Large Uniaxial Magnetic Anisotropy Research Articles

Stabilization of a Metastable Fibrous Bi21.2(1)(Mn1–xCox)20 Phase with Pseudo-Pentagonal Symmetry Prepared Using a Bi Self-Flux

Communication pubs.acs.org/cm Stabilization of a Metastable Fibrous Bi 21.2(1) (Mn 1−x Co x ) 20 Phase with Pseudo-Pentagonal Symmetry Prepared Using a Bi Self-Flux Srinivasa Thimmaiah,* ,† Valentin Taufour, †,§ Scott Saunders, § Stephen March, § Yuemei Zhang, ‡ Matthew J. Kramer, †,∥ Paul C. Canfield, †,§ and Gordon J. Miller †,‡ The Ames Laboratory, U.S. Department of Energy, Iowa State University, Ames, Iowa 50011, United States Department of Chemistry, Iowa State University, Ames, Iowa 50011, United States Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, United States Department of Materials Science and Engineering, Iowa State University, Ames, Iowa 50011, United States S Supporting Information P ermanent magnets are exceptionally critical for many environmentally friendly, energy harvesting technologies in our energy demanding modern world. 1,2 In particular, rare earth (RE) based permanent magnets such as Nd 2 Fe 14 B 23−5 and SmCo 56,7 are extensively used for energy conversion purposes, mainly in high-power generators and motors found in wind turbines and electric vehicles, due to their very high energy product (BH max ) and lightweight. In particular, demand for RE-based permanent magnets has been growing exponen- tially in recent years. Therefore, to ease reliance on RE-based permanent magnets, development of low cost alternative materials that have high energy products and high Curie temperatures are critical for future sustainability. Mn-based 8 intermetallic compounds are examples gaining ground as an effective alternative, especially the ferromagnetic, low temper- ature (LT) BiMn phase adopting the NiAs-type structure and exhibiting a large uniaxial magnetic anisotropy (K = 2.2 × 10 7 erg cm −3 at 500 K). At temperatures exceeding 300 K, LT- BiMn shows remarkably high coercivity, which is even larger than that for Nd 2 Fe 14 B 2 magnets, making LT-BiMn suitable for high temperature applications. 9−12 However, at 633 K ferromagnetic LT-BiMn transforms to a paramagnetic, high- temperature phase (HTP), 13,14 which, upon rapid quenching, results in a ferromagnetic phase that shows an interesting magneto-optical property 15−18 applicable for magneto-optical memory devices. There are two structural transitions reported for LT-BiMn: one at ca. 100 K where spin reorientation occurs; 19 and another above its Curie or decomposition temperature (633 K). Theoretical calculations suggested that partial replacement of Mn by other transition metals could stabilize its structure in the hexagonal NiAs-type, which is essential for retaining the magnetic properties as well as increasing the magnetic anisotropy, 20,21 but experimental results reveal a change in crystal structure upon doping. 22 Herein, we report a new phase Bi 21.2(1) (Mn 1−x Co x ) 20 (x ∼ 0.15) that was discovered during systematic substitution of 3d and 4d 22 transition metals for Mn in LT-BiMn (NiAs-type) as a theoretically predicted strategy to increase magnetic anisotropy and stabilization of NiAs-type structure at elevated temperature. Crystals of a new metastable Co-doped BiMn phase were grown using Bi as a self-flux at 280 °C. 23 Figure 1 shows the soft and highly fibrous nature of these crystals, which split into submicron-size strands upon applied pressure. According to © 2016 American Chemical Society Figure 1. SEM micrographs of Bi 21.2(1) (Mn 1−x Co x ) 20 crystals showing fibrous morphology. DSC, the Co-doped BiMn phase decomposes endothermically on heating around 168 °C, with no evidence of formation of a new phase during cooling (see Supporting Information, Figure S2). This clearly indicates the metastable nature of the new phase. On the contrary, typical high-temperature reaction conditions of a sample with nominal composition of Mn 43 Co 7 Bi 50 resulted in the hexagonal NiAs-type structure (a = 4.2907(1) A, c = 6.1199(3) A), 24 which decomposes around 355 °C (see Supporting Information, Figure S4) upon heating. Single crystal X-ray diffraction revealed that the new phase crystallizes in orthorhombic symmetry, space group Imma, and the refined composition is Bi 21.2(1) (Mn 1−x Co x ) 20 (x ∼ 0.15). However, the positions of Co atoms in the crystal structure could not be established precisely due to the similar X-ray scattering contrasts of Mn and Co. Thus, wavelength-dispersive X-ray spectroscopy was employed to estimate the amount of Co to be 7(1) at.% in the structure. To evaluate any possible chemical ordering of Co in the structure, we have calculated total energies for two different compositions, each with three different coloring schemes, using VASP. The calculated energy differences among these various coloring models were found to be negligible (11−64 meV/cell or 0.26−1.52 meV/atom) (see Supporting Information, Figure S9), indicating low probability for chemical ordering of Co atoms in the structure under these synthetic conditions. Therefore, we can conclude that Co atoms are essentially randomly mixed with Mn atoms at the 3d metal positions. Received: October 21, 2016 Revised: November 15, 2016 Published: November 15, 2016 DOI: 10.1021/acs.chemmater.6b04505 Chem. Mater. 2016, 28, 8484−8488

Dynamic magnetic characteristics of obliquely sputtered Co2FeSi film with ultrahigh in-plane uniaxial anisotropy

We performed systematically studies on the magnetostatic and magnetodynamic properties of obliquely deposited Co2FeSi thin films. By varying the oblique deposition angle of Co2FeSi films, we induced a large in-plane uniaxial magnetic anisotropy field with the maximum value of 626.7 Oe at oblique angle of 39°, which significantly enhanced its zero-field ferromagnetic resonance (FMR) frequency up to 10.8 GHz. The magnetodynamic parameters such as Gilbert damping factor and exchange stiffness of Co2FeSi films were determined from the uniform FMR mode and the first perpendicular standing spin-wave, respectively. It is found that the Gilbert damping factor can be easily tuned from 0.006 to 0.013 by changing oblique deposition angle, whereas the exchange stiffness stays almost constant. These Co2FeSi films with tunable magnetic properties might be a good candidate for applications in both conventional and spintronics microwave devices.

Element specific spin and orbital moments of nanoscale CoFeB amorphous thin films on GaAs(100)

CoFeB amorphous films have been synthesized on GaAs(100) and studied with X-ray magnetic circular dichroism (XMCD) and transmission electron microscopy (TEM). We have found that the ratios of the orbital to spin magnetic moments of both the Co and Fe in the ultrathin amorphous film have been enhanced by more than 300% compared with those of the bulk crystalline Co and Fe, and specifically a large orbital moment of 0.56 μB from the Co atoms has been observed and at the same time the spin moment of the Co atoms remains comparable to that of the bulk hcp Co. The results indicate that the large uniaxial magnetic anisotropy (UMA) observed in the ultrathin CoFeB film on GaAs(100) is related to the enhanced spin-orbital coupling of the Co atoms in the CoFeB. This work offers experimental evidences of the correlation between the UMA and the element specific spin and orbital moments in the CoFeB amorphous film on the GaAs(100) substrate, which is of significance for spintronics applications.

Growth of L10–FeNi thin films on Cu(001) single crystal substrates using oxygen and gold surfactants

We alternately deposited Ni and Fe monatomic layers on Cu(001) single crystal substrates and also performed surfactant-mediated growth. FeNi films exhibited a large uniaxial magnetic anisotropy (Ku) of over 0.1MJ/m3 by using an oxygen or a gold surfactant, although Ku was almost zero without surfactants. Reflection high-energy electron diffraction and atomic force microscopy indicated surfactant effects of oxygen and gold. Synchrotron X-ray diffraction indicated a formation of an L10-type ordered FeNi phase having a large Ku. We have considered that the surfactants enhanced layer-by-layer growth leading to the L10-ordering by changing a diffusion scenario of Fe and Ni atoms.

Large Negative Magnetic Anisotropy of W/Fe/W (001) Epitaxial Trilayers

In this paper, we report that W/Fe/W (001) epitaxial trilayer films exhibit very large negative uniaxial magnetic anisotropy $K_{u}$ . The magnitude of $K_{u}$ increases with decreasing the Fe layer thickness and also exhibits significant temperature dependence varying from $-2.1 \times 10^{7}$ erg/cc at 300 K to $-3.5 \times 10^{7}$ erg/cc at 10 K for the Fe layer thickness of 0.5 nm. The large negative $K_{u}$ is attributable to interface magnetic anisotropy in nature at the W/Fe interface and can be quantitatively reproduced by the firstprinciples calculation. The X-ray magnetic circular dichroism measurement at room temperature has revealed that the interfacial W atom is polarized in antiparallel with the magnetic moment of Fe. The induced magnetic moment of the interfacial W is almost the half of the values obtained by the first-principles calculation and the previous experiment at low temperature. This small induced W moment may be ascribed to the significant temperature dependence of the negative $K_{u}$ .

Epitaxial Growth of Hard Ferrimagnetic Mn3Ge Film on Rhodium Buffer Layer

Mn\(_3\)Ge has a tetragonal Heusler-like D0\(_{22}\) crystal structure, exhibiting a large uniaxial magnetic anisotropy and small saturation magnetization due to its ferrimagnetic spin structure; thus, it is a hard ferrimagnet. In this report, epitaxial growth of a Mn\(_3\)Ge film on a Rh buffer layer was investigated for comparison with that of a film on a Cr buffer layer in terms of the lattice mismatch between Mn\(_3\)Ge and the buffer layer. The film grown on Rh had much better crystalline quality than that grown on Cr, which can be attributed to the small lattice mismatch. Epitaxial films of Mn\(_3\)Ge on Rh show somewhat small coercivity (\(H_{\rm c}\) = 12.6 kOe) and a large perpendicular magnetic anisotropy (\(K_{\rm u}\) = 11.6 Merg/cm\(^3\)), comparable to that of the film grown on Cr.

A cascade approach to hetero-pentanuclear manganese-oxide clusters in polyoxometalates and their single-molecule magnet properties.

Structurally well-defined hetero-pentanuclear manganese-oxide clusters {MMn4} were successfully synthesized in TBA7Hn[MMn4(OH)2(A-α-SiW9O34)2]·2H2O·C2H4Cl2 (, M = Fe(iii), Co(ii), Ni(ii), Cu(ii), Ga(iii)) by sequential introduction of metal cations into the trivacant lacunary polyoxometalates (POMs). The pentanuclear manganese-oxide cluster {Mn5} showed a small spin ground state and a low energy barrier for magnetization relaxation. In contrast, the magnetic interactions in the hetero-pentanuclear clusters could be controlled by the arrangements of metals, and the clusters showed large magnetic anisotropy and single-molecule magnet behavior. In particular, the cluster {FeMn4} in (S = 11/2) showed the slowest relaxation and the highest energy barrier among the previously reported transition metal-containing POMs.

Strong Perpendicular Uniaxial Magnetic Anisotropy in Tetragonal Fe<sub>0.5</sub>Co<sub>0.5</sub> Films of Artificially Ordered B2 State

We studied the relationship between the magnetic anisotropy properties and crystal structure of an artificial B2 state of Fe 0.5 Co 0.5 epitaxial films on a Rh(001) seed layer with a tetragonal distortion structure, which exhibited large perpendicular uniaxial magnetic anisotropy over a wide range of thicknesses and c/a ratios at room temperature. We obtained the bulk anisotropy energy of the Fe 0.5 Co 0.5 films, which reached 2.5× 10 7 erg/cm 3 at c/a=1.20. The magnetization easy axis of the Fe 0.5 Co 0.5 films transferred from out-of-plane to in-plane when the thickness was increased from 24 to 30 monolayers. This is due to the lattice relaxation caused by the increase of the total thickness. We observed that the cubic anisotropy component around the in-plane of the films was much smaller than the perpendicular effective magnetic anisotropy, which demonstrates that our films exhibited the uniaxial anisotropy property. Furthermore, we found the bulk anisotropy values of our artificial B2-state Fe 0.5 Co 0.5 to be larger than those of the A2 state under the same lattice distortion. Thus, we suggest that tuning the degree of crystal order of FeCo alloys is an effective way to experimentally enhance their perpendicular uniaxial magnetic anisotropy.

Addition of Co to L10-ordered FeNi films: influences on magnetic properties and ordered structures

L10-type ordered FeNi alloy is a good candidate material for rare-earth-free permanent magnets due to its large saturation magnetization, large uniaxial magnetic anisotropy (Ku) and high Curie temperature. Previously, we investigated the fundamental properties of L10–FeNi by preparing single crystal films by alternate deposition of Fe and Ni monatomic layers. In the present study, we investigated the influences of the addition of Co on the magnetic and structural properties of L10-type ordered FeNi films. Three types of Co addition, with the substitution of Fe layers, Ni layers and both layers, were performed. X-ray diffraction using synchrotron radiation indicated that Co atoms were randomly distributed in films in all addition processes. A long-range order parameter of the L10–FeNi phase and Ku showed a tendency to decrease with the amount of Co for additions substituting Fe layers and both layers. However, Ku showed no decrease with Co addition substituting ⩽10% of Ni layers, even though the order parameter notably decreased. This indicates that an achievement of the Co addition substituting Ni atoms without any reduction of the order parameter enhances Ku in L10–FeNi.

Enhancement of ferromagnetic resonance in Al2O3-doped Co2FeAl Heusler alloy film prepared by oblique sputtering

Large and variable in-plane uniaxial magnetic anisotropy in a nanocrystalline (Co2FeAl)97.8(Al2O3)2.2 soft magnetic thin film is obtained by an oblique sputtering method without being induced by magnetic field or post annealing. The in-plane uniaxial magnetic anisotropy varies from 50 Oe to 180 Oe (1 Oe = 79.5775 Am−1) by adjusting the sample's position. As a result, the ferromagnetic resonance frequency of the film increases from 1.9 GHz to 3.75 GHz.

Atomic Resolution Tomography of Magnetically Anisotropic FePt Nanoparticles

Bimetallic FePt nanoparticles have found application in high-density magnetic memory devices due to their large uniaxial magnetic anisotropy and overall stability [1]. These particles form an ordered L10 state, in which the Fe and Pt segregate into alternately stacked (100) planes. While it is well known that the magnetic anisotropy in this material arises from the L10 phase [1], to truly understand the functionality of this material it is necessary to determine the atomic arrangement and defect structures in the interior of the particles. To do this, we have undertaken an atomic resolution tomographic study of 8-10nm bimetallic FePt nanoparticles in the L10 state.

Fe–Ni composition dependence of magnetic anisotropy in artificially fabricated L1 0-ordered FeNi films

We prepared L10-ordered FeNi alloy films by alternate deposition of Fe and Ni monatomic layers, and investigated their magnetic anisotropy. We employed a non-ferromagnetic Au–Cu–Ni buffer layer with a flat surface and good lattice matching to L10-FeNi. An L10-FeNi film grown on Au6Cu51Ni43 showed a large uniaxial magnetic anisotropy energy (Ku = 7.0 × 106 erg cm−3). Ku monotonically increased with the long-range order parameter (S) of the L10 phase. We investigated the Fe–Ni composition dependence by alternating the deposition of Fe 1 − x and Ni 1 + x monatomic layers (− 0.4 < x < 0.4). Saturation magnetization (Ms) and Ku showed maxima (Ms = 1470 emu cm−3, Ku = 9.3 × 106 erg cm−3) for Fe60Ni40 (x = −0.2) while S showed a maximum at the stoichiometric composition (x = 0). The change in the ratio of lattice parameters (c/a) was small for all compositions. We found that enrichment of Fe is very effective to enhance Ku. The large Ms and Ku of Fe60Ni40 indicate that Fe-rich L10-FeNi is promising as a rare-earth-free permanent magnet.

Recent Developments in Magneto-optic Garnet-type Thin-film Materials Synthesis

Magneto-optic (MO) garnets are used in a range of applications in nanophotonics, integrated optics, communications and imaging. Bi-substituted iron garnets of different compositions are the most useful class of materials in applied magneto- optics due to their excellent MO properties (large Faraday effect) and record-high MO figure of merit among all semi- transparent dielectrics. It is highly desirable to synthesise garnets which possess simultaneously a high MO figure of merit and large uniaxial magnetic anisotropy. However, the simultaneous optimization of several material properties and parameters can be difficult in single-layer garnet thin films, and it is also challenging to prepare films with high bismuth content using physical vapor deposition technologies. To meet the current challenge of developing next-generation functional MO materials, we design, develop and demonstrate the functionality of new magnetostatically-altered all-garnet multilayer heterostructures using two different garnet materials of dissimilar anisotropy types (out-of-plane and almost-in- plane). The multilayer structures possess simultaneously a high MO figure of merit and large uniaxial magnetic anisotropy together with low coercivity, if each of the layers is optimized in composition and annealed correctly. We prepare thin-film heterostructures by sandwiching a MO garnet layer with almost in-plane magnetization in-between two MO garnet layers with out-of-plane magnetization using RF sputtering. We apply customised high-temperature oven annealing processes (optimized in temperature and process durations after running many trials) for the as-deposited (amorphous) garnet multilayers to obtain the crystalline garnet phase in every layer. These structures then possess simultaneously a high optical/MO quality and low coercivity, which is very attractive for the development of magnetic photonic crystals, sensing devices and ultra-fast switches. Based on Bi-substituted ferrite garnets grown on garnet substrates, this new and unique method for the development of new magnetic materials, enables customized magnetic properties to be attained, and can be used to develop novel types of synthetic garnet materials.

Effect of anisotropic incidence and atomic arrangement of CoFe system magnetic thin films for the gigahertz frequency in the carousel sputtering method

AbstractTo increase the uniaxial magnetic anisotropy in thin‐film materials is a key issue for micro‐magnetic devices driven at gigahertz frequencies. It is known that an especially large uniaxial magnetic anisotropy is induced only by the carousel sputtering method. In this paper, the atomic deposition process of CoFeB and CoFe films was analyzed by using a simulation of kinetic theory of gases. The results showed that the sputtered particles had markedly anisotropic incidence on the substrate, leading to different atomic distances with respect to the direction of incidence, resulting in a magnetioelastic effect. These sputtered particles with anisotropic incidence are a main cause of the higher uniaxial magnetic anisotropy of the films. © 2012 Wiley Periodicals, Inc. Electron Comm Jpn, 95(6): 58–65, 2012; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ecj.10426

Composition dependence of magnetic properties in perpendicularly magnetized epitaxial thin films of Mn-Ga alloys

Mn-Ga binary alloys show strong magnetism and large uniaxial magnetic anisotropy even though these alloys do not contain any noble, rare-earth metals or magnetic elements. We investigate the composition dependence of saturation magnetization ${M}_{\mathrm{S}}$ and uniaxial magnetic anisotropy ${K}_{\mathrm{u}}$ in epitaxial films of Mn${}_{x}$Ga${}_{1\ensuremath{-}x}$ alloys ($x\ensuremath{\sim}0.5$--0.75) grown by magnetron sputtering. The ${M}_{\mathrm{S}}$ values decrease linearly from approximately 600 to 200 emu/cm${}^{3}$ with increasing $x$, whereas the ${K}_{\mathrm{u}}$ values decrease slightly from approximately 15 to 10 Merg/cm${}^{3}$ with increasing $x$. These trends are distinct from those for known tetragonal hard magnets obtained in a limited composition range in Mn-Al and Fe-Pt binary alloys. These data are analyzed using a localized magnetic moment model.

Magnetic properties of L1-FePt/permalloy exchange-spring films

In-plane magnetized L10-FePt/permalloy (Py) bilayer films were prepared on an MgO (110) single crystal substrate, and their exchange-spring behavior was investigated. The FePt single layer deposited at the substrate temperature (Ts) of 300 °C showed large uniaxial magnetic anisotropy in the in-plane direction. However, the coercivity (Hc) was reduced remarkably when the FePt layer of Ts = 300 °C was combined with the Py layer, resulting in no observation of exchange-spring behavior. By elevating Ts up to 450 °C, on the other hand, Hc for the FePt layer was enhanced. Owing to the increase in Hc for Ts = 450 °C, spring back was definitely observed in the minor magnetization curves for the FePt/Py bilayers. This suggests the existence of the twisted magnetic structure in the Py layer. The dependence of exchange-spring behavior on the Py layer thickness was systematically examined. The experimental results were also discussed in comparison with numerical calculations.

Assembly and patterning of ferromagnetic nanoparticles from solution: A novel low-temperature liquid-phase annealing approach

Ferromagnetic L1 0-phase FePtAu nanoparticles (5–6 nm) were prepared using a low-temperature, liquid phase annealing approach. The chemically ordered, ferromagnetic L1 0-phase onset occurs for annealing temperatures as low as 150 °C. Large uniaxial magnetic anisotropy ( K u ∼ 10 7 erg/cm 3) and a large long-range ordering parameter have been obtained. Significantly, the low-temperature liquid-phase annealing preserves the organic functionalization groups of the nanoparticles. Hence, these nanoparticles are flexible for further assembly and patterning. In addition, we present the assembly of FePt nanoparticles on ferromagnetic substrates with a thin alumina capping layer. This enables us to investigate spin dependent transport through the FePt nanoparticles by scanning tunneling microscopy.

Low-Temperature Solution Synthesis of Chemically Functional Ferromagnetic FePtAu Nanoparticles

Magnetic nanoparticles are of great scientific and technological interest. The application of ferromagnetic nanoparticles for high-density data storage has great potential, but energy efficient synthesis of uniform, isolated, and patternable nanoparticles that remain ferromagnetic at room temperature is not trivial. Here, we present a low-temperature solution synthesis method for FePtAu nanoparticles that addresses all those issues and therefore can be regarded as an important step toward applications. We show that the onset of the chemically ordered face-centered tetragonal (L1(0)) phase is obtained for thermal annealing temperatures as low as 150 degrees C. Large uniaxial magnetic anisotropy (10(7) erg/cm(3)) and a high long-range order parameter have been obtained. Our low-temperature solution annealing leaves the organic ligands intact, so that the possibility for postanneal monolayer formation and chemically assisted patterning on a surface is maintained.

Dot arrays of L11 type Co–Pt ordered alloy perpendicular films

Magnetic properties of dot arrays of L11 type Co–Pt ordered alloy perpendicular films were studied. L11-Co–Pt films with a large uniaxial magnetic anisotropy Ku of the order of 107 erg/cm3 were fabricated at a substrate temperature of 360 °C using ultrahigh vacuum sputter film deposition. Dot patterns with dot diameters of 70–200 nm were made using high resolution e-beam lithography and reactive ion etching (RIE). The values of Ku were measured by the GST method using the Anomalous Hall Effect; we observed the averaged signals of 6000 dots. The values of Ku for dot arrays of 10-nm-thick L11-Co50Pt50 films deposited on MgO(111) substrates (single crystal films) and glass disks (polycrystalline films) were nearly the same as those of the original films independent of D, indicating no significant etching damage by the RIE process. Magnetic force microscopy images revealed that all dots were single domains in the present D region. The coercivity Hc of the dot arrays was 25.0 kOe [MgO(111) substrate, D=70 nm] and 14.3 kOe (glass disks, D=80 nm). The switching field distribution σ/Hc was relatively small, σ/Hc=0.15, even for dot arrays fabricated on glass disks, indicating the homogeneous formation of a L11 type ordered structure in the Co50Pt50 layers.

Formation of Pancakelike Ising Domains and Giant Magnetic Coercivity in FerrimagneticLuFe2O4

We have studied quasi-two-dimensional multiferroic LuFe2O4 with strong charge-spin-lattice coupling, in which low-temperature coercivity approaches an extraordinary value of 9 T in single crystals. The enhancement of the coercivity is connected to the collective freezing of nanoscale pancakelike ferrimagnetic domains with large uniaxial magnetic anisotropy ("Ising pancakes"). Our results suggest that collective freezing in low-dimensional magnets with large uniaxial anisotropy provides an effective mechanism to achieve enhanced coercivity. This observation may help identify novel approaches for synthesis of magnets with enhanced properties.