2490 IEEE TRANSACTIONS ON MAGNETICS, VOL. 47, NO. 10, OCTOBER 2011

Glass-Forming Ability and Magnetocaloric Effect in ����������� ��

Bulk Metallic GlassQian Li and Baolong Shen

Zhejiang Province Key Laboratory of Magnetic Materials and Application Technology, Key Laboratory of Magnetic Materialsand Devices, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China

In this letter, we investigated the effect of Si addition on glass forming ability and magnetocaloric effect in Gd-Co-Al bulk glassy alloys.It is found that the addition of only 1 at.% or 2 at.% Si is effective for extension of the supercooled liquid region �� � and decreaseof the fragility parameter . As a result, Gd-based glassy alloys with diameters up to 5 mm were successfully synthesized. The alloysalso exhibit large MCE, i.e., having the magnetic entropy change � � �� of 9.2 � �� � � �, the full width at half maximum of the� � � ����� of 87 K, and the refrigerant capacity (RC) of 800 � �� �.

Index Terms—Gd-based bulk glassy alloys, glass-forming ability, magnetic entropy change, magnetocaloric effect.

I. INTRODUCTION

M AGNETIC materials exhibiting large magnetocaloriceffect (MCE) have been drawing increasing attention

due to their potential application as magnetic refrigerants [1],[2]. Compared with traditional vapor-compression refrigera-tion, the magnetic refrigeration exhibits advantages both ofenvironmental friendliness and relatively high efficiency [3].In recent years, the study of magnetic refrigeration has beenincreasingly focused on amorphous magnetic materials dueto their intrinsic nature, such as the tailorable nature of theordering temperature, high electrical resistivity, low hysteresisloss, and high corrosion resistance [4]. Previous work exhibitsthat amorphous magnetic materials can have large magneticentropy changes comparable to that of some known crystallinemagnetic refrigerants [5], [6]. Furthermore, the amorphousmaterials have a larger temperature range of the half maximumpeak of and that lead to a much higher refrigerant ca-pacity (RC), which is another important parameter to evaluatethe technological interest in a refrigerant material [7]. Veryrecently, a series of Gd-based bulk glassy alloys (BGAs) withlarge MCE have been developed, such as [5],

[8], [4], etc. However,their glass-forming ability (GFA) is not large enough. For largescale production and processing, an improvement of GFA isrequired.

Recently, the microalloying approach has proved to be an ef-fective method to enhance the GFA of BGAs. For instance, with1 at.% Co addition to , the critical size increasesfrom 2 to 10 mm [9], with 1 at.% Gd addition to ,the critical size also increases from 3 mm to 10 mm. However,little work has been found in Gd-based BGAs [10]. In this letter,based on above considerations, we investigate the effect of Simicroalloying on glass-forming ability and magnetocaloric ef-fect in GdCoAl bulk glassy alloys. It is found that the addi-

Manuscript received February 21, 2011; accepted June 12, 2011. Date ofcurrent version September 23, 2011. Corresponding author: B. Shen (e-mail:blshen@nimte.ac.cn).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TMAG.2011.2160332

Fig. 1. Powder XRD patterns of the cast �� �� �� �� (

�� �� � �) glassy alloy rods.

tion of only 1 or 2 at.% Si is efficiently to extend the GFA of, and the MCE is also fully preserved.

II. EXPERIMENT

The Gd-based BGAs with a nominal composition of( , 1, 2, 3) were prepared by

arc-melting Gd (99.9 wt%), Co (99.99 wt%), Al (99.99 wt%),Si (99.99 wt%) in Ti-gettered argon atmosphere. The ingotswere remelted five times to ensure homogeneity of the samples.The ingots were then suck-cast into a copper mold to yieldcylindrical rods. Their structure was ascertained by powderX-ray diffraction (XRD) with radiation at 40 kV.Thermal analysis was carried out by differential scanningcalorimetry (DSC) at a heating rate of 40 . Thetemperature and field dependences of magnetization were mea-sured with physical properties measurement system (ppms).

III. RESULTS AND DISCUSSION

The powder XRD patterns of as-cast( ) rods are shown in Fig. 1. Only

broad peaks without crystalline peak can be seen for the bulksamples, which illustrate the glassy character of these samples.

0018-9464/$26.00 © 2011 IEEE

LI AND SHEN: GLASS-FORMING ABILITY AND MAGNETOCALORIC EFFECT IN BULK METALLIC GLASS 2491

Fig. 2. DSC curves of the cast �� �� �� �� ( �� �� � �) glassyalloy rods (the heating rate is 40 � ��� ).

TABLE ITHERMAL PROPERTIES OF �� �� �� �� GLASSY ALLOYS

The maximum diameters of BGAs withare 2 mm, 5 mm, 5 mm, and 4.5 mm, respectively.

Only 1 at.% and 2 at.% Si substitution for Al dramaticallyincreases the critical size form 2 mm to 5 mm. However,increasing the Si content to 3 at.%, the critical size decreasesfrom 5 mm to 4.5 mm. Further increasing the Si content to 4at.%, the critical size decreases sharply to only 1 mm.

The glass nature of ( )was confirmed by the continuous DSC at a heating rate of 40

and exhibited in Fig. 2. The distinct glass transitionand sharp crystallization verify the glassy structure of the al-loys. The values of glass transition temperature , the onsetcrystallization temperature and the supercooled liquidregion are summarized in Table I. As theSi content increases from 0 to 1 at.%, the increases from72 K to 79 K. When the Si content increases to 2 at.%, the ,

and remain nearly the same with that of the 1 at.%of Si content. With the Si content further increasing to 3 at.%,the rapidly reduce to 64 K. As an indicator of thermalstability of the BGAs, the of 1 at.% and 2 at.% Si are thegreatest among the samples, agreeing well with the best GFA.

Fig. 3 shows the DSC curves of the( ) alloys showing the cooling behavior of thisGd-based glassy alloy system. The liquidus temperature, , re-duced glass transition temperature are alsosummarized in Table I. The increases from 1000 K to 1102K with the Si content from 0 to 3 at.%, resulting in the decrease

Fig. 3. DSC curves of the cast �� �� �� �� ( �� �� � �) glassyalloy rods with a cooling rate of 4 � ��� .

Fig. 4. VFT relationship between � and � of �� �� �� �� BGA.

of from 0.597 to 0.548. It is seen that the comparison of thecritical diameters of these alloys does not follow the clas-sical criteria [11]. Another disagreement of our results withthe common criteria is that although the alloys with Si additiondeviate from the eutectic point, the GFA of them are much largerthan that of .

To better understand the effect of the Si addition, the fragilityof the alloys was investigated. In this letter, we try to fit therelationship of and with the Vogel-Fulcher equation [12]

(1)

where is the different heating rate, is a parameter rep-resenting the time scale in the glass-forming system [12],is the strength parameter in the VFT equation, and is theasymptotic value of usually approximated as the onset of theglass transition within the limit of infinitely slow cooling andheating rate. The best fit ranging from 2 K to 50 of

BGA sample as a representative is shown inFig. 4 and the fitting parameters , , and are 8.66, 0.60,and 535 K, respectively. These parameters of the other samples

2492 IEEE TRANSACTIONS ON MAGNETICS, VOL. 47, NO. 10, OCTOBER 2011

TABLE IITHE PARAMETERS ���, �, AND � OF �� �� �� �

GLASSY ALLOYS

Fig. 5. Temperature dependence of the magnetization in a magnetic field of200 Oe for the �� �� �� � ( � �, 1, 2, 3) BGAs.

are summarized in Table II. From the VFT fit, the value of ata particular can be calculated from [13]

(2)

The of the BGAs evaluated at a heatingrate of 20 are 35, 28, 27, and 27 with the Si con-tent from 0 to 3 at.%. It is seen that the values of of theglassy alloys with Si addition are smaller than that without Si.According to Agell’s classification [14], strong liquids oftenhave small values of . The small values of demonstratesthat the Gd-based BGAs with Si addition become strong liquidsbeing less sensitive to the temperature changes and more stable.Therefore, we suppose our superior GFA of the alloys correlateswith the strong liquid behavior, which has also been reported inother BGAs [9], [15].

Fig. 5 shows the temperature dependence of the magnetiza-tion for alloys determined in an ap-plied field of 200 Oe. The values of Curie temperature ,determined as the temperature corresponding to the minimumin , are near 105 K for all four samples. This is ex-pected given the concentrations of the magnetic species Gd andCo do not change with the Si addition. However, the of

increased slightly to 108 K. We consider thisphenomena could be explained by the next reason. It is provedthat the low of Gd-based glassy alloys could be attributedto the strong structural disordering. We suppose that as the GFA

Fig. 6. Isothermal magnetization as a function of magnetic field at various tem-peratures of �� �� �� � BGA.

decreases, there may be more clusters in thanin the other three glassy alloys. The existence of the clusters de-creases the structural disordering of , whichleads a slight higher . As it is noted from Fig. 5, the magneti-zation varies rapidly at the magnetic-ordering temperature.

The isothermal M(H) curves of as a rep-resentative with an increasing field at different temperatures aredisplaced in Fig. 6. In the vicinity of the temperature stepof 5 K was chosen, and a step of 10 K for the regions far awayfrom . The sweeping rate of the field was slow enough to en-sure that the data were recorded in an isothermal process. The

curves show typical ferromagnetic behavior and para-magnetic behavior below and above .

The magnetic entropy of alloy iscalculated by Maxwell relation

(3)

where ( is usually fixed to 0) and are the ini-tial and final values of magnetic field, respectively, and maximalvalue is 50 kOe in our experiments. The relative cooling poweror refrigeration capacity (RC), which is another important pa-rameter for evaluating magnetic reftigerants, is calculated byGschneidner’s method [16] as , where

is the full-width at half-maximum. By (3), the temper-ature dependence of for alloys in theapplied field of 50 kOe are obtained (shown in Fig. 7). The peakvalues of and the values of RC are 8.9 , 9.2

, 8.9 , 8.9 and 758 ,800 , 774 , 770 , respectively, with Si con-tent from 0 to 3 at.%. It is seen that the Si addition does not causedecrease in and RC. The large of the alloys isdue to the large magnetic moment of Gd, and the largeand RC is caused by the structural disorder because the fluctua-tion of exchange integral which is caused by local concentrationfluctuation which always exists in amorphous alloys [17]. In ad-dition, the peak values of and the values of RC of these

LI AND SHEN: GLASS-FORMING ABILITY AND MAGNETOCALORIC EFFECT IN BULK METALLIC GLASS 2493

Fig. 7. Magnetic entropy change as a function of temperature for�� �� �� �� ( �, 1, 2, 3) BGAs from 0 to 50 kOe.

alloys are also lager or comparable with those of other glassyalloys [18]–[20].

IV. CONCLUSION

Si addition with a micro content of 1 at.% or 2 at.% can dra-matically extend the the supercooled liquid region anddecrease of the fragility parameter of Gd-based glassy alloys.As a result, the Gd-based glassy alloys with diameters up to 5mm were successfully synthesized. We suppose that our supe-rior GFA of the alloys correlates with the strong liquid behavior.The MCE of the alloys has also been preserved. The magneticentropy change and the refrigerant capacity (RC) aremore than 8.9 and 750 , which make thesematerials candidates for magnetic refrigeration applications.

ACKNOWLEDGMENT

This work was supported by the National Science Fundfor Distinguished Young Scholars under Grant No. 50825103and the “Hundred of Talents Program” under Grant No.KGCX-2-YW-803 by Chinese Academy of Sciences.

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