Perpendicular magnetic anisotropy induced by tetragonal distortion of FeCo alloy films grown on Pd(001)

The Future of Permanent-Magnet-Based Electric Motors: How Will Rare Earths Affect Electrification?

In this review article, we focus on the relationship between permanent magnets and the electric motor, as this relationship has not been covered in a review paper before. With the increasing focus on battery research, other parts of the electric system have been neglected. To make electrification a smooth transition, as has been promised by governing bodies, we need to understand and improve the electric motor and its main component, the magnet. Today's review papers cover only the engineering perspective of the electric motor or the material-science perspective of the magnetic material, but not both together, which is a crucial part of understanding the needs of electric-motor design and the possibilities that a magnet can give them. We review the road that leads to today's state-of-the-art in electric motors and magnet design and give possible future roads to tackle the obstacles ahead and reach the goals of a fully electric transportation system. With new technologies now available, like additive manufacturing and artificial intelligence, electric motor designers have not yet exploited the possibilities the new freedom of design brings. New out-of-the-box designs will have to emerge to realize the full potential of the new technology. We also focus on the rare-earth crisis and how future price fluctuations can be avoided. Recycling plays a huge role in this, and developing a self-sustained circular economy will be critical, but the road to it is still very steep, as ongoing projects show.

High and Ultra-High Coercive Materials in Spring-Exchange Systems-Review, Simulations and Perspective

The paper refers to the spring-exchange magnetic systems containing magnetically soft and hard phases. This work consists of two parts. The first part is a brief review of hard magnetic materials, with special attention paid to ultra-high coercive compounds, as well as selected spring-exchange systems. The second part is a theoretical discussion based on the Monte Carlo micromagnetic simulations about the possible enhancement of the hard magnetic properties of systems composed of magnetically soft, as well as high and ultra-high coercive, phases. As shown, the analyzed systems reveal the potential for improving the |BH|_max parameter, filling the gap between conventional and Nd-based permanent magnets. Moreover, the carried-out simulations indicate the advantages and limitations of the spring-exchange composites, which could lead to a reduction in rare earth elements in permanent magnet applications.

From Termination Dependent Chemical Sensitivity of Spin Orientation in All-bcc Fe/Co Magnetic Superlattices toward the Concept of an Artificial Surface of a Ferromagnet

Adsorption of gases on the surface of all-bcc (Fe/Co)_N superlattices drives the in-plane, 90° magnetization rotation of the bulk-like Fe(110) supporting ferromagnet. Both experimental and theoretical results prove that terminating the surface of (Fe/Co)_N superlattices either by Co or by Fe switches "ON" or "OFF" the spin orientation sensitivity to adsorption. Results indicate that purely surface limited adsorption processes strongly modify the magnetic anisotropy of the entire (Fe/Co)_N superlattice, which acts as a kind of "artificial" surface of the bulky Fe(110) ferromagnet. Such an artificial magnetic surface anisotropy concept not only enhances the surface contribution in classical surface-bulk competition but also provides its additional chemical sensitivity.

Simultaneous tuning of the magnetic anisotropy and thermal stability of [Formula: see text]-phase Fe[Formula: see text]N[Formula: see text]

Simultaneously enhancing the uniaxial magnetic anisotropy ([Formula: see text]) and thermal stability of [Formula: see text]-phase Fe[Formula: see text]N[Formula: see text] without inclusion of heavy-metal or rare-earth (RE) elements has been a challenge over the years. Herein, through first-principles calculations and rigid-band analysis, significant enhancement of [Formula: see text] is proposed to be achievable through excess valence electrons in the Fe[Formula: see text]N[Formula: see text] unit cell. We demonstrate a persistent increase in [Formula: see text] up to 1.8 MJ m[Formula: see text], a value three times that of 0.6 MJ m[Formula: see text] in [Formula: see text]-Fe[Formula: see text]N[Formula: see text], by simply replacing Fe with metal elements with more valence electrons (Co to Ga in the periodic table). A similar rigid-band argument is further adopted to reveal an extremely large [Formula: see text] up to 2.4 MJ m[Formula: see text] in (Fe[Formula: see text]Co[Formula: see text])[Formula: see text]N[Formula: see text] obtained by replacing Co with Ni to Ga. Such a strong [Formula: see text] can also be achieved with the replacement by Al, which is isoelectronic to Ga, with simultaneous improvement of the phase stability. These results provide an instructive guideline for simultaneous manipulation of [Formula: see text] and the thermal stability in 3d-only metals for RE-free permanent magnet applications.

Giant Vertical Magnetization Shift Caused by Field-Induced Ferromagnetic Spin Reconfiguration in Ni50Mn36Ga14 Alloy

Vertical magnetization shift (VMS) is a special type of exchange bias effect that may lead to a revolution in future ultrahigh-density magnetic recording technology. However, there are very few reports focusing on the performance of VMS due to the unclear mechanism. In this paper, a giant vertical magnetization shift (M_E) of 6.34 emu/g is reported in the Ni₅₀Mn₃₆Ga₁₄ alloy. The VMS can be attributed to small ferromagnetic ordered regions formed by spin reconfiguration after field cooling, which are embedded in an antiferromagnetic matrix. The strong cooling-field dependence, temperature dependence, and training effect all corroborate the presence of spin reconfiguration and its role in the VMS. This work can enrich VMS research and increase its potential in practical applications as well.

Rare-earth-free magnetically hard ferrous materials

Permanent magnets, especially rare-earth based magnets, are widely used in energy-critical technologies in many modern applications, involving energy conversion and information technologies. However, the environmental impact and strategic supplies of rare-earth elements hamper the long-term development of permanent magnets. Hence, there is a surge of interest to expand the search for rare-earth-free magnets with a large energy product (BH)_max. Among these rare-earth-free magnets, iron-based permanent magnets emerge as some of the most promising candidates due to their abundance and magnetic performance. In this review, we present a summary of iron-based permanent magnets from materials synthesis to their magnetic properties.

Three-dimensional superlattice engineering with block copolymer epitaxy

Three-dimensional (3D) structures at the nanometer length scale play a crucial role in modern devices, but their fabrication using traditional top-down approaches is complex and expensive. Analogous to atomic lattices, block copolymers (BCPs) spontaneously form a rich variety of 3D nanostructures and have the potential to substantially simplify 3D nanofabrication. Here, we show that the 3D superlattice formed by BCP micelles can be controlled by lithographically defined 2D templates matching a crystallographic plane in the 3D superlattice. Using scanning transmission electron microscopy tomography, we demonstrate precise control over the lattice symmetry and orientation. Excellent ordering and substrate registration can be achieved, propagating through 284-nanometer-thick films. BCP epitaxy also showed exceptional lattice tunability, with a continuous Bain transformation from a body-centered cubic to a face-centered cubic lattice. Lattice stability was mediated by molecular packing frustration, and surface-induced lattice reconstruction was observed, leading to the formation of a unique honeycomb lattice.

Nano-imprinting potential of magnetic FeCo-based metallic glass

Fabrication of magnetic nanostructures at low cost is strongly desired for applications such as sensors, actuators, magnetic memory, etc. In conventional nano-patterning techniques, the magnetic field of a magnetic material interferes with the patterning process, making nano-patterning challenging. Here, we report on the low cost patterning potential of FeCo-based magnetic metallic glass using a nano-imprinting technique. We show that out of a large number of magnetic metallic glasses, Fe₄₀Co₃₅P₁₀C₁₀B₅ glassy alloy exhibits high saturation magnetic flux density (B _s  ∼ 1.24 T), a large super-cooled liquid temperature range (ΔT _x  ∼ 49 °C), and a relatively low glass transition temperature (T _g  ∼ 430 °C) with good thermal stability. The quasi-static viscosity (∼10⁸ Pa.s at a heating rate of ∼40 °C min^-1) in ΔT _x , which is one of the most important parameters for nano-imprinting, is lowest among the reported magnetic metallic glasses. The deformability of this magnetic alloy is similar to the well-known non-magnetic metallic glasses, which can be patterned to a few tens of nanometers. Crystallization of Fe₄₀Co₃₅P₁₀C₁₀B₅ glassy alloy leads to the precipitation of a high B _s FeCo phase that may exhibit high magnetocrystalline anisotropy. Based on detailed investigations of structural, thermal, and magnetic behavior, along with imprinting experiments, we show that the Fe₄₀Co₃₅P₁₀C₁₀B₅ glassy alloy is the most desirable material for making various nano-patterns with tailorable magnetic properties.

Stabilisation of tetragonal FeCo structure with high magnetic anisotropy by the addition of V and N elements

The development of magnetic materials with high saturation magnetization (M_s) and uniaxial magnetic anisotropy (K_u) is required for the realisation of high-performance permanent magnets capable of reducing the power consumption of motors and data storage devices. Although FeCo-based materials with the body-centred cubic structure (bcc) exhibit the highest M_s values among various transition metal alloys, their low K_u magnitudes makes them unsuitable for permanent magnets. Recent first-principles calculations and experimental studies revealed that the epitaxial FeCo thin films with the body-centred tetragonal (bct) structure and thicknesses of several nanometres exhibited K_u values of 10⁶ J·m⁻³ due to epitaxial stress, which required further stabilisation. In this work, the FeCo lattice stabilised via VN addition were characterised by high K_u magnitudes exceeding 10⁶ J·m⁻³. The obtained bct structure remained stable even for the films with thicknesses of 100 nm deposited on an amorphous substrate, suggesting its possible use in bulk systems.

Tuning the Magnetic Properties of FeCo Thin Films through the Magnetoelastic Effect Induced by the Au Underlayer Thickness

Tuning the magnetic properties of materials is a demand of several technologies; however, our microscopic understanding of the process that drives the enhancement of those properties is still unsatisfactory. In this work, we combined experimental and theoretical techniques to investigate the handling of magnetic properties of FeCo thin films via the thickness-tuning of a gold film used as an underlayer. We grow the samples by the deposition of polycrystalline FeCo thin films on the Au underlayer at room temperature by a magnetron sputtering technique, demonstrating that the lattice parameter of the sub-20 nm thickness gold underlayer is dependent on its thickness, inducing a stress up to 3% in sub-5 nm FeCo thin films deposited over it. Thus, elastic-driven variations for the in-plane magnetic anisotropy energy, K_u, up to 110% are found from our experiments. Our experimental findings are in excellent agreement with ab initio quantum chemistry calculations based on density functional theory, which helps to build up an atomistic understanding of the effects that take place in the tuning of the magnetic properties addressed in this work. The handling mechanism reported here should be applied to other magnetic films deposited on different metallic underlayers, opening possibilities for large-scale fabrication of magnetic components to be used in future devices.

High-Magnetization Tetragonal Ferrite-Based Films Induced by Carbon and Oxygen Vacancy Pairs

Herein, a low-temperature thermal decomposition method is utilized to grow new stable tetragonal Fe₃O₄-based thick ferrite films. The tetragonal Fe₃O₄-based film possesses high saturation magnetization of ∼800 emu/cm³. Doping with approximately 10% Co results in a high-energy product of ∼10.9 MGOe with perpendicular magnetocrystalline anisotropy, whereas doping with Ni increases electrical resistivity by a factor of 6 and retains excellent soft magnetic properties (high saturation magnetization and low coercivity). A combined experimental and first-principles study reveals that carbon interstitials (C_i^B) and oxygen vacancies (V_O) form C_i^B-V_O pairs which stabilize the tetragonal phase and enhance saturation magnetization. The magnetization enhancement is further attributed to local ferromagnetic coupling between Fe_A and Fe_B induced by C_i^B-V_O pairs in a tetragonal spinel ferrite lattice.

Conversion of FeCo from soft to hard magnetic material by lattice engineering and nanopatterning

The development of magnetic materials with large uniaxial magnetic anisotropy (K u) and high saturation magnetization has attracted much attention in various areas such as high-density magnetic storage, spintronic devices, and permanent magnets. Although FeCo alloys with the body-centred cubic structure exhibit the highest M s among all transition metal alloys, their low K u and coercivity (H c) make them unsuitable for these applications. However, recent first-principles calculations have predicted large K u for the FeCo films with the body-centred tetragonal structure. In this work, we experimentally investigated the hard magnetic properties and magnetic domain structures of nanopatterned FeCo alloy thin films. As a result, a relatively large value of the perpendicular uniaxial magnetic anisotropy K u = 2.1 × 106 J·m-3 was obtained, while the H c of the nanopatterned FeCo layers increased with decreasing dot pattern size. The maximum H c measured in this study was 4.8 × 105 A·m-1, and the corresponding value of μ 0 H c was 0.60 T, where μ 0 represented the vacuum permeability.

Impact of orthogonal exchange coupling on magnetic anisotropy in antiferromagnetic oxides/ferromagnetic systems

The influence of interface exchange coupling on magnetic anisotropy in the antiferromagnetic oxide/Ni system is investigated. We show how interfacial exchange coupling can be employed not only to pin the magnetization of the ferromagnetic layer but also to support magnetic anisotropy to orient the easy magnetization axis perpendicular to the film plane. The fact that this effect is only observed below the Néel temperature of all investigated antiferromagnetic oxides with significantly different magnetocrystalline anisotropies gives evidence that antiferromagnetic ordering is a source of the additional contribution to the perpendicular effective magnetic anisotropy.

From soft to hard magnetic Fe-Co-B by spontaneous strain: a combined first principles and thin film study

In order to convert the well-known Fe-Co-B alloy from a soft to a hard magnet, we propose tetragonal strain by interstitial boron. Density functional theory reveals that when B atoms occupy octahedral interstitial sites, the bcc Fe-Co lattice is strained spontaneously. Such highly distorted Fe-Co is predicted to reach a strong magnetocrystalline anisotropy which may compete with shape anisotropy. To probe this theoretical suggestion experimentally, epitaxial films are examined. A spontaneous strain up to 5% lattice distortion is obtained for B content up to 4 at%, which leads to uniaxial anisotropy constants exceeding 0.5 MJ m(-3). However, a further addition of B results in a partial amorphisation, which degrades both anisotropy and magnetisation.

Atomic-scale engineering of magnetic anisotropy of nanostructures through interfaces and interlines

The central goals of nanoscale magnetic materials science are the self-assembly of the smallest structure exhibiting ferromagnetic hysteresis at room temperature, and the assembly of these structures into the highest density patterns. The focus has been on chemically ordered alloys combining magnetic 3d elements with polarizable 5d elements having high spin–orbit coupling and thus yielding the desired large magneto-crystalline anisotropy. The chemical synthesis of nanoparticles of these alloys yields disordered phases requiring annealing to transform them to the high-anisotropy L1₀ structure. Despite considerable efforts, so far only part of the nanoparticles can be transformed without coalescence. Here we present an alternative approach to homogeneous alloys, namely the creation of nanostructures with atomically sharp bimetallic interfaces and interlines. They exhibit unexpectedly high magnetization reversal energy with values and directions of the easy magnetization axes strongly depending on chemistry and texture. We find significant deviations from the expected behaviour for commonly used element combinations. Ab-initio calculations reproduce these results and unravel their origin.

The design and assembly of nanostructures exhibiting ferromagnetic hysteresis at room temperature are recognized goals for high-density data storage. Here, the authors engineer nanostructures with atomically sharp bimetallic interfaces and interlines, which exhibit large magnetic anisotropy and high temperature hysteresis.

Effect of large mechanical stress on the magnetic properties of embedded Fe nanoparticles

Magnetic nanoparticles are promising candidates for next generation high density magnetic data storage devices. Data storage requires precise control of the magnetic properties of materials, in which the magnetic anisotropy plays a dominant role. Since the total magneto-crystalline anisotropy energy scales with the particle volume, the storage density in media composed of individual nanoparticles is limited by the onset of superparamagnetism. One solution to overcome this limitation is the use of materials with extremely large magneto-crystalline anisotropy. In this article, we follow an alternative approach by using magneto-elastic interactions to tailor the total effective magnetic anisotropy of the nanoparticles. By applying large biaxial stress to nanoparticles embedded in a non-magnetic film, it is demonstrated that a significant modification of the magnetic properties can be achieved. The stress is applied to the nanoparticles through expansion of the substrate during hydrogen loading. Experimental evidence for stress induced magnetic effects is presented based on temperature-dependent magnetization curves of superparamagnetic Fe particles. The results show the potential of the approach for adjusting the magnetic properties of nanoparticles, which is essential for application in future data storage media.

Structure, morphology, and magnetic properties of Fe nanoparticles deposited onto single-crystalline surfaces

BACKGROUND

Magnetic nanostructures and nanoparticles often show novel magnetic phenomena not known from the respective bulk materials. In the past, several methods to prepare such structures have been developed - ranging from wet chemistry-based to physical-based methods such as self-organization or cluster growth. The preparation method has a significant influence on the resulting properties of the generated nanostructures. Taking chemical approaches, this influence may arise from the chemical environment, reaction kinetics and the preparation route. Taking physical approaches, the thermodynamics and the kinetics of the growth mode or - when depositing preformed clusters/nanoparticles on a surface - the landing kinetics and subsequent relaxation processes have a strong impact and thus need to be considered when attempting to control magnetic and structural properties of supported clusters or nanoparticles.

RESULTS

In this contribution we focus on mass-filtered Fe nanoparticles in a size range from 4 nm to 10 nm that are generated in a cluster source and subsequently deposited onto two single crystalline substrates: fcc Ni(111)/W(110) and bcc W(110). We use a combined approach of X-ray magnetic circular dichroism (XMCD), reflection high energy electron diffraction (RHEED) and scanning tunneling microscopy (STM) to shed light on the complex and size-dependent relation between magnetic properties, crystallographic structure, orientation and morphology. In particular XMCD reveals that Fe particles on Ni(111)/W(110) have a significantly lower (higher) magnetic spin (orbital) moment compared to bulk iron. The reduced spin moments are attributed to the random particle orientation being confirmed by RHEED together with a competition of magnetic exchange energy at the interface and magnetic anisotropy energy in the particles. The RHEED data also show that the Fe particles on W(110) - despite of the large lattice mismatch between iron and tungsten - are not strained. Thus, strain is most likely not the origin of the enhanced orbital moments as supposed before. Moreover, RHEED uncovers the existence of a spontaneous process for epitaxial alignment of particles below a critical size of about 4 nm. STM basically confirms the shape conservation of the larger particles but shows first indications for an unexpected reshaping occurring at the onset of self-alignment.

CONCLUSION

The magnetic and structural properties of nanoparticles are strongly affected by the deposition kinetics even when soft landing conditions are provided. The orientation of the deposited particles and thus their interface with the substrate strongly depend on the particle size with consequences regarding particularly the magnetic behavior. Spontaneous and epitaxial self-alignment can occur below a certain critical size. This may enable the obtainment of samples with controlled, uniform interfaces and crystallographic orientations even in a random deposition process. However, such a reorientation process might be accompanied by a complex reshaping of the particles.

Field Dependence of the Spin Relaxation Within a Film of Iron Oxide Nanocrystals Formed via Electrophoretic Deposition

The thermal relaxation of macrospins in a strongly interacting thin film of spinel-phase iron oxide nanocrystals (NCs) is probed by vibrating sample magnetometry (VSM). Thin films are fabricated by depositing FeO/Fe(3)O(4) core-shell NCs by electrophoretic deposition (EPD), followed by sintering at 400°C. Sintering transforms the core-shell structure to a uniform spinel phase, which effectively increases the magnetic moment per NC. Atomic force microscopy (AFM) confirms a large packing density and a reduced inter-particle separation in comparison with colloidal assemblies. At an applied field of 25 Oe, the superparamagnetic blocking temperature is T(B) (SP) ≈ 348 K, which is much larger than the Néel-Brown approximation of T(B) (SP) ≈ 210 K. The enhanced value of T(B) (SP) is attributed to strong dipole-dipole interactions and local exchange coupling between NCs. The field dependence of the blocking temperature, T(B) (SP)(H), is characterized by a monotonically decreasing function, which is in agreement with recent theoretical models of interacting macrospins.

Full tunability of strain along the fcc-bcc bain path in epitaxial films and consequences for magnetic properties

Strained coherent film growth is commonly either limited to ultrathin films or low strains. Here, we present an approach to achieve high strains in thicker films, by using materials with inherent structural instabilities. As an example, 50 nm thick epitaxial films of the Fe70Pd30 magnetic shape memory alloy are examined. Strained coherent growth on various substrates allows us to adjust the tetragonal distortion from c/a{bct}=1.09 to 1.39, covering most of the Bain transformation path from fcc to bcc crystal structure. Magnetometry and x-ray circular dichroism measurements show that the Curie temperature, orbital magnetic moment, and magnetocrystalline anisotropy change over broad ranges.

Direct evidence of a nonorthogonal magnetization configuration in single crystalline Fe(1-x)Co(x)/Rh/Fe/Rh(001) System

Tetragonal distortion in Fe1-xCox alloy films grown epitaxially on Rh(001) substrates results in a strong perpendicular magnetic anisotropy. Since the perpendicular magnetic anisotropy varies with the Fe1-xCox film composition, one can grow multilayer structures with ferromagnetic films sequentially showing either an in-plane (e.g., Fe) or out-of-plane (e.g., Fe0.5Co0.5) easy-magnetization axis. The Rh spacers mediate an interlayer coupling which couples the magnetizations either ferromagnetically or antiferromagnetically, depending on the spacer thickness. When the anisotropy energy is compatible to the coupling, it produces nonorthogonal magnetization configurations which vary under a small change of the external magnetic field.

Strongly enhanced orbital moment by reduced lattice symmetry and varying composition of Fe1-xCox alloy films

We studied tetragonally distorted Fe(1-x)Co(x) alloy films on Rh(001), which show a strong perpendicular anisotropy in a wide thickness and composition range. Analyzing x-ray magnetic circular dichroism spectra at the L_(3,2) edges we found a dependence of the Co magnetic orbital moment on the chemical composition of the Fe(1-x)Co(x) alloy films, with a maximum at x=0.6. For this composition, we observed an out-of-plane easy axis of magnetization at room temperature for film thickness up to 15 monolayers. Since both the magnetic orbital moment and the anisotropy energy show similar composition dependence, it confirms that both quantities are directly related. Our experiments show that the adjustment of the Fermi level by a proper choice of the alloy composition is decisive for the large magnetic orbital moment and for a large magnetic anisotropy in a tetragonally distorted lattice.