Abstract:
A magnetic tunnel junction having a first electrode separated from a second electrode by a tunneling barrier is provided. The tunneling barrier is a ferromagnetic insulator that provides a spin dependent barrier energy for tunneling. The first electrode includes a ferromagnetic, electrically conductive layer. Electrons emitted from the first electrode toward the tunneling barrier are partially or completely spin-polarized according to the magnetization of the ferromagnetic electrode layer. The electrical resistance of the tunnel junction depends on the relative orientation of the electrode layer magnetization and the tunneling barrier magnetization. Such tunnel junctions are widely applicable to spintronic devices, such as spin valves, magnetic tunnel junctions, spin switches, spin valve transistors, spin filters, and to spintronic applications such as magnetic recording, magnetic random access memory, ultrasensitive magnetic field sensing (including magnetic biosensing), spin injection and spin detection.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of U.S. application Ser. No. 11/520,489, filed on Sep. 12, 2006, and entitled “Spin Filter Junction and Method of Fabricating the Same”. U.S. application Ser. No. 11/520,489 claims the benefit of U.S. provisional application 60/717,043, filed on Sep. 13, 2005, entitled “Spin Filter Junction and Method of Fabricating the Same”, and hereby incorporated by reference in its entirety. 
     
    
     GOVERNMENT SPONSORSHIP 
       [0002]    This invention was made with Government support under grant number ECS-0103302 from the National Science Foundation. The Government has certain rights in this invention. 
     
    
     FIELD OF THE INVENTION 
       [0003]    This invention relates to magnetic spin filtering, and to associated spintronic devices. 
       BACKGROUND 
       [0004]    Spintronics is a field of electronics based on manipulating electron spin within devices. Spintronics is of interest because of the relatively small amount of energy required to manipulate spins, as well as the possibilities inherent in exploiting the quantum nature of single spins. Methods for generating and detecting spin-polarized electrons are basic building blocks for spintronic devices, and various proposals have been considered in the art. 
         [0005]    For example, spin filtering by electron tunneling through a ferromagnetic insulating tunnel barrier has been experimentally demonstrated in an EuS barrier. Such a barrier provides different barrier energies for spin up and spin down electrons, an effect referred to as exchange splitting. Since the tunneling probability through a barrier depends sensitively on barrier energy, such an arrangement can act as a spin filter by preferentially passing electrons having the energetically favored spin. However, the Curie temperature of EuS is only 16.8 K. At temperatures above the Curie temperature, EuS is not ferromagnetic, so an EuS tunneling barrier does not provide exchange splitting and therefore does not act as a spin filter. Thus this early work on spin filtering does not readily lead to room temperature spintronic devices. 
         [0006]    In US 2002/0064004 by Worledge, a double spin filter tunnel junction is considered. In this work, the tunneling barrier has two layers with independently controllable magnetization. Such an arrangement can be regarded as two spin filters in series. Although this structure is expected to provide sensitive magnetoresistive sensors and related devices, there are practical challenges in realizing such a device. In particular, the requirement that the two layers of the tunneling barrier have independently controllable magnetization presents difficulties. The known remedy of placing a non-magnetic decoupling layer between the two layers of the tunneling barrier to decouple them undesirably increases the tunneling barrier thickness, which can decrease device performance. 
         [0007]    In the preceding examples, the relevant physical effect is quantum-mechanical electron tunneling through a barrier having a different barrier energy for spin up electrons than for spin down electrons. Tunneling from one ferromagnetic electrode to another ferromagnetic electrode through a non-magnetic insulating tunneling barrier has also been considered, and the resulting effect is often referred to as tunnel magnetoresistance (TMR). Although the barrier energy does not depend on spin in a TMR device, the density of final states available for tunneling does depend on the relative orientation of the magnetizations of the two ferromagnetic electrodes, thereby providing a magnetization-dependent resistance. U.S. Pat. No. 5,629,922 considers a TMR-based magnetoresistive sensor. U.S. Pat. No. 6,781,801 considers a TMR device where a spin filter is employed to spin-polarize the TMR device sense current, thereby increasing the magnetoresistance (MR) ratio. However, it is expected that devices based on a spin-dependent tunneling barrier energy should outperform TMR devices, since the tunneling current depends more sensitively on barrier energy than on the density of final states. 
         [0008]    Accordingly, it would be an advance in the art to provide tunneling spin filter junctions suitable for operation at room temperature and providing high performance. 
       SUMMARY 
       [0009]    A magnetic tunnel junction having a first electrode separated from a second electrode by a tunneling barrier is provided. The tunneling barrier is a ferromagnetic insulator that provides a spin dependent barrier energy for tunneling. The first electrode includes a ferromagnetic, electrically conductive layer. Electrons emitted from the first electrode toward the tunneling barrier are partially or completely spin-polarized according to the magnetization of the ferromagnetic electrode layer. The electrical resistance of the tunnel junction depends on the relative orientation of the electrode layer magnetization and the tunneling barrier magnetization. Such tunnel junctions are widely applicable to spintronic devices, such as spin valves, magnetic tunnel junctions, spin switches, spin valve transistors, spin filters, and to spintronic applications such as magnetic recording, magnetic random access memory, ultrasensitive magnetic field sensing (including magnetic biosensing), spin injection and spin detection. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIGS. 1   a - c  show a first embodiment of the invention in various operating states. 
           [0011]      FIG. 2  shows a second embodiment of the invention. 
           [0012]      FIG. 3  shows a third embodiment of the invention. 
           [0013]      FIG. 4  shows a two terminal semiconductor device according to an embodiment of the invention. 
           [0014]      FIG. 5  shows a three terminal semiconductor device according to an embodiment of the invention. 
           [0015]      FIG. 6  shows measured I-V curves from an embodiment of the invention. 
           [0016]      FIG. 7  shows measured magnetoresistance ratios from an embodiment of the invention. 
           [0017]      FIGS. 8   a - b  show calculated magnetoresistance ratios for various embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]      FIGS. 1   a - c  show a first embodiment of the invention in various operating states. On  FIG. 1   a , a tunnel junction  120  includes a first electrode  102  having a first magnetization direction  104  separated from a second electrode  110  by a tunneling barrier  106  having a second magnetization direction  108 . In this example, first electrode  102  is an electrically conductive ferromagnetic layer, tunneling barrier  106  is a ferromagnetic electrically insulating layer, and second electrode  110  is electrically conductive and non-magnetic. Second electrode  110  can include any electrically conductive material (e.g., Au). In some embodiments of the invention, second electrode  110  is non-magnetic. Magnetoresistance is observed regardless the direction of current or spin flow. In preferred embodiments of the invention, second electrode  110  is magnetic or spin-polarized. In these embodiments, spin polarization of second electrode  110  can further enhance the spin-dependent tunneling process described below. Such enhancement is analogous to the behavior of conventional magnetic tunnel junctions. In cases where second electrode  110  is magnetic, its magnetization can be coupled to the magnetization of barrier  106  or it can be independent of the magnetization of barrier  106 . 
         [0019]    An electron energy band diagram  130  shows features of importance for device operation. In particular, exchange splitting in tunneling barrier  106  provides tunneling barriers having different barrier energies for spin up electrons (barrier  114 ) than for spin down electrons (barrier  112 ). In this example, when magnetization direction  108  is “up”, the spin down energy barrier (barrier  112 ) is higher than the spin up energy barrier (barrier  114 ). It is also possible for this relation between magnetization  108  and the relative heights of the spin up and spin down energy barrier to be reversed, depending on properties of the material selected for tunneling barrier  106 . Device operation does not critically depend on whether the spin up barrier or the spin down barrier is higher for “up” magnetization. 
         [0020]    Electrons emitted from first electrode  102  toward tunneling barrier  106  are substantially spin polarized according to magnetization direction  104 . The example of  FIGS. 1   a - c  shows negative spin polarization, where the spin-down current density J↓ is greater than the spin-up current density J↑ for “up” magnetization  104 . Thus negative spin polarization relates to situations where electron spin tends to be anti-parallel to the magnetization. Positive spin-polarization, where electron spin tends to be parallel to magnetization direction  104 , is also possible, depending on the composition and/or structure of first electrode  102 . Device operation does not depend critically on whether the spin polarization provided by first electrode  102  is positive or negative. The degree of spin polarization can be defined as the ratio of the difference of spin up electrons and spin down electrons over their sum. Usually, only electrons at the Fermi level are relevant for the calculation of spin polarization, since tunneling primarily involves electrons at or near the Fermi level. Preferably, this ratio is greater than 25%, and more preferably this ratio is closer to 100% (e.g., &gt;85%). 
         [0021]    A key aspect of the invention is that the combination of a spin-polarized first electrode with a spin-dependent tunneling barrier provides magnetoresistance in a relatively simple device configuration. A single spin-dependent tunneling barrier by itself does not provide magnetoresistance. Although a double spin-dependent tunneling barrier can provide magnetoresistance, significant complications arise in practice, as described above. In the example of  FIGS. 1   a - c , the electrical resistance of tunnel junction  120  between the first and second electrodes depends on the relative orientation of first and second magnetization directions  104  and  108 . 
         [0022]      FIG. 1   a  shows a relatively high-resistance state, since most of the current provided by electrode  102  is spin-down, and the spin-down tunneling barrier  112  is higher than the spin-up tunneling barrier  114 . If the magnetization of first electrode  102  is switched to “down” as shown by  104 ′ on  FIG. 1   b , the relative proportion of spin-up and spin-down current provided to tunneling barrier  106  is switched. In this case, most of the current provided to tunneling barrier  106  is spin-up, which has the lower energy barrier. Thus  FIG. 1   b  shows a relatively low resistance state. If the magnetization of tunneling barrier  106  is switched to “down”, as shown by  108 ′ on  FIG. 1   c , the barrier heights for spin-up and spin-down electrons are switched compared to  FIG. 1   a . Thus barrier  114 ′ for spin-up electrons is higher than barrier  112 ′ for spin-down electrons on  FIG. 1   c . Since most of the current on  FIG. 1   c  is spin-down, which has the lower energy tunneling barrier,  FIG. 1   c  also shows a relatively low resistance state. 
         [0023]    Since tunneling barrier  106  must provide a tunneling barrier to electrons, it can be an electrical insulator (or semiconductor) that acts as an electrical insulator in tunnel junction  120 . Tunneling barrier  106  is also ferromagnetic, and preferably its Curie temperature is well above room temperature, so that device operation at or near room-temperature will not be impaired by approaching too closely to, or crossing, the ferromagnetic-nonmagnetic phase transition. Suitable tunneling barrier materials include, but are not limited to, ferrites such as CoFe 2 O 4 , NiFe 2 O 4 , and MnFe 2 O 4 , and ferromagnetic semiconductors such as Co-doped TiO 2 , Mn-doped GaN, Al and Cr doped GaN, etc. (see S. A. Wolf et al., IBM Journal of Research &amp; Development, vol. 50(1), p. 101.). 
         [0024]    In this example, first electrode  102  is an electrically conductive ferromagnet having substantial spin polarization. The Curie temperature of first electrode  102  is also preferably well above room temperature. Half-metallic ferromagnets should provide ˜100% spin polarization, and are therefore attractive candidate materials for first electrode  102 . Although these materials tend to be difficult to grow in thin film form at this time, they may become more readily available in the future. Other suitable materials for first electrode  102  that can provide substantial spin polarization include, but are not limited to Fe 3 O 4 , La 2/3 Sr 1/3 MnO 3 , CrO 2 , Co doped ZnO, and any ferromagnetic alloy containing Co, Fe, and/or Ni. First electrode  102  can also be a multilayer structure designed to provide spin-polarized current to tunnel barrier  106 , as described below in connection with  FIG. 3 . 
         [0025]    Since the resistance of tunnel junction  120  depends on the relative orientation of magnetization directions  104  and  108 , sensing an external magnetic field relies on keeping one of magnetization directions  104  and  108  fixed and independent of the external field, while the other of magnetization directions  104  and  108  is free to follow the external field. A layer having a fixed magnetization direction is customarily referred to as a pinned layer, while a layer having a magnetization that can follow an external magnetic field is customarily referred to as a free layer. Thus one of layers  102  and  106  should be pinned and the other should be free, in order to provide a MR sensor. Electrode  102  can be free and barrier  106  can be pinned ( FIG. 1   b ), or electrode  102  can be pinned and barrier  106  can be free ( FIG. 1   c ). Device operation does not depend critically on which layer is pinned and which layer is free. 
         [0026]    In some cases, the coercivity of the pinned layer is sufficiently high that pinning is inherently provided by the high coercivity. In other cases, a high-coercivity pinning layer can be disposed in proximity to the pinned layer in order to pin it. Such use of a pinning layer to fix the magnetization direction in a pinned layer is well known in the art in connection with various conventional MR sensors, and the same pinning principles are applicable in connection with the present invention. 
         [0027]    The free layer should have a sufficiently low coercivity that it can respond to the external magnetic field to be sensed. In addition, it may be necessary to magnetically decouple the free layer from other nearby layers. For example, if barrier  106  on  FIG. 1   a  is pinned, magnetic coupling between barrier  106  and electrode  102  undesirably tends to fix magnetization direction  104  with respect to magnetization direction  108 , thereby degrading MR sensor performance. 
         [0028]      FIG. 2  shows an embodiment of the invention where a decoupling layer is introduced in order to reduce undesirable magnetic coupling between free and pinned layers. More specifically, a decoupling layer  202  is sandwiched between first electrode  102  and tunneling barrier  106  to reduce magnetic coupling between these two layers. Decoupling layer  202  is a thin layer of a non-magnetic material. The use of such magnetic decoupling layers is well known in the art in connection with various conventional MR sensors, and the same decoupling principles are applicable to the present invention. Typical decoupling layer thicknesses are less than about 3 nm. A decoupling layer of MgAl 2 O 4  has been employed in experiments relating to the invention, but other non-magnetic materials are also suitable for use as decoupling layers with the invention. The decoupling layer can be insulating (e.g., MgAl 2 O 4 , CoCr 2 O 4 , MgO, Al 2 O 3 , etc.), semiconducting (e.g., Si, Ge, SiGe, GaAs, etc.), or metallic (Ru, V, Pt, Pd, Au, Cu etc.). 
         [0029]    As indicated above, provision of spin polarized electrons from the first electrode is a key aspect of the invention. Some ferromagnetic electrical conductors (e.g., half metals and other materials described above) inherently provide spin-polarized electrons. Spin polarized electrons can also be provided by a first electrode including two or more layers, at least one layer being a ferromagnetic electrical conductor. For example,  FIG. 3  shows one such embodiment of the invention. In this example, the first electrode includes a ferromagnetic electrically conductive layer  102   a  and a non-magnetic electrically insulating layer  102   b . Such ferromagnet-insulator bilayers can provide a high degree of spin polarization. For example, a spin polarization of 85% has been inferred for a CoFe—MgO ferromagnet-insulator bilayer, based on superconductor spin analyzer measurements from a CoFe/MgO/superconductor junction (Parkin et al., Nature Materials, 3 862 (2004)). 
         [0030]    The CoFe layer of the above example can be replaced by any spin-polarized material such as a ferromagnetic alloy including Co, Fe, and/or Ni. The MgO layer can be replaced by any material whose presence enhances the spin polarization of the first electrode. 
         [0031]    The invention is applicable to a wide variety of spintronic devices and application, in addition to the magnetoresistive sensing application considered above. Tunnel junctions according to embodiments of the invention can be included in any kind of spintronic device, including but not limited to spin valves, magnetic tunnel junctions, spin switches, spin valve transistors, and spin filters. 
         [0032]      FIG. 4  shows a two terminal semiconductor device according to an embodiment of the invention. In this device, a first terminal  402   a  makes contact to a semiconductor channel  406  on a substrate  408  via a first tunnel junction. The first tunnel junction includes a first electrode  402   b  and a tunneling barrier  402   c . Similarly, a second terminal  404   a  makes contact to the semiconductor channel  406  via a second tunnel junction. The second tunnel junction includes a first electrode  404   b  and a tunneling barrier  404   c . The first and second tunnel junctions both operate as described above (i.e., the first electrodes  402   b  and  404   b  provide spin-polarized electrons, and the ferromagnetic tunneling barriers  402   c  and  404   c  provide spin-dependent tunneling barriers.). For both tunnel junctions, semiconductor channel  406  acts as the second electrode (e.g., electrode  110  on  FIG. 1   a ). Thus current provided to semiconductor channel  406  and/or current received from channel  406  can be spin-filtered. 
         [0033]      FIG. 5  shows a three terminal semiconductor device according to an embodiment of the invention. This embodiment is similar to the embodiment of  FIG. 4 , except that a gate terminal  502  is added. An electrical signal applied to gate terminal  502  can modulate current flow through channel  406  (e.g., as in a field effect transistor), thereby modulating spin transport in the channel. 
         [0034]    In a preferred embodiment semiconductor channel  406  can be magnetic to provide additional gains in device performance. It can also be made of multiferroic materials which display ferromagnetism and ferroelectricity simultaneously and have a magnetization responsive to an applied electrical voltage. Similarly, the first electrode and/or second electrode of a tunnel junction according to the invention can include a multiferroic material having a magnetization responsive to an applied electrical voltage. 
         [0035]    Modeling and experiments have been done to investigate the performance of various embodiments of the invention. In one experiment, a Fe 3 O 4  first electrode  102  was separated from a CoFe 2 O 4  tunneling barrier  106  by a MgAl 2 O 4  decoupling layer  202 , as shown on  FIG. 2 . The tunnel junction of this experiment was grown on an (001) oriented MgAl 2 O 4  substrate by pulsed laser deposition (PLD). A focused KrF excimer laser (248 nm) with a 10 Hz repetition rate and a target fluence of ˜3 J/cm 2  was employed. A CoCr 2 O 4  buffer layer was first grown on the substrate (typical growth conditions were 650° C., 10 mTorr O 2  atmosphere, 2 nm/min deposition rate). The Fe 3 O 4 , MgAl 2 O 4  and CoFe 2 O 4  layers were grown on top of the CoCr 2 O 4  buffer layer in sequence, typically at a growth rate of 0.6 nm/min. The Fe 3 O 4  layer was deposited at 350° C. in a 10 −6  Torr O 2  atmosphere, while the MgAl 2 O 4  and CoFe 2 O 4  layers were deposited at 350° C. in a 10 −5  Torr O 2  atmosphere. Second electrode  110  was formed by e-beam evaporation of 25 μm×25 μm Au contact pads through a shadow mask. 
         [0036]    High quality and near-perfect stoichiometry of the Fe 3 O 4  layers grown as above was verified by observation of the Verwey transition for film thicknesses as low as 20 nm. The MgAl 2 O 4  and CoFe 2 O 4  layers were grown under conditions that did not oxidize the Fe 3 O 4  surface. This was confirmed by X-ray photoelectron spectroscopy (XPS) and by observation of the Verwey transition. XPS was also employed to determine the composition of the CoFe 2 O 4  layer. A Fe to Co ratio very close to 2 was measured, indicating near-perfect stoichiometry. The spectra also indicate the Co ions are in the +2 formal oxidation state and nearly all of the Fe ions are in the +3 formal oxidation state. 
         [0037]    In this structure, the MgAl 2 O 4  and CoFe 2 O 4  layers both act as tunneling barriers, with barrier heights of 0.8 eV and 0.29 eV respectively. These barrier heights were determined from independent experiments on Fe 3 O 4 /MgAl 2 O 4  and Fe 3 O 4 /CoFe 2 O 4  samples. Tunneling measurements performed on a MgAl 2 O 4 /CoFe 2 O 4  double barrier structure provided results consistent with the barrier heights obtained from single barrier structures. 
         [0038]      FIG. 6  shows measured I-V curves from a Fe 3 O 4 (30 nm)/MgAl 2 O 4 (1 nm)/CoFe 2 O 4 (3 nm)/Au tunnel junction for parallel (↑↑) and anti-parallel (↑↓) magnetization directions. Since the coercivity of CoFe 2 O 4  is higher than that of Fe 3 O 4 , the CoFe 2 O 4  and Fe 3 O 4  layers in this tunnel junction act as the pinned and free layers respectively. The sample was initially magnetized in a 12 kOe magnetic field to set the magnetization direction in the pinned layer. Subsequent application of a small external magnetic field of 550 Oe or less was employed to characterize magnetoresistance in this structure. The magnetization direction of the CoFe 2 O 4  layer is unaffected by fields of 550 Oe or less, while the Fe 3 O 4  layer is free to follow the direction imposed by the small external field. A different resistance is clearly seen on  FIG. 6  for parallel and anti-parallel magnetization directions. An MR ratio of about 70% near zero bias is obtained in this case. Lower resistance is observed for anti-parallel magnetization, which is consistent with the CoFe 2 O 4  layer as having a partial inverse structure with ˜7-20% of the Co ions in tetrahedral A sites. Based on this analysis, an exchange splitting on the order of 0.1 eV is inferred, which is also consistent with experimental tunnel junction observations. 
         [0039]      FIG. 7  shows a typical plot of the magnetoresistance ratio (R−R −550Oe )/R −550Oe  versus applied magnetic field. Hysteresis is apparent, with a sharp change corresponding to the switching field of the free Fe 3 O 4  layer. In this experiment, estimated spin polarizations from the first electrode were in a range from about 10% to about 36%, based on results from several samples. The net spin polarization of electrons emitted from the tunnel junction was calculated to have exceeded 70% for most samples. MR ratios as large as 75% have been experimentally observed. 
         [0040]    Increasing the exchange splitting provided by barrier  106  and/or the spin polarization provided by first electrode  102  can improve device performance.  FIG. 8   a  shows how the MR ratio for a 3 nm thick insulating barrier having an average barrier height of 0.3 eV varies as a function of spin polarization provided by first electrode  102  for several different values of exchange splitting J. Extremely high MR ratios can be obtained as the spin polarization approaches 100%, which may be difficult to achieve in practice.  FIG. 8   b  shows how the MR ratio for a 3 nm thick insulating barrier varies as a function of exchange splitting J for several values of average barrier height, assuming an incident spin polarization from the first electrode of 85%. Very high MR ratios greater than 10 (i.e., &gt;1,000%) can be obtained in some cases, even though the assumed incident spin polarization is only 85%.