Study of the morphological and magnetic microstructure of SmCo5 magnets

In this paper, high-coercivity sintered SmCo₅ permanent magnets were investigated. The study of the morphological microstructure was performed by scanning electron microscopy (SEM) using backscattered electron imaging and energy dispersive X-ray (EDX) microanalysis, and the magnetic microstructure was observed with magnetic force microscopy (MFM) in the thermally demagnetized state of the magnets at the surface perpendicular to the alignment axis. In addition to the main SmCo₅ phase, an appreciable amount of a mixture of the Sm₂Co₇ and Sm₅Co₁₉ phases, pores, Sm oxides and carbon was detected. The grains exhibited magnetic domains. Except for rare cases, the domains were continuous from grain to grain, indicating good magnetic alignment of the individual grains.     

SEM investigation of the magnetic domain structure of sintered SmCo5 permanent magnets

The paper presents scanning electron microscopy (SEM) study of the magnetic microstructure of anisotropic sintered SmCo₅ permanent magnets. Observations were made in the thermally demagnetized state of the magnets at the surfaces both perpendicular and parallel to the alignment axis. Magnetic domains were revealed using the technique of type-I magnetic contrast (for the first time) and the colloid-SEM method. The domain structure consists of main domains (which extend through the whole grain thickness) and surface domains of reverse magnetization (reverse spikes). The main domains form a maze pattern near the surface perpendicular to the alignment axis. The reason for the presence of the maze domain structure and reverse spikes at the surface perpendicular to the alignment axis is the reduction in the magnetostatic energy at the cost of a larger total Bloch wall area. Investigations carried out on the surface parallel to the alignment axis allowed to obtain much better insight into the orientation of grains.     

Effect of thermal stress on magnetic domain patterns of multiferroic Bi1-xDyxFeO3 thin films

Effect of temperature on magnetic domain structure of Bi_0.7Dy_0.3FeO₃ (BDFO) multiferroic thin films is studied in situ using magnetic force microscopy (MFM). Initially, as the temperature increases the domains start aligning from irregular to more distinct stripe pattern. However, above 250 °C, the domain alignment is disturbed. The systematic change in the domain configuration with temperature, suggests a strong thermal history of the system. The randomness in domain alignment caused above 250 °C is correlated to internal stress developed during ferromagnetic to paramagnetic phase transition occurring in BDFO. Indirect experimental evidence is given to support the explanation based on stress.     

快淬粉爆炸压制Nd-Fe-B永磁体的性能与结构特征

利用爆炸压制方法制备的快淬永磁体,其磁性能、压缩强度和密度都比相同粉料制备的粘结磁体有明显提高,其中最大磁能积（BH）max,提高了30％,压缩强度σbc增加了40%。扫描电镜观察显示,爆炸压制磁体的粉体颗粒表面出现了局部的熔融区域与大量的微裂纹。借助于场发射扫描电镜进一步观察发现,爆炸样品粉体颗粒表面出现纳米数量级的蜂窝状组织。磁力显微镜观察表明,爆炸压制不仅保持了原始粉末细小的晶粒尺寸,还保持了原始快淬粉细小的磁畴结构,磁体粉体颗粒中存在大量的微裂纹,但是微裂纹对快淬粉爆炸压制磁体的磁性能几乎没有影响。     

Magneto-microstructural coupling during stress-induced phase transformation in Co49Ni21Ga30 ferromagnetic shape memory alloy single crystals

The present study reports on direct magneto-microstructural observations made during the stress-induced martensitic transformation in Co₄₉Ni₂₁Ga₃₀ alloy single crystals with optical, scanning electron, and magnetic force microscopy (MFM). The evolution of the microstructure and the associated magnetic domain morphology as a function of applied strain were investigated in the as-grown condition and after thermo-mechanical training. The results demonstrated that the stress-induced martensite (SIM) evolves quite differently in the two conditions and depending on the martensite formation mechanisms, the magnetic domain configuration was dissimilar. In the as-grown crystals two twin-related martensite variants were formed and the growth of these twin variants resulted in large strain. After thermo-mechanical training a morphology similar to a self-accommodating martensite structure was present at the initial stages of the transformation and thereafter martensite reorientation (MR) was the main transformation mechanism. The magnetic domains were found to be superimposed on the nano-scaled martensite twins in the as-grown condition, whereas training brought about the formation of domains on the order of a few microns without showing the one-to-one correspondence between domains and the twin structure. After the thermo-mechanical training detwinning at high-strain levels led to the formation of stripe-like domain structures. The ramifications of the results with respect to the magneto-microstructural coupling that may cause the magnetic shape memory effect (MSME) in Co–Ni–Ga alloys under constant external stress is addressed.     

GISAXS studies of self-assembling of colloidal Co nanoparticles

In this work we report on the formation of ordered monolayers (2-D) and arrays of rods (3-D) of magnetic Co nanoparticles in magnetic field perpendicular to the substrate surface. Samples were prepared by drying a droplet of colloidal solution of Co nanoparticles (10 nm diameter) on Si/Si₃N₄ substrates in magnetic field between 0.2 and 0.9 T. The samples were characterized by high resolution scanning electron microscopy (SEM), atomic and magnetic force microscopy (AFM/MFM) and grazing incidence small angle X-ray scattering (GISAXS). SEM studies of monolayers show well-ordered 2-D arrays with hexagonal symmetry of 200 nm × 500 nm in size forming a mosaic structure. Rods, about 500 nm in diameter, aligned with the field direction and forming a hexagonal pattern were obtained when higher concentration of colloid and low evaporation rate of the solvent were used. The ordering of nanoparticles in the monolayer analyzed by GISAXS is described by the local order with hexagonal symmetry. The model of close packing of hard spheres is used for ordering of particles inside the rods. Magnetic features corresponding to the 3-D arrays have been observed by MFM pointing out that all magnetic moments in the rod are oriented along the field direction.     

Magnetic Domain Evolution in Sintered Nd–Fe–B Magnet during Magnetization Process

In the present study, the magnetic domains and their evolution during magnetization process have been investigated for sintered Nd–Fe–B permanent magnets with Kerr microscopy. Observation of the magnetic domain evolution process during magnetization process shows that some domain walls were pinned at the grain boundary area under magnetic field up to 5 kOe. It is suggested that magnetic interaction between individual Nd₂Fe₁₄B grains contacting to each other leads to appearance of small closed domains near the grain boundary area, which are responsible for the pinning effect.     

High resolution magnetic force microscopy of patterned L1(0)-FePt dot arrays by nanosphere lithography

High resolution magnetic force microscopy (MFM) has been carried out on L1(0)-FePt dot arrays patterned by plasma modified nanosphere lithography. An ex situ tip magnetization reversal experiment is carried out to determine the magnetic domains and verify the imaging stability of MFM and the mutual perturbations between the magnetic tip and the sample. We have identified that the critical size for the single domain region is about 90?nm across. Comparison with MFM image simulation also suggests that the magnetizations of the triangular dots in both single and double domain states are parallel to one edge of the dots, indicating the large uniaxial magnetocrystalline anisotropy of the L1(0)-FePt phase and the need for decreasing the magnetostatic energy.     

Dielectric properties of (Ba,Ca)(Zr,Ti)O3/CaRuO3 heterostructure thin films prepared by pulsed laser deposition

(Ba_0.90Ca_0.10)(Zr_0.25Ti_0.75)O₃ (BCZT) thin films were grown on Pt/Ti/SiO₂/Si substrates without and with a CaRuO₃ (CRO) buffer layer using pulsed laser deposition (PLD). The structure and surface morphology of the films have been characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). At room temperature and 1 MHz, the dependence of dielectric constant and tunability of the films with electric field were investigated; the dielectric constant and tunability are 725 and 47.0%, 877 and 50.4%, respectively, for the BCZT film on Pt/Ti/SiO₂/Si substrates without and with the CRO buffer layer at 400 kV/cm. The tunability of the BCZT/CRO heterostructure thin films on Pt/Ti/SiO₂/Si substrates was higher than that of the BCZT thin films on Pt/Ti/SiO₂/Si substrates. The high constant likely results from the oxide electrode (CRO).     

Investigation of antiperovskite Mn3CuNx film prepared by DC reactive magnetron sputtering

Antiperovskite Mn₃CuN_x film was prepared by dc reactive magnetron sputtering. It is the first time to report an antiperovskite ternary nitride film. The composition and crystal structure were characterized by energy dispersive spectroscope (EDS), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). From the XRD pattern, it displays a (1 0 0) preferential orientation. A comparative study on the properties of Mn₃CuN_x film and the bulk sample was presented. The film exhibits an antiferromagnetic to paramagnetic transition around 135 K, similar with the bulk sample. With temperature, the resistivity of the film shows semiconductor-like behavior throughout the measured temperature region, whereas there is an abrupt drop around the magnetic transition for the bulk. The variable temperature XRD results indicate that the film did not display any structure transition and shows a normal linear thermal expansion property around the magnetic transition.     

Study of magnetic properties of thin cobalt films deposited by chemical vapour deposition

Chemical Vapour Deposition (CVD) of cobalt was deposited from a liquid source precursor of cobalt tricarbonyl nitrosyl (Co(CO)₃NO) on to oxidised < 100 > silicon wafers. The cobalt layers were deposited at 450∘C at 1.5 torr chamber pressure of hydrogen for 15 min processing time with various precursor flow rates. X-ray diffraction studies of the cobalt films reveal both hcp and fcc peaks. The vibrating sample magnetometer (VSM) yields coercivity (Hc) 167 Oe and 364 Oe for 46 nm and 30 nm thickness layers respectively at room temperature and squareness (S) M_r/M_s (remanence/saturation of magnetisation) value of ∼ 1. The study of magnetic properties of the cobalt suggests that magnetisation is dependent on grain size and therefore thickness. The grain size was observed by atomic force microscopy (AFM). Magnetic images were observed by magnetic force microscopy (MFM) and analyzed in terms of domain structure. The surface domain structure was recorded with the tip lift height 100 nm so that the magnetic interactions arising produced the topography effect. Where there is repulsive interaction the intensity is recorded as a bright region and where the interaction is attractive the intensity is recorded as a dark region.     

Magnetic,morphological and structural investigations of CoFe/Si interfacial structures

CoFe/Si interfacial structures have been realised by electron beam evaporation of CoFe magnetic alloy on p- and n-Si substrates. These realised interfacial structures have been characterised from X-ray diffraction (XRD), atomic force microscopy (AFM), magnetic force microscopy (MFM) and magnetisation (M–H) characteristics. XRD data have shown the presence of CoFe (bcc phase) and β-FeSi₂ phases for CoFe/p-Si interfacial structures, whereas CoFe/n-Si interfacial structures have shown the presence of CoFe (bcc and fcc) phases along with Fe₃Si, ?-FeSi and β-FeSi₂ silicide phases having nanodimension crystallites. The M–H characteristics have shown the superparamagnetic type behaviour for CoFe/p-Si structure, whereas CoFe/n-Si structure shows a feature of interfacial antiferromagnetic (AF) coupling. The observed magnetic behaviour has been understood due to the presence of various magnetic phases and their nanosized grains. The MFM data of domain size have also been correlated with the observed magnetic behaviour. CoFe/n-Si structures have shown a significant feature of giant magnetoresistance (GMR) as compared to CoFe/p-Si structures. It has been found that CoFe/p-Si structures show a distinctly different behaviour than CoFe/n-Si structures for their chemical structure, morphology and magnetic behaviour.     

3D magnetic characterization of nanocrystalline cobalt thin films by combined magnetic force and Lorentz microscopy

Thin films of cobalt and cobalt-based compounds are recently popular for magnetic recording media because of their high recording density and great magnetic properties. Many techniques exist to image magnetic structures in thin films, nevertheless, none of them can furnish complete information about the magnetic details. In the present work the combined use of the information obtainable with Lorentz microscopy, performed in a transmission electron microscope (TEM), and of an atomic force microscope (AFM) working in the magnetic mode (MFM, magnetic force microscopy), both performed on the same specimen area, enabled, in a easy way, the study of the 3D magnetic structure of domains, of single cross-ties, the location of Bloch lines within a domain wall and the magnetic structure of magnetisation ripples. The 3D magnetic structure and contrast of nanocrystalline thin films of cobalt (100 nm thick), prepared by evaporation in high vacuum, were investigated at a spatial resolution of tens of nanometers.     

Study on microstructure and magnetic domain structure in sputtered (Ni66Fe22Co12) x C1−x nanocomposite films

Composition and annealing temperature dependence of microstructure and magnetic domain structure in sputtered (Ni₆₆Fe₂₂Co₁₂) _x C_1–x nanocomposite films with x=10–75 at % were studied by X-ray diffraction and magnetic force microscopy (MFM). Films with x20 at % showed amorphous structures, and no domain structure could be found due to the disappearance of magnetocrystalline anisotropy. For the films with x=30–55 at %, face-centered cubic (fcc) NiFeCo nanocrystals encapsulated in graphite-like carbon could be found in the samples annealed beyond 400 °C, and stripe domains with typical dimension of 120–150 nm were observed. For the films with x62 at %, the as-deposited films went through a meta-stable stage at which a rhombohedral Ni₃C phase and fcc NiFeCo co-existed after annealing to a temperature between about 300–400 °C (dependent on composition). Upon further annealing to a sufficiently high temperature between about 350–500 °C, the carbide phase decomposed into fcc NiFeCo and graphite. While short-range domain structures were observed in the samples before the formation of carbide phase, long-range domain structure with dispersed domains in the meta-stable stage were observed. After the decomposition of carbide, large domains with typical size of 500–700 nm were observed due to the formation of large grain aggregators.     

Direct and simultaneous observation of ferroelectric and magnetic domains

The ferroelectric/magnetic domain structure in a magnetoelectric binding BaTiO₃/Fe₈₁Ga₁₉ and the ferroelectric/crystallographic domain structure in a magnetoelectric binding PMN-34PT/Mn₅₀Ni₂₈Ga₂₂ were observed successfully by scanning electron acoustic microscopy (SEAM). Both the stripe ferroelectric domains in single crystals and the stripe magnetic domains in polycrystalline grains are obtained simultaneously, which exhibits that the scanning electron acoustic microscopy is a unique imaging technique. In addition, the experimental results show that the SEAM technique may be used in multiferroics to character two or more ferroic domains simultaneously. The imaging mechanisms of ferroelectric domains, magnetic domains, and crystallographic domains are attributed to piezoelectric coupling mechanism, magneto–elastic coupling mechanism, and thermo–wave coupling mechanism, respectively.     

Measurement of an Iso‐Curie Temperature Line of a Co?Cr?Mo Solid Solution by Magnetic Force Microscopy Imaging on a Diffusion Multiple

The 24 °C iso‐Curie temperature line of a Co? Cr? Mo fcc solid solution is obtained by performing magnetic force microscopy (MFM) imaging on solid solution compositions created in a diffusion multiple. The MFM imaging clearly reveals the boundary that separates the paramagnetic region without magnetic domains from the ferromagnetic region with domains. Compositional analysis along the boundary yields a constant Curie temperature (24 °C) composition line. Such a measurement is more efficient than one‐alloy‐at‐a‐time tests and can be used to screen new ferromagnetic materials.     

Strain Anisotropy and Magnetic Domains in Embedded Nanomagnets

Nanoscale modifications of strain and magnetic anisotropy can open pathways to engineering magnetic domains for device applications. A periodic magnetic domain structure can be stabilized in sub‐200 nm wide linear as well as curved magnets, embedded within a flat non‐ferromagnetic thin film. The nanomagnets are produced within a non‐ferromagnetic B2‐ordered Fe₆₀Al₄₀ thin film, where local irradiation by a focused ion beam causes the formation of disordered and strongly ferromagnetic regions of A2 Fe₆₀Al₄₀. An anisotropic lattice relaxation is observed, such that the in‐plane lattice parameter is larger when measured parallel to the magnet short‐axis as compared to its length. This in‐plane structural anisotropy manifests a magnetic anisotropy contribution, generating an easy‐axis parallel to the short axis. The competing effect of the strain and shape anisotropies stabilizes a periodic domain pattern in linear as well as spiral nanomagnets, providing a versatile and geometrically controllable path to engineering the strain and thereby the magnetic anisotropy at the nanoscale.     

Nanoscale magneto-structural coupling in as-deposited and freestanding single-crystalline Fe7Pd3 ferromagnetic shape memory alloy thin films

Ferromagnetic shape memory alloys are characterized by strong magneto-mechanical coupling occurring at the atomic scale causing large magnetically inducible strains at the macroscopic level. Employing combined atomic and magnetic force microscopy studies at variable temperature, we systematically explore the relation between the magnetic domain pattern and the underlying structure for as-deposited and freestanding single-crystalline Fe₇Pd₃ thin films across the martensite–austenite transition. We find experimental evidence that magnetic domain appearance is strongly affected by the presence and absence of nanotwinning. While the martensite–austenite transition upon temperature variation of as-deposited films is clearly reflected in topography by the presence and absence of a characteristic surface corrugation pattern, the magnetic domain pattern is hardly affected. These findings are discussed considering the impact of significant thermal stresses arising in the austenite phase. Freestanding martensitic films reveal a hierarchical structure of micro- and nanotwinning. The associated domain organization appears more complex, since the dominance of magnetic energy contributors alters within this length scale regime.     

Behavior of magnetic Fe3O4 nano-particles in magnetically assisted gas-fluidized beds

Improved magnetically assisted fluidization of Fe₃O₄ nano-particles by immersed coarse magnets has been investigated. The coarse magnetic particles work as aggregate breakers while the external field controls the fluidization of the nano-particles and the overall bed structure. The general outcome of this magnetically assisted fluidization techniques is that the enhancement of the bed internal structure and fluidization characteristics. Two basic cases of immersed bodies have been considered to improve the bed fluidization performance: coarse magnetic spheres as aggregate breakers and fine non-magnetic nano-particles forming an admixture and working as glidants.     

Magnetic force microscopy of superparamagnetic nanoparticles

The use of magnetic force microscopy (MFM) to detect probe-sample interactions from superparamagnetic nanoparticles in vitro in ambient atmospheric conditions is reported here. By using both magnetic and nonmagnetic probes in dynamic lift-mode imaging and by controlling the direction and magnitude of the external magnetic field applied to the samples, it is possible to detect and identify the presence of superparamagnetic nanoparticles. The experimental results shown here are in agreement with the estimated sensitivity of the MFM technique. The potential and challenges for localizing nanoscale magnetic domains in biological samples is discussed.