High Perpendicular Magnetic Anisotropy Research Articles

Effect of deposition temperature on morphological, magnetic and elastic properties of ultrathin Co 49 Pt 51 films

Abstract Morphological, magnetic and elastic properties of 5 nm-thick Co 49 Pt 51 films, sputtered on glass substrates, with 20 nm-thick Ta (seed) and Pt (buffer) layers were studied as function of the deposition temperature T d ranging between room temperature and 350° C. Atomic and magnetic force microscopy, vibrating sample magnetometer and Brillouin light scattering techniques were used to investigate the root mean square (RMS) roughness, the magnetic domain configuration, the coercive field (H c ), the perpendicular magnetic anisotropy (PMA), and the dynamic magnetic and elastic properties of the films with T d . The results show that surface uniformity was enhanced since the RMS roughness decreases with T d while magnetic domains typical of films with high PMA are observed. H c and PMA are found to sensibly increase with T d . The dynamic magnetization behavior is characterized by magnetic modes related with the co-existence of hard and soft magnetic areas within the samples. The elastic properties of the stack were first analyzed by means of a model describing the main variation of the elastic wave frequencies within the frame of weighted average thickness, density, Young’s modulus and Poisson coefficient of all the layers constituting the stacks. However, while H c and PMA keep increasing with T d , a more precise experimental analysis of the mechanical behavior shows that the group velocity starts increasing and finally decreases with T d , suggesting that knowledge of the influence of T d on the mechanical properties of each individual layer composing the stack is required to obtain a more accurate analysis.

Tailoring magnetization and anisotropy of tetragonal Mn3Ga thin films by strain-induced growth and spin orbit coupling

Tetragonal Mn3Ga thin films were epitaxially grown with and without strain on Cr and Mo crystalline buffer layers, respectively, using rf-magnetron sputtering. Epilayers grown on Cr with a lattice mismatch of 4.16%, exhibit a high magnetization of 220 kAm−1 and high perpendicular magnetic anisotropy. These characteristics are attributed to interfacial strain. Additionally, a soft ferromagnetic component is observed in these films but not in relaxed layers grown on Mo, where Δa/a is −0.1%. These latest films exhibit a low magnetization of 80 kAm−1 and both perpendicular and in-plane magnetic anisotropies. We propose that high spin orbit coupling of Mo-5s14d5 orbitals from the buffer layer and strong hybridization with Mn3+-3d4 orbitals from the magnetic layer are at the origin of in-plane anisotropy at the interface, while Mn3Ga magnetocrystalline anisotropy leads to perpendicular anisotropy on the rest of the film.

高垂直磁気異方性Co-Cr-Pt薄膜

高垂直磁気異方性Co-Cr-Pt薄膜

Perpendicular magnetic anisotropy of TiN buffered Co2FeAl/MgO bilayers

We investigated crystallographic and magnetic properties of TiN buffered Co2FeAl thin films. Room temperature deposition of the Heusler compound led to high crystalline ordering. The layer thickness and post annealing temperature dependence of the coercivity, squareness ratio, and the anisotropy constant of the thin Co2FeAl films were measured. High perpendicular magnetic anisotropy was observed for the 0.9 nm thin Co2FeAl film. A squareness ratio of one was reached at 325 °C post annealing temperature, which is lower compared to previous studies. It remained constant for temperatures up to 500 °C. Surface sensitive x-ray absorption and x-ray magneto circular dichroism measurements were carried out, in order to investigate the element specific magnetic moments. A magnetic moment of 5.4 ± 0.4 μB/f.u. for the 1 nm thin Co2FeAl was determined, which is in good agreement with the previously reported values for Cr or MgO buffered Co2FeAl.

Origin of the Tetragonal Ground State of Heusler Compounds

Spin-transfer-torque magnetic random-access memory (STT-MRAM) is one of the leading emerging technologies for nonvolatile memory. Key to its development is identifying magnetic materials with sufficient perpendicular magnetic anisotropy (PMA). The authors describe the general mechanism of tetragonal distortion in Heusler compounds, a class of promising materials for STT-MRAM, with over 2000 members. This study offers guidance for finding tetragonal Heusler alloys with high PMA.

Room temperature deposition of perpendicular magnetic anisotropic Co3Pt thin films on glass substrate

Co3Pt alloy thin films were deposited on the glass substrate at room temperature (RT) and 300°C, which showed high perpendicular magnetic anisotropy (PMA) and isotropy magnetic behaviors, respectively. Co3Pt HCP (0002) planes grew along the substrate plane for the films deposited at RT. The easy axis [0001] was consequently vertical to the substrate surface and obtained the predominant PMA. Large magnetic domains and sharp boundary also supported high PMA in RT-deposited samples. On the other hand, the PMA was significantly decreased with increasing the deposition temperature from RT to 300°C. Hard HCP(0002) and soft A1(111) co-existed in the film and the magnetic exchanged coupling between these two phases induced isotropy magnetic behavior. In addition, the various thicknesses (t) of the RT-deposited Co3Pt films were deposited with different base pressures prior to sputtering. The Kerr rotation loops showed high PMA and out-of-plane squareness (S⊥) of ~0.9 were found in low base pressure chamber. Within high base pressure chamber, Co3Pt films just show magnetic isotropy behaviors. This study provides a fabrication method for the preparation of high PMA HCP-type Co3Pt films on the glass substrate without any underlayer at RT. The results could be the base for future development of RT-deposited magnetic alloy thin film with high PMA.

Tuning the magnetic properties and surface morphology of D22 Mn3-δGa films with high perpendicular magnetic anisotropy by N doping

We report the tunable magnetic properties and the smoothened surface morphology of epitaxial D022 Mn-Ga (Mn3Ga and Mn2.5Ga) films by N doping using reactive sputtering at 480 °C. The 50 nm thick Mn-Ga films grown with the N2/Ar gas flow rate (η) up to 0.66% showed 33%–50% reduction in the saturation magnetization compared to non-doped Mn-Ga. In particular, a single D022 phase was obtained in an optimal η range for Mn2.5Ga, resulted in the perpendicular magnetic anisotropy energy density of ∼1 MJ/m3 with 33% reduction in magnetization. Furthermore, the introduction of N provided the smoothened surface morphology at 50 nm thickness despite its high growth temperature, which is advantageous for thin film device applications.

Effects of annealing on the magnetic properties and microstructures of Ta/Mo/CoFeB/MgO/Ta films

We report a study on the evolution of perpendicular magnetic anisotropy (PMA) in Ta/Mo/CoFeB/MgO/Ta junctions annealed at 350 °C for different periods of time. The PMA quickly developed after a short annealing time followed by a slow approach to saturation. This evolution of PMA was accompanied by structural changes. The surface roughness (Rq) gradually increased with increasing annealing time to 30 min; further increases in annealing time resulted in a dramatic increase in Rq. The effects of film composition and chemical state on magnetic anisotropy in the as-deposited Ta/Mo/CoFeB/MgO/Ta films and annealed films were studied. The X-ray photoelectron spectroscopy results demonstrated that more Fe oxides were formed at the CoFeB/MgO interface after 1 min of annealing compared to in the as-deposited sample, which is critical to obtain the PMA. High-resolution transmission electron microscopy analysis revealed that the CoFeB and MgO layers are predominantly amorphous in sample as-deposited and after 1min annealing. However, the CoFeB and MgO layers are crystallized in sample after 30 and 60 min annealing, which is beneficial to get high PMA.

Influence of the Reference Layer Composition on the Back-End-of-Line Compatibility of Co/Ni-Based Perpendicular Magnetic Tunnel Junction Stacks

Spin-transfer torque magnetic random access memory (STT-MRAM) is currently explored to challenge the dynamic random access memory and embedded memory applications. Perpendicular magnetic tunnel junction (p-MTJ) stacks used in the STT-MRAM must be compatible with CMOS back-end-of-line processing, such as high perpendicular magnetic anisotropy, high tunnel magnetoresistance (TMR), and intact resistance area (RA) product at temperatures, as high as 400°C. In this paper, we deposited a bottom-pinned Co/Ni-based p-MTJ stack with a trilayered Co/spacer/CoFeB as a reference layer and a CoFeB film as the storage layer. By replacing the standard Ta spacer material with CoFeBTa, the thickness of the spacer layer can be increased from 4 to 8 Å without TMR and RA penalty, which is beneficial to process controllability. Moreover, the TMR increases for an 8 Å CoFeB polarizer from 125% to 145% for a similar RA product after a 300°C 30 min anneal. In addition, the thermal budget of the p-MTJ stack containing a CoFeBTa spacer was improved. The p-MTJ stack with a 12 Å CoFeBTa reference layer spacer can even withstand annealing at 400°C for 90 min and can retain a TMR value of 100% at RA 10 Ω · μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , while TMR dropped below 40% for the p-MTJ stack containing a pure Ta spacer layer.

B2O3: Grain boundary material for high-Ku CoPt–oxide granular media with low degree of intergranular exchange coupling

The magnetic properties and structures of Co80Pt20–30 vol % oxide (ZrO2, Cr2O3, Y2O3, Al2O3, MnO, TiO2, WO2, SiO2, Mn3O4, WO3, Co3O4, MoO3, and B2O3) granular media deposited at room temperature were investigated. As a result, the following were found. 1) By employing oxides with low melting point temperatures as the granular media, magnetic grains with high saturation magnetization () and perpendicular magnetic anisotropy () are obtained; the increases in and are due to the promotion of the columnar growth of magnetic grains and phase separation or two-phase precipitation between magnetic grains and oxides. 2) The increase in the of the granular media followed by the decrease in the melting point of oxides is due to the reduction in the amount of stacking faults. 3) Among these granular media, the CoPt–B2O3 granular medium has the highest coercivity (Hc) and ratio of Hc to the magnetic anisotropy field () of 8.0 kOe and 0.4, with and of 1115 emu/cm3 and 1.1 × 107 erg/cm3, respectively. 4) The CoPt–B2O3 granular medium has well-isolated and columnar growth magnetic grains with an average grain size of 6.5 nm.

Tunnelling magnetoresistance of the half-metallic compensated ferrimagnet Mn2RuxGa

Tunnel magnetoresistance ratios of up to 40% are measured between 10 K and 300 K when the highly spin-polarized compensated ferrimagnet, Mn2RuxGa, is integrated into MgO-based perpendicular magnetic tunnel junctions. Temperature and bias dependences of the tunnel magnetoresistance effect, with a sign change near −0.2 V, reflect the structure of the Mn2RuxGa interface density of states. Despite magnetic moment vanishing at a compensation temperature of 200 K for x≈0.8, the tunnel magnetoresistance ratio remains non-zero throughout the compensation region, demonstrating that the spin-transport is governed by one of the Mn sub-lattices only. Broad temperature range magnetic field immunity of at least 0.5 T is demonstrated in the same sample. The high spin polarization and perpendicular magnetic anisotropy make Mn2RuxGa suitable for applications in both non-volatile magnetic random access memory cells and terahertz spin-transfer oscillators.

Growth of L 1 1 structured CoPt thin films by alternate sputtering method on glass substrate

Abstract L11 structured [Cox/Ptx]n multilayer films (x = 0.1–2 nm and n = 1–20) were prepared by alternate sputtering of Co and Pt layers on glass substrate with textured Pt(111) underlayer. The [Cox/Ptx]n multilayer films showed higher perpendicular magnetic anisotropy (PMA) and out-of-plane coercivity (Hc ⊥) for x = 0.3–0.5 and n = 4–6. Poor PMA was found when x was below 0.3. Further increase in x and n, the magnetic properties of [Co/Pt]n multilayer films significantly deteriorated. In this study, [Co0.5/Pt0.5]4 multilayer film showed high PMA, high magnetocrystalline anisotropy constant (Ku ~ 2.1 × 106 J/m3), and larger Hc ⊥ (~ 160 kA/m). It is demonstrated that L11 structured [Cox/Ptx]n multilayer films with better perpendicular magnetic properties can be fabricated by alternate sputtering method. The correlation between magnetic properties and microstructures of [Cox/Ptx]n multilayer films with different x and n was also investigated.

Termination layer compensated tunnelling magnetoresistance in ferrimagnetic Heusler compounds with high perpendicular magnetic anisotropy.

Although high-tunnelling spin polarization has been observed in soft, ferromagnetic, and predicted for hard, ferrimagnetic Heusler materials, there has been no experimental observation to date of high-tunnelling magnetoresistance in the latter. Here we report the preparation of highly textured, polycrystalline Mn3Ge films on amorphous substrates, with very high magnetic anisotropy fields exceeding 7 T, making them technologically relevant. However, the small and negative tunnelling magnetoresistance that we find is attributed to predominant tunnelling from the lower moment Mn–Ge termination layers that are oppositely magnetized to the higher moment Mn–Mn layers. The net spin polarization of the current reflects the different proportions of the two distinct termination layers and their associated tunnelling matrix elements that result from inevitable atomic scale roughness. We show that by engineering the spin polarization of the two termination layers to be of the same sign, even though these layers are oppositely magnetized, high-tunnelling magnetoresistance is possible.

Enhanced interlayer exchange coupling in antiferromagnetically coupled ultrathin (Co70Fe30/Pd) multilayers

We report the studies of magnetization reversal and magnetic interlayer coupling in synthetic antiferromagnetic (SAF) [Pd/Co70Fe30]9/Ru(tRu)/Pd(tPd)/[Co70Fe30/Pd]9 structure as functions of inserted Pd layer (tPd) and Ru layer (tRu) thicknesses. We found the exchange coupling field (Hex) and perpendicular magnetic anisotropy (PMA) can be controlled by both the tPd and tRu, The Hex shows a Ruderman-Kittel-Kasuya-Yosida-type oscillatory decay dependence on tRu and a maximum interlayer coupling strength Jex = 0.522 erg/cm2 is achieved at tPd + tRu ≈ 0.8 nm in the as-deposited sample. As it is known that a high post-annealing stability of SAF structure is required for magnetic random access memory applications, the dependence of Hex and PMA on the post-annealing temperature (Ta) is also investigated. We found that both high PMA of the top Co70Fe30/Pd multilayer is maintained and Hex is enhanced with increasing Ta up to 350 °C for tRu &amp;gt; 0.7 nm in our SAF structure.

Perpendicular magnetic tunnel junction with enhanced anisotropy obtained by utilizing an Ir/Co interface

A highly scalable perpendicularly magnetized storage layer of a spin-torque-switching magnetic random-access memory (STT-MRAM) was developed. This storage layer attains a perpendicular magnetic anisotropy (PMA) of above 0.9 erg/cm2 at a thickness of 2 nm. Such high PMA is suitable for pushing STT-MRAM technology beyond the 20 nm node. The key was to realize dual interfacial PMA at both the Ir/Co and FeB/MgO interfaces in the united structure of the storage layer. While a high PMA was retained, a high magnetoresistance ratio (100%) and a low resistance–area product (3.0 Ω µm2) were also achieved.

High tunneling magnetoresistance ratio in perpendicular magnetic tunnel junctions using Fe-based Heusler alloys

Heulser alloys Fe2Cr1−xCoxSi (FCCS) with different Co compositions x have been predicted to have high spin polarization. High perpendicular magnetic anisotropy (PMA) has been observed in ultra-thin FCCS films with magnetic anisotropy energy density up to 2.3 × 106 erg/cm3. The perpendicular magnetic tunnel junctions (p-MTJs) using FCCS films with different Co compositions x as the bottom electrode have been fabricated and the post-annealing effects have been investigated in details. An attractive tunneling magnetoresistance ratio as high as 51.3% is achieved for p-MTJs using Fe2CrSi (FCS) as the bottom electrode. The thermal stability Δ can be as high as 70 for 40 nm dimension devices using FCS, which is high enough to endure a retention time of over 10 years. Therefore, Heusler alloy FCS is a promising PMA candidate for p-MTJ application.

Analysis and Control of the Initial Electrodeposition Stages of Co-Pt Nanodot Arrays

We have fabricated Co-Pt nanodot arrays by combining electrodeposition with electron beam lithography (EBL) for applications in ultra-high density magnetic recording media, such as bit-patterned media (BPM). In this work, we analyzed the initial nucleation and growth of Co-Pt inside nanopores to achieve nanodot arrays with high deposition uniformity, as well as magnetic properties. At −900mV (vs. Ag/AgCl), multiple nuclei of 2.0–3.0nm in size were randomly distributed, even in nanopores with a 10nm diameter, which could result in a lack of uniformity in the magnetic properties. The number of nuclei was then reduced by applying a less negative potential (>−700mV vs. Ag/AgCl) to deposit a single nucleus inside each nanopore. As a result, a single grain of 5.0–10nm in size was successfully deposited inside the nanopore, which could induce uniform magnetic properties in each nanodot. In addition, at less negative potentials, the coercivity of the Co-Pt films increased, which was induced by the epitaxial-like growth of Co-Pt from the Ru substrate. Cross-sectional TEM analysis suggested that Co-Pt deposited with a less negative potential was single crystalline with uniform hcp lattice fringes in the direction perpendicular to the Ru interface, indicating the formation of highly uniform nanodot arrays with high perpendicular magnetic anisotropy.

&lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;${L}{\text {1}}_{\text {0}}$ &lt;/tex-math&gt;&lt;/inline-formula&gt;-Ordered MnAl Thin Films With High Perpendicular Magnetic Anisotropy Using TiN Underlayers on Si Substrates

Ferromagnetic L1 0 -ordered τ-phase MnAl thin films have been shown to exhibit high perpendicular magnetic anisotropy (K u > 107 ergs/cm3), moderate saturation magnetization (M 0 -ordering for perpendicular anisotropy in MnAl thin films has proven challenging and highly sensitive to deposition conditions and underlayers used [2]. Using MgO underlayers, we previously demonstrated MnAl thin films with high perpendicular magnetic anisotropy (PMA) on Si substrates for the first time [3]. However, MgO is an insulator, which presents two major challenges. First, MgO must be RF-sputtered, which results in low deposition rates, re-sputtering, and therefore the potential for MgO contamination in undesirable locations. Second, since it is an insulator, MgO impedes the use of MnAl film stacks as the bottom electrode in a magnetic tunnel junction (MTJ) for spintronic applications.

Magnetic Properties and Microstructure of FePt(MoC, C) Granular Films

High magnetocrystalline anisotropy (Ku) material was required for future heat assisted magnetic recording media. The ordered L1 0 FePt film can be prepared to have high [001] texture which shows high perpendicular magnetic anisotropy. Epitaxial growth of FePt films on MoC/CrRu/glass have been proved to promote chemical ordering and texturing. Dual-segregants such as transition metal-oxide with carbon have been used to obtain FePt granular structure and columnar grains. In this study, the magnetic properties and microstructure of FePt(MoC, C) films were discussed and the segregant materials were MoC and C. The Mo 40 C 60 was co-sputtered with FePt and the carbon was diffused from the MoC intermediate layer. The MoC phase is formed at equal atomic ratio or higher C concentration. The Mo 50 C 50 film with equal atomic ratio has been studied previously [1] and chemical composition of Mo 40 C 60 film was discussed in this study. When the atomic concentration of C is above 50%, the MoC and C phases are co-existed. The excess C was used as one of the segregant to diffuse up to separate FePt grains from intermediate layer.

Perpendicular magnetic anisotropy properties of tetragonal Mn3Ga films under various deposition conditions

The tetragonal Mn3Ga films exhibited high perpendicular magnetic anisotropy, low saturation magnetization, and high spin polarization, which satisfy the criteria of spin-transfer-torque based devices. For practical device applications, it is necessary to improve the interface nature and optimize the deposition conditions. We fabricated thin films of tetragonal Mn3Ga directly on MgO(100) without any buffer layer by using DC/RF magnetron sputtering method. We investigated the crystallinity, microstructure, and magnetic properties with varying the deposition conditions; such as deposition temperature (350–450 °C), RF power (25–40 W), and Ar gas pressure (2–7 mTorr). X-ray diffraction data revealed that the growth direction is perpendicular to the film plane, i.e., the c axis. Scanning electron microscope images showed that the top surface is flat with a maximum thickness of 290 nm. The optimal deposition conditions are 400 °C, 35 W, and 5 mTorr in our sputtering system. For the field perpendicular to the film plane, clear hysteresis loop was observed with the saturation magnetization MS = 100 emu/cc at room temperature. By extrapolating the hard magnetization data for the field parallel to the film plane, the anisotropic energy was estimated about K1 = 1 × 106 J/m3.