Properties of Materials. Chapter Two Magnetic Properties of Materials

Transcription

1 Properties of Materials Chapter Two Magnetic Properties of Materials

2 Key Magnetic Parameters

3 How does M respond to H? Ferromagnetic Fe, Co, Ni Ferrimagnetic Fe 3 O 4 Antiferromagnetic Paramagnetic Diamagnetic Diamagnetic Linear,χ<0,~ 10-5 ~ 10-6 Paramagnetic Linear, χ>0,~10-3 ~ 10-6 Ferromagnetic nonlinearity, χ>0, χ is the largest Ferrimagnetic nonlinearity, χ>0, χ is relatively large Antiferromagnetic Linear, χ>0, χ is relatively small All materials have response to external magnetic field to some extent.

4 Ferromagnetism M = χ H Possess a permanent magnetic moment in the absence of an external field and manifest very large and permanent magnetizations; Occurs in transition metals iron (as BCC -ferrite), cobalt, nickel, and some rare earth metals such as gadolinium (Gd); χ can be 10 6

5 Origin of Ferromagnetism Permanent magnetic moments results from atomic magnetic moments due to uncancelled electron spins Small contribution from orbital magnetic moment Quantum mechanical exchange interactions favour parallel alignment of moments Alignment within a domain

6 Ferrimagnetism Macroscopically similar to ferromagntism Antiferromagnetic exchange interactions Different sized moments on each sublattice results in net magnetization (incomplete cancellation of spin moments) Example: MFe 2 O 4, magnetite, maghemite

7 Anti-ferromagnetism In some materials, exchange interactions favor antiparallel alignment of atomic magnetic moments Materials are magnetically ordered but have zero remnant magnetization and very low Many metal oxides are antiferromagnetic

8 Anti-ferromagnetic Materials Transition metal compounds, especially oxides: hematite, metals such as chromium, alloys such as iron manganese (FeMn), and oxides such as nickel oxide (NiO). Above the Neel temperature they become paramagnetic Thermal energy can be used to overcome exchange interactions Magnetic order is broken down at the Néel temperature (c.f. Curie temp)

9 Ferro/Ferri/Antiferro-magnetism

10 Spontaneous Magnetization When the two atoms are close to each other, their electrons in 3d layer and 4s layer can exchange positions, and such interaction forces the spin magnetic moments ( Antiferromagnetic S ) in the adjacent atoms to align in order. Iron group Rare earth The exchange interaction energy: E ex 2 2AS1S 2 2AS cos A-Exchange interaction constant Ferromagnetic when A>0,spontaneous magnetization; when A<0,no spontaneous magnetization;

11 Magnetic Domain Theory

12 Magnetic Domain Theory Utotal Uinternal Uwall Uexternal

13 Magnetic Domain Theory Magnetic domain structure in a single crystals of Fe - 3%Si alloy photomicrographs Magnetic domains change shape as a magnetic field (H) is applied. Domains favorably oriented with the field grow at the expense of the unfavorably oriented domains.

14 Magnetic Hysteresis Loop M s, B s

15 Single crystal Magnetic Anisotropy

16 Soft/Hard Magnetic Materials Size and shape of hysteresis loop for ferromagnetic and ferrimagnetic materials are of considerable practical importance. Loop area: energy loss per unit volume per magdemag cycle (heat)

17 Soft Magnetic Materials Used in devices subject to alternating magnetic fields and energy loss must be low, such as transformer Features: easy to be magnetized and demagnetized. Properties: low Bs, Br, Hs, Hc; high μ Application: magnetic conductors, such as: transformer, relay, induction coil, iron core; motor rotor, stator; magnetic circuit connection, magnetic screen, switch, storage element. Material: Industrial pure iron, silicon steel Fe-Ni alloy Soft magnetic ferrite Large Mr and small Hc desirable for transformer and motor cores to minimize energy dissipation with AC fields.

18 Transformer Industrial pure iron/silicon steel

19 Hard Magnetic Material Features: Maintain high magnetization intensity with no external magnetic field. Properties: high B s, B r, H s, H c. Applications:electrical meter, motor, telephone, radio, tape recorder... Materials: Maraging steel; Cast aluminum nickel; Oxide ferrite; Rare earth cobalt, neodymium iron boron Large Mr and Hc desirable for permanent magnets and magnetic recording and memory devices.

20 Application of Hard Magnetic Materials Electrical Motor

21 Magnetic Data Storage Magnetic Tape easy to be magnetized (low H s ) high remanence (high B r ) easy to be demagnetized (low H c )

22 Classification of Magnetic Materials All are technologically important External field H = 0

23 Classification of Magnetic Materials

24 Magnetostrction Magnetostriction -- When a ferromagnet is magnetized, the length changes along the magnetic field direction (elongation or shortening). Magnetostrictive coefficient: l l l 0 0 when λ> 0,Positive magnetostriction(elongation) when λ< 0,Negative magnetostriction(shorten)

25 Magnetostrictive Materials Saturation magnetostriction With the strength of the external magnetic field (H) increasing, the magnetization intensity of ferromagnet (M) also increases, and the absolute value of λ increases. When the magnetization reaches the saturation value M s, λ λ s (saturation magnetostriction coefficient)

26 Magnetoresistance Ordinary Magnetoresistance (OMR) discovered in 1856 by Lord Kelvin Magnetoresistance (MR) is the change of resistance of a conductor in an external magnetic field. In typical metal, at room temperature, OMR effects are very small, at most of the order of a few per cent. In response to the Lorentz force, the carrier velocity v: Corbino disc

27 Direction of force on conduction electrons Magnetic field pointing into page (screen) Current-Carrying Wire Direction of velocity v of electrons Direction of qv of (negative) electrons

28 Giant Magnetoresistance In 1988 two research groups independently discovered materials showing a very large MR, now known as giant magnetoresistance (GMR). Superlattice

29 Magneto-Optic Effect Zeeman Effect by Dutch physicist Pieter Zeeman, is the effect of splitting a spectral line into several components in the presence of a static magnetic field. Applications Since the distance between the Zeeman sub-levels is a function of the magnetic field, this effect can be used to measure the magnetic field, e.g. that of the Sun and other stars or in laboratory plasmas. The Zeeman effect is very important in applications such as nuclear magnetic resonance spectroscopy, electron spin resonance spectroscopy, magnetic resonance imaging (MRI) and Mössbauer spectroscopy.

30 Magneto-Optic Effect Farady Effect Causes a rotation of the plane of polarization which is linearly proportional to the component of the magnetic field in the direction of propagation Caused by left and right circularly polarized waves propagating at slightly different speeds θ F = F L (M/M s ) Typical materials: Y 3 Fe 3 O 12 (YIG); (Yb 0.3 Tb 1.7 Bi 1 )Fe 5 O 12 Applications Needs high optical transmission applications in measuring instruments: measure optical rotatory power and for remote sensing of magnetic fields; amplitude modulation of light, and are the basis of optical isolators and optical circulators (Faraday Rotator).

31 Magneto-Optic Effect Cotton-Mouton Effect: birefringence induced by magnetic field When the applied magnetic field is perpendicular to incident light

32 Magneto-Optic Effect Kerr Effect Light that is reflected from a magnetized surface can change in both polarization and reflected intensity

33 Magneto-Optic Effect Applications Magneto-optical memory

34 Magnetocaloric Effect the heating or cooling (i.e., the temperature change) of a magnetic material due to the application of a magnetic field where T is the temperature, H is the applied magnetic field, C is the heat capacity of the working magnet (refrigerant) and M is the magnetization of the refrigerant How to enhancemagnetocaloric effect? applying a large field using a magnet with a small heat capacity using a magnet with a large change in magnetization vs. temperature, at a constant magnetic field

35 Thermodynamic Cycle Magnetic Refrigeration Adiabatic magnetization Isomagnetic enthalpic transfer Heat removed by liquid or gas Adiabatic demagnetization Isomagnetic entropic transfer In thermal contact with environment to be refrigerated Magnetic refrigeration VS Conventional Carnot refrigeration cycle

36 Magnetic refrigeration adiabatic demagnetization refrigerator Magnetic refrigeration advantages: Safer & Environmental friendly No harmful, ozone-depleting coolant gases Compact, quieter Lower power consumption Higher cooling efficiency Magnetocaloric refrigeration system prototype developed by GE Can reach extremely low T

37 Nanomagnets A nanomagnet is a submicrometric system that prevents spontaneous magnetic order (magnetization) at zero applied magnetic field (remanence). Like the paramagnet, the superparamagnet returns to zero magnetization when the field is removed. It does so for a different reason: small size, not intrinsically weak exchange between the individual moments.

38 Nanomagnets Ferromagnetic Superparamagnetic

39 Size Effects Superparamagnetism

40 Nanomagnets Nano scale has a big impact on the magnetic properties! In a normally ferromagnetic material, nano scale reduces the moment, but it can be restored by applying a magnetic field. The good news: switchable interactions! (medical application) The bad news: There would seem to be a lower limit to the size of a magnetic particle that can hold an alignment for data storage. Beating the superparamagnetic limit by developing synthesis routes for NPs with high anisotropy constants is one way to try to compensate for thermal fluctuations that become dominant at small particle volumes.

41 Ferrofluid: Flowable Magnets In the 1960 s Stephen Pappell at NASA first developed ferrofluids as a method for controlling fluids in space; Colloidal liquids made of nanoscale ferromagnetic, or ferrimagnetic, particles suspended in a carrier fluid; Each tiny particle is thoroughly coated with a surfactant to inhibit clumping; The magnetic attraction of nanoparticles is weak enough that the surfactant's Van der Waals force is sufficient to prevent magnetic clumping or agglomeration

42 Ferrofluid Dispersion stability of ferrofluid Surfactantstabilized nanoparticles Magnetorheological fluid Micro-particles

43 Properties of Ferrofluid Nanomagents Viscosity Magnetic pressure Interfacial control Induced heat convection Normal-field instability

44 Cutting-edge Research Example-1 Switchable Static and Dynamic Self-Assembly of Magnetic Droplets on Superhydrophobic Surfaces Science, 2013, 341,

45 Cutting-edge Research Example-1 Switchable Static and Dynamic Self-Assembly of Magnetic Droplets on Superhydrophobic Surfaces Science, 2013, 341,

46 The key concepts in this chapter Magnetic moment Nanomagnet Permeability Magnetic torque Magnetic field strength Magnetic induction Francis Bitter pattern Magnetization Magnetic medium Magnetic susceptibility Ferromagnetism Ferrimagnetism Antiferromagnetism Paramagnetism Diamagnetism Magnetization curve Hysteresis loop Magnetoresistance Curie s law Curie Weiss law Coercivity Hard magnets Soft magnets Magnetic domain Magnetocrystalline anisotropy Magnetostriction Spontaneous magnetization Ferrofluid

47 Homework Read one paper (related to application of magnetic materials) that s published in Science, Nature, Advanced Materials, Nano Letters or other top English journals. Write one or two paragraphs of your learning after reading the paper.