Giant magnetoresistance - Sideversjonshistorikk

Gatti: /* GMR-based hard-drives */

2009-03-27T16:04:33Z

GMR-based hard-drives

← Eldre sideversjon		Sideversjonen fra 27. mar. 2009 kl. 16:04
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	If the nonmagnetic layer gets too thin, pinholes will cause a strong ferromagnetic coupling of the magnetic layers, which reduces the GMR effect dramatically. Normally, therefore, the nonmagnetic layer must be at least 15Å thick. <ref name="Dieny"> B. Dieny, Journal of Magnetism and Magnetic Materials 136 (1994) 335-359</ref>		If the nonmagnetic layer gets too thin, pinholes will cause a strong ferromagnetic coupling of the magnetic layers, which reduces the GMR effect dramatically. Normally, therefore, the nonmagnetic layer must be at least 15Å thick. <ref name="Dieny"> B. Dieny, Journal of Magnetism and Magnetic Materials 136 (1994) 335-359</ref>

	== GMR-based hard-drives ==		== GMR-based hard drives ==

Gatti: /* GMR in hard-drives */

2009-03-27T16:04:13Z

GMR in hard-drives

← Eldre sideversjon		Sideversjonen fra 27. mar. 2009 kl. 16:04
Linje 78:		Linje 78:
	If the nonmagnetic layer gets too thin, pinholes will cause a strong ferromagnetic coupling of the magnetic layers, which reduces the GMR effect dramatically. Normally, therefore, the nonmagnetic layer must be at least 15Å thick. <ref name="Dieny"> B. Dieny, Journal of Magnetism and Magnetic Materials 136 (1994) 335-359</ref>		If the nonmagnetic layer gets too thin, pinholes will cause a strong ferromagnetic coupling of the magnetic layers, which reduces the GMR effect dramatically. Normally, therefore, the nonmagnetic layer must be at least 15Å thick. <ref name="Dieny"> B. Dieny, Journal of Magnetism and Magnetic Materials 136 (1994) 335-359</ref>

	== GMR in hard-drives ==		== GMR-based hard-drives ==

Einarh: /* Ferromagnetic layer thickness */

2009-03-26T13:49:06Z

Ferromagnetic layer thickness

← Eldre sideversjon		Sideversjonen fra 26. mar. 2009 kl. 13:49
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	=====Ferromagnetic layer thickness=====		=====Ferromagnetic layer thickness=====
	[[Bilde:Thicknessofgmr.PNG\|300px\|thumb\|right\|Thickness dependence of relative GMR effect.<ref name="Sato">M. Sato, S. lshio and T. Miyazaki, J. Magn. Magn. Mater. 126 (1993) 460.</ref><ref name="Dieny2">B. Dieny, V.S. Speriosu, S. Metin, S.S.P. Parkin, B.A.		[[Bilde:Thicknessofgmr.PNG\|300px\|thumb\|right\|Thickness dependence of relative GMR effect. NM = nonmagnetic, FM = ferromagnetic. <ref name="Sato">M. Sato, S. lshio and T. Miyazaki, J. Magn. Magn. Mater. 126 (1993) 460.</ref><ref name="Dieny2">B. Dieny, V.S. Speriosu, S. Metin, S.S.P. Parkin, B.A.
	Gurney, P. Baumgart and D. Wilhoit, J. Appl. Phys. 69		Gurney, P. Baumgart and D. Wilhoit, J. Appl. Phys. 69
	(1991) 4774</ref> ]]		(1991) 4774</ref> ]]

Einarh: /* Ferromagnetic layer thickness */

2009-03-26T13:48:31Z

Ferromagnetic layer thickness

← Eldre sideversjon		Sideversjonen fra 26. mar. 2009 kl. 13:48
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	(1991) 4774</ref> ]]		(1991) 4774</ref> ]]

	As the ferromagnetic layer thickness is ''reduced'', there will obviously be less bulk scattering events. The critical thickness is at the mean free path of the strongly scattered electrons. If the layer is any thinner than this, the GMR effect drops dramatically. Experiments show that interface scattering ~~also~~ depends on the layer thickness. If the layer is thinner than approximately 2 monolayers, the scattering is not optimal. <ref name="Parkin"> S.S.P. Parkin, Phys. Rev. Lett. 71 (1993) 1641</ref>		As the ferromagnetic layer thickness is ''reduced'', there will obviously be less bulk scattering events. The critical thickness is at the mean free path of the strongly scattered electrons. If the layer is any thinner than this, the GMR effect drops dramatically. Experiments show that not only bulk scattering, but also interface scattering depends on the layer thickness. If the layer is thinner than approximately two monolayers, the scattering is not optimal. <ref name="Parkin"> S.S.P. Parkin, Phys. Rev. Lett. 71 (1993) 1641</ref>

	As the ferromagnetic layer thickness is ''increased'', current will be shunted towards the center of the channel. This leads to a higher current density, with a resultant inrease in resistance and a drop of GMR. <ref name="Dieny"> B. Dieny, Journal of Magnetism and Magnetic Materials 136 (1994) 335-359</ref>		As the ferromagnetic layer thickness is ''increased'', current will be shunted towards the center of the channel. This leads to a higher current density, with a resultant inrease in resistance and a drop of GMR. <ref name="Dieny"> B. Dieny, Journal of Magnetism and Magnetic Materials 136 (1994) 335-359</ref>

	=====Nonferromagnetic layer thickness=====		=====Nonferromagnetic layer thickness=====

Einarh: /* Purity */

2009-03-26T13:46:52Z

Purity

← Eldre sideversjon		Sideversjonen fra 26. mar. 2009 kl. 13:46
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	=====Purity=====		=====Purity=====
	~~The high~~ impurity or high internal strains in a ferromagnet increases its coercivity. Recall that a high coercivity means that a high magnetic field must be applied to change the direction of magnetization. Good purity control is therefore essential ~~in synthesis~~ to tailor the properties.<ref name="Kittel">C. Kittel, "Introduction to Solid State Physics", John Wiley & Sons; 8th Edition, 2004</ref>		High impurity or high internal strains in a ferromagnet increases its coercivity. Recall that a high coercivity means that a high magnetic field must be applied to change the direction of magnetization. Good purity control is therefore essential to tailor a structure with the desired properties <ref name="Kittel">C. Kittel, "Introduction to Solid State Physics", John Wiley & Sons; 8th Edition, 2004</ref>

	In an ideal GMR structure, one type of spin electrons ~~have 0~~ resistance while the other has infinite resistance. Hence, in order to maximize the GMR effect, we generally want to minimize the spin-independent scattering events and maximize the spin-dependent scattering of either spin-up or spin-down electrons. ~~The most fundamental type of spin-dependent scattering, namely interface scattering, has already been discussed. However, bulk impurities~~ such as dislocations, stacking faults and grain boundaries can ~~also show~~ spin-dependent scattering. ~~This~~ can be a severe disadvantage or a powerful tool, depending on whether ~~those imperfections~~ are present by design or by random. <ref name="Gurney et.al.">B.A. Gurney, V.S. Speriosu, J.P. Nozi~res, H. Lefakis, D.R. Wilhoit and O.U. Need, Phys. Rev. Lett 71 (1993) 4023.</ref>		In an ideal GMR structure, one type of spin electrons experience zero resistance while the other has infinite resistance. Hence, in order to maximize the GMR effect, we generally want to minimize the spin-independent scattering events and maximize the spin-dependent scattering of either spin-up or spin-down electrons. Material imperfections such as dislocations, stacking faults and grain boundaries can lead to both spin-dependent and spin-independnet scattering, depending on a number of factors. Therefore, imperfections can be a severe disadvantage or a powerful tool, depending on whether they are present by design or by random. <ref name="Gurney et.al.">B.A. Gurney, V.S. Speriosu, J.P. Nozi~res, H. Lefakis, D.R. Wilhoit and O.U. Need, Phys. Rev. Lett 71 (1993) 4023.</ref>

	=====Ferromagnetic layer thickness=====		=====Ferromagnetic layer thickness=====

Einarh: /* Band structure */

2009-03-26T13:43:36Z

Band structure

← Eldre sideversjon		Sideversjonen fra 26. mar. 2009 kl. 13:43
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	=====Band structure=====		=====Band structure=====
	Above, spin dependent scattering within the ferromagnetic layer has been described by the Mott model. However, in addition to this, there is also spin dependent scattering at the interface between FM and NM layers.		Above, spin dependent scattering within the ferromagnetic layer has been described by the Mott model. However, in addition to this, there is also spin dependent scattering at the interface between magnetic and nonmagnetic layers.

	The electical conductivity across a junction between two metals will depend on the difference in band structure. A large difference in band structure leads to a high potential barrier, which increases the scattering probability and hence the resistance. Since the spin-up and spin-down electrons in a ferromagnet have different band structures due to the exchange splitting effect, as explained earlier, the transmission probability across the ~~FM/NM~~ junction will be different for spin-up and spin-down electrons <ref name="H. Ehrenreich"> H.Ehrenreich, F. Spaepen, Solid State Physics Vol 56, pp. 113-237</ref>		The electical conductivity across a junction between two metals will depend on the difference in band structure. A large difference in band structure leads to a high potential barrier, which increases the scattering probability and hence the resistance. Since the spin-up and spin-down electrons in a ferromagnet have different band structures due to the exchange splitting effect, as explained earlier, the transmission probability across the junction will be different for spin-up and spin-down electrons <ref name="H. Ehrenreich"> H.Ehrenreich, F. Spaepen, Solid State Physics Vol 56, pp. 113-237</ref>
	Thus, in order to achieve high transmission of spin-up and not spin-down, for instance, it is necessary to select a nonmagnet whose band structure closely resembles the band structure of spin-up electrons in the ferromagnet. <ref name="Nobel article">"Scientific background on the Nobel Prize in Physics 2007. The Discovery of Giant Magnetoresistance", The Royal Swedish Academy of Sciences 2007</ref>		Thus, in order to achieve high transmission of spin-up and not spin-down, for instance, it is necessary to select a nonmagnet whose band structure closely resembles the band structure of spin-up electrons in the ferromagnet. <ref name="Nobel article">"Scientific background on the Nobel Prize in Physics 2007. The Discovery of Giant Magnetoresistance", The Royal Swedish Academy of Sciences 2007</ref>

Einarh: /* Mott's model and spin dependent transport */

2009-03-26T13:38:12Z

Mott's model and spin dependent transport

← Eldre sideversjon		Sideversjonen fra 26. mar. 2009 kl. 13:38
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	==== Mott's model and spin dependent transport====		==== Mott's model and spin dependent transport====
	GMR can be qualitatively understood using the Mott model, which was introduced as early as 1936 to explain the sudden increase in resistivity of ferromagnetic metals as they are heated above the Curie temperature, where the material is no longer ferromagnetic.<ref name="West">A. West, "Basic solid state Chemistry", 2nd edition, John Wiley & Sons 1999</ref>
	In most metals, such as for instance copper, there is an equal number of spin-up and spin-down electrons. This causes the atom to be spin neutral. The ferromagnetic materials used most frequently in GMR applications are metals of the first transition series. For these elements it is energetically favourable for the d-electrons to have parallel spins in order to reduce their Coulomb repulsion. This is known as exchange splitting, and it gives rise to a net magnetic moment which makes the metal ferromagnetic. Moreover, exchange splitting means that the density of states is not the same for the spin-up and spin-down electrons at the Fermi energy.<ref name="H. Ehrenreich"> H.Ehrenreich, F. Spaepen, Solid State Physics Vol 56, pp. 113-237</ref>		In most metals, such as for instance copper, there is an equal number of spin-up and spin-down electrons. This causes the atom to be spin neutral. The ferromagnetic materials used most frequently in GMR applications are metals of the first transition series. For these elements it is energetically favourable for the d-electrons to have parallel spins in order to reduce their Coulomb repulsion. This is known as exchange splitting, and it gives rise to a net magnetic moment which makes the metal ferromagnetic. Moreover, exchange splitting means that the density of states is not the same for the spin-up and spin-down electrons at the Fermi energy.<ref name="H. Ehrenreich"> H.Ehrenreich, F. Spaepen, Solid State Physics Vol 56, pp. 113-237</ref>

	Mott proposed that the resistivity of spin-up and spin-down electrons is different because of the exchange splitting effect. This is because the scattering rates of the spin-up and spin-down electrons are quite different in a ferromagnetic material. For this reason, conductivity in		GMR can be qualitatively understood using the Mott model, which was introduced as early as 1936 to explain the sudden increase in resistivity of ferromagnetic metals as they are heated above the Curie temperature, where the material is no longer ferromagnetic.<ref name="West">A. West, "Basic solid state Chemistry", 2nd edition, John Wiley & Sons 1999</ref>
			Mott proposed that the resistivity of spin-up and spin-down electrons in a ferromagnet is different because of the exchange splitting effect. This is because the scattering rates of the spin-up and spin-down electrons are quite different in a ferromagnetic material. For this reason, conductivity in
	metals can be described in terms of two largely independent conducting channels, corresponding to the spin-up and spin-down electrons.		metals can be described in terms of two largely independent conducting channels, corresponding to the spin-up and spin-down electrons.

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	[[Bilde:Exchangecoupling.PNG\|200px\|thumb\|right\|Exchange coupling<ref name="H. Ehrenreich"> H.Ehrenreich, F. Spaepen, Solid State Physics Vol 56, pp. 113-237</ref>]]		[[Bilde:Exchangecoupling.PNG\|200px\|thumb\|right\|Exchange coupling<ref name="H. Ehrenreich"> H.Ehrenreich, F. Spaepen, Solid State Physics Vol 56, pp. 113-237</ref>]]

	==== GMR structures====		==== GMR structures====
	In GMR multilayer materials, thin layers of of magnetic materials are separated by layers of non-magnetic materials. Depending on the thickness of the non magnetic layers, the magnetic layers couple either ferromagnetically (parallel) or antiferromagnetically (antiparallel). This is known as exchange coupling. Giant magnetoresistance occurs when the thicknesses are chosen such that the adjacent layers are antiferromegnetic in zero applied field. This results in a high-resistance state as spin-up electrons are scattered by regions of down spin magnetization and vice versa, as explained by the Mott model.		In GMR multilayer materials, thin layers of of magnetic materials are separated by layers of non-magnetic materials. Depending on the thickness of the non magnetic layers, the magnetic layers couple either ferromagnetically (parallel) or antiferromagnetically (antiparallel). This is known as exchange coupling. Giant magnetoresistance occurs when the thicknesses are chosen such that the adjacent layers are antiferromegnetic in zero applied field. This results in a high-resistance state as spin-up electrons are scattered by regions of down spin magnetization and vice versa, as explained by the Mott model.

Einarh: /* GMR structures */

2009-03-26T13:36:51Z

GMR structures

← Eldre sideversjon		Sideversjonen fra 26. mar. 2009 kl. 13:36
Linje 27:		Linje 27:
	Using Mott’s arguments it is straightforward to explain GMR in magnetic multilayers. We consider collinear magnetic configurations, and assume that the scattering is strong for electrons with spin antiparallel to the magnetization direction, and weak for electrons with spin parallel to the magnetization direction. For the parallel-aligned magnetic layers, the spin-up electrons pass through the structure almost without scattering, because their spin is parallel to the magnetization of the layers. On the other hand, the spin-down electrons are scattered strongly within both ferromagnetic layers, because their spin is antiparallel to the magnetization of the layers. Since conduction occurs in parallel for the two spin channels, the total resistivity of the multilayer is determined mainly by the highly-conductive spin-up electrons and appears to be low. For the antiparallel-aligned multilayer, both the spin-up and spin-down electrons are scattered strongly within one of the ferromagnetic layers, because within the one of the layers the spin is antiparallel to the magnetization direction. Therefore, in this case the total resistivity of the multilayer is high. [need a figure!]		Using Mott’s arguments it is straightforward to explain GMR in magnetic multilayers. We consider collinear magnetic configurations, and assume that the scattering is strong for electrons with spin antiparallel to the magnetization direction, and weak for electrons with spin parallel to the magnetization direction. For the parallel-aligned magnetic layers, the spin-up electrons pass through the structure almost without scattering, because their spin is parallel to the magnetization of the layers. On the other hand, the spin-down electrons are scattered strongly within both ferromagnetic layers, because their spin is antiparallel to the magnetization of the layers. Since conduction occurs in parallel for the two spin channels, the total resistivity of the multilayer is determined mainly by the highly-conductive spin-up electrons and appears to be low. For the antiparallel-aligned multilayer, both the spin-up and spin-down electrons are scattered strongly within one of the ferromagnetic layers, because within the one of the layers the spin is antiparallel to the magnetization direction. Therefore, in this case the total resistivity of the multilayer is high. [need a figure!]

			[[Bilde:Exchangecoupling.PNG\|200px\|thumb\|right\|Exchange coupling<ref name="H. Ehrenreich"> H.Ehrenreich, F. Spaepen, Solid State Physics Vol 56, pp. 113-237</ref>]]
	==== GMR structures====		==== GMR structures====
	In GMR multilayer materials, thin layers of of magnetic materials are separated by layers of non-magnetic materials. Depending on the thickness of the non magnetic layers, the magnetic layers couple either ferromagnetically (parallel) or antiferromagnetically (antiparallel). This is known as exchange coupling. Giant magnetoresistance occurs when the thicknesses are chosen such that the adjacent layers are antiferromegnetic in zero applied field. This results in a high-resistance state as spin-up electrons are scattered by regions of down spin magnetization and vice versa, as explained by the Mott model.		In GMR multilayer materials, thin layers of of magnetic materials are separated by layers of non-magnetic materials. Depending on the thickness of the non magnetic layers, the magnetic layers couple either ferromagnetically (parallel) or antiferromagnetically (antiparallel). This is known as exchange coupling. Giant magnetoresistance occurs when the thicknesses are chosen such that the adjacent layers are antiferromegnetic in zero applied field. This results in a high-resistance state as spin-up electrons are scattered by regions of down spin magnetization and vice versa, as explained by the Mott model.

Einarh: /* Giant magnetoresistance */

2009-03-26T13:36:32Z

Giant magnetoresistance

← Eldre sideversjon		Sideversjonen fra 26. mar. 2009 kl. 13:36
Linje 16:		Linje 16:

	=== Giant magnetoresistance ===		=== Giant magnetoresistance ===
	~~[[Bilde:Exchangecoupling.PNG\|200px\|thumb\|right\|Exchange coupling<ref name="H. Ehrenreich"> H.Ehrenreich, F. Spaepen, Solid State Physics Vol 56, pp. 113-237</ref>]]~~

	The magnetoresistive component in modern read heads operates on the giant magnetoresistive (GMR) effect, extremely larger than the previous effects.		The magnetoresistive component in modern read heads operates on the giant magnetoresistive (GMR) effect, extremely larger than the previous effects.

Einarh: /* Lattice considerations */

2009-03-26T13:35:18Z

Lattice considerations

← Eldre sideversjon		Sideversjonen fra 26. mar. 2009 kl. 13:35
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	When building a superlattice of two or more crystalline materials it is important to consider the lattice parameters of the materials. If the lattice parameters differ significantly there would be significant strain at and near the interface, causing dislocations and imperfections which would scatter both the spin-up and spin-down electrons equally. This would be derogative to the total GMR effect. Hence, it is imperative that the lattice parameters are very closely matched.		When building a superlattice of two or more crystalline materials it is important to consider the lattice parameters of the materials. If the lattice parameters differ significantly there would be significant strain at and near the interface, causing dislocations and imperfections which would scatter both the spin-up and spin-down electrons equally. This would be derogative to the total GMR effect. Hence, it is imperative that the lattice parameters are very closely matched.

	Another lattice consideration is that the magnetization direction in a ferromagnet is related to crystallographic directions within the lattice. This is something which must be taken into consideration in the synthesis process.		Another lattice consideration is that the magnetization direction in a ferromagnet is related to crystallographic directions within the lattice. This is something which must be taken into consideration in the synthesis process.<ref name="West">A. West, "Basic solid state Chemistry", 2nd edition, John Wiley & Sons 1999</ref>

	=== Other important factors for GMR effect ===		=== Other important factors for GMR effect ===