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antiferromagnetic microwires with excellent magnetocaloric properties
Journal Pre-proof Melt-extracted Gd73.5Si13B13.5/GdB6 ferromagnetic/antiferromagnetic microwires with excellent magnetocaloric properties N.T.M. Duc, H.X. Shen, O. Thiabgoh, N.T. Huong, J.F. Sun, M.H. Phan PII:
S0925-8388(19)34579-7
DOI:
https://doi.org/10.1016/j.jallcom.2019.153333
Reference:
JALCOM 153333
To appear in:
Journal of Alloys and Compounds
Received Date: 19 September 2019 Revised Date:
5 December 2019
Accepted Date: 7 December 2019
Please cite this article as: N.T.M. Duc, H.X. Shen, O. Thiabgoh, N.T. Huong, J.F. Sun, M.H. Phan, Melt-extracted Gd73.5Si13B13.5/GdB6 ferromagnetic/antiferromagnetic microwires with excellent magnetocaloric properties, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/ j.jallcom.2019.153333. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Author Contribution Statement N.T.M. D, H.X.S and M.H.P initiated the concept. N.T.M. D performed magnetic measurements and analysis. N.T.M. D and N.T.H performed XRD and DSC measurements. H.X.S and J.F.S performed TEM imaging and analysis. O.T. performed SEM imaging and analysis. N.T.M. D wrote the manuscript with inputs from all the other authors. M.H.P led the entire project. All the authors approved the final manuscript.
Melt-extracted Gd73.5Si13B13.5/GdB6 ferromagnetic/antiferromagnetic microwires with excellent magnetocaloric properties N.T.M. Duc,a,b,c,* H.X. Shen,a,d,** O. Thiabgoh,e N.T. Huong,b J.F. Sun,d and M.H. Phana,*** a
b
c
Department of Physics, University of South Florida, Tampa, Florida 33620, USA
Department of Physics, VNU University of Science, 334 Nguyen Trai, Hanoi, Vietnam
The University of Danang, University of Science and Education, 459 Ton Duc Thang, Danang, Vietnam
d
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China e
Department of Physics, Faculty of Science, Ubon Ratchathani University, Warin Chamrap, Ubon Ratchathani 34190, Thailand
A novel class of magnetocaloric microwires composed of antiferromagnetic GdB6 nanocrystals embedded in an amorphous ferromagnetic Gd73.5Si13B13.5 matrix is reported. These composite microwires were created directly from a melt-extraction process. XRD, TEM, HRTEM and SAED confirm the formation of GdB6 nanocrystals (~10 nm) in the amorphous Gd73.5Si13B13.5 matrix. As a second-order magnetic material, the Gd73.5Si13B13.5/GdB6 microwires exhibit an excellent magnetocaloric response, with a large magnetic entropy change ∆SM ~6.4 J kg–1 K–1 at ~120 K, a large temperature interval δTFWHM ~130 K, and the large refrigerant capacity RC ~ 890 J kg-1 for a field change of 5 T. Such microwires are a promising candidate for active magnetic refrigeration in the temperature range of 10 – 200 K.
Keywords: melt-extraction; magnetic microwires; magnetocaloric effect; magnetic refrigeration *Corresponding author: [email protected] (N.T.M. Duc) **Corresponding author: [email protected] (H. Shen) ***Corresponding author. [email protected] (M.H. Phan)
1. Introduction Current refrigerators, such as air conditioners and refrigerators are based mostly on an environmentally unfriendly gas compression technique, which emits harmful gases such as chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFC) to the environment. Magnetic refrigeration (MR) based on the magnetocaloric effect (MCE) using a magnetic solid substance as a refrigerant represents a promising alternative, due to its higher cooling efficiency, compactness and environmental friendliness [1-8]. In addition to the adiabatic temperature change (∆Tad), there are other two important parameters that are usually employed in assessing the performance of a magnetocaloric material for MR. The first parameter is a magnetic entropy change (∆SM), which should be as large as possible. The largest ∆SM typically occurs in the vicinity of a magnetic phase transition and depends on the characteristics of the magnetic phase transition. As the first-order magnetic transition (FOMT) is typically sharp, the resulting ∆SM is large in this type of FOMT material [1]. However, FOMT materials usually possess narrow phase transition regions, leading to narrow cooling temperature ranges. Thermal and magnetic hystereses associated with the FOMT nature are also large, which are detrimental to magnetic refrigeration [9]. By contrast, second-order magnetic transition (SOMT) materials possess smaller ∆SM values, but extend ∆SM (T) over wider operating temperature ranges around their transition temperatures. Thermal and magnetic hystereses are often negligible in these SOMT materials. The second parameter is the refrigerant capacity (RC), which is generally considered a measure of the amount of heat transfer between the cold and hot sinks in an ideal refrigeration cycle [9-10]. In the SOMT materials, the broadening of the phase transition has been reported to result in a large RC, which is even greater than those of some FOMT materials [1]. A benchmark
SOMT material for sub-room temperature magnetic cooling application is gadolinium (Gd) [2,3,6]. From a practical cooling perspective, the form of a magnetocaloric material plays a crucial role in the performance of a magnetic cooler [11-28]. While a magnetocaloric material can be synthesized in different forms, such as powder particles, thin films or wires, when it is used in an actual cooling device, it is essential to design the material in a form so that it can transfer/release heat most effectively to a surrounding environment (e.g. water) [23]. It has been shown theoretically and experimentally that magnetocaloric materials in the form of wires are desirable for use in actual magnetic cooling devices [11-28]. In the wire form, it can release the heat faster due to its enhanced surface area, and yield a good mechanical response that reduces the relaxation time and consequently increases the operation frequency of the cooling device [27,28]. As a result, the wire-shaped magnetocaloric materials have exhibited the highest cooling efficiencies as compared to their particle- and thin film-shaped counterparts [19,21]. Gd-alloy microwires prepared by the melt-extraction method have been extensively explored by us for active magnetic refrigeration as they exhibit excellent magnetocaloric and mechanical properties. A large number of MCE studies on these Gd-based microwires have been reported [11-28]. The wire shape with increased surface-to-volume areas allows for a higher heat transfer between the magnetic refrigerant and the surrounding liquid. Relative to their bulk counterparts, these microwires have shown the larger ∆SM and RC values [11-26]. To improve the RC in these microwires, different approaches have been proposed [2426]. As such, a low-temperature and short-time annealing of Gd53Al24Co20Zr3 amorphous wires to create nanocrystals embedded in an amorphous matrix has been demonstrated to enhance both the ∆SM and RC, while preserving the good mechanical strength [26]. Another effective approach
has been proposed as to create a biphase amorphous/nanocrystalline structure in Gd-Al-Co wires directly through a controlled melt-extraction process, resulting in the enhanced RC values [24, 25]. It has been shown that reducing size of anti-ferromagnetic (AFM) particles to the nanoscale could weaken the AFM coupling significantly so that a moderate magnetic field could convert the AFM into the ferromagnetic (FM), leading to a large change in magnetization and hence the MCE [29]. Recent studies have revealed that reducing size of antiferromagnetic material GdB6 to the nanoscale induces the FM state [30-32]. From a metallurgy perspective, the GdB6 nanophase may be formed in Gd-Si-B wires through the melt-extraction process. In the present study, we have demonstrated this as a new, effective approach for creating magnetocaloric composite microwires composed of antiferromagnetic GdB6 nanocrystals embedded in an amorphous ferromagnetic Gd73.5Si13B13.5 matrix. This class of microwires exhibits an enhanced RC over a large temperature range, making it a promising candidate for active magnetic refrigeration. 2. Experiment Amorphous Gd73.5Si13B13.5 microwires were fabricated via the melt-extraction technology by using a spinning copper wheel of 160 mm in diameter with a 60o tapered edge. To obtain a uniform diameter for melt-extracted microwires, the wheel was keep at a fixed rotation speed of 30 m/min while the melted material was fed at a constant rate of 90 µm/s. The purity of the starting elements used to prepare the induction-melted alloys was as follows: Gd (99.5%), Si (99.99%), and B (99.99%).
The structure of the wire samples was examined in a Bruker D5005 X-ray diffractometer (Cu/Kα). Differential scanning calorimetry (DSC) was studied on DSC Labsys Evo S60/58988 system (France) at a heating rate of 10 K/min. Energy dispersive X-ray spectroscopy (EDS) and scanning electron microscopy (SEM) were performed with a Jeol model JSM-6390LV scanning electron microscope equipped with an INCAEnergy EDS system from Oxford instruments. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were recorded on a FEI TECNAI G2 F30 system. Selected area-electron diffraction (SAED) was also used to analyze the microwires. The magnetization of a single wire was measured by means of the vibrating sample magnetometer option of a Quantum Design commercial physical property measurement system (PPMS). DC magnetic fields up to 5 T were applied longitudinally along the wire as temperature was varied from 4 K to 256 K with an increment of 4 K. The molecular weight of the Gd73.5Si13B13.5 microwire is 120.69 g/mol. 3. Results and Discussion 3.1. Structural and morphological characterizations The micromorphology of a Gd73.5Si13B13.5 microwire was obtained by SEM, which reveals a homogeneous and smooth surface (Fig. 1a). The uniform diameter of the microwire was determined directly from planar SEM micrographs to be ~63 µm with a circular cross section. The EDS measured at room temperature is displayed in the inset of Fig. 1a, which confirms the presence of raw chemical elements Gd and Si (B cannot be detected using EDS). The EDS analysis was performed to determine the percentage of these elements in the sample. The atomic percentages are estimated to be 73.50%, 12.73% and 13.77% for Gd, Si and B respectively, which are expected for the desired nominal composition Gd73.5Si13B13.5. The data of
the atomic percentages collected at three locations on the surface of the microwire are reported in Table 1, confirming the unchanged compositions in the microwire. The structural characterizations of the melt-extracted Gd73.5Si13B13.5 microwires have been examined by XRD, DSC and TEM. The results are displayed in Figure 1b and Figure 2. The XRD pattern measured from 20° to 100° (2θ) with a scanning speed of 0.9°/min is shown in Fig. 1b. The typical broad halo pattern with a broad diffuse peak is observed at near angle of 33°. This typical peak has also been found in the XRD spectra of previously studied Gd-based materials [11-19], demonstrating the amorphous characteristic of the presently studied Gd73.5Si13B13.5 microwires. It is worth mentioning here that a crystalline peak is clearly detected at ~31° which perfectly coincides with the XRD peak (110) of GdB6 nanowires [31]. This indicates the presence of GdB6 nanocrystals in an amorphous Gd73.5Si13B13.5 matrix, and such a combination of these two phases would result in the superior magnetocaloric response of the microwires. To explore the thermal stability of the melt-extracted Gd73.5Si13B13.5 microwires, the DSC analysis was carried out through the heating process with a heating rate of 10 K/min at temperatures between 300 K and 750 K. The DSC spectrum is shown in the inset of Fig. 1b. The three exothermic peaks appear to occur, demonstrating the three-step crystallization of a meltextracted Gd73.5Si13B13.5 microwire during the heating process. The peak intensity of these three peaks decreased with increasing temperature, indicating that the exothermic process became more and more slowly. As shown in the inset of Fig. 1b, the glass transition temperature Tg is around 330 K and the first crystallization temperature Tx1 is about 350 K. The temperature range between Tg and Tx1 is called a supercooled liquid region or a glass transition temperature region
[11-19], which was calculated as ∆Tx = Tx1 – Tg = 20 K. Values of ∆Tx have been reported to influence on the glass forming ability (GFA) of Gd alloy microwires [11-19]. To independently confirm the presence of GdB6 nanocrystals in an amorphous matrix in these fabricated alloy microwires, TEM and HRTEM images were taken in scale of 2 µm, 200 nm, 10 nm and 20 nm as displayed in Fig. 2a-d, respectively. The HRTEM images show that GdB6 nanocrystals of ~10 nm are homogeneously distributed in the amorphous Gd73.5Si13B13.5 matrix (see, Fig. 2b and 2d). The SAED patterns are shown in the inset of Fig. 2b and Fig. 2c, in order to confirm details of the reflections discretely by these nanocrystals. The high-angle annular dark field (HAADF) images of the microwire confirmed the presence of the chemical elements in the sample (see Fig. 2e-h). The combination of XRD and HR-TEM allows us to confirm the presence of GdB6 nanocrystals (~10 nm) in an amorphous Gd-Si-B matrix. The EDS-Line scanning was also performed, and the results obtained (Fig. 3) show the dependence of mass percent of the elements on distance. As can be seen in Fig. 3, the nanocrystals area (dark area) has more mass percentages of Gd and B as compared to the amorphous matrix (light area). The presence of GdB6 nanocrystals in an amorphous Gd73.5Si13B13.5 matrix would impact the magnetic and magnetocaloric properties of the microwires, and this effect will be examined in detail in the following sections. 3.2. Magnetic properties Figure 4a (red curve on left hand) presents the temperature dependence of magnetization (M-T) for these microwires measured while cooing under an applied DC magnetic field of µ0H = 10 mT. As shown in Fig. 4a, the M-T curve exhibits a broad paramagnetic to
ferromagnetic (PM-FM) phase transition around the Curie temperature TC. Figure 4c also shows the (M-T) of this microwires sample but measured under higher magnetic field of µ0H = 5 T. Comparing these two cases together in Figs. 4a and 4c, we find that, as the magnetic field increases, the magnetic phase transition becomes more and more broaden. This is not only a typical behavior for SOMT materials (long-range ferromagnetic order) and contributing to the structural disorder caused by the amorphous structure, but also a testament to the existence of the GdB6 nanocrystals in the amorphous Gd73.5Si13B13.5 matrix [20]. If the interaction is only ferromagnetic, the M-T curve should remain sharp when it is in low magnetic field regimes. As shown in Fig. 4c, the M-T curve is significantly broadened; this demonstrates the presence of AFM GdB6 nanocrystals. It is the presence of the AFM GdB6 nanocrystals that caused the broadened PM-FM phase transition, upon the application of a sufficiently high magnetic field. This consequently results in the broadened ∆SM(T) and hence the large RC. Once again, Figure 4d shows the temperature dependence of magnetization (M-T) at different applied magnetic fields from µ 0H ~ 0.01 – 5 T in the filled 2D contour plot of the temperature and magnetic field dependence of the magnetization. The magnetization value varies greatly around the Curie temperature (TC), which leads to an expectation that there will be a large change in magnetic entropy. The TC is determined to be ~116 K from the minimum of the derivative dM/dT vs. T curve, as displayed in Fig. 4b. This value of TC is larger than that reported for Gd59Si41 (~64.6 K), but smaller than that of Gd87Si13 (~149 K) [33]. This suggests that the TC value can be increased by increasing the concentration of Gd [33] or adding B to create GdB6 nanocrystals in an amorphous matrix as we have demonstrated in this study. The inverse susceptibility as a function of temperature, χ-1(T) = µ 0H/M, generated from the M(T) curve in the paramagnetic region is also shown in Fig. 4a (the blue curve on right
hand). Because of the broadened phase transition of M-T, the χ-1(T) is linear at high temperature region above 245 K. Based on the Curie-Weiss law in the paramagnetic region: χ = Curie constant defined by =
with the
μ ,where NA = 6.022x1023 mol-1 is Avogadro’s number, µ B
= 9.274x10-21 emu is the Bohr magneton, and kB = 1.38016x10-16 erg/K (in the CGS system of units) is Boltzmann constant, the Curie-Weiss temperature θ and the effective magnetic moment values
will be determined. The results of fitting linear region of the χ-1(T) are θ = 202 K and
C = 7.81 emu K mol-1. The difference between θ and TC caused by the broaden phase transition μ , the effective magnetic moment µ eff
of these microwires. From the Curie constant =
of the sample will be calculated via
=
/
= √8
. The value of
is
= 7.90
µ B, is close to the theoretically calculated value of pure Gd (~7.94µ B). The interaction of magnetic moments between transition metal (TM) elements with 3d-electrons and rare-earth (RE) element with 4f-electrons [34] is the main cause of decreasing the effective magnetic moment µ eff. As we know, there are no elements with 3d-electrons such as Fe, Co, Ni,… in the presently fabricated Gd73.5Si13B13.5 microwires. So, the effective magnetic moment µ eff of the Gd73.5Si13B13.5 microwires is close to pure Gd. 3.3 Magnetocaloric properties A set of isothermal magnetization M-H curves of the Gd73.5Si13B13.5 microwires taken at different temperatures ranging around TC from 4 to 256 K, with a temperature interval of 4 K, was obtained under applied magnetic fields of µ 0∆H = 0 – 5 T, in order to investigate the magnetocaloric properties of these microwires. The sweeping rate of the field was slow enough to ensure that the magnetization curves are measured in an isothermal process. The results are
shown in Fig. 5a. As can be seen in Fig. 5a, a typical ferromagnetic transition is obvious in the vicinity of TC. The largest saturation magnetization (MS) value for the largest magnetic field of ~5 T is determined to be about ~220 A m2 kg-1. This MS value is equivalent to those of previously reported Gd-based alloys [11-19]. When compared with Gd-Si alloys (e.g. Gd87Si13, Gd59Si47, Gd18Si82 [33] without B element), however, this MS value of the Gd-Si-B microwires is much larger. MS ~ 220 A m2 kg-1 at T = 4 K for Gd73.5Si13B13.5, while it is ~145 A m2 kg-1 (T < 20 K), ~120 A m2 kg-1 (T < 12 K) and ~110 A m2 kg-1 (T = 4.2 K) for Gd87Si13, Gd59Si47 and Gd18Si82, respectively [33]. It indicates that when the element B is present, leading to a combination of Gd-B, the MS increased significantly. In other words, the larger MS at low temperature implies that anti-ferromagnetic interaction within GdB6 nanocrystals is likely reduced and converted into the ferromagnetic order. To confirm the nature of the magnetic phase transition of the Gd73.5Si13B13.5/GdB6 microwires, a set of isothermal magnetization M-H is converted to the modified Arrott plots µ0H/M vs. M2 as shown in Fig. 5b. According to Banerjee criterion, a magnetic transition is considered as FOMT when the slope of Arrott plot is negative; otherwise, it is expected to be SOMT when the slope is positive [35]. As can be seen in Fig. 5b, all slopes remain positive, indicating that the FM-PM transition of the Gd73.5Si13B13.5/GdB6 microwires is a type of SOMT, which is in full agreement with the previous reports on Gd-based alloy microwires [11-19]. It is also one of the reasons for broadening the magnetic phase transition that consequently enhances the RC value. From the isothermal magnetization M-H curves, the magnetic entropy change –∆SM can be calculated based on thermal-dynamic Maxwell equation [1]:
Δ
, μ" # $ = μ% &%
),-. '
'( )
dH
(1)
where M is the magnetization, µoH is the applied magnetic field, and T is the temperature, by integrating over the magnetic field. Figure 6a shows the magnetic entropy change –∆SM as a function of temperature for different applied magnetic field of µ 0∆H = 0.1 – 5 T by (1). As can be seen in Fig. 6a, the magnitude of –∆SM significantly increased along with an increase of applied magnetic field µ0H and reached a maximum value –∆SMmax near TC. One can also see that the –∆SM(µoH,T) curves across the PM-FM phase transition are broad, which is attributed to the broadened PM-FM transition as seen in the M(T) curves under high applied magnetic fields (see Fig. 4c). This effect is likely associated with the distribution of GdB6 nanocrystals within an amorphous Gd73.5Si13B13.5 matrix [25]. As reported in a previous work [29], bulk La0.35Pr0.275Ca0.375MnO3 possesses an AFM property. When reducing the size of the material to the nanoscale (below 100 nm), the AFM pairing energy was greatly weakened, and it became a pseudo-FM-state. As a result, a moderate magnetic field was sufficient to transform the material from the AFM state to the FM state. This intriguing property depends greatly on the size of the nanoparticles and the nature of magnetic systems [29,36]. For the La0.35Pr0.275Ca0.375MnO3 system [29], although the nanoparticle size was 50 nm, the transformation from the AFM to FM state already appeared to occur at moderate magnetic fields (~0.5 T). When a single AFM material is reduced in size to the nanoscale, its surface effect becomes significant, causing spins to transform from AFM order (M = 0) to magnetic disorder (weakly magnetic ordering, M ≠ 0). However, for composite systems of similar size, when the AFM nanoparticle size is reduced below 100 nm, this property is rather decent, and it is only pronounced when the particle size is in the range of 5 - 20 nm [25,26]. In
the present work, the size of GdB6 nanocrystals is ~10 nm, as has been determined from the HRTEM image (Fig. 2d). The application of a sufficiently large magnetic field could not only align the magnetic moments in the Gd-Si-B matrix but also convert the AFM into the FM state within the GdB6 nanocrystals, leading to the overall enhancement and broadening of -∆SM(T) and the large RC. Figure 6c (on the left hand, in black color) displays the applied magnetic field dependence of –∆SMmax in which –∆SMmax increases in an almost linear manner with increasing magnetic field. For µ 0∆H = 5 T, –∆SMmax was obtained to be ~6.4 J kg–1 K–1 at ~120 K. This – ∆SMmax value is quite large, although it is slightly smaller than those of some Gd-based amorphous alloy microwires in previous reports [11-22]. A remarkable feature is that the full width at half maximum δTFWHM of the –∆SM(µ 0H) curve reaches about ~130 K under the field change of 0 – 5 T for the Gd73.5Si13B13.5/GdB6 microwires, which is much larger as compared to Gd-based BMGs [37,38] and Gd-based amorphous microwires [11-15]. A large δTFWHM is advantageous to yield a large RC, which is an important figure-of-merit for assessing the performance of a magnetic refrigerator. To confirm this, the RC values are calculated by integrating the area under the –∆SM-T curves by using the temperatures at half maximum of the peaks [2]: RC = &( 3456 −∆ (
748
$d
(2)
where Tcold and Thot are the onset and offset temperatures of δTFWHM, respectively. The other method, using the relative cooling power (RCP), represents an amount of heat transfer between the hot and cold sides in an ideal refrigeration cycle, which can be defined as Wood and Potter’s method [39]: RCP = –∆SMmax δTFWHM
(3)
where δTFWHM = Thot – Tcold is the temperature difference at the full width at half maximum of the ∆SM (T) curve. Equation (3) indicates that a large value of RCP can be obtained by a broadening of magnetic phase transition, on the other hand, a large δTFWHM. Figure 6c (on the right hand, in blue color) is the result of the applied magnetic field dependence of RCP, namely (RCP(µ0H)). When increasing magnetic field, the RCP values increase significantly, associated with the increase in –∆SM(µ 0H), so they are represented on the same graph for comparison. For µ 0∆H = 5 T, the RC and the RCP values for the Gd73.5Si13B13.5/GdB6 microwires are calculated to be ~695 J kg-1 and ~890 J kg-1, respectively. The RC and RCP values are slightly larger than those previously reported for the biphase nanocrystalline/amorphous Gd-Al-Co alloy microwires [25], as well as other Gd-based amorphous alloys microwires [11-28]. For comparison, Table 2 summarizes TC, -∆SMmax, RC and RCP of the Gd73.5Si13B13.5/GdB6 microwires, as along with other Gd-based alloy microwire candidates. To probe the influence of the applied magnetic field on the RC values in the low (2 T) and high (5T) magnetic field regimes, the ratio of RC in these two regions is considered. In particular, together with the MCE parameters (∆SM, RC…), the RC5T/RC2T ratio values (at 5 T and 2 T, respectively) of the present microwires and other materials are presented in Tab. 2. We consider RC5T/RC2T as an additional factor that allows us to compare and evaluate the magnetocaloric figure-of-merits of these materials. From Tab. 2, it appears that if the phase of the material is purely amorphous or crystalline, the RC5T/RC2T ratio is typically smaller compared to those of materials with two structural phases (e.g., a mixture of amorphous and crystalline phases). In case of materials with coexisting AFM and FM phases, the RC values in the low magnetic field regime (< 2T) can be quite small as the AFM state has not been transformed into the FM state at these applied fields. When the applied magnetic field is
sufficiently large (~5 T), however, it transforms the AFM spins into the FM spins within the GdB6 nanocrystals, giving rise to the enhanced RC values. For the Gd73.5Si13B13.5/GdB6 composite microwires, the RC5T/RC2T ratio is 2.81, which is greater than those of the single phase Gd-alloy wires. This once again proves the contribution of the AFM GdB6 nanocrystals to the enhancement and
broadening of –∆SM(T) and consequently the RC of the
Gd73.5Si13B13.5/GdB6 microwires. In addition to Banerjee’s criterion based Arrott plots for determining the type of magnetic phases, we have used a second method based on the universal curves of –∆SM (T) as proposed by Franco et al. [40]. For SOMT materials, a universal curve can be built up to depict –∆SM(T) at different magnetic fields, µ0H. Then, all the –∆SM(T) curves at different values of µ 0∆H should be collapsed onto a single universal curve when ∆SM is normalized to ∆SMmax and the temperature axis needs to be rescaled as [40-43]: 9=
=− ; < ; :
?
− >$ ≤ ? − > − >$ ≥ − >
> >
4$
where Tr1 and Tr2 are two reference temperatures below and above TC satisfying the relation ∆SM(Tr1) = ∆SM(Tr2) = f x ∆SMmax (4) with f = 0.5 for this study. Figure 6b shows the universal ∆SM/∆SMmax vs. θ curve in which all the –∆SM(T) curves from 1 – 5 T collapses onto a universal curve. It once again confirms that the magnetic phase transition of the Gd73.5Si13B13.5/GdB6 microwires is indeed of SOMT type, which is also fully consistent with that based on Banerjee’s criterion. For SOMT materials, the magnetic field dependence of the maximum magnetic entropy change (∆SMmax) follows a scaling law [44]:
∆
CDE
∝ μ% # G
(5)
where n is a scaling exponent for the field dependence of the peak in the magnetic entropy change.
The ∆
CDE
and μ% # values are rescaled to ln(∆
CDE
) and ln μ% #$ and n is extracted
from the slope of the linear fit (not shown here), as n = 0.8637 ± 0.0058 (for μ% # = 0 – 1 T) and n = 0.8144 ± 0.0029 (for μ% # = 1 – 5 T). Rescaling the μ% # axis to obtain ∆SMmax vs. (µ 0H)n
with n = 0.8637 (for μ% # = 0 – 1 T, see in Fig. 6d with red color) and n = 0.8114 (for μ% # = 1 – 5 T, see in Fig. 6d with blue color) reveals an expected linear relationship. These n values are higher than the value of 0.67 corresponding to the mean field theory [45]. It is likely due to, either the local inhomogeneities in amorphous materials [11-19], or the presence of nanocrystalline structures embedded in a disordered matrix. The n value is field independent at TC for single phase materials, but n will alter with the change of magnetic field at any temperatures for multiphase materials, so the presence of these different magnetic phases could affect the n value [46,47]. This is the case for the presently studied Gd73.5Si13B13.5/GdB6 microwires. 4. Conclusion In summary, we have studied the structural, magnetic properties and magnetocaloric effect of the melt-extracted Gd73.5Si13B13.5/GdB6 microwires, with GdB6 nanocrystals (~10 nm) embedded in an amorphous Gd73.5Si13B13.5 matrix. The microwires undergo a second-order paramagnetic to ferromagnetic transition around 116 K. The microwires exhibited the large values of –∆SMmax ~6.4 J kg–1 K–1 at ~120 K, δTFWHM ~130 K, and RCP ~ 890 J kg-1 for a field change of 5 T. The enhanced RC and RCP make the composite microwires an attractive candidate for active magnetic cooling application.
Acknowledgments Work at USF was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award No. DE-FG02-07ER 46438 (Magnetic and magnetocaloric studies). Work at HIT was supported by the National Natural Science Foundation of China (NSFC, Nos. 51371067) (Sample fabrication and TEM characterization). Dr. J.L. Sanchez Llamazares is acknowledged for his assistance with some magnetic measurements. The authors also thank Prof. Hari Srikanth for useful discussions.
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Tables Table 1. EDS at three locations in the Gd73.5Si13B13.5/GdB6 wire. Spectrum
Gd
Si
B
Spectrum 1
73.50 12.47 14.03
Spectrum 2
73.46 12.88 13.66
Spectrum 3
73.53 12.85 13.62
Average
73.50 12.73 13.77
Table 2. Curie temperature TC, applied magnetic field µ0∆H, maximum magnetic entropy change |∆SMmax|, refrigerant capacity RC, and relative cooling power RCP for the fabricated
Gd73.5Si13B13.5/GdB6 microwires in comparison with other Gd-based microwires reported previously in the literature. AW, NC, BMGs and GR stand for amorphous wires, nanocrystals, bulk metallic glass, and glass ribbons, respectively.
Composition
Structure
TC (K)
µ0∆ H
-∆Smmax -1
(T)
RC
-1
RCP
-1
I I
J( (
Ref.
-1
(J·kg ·K ) (J·kg ) (J·kg )
Gd73.5Si13B13.5/GdB6
AW+NC
106
5
6.4
790
885
2.81
This work
Gd50Al30Co20
AW+NC
86
5
10.09
672
861
-
[25]
Gd55Al25Co20
AW+NC
100
5
9.67
652
861
2.55
[12]
Gd60Al20Co20
AW+NC
109
5
10.11
681
915
2.52
[11]
Gd60Al25Co15
AW
101
5
9.73
732
973
2.49
[13]
Gd55Al20Co25
AW
110
5
9.69
580
804
2.61
[14]
AW
170
5
6.56
625
826
2.72
[15]
AW
174
5
5.90
-
-
-
[16]
Gd60Fe20Al20
AW
202
5
4.8
687
900
2.55
[17]
Gd95Fe2.8Al2.2
GR+NC
232
5
4
551
-
2.75
[18]
Gd50(Co69.25Fe4.25Si13B13.5)50 Gd50(Co69.25Fe4.25Si13B13.5)50
Gd53Al24Co20Zr3
AW
96
5
10.3
733
-
-
[19]
Gd53Al24Co20Zr3
AW
96
5
10.3
-
733.4
-
[21]
Gd53Al24Co20Zr3
AW
94
5
8.8
600
774
2.73
[26]
AW+NC
94
5
9.5
687
893
2.41
[26]
AW+NC
94
5
8.0
629
744
2.59
[26]
AW+NC
94
5
5.1
396
525
2.75
[26]
AW
113
5
10.33
748.22
1006
2.74
[22]
Gd53Al24Co20Zr3 (Annealed at 100 oC) Gd53Al24Co20Zr3 o
(Annealed at 200 C) Gd53Al24Co20Zr3 o
(Annealed at 300 C) Gd59.4Al19.8Co19.8Fe1
Figure captions Figure 1: (a) SEM image of a Gd73.5Si13B13.5/GdB6 microwire. Inset shows the EDS of a local region selected in the SEM image; (b) XRD pattern of the Gd73.5Si13B13.5/GdB6 microwires. Inset shows the DSC curve of the Gd73.5Si13B13.5 microwires. Figure 2: (a) to (d) TEM images and insets of (b) and (c) show SAED of the nanocrystalline structure in an amorphous matrix; (e) to (h) high-angle annular dark field (HAADF) images showing the presence of the desired chemical elements in the Gd73.5Si13B13.5/GdB6 microwires. Figure 3: EDS-Line scanning of the Gd73.5Si13B13.5/GdB6 microwires. Figure 4: (a) The temperature dependence of the magnetization (left hand, in red color) and magnetic susceptibility (right hand, in blue color) at a field of 100 Oe; (b) The dM/dT vs. T curve; (c) The temperature dependence of magnetization at a field of 5 T; (d) The temperature dependence of magnetization at various magnetic fields from 0 to 5T for the Gd73.5Si13B13.5/GdB6 microwires. Figure 5: (a) Isothermal magnetization curves of the Gd73.5Si13B13.5/GdB6 microwires in a temperature range of 4–256 K with ∆T = 2 K in magnetic fields up to 5 T. (b) Arrott-Noakes plot of magnetization isotherms using the mean-field model (β = 0.5; γ = 1). Figure 6: (a) The temperature dependence of magnetic entropy change (∆SM) for different applied fields from 0.1 to 5 T; (b) The universal ∆SM/∆SMmax vs. θ curve; (c) Peak magnetic entropy change vs. (µ0H) (left hand, in black color) and the refrigerant capacity (RC) of the Gd73.5Si13B13.5/GdB6 microwires (right hand, in blue color); (d) Peak magnetic entropy change
vs. (µ0H)n, where n = 0.8637 is the prediction of the scaling relation at low magnetic field from 0 to 1 T (red color) and high magnetic field from 1 to 5 T (blue color).
Research Highlights
• • •
Composite melt-extracted Gd73.5Si13B13.5/GdB6 microwires Large magnetic entropy change and refrigerant capacity Magnetic refrigerator for nitrogen liquefaction
Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
S0925-8388(19)34579-7
DOI:
https://doi.org/10.1016/j.jallcom.2019.153333
Reference:
JALCOM 153333
To appear in:
Journal of Alloys and Compounds
Received Date: 19 September 2019 Revised Date:
5 December 2019
Accepted Date: 7 December 2019
Please cite this article as: N.T.M. Duc, H.X. Shen, O. Thiabgoh, N.T. Huong, J.F. Sun, M.H. Phan, Melt-extracted Gd73.5Si13B13.5/GdB6 ferromagnetic/antiferromagnetic microwires with excellent magnetocaloric properties, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/ j.jallcom.2019.153333. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Author Contribution Statement N.T.M. D, H.X.S and M.H.P initiated the concept. N.T.M. D performed magnetic measurements and analysis. N.T.M. D and N.T.H performed XRD and DSC measurements. H.X.S and J.F.S performed TEM imaging and analysis. O.T. performed SEM imaging and analysis. N.T.M. D wrote the manuscript with inputs from all the other authors. M.H.P led the entire project. All the authors approved the final manuscript.
Melt-extracted Gd73.5Si13B13.5/GdB6 ferromagnetic/antiferromagnetic microwires with excellent magnetocaloric properties N.T.M. Duc,a,b,c,* H.X. Shen,a,d,** O. Thiabgoh,e N.T. Huong,b J.F. Sun,d and M.H. Phana,*** a
b
c
Department of Physics, University of South Florida, Tampa, Florida 33620, USA
Department of Physics, VNU University of Science, 334 Nguyen Trai, Hanoi, Vietnam
The University of Danang, University of Science and Education, 459 Ton Duc Thang, Danang, Vietnam
d
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China e
Department of Physics, Faculty of Science, Ubon Ratchathani University, Warin Chamrap, Ubon Ratchathani 34190, Thailand
A novel class of magnetocaloric microwires composed of antiferromagnetic GdB6 nanocrystals embedded in an amorphous ferromagnetic Gd73.5Si13B13.5 matrix is reported. These composite microwires were created directly from a melt-extraction process. XRD, TEM, HRTEM and SAED confirm the formation of GdB6 nanocrystals (~10 nm) in the amorphous Gd73.5Si13B13.5 matrix. As a second-order magnetic material, the Gd73.5Si13B13.5/GdB6 microwires exhibit an excellent magnetocaloric response, with a large magnetic entropy change ∆SM ~6.4 J kg–1 K–1 at ~120 K, a large temperature interval δTFWHM ~130 K, and the large refrigerant capacity RC ~ 890 J kg-1 for a field change of 5 T. Such microwires are a promising candidate for active magnetic refrigeration in the temperature range of 10 – 200 K.
Keywords: melt-extraction; magnetic microwires; magnetocaloric effect; magnetic refrigeration *Corresponding author: [email protected] (N.T.M. Duc) **Corresponding author: [email protected] (H. Shen) ***Corresponding author. [email protected] (M.H. Phan)
1. Introduction Current refrigerators, such as air conditioners and refrigerators are based mostly on an environmentally unfriendly gas compression technique, which emits harmful gases such as chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFC) to the environment. Magnetic refrigeration (MR) based on the magnetocaloric effect (MCE) using a magnetic solid substance as a refrigerant represents a promising alternative, due to its higher cooling efficiency, compactness and environmental friendliness [1-8]. In addition to the adiabatic temperature change (∆Tad), there are other two important parameters that are usually employed in assessing the performance of a magnetocaloric material for MR. The first parameter is a magnetic entropy change (∆SM), which should be as large as possible. The largest ∆SM typically occurs in the vicinity of a magnetic phase transition and depends on the characteristics of the magnetic phase transition. As the first-order magnetic transition (FOMT) is typically sharp, the resulting ∆SM is large in this type of FOMT material [1]. However, FOMT materials usually possess narrow phase transition regions, leading to narrow cooling temperature ranges. Thermal and magnetic hystereses associated with the FOMT nature are also large, which are detrimental to magnetic refrigeration [9]. By contrast, second-order magnetic transition (SOMT) materials possess smaller ∆SM values, but extend ∆SM (T) over wider operating temperature ranges around their transition temperatures. Thermal and magnetic hystereses are often negligible in these SOMT materials. The second parameter is the refrigerant capacity (RC), which is generally considered a measure of the amount of heat transfer between the cold and hot sinks in an ideal refrigeration cycle [9-10]. In the SOMT materials, the broadening of the phase transition has been reported to result in a large RC, which is even greater than those of some FOMT materials [1]. A benchmark
SOMT material for sub-room temperature magnetic cooling application is gadolinium (Gd) [2,3,6]. From a practical cooling perspective, the form of a magnetocaloric material plays a crucial role in the performance of a magnetic cooler [11-28]. While a magnetocaloric material can be synthesized in different forms, such as powder particles, thin films or wires, when it is used in an actual cooling device, it is essential to design the material in a form so that it can transfer/release heat most effectively to a surrounding environment (e.g. water) [23]. It has been shown theoretically and experimentally that magnetocaloric materials in the form of wires are desirable for use in actual magnetic cooling devices [11-28]. In the wire form, it can release the heat faster due to its enhanced surface area, and yield a good mechanical response that reduces the relaxation time and consequently increases the operation frequency of the cooling device [27,28]. As a result, the wire-shaped magnetocaloric materials have exhibited the highest cooling efficiencies as compared to their particle- and thin film-shaped counterparts [19,21]. Gd-alloy microwires prepared by the melt-extraction method have been extensively explored by us for active magnetic refrigeration as they exhibit excellent magnetocaloric and mechanical properties. A large number of MCE studies on these Gd-based microwires have been reported [11-28]. The wire shape with increased surface-to-volume areas allows for a higher heat transfer between the magnetic refrigerant and the surrounding liquid. Relative to their bulk counterparts, these microwires have shown the larger ∆SM and RC values [11-26]. To improve the RC in these microwires, different approaches have been proposed [2426]. As such, a low-temperature and short-time annealing of Gd53Al24Co20Zr3 amorphous wires to create nanocrystals embedded in an amorphous matrix has been demonstrated to enhance both the ∆SM and RC, while preserving the good mechanical strength [26]. Another effective approach
has been proposed as to create a biphase amorphous/nanocrystalline structure in Gd-Al-Co wires directly through a controlled melt-extraction process, resulting in the enhanced RC values [24, 25]. It has been shown that reducing size of anti-ferromagnetic (AFM) particles to the nanoscale could weaken the AFM coupling significantly so that a moderate magnetic field could convert the AFM into the ferromagnetic (FM), leading to a large change in magnetization and hence the MCE [29]. Recent studies have revealed that reducing size of antiferromagnetic material GdB6 to the nanoscale induces the FM state [30-32]. From a metallurgy perspective, the GdB6 nanophase may be formed in Gd-Si-B wires through the melt-extraction process. In the present study, we have demonstrated this as a new, effective approach for creating magnetocaloric composite microwires composed of antiferromagnetic GdB6 nanocrystals embedded in an amorphous ferromagnetic Gd73.5Si13B13.5 matrix. This class of microwires exhibits an enhanced RC over a large temperature range, making it a promising candidate for active magnetic refrigeration. 2. Experiment Amorphous Gd73.5Si13B13.5 microwires were fabricated via the melt-extraction technology by using a spinning copper wheel of 160 mm in diameter with a 60o tapered edge. To obtain a uniform diameter for melt-extracted microwires, the wheel was keep at a fixed rotation speed of 30 m/min while the melted material was fed at a constant rate of 90 µm/s. The purity of the starting elements used to prepare the induction-melted alloys was as follows: Gd (99.5%), Si (99.99%), and B (99.99%).
The structure of the wire samples was examined in a Bruker D5005 X-ray diffractometer (Cu/Kα). Differential scanning calorimetry (DSC) was studied on DSC Labsys Evo S60/58988 system (France) at a heating rate of 10 K/min. Energy dispersive X-ray spectroscopy (EDS) and scanning electron microscopy (SEM) were performed with a Jeol model JSM-6390LV scanning electron microscope equipped with an INCAEnergy EDS system from Oxford instruments. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were recorded on a FEI TECNAI G2 F30 system. Selected area-electron diffraction (SAED) was also used to analyze the microwires. The magnetization of a single wire was measured by means of the vibrating sample magnetometer option of a Quantum Design commercial physical property measurement system (PPMS). DC magnetic fields up to 5 T were applied longitudinally along the wire as temperature was varied from 4 K to 256 K with an increment of 4 K. The molecular weight of the Gd73.5Si13B13.5 microwire is 120.69 g/mol. 3. Results and Discussion 3.1. Structural and morphological characterizations The micromorphology of a Gd73.5Si13B13.5 microwire was obtained by SEM, which reveals a homogeneous and smooth surface (Fig. 1a). The uniform diameter of the microwire was determined directly from planar SEM micrographs to be ~63 µm with a circular cross section. The EDS measured at room temperature is displayed in the inset of Fig. 1a, which confirms the presence of raw chemical elements Gd and Si (B cannot be detected using EDS). The EDS analysis was performed to determine the percentage of these elements in the sample. The atomic percentages are estimated to be 73.50%, 12.73% and 13.77% for Gd, Si and B respectively, which are expected for the desired nominal composition Gd73.5Si13B13.5. The data of
the atomic percentages collected at three locations on the surface of the microwire are reported in Table 1, confirming the unchanged compositions in the microwire. The structural characterizations of the melt-extracted Gd73.5Si13B13.5 microwires have been examined by XRD, DSC and TEM. The results are displayed in Figure 1b and Figure 2. The XRD pattern measured from 20° to 100° (2θ) with a scanning speed of 0.9°/min is shown in Fig. 1b. The typical broad halo pattern with a broad diffuse peak is observed at near angle of 33°. This typical peak has also been found in the XRD spectra of previously studied Gd-based materials [11-19], demonstrating the amorphous characteristic of the presently studied Gd73.5Si13B13.5 microwires. It is worth mentioning here that a crystalline peak is clearly detected at ~31° which perfectly coincides with the XRD peak (110) of GdB6 nanowires [31]. This indicates the presence of GdB6 nanocrystals in an amorphous Gd73.5Si13B13.5 matrix, and such a combination of these two phases would result in the superior magnetocaloric response of the microwires. To explore the thermal stability of the melt-extracted Gd73.5Si13B13.5 microwires, the DSC analysis was carried out through the heating process with a heating rate of 10 K/min at temperatures between 300 K and 750 K. The DSC spectrum is shown in the inset of Fig. 1b. The three exothermic peaks appear to occur, demonstrating the three-step crystallization of a meltextracted Gd73.5Si13B13.5 microwire during the heating process. The peak intensity of these three peaks decreased with increasing temperature, indicating that the exothermic process became more and more slowly. As shown in the inset of Fig. 1b, the glass transition temperature Tg is around 330 K and the first crystallization temperature Tx1 is about 350 K. The temperature range between Tg and Tx1 is called a supercooled liquid region or a glass transition temperature region
[11-19], which was calculated as ∆Tx = Tx1 – Tg = 20 K. Values of ∆Tx have been reported to influence on the glass forming ability (GFA) of Gd alloy microwires [11-19]. To independently confirm the presence of GdB6 nanocrystals in an amorphous matrix in these fabricated alloy microwires, TEM and HRTEM images were taken in scale of 2 µm, 200 nm, 10 nm and 20 nm as displayed in Fig. 2a-d, respectively. The HRTEM images show that GdB6 nanocrystals of ~10 nm are homogeneously distributed in the amorphous Gd73.5Si13B13.5 matrix (see, Fig. 2b and 2d). The SAED patterns are shown in the inset of Fig. 2b and Fig. 2c, in order to confirm details of the reflections discretely by these nanocrystals. The high-angle annular dark field (HAADF) images of the microwire confirmed the presence of the chemical elements in the sample (see Fig. 2e-h). The combination of XRD and HR-TEM allows us to confirm the presence of GdB6 nanocrystals (~10 nm) in an amorphous Gd-Si-B matrix. The EDS-Line scanning was also performed, and the results obtained (Fig. 3) show the dependence of mass percent of the elements on distance. As can be seen in Fig. 3, the nanocrystals area (dark area) has more mass percentages of Gd and B as compared to the amorphous matrix (light area). The presence of GdB6 nanocrystals in an amorphous Gd73.5Si13B13.5 matrix would impact the magnetic and magnetocaloric properties of the microwires, and this effect will be examined in detail in the following sections. 3.2. Magnetic properties Figure 4a (red curve on left hand) presents the temperature dependence of magnetization (M-T) for these microwires measured while cooing under an applied DC magnetic field of µ0H = 10 mT. As shown in Fig. 4a, the M-T curve exhibits a broad paramagnetic to
ferromagnetic (PM-FM) phase transition around the Curie temperature TC. Figure 4c also shows the (M-T) of this microwires sample but measured under higher magnetic field of µ0H = 5 T. Comparing these two cases together in Figs. 4a and 4c, we find that, as the magnetic field increases, the magnetic phase transition becomes more and more broaden. This is not only a typical behavior for SOMT materials (long-range ferromagnetic order) and contributing to the structural disorder caused by the amorphous structure, but also a testament to the existence of the GdB6 nanocrystals in the amorphous Gd73.5Si13B13.5 matrix [20]. If the interaction is only ferromagnetic, the M-T curve should remain sharp when it is in low magnetic field regimes. As shown in Fig. 4c, the M-T curve is significantly broadened; this demonstrates the presence of AFM GdB6 nanocrystals. It is the presence of the AFM GdB6 nanocrystals that caused the broadened PM-FM phase transition, upon the application of a sufficiently high magnetic field. This consequently results in the broadened ∆SM(T) and hence the large RC. Once again, Figure 4d shows the temperature dependence of magnetization (M-T) at different applied magnetic fields from µ 0H ~ 0.01 – 5 T in the filled 2D contour plot of the temperature and magnetic field dependence of the magnetization. The magnetization value varies greatly around the Curie temperature (TC), which leads to an expectation that there will be a large change in magnetic entropy. The TC is determined to be ~116 K from the minimum of the derivative dM/dT vs. T curve, as displayed in Fig. 4b. This value of TC is larger than that reported for Gd59Si41 (~64.6 K), but smaller than that of Gd87Si13 (~149 K) [33]. This suggests that the TC value can be increased by increasing the concentration of Gd [33] or adding B to create GdB6 nanocrystals in an amorphous matrix as we have demonstrated in this study. The inverse susceptibility as a function of temperature, χ-1(T) = µ 0H/M, generated from the M(T) curve in the paramagnetic region is also shown in Fig. 4a (the blue curve on right
hand). Because of the broadened phase transition of M-T, the χ-1(T) is linear at high temperature region above 245 K. Based on the Curie-Weiss law in the paramagnetic region: χ = Curie constant defined by =
with the
μ ,where NA = 6.022x1023 mol-1 is Avogadro’s number, µ B
= 9.274x10-21 emu is the Bohr magneton, and kB = 1.38016x10-16 erg/K (in the CGS system of units) is Boltzmann constant, the Curie-Weiss temperature θ and the effective magnetic moment values
will be determined. The results of fitting linear region of the χ-1(T) are θ = 202 K and
C = 7.81 emu K mol-1. The difference between θ and TC caused by the broaden phase transition μ , the effective magnetic moment µ eff
of these microwires. From the Curie constant =
of the sample will be calculated via
=
/
= √8
. The value of
is
= 7.90
µ B, is close to the theoretically calculated value of pure Gd (~7.94µ B). The interaction of magnetic moments between transition metal (TM) elements with 3d-electrons and rare-earth (RE) element with 4f-electrons [34] is the main cause of decreasing the effective magnetic moment µ eff. As we know, there are no elements with 3d-electrons such as Fe, Co, Ni,… in the presently fabricated Gd73.5Si13B13.5 microwires. So, the effective magnetic moment µ eff of the Gd73.5Si13B13.5 microwires is close to pure Gd. 3.3 Magnetocaloric properties A set of isothermal magnetization M-H curves of the Gd73.5Si13B13.5 microwires taken at different temperatures ranging around TC from 4 to 256 K, with a temperature interval of 4 K, was obtained under applied magnetic fields of µ 0∆H = 0 – 5 T, in order to investigate the magnetocaloric properties of these microwires. The sweeping rate of the field was slow enough to ensure that the magnetization curves are measured in an isothermal process. The results are
shown in Fig. 5a. As can be seen in Fig. 5a, a typical ferromagnetic transition is obvious in the vicinity of TC. The largest saturation magnetization (MS) value for the largest magnetic field of ~5 T is determined to be about ~220 A m2 kg-1. This MS value is equivalent to those of previously reported Gd-based alloys [11-19]. When compared with Gd-Si alloys (e.g. Gd87Si13, Gd59Si47, Gd18Si82 [33] without B element), however, this MS value of the Gd-Si-B microwires is much larger. MS ~ 220 A m2 kg-1 at T = 4 K for Gd73.5Si13B13.5, while it is ~145 A m2 kg-1 (T < 20 K), ~120 A m2 kg-1 (T < 12 K) and ~110 A m2 kg-1 (T = 4.2 K) for Gd87Si13, Gd59Si47 and Gd18Si82, respectively [33]. It indicates that when the element B is present, leading to a combination of Gd-B, the MS increased significantly. In other words, the larger MS at low temperature implies that anti-ferromagnetic interaction within GdB6 nanocrystals is likely reduced and converted into the ferromagnetic order. To confirm the nature of the magnetic phase transition of the Gd73.5Si13B13.5/GdB6 microwires, a set of isothermal magnetization M-H is converted to the modified Arrott plots µ0H/M vs. M2 as shown in Fig. 5b. According to Banerjee criterion, a magnetic transition is considered as FOMT when the slope of Arrott plot is negative; otherwise, it is expected to be SOMT when the slope is positive [35]. As can be seen in Fig. 5b, all slopes remain positive, indicating that the FM-PM transition of the Gd73.5Si13B13.5/GdB6 microwires is a type of SOMT, which is in full agreement with the previous reports on Gd-based alloy microwires [11-19]. It is also one of the reasons for broadening the magnetic phase transition that consequently enhances the RC value. From the isothermal magnetization M-H curves, the magnetic entropy change –∆SM can be calculated based on thermal-dynamic Maxwell equation [1]:
Δ
, μ" # $ = μ% &%
),-. '
'( )
dH
(1)
where M is the magnetization, µoH is the applied magnetic field, and T is the temperature, by integrating over the magnetic field. Figure 6a shows the magnetic entropy change –∆SM as a function of temperature for different applied magnetic field of µ 0∆H = 0.1 – 5 T by (1). As can be seen in Fig. 6a, the magnitude of –∆SM significantly increased along with an increase of applied magnetic field µ0H and reached a maximum value –∆SMmax near TC. One can also see that the –∆SM(µoH,T) curves across the PM-FM phase transition are broad, which is attributed to the broadened PM-FM transition as seen in the M(T) curves under high applied magnetic fields (see Fig. 4c). This effect is likely associated with the distribution of GdB6 nanocrystals within an amorphous Gd73.5Si13B13.5 matrix [25]. As reported in a previous work [29], bulk La0.35Pr0.275Ca0.375MnO3 possesses an AFM property. When reducing the size of the material to the nanoscale (below 100 nm), the AFM pairing energy was greatly weakened, and it became a pseudo-FM-state. As a result, a moderate magnetic field was sufficient to transform the material from the AFM state to the FM state. This intriguing property depends greatly on the size of the nanoparticles and the nature of magnetic systems [29,36]. For the La0.35Pr0.275Ca0.375MnO3 system [29], although the nanoparticle size was 50 nm, the transformation from the AFM to FM state already appeared to occur at moderate magnetic fields (~0.5 T). When a single AFM material is reduced in size to the nanoscale, its surface effect becomes significant, causing spins to transform from AFM order (M = 0) to magnetic disorder (weakly magnetic ordering, M ≠ 0). However, for composite systems of similar size, when the AFM nanoparticle size is reduced below 100 nm, this property is rather decent, and it is only pronounced when the particle size is in the range of 5 - 20 nm [25,26]. In
the present work, the size of GdB6 nanocrystals is ~10 nm, as has been determined from the HRTEM image (Fig. 2d). The application of a sufficiently large magnetic field could not only align the magnetic moments in the Gd-Si-B matrix but also convert the AFM into the FM state within the GdB6 nanocrystals, leading to the overall enhancement and broadening of -∆SM(T) and the large RC. Figure 6c (on the left hand, in black color) displays the applied magnetic field dependence of –∆SMmax in which –∆SMmax increases in an almost linear manner with increasing magnetic field. For µ 0∆H = 5 T, –∆SMmax was obtained to be ~6.4 J kg–1 K–1 at ~120 K. This – ∆SMmax value is quite large, although it is slightly smaller than those of some Gd-based amorphous alloy microwires in previous reports [11-22]. A remarkable feature is that the full width at half maximum δTFWHM of the –∆SM(µ 0H) curve reaches about ~130 K under the field change of 0 – 5 T for the Gd73.5Si13B13.5/GdB6 microwires, which is much larger as compared to Gd-based BMGs [37,38] and Gd-based amorphous microwires [11-15]. A large δTFWHM is advantageous to yield a large RC, which is an important figure-of-merit for assessing the performance of a magnetic refrigerator. To confirm this, the RC values are calculated by integrating the area under the –∆SM-T curves by using the temperatures at half maximum of the peaks [2]: RC = &( 3456 −∆ (
748
$d
(2)
where Tcold and Thot are the onset and offset temperatures of δTFWHM, respectively. The other method, using the relative cooling power (RCP), represents an amount of heat transfer between the hot and cold sides in an ideal refrigeration cycle, which can be defined as Wood and Potter’s method [39]: RCP = –∆SMmax δTFWHM
(3)
where δTFWHM = Thot – Tcold is the temperature difference at the full width at half maximum of the ∆SM (T) curve. Equation (3) indicates that a large value of RCP can be obtained by a broadening of magnetic phase transition, on the other hand, a large δTFWHM. Figure 6c (on the right hand, in blue color) is the result of the applied magnetic field dependence of RCP, namely (RCP(µ0H)). When increasing magnetic field, the RCP values increase significantly, associated with the increase in –∆SM(µ 0H), so they are represented on the same graph for comparison. For µ 0∆H = 5 T, the RC and the RCP values for the Gd73.5Si13B13.5/GdB6 microwires are calculated to be ~695 J kg-1 and ~890 J kg-1, respectively. The RC and RCP values are slightly larger than those previously reported for the biphase nanocrystalline/amorphous Gd-Al-Co alloy microwires [25], as well as other Gd-based amorphous alloys microwires [11-28]. For comparison, Table 2 summarizes TC, -∆SMmax, RC and RCP of the Gd73.5Si13B13.5/GdB6 microwires, as along with other Gd-based alloy microwire candidates. To probe the influence of the applied magnetic field on the RC values in the low (2 T) and high (5T) magnetic field regimes, the ratio of RC in these two regions is considered. In particular, together with the MCE parameters (∆SM, RC…), the RC5T/RC2T ratio values (at 5 T and 2 T, respectively) of the present microwires and other materials are presented in Tab. 2. We consider RC5T/RC2T as an additional factor that allows us to compare and evaluate the magnetocaloric figure-of-merits of these materials. From Tab. 2, it appears that if the phase of the material is purely amorphous or crystalline, the RC5T/RC2T ratio is typically smaller compared to those of materials with two structural phases (e.g., a mixture of amorphous and crystalline phases). In case of materials with coexisting AFM and FM phases, the RC values in the low magnetic field regime (< 2T) can be quite small as the AFM state has not been transformed into the FM state at these applied fields. When the applied magnetic field is
sufficiently large (~5 T), however, it transforms the AFM spins into the FM spins within the GdB6 nanocrystals, giving rise to the enhanced RC values. For the Gd73.5Si13B13.5/GdB6 composite microwires, the RC5T/RC2T ratio is 2.81, which is greater than those of the single phase Gd-alloy wires. This once again proves the contribution of the AFM GdB6 nanocrystals to the enhancement and
broadening of –∆SM(T) and consequently the RC of the
Gd73.5Si13B13.5/GdB6 microwires. In addition to Banerjee’s criterion based Arrott plots for determining the type of magnetic phases, we have used a second method based on the universal curves of –∆SM (T) as proposed by Franco et al. [40]. For SOMT materials, a universal curve can be built up to depict –∆SM(T) at different magnetic fields, µ0H. Then, all the –∆SM(T) curves at different values of µ 0∆H should be collapsed onto a single universal curve when ∆SM is normalized to ∆SMmax and the temperature axis needs to be rescaled as [40-43]: 9=
=− ; < ; :
?
− >$ ≤ ? − > − >$ ≥ − >
> >
4$
where Tr1 and Tr2 are two reference temperatures below and above TC satisfying the relation ∆SM(Tr1) = ∆SM(Tr2) = f x ∆SMmax (4) with f = 0.5 for this study. Figure 6b shows the universal ∆SM/∆SMmax vs. θ curve in which all the –∆SM(T) curves from 1 – 5 T collapses onto a universal curve. It once again confirms that the magnetic phase transition of the Gd73.5Si13B13.5/GdB6 microwires is indeed of SOMT type, which is also fully consistent with that based on Banerjee’s criterion. For SOMT materials, the magnetic field dependence of the maximum magnetic entropy change (∆SMmax) follows a scaling law [44]:
∆
CDE
∝ μ% # G
(5)
where n is a scaling exponent for the field dependence of the peak in the magnetic entropy change.
The ∆
CDE
and μ% # values are rescaled to ln(∆
CDE
) and ln μ% #$ and n is extracted
from the slope of the linear fit (not shown here), as n = 0.8637 ± 0.0058 (for μ% # = 0 – 1 T) and n = 0.8144 ± 0.0029 (for μ% # = 1 – 5 T). Rescaling the μ% # axis to obtain ∆SMmax vs. (µ 0H)n
with n = 0.8637 (for μ% # = 0 – 1 T, see in Fig. 6d with red color) and n = 0.8114 (for μ% # = 1 – 5 T, see in Fig. 6d with blue color) reveals an expected linear relationship. These n values are higher than the value of 0.67 corresponding to the mean field theory [45]. It is likely due to, either the local inhomogeneities in amorphous materials [11-19], or the presence of nanocrystalline structures embedded in a disordered matrix. The n value is field independent at TC for single phase materials, but n will alter with the change of magnetic field at any temperatures for multiphase materials, so the presence of these different magnetic phases could affect the n value [46,47]. This is the case for the presently studied Gd73.5Si13B13.5/GdB6 microwires. 4. Conclusion In summary, we have studied the structural, magnetic properties and magnetocaloric effect of the melt-extracted Gd73.5Si13B13.5/GdB6 microwires, with GdB6 nanocrystals (~10 nm) embedded in an amorphous Gd73.5Si13B13.5 matrix. The microwires undergo a second-order paramagnetic to ferromagnetic transition around 116 K. The microwires exhibited the large values of –∆SMmax ~6.4 J kg–1 K–1 at ~120 K, δTFWHM ~130 K, and RCP ~ 890 J kg-1 for a field change of 5 T. The enhanced RC and RCP make the composite microwires an attractive candidate for active magnetic cooling application.
Acknowledgments Work at USF was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award No. DE-FG02-07ER 46438 (Magnetic and magnetocaloric studies). Work at HIT was supported by the National Natural Science Foundation of China (NSFC, Nos. 51371067) (Sample fabrication and TEM characterization). Dr. J.L. Sanchez Llamazares is acknowledged for his assistance with some magnetic measurements. The authors also thank Prof. Hari Srikanth for useful discussions.
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Tables Table 1. EDS at three locations in the Gd73.5Si13B13.5/GdB6 wire. Spectrum
Gd
Si
B
Spectrum 1
73.50 12.47 14.03
Spectrum 2
73.46 12.88 13.66
Spectrum 3
73.53 12.85 13.62
Average
73.50 12.73 13.77
Table 2. Curie temperature TC, applied magnetic field µ0∆H, maximum magnetic entropy change |∆SMmax|, refrigerant capacity RC, and relative cooling power RCP for the fabricated
Gd73.5Si13B13.5/GdB6 microwires in comparison with other Gd-based microwires reported previously in the literature. AW, NC, BMGs and GR stand for amorphous wires, nanocrystals, bulk metallic glass, and glass ribbons, respectively.
Composition
Structure
TC (K)
µ0∆ H
-∆Smmax -1
(T)
RC
-1
RCP
-1
I I
J( (
Ref.
-1
(J·kg ·K ) (J·kg ) (J·kg )
Gd73.5Si13B13.5/GdB6
AW+NC
106
5
6.4
790
885
2.81
This work
Gd50Al30Co20
AW+NC
86
5
10.09
672
861
-
[25]
Gd55Al25Co20
AW+NC
100
5
9.67
652
861
2.55
[12]
Gd60Al20Co20
AW+NC
109
5
10.11
681
915
2.52
[11]
Gd60Al25Co15
AW
101
5
9.73
732
973
2.49
[13]
Gd55Al20Co25
AW
110
5
9.69
580
804
2.61
[14]
AW
170
5
6.56
625
826
2.72
[15]
AW
174
5
5.90
-
-
-
[16]
Gd60Fe20Al20
AW
202
5
4.8
687
900
2.55
[17]
Gd95Fe2.8Al2.2
GR+NC
232
5
4
551
-
2.75
[18]
Gd50(Co69.25Fe4.25Si13B13.5)50 Gd50(Co69.25Fe4.25Si13B13.5)50
Gd53Al24Co20Zr3
AW
96
5
10.3
733
-
-
[19]
Gd53Al24Co20Zr3
AW
96
5
10.3
-
733.4
-
[21]
Gd53Al24Co20Zr3
AW
94
5
8.8
600
774
2.73
[26]
AW+NC
94
5
9.5
687
893
2.41
[26]
AW+NC
94
5
8.0
629
744
2.59
[26]
AW+NC
94
5
5.1
396
525
2.75
[26]
AW
113
5
10.33
748.22
1006
2.74
[22]
Gd53Al24Co20Zr3 (Annealed at 100 oC) Gd53Al24Co20Zr3 o
(Annealed at 200 C) Gd53Al24Co20Zr3 o
(Annealed at 300 C) Gd59.4Al19.8Co19.8Fe1
Figure captions Figure 1: (a) SEM image of a Gd73.5Si13B13.5/GdB6 microwire. Inset shows the EDS of a local region selected in the SEM image; (b) XRD pattern of the Gd73.5Si13B13.5/GdB6 microwires. Inset shows the DSC curve of the Gd73.5Si13B13.5 microwires. Figure 2: (a) to (d) TEM images and insets of (b) and (c) show SAED of the nanocrystalline structure in an amorphous matrix; (e) to (h) high-angle annular dark field (HAADF) images showing the presence of the desired chemical elements in the Gd73.5Si13B13.5/GdB6 microwires. Figure 3: EDS-Line scanning of the Gd73.5Si13B13.5/GdB6 microwires. Figure 4: (a) The temperature dependence of the magnetization (left hand, in red color) and magnetic susceptibility (right hand, in blue color) at a field of 100 Oe; (b) The dM/dT vs. T curve; (c) The temperature dependence of magnetization at a field of 5 T; (d) The temperature dependence of magnetization at various magnetic fields from 0 to 5T for the Gd73.5Si13B13.5/GdB6 microwires. Figure 5: (a) Isothermal magnetization curves of the Gd73.5Si13B13.5/GdB6 microwires in a temperature range of 4–256 K with ∆T = 2 K in magnetic fields up to 5 T. (b) Arrott-Noakes plot of magnetization isotherms using the mean-field model (β = 0.5; γ = 1). Figure 6: (a) The temperature dependence of magnetic entropy change (∆SM) for different applied fields from 0.1 to 5 T; (b) The universal ∆SM/∆SMmax vs. θ curve; (c) Peak magnetic entropy change vs. (µ0H) (left hand, in black color) and the refrigerant capacity (RC) of the Gd73.5Si13B13.5/GdB6 microwires (right hand, in blue color); (d) Peak magnetic entropy change
vs. (µ0H)n, where n = 0.8637 is the prediction of the scaling relation at low magnetic field from 0 to 1 T (red color) and high magnetic field from 1 to 5 T (blue color).
Research Highlights
• • •
Composite melt-extracted Gd73.5Si13B13.5/GdB6 microwires Large magnetic entropy change and refrigerant capacity Magnetic refrigerator for nitrogen liquefaction
Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.