Lecture 1. Fundamentals of Magnetism

Transkrypt

1 Lecture 1 Fundamentals of Magnetism

2

3

4 Magnetic domain image of iron from Principia Rerum Naturalium (1734) given by E.Swedenborg Magnetized state of Fe Demagnetized state of Fe

5

6

7 Definitions of magnetic fields r Induction: B μ ( H + M ) = 0 External magnetic field: Magnetization r M r H average magnetic moment of magnetic material Susceptibility χ tensor representing anisotropic material B M = χ H ( χ + ) = μ H 1 = μ H 0 where: μ = μ ( 1+ χ ) 0 permability of the material

8

9 Maxwell s equations r r r o B = divb = 0 r r r r H = roth = j r r H o dl = i l r r r r B E = rote = t r r r r E o dl = B o ds t S = φ = U t H = i 2πr [A/m] [oe] [oe] H = in l [A/m]

10 r H d r = NM Demagnetization field when magnetic materials becomes magnetized by application of external magnetic field, it reacts by generating an opposing field. To compute the demagnetization field, the magnetization at all points must be known. ρ m r r = o M dh H = 0.2s / dm = dx r = x + 4πρ dv 2 r dm dy y + dm dz z [emu/cm 4 ] The magnetic field caused by magnetic poles can be obtained from: The fields points radially out from the positive or north poles of long line. The s is the pole strength per unit length [emu/cm 2 ] [oe= emu/cm 3 ]

11

12 Demagnetization field poles density, magnetic charge density o r r B μ M μ 0 0 = o M = ρ m

13 xx yx zx xy yy zy xz yz zz Demagnetization tensor N For ellipsoids, the demagnetization tensor is the same at all the points within the given body. The demagnetizing tensors for three cases are shown below: π 4π / π / π /3 The flat plate has no demagnetization within its x-y plane but shows a 4π demagnetizing factor on magnetization components out of plane. A sphere shows a 4/3 π factor in all directions. A long cylinder has no demagnetization along its axis, but shows 2π in the x and y directions of its cross sections. H total = H S H 0 D 0 2π π H S - the solenoid field

14 (4π)

15

16

17 Electron spin Orbital momentum Magnetic moment of electron L r r r r 2 L = p L = rmv = r mω μ = i S = L ω = 2π T e πr T μ 2 L = 2 eπr ω 2π r μ p r L i μ μ L L L e = 2m eh = l( l 4πm h L = l( l 2π + 1) + 1)

18

19 B Electron Spin The magnetic moment of spining electron is called the Bohr magneton μ eh = 4 π m = emu 3d shells of Fe are unfilled and have uncompensated electron spin magnetic moments when Fe atoms condense to form a solid-state metallic crystal, the electronic distribution (density of states), changes. Whereas the isolated atom has 3d: 5+, 1-; 4s:1+, 1-, in the solid state the distribution becomes 3d: 4.8+, 2.6-; 4s: 0.3+,0.3-. Uncompensated spin magnetic moment of Fe is 2.2 μ B.

20 Electron spin

21 Exchange coupling The saturation of magnetization M S for body-centered cubic Fe crystal can be calculated if lattice constant a=2.86 Å and two iron atoms per unit cell. M 2 2.2μB S T = 0) = = 8 3 ( 1700emu / cm ( ) 3

22

23

24 Magnetyczny (analogowy) zapis dźwięku T. Edison nagranie i odtworzenie dźwięku z woskowego cylindra zapis niemagnetyczny V. Poulsen telegrafon zapis na drucie stalowym (Φ = 1mm) prędkość zapisu 2m/s Prezentacja telegrafonu na Światowej Wystawie w Paryżu. Lata dwudzieste XX wieku -L. Blattner Blattnerphone - zapis na taśmie stalowej (grubość 0.05mm, szer. 3mm) prędkość zapisu 1m/s F. Pflumer zapis na taśmie papierowej pokrytej klejem z opiłkami żelaza. Lata trzydzieste XX wieku -BASF pierwsze taśmy z tworzyw sztucznych pokryte tlenkami żelaza.

25 Historia pierwszy zapis dźwięku 1898 Valdemar Poulsen Nośnik informacji struna fortepianowa (drut stalowy). Zapis magnetyczny Fala dźwiękowa Głowica zapisu Elektromagnes Mikrofon bla bla. la.. 2 m/s Sygnał elektryczny

26 Odczyt informacji (głowica indukcyjna) : Do odczytu informacji wykorzystywane jest zjawisko indukcji magnetycznej generowanie siły elektromotorycznej w obwodzie prądu pod wpływem zmian strumienia magnetycznego przecinania linii sił rozproszonego pola magnetycznego pochodzącego od różnie zorientowanych magnetycznie obszarów. Głowica odczytu Elektromagnes Głośnik Bla bla... 2 m/s

27

28

29 Jak zwiększyć gęstość zapisu informacji? Zastąpić materiał lity (stalowy drut lub taśmę) drobinami materiału ferromagnetycznego naniesionymi na niemagnetyczne podłoże (wpierw papier później tworzywa sztuczne taśmy magnetofonowe).

30 Pierwsze magnetofony firmy AEG prezentowane na światowej wystawie sprzętu radiowego (Berlin 1935). In the early 1930s researchers at the Ludwigshafen works created a sensation with another pioneering invention: the Magnetophon, which had been developed in cooperation with AEG. It was presented at the Berlin Radio Exhibition in 1935.

31 Magnetyczny zapis w informatyce Zapis binarny (0, 1) kierunek namagnesowania / 350 nm szer. -H C 1 M H C H zapis 45 nm długość 20 nm dysk odczyt Gęstość zapisu przy podanych rozmiarach bitów wynosi około 30Gbit/inch 2. 0 Warunek stabilności pole koercji (pole potrzebne do przemagnesowania) materiału ferromagnetycznego H C musi być dostatecznie duże im węższe bity tym większe musi być pole H C Im większe pole H C tym trudniej zapisać informację wymagane są bardzo małe odległości pomiędzy głowicą zapisu i dyskiem (obecnie 0.1μm, większe wartości prądu płynące przez elektromagnes... 1nm = 10-9 m 1nm = 10-6 mm 1nm = mm 1Å = 10-7 mm

32 Dążymy do uzyskania maksymalnej gęstości zapisu!!!!!!! Co oznacza gęsty zapis? Bity obszary o namagnesowaniu / powinny mieć jak najmniejszą długość i szerokość. Szerokość bitu Długość bitu Co ogranicza gęstość zapisu? Nośnik informacji materiał magnetyczny Zapis informacji głowica zapisu Odczyt informacji głowica odczytu

33 History of HDD 1956 HDD of IBM, random access method of accounting and control (RAMAC) 1980 induction thin film head 1990 write induction coil, read AMR sensor 1996 GMR sensor

34 HDD for 50 years and now First Hard Disk Drive with 24" Diameter Disks Compared with Modern 2.5" HDD. The first HDD was introduced in 1956 with 50 disks of 24" diameter holding a total of 4.4 Mbytes of data. The purchase price of this HDD was $10,000,000 per Gbyte. For comparison in the foreground a modern HDD is shown holding 160 Gbyte of data on two 2.5" diameter disks at a purchase price of less than $1 per Gbyte.

35 Dimensions scaling

36

37 Areal data storage density vs. time for inductive and MR read heads

38 Disc drive The slider carrying the magnetic write/read head. The slider is mounted on the end of head gimbal assembly (HGA) The magnetic disks (up to 10) in diameter inches RPM it is related to about 100 km/h The air-bearing surface (ABS) allowing the head to fly at a distance above the medium about 10 nm

39 Thin film disks Substrate Al Mg (or glass) + electroplated Ni 80 P 20 (T c <T room ). NiP undercoat layer make disk hard and smooth. Cr underlayer is used to control microstructure and magnetic properties the main magnetic recording layer of CoPtCr doped with B. The magnetic layer is covered by a carbon overcoat layer and lubricant. The last two layers are necessary for the tribological performance of the head-disk interface and for the protection of the magnetic layer.

40 Disk layer structure

41 Evolution of bit size

42 Macroscopic properties of the disk For high density recording the macroscopic properties such as coercivity (H c ), remanence (M r ), coercive squerness (S * ) and remanence squerness (S) determine read-back signal variables such as pulse shape, amplitude and resolution arising from magnetic transitions. 2 M ( x) = M r arctan π The parameter (f ) is called as transition slope parameter. In an ideal situation, in the absence of demagnetizing fields (arising from adjacent bit cells), the transition would be abrupt. Assuming that the contributions to the transition parameter from characteristics of the write head are negligible and that the recording medium has a coercive squerness S * = 1 a simplified relation can be derived from the Williams Comstock model: f M r x f ( ) d + δ To minimize the transition slope f, Eq.5 indicates that this can be achieved by: decreasing the magnetic spacing d and thickness of recording medium δ, increasing the H c of the medium, smaller M r reduce the read signal detected by AMR or GMR. = δ πh c 2 (5) (4)

43 Tradycyjne media jeden bit złożony z 1000 ziaren

44 Grains in bit cell

45 Microscopic properties Coercivity H c - control and modification: magnetocrystalline anisotropy (grain shape anisotropy), selection of alloying elements (Al, Cr, Pt, Ta, B,...) determination of influence: deposition conditions and parameters: substrate temperature, bias voltage, sputtering power (deposition rate), sputtering gas pressure (Ar) microstructure: film stresses, grain size, texture (grain orientation), grain boundaries, crystal defects. If the grain structure is noticably voided, leading to reduced magnetic intractions and lower transition noise.

46 Superparamagnetic effect The superparamagnetic effect originates from the shrinking volume of magnetic grains that compose the storage properties of hard disk media. The magnetic grains represent the data bits that are stored as alternating magnetic orientations. To increase data-storage densities while maintaining acceptable performance, designers have shrunk the media's grain diameters and decreased the thickness of the media. The resulting smaller grain volume makes them increasingly susceptible to thermal fluctuations, which decreases the signal sensed by the drive's read/write head. If the signal reduction is great enough, data could be lost over time to this superparamagnetic effect. TEM of the grain structures in magnetic media. (magnification = 1 million) The pictures are transmission electron micrographs (TEM) of two different disk media which illustrates how the grain structure has evolved over time. The TEM on the left is a magnetic media that supports a data density of about 10 gigabits/inch2 with an average grain diameter of about 13 nanometers. The magnetic media on the right supports a data density of 25 gigabit/inch2 with an average grain diameter of about 8.5 nanometers. Historically, disk drive designers have had only two ways to maintain thermal stability as the media's grain volume decreases with increasing areal density: 1) Improve the signal processing and error-correction codes (ECC) so fewer grains are needed per data bit, and 2) develop new magnetic materials that resist more strongly any change to their magnetization, known technically as higher coercivity. The later is complicated by the laws of physics, as higher coercivity alloys are more difficult to write on. While improvements in coding and ECC are ongoing, Hitachi GST's discovery of AFC media is a major advancement because it allows disk-drive designers to write at very high areal densities on a surface that offers greater stability than conventional media.

47 Nośnik informacji ferromagnetyczna cienka warstwa (z anizotropią w płaszczyźnie warstwy) o granularnej (ziarnistej) strukturze. Takie dyski są obecnie powszechnie stosowane. 1 bit (obszar, w którym ziarna mają określony kierunek magnetyzacji) składa się z około 10 3 ziaren. Rozmycie granicy pomiędzy bitami nie może być duże. Tak więc wzrost gęstości zapisu można uzyskać poprzez zmniejszenie rozmiaru ziaren. Czy można bezkarnie zmniejszać rozmiar cząstek? Dla małych cząstek (d<3nm w temperaturze pokojowej) kierunek namagnesowania cząstki ferromagnetycznej w wyniku wzbudzeń termicznych fluktuuje (zmienia kierunek) cząstka superparamagnetyczna. Oznacza to utratę zapisanej informacji (limit superparamagnetyczny).

48 Signal to noise (S/N) ratio Highly intergranular coupled magnetic thin films with long correlation lengths tends to form zizag domain walls in recorded transitions (bits) which results in noises. Threrefore the reduction of intergranular exchange coupling becomes important. There are three major approaches to noise reduction: physical grain segregation, compositional segregation, Small grains with very narrow size distributions. For example higher Cr content and higher sputter temperatures leads to Cr segregation to grain boundaries, forming non-magnetic phases in CoPtCrTa and CoPtCrTB. The low solubility of B in the cobalt alloys leads to compositional segregation. The ideal thin-recording-magnetic layer should be composed of grains with high-anisotropy, which are smaller than the recording bit cell, uniform in size and magnetically isolated.

49 Thermal stability For high density recording the grains are small in comaprison to the bit cell. In a simplified model, assuming isolated grains, the thermally induced switching of magnetization has to overcome an energy barier. The switching probability f is given by an Arrhenius equation: Δ W f = f 0 exp where ΔW = K u V (6) kt ΔW is energy barier, K u is the uniaxial anisotropy constant, V is grain volume. If the grains become very small, the magnetization switch very easily which leads to superparamagnetic efect. Estimation of minimum grain size (example): K u = J/m 3. Bit stored 10 years at room temperature (f< Hz at T=300 K), than diameter of spherical grain is 9 nm.

50 Granular media vs. patterned media

51 Self organized particle media Chemical method for preparation FePt nanoparticles TEM micrograph of self-assembled FePt nanoparticles (size 3 to 10nm)

52 Write/read head of HDD

53 Inductive write head The yoke consists of structured Ni 81 Fe 19 (permalloy) films P 1 and P 2.These films are all deposited on the top of substrate which consists of insulators (Al 2 O 3 and TiC). The gap width is defined by the thickness of Al 2 O 3 insulation layer between P 1 and P 2 hich is below 100 nm. Micrograph of the write/read head taken by SEM from the ABS side.

54

55

56 Aim for application Magnetic properties optimization of ML (Fe 97 Al 3 ) 85 N 15 /Al 2 O 3 for shields and poles of HDD heads

57 SEM cross section of the head

58 Schematic representation of a longitudinal recording process Magnetic force micrograph (MFM)of recorded bit patterns. Track width is 350 nm recorded in antiferromagnetic coupled layers (AFC media)

59

60

61

62

63

64

65

66