Effective Perpendicular Anisotropy Research Articles

Magnetic properties of amorphous ferrromagnetic Co2MnSi/Pt multilayers

Ferrromagnetic Co2MnSi (CMS)/Pt multilayer with perpendicular magnetic anisotropy (PMA) is investigated. A series of amorphous CMS/Pt multilayers with various interface numbers are fabricated by sputtering method. PMA is found to be strongly related to the individual thickness of the CMS/Pt bilayer and the repetition number. For the intermediate Pt spacer thicker than 1.2 nm PMA is demonstrated in the multilayers with CMS thickness in the range of 0.3∼0.7 nm. The effective perpendicular anisotropy energy is measured to be in the order of 102 kJ/m3 in magnitude. The interfacial anisotropy energy density is determined to be 0.58 mJ/m2 per interface. With increasing repetition number wasp-waist shaped magnetization hysteresis appears, showing the switching mechanism with multi-domain feature. Our findings provide some useful information to explore CMS/Pt multilayer-based structures with large PMA for next-generation spintronic devices.

Design and fabrication of Co2FeSi/Pt multilayers with perpendicular magnetic anisotropy

A series of multilayers consisting of Co2FeSi (CFS) and Pt are sputtered. Perpendicular magnetic anisotropy (PMA) is demonstrated in the CFS/Pt multilayers with CFS thickness in the range of 0.4 ∼ 1.0 nm. The individual thicknesses of Pt spacer and Co2FeSi layer as well as interface numbers are shown to strongly affect PMA. Two-step switching is observed in the multilayers with 8 nm Pt spacer, showing typically decoupled feature. The effective perpendicular anisotropy energy is determined to be in the order of 106 erg/cm3. The interfacial perpendicular anisotropy energy density is estimated to be 0.32 erg/cm2. The magnetic reversal with multi-domain feature is observed with increasing interface number. Sufficiently large saturation magnetization is derived from the multilayers with various repetitions. The thickness of magnetic dead layer is found to increase with the number of interface, which may be ascribed to more interdiffusion occurring at the multi-interfaces. Our results provide some useful information for the design and development of spintronic devices based on CFS/Pt multilayers.

Twofold and fourfold anisotropies in zinc ferrite thin films investigated by ferromagnetic resonance

By means of the ferromagnetic resonance (FMR) technique (at the $X$ band) we investigated the magnetic properties of ${\mathrm{ZnFe}}_{2}{\mathrm{O}}_{4}$ (ZFO) films grown on different substrates. Angular dependence of the FMR field showed the existence of twofold and fourfold anisotropies for the same material, grown on Si(111), MgO(100), and STO(100), respectively. By using a model that takes into consideration the relevant contributions to the energy, we were able to extract the following parameters: effective magnetization ${M}_{\mathrm{eff}}$, cubic magnetocrystalline anisotropy field, in-plane and out of plane uniaxial anisotropy fields, and the $g$ factor. The samples deposited on Si showed a twofold in-plane anisotropy while a fourfold magnetic anisotropy was observed for the ZFO/MgO and ZFO/${\mathrm{SrTiO}}_{3}$ samples. All samples showed a strong perpendicular uniaxial anisotropy above 1 kOe and low in-plane uniaxial and cubic anisotropy field, on the order of tens of oersteds. The FMR linewidth ($\mathrm{\ensuremath{\Delta}}H$) fitted from FMR data shows $\mathrm{\ensuremath{\Delta}}H$ values above 190 Oe for all samples, indicating high damping of this material to be $\ensuremath{\sim}{10}^{\ensuremath{-}2}$ for the ZFO/MgO. The present analysis compared the FMR data obtained for the in-plane and out of plane measurements. Likewise, the effective magnetization and perpendicular uniaxial anisotropy field by FMR were compared with the magnetic response obtained by a vibrating sample magnetometer.

Unconventional Seedless Multilayers with Large Perpendicular Anisotropy for Back-End-of-Line Compatible Spintronic Devices

An unconventional class of hard and out-of-plane magnetized seedless multilayers (SL-MLs) of the form [Co/insertion layer/Pt]n, grown as a top reference layer in magnetic tunnel junctions (MTJ), was investigated. Among different non-ferrous insertion layers (Ta, Cu, Mg, W, Ru, and Al), Ta produces the highest perpendicular magnetic anisotropy. A back-end-of-line (BEOL) compatible hard top reference layer using Ta-inserted SL-MLs was achieved, exhibiting more than twice effective perpendicular anisotropy compared to a conventional top reference layer. Using this top reference layer, we developed three types of spintronic memory stacks: (a) top-pinned MTJ stacks that can withstand annealing up to 425 °C, (b) BEOL compatible double-MTJ memory cells with read/write mode control layer enabling a faster readout and low-voltage writing, and (c) 2-bit MTJ stacks combining spin-transfer torque and spin-orbit torque writing. The magnetic properties of these three types of memory stacks are discussed.

Theory of relaxation time of stochastic nanomagnets

Reducing the relaxation time between binary spin states is crucial to enhance the speed and accuracy of a spintronics-based approach for a probabilistic computer, as envisioned by R.P. Feynman. Contrary to previous perception that the reduction of the energy barrier between the states is the most efficient way, the authors discover here that the relaxation time in in-plane easy-axis nanomagnets is widely controllable by changing the effective perpendicular anisotropy field, even though it has nothing to do with the energy barrier.

Epitaxial growth of Ho<sub>3</sub>Fe<sub>5</sub>O<sub>12</sub> films with perpendicular magnetic anisotropy and spin transport properties in Ho<sub>3</sub>Fe<sub>5</sub>O<sub>12</sub>/Pt heterostructures

Rare-earth iron garnet films with perpendicular magnetic anisotropy could open new perspectives for spintronics. Holmium iron garnet (Ho<sub>3</sub>Fe<sub>5</sub>O<sub>12</sub>, HoIG) films with thickness ranging from 2 to 100 nm are epitaxially grown on (111) orientated gadolinium gallium garnet single crystal substrate doped with yttrium and scandium (Gd<sub>0.63</sub>Y<sub>2.37</sub>Sc<sub>2</sub>Ga<sub>3</sub>O<sub>12</sub>, GYSGG) by ultra-high vacuum magnetron sputtering. A 3-nm Pt film is further deposited on each of the HoIG films. The magnetic anisotropy and magneto-transport properties of heterostructures at room temperature are investigated. It is shown that the HoIG film as thin as 2 nm (less than two unit cells in thickness) exhibits the ferromagnetic properties at room temperature, and perpendicular magnetic anisotropy is achieved in the 2-60 nm thick films, and a maximum effective perpendicular anisotropy field reaches 350 mT due to the strain induced magnetoelastic anisotropy. The HoIG/Pt heterostructure shows significant anomalous Hall effect (AHE) and appreciable spin-Hall magnetoresistance (SMR) and/or anisotropic magnetoresistance (AMR). Remarkably, the AHE starts to decline gradually when the HoIG thickness is less than 4 nm, but the magnetoresistance decreases rapidly with the HoIG layer becoming less than 7 nm in thickness. The fact that the AHE in the heterostructure is less sensitive to the HoIG thickness suggests that the interface effect is more dominant in the AHE mechanism, whereas the bulk magnetic properties of the HoIG plays a more important role for the observed magnetoresistance. In addition, the spin Seebeck effect decreases exponentially with the decrease of HoIG thickness till the ultrathin limit, which was previously validated in the micrometer-thick YIG/Pt stacks in the frame of thermally excited magnon accumulation and propagation. The present results show that the nanometer HoIG/Pt heterostructure with tunable perpendicular magnetic anisotropy and efficient interfacial spin exchange interaction could be a promising candidate for insulating magnet based spintronic devices.

Impact of Fe80B20 insertion on the properties of dual-MgO perpendicular magnetic tunnel junctions

We explore the impact of Fe80B20 inserted at both Co20Fe60B20/MgO interfaces of dual-MgO free layers (FLs) in bottom-pinned magnetic tunnel junctions (MTJs). MTJ stacks are annealed for 30 min at 350 °C and 400 °C in a vacuum after film deposition. Current-in-plane tunneling measurements are carried out to characterize magnetotransport properties of the MTJs. Conventional magnetometry measurements and ferromagnetic resonance (FMR) are conducted to estimate the saturation magnetization, the effective perpendicular anisotropy field and the Gilbert damping of dual-MgO FLs as a function of the Fe80B20 thickness and annealing temperatures. With ultrathin Fe80B20 (0.2–0.4 nm) inserted, perpendicular magnetic anisotropy (PMA) of FLs increases with similar tunnel magneto-resistance (TMR) and low damping values. As Fe80B20 layer thickness further increases (0.6–1.2 nm), both TMR and PMA degrade, and damping increases dramatically. This study demonstrates a novel approach to tune properties of MTJ stacks with dual-MgO FLs up to 400 °C annealing, which enables MTJ stacks for various applications.

Spin–orbit torque driven multi-level switching in He+ irradiated W–CoFeB–MgO Hall bars with perpendicular anisotropy

We have investigated the spin–orbit torque-driven magnetization switching in W/CoFeB/MgO Hall bars with perpendicular magnetic anisotropy. He+ ion irradiation through a mask has been used to reduce locally the effective perpendicular anisotropy at a Hall cross. Anomalous Hall effect measurements combined with Kerr microscopy indicate that the switching process is dominated by domain wall (DW) nucleation in the irradiated region followed by rapid domain propagation at a current density as low as 0.8 MA/cm2 with an assisting in-plane magnetic field. Thanks to the implemented strong pinning of the DW at the transition between the irradiated and the non-irradiated region, an intermediate Hall resistance state is induced, which is further verified by finite element simulations. Such a method to control electrically multi-level resistances using He+ ion irradiation shows great potential in realizing neuromorphic and memristor devices.

Bidirectional switching assisted by interlayer exchange coupling in asymmetric magnetic tunnel junctions.

We study the combined effects of spin transfer torque, voltage modulation of interlayer exchange coupling and magnetic anisotropy on the switching behavior of perpendicular magnetic tunnel junctions (p-MTJs). In asymmetric p-MTJs, a linear-in-voltage dependence of interlayer exchange coupling enables the effective perpendicular anisotropy barrier to be lowered for both voltage polarities. This mechanism is shown to reduce the critical switching current and effective activation energy. Finally, we analyze the possibility of having switching via interlayer exchange coupling only.

Non-linear variation of domain period under electric field in demagnetized CoFeB/MgO stacks with perpendicular easy axis

We investigate a variation of magnetic domain pattern in demagnetized perpendicular-easy-axis CoFeB/MgO stacks with a wide-range electric field and discuss the electric-field effect on the exchange stiffness constant AS. We observe a non-linear variation of domain period DP with the applied electric field, in contrast to the effective perpendicular anisotropy field HKeff that is evaluated from a ferromagnetic resonance and shows a linear response. An analysis based on a model describing the relation of DP to AS and HKeff suggests that AS varies non-linearly with the electric field, implying an involved characteristic mechanism of the exchange stiffness in the studied systems. A possible scenario for explaining the non-linear behavior of AS is discussed.

Spin-orbit torque-induced switching in ferrimagnetic alloys: Experiments and modeling

We investigate spin-orbit torque (SOT)-induced switching in rare-earth-transition metal ferrimagnetic alloys using W/CoTb bilayers. The switching current is found to vary continuously with the alloy concentration, and no reduction in the switching current is observed at the magnetic compensation point despite a very large SOT efficiency. A model based on coupled Landau-Lifschitz-Gilbert (LLG) equations shows that the switching current density scales with the effective perpendicular anisotropy which does not exhibit strong reduction at the magnetic compensation, explaining the behavior of the switching current density. This model also suggests that conventional SOT effective field measurements do not allow one to conclude whether the spins are transferred to one sublattice or just simply to the net magnetization. The effective spin Hall angle measurement shows an enhancement of the spin Hall angle with the Tb concentration which suggests an additional SOT contribution from the rare earth Tb atoms.

Domain-width model for perpendicularly magnetized systems with Dzyaloshinskii-Moriya interaction

The influence of the Dzyaloshinskii-Moriya interaction (DMI) on stripe domains in perpendicularly magnetized thin ferromagnetic films is theoretically and experimentally investigated. We develop a domain spacing model describing the dependence of the stripe domain width on the magnetic properties of the sample. By including the magnetostatic energy of the domain walls the model correctly describes the transition from Bloch to N\'eel walls with increasing DMI constant. An approach to determine the magnitude of the DMI constant by fitting the stripe domain width as a function of the effective perpendicular anisotropy of wedge-shaped samples is developed and applied to several ultrathin multilayer samples based on Ni/Fe/Cu(001). The magnitude of the DMI constant arising from Fe/Ni and Ni/Fe interfaces is $0.3\ifmmode\pm\else\textpm\fi{}0.14$ meV/atom, indicating that the domain walls are in a pure chiral N\'eel state. Furthermore, phase diagrams of the skyrmionic bubble domain phase are recorded for two samples with different DMI constants, and by scaling the magnetic field a universal phase diagram for perpendicularly magnetized systems is obtained.

Magnetic properties tailored by inserting ultrathin Cr layer in Gd/Ni80Fe20 film

The magnetic properties of Gd(4 nm)/Cr()/(12 nm) films with different thicknesses of Cr layer prepared by magnetron sputtering were investigated. X-ray diffraction proves the coexistence of hexagonal closed-packed (hcp) (0 0 2) and face-centred cubic (fcc) (1 1 1) peaks in the Gd layer. Magnetic measurements show a modulation akin to RKKY coupling in the saturation magnetization () as a result of the coupling sign between Gd and Py layers oscillating with . The strained hcp phase and, in particular, the fcc phase lead to poor magnetic moments and strongly reduced saturation of Gd, which can also be affected by the possible magnetically dead layers, the intermixing at interfaces and a presumably more gradual transition to adjacent phases. Theoretical fittings of the relationship between the ferromagnetic resonance field () and the microwave frequency yield the effective magnetization (4) and perpendicular anisotropy constant (), which are all enhanced by introducing Gd and Cr layers as a result of the increased interfaces. Interestingly, with the increasing thickness of Cr spacer layer, an obvious decrease in the magnetic damping coefficient () is observed, from which we infer that both the spin–orbit interaction due to the interlayer couplings and the electron density of states (DOS) at the Fermi energy (EF) resulting from the intermixing at the interfaces of Gd/Cr/ film are the major origins. In addition, all magnetic parameters are approaching to those of a Py film with increasing to 3 nm.

Enhanced annealing stability and perpendicular magnetic anisotropy in perpendicular magnetic tunnel junctions using W layer

The magnetic properties of the perpendicular storage electrode (buffer/MgO/FeCoB/Cap) were studied as a function of annealing temperature by replacing Ta with W and W/Ta cap layers with variable thicknesses. W in the cap boosts up the annealing stability and increases the effective perpendicular anisotropy by 30% compared to the Ta cap. Correspondingly, an increase in the FeCoB critical thickness characterizing the transition from perpendicular to in-plane anisotropy was observed. Thicker W layer in the W(t)/Ta 1 nm cap layer makes the storage electrode highly robust against annealing up to 570 °C. The stiffening of the overall stack resulting from the W insertion due to its very high melting temperature seems to be the key mechanism behind the extremely high thermal robustness. The Gilbert damping constant of FeCoB with the W/Ta cap was found to be lower when compared with the Ta cap and stable with annealing. The evolution of the magnetic properties of bottom pinned perpendicular magnetic tunnel junctions (p-MTJ) stack with the W2/Ta1 nm cap layer shows back-end-of-line compatibility with increasing tunnel magnetoresistance up to the annealing temperature of 425 °C. The pMTJ thermal budget is limited by the synthetic antiferromagnetic hard layer which is stable up to 425 °C annealing temperature while the storage layer is stable up to 455 °C.

Giant perpendicular exchange bias with antiferromagnetic MnN

We investigated an out-of-plane exchange bias system that is based on the antiferromagnet MnN. Polycrystalline, highly textured film stacks of Ta/MnN/CoFeB/MgO/Ta were grown on SiOx by (reactive) magnetron sputtering and studied by x-ray diffraction and Kerr magnetometry. Nontrivial modifications of the exchange bias and the perpendicular magnetic anisotropy were observed as functions of both film thicknesses and field cooling temperatures. In optimized film stacks, a giant perpendicular exchange bias of 3600 Oe and a coercive field of 350 Oe were observed at room temperature. The effective interfacial exchange energy is estimated to be Jeff = 0.24 mJ/m2 and the effective uniaxial anisotropy constant of the antiferromagnet is Keff = 24 kJ/m3. The maximum effective perpendicular anisotropy field of the CoFeB layer is Hani = 3400 Oe. These values are larger than any previously reported values. These results possibly open a route to magnetically stable, exchange biased perpendicularly magnetized spin valves.

Electric-field control of magnetic property in perpendicularly magnetized Co0.25Pd0.75/PMN-PT heterostructure

We report about the electric-field control of magnetic properties in a perpendicularly magnetized Co0.25Pd0.75 film deposited on PMN-PT (lead magnesium niobate-lead titanate) substrate. The effective perpendicular anisotropy coefficient Keff is efficiently tuned via the electric field up to 2 MV/m from 2.5 × 104 to 0.8 × 104 J/m3, which leads to the change of the effective magnetic anisotropy field μ0ΔHeff = −45 mT. With a time-resolved magneto-optical Kerr effect, we observed the precession frequency change Δf = 0.8 GHz. Our simulation estimates that this value corresponds to Δf = −3 GHz and Δf/f = −0.23 in a scheme of ferromagnetic resonance measurement.

Seed layer impact on structural and magnetic properties of [Co/Ni] multilayers with perpendicular magnetic anisotropy

[Co/Ni] multilayers with perpendicular magnetic anisotropy (PMA) have been researched and applied in various spintronic applications. Typically, the seed layer material is studied to provide the desired face-centered cubic (fcc) texture to the [Co/Ni] to obtain PMA. The integration of [Co/Ni] in back-end-of-line processes also requires the PMA to survive post-annealing. In this paper, the impact of NiCr, Pt, Ru, and Ta seed layers on the structural and magnetic properties of [Co(0.3 nm)/Ni(0.6 nm)] multilayers is investigated before and after annealing. The multilayers were deposited in-situ on different seeds via physical vapor deposition at room temperature. The as-deposited [Co/Ni] films show the required fcc(111) texture on all seeds, but PMA is only observed on Pt and Ru. In-plane magnetic anisotropy is obtained on NiCr and Ta seeds, which is attributed to strain-induced PMA loss. PMA is maintained on all seeds after post-annealing up to 400 °C. The largest effective perpendicular anisotropy energy (KUeff≈2×105 J/m3) after annealing is achieved on the NiCr seed. The evolution of PMA upon annealing cannot be explained by further crystallization during annealing or strain-induced PMA, nor can the observed magnetization loss and the increased damping after annealing. Here, we identify the diffusion of the non-magnetic materials from the seed into [Co/Ni] as the major driver of the changes in the magnetic properties. By selecting the seed and post-annealing temperature, the [Co/Ni] can be tuned in a broad range for both PMA and damping.

Thickness dependent structural, magnetic and magneto-dynamic properties of Mn rich Ni-Mn-Sn films

Off-stoichiometric Mn-rich Ni-Mn-Sn films of different thicknesses were deposited on Si (100) substrate by dc magnetron sputtering at ambient temperature and then annealed ex situ at 550 °C for 1 h under high vacuum. X-ray diffraction patterns revealed that as deposited films were amorphous, whereas the annealed films exhibited room temperature cubic structure with space group Fm3¯m (space group 225) with high degree of L21 ordering. Magnetic study of the annealed film disclosed room temperature ferromagnetic order with easy axis of magnetization along the plane of the film. Magneto-dynamic study of the annealed films was carried out using a micro strip-ferromagnetic resonance (FMR) spectrometer. The deduced values of saturation magnetization from frequency dependence of the resonance field of the FMR spectra were in close agreement with those obtained from dc magnetization measurement. The analysis of frequency variation of linewidth shows the presence of low Gilbert damping constant of 0.007 for the 500 nm thick film. Gilbert damping constant was found to increase slightly with film thickness. The polar angle variation FMR study for 1500 nm film revealed the presence of high effective perpendicular anisotropy (K1 ≈ −9.0 ± 0.5 × 105 erg/cm3) and easy magnetization along its plane.

Magnetic Properties and Interfacial Anisotropies of Pt/Co/AlOxPerpendicularly Magnetized Thin Films

The thin films of Pt/Co/AlO x , showing perpendicular magnetic anisotropy, were grown by magnetron sputtering with AlO x formed by the oxidation of thin Al layers using an oxygen atom source. Films were studied as a function of Pt thickness and Al oxidation, and films that showed full remanence and sharp switching coercivity were achieved. In order to prevent further oxidation of the interface in ambient conditions, we use a double Al growth and oxidation process. The magnetooptical Kerr effect and vibrating sample magnetometry were used to analyze these films. We find an effective perpendicular anisotropy of $2 \times 10^{6}$ erg/cm3, with the majority of the perpendicular anisotropy coming from the Pt/Co interface. From the sweep rate dependence on the coercivity, we are able to extract an activation volume of 4.3 $\pm 0.5 \times 10^{-18}$ cm3, similar to other Co-based perpendicular systems.

Large Perpendicular Exchange Bias in CoFeB/MgO Systems Pinned by a Bottom IrMn Layer via an Interfacial CoFe/Ta Composite Layer

CoFeB/MgO films with perpendicular magnetization pinned by an antiferromagnetic IrMn layer have been explored. CoFeB/MgO films directly grown on an antiferromagnetic IrMn layer exhibit insufficient perpendicular magnetic anisotropy (PMA) to overcome shape anisotropy. With a Ta dusting layer inserted into the CoFeB layer, perpendicular magnetization and perpendicular exchange bias (PEB) are simultaneously acquired in IrMn/CoFeB/MgO films. More significantly, when the CoFeB sublayer between IrMn and Ta is replaced by a thin CoFe layer, the PEB is drastically enhanced with large PMA sustained. Through optimizing the thicknesses of CoFe and Ta, a PEB field of about 540 Oe is achieved in the CoFeB films with an effective perpendicular anisotropy field of about 2.5 kOe. The present results suggest that the interfacial CoFe/Ta composite layer insertion paves the way to establish large PEB in the IrMn/CoFeB/MgO system.