Dipanjan Mazumdar

In situ detection of single micron-sized magnetic beads using magnetic tunnel junction sensors

We have demonstrated the use of highly sensitive magnetic tunnel junction (MTJ) sensors for the detection of individual micron-sized magnetic labels. By integrating the MTJ sensor into a microfluidic channel, we were able to detect the presence of moving superparamagnetic beads (Dynabeads® M-280) in real time by direct measurement of the magnetic dipole fields associated with single beads. The dipolar fields of a single bead were sufficient to obtain a signal of 80μV with signal to noise ratio of 24dB in an applied field of 15Oe. Our data show conclusively that MTJ sensors are very promising candidates for future applications involving the accurate detection and identification of biomolecules with magnetic labels.

Nanoscale Switching Characteristics of Nearly Tetragonal BiFeO3 Thin Films

We have investigated the nanoscale switching properties of strain-engineered BiFeO(3) thin films deposited on LaAlO(3) substrates using a combination of scanning probe techniques. Polarized Raman spectral analysis indicates that the nearly tetragonal films have monoclinic (Cc) rather than P4mm tetragonal symmetry. Through local switching-spectroscopy measurements and piezoresponse force microscopy, we provide clear evidence of ferroelectric switching of the tetragonal phase, but the polarization direction, and therefore its switching, deviates strongly from the expected (001) tetragonal axis. We also demonstrate a large and reversible, electrically driven structural phase transition from the tetragonal to the rhombohedral polymorph in this material, which is promising for a plethora of applications.

Effect of film roughness in MgO-based magnetic tunnel junctions

We have systematically investigated the dependence of tunnel magnetoresistance in MgO-based magnetic tunnel junctions as a function of Ar pressure during sputtering. The MgO surface roughness, and therefore device magnetoresistance, depends strongly on Ar gas pressure. Magnetoresistance of up to 236% was achieved at room temperature after thermal annealing at 425°C and with optimal sputtering conditions. The long mean free path of target atoms at low background pressures increases their kinetic energy at the substrate surface, resulting in smooth surface morphology and correspondingly improved device performance.

Chemical tuning of the optical band gap in spinel ferrites: CoFe2O4 vs NiFe2O4

We measured the optical properties of epitaxial CoFe2O4 thin films and compared our findings with complementary electronic structure calculations and similar studies on the Ni analog. Our work reveals CoFe2O4 to be an indirect band gap material (1.2 eV, X → Γ in the spin-down channel) with a direct gap at 2.7 eV. The latter is robust up to 800 K. Compared to NiFe2O4, the indirect gap is ≈0.5 eV lower, a difference we discuss in terms of size and covalency effects in spinel ferrites.

Reduced Coercive Field in BiFeO3 Thin Films through Domain Engineering

The bismuth ferrite (BiFeO3) material offers a comprehensive package of multifunctionality. In addition to the multiferroic behavior, i.e. coexistence of electric and magnetic orderings,[1] it also exhibits photovoltaic effect, [2] metal-insulator transition,[3] electric modulation of conduction,[4] and terahertz radiation emission.[5] The possibility of joint control of electric, magnetic, and optical properties provides several degrees of freedom to design exotic devices. It is a green energy material in the sense that it is lead-free and energy-efficient due to cryogen-less functionality. Therefore, a wide variety of applications in terms of sensors, memories and spintronic devices are foreseen.[6] However, the incipient lower value of magneto-electric coupling has raised skepticism regarding its multiferroic capabilities and allied applications.[6] Nevertheless, the highest value of ferroelectric polarization (~ 100 C.cm-2) is very promising for next generation ferroelectric random access memory devices.

Polarized Raman spectroscopy of nearly tetragonal BiFeO3 thin films

BiFeO

Thermal stability of magnetic tunneling junctions with MgO barriers for high temperature spintronics

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Formation of antiphase domains in NiFe2O4 thin films deposited on different substrates

thin films can be epitaxially stabilized in a nearly-tetragonal phase under a high biaxial compressive strain. Here we investigate the polarized Raman spectra of constrained BiFeO

Nanoscale electroresistance properties of all-oxide magneto-electric tunnel junction with ultra-thin barium titanate barrier

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Colloidal Synthesis of Magnetic CuCr2S4 Nanocrystals and Nanoclusters

films with tetragonal-like (BFO-T), rhombohedral-like (BFO-R) and multiphase (BFO-T+R) structure. Based on analysis of the number and symmetry of the Raman lines, we provide strong experimental evidence that the nearly-tetragonal films are monoclinic (