Giant Magnetoresistance

Transcription

1 Giant Magnetoresistance This is a phenomenon that produces a large change in the resistance of certain materials as a magnetic field is applied. It is described as Giant because the observed effect is much larger than the effect had ever been previously seen in metals. It has generated interest from both physicists & device engineers, as there is both new physics to be investigated and overall there are huge technological applications in magnetic recording and sensors. Due to this direct application in the magnetic data storage technology there is a huge international research effort ongoing. The largest is in the data storage industry and IBM were first to market with hard disks based on GMR technology although today all disk drives make use of this technology. Other applications are as diverse as solid-state compasses, automotive sensors, and non-volatile magnetic memory. The effect is most usually seen in magnetic multilayered structures, where two magnetic layers are closely separated by a thin spacer layer a few nm thick. It is analogous to a polarization experiment, where aligned polarizers allow light to pass through, but crossed polarizers do not. The first magnetic layer allows electrons in only one spin state to pass through easily - if the second magnetic layer is aligned then that spin channel can easily pass through the structure, and the resistance is low. If the second magnetic layer is misaligned then neither spin channel can get through the structure easily and the electrical resistance is high. This is illustrated in this diagram: The phenomenon is based in a different scattering rate for electrons with different spins. This can be understood if we consider that the band structure in a ferromagnet is split, so that the density of states is not the same for spin up and down electrons at the Fermi level. Quantum mechanics describes the probability of scattering if one electron with a given spin in a material with the Fermi's golden rule. This law states that scattering rates are proportional to the density of states at the state being scattered into (in this case the Fermi level), so the scattering rates are different for electrons of different spin. This law explains for example the sudden decrease in resistivity of ferromagnetic metals as they are cooled through the Curie point. The GMR effectively measures the difference in angle between the two magnetisations in the magnetic layers. Small angles (parallel alignment) gives a low resistance, large angles

2 (antiparallel alignment) gives a higher resistance. It is easy to produce the state where the two magnetic layers are parallel - simply apply a field large enough to magnetically saturate both layers. But how do we achieve an antiparallel state? There are three basic ideas used here: Antiferromagnetic coupling: When the spacer layer is extremely thin the sense of the coupling between the two layers oscillates. In other words, for certain specific values of the spacer thicknesses the magnetizations of neighboring layers will lie in opposite directions. By applying a large enough magnetic field we can force the magnetizations of the layers to all line up along the field direction. In this way it is possible to get the two different resistance states. It's possible to build up multilayered structures called superlattices which work on this principle - they are samples where you have a magnetic layer and a non-magnetic spacer repeated many times, e.g. {Co/Cu} x 30. Each magnetic Co layer is separated from it's neighbors by a thin Cu spacer. The GMR depends on the thickness of the Cu layers. The figure shows how the resistivity of the films change as a function of the thickness of the spacer layer. There are 3 peaks visible, although only the first two are very strong. At large thicknesses the samples are decoupled, and small random differences between the layers give a small GMR. When the layers are ferromagnetically coupled, the GMR is zero. Notice that the first peak is at a Cu thickness of only 9Å - a copper atom is about 3Å across. Depositing such thin layers without pinholes has only become possible recently with advances in UHV deposition technology. Samples like this have the largest GMR of all - the reason is to do with the mean free path of the electrons which carry the current. (mean free path is the average distance an electron will travel between scattering events - the longer it is the lower the resistance). It possible for electrons of appropriate spin to pass through many aligned magnetic layers and have a very long mean free path. This means that the distance between scatters is increased the most when the layers become magnetically parallel. However in most cases they are inappropriate for senor applications, as the fields required to observe the magnetoresistance are very large. Different Coercivities: If we use two different materials with different switching fields then as we apply the reverse field one layer will switch before the other - we then have the desired anti-parallel alignment. In practice the contrast between the layers has to be good, and most materials do not switch sharply enough to get the full benefit from this technique. We have grown {Co/Cu/Fe/Cu}xN multilayers by MBE which show this

3 sharp switching behaviour. These types of structures are sometimes called 'pseudo' spin valves. Exchange biasing & spin-valves: These are structures where one layer moves in a field, whilst the other does not, and is used as a reference magnetic moment. This will give us a bipolar output with a very high sensitivity in an optimized device. There are a number of different schemes in which it is possible to do this. It is possible to exchange couple one of a pair of magnetic layers to another back layer of antiferromagnetic material. In the diagram below a FeMn layer is used to 'pin' the Co layer magnetization in a certain direction. This layer is used as a reference layer. The NiFe layer, which is very magnetically soft, can now be aligned parallel or antiparallel by very tiny fields. There is a thick enough Cu spacer between these two to stop there from being any magnetic coupling between the layers. The Ta layers are a buffer (to give a good surface to grow on) and a cap (to stop the sample from being oxidized in air). The whole sample is deposited on a piece of Si wafer, which is in fact many thousands of times thicker than the whole multilayer structure. The GMR active region will be only about 100Å thick, with the whole structure being about 300Å, on top of a 1mm thick piece of Si. The magnetisation in the Co layer is pinned by the last plane of spins in the antiferromagnet, causing it to be unaffected by small fields applied in this direction. This means that there is effectively a local magnetic field applied to the pinned layer, causing it to be fully saturated at zero field. The magnetization in the layer actually reverses about some point biased away from zero, due to the effective field.

4 These type of structures are very sensitive to magnetic fields, and are being researched heavily by the magnetic recording industry for use in high density disk and tape playback heads. Dynamic of polarization induced by a current of polarized electrons This is a method to control the magnetization. The idea is to exploit the interaction between the electron spins and the magnetization to drive the orientation of M in the material. In a ferromagnetic metal F the spin of the conduction electrons interacts with the global magnetization M of the material. In standard electron optics, if an electron with spin aligned along the orientation p i that is not parallel to the magnetization M is injected into F this interaction generates a couple which gradually lines up the electron spin with the magnetization M. As this happens the electrons abandon the transverse component of the angular momentum which is transferred to the magnetization by the principle of action and reaction. This transverse angular momentum acts on M to align the it with p i, a process referred as the spin transfer mechanism. The figure below

5 shows schematically this effect. The conclusion is that it is possible to act on the magnetization using an electron current, if this current has spin polarization, i.e., if there are more electrons with spin parallel to p i than antiparallel to it. A realistic calculation of this effect is a complex process and is still subject of some debate. It can be shown that in the Landau-Lifschitz-Gilbert equation applied to the magnetization M the effect shows up through an extra effective field given by [ ] H = χ M p + β χ M p inj i S i The factor χ measures the intensity of this effect. It is of course proportional to the number of injected electrons and hence the current, and depends on the angle between p i and M. Moreover it can be shown that the effect extends over a very short range (<1nm) in the magnetic layer. The factor β is estimated to be less than unity and the first term of the equation dominates. This concept is demonstrated in the following example. A thin film Co nanomagnet in the shape of an elongated hexagon is incorporated in a vertical device structure consisting of the nanomagnet and a thin Cu spacer layer formed on top of a thick Co film. the spin polarized current flowing between the nanomagnet and the Co film is used to abruptly switch the magnetic alignment of the nanomagnet relative to the thick Co layer by the transfer of spin angular momentum from the conduction electrons to the nanomagnet moment. The anisotropy of the nanomagnet promotes the single domain behavior required for non volatile memory applications. The figure shows the results in this experiment. In figure (a) is a scheme of the structure. The top Cu layers act as contacts to polarize the device and produce the current through the nanopillar. In figure (b) is a SEM iamge of the nanopillar. Figure (c) indicates the relative change in the resistance of the device as the magnetic filed is reversed. The change in the differential resistance dv characteristic is produced by di the spin transfer switching provoked by the current. The importance of this effect that demonstrate the possibility to orient magnetic moments utilizing electric currents is of instrumental importance in applications to magnetic memories.