4 tt luan an ha en

MINISTRY OF EDUCATION AND TRAINING VIETNAM ACADEMY OF SCIENCE AND TECHNOLOGY GRADUATE UNIVERSITY OF SCIENCE AND TECHNOLOGY  NGUYEN HOANG HA INVESTIGATION OF FABRICATION, MAGNETIC PROPERTIES AND MAGNETOCALORIC EFFECT ON Fe-Zr BASE AMORPHOUS ALLOYS Major: Electronic materials Code: 9.44.01.23 SUMMARY OF DOCTORAL THESIS IN MATERIALS SCIENCE HA NOI - 2022 The thesis was completed at the Key Laboratory for Electronic Materials and Devices, and Laboratory of Magnetism and Superconductivity, Institute of Materials Science, Vietnam Academy of Science and Technology Supervisor: Assoc.Prof.PhD Nguyen Huy Dan Assoc.Prof.PhD Nguyen Manh An Reviewer 1: Reviewer 2: Reviewer 3: The thesis will be defended at Graduate University of Science and Technology, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Ha Noi, Vietnam Time: ., , 2022 Thesis can be further referred at: - National Library of Vietnam - Library of Graduate University of Science and Technology - Library of Institute of Materials Science MỞ ĐẦU Today, global warming and rising costs of energy require the development of new cooling technologies to replace the conventional gascompression/expansion refrigeration In response to this demand, the magnetic cooling technology based on the magnetocaloric effect of the material is a good candidate This technology can be used to obtain extremely low temperatures, as well as application in refrigerators in the room temperature region The magnetic refrigerator is more efficient than the conventional vapour-cycle one The cooling efficiency of magnetic refrigerators can achieve up 70% of the ideal Carnot cycle The gas-compression refrigerators, meanwhile, just only obtain an efficiency of about 40% Moreover, magnetic refrigeration is an environmentally friendly cooling technology It does not use ozone-depleting chemicals or greenhouse gases Magnetocaloric effect (MCE) is the heating or cooling of a magnetic material under variation of a magnetic field This effect holds a potential for magnetic refrigeration applications As compared to conventional gas compression refrigeration techniques, the magnetic refrigeration has advantages of avoiding environmental pollution and saving energy Many scientists have focused on finding magnetocaloric materials for magnetic refrigeration at ambient temperature To explore magnetic refrigeration in the room temperature region, magetocaloric materials must have a magnetic phase transition at room temperature, because the MCE (the isothermal magnetic entropy change or the adiabatic temperature change) is often maximal at the magnetic phase transition temperature (TC) In most cases, the MCE of a material can be assessed through its magnetic entropy change (ΔSm) and the refrigeration capacity (RC), which is the product of the magnetic entropy change and the working temperature range In addition, some other properties such as the low specific heat capacity (the large adiabatic temperature change), high thermal conductivity (for fast heatexchange), low electrical conductivity (reducing power losses due to Foucault currents), high durability and low magnetic hysteresis are also needed for application of the magnetocaloric materials A large number of magnetocaloric materials for room-temperature magnetic refrigeration have been developed, including Gd-containing alloys (GdSiGe, GdCo), Ascontaining alloys (MnAsSb, MnFePAs), and La-containing alloys (LaFeSi) Many researchers have focused on the exploitation of magnetocaloric materials with an amorphous or a nanocrystalline structure The main advantages of amorphous materials are low coercivity, high resistivity, room temperature magnetic phase transition, and low cost, which are necessary for practical applications Among these materials, Fe-Zr based alloys have attracted growing attention from scientists By adding other elements to the alloys, their ferromagnetic-paramagnetic (FM-PM) transition temperature (Curie temperature, TC) can be easily tuned to a desired temperature range while retaining the good magnetocaloric properties.Although the Fe-Zr based amorphous alloys have the maximum magnetic entropy smaller than that of other GMCE materials (such as Gd-containing alloys, La-Fe-Si alloys, Heusler alloys…) but they own a wide working temperature range leading to large RC, which are necessary for practical application To change the Curie temperature and improve glass forming ability (GFA) of this material, other elements such as Co, Ni, B, Y, Cr, Mn have been added However, the effect of the additional elements on the GFA and TC of the alloy is widely various For example, the Curie temperature of the Fe 90xMnxZr10 system is decreased from 210 K (for x = 8) to 185 K (for x = 10) with increasing Mn concentration While that of the Fe89-xBxZr11 is increased from  310 K (for x = 2.5) to 370 K (for x = 10) with increasing B concentration Therefore, with the goal of adjusting the working temperature of the alloy to room temperature, the study of the influence of additional elements is very necessary for this alloy system In Vietnam, there have been some research groups interested in magnetocaloric materials such as University of Natural Sciences, University of Engineering and Technology, Hanoi University of Science and Technology, Institute of Materials Science These research groups have had a number of scientific publications in the domestic and international journals The research on the magnetocaloric materials in Vietnam is relatively close to the progress in the world However, due to insufficience of equipment, funding and human resources to the research, the results including both basic and applied researches are limited Therefore, structure, magnetic properties and MCE of the magnetocaloric materials are still necessary to study From the above reasons, we have chosen the research topic of the thesis as following: “Investigation of fabrication, magnetic properties and magnetocaloric effect of Fe-Zr amorphous alloys” The research objectives of the thesis: The Fe-Zr amorphous alloy systems: Fe-(Pr,La,Nd)-Zr, Fe-(Cr,Cu)(Nd,Gd,Co)-Zr The goal of the thesis: Fabrication, investigation of structure, magnetic properties and magnetocaloric effect of Fe-Zr amorphous alloys, in order to find the magnetocaloric materials that have applicability for magnetic refrigeration in room temperature region The research contents of the thesis: - Investigation of fabrication of Fe-(Pr,La,Nd)-Zr, Fe-(Cr,Cu)(Nd,Gd,Co)-Zr possessing large magnetocaloric effect - Studying the relationship between structure, magnetocaloric properties and magnetocaloric effect of alloys - Taking working temperature of the alloys to room temperature range Research Methods: The thesis was carried out by experimental methods The samples were prepared by melt-spinning method Some ribbon samples were annealed to stabilize or create desired structure phases The structure of the ribbons was analyzed by X-ray diffraction (XRD) Magnetic properties of the alloys were investigated by magnetic hysteresis and thermomagnetization measurement Magnetocaloric effect is assessed indirectly through determination of the magnetization versus magnetic field, M(H), at various temperatures Scientific meanings of the thesis: The results of the thesis contribute to the search for magnetocaloric materials that used in magnetic refrigeration technology at room temperature This is an advanced technology that can be applied in practice and has been attracting a lot of scientists The clarification of the relationship between GMCE and structural and magnetic phase transitions in the magnetocaloric materials is also an interesting object for fundamental research The layout of the thesis: The main content of the thesis is presented in four chapters The first chapter is an overview of magnetocaloric effect on rapidly quenched alloys The second chapter displays the experimental techniques for fabrication methods, structural characteristics and magnetic properties of the materials The remaining chapters present the obtained research results of Fe-(Pr,La,Nd)-Zr, Fe(Cr,Cu)-(Nd,Gd,Co)-Zr alloys Main results of the thesis: Successfully synthesized the sample systems: Fe90-xNdxZr10 (x = - 5), Fe90-xPrxZr10 (x = - 3), Fe90-xLaxZr10 (x = - 3), Fe84-xCr2+xB2Co2Zr10 (x = - 5), Fe90-xCoxZr7Cu1B2 (x = - 4), Fe81-xCr4+xB2Nd3Zr10 (x = - 5) Fe82xCr4+xB2Gd2Zr10 (x = - 5) The structure, magnetic properties and magnetocaloric effects of Fe-Zr-based amorphous alloys were systematically investigated Almost the samples have a nearly amorphous structure All the ribbons reveal soft magnetic behavior with low coercive force.The Curie temperature of the Fe-Zr base alloys can be controlled to be near room temperature by changing concentration of Pr, La, Nd, Gd, Co, Cu, Cr and B The quite high maximum magnetic entropy change (Smmax > 1,5 J.kg-1.K-1 with H = 12 kOe), large refrigerant capacity (RC > 130 J.kg-1) and wide working range around room temperature (δTFWHM > 100 K) reveal application potential in magnetic refrigeration technology of this alloy The thesis was carried out at the Key Laboratory for Electronic Materials and Devices, and Laboratory of Magnetism and Superconductivity, Institute of Materials Science, Vietnam Academy of Science and Technology CHAPTER OVERVIEW OF MAGNETOCALORIC EFFECT OF RAPIDLY QUENCHED ALLOYS 1.1 Overview of magnetocaloric effect The magnetocaloric effect is defined as the change in the adiabatic temperature of magnetic materials as the external magnetic field apply on them changes In the case of ferromagnetic materials, they heat-up as magnetized and cool-down as demagnetized Basically, the MCE is directly related to the magnetic entropy change, ∆Sm(T, H), and the adiabatic temperature change, ΔTad(T, H) They are determined by the following equations: H  M (T , H )   dH ∆Sm(T, H) = Sm(T, H) – Sm(T, 0) =   (1.15)  T  H  0  T   M T , H   Tad T , H         dH (1.20) C T , H  T   [ H ] 0  In fact that, if using the magnetic materials for magnetic refrigeration application, another useful parameter is relative cooling power (RC) defined by: H RC = |ΔSM|×δTFWHM, (1.22) where δTFWHM = T2 – T1 is the full-width-at-half maximum of the ΔSm(T) curve and it corresponds to the amount of heat that can be transferred between the cold and hot parts of the refrigerator in an ideal thermodynamic cycle For second-order phase transition ferromagnetic materials, during the transition phase FM-PM, at vicinity of the critical temperature TC, the variations of MS(T) and 0-1(T) versus temperature thus obey the asymptotic relations: MS(T) = M0(−ε), ε < 0; (2.15) 0-1(T) = (H0/M0)ε, ε > 0; (2.16) 1/δ M(TC) = DH , ε = (2.17) where M0, H0 and D are critical amplitudes, and ε = (T - TC)/TC is the reduced temperature Table 1.1 Values of the critical parameters according to the theory Model β γ  Mean field theory 0,5 1,0 3,0 3D Heisenberg 0,365 1,336 4,8 3D Ising 0,325 1,241 4,82 1.2 Overview of magnetocaloric material Magnetocaloric materials have been used and developed since the early 20th century Since then, the study of this material has concentrated on two areas of application The first area studies materials that have high MCE at low temperatures to use in very low temperature techniques The second area studies materials that high MCE around room temperatures to use in chillers Nowadays, a number of magnetic materials having large MCE have been discovered, such as Gd-containing alloys, As-containing alloys, La-Fe-Si alloys, Heusler alloys, Fe and Mn based rapidly quenched alloys, the ferromagnetic perovskite maganites… Although the Fe-Zr based amorphous alloys have the maximum magnetic entropy smaller than that of other GMCE materials, they own a wide working temperature range leading to large RC, which are necessary for practical application CHAPTER EXPERIMENTAL TECHNIQUES 2.1 Fabrication of the samples Alloy ingots with nominal compositions of Fe90-xNdxZr10 (x = - 5), Fe90-xPrxZr10 (x = - 3), Fe90-xLaxZr10 (x = - 3), Fe84-xCr2+xB2Co2Zr10 (x = - 5), Fe90-xCoxZr7Cu1B2 (x = - 4), Fe81-xCr4+xB2Nd3Zr10 (x = - 5) Fe82xCr4+xB2Gd2Zr10 (x = - 5) were prepared from pure (> N) components of Fe, Pr, La, Nd, Gd, Co, Cr, Cu and Zr on an arc-melting furnace A meltspinning method was then used to fabricate the ribbon samples 2.2 Methods of struture analysis, magnetic properties and magnetocaloric effect 2.2.1 Structure analysis by X-ray diffraction Powder X-ray diffraction (XRD) was used to study the structure of samples Through XRD patterns, we cuold determine the structural characteristics of the lattice such as: lattice type, crystalline phase and lattice parameters From the XRD schema, we could also evaluate the amorphous and crystalline crystal fraction of samples 2.2.2 Study of magnetic properties and magnetocaloric effect by hysteresis and thermomagnetic measures The dependences of magnetization on the temperature and magnetic field were investigated by a vibrating sample magnetometer (VSM) and a superconducting quantum interference device (SQUID) The values of magnetic entropy change (Sm) caused by a variation of applied magnetic field was calculated by using the formula:  M  H   MdH  S m   dH  T  0 T  H In order to assess the applicability of the magnetocaloric material, the refrigerant capacity (RC) of the material usually is usually used: RC = |Sm|max  TFWHM where TFWHM is full width at half maximum of entropy change peak CHAPTER STRUCTURE, MAGNETIC PROPERTIES AND MAGNETOCALORIC EFFECT IN Fe-(Pr,Nd,La)-Zr THREECOMPONENTS ALLOYS 3.1 The Fe90-xPrxZr10 alloy system Figure 3.1 shows XRD patterns of Fe90-xPrxZr10 (x = 1, and 3) ribbons We can see that, all the patterns appear an XRD peak corresponding to FeZr2 phase at 2 of 43.2o However, intensity of this XRD peak is low That means volume fraction of the crystalline phase in the ribbons is small Except the XRD peak of the sample with x = 3, which is a litle bit sharp, the other ones are broad, characterizing for nearly-full amouphous structure in the alloy ribbons Cuong (d v t y) * 35 40 * FeZr 45 50 55 2 ) 60 65 70 Figure 3.1 XRD patterns of Fe90-xPrxZr10 (x = 1, and 3) rapidly quenched alloy ribbons The measurements of magnetization versus temperature are carried out and illustrated in Figure 3.2 As seen from the graph, ferromagneticparamagnetic transition (FM-PM) temperature of the alloy ribbons is depended on Pr concentration With x = 3, no magnetic phase transition is observed in the thermomagnetization curves M(T) While, the M(T) curves of samples with x = and demonstrate a quite sharp FM-PM phase transition at 282 K and 302 K, respectively Thus, for the x = sample, the phase transition temperature is in room temperature region M(emu/g) 100 x=1 x=2 x=3 150 200 250 300 350 T(K) 400 Figure 3.2 Thermomagnetization curves of Fe90-xPrxZr10 (x = 1, and 3) alloy ribbons in an applied magnetic field of 100 Oe Temperature dependence of the magnetic entropy change ΔS m(T) in magnetic change ∆H = 4, 6, 8, 10 and 12 kOe are depicted in Figure 3.6 The |ΔSm|max determined for the sample with x = is 0.92 J.kg-1.K-1 at 282 K (with ΔH = 12 kOe) The working temperature range (FWHM), which is defined by full width at half maximum (FWHM) of magnetic entropy change peak, of this ribbon is 69 K As for the sample with x = 2, |ΔSm|max is 0.99 J.kg-1.K-1 at 302 K (with ΔH = 12 kOe), and the working temperature range is 70 K Refrigerant capacity (RC) of the samples, which is defined as product of maximum magnetic entropy change and working temperature range (FWHM), is determined (Table 1) We can realize that, the working temperature of the these alloy ribbons is about 70 K and their refrigerant capacity RC is larger than 64 J/kg at near room temperature with Pr concentration of - 2% 12 kOe kOe 10 kOe 0.8 -1 S | (J Kg K ) -1 0.4 4k Oe 6k Oe 8k Oe 10kOe 12kOe 0.6 0.4 m m -1 0.6 0.2 (a) kOe -1 S | (J Kg K ) 0.8 kOe 0.2 200 250 T(K) 300 350 (b) 200 250 T(K) 300 350 Cuong (d v t y) Figure 3.6 Temperature dependence of magnetic entropy change of Fe90xPrxZr10 alloy ribbons with x = (a) and (b) in various magnetic field change 3.2 The Fe90-xLaxZr10 alloy system Figure 3.2 shows the XRD diffraction pattern of Fe90-xLaxZr10 alloy ribbons at room temperature The results reveal that characteristic of the XRD o -Fe + + Fe2Zr patterns of the samples is quite similar o x=3 All the ribbons have a coexistence amorphous and crystalline phases x=2 Diffraction peaks corresponding to the x=1 crystalline phase of -Fe and Fe2Zr are 20 30 40 50 60  observed in these patterns However, 2 ) these diffraction peaks are very weak Figure 3.2 XRD patterns of That means the alloy ribbons are almost Fe90-xLaxZr10 alloy ribbons amorphous Figure 3.9 shows the reduced thermomagnetization curves in magnetic field of 100 Oe of the Fe90-xLaxZr10 (x = 1, and 3) ribbons The results indicate that La-concentration clearly influences the Curie phase transition 12 -1 200 250 300 T (K) -1 m (c) 200 250 T (K) 300 x=1 x=2 x=3 x=4 x=5 ke -1 m 240 280 320 360 400 (e) T (K) 350 -1 -1 -1 m 150 kOe kOe kOe 10 kOe 12 kOe kOe m 0.6 -1 -1 kOe kOe 10 kOe 12 kOe S | (J Kg K ) -1 -1 200 250 300 350 400 450 (b) T (K) 1.3 kOe kOe kOe 10 kOe 12 kOe 1.5 S | (J Kg K ) 1.2 S | (J Kg K ) m 200 240 280 320 360 400 (a) T (K) (d) kOe kOe kOe 10 kOe 12 kOe S | (J Kg K ) S | (J Kg K ) 1.5 4kOe 6kOe 8kOe 10kOe 12kOe m S | (J Kg-1 K-1) 240 (f) 300 360 T (K) 420 Figure 3.19 Temperature dependence of the magnetic entropy change of Fe90-xNdxZr10 alloy ribbons with x = (a) and (b), (c), (d) and (e) for various magnetic field changes T =262.68 K =0.29269 =0.84197 C C 220 240 260 280 300 T(K) 200 45 S M (emu/g) S 10 T =260.19 K 50 40 150 35 100 30 100 T =280.8 K T =280.82 K =0.39437 =0.88922 20 500 50 C C 25 600 60 50 400 40 300 30 20 10 240 260 280 300 320 340 T(K) 200 T =300.71 K T =303.26 K =0.35394 =0.98826 260 280 600 60 500 10 260 T =307.97 T =306.3 K =0.60911 =1.101 C 280 C 300 320 T(K) 340 100 360 200 50 160 40 120 S M (emu/g) S M (emu/g) 20 200 30 20 10 300 80 T =361.49 K T =360.41 K C C =1.162 320 =1.1329 340 T(K) 360 380 -1 30 340  (Oe.g/emu) 300 -1 40 T (K) 320 240 60  (Oe.g/emu) 400 300 100 280 70 50 C C 40 400 Figure 3.22 Temperature dependence of spontaneous magnetization MS(T) and inverse initial susceptibility χ0-1(T) of the Fe90-xNdxZr10 ribbons with x = (a), x = (b), x = (c), x = (d) and x = (e) -1 20 250  (Oe.g/emu) 30 55 -1 200 300  (Oe.g/emu) 40 -1 300 50  (Oe.g/emu) M (emu/g) 400 60 60 S 500 M (emu/g) 80 70 13 Cuong (d v t y) The values of Ms(T) and -10(T) as functions of temperature, T, are plotted for the Fe90-xNdxZr10 ribbons (Fig 3.22) All the obtained critical parameters were caculated One realizes that the TC values of the alloys obtained from the fittings are mostly equal to those directly determined from the M-T measurements This means that the procedures of deducing and fitting the magnetic data are correct In comparison with some theory, our critical parameters obtained with this method for the Fe90-xNdxZr10 ribbons are close to those of the mean field theory of long-range ferromagnetic order CHAPTER STRUCTURE, MAGNETIC PROPERTIES AND MAGNETOCALORIC EFFECT IN Fe-(Cr,Cu)-(Nd,Gd,Co)-B-Zr FIVE-COMPONENTS ALLOYS 4.1 The Fe81-xCrx+4Nd3B2Zr10 alloy system Figure 4.1 shows the XRD patterns of the Fe81-xCrx+4Nd3B2Zr10 (với x = 1, 2, and 4) alloy ribbons One can see that there is only one lowintensity broad peak near 2 = 43o, indicating the almost amorphous characteristics of this alloys Besides, the obtained peak was (110) broadened with the increase of x=4 Cr-concentration It means that x=3 the ribbon samples are almost x=2 amorphous By qualitative analysis of the crystal phase, x=1 we get the diffraction line at 2θ 20 30 40 60 70 o 50 = 43o which is very similar to 2 ( ) the (110) line of the α-Fe Figure 4.1 XRD patterns of structural phase This may be Fe Cr Nd B Zr (x = 1, 2, and 4) 81-x x+4 10 evidence for the existence of ribbons some nanocrystalline germ with structure close to that of α-Fe on the Fe81-xCrx+4Nd3B2Zr10 amorphous When the concentration of Cr increased, we saw that the intensity of the diffraction line at position 2θ = 43o gradually decreased, showing that the fraction of the crystal phase gradually decreased In other words, the fraction of amorphous phase Fe81-xCrx+4Nd3B2Zr10 gradually increased with increasing Cr concentration 14 100 (a) M (emu/g) M (d v t y) H = 100 Oe x=1 x=2 x=3 x=4 0.5 150 225 x=1 x=2 x=3 x=4 80 60 40 20 T (K) 300 375 150 (b) H = 12 kOe 200 250 300 T (K) 350 400 Figure 4.2 Thermomagnetization curves of Fe81-xCrx+4Nd3B2Zr10 (x = 1, 2, and 4) alloy ribbons at field of 100 Oe (a) and 12 kOe (b) The magnetic phase transition of the Fe81-xCrx+4Nd3B2Zr10 alloy ribbons were investigated through thermomagnetic lines measured in the 100 Oe and 12 kOe magnetic fields shown in Figures 4.2 (a) and (b) The TC values of the samples ranged from 277 to 302,5 K, depending on the Cr concentration In the 12 kOe magnetic field, the M(T) curves show that this material system has a rather high magnetic strength and tends to decrease gradually when the Cr concentration in the sample increases from to 4% Although sample x = has the highest fraction of amorphous phase, the T C of this sample is quite far from room temperature Meanwhile, sample x = with TC is quite suitable for studying MCE at room temperature, but the fraction of crystalline phase in this sample is the largest Moreover, the M(T) curve measured at 100 Oe of this sample shows that in the region above TC, the magnetization of the sample is quite large, indicating that the secondary magnetic phase in this sample has a rather high concentration Therefore, among the fabricated samples, we choose two samples with x = and that have the TC closest to room temperature and high amorphous phase fraction to continue studying MCE Figure 4.4 performs the temperature dependences of -Sm of Fe81xCrx+4Nd3B2Zr10 alloy ribbons for magnetic field changes of 4, 6, 8, 10, and 12 kOe Clearly, ΔSm(T) curves exhibit the maxima corresponding to the FM-PM transition The Smmax values determined for the samples with x = is 0.96 J.kg-1.K-1at 298 K and 0.85 J.kg-1.K-1 at 280 K (ΔH = 12 kOe) with x =3 Therefore, the RC value achieved for both the samples with x = and are as high as 72and 70 J.kg-1, respectively 15 0.9 0.3 (a) -1 x=2 250 300 T (K) 350 kOe kOe kOe kOe 10 kOe 12 kOe 0.6 -1 m m -1 -1 -S (J Kg K ) 0.6 -S (J Kg K ) kOe kOe kOe kOe 10 kOe 12 kOe 0.9 0.3 x=3 400 (b) 250 300 350 400 T (K) Cuong (d.v.t.y) Figure 4.4 Temperature dependence of magnetic entropy change of Fe81-xCrx+4Nd3B2Zr10 alloy ribbons with x = (a) and x = (b) in various magnetic field change 4.2 The Fe82-xCr4+xGd2B2Zr10 alloy system Figure 4.9 shows the XRD patterns of the samples The obtained result shows that the only one broad diffraction peak was obser ved x=4 at the 2θ angle in the range of 40o and 50o, no diffraction peak x=3 corresponding to the crystalline x=2 phase was found Besides, the x=1 obtained peak was broadened with the increase of Cr30 40 50 60 70  2 ) concentration It means that the ribbon samples are almost Figure 4.9 XRD patterns of amorphous Fe82-xCr4+xB2Gd2Zr10 (x = 1, 2, and 4) Figure 4.10 represents the ribbons reduced thermomagnetization curves of the alloy ribbons with an applied magnetic field of 100 Oe We can see that these ribbons have a magnetic phase transition in the range of 250 - 350 K The TC values determined for the samples with x = 1, 2, and are 306, 297, 288 and 282 K, respectively When the concentration of Cr increases from to at %, the TC of the alloy ribbons is reduced (see inset of figure 4.10) Thus, Cr-concentration considerably influences on the TC of Fe82-xCr4+xB2Gd2Zr10 (x = 1, 2, and 4) alloy ribbons 16 1.2 x=1 x=2 x=3 x=4 0.8 0.6 300 TC (K) M(emu/g) 0.4 285 0.2 100 x (%) 200 300 T(K) 400 Figure 4.10 Reduced thermomagnetization curves of Fe82-xCr4+xB2Gd2Zr10 (x = 1, 2, and 4) alloy ribbons in the applied magnetic field of 100 Oe The inset shows the Curie temperature TC vs Cr-concentration of these samples 210 240 270 T (K) 300 330 kOe kOe kOe 10 kOe 240 -1 270 300 T (K) 0.9 12 kOe (d) -1 -1 S | (J Kg K ) 0.6 x=4 -1 x=3 0.3 210 -1 0.3 S | (J Kg K ) 0.3 0.6 (c) m x=2 m x=1 0.6 kOe kOe kOe 10 kOe 12 kOe 0.9 (b) -1 -1 -1 S | (J Kg K ) m -1 -1 S | (J Kg K ) 0.6 kOe kOe kOe 10 kOe 12 kOe 0.9 (a) S | (J Kg K ) kOe kOe kOe 10 kOe 12 kOe 0.9 210 x=1 x=2 x=3 x=4 240 270 T (K) 300 330 (e) 0.6 m m 0.3 330 0.3 ke 210 240 270 T (K) 300 330 250 T(K) 300 350 Figure 4.14 Temperature dependence of magnetic entropy change of Fe82xCr4+xB2Gd2Zr10 alloy ribbons with x = (a), x = (b), x = (c) and x = (d) in various magnetic field changes and magnetic field change of 12 kOe (e) The temperature dependences of the magnetic entropy change of Fe 82xCr4+xB2Gd2Zr10 (x = 1, 2, and 4) alloy ribbons have investigated under magnetic field changes of 4, 6, 8, 10 and 12 kOe The -Sm(T) curves for all the samples are represented in figure 4.14 The variation of the magnetic 17 entropy change for each applied magnetic field change is a function of temperature With increasing of temperature, the - Sm increases and reaches a maximum value |Sm|max around the FM-PM phase transition and then decreases For each sample, the |Sm|max reduces with reducing of the magnetic field change, but the peak temperature of - Sm(T) curves is unchanged due to nature of the second-order phase transition With H = 12 kOe, the Smmax determined for the samples with x = 1, 2, and are 0.88, 0.82, 0.78 and 0.53 J∙kg-1K-1, respectively (table 1) It reveals that when Crconcentration increases, the Smmax value of the alloy reduces The sample with x = has the highest |Sm|max at room temperature 70 30  = 1.004±0.016 300 325 T(K) 350 21 400 (c) T = 287.6 ±0.6 C  = 1.256±0.126 240 260 280 300 320 340 T(K) S 1000 50 800 600 40 S 400 30 T = 282.4±0.2 (d) C 200 -1 S T = 281.8±0.3 350  (Oe.g/emu) 800 325  = 0.394±0.011 -1 35 300 T (K) C 1200 180 C  = 1.139±0.121 275 60  (Oe.g/emu) M (emu/g) 10 C 28 T = 297.7 ±0.2 (b) T = 286.7±0.3  = 0.438±0.014 14 360 20 C 275 42 540 T = 306.8 ±0.1 M (emu/g) 10 100 M (emu/g) S 200 -1 -1 40 720 30  (Oe.g/emu) 300 50  (Oe.g/emu) M (emu/g) C  = 0.458±0.021  = 0.486±0.032 (a) 900 T = 297.2±0.6 C 60 20 40 400 T = 306.3±0.3  = 1.296±0.121 20 240 260 280 300 320 340 T(K) Figure 4.16 Temperature dependence of spontaneous magnetization MS(T) and inverse initial susceptibility χ0-1(T) of the Fe82-xCr4+xB2Gd2Zr10 ribbons with x = (a), x = (b), x = (c) and x = (d) Figure 4.16 shows the temperature dependence of spontaneous magnetization MS(T) and inverse initial susceptibility χ0-1(T) of the Fe82-1 xCr4+xB2Gd2Zr10 ribbons These MS(T) and o (T) data are then fitted to (1.23) and (1.25), respectively, to achieve new ,  and TC our critical parameters obtained for Fe82-xCr4+xB2Gd2Zr10 ribbons are fall between those of mean-field model and 3D Heisenberg model This result indicates that a part of short-range order coexists with long-range order of ferromagnetic 18 interactions in the alloy 4.3 The Fe84-xCr2+xB2Co2Zr10 alloy system Figure 4.18 shows XRD patterns of Fe84-xCr2+xB2Co2Zr10 (x = 1, 2, 3, 4, 5, and 6) alloy ribbons There is a broad peak located at 2θ ≈ 45o on XRD patterns suggesting a coexistence of the amorphous and nanocrystalline phases However, the intensity of this XRD peak is quite low This proves that the volume fraction of the Figure 4.18 XRD patterns of nanocrystalline phase in these Fe Cr B Co Zr (x = 1, 2, 3, 4, 84-x 2+x 2 10 samples is insignificant and 6) ribbons Figure 4.20 represents the reducedthermomagnetization curves of the ribbons at 100 Oe We can see that these ribbons have a magnetic phase transition in a range of 250 350 K It should be noted that Cr-concentration considerably influences on TC value of Fe84-xCr2+xB2Co2Zr10 ribbons, decreasing from 330 K for x = to 290 K for x = Figure 4.20 Thermomagnetization curves of Fe84-xCr2+xB2Co2Zr10 (x = 1, 2, 3, 4, and 6) alloy ribbons in an applied magnetic field of 12 kOe The inset shows the Curie temperature TC vs concentration of Cr of these samples Figure 4.22 performs the temperature dependences of -Sm for the samples with x = and for magnetic field changes of 4, 6, 8, 10, and 12 kOe Clearly, ΔSm(T) curves exhibit the maxima corresponding to the FMPM transition The position of these maxima shifts towards lower 19 temperatures with increasing Cr-concentration As Cr reduces the saturation magnetization of the alloy, a little drop of Smmax is observed The Smmax values determined for the samples with x = and are 0.85 and 0.8 J.kg-1.K1 at room temperature, respectively the working temperature range is found to be quite large, T > 90 K Therefore, the RC value achieved for both the samples with x = and is as high as 81 and 74 J.kg-1, respectively Figure 4.22 ΔSm(T) curves for magnetic field changes of 4, 6, 8, 10 and 12 kOe of Fe84-xCr2+xB2Co2Zr10 with x = (a) and x = (b) Figure 4.24 Temperature dependence of spontaneous magnetization Ms(T) and inverse initial susceptibility -10(T) along with fittings to Arrott-Noakes relations for Fe84-xCr2+xB2Co2Zr10 with x = (a) and x = (b) Figure 4.24 shows the final step of the modified Arrott plots, MS(T) and o-1(T) curves, for samples with x = and In comparison with some theory models, our critical parameters obtained for Fe84-xCr2+xB2Co2Zr10 ribbons are quite close to those of the mean-field theory of the long-range ferromagnetic orders It suggests that the long-range ferromagnetic orders are existence in these samples 20 4.4 The Fe90-xCoxCu1B2Zr7 alloy system Figure 4.26 displays the XRD patterns of Fe90-xCoxCu1B2Zr7 (x = 0, 1, 2, and 4) alloy ribbons measured at room temperature The obtained results show that no obvious crystalline peaks were observed in the XRD patterns It means that all the samples are almost amorphous Figure 4.28 shows the reduced thermomagnetization curves (M/M100 Figure 4.26 XRD patterns of Fe84K) of Fe90-xCoxZr7Cu1B2 (x = 0, 1, 2, 3, and 4) alloys in an applied magnetic xCr2+xB2Co2Zr10 Fe90-xCoxCu1B2Zr7 (x = 0, 1, 2, and 4) field of 100 Oe One can see that the samples undergo a ferromagnetic (FM) - paramagnetic (PM) phase transition in the range of 250 - 400 K The TC values are found to be 242, 262, 296, 320, and 342 K for x = 0, 1, 2, 3, and 4, respectively Thus, when the concentration of Co increases from to at %, the TC of the alloy ribbons can be adjusted in a wide temperature range around 300 K Figure 4.28 Reduced thermomagnetization curves of Fe90-xCoxZr7Cu1B2 (x = 0, 1, 2, and 4) alloy ribbons in the applied magnetic field of 100 Oe (a), and the Curie temperature TC vs Cr-concentration of these samples (b) The insets of Fig 4.28a respectively show the ways to determine the Curie temperatures of the ribbons Figure 4.31 displays the temperature dependences of - Sm for Fe90xCoxZr7Cu1B2 (x = 0, 1, 2, 3, and 4) alloy ribbons in various magnetic field 21 changes of - 12 kOe As a function of temperature, - Sm(T) curves reach a maximum value, Smmax, around the phase transition temperature for each alloy The Smmax increases with increasing the magnetic field change With a magnetic field change of 12 kOe, the Smmax values are found to be 0.70, 0.74, 0.82, 0.84, and 0.86 J·kg-1·K-1 for x = 0, 1, 2, 3, and respectively It reveals that the Smmax of the alloys slightly increases with increasing Coconcentration The Fe86Co4Zr7Cu1B2 alloy ribbon has the highest |Sm|max at room temperature region Figure 4.31 Temperature dependence of magnetic entropy change of Fe90xCoxZr7Cu1B2 alloy ribbons with x = (a), x = (b), x = (c), x = (d), and x = (e) in various magnetic field changes and magnetic field change of 12 kOe (f) The insets show universal master curves of Sm/(Sm)max versus θ in various magnetic field changes 22 =0.545 0.0411 100 TC=296.9K 0.176 (a) 280 290 300 310 T (K) 320 -10 12 -20 TC=297.2K 0.152 -30 =1.109 0.018 16 =0.547 0.005 Ms(T)/(dMs/dT) (g/Oe/emu) Ms (emu/g) 200 20 10 TC=296.6 0.163 300 (T)/(d/dT) TC=296.7 0.142 30 =1.105 0.016 (b) 260 280 300 T (K) χ0-1(T) 320 Figure 4.35 Temperature dependence of Ms(T) and along with fittings to Arrott-Noakes relations (a), and Kouvel-Fisher plots (b) for Fe88Co2Zr7B2Cu1 ribbon With the purpose of determining the critical exponents in the amorphous Fe88Co2Zr7B2Cu1 alloy ribbon, we analyzed the M(H, T) data around its TC through the modified Arrott plot (MAP) method Figure 4.35 showTemperature dependence of MS(T) and χ0-1(T) along with fittings to Arrott-Noakes relations and Kouvel-Fisher plots for Fe88Co2Zr7B2Cu1 ribbon The critical parameters are found to be β = 0.545 ± 0.041, T C = 296.7 K ± 0.142 (from Eq 1.23) and γ = 0.952 ± 0.018, TC = 296.9 K ± 0.176 (from Eq 1.25) These critical parameters are much close to the meanfield theory of long-range ferromagnetic orders 23 CONCLUSION Successfully prepared the alloy systems by melt-spinning method: Fe90-xPrxZr10 (x = 1, and 3), Fe90-xLaxZr10 (x = 1, and 3), Fe90-xNdxZr10 (x = 1, 2, 3, and 5), Fe81-xCr4+xB2Nd3Zr10 (x = 1, 2, 3, and 5), Fe82-xCr4+xB2Gd2Zr10 (x = 1, 2, 3, and 5), Fe84-xCr2+xB2Co2Zr10 (x = 1, 2, 3, and 5), Fe90-xCoxCu1B2 Zr7 (x = 0, 1, 2, and 4) Investigated structure of the fabricated samples The results show that almost of alloy ribbons are amorphous structure Investigated magnetic properties of the obtained samples All the ribbons have a soft magnetic feature with small coercive force, Hc, (Hc < 100 Oe) Some samples have a rather sharp FM-PM transition and this Curie temperature in room temperature region The Fe-(Pr,La,Nd)-Zr three-components alloy ribbons have quite high maximum magnetic entropy change, |∆Sm|max > 1.5 J.Kg-1.K-1 (with H = 12 kOe) has been obtained on the Fe89Nd1Zr10 alloy system The Fe(Cr,Cu)-(Nd,Gd,Co)-B-Zr five-components alloy ribbons give a large refrigerant capacity, the RC > 130 J.kg-1 (with H = 12 kOe) has been obtained on the Fe88Co2Zr7Cu1B2 alloy system These results show that, the high applicability of the alloys to magnetic refrigeration technology The critical parameters of some samples were determined by Modified Arrott Plots (MAP) and Kouvel-Fisher (K-F) method The TC values obtained by using this method are in good agreement with those achieved from data of M(T) curves The critical parameters of almost Fe-Zr based alloy ribbons were determined to be close to those critical parameters of mean field theory of long-range ferromagnetic order From the above results, we see that this subject can be continuously studied in the following directions: - Investigation of electrical and thermal conductivity of alloy tapes with large MCE 24 - Investigation of mechanical properties and corrosion resistance of alloy tapes - Investigate the change of adiabatic temperature ΔTad by direct measurement to evaluate the practical applicability of alloy bands LIST OF REPORTED PUBLICATIONS * List of publications used for the thesis: Hoang Ha Nguyen, Hai Yen Nguyen, Thi Thanh Pham, Mau Lam Nguyen, Chi Linh Dinh, Manh An Nguyen and Huy Dan Nguyen, Magnetic properties and Magnetocaloric Effect of Fe90-xNdxZr10 Rapidly queenched alloys, IEEE Transactions On Magnetics, 54 (2018) 2000904 Nguyen Hai Yen, Nguyen Hoang Ha, Pham Thi Thanh, Tran Dang Thanh, Nguyen Huy Ngoc, Nguyen Huy Dan, Influence of Cr-Addition on Magnetic Properties and Magnetocaloric Effect of Fe-Cr-B-Gd-Zr Rapidly Quenched Alloys, Journal of Electronic Materials, 48 (2019) 7282-7291 Nguyen Hai Yen, Nguyen Hoang Ha, Pham Thi Thanh, Tran Dang Thanh, and Nguyen Huy Dan, Magnetocaloric, and Critical Properties of Fe84-xCr2+xB2Co2Zr10 Melt-Spun Ribbons, Journal of Superconductivity and Novel Magnetism, 33 (2020) 3443-3449 Nguyen Hai Yen, Nguyen Hoang Ha, Pham Thi Thanh, Nguyen Huy Ngoc, Tran Dang Thanh and Nguyen Huy Dan, Influence of Co-doping on magnetic properties and magnetocaloric effect of Fe-Co-Zr-Cu-B melt-spun ribbons, Journal of Nanoscience and Nanotechnology, 21 (2021) 2552-2557 Nguyen Hai Yen, Nguyen Trung Hieu, Nguyen Hoang Ha, Nguyen Mau Lam, Pham Thi Thanh, and Nguyen Huy Dan, Large magnetocaloric effect and critical parameters around room temperature in the Fe79Cr6B2Nd3Zr10 alloy ribbon Journal of Materials Science: Materials in Electronics, 32 (2021) 18862-18872 Kieu Xuan Hau, Nguyen Hoang Ha, Nguyen Le Thi, Nguyen Hai Yen, Pham Thi Thanh, Pham Duc Huyen Yen, Nguyen Huy Ngoc, Tran Dang Thanh, Victor V Koledov, Dong Hyun Kim, Seong-Cho Yu, Nguyen Huy Dan, 25 Magnetocaloric effect and critical behavior in Fe-La-Zr rapidly quenched ribbons, Journal of Science:Advanced Materials and Devices, (2018) 406-411 Nguyen Hoang Ha, Nguyen Hai Yen, Pham Thi Thanh, Nguyen Mau Lam, Nguyen Le Thi, Dinh Chi Linh, Nguyen Manh An and Nguyen Huy Dan, Magnetic properties and Magnetocaloric Effect of Fe90xPrxZr10 Rapidly queenched alloys, Vietnam Journal of Science and Technology, 56 (2018) 59-64 Nguyen Hai Yen, Nguyen Hoang Ha, Pham Thi Thanh, Kieu Xuan Hau, Tran Dang Thanh, Nguyen Huy Dan, Study of critical behavior in Fe88Co2Zr7B2Cu1 alloy ribbons, Dalat University Journal of Science, 11 (2021) 3-12 Nguyen Hai Yen, Nguyen Hoang Ha, Pham Thi Thanh, Nguyen Huy Ngoc, Tran Dang Thanh, Dinh Thi Kim Oanh and Nguyen Huy Dan, Influence of Cr-addition on magnetic properties and magnetocaloric effect of Fe-Cr-Gd-Zr-B rapidly quenched alloys, Proceedings of The 9th International Workshop on Advanced Materials Science and Nanotechnology, 2018, 134-142 10 Nguyễn Hải Yến, Nguyễn Hoàng Hà, Nguyễn Huy Ngọc, Phạm Thị Thanh Nguyễn Huy Dân, Tính chất từ hiệu ứng từ nhiệt hệ hợp kim nguội nhanh Fe81-XCrX+4B2Nd3Zr10, Kỷ yếu hội nghị Vật lý chất rắn khoa học vật liệu toàn quốc lần thứ 11, 2019, 68-72 * List of publications related to the thesis: 11 Nguyễn Huy Dân, Nguyễn Hải Yến, Phạm Thị Thanh, Nguyễn Hữu Đức, Đỗ Trần Hữu, Nguyễn Mạnh An, Nguyễn Hoàng Hà, Nguyễn Lê Thi, Đinh Chí Linh, Phạm Khương Anh, Nguyễn Thị Thanh Huyền, Nghiên cứu hiệu ứng từ nhiệt lớn số hợp kim nguội nhanh, Tạp chí Khoa học Công nghệ, 52 (2014) 1-7 12 Nguyễn Huy Dân, Nguyễn Hải Yến, Phạm Thị Thanh, Nguyễn Hữu Đức, Nguyễn Mạnh An, Nguyễn Lê Thi, Nguyễn Hoàng Hà, Nguyễn Thị Mai, Ảnh hưởng trình xử lý nhiệt lên cấu trúc, tính chất từ hiệu ứng từ nhiệt hợp kim Ni50Mn37Sn13, Tạp chí Khoa học Công nghệ tập, 52 (2014) 84-89 26 13 Nguyễn Hoàng Hà, Nguyễn Mạnh An, Nguyễn Huy Dân, Nguyễn Hải Yến, Phạm Thị Thanh, Đinh Chí Linh, Nguyễn Lê Thi, Tạo pha hiệu ứng từ nhiệt hợp kim nguội nhanh (Pr, Nd)-Fe, Kỷ yếu hội nghị Vật lý chất rắn khoa học vật liệu toàn quốc lần thứ 9, 2015, 28-31 14 Hoang Ha Nguyen, Hai Yen Nguyen, Thi Thanh Pham, Mau Lam Nguyen, Le Thi Nguyen, Manh An Nguyen, Dang Thanh Tran, Xuan Hau Kieu and Huy Dan Nguyen Phase formation, magnetic properties and magnetocaloric effect of (Pr,Nd)-Fe alloys The 8th International Workshop on Advanced Materials Science and Nanotechnology, 2016, 52-56 15 Dan Nguyen, Ha Nguyen, Yen Nguyen, Thanh Pham, Victor Koledov, Alexander Kamantsev, Alexey Mashirov, Thanh Tran, Hau Kieu and Seong Yu, Phase formation and magnetocaloric effect in (Pr,Nd)-Fe alloys prepared by rapidly quenched method, Proceedings of Moscow International Symposium on Magnetism, 2017, 15-20 16 Hai Yen Nguyen, Thi Mai Nguyen, Manh Quang Vu, Thi Thanh Pham, Dang Thanh Tran, Huu Duc Nguyen, Le Thi Nguyen, Hoang Ha Nguyen, Victor Koledov, Alexander Kamantsev, Alexey Mashirov and Huy Dan Nguyen, Influence of Al on structure, magnetic properties and magnetocaloric effect of Ni50Mn37-xAlxSn13 ribbons, Advances in Natural Sciences: Nanoscience and Nanotechnology, (2018) 025007: 1-6