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Fabricating highly ordered tall L10–FePt with a small grain pitch distance on a commercially available glass substrate is crucial to realize heat-assisted magnetic recording (HAMR) media for industrial manufacture. We have realized tall FePt grains surrounded by crystalized h–BN on Corning NXT™ glass deposited at elevated temperatures in the presence of radio frequency (RF) bias. In this paper, we discuss the effect of deposition temperature on the order parameter of L10–FePt–BN granular media. Well-isolated L10–FePt–BN granular media with a grain diameter of 6.5 nm and height of 11 nm is achieved. These films exhibit a high order parameter of 0.85 with a perpendicular coercivity of 35 kOe.

Bulk (Mn1-xCox)2VAl (x = 0, 0.25, 0.50) alloys in highly ordered L21 structure with a very small amount of B2 disorder have been prepared and their magnetic properties have been measured. The value of saturation magnetizations of the alloys with x = 0, 0.25 and 0.50 are 1.88 μB, 0.84 μB and 0.07 μB, respectively, being consistent with 2.00 μB, 1.00 μB and 0 μB, respectively, predicted by the Slater–Pauling rule. This indicates that the stoichiometric MnCoVAl alloy is a fully compensated ferrimagnet (FCF). Spin polarization measurements using point contact Andreev reflection technique showed that the quaternary alloys exhibited higher intrinsic spin polarization than the parent ternary composition. The spin polarization of 0.60 deduced for the MnCoVAl alloy suggests that spin-polarized current can be extracted from FCF. Curie temperature (TC) and the effective anisotropy constant of (Mn1-xCox)2VAl alloys decrease with increase in Co content.

Systematic investigations for the optimization of FePt–C nanogranular films for heat-assisted magnetic recording media are overviewed. FePt–C films prepared by using compositionally graded process exhibit binomial distribution of grains with a maximum size distribution of 16%. In addition, the average grain size of FePt could be controlled well below 7 nm. Easy-axis distribution can be substantially reduced by growing FePt–C layers on a single crystalline substrate instead of a (001)-textured MgO seed layer, indicating the improvement of (001) texture in the seed layer would reduce the in-plane component. Finite-element micromagnetic simulations incorporating experimentally determined misorientation of FePt grains do not reproduce the experimentally observed demagnetization curve, suggesting the presence of large distribution in the anisotropy fields of FePt grains. A typical growth model of the FePt–C nanogranular films has been proposed on the basis of the thickness-dependent microstructure of the FePt–C nanogranular films, and the suitability of various seed layers has been discussed. The observed results suggest that the suppression of the coarsening of FePt grains is essential to keep the grain size smaller than 6 nm.

We attempted a compositionally graded sputtering process to overcome the phase separation in the growth direction in the FePt–C granular film in order to realize the columnar structure of FePt grains with an aspect ratio larger than 1.5. In order to overcome the problem of the formation of a second FePt–C layer for the thickness larger than 6 nm, we deposited the FePt–C film by changing the carbon concentration during the sputtering process. The formation of the second FePt–C layer was suppressed, which led to the growth of columnar grains with an aspect ratio of 1.5. The overall volume fraction of carbon is a decisive factor on the size of the FePt grains. Using the compositionally graded process, FePt–C nanogranular films with an average grain size of 7.8 nm, a thickness of 10 nm, and a perpendicular coercivity of 40 kOe were obtained.

We report electron transport mechanism in FePt-C granular films as a function of temperature by varying intergranular separation. FePt-C nanogranular films were prepared by sputtering on MgO substrates. From magnetic measurement of the sample, a coercivity of about 3T was found in the perpendicular direction. Above 25 K, the electrical resistivity of the films were found to obey Mott variable range hopping, Efros-Shklovskii variable range hopping and extended critical regime depending on the intergranular separation. However, at lower temperatures it deviates from the above behaviour showing an increase in conductance. Reduced activation energy calculated from resistivity data of these films shows metal-insulator transition. The metallic nature observed at low temperature was attributed to the intergranular ferromagnetic type ordering between granules that enhances the transport of electrons. Intergranular separation, thus, can be used as a tool to engineer the electron transport mechanism to different hopping regimes or extended critical regime in these films.

A transmission electron microscope (TEM) based orientation mapping technique and micromagnetic simulations were applied to study the influence of easy axis distribution (EAD) on magnetic properties of FePt–C granular films which were deposited on a single crystalline MgO (0 0 1) substrate and a (0 0 1)-textured poly-crystalline MgO underlayer. The FePt–C film on the polycrystalline MgO underlayer shows smaller perpendicular coercivity, broader switching field distribution and visible in-plane minor loop compared with that deposited on the single crystalline MgO (0 0 1) substrate. Although the grain sizes and their distributions in both films look similar in TEM, orientation mapping and texture analysis revealed that the polycrystalline MgO underlayer introduces significant misorientation in the (0 0 1)-textured FePt grains. Micromagnetic simulations successfully reproduced the large hysteresis in the in-plane magnetization by introducing the specific misorientation distribution of the FePt grains obtained from the texture analysis. The misoriented FePt grains were found to be grown from misoriented MgO grains, indicating that the improvement of the (0 0 1) texture of the MgO underlayer is essential to reduce the in-plane component of FePt based recording media.

By using TiC as the segregate, FePt–TiC and FePt–TiC–C thin films were prepared by co-sputtering onto strongly (002)-textured MgO/NiTa underlayer on thermally-oxidized Si substrates. The TiC effects on the microstructure and magnetic properties of L10 ordered FePt and FePt–C thin films were studied. The TiC segregate can refine the grain size of L10 ordered FePt. Coercivities and the degree of L10 ordering were degraded with increasing TiC concentration. By properly tuning the composition of TiC and C, the granular microstructure with average grain size of 10.3 nm and standard deviation of 1.86 nm can be achieved with the coercivity of 1.4 T. The balance between microstructure and magnetic properties indicates that mixture addition as the segregates is promising.

L10-ordered FePt nanogranular film is one of the promising candidates for the next-generation high areal density media for heat-assisted magnetic recording (HAMR). To realize a suitable microstructure for the HAMR media, the granular structure composed of uniformly dispersed L10-FePt nanoparticles with various segregants has been optimized using various combinations of segregants and seed layer materials. Although the microstructure of FePt-C came so close to the ideal one for HAMR, it would not be the final answer as the growth of the columnar grain structure is not possible due to too strong phase separation between FePt and C. In this article, we review recent investigations aiming at achieving the ideal microstructure for HAMR media.

The spin polarization ( P ) of ferromagnetic Heusler alloys, Co 2 Fe(Ga x Ge 1− x ) ( x = 0, 0.25, 0.5, 0.75, and 1), is investigated by point contact Andreev reflection (PCAR) measurements. While the P values of the ternary Co 2 FeGe and Co 2 FeGa alloys are 0.58 and 0.59, respectively, the corresponding value for Co 2 Fe(Ge 0.5 Ga 0.25 ) is as high as 0.69. Co 2 Fe(Ge 0.5 Ga 0.5 ) alloy shows a strong tendency for L2 1 ordering and a high Curie temperature of 807 °C. Ab initio calculations indicate that the band structures of the Co 2 FeGe and Co 2 Fe(Ge 0.5 Ga 0.5 ) alloys with L2 1 or B2 structures are half-metallic. Thin films of the quaternary Co 2 Fe(Ge 0.5 Ga 0.5 ) alloy grown on MgO(0 0 1) substrates order to the L2 1 structure upon annealing at 500 °C, giving rise to a high P of 0.75. This is the highest P value reported for Heusler alloy thin films using the PCAR technique. Ferromagnetic resonance measurements show that the Gilbert damping constant of the film is ∼0.008. All these indicate that the Co 2 Fe(Ge 0.5 Ga 0.5 ) alloy is promising as a spin polarized current source for spintronics devices.

Co2MnGa0.5Sn0.5 (CMGS) thin films were epitaxially grown on MgO (0 0 1) substrates by magnetron sputtering and the current spin polarizations of the films with different post annealing conditions were measured by the point contact Andreev reflection method. The film deposited at a substrate temperature of 150 °C had a B2 structure and its spin polarization was estimated to be 59%. The film was ordered to the L21 structure by annealing at 600 °C, and the spin polarization was enhanced to 66%. The spin polarization and the intensity of the L21 diffraction showed clear correlation, suggesting L21 ordering is essential to achieve higher spin polarization of this quaternary Heusler alloy.

Grain-to-grain Curie temperature (Tc) variation in media reduces the signal-to-noise ratio due to its contribution to the transition jitter noise, especially when the average grain size is decreased to increase the area storage capacity. Thermally insulating magnetic grain boundaries may suppress such grain-to-grain Tc variation, especially at small grain sizes. In this paper, we discuss the effect of adding a thermally insulating magnetic oxide, in particular, BaFexOy, to the grain boundaries of granular FePt-C heat-assisted magnetic recording media. It is found that BaFexOy is chemically inert with respect to FePt and the chemical ordering of FePt-BaFexOy-C media is similar to that of FePt-C media. By tuning the volume fraction of BaFexOy and C, well-separated FePt grains (average grain size = 6.8 nm) surrounded by BaFexOy shells enable the attainment of perpendicular Hc greater than 35 kOe. Chemical mapping by transition electron microscopy shows that the magnetic oxide appears to be crystalline and completely surrounds the FePt grains. Temperature-dependent magnetization measurements indicate an effective increase in the magnetic grain size at temperatures below the FePt Curie temperature. The pulsed laser pump-probe measurement indicates a measurable reduction of the Curie temperature variation for the FePt-BaFexOy-C media compared to the reference FePt-C media.

Fabrication of composite materials by in situ generation of gamma- and epsilon-Fe(2)O(3) nanoparticles in a SiO(2) matrix through sol-gel process is reported. The process involves the hydrolysis and condensation of 1:3:10:x (x = 0.05, 0.1 and 0.2) molar ratios of tetraethoxysilane, absolute ethanol, nitric acid (0.16 N) and ferric nitrate, respectively, and subsequent thermal-treatment at temperatures ranging from 110 to 1000 degrees c. The in situ generation and growth of gamma- and epsilon-Fe(2)O(3) nanoparticles, and their distribution in SiO(2) matrix strongly depend on the concentration of Fe(3+) ions and thermal-treatment temperatures. The restricted growth of Fe(2)O(3) in SiO(2) matrix seems to stabilize the metastable epsilon-Fe(2)O(3) phase and prevent the formation of alpha-Fe(2)O(3) even at 1000 degrees C. Further, the presence of Fe(2)O(3) nanoparticles in SiO(2) matrix modified the gel morphology on thermal-treatment, leading to strong structural and chemical changes which influence the magnetic properties to a large extent. The concentration of individual magnetic phase (gamma- and epsilon-Fe(2)O(3)) in the samples, the particle size and distribution, and thermal-treatment temperature determine the net magnetic moment, shape of the hysteresis loop (symmetric or concentric), coercivity and magnetic phase transition. (C) 2009

Point contact Andreev reflection measurements of Co2MnSn1−xGax alloys showed that the spin polarization of L21 ordered quaternary Co2MnSn0.5Ga0.5 compound increased to 0.72 compared to 0.6 for Co2MnSn and Co2MnGa ternary alloys. The L21 Co2MnSn0.5Ga0.5 phase was found to be an intermetallic compound, in which the L21 structure was stable up to the melting temperature. The increase in spin polarization was attributed to the high degree of L21 order as well as an increase in the spin-up density of states near the Fermi level.

In this paper, we investigate CoFeCrAl alloy by means of various experimental techniques and ab-initio calculations to look for half-metallic nature. The alloy is found to exist in the cubic Heusler structure, with presence of B2 ordering. Saturation magnetization (MS) value of about 2 Bohr magneton/f.u. is observed at 8 K under ambient pressure, which is in good agreement with the Slater-Pauling rule. MS values are found to be independent of pressure, which is a prerequisite for half-metals. The ab-initio electronic structure calculations predict half-metallic nature for the alloy with a spin slitting energy of 0.31 eV. Importantly, this system shows a high current spin polarization value of 0.67 [with error of 0.02], as deduced from the point contact Andreev reflection (PCAR) measurements. Linear dependence of electrical resistivity with temperature indicates the possibility of reasonably high spin polarization at elevated temperatures (~150 K) as well. All these suggest that CoFeCrAl is a promising material for the spintronic devices., 11 pages, 5 figures, regular article

A detailed study of the Fe2-xCoxMnSi (0 = 0.2 as revealed by the magnetization and resistivity data. The Curie temperature and saturation magnetization values increase with increase in the Co concentration. Anomalous, non metallic-like behaviour is observed for x = 0.4. Current spin polarization values of 0.61 +/- 0.01 and 0.66 +/- 0.01 were deduced for x = 0.2 and x = 0.4 respectively, using the point contact Andreev reflection spectroscopy. Considering the high spin polarization and Curie temperature, materials with x >= 0.2 appear to be very promising for spintronic devices.

Spin gapless semiconductors (SGS) form a new class of magnetic semiconductors, which has a band gap for one spin sub band and zero band gap for the other, and thus are useful for tunable spin transport based applications. In this paper, we report the first experimental evidence for spin gapless semiconducting behavior in CoFeMnSi Heusler alloy. Such a 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 3.7 Bohr's magneton/f.u.. and Curie temperature of 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 is observed from 5 to 300 K. An anomalous Hall coefficient of 162 S/cm is obtained at 5 K. Point contact Andreev reflection data has yielded the current spin polarization value of 0.64, which is found to be robust against the structural disorder. All these properties are quite promising for the spintronic applications such as spin injection and can bridge a gap between the contrasting behavior of half-metallic ferromagnets and semiconductors., 14 pages, 7 figures, regular article

Transparent crack-free and bubble-free Fe3+ doped SiO2 nanostructured gel-glasses were obtained by the sol-gel process. The process involves the hydrolysis and condensation of an appropriate molar ratio of tetraethoxysilane (TEOS), absolute ethanol, nitric acid and ferric nitrate, followed by stepwise annealing at temperatures ranging from 110 degrees C to 1000 degrees C. The structural variation of the gel-glasses and their influence on physical properties during annealing has been studied. It has been observed that monolithicity and chemical environment around Fe3+ in the gel-glasses are strongly dependent on the annealing temperatures. The colour of gel-glass samples is different for different annealing temperatures, mainly due to the different co-ordination state of Fe3+ and the generation of Fe2O3 colloids of size 20-60 nm in the silica matrix. The annealing process facilitates the tuning of the UV-visible transmission cut-off edge in high optical quality Fe3+ doped silica gel-glasses. A marked difference in the magnetic properties of these glasses is, also observed with annealing temperatures. (c) 2005

Currently, the most common recording media used for heat assisted magnetic recording is granular L10 based FePt-C directly deposited on a highly textured polycrystalline MgO underlayer. In this study, a thin granular MgO-C interlayer is inserted between the MgO underlayer and the granular L10-FePt-C magnetic layer to control the grain size of the magnetic media. By varying the deposition conditions of the MgO-C interlayer, we can vary the size of the L10-ordered FePt grains from about 10.5 nm to 7.6nm, while keeping the carbon composition in the magnetic layer unchanged. With the optimized interlayer, the L10-FePt-C media of grain size to 7.6 nm shows good squareness and perpendicular coercivity of 47 kOe as well as an order parameter of S = 0.85.

The interplay of light and magnetism has been a topic of interest since the original observations of Faraday and Kerr where magnetic materials affect the light polarization. While these effects have historically been exploited to use light as a probe of magnetic materials there is increasing research on using polarized light to alter or manipulate magnetism. For instance deterministic magnetic switching without any applied magnetic fields using laser pulses of the circular polarized light has been observed for specific ferrimagnetic materials. Here we demonstrate, for the first time, 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. These results challenge the current theoretical understanding and will have a major impact on data memory and storage industries via the integration of optical control of ferromagnetic bits., Comment: 21 pages, 11 figures

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. (C) 2014 AIP Publishing LLC.

In order to optimize the nanogranular structure of FePt-C for heat-assisted magnetic recording media, we investigated the influence of MgO underlayers on the growth of FePt grains in the FePt-C layer. The FePt-C layer was deposited by using the alternating sputtering method, by which FePt and FePt-C layers were alternately deposited. To understand the growth mechanism of the FePt-C layer on the MgO underlayers deposited under various conditions, detailed plan-view and cross sectional transmission electron microscopy observations were made for different film thicknesses. We found that columnar FePt grains grow only when the deposition conditions of the MgO underlayer are optimal. Direct TEM observation of the growth process of the FePt-C layer revealed that the number density of nuclei is sufficient in the initial stage of the film deposition; however, coarsening of the grains after grain impingement causes a substantial decrease in the number density of the FePt grains.

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.

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.

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.

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.

Current-perpendicular-to-plane spin valves (SVs) with a new Heusler alloy of Co2Mn(Ga0.5Sn0.5) (CMGS) as magnetic layers and an Ag spacer have been investigated. Magnetoresistance (MR) values of 8.8% and 17.2% and resistance change-area product (ΔRA) of 4.0 mΩ μm2 and 6.5 mΩ μm2 were measured at 300 K and 12 K, respectively. Transmission electron microscopy observations have revealed that the CMGS magnetic layer was ordered to the B2 structure for the SV annealed at 400 °C. Higher annealing temperature led to the degradation of the multilayer structure, resulting in lower MR and ΔRA values regardless of the L21 structure of the CMGS layer. This suggests that MR properties may be enhanced further if the thermal stability of the multilayer structure is improved.

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.