Ferrimagnetic Layer Research Articles

Magnetization Reversal of Strongly Exchange-Coupled Double Nanolayers for Spintronic Devices

Contemporary spintronics devices, notably GMR and TMR-sensors with high linear ranges, stand to benefit from large bias fields in excess of 1T that have been observed in exchange-coupled double layers using rare earth-transition metal ferrimagnets as pinning layers. However, before exploiting the strong coupling the reversal process must be understood at the scales of the smallest devices, where it would be effective. Little is known about these processes to-date, owing to the technical difficulty of imaging magnetic structures smaller than 20nm in fields of several Tesla. Here we image the nanoscale magnetic structures of a double layer comprising a polycrystalline Co/Pt-multilayer (Co/Pt) which is strongly exchange-coupled to an amorphous and magnetically-hard ferrimagnetic TbFe alloy layer. High resolution magnetic force microscopy provides snapshots of the magnetic structures along various applied fields between 0 and 7T as the magnetization reversal takes place. In contrast to a magnetization reversa...

High-Frequency Current-Controlled Vortex Oscillations in Ferrimagnetic Disks

Recently it was shown that ferrimagnetic nanodisks support a magnetic vortex state that can oscillate at frequencies up to 30 GHz, suggesting that a ferrimagnetic free layer in a spin-torque-driven nanopillar oscillator would have a significantly higher frequency than the ferromagnetic oscillator. Here theory shows that a nanopillar spin-torque oscillator with a current perpendicular to the ferrimagnetic free-layer will indeed drive high-frequency vortex oscillations. This work indicates that it is feasible to construct ferrimagnetic spin-torque oscillators with frequencies increased tenfold, or more. Moreover, the Oersted field from the current provides enhanced frequency control.

Effect of surface modification treatment of buffer layer on thermal tolerance of synthetic ferrimagnetic reference layer in perpendicular-anisotropy magnetic tunnel junctions

We improve thermal tolerance of a Co/Pt based synthetic ferrimagnetic (SyF) reference layer in a CoFeB/MgO-based perpendicular anisotropy magnetic tunnel junction by employing a surface modification treatment (SMT) using Ar plasma on a Pt buffer layer underneath the SyF layer. A higher perpendicular anisotropy and more robustness against annealing for the SyF reference layer is observed by the employment of SMT. From the cross-sectional scanning transmission electron microscope of the magnetic tunnel junction and atomic force microscopy on the Pt buffer layer, SMT has no significant effect on the surface roughness of the Pt buffer layer. On the other hand, X-ray diffraction analysis reveals that SMT increases the lattice spacing of the Pt buffer layer and shrinks that of the Co/Pt multilayer, which are maintained after annealing at 400 °C. Those structural changes could lead to the improvement of perpendicular anisotropy and thermal tolerance against annealing. A high-resolution Rutherford backscattering analysis reveals that Ar ions used for SMT penetrate to the Pt buffer layer, resulting in variation of the lattice spacing for the Pt buffer layer and Co/Pt multilayer. In addition, suppression of Fe diffusion from the CoFeB reference layer to the bottom Co/Pt multilayer is observed by using energy dispersive X-ray spectrometry, which also contributes to the higher thermal tolerance against annealing. The present observation reveals that the employment of SMT enables one to obtain both high perpendicular magnetic anisotropy and high robustness against annealing through crystallographic change of the Co/Pt multilayer and suppression of Fe diffusion from CoFeB layer into the Co/Pt multilayer.

Room temperature electric field control of magnetic properties for the α-Fe2O3/Fe3O4 composite structure

Abstract The raw nano α-Fe2O3 powders consisting of nanorods with 50–100 nm in length and 8–14 nm in width exhibit superparamagnetism. The vacuum annealing of the raw nano α-Fe2O3 at 400 °C for 3 h leads to ferromagnetic-like behaviors at room temperature (RT), due to the appearance of ferrimagnetic Fe3O4. The α-Fe2O3/Fe3O4 composite structure has an antiferromagnetic α-Fe2O3 layer and a ferrimagnetic Fe3O4 layer contains a certain amount of Fe2+ and oxygen vacancies. In addition, nano α-Fe2O3 annealed in H2 at 300 °C for 3 h completely become Fe3O4. Some magnetic properties of the α-Fe2O3/Fe3O4, such as magnetization, magnetic remanence (Mr), coercivity (Hc) and exchange bias (EB), can be modulated almost reversibly and non-volatilely by applied external RT electric field without loop current. However, for both the raw nano α-Fe2O3 and the Fe3O4, these magnetic properties cannot be modulated by applied external RT electric field. The electric field control of magnetic properties for the α-Fe2O3/Fe3O4 should be due to the mechanism of phase transition induced by migration and redistribution of oxygen vacancies within the interface of α-Fe2O3/Fe3O4.

Effect of Dzyaloshinskii–Moriya Interaction Energy Confinement on Current‐Driven Dynamics of Skyrmions

Magnetic skyrmions, which are topological spin configuration, have gained interest in the past few years. However, skyrmions suffer from the skyrmion Hall effect, a phenomenon where skyrmions deflect from the path of the electron flow and annihilate at the edge of the track. There are attempts to overcome this effect using synthetic antiferromagnetic layer structures and ferrimagnetic layer structures. Herein, a new approach based on introducing Dzyaloshinskii–Moriya interaction (DMI) energy wells is reported to direct the motion of skyrmions in a nanowire. It is shown through simulations that by creating DMI energy wells with a critical DMI value, a skyrmion moves within the boundaries set by the energy wells. It is also shown that the diameter of a skyrmion can be reduced by decreasing the space between the DMI energy wells. The proposed DMI energy well enables packing of skyrmion tracks at a high density. In this view, the effect of magnetostatic interactions between multiple skyrmions on their motion in parallel tracks is also investigated. Furthermore, the effect of changing the distance between skyrmions on magnetostatic interactions is studied. These results offer a new path toward maneuvering the skyrmion motion for racetrack memory or logic devices.

Bulk Dzyaloshinskii-Moriya interaction in amorphous ferrimagnetic alloys.

Symmetry breaking is a fundamental concept that prevails in many branches of physics1-5. In magnetic materials, broken inversion symmetry induces the Dzyaloshinskii-Moriya interaction (DMI), which results in fascinating physical behaviours6-14 with the potential for application in future spintronic devices15-17. Here, we report the observation of a bulk DMI in GdFeCo amorphous ferrimagnets. The DMI is found to increase linearly with an increasing thickness of the ferrimagnetic layer, which is a clear signature of the bulk nature of DMI. We also found that the DMI is independent of the interface between the heavy metal and ferrimagnetic layer. This bulk DMI is attributed to an asymmetric distribution of the elemental content in the GdFeCo layer, with spatial inversion symmetry broken throughout the layer. We expect that our experimental identification of a bulk DMI will open up additional possibilities to exploit this interaction in a wide range of materials.

Topological Hall effect in ferrimagnetic CoTb single layer

Abstract Hall transport properties of single ferrimagnetic CoTb thin films were systematically studied. We report on the extraordinary Hall signals near the magnetization compensation point. The Hall “anomaly”, which is reminiscent of the Topological Hall effect (THE), is distinguished from the anomalous Hall effect (AHE) by fitting the overall Hall resistance signal. At the Hall anomaly regime, magnetic bubbles are observed by optical magnetometry. We propose the observed Hall anomalies originate from the noncollinear spin texture stabilized by Dzyaloshinskii–Moriya interaction (DMI), which is closely related to the anisotropic antiferromagnetic coupling between Co and Tb moments. The novel phenomena found in CoTb thin films broaden the physic understanding of THE and may stimulate on further studies of THE in amorphous materials.

Exchange Bias Enhancement and Magnetic Proximity Effect in FeVO4-Fe3O4 Nanoparticles

We study the behavior of the exchange bias (EB) and the blocking temperature in an antiferromagnetic FeVO4–ferrimagnetic Fe3O4 nanocomposite system upon annealing in Ar atmosphere. Surprisingly, the blocking temperature of post-annealed samples increased to ∼ 50 K, more than two-fold compared the Neel temperature (TN = 22 K) of individual FeVO4 nanoparticles. This significant enhancement of the blocking temperature was accompanied by the corresponding increase of EB, from ∼ 50 Oe in as-prepared samples to ∼ 110 Oe in post-annealed samples. The temperature dependence of EB can be described by two approximately linear regions with different slopes, with an inflection point at T ∼ 21 K coinciding with the Neel temperature of FeVO4 nanoparticles. The region above the inflection point with non-zero EB is characterized by a weaker temperature dependence and is expanded well beyond TN. The x-ray photoemission spectroscopy measurements indicate that the surface of post-annealed Fe3O4 particles becomes oxygen deficient, which leads to a modification of the electronic, magnetic and morphological properties of the FeVO4/Fe3O4 interface. We associate this unusual behavior with a magnetic proximity effect, in which the ordering temperature of the antiferromagnetic FeVO4 nanoparticles and the corresponding exchange bias are strongly affected by the adjacent ferrimagnetic Fe3O4 layer.

Tetragonal Mn3Sn Heusler films with large perpendicular magnetic anisotropy deposited on metallic MnN underlayers using amorphous substrates

Tetragonal Heusler compounds that exhibit large perpendicular magnetic anisotropy are promising materials for advanced spintronic devices. A prerequisite are thin films whose tetragonal axis is oriented perpendicular to the plane of the films. Here we show that highly textured, (001) oriented, tetragonal Mn3Sn layers can be prepared using metallic zinc-blende (ZB) MnN as underlayers. Moreover, we show that these layers can be deposited on amorphous substrates using reactive magnetron sputtering. The ferrimagnetic Mn3Sn layers exhibit perpendicularly magnetized hysteresis loops with coercive fields of ∼2 T. Stoichiometric ZB-MnN underlayers share an “equivalent” Mn-Mn layer at the interface with Mn3Sn, thus promoting their oriented growth. Other nitride underlayers are not effective due to their rock-salt (RS) crystal structure and the absence of Mn. Density functional theory calculations confirm that tetragonal Mn3Sn Heusler films are energetically stable when interfaced with ZB-MnN underlayers and not with any of the other RS nitride underlayers considered here. Such Heusler compounds have much promise as electrodes for magnetic tunnel junction memory elements for deeply scaled magnetic random access memories.

Critical Role of W Insertion Layer Sputtering Condition for Reference Layer on Magnetic and Transport Properties of Perpendicular-Anisotropy Magnetic Tunnel Junction

We investigated an effect of sputtering gas species Ar, Kr, and Xe for deposition of a W insertion layer W(Ar, Kr, and Xe) in the synthetic ferrimagnetic reference layer consisting of [Co/Pt] multilayer/Ru/[Co/Pt] multilayer/W/CoFeB on magnetic properties of the reference layer and tunnel magnetoresistance ratio (TMR ratio) of magnetic tunnel junction (MTJ) stacks with the reference layer. In all the cases, the TMR ratio increased with the increase of the W insertion layer thickness $t_{\mathrm {W}}$ and showed a maximum at certain $t_{W}$ . Although the maximum TMR ratio is almost the same for all the cases, $t_{\mathrm {W}}$ range in which we observed the maximum TMR ratio is dependent on the gas species; $t_{W} = 0.2$ nm for Ar, $t_{\mathrm {W}} = 0.2$ –0.4 nm for Kr, and $t_{\mathrm {W}} = 0.2$ –0.5 nm for Xe, indicating that $t_{\mathrm {W}}$ margin giving high-TMR ratio becomes wider with increasing atomic number of the gas species. The lower TMR ratio for W (Ar) at larger $t_{\mathrm {W}}$ region is due to the degradation of ferromagnetic coupling between the CoFeB and Co/Pt multilayer sandwiching W, resulting in canting of magnetization in the CoFeB layer. We found that an intermixing of Pt atoms in the Co/Pt multilayer occurred in the MTJ stack with W (Ar), whereas it did not intermix with W (Kr), indicating that the Pt interdiffusion caused the degradation of ferromagnetic coupling through W layer.

Effect of annealing on magnetic properties of Fe/Fe3O4 soft magnetic composites prepared by in-situ oxidation and hydrogen reduction methods

Iron-based soft magnetic composites (SMCs) coated with the ferrimagnetic Fe3O4 layer have been fabricated by in-situ oxidation and hydrogen reduction methods, and then the effects of annealing process on the soft magnetic properties were investigated. It showed that the Fe3O4 insulating layer was coated on the surface of the iron powders, which effectively reduced the magnetic dilution and decreased the core loss of the composites. In addition, the Fe/Fe3O4 core-shell composites exhibited high permeability and frequency stability of permeability after annealing. The real part of permeability of the soft magnetic composites could reach a maximal value of 142.9 and a rather low core loss of 274.9 W/kg (measured at 50 mT and 100 kHz). Based on the magnetic loss separation model, the hysteresis loss and eddy current loss coefficient was obtained. It was indicated that the annealing process enhanced the eddy current loss coefficient while the hysteresis loss coefficient is close to each other for all the samples. Therefore, this work can provide a method to prepare soft magnetic composites with excellent magnetic properties and low core loss.

Order Parameter Profiles near Surfaces of Super-weak Ferrimagnetic Materials Exhibiting a Paramagnetic-Ferrimagnetic Transition

The aim of this paper is to determine the full order parameter profiles close to the surface for a system made of two strongly coupled paramagnetic sublattices of respective moments and . The material exhibits a para-ferrimagnetic transition at some critical temperature Tc greater than the room temperature. The free energy describing the physics of the system is of Landau type, and involves, beside quadratic and quartic terms in both and , a lowest-order coupling, -Co where CoL, and the free energy is then a sum of bulk and surface contributions expanded in terms of the local order parameters. The magnetization at the surface are s and s. As in a recent paper (whom the present work is an extension), we first reduce the model to an effective 4 -theory written in terms of the overall magnetization =+ and the associated fraction of magnetization a=/(+). For the ordinary transition where the extrapolation length is positive s>0, we determine the order parameter profile near surfaces. We show, in particular, that the magnetization behavior is governed, in addition of s and the bulk correlation length b-, by a new length La. We have interpreted this latter as the width of the ordered ferrimagnetic layer close to the surface. Finally, we examine the extraordinary (ss=) and surface (at Tcs>Tc) transitions.

Structure and magnetic properties of Tb-Co/Ti and Tb-Co/Al2O3 multilayers

The influence of the nanoscale thicknesses of the ferrimagnetic Tb-Co layers on the structure and magnetic properties of magnetron sputtered Tb-Co/Ti and Tb-Co/Al2O3 magnetic multilayers were comparatively studied. Low angle X-ray diffraction patterns testified to the existence of a well defined layered structure in all the cases under consideration. For all samples, the magnetization was positioned in the plane of the films in the entire investigated temperature interval from 5 to 350 K. The temperature dependence of magnetization of the multilayered structures is the function of the Tb-Co layer thickness. The material of the spacers was shown to significantly influence the magnetic properties of the multilayers.

CsFe3(SeO3)2F6 with S = 5/2 Cube Tile Lattice.

A layered iron selenite fluoride CsFe3(SeO3)2F6 1 was hydrothermally synthesized. Single-crystal X-ray diffraction studies show that 1 has a trigonal ( P3̅ m1) lattice, where [Fe3(SeO3)2F6]- blocks of three iron sublayers are separated by Cs cations. Within the block, only Fe(2)F6 and Fe(1)O3F3 octahedra are magnetically connected via superexchange Fe(1) -F -Fe(2) pathways, giving an S = 5/2 cube tile (dice) lattice. At low magnetic field, 1 exhibits an antiferromagnetic transition at ∼130 K, where ferrimagnetic cube tile layers are arranged in a staggered manner. At low temperatures, we observed a field-induced transition to a ferrimagnetic state with a one-third magnetization plateau.

Comparative study on spin-orbit torque efficiencies from W/ferromagnetic and W/ferrimagnetic heterostructures

It has been shown that W in its resistive form possesses the largest spin-Hall ratio among all heavy transition metals, which makes it a good candidate for generating efficient dampinglike spin-orbit torque (DL-SOT) acting upon adjacent ferromagnetic or ferrimagnetic (FM) layer. Here we provide a systematic study on the spin transport properties of W/FM magnetic heterostructures with the FM layer being ferromagnetic Co$_{20}$Fe$_{60}$B$_{20}$ or ferrimagnetic Co$_{63}$Tb$_{37}$ with perpendicular magnetic anisotropy. The DL-SOT efficiency $|\xi_{DL}|$, which is characterized by a current-induced hysteresis loop shift method, is found to be correlated to the microstructure of W buffer layer in both W/Co$_{20}$Fe$_{60}$B$_{20}$ and W/Co$_{63}$Tb$_{37}$ systems. Maximum values of $|\xi_{DL}|\approx 0.144$ and $|\xi_{DL}|\approx 0.116$ are achieved when the W layer is partially amorphous in the W/Co$_{20}$Fe$_{60}$B$_{20}$ and W/Co$_{63}$Tb$_{37}$ heterostructures, respectively. Our results suggest that the spin Hall effect from resistive phase of W can be utilized to effectively control both ferromagnetic and ferrimagnetic layers through a DL-SOT mechanism.

In-plane current-induced magnetization switching in CoGa/MnGa/MgO films

In-plane current-induced magnetization switching was investigated in a micron-sized Hall bar for the paramagnetic CoGa-buffered and MgO-capped 2-nm-thick ferrimagnetic L10 MnGa layer with perpendicular magnetic anisotropy. Current-induced switching was clearly observed when the in-plane longitudinal magnetic field applied was sufficiently large and at an injection current density of 1011 A/m2. The in-plane field direction and CoGa thickness dependence indicate that the deterministic bipolar switching primarily originates from a damping-like spin–orbit torque (SOT) arising from the spin-Hall effect in CoGa with a positive spin-Hall angle. The current pulse response suggested that the SOT switching was thermally assisted by Joule heating.

Tailoring of switching field in GdCo-based spin valves by inserting Co layer

GdCo(35nm)/Co(1÷7nm)/Cu(4nm)/Co(7nm) spin valves were prepared by magnetron sputtering. The Gd23Co77 alloy with rare earth rich composition used in this study has a magnetization compensation temperature above room temperature. The effective composition of GdCo/Co exchange coupled bilayers changes from rare earth rich to transition metal rich compositions with the increase of Co layer thickness, leading to the change of giant magnetoresistance sign. The coercivity of GdCo/Co composite layer also changes. Consequently, the coercivity and the switching field of ferrimagnetic layer can be tailored by the insertion of thin Co layers of different thickness between the GdCo and Cu layers. In the present case, the switching field of GdCo/Co composite layer varied from 40 to 400 Oe.

Oxidation fabrication and enhanced soft magnetic properties for core-shell FeCo/CoFe2O4 micron-nano composites

FeCo/CoFe2O4 micron-nano composites have been fabricated by controllably oxidizing micron-sized FeCo particles. High-purity 60-nm-thick CoFe2O4 layers can be formed in situ on the surface of the unoxidized FeCo particles to form a dense FeCo/CoFe2O4 core-shell structure. The good intrinsic magnetic properties, including a high saturation magnetization of 210A·m2·kg−1 and a low coercivity of 11.7Oe, are obtained in these FeCo/CoFe2O4 composites due to the nanoscale thickness of the ferrimagnetic CoFe2O4 layers. For traditional soft magnetic composites, low core loss can only be obtained by seriously sacrificing their magnetic induction. In this work, balanced soft magnetic properties with low core loss and high magnetic induction are obtained in the fully compacted FeCo/CoFe2O4 composites, which promise great potential applications in high-power and high-frequency magnetic devices. This controlled surface oxidation may also provide an effective surface treatment for metal-based soft magnets to improve their overall performance.

Origin of variation of shift field via annealing at 400°C in a perpendicular-anisotropy magnetic tunnel junction with [Co/Pt]-multilayers based synthetic ferrimagnetic reference layer

We investigated properties of perpendicular-anisotropy magnetic tunnel junctions (p-MTJs) with [Co/Pt]-multilayer based synthetic ferrimagnetic reference (SyF) layer at elevated annealing temperature Ta from 350°C to 400°C. Shift field HS defined as center field of minor resistance versus magnetic field curve of the MTJs increased with increase of Ta from 350°C to 400°C. The variation of HS is attributed to the variation of saturation magnetic moment in the SyF reference layer. Cross sectional energy dispersive X-ray spectroscopy analysis revealed that Fe element of CoFeB in the reference layer diffuses to Co/Pt multilayers in the SyF reference layer.

A Novel Ternary CoFe2O4/CuO/CoFe2O4 as a Giant Magnetoresistance Sensor

This paper reports the results of a study relating to the synthesis of a novel ternary CoFe 2 O 4 /CuO/CoFe 2 O 4 thin film as a giant magnetoresistance (GMR) sensor. The CoFe 2 O 4 /CuO/CoFe 2 O 4 thin film was prepared onto silicon substrate via DC magnetron sputtering with the targets facing each other. X-ray diffraction was used to determine the structure of the thin film and a 4-point method was used to measure the MR ratio. The GMR ratio is highly dependent on the ferrimagnetic (CoFe 2 O 4 ) and nonmagnetic (CuO) layer thickness. The maximum GMR ratio at room temperature obtained in the CoFe 2 O 4 /CuO/CoFe 2 O 4 thin film was 70% when the CoFe 2 O 4 and the CuO layer had a thickness of 62.5 nm and 14.4 nm respectively.