Thin Films and Physics of Nanostructures
  

Research: Electronic structure of spinelectronic materials

Spin Electronics

The term spin electronics or spintronics refers to a field, which aims to exploit the electrons' spin degree of freedom within electronic applications. Thanks to the well-known Hall effect and the anisotropic magnetoresistance (AMR), magnetic fields can be sensed electronically. Both effects are, however, limited when it comes to extreme miniaturization of the sensors. Such miniaturization is required, e.g., for modern high density hard disk drives. Here, one uses multilayer systems which exploit the giant magnetoresistance (GMR) or tunnel magnetoresistance (TMR). Devices built from such multilayers of just a few nanometers thickness allow in principle for the required downsizing.

Based on the TMR in magnetic tunnel junctions (MTJs) it is also possible to realize non-volatile magnetic random access memory (MRAM), which has some similarity to the ancient magnetic-core memory. An MTJ basically consists of two ferromagnetic films, separated by a thin insulating film. By pinning one of the magnetic electrodes with an antiferromagnet, a so-called "spin valve" is created, which usually has low resistance in the parallel magnetic configuration and high resistance in the antiparallel configuration. The magnetic configuration can be switched by external fields (or can be used for sensing them) or by strong current pulses, which is called spin transfer torque switching (STT). Making the magnetization perpendicular to the film plane allows for further reduction in size without compromising on the thermal stability of the junctions.

Being both fast and non-volatile, the MRAM technology holds the promise of merging today's RAM and hard disk or solid state drive technologies. This will make data access faster and allow for considerably lower power consumption.

Spin-Orbitronics

Recent research revealed that spin-orbit coupling in certain materials leads to non-equilibrium spin accumulations within the materials or at their surfaces when charge current are driven. The subfield which aims for understanding and exploiting of spin-orbit effects in spintronics is now often referred to as spin-orbitronics. Among the most important effects in the field is the spin Hall effect, which converts a longitudinal charge current into a transverse spin current. This spin current can be used to exert a spin-transfer torque on an adjacent ferromagnetic layer, thereby inducing magnetization precession, domain wall motion, or magnetization switching. The magnetization switching is at the heart of a new magnetic memory concept, the spin-orbit torque MRAM (SOT-MRAM).

Another fascinating phenomenon is the Néel-order spin-orbit torque (NSOT), which is observed in antiferromagnets with a special combination of spatial inversion and time reversal  symmetry. When a charge current is driven through such a material, local magnetic fields emerge at the positions of the magnetic atoms. These fields have the same staggered order as the magnetic moments of the antiferromagnet, which allow to effectively manipulate the magnetic order of the material. The most intriguing effect here is the switching of the magnetic order by 90° with intense current pulses. Due to the probabilistic nature of the process, the devices built from these materials show memristive properties and applications related to neuromorphic computing are envisaged.

Materials Science for Spin Electronics

The demands for many applications of spin electronic devices pose a serious challenge for materials science. Intrinsic properties related to the band structure and extrinsic properties of real thin films, such as grain sizes, film roughnesses, and chemical disorder, need to be carefully balanced. The interplay between interfacial and volume effects must be understood and controlled. Presently, I'm working on the following subjects.


Néel-order spin-orbit torque.
At present, we know two metallic materials that combine the necessary symmetries for the NSOT, namely Mn2Au and CuMnAs. We study thin films of both materials, investigate the influence of film growth parameters on their properties, and study the electrical switching. A key aspect is the thermal activation, which allows to switch the magnetic order at room temperature. Despite the fundamental mechanism of the creation of the non-equilibrium local magnetic fields is essentially understood, many aspects of the electrical switching are tightly linked to the actual meso- and microscopic structure of the samples and are yet to be understood.

Electrical Switching of Antiferromagnetic Mn2Au and the Role of Thermal Activation, M. Meinert, D. Graulich, and T. Matalla-Wagner, Phys. Rev. Appl. 9, 064040 (2018), [LINK]

Spin Hall Effect. The pivotal quantity of the spin Hall effect is the spin Hall angle, which depends on the spin Hall conductivity and the resistivity of a material. To maximize the spin Hall angle, one needs to find materials with large intrinsic spin Hall conductivity and simultaneously maximize the resistivity. The latter can be achieved by doping, by controlling the grain size, or by making the material amorphous. All these mechanisms to maximize the resistivity may be detrimental to the band structure, and thereby reduce the intrinsic spin Hall conductivity. Thus, a careful control of the material growth is a fundamental prerequisite to maximize the spin Hall effect. Once we have a material with large spin Hall angle, we demonstrate its utility for current-induced domain wall motion and magnetization switching with good efficiency.

Influence of the Hall-bar geometry on harmonic Hall voltage measurements of spin-orbit torques, L. Neumann and M. Meinert, AIP Adv. 8, 095320 (2018). [LINK]

Large spin Hall effect in an amorphous binary alloy, K. Fritz, S. Wimmer, H. Ebert, and M. Meinert, Phys. Rev. B 98, 094433 (2018). [LINK]

Exchange Bias. To stabilize the magnetization of one magnetic layer, the so-called fixed or pinned layer,  one couples it to a thin antiferromagnetic layer. Due to the exchange bias effect, a shift of the magnetic hysteresis away from zero field is acieved. This allows us to construct MTJs that have a well-defined anti-parallel magnetic state, which is necessary to construct magnetic sensors or to build magnetic memories. Nowadays, materials containing Pt or Ir are used as antiferromagnets, which are costly and their production causes significant pollution. Alternatives without noble metals or rare earth element are necessary for a large-scale usage of spintronics.

High-throughput screening for antiferromagnetic Heusler compounds using density functional theory, J. Balluff, K. Diekmann, G. Reiss, and M. Meinert, Phys. Rev. Mater. 1, 034404 (2017). [LINK]

Giant perpendicular exchange bias with antiferromagnetic MnN, P. Zilske, D. Graulich, M. Dunz, and M. Meinert, Appl. Phys. Lett. 110, 192402 (2017). [LINK]

Large exchange bias in polycrystalline MnN/CoFe bilayers at room temperature, M. Meinert, B. Büker, D. Graulich, and M. Dunz, Phys. Rev. B 92, 144408 (2015). [LINK]

Methods

To investigate the properties of new materials, we deposit thin films of promising intermetallic compounds by magnetron sputtering. Our co-sputtering tools allow us to create virtually any quaternary intermetallic compound. By carefully choosing the substrates and seed layers, the crystal structure, strain, and crystal orientation can be manipulated.

The films are characterized with various techniques, such as x-ray diffraction, magnetization measurements, optical absorption (from IR to UV), and electrical measurements. With synchrotron techniques such as soft x-ray absorption it is possible to investigate the magnetic properties of a film for each element individually.

Electrical transport measurements are used to characterize the spin Hall effect with the Harmonic Hall voltage detection method, and experiments with current pulses serve to investigate the electrical switching of antiferromagnets and ferromagnets via intrinsic or induced spin-orbit torques.

Based on the known crystal structure of a material, we can compute its band structure and many more properties with density functional theory (DFT) and many-body perturbation theory. Comparison of measured data with calculations allows us to obtain a deeper understanding of the materials and the measurements, particularly of spectroscopic data. Computational materials science using DFT helps to find new useful materials and predict their phase diagrams. The stability of new compounds can be predicted and the search can be narrowed to materials which are expectedly stable, thus saving resources in the laboratory.

Bachelor's and Master's Theses

Research is moving on every day, just visit me in my office and ask for the latest opportunities for bachelor's and master's theses. I will be happy to give you an introduction into my field and answer your questions!

meinert@physik.uni-bielefeld.de


Selected Publications

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 Publications of Markus Meinert can also be found here.

Vita

Markus Meinert
Dr. Markus Meinert studied Physics in Bielefeld, where he also received his PhD. Since 2014 he is a Junior Professor. His work is focused on the electronic structure of functional materials for spinelectronics.
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