My current Research interest includes the following:
Coupled and decoupled Magnetic and structural transitions
Magnetic materials often undergo magnetic phase transitions that can be induced by either or both tempearture and magnetic field.
In special cases stractural phase transitions may accompany these magnetic transitions.Considering this, the ferromagnetic martensitic transition (FMMT), a coinciding crystallographic and magnetic transition mainly found selected ferromagnetic Heusler alloys is of great interest.
Magnetic Refrigeration and Magnetocaloric Effects
The worldwide cooling applications that employ CFC gases as the coolant material are one of the major consumers of energy.
In addition to the consumption of huge amount of energy, the gases are the main source of ozone layer destruction that enhances
global warming. Recent researches and discoveries have shown that the magnetocaloric cooling technology that utilizes magnetic
materials as the coolant is a potential alternative to the environmental unfriendly gas technology. When compared to the gas technology,
the magnetocaloric cooling technology has noticeably improved efficiency, and therefore the recent discoveries and continuing research on
magnetocaloric materials may lead to intense universal consequences.
For near room temperature magnetic refrigeration, materials with magnetic phase transition temperatures close to room temperature are needed.
The materials must also exhibit giant magnetocaloric effects (MCE) in the vicinity of the phase transition. Usually materials undergoing a
first order magnetic transition are found to exhibit very large MCE's. However, for their proper functionality, the properties of the materials must be controllable.
One of our research goal is to develop
such materials.
Large magnetoresistance
Upon the application of an external magnetic field, resistivity of certain materials decreases significantly. Such materials are known as giant magnetoresistence (GMR) materials.
Discovery of GMR materials have opened doors of developing new class of sensors and therefore from application point of view, development of new materials exhibiting GMR
effects are of intense interest. GMR was first discovered in Fe/Cr multilayers that are coupled antiferromagnetically.Although not giant, large MR has are also observed in a slected group of bulk materials including Heusler alloys. We mainly explore pollycrystalline bulk materials for large MR effects.
Exchange bias effects
The exchange bias effect usually takes place when the ferromagnetic film layered with the antiferromagnetic film is cooled down to a temperature, T,
lower than the Neel temperature, TN, of the antiferromagnetic layer. The basic idea can be visualized by looking at Figure on the right. When an external
magnetic field is applied in the temperature range TN < T < TC, the spins of the FM layer gets aligned with the field, while the spins of the AFM
layer remain randomly oriented (see Figure(i)). As the bilayer is cooled down to T < TN in the presence of the applied magnetic field,
the AFM spins that are close to the FM layer align ferromagnetically to those of the FM. The remaining AFM spins align antiferromagnetically
producing a net zero magnetization (Figure (ii)). When the field is reversed, the FM spins starts rotating, but due to the large AFM anisotropy the
AFM spins remains unaffected (Figure (iii)). However the AFM spins at the interface that previously aligned with the spins of the FM layer, try to
stop the FM spins from rotating. To be more specific, the AFM spins at the interface exert a microscopic torque on the FM spins to keep them in original position.
As a result the field required for the complete reversal of the FM spins is larger when it is in contact with an AFM layer, because an extra field is needed
to overcome the torque exerted by the AFM spins at the interface. However, once the complete rotation takes place and the field then is rotated back to its
original direction, the FM spins will start to rotate even due to a smaller field. This is because, the AFM spins at the interface now exerts a torque in the
same direction as the field. Thus a shift in the hysteresis loop takes place.
Permanent magnetic materials
Permanent magnets are critical components for numerous technologies like miniature speakers, disk drives, motors for hybrid vehicles and wind generators.
The most powerful permanent magnets that currently exist are rare earth based. Because of the high cost and limited availability of rare-earth elements,
there is a growing interest in developing new magnetic materials that are free from these critical elements. One of the key properties of high performance magnets is
the existence of large magnetocrystalline anisotropy (MCA). Therefore for the development of rare-earth-free magnets it is of great interest to investigate ferromagnetic
materials that exhibit large MCA. A selected group of intermetallic alloys based on Mn-Bi, Mn-Al and Mn-Ga are example materials that are potential candidates in this regard.
However, in order to fully utilize the large MCA in these materials and make them suitable for applications they must be synthesized with an appropriate composition and structure at the nanoscale level.
My reasrch explores TM based nanostructured materials with the aim of developing rare-earth free and low cost high performance permanent magnets.
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