本页使用了标题或全文手工转换

居里点

维基百科,自由的百科全书
跳到导航 跳到搜索

居里点(Curie point)又作居里温度(Curie temperature,Tc)或磁性转变点。是指磁性材料中自发磁化强度降到零时的温度,是铁磁性亚铁磁性物质转变成顺磁性物质的临界点。低于居里点温度时该物质成为铁磁体,此时和材料有关的磁场很难改变。当温度高于居里点时,该物质成为顺磁体,磁体的磁场很容易随周围磁场的改变而改变。这时的磁敏感度约为10-6。居里点由物质的化学成分和晶体结构决定。

居里點的溫度可以用平均場理論估計。

材料的居里温度[1][2][3]
材料 居里
温度 (K)
(Fe) 1043
(Co) 1400
(Ni) 627
(Gd) 292
(Dy) 88
铋化锰 (MnBi) 630
锑化锰(MnSb) 587
二氧化铬 (CrO2) 386
砷化锰 318
氧化铕 69
氧化铁 (Fe2O3) 948
四氧化三铁 (FeOFe2O3) 858
氧化镍-氧化铁NiO–Fe2O3 858
氧化铜-氧化铁CuO–Fe2O3 728
氧化镁-氧化铁MgO–Fe2O3 713
氧化锰-氧化铁MnO–Fe2O3 573
钇铁石榴石 (Y3Fe5O12) 560
钕磁铁 583~673
铝镍钴合金 973~1133
钐钴磁铁 993~1073
锶铁氧体 723

磁矩[编辑]

磁矩是原子内的永久偶极矩,包含电子的角动量和自旋[4],他们之间的关系是 μl = el/2me, me 是电子质量, μl 是磁矩, l是角动量; 这个比例被称作 gyromagnetic ratio旋磁比).

原子中的电子从它们自己的角动量和它们围绕原子核的轨道动量贡献磁矩。与来自电子的磁矩相比,来自原子核的磁矩是微不足道的。[5] 热作用在更高能量的电子上结果就是扰乱了秩序,并破坏了偶极子之间的对齐。


铁磁性, 顺磁性, 亚铁磁性反铁磁性材料有不同的固有磁矩结构。在材料特定的居里温度下,这些属性会发生变化。从反铁磁性到顺磁性(或反之亦然)的过渡发生在奈尔温度, 这与居里温度类似。

低于TC 高于TC
铁磁性 顺磁性
亚铁磁性 顺磁性
反铁磁性 顺磁性

在居里温度下改变特性的具有磁矩的材料[编辑]

铁磁性,顺磁性,亚铁磁性和反铁磁性结构由固有磁矩组成。 如果结构中的所有电子都配对,则由于它们的相反自旋和角动量,这些力矩会抵消。 因此,即使施加磁场,这些材料也具有不同的性质,并且没有居里温度。[6][7]

顺磁性[编辑]

A material is paramagnetic only above its Curie temperature. Paramagnetic materials are non-magnetic when a 磁场 is absent and magnetic when a magnetic field is applied. When a magnetic field is absent, the material has disordered magnetic moments; that is, the atoms are asymmetrical and not aligned. When a magnetic field is present, the magnetic moments are temporarily realigned parallel to the applied field;[8][9] the atoms are symmetrical and aligned.[10] The magnetic moments being aligned in the same direction are what causes an induced magnetic field.[10][11]

For paramagnetism, this response to an applied magnetic field is positive and is known as magnetic susceptibility.[6] The magnetic susceptibility only applies above the Curie temperature for disordered states.[12]

Sources of paramagnetism (materials which have Curie temperatures) include:[13]

  • All atoms that have unpaired electrons;
  • Atoms that have inner shells that are incomplete in electrons;
  • Free radicals;
  • Metals.

Above the Curie temperature, the atoms are excited, and the spin orientations become randomized[7] but can be realigned by an applied field, i.e., the material becomes paramagnetic. Below the Curie temperature, the intrinsic structure has undergone a phase transition,[14] the atoms are ordered and the material is ferromagnetic.[10] The paramagnetic materials' induced magnetic fields are very weak compared with ferromagnetic materials' magnetic fields.[14]

铁磁性[编辑]

Materials are only ferromagnetic below their corresponding Curie temperatures. Ferromagnetic materials are magnetic in the absence of an applied magnetic field.

When a magnetic field is absent the material has spontaneous magnetization which is a result of the ordered magnetic moments; that is, for ferromagnetism, the atoms are symmetrical and aligned in the same direction creating a permanent magnetic field.

The magnetic interactions are held together by exchange interactions; otherwise thermal disorder would overcome the weak interactions of magnetic moments. The exchange interaction has a zero probability of parallel electrons occupying the same point in time, implying a preferred parallel alignment in the material.[15] The Boltzmann factor contributes heavily as it prefers interacting particles to be aligned in the same direction.[16] This causes ferromagnets to have strong magnetic fields and high Curie temperatures of around 1,000 K(730 °C).[17]

Below the Curie temperature, the atoms are aligned and parallel, causing spontaneous magnetism; the material is ferromagnetic. Above the Curie temperature the material is paramagnetic, as the atoms lose their ordered magnetic moments when the material undergoes a phase transition.[14]

亚铁磁性[编辑]

Materials are only ferrimagnetic below their corresponding Curie temperature. Ferrimagnetic materials are magnetic in the absence of an applied magnetic field and are made up of two different 离子.[18]

When a magnetic field is absent the material has a spontaneous magnetism which is the result of ordered magnetic moments; that is, for ferrimagnetism one ion's magnetic moments are aligned facing in one direction with certain magnitude and the other ion's magnetic moments are aligned facing in the opposite direction with a different magnitude. As the magnetic moments are of different magnitudes in opposite directions there is still a spontaneous magnetism and a magnetic field is present.[18]

Similar to ferromagnetic materials the magnetic interactions are held together by exchange interactions. The orientations of moments however are anti-parallel which results in a net momentum by subtracting their momentum from one another.[18]

Below the Curie temperature the atoms of each ion are aligned anti-parallel with different momentums causing a spontaneous magnetism; the material is ferrimagnetic. Above the Curie temperature the material is paramagnetic as the atoms lose their ordered magnetic moments as the material undergoes a phase transition.[18]

反铁磁性与奈尔温度[编辑]

Materials are only antiferromagetic below their corresponding 奈尔温度. This is similar to the Curie temperature as above the Néel Temperature the material undergoes a 相变 and becomes paramagnetic.

The material has equal magnetic moments aligned in opposite directions resulting in a zero magnetic moment and a net magnetism of zero at all temperatures below the Néel temperature. Antiferromagnetic materials are weakly magnetic in the absence or presence of an applied magnetic field.

Similar to ferromagnetic materials the magnetic interactions are held together by exchange interactions preventing thermal disorder from overcoming the weak interactions of magnetic moments.[15][19] When disorder occurs it is at the Néel temperature.[19]

居里 - 韦斯定律[编辑]

The Curie–Weiss law is an adapted version of 居里定律.

The Curie–Weiss law is a simple model derived from a mean-field approximation, this means it works well for the materials temperature, T, much greater than their corresponding Curie temperature, TC, i.e. TTC; however fails to describe the magnetic susceptibility, χ, in the immediate vicinity of the Curie point because of local fluctuations between atoms.[20]

Neither Curie's law nor the Curie–Weiss law holds for T < TC.

Curie's law for a paramagnetic material:[21]

Definition
χ the magnetic susceptibility; the influence of an applied magnetic field on a material
M the magnetic moments per unit volume
H the macroscopic magnetic field
B the magnetic field
C the material-specific Curie constant
[22]
Avagadro常数
µ0 the permeability of free space. Note: in CGS units is taken to equal one.[23]
g the Landé g-factor
J(J + 1) the eigenvalue for eigenstate J2 for the stationary states within the incomplete atoms shells (electrons unpaired)
µB the Bohr Magneton
kB Boltzmann's constant
total magnetism is N number of magnetic moments per unit volume

The Curie–Weiss law is then derived from Curie's law to be:

where:

λ 是Weiss分子场常数。[22][24]

For full derivation see 居里-韦斯定律.

物理[编辑]

从上方接近居里温度[编辑]

As the Curie–Weiss law is an approximation, a more accurate model is needed when the temperature, T, approaches the material's Curie temperature, TC.

Magnetic susceptibility occurs above the Curie temperature.

An accurate model of critical behaviour for magnetic susceptibility with critical exponent γ:

The critical exponent differs between materials and for the mean-field model is taken as γ = 1.[25]

As temperature is inversely proportional to magnetic susceptibility, when T approaches TC the denominator tends to zero and the magnetic susceptibility approaches infinity allowing magnetism to occur. This is a spontaneous magnetism which is a property of ferromagnetic and ferrimagnetic materials.[26][27]

从下方接近居里温度[编辑]

Magnetism depends on temperature and spontaneous magnetism occurs below the Curie temperature. An accurate model of critical behaviour for spontaneous magnetism with critical exponent β:

The critical exponent differs between materials and for the mean-field model as taken as β = 1/2 where TTC.[25]

The spontaneous magnetism approaches zero as the temperature increases towards the materials Curie temperature.

接近绝对零度(0开尔文)[编辑]

The spontaneous magnetism, occurring in ferromagnetic, ferrimagnetic and antiferromagnetic materials, approaches zero as the temperature increases towards the material's Curie temperature. Spontaneous magnetism is at its maximum as the temperature approaches 0 K.[28] That is, the magnetic moments are completely aligned and at their strongest magnitude of magnetism due to no thermal disturbance.

In paramagnetic materials temperature is sufficient to overcome the ordered alignments. As the temperature approaches 0 K, the decreases to zero, that is, the disorder decreases and becomes ordered. This occurs without the presence of an applied magnetic field and obeys the 热力学第三定律.[15]

Both Curie's law and the Curie–Weiss law fail as the temperature approaches 0 K. This is because they depend on the magnetic susceptibility which only applies when the state is disordered.[29]

硫酸钆 continues to satisfy Curie's law at 1 K. Between 0 and 1 K the law fails to hold and a sudden change in the intrinsic structure occurs at the Curie temperature.[30]

Ising相变模型[编辑]

The Ising model is mathematically based and can analyse the critical points of phase transitions in ferromagnetic order due to spins of electrons having magnitudes of ±1/2. The spins interact with their neighbouring dipole electrons in the structure and here the Ising model can predict their behaviour with each other.[31][32]

This model is important for solving and understanding the concepts of phase transitions and hence solving the Curie temperature. As a result, many different dependencies that affect the Curie temperature can be analysed.

For example, the surface and bulk properties depend on the alignment and magnitude of spins and the Ising model can determine the effects of magnetism in this system.

Weiss磁畴和表面和体积居里温度[编辑]

Materials structures consist of intrinsic magnetic moments which are separated into domains called Weiss domains.[33] This can result in ferromagnetic materials having no spontaneous magnetism as domains could potentially balance each other out.[33] The position of particles can therefore have different orientations around the surface than the main part (bulk) of the material. This property directly affects the Curie temperature as there can be a bulk Curie temperature TB and a different surface Curie temperature TS for a material.[34]

This allows for the surface Curie temperature to be ferromagnetic above the bulk Curie temperature when the main state is disordered, i.e. Ordered and disordered states occur simultaneously.[31]

The surface and bulk properties can be predicted by the Ising model and electron capture spectroscopy can be used to detect the electron spins and hence the magnetic moments on the surface of the material. An average total magnetism is taken from the bulk and surface temperatures to calculate the Curie temperature from the material, noting the bulk contributes more.[31][35]

The angular momentum of an electron is either +ħ/2 or −ħ/2 due to it having a spin of 1/2, which gives a specific size of magnetic moment to the electron; the Bohr magneton.[36] Electrons orbiting around the nucleus in a current loop create a magnetic field which depends on the Bohr Magneton and magnetic quantum number.[36] Therefore, the magnetic moments are related between angular and orbital momentum and affect each other. Angular momentum contributes twice as much to magnetic moments than orbital.[37]

For terbium which is a rare-earth metal and has a high orbital angular momentum the magnetic moment is strong enough to affect the order above its bulk temperatures. It is said to have a high anisotropy on the surface, that is it is highly directed in one orientation. It remains ferromagnetic on its surface above its Curie temperature while its bulk becomes ferrimagnetic and then at higher temperatures its surface remains ferrimagnetic above its bulk Néel Temperature before becoming completely disordered and paramagnetic with increasing temperature. The anisotropy in the bulk is different from its surface anisotropy just above these phase changes as the magnetic moments will be ordered differently or ordered in paramagnetic materials.[34]

更改材料的居里温度[编辑]

Composite materials[编辑]

Composite materials, that is, materials composed from other materials with different properties, can change the Curie temperature. For example, a composite which has silver in it can create spaces for oxygen molecules in bonding which decreases the Curie temperature[38] as the crystal lattice will not be as compact.

The alignment of magnetic moments in the composite material affects the Curie temperature. If the materials moments are parallel with each other the Curie temperature will increase and if perpendicular the Curie temperature will decrease[38] as either more or less thermal energy will be needed to destroy the alignments.

Preparing composite materials through different temperatures can result in different final compositions which will have different Curie temperatures.[39] Doping a material can also affect its Curie temperature.[39]

The density of nanocomposite materials changes the Curie temperature. Nanocomposites are compact structures on a nano-scale. The structure is built up of high and low bulk Curie temperatures, however will only have one mean-field Curie temperature. A higher density of lower bulk temperatures results in a lower mean-field Curie temperature and a higher density of higher bulk temperature significantly increases the mean-field Curie temperature. In more than one dimension the Curie temperature begins to increase as the magnetic moments will need more thermal energy to overcome the ordered structure.[35]

Particle size[编辑]

The size of particles in a material's crystal lattice changes the Curie temperature. Due to the small size of particles (nanoparticles) the fluctuations of electron spins become more prominent, this results in the Curie temperature drastically decreasing when the size of particles decrease as the fluctuations cause disorder. The size of a particle also affects the anisotropy causing alignment to become less stable and thus lead to disorder in magnetic moments.[31][40]

The extreme of this is superparamagnetism which only occurs in small ferromagnetic particles and is where fluctuations are very influential causing magnetic moments to change direction randomly and thus create disorder.

The Curie temperature of nanoparticles are also affected by the crystal lattice structure, body-centred cubic (bcc), face-centred cubic (fcc) and a hexagonal structure (hcp) all have different Curie temperatures due to magnetic moments reacting to their neighbouring electron spins. fcc and hcp have tighter structures and as a results have higher Curie temperatures than bcc as the magnetic moments have stronger effects when closer together.[31] This is known as the coordination number which is the number of nearest neighbouring particles in a structure. This indicates a lower coordination number at the surface of a material than the bulk which leads to the surface becoming less significant when the temperature is approaching the Curie temperature. In smaller systems the coordination number for the surface is more significant and the magnetic moments have a stronger affect on the system.[31]

Although fluctuations in particles can be minuscule, they are heavily dependent on the structure of crystal lattices as they react with their nearest neighbouring particles. Fluctuations are also affected by the exchange interaction[40] as parallel facing magnetic moments are favoured and therefore have less disturbance and disorder, therefore a tighter structure influences a stronger magnetism and therefore a higher Curie temperature.

Pressure[编辑]

Pressure changes a material's Curie temperature. Increasing pressure on the crystal lattice decreases the volume of the system. Pressure directly affects the kinetic energy in particles as movement increases causing the vibrations to disrupt the order of magnetic moments. This is similar to temperature as it also increases the kinetic energy of particles and destroys the order of magnetic moments and magnetism.[41]

Pressure also affects the density of states (DOS).[41] Here the DOS decreases causing the number of electrons available to the system to decrease. This leads to the number of magnetic moments decreasing as they depend on electron spins. It would be expected because of this that the Curie temperature would decrease however it increases. This is the result of the exchange interaction. The exchange interaction favours the aligned parallel magnetic moments due to electrons being unable to occupy the same space in time[15] and as this is increased due to the volume decreasing the Curie temperature increases with pressure. The Curie temperature is made up of a combination of dependencies on kinetic energy and the DOS.[41]

The concentration of particles also affects the Curie temperature when pressure is being applied and can result in a decrease in Curie temperature when the concentration is above a certain percent.[41]

Orbital ordering[编辑]

Orbital ordering changes the Curie temperature of a material. Orbital ordering can be controlled through applied strains.[42] This is a function that determines the wave of a single electron or paired electrons inside the material. Having control over the probability of where the electron will be allows the Curie temperature to be altered. For example, the delocalised electrons can be moved onto the same plane by applied strains within the crystal lattice.[42]

The Curie temperature is seen to increase greatly due to electrons being packed together in the same plane, they are forced to align due to the exchange interaction and thus increases the strength of the magnetic moments which prevents thermal disorder at lower temperatures.

铁电材料中的居里温度[编辑]

In analogy to ferromagnetic and paramagnetic materials, the term Curie temperature (TC) is also applied to the temperature at which a ferroelectric material transitions to being paraelectric. Hence, TC is the temperature where ferroelectric materials lose their spontaneous polarisation as a first or second order phase change occurs. In case of a second order transition the Curie Weiss temperature T0 which defines the maximum of the dielectric constant is equal to the Curie temperature. However, the Curie temperature can be 10 K higher than T0 in case of a first order transition.[43]

Figure 4. (Below T0) Ferroelectric polarisation P in an applied electric field E
Figure 5. (Above T0) Dielectric polarisation P in an applied electric field E
Below TC Above TC[44]
Ferroelectric ↔ Dielectric (paraelectric)
Antiferroelectric ↔ Dielectric (paraelectric)
Ferrielectric ↔ Dielectric (paraelectric)
Helielectric ↔ Dielectric (paraelectric)

Ferroelectric and dielectric[编辑]

Materials are only ferroelectric below their corresponding transition temperature T0.[45] Ferroelectric materials are all pyroelectric and therefore have a spontaneous electric polarisation as the structures are unsymmetrical.

Ferroelectric materials' polarization is subject to hysteresis (Figure 4); that is they are dependent on their past state as well as their current state. As an electric field is applied the dipoles are forced to align and polarisation is created, when the electric field is removed polarisation remains. The hysteresis loop depends on temperature and as a result as the temperature is increased and reaches T0 the two curves become one curve as shown in the dielectric polarisation (Figure 5).[46]

相对介电常数[编辑]

A modified version of the Curie–Weiss law applies to the dielectric constant, also known as the relative permittivity:[43][47]

应用[编辑]

A heat-induced ferromagnetic-paramagnetic transition is used in magneto-optical storage media, for erasing and writing of new data. Famous examples include the Sony Minidisc format, as well as the now-obsolete CD-MO format. Curie point electro-magnets have been proposed and tested for actuation mechanisms in passive safety systems of fast breeder reactors, where control rods are dropped into the reactor core if the actuation mechanism heats up beyond the material's curie point.[48] Other uses include temperature control in soldering irons,[49] and stabilizing the magnetic field of tachometer generators against temperature variation.[50]

参见[编辑]

注释[编辑]

  1. ^ Buschow 2001,p5021, table 1
  2. ^ Jullien & Guinier 1989,第155页
  3. ^ Kittel 1986
  4. ^ Hall & Hook 1994,第200页
  5. ^ Jullien & Guinier 1989,第136–38页
  6. ^ 6.0 6.1 Ibach & Lüth 2009
  7. ^ 7.0 7.1 Levy 1968,第236–39页
  8. ^ Dekker 1958,第217–20页
  9. ^ Levy 1968
  10. ^ 10.0 10.1 10.2 Fan 1987,第164–65页
  11. ^ Dekker 1958,第454–55页
  12. ^ Mendelssohn 1977,第162页
  13. ^ Levy 1968,第198–202页
  14. ^ 14.0 14.1 14.2 Cusack 1958,第269页
  15. ^ 15.0 15.1 15.2 15.3 Hall & Hook 1994,第220–21页
  16. ^ Palmer 2007
  17. ^ Hall & Hook 1994,第220页
  18. ^ 18.0 18.1 18.2 18.3 Jullien & Guinier 1989,第158–59页
  19. ^ 19.0 19.1 Jullien & Guinier 1989,第156–57页
  20. ^ Jullien & Guinier 1989,第153页
  21. ^ Hall & Hook 1994,第205–06页
  22. ^ 22.0 22.1 Levy 1968,第201–02页
  23. ^ Kittel 1996,第444页
  24. ^ Myers 1997,第334–45页
  25. ^ 25.0 25.1 Hall & Hook 1994,第227–28页
  26. ^ Kittel 1986,第424–26页
  27. ^ Spaldin 2010,第52–54页
  28. ^ Hall & Hook 1994,第225页
  29. ^ Mendelssohn 1977,第180–81页
  30. ^ Mendelssohn 1977,第167页
  31. ^ 31.0 31.1 31.2 31.3 31.4 31.5 Bertoldi,Bringa & Miranda(2012)
  32. ^ Brout 1965,第6–7页
  33. ^ 33.0 33.1 Jullien & Guinier 1989,第161页
  34. ^ 34.0 34.1 Rau,Jin & Robert(1988)
  35. ^ 35.0 35.1 Skomski & Sellmyer 2000
  36. ^ 36.0 36.1 Jullien & Guinier 1989,第138页
  37. ^ Hall & Hook 1994
  38. ^ 38.0 38.1 Hwang et al. 1998
  39. ^ 39.0 39.1 Paulsen et al. 2003
  40. ^ 40.0 40.1 López Domínguez et al. 2013
  41. ^ 41.0 41.1 41.2 41.3 Bose et al. 2011
  42. ^ 42.0 42.1 Sadoc et al. 2010
  43. ^ 43.0 43.1 Webster 1999
  44. ^ Kovetz 1990,第116页
  45. ^ Myers 1997,第404–05页
  46. ^ Pascoe 1973,第190–91页
  47. ^ Webster 1999,第6.55–6.56页
  48. ^ Takamatsu. Demonstration of Control Rod Holding Stability of the Self Actuated Shutdown System in Joyo for Enhancement of Fast Reactor Inherent Safety. Journal of Nuclear Science and Technology. 
  49. ^ TMT-9000S
  50. ^ Pallàs-Areny & Webster 2001,第262–63页

参考文献[编辑]