Projects

1.1-A New materials research for high spin polarization (Naganuma, Tezuka – Sendai)

Controlling high spin polarized currents is most important factor for spintronics devices. For this purpose, magnetic tunnel junctions using Co based full Heusler alloy as electrode and spin-filter devices using oxide materials (includes CoFe₂O₄, Fe₃O₄, γ-Fe₂O₃, BiFeO₃, etc.) as barrier are attracted much attention. It has been reported that the atomic disordering, defect, and interfacial structure have a significant influence on the electronic structure and their magneto-transport properties. Thus, knowing atomic ordering, defects, and interfacial structure in conjunction with magneto-transport properties is of importance to obtain a high spin polarization current. We examine the influence of electric structure on the atomic ordering, defect, and interfacial structure in these alloys films, to realize the control of highly spin polarization current.

1.2-A Theory of the electronic and magnetic structure of advanced spintronic materials with emphasis on spectroscopy and properties (Chadov, Fecher – Mainz)

This project develops and applies theoretical methods to investigate the electronic structure of advanced spintronic materials and calculates their relevant physical properties that are based on the band structure and density of states that are magnetic moments, Curie temperatures, phonon spectra, exchange coupling and magnetic anisotropies including reorientation transitions, magnon spectra, specific heat, conductivity, magnetoresistance, optical, photoemission and photoabsorption spectra. Extensions of LSDA to include dynamical correlations will be of importance to explain the results of all experimental groups working with spectroscopic methods. Stepwise, the methods will be improved and applied to the materials investigated by ASPIMATT. Attention will be paid on the design of new semiconducting as well as tetragonal materials with high Z atoms (e.g.: Mn_2-xYGa, Y=Fe, Pt) exhibiting perpendicular anisotropy for CPP-GMR and other devices for spintransport. Special emphasis will be given on interfaces, spin dependent transport phenomena, spectroscopic properties and photoemission.

1.3-A Investigation of the electronic structure of buried layers and interfaces using high energy photoemission (Balke, Fecher, Felser – Mainz)

This project will produce thin films of advanced spintronic materials and performs in-situ investigation by photoelectron spectroscopy. The investigation of the bulk electronic structure by photoemission is already challenging and even higher demands occur in the case of buried layers and interfaces. Photoemission with hard X-ray excitation (HAXPES) in combination with spin resolution is, at present, the only method that is able to solve that problem for spintronic materials. HAXPES will be used to investigate the electronic structure of buried layers, interfaces, and devices. The in-situ study of the thin films allows to investigate the electronic structure step-by-step during the growth of devices (eg. for CPP-GMR) to optimize their physical properties. Besides the materials used in other groups of ASPIMATT (e.g.: Co2YZ) new tetragonal Heusler compounds (e.g.: Mn2-xYGa, Y=Fe, Pt) with perpendicular anisotropy will be investigated. The combination of thin film production with HAXPES in a laboratory system will allow the in-situ investigation of the electronic structure. High resolution valence band spectroscopy of the materials will be performed at synchrotron facilities.

1.4-B Theoretical studies on spin-dependent transport phenomena in heterostructures based on half-metallic Heusler alloys (Shirai – Sendai)

We investigate spin-dependent transport phenomena in heterostructures composed of half-metallic Heusler alloys (Co₂CrAl, Co₂FeSi, Co₂MnSi) and non-magnetic metals (Cr, Ag, Au) or semiconductors (Si, Ge, GaAs) by using first-principles calculations on the basis of the density functional theory.  The goals of our project are as follows: (1) theoretical assessment of half-metallic Heusler alloys as a spin polarizer in current perpendicular to plane (CPP) giant magnetoresistance (GMR) and other spintronic devices, and (2) providing guidelines for realizing efficient spin injection from half-metallic Heusler alloys into semiconductors. In particular, the influences of thermal spin fluctuation as well as atomic disorder on spin-dependent transport phenomena should be inspected extensively in order to confirm the usefulness of half-metallic Heusler alloys.  The theoretical findings could offer an insight into microscopic interpretations of observed phenomena and also useful guidelines for focusing target materials in realizing required properties.

1.5-B Direct measurement of spin-injection efficiency and dynamics in model systems for advanced Heusler-based spintronics devices (Cinchetti, Aeschlimann – K’lautern)

This project addresses the fundamental issues connected to the realization of advanced pure Heusler-based room temperature spintronics devices. Focus will be put on the basic building block of every such device, namely the heterojunction between a spin injector (SI) and a spin conductor (SC) material. The specific functionality of SI/SC building blocks will be characterized in terms of two fundamental properties: (i) the spin-injection efficiency at the SI-SC interface; and (ii) the spin diffusion length in the SC itself. Experimentally, the quantities (i) and (ii) will be accessible by implementing an established experimental strategy based on the spin-and time-resolved two-photon photoemission (STR-2PPE). It allows to correlate the efficiency of spin-injection with the static spin-dependent properties of the considered interface as well as to distinguish unambiguously between interface and bulk spin-dependent properties of the considered SI/SC heterojunction. The provided experimental evidence will allow identifying the best suited SI/SC building blocks for developing the foundations of future advanced spintronics devices.

Task 1: New materials for spin filter effect and perpendicular transport

1.1-A New materials research for high spin polarization (Naganuma, Tezuka – Sendai)

Controlling high spin polarized currents is most important factor for spintronics devices. For this purpose, magnetic tunnel junctions using Co based full Heusler alloy as electrode and spin-filter devices using oxide materials (includes CoFe2O4, Fe3O4, γ-Fe2O3, BiFeO3, etc.) as barrier are attracted much attention. It has been reported that the atomic disordering, defect, and interfacial structure have a significant influence on the electronic structure and their magneto-transport properties. Thus, knowing atomic ordering, defects, and interfacial structure in conjunction with magneto-transport properties is of importance to obtain a high spin polarization current. We examine the influence of electric structure on the atomic ordering, defect, and interfacial structure in these alloys films, to realize the control of highly spin polarization current.

1.2-A Theory of the electronic and magnetic structure of advanced spintronic materials with emphasis on spectroscopy and properties (Chadov, Fecher – Mainz)

This project develops and applies theoretical methods to investigate the electronic structure of advanced spintronic materials and calculates their relevant physical properties that are based on the band structure and density of states that are magnetic moments, Curie temperatures, phonon spectra, exchange coupling and magnetic anisotropies including reorientation transitions, magnon spectra, specific heat, conductivity, magnetoresistance, optical, photoemission and photoabsorption spectra. Extensions of LSDA to include dynamical correlations will be of importance to explain the results of all experimental groups working with spectroscopic methods. Stepwise, the methods will be improved and applied to the materials investigated by ASPIMATT. Attention will be paid on the design of new semiconducting as well as tetragonal materials with high Z atoms (e.g.: Mn_2-xYGa, Y=Fe, Pt) exhibiting perpendicular anisotropy for CPP-GMR and other devices for spintransport. Special emphasis will be given on interfaces, spin dependent transport phenomena, spectroscopic properties and photoemission.

1.3-A Investigation of the electronic structure of buried layers and interfaces using high energy photoemission (Balke, Fecher, Felser – Mainz)

This project will produce thin films of advanced spintronic materials and performs in-situ investigation by photoelectron spectroscopy. The investigation of the bulk electronic structure by photoemission is already challenging and even higher demands occur in the case of buried layers and interfaces. Photoemission with hard X-ray excitation (HAXPES) in combination with spin resolution is, at present, the only method that is able to solve that problem for spintronic materials. HAXPES will be used to investigate the electronic structure of buried layers, interfaces, and devices. The in-situ study of the thin films allows to investigate the electronic structure step-by-step during the growth of devices (eg. for CPP-GMR) to optimize their physical properties. Besides the materials used in other groups of ASPIMATT (e.g.: Co₂YZ) new tetragonal Heusler compounds (e.g.: Mn_2-xYGa, Y=Fe, Pt) with perpendicular anisotropy will be investigated. The combination of thin film production with HAXPES in a laboratory system will allow the in-situ investigation of the electronic structure. High resolution valence band spectroscopy of the materials will be performed at synchrotron facilities.

1.4-B Theoretical studies on spin-dependent transport phenomena in heterostructures based on half-metallic Heusler alloys (Shirai – Sendai)

We investigate spin-dependent transport phenomena in heterostructures composed of half-metallic Heusler alloys (Co₂CrAl, Co₂FeSi, Co₂MnSi) and non-magnetic metals (Cr, Ag, Au) or semiconductors (Si, Ge, GaAs) by using first-principles calculations on the basis of the density functional theory.  The goals of our project are as follows: (1) theoretical assessment of half-metallic Heusler alloys as a spin polarizer in current perpendicular to plane (CPP) giant magnetoresistance (GMR) and other spintronic devices, and (2) providing guidelines for realizing efficient spin injection from half-metallic Heusler alloys into semiconductors. In particular, the influences of thermal spin fluctuation as well as atomic disorder on spin-dependent transport phenomena should be inspected extensively in order to confirm the usefulness of half-metallic Heusler alloys.  The theoretical findings could offer an insight into microscopic interpretations of observed phenomena and also useful guidelines for focusing target materials in realizing required properties.

1.5-B Direct measurement of spin-injection efficiency and dynamics in model systems for advanced Heusler-based spintronics devices (Cinchetti, Aeschlimann – K’lautern)

This project addresses the fundamental issues connected to the realization of advanced pure Heusler-based room temperature spintronics devices. Focus will be put on the basic building block of every such device, namely the heterojunction between a spin injector (SI) and a spin conductor (SC) material. The specific functionality of SI/SC building blocks will be characterized in terms of two fundamental properties: (i) the spin-injection efficiency at the SI-SC interface; and (ii) the spin diffusion length in the SC itself. Experimentally, the quantities (i) and (ii) will be accessible by implementing an established experimental strategy based on the spin-and time-resolved two-photon photoemission (STR-2PPE). It allows to correlate the efficiency of spin-injection with the static spin-dependent properties of the considered interface as well as to distinguish unambiguously between interface and bulk spin-dependent properties of the considered SI/SC heterojunction. The provided experimental evidence will allow identifying the best suited SI/SC building blocks for developing the foundations of future advanced spintronics devices.

Task 2: Heusler alloys with large perpendicular anisotropy

2.1-A Fabrication and characterization of half-metallic Heusler alloy films with large perpendicular magnetic anisotropy (Mizukami, Miyazaki – Sendai)

Tunnel magnetoresistance (TMR) and spin transfer effect opened up possibilities of novel “active” spintronics devices, such as an electrically switchable magnetic tunnel junction for memory application and an electrically tuneable nano-scaled microwave oscillator for rf-application. The device performance depends not only on TMR ratio and critical current density for spin transfer but also on magnetic anisotropy of electrode material which determines thermal stability as well as oscillation frequency. Thus, it is of quite importance to realize new materials which have low saturation magnetization, low magnetic damping, high spin polarization, and high magnetic anisotropy. Candidates are in the class of half-metallic Heusler alloy with large magnetic anisotropy. The goal of this project is the search for such Heusler alloys, the growth of its high-quality films, and the characterization experimentally to understand the underlying physics in this new class of materials.

2.2-A Half-metallic Heusler alloys combined with L10 ordered alloy films (Sakuraba, Takanashi – Sendai)

The materials having large uniaxial magnetic anisotropy such as L10-ordered alloys and RE-TM alloys are extensively studied in the spintronics research field in recent years, because they show a high thermal stability even on the nanometer scale, and they can also be utilized as a perpendicular spin injector into non-magnetic metals/semiconductors. Generally, however, these materials do not have high spin-polarization at the Fermi level. Therefore, it is difficult to extract a highly spin-polarized current from these materials. The central goals of this project are (i) to fabricate half-metallic Heusler alloy films with large perpendicular magnetic anisotropy by combining them with L10-ordered alloys, and (ii) investigate spin-transport properties using them as a spin injector.

2.3-A New compounds with large perpendicular magnetic anisotropy and for spin-torque application (Felser – Mainz)

Goal of this project is the design of new tetragonal half-metallic compounds for spin torque application. Heusler compounds can be designed as low-damping materials, with damping values smaller than for conventional ferromagnetic metals such as CoFe. It is well established that the Co₂YZ Heusler compounds are the only theoretical half metallic compounds for which high spin polarisation was realized in tunnel junctions. A reduced saturation magnetization despite a high Curie temperature can be found for ferrimagnetic Heusler compounds such as CoMn2Z and related compounds such as Mn3Ga. However, tetragonal Heusler compounds are of special interest due to their large perpendicular magnetic anisotropy. Especially Heusler compounds with Gallium and Tin show the structural instabilities. Several possible ferro- and ferrimagnetic candidates with tetragonal distortion will be synthesized and investigated within the project such as Mn₂FeZ, CoMn₂Z, XCr₂Z, and XMn₂Z.

Task 3: Gilbert damping

3.1-A Gilbert damping in half metal Heusler alloy films (Oogane, Mizukami – Sendai)

From recent theoretical and experimental researches on spin torque transfer switching, the magnitude of the critical current density JC0 is considered to be proportional to the damping constant (G) and saturation magnetization (MS), and it is inversely proportional to the spin injection efficiency. Thus one needs materials showing low G, low MS, and high-spin polarization. According to theoretical models, the local band structure determines the damping properties. The underlying microscopic process for the relaxation of magnetization dynamics is transfer of angular momentum from the electronic system to the lattice occurring via inter- and intra- band transitions near the Fermi level. Half-metallic materials thus offer an ideal basis for tailoring the damping properties towards small G value because the energy gap for minority electrons closes the spin-flip channel as one of the important transfer mechanisms. Such a situation is considered to be realized in the some types of the Heusler alloys, which shows a very low damping constant as previously reported by our group. While the mechanism of damping in Heusler alloys is poorly understood, much investigation is required. Thus, the central goals of this experimental project are a better understanding of the fundamental origin and a control of the damping in half-metallic Heusler alloy films. Finally, we will realize high quality ultra-thin films of Heusler alloy films showing low-damping constant.

3.3-A Theoretical study of Gilbert damping in half metal Heusler alloy films (Sakuma, Tsuchiura – Sendai)

One of the serious problems of MTJ using the Heusler alloy film is the strong temperature dependence of TMR ratio, that is, remarkable decrease of MR ratio with increasing temperature.  To improve the MR ratio of this system, it is of great importance to make clear of the mechanism of such strong temperature dependence.  Thus, one of the aims of this project is to calculate the exchange constants or the exchange stiffness constants of Heusler alloy films especially around the interfaces of MTJ. The goal of this subject is to propose microscopic structure improving the MR ratio at room temperature.  Another aim of this project is to provide microscopic description of the Gilbert damping of both bulk and multilayer systems.  The microscopic origin of the Gilbert damping is still unclear as well as that of the temperature dependence of MR ratio.  Based on the theory developed by Kamberský,1)  we extend the theory to investigate the damping at the interface and will perform the first principles calculation of the Gilbert damping of Heusler alloys and MTJs in order to support the development of current induced spin transfer torque in MRAM.

Task 4: Lateral spin transport and spin waves including devices

4.1-B Novel devices based on Heusler films with GMR and MTJ nanocontacts (Ando, Naganuma, Oogane – Sendai)

This project focuses on the preparation of GMR and TMR nano-contacts with diameters significantly lower than 100 nm to films of Heusler compounds. Some of the Heusler alloys show very low magnetic damping compared to conventional materials. When we use the Heusler alloys as the bottom electrode, it must be an advantage to transport spin waves on it. The structures can generate magnetization precession under certain conditions when a dc current is flowing through the GMR/TMR stack. In order to confirm the propagation of spin waves in the bottom electrode, we prepare nano-contacts with various distances between them. We can check the mode locking of the magnetization oscillation of the contacts. The goals of this project is to obtain the appropriate stacking structure and the arrangement of the GMR and TMR nano-contacts in order to achieve strong and sharp signal output from this device.

4.3-B Lateral spin transport in semiconductors (Matsukura, Kohda, Nitta – Sendai)

This project focuses on the establishment of efficient generation and sensitive detection of spin current, and the investigation of spin dynamics in semiconductor lateral devices combined with ferromagnetic semiconductors and/or Heusler materials. The advantage of using semiconductors for spintronic devices is that the spin states can be manipulated by electrical and optical means. The systematic study on gate voltage dependence of spin states is expected to provide useful information, such as the effect of the Rashba and the Dresselhaus spin-orbit interactions on spin dynamics, which is important to realize the long spin coherence with the short spin flip time. In order to demonstrate new functional device operation, the optimization of device structures by selecting appropriate combination of semiconducting and Heusler materials will be done.

4.5-A Heusler materials for new spin transport applications (Jakob, Felser – Mainz)

This project combines the design of new semiconducting Heusler compounds with new transport phenomena. The goal is to design new materials usable in devices for lateral spin transport and the realization of such devices. The materials for new magneto resistive devices are based on half metallic Heusler compounds combined with new semiconducting Heusler compounds. The combination of half metallic and semiconducting Heusler films will render a pure spin current possible. For lateral spin transport, materials with a large spin diffusion length are essential. The diffusion length is expected to be large in Heusler compounds providing a band gap at the Fermi energy and possessing a low spin orbit coupling. With ever increasing miniaturization non-local effects will become important in next generation devices. The final aim will be to demonstrate spin accumulation effects in lateral devices using new materials.

4.6-B Lateral spin transport in Heusler alloy systems (Takanashi, Sakuraba – Sendai)

Compared with a current-perpendicular-to-plane (CPP) structure, a lateral structure consisting of ferromagnetic material (FM) and nonmagnetic material (NM) has a high degree of freedom in the device structure to create and control spin-current. For example, spin-MOSFET using FM source and drain and Si channel has been proposed and attracted much interest as a useful application. Therefore, spin-injection into NM including semiconductor is a challenging subject in recent and future spintronics field. Half-metallic Heusler alloys are promising candidates as an injector and detector of spin-current because of their large spin-polarization. In addition, Heusler-type semiconductors, such as CoTiSb and FeVSb, are expected as novel, potential NM materials, because of their small structural and electronic mismatch with half-metallic Heusler alloys. The central goals of this project are (i) to fabricate lateral spin-valve structure with half-metallic Heusler alloy films with non-magnetic metals and semiconductors and (ii) investigate the lateral spin-transport properties. One of the important challenges in this project is to inject spin-current to a Heusler-type semiconductor from a half-metallic Heusler alloy.

4.7-B Nonlinear spin-wave dynamics and radiation properties of small Heusler devices (Serga, Hillebrands - Kaiserslautern)

This project explores fundamental issues of nonlinear spin dynamics and spin-wave radiation in Heusler compound based structures. Some novel Heusler materials developed in the ASPIMATT Reseach Unit and the preceding FG559 Research Unit exhibit very low magnetic damping and high spin polarization compared with ordinary 3d magnetic metals and alloys (e.g. Permalloy and CoFe). These special material properties permit increased spin-wave propagation distances and give rise to lower nonlinear thresholds, opening doors to new types of spin-wave devices. Moreover, Heusler compounds offer new possibilities for spin-torque based systems operating at reduced driving currents. Using the technique of Brillouin light scattering microscopy, this project will investigate two interconnected issues: a) nonlinear spin-wave mode formation and coupling in patterned Heusler films externally driven by microwave magnetic fields, and b) nonlinear spin-wave excitation, radiation, and propagation (including magnetic bullet formation) in Heusler-based spin-valve nanocontact spin-torque oscillators prepared in Project 4.1-B  (Ando, Naganuma, Oogane). These processes will be studied in the context of potential device applications such as microwave oscillators for chip-to-chip or intra-chip communication, and direct spin-wave based information transfer.