VIETNAM NATIONAL UNIVERSITY, HANOI VNU UNIVERSITY OF SCIENCE FACULTY OF PHYSICS Vu Thi Huong THE INFLUENCE OF COBALT ON CRYSTAL STRUCTURE AND MAGNETIC PROPERTIES OF NiMnSbB2 HEUSLER ALLOYS Submitted in partial fulfillment of the requirements for the degree of Bachelor of Science in International Physics ( International Standard Program) Supervisor: Prof.Dr Luu Tuan Tai Hanoi - 2016 Thesis Vu Thi Huong ACKNOWLEDMENT First, I would like to say my sincere thanks to our teachers in the Department of Low Temperature and Faculty of Physics, Hanoi university of science, Vietnam National university, who have equipped me with valuable knowledge in the past and help me a lots to have the best result I would like to express my sincere thanks to Professor Luu Tuan Tai who has enthusiastically instructed me to complete the thesis After all, I would like to send many thanks to my friends, my colleges and my closed people who did not hesitate to spend time on support, give comments and help me during my study Hanoi, May 25th,2016 Huong Vu Thi Huong Thesis Vu Thi Huong List of Figures Figure 1.1 – Schematic picture that shows the two basic processes of the magnetocaloric effect when a magnetic field is applied or removed in a magnetic system: the isothermal process, which lead to an entropy change, and the adiabatic process, which yields a variation in temperature Figure 1.2 Theoretical molar magnetic entropy, SM, (left scale) and effective magnetic moment of free R3+ ions, peff, (right scale) of the lanthanide elements Figure 1.3 The isothermal magnetization curves at different temperatures of a material with magnetic refrigeration 10 Figure 1.4 Analogy between magnetic refrigeration and vapor cycle or conventional refrigeration H = externally applied magnetic field; Q = heat quantity; P = pressure; ΔTad = adiabatic temperature variation 11 Figure 1.5 Crystalline structure of Perovskite familiar ABO3 14 Figure 1.6 Comparing the crystalline structure of Perovskite with another group 15 Figure 1.7 SEM image (a) and magnetic cooling curve(b) when magnetic field is applied (FC) and there isn’t magnetic applied field (ZFC) of La0,2Ca0,8MnO3 17 Figure 1.8 Variation of entropy following the changing of temperature of Ga5Si2Ge2 19 Figure 1.9 The dependence of magnetization M on temperature of I1 system and the variation of entropy for both systems I1, I2 21 Figure 1.10 Five closed order structures following Berna model 22 Figure 1.11 Radial distribution function of liquid sodium (a) crystals sodium (b) and density function (c) 23 Figure 2.1 Acr meting system 28 Figure 2.2 Diagram describing operation principles X-ray diffraction method 30 Figure 2.3 X-ray measurement’s system at the Faculty of Physics, University of Sciences, VNU 31 Figure 3.1 X-ray diffraction of NiMnSbB2 at 291K 33 Thesis Vu Thi Huong Figure 3.2 X-ray diffraction of NiMnSbCo1B2 at 77K 35 Figure 3.3 X-ray diffraction of NiMnSbCo1B2 at 300K 34 Figure 3.4 The temperature dependence of magnetization under a magnetic field of 0.01 T (ZFC and FC) for the NiMnSbB2 sample 36 Figure 3.5 The temperature dependence of magnetization under a magnetic field of 0.01 T (ZFC and FC) for the NiMnSbCo1B2 sample 38 Figure 3.6 The temperature dependence of entropy’s variation under magnetic fields of 0.3 T, 0.5T and 0.8T of NiMnSbB2 39 Figure 3.7 The temperature dependence of entropy’s variation under magnetic fields of 0.3 T, 0.5T and 0.8T of NiMnSbCo1B2 40 Thesis Vu Thi Huong Contents INTRODUCTION CHAPTER I: OVERVIEW 1.1 Magnetocaloric effect 1.1.1 Concept of magnetocaloric effect 1.2.2 Magnetocaloric mechanism 1.1.3 Measurement of the magnetocaloric effect 1.1.4 Application of the magnetocaloric effect 10 1.2 Magnetocaloric materials 12 1.2.1 Some types of magnetocaloric materials 13 1.2.2 Applications of magnetocaloric materials 25 CHAPTER II: THE EXPERIMENTAL METHOD 28 2.1 Sample preparation using arc melting method 28 2.1.1 Weighting out samples 28 2.1.2 Melting samples 28 2.2 Measurements 29 2.2.1 Characteristic crystals measure 29 2.2 Magnetic characteristic and magnetic refrigeration measurement 32 CHAPTER III: RESULTS AND DISCUSSION 33 3.1 Crystalline structure of NiMnSbB2 and NiMnSbCo1B2 33 3.2 Magnetic properties of NiMnSbB2 and NiMnSbCoB2 alloys 35 CONCLUSION 40 Thesis Vu Thi Huong INTRODUCTION Although the magnetocaloric effect [4], which displays itself in the changing of a magnetic material temperature under magnetization in adiabatic conditions, was discovered a long time ago, in recent years it has attracted the attention of investigators and the amount of papers in this direction increases practically exponentially First of all this is related to practical application of the MCE and magnetic materials in refrigeration devices and, especially, in magnetic refrigerators, which work on magnetic refrigeration cycles instead of conventional vapour-gas cycles Recently a series of acting magnetic refrigerator prototypes have been developed and created The particular importance is that the created prototypes work at room temperature and have significant potential to be incorporated into the marketplace Magnetic refrigerators are characterized by compactness, high effectiveness, low energy consumption and environmental safety The further development of such devices is related to progress in permanent magnets, which can replace such cumbersome sources of magnetic fields requiring liquid helium superconducting magnets It is expected that in the near future the energy product of permanent magnets will at least double We suggest that the first commercially available models of magnetic refrigerators will appear rather sooner in spite of essential competition which they will have to withstand from existing vapour–gas refrigeration technology [4] One of the most important parts of the magnetic refrigerator is its working body- a magnetic material- which should have high magnetocaloric properties, in particular, high MCE value The working body in ix many respects determines the characteristics of the whole refrigeration device At the present time the energies of many scientific groups working in the field of MCE are directed on searching for the most effective magnetic working body This is the field where the scientific interests of investigators intersect with practical applications In recent years, scientists have discovered giant magnetic refrigeration[7] (giant MCE) in the ambient temperature above room temperature material systems Gd5Si2Ge2, this material system opens up Thesis Vu Thi Huong possibilities for applications in set cooled at room temperature with environmental friendly technologies Magnetic refrigeration is near the critical state of magnetic transition Transition from second grade showed clear peaks of MCE and small MCE effects Transition from a point out of the top categories MCE sharp and big MCE effects The scientists also discovered the coexistence of Gd5Si1.7Ge2.3’s structure, phase and temperature of transition (239 K) Transfer in this phase has the phase transition temperature of a kind with a delay of about 7.4 K, in about 1T The magnetic material is magnetic refrigeration is quite common, these are pretty good material for applications but the components Gd has a very high cost MnFeP 1-xAsx system (0.25 [...]... Because of the flexibility of bond angles inherent in the perovskite structure there are many different types of distortions which can occur from the ideal structure These include tilting of the octahedra, displacements of the cations out of the centers of their 17 Thesis Vu Thi Huong coordination polyhedra, and distortions of the octahedra driven by electronic factors (Jahn-Teller distortions) 1.2.1.2... crystals measure To determine the formation and transformation of crystalline phase materials we use the X-ray diffraction General principles of X-ray diffraction method of determining the crystal structure bases on the different effects of crystal size to diffraction In each crystalline, the position of the atoms are arranged in the Bragg plane For Bragg plane, the X-ray obeys the law of reflection... be closer than in the bulk of the alloy and, for non-stoichiometric alloys with an excess of copper (e.g Cu2.2MnAl0.8), an antiferromagnetic layer forms on every thermal APB These antiferromagnetic layers completely supersede the normal magnetic domain structure and stay with the APBs if they are grown by annealing the alloy This significantly modifies the magnetic properties of the nonstoichiometric... of the maximum magnetocaloric effect in a properly designed active magnetic regenerator cycle If both the magnetization and entropy are continuous functions of the temperature and magnetic field, then the infinitesimal isobaric-isothermal 6 Thesis Vu Thi Huong magnetic entropy change can be related to the magnetization (M), the magnetic field strength (H ), and the absolute temperature (T ) using one... Figure 1.5 Crystalline structure of Perovskite familiar ABO3 14 Thesis Vu Thi Huong Figure 1.6 Comparing the crystalline structure of Perovskite with another group b Material’s properties Perovskites have a cubic structure with general formula of ABO3 In this structure, an A-site ion, on the corners of the lattice, is usually an alkaline earth or rare earth element B site ions, on the center of the lattice,... neighboring magnetic ions The latter are usually manganese ions, which sit at the body centers of the cubic structure and carry most of the magnetic moment of the alloy (See the BetheSlater curve for more info on why this happens.) The term is named after a German mining engineer and chemist Friedrich Heusler, who studied such an alloy in 1903 It contained two parts copper, one part manganese, and one part... wavelength of X-rays n is the degree of reflection Set of Bragg diffraction peaks under different angles 2 can be recorded using the detector Figure 2.2 Diagram describing operation principles X-ray diffraction method And we will observe the maximum peak value at 2 Based on the position of the maximum peak at diffraction plane (hkl) we will determine the size of the crystalline and the lattice constant... threefold the strength of conventional steel alloys have been produced 23 Thesis Vu Thi Huong 1.2.1.4 Magnetocaloric materials based on Heusler alloys NiMnX (X=Ga, Sn, Sb, In) A Heusler alloy is a ferromagnetic metal alloy based on a Heusler phase Heusler phases are intermetallics with particular composition and facecentered cubic crystal structure [8] They are ferromagnetic as a result of the double-exchange... lower In England there was a company called Cambridge Magnetic Refrigeration design cooling systems use magnetic refrigeration and material from the heat On 20 - 8-2007 National Laboratory at the Technical University of Denmark claims reached a milestone in the study of magnetic refrigeration They hope to research commercial applications of this in the coming years Some projected applications of magnetocaloric... T1) to the hot end (at T2) of the refrigerator in one ideal thermodynamic cycle The second is an adiabatic process that occurs when the magnetic field is modified but the material is isolated from the surroundings and, therefore, the total entropy of a solid remains constant The temperature of a magnetic material is then changed by ∆Tad(T)∆H = (T(S)Hf – T(S)Hi)S (1.3) and ∆Tad(T )∆H is conventionally