Chapter One Spin-Dependent Tunneling in Magnetic Junctions

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Abstract

This chapter reviews the physics of spin-dependent tunneling in magnetic tunnel junctions, i.e. ferromagnetic layers separated by an ultrathin, insulating barrier. In magnetic junctions the tunneling current between the ferromagnetic electrodes depends strongly on an external magnetic field, facilitating a wealth of applications in the field of magnetic media and storage. After a short introduction on the background and elementary principles of magnetoresistance and tunneling spin polarization in magnetic tunnel junctions, the basic magnetic and transport phenomena are discussed emphasizing the critical role of the preparation and properties of (mostly Al2O3) tunneling barriers. Next, key ingredients to understand tunneling spin polarization are introduced in relation to experiments using superconducting probe layers. This is followed by discussing a number of crucial results directly addressing the physics of spin tunneling, including the role of the polarization of the ferromagnetic electrodes, the interfaces between barrier and electrodes and quantum-well formation, and the successful use of alternative crystalline barriers such as SrTiO3 and MgO.

Introduction

This review is focusing on the fundamental aspects of magnetic tunnel junctions or shortly MTJs. It will cover the preparation and experimental aspects of MTJs, and most of the crucial experiments that were performed to unravel their basic physics. In the last section, new promising directions for further research will be reviewed.

In this introductory section the following subjects will be covered:

  • the breakthrough towards magnetoresistance in layered magnetic structures, more specifically in metallic multilayers and subsequently in magnetic tunnel junctions

  • phenomenology of magnetoresistance in MTJs using the Julliere model, including the concept of so-called tunneling spin polarization

  • the shortcomings of elementary models via an introduction to some crucial experimental observations and advanced theoretical approaches.

It should be noted that several other reviews exist also partially covering the physics and applications of spin-polarized tunneling in tunnel junctions; see Meservey and Tedrow (1994), Moodera et al., 1999a, Moodera et al., 2000, Moodera and Mathon (1999), Dennis et al. (2002), Ziese (2002), Maekawa et al. (2002), Miyazaki (2002), Tsymbal et al. (2003), Zhang and Butler (2003), Zutic et al. (2004), Shi (2005), and LeClair et al. (2005). In most cases, however, the focus is different as compared to the present paper, and some recent developments in this rapidly evolving field may not be included. To assist the reader, the last part of this introduction will briefly explain the scope of the present review.

Magnetic tunnel junctions are within the florishing field of magnetoelectronics or spin electronics, shortly spintronics. In this area, nanostructured magnetic materials are used for functional devices explicitly exploiting both charge and spin in electron transport, the so-called spin-polarized transport. As we will see later on, magnetic junctions are offering several unique opportunities for studying new, sometimes unexpected effects in physics, and, furthermore, they have opened up a number of new research directions within spintronics. Apart from that, magnetic junctions are superb materials for exploring novel device options, such as improved read-head sensors, magnetic memories or magnetic biosensors. Before a more detailed insight in the principles of magnetic tunneling will be given, it is instructive to first shortly review the field of spin-polarized transport and the ongoing increasing role of tunneling transport.

In the mid-eighties the first crucial steps are made towards the exploitation of magnetic nanostructures for new electrical effects. These breakthroughs were strongly stimulated by the progress in ultra-high vacuum deposition and characterization techniques, enabling full control of layer-by-layer growth of metallic magnetic (multi-)layers. One of the first intriguing observations by Carcia et al. (1985) is the presence of perpendicular magnetic anisotropy in ultrathin magnetic (multi-)layers due to strong magnetic surface anisotropies, see, e.g., also Parkin (1994) and Johnson et al. (1996). Due to perpendicular anisotropy, the magnetization can be pointing out of the plane of a magnetic thin film, a novel way of engineering the direction of magnetization in ferromagnetic films. To illustrate the technological relevance, this phenomenon is now used in magnetic media to increase the data density as compared to in-plane magnetized (longitudinal) magnetic disks. The subsequent discovery of magnetic interaction across ultrathin nonmagnetic spacers has been critically important for the field of spin-polarized transport. It is shown by Grünberg et al. (1986) that this so-called interlayer coupling may favor an antiparallel, in-plane alignment of two neighboring magnetic layers separated by only a few atomic planes of a nonmagnetic element. It is now well accepted that the driving mechanism for the interaction is spin-dependent electron reflection and transmission at the interfaces between the magnetic and nonmagnetic layers (for a review, see Bürgler et al., 1999). The first observation of remarkable, unexpected electrical effects in these magnetic nanostructures is independently reported by the research groups of Fert and Grünberg (Baibich et al., 1988, Binasch et al., 1989). They have demonstrated that the resistance of a multilayered stack of magnetic layers separated by nonmagnetic spacers strongly depends on the mutual orientation of the layer magnetization. Due to the presence of antiferromagnetic coupling, the magnetization of these layers can be engineered between parallel and anti-parallel via an externally applied magnetic field. The enormous magnitude of the magnetoresistance at room temperature explains the term giant magnetoresistance or GMR used since then.

The observation of GMR has initiated an intensive research effort. Fundamentally, the physics of the underlying spin-polarized transport is studied extensively using magnetic engineering tools, novel material combinations, and a variety of theoretical approaches (Coehoorn, 2003). Along with the fundamental interest, the application potential of this effect has been immediately recognized by the magnetic recording industries. As a well-known achievement in this area, the concerted scientific and industrial effort led to the introduction of a GMR read head already in 1997, just nine years after the pioneering, curiosity-driven experiments. A similar strong interplay between scientific discovery and subsequent device implementation can be observed in the field of magnetic tunnel junctions (MTJs). Although junctions were already studied for a long time (e.g. in the case of one superconducting and one metallic electrode), especially in the beginning of the nineties an increasing number of contributions are devoted to full magnetic junctions with two ferromagnetic electrodes. Although these experiments are certainly inspired by the original work of Julliere (1975) and Maekawa and Gafvert (1982) on Fe-Ge-Co and Ni-NiO-Ni(Co,Fe), respectively, the booming interest for GMR in metallic systems has also fuelled the renewed interest. For some of these pioneering experiments on MTJs in the beginning of the nineties, see Miyazaki et al. (1991), Nowak and Raułuszkiewicz (1992), Suezawa et al. (1992), Yaoi et al. (1993), and Plaskett et al. (1994). The final breakthrough in this field takes place in 1995 when unprecedented large magnetoresistance effects are discovered at room temperature. Moodera et al. (1995) as well as Miyazaki and Tezuka (1995a) are the first to show that a system of two magnetic layers separated by a very thin nonmagnetic oxide layer displays a huge tunnel magnetoresistance or TMR effect, substantially larger than GMR in a similar system with a metal spacer (for a review on exchange-biased spin-valves, see, e.g., Coehoorn, 2003). To illustrate the order of magnitude of GMR versus TMR, Fig. 1.1 shows the chronology of these developments. It is clear from the graph that the TMR data on Al2O3-based MTJs have shown a steady increase and are always well above GMR data. In more recent years, the use of MgO as a barrier (as well as other oxide and ferromagnetic material combinations) have undoubtedly demonstrated the record-high magnitude of TMR effects. In comparing these data, one should realize that the physics behind the magnetoresistance in tunnel junctions is completely different from that in all-metallic GMR structures, since quantum-mechanical tunneling is now the fundamental process governing the electrical transport. We will return to that in section 1.2. In Fig. 1.2 an experimental example of tunnel magnetoresistance is shown from the group of Moodera, using two magnetic layers of different coercivity separated by a thin alumina barrier. It clearly demonstrates a large resistance change when the two magnetic layers are switched from a parallel to an anti-parallel orientation by an external magnetic field.

The magnetoresistance in MTJ's can be exploited in a novel solid-state memory. It consists of (sub)micron-sized tunneling elements connected via word and bit lines in a two-dimensional architecture, a similar layout as in macroscopic ferrite core memories invented in the fifties; see Livingston (1997) and references therein. The fact that the electrical current flows perpendicular to the layers in an MTJ (due to the quantum-mechanical tunneling process across the insulator) rather than in the plane of the layers (as in GMR) allows for an efficient use of word and bit lines addressing individual bits. This, together with the huge magnetoresistances of MTJs paved the way to a fast implementation in memory applications. In fact, new nonvolatile solid-state memories based on magnetic tunnel junctions have entered the market in the beginning of the new millennium. In Fig. 1.3a a schematics is shown of one bit cell within a so-called magnetic random access memory or MRAM. It is shown how to use the magnetoresistance effect (as displayed in Fig. 1.2) to store information in a solid-state device. In this system one of the layers, the reference layer, is always pointing in one direction (in Fig. 1.3b to the right), which means that the applied magnetic fields created by the orthogonal word and bit line should never exceed its coercivity. On the other hand, the softer magnetic layer is used to actually store the information, and is switched by a small magnetic field to create a zero-field state with low or high resistivity, corresponding to a logical “0” or “1”. The reader is referred to Tehrani et al., 2000, Tehrani et al., 2003, de Boeck et al. (2002), Parkin et al. (2003), DeBrosse et al. (2004), Shi (2005), and references therein, for papers on MRAM technology.

Although the magnetoresistance effects in MTJs have been reproducibly reported by many groups, and applications are being developed since then, the fundamental issues in explaining the observed effects are far from fully understood, and need a careful introduction. In the following, it is explained how the existence of TMR can be predicted in the most elementary phenomenological model capturing some of the basic fundamental properties of these devices. This will serve as a starting point for a further exploration of the underlying physics, which is addressed later on in the review.

In elementary textbooks on quantum mechanics, the tunneling current through a potential barrier is extensively treated, illustrating the finite probability for an electron to tunnel through energetically forbidden barriers. Within the Wentzel-Kramers-Brillouin (WKB) approximation, which is valid for potentials U varying slowly on the scale of the electron wavelength, the transmission probability across a potential barrier is in one dimension proportional to:T(E)exp(20t2me[U(x)E]/2dx) with E the electron energy, me the electron mass, and x the direction perpendicular to the barrier plane. This equation directly shows the well-known exponential dependence of tunnel transmission on the thickness t and energy barrier U(x)E. Note that the electron momentum in the plane of the layers is assumed to be absent, i.e., k=0. In fact, when electrons are impinging the barrier under an off-normal angle (k0), the tunneling probability rapidly decreases with increasing k since in that case the term 2m[U(x)E]/2 in the exponent of the transmission should be replaced by 2m[U(x)E]/2+k2.

In an experimental situation, this tunneling process can be measured in a metal-oxide-metal structure, a trilayered structure of two metals or electrodes separated by an insulating spacer. The thickness of the spacer is in the order of just 1 nanometer, a few atomic distances, otherwise the exponentially decaying tunneling current (proportional to the transmission in Eq. (1)) becomes immeasurably small. The metal-oxide-metal junction is drawn in Fig. 1.4 where the potential of the barrier U(x) is assumed to be constant across the barrier and located at an energy ϕ above the Fermi energy EF of the metals. Without a voltage difference between the metals layers, the Fermi levels will be equal on either side of the barrier, and the tunnel current is zero. When a finite bias voltage V is applied, the Fermi level is lowered at the right-hand side of the barrier, and electrons are now able to elastically tunnel from filled electron states (left) towards unoccupied states in the second (right) electrode. Note that in this case the electrode at right is at a higher electrical potential as compared to the left electrode, yielding a net electrical current from right to left. As a result, the amount of current will be proportional to the product of the available, occupied electron states on the left, and the number of empty states at the right electrode, multiplied by the barrier transmission probability. Therefore, the tunneling current is directly proportional to the density-of-states of each electrode (at a specific energy E) multiplied by the Fermi–Dirac factors f(E) and 1f(E) to account for the amount of occupied and unoccupied electron states, respectively.

To analytically calculate the net tunneling current in the metal-oxide-metal structure, we first write the current due to electrons tunneling from left to right assuming an elastic (energy-conserving) electron tunneling process from occupied states on the left to empty states at the right (see the figure):ILR(E)NL(EeV)f(EeV)T(E,V,ϕ,t)NR(E)[1f(E)]. As indicated by Eq. (1), the transmission T(E,V,ϕ,t) depends on the electron energy and barrier thickness and potential, but it is also affected by the bias voltage V that effectively reduces the barrier height ϕ. For the opposite current we write a similar equation, by which the total current I is obtained by integrating ILRIRL over all energies:I+NL(EeV)T(E,V,ϕ,t)NR(E)[f(EeV)f(E)]dE. For small voltages eVϕ only the electrons at (or close to) the Fermi level EF contribute to the tunneling current, by which the transmission no longer depends on energy E. Moreover, in this limit also the density-of-states factors are in principle independent of E, which reduces the current to:INL(EF)NR(EF)T(ϕ,t)+[f(EeV)f(E)]dE. For low enough temperature (kBTeV) the integral over the Fermi functions simply yields eV, by which we end up with a transparant expression for the tunnel conductance:GdI/dVNL(EF)NR(EF)T(ϕ,t).

It shows that in this simple model the tunnel conductance is proportional to the transmission probability and the density-of-states of the two electron systems. The explicit dependence of the density-of-states factors is originally proposed by the pioneering theoretical work of Bardeen (1961), now referred to the transfer-Hamiltonian method (see Wolf, 1985). Note that usually in this method the probability T(ϕ,t) is written as |M|2, which is the squared transfer matrix element that determines the tunneling transition rate between an initial and final state.

Now we can proceed with evaluating the current in a magnetic junction, that is, two magnetic electrodes separated by a nonmagnetic insulator (see Fig. 1.5). The density-of-states of a ferromagnetic material is represented by a simple majority and minority electron band, shifted in energy due to exchange interactions. First, we consider two identical ferromagnetic electrodes with parallel magnetization orientations, separated by an insulating barrier. Assuming that the electron spin is conserved in these processes (Tedrow and Meservey, 1971a), tunneling may only occur between bands of the same spin orientation in either electrode, i.e., from a spin majority band to a spin majority band, and similar for the minorities. Using Eq. (5) and assuming equal transmission for both spin species, we write the conductance for parallel magnetization as:GP=G+GNmaj2(EF)+Nmin2(EF), where G() is the conductance in the up- (down-) spin channel, and Nmaj(EF) (Nmin(EF)) is the majority (minority) density-of-states at EF. When we switch the magnetization orientation of one ferromagnetic electrode relative to that of the other ferromagnetic electrode, the axis of spin quantization is also changed in that electrode. Tunneling between like spin orientations now means tunneling from a majority to a minority band, and vice versa. The conductance for antiparallel aligned magnetization is then simply:GAP=G+G2Nmaj(EF)Nmin(EF). It is immediately clear that conductances are different for parallel and antiparallel magnetizations. In other words, ferromagnetic tunnel junctions display a magnetoresistance when an external field is used to switch between these magnetic orientations. This tunnel magnetoresistance (TMR) is usually defined as the difference in conductance between parallel and antiparallel magnetizations, normalized by the antiparallel conductance, or, alternatively, as the resistance change normalized by the parallel resistance:TMRGPGAPGAP=RAPRPRP. Note that the equality of the two definitions for TMR is only valid for very small bias voltage, since in that case the inverse tunnel resistance R−1=I/V is identical to the conductance dI/dV. In literature on MTJs, another, more pessimistic definition of TMR is used as well, normalizing the resistance change by the resistance in antiparallel instead of parallel orientation. However, throughout the review, Eq. (8) will be strictly applied to quantify the magnetoresistance ratio in magnetic junctions. Using Eqs. (6), (7), it is easily derived that TMR is equal to [Nmaj(EF)Nmin(EF)]2/[2Nmaj(EF)Nmin(EF)]. We can generalize this for two different magnetic electrodes, resulting in the well-known Julliere-formula for the magnetoresistance of MTJ's (Julliere, 1975):TMR=2PLPR1PLPR, where PL(R) is the tunneling spin polarization in the left (right) ferromagnetic electrode. The tunneling spin polarization of each electrode is defined asP=Nmaj(EF)Nmin(EF)Nmaj(EF)+Nmin(EF), and is simply the normalized difference in majority and minority density-of-states at the Fermi level. From these equations it is immediately seen that in the limit of zero polarization of one of the electrodes, no TMR is expected. On the other hand, for a full polarization of ±1, the TMR becomes infinitely high. These fully polarized materials (one spin channel is absent at the Fermi level) are referred to as being half-metallic, and have been intensively investigated in this field; see also section 4.4.

In an experimental study, Julliere (1975) is the first to use Eqs. (9), (10) for TMR in Fe-Ge-Co junctions, although in principal with a different interpretation of tunneling spin polarization. N(EF) is defined as an effective number of tunneling electrons to stress the fact that the tunneling process is not only governed by the (static) density-of-states at EF. We will return to this crucial point later on. Nevertheless, it should be emphasized that the Julliere equation in its simplest form demonstrates the fundamental role of the tunneling spin polarization of the ferromagnetic electrode in understanding the observed TMR in magnetic junctions. The tunneling spin polarization of individual magnetic electrodes can be measured with a so-called superconducting tunneling spectroscopy (STS) technique that uses a superconductor (in most cases Al) to probe the spin imbalance in tunneling currents. In more detail, in a ferromagnetic-Al2O3-Al junction a magnetic field splits up the sharply-peaked density-of-states of the superconducting Al electrode, which leads to an asymmetry in the conductance G(V) that reflects the amount of spin polarization. In section 3 this will be further introduced, here only a numerical example will be given. The tunneling spin polarization for Co is experimentally determined to be around +0.42, which via Eq. (9) corresponds to a TMR effect of more than 40% for Co-Al2O3-Co MTJs. This is only slightly above the observed (low-temperature) value. For the moment, it seems that we can use this formula as a phenomenological equation that nicely connects tunneling polarization P to the magnitude of the magnetoresistance. However, as we will see below, the physics of spin-polarized tunneling is much more complex and needs a dramatic reconsideration of these phenomena.

Although the model we have introduced captures some of the basic physics in magnetic tunnel junctions and is rather illustrative on a tutorial level, it fails to predict a number of experimental observations. These observations for TMR include, for instance:

  • strong dependence of TMR on the applied bias voltage V and temperature T

  • sensitivity of TMR on the electronic structure of the barrier-ferromagnetic interface region, not just the bulk density-of-states (as suggested by Eqs. (9), (10))

  • relevance of the electronic structure of the barrier, in some cases even leading to an inversion of TMR.

Here we will briefly introduce some of the advanced theories to better appreciate these observations, focusing at this point on the tunneling spin polarization for its fundamental role in the physics of magnetic tunnel junctions. A more detailed treatment will be postponed for sections 3 Tunneling Spin Polarization, 4 Crucial Experiments on Spin-Dependent Tunneling.

Later on in this review (Table 1.2 in section 3) we will show that the tunneling spin polarization of the 3d ferromagnetic metals are all positive, and in the range of 40–60%. According to the definition of Eq. (10), the positive sign of the polarization relates to a dominant majority density-of-states at the Fermi level. If one considers the band structure and density-of-states of the 3d metals, however, the situation is completely reversed. As an example, Fig. 1.6 shows the (calculated) density-of-states of Co and Ni, both having a surplus of minority states of the Fermi level. This would suggest a negative tunneling spin polarization, and completely contradicts the experimental observations. This dichotomy was recognized already in the seventies when pioneering experiments in the field of superconducting tunneling spectroscopy were reported on ferromagnetic-superconducting junctions (Tedrow and Meservey, 1971a, Tedrow and Meservey, 1971b, Tedrow and Meservey, 1975). Theoretically, Stearns (1977) has shown that the conductance in a tunnel junction is not simply determined by the electron density-of-states at the Fermi level, but should include the probability for them to tunnel across an ultrathin barrier. Especially the most mobile s-like electron states are able to tunnel with a much larger probability as compared to the d electrons due to their different effective mass. Based on this, Stearns could explain the positive spin polarization by considering the spin asymmetry of the s-like energy bands, thereby neglecting the contribution from the rapidly decaying d-like wave functions in tunneling experiments.

More recently, another advanced aspect of spin-polarized tunneling is reported. Slonczewski (1989) emphasizes that spin-dependent tunneling is not a process solely related to the (complex) electronic properties of the ferromagnetic electrodes. He has analytically calculated the tunneling current between free-electron ferromagnetic metals within the WKB approximation (see Eq. (1)), assuming that tunneling electrons have a very small parallel wave vector, close to k=0. By explicitly matching the electron wave functions at the barrier interfaces, the tunneling spin polarization is calculated as:P=P0×κ2kF,majkF,minκ2+kF,majkF,min, where kF,maj and kF,min are the Fermi wave vectors, and κ the imaginary component of the wave vector of electrons in the barrier with k=0 at the Fermi level, corresponding to κ=(2meϕ/2)1/2 with ϕ the height of the barrier. The first term P0 is equal to the earlier result in Eq. (10). The second term, however, contains the properties of the barrier as well, and is due to the discontinuous change of the potential at the interface with the barrier. As a result of this interface factor, the polarization becomes greatly dependent on the band parameters in relation to the height of the barrier, with the possibility to even change the sign of P. This is in fact a first demonstration that tunneling spin polarization is not an intrinsic property solely determined by the ferromagnetic electrode. A similar conclusion is reached in free-electron calculations where the conductance is analytically obtained by matching the free-electron wave functions (and its derivatives) at the two interfaces (MacLaren et al., 1997). In this free-electron calculation, also electrons with k0 are considered, although k is assumed to be strictly conserved upon tunneling. In Fig. 1.7 the free-electron magnetoresistance calculated by Slonczewski (1989) and MacLaren et al. (1997) is plotted as a function of polarization P=(kF,majkF,min)/(kF,maj+kF,min), which is equivalent to P0 in Eq. (11). For thick barriers, the solutions in the calculation of MacLaren et al. (1997) approach the model of Slonczewksi based on the WKB approximation, whereas no correspondence is found with the Julliere expression. However, it should be stressed that the predictability of this elementary, simplified free-electron model is rather poor. As already pointed out by Harrison (1961), this is related to the suspicious absence of density-of-states factors in the transport characteristics. MacLaren et al. (1997) and Zhang and Levy (1999) emphasize that, generally, these free-electron calculations (including the Julliere model) fail to predict the observed magnetoresistance behavior in magnetic junctions, and its dependencies on, e.g., barrier thickness, barrier height, and bias voltage. Nevertheless, there are some attempts to directly use Slonczewski's or other free-electron calculations to investigate how TMR behaves as a function of the model parameters. For an example, see the work of Tezuka and Miyazaki (1998) on the variation of TMR with the Al2O3 barrier height.

After the work of Slonczewski (and free-electron calculations by others), a great number of advanced theoretical investigations have been published to further explore the physics of TMR and tunneling spin polarization; see for example the review paper of Zhang and Butler (2003). Along with that, experimental evidence has become gradually available that shows, e.g., the decisive role of the barrier-electrode combination for spin-polarized tunneling. Other exciting observations have been reported, such as the role of crystallinity and orientation of the magnetic electrode, oscillations in TMR due to the presence of nonmagnetic layers favoring quantum well states, and unprecedented, giant TMR in junctions when incorporating half-metallic electrodes or, more recently, crystalline MgO barriers. Especially in sections 3 Tunneling Spin Polarization, 4 Crucial Experiments on Spin-Dependent Tunneling these developments will be extensively addressed.

It is the purpose of this review to introduce the reader to the most important aspects of spin-polarized tunneling. We have just seen that spin polarization in MTJs is a complex parameter heavily dependent on the details of the potential the electrons experience when crossing the barrier region. This in turn strongly influences the fabrication process of MTJs, where obviously utmost care should be taken in designing and characterizing the barrier and the interface regions with the ferromagnetic metals. Barriers in MTJs are traditionally made out of oxidized Al for their relative ease to create superior coverage of the metallic electrode, together with the observation of large magnetoresistances. A huge research effort could be witnessed in the late nineties to optimize the oxidation process for enhancing the functionality and reliability of MTJs. Furthermore, a number of oxidation methods have been explored in great detail, in particular the use of an oxygen plasma to gradually oxidize a previously deposited Al layer. The physical properties and optimization of the barrier and adjacent ferromagnetic layers are the topic of section 2. Also in this section the basic design rules for a magnetic junction will be discussed together with elementary transport properties, such as the dependence of TMR on bias voltage, barrier thickness, and temperature.

In section 3 we return to the physics of tunneling spin polarization. Details will be given on the experimental method involving superconducting probe layers, followed by a more in-depth discussion on the basic fundamental ingredients. Topics of interest are the relation between tunneling spin polarization and the ferromagnetic magnetization, the relevance of the barrier-electrode interface region including the local chemical bonding, and the relevance of the symmetry of the wave functions of tunneling electrons. Section 4 reviews a number of crucial experiments in the field of TMR in magnetic junctions. Especially those topics will be highlighted that have contributed to the understanding of the underlying physics of spin tunneling, e.g., addressing the role of the interfaces with the barrier, the (local) density-of-states of the magnetic layers, half-metallic or epitaxial ferromagnetic electrodes, and tunneling across crystalline barriers (such as MgO or SrTiO3). The review will be concluded by briefly considering some of the promising directions within this field of magnetic junctions or, in a wider perspective, the field of hybrid devices where tunnel barriers are often combined with new materials to create new physics or functionality. This includes, for example, the development of all-semiconductor MTJs, the use of magnetic semiconductors as (spin-filter) barriers, and the realization of three-terminal magnetic tunnel transistors.

Section snippets

Basis Phenomena in MTJs

The fabrication of a properly operating insulating tunnel barrier, separating the magnetic electrodes, has developed as a wide and very active research field where many aspects on oxide growth, characterization, magnetism, and transport are being considered. Although there are several ways to fabricate barriers for MTJs, a clear distinction can be noticed between crystalline and amorphous barriers. The amorphous Al2O3 barriers are most extensively studied due to the ability to serve as an

Tunneling Spin Polarization

As we have seen in the introduction (section 1), the degree of spin polarization is the key ingredient for the magnetoresistance effect in magnetic junctions. Generally, however, in literature the physical property of spin polarization is defined in several different ways. To start with, spin polarization is sometimes related directly to magnetization of ferromagnetic metals, i.e. the difference between the number of spin-up and spin-down electrons. In transport experiments, it is clear that

Crucial Experiments on Spin-Dependent Tunneling

This section deals with a number of key experiments in the area of magnetic tunnel junctions. Although the field of magnetic tunneling is not settled and is still further developing, contributions are selected that are believed to be critically important for better understanding the physics behind magnetoresistance effects in MTJs. Through the direct relation between tunneling spin polarization and TMR, experiments are to a great extent focused on similar aspects as introduced in the previous

Outlook

In the foregoing sections, the physics of spin-dependent tunneling is addressed focusing on a number of critical scientific breakthroughs in the field of MTJs. Since the first discoveries in the mid-nineties, the field is rapidly expanding in many other directions, related to various alternative hybrid material combinations often motivated by new application potential for solid-state devices. A few prominent examples are:

  • hybrid semiconductor magnetic tunnel junctions

  • tunnel barriers used for

Acknowledgements

Wim de Jonge, Corné Kant, Karel Knechten, Jürgen Kohlhepp, Bert Koopmans, Patrick LeClair, and Paresh Paluskar are acknowledged for many useful discussions and for critically reading this manuscript. Some of the results presented here are embedded in research programs of the Technology Foundation STW and the “Stichting voor Fundamenteel Onderzoek der Materie (FOM)”, both financially supported by the “Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO)”.

References (497)

  • J.A. Appelbaum et al.

    Phys. Rev.

    (1969)
  • J.A. Appelbaum et al.

    Phys. Rev. B

    (1970)
  • J.Y. Bae et al.

    Jpn. J. Appl. Phys.

    (2005)
  • M.N. Baibich et al.

    Phys. Rev. Lett.

    (1988)
  • J. Bardeen

    Phys. Rev. Lett.

    (1961)
  • F. Bardou

    Europhys. Lett.

    (1997)
  • K.D. Belashchenko et al.

    Phys. Rev. B

    (2005)
  • K.D. Belashchenko et al.

    Phys. Rev. B

    (2004)
  • K.D. Belashchenko et al.

    Phys. Rev. B

    (2005)
  • R. Bertacco et al.

    Phys. Rev. B

    (2004)
  • M. Bibes et al.

    Appl. Phys. Lett.

    (2003)
  • M. Bibes et al.

    Appl. Phys. Lett.

    (2003)
  • G. Binasch et al.

    Phys. Rev. B

    (1989)
  • H. Boeve et al.

    IEEE Trans. Magn.

    (1999)
  • H. Boeve et al.

    J. Appl. Phys.

    (2001)
  • H. Boeve et al.

    Appl. Phys. Lett.

    (2000)
  • H. Boeve et al.

    J. Appl. Phys.

    (1998)
  • M. Bowen et al.

    Phys. Rev. Lett.

    (2005)
  • M. Bowen et al.

    J. Phys.: Condens. Matter

    (2005)
  • M. Bowen et al.

    Appl. Phys. Lett.

    (2003)
  • M. Bowen et al.

    Appl. Phys. Lett.

    (2001)
  • R.M. Bozorth

    Ferromagnetism

    (1993)
  • A.M. Bratkovsky

    Phys. Rev. B

    (1997)
  • A.M. Bratkovsky

    Phys. Rev. B

    (1998)
  • L. Brey et al.

    Appl. Phys. Lett.

    (2004)
  • W.F. Brinkman et al.

    J. Appl. Phys.

    (1970)
  • H. Bruckl et al.

    Appl. Phys. Lett.

    (2001)
  • P. Bruno

    Phys. Rev. B

    (1995)
  • R. Bubber et al.

    IEEE Trans. Magn.

    (2002)
  • D.E. Bürgler et al.

    Interlayer exchange coupling in layered magnetic structures

  • W.H. Butler et al.

    Phys. Rev. B

    (2001)
  • W.H. Butler et al.

    Phys. Rev. B

    (2001)
  • W.H. Butler et al.

    IEEE Trans. Magn.

    (2005)
  • P.F. Carcia et al.

    Appl. Phys. Lett.

    (1985)
  • S. Cardoso et al.

    J. Appl. Phys.

    (2005)
  • S. Cardoso et al.

    IEEE Trans. Magn.

    (2004)
  • S. Cardoso et al.

    Appl. Phys. Lett.

    (2000)
  • S. Cardoso et al.

    J. Appl. Phys.

    (2000)
  • S. Cardoso et al.

    Appl. Phys. Lett.

    (2000)
  • S. Cardoso et al.

    J. Appl. Phys.

    (2001)
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