Device properties of the spin-valve transistor and the magnetic tunnel transistor

Spin electronics is a new research area, which not only uses the electron’s charge but also its spin. By using the electron spin dependent properties of magnetic materials one can make devices with a new functionality. This has lead to magnetoresistive devices that can change their resistance by 10 to 50% in small magnetic fields, such as giant magnetoresistance (GMR) devices and the magnetic tunnel junction (MTJ). This large resistance change can be used in applications such as read heads or serve as memory elements in a magnetic random access memory (MRAM). This thesis describes two devices: the spin-valve transistor (SVT) and the magnetic tunnel transistor (MTT). The SVT has an unique property, namely its huge relative collector current change of more than 300% in small magnetic fields at room temperature. This unique property by itself is not enough to warrant the applicability of the SVT. The other properties that are important for the applicability of the SVT are described in this thesis. An alternative to the SVT, the MTT, will also be discussed. The SVT is a hybrid device that generally has an n-Si/ Pt / Ni82Fe18/ Au/ Co/ Au/ n-Si structure. The Pt / Ni82Fe18/ Au/ Co/ Au multi layer is the base and the two semiconductors on each side are the emitter and the collector respectively. The SVT is used in the common base configuration, where the emitter barrier (Si/Pt) is forward biased and the collector diode is zero or reversed biased. A flow of electrons from the silicon over the Schottky barrier into the metal base starts when the emitter is forward biased. These electrons have an excess energy compared to the Fermi level of the base and move in the direction of the collector. The electrons that are scattered in the base will lose their energy or momentum and make up the base current. Only those electrons that reach the collector with the right momentum and a high enough energy can enter the collector. The collector current is thus extremely sensitive to the scattering conditions in the base. The scattering conditions in the Ni82Fe18 and the Co layer are different for the spin-up and spin-down electrons. This makes the total scattering dependent on the relative magnetizations of the Ni82Fe18 and the Co layer. The collector current is largest when the magnetizations are parallel (I P C ) and smallest when the magnetizations are anti-parallel aligned (I AP C ). The relative change in collector current is called the magnetocurrent (MC = (I P C - I AP C )/I AP C ). This PhD research started with the development of a reliable process for fabricating spin-valve transistors. The introduction of this process together with the introduction of an ultra-high vacuum metal-evaporation system and the right choice of materials resulted in the SVT’s that exhibit an MC of more than 300% at room temperature. This thesis starts with a study on the size dependence of the magnetic and electrical properties of the SVT. We extended the previously mentioned process by using silicon on insulator (SOI) wafers, a combination of dry and wet etching techniques and SU8 (a negative tone photoresist) as an insulator layer. We were successful in producing SVTs with lateral dimensions that ranged from 300µm by 300µm to 10µm by 10µm. As expected, we saw no influence of the dimensions on the Schottky barrier height. Moreover the reverse current scaled down linearly with area. Both observations show that we have high-quality Schottky diodes. The key property of the SVT, its MC, showed no size dependence and remained constant around 240% for all dimensions. The transfer ratio is the ratio between the applied emitter current and the measured collector current. This ratio showed a slight decrease for transistors with dimensions below 25µm by 25µm. This is attributed to a deterioration of the emitter efficiency. The maximum possible emitter current decreases with transistor dimensions. The limiting factor is the maximum possible current density in the spin-valve base, which is 1.7 × 10 7 A/cm 2 . This value agrees with electromigration failure of spin valves. We have shown that it is possible to scale the lateral dimensions of the SVT down to 10µm by 10 µm. In my view further scaling down is limited to the physical height needed for the emitter, which includes the depletion width for the Schottky barrier and the doping profile needed for the Ohmic contact. To characterize the noise sources of the SVT we studied the frequency spectrum of three types of transistors that differed only in the type of metal base. The measurement showed that the frequency spectrum of the transistor with only non-magnetic layers in the base was completely dominated by shot noise in the frequency range of the measurement (10 Hz to 100kHz). The inclusion of one or more magnetic layers lowered the collector current and thus the level of the shot noise. It did not however change the nature of the noise or add noise (of magnetic origin) to the collector current. The collector current spectral density (SI) changes linearly with IC in a quasi-static magnetic field as expected for shot noise. We have however not observed 1/f noise in our measurements, not even at the switching fields of the spin valve. With this knowledge we can calculate the signal to noise ratio (SNR) of the SVT. The SNR increases with increasing MC and also with the absolute value collector current. From the basic relation IC = α IE we see that we can increase the collector current by either increasing the emitter current (IE) or the transfer ratio (alpha). We saw before that the emitter current has an upper limit imposed by device breakdown, therefore the way to enlarge IC is to improve the transfer ratio. We started to improve α by enlarging the energy difference between the emitter and collector barrier. The transfer ratio increased with increasing energy difference due to the larger number of states available at the collector semiconductor when electrons arrive with a higher energy. The transfer ratio also improves when materials with longer attenuation lengths are used in the base, i.e. Au instead of Pt. The influence of the SVT’s structural quality on the transfer ratio is demonstrated by the optimum in collector current versus Pt layer thickness. Furthermore, by varying the thickness of the NiFe layer we were able to prove that there is a maximum in the absolute current change for a certain thickness, due to the trade-off between transfer ratio and MC. The same study yielded a value for the attenuation of an interface, which is a factor of 0.55. The influence of crystal orientation on the transfer ratio was found to be negligible. Temperature effects on the transfer ratio are weak and are due to the spatial distribution of Schottky barrier heights and thermal spin wave scattering. Summarizing, we improved the transfer ratio by a factor of 118 from a Si/Pt/NiFe/Au/Co/Pt/Si SVT compared with a Si/Au/NiFe/Au/Co/Cu/Si transistor, while the MC remained constant above 200% and showed only small and non-systematic changes. The latter implies that the collection of both the spin-up and spin-down electrons can be improved, resulting in an increase in collector current without affecting the MC. The best results so far for SVTs are with a Si/ Au (20Å)/ Ni82Fe18 (30Å)/ Au (70Å)/ Co (30Å)/ Au (40Å)/ Si SVT, it has a transfer ratio of 1.2 × 10 4 and an MC of 230%. Further improvement of the transfer ratio might result from better control over the quality of the complete SVT structure. Another option is to use a tunnel barrier on the emitter side. This not only allows one to further enlarge the energy difference between the injected electrons and the collector Schottky barrier, but also opens up the possibility to remove layers from the base if a ferromagnetic emitter electrode is used, as in an MTT. Magnetic tunnel transistors have been successfully realized with the use of in situ shadow mask technology. Already we achieved a transfer ratio equal to that of SVTs, while the MCof the MTT is above 100%. The MTT has a Si/ Co/ Al2O3/ CoFe/ IrMn/ Ta structure. We have shown that the MTT can be used to determine a lower limit for the tunnel spin polarization of a ferromagnet/insulator interface. With a MTT this lower limit can be determined in a large temperature and tunnel-barrier bias range. The transfer ratio measured versus tunnel-barrier bias continues to increase, due to the larger number of available states at the collector at higher energies. More research is needed to explain the tunnelbarrier bias dependence of the MC. We expect that MTTs can be improved by using evaporation techniques rather than sputter techniques. Furthermore the quality of the collector diode can be improved with a corresponding increase in transfer ratio by choosing the right materials. The comparison of the SVT and MTT with tunnel junctions in terms of signal, noise, scalability, frequency response, robustness and of course the ability to study the properties of spin-polarized hot-electrons in magnetic materials justifies the further research of SVTs and MTTs. Last modified: May 16, 2002 by Hans

Direct comparison of current-induced spin polarization in topological insulator Bi2Se3 and InAs Rashba states

Three-dimensional topological insulators (TIs) exhibit time-reversal symmetry protected, linearly dispersing Dirac surface states. Band bending at the TI surface may also lead to coexisting trivial two-dimensional electron gas (2DEG) states with parabolic energy dispersion that exist as spin-split pairs due to Rashba spin-orbit coupling (SOC). A bias current is expected to generate spin polarization in both systems arising from their helical spin-momentum locking. However, their induced spin polarization is expected to be different in both magnitude and sign. Here, we compare spin potentiometric measurements of bias current-generated spin polarization in Bi2Se3(111) films where Dirac surface states coexist with trivial 2DEG states, with identical measurements on InAs(001) samples where only trivial 2DEG states are present. We observe spin polarization arising from spin-momentum locking in both cases, with opposite signs of the spin voltage. We present a model based on spin dependent electrochemical potentials to directly derive the signs expected for the TI surface states, and unambiguously show that the dominant contribution to the current-generated spin polarization measured in the TI is from the Dirac surface states. This direct electrical access of the helical spin texture of Dirac and Rashba 2DEG states is an enabling step towards the electrical manipulation of spins in next generation TI and SOC based quantum devices

Emergent electric field control of phase transformation in oxide superlattices.

Electric fields can transform materials with respect to their structure and properties, enabling various applications ranging from batteries to spintronics. Recently electrolytic gating, which can generate large electric fields and voltage-driven ion transfer, has been identified as a powerful means to achieve electric-field-controlled phase transformations. The class of transition metal oxides provide many potential candidates that present a strong response under electrolytic gating. However, very few show a reversible structural transformation at room-temperature. Here, we report the realization of a digitally synthesized transition metal oxide that shows a reversible, electric-field-controlled transformation between distinct crystalline phases at room-temperature. In superlattices comprised of alternating one-unit-cell of SrIrO3 and La0.2Sr0.8MnO3, we find a reversible phase transformation with a 7% lattice change and dramatic modulation in chemical, electronic, magnetic and optical properties, mediated by the reversible transfer of oxygen and hydrogen ions. Strikingly, this phase transformation is absent in the constituent oxides, solid solutions and larger period superlattices. Our findings open up this class of materials for voltage-controlled functionality

Large magneto-optical Kerr effect and imaging of magnetic octupole domains in an antiferromagnetic metal

When a polarized light beam is incident upon the surface of a magnetic material, the reflected light undergoes a polarization rotation. This magneto-optical Kerr effect (MOKE) has been intensively studied in a variety of ferro- and ferrimagnetic materials because it provides a powerful probe for electronic and magnetic properties as well as for various applications including magneto-optical recording. Recently, there has been a surge of interest in antiferromagnets (AFMs) as prospective spintronic materials for high-density and ultrafast memory devices, owing to their vanishingly small stray field and orders of magnitude faster spin dynamics compared to their ferromagnetic counterparts. In fact, the MOKE has proven useful for the study and application of the antiferromagnetic (AF) state. Although limited to insulators, certain types of AFMs are known to exhibit a large MOKE, as they are weak ferromagnets due to canting of the otherwise collinear spin structure. Here we report the first observation of a large MOKE signal in an AF metal at room temperature. In particular, we find that despite a vanishingly small magnetization of $M \sim$0.002 $\mu_{\rm B}$/Mn, the non-collinear AF metal Mn$_3$Sn exhibits a large zero-field MOKE with a polar Kerr rotation angle of 20 milli-degrees, comparable to ferromagnetic metals. Our first-principles calculations have clarified that ferroic ordering of magnetic octupoles in the non-collinear Neel state may cause a large MOKE even in its fully compensated AF state without spin magnetization. This large MOKE further allows imaging of the magnetic octupole domains and their reversal induced by magnetic field. The observation of a large MOKE in an AF metal should open new avenues for the study of domain dynamics as well as spintronics using AFMs.Comment: 30 pages, 4 figure

Paleomagnetism indicates that primary magnetite in zircon records a strong Hadean geodynamo.

Determining the age of the geomagnetic field is of paramount importance for understanding the evolution of the planet because the field shields the atmosphere from erosion by the solar wind. The absence or presence of the geomagnetic field also provides a unique gauge of early core conditions. Evidence for a geomagnetic field 4.2 billion-year (Gy) old, just a few hundred million years after the lunar-forming giant impact, has come from paleomagnetic analyses of zircons of the Jack Hills (Western Australia). Herein, we provide new paleomagnetic and electron microscope analyses that attest to the presence of a primary magnetic remanence carried by magnetite in these zircons and new geochemical data indicating that select Hadean zircons have escaped magnetic resetting since their formation. New paleointensity and Pb-Pb radiometric age data from additional zircons meeting robust selection criteria provide further evidence for the fidelity of the magnetic record and suggest a period of high geomagnetic field strength at 4.1 to 4.0 billion years ago (Ga) that may represent efficient convection related to chemical precipitation in Earth's Hadean liquid iron core

Magnetic Field-Induced Spin Nematic Phase Up to Room Temperature in Epitaxial Antiferromagnetic FeTe Thin Films Grown by Molecular Beam Epitaxy

Electronic nematicity, where strong correlations drive electrons to align in a way that lowers the crystal symmetry, is ubiquitous among unconventional superconductors. Understanding the interplay of such a nematic state with other electronic phases underpins the complex behavior of these materials and the potential for tuning their properties through external stimuli. Here, we report magnetic field-induced spin nematicity in a model system tetragonal FeTe, the parent compound of iron chalcogenide superconductors, which exhibits a bicollinear antiferromagnetic order. The studies were conducted on epitaxial FeTe thin films grown on SrTiO3(001) substrates by molecular beam epitaxy, where the bicollinear antiferromagnetic order was confirmed by in situ atomic resolution scanning tunneling microscopy imaging. A 2-fold anisotropy is observed in in-plane angle-dependent magnetoresistance measurements, indicative of magnetic field-induced nematicity. Such 2-fold anisotropy persists up to 300 K, well-above the bicollinear antiferromagnetic ordering temperature of 75 K, indicating a magnetic field-induced spin nematic phase up to room temperature in the antiferromagnet FeTe

Room-Temperature Spin Filtering in Metallic Ferromagnet–Multilayer Graphene–Ferromagnet Junctions

We report room-temperature negative magnetoresistance in ferromagnet–graphene–ferromagnet (FM|Gr|FM) junctions with minority spin polarization exceeding 80%, consistent with predictions of strong minority spin filtering. We fabricated arrays of such junctions <i>via</i> chemical vapor deposition of multilayer graphene on lattice-matched single-crystal NiFe(111) films and standard photolithographic patterning and etching techniques. The junctions exhibit metallic transport behavior, low resistance, and the negative magnetoresistance characteristic of a minority spin filter interface throughout the temperature range 10 to 300 K. We develop a device model to incorporate the predicted spin filtering by explicitly treating a metallic minority spin channel with spin current conversion and a tunnel barrier majority spin channel and extract spin polarization of at least 80% in the graphene layer in our structures. The junctions also show antiferromagnetic coupling, consistent with several recent predictions. The methods and findings are relevant to fast-readout low-power magnetic random access memory technology, spin logic devices, and low-power magnetic field sensors