Plane Magnetocrystalline Anisotropy Research Articles

Effect of oxidation on the microwave absorption properties of rare-earth intermetallics La2Fe4Co10B

Microwave absorbers are increasingly being used to enhance shielding performance at higher frequencies. Great effort has been made to develop materials with superior reflection loss (RL), thin thickness, wide bandwidth, and low density to improve their performance in electromagnetic microwave absorption. In this work, the rare-earth intermetallics La2Fe4Co10B fine powders with planar magnetocrystalline anisotropy and high magnetization were prepared using the hydrogenation desorption (HD) technique. By heating the obtained magnetic powders in the air to slowly form an oxide layer on their surfaces, its complex permittivity can be tuned without changing its complex permeability, and its microwave absorption performance can be dramatically enhanced due to the improved impedance matching condition. For the La2Fe4Co10B/paraffin composite, the real part of the complex permittivity at 10 GHz can be reduced from 20.1 to 12.8, decreasing 36.3%. Before the oxidation, the RL of La2Fe4Co10B/paraffin composite is only −14.8 dB at 3.3 GHz under 3.4 mm thickness with an effective absorption bandwidth (EAB) of 0.9 GHz. However, after the 10-hour oxidation, the RL changes to −15.6 dB at 13.4 GHz under 1.4 mm thickness with an EAB of 6.2 GHz. As an efficient and lightweight absorber, La2Fe4Co10B/paraffin composite has potential application value in constructing a new microwave absorber.

Microwave absorbing properties of Y2Fe16Si micropowders with broad bandwidth and strong absorption

The rare earth-Fe intermetallic compounds with planar magnetocrystalline anisotropy were deemed as hopeful candidates for microwave absorption owing to their higher Snoek’s limit as compared to the cubic spinel-type ferrites. In this work, The Y2Fe16Si alloy was massively produced through strip casting technique, and the microwave absorbing properties of Y2Fe16Si-paraffin composites were studied. The Y2Fe16Si alloy exhibits a single phase of hexagonal Th2Ni17-type structure with a planar magnetocrystalline anisotropy and has a saturation magnetization of 121 emu g−1. It was found that electromagnetic dissipation contributes to microwave absorption but is not the determinative factor in Y2Fe16Si-paraffin composites. Controlling the values of the complex permittivity and the complex permeability in a reasonable range can lead to proper impedance matching. Thus excellent microwave absorbing properties can be obtained. The sample with the mass ratio of 3:1 between Y2Fe16Si and paraffin displays high performance ascribed to its appropriate values of complex permittivity and permeability. A minimum reflection loss (RL) of  −51.6 dB is obtained with a thickness of 1.8 mm. Full covers of the effective bandwidth (RL  <  −10 dB) on the X-band and the Ku-band are achieved when the thicknesses are 1.9 mm and 1.38 mm, respectively. Hence, Y2Fe16Si exhibits excellent potential as a high-performance microwave absorbing material.

Extended exchange interactions stabilize long-period magnetic structures in Cr1∕3NbS2

The topologically protected, chiral soliton lattice is a unique state of matter offering intriguing functionality, and it may serve as a robust platform for storing and transporting information in future spintronic devices. While the monoaxial chiral magnet Cr1∕3NbS2 is known to host this exotic state in an applied magnetic field, its detailed microscopic origin has remained a matter of debate. Here, we work towards addressing this open question by measuring the spin wave spectrum of Cr1∕3NbS2 over the entire Brillouin zone with inelastic neutron scattering. The well-defined spin wave modes allow us to determine the values of several microscopic interactions for this system. The experimental data are well-explained by a Heisenberg Hamiltonian with exchange constants up to the third nearest neighbor and an easy plane magnetocrystalline anisotropy term. Our work shows that both the second and third nearest neighbor exchange interactions contribute to the formation of the helimagnetic and chiral soliton lattice states in this robust three-dimensional magnet.

Magnetic properties of Ni-Mn-Ga-Co-Cu tetragonal martensites exhibiting magnetic shape memory effect

Magnetic properties of Ni46Mn24Ga22Co4Cu4, (Ni45Mn23Ga22Co5Cu5, Ni46Mn22.5Ga23Co5Cu3.5) tetragonal martensites with c/a≈1.14 exhibiting near 10% magnetic field-induced strain (MFIS) were investigated between 10 and 500K and up to 9T field. The (001) easy plane magnetocrystalline anisotropy with K1(300K)=−1.95∙105J∙m−3 (K2<6%|K1|, K3<2%|K1|, MS(300K)=63Am2/kg) was about 30% lower than in the non-doped Ni50.5Mn30.4Ga19.1 reference, which can be related to a reduced c/a ratio in the doped material. In spite of different type of magnetocrystalline anisotropy compared to 10M martensite, the mechanism of MFIS is the same, i.e. with the typical first-quadrant magnetization hysteresis and {101} twin boundary motion.

Electronic and magnetic properties of 2H−NbS2 intercalated by 3d transition metals

The electronic structure and magnetic properties of the 2H-NbS$_2$ compound intercalated by Cr, Mn and Fe, have been investigated by means of the Korringa-Kohn-Rostoker (KKR) method. The calculations demonstrate easy plane magneto-crystalline anisotropy (MCA) of Cr$_{1/3}$NbS$_2$ monotonously decreasing towards the Curie temperature in line with the experimental results. The modification of the electronic structure results in a change of the easy axis from in-plane to out-of-plane. It is shown, that for Cr$_{1/3}$NbS$_2$ and Mn$_{1/3}$NbS$_2$ the in-plane MCA and Dzyaloshinskii-Moriya interactions results in a helimagnetic structure along the hexagonal $c$ axis, following the experimental observations. The negative exchange interactions in the Fe$_{1/3}$NbS$_2$ compound results in a non-collinear frustrated magnetic structure if the MCA is not taken into account. It is shown, however, that a strong MCA along the hexagonal $c$ axis leads to a magnetic ordering referred to as an ordering of the third kind, which was observed experimentally.

Room-temperature tetragonal non-collinear Heusler antiferromagnet Pt2MnGa.

Antiferromagnetic spintronics is a rapidly growing field, which actively introduces new principles of magnetic storage. Despite that, most applications have been suggested for collinear antiferromagnets. In this study, we consider an alternative mechanism based on long-range helical order, which allows for direct manipulation of the helicity vector. As the helicity of long-range homogeneous spirals is typically fixed by the Dzyaloshinskii–Moriya interactions, bi-stable spirals (left- and right-handed) are rare. Here, we report a non-collinear room-temperature antiferromagnet in the tetragonal Heusler group. Neutron diffraction reveals a long-period helix propagating along its tetragonal axis. Ab-initio analysis suggests its pure exchange origin and explains its helical character resulting from a large basal plane magnetocrystalline anisotropy. The actual energy barrier between the left- and right-handed spirals is relatively small and might be easily overcome by magnetic pulse, suggesting Pt2MnGa as a potential candidate for non-volatile magnetic memory.

Magnetocrystalline anisotropy in Gd(Co,Fe)12B6: A first-principles study

The electronic structure, preferential site occupancy for Fe, and magnetocrystalline anisotropy (MCA) have been investigated in Gd(Co12−xFe)xB6via a DFT (density functional theory) plus Hubbard U approach (DFT + U). For Gd atoms, the spin-up 4f bands are fully occupied while the spin-down 4f hole levels are completely empty. The d-like states dominate at the Fermi Level, mainly contributed by Co(Fe) 3d electrons. Co atoms in GdCo12B6 carry magnetic moments of − 0.3 μB and − 0.7 μB at the 18g and 18h sites respectively, while the 7.3 μB Gd moments are ordered antiparallel to the Co(Fe) sublattice. The DFT total energy calculations show that the Fe atoms prefer to replace Co at the 18h site rather than the 18g site, with an energy gain of 0.16 eV/f.u. and they carry a magnetic moment of 1.5 μB/Fe. GdCo12B6 shows a weak easy c− axis MCA (Emca = 90 μeV/f.u.) while GdCo11FeB6 displays a moderate easy ab− plane MCA (Emca = −2690 μeV/f.u.). The spin reorientation induced by Fe doping is related to the electronic structure change near Fermi Level and the different contributions of Co and Fe to the MCA in GdCo12−xFexB6.

Integration of Ni-Cu-Zn and Hexagonal Ferrites into LTCC Modules: Cofiring Strategies and Magnetic Properties

We have studied the integration of Ni-Cu-Zn ferrite spinels as well as substituted hexagonal Co2Y-and M-type ferrites into LTCC (Low Temperature Ceramic Co-firing) modules. The cofiring behavior and the magnetic properties of these materials were investigated and evaluated for multilayer applications. Ni-Cu-Zn ferrites exhibit permeabilities of µ=300–500 for operating frequencies in the MHz range. Cu-substituted Y-type ferrites Ba2Co2-x-yZnxCuyFe12O22 in combination with sintering additives display sufficient shrinkage and densification at 900°C. A permeability of µ=10 is observed; however, substituted Co2Y-type ferrites do not exhibit long-term stability at 900°C. Co/Ti-substituted M-type ferrites BaFe12-2yCoyTiyO19 (y=1.2) with planar magneto-crystalline anisotropy exhibit excellent soft magnetic behavior. Using sintering additives, complete densification is reached at 900°C and a permeability of µ=15 and a resonance frequency of larger than 1?GHz are observed. Integration of ferrite multilayer inductor components into LTCC modules using free and constrained cofiring technologies is demonstrated.

Low-fired Y-type hexagonal ferrite for hyper frequency applications

Y-type hexagonal ferrite with planar magnetocrystalline anisotropy has ultrahigh cut-off frequency up to GHz and excellent magnetic properties in hyper frequency range, so that is regarded as the most suitable material in correpongding inductive devices and components. The technology of low temperature cofired ceramics for surface-mounted multilayer chip components needs ferrite to be sintered well under 900 ℃ to avoid the melting and diffusion of Ag inner electrode during the cofiring process. To lower the sintering temperature of Y-type hexagonal ferrite, there are several methods, (1) using nano-sized starting powders, (2) substitution by low-melting elements, (3) adding sintering additives, and (4) introducing lattice defect. In this paper, the effects of different methods on the sintering behavior and the magnetic properties were discussed in detail.

Magnetic and structural characterization of nanosized BaCoxZn2−xFe16O27 hexaferrite in the vicinity of spin reorientation transition

Numerous applications of hexagonal ferrites are related to their easy axis or easy plane magnetocrystalline anisotropy configurations. Certain W-type ferrites undergo spin reorientation transitions (SRT) between different anisotropy states on magnetic field or temperature variation. The transition point can be tuned by modifying the chemical composition, which suggests a potential application of hexaferrites in room temperature magnetic refrigeration. Here we present the results of structural and magnetic characterization of BaCoxZn2−xFe16O27 (0.7 ≤ x ≤ 2) doped barium ferrites. Fine powders were prepared using a sol-gel citrate precursor method. Crystal structures and particle size distributions were examined by X-ray diffraction and transmission electron microscopy. The optimal synthesis temperature ensuring complete formation of single W-phase with limited grain growth has been determined. Spin reorientation transitions were revealed by thermomagnetic analysis and AC susceptibility measurements.

High frequency magnetic mechanism of Ni-substituted Co 2Z hexagonal ferrite

The magnetic properties, especially the high frequency magnetic mechanism, of Ni-substituted Co 2Z hexagonal ferrite were studied. The polycrystalline Z-type hexagonal ferrite of Ba 3Ni x Co 2− x Fe 24O 41 (0 ≤ x ≤ 2) were prepared by solid-state reaction. The results indicate that Ni-substituted Co 2Z samples all exhibit typical soft magnetic character. Substitution of Ni for Co will turn the planar magnetocrystalline anisotropy of Co 2Z to uniaxial anisotropy when x ≥ 1, so that the permeability drops dramatically and domain wall resonance appears in the frequency spectra. With the rise of Ni amount or sintering temperature, domain wall resonance strengthens gradually.

Crystal structure and spin-reorientation transition in TbFe 10− xNi xSi 2 compounds

The crystal structure and magnetic properties of TbFe 10− x Ni x Si 2 compounds have been investigated by means of X-ray diffraction and magnetic measurements. It is revealed that Ni substitution for Fe results in a slight variation of the Curie temperature, but a salient decrease of the spin-reorientation temperature of the compounds. A spin-phase diagram of TbFe 10− x Ni x Si 2 is constructed. At room temperature the compounds show planar magnetocrystalline anisotropy for x⩽1.5, but easy-axis anisotropy for x>1.5. Temperature dependent X-ray diffraction indicates a noticeable magnetostriction around the spin-reorientation temperature.

Neutron diffraction and magnetic investigations of the TbCo4M compounds (M = Al and Ga)

The structural and magnetic features of the TbCo4M compounds (M = Al and Ga)have been studied by means of neutron powder diffraction and magneticmeasurements. X-ray diffraction experiments show that the samples are singlephased, crystallizing in the CaCu5-type structure. Neutron powder diffractionmeasurements demonstrate a preferential substitution of Al and Ga atthe 3g crystallographic site of the CaCu5-type structure. Together withneutron powder diffraction investigations, magnetization measurements andx-ray powder diffraction studies performed on field-oriented samples arecombined in order to clarify the effects of the Al and Ga substitutions on themagnetic features of these compounds. The modifications of the magneticproperties upon substitution are significant: a drastic decrease of the mean Comagnetic moment and of the Curie temperature is observed. Unlike whatwas found earlier for TbCo5, no sign of a spin reorientation transitionhas been observed in the present TbCo4M compounds. Moreover, bothcompounds present a planar magnetocrystalline anisotropy over the wholetemperature range. Finally, the thermal evolutions of the magnetization ofterbium and cobalt sublattices in TbCo4Al and TbCo4Ga are presented.

Thickness dependence of magnetic and transport properties of chromium dioxide (CrO/sub 2/) strained epitaxial thin films

Chromium dioxide (CrO/sub 2/) thin films of different thicknesses (110-2600 /spl Aring/) have been grown epitaxially on [100]-oriented TiO/sub 2/ substrates by the chemical vapor deposition technique. The thicker films, with a Curie temperature of 395 K, exhibit in plane magnetocrystalline anisotropy with the magnetic easy axis along the c-axis direction. At low temperatures, the easy axis is deflected from the c- toward the b-axis in the thickness range 1500-600 /spl Aring/. For even smaller thicknesses, the c-axis and the magnetic easy axis coincide again. However, at 300 K, the c-axis is always the magnetic easy axis in every film studied. Resistivity measurement in the temperature range of 4.2 to 420 K has also been performed. Temperature dependence of the resistivity is highly anisotropic between the b- and the c-axis, due to thickness induced anisotropic strain in the CrO/sub 2/ films.

Spin wave instability in single crystal Zn–Y hexagonal ferrite at 8.93 GHz

Resonance saturation (RS), subsidiary absorption (SA), and parallel pump (PP) spin wave instability threshold measurements have been made on single crystal easy plane disks of Mn substituted Zn–Y type hexagonal ferrite materials at 8.93 GHz and room temperature. For each configuration, “butterfly curves” of the spin wave instability threshold microwave field amplitude hcrit as a function of the static field applied in the disk plane were obtained. The previous theory for these instability processes was also extended to include planar magnetocrystalline anisotropy and a wave vector k dependent spin wave linewidth, ΔHk. The RS butterfly curve had a characteristic “V” shape with a rounded minimum at the ferromagnetic resonance (FMR) field. The nominal ΔHk needed to fit the data at the ferromagnetic resonance field was 7 Oe, but the butterfly curve shape indicated a k-dependent ΔHk. The butterfly curves for the PP configuration were flat at low field and then diverged rapidly at the cutoff field for first order instability processes, Hcut. The SA butterfly curves were also flat over the field interval for first order processes, but then decreased as the field was increased above Hcut. This decrease is attributed to the onset of second order processes due to the proximity of the FMR and additional magnetostatic mode peaks as one moves to and then above Hcut. The flat portions of the PP and SA butterfly curves could be fitted with a single k-independent ΔHk value of 18 Oe.

Structure and magnetic properties of Gd3Fe28.2W0.8

The ternary intermetallic compound Gd3Fe28.2W0.8 with a Nd3Fe29−xTix-type structure was synthesized with W as a stabilizing element, and its crystal structure and magnetic properties were investigated by means of x-ray powder diffraction and magnetic measurements. The space group of Gd3Fe28.2W0.8 is A2/m with lattice parameters a=10.5812 Å, b=8.5338 Å, c=9.6986 Å, and β=96.86°. Rietveld refinement of the crystal structure reveals that W atoms occupy exclusively three crystallographic positions (4g, 4i3, 4i4) with only one neighboring Gd atom. The preferential occupation of W atoms can be understood in terms of the site volume and enthalpy of mixing. The compound exhibits a planar magnetocrystalline anisotropy perpendicular to the [102] direction. The Curie temperature Tc, saturation moment Ms, and anisotropy field HA are 502 K, 38.6 μB/f.u., and 7.5 T, respectively.

Superparamagnetic magnetization equation in two dimensions

An equation for the dependence of magnetization on magnetic field in the case of two-dimensional (base plane) anisotropy has been derived. The resulting equation is expressed as an infinite series of modified Bessel functions, unlike the elementary function expressions that are applicable to the one-dimensional (axially anisotropic) and three-dimensional (isotropic) cases. Nevertheless, in the low-field limit, the series can be effectively truncated to give an approximate solution, while, in the high-field limit, an alternative expression has been derived which represents the limiting function as the field strength tends to infinity. The resulting expressions can be used to describe the superparamagnetic magnetization and susceptibility as a function of magnetic field in situations where the magnetic moments are constrained to lie in a plane, with no preferred direction within the plane. This can therefore be applied to two-dimensional structures, such as magnetic thin films, where magnetostatic energy confines the moments to the plane of the film, or to three-dimensional structures with planar magnetocrystalline anisotropy.

Magnetic properties of Sm 2(Fe,Ti) 17 compounds and their nitrides with Th 2Zn 17 and Th 2Ni 17 structures

In Sm 2(Fe 1− x Ti x ) 17 compounds ( x=0.02, 0.03, 0.04) the hexagonal Th 2Ni 17 structure has been stabilized at high temperatures. The type of crystal structure affects the magnetic properties of Sm 2(Fe,Ti) 17. For the Th 2Ni 17 structure, higher values of the Curie temperature and constants of planar magnetocrystalline anisotropy than those for the Th 2Zn 17 structure are observed. For both structures, the substitution of Ti for Fe leads to growing importance of the anisotropy constants of orders higher than second. No change in the type of structure is found upon nitrogenation. The hexagonal Sm 2(Fe,Ti) 17 nitrides as well as the rhombohedral nitrides show uniaxial anisotropy at room temperature.

Magnetocrystalline anisotropy of of R 2Fe 10Ga 7 compounds with R=Y, Sm, Gd, Tb, Dy, Ho, Er and Tm

Samples with compositions of R 2Fe 10Ga 7 (R=Y, Sm, Gd, Tb, Dy, Ho, Er and Tm) were prepared by arc-melting. The prepared samples are single phase with the rhombohedral Th 2Zn 17-type structure. The substitution of Ga in the R 2Fe 17 results in the increase in unit cell volume. Curie temperatures of R 2Fe 10Ga 7, except for Gd compound, are higher than that of R 2Fe 17. For R 2Fe 10Ga 7 compounds with R=Dy and Ho, the Fe sublattice moment and the R sublattice moment cancel occurs at a certain temperature. The compensation temperature T comp of Dy 2Fe 10Ga 7 and Ho 2Fe 10Ga 7 compounds are found to be 60 and 101 K, respectively. The R 2Fe 10Ga 7 compounds with R=Y, Gd, Tb, Dy, Ho, Tm are found to have a uniaxial magnetocrystalline anisotropy at room-temperature, whereas the situation is the reverse for R 2Fe 10Ga 7 (R=Sm and Er) which exhibit an easy plane magnetocrystalline anisotropy.

Magnetization curve and magnetic characterization of a system of fine single domain particles with oblate ellipsoidal shape and large planar magnetocrystalline anisotropy

The magnetization curve of a magnetic fine particle ensemble is calculated starting from the following assumptions: oblate ellipsoidal particle shape with the short axis perpendicular to the easy plane of the magnetocrystalline anisotropy energy, homogeneous particle magnetization, magnetization reversal by coherent rotation, and the anisotropy field strength much larger than the strength |H| of the microscopic magnetic field H. Furthermore, a particle arrangement resulting in macroscopically isotropic ensemble properties, and an exponential distribution of the shape anisotropy of the particles with respect to its easy plane is presumed. The interaction between the particles is described by treating H as a stochastic field which is determined by the symmetry of the short-range order of the particles. For packing densities ≦ 0.3, the mean value of H equals the macroscopic internal field Hi in zeroth approximation. The magnetic properties of the fine particle ensemble studied are determined by the rotation of the magnetization vector of each particle within its easy plane. The system is characterized by a low coercivity Hc of about M0/4π (M0: bulk magnetization) which is governed by the easy plane-shape anisotropy of the particles, as well as by the approach to a “pseudo-saturation state” of the magnetization rotation within the easy plane of the particles, which occurs for |Hi| > 3Hc and results in a characteristic functional dependence of the magnetization curve. It is shown that large fluctuations of |H| of about Hc/2 cause only a small increase of Hc of about 10%. Die Magnetisierungskurve eines Ensembles magnetischer Teilchen wird unter folgenden Voraussetzungen berechnet: Abgeplattet-ellipsoidförmige Teilchengeometrie mit kurzer Achse senkrecht zur leichten Ebene der magnetokristallinen Anisotropieenergie, homogene Teilchenmagnetisierung, Ummagnetisierung durch kohärente Rotation, sowie Stärke des Anisotropiefeldes viel größer als die Stärke |H| des mikroskopischen Magnetfeldes H. Außerdem wird eine Partikelanordnung angenommen, die zu makroskopisch isotropen Ensembleeigenschaften führt, sowie eine Exponentialverteilung der Formanisotropie der Partikel bezüglich ihrer leichten Ebene vorausgesetzt. Die Wechselwirkung zwischen den Partikeln wird dadurch berücksichtigt, daß H als stochastisches Feld angesehen wird, das durch die Symmetrie der Nahordnung der Partikel bestimmt wird. Für Packungsdichten ≦ 0,3 ist der Mittelwert von H in nullter Näherung gleich dem makroskopischen inneren Magnetfeld Hi. Die magnetischen Eigenschaften des Ensembles werden bestimmt durch die Drehprozesse des Magnetisierungsvektors von jedem Teilchen in dessen leichter Ebene. Charakteristische Eigenschaften der untersuchten Partikelsysteme sind niedrige Koerzitivfeldstärke Hc von der Größenordnung M0/4π (M0: Sättigungsmagnetisierung des massiven Materials), die durch die Formanisotropie der Partikel in der leichten Ebene bestimmt wird, sowie die Annäherung an einen „Pseudosättigungszustand”︁ der Drehprozesse in der leichten Ebene, die für |Hi| > 3Hc auftritt und zu einer charakteristischen funktionalen Abhängigkeit der Magnetisierungskurve führt. Es wird gezeigt, daß große Fluktuationen von |H| von etwa Hc/2 nur zu einer kleinen Vergrößerung von Hc von etwa 10% Anlaß geben.