Anisotropic Ferromagnet Research Articles

Quantification of magneto-optically active nanorods and inactive aggregates in nickel nanorod colloids

Nickel nanorods of sufficiently small size exhibit collinear ferromagnetic and optical anisotropy which allows the measurement of their field-dependent orientation in colloidal suspension by optical transmission of linearly polarized light. Coagulation of such extended dipolar particles favors the formation of flux-closing structures such as nanorod dimers with anti-parallel magnetic moments. We present an experimental approach to separate and quantify the fraction of magneto-optically active nanorods and inactive non-magnetic aggregates. The analysis was applied to monitor the coagulation of nanorod colloids which were deliberately destabilized by addition of NaCl.

Structural, optical and magnetic properties of aluminum doped MnZnO films deposited by dip coating

Al doped MnZnO films were synthesized by sol–gel dip coating method on glass substrates. Films were annealed between 250 and 550 °C in an air furnace. The X-ray diffraction analysis showed that the films are polycrystalline having the hexagonal structure and a preferred orientation along (100) for films annealed for 2 h between 250 and 450 °C. Further increase in annealing temperature shifted the preferred orientation to (002) direction. Optical characteristics in the UV–Visible region showed increase in transmission from 70 to 85% when increasing the wavelength from 300 to 900 mm. All films showed room temperature anisotropic ferromagnetism. The annealing temperature had a prominent effect on the magnetic properties of the films which in turn increased saturation magnetization. Al doped MnZnO films exhibited a dense homogenous microstructure with grain sizes less than 80 nm as evaluated by SEM micrographs.

Anisotropic defect-induced ferromagnetism and transport in Gd-doped GaN two-dimensional electron gasses

Here we report on the effect of rare earth Gd-doping on the magnetic properties and magnetotransport of GaN two-dimensional electron gasses (2DEGs). Samples are grown by plasma-assisted molecular beam epitaxy and consist of AlN/GaN heterostructures where Gd is delta-doped within a polarization-induced 2DEG. Ferromagnetism is observed in these Gd-doped 2DEGs with a Curie temperature above room temperature and an anisotropic spontaneous magnetization preferring an out-of-plane (c-axis) orientation. At magnetic fields up to 50 kOe, the magnetization remains smaller for in-plane configuration than for out-of-plane, which is indicative of exchange coupled spins locked along the polar c-axis. The sample with the lowest Gd concentration (2.3 $\times$ $10^{14}$ cm$^{-2}$) exhibits a saturation magnetization of 41.1 $\mu_B/Gd^{3+}$ at 5 K revealing that the Gd ion spins (7 ${\mu}_B$) alone do not account for the magnetization. Surprisingly, control samples grown without any Gd display inconsistent magnetic properties; in some control samples weak ferromagnetism is observed and in others paramagnetism. The ferromagnetic 2DEGs do not exhibit the anomalous Hall effect; the Hall resistance varies non-linearly with the magnetic field, but does not track the magnetization indicating the lack of coupling between the ferromagnetic phase and the conduction band electrons within the 2DEG.

Real-space representation of uniaxial magnetic anisotropy of ferromagnetic metals with non-periodic structures

A real-space representation of magnetic anisotropy (MA) in metallic ferromagnets is formulated in a d-orbital tight-binding model. By adopting the second-order perturbation for the spin–orbit interaction (SOI), which takes into account the direction of magnetisation, the lowest order of the uniaxial MA constant is expressed in terms of non-local Green’s functions and the matrix elements of the SOI. The non-local Green’s functions are calculated using a symmetry-conserving recursive method. The validity of the method is examined by comparing the results obtained with those calculated by the first-principles method. The method is applied to calculate layer- or site-resolved near body-centred cubic (bcc) Fe and face-centred cubic Ni surfaces with various surface structures. We find that surface resonant states contribute considerably to the uniaxial MA of bcc Fe thin films. Moreover, it is shown that the uniaxial MA is determined by rather short-range atomic configurations.

Roadmap for Emerging Materials for Spintronic Device Applications

The Technical Committee of the IEEE Magnetics Society has selected 7 research topics to develop their roadmaps, where major developments should be listed alongside expected timelines; (i) hard disk drives, (ii) magnetic random access memories, (iii) domain-wall devices, (iv) permanent magnets, (v) sensors and actuators, (vi) magnetic materials and (vii) organic devices. Among them, magnetic materials for spintronic devices have been surveyed as the first exercise. In this roadmap exercise, we have targeted magnetic tunnel and spin-valve junctions as spintronic devices. These can be used for example as a cell for a magnetic random access memory and spin-torque oscillator in their vertical form as well as a spin transistor and a spin Hall device in their lateral form. In these devices, the critical role of magnetic materials is to inject spin-polarised electrons efficiently into a non-magnet. We have accordingly identified 2 key properties to be achieved by developing new magnetic materials for future spintronic devices: (1) Half-metallicity at room temperature (RT); (2) Perpendicular anisotropy in nano-scale devices at RT. For the first property, 5 major magnetic materials are selected for their evaluation for future magnetic/spintronic device applications: Heusler alloys, ferrites, rutiles, perovskites and dilute magnetic semiconductors. These alloys have been reported or predicted to be half-metallic ferromagnets at RT. They possess a bandgap at the Fermi level EF only for its minority spins, achieving 100% spin polarisation at EF. We have also evaluated L10-alloys and D022-Mn-alloys for the development of a perpendicularly anisotropic ferromagnet with large spin polarisation. We have listed several key milestones for each material on their functionality improvements, property achievements, device implementations and interdisciplinary applications within 35 years time scale.

Phase separation and exchange bias effect in Ca doped EuCrO3

The rare-earth chromites have attracted increasing interests in recent years, as a member of a few single-phase multiferroic materials. We studied the structure and magnetic property of a series of Ca-doped EuCrO3 samples by using X-ray powder diffraction and Physical Property Measurement System. Phase separation, rotation of magnetization in M(T) curve and exchange bias effect have been identified. The Eu0.7Ca0.3CrO3 polycrystalline sample may be intrinsically phase-separated, with Cr3+-rich, Cr4+-rich canted antiferromagnetic regions surrounded by spin glass-like frustrated phase, resulting in several magnetic features including: (1) a broad and slow increase of M(T) curve with the decrease of temperature; (2) rotation of magnetization with increasing cooling field; (3) exchange bias and glassy magnetism. The rotation of magnetization is ascribed to the rotation of the moment of Cr4+-rich regions, arising from the competition between exchange coupling energy and magnetostatic energy. The exchange bias effect suggests the formation of weak ferromagnetic unidirectional anisotropy during field cooling, due to the exchange coupling among weak ferromagnetic domains and surrounding spin glass-like regions. This result helps understanding the interaction among different magnetic domains and phases in a complex system.

Perpendicular magnetic anisotropy and complex magnetotransport behavior of cobalt nanoparticles in silver matrix

A cobalt concentration dependent transition from usual magnetoresistance (MR) to complex MR behavior is reported in the co-sputtered Co-Ag thin films containing cobalt nanoparticles. At 20 K, the MR behavior of 32.6% film consists of three contributions superparamagnetic (SPM), ferromagnetic (FM), and perpendicular magnetic anisotropy (PMA), which are, respectively, operative at high, intermediate, and low fields. Magnetic force microscopy (MFM) provides the supporting evidence for the existence of a magnetic microstructure with magnetization perpendicular to the film plane for a threshold of 32.6% cobalt concentration. MFM images show the presence of regions with perpendicular magnetization having sizes much larger than the particle size. In addition, the magnetization measurements also substantiate the presence of PMA in films above 25.2% cobalt. The observed complex magnetotransport behavior and associated PMA are interpreted in terms of inter-particle (FM-SPM-FM) interactions in which local magnetic fields of the nearby FM particles align the SPM particles out of film plane.

Real-space presentation of the magnetic anisotropy of metallic ferromagnets

A real-space representation of the uniaxial magnetic anisotropy (MA) of metallic ferromagnets is formulated using the full-orbital tight-binding model. By adopting a second-order perturbation for the spin-orbit interaction, numerical calculations of the uniaxial MA are performed for the L10 FePt and Y2Fe14B compounds to evaluate the validity and applicability of the present method.

Magnetic anisotropy in clinopyroxene and orthopyroxene single crystals

AbstractPyroxenes constitute an important component in mafic igneous and metamorphic rocks. They often possess a prismatic habit, and their long axis, the crystallographic c axis, helps define a lineation in a textured rock. Anisotropy of magnetic susceptibility (AMS) serves as a fabric indicator in igneous and metamorphic rocks. If a rock's AMS is carried by pyroxenes, it can be related to their crystallographic preferred orientation and degree of alignment. This requires knowing the intrinsic AMS of pyroxene single crystals. This study provides a comprehensive low‐field and high‐field AMS investigation of chemically diverse orthopyroxene and clinopyroxene crystals in relation to crystal structure, chemical composition, oxidation state of Fe, and the possible presence of ferromagnetic inclusions. The paramagnetic anisotropy, extracted from high‐field data, shows clear relationships to crystallographic directions and Fe concentration both in clinopyroxene and orthopyroxene. In the diopside‐augite series, the intermediate susceptibility is parallel to b, and the maximum is at 45° to the c axis. In aegirine, the intermediate axis remains parallel to b, while the maximum susceptibility is parallel to c. The AMS of spodumene depends on Fe concentration. In enstatite, the maximum susceptibility aligns with c and the minimum with b, and in the case of hypersthene, the maximum susceptibility is normal to the exsolution lamellae. Magnetite inclusions within augite possess a ferromagnetic anisotropy with consistent orientation of the principal susceptibilities, which dominates the low‐field anisotropy. These results provide better understanding of magnetic anisotropy in pyroxenes and form a solid basis for interpretation of magnetic fabrics in pyroxene‐bearing rocks.

Very large magnetoresistance inFe0.28TaS2single crystals

Magnetic moments intercalated into layered transition metal dichalcogenides are an excellent system for investigating the rich physics associated with magnetic ordering in a strongly anisotropic, strong spin-orbit coupling environment. We examine electronic transport and magnetization in Fe$_{0.28}$TaS$_{2}$, a highly anisotropic ferromagnet with a Curie temperature $T_{\mathrm{C}} \sim 68.8~$K. We find anomalous Hall data confirming a dominance of spin-orbit coupling in the magnetotransport properties of this material, and a remarkably large field-perpendicular-to-plane MR exceeding 60% at 2 K, much larger than the typical MR for bulk metals, and comparable to state-of-the-art GMR in thin film heterostructures, and smaller only than CMR in Mn perovskites or high mobility semiconductors. Even within the Fe$_x$TaS$_2$ series, for the current $x$ = 0.28 single crystals the MR is nearly $100\times$ higher than that found previously in the commensurate compound Fe$_{0.25}$TaS$_{2}$. After considering alternatives, we argue that the large MR arises from spin disorder scattering in the strong spin-orbit coupling environment, and suggest that this can be a design principle for materials with large MR.

Current-induced magnetic soliton solutions in a perpendicular ferromagnetic anisotropy nanowire**Project supported by the Natural Science Foundation of Hebei Province, China (Grant Nos. A2012202022 and A2012202023), the Aid Program for Young Teacher of Hunan University, China, the Project-sponsored by SRF for ROCS, SEM, China, and the Aid Program for Science and Technology Innovative Research Team in Higher Educational Instituions of Hunan

Magnon density distribution can be affected by the spin-transfer torque in a perpendicular ferromagnetic anisotropy nanowire. We obtain the analytical expression for the critical current condition. For the cases of below and above the critical value, the magnon density distribution admits bright and dark soliton states, respectively. Moreover, we discuss two-soliton collision properties that are modulated by the current. Each magnetic soliton exhibits no changes in both velocity and width before and after the collision.

Magnetic anisotropy and deep ultraviolet photoresponse characteristics in Ga2O3:Cr vermicular nanowire thin film nanostructure

The Ga2O3:Cr vermicular nanowire thin film nanostructure with a big ferromagnetic anisotropy and obvious deep ultraviolet photoresponse was obtained by PLD.

UV irradiations enhance anisotropy of ZnO nanorods in crystal growth and ferromagnetism

Abstract The crystalline growth of the preferred orientation in the ZnO nanorods will be profoundly enhanced as we irradiate a specific wave length of ultra-violate (UV) during the sample fabrication. The specific UV photon energy is equivalent to the ZnO band gap. Both samples, with and without UV irradiated undoped nanorods, exhibit the anisotropic ferromagnetism. The electric polarization yields the ferromagnetic anisotropy along the c -axis, which suppresses the ferromagnetism in the same polarized direction and creates a similar multiferroic characteristic. Besides, the specific UV irradiation significantly enhances the ferromagnetic anisotropy.

Symmetry-lowering lattice distortion at the spin reorientation in MnBi single crystals

Structural and physical properties determined by measurements on large single crystals of the anisotropic ferromagnet MnBi are reported. The findings support the importance of magneto-elastic effects in this material. X-ray diffraction reveals a structural phase transition at the spin reorientation temperature $T_{SR}$ = 90 K. The distortion is driven by magneto-elastic coupling, and upon cooling transforms the structure from hexagonal to orthorhombic. Heat capacity measurements show a thermal anomaly at the crystallographic transition, which is suppressed rapidly by applied magnetic fields. Effects on the transport and anisotropic magnetic properties of the single crystals are also presented. Increasing anisotropy of the atomic displacement parameters for Bi with increasing temperature above $T_{SR}$ is revealed by neutron diffraction measurements. It is likely that this is directly related to the anisotropic thermal expansion in MnBi, which plays a key role in the spin reorientation and magnetocrystalline anisotropy. The identification of the true ground state crystal structure reported here may be important for future experimental and theoretical studies of this permanent magnet material, which have to date been performed and interpreted using only the high temperature structure.

Magnetocaloric effect (MCE): Microscopic approach within Tyablikov approximation for anisotropic ferromagnets

The aim of this paper is to generalize the microscopic approach to the description of the magnetocaloric effect (MCE) started by Kokorina and Medvedev (E.E. Kokorina, M.V. Medvedev, Physica B 416 (2013) 29.) by applying it to the anisotropic ferromagnet of the “easy axis” type in two settings—with external magnetic field parallel and perpendicular to the axis of easy magnetization. In the last case there appears the field induced (or spin-reorientation) phase transition which occurs at the critical value of the external magnetic field. This value is proportional to the exchange anisotropy constant at low temperatures, but with the rise of temperature it may be renormalized (as a rule, proportional to the magnetization). We use the explicit form of the Hamiltonian of the anisotropic ferromagnet and apply widely used random phase approximation (RPA) (known also as Tyablikov approximation in the Green function method) which is more accurate than the well known molecular field approximation (MFA). It is shown that in the first case the magnitude of MCE is raised whereas in the second one the MCE disappears due to compensation of the critical field renormalized with the magnetization.

Phase transition of two-dimensional doped mixed-spin anisotropic ferromagnets

The two-dimensional doped mixed-spin anisotropic ferromagnets is investigated by means of the double-time Green׳s function based on the random phase approximation and Anderson–Callen׳s decoupling approximation. The analytic expressions of the critical temperature and the high-temperature zero-field susceptibility are obtained. The phase diagrams in which the critical temperature, the reorientation temperature and the reorientation magnetic field are shown as a function of single-ion anisotropic parameter are discussed.

Spin discrete breathers in two-dimensional square anisotropic ferromagnets

We present an analytical study of spin discrete breathers in two-dimensional ferromagnets with easy-plane anisotropy by means of the coherent-state method combined with the Dyson–Maleev representation of spin operators. Making use of the semidiscrete multiple-scale method, the equation of motion for the coherent amplitude is reduced to the two-dimensional nonlinear Schrödinger equation. We show that the system can produce the two-dimensional spin discrete breathers whose eigenfrequency lies above the top of the linear spin-wave spectrum. In addition, we analyze the existence conditions of such spin discrete breathers and discuss their properties.

Tuning of soft magnetic properties in FeCoHfO thin films by various sputtering power and pressure

FeCoHfO thin films with soft magnetic properties were deposited on Si (100) substrates using radio frequency reactive magnetron sputtering in Ar+O2 gases. During the deposition process, the partial pressure of oxygen was fixed at 3%. Sputtering power and sputtering pressure varied from 50 to 350 W and from 0·2 to 1 Pa respectively. In the condition of stationary sputtering gases pressure of 0·4 Pa, it was found that the film electrical resistivity ρ decreases steeply with the increasing sputtering power, and the natural ferromagnetic resonant frequency fr and static anisotropy field Hk increase with increase in power from 50 to 250 W, and then decrease when power exceeded 250 W. The coercivity of films had a contrary trend compared with fr. At 250 W sputtering powers, the optimal properties with fr≈3·02 GHz, saturation magnetisation 4πMs≈19·6 kGs, coercivity Hc≈2 Oe and ρ≈2200 μΩ cm were obtained. In the condition of fixed sputtering power of 250 W, fr, 4πMs and Hk decreased with the increases in sputtering pressure; Hc and ρ had an opposite trend. The physical nature of the effect was suggested to be related to the structure and composition changes in films.

Micromagnetic Simulation of the Depinning Field Domain Wall on Symmetric Double Notch Ferromagnetic Wires

In this paper, we investigate the depinning field domain wall on symmetric double notch ferromagnetic wires by means of micromagnetic simulation for Permalloy (Py), Cobalt (Co), and Nickel (Ni) materials. T he depinning field domain wall increases as the size of the notch decreases. At a lower depinning field, the domain wall inner structure exhibited a transverse wall (TW), while at a higher depinning field, there was a transformation of the domain wall inner structure from transverse wall to antivortex wall (AVW). We also observed that the magnetization energy increased as the size of the notch decreased. This means that more energy was needed to release the domain wall from a smaller notch. Micromagnetic simulation showed that the depinning field domain wall depends on the size of the notch and on the ferromagnetic anisotropy.

Modification of the ferromagnetic anisotropy and exchange bias field of NiFe/CoO/Co trilayers through the CoO spacer thicknesses

We have investigated the magnetism of NiFe/CoO/Co trilayers with different CoO spacer thicknesses. The dependence of the coercivity (Hc) and exchange bias field (Hex) on the CoO thicknesses indicated that different pinning strengths from the CoO were acting on the top NiFe and bottom Co layers, respectively. DC susceptibility indicated the different interlayer coupling energies and showed that the anisotropy of CoO layer strongly affected the temperature dependence of the magnetization.