consistent with typical data for amorphous structures.

3.6. F. Discussion

Different from crystalline alloys, BMG alloys are composed of different short-range ordered zones regarded as clusters, and show highly disordered behaviors [13]. The extensive structure disorder usually reinforces the magnetic frustration in BMGs.

In fact, the suppression of long-range spin order by structure disorder was also demonstrated in rare-earth doped manganites[19]. For the present Sm-based magnetic BMGs, it has been revealed that the strong magnetic frustration is associated with the highly disordered structure, and Co content plays an important role in modulating magnetic properties. With increas- ing Co contenty, the frustration of magnetic competition would be enhanced, giving rise to a signiﬁcant reduction of Mat 5 K under 100 Oe in the FC condition and disappearance of the FM ordering signature at about 39 K. This frustration of magnetic competing provided by the atomic disorder should be responsible to the low T cluster SG state. Also the intrinsic inhomogeneity should be take into account as the weak FM behavior indicates some Co clusters survived atT¼300 K. In addition, different from the long-range FM ordering, the associated magnetic entropy of a SG transition would be small[28], which is in agreement with the weak anomaly inC(T) curve of current case.

4. Conclusion

In conclusion, based on the presented magnetic and speciﬁc heat studies on the Sm-based ternary BMGs, it has been demonstrated that: (i) A low temperature a cluster SG state exists belowTf25 K for all samples, which is ascribed to multi- fold magnetic competitions and the intrinsic structural inhomogeneity. (ii) The Co atoms play an important role in modulating

the magnetism, and reinforce strong magnetic frustration which is responsible for the cluster SG behavior.

Acknowledgements

This work was supported by the Natural Science Foundation of China (50601013, 50832002), and the National Key Projects for Basic Research of China (2006CB921802).

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Fig. 9.Measured specific heatCpas a function of temperature for (a) Sm54Al23Co23, (b) Sm50Al25Co25. LowT specific heat up to 10 K fitted usingCp¼gT+bT³ for Sm54Al23Co23and Sm50Al25Co25are shown in the inset of (a) and (b), respectively.