Somnath Jana

Origin of the unconventional magnetoresistance in Sr2FeMoO6

The unusual magnetoresistance (MR) behavior in Sr2FeMoO6, recently termed as spin-valve?type MR (SVMR), presents several anomalies that are little understood so far. The difficulty in probing the origin of this phenomenon, arising from the magnetic property of only a small volume fraction of the ferromagnetic bulk, is circumvented in the present study by the use of ac susceptibility measurements that are sensitive to the slope rather than the magnitude of the magnetization. The present study unravels a spin-glass (SG) like surface layer around each soft ferromagnetic (FM) grain of Sr2FeMoO6. It is also observed that there is a very strong exchange coupling between the two, generating exchange bias effect, which consequently creates the valve, responsible for the unusual MR effects.

Surface spin-glass and exchange bias in Sr2FeMoO6 nanoparticle

Tunneling magnetoresistance in polycrystalline double perovskite Sr2FeMoO6 exhibits many unusual features, which can be efficiently probed by manipulating the tunnel barriers/grain surfaces. Accordingly, many experimental reports appeared on nanosized particles of Sr2FeMoO6 with largely enhanced grain boundary contributions. However, for the first time we report the existence of a spin-glasslike component, along with conventional ferromagnetism, in well-characterized Sr2FeMoO6 nanoparticles, which has been critically confirmed by the perceptible exchange bias effect, observed in these nanoparticles. Our results suggest that the spin-glass component is likely to reside on the surface of each particle, which probably provides useful clues about the unusual tunneling magnetoresistance responses, always exhibited by nanocrystalline Sr2FeMoO6.

Signature of an antiferromagnetic metallic ground state in heavily electron-doped Sr2FeMoO6

Sr2FeMoO6 is a well-known double perovskite with exciting high-temperature magnetic properties. Through various magnetic and spectroscopic measurements, we collect compelling evidence here that thi ...

Exponentially decaying magnetic coupling in sputtered thin film FeNi/Cu/FeCo trilayers

Magnetic coupling in trilayer films of FeNi/Cu/FeCo deposited on Si/SiO2 substrates have been studied. While the thicknesses of the FeNi and FeCo layers were kept constant at 100 angstrom, the thic ...

Atomic-scale chemical fluctuation in LaSrVMoO6, a proposed half-metallic antiferromagnet

Half-metallic antiferromagnets (HMAFMs) have been proposed theoretically long ago but have not been realized experimentally yet. Recently, a double perovskite compound, LaSrVMoO6, has been claimed to be an almost real HMAFM system. Here, we report detailed experimental and theoretical studies on this compound. Our results reveal that the compound is neither a half-metal nor an ordered antiferromagnet. Most importantly, an unusual chemical fluctuation is observed locally, which finally accounts for all the electronic and magnetic properties of this compound.

Magnetic coupling in asymmetric FeCoV/Ru/FeNi trilayers

We have investigated the magnetic anisotropy and interlayer coupling in trilayer films of permendur(100 \AA)/Ru/permalloy(100 \AA), with the thickness of the Ru spacer varying from 0 to 200 \AA. Wh ...

Magnetic and nonmagnetic tunnel barriers in Sr2FeMoO6

Tunnelling magnetoresistance (TMR) in polycrystalline Sr2FeMoO6 has been a matter of intense interest for more than a decade now, where the nature of the insulating tunnel barrier turned out to be the core issue of debate. Other than the nonmagnetic grain boundaries as conventional tunnel barriers, intragrain magnetic anti-phase boundaries (APB) as well as magnetically frustrated grain surfaces have also been proposed to act as tunnel barriers and influence the TMR response of Sr2FeMoO6 in different ways. In this paper, it is shown that depending upon the physical state of the sample, at least three different types of tunnelling mechanism may persist in polycrystalline Sr2FeMoO6 and a purely conventional TMR response, seldom reported in this important magnetoresistive material, is also possible.

A setup for element specific magnetization dynamics using the transverse magneto-optic Kerr effect in the energy range of 30-72 eV

In this paper, we present a spectrometer that is designed for element-specific and time-resolved transverse magneto-optic Kerr effect experiments at the high-harmonic generation pump-probe facility High Energy Laser Induced Overtone Source (HELIOS) laboratory. HELIOS delivers photons with energies between 30 eV and 72 eV with an overall time resolution of less than 40 fs. The spectrometer is based on a Rowland-circle geometry and allows for simultaneous measurements of all magnetic transition-metal elements. The setup also features easy sample transfer and alignment, and it combines high photon throughput, optimized data acquisition, and a fast switching of the magnetic field at the sample. The spectrometer performance is demonstrated by measuring the ultrafast demagnetization of permalloy. Our data are, for all practical purposes, identical to what have been reported in the earlier high-order harmonic generation work of a similar sample by Mathias et al. [Proc. Natl. Acad. Sci. U. S. A. 109, 4792-4797 (2012)], however, obtained within 15% of the acquisition time compared to their study. Furthermore, our data show a shift of the demagnetization curve of Ni relative to Fe, which has previously been interpreted as a delay of the Ni demagnetization to that of Fe [S. Mathias et al., Proc. Natl. Acad. Sci. U. S. A. 109, 4792-4797 (2012)].

Anti-phase boundary free two-dimensional epitaxial Fe3O4 thin films: evidence of an unquenched orbital magnetic moment at room temperature

Two-dimensional (2-D) epitaxial ferrimagnetic Fe3O4 thin films are attractive choices for the next generation of spin devices due to their half-metallicity, high Curie temperature and high electrical conductivity. Despite having profound spin device compatibility, the use of Fe3O4 thin films has not been exploited to date due to the presence of a magnetic disorder known as anti-phase boundaries (APBs). Here we demonstrate the growth of 2D single crystalline APB free Fe3O4(100) thin films on TiN buffered Si(100). The epitaxial orientation relationship, Si(400)//TiN(200)//Fe3O4(400), was confirmed by reflection high-energy electron diffraction and polarized Raman analysis. The Fe3O4(100) thin films possess large in-plane magnetic domains in its remanent magnetization state and cubic in-plane magnetic anisotropy as confirmed using magnetic force microscopy and magneto-optic Kerr effect measurements, respectively. The orbital to spin angular moment ratio, ml/ms = 0.144, and the total magnetic moment extracted from X-ray magnetism circular dichroism (XMCD) measurements are close to the corresponding bulk value.

An agglomeration induced glassy magnetic state in a carbon nanotube/NiO nanocomposite system

A series of nanocomposite materials were synthesized using multi-walled carbon nanotubes (MWCNTs) and NiO nanoparticles by varying the concentration of NiO in the MWCNT host matrix. Such an increment in the NiO particle density actually tunes the degree of isolation among the magnetic nanoparticles. Careful investigation by transmission electron microscopy shows that particle agglomeration increases substantially with NiO particle density. Field dependence of magnetization measurements depict a gradual enhancement of coercivity with increasing NiO concentration, signifying the enhancement of magnetic anisotropy in this nanocomposite system. Furthermore, field cooled and zero field cooled memory effect as well as magnetization relaxation measurements show that a glassy magnetic state gradually develops when the concentration increases. Analysis based on the result of high resolution transmission electron microscopy along with the magnetization data reveals that interparticle magnetic exchange interaction in the presence of interfacial disorders plays the major role in the emergence of the glassy magnetic state in this nanocomposite system.