Ferromagnetism and Antiferromagnetism

Transcription

1 Chapter Twelve Ferromagnetism and Antiferromagnetism In some materials, there are important interactions between magnetic moments resulting in ordered magnetic phases in the absence of H. >> 0 Parallel alignment Ferromagnetism > 0 Anti-parallel Ferrimagnetism 0 Anti-parallel alignment Anti-Ferromagnetism What are the interactions? (1) Dipole-dipole interaction Not Important µ µ produces local field align adjacent () Exchange interaction Q interaction due to overlap of electron wavefunctions (Spin dependent) U J S i S j ij very weak

2 Ferromagnetism T300K s H c H A hysteresis loop of magnetization is present. s : saturation magnetization r (0) residual magnetization H c coercive field (± H c )0 The typical magnetization curve of the ferromagnet

3 s χ χ H ~ 1 T θ Curie-Weiss law Ferromagnet T c High T (T>Tc) : paramagnetic Low T (T<Tc) : ferromagnetic Paramagnet T Curie temperature 1473K 1388K 173K FCC s (emu/g) o 1043K 1073K 67K T(K) 73K

4 Approaching T c (Curie temperature) High T (T>T c ) : paramagnetic Low T (T<T c ) : ferromagnet χ s ( T T ) γ c ( T T) β In mean field approximation : γ1 and β1/. c Experimental data γ β T c (K) Fe Co Ni Gd CrO Cur 3 EuS Ni T c 354 o C

5 How will s vary with temperature? What is T c? Recall Langevin function o where L( a) a coth µ H k T ( a) 1 a in the theory of paramagnetism s / o Weiss suggested that a molecular field H in addition to H is acting on material. : H + H H + γ When H0, a µ H µγ T/T c o a k T k T o o µγ o / o is a linear function of a with a slope proportional to T. k T

6 / o T 3 T aµh/k T The Curie temperature T c : Hence, T 1 P Three straight lines: w/. T 3 >T >T 1 o k T µγ o a The intersection P gives spontaneous fractional magnetization s / o achieved at that temperature. s decreases with increasing T and reaches 0 at T. When T is higher than T, the spontaneous magnetization vanishes implying that T is T c. k T 1 µγ c o Tc µγo 3 3k k T T a a o o 3T intersects w/. L( a) µγ c o determining s / o. at P

7 The law of corresponding states : All ferromagnetic materials, which naturally have different values of o and T c have the same value of s / o for any particular value of T/T c. Very nearly, but not exactly, correct in the calculation is magnetic moment per unit volume, emu/cm 3. n: # of atoms per unit volume, changes with temperature. Exact statement : take [emu/g] [emu/cm 3 ] / ρ[g/cm 3 ] instead of. All materials have the same values of s / o for the same value of T/T c. Μ o k T µγρ o a T 3T c a The Weiss s prediction of a law of corresponding states is verified. s / o ut the shape of the curve is wrong. Weiss-Langevin : J T/T c

8 As discussed in the previous chapter, Ch.11. Q need to be taken into accounted for the source of. o where ( J ) J a' coth J J Jµ H H a' µ H g k T k T J (a' ) The straight line : o k T µ Hγρ The slope of J (a ) at the origin is : Curie temperature Μ o J + 1 J T 3T c T c a' o 1 J a' coth 1 J J a' J ( J 1) µ Hγρ o J + 1 g + 3k J J1/ Μ s o µ γρ 3k s / tanh T/T c in fairly good agreement with experiment o o

9 J1/ implies that the magnetic moment is due entirely to spin, g and there is no orbital contribution. Fe Co Ni Experimental data g* g + Ferromagnetism is due essentially to electron spin Cu nal Ni 78 Fe Ni 79 o 5 Fe with little or no contribution from orbital motion of the electrons. At 0K, the spins on all the atoms are parallel, in one direction. At a higher temperature, a certain fraction of the total, determined by the rillouin function, flip over into the other direction; the value of that fraction determines the value of s. At 0K Reduction of s (T 0) is due to excitation of spin waves. At T 0

10 In the calculation shown above, we only consider the effect of the molecular field and put the applied field H equals to zero. Now we apply a magnetic field H, µ ( H + H ) µ ( H + γρ ) Μ o k T µ Hγρ o k T a' µ Hγρ o a' µ HH k T H H k T k T k T H a' µ Hγρ o γρ o / o (a ) J1/ Curve is the rillouin function for J1/. Lines & 4 represent the molecular field alone. The dashed lines & 4 represent the molecular and a applied field. H γρ o a

11 Above the Curie temperature T>T c For instance, T1.T c, the effect of the applied field move the intersection point of the field line and the magnetization curve from the origin to point. Near the origin Susceptibility Μ o J + 1 a' 3J Μ µ HΜ o( J + 1) / 3k J χ H T µ HγρΜ o( J + 1) / 3k J H Μ o( J + 1) C µ µ ( J 1 ) H γρμ o + and θ ' 3k J θ is the temperature at which susceptibility becomes infinite. 3k J C' T θ T c is the temperature at which the spontaneous magnetization becomes 0. T c µ Hγρ 3k o J + 1 J identical

12 elow the Curie temperature T<T c For instance, T0.5T c, the effect of the applied field shift the intersection point of the field line and the magnetization curve from point P to point P. Since the magnetization curve is nearly flat in this region, the increase in relative magnetization from P to P is very slight. ferromagnetic paramagnetic spin molecular field H : γρ s H γρ s k µ H Tc For Fe, µ o 1.9emu/g o s J1/ / o -16 ( erg/k )( 1043K) ( )( ) ( 18emu/g ) erg/oe 1.9emu/g Oe T(K) Co Ni H H Oe Oe

13 These fields are very much larger than any continuous field yet produced in the laboratory. However, the molecular field is in no sense a real field, but rather a force tending to make adjacent atomic moments parallel to one another. Fe Co Ni Ferromagnetic o [emu/g] µ H [µ ] o μ H N A N : Avogadro s number A : Atomic mass µ erg/oe The huge difference between a ferromagnetic and paramagnetic is due to the degree of alignment achieved and not to any large difference in the size of moment per atom.

14 Exchange energy Play an important part of the total energy of many molecules and of the covalent bond in many solids. Heisenberg showed that it also played a decisive role in ferromagnetism. Ferromagnetism is due essentially to electron spin and reduction of s (T 0) is due to excitation of spin waves. In general, consider N spins U J N i,j 1 S i S j Quantized spin waves : agnons where J is called exchange integral and spin angular momentum of atom i : S i If J is positive, U is a minimum when the spins are parallel (φ ij 0) and a maximum when they are anti-parallel (φ ij π). If J is negative, U is a maximum when the spins are parallel (φ ij 0) and a minimum when they are anti-parallel (φ ij π)

15 As we have already seen, ferromagnetism is due to the alignment of spin moments on adjacent atoms. J>0 for ferromagnetism The ground state of a simple ferromagnet has all spins parallel U J N i 1 S i S i + 1 Treat S i as classical vector What is the energy of the first excited state? OR U U + o 8JS U JNS One particular spin is reversed. we can form an excitation of much lower energy if all spins share the reversal o The elementary excitations of a spin system have a wave like form and called magnons. Analog to lattice vibrations or phonons

16 A spin wave on a line of spins Side view Top view A classical derivation of the magnon dispersion relation ( ) For the p th spin JS S S + p p 1 p+ 1 The magnetic moment at site p : µ p - gµ S p µ p J J Sp 1 + Sp+ 1 µ p gµ gµ ( ) ( ) S + S p 1 p+ 1 Classical mechanics dsp µ p dt p Effective magnetic field or exchange field on the p th spin p.

17 ds dt p J gµ Sp p-1 p+ 1 p p-1 p gµ ( S + S ) J( S S + S S ) x y z In Cartesian coordinate Assuming that S S, S S, and S S ds x p dt ds dt ds dt y p z p J ( y z y z z y z y S ) psp-1 + SpSp+ 1 SpSp-1 SpSp+ 1 J ( z x z x x z x z S ) psp-1 + SpSp+ 1 SpSp-1 SpSp+ 1 J ( x y x y y x y x S S + S S S S S S ) p p-1 A trial solution set S S S p x p y p z p p+ 1 p p-1 S u exp vexp p p+ 1 p << p << p p+ 1 [ i( kpa ωt) ] [ i( kpa ωt) ] JS ( y y y S ) p Sp-1 Sp+ 1 JS ( x x x S S S ) 0 p p-1 p+ 1

18 JS iωu JS iωv For a set of solution 4JS ( ) v 4JS ( )u ( ika ika e e ) v 1 cos( ka) ( ika ika e e ) u 1 cos( ka) 4JS iω ( 1 cos( ka) ) 4JS ( 1 cos( ka) ) iω 0 ω 4JS ω 4JS + ( 1 cos( ka) ) 0 ( 1 cos( ka) ) and u iv S S S x p y p z p u cos u sin S ( kpa ωt) ( kpa ωt) At long wavelength ka<<1 ω 4 JS ( ka) ( ) JSa k

19 Dispersion relation for magnons in a ferromagnet in 1D w/. nearest-neighbor interactions ω/4js ω 4JS ka/π cos( ka) k may be determined accurately by neutron scattering or by spin wave resonance in thin films. At long wave length limit, ω ( ) JSa k Dk Fe Co Ni D mevå T 95K by Neutron scattering

20 agnon spectrum obtained by neutron scattering experiment k n k ' k n k n n k ' n n + ω k

21 Quantization of spin waves k k k 1 n ω ε + In thermal equilibrium the average value of the number of magnons ( ) 1 T k / exp 1 n k k ω Planck distribution The number of magnons from ω to ω+dω ( ) dk k 4 1 d D 3 π π ω ω ( ) ω π ω π π ω π π ω 3/ 3 JSa 4 1 4JSa JSa 1 k JSa k 1 d dk k 4 1 D ( ) k JSa dk d k JSa ω ω ( ) 0 3/ 0 k 1 T k / exp JSa 4 1 d n ) D( d ω ω π ω ω ω Total number of magnons

22 Let x ω k T and dx 0 e x x ( π ) 0 dωd( ω) n k 3/ 3/ 1 k T 4π JSa 3/ k T JS 1 a The number N of atoms per unit volume: Q/a 3 Fractional change of magnetization 3 dω k T exp 0 ω / k T ( ω / k T) 1 ( 4 ) π SC CC FCC 1/a 3 /a 3 4/a 3 (0) n NS k loch T 3/ law k T SQ JS 3/ In agreement with experimental data

23 Ni T c 67K o 510 gauss At T60K ~ 0.1T c o 10 3 loch T 3/ law A(7.5±0.)x10-6 K -3/ Ni o AT 3/ A(3.4±0.)x10-6 K -3/ Fe

24 This application of band theory to magnetic problems was made by Stoner, ott, and Slater in (the collective-electron theory) Why µ H (0K) are., 1.7, and 0.6 µ for Fe, Co, and Ni, respectively? Electron distributions in free atoms K Ca Sc Ti V Cr n Fe Co Ni Cu Zn 3d s d+4s The occupation of energy levels is in accordance with the Pauli exclusion principle. When atoms are brought close together to form a solid, the position of energy levels are profoundly modified. overlapping of electron clouds

25 Splitting of levels When dd o, the 3d levels are spread into a band extending from to C, and the 4s levels are spread into a much wider band extending from A to D. Why does the band spread wider in 4s than 3d? Interatomic distance d A: 4s electrons are farther from the nucleus. An important and difficult problem of the band theory is to calculate the shape of energy bands, i.e., DOS : N(E) for the band.

26 Density of states N(E)dN/dE The states are full of electrons, up to the Fermi energy at 0K. The total number of spins in the subband with spins up is different from that in the subband with spins down. Net magnetization olecular field or exchange interaction The nonintegral values of the magnetic moments are easily explained by the complex shapes of the bands.

27 Ni dn de dn de dn de dn de Co Fe

28 4s and 3d bands in Cu 7.1eV Fermi surface.ev 3.46eV 4s 3d Filled-10electrons 3d 3d 5 electrons 5 electrons 4s and 3d bands in Ni above Tc 0.7 hole 0.54 hole below Tc 0.54 electron 4s 3d 3d 4.73 electron 0.54 electron 3d 4.46 electron 3d 5 electron

29 For Ni, µ0.6µ, Hence, n x n x number of number of number of x0.6 3d + 4s electrons 4s electrons 3d electrons per atom per atom per atom The magnetic moment per atom for this series of transition metal is predicted to increase as number of n-x of 3d electrons increase up to n-x5 and then decrease to zero as number reach 10. At saturation (T0K), the net moment is the difference between spin-up states (n-x5) and the spin-down states, n-x-5: [ 5 ( n x 5) ] µ [ 10 n x] µ µ + [ 10.6 n] µ µ The agreement is generally good for Fe, Co, and Ni.

30 agnetic alloys : Alloys of the transition metals or rare earth metals Chemical valence Z N + N Valence electrons ( ) µ N N µ ( ) agnetization of the atom µ N Z µ For the transition metals, N is determined from the fact that the spin-up d bands lie entirely above or below the Fermi level. agnetic valence Z m N d Z Other contribution from the 4s band (actually 4sp hybrid band) The magnetic moment per atom N µ s 0. 3 ( ) Z m + N s µ ( Z m 0. 6) µ µ + Ex. Fe 0.8 Co 0. alloy : Z m ( ) µ.4µ µ The average magnetic valence concept can then be used to predict the resulting magnetic moment when transition elements are alloyed.

31 The Slater-Pauling curve : The curve of magnetic moment per atom versus electron-to-atom ratio. Assuming 0.3µ for the sp band spin-up electrons. agnetic valence Z m N d+ -Z

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33 and model contribution to magnetic moment per atom Ericksson et al. considered the contributions of the 3d and 4sp hybrid bands and obtained the magnetic moment resulting from the excess holes (N h ) in the spin-up and spin-down 3d bands.. Parameter Fe Co Ni Excess holes (N h ) µ spin 3d µ spin 4sp µ spin total µ orbit 3d µ total Experimental value

34 Criteria for the existence of ferromagnetism in a metal : The electrons responsible must lie in partially filled bands in order that there may be vacant energy levels available for electrons with unpaired spins to move into. Ruling out inner core electrons The density of states in the band must be high, so that the increase in energy caused by spin alignment will be small. Ruling out valence electrons The atoms must be the right distance apart so that exchange force can cause the d-electron spins in one atom to align the spins in the neighboring atom. Only Fe, Co, and Ni of transition metals are ferromagnetic. any of rare earths are ferromagnetic below room temperature due to spin imbalance in their 4f bands.

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36 ethe-slater curve: postulated variation of J with the ratio r a /r 3d. r a :the radius of an atom r 3d : the radius of its 3d shell of electrons Exchange forces are responsible for magnetism, as well for antiferromagnetism, and ferrimagnetism.

37 Antiferromagnetism 0 Spins are ordered in an antiparallel arrangement with zero net moment. elow T N :Néel temperature A small positive susceptibility present at all temperatures but varies in a peculiar way with temperature. a weak cusp at TT N T > T N, Anomalous paramagnetic C χ T + θ like Curie temperature w/. T c -T N Paramagnetic T < T N, anti-ferromagnetic order

38 Some antiferromagnetic materials material T N (K) θ(k) χ(0)/χ(t N ) no FeO CoO NiO Fe O Cr O FeS FeCl 4-48 <0. FeF no α-cr 310 ost are ionic compounds :oxides, sulphides, chlorides, and the like.

39 anganese Oxide no: Ordered arrangements of spins of the n + ions. A A Antiferromagnetic arrangement of A and sublattices in D

40 H H Considering only the nearest neighbor interaction A ma m γ olecular field acts on the ion A due to magnetization of γ Above T N, Therefore, A olecular field acts on the ion due to magnetization of A Μ χ ρ H C T A T C' ρ T C' ρ ( H γ ) ( H γ ) Curie law ( + ) T C' ρ ( H γ ( + )) χ Μ ρh A T C' ρh C' ργ ( T + C' ργ ) C' ρh C' T + C' ργ When a field is applied above T N, each sublattice becomes magnetized in the same direction as the field, but each sublattice set up a molecular field in the opposite direction to the applied field, tending to reduce both A and. Hence, the susceptibility χ is smaller than that of an ideal paramagnetic in which the molecular field is zero. A A

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42 elow T N, Each sublattice spontaneously magnetized, in zero applied field, by the molecular field created by the other sublattice. + A 0 ATN C' ργ A C' ργ TN T At TT N, where H0 The Néel temperature at which χ(t) is the maximum equals to θ. elow T N, each sublattice is spontaneously magnetized to saturation just as a ferromagnetic is. A µ H rillouin function oa N J, k For the spontaneous magnetization in the absence of applied field, H ma γ γ γρ A oa A A µ Hγρ J, k T A T

43 A oa nf T T N o The net spontaneous magnetization is zero below T N. However, an applied field can produce a small magnetization. χ depends on the angle bet. the applied field and its spin axis. H χ A A Spin axis H χ χ

44 Field at right angle to spin axis Assuming that the applied field turn each sublattice magnetization away from spin axis by a small angle α. α A H This rotation immediately creates a magnetization in the direction of H. The spins will rotate until that cancel the applied field H. H ma + H m Hm H m H ma H ma sin α H and γρ A A sin α sin α H γρ Therefore, χ 1 C H γρ θ Independent of temperature a constant

45 Field parallel with spin axis Assuming that the applied field H increase the zero-field value of the A- sublattice magnetization by A and decreases the corresponding value of the -sublattice by. H A A A net magnetization in the direction of H A A + / o o a µ H H/k T (J,a ) A oa nearly linear A a' n g k µ H T a' A oa µ H k T µ H k T ' (J, a ( H γρ ) a ( H γρ ) a A o ') ( H γρ ) ' (J, a ') a A o

46 Therefore, χ 11 H a H a A k n g T + n µ ' (J, a g H µ H o ') γρ' (J, a o ') temperature dependence C It becomes χ θ at TT N. C It reduces to χ at high temperatures. T + θ It approaches to zero as T approaches 0. For a powder specimen, there is no preferred orientation of the crystals. Theoretical calculation for J1 χ p χ 11 1 χ 3 cos 11 θ + χ sin + χ 3 θ

47 Ferrimagnetism Ferrimagnetic materials exhibit the phenomena of magnetic saturation and hysteresis, like ferromagnetics. The most important ferrimagnetic materials are certain double oxides of iron and another metal, called ferrites. The ferrites were developed into commercially useful materials Si 0.03 Fe 0.97 (Silicon ferrite). Until 1948, ferrimagnetics were separated from ferromagnetics. Néel provided the theoretical key to understand the ferrites. They fall mainly into two groups with different crystal structures : Cubic : O Fe O 3 where is a divalent metal ion, n, Ni, Co, Fe, Ironferrite FeO Fe O 3 is the oldest magnetic material to man. Hexagonal : ao 6Fe O 3 arium ferrite

48 Cubic ferrite Hexagonal ferrite metal ion Oxygen ion metal ion Tetrahedral A site Octahedral site arium ferrite

49 Key: Lattice with bases of two magnetic atoms oments anti-align, but do not cancel Eg. agnetite Fe 3 O 4 FeO Fe O 3 parallel Fe + Fe 3+ antiparallel 0 Why is it different from ferromagnetic? How to know this fact? Fe 3+ ions have a spin of 5/ and should contribute 5µ Fe + ions have a spin of and should contribute 4µ expectation Effective moment ( 5+4) µ 14 µ at T0K results It show only 4.1 µ only from Fe + ions confirmed by Neutron diffraction

50 agnetization : complex behaviors FeO Fe O 3 T > T c, paramagnetic H A ρ χ + Let C A and C be Curie constants for ions A and. ( ) ( ) A A A H T H T γ ρ γ ρ C C For non-zero solutions A and At H0 A c C C T ργ ( ) c A A T T C C T C C + ργ χ

51 elow T c O Fe O 3 (emu/g) Cubic ferrite T( o C) / o

52 elow T c 3 Fe 5 O 1 Iron Garnets 3+ is a trivalent metal ion. 3 Fe 3+ ion on tetrahedral site d Fe 3+ ion on octahedral site a 3 3+ ion on site c Y 3+ is a diamagnetic ion At T0K, resulting in 5µ decreases w/. increasing T and reaches to zero at TT c. The compensation temperature at which the magnetization crosses zero. Rare earth 3+ :are paramagnetic and magnetized (c) opposite to the resultant of the Fe 3+ ions (a+d). drops rapidly w/. increasing T Due to weak c-a and c-d coupling. Then passes through zero and increase again due to Fe 3+.

53 T/T c T/T c

54 Neutron magnetic scattering An X-ray photon see the spatial distribution of electronic charge, whether the charge density is magnetized or unmagnetized. A neutron sees two aspects of a crystal: the distribution of nuclei and the distribution of electronic magnetization. Neutron k n k n agnon A neutron can be inelastically scattered by the magnetic structure, With creation or annihilation of a magnon. magnon spectra k n n k ' n n + ω Neutrons are uncharged, easily penetrate electron cloud, and are scattered only by nucleus. If the scattering atom or ion has a net magnetic moment, that moment will interact with neutron beam, because the neutron has a small magnetic moment, ~0.3µ. k

55 Neutrons λ( A) 1.67x10-7 kg 0.8 When E80meV, λ1å E(eV) µ µ

56 ody-centered tetragonal CT structure nf T3K T300K nf T N 67K

57 T80K T N 1K no no T93K The first substance to be clearly recognized as antiferromagnetic in However, the first direct evidence came from Neutron diffraction experiment by Shull and Smart in 1949.

58 agnetic Domains Ferromagnetic materials are not uniformly magnetized, but break up into regions called domains. The magnetization for each domain will, in general, have a different orientation.

59 Origin of domains magnetic, exchange, and anisotropy energies agnetic field energy Introducing more domains E µ o H dv decreasing the magnetic field energy However, it increases the wall energy.

60 a: lattice constant For a 180 o magnetization reversal in one step, an exchange energy per wall area has to be overcome: W ex a JS For a 180 o magnetization reversal in N steps, an exchange energy per wall area is reduced: W ex N a JS π N Domain wall orientations loch Wall Néel Wall

61 Anisotropy Energy The magnetic moments in a magnetic material tend to line up preferentially along certain crystallographic direction. The exchange energy depends on the orbital overlap of electronic wavefunctions between electronic orbits on different sites. With crystal anisotropy the rotation away from the easy axis costs extra energy U K α α + α α + α α + K α α K 1 ( ) α 1 where k 1 and k are the magnetic anisotropy constants

62 N spins in a loch wall with lattice constant a U K KNa N π Wex JS a N The energy per unit area of the wall inimizing σ W, N π σ W Wex + UK JS + a N σ W π JS + Ka 0 N Na KNa N π JS 3 Ka and σ W π KJS a For Iron, σ W 1 erg/cm (180 o wall, N 300)

63 agnetic hysteresis Remanent magnetization Saturation magnetization Saturation magnetization Demagnetized state Coercive field

64 1/χ 1/χ T T Diamagnetism Paramagnetism Ferromagnetism s χ χ H ~ 1 T θ Curie-Weiss law Ferromagnet T c Paramagnet T Curie temperature

65 A A Anti-ferromagnetism T N Néel temperature A s 1/χ A Ferrimagnet Paramagnet T Ferrimagnetism T c Curie temperature

66 agnetic Devices Permanent agnets Transformers agnetic Amplifiers Data Storage

67 Spin Spin + Electronics agneto- Electronics Quantum Electronics Spin Photonics a multidisciplinary field including magnetism, semiconductor physics, optics, mesoscopic physics, superconductivity and new connections to other fields The central theme is how to manipulate the spin degree of freedom which interact with the solid-state environment.

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69 Spin-dependent transport Conventional transistors make use of current or voltage to control the transmitted current. Electronics Nature 404,918 (000) Use the spin configurations to control current Spintronics

70 The most commonly built structures for spin-dependent transport make use of (1) Giant magnetoresistance effect (GR) () Tunneling magnetoresistance effect (TR).N. aibich et al., Phys. Rev. Lett. 61, 47 (1988).

71 Tunneling magnetoresistance effect (TR) Tunneling conductance across the barrier is proportional to the product of density of states on both sides. G N N + N N 1,, 1,, Rparallel < Ranti parallel

72 GanAs/AlAs/GanAs tunneling junction. Tanaka and Y. Higo, Phys. Rev. Lett. 87, 0660 (001).

73 Datta-Das Spin field-effect transistor Datta and Das, Appl. Phys. Lett. 56, 665 (1990). gate How to maintain spin coherence?

74 RA agnetic RA chips use magnetic rather than electrical structures to store information, so they do not need to be constantly powered to retain data, like current RA technologies. much faster and less expensive I RA images I scientists made a string of key discoveries about the "giant magnetoresistive" effect in thin-film structures I and Infineon established a joint RA development program NVE Announces Technology Exchange with Cypress Semiconductor A 18K bit RA chip was introduced, manufactured win 0.18 technology. 004 June - Infineon unveiled a 16 bit prototype based on 0.18 September - RA becomes a standard product in Freescale, which has began sampling R

75 NEC, Toshiba claim agnetorestitive RA could replace flash memory and DRA by as early as 010 One issue involves the size of RA cells, which tend to be bigger than those of other memory types. New Technique could allow them to develop 56 bit RAs by early 006