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Two-step grain boundary diffusion processes (GBDP) were carried out to obtain the diffused magnets with excellent magnetic properties and high thermal stability, using Dy and Al coated films as diffused sources in turn, prepared by magnetron sputtering. It is found that through the two-step diffusion with Dy/Al films, the highest coercivity, up to 1382 kA/m is achieved, increasing by 42.3% compared to as-prepared magnet, moreover, the maximum energy product is improved significantly as well. Besides, whether for as-prepared magnets or one-step diffused magnets with Al film and Dy film, respectively, the thermal stability and magnetic properties of two-step diffused magnet with Dy/Al film, are the highest among magnets. Microstructural investigation reveals that Dy is preferred to diffuse along the RE-rich phase and forming the “shell” of (Nd, Pr, Dy)2Fe14B phase around the outer region of main phase, restraining the nucleation of anti-magnetization domain, while the Al tends to locate in the RE-rich region, playing the role of modifying the interface between RE-rich phase and 2:14:1 main phase. Eventually, the temperature coefficient of coercivity (|β|) and remanence (|α|) with 0.65%/K and 0.12%/K, can be obtained, respectively.

Elements of alnico 8 are added into Nd-Fe-B alloys fabricated by rapid solidification method. It is observed that the magnetic properties at high temperature improved by small addition of alnico elements. The Curie temperature of the alloys increased from 580 K for standard alloy to 639 K by 20% addition of alnico 8 elements. The spin reorientation temperature decreased from 133 K to 104 K. The TEM analysis showed that elements of alnico 8 refine the microstructure of Nd-Fe-B ribbons. The STEM analysis confirmed the heterogeneous distribution of Nd, Fe, Cu, Al, Ni and homogeneous distribution of Ti, Nb and Co. The boundaries of nano grains contain more than 70% ferromagnetic elements, ensuring strong inter-grain coupling among the grains.

Alnico 8 elements are added into Ce-Fe-B ribbons and the effects of the multi elements on the magnetic properties, phase transition temperatures and microstructure are investigated. It is found that the addition of alnico elements is helpful for enhancing the magnetic properties of the Ce-Fe-B alloys. The Curie temperature of Ce2Fe14B (2:14:1) main phase increased from 422 K for the standard alloy to 490 K by 20 wt% addition of alnico, while the Curie temperature of CeFe2 (1:2) phase decreased from 223 K to 122 K. The transmission electron microscopy (TEM) analysis showed that the nano grains of the alloys are refined which is attributed to the refractory character of Ti. The intergranular exchange interactions measured by applying Henkel plots also enhanced by adding alnico elements into Ce-Fe-B ribbons. The magnetic properties Hcj, Br and (BH)max improved significantly for the optimized addition of alnico. Magnetic properties of Hcj = 5.16 kOe, Br = 4.59 kG and (BH)max = 4.14 MGOe are obtained in Ce35Fe56.5(alnico)7.5B ribbon. The improved magnetic properties in alnico-doped Ce-Fe-B alloys are attributed to the refined microstructure and enhanced intergranular exchange interactions.

The microstructure, crystalline structure, martensitic transformation, and magnetocaloric effect are systematically studied in all-d-metal Ni36.0Co14.0Mn35.7Ti14.3 alloy ribbons fabricated by melt-spinning technique. Five-layer modulated martensite (5 M) and B2 austenite coexist in both melt-spun and annealed ribbons. Interestingly, all ribbons show a staircase-like magnetic-field-induced metamagnetic reverse martensitic transformation. Accordingly, large magnetocaloric effect near room temperature is obtained, including large magnetic entropy changes of 10 and 13 Jkg−1K−1 and large effective refrigerant capacity of 62 and 63 Jkg-1 under the field change of Δμ0H = 0–2 T, for melt-spun and annealed ribbons, respectively, making the all-d-metal Ni-Mn-based alloy ribbons promising for room temperature magnetic refrigeration.

In this paper, FeCo and FeCo@C nanoparticles are synthesized by using plasma arc-discharging technology. FeCo@C nanoparticles are fabricated by particularly applying methane as a carbon source in the plasma arc-discharging facility. The influences of core-shell nanostructure on magnetic and absorbing properties are investigated. Compared with FeCo nanoparticles, FeCo@C nanoparticles in a typical core-shell structure exhibit lower saturation magnetization, excellent impedance matching and reflection loss (RL) in the GHz frequency range. The optimal RL of −16.8 dB is obtained at 9.9 GHz with the absorber thickness of 3.0 mm. The excellent microwave absorption is attributed to the effective complementarities between dielectric loss and magnetic loss in FeCo@C core-shell nanoparticles.

Corrosion behavior of a material is complicated and can affect strongly its surface physical and chemical properties. In this study, the effect of corrosion on surface morphology and electron work function (EWF) of various Al alloys was comparatively investigated using a scanning electron microscopy and a scanning Kelvin probe. The experimental results showed that the change in EWF with corrosion time was not linear, but firstly increased and then decreased. The theoretical treatments demonstrated that such a change in EWF did not only depend on the surface roughness, but also correlate with oxide layers formed at the surfaces of Al alloys. The present results can also be used to reasonably explain the correlation between surface roughness and EWF reported previously.

It is very important to understand the interfacial behavior for effective evaluation of welding quality. This letter reports a simple nondestructive characterization method of the interfacial strength of dissimilar metallic joints. Using a scanning Kelvin probe, the electron work function of Cu–steel joints was measured and a strong correlation between the electron work function and the interfacial tensile strength was found.

In Ni-Mn-X (X =In, Sn, Sb) ferromagnetic shape memory alloys, a ferromagnetic transition from paramagnetic to ferromagnetic austenite and a martensitic transformation from ferromagnetic austenite to weak magnetic martensite occur in some particular composition ranges, in which abundant physical properties have been observed by the abrupt change of magnetization and resistivity around their transition temperatures in these alloys. Therefore, tuning the martensitic transformation temperature (TM) and enlarging the working-temperature interval for Ni-Mn-X (X=In, Sn, Sb) alloys, are of great importance. In the present paper, we will focus on the effect of external factors, including pre-deformation, annealing, and high pressure annealing, on the magnetic transitions and the related magnetocaloric properties in Ni-Mn-Co-Sn ferromagnetic shape memory alloys. Our approaches and the main results in this particular field will be reviewed.

Nd2Fe14B-type permanent magnets have been widely applied in various fields such as wind power, voice coil motors, and medical instruments. The large temperature dependence of coercivity, however, limits their further applications. We have systematically investigated the magnetic properties, thermal stabilities and coercivity mechanisms of the (Pr0.2Nd0.8)13Fe81-xB6Hfx (x=0, 0.5) nanocrystalline magnets fabricated by a spark plasma sintering (SPS) technique. The results indicate that the influence of Hf addition is significant on magnetic properties and thermal stabilities of the (PrNd)2Fe14B-type sintered magnets. It is shown that the sample with x = 0.5 at 300 K has much higher coercivity and remanent magnetization than those counterparts without Hf. The temperature coefficients of remanence (α) and coercivity (β) of the (Pr0.2Nd0.8)13Fe81-xB6Hfx magnets are improved significantly from -0.23 %/K, -0.57 %/K for the sample at x = 0 to -0.17 %/K, -0.49 %/K for the sample at x = 0.5 in the temperature range of 300-400 K. Furthermore, it is found out that the domain wall pinning mechanism is more likely responsible for enhancing the coercivity of the (Pr0.2Nd0.8)13Fe81-xB6Hfx magnets.

MnBi magnets with a high content of low temperature phase (LTP) and excellent magnetic properties were prepared by spark plasma sintering (SPS) using ball milling powders as precursors without magnetic purification. A complicated intergranular phase, which contains Mn phase, Bi phase, MnO phase, and even amorphous phase in MnBi magnets, was characterized and reported systematically. It was found that the formation of intergranular phase which was contributed by ball milling precursors and sintering mechanism, jointly, had important influence on the magnetic properties. The appropriate content of intergranular phase was beneficial in improving the coercivity due to the strong magnetic isolation effects. The optimum magnetic properties with Mr=26.0 emu/g, Hci= 7.11 kOe and (BH)max=1.53 MGOe at room temperature, and a maximum value Hci= 25.37 kOe at 550 K can be obtained. Strongly favorable magnetic properties make SPSed MnBi magnets an attractive candidate material for small permanent magnets used in high-temperature applications.

Diffusion process by Nd-Al and Pr-Al alloys was compared and investigated in Nd-Fe-B thin films. Enhanced coercivity 2.06T and good squareness was obtained by using Pr85Al15 and Nd85Al15 alloys as diffusion sources. But the coercivity of diffusion-processed thin films by Pr70Al30 and Pr55Al45 alloys decreased to 2.04T and 1.82T. High ambient coercivity of 2.26T was achieved in diffusion-processed thin film by Nd70Al30 leading to an improved coercivity thermal stability because Nd2Fe14B grains were enveloped by Nd-rich phase as seen by transmission electron microscopy Nd-loss image. Meanwhile, microstructure-dependent parameters α and Neff were improved. However, high content of Al in diffusion-processed thin film by Nd55Al45 lead to degraded texture and coercivity.

The microstructure, magnetic and magnetocaloric properties are investigated in the melt-spun and annealed Ni42.9Co6.9Mn38.3Sn11.9 ribbons. The columnar grains grow perpendicular to ribbon surfaces. After annealing, the grain size increases greatly. Meanwhile, the parent phase is suppressed and therefore L10 martensite predominates, indicating obvious shift of martensitic transformation to high temperature. More interestingly, the martensite variants are distinctly observed on the fractured cross-section of annealed ribbons, not just on the free surface in general. The significant enhancement of magnetic entropy change and effective refrigerant capacities with relatively smaller thermal hysteresis make annealed ribbons potential candidate in magnetic refrigeration around room temperature.

The antiferromagnetic (AFM)-ferromagnetic (FM) conversion in martensite was observed in Mn/Ni-substitution upon FM elements, such as Fe or Co, in MnNiGe helical antiferromagnets. Here, we report an AFM-FM conversion and consequently a sharp magnetic-field-driven metamagnetic martensitic transformation from paramagnetic (PM) austenite to FM martensite in the Ni- and Mn-substituted MnNiGe alloys with indium, a non-magnetic and large-sized main group element. Accordingly, a giant magnetocaloric effect such that a twofold increase of the magnetic entropy change in MnNi0.92GeIn0.08 and even a nearly threefold increase in the Mn0.92NiGeIn0.08 alloy is obtained with respect to the MnNiGe0.95In0.05 alloy. The origin of AFM-FM conversion and resultantly sharp magnetic-field-induced PM-FM metamagnetic transformation is discussed based on the first-principles calculations and X-ray absorption spectroscopic results.

The sharp metamagnetic martensitic transformation (MMT) triggered by a low critical field plays a pivotal role in magnetoresponsive effects for ferromagnetic shape memory alloys (FSMAs). Here, a sharper magnetic-field-induced metamagnetic martensitic transformation (MFIMMT) is realized in Mn1−xCo1+xGe systems with a giant magnetocaloric effect around room temperature, which represents the lowest magnetic driving and completion fields as well as the largest magnetization difference around MFIMMT reported heretofore in MnCoGe-based FSMAs. More interestingly, a reversible MFIMMT with field cycling is observed in the Mn0.965Co0.035Ge compound. These results indicate that the consensus would be broken that the magnetic field is difficult to trigger the MMT for MnCoGe-based systems. The origin of a higher degree of sensitivity of martensitic transformation to the magnetic field is discussed based on the X-ray absorption spectroscopic results.

We have set up successfully two experimental systems during the past time of the project. The first system is sol-gel chemical method for preparing {gamma}-Al{sub 2}O{sub 3}, SiO{sub 2}, Cr{sub 2}O{sub 3} granular support particles. The second system is the laser-induced solution deposition (LISD) technique for nanoparticle catalysts containing Fe/Cu, and Co/Cu on the granular support. We have successfully deposited {gamma}-Al{sub 2}O{sub 3}, SiO{sub 2}, Cr{sub 2}O{sub 3} granular support particles by sol-gel method and Co and CoO nanoparticles by LISD novel fabrication technique. The characterization methods we have used include scanning electron microscope (SEM), high-resolution transmission electron microscope (HRTEM) and X-Ray diffraction (XRD). The research toward to the proposed direction is in good progress. We have given three presentations in national and local materials meetings and have submitted another two papers in another two key national meetings in nanotechnology and American Physical Annual March Meeting 2002. A couple of papers are in preparation.

Two Mn-Ni-Fe-Ge ribbon samples are prepared. The ribbons reveal the complicated magnetization process with the variation of temperature and magnetic field. Strikingly, the antiferromagnetic-ferromagnetic conversion and competition are observed in the TiNiSi-type phase in these ribbons. The required field to complete the conversion decreases with increasing temperature. When completing conversion, the coupled magnetostructural transformation from the ferromagnetic TiNiSi-type to paramagnetic Ni2In-type state with large magnetic entropy changes near room temperature is achieved for these ribbons. However, it is found that the magnetic field would be difficult to drive ferromagnetic-paramagnetic magnetostructural transformation for these ribbons unlike temperature.

Laser Induced Solution Deposition (LISD) is essentially a laser chemical processing with similar mechanism of the counterpart – laser chemical vapor deposition (LCVD). It is a novel method for synthesizing nanocsale particles or thin films with the combined advantages of laser chemical vapor deposition and electroless chemical deposition in solution (electrolyte). It is simple and efficient, and it can produce high quality nanoparticles or patterned uniform thin films. In this paper the nanostructured Co/CoOx particles have been fabricated by LISD technique. We have characterized the deposited materials by using scanning electron microscope (SEM), the high-resolution transmission electron microscope (HRTEM) and X-ray diffraction technique (XRD). We have found that the sizes of the deposited particles are smaller than 5 nm uniformly suspended in the solution used in the LISD deposition, and the sizes are smaller than 500 nm on the silicon substrates inserted in the deposited solution. The results by energy dispersive X-ray analysis (EDX) have indicated that there are Co peaks and O peaks, which meant there were oxygen contamination or the deposited products were cobalt oxide rather than pure cobalt nanoparticles. The X-ray diffraction has shown that we have obtained pure Co nanoparticles in some cases but we have obtained cobalt oxides in other cases, which mainly depended on the experimental conditions such as the selection of solvents and solutes as well as the selection of the lasers including the wavelength and the power of the laser beams. The studies of the magnetic properties, catalytic properties as well as their relationship of pure cobalt nanoparticles and cobalt oxide nanoparticles are in good progress and we will publish the useful results elsewhere.

Well-dispersed nanoscale textured chromium oxide particles/thin films can be fabricated under certain conditions by laser-induced solution deposition (LISD) from organic solutions and by using selective organometallic chemical vapor deposition (OMCVD). The fabricated nanoparticles/thin films are characterized by scanning electron microscope (SEM), EDX, X-ray diffraction, and magnetic measurements. We have successfully demonstrated that the LISD and OMCVD are unique techniques for fabricating uniformly-distributed thin films but anistropic chromium oxide particles, which can be used in electro-magnetic devices. The magnetization measurements show that both types of chromium oxides are presented and that the Curie temperature Tc and the saturation magnetization field may be adjustable by controlling the stoichiometry.

We have demonstrated that well-dispersed sub-micron chromium oxide particles can be fabricated by laser-induced solution deposition (LISD) from the solution of CrCl2 solute dissolved in organic solvents containing THF and cyclohexane. The particles are uniform in composition and contain little contamination. The distribution of the particles is narrow in three sizes: 230 nm, 350 nm and 400 nm. We have discussed the kinetics of forming these three kinds of sub-micron particles and the difficulty in obtaining single-phase chromium oxide. We have successfully shown that LISD is a unique technique for fabricating submicron chromium oxide particles with uniform composition and controllable size with narrow particle size distribution. The kinetics (nucleation and growth mechanism) of forming three sizes of particles is discussed.

Magnetic and transitional behaviors are investigated in Mn1.9Co0.1Sb intermetallic compound. Mn2Sb is a simple ferrimagnet with Neel temperature around 550 K. The Co-introduction results in the appearance of antiferromagnetic state, and therefore, the first-order magnetic transition occurs between antiferromagnetic and ferrimagnetic state, which can be induced by temperature/magnetic field, in Mn1.9Co0.1Sb alloy. Accordingly, a magnetic entropy change as high as 5 J kg−1 K−1 and a large magnetoresistance of 46% under the field change of 10 and 50 kOe are achieved, respectively.

Magnetic and resistance measurements have been carried out on a Ni43Mn41Co5Sn11 alloy annealed under high-pressure. Due to the existence of the intermediate phase, a change of slope in the temperature dependence of resistivity curves has been observed prior to the martensitic transformation for the high-pressure annealed alloy. As a consequence, two successive magnetoresistance peaks with the same sign are achieved around room-temperature, corresponding to the martensitic transformation and intermediate phase transition, respectively. The origin of the peculiarity of magnetoresistance properties in high-pressure annealed Ni43Mn41Co5Sn11 alloy is discussed.