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.
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.
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]
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
absorption (from IR to UV), and electrical measurements.
techniques such as soft x-ray absorption it is possible
to investigate the magnetic properties of a film for each
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.
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 email@example.com