High perpendicular hard magnetic properties of nanocomposite Co-rich Co-Pt/Pt double-layered films by epitaxial deposition without capped layer

The HRTEM cross-sectional lattice image shows that a well epitaxial growth of hcp Co-rich Co-Pt (002) on Pt (111) underlayer leads to good perpendicular magnetic anisotropy of Co-rich Co-Pt film. It is found that both the perpendicular coercivity (Hc_⊥) and perpendicular squareness (S_⊥) of Co-rich Co-Pt films without Pt capped layer are larger than that of Co-rich Co-Pt films with Pt capped layer. The cross-sectional TEM-EDS and AES analysis confirm that the oxygen atoms will diffuse from film surface into the Co-rich Co-Pt film without adding Pt capped layer, and it react with cobalt atoms to form CoO, which is detected by XPS analysis. The increase in perpendicular hard magnetic properties of Co-rich Co-Pt film without Pt capped layer is mainly due to form CoO in the Co-rich Co-Pt film.     

Effects of AlN layer thickness on magnetic anisotropy and transport phenomena of CoPt/AlN layered structure

The magnetic anisotropy was studied as a function of the AlN layer thickness in [AlN(x nm)/CoPt(2 nm)]5/AlN(x nm) layered structure (x is AlN layer thickness, and 5 is the number of multilayer series). The multilayered film was deposited by a sputtering apparatus equipped with two pairs of facing targets. It was found that, in the range of AlN layer thickness below 30 nm, CoPt/AlN multilayers transform from an enhanced in-plane magnetic anisotropy to perpendicular magnetic anisotropy (PMA) through thermal annealing in vacuum, with an optimized AlN thickness of 10 nm for strong PMA. However, beyond this thickness range, the PMA did not occur, and thermal annealing only results in magnetic isotropy in both parallel and perpendicular directions. The related structure analysis revealed that smooth interface and good texture of CoPt (111) make positive contributions to interface anisotropy energy and magnetocrystalline anisotropy energy for producing PMA in CoPt/AlN layered structure. In addition, the transport phenomena were also studied by using a four-probe method.     

Perpendicular magnetic anisotropy in amorphous ferromagnetic CoSiB/Pt multilayers

Magnetic anisotropy properties of amorphous ferromagnetic CoSiB/Pt multilayers with perpendicular magnetic anisotropy (PMA, K(u)) were systematically investigated as a function of CoSiB layer thickness (t(coSiB)) and Pt layer thickness (t(Pt)). In two series of [CoSiB t(coSiB)Pt t(P1)]5 multilayers, the perpendicular coercivity (H(c)) increased to reach a maximum and then decreased with further increase in both t(coSiB) and t(Pt), due to intermixing of CoSiB/Pt interfaces. Particularly, using the amorphous soft magnetic CoSiB, the coercivity became very sensitive to the CoSiB thickness. These multilayer films exhibited a high K(u) of 2 x 10(6) erg/cc and a high H(c) of 360 Oe with marked squareness. It was found that even after annealing at 350 degrees C, the CoSiB/Pt multilayers had a high PMA and their H(c) increased.     

Optical and magnetic properties of 30 and 60 nm Ni nanowires

Ni nanowire arrays of high aspect ratio with the diameters of about 30 nm and 60 nm were prepared by DC applied AC electrodeposition. We observe the different preferred orientation and various magnetic behaviors of 30 and 60 nm diameter nanowires. In addition, the coercivity H_c(||), squareness S(||) and the ratio H_c(||)/H_c(⊥) where the applied field is parallel (||) and perpendicular (⊥) to the long axis of nanowires increase with decreasing wire diameter. This is the first time that optical results of Ni nanowires were presented.     

Deterministic Magnetic Switching in Perpendicular Magnetic Trilayers Through Sunlight-Induced Photoelectron Injection

Finding an energy-efficient way of switching magnetization is crucial in spintronic devices, such as memories. Usually, spins are manipulated by spin-polarized currents or voltages in various ferromagnetic heterostructures; however, their energy consumption is relatively large. Here, a sunlight control of perpendicular magnetic anisotropy (PMA) in Pt (0.8 nm)/Co (0.65 nm)/Pt (2.5 nm)/PN Si heterojunction in an energy-efficient manner is proposed. The coercive field (H_C) is altered from 261 to 95 Oe (64% variation) under sunlight illumination, enabling a nearly 180° deterministic magnetization switching reversibly with a 140 Oe magnetic bias assistant. The element-resolved X-ray circular dichroism measurement reveals different L3 and L2 edge signals of the Co layer with or without sunlight, suggesting a photoelectron-induced redistribution of the orbital and spin moment in Co magnetization. The first-principle calculations also reveal that the photo-induced electrons shift the Fermi level of electrons and enhance the in-plane Rashba field around the Co/Pt interfaces, leading to a weakened PMA and corresponding H_C decreasing and magnetization switching accordingly. The sunlight control of PMA may provide an alternative way for magnetic recording, which is energy efficient and would reduce the Joule heat from the high switching current.     

Investigations on the interlayer coupling in Co/Pt multilayers with perpendicular anisotropy via the extraordinary Hall effect

The interlayer coupling in Co/Pt multilayers has been investigated via the measurements of extraordinary Hall effect. In the conventional [Co/Pt]_n multilayer, the coercivity H_C has been observed to increase exponentially for n < 6 and almost linearly for n > 6 with the decrease of temperature. Surprisingly, H_C for n = 1 shows the fastest increase at low temperatures, and becomes the largest one at T < 40 K. As a function of the repetition number n, the almost temperature-independent oscillation of H_C has been observed, being indicative of the Ruderman-Kittel-Kasuya-Yosida type ferromagnetic interlayer coupling in the [Co/Pt]_n multilayer. However, the antiferromagnetic interlayer coupling can be realized via the spin-valve configuration of the Co/Pt multilayer. In the Co/Pt/[Co/Pt]_n multilayer, the antiferromagnetic-to-ferromagnetic transition of the interlayer coupling has been observed at low temperatures. These observed phenomena are strongly related to the temperature-dependent polarization in the Co/Pt multilayers.     

Magnetic properties and microstructures of single-layered Fe100 − xPtx films deposited onto heated substrates

The single-layered Fe_100 − xPt_x films of 30 nm thick with Pt contents (x) of 35-57 at.% are deposited on heated Si (100) substrate at a temperature (Ts) of 620 °C by magnetron co-sputtering. When the Pt content in the Fe-Pt alloy film is 35 at.%, the value of in-plane coercivity (Hc_//) is close to perpendicular coercivity (Hc_⊥) and both values are about 800 kA/m. The FePt films exhibit perpendicular magnetic anisotropy when the Pt content increases to the values of between 45 and 51 at.%. The perpendicular coercivity, saturation magnetization (Ms) and perpendicular squareness (S_⊥) for Fe₅₄Pt₄₆ film are as high as 1113 kA/m, 0.594 Wb/m² and 0.96, respectively. These magnetic properties reveal its significant potential as perpendicular magnetic recording media. Upon further increasing the Pt content to 57 at.%, the coercivity of the Fe-Pt film decreases drastically to below 230 kA/m and tends to be closer to in-plane magnetic anisotropy.     

Magnetic properties and microstructure of TbCo/(SiNx/Co)n films

The Tb₃₂Co₆₈/(SiN_x/Co)_n films (n = 0 – 3) were prepared by magnetron sputtering. The magnetic anisotropy of all Tb₃₂Co₆₈/(SiN_x/Co)_n films are perpendicular to the film plane. It is found that the saturation magnetization (M_s) and perpendicular coercivity (H_c⊥) of the Tb₃₂Co₆₈/(SiN_x/Co)₃ film are 263 emu/cm³ and 3592 Oe, respectively. This film appears to be a promising material as a heat-assisted magnetic recording (HAMR) medium. The cross-sectional high resolution transmission electron microscope (HRTEM) images show that the interface roughness between the (SiN_x/Co)_n layers and TbCo layer increases as n is increased. The rough surface provides more obstacles and pinning sites that hinder the motion of the domain walls at interface between the (SiN_x/Co)_n layers and TbCo layer. Therefore, the H_c⊥ values are profoundly influenced by the interface roughness.     

Onset of hard magnetic L10 FePt phase with (001) texture

The perpendicular anisotropic magnetic properties of in-situ deposited FePt/Pt/Cr trilayer films were elucidated as functions of the deposition temperature and the sputtering rate of the FePt magnetic layer. Ordered L1₀ FePt thin films with perpendicular anisotropy and a (001) texture can be developed at a temperature as low as 300 °C with the sputtering of a FePt layer at a low rate. The larger Pt(001)[100] lattice induced an expansion of the FePt a- and b-axis, leading to the contraction of the FePt c-axis, enabling the epitaxial growth of the L1₀ FePt(001) texture to occur. A low rate of sputtering of the FePt thin film promotes the formation of the magnetically hard FePt(001) texture on the surface of the Pt(001) buffer layer at low temperature, while the high sputtering rate of FePt layer suppresses the phase transformation.     

Effect of capped layer on microstructure and magnetic properties of FePt films

Sputter-deposited FePt films exhibit an in-plane magnetic anisotropy when MgO is used as the capped layer. The perpendicular magnetic anisotropy of FePt films can be enhanced by introducing a Ag capped layer instead of a MgO capped layer. Although the in-plane coercivity (Hc_//) of FePt films decreases slightly after introducing a Ag capped layer instead of a MgO capped layer, the perpendicular coercivity (Hc_⊥) is increased significantly from 3169 Oe to 6726 Oe. Auger electron spectroscopy analysis confirms that Ag atoms diffuse from the capped layer into the FePt magnetic layer and are mainly distributed at the grain boundary of FePt. This phenomenon results in enhancement of the grain boundary energy and inhibition of grain growth, thus increasing the perpendicular coercivity and reducing the grain size of the FePt film.     

Influence of thin platinum layer on the magnetic properties of multiple layers of CVD cobalt thin films

This paper presents the results obtained on the multiple layers of cobalt (Co)/platinum (Pt) and cobalt (Co)/platinum (Pt)/cobalt (Co) on the oxidised silicon substrate. The cobalt layers were deposited by metal-organic chemical vapour deposition on oxidised-silicon substrates at 450?°C, in H₂ ambient with 2-torr processing pressure. The platinum layers were deposited by E-beam evaporation in a separate vacuum system. The magnetic properties of Co/Pt/Co and Co/Pt multilayer were compared with the single cobalt layers of similar thicknesses on the oxidised silicon substrate. From the hysteresis loops it was observed that thin cobalt layers on oxidised-silicon substrate shows hard magnetic property with coercivity H _c values of 360 Oe and 500 Oe respectively for the 30- and 15-nm cobalt layers. The multiple layers of Co/Pt/Co and Co/Pt of cobalt thickness 15- and 30-nm with platinum 1.5-nm spacer-layer show significant change in magnetic properties (i.e. coercivity H _c and magnetisation M _s ) and, gave soft magnetic properties with H _c values 51 and 49 Oe respectively, which are significantly less than the H _c values of single cobalt layers on oxidised silicon. Also, single and multiple layers cobalt with platinum were annealed and compared with the as-deposited layer structures. From the microstructure analysis by SEM, and AFM it was found that the single and multilayer had similar roughness. Magnetic images were observed by MFM and analyzed in terms of domain structure.     

Microstructure,Magnetic and Magnetoresistance Properties of Electrodeposited [Fe/Pt] Granular Multilayers

In this paper, we report experimental results concerning the magnetic properties and the magnetoresistance effect of [Fe/Pt] _n and [Pt/Fe] _n electrodeposited multilayers. Two series of multilayers starting with Pt and Fe layers, respectively, were grown onto glass substrate covered with electroless deposited amorphous Ni. We investigated the effect of the seed layer (Pt or Fe) and Pt layer thicknesses on the magnetic and magnetoresistance properties of electrodeposited multilayers. The structure and morphology of the samples were characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM). The hysteresis loops of the [Fe/Pt] _n and [Pt/Fe] _n multilayers showed that the magnetic properties strongly depend on the Pt thickness and on the seed layer (the first layer deposited onto the glass substrate). [Fe/Pt] _n and [Pt/Fe] _n electrodeposited multilayers display magnetoresistance (∼15%) effect which can be explained mainly by the spin-dependent scattering of conduction electrons between Fe layers through a Pt layer and by the existence of anti-ferromagnetic coupling between subsequent Fe layers. The existence of a GMR effect in Fe/Pt multilayers is very promising for technological applications (e.g., magnetoresistance sensors).     

Tuning the perpendicular magnetic anisotropy of co-based layers in multilayered systems

The combination of Pt with Co either in alloy or in multilayer form is widely studied among the potential magnetic media for ultrahigh density magnetic recording. On the other hand the combination of Co with Cr in alloy form is currently providing commercial magnetic media. In an effort to further exploit and benefit from both systems, we fabricated Co(1-x)Cr(x)/Pt multilayers with two adjustable parameters. The first one is the Cr concentration on CoCr layer (x = 0, 5, 30), which modulates segregation effects on Co grains, thus tunes macroscopic magnetic features such as saturation magnetization and coercive field. The second one is the small layer thickness (< or = 0.6 nm) that affects interlayer coupling, perpendicular magnetic anisotropy and magnetization enhancement through spin polarization of Pt atoms in a ferromagnetic environment. The X-ray diffraction patterns verified the existence of multilayered structures following a preferable face-centered-cubic stacking. The Pt thickness and Cr concentration are found to significantly affect the macroscopic magnetic behavior. It is remarkable the fact that, samples present perpendicular anisotropy that scales with Pt thickness and temperature, even in the case of significant Cr concentration (30% in the alloy) when ferromagnetic behavior is expected to diminish according to relevant studies in alloys and in bulk films. Such an effect may be attributed to spin-polarization of Pt interlayers and was evidenced by X-ray magnetic circular dichroism. The spin-polarization of Pt is also the drive for the strong magneto-optic enhancement in the ultra-violet region between 4.5 and 5 eV shown by magnetooptic Kerr spectroscopy.     

Tunable coercivity in perpendicular (CoFe/Tb)n/CoFe multilayers

Magnetic coercivity and anisotropy have been investigated in amorphous Co₅₀Fe₅₀t/Tb t multilayers with perpendicular anisotropy. The thin CoFe layer does not crystallize when sandwiched between Tb layers. The saturation magnetization M_s (~ 10⁶ A/m) and perpendicular magnetic anisotropy constant K_u (~ 10⁵ J/m³) combined with low coercivity μ₀H_c (10-10² mT) have been obtained. These magnetic properties can be tuned. These amorphous multilayers could be designed to suit specific spintronic applications.     

Influence of metal spacer on the properties and microstructure of multilayer films and analyses

Ta/NiFe/nonmagnetic metal spacer/FeMn/Ta films were prepared by magnetron sputtering. The dependence of the exchange coupling field (H_ex) between the FeMn and NiFe layers on the thickness of a nonmagnetic metal spacer layers was investigated systematically. The results show that H_ex decreases rapidly with increasing thicknesses of the Bi and Ag spacer layers. It decreases gradually, however, with an increase in the thickness of the Cu spacer layer. We found empirically that H_ex corresponds to the lattice match between spacer layer atoms and NiFe layer atoms. However, the results of X-ray photoelectron spectroscopy show that when a small amount of Bi atoms are deposited on the NiFe/FeMn interface, they migrate to the FeMn layer surface and hardly influence H_ex.     

Perpendicular exchange bias in IrMn/Pt/[Co/Pt]3 multilayers with cone magnetization

The perpendicular exchange bias and magnetic anisotropy were investigated in IrMn/Pt/[Co/Pt]₃ multilayers through the analysis of in-plane and out-of-plane magnetization hysteresis loops. A phenomenological model was used to simulate the in-plane curves and the effective perpendicular anisotropies were obtained employing the area method. The canted state anisotropy was introduced by taking into account the first and second uniaxial anisotropy terms of the ferromagnet with the corresponding uniaxial anisotropy direction allowed to make a nonzero angle with the film's normal. This angle, obtained from the fittings, was of approximately 15° for IrMn/[Co/Pt]₃ film and decreases with the introduction of Pt in the IrMn/Pt/ [Co/Pt]₃ system, indicating that the Pt interlayer leads to a predominant perpendicular anisotropy. A maximum of the out-of-plane anisotropy was found between 0.5 and 0.6 nm of Pt, whereas a maximum of the perpendicular exchange bias was found at 0.3 nm. These results are very similar to those obtained for IrMn/Cu/[Co/Pt]₃ system; however, the decrease of the exchange bias with the spacer thickness is more abrupt and the enhacement of the perpendicular anisotropy is higher for the case of Cu spacer as compared with that of Pt spacer. The existence of a maximum in the perpendicular exchange bias as a function of the Pt layer thickness was attributed to the predominance of the enhancement of exchange bias due to more perpendicular Co moment orientation over the exponential decrease of the ferromagnetic/antiferromagnetic exchange coupling and, consequently, of the exchange-bias field.     

Granular L10 FePt nanocomposite films fabricated by rapid thermal annealing

Granular (FePt)_100−x-(NiO)_x nanocomposite thin films with x = 0 − 42 vol% were fabricated on a natural-oxidized Si(100) substrate. It is found that both the coercivity and FePt domain size decrease with increasing NiO content for the (FePt)_100−x-(NiO)_x films. When the FePt-NiO composite film with NiO content of 10.4 vol% is post-annealed at 750 °C with a high heating ramp rate of 100 °C/s, the in-plane coercivity (H_c//) and perpendicular coercivity (H_c⊥) of the FePt films are 513 and 430 kA/m, respectively. On the other hand, we used conductive atomic force microscope (CAFM) to confirm that the NiO compound is distributed at boundary of FePt particles that will constrain the domain size of FePt and decrease the exchange coupling interactions between FePt magnetic particles.     

Magnetic properties of Nd–Fe–B/FeMn multilayer films

[Nd–Fe–B(x nm)/FeMn(d nm)]_n thin films were deposited by magnetron sputtering on Si (100) substrates heated at 650 °C. The influence of the composition and thickness of FeMn layer on the structure and magnetic properties of Nd–Fe–B films are investigated. The Nd–Fe–B/FeMn multilayer films present an enhanced coercivity and a reduced saturation magnetization, in comparison with those of a Nd–Fe–B single layer. The coercivity of [Nd–Fe–B(x nm)/FeMn(5 nm)]_n films increases with increasing the period number of FeMn layer for the same thickness of magnetic layer, while the coercivity in [Nd–Fe–B(50 nm)/FeMn(5 nm)]_n films increases with decreasing the period number of Nd–Fe–B/FeMn bilayers. The coercivity H_c of about 17.2 kOe is achieved in the Nd–Fe–B(50 nm)/FeMn(5 nm) film.     

Nanocomposite magnetic films for high-density perpendicular magnetic recording media

Multi-layer nanocomposite structures of Ta/Ru/CoCr₁/FeCoTaCr(soft magnetic layer)/CoCr₂/CoCrPt-SiO₂(hard magnetic layer or recording layer)/C and Ta/Ru/CoCr₁/CoCrPt-SiO₂/CoCr₂/FeCoTaCr/C were proposed. This exchange coupled composite (ECC) media consisting of hard/soft stacked magnetic layers were promising in improving the writability of perpendicular magnetic recording media. A small CoCrPt c-axis orientation dispersion of about 3° was achieved with the optimized sputter conditions. The CoCrPt-SiO₂ grains were well segregated by SiO₂ at grain boundaries. The macro-magnetic properties showed that the stacked magnetic grains switched in a coherent mode and that switching field decreased with increasing the thickness of the soft magnetic layer.     

Fabrication and characterization of thin films with perpendicular magnetic anisotropy for high-density magnetic recording

Perpendicular magnetic anisotropy (PMA) was first observed in thin films of cobalt-chromium alloys in 1974, and perpendicular magnetic recording was proposed in 1977. After less than ten years, a new technology for high-density magnetic recording is firmly established. This breakthrough of the science and technology of magnetic recording has been made possible mainly through the ingenuity and concerted efforts of Iwasaki and other researchers. The preparation, characterization, and application of the Co-Cr films featuring PMA have been extensively studied. This paper reviews the large number of reports on PMA films with emphasis in three areas: (1) processing of PMA films; (2) correlation of magnetic properties and microstructures of PMA films; and (3) state-of-the-art techniques for fabricating PMA films.Nomenclature PMA Perpendicular magnetic anisotropy - PMR Perpendicular magnetic recording - B Magnetic induction - H Magnetic field - H _c Coercivity - H _c, Perpendicular coercivity - H _d Demagnetizing field - H _K Anisotropy field - H Perpendicular anisotropy constant - M _r Remanent magnetization - M _s Saturation magnetization - P _Ar Argon pressure - T _s Substrate temperature - V _b Substrate bias voltage - Incidence angle - ₅₀ Half-width dispersion angle in the rocking curve - _c Curie temperature - _o Internal stress