B. S. D. Ch. S. Varaprasad

All-optical control of ferromagnetic thin films and nanostructures

All-optical magnetic state switching Magneto-optical memory storage media, such as hard drives, use magnetic fields to change the magnetization of memory bits, but the process is slow. Light can often reveal information about the magnetization state of a sample, such as its field direction. Lambert et al. show that under the right circumstances, light can also switch the magnetization state of a thin ferromagnetic film. Using light pulses instead of magnetic fields led to ultrafast data memory and data storage. Science, this issue p. 1337 The all-optical control of magnetization in thin ferromagnetic films is demonstrated. The interplay of light and magnetism allowed light to be used as a probe of magnetic materials. Now the focus has shifted to use polarized light to alter or manipulate magnetism. Here, we demonstrate optical control of ferromagnetic materials ranging from magnetic thin films to multilayers and even granular films being explored for ultra-high-density magnetic recording. Our finding shows that optical control of magnetic materials is a much more general phenomenon than previously assumed and may have a major impact on data memory and storage industries through the integration of optical control of ferromagnetic bits.

Spin gapless semiconducting behavior in equiatomic quaternary CoFeMnSi Heusler alloy

In this paper, we report the signature of spin gapless semiconductor (SGS) in CoFeMnSi that belongs to the Heusler family. SGS is a new class of magnetic semiconductors which have a band gap for one spin subband and zero band gap for the other, and thus are useful for tunable spin transport based applications. We show various experimental evidences for SGS behavior in CoFeMnSi by carefully carrying out the transport and spin-polarization measurements. SGS behavior is also confirmed by first-principles band-structure calculations. The most stable configuration obtained by the theoretical calculation is verified by experiment. The alloy is found to crystallize in the cubic Heusler structure (LiMgPdSn type) with some amount of disorder and has a saturation magnetization of

Mechanism of coercivity enhancement by Ag addition in FePt-C granular films for heat assisted magnetic recording media

3.7\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{B}/\mathrm{f}.\mathrm{u}.

High spin polarization in CoFeMnGe equiatomic quaternary Heusler alloy

and Curie temperature of \ensuremath{\sim}620 K. The saturation magnetization is found to follow the Slater-Pauling behavior, one of the prerequisites for SGS. Nearly-temperature-independent carrier concentration and electrical conductivity are observed from 5 to 300 K. An anomalous Hall coefficient of 162 S/cm is obtained at 5 K. Point contact Andreev reflection data have yielded the current spin-polarization value of 0.64, which is found to be robust against the structural disorder. All these properties strongly suggest SGS nature of the alloy, which is quite promising for the spintronic applications such as spin injection as it can bridge the gap between the contrasting behaviors of half-metallic ferromagnets and semiconductors.

Electrically conductive (Mg0.2Ti0.8)O underlayer to grow FePt-based perpendicular recording media on glass substrates

We investigated the Ag distribution in a FePtAg-C granular film that is under consideration for a heat assisted magnetic recording medium by aberration-corrected scanning transmission electron microscope-energy dispersive X-ray spectroscopy and X-ray absorption fine structure. Ag is rejected from the core of FePt grains during the deposition, forming Ag-enriched shell surrounding L10-ordered FePt grains. Since Ag has no solubility in both Fe and Pt, the rejection of Ag induces atomic diffusions thereby enhancing the kinetics of the L10-order in the FePt grains.

Structure and magnetoresistance of current-perpendicular-to-plane pseudo spin valves using Co2Mn(Ga0.25Ge0.75) Heusler alloy

We report the structure, magnetic property, and spin polarization of CoFeMnGe equiatomic quaternary Heusler alloy. The alloy was found to crystallize in the cubic Heusler structure (prototype LiMgPdSn) with considerable amount of DO3 disorder. Thermal analysis result indicated the Curie temperature is about 750 K without any other phase transformation up to melting temperature. The magnetization value was close to that predicted by the Slater-Pauling curve. Current spin polarization of P = 0.70 ± 0.01 was deduced using point contact andreev reflection measurements. The temperature dependence of electrical resistivity has been fitted in the temperature range of 5–300 K in order to check for the half metallic behavior. Considering the high spin polarization and Curie temperature, this material appears to be promising for spintronic applications.

Enhanced Spin Polarization of Co2MnGe Heusler Alloy by Substitution of Ga for Ge

We have explored a new electrically conductive underlayer material to grow (001) textured L10-FePt films on glass substrates for potential application in ultrahigh density magnetic recording media. We found (Mg0.2Ti0.8)O (MTO) polycrystalline film grows with a strong (001) texture on Cr buffer layer, which induces strong (001) texture of L10-FePt polycrystalline and granular films. Strong perpendicular anisotropy of 3.8 × 107 erg/cm3 and coercivity of 15 kOe of a FePt-C film has been demonstrated using the MTO underlayer on glass substrates.

⟨001⟩ textured polycrystalline current-perpendicular-to-plane pseudo spin-valves using Co2Fe(Ga0.5Ge0.5) Heusler alloy

We have investigated the structure and magnetoresistance of current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) pseudo spin valves (PSVs) using Co2Mn(Ga0.25Ge0.75) (CMGG) Heusler alloy ferromagnetic layers annealed at different temperatures. Relatively large ΔRA of 6.1 mΩμm2 and MR ratio of 40.2% were obtained in the CPP-PSV annealed at 400 °C. Annealing at higher than 400 °C led to a significant deterioration of MR outputs. Detailed structure analysis using transmission electron microscopy, three-dimensional atom probe, and synchrotron x-ray diffraction showed that the degradation of MR in the over-annealed sample was due to the increase of DO3 disorder in the CMGG layer, while the layered structure of the film is fully preserved.

Current-perpendicular-to-plane spin valves with a Co2Mn(Ga0.5Sn0.5) Heusler alloy

Point contact Andreev reflection (PCAR) measurements indicated that the conductive spin polarization of the Co2MnGe Heusler alloy was enhanced substantially by the substitution of Ga for Ge. While the spin polarization of the conductive electrons of Co2MnGe and Co2MnGa was estimated to be around 0.6, that of the Co2MnGe0.75Ga0.25 quaternary alloy was 0.74, the highest value measured by PCAR for Heusler alloys. Moreover, the alloy was found to be an L21 intermetallic compound having a high Curie temperature of 895 K.

Columnar Structure in FePt–C Granular Media for Heat-Assisted Magnetic Recording

The orientation dependence of current-perpendicular-to-plane giant-magnetoresistance (CPP-GMR) was investigated by fabricating ⟨001⟩ textured polycrystalline pseudo spin valves (PSVs) with Co2Fe(Ga0.5Ge0.5) (CFGG) Heusler alloy ferromagnetic layers and a Ag spacer. The PSV with 10 nm CFGG annealed at 400 °C exhibited the resistance-change area product (ΔRA) of 5.8 mΩ (MR ratio of 16%), the largest value reported for polycrystalline CPP-(P)SVs. The ⟨001⟩ textured CFGG PSV outperforms the ⟨011⟩ textured CFGG PSV possibly due to the reduced lattice mismatch or improved band matching at the CFGG/Ag interface.