magnetic materials [자성 재료] -...

The Science & Engineering of Materials

Magnetic Materials

[자성 재료]

Chapter 19. Magnetic Materials

1. 오디오와 비디오카세트는 어떤 재료로 만들어지는가? 2. 무엇이 자석의 힘에 영향을 주는가? 3. 연자석과 경자석 재료는 어떻게 구분합니까? 4. 비자성 재료가 존재하는가? 5. 자기장을 가할때 기계적인 변형이 일어나는 재료가 있는가?


Introduction

Magnetic behavior is determined primary by the electronic structure of a material, which provide magnetic dipole.

Interactions between these dipoles determine the type of magnetic.

Magnetic behavior can be controlled by composition, microstructure, and the processing of basic materials.

이 세상의 모든 재료는 magnetic field에 반응을 한다. 자성재료는 electric motors, generators, transformers 등에 사용된다. 또한 computer hard disks, computer disks, video and audio cassettes 등의 data storage technology에 magnetic particles가 사용된다. 이 외에도 loudspeakers, telephones, CD players, telephones, televisions, video recorders 등에도 magnetic materials가 사용된다.


19-1 Classification of Magnetic Materials

Ferromagnetic materials – Materials such as Fe, Ni, Co, and some of their alloy.

Ferrimagnetic materials – Materials include many ceramic materials such as

nickel zinc ferrite and manganese zinc ferrite. Diamagnetic or paramagnetic materials Ferromagnetic and ferrimagnetic materials are usually

further classified as either soft or hard magnetic materials

엄밀한 의미에서 이 세상에 nonmagnetic materials는 없다. 왜냐하면, 모든 재료는 원자로 이루어져 있고, 전자가 핵주변을 공전하기 때문 Magnetic materials 는 대개 아래의 4가지 type으로 구분함:


These two motions (i,e., spin and orbital) contribute to the magnetic behavior of materials.

The magnetic moment of an electron due to its spin is known as the Bohr magnetron (MB).

19-2 Magnetic Dipoles and Magnetic Moments

224

eB mA109.274

m 4πhMagnetronBohr M ⋅×=== −q

h : Planck’s constant, me : mass of electron

The magnetic behavior of materials can be traced to the structure of atoms.

전자의 스핀

전자의 퀘도


Two effect that make most materials in the world not be magnetic – The magnetic moment of atoms(electron spin):

• According to the exclusion principle, two electrons within the same energy level (orbital state) must have opposite spins [Up-, Down -].

• This means their electron-spin-derived magnetic moments are opposite and cancel out.

This is why atoms of most elements do not have a net magnetic moment

– The orbital moments of electrons also cancel each other out. Thus, in a completely filled shell, all electron-spin and orbital moments will cancel out.

• Some elements, such as transition elements (3d, 4d, 5d partially filled), the lanthanides (4f partially filled), and actinides (5f partially filled), have a net magnetic moments since some of their levels have an unpaired electron.


대부분 재료에서 magnetic 특성이 안 나타나고, 특정한 재료에서만 나타나는 이유?



각 원소의 원자 1개에 대한 electronic structure 3차원 solid는 달라짐

1 Bohr magnetron (MB): ??


Fe, Ni and Co, the magnetic moments of the atoms line up in the same directions, and these materials are known as ferromagnetic materials.

Materials(Cr) in which there is a complete cancellation of the magnetic moments of atoms or ions are known as anti-ferromagnetic.[주변의 다른 원자와 상호작용으로 상쇄]

Materials (ferrite)in which magnetic moments of different atoms or ions do not completely cancel out are known as ferrimagnetic materials.


3차원 고체 재료: ferromagnetic materials의 경우 exchange interaction 이 지배, anti-ferromagnetic materials 경우는 unit cell 내에서 up-down cancellation, ferrimagnetic materials 에서는 unit cell에서 불완전한 cancellation 발생함.


19-3 Magnetization, Permeability and the Magnetic Field

lnIH =

n : the number of turns l : length of coil (m) I : current (A)

Magnetic field (H)와 inductance( B) 및 magnetization (M)의 관계

코일내부에 magnetic materials가 존재하면 flux density(inductance) B는 달라짐 Permeability µ(= B/H)가 크다는 것 magnetic flux가 쉽게 전달(작은 전기저항)함을 의미 Magnetization M: represents the increase in the inductance due to the core materials

[자속밀도, 자기유도]

B = H + 4πM [CGS unit]



MμBHμMμ

χ1μHMχ

χlity susceptibi magneticmaterial) core the todue inductance in the increase theon,magnetzati :(M MμHμB

field(H) magnetic applied theof valueon the depends andconstant not is materials neticorferrimag ticferromagne ofty permeabili The

field. magnetic theofeffect theamplifies materials that themeansty permeabili relative largeA μμμ

)ty permeabili relative the:μ field, magnetic e(within th μHB vacuum)ofty permeabili magnetic :(μ HμB

B inductance dendity,flux thel

nIH

0

00

mr

m

m

00

0r

r

00

≅∴>>+=

=

+=

⋅⋅

=

==

=

[카이, 자기장, 자속밀도, 자기유도, 비투자율, 자화, 자화율,..

[각 용어들의 의미를 이해!!]


19-4 Diamagnetic, Paramagnetic, Ferrimagnetic and (Super)paramagnetic Materials

Magnetic field H에 대한 spin의 방향이 중요함!! 4가지 type를 구분 설명

** 기울기와 절대값 크기!!


Diamagnetic Behavior – A magnetic field acting on any atom include a magnetic dipole for the entire atom

by influencing the magnetic moment caused by the orbiting electron. – These dipoles opposed the magnetic field, causing the magnetization to be less than

zero. – Materials such as Cu, Ag, Si, Au and Al are diamagnetic at room temperature.

Paramagnetism – When materials have unpaired electrons, a net magnetic moment due to electron

spin is associated with each atom. – When a magnetic field is applied, the dipoles line up with the filed, causing a

positive magnetization. – However, because the dipoles do not interact, extremely large magnetic filed are

required to align all of the dipoles. – In addition, the effect is lost as soon as the magnetic filed is removed. – This effect, called paramagnetism, is found in metal such as Al, Ti and alloy of Cu.

19-4 Diamagnetic, Paramagnetic, Ferrimagnetic and Superparamagnetic Materials


Ferromagnetism – Ferromagnetism behavior is caused by the unfilled energy

levels in the 3d level of Fe, Ni and Co. – Above the Curie temperature, ferromagnetic materials behave

as paramagnetic materials and their susceptibility Kai is given the following equation known as the Curie-Weiss law

)( cm TT

C−

=χ

Tc : Curie temperature, T : the temperature above Tc



Antiferromagnetism – The magnetic moment produced in neighboring dipoles line

up in opposition to one another in magnetic field, even though the strength of each dipole is very high.

– Ex) Mn, Cr, MnO and NiO



Ferrimagnetism – In ceramic materials, different ions have different magnetic moment. – Most ferrimagnetic materials are ceramics and are good insulators of

electricity. – Electrical loss (known as eddy current losses) are much smaller compared

to those on metallic ferromagnetic materials. – Therefore, ferrites are used in many high-frequency application.

Superparamagnetism

– When the size or grain size of ferromagnetic and ferrimagnetic materials gets below a certain critical size, these materials behave as if they are paramagnetic.

– The magnetic dipole energy of each particle becomes comparable to the thermal energy.

– This small magnetic moment changes its direction randomly (as a result of the thermal energy).

– Thus, the material behaves as if it has no net magnetic moment. – This is known as superparamagnetism.



19-5 Domain Structure and the Hysteresis Loop

• Materials ordinary do not show a net magnetization. This happen because the presence of many domains in the material, arranged so that the net magnetization is zero, minimizes the magneto-static energy.

• Domains are region in the material which all of the dipoles are aligned in a certain direction.

• In a materials that has never been exposed to a magnetic field, the individual domains have a random orientation.

• The net magnetization in the virgin ferromagnetic or ferrimagnetic materials as a whole zero.

• Similar to ferroelectrics application of a magnetic field (poling) will coerce many of the magnetic domains to line up ling with the magnetic field direction.

• Boundaries, called Bloch walls, separate the individual magnetic domain.

• The Bloch walls are narrow zones in which the direction of the magnetic moment gradually and continuously changed from that of one domain to that of next.

• Grain boundary와 domain boundary(전자spin) 의 구분 설명 요망!!



Magnetic Domain: 전자의 스핀방향이 3D로 일정한 영역


Movement of Domain in a magnetic Field – When a magnetic field is imposed on the material, domains that are

nearly lined up with the field grow at the expense of unaligned domains. – In order for the domains to grow, the Bloch walls must move; the field

provides the force required for this movement.



19-5 Domain Structure and the Hysteresis Loop 그림의 작성 원리? M-H curve와 B-H curve 영구자석의 세기?



Effect of Removing the Field – The magnetic field needed to bring the induced

magnetization to zero is the coercivity of the materials. This is a microstructure sensitivity property.

Effect of Reserving the Field – The shaded area shown in figure 19-7(b) is the largest B-H

product and is known as the power of magnetic material. [영구자석의 세기 평가 기준]

– Hard magnet와 soft magnet 구분 Hc



19-6 The Curie Temperature

그림 (a)와 (b)의 의미?


19-6 The Curie Temperature

• When the temperature of a ferromagnetic or ferrimagnetic material is increased, the added thermal energy increases the mobility of the domains, making it easier for them to become aligned, but also preventing them remaining aligned when the field is removed.

• The dipole can still be aligned in magnetic field above the Curie temperature, but they become randomly aligned when the field is removed.

욧점: Thermal energy와 Curie temperature의 관계??

숙제: Example 19-1,2,3 작성후 제출


19-7 Application of Magnetic Materials

(a) Comparison of the hysteresis loops for three application of ferromagnetic and ferrimagnetic materials. (b) Saturation magnetization and coercivity values for different magnetic materials.

• Note that the coercivity is a very strongly microstructure-sensitive property, however, for a material of a given composition, the saturation magnetization is constant.

• Many factors such as the structure of grain boundaries, and the presence of pores or surface layers on particles, affect the coercivity values(Hc).

• The coercivity of single crystal depends strongly on crystallographic direction.

2개의 그림에서 응용처 이해


Soft Magnetic Materials – Ferromagnetic materials are often used to enhance the magnetic flux density

(B) produced when an electric current is passed through the material. – The magnetic field is then expected to do work. Applications include cores

for electromagnets, electric motors, transformers, generators and other electric equipment.

– These materials often have the following characteristics: 1. High-saturation magnetization. 2. High permeability 3. Small coercive field 4. Small remanance(Mr) 5. Small hysteresis loop 6. Rapid response to high-frequency magnetic fields 7. High-electrical resistivity



• High saturation magnetization Ms permits a material to do work, while high permeability permits saturation magnetization to be obtained with small imposed magnetic field.

• A small coercive Hc field also indicates that domains can be reoriented with small magnetic field.

• A small remanance Mr is desired so that almost no magnetization remains when the external field is removed.

• For high-frequency application, materials must permit the dipoles to be aligned at exceptionally rapid rates.

• Eddy current losses are particularly serve when the material operates at high frequency.

• If the electrical resistivity is high, eddy current losses can be held to a minimum. [ Loss = I2 R under the same power transformation]



Data Storage Materials – Memory is stored by magnetizing the material in a certain direction. – Materials with a square hysteresis loop, a low remanance, a low

saturation magnetization and a low coercive field are preferable. – The square loop assures that a bit of information placed I the materials by

a field remains stored; a steep and abrupt change in magnetization is require to remove the information from storage in the ferromagnet.

– Furthermore, the magnetization produced by small external fields keeps the coercive field (Hc), saturation magnetization, and remanance (Br) low.

19-7 Application of Magnetic Materials 각형성!!!



Permanent Magnets – Strong permanent magnets, often called hard magnets, require the following:

1. High remanance (stable domains) 2. High permeability. 3. High coercive field. 4. Large hysteresis loop. 5. High power (or BH product)

– Lifting power(부양력):

2

20 AMF µ

=

영구자석!!


2

20 AMF µ

= A : the area of cross section, M : the magnetization, μo: the magnetic permeability of free space

19-7 Application of Magnetic Materials 영구자석의 세기


19-8 Metallic and Ceramic Magnetic Materials

• Some polymeric materials have shown magnetic activity, however the Curie transition temperature of these materials are too low compared to those for metallic and ceramic magnetic materials.


Magnetic Metals – Pure iron, nickel and cobalt are not usually used for electrical application

because they have high electrical conductivities and relatively large hysteresis loops, leading to excessive power loss.

Iron-Nickel Alloy – Some iron-nickel alloy, such as Permalloy, have high permeabilities,

making them useful as soft magnets.


[CD에서 기록과 재생 원리]

store retrieve write read


Silicon Iron [Fe-3%Si] – We take advantage of the anisotropic magnetic behavior of

silicon iron to obtain the best performance. Composite Magnet

– Composite materials are used to reduce eddy current losses


Easy direction의 의미??


Metallic Glasses [장점은?] – These materials behave as soft magnets with a high-magnetic

permeability; the absence of grain boundaries avoids magnetocrystalline anisotropy and permits easy movement of the domains, while a high electrical resistivity minimizes eddy current losses.

Magnetic Tape [요구조건과 응용사례?] – Magnetic materials for information storage must have a square

loop and a low coercive field, permitting very rapid transition of information.

– Magnetic tape for audio or video applications is produced by evaporating, sputtering or plating particles of a magnetic materials such as γ-Fe2O3 or CrO2 onto a polyester tape.



Complex Metallic Alloy for Permanent Magnet – Improved permanent magnets are produced by making the grain

size so small that only one domain is present in each grain. – Now the boundaries between domains are grain boundaries rather

than Bloch walls. – The domain can change their orientation only by rotating, which

requires greater energy than domain growth.


Single domain particle!!

High Hc

Low Hc


Ferrimagnetics Ceramic Materials


Fe(26) : 3d6 4s2

4s2

25, 24, 23


Magnetostriction [의미와 중요성]

– Certain materials can develop strain when their magnetic state is changed.

– This effect is used in actuators. – The magnetostrictive effect can be seen either by

changing the magnetic field or by changing temperature.



1. 다음의 Example 19-4, 5, 6, 7, 8 작성후 제출하시오. 2. 다음의 용어를 정의하시오. Permeability(µ), Susceptibility(χ), Magnetization(M), Eddy current loss(철손).

숙 제

Chapter 19. Magnetic Materials


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