Strong Perpendicular Anisotropy Research Articles

On the Origin of the Above-Room-Temperature Magnetism in the 2D van der Waals Ferromagnet Fe3GaTe2.

2D magnetic materials have attracted growing interest driven by their unique properties and potential applications. However, the scarcity of systems exhibiting magnetism at room temperature has limited their practical implementation into functional devices. Here we focus on the van der Waals ferromagnet Fe3GaTe2, which exhibits above-room-temperature magnetism (Tc = 350-380 K) and strong perpendicular anisotropy. Through first-principles calculations, we examine the magnetic properties of Fe3GaTe2 and compare them with those of Fe3GeTe2. Our calculations unveil the microscopic mechanisms governing their magnetic behavior, emphasizing the pivotal role of ferromagnetic in-plane couplings in the stabilization of the elevated Tc in Fe3GaTe2. Additionally, we predict the stability, substantial perpendicular anisotropy, and high Tc of the single-layer Fe3GaTe2. We also demonstrate the potential of strain engineering and electrostatic doping to modulate its magnetic properties. Our results incentivize the isolation of the monolayer and pave the way for the future optimization of Fe3GaTe2 in magnetic and spintronic nanodevices.

Magnetoresistance and magnetic field-induced phase transition in two-dimensional antiferromagnet Fe1/3NbS2

We perform systematic studies on the magnetic and magnetotransport properties of Fe1/3NbS2 single crystal. Results show that Fe1/3NbS2 is an antiferromagnet with strong perpendicular magnetocrystalline anisotropy even in the paramagnetic state. The Fe1/3NbS2 nanoplate exhibits exotic magnetotransport characteristics with variation of temperature and magnetic field in the vicinity of Néel temperature. The temperature dependence of resistivity indicates that the magnetic field has a substantial effect on the Néel order of Fe1/3NbS2, and an intermediate phase occurs before the sample enters into the antiferromagnetic state. It is the field- or temperature-induced antiferromagnetic–intermediate phase and intermediate phase–paramagnetic phase transitions that lead to the novel magnetotransport characteristics in the vicinity of Néel temperature. The findings in this work will promote future studies based on two-dimensional antiferromagnetic spintronics.

Temperature-dependent anisotropy variation in quasi-two-dimensional ferromagnetic Cr5Te8

The quasi-two-dimensional ferromagnet Cr5Te8 is a promising material for spintronic devices due to its near-room-temperature Curie temperature (TC), strong magnetic anisotropy, and easily controllable properties. In this study, the anisotropic magnetization of trigonal (T-) Cr5Te8 single crystals is investigated. Our magnetization study reveals that a ferromagnetic transition occurs when the magnetic field is aligned parallel to the c-axis (H//c), while an antiferromagnetic transition appears for H//ab, with a strong perpendicular anisotropy in the ground state. However, electron spin resonance (ESR) spectroscopy indicates that the direction of the easy-axis changes from the c-axis to the ab-plane at TV∼ 190 K as the temperature increases. Furthermore, angle-dependent ESR spectra demonstrate a characteristic of two-dimensional magnetism in the bulk Cr5Te8 single crystal. It is suggested that the temperature-dependent variation in anisotropy is caused by changes in lattice structure through the tuning of the dominant direct-exchange between the intralayers and the super-exchange within the interlayers. The temperature- and field-dependent microwave responses of T-Cr5Te8 are advantageous for the application of this material as a microwave-based spintronic device.

Direct observation of Néel-type skyrmions and domain walls in a ferrimagnetic DyCo3 thin film

Isolated magnetic skyrmions are stable, topologically protected spin textures that are at the forefront of research interests today due to their potential applications in information technology. A distinct class of skyrmion hosts are rare earth - transition metal (RE-TM) ferrimagnetic materials. To date, the nature and the control of basic traits of skyrmions in these materials are not fully understood. We show that for an archetypal ferrimagnetic material DyCo3 that exhibits a strong perpendicular anisotropy, the ferrimagnetic skyrmion size can be tuned by an external magnetic field. Moreover, by taking advantage of the high spatial resolution of scanning transmission X-ray microscopy (STXM) and utilizing a large x-ray magnetic linear dichroism (XMLD) contrast that occurs naturally at the RE resonant edges, we resolve the nature of the magnetic domain walls of ferrimagnetic skyrmions. We demonstrate that through this method one can easily discriminate between Bloch and Néel type domain walls for each individual skyrmion. For all isolated ferrimagnetic skyrmions, we observe that the domain walls are of Néel-type. This key information is corroborated with results of micromagnetic simulations and allows us to conclude on the nature of the Dzyaloshinskii-Moriya interaction (DMI) which concurs to the stabilisation of skyrmions in this ferrimagnetic system. Establishing that an intrinsic DMI occurs in RE-TM materials will also be beneficial towards a deeper understanding of chiral spin texture control in ferrimagnetic materials.

Strong perpendicular anisotropic ferromagnet Fe3GeTe2/graphene van der Waals heterostructure

Two-dimensional magnets offer a new platform for exploring fundamental properties in van der Waals (vdW) heterostructures and their device applications. Here, we investigated heterostructure devices of itinerant metallic vdW ferromagnet Fe3GeTe2 (FGT) with monolayer chemical vapor deposited graphene. The anomalous Hall effect measurements of FGT Hall-bar devices exhibit robust ferromagnetism with strong perpendicular anisotropy at low temperatures. The electrical transport properties measured in FGT/graphene heterostructure devices exhibit a tunneling transport with weak temperature dependence. We assessed the suitability of such FGT/graphene heterostructures for spin injection and detection and investigated the presence of FGT on possible spin absorption and spin relaxation in the graphene channel. These findings will be useful for engineering spintronic devices based on vdW heterostructures.

Rhombic Fermi surfaces in a ferromagnetic MnGa thin film with perpendicular magnetic anisotropy

Mn$_{1-x}$Ga$_x$ (MnGa) with the $L1_0$ structure is a ferromagnetic material with strong perpendicular magneto-crystalline anisotropy. Although MnGa thin films have been successfully grown epitaxially and studied for various spintronics devices, fundamental understandings of its electronic structure are still lacking. To address this issue, we have investigated $L1_0$-MnGa thin films using angle-resolved photoemission spectroscopy (ARPES). We have observed a large Fermi surface with a rhombic shape in the $k_x$-$k_y$ plane overlapping neighboring Fermi surfaces. The $k_z$ dependence of the band structure suggests that the band dispersion observed by ARPES comes from the three-dimensional band structure of MnGa folded by a $\sqrt{2} \times \sqrt{2}$ reconstruction. The band dispersion across the corner of the rhombic Fermi surface forms an electron pocket with a weak $k_z$ dependence. The effective mass and the mobility of the bands crossing the Fermi level near the corner are estimated from the ARPES images. Based on the experimental findings, the relationship between the observed band structure and the spin-dependent properties in MnGa-based heterostructures is discussed.

First-Principles Calculations of the Exchange Interaction of the CrGeTe3/NiO Interface

Two-dimensional (2D) magnetic materials have recently attracted a great attention due to their potential applications in information processing and storage. It was observed in an experiment that the CrGeTe3/NiO heterostructure shows a higher Curie temperature and a stronger perpendicular anisotropy. This suggests that antiferromagnet NiO can be coupled with ferromagnetic CrGeTe3 by the proximity effect, which enhances ferromagnetism. However, the mechanism behind the coupling is unknown. In this work, we built a CrGeTe3/NiO heterostructure model and investigated the electronic structure and magnetic properties of the CrGeTe3/NiO interface by first-principles calculations. It is shown that the intralayer exchange interaction between the Cr atoms is ferromagnetic, and the interlayer exchange interaction between the Ni atoms and Cr atoms at the interface is antiferromagnetic. The interlayer interaction is equivalent to a magnetic field as strong as B=100.3 T applied by the NiO substrate to the CrGeTe3 layer, which results in the higher Curie temperature of CrGeTe3 observed in the experiment.

Phase engineering of Cr5Te8 with colossal anomalous Hall effect

Two-dimensional materials that are intrinsically ferromagnetic are crucial for the development of compact spintronic devices. However, most non-layered 2D magnets with a strong ferromagnetic order are difficult to synthesize. Here we show that the flakes of trigonal and monoclinic Cr5Te8 can be grown via a chemical vapour deposition method. Using magneto-optical and magnetotransport measurements, we show that both phases exhibit robust ferromagnetism with strong perpendicular anisotropy at thicknesses of a few nanometres. A high Curie temperature of up to 200 K can be obtained by manipulating the phase structure and thickness. We also observe a colossal anomalous Hall effect in the more structurally distorted monoclinic Cr5Te8, with an anomalous Hall conductivity of 650 Ω−1 cm−1 and anomalous Hall angle of 5%. Few-nanometre-thick flakes of trigonal and monoclinic Cr5Te8 can be grown using chemical vapour deposition, with the monoclinic phase exhibiting an anomalous Hall conductivity of 650 Ω–1 cm–1 and anomalous Hall angle of 5%.

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.

Size-dependent enhancement of passive microwave rectification in magnetic tunnel junctions with perpendicular magnetic anisotropy

Spintronic rf detectors are efficient nanoscale counterparts to conventional semiconductor-based components for energy harvesting and wireless communication at low input power. Here, we report on the optimization of the rectified output dc voltage using magnetic tunnel junctions (MTJs) with strong perpendicular anisotropy of both the polarizing and free layers. The magnetization of the polarizing layer is fixed out-of-plane, while the free layer thickness is adjusted so that its magnetization orientation changes from in-plane to out-of-plane. Rectification dc output voltages in the mV range are obtained for moderate rf source powers with a signal-to-noise ratio of 26–39 dB for Prf = −25 dBm and a sensitivity ε of 300 mV/mW. The rectified signal shows a strong dependence on MTJ dimensions: it increases by a factor of 5–6 when reducing the diameter from 180 to 50 nm. Furthermore, this enhancement can be doubled when reducing the free layer thickness from 1.8 to 1.6 nm. This size-related enhancement is attributed to several jointly acting effects: the amplitude of the spin transfer torque that depends inversely on the diameter, the effective anisotropy that depends on the thickness of the excited layer, and the tunneling magneto-resistance ratio that for the devices studied here depends on diameter. The obtained results indicate that the geometry of the MTJ can be used to design spintronic based rf detectors with optimized sensitivity.

Manipulation of crystalline structure, magnetic performance, and topological feature in Mn3Ge films

The Mn3X (where X = Ga, Ge, Sn, etc.) compounds have appealing prospects for spintronic applications due to their various crystal structures and magnetic properties for the design of reliable high-density memories. However, controlled growth of high-quality Mn3X thin films remains challenging in material science. Here, we reported the controlled film growth of Heusler alloy Mn3Ge, which could crystallize in respective tetragonal and hexagonal structures. The tetragonal D022-type Mn3Ge film exhibits strong perpendicular ferromagnetic anisotropy, while the hexagonal D019-type Mn3Ge film indicates non-collinear triangular antiferromagnetic order. From our experimental observations of structure characterizations, magnetic properties, anomalous Hall effect, and magnetoresistance measurements, we realized the manipulation of spin orientations and topological features. Majority/minority spin polarized Fermi surface and density of states of both tetragonal and hexagonal Mn3Ge structures were investigated by density functional theory calculations. Our work not only opens up technology routes toward the development of Mn3X-based devices for applications in topological spintronics and spin-torque memories but also leads to engineer the physical properties for fundamental study.

Transverse Rashba effect and unconventional magnetocrystalline anisotropy in graphene-nanoribbon-based centrosymmetric antiferromagnet

The transverse Rashba effect in a centrosymmetric antiferromagnet materialized by double-Gd-adsorbed graphene nanoribbon is investigated via the first-principle calculations. The Rashba effect is associated with not only the local transverse dipole fields induced by the off-center adsorption of the Gd adatoms but also the local magnetic moments. It is perfectly realized that the transverse Rashba effect (Rashba parameter αx reaches to 2.205 eV Å) and strong perpendicular magnetocrystalline anisotropy energy (MAE) coexist in antiferromagnetic (AFM) ground-state structure. However, the global Rashba effect disappears in ferromagnetic structure. The origin of the strong MAE is elaborated in k − space. It is intriguing that the first-order perturbation of the orbit and spin angular momentum coupling is the major source of the MAE due to the specific one-dimensional band structure. The transverse Rashba effect and the strong perpendicular magnetization hosted simultaneously by the proposed centrosymmetric antiferromagnet lock the up- (or down-) spin quantization direction (SQD) to the backward (or forward) movement, and the SQD is related to the chirality of the AFM structure. This finding offers a magnetic approach to a high coherency spin propagation in one-dimensionality, and open a new door to manipulating spin transportation in graphene nanoribbon-based spintronics.

L-shaped electrode design for high-density spin–orbit torque magnetic random access memory with perpendicular shape anisotropy

Magnetic tunneling junctions with strong perpendicular shape anisotropy attract attention due to their high-density magnetic random access memory. As thermal stability increases, the power consumption also increases. To solve this problem, devices are made to be driven by spin–orbit torque (SOT) instead of spin-transfer torque. However, the assisting field needed for deterministic switching is a major obstacle for SOT devices. In this work, we demonstrate an L-shaped electrode structure attached to the magnetic recording layer to induce a composite SOT, achieving high-speed and field-free magnetization switching. Meanwhile, a comparative study between L-shaped and sidewall electrode structure demonstrates that the L-shaped structure leads to fast and low-power switching. Finally, the switching characteristic at various current densities and spin Hall angles is studied and it turns out that to achieve high-speed reversal, the current density and the spin Hall angle need to be optimized, which might be attributed to strong in-plane effective field component disturbance. The novel L-shaped structure is feasible for high-speed, low-power and deterministic switching and has great potential in spintronic applications.

CVD Growth of Large‐scale and Highly Crystalline 2D Chromium Telluride Nanoflakes

AbstractRecently, 2D magnetic materials have attracted much attention due to their potential applications in spintronic devices. Chromium telluride (CrTe) flakes have been reported for hard magnetism with a strong perpendicular anisotropy. However, the synthesis of large‐scale CrTe nanoflakes on SiO2/Si substrate is still a challenge. Here, we report the synthesis of large‐scale CrTe nanoflakes with a size of up to ∼250 μm on a SiO2/Si substrate via chemical vapor deposition (CVD). Our work demonstrated an effective pathway to synthesize large‐scale 2D transition metal tellurides.

A New Highly Anisotropic Rh-Based Heusler Compound for Magnetic Recording.

The development of high-density magnetic recording media is limited by superparamagnetism in very small ferromagnetic crystals. Hard magnetic materials with strong perpendicular anisotropy offer stability and high recording density. To overcome the difficulty of writing media with a large coercivity, heat-assisted magnetic recording was developed, rapidly heating the media to the Curie temperature Tc before writing, followed by rapid cooling. Requirements are a suitable Tc , coupled with anisotropic thermal conductivity and hard magnetic properties. Here, Rh2 CoSb is introduced as a new hard magnet with potential for thin-film magnetic recording. A magnetocrystalline anisotropy of 3.6 MJ m-3 is combined with a saturation magnetization of μ0 Ms = 0.52 T at 2 K (2.2 MJ m-3 and 0.44 T at room temperature). The magnetic hardness parameter of 3.7 at room temperature is the highest observed for any rare-earth-free hard magnet. The anisotropy is related to an unquenched orbital moment of 0.42 μB on Co, which is hybridized with neighboring Rh atoms with a large spin-orbit interaction. Moreover, the pronounced temperature dependence of the anisotropy that follows from its Tc of 450 K, together with a thermal conductivity of 20 W m-1 K-1 , make Rh2 CoSb a candidate for the development of heat-assisted writing with a recording density in excess of 10Tb in.-2 .

Strong interfacial perpendicular anisotropy and interfacial damping in Ni0.8Fe0.2 films adjacent to Ru and SiO2

We report detailed investigations of a strong perpendicular magnetic anisotropy in Ni0.8Fe0.2 thin films originating from interfaces with adjacent Ru and SiO2 layers. The anisotropy is quantified using broadband ferromagnetic resonance measurements for three types of layered heterostructures, namely an asymmetric structure of SiO2/Ni0.8Fe0.2/Ru and symmetric structures of Ru/Ni0.8Fe0.2/Ru and SiO2/Ni0.8Fe0.2/SiO2. The results show a stronger perpendicular anisotropy at the Ni0.8Fe0.2/SiO2 interface than at the Ni0.8Fe0.2/Ru interface and a lower interfacial damping contribution for the Ni0.8Fe0.2/SiO2 interface compared to the Ni0.8Fe0.2/Ru interface.

Ultrathin Scale Tailoring of Anisotropic Magnetic Coupling and Anomalous Magnetoresistance in SrRuO3-PrMnO3 Superlattices.

A strong perpendicular magnetocrystalline anisotropy (PMA) in antiferromagnetically coupled SrRuO3(17 uc (unit cell))/PrMnO3( n uc) superlattices effectively reconstructs the interfacial spin ordering. The occurrence of significant anisotropic interfacial antiferromagnetic coupling between the Ru and Mn ions is systematically tuned by varying the PrMnO3 layer thickness in ultrathin scale from 3 to 12 uc, which is associated with a rise in PMA energy from 0.28 × 106 to 1.60 × 106 erg/cm3. The analysis using the Stoner-Wohlfarth model and density functional theory confirm that the exchange anisotropy is the major contribution to the PMA. The superlattices with PrMnO3 layer thickness ≥7 uc exhibit the tunneling-like transport of Ru 4d electrons, which is rather expected in the stronger antiferromagnetically coupled superlattices with thinner PrMnO3 layer. Tunneling-like transport at thicker spacer layer in the SrRuO3-PrMnO3 superlattice system is an unique feature of two ferromagnet-based superlattices. Our investigations show that the technologically important interfacial magnetic coupling, PMA, and tunneling magnetoresistance could be achieved in a periodically stacked bilayer and can be precisely manipulated by the size effect in ultrathin scale.

Spin–Orbit Torque Switching in a Nearly Compensated Heusler Ferrimagnet

Ferrimagnetic materials combine the advantages of the low magnetic moment of an antiferromagnet and the ease of realizing magnetic reading of a ferromagnet. Recently, it was demonstrated that compensated ferrimagnetic half metals can be realized in Heusler alloys, where high spin polarization, zero magnetic moment, and low magnetic damping can be achieved at the same time. In this work, by studying the spin-orbit torque induced switching in the Heusler alloy Mn2 Ru1- x Ga, it is found that efficient current-induced magnetic switching can be realized in a nearly compensated sample with strong perpendicular anisotropy and large film thickness. This work demonstrates the possibility of employing compensated Heusler alloys for fast, energy-efficient spintronic devices.

Two-dimensional itinerant ferromagnetism in atomically thin Fe3GeTe2.

Discoveries of intrinsic two-dimensional (2D) ferromagnetism in van der Waals (vdW) crystals provide an interesting arena for studying fundamental 2D magnetism and devices that employ localized spins1-4. However, an exfoliable vdW material that exhibits intrinsic 2D itinerant magnetism remains elusive. Here we demonstrate that Fe3GeTe2 (FGT), an exfoliable vdW magnet, exhibits robust 2D ferromagnetism with strong perpendicular anisotropy when thinned down to a monolayer. Layer-number-dependent studies reveal a crossover from 3D to 2D Ising ferromagnetism for thicknesses less than 4 nm (five layers), accompanied by a fast drop of the Curie temperature (TC) from 207 K to 130 K in the monolayer. For FGT flakes thicker than ~15 nm, a distinct magnetic behaviour emerges in an intermediate temperature range, which we show is due to the formation of labyrinthine domain patterns. Our work introduces an atomically thin ferromagnetic metal that could be useful for the study of controllable 2D itinerant ferromagnetism and for engineering spintronic vdW heterostructures5.

Effects of overlayer capping and lattice strain on perpendicular magnetic anisotropy of TM|FePt|MgO heterostructures

Magnetic tunnel junctions (MTJs) with ferromagnetic electrodes possessing the strong perpendicular magnetocrystalline anisotropy (PMA) are of great interest as they have a potential for realizing next-generation high-density non-volatile memory and logic chips. To date, it is an urgent and critical issue to continuously promote the PMAs through feasible modifications such as the substitution of ferromagnetic layers as well as the overlayer coating on them. Here, we perform the relativistic first principles calculations of TM|L10-FePt|MgO sandwich systems, and demonstrate that the changes in PMAs by capping TM layers are always giant and positive, e.g., PMA of Fe|FePt|MgO, the largest one among all our studied systems, is about 2 times larger than that of FePt|MgO. The interfacial PMAs at TM|FePt and FePt|MgO interfaces are extracted to be 3.31~9.40 meV and 3.32 meV, respectively, which are at least 3 times larger than 0.93 meV/ML of interior FePt layer. We illustratively verify that PMAs of TM|FePt|MgO can be turned in a large range by varying the TM layer and in-plane strain. Our results and model analyses provide useful insights for how these magnetic quantities are linked, and pave a way for the promotion of PMAs of FePt-based heterostructures via contact with TM overlayers.