Excellent Perpendicular Magnetic Anisotropy Research Articles

The Success of Fabrication of Pure SmFe2 Phase Film with Outstanding Perpendicular Magnetic Anisotropy.

This study used DC magnetron sputtering technology to fabricate Sm-Fe films and systematically investigated the phase transition behavior of Sm-Fe films with different Fe ratios. It was found that at higher Fe content, the films consisted of Sm2Fe17 or SmFe7 phases; as Fe content decreased, the films were mainly composed of SmFe3 or SmFe2 phases; at higher Sm content, the films primarily consisted of Sm phase. Sm is prone to volatilization at high temperatures, so Ta was used as a capping layer to effectively suppress Sm volatilization, successfully synthesizing pure SmFe2 phase films at a nearly 1:2 ratio. The magnetic properties and magnetostrictive behavior of the SmFe2 films were investigated, revealing that pure-phase SmFe2 films exhibit good perpendicular magnetic anisotropy and magnetostriction properties. The larger stress along the perpendicular-to-film direction, resulting from the absence of substrate-induced constraints, contributes to the excellent perpendicular magnetic anisotropy of the films. This study successfully synthesized pure-phase SmFe2 films and discovered a new method for fabricating perpendicularly anisotropic films. The research findings are of great significance for the efficient synthesis of desired films with high phase formation temperatures containing volatile elements.

Efficient current-induced spin torques and field-free magnetization switching in a room-temperature van der Waals magnet

The discovery of magnetism in van der Waals (vdW) materials has established unique building blocks for the research of emergent spintronic phenomena. In particular, owing to their intrinsically clean surface without dangling bonds, the vdW magnets hold the potential to construct a superior interface that allows for efficient electrical manipulation of magnetism. Despite several attempts in this direction, it usually requires a cryogenic condition and the assistance of external magnetic fields, which is detrimental to the real application. Here, we fabricate heterostructures based on Fe 3 GaTe 2 flakes that have room-temperature ferromagnetism with excellent perpendicular magnetic anisotropy. The current-driven nonreciprocal modulation of coercive fields reveals a high spin-torque efficiency in the Fe 3 GaTe 2 /Pt heterostructures, which further leads to a full magnetization switching by current. Moreover, we demonstrate the field-free magnetization switching resulting from out-of-plane polarized spin currents by asymmetric geometry design. Our work could expedite the development of efficient vdW spintronic logic, memory, and neuromorphic computing devices.

Chiral domain wall motion in unit-cell thick perpendicularly magnetized Heusler films prepared by chemical templating

Heusler alloys are a large family of compounds with complex and tunable magnetic properties, intimately connected to the atomic scale ordering of their constituent elements. We show that using a chemical templating technique of atomically ordered X′Z′ (X′ = Co; Z′ = Al, Ga, Ge, Sn) underlayers, we can achieve near bulk-like magnetic properties in tetragonally distorted Heusler films, even at room temperature. Excellent perpendicular magnetic anisotropy is found in ferrimagnetic X3Z (X = Mn; Z = Ge, Sn, Sb) films, just 1 or 2 unit-cells thick. Racetracks formed from these films sustain current-induced domain wall motion with velocities of more than 120 m s−1, at current densities up to six times lower than conventional ferromagnetic materials. We find evidence for a significant bulk chiral Dzyaloshinskii–Moriya exchange interaction, whose field strength can be systematically tuned by an order of magnitude. Our work is an important step towards practical applications of Heusler compounds for spintronic technologies.

Perpendicular magnetic anisotropy study of CoFeB/Ni multilayers by anomalous Hall effect

The CoFeB/Ni multilayers with Pt underlayer are prepared by magnetron sputtering technique and the perpendicular magnetic anisotropy (PMA) of each of the samples is studied by anomalous Hall effect (AHE) method. The PMA of CoFeB/Ni multilayer is dependent on the thickness of Pt, Co, CoFeB and the number of CoFeB/Ni bilayers strongly. It is found that the sample structured as Pt(4)/[CoFeB(tCoFeB)/Ni(0.3)]2/Pt(1.0) has a good PMA when the CoFeB thickness is 0.4 nm for the interface anisotropy dominated in the multilayer. So the CoFeB thickness is fixed at 0.4 nm. The effect of Ni thickness on multilayer PMA is also studied. The PMA of the sample is kept relatively well and the Hall resistance (RHall) decreases as the Ni thickness increases. Meanwhile the coercivity (HC) fluctuates in a small range. When the Ni thickness is 0.3 nm, the remanence squareness of the sample is very good and the Hall effect is strongest. The influence of period number n on the sample PMA is significant for it changes the interface of the sample. When n is 3, the sample has a very good remanence squareness, for the interface effect is obvious and the magnetization reversal process is consistent. The Pt underlayer shows a great effect on the PMA performance of the sample, for it can change the (111) texture of the multilayer. The results show that when the Pt thickness is 4 nm, the remanence squareness is good and the sample has a suitable HC. So the optimum CoFeB/Ni multilayer with an excellent performance of PMA is structured as Pt(4)/[CoFeB(0.4)/Ni(0.3)]3/Pt(1.0). Its anisotropy constant Keff is 2.2106 erg/cm3 (1 erg/cm3=10-1 J/m3) which indicates that the sample has an excellent PMA and its interface anisotropy is the main reason for making the Keff have a larger value. The magnetic layer thickness of the optimum sample is 2.1 nm and the total thickness of it is less than 8 nm. The integration with device can be studied further. Furthermore, HC of the CoFeB/Ni multilayer is relatively small and can be increased by inserting the oxidation layer or other ways.

Enhancement of perpendicular magnetic anisotropy and thermal stability in Co/Ni multilayers by MgO/Pt interfaces

Co/Ni multilayers with Pt and MgO/Pt underlayer have been grown by means of magnetron sputtering and the perpendicular magnetic anisotropy (PMA) of the samples is studied using anomalous Hall effect (AHE). The Co/Ni multilayer has to be thermally stable to stabilize the PMA, which is studied by annealing treatment. In early researches of Co/Ni multilayes, the optimum sample with Pt underlayer was obtained as Pt(2 nm)/Co(0.2 nm)/Ni(0.4 nm)/Co(0.2 nm)/Pt(2 nm) with PMA in good performance. Thermal stability of the sample is studied in this paper by the Hall loop measurement of it after annealing. Results show that the remanence ratio and rectangular degree of the sample are kept well and the Hall resistance (RHall) has little change at the annealing temperature of 100 ℃. As the annealing temperature rising above 100 ℃, the PMA of Pt(2 nm)/Co(0.2 nm)/Ni(0.4 nm)/Co(0.2 nm)/Pt(2 nm) becomes weakened. Its coercivity (Hc) decreases rapidly and RHall reduces greatly. So the thermal stability of Pt(2 nm)/Co(0.2 nm)/Ni(0.4 nm)/Co(0.2 nm)/Pt(2 nm) will be poor and the PMA cannot be enhanced by annealing treatment. A series of samples with MgO/Pt underlayer are prepared with the thickness of Pt being fixed at 2 nm and that of MgO ranging from 1 to 5 nm. Thus the interface between amorphous insulation layer and metal layer is added to be used to enhance the PMA of the sample for the strong electron additive scattering. Magnetization reversal can be very rapid and the rectangular degree is kept very well, and furthermore, the remanence ratio of the samples can reach 100% so they all show good PMA.The Hc increases with increasing MgO underlayer and reaches the maximum value as the MgO thickness arrives at 4 nm, and the Hc of the sample MgO(4 nm)/Pt(2 nm)/Co(0.2 nm)/Ni(0.4 nm)/Co(0.2 nm)/Pt(2 nm) is 2.3 times that of Pt(2 nm)/Co(0.2 nm)/Ni(0.4 nm)/Co(0.2 nm)/Pt(2 nm), the RHall is up to 9% correspondingly. The roughnesses of Pt(2 nm)/Co(0.2 nm)/ Ni(0.4 nm)/Co(0.2 nm)/Pt(2 nm) and MgO(4 nm)/Pt(2 nm)/Co(0.2 nm) /Ni(0.4 nm)/Co(0.2 nm)/Pt(2 nm) are 0.192 nm and 0.115 nm respectively, as tested by AFM. Result shows that the roughness of the Co/Ni multilayer is greatly reduced so the PMA of the Co/Ni multilayer is enhanced remarkably after the addition of 4 nm MgO. The thermal stability of MgO(4 nm)/Pt(2 nm)/Co(0.2 nm)/Ni(0.4 nm)/Co(0.2 nm)/Pt(2 nm) is also studied. When the annealing temperature rises up to 200 ℃, the Hc reaches its maximum value i.e. 1.5 times that of the sample without MgO, and it is 3.5 times that of the sample with Pt underlayer only. This sample also show good thermal stability. Higher temperatures will result in intermixing of Co and Ni and diminish the PMA. After annealing at 400 ℃, the easy axis of the sample becomes in-plane. The anisotropy constant Keff of MgO(4 nm)/Pt(2 nm)/Co(0.2 nm)/Ni(0.4 nm)/Co(0.2 nm)/Pt(2 nm) is 8.2106 erg/cm3, and it has an increase of 15% in Pt(2 nm)/Co(0.2 nm)/ Ni(0.4 nm)/Co(0.2 nm)/Pt(2 nm), which shows that the sample has an excellent PMA.

Perpendicular magnetic anisotropy in Co/Ni multilayers studied by anomalous Hall effect

Co/Ni multilayers with Pt underlayers have been prepared by magnetron sputtering technique, and their perpendicular magnetic anisotropy (PMA) was studied by the anomalous Hall effect (AHE). The PMA of the samples can be studied by the intensity of Hall signal (RHall), remanence ratio (Mr/Ms), coercivity (HC) and the squarefless of the samples in the Hall hysteresis loops. A clear PMA is observed in the as-deposited amorphous Co/Ni multilayers. The PMA of Co/Ni multilayers is strongly dependent on the thicknesses of Pt, Co, and Ni, and the number of Co/Ni bilayers. After testing, the thicknesses of Pt, Co, and Ni, and the periodic number (n) of Co/Ni bilayers are determined to be 2 nm, 0.2 nm, 0.4 nm and 1 respectively. The optimum Co/Ni multilayer with excellent performance of PMA has a structureflexpressed as Pt(2)/Co(0.2)/Ni(0.4)/Co(0.2)/Pt(2). The hysteresis loop of the sample with the field applied in plane is tested, showing the characteristics of hard axis typically. PMA can be measured by the anisotropy constant Keff which is determined by the competition of the interface anisotropy to the volume anisotropy. If the interface anisotropy is dominant, the sample will have PMA. The anisotropy constant Keff of Pt(2)/Co(0.2)/Ni(0.4)/Co(0.2)/Pt(2) is 3.6×105 J/m3, illustrating that it has an excellent PMA, and the interface anisotropy of Co/Ni is the main factor that makes Keff a larger value. Since the thickness of magnetic layer in the optimum sample is only 0.8 nm and the total thickness of it less than 5 nm, the integration of the device can be studied further. Furthermore, the coercivity of an optimum Co/Ni multilayered sample is relatively small and can be increased by inserting an oxidation layer or by other ways.

Improved perpendicular magnetic anisotropy of Pr–Fe–B films with Ta underlayer

Abstract Perpendicular magnetic anisotropy (PMA) of sputter-prepared Pr–Fe–B films on the glass substrates were improved by inserting a Ta underlayer. Pr–Fe–B directly sputtered on glass substrate exhibits soft in-plane magnetic property due to the amorphous phase or poor crystallization. When Ta layer is introduced as an underlayer, strong PMA with attractive hard magnetic properties are obtained. Their magnetic properties highly depend on the thickness of Ta underlayer ( t ). Large out-of-plane coercivity ( H c⊥ > 800 kA/m) is attained for t in the range of 10–75 nm, where the Pr–Fe–B film sputtered on 50-nm-thick Ta underlayer shows stronger PMA due to promotion of the formation for the Pr 2 Fe 14 B phase with Ta underlayer. Therefore, it was demonstrated that inserting a Ta underlayer with suitable t (∼50 nm) between magnetic layer and glass substrate promotes to obtain Pr–Fe–B film with excellent PMA.

Magnetization reversal mechanism of Nd-Fe-B films with perpendicular magnetic anisotropy

The microstructure and magnetic properties of Nd-Fe-B films with thicknesses from 100 nm to 3 nm have been investigated. All the films show excellent perpendicular magnetic anisotropy with a squareness ratio of 1 in the perpendicular direction and almost zero coercivity in the in-plane direction. Of particular interest is that the initial magnetization curves sensitively depended on the film thickness. Films thicker than 15 nm show steep initial magnetization curve. Although the films have coercivities larger than 21 kOe, the films can be fully magnetized from the thermally demagnetized state with a field as small as 5 kOe. With the decrease of film thickness to 5 nm, the initial magnetization curve becomes flat. The evolution of initial magnetization curves with film thickness can be understood by the microstructure of the films. Films with thickness of 15 nm show close-packed grains without any intergranular phases. Such microstructures lead to steep initial magnetization curves. On the other hand, when the film thickness decreased to 3 nm, the film thickness became nonuniform. Such microstructure leads to flat initial magnetization curves.

Preparation of L10-FePt film for perpendicular magnetic recording media by using FePt/Au mutilayers

FePt/Au mutilayers were deposited on 100℃ MgO（001） single crystal substrate by magnetron sputtering. The influence of the FePt/Au multilayer structure on the ordering temperature, coercivity （ＨC）, perpendicular magnetic anisotropy （PMA）, FePt grain size and interparticle exchange coupling （IEC） of films were studied. Results show that the film possesses high ＨC, excellent PMA, small grain size and weak interparticle exchange coupling after annealing. The results of HRTEM reveal that Au can relieve mismatch between MgO and FePt lattices, leading to excellent PMA. Moreover, the FePt/Au interface energy, the stress energy due to the mismatch between FePt and Au lattices and the diffusion of Au atoms promote the ordering of FePt film. This leads to the decrease of ordering temperature and considerable increase of ＨC. Au atoms partly diffuse to the boundaries of FePt phase and function as grain finer and particle isolator, which decrease the FePt grain size and IEC.

Magnetic properties and microstructure of FePt/Au multilayers with high perpendicular magnetocrystalline anisotropy

FePt/Au multilayers were deposited on preheated 100 °C MgO (001) single crystal substrates by magnetron sputtering. Magnetic properties and microstructure of the films were studied. Results show that the film possesses high coercivity HC, excellent perpendicular magnetic anisotropy (PMA), and weak interparticle exchange coupling (IEC) after annealing. FePt and Au can grow epitaxially on MgO substrates along the [001] direction, leading to excellent PMA. Au atoms partly diffuse to the boundaries of FePt phases and function as grain refiners and particle isolators, which markedly decrease the FePt grain size and IEC. Moreover, the additional FePt/Au interface energy, the stress energy due to the mismatch between FePt and Au lattices, and the diffusion of Au atoms all promote the ordering process of FePt film. This leads to the decrease in ordering temperature by 150 °C and a considerable increase in HC.

Magnetic properties and microstructure of FePt∕BN nanocomposite films with perpendicular magnetic anisotropy

We have prepared L10-FePt∕BN nanocomposite films by depositing the FePt∕BN mutilayers on the MgO(100) substrates using magnetron sputtering followed by a postannealing process. We found the excellent perpendicular magnetic anisotropy of [FePt(2nm)∕BN(0.5nm)]10 film after annealing at 700°C for 1h x-ray diffraction patterns also indicate an excellent (001) texture of the film. This film shows a perpendicular coercivity of 7.48kOe and the ratio of the remnant magnetization Mr to the saturation one MS of as high as 0.93. We also found the good epitaxial growth of FePt grains on the MgO(100) substrate by cross-section high resolution transmitting microscopy.

磁性材料の化学 スパッタ法・めっき法によるフェライト薄膜の堆積技術と構造・物性の比較

The present paper describes synthesis techniques of ferrite thin films by the facing-targets sputtering and the ferrite-plating methods. The difference in microstructure, crystallinity and magnetic properties of the ferrite films deposited by the two deposition methods is caused by the difference in energy which atoms and ions have in order to migrate on and around substrates. Co-M ferrite films (M=Zn, Ni) deposited by the two methods for magnetic recording media were investigated. Since the sputtered atoms have higher migration energy than the plating ions, the sputter-deposited Co-Zn ferrite films exhibited closely packed finer microstructure, better crystallinity, and higher mechanical strength. Co-Ni ferrite-plated films had excellent perpendicular magnetic anisotropy though the microstructure must be improved to apply to the magnetic recording media. The ferrite plating has the advantages in coating micrometersize particles used as magnetically separable carriers such as for medical applications because it is a wet process.

Co ferrite films with excellent perpendicular magnetic anisotropy and high coercivity deposited at low temperature

The present article describes CoxFe3−xO4 films, which were fabricated at 90 °C by the ferrite-plating method and have the excellent perpendicular magnetic anisotropy and the perpendicular coercivity Hc⊥ above 2 kOe. The films were composed of Co ferrite and Fe3O4 grains. The highest Hc⊥ reached 3 kOe at the composition of Co0.43Fe2.57O4 and the saturation magnetization Ms was 500–550 emu/cc. Hc⊥ was over 2 kOe even for a film thickness of 35 nm without any underlayers. ΔM⊥ evaluation that is measured in the perpendicular direction to the film plane showed that the interactions in the films supported the magnetized state, stabilizing the direct current remanent state. The Mössbauer analyses revealed that the Co0.43Fe2.57O4 films have excellent perpendicular magnetic anisotropy and the magnetization direction inclined from the perpendicular axis due to the demagnetization field. This achievement will open the way to develop the high-density perpendicular recording media even when plastic disks and sheets are used as substrates.

Low-temperature fabrication of Co ferrite thin films with high coercivity for perpendicular recording disks by wet process

Perpendicular anisotropic Co x Fe 3− x O 4 films of thickness 25–200 nm for perpendicular magnetic recording media were successfully obtained by the spin-spray ferrite-plating method at 90°C. The maximum perpendicular coercivity H c⊥ of the films was found to be around x=0.5, and the value of H c⊥ was over 2000 Oe even for a film thickness of 25 nm. By optimizing the concentration of NaNO 2 in the oxidizing solution, the in-plane coercivity H c‖ was reduced to 300 Oe, while H c⊥ was kept at over 2000 Oe. Mössbauer analyses also show that the films have excellent perpendicular magnetic anisotropy. The angular dependence of the coercivity shows that a reversal of the magnetization takes place by rotation as in Co–Cr sputter-deposited films.

Coercivity mechanism in CoPt alloy films with perpendicular magnetic anisotropy (abstract)

The coercivity is one of the key parameters for defining a suitable magneto-optic recording material. In order to get insight into coercivity mechanisms we have studied the magnetization reversal processes by means of magneto-optic methods in CoPt alloy films. These films were prepared in UHV by coevaporation with a typical composition ratio of Co28Pt72 and show excellent perpendicular magnetic anisotropy at elevated substrate temperatures during deposition. STM and x-ray measurements reveal characteristic grain size of about 15–20 nm and dispersion of crystallite axis of the order of 0.5°, respectively, which varies only slightly with total film thicknesses of 5–50 nm. The magnetization reversal processes are investigated by using polar Kerr microscopy and were found to be dominated by domain formation and domain wall propagation. A micromagnetic model was developed which describes the film as an ensemble of single domain particles. The size of these single domain particles was chosen to be of the order of the average grain size. In the numerical simulation we include dipolar interaction and domain wall energy contribution. The comparison of the results of the simulation with the experimental results clearly points out that the microstructure is responsible for the magnetization reversal behavior. A thermal as well as thermally activated magnetization processes contribute to the reversal mechanism. The coercivity is predominantly determined by pinning on grain boundaries.

Dynamic Properties of Perpendicular Magnetic Recording Tape Coated with Acicular Ba-Ferrite Media

Magnetic properties of a particulate film fabricated from acicular Ba-ferrite microparticles, coated onto a plastic tape, were evaluated. Squareness ratio measurements showed that this fine dispersion of particles in paint 1) was clearly oriented in the direction of their easy magnetization axis after orientation in a magnetic field, 2) is much more readily formed into a coating film than hexagonal-plate-form Ba-ferrite particles, and 3) has a smooth magnetic field orientation. The hysteresis measurements showed that acicular Ba-ferrite particles have an excellent perpendicular magnetic anisotropy.