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The effects of chemical disorder on the transport properties of the spin-filter material CrVTiAl are investigated experimentally and theoretically. Synchrotron X-ray diffraction experiments on bulk CrVTiAl and the associated Rietveld analysis indicate that the crystal structure consists primarily of a mixture of a partially ordered B2 phase, a fully disordered A2 phase and a small component of an ordered L2\textsubscript{1} or Y phase. High temperature resistivity measurements confirm the existence of a band gap. First-principles, all-electron, self-consistent electronic structure computations show that the chemically disordered A2 and B2 phases are metallic, while the spin-filter properties of the ideal Y-type phase are preserved in the presence of L2\textsubscript{1} disorder. The Hall coefficient is found to decrease with increasing temperature, similar to the measured increase in the conductivity, indicating the presence of thermally activated semiconductor-like carriers., Accepted to Journal of Applied Physics

We report detailed measurements of the magnetic behavior of La1−xBaxMnO3 bulk samples over a wide range of composition (0⩽x⩽1), temperature (4.2–775 K), and magnetic field strength (1 G–50 kG). The end members of this series namely LaMnO3+δ (with δ⩾0) and BaMnO3 crystallize into pseudocubic perovskite and hexagonal lattices, respectively. The perovskite structure is stable for x⩽0.35 while a phase separation into La0.65Ba0.35MnO3 and BaMnO3 occurs for x>0.35. The ordered moment on LaMnO3+δ depends sensitively on the processing conditions and the lowest value of 0.006 μB per Mn ion at 4.2 K is realized in the antiferromagnetic compound of δ=0. The complex magnetic behavior of BaMnO3 suggests antiferromagnetic ordering below ∼150 K. The Ba-substituted compounds order ferromagnetically with a monotonic increase of TC and saturation magnetization Ms with x

The microstructure and the room-temperature hysteretic magnetic properties of sputtered, 10 nm thin films of equiatomic binary alloys of CoPt and FePt were characterized using transmission electron microscopy (TEM) and a superconducting quantum interference device (SQUID) magnetometer. A transformation from an atomically disordered, face-centered-cubic structure to the L10 ordered structure occurred during postdeposition annealing and was characterized using digital analysis of dark-field TEM images. The transformation was observed to follow first-order nucleation and growth kinetics, and the ordered volume fraction transformed was quantified at numerous points during the transformation. The ordered volume fraction was then compared to the magnetic coercivity data obtained from the SQUID magnetometer. In contrast to the relationship most commonly described in the literature, that the highest coercivity corresponds to a two phase ordered/disordered mixture, the maximum value for coercivity in this study wa...

Several permanent magnet alloys based on the ternary Nd2Fe14B (2-14-1) composition have been prepared by inert gas atomization (IGA). The microstructure and magnetic properties of these alloys have been studied as a function of particle size, both before and after heat treatment. Different particle sizes have characteristic properties due to the differences in cooling rate experienced during solidification from the melt. These properties are also strongly dependent on the alloy composition due to the cooling rate’s effect on the development of the phase structure; the use of rare earth rich compositions appears necessary to compensate for a generally inadequate cooling rate. After atomization, a brief heat treatment is necessary for the development of the optimal microstructure and magnetic properties, as seen from the hysteresis loop shape and improvements in key magnetic parameters (intrinsic coercivity Hci, remanence Br, and maximum energy product BHmax). By adjusting alloy compositions specifically fo...

Melt‐quenched and thermomechanically deformed samples with the nominal compositions Pr18Co82, Pr18Co81C, and Pr18Co76C6 were examined with optical and electron microscopy, differential thermal analysis, and superconducting quantum interference device magnetometry that was performed in the temperature range 20 K≤T≤300 K. At room temperature, Pr18Co82 exhibits poor coercivity and remanence, 600 Oe and 2.9 kG, respectively. Pr18Co81C exhibits relatively superior remanence, 7.7 kG, but poor coercivity, 2.7 kOe, while Pr18Co76C6 exhibits the opposite trend, BR=4.5 kG and Hci=12 kOe. The main phase present in all samples, PrCo5, has basically the same character and morphology for all three samples and shows no evidence of intragranular carbon, which is demonstrated to reside in the impurity phases. The superior coercivity found in Pr18Co76C6 is attributed to a previously unknown triclinic phase, Pr3Co4Cx (x≊3–4) that appears to undergo a magnetic–nonmagnetic transition with increasing temperature around T=80 K....

A first-order phase transition in mechanically soft, nonstoichiometric Ni2MnGa oriented single crystals was achieved by the application of a magnetic field lower than 1 T on a sample with a forward martensitic transformation temperature TM of 314 K. The observed change in magnetization corresponds to a temperature decrease of 1 K. The entropy variation, corresponding to this partial phase rearrangement, is of the order of −1.5 J/kg K at 1 T, and reaches a value of −18 J/kg K at 5 T.

The order parameters and anisotropy constants of a series of epitaxial, L1{sub o} FePt films with compositions in the range of 45-55 at% Fe and nominal thicknesses of 50 nm have been characterized. The films were made by cosputtering the elements onto single crystal MgO(001) substrates. The substrates were coated with 1 nm Pt/1 nm Fe bilayer seeds prior to alloy deposition. Both the bilayer seed and the alloy film were deposited at 620 C. Lattice and order parameters were obtained by x-ray diffraction. Film compositions and thicknesses were determined by Rutherford Backscattering Spectrometry, and room-temperature magnetocrystalline anisotropies were determined with a torque magnetometer. It is found that the order parameter has a maximum for the film composition closest to the equiatomic composition, whereas the magnetocrystalline anisotropy increases as the Fe content is increased from below to slightly above the equiatomic composition. These results imply that non-stoichiometric FePt compositions, with a slight excess of Fe, may in fact be preferred for applications that require high anisotropy.

100 nm coatings of Fe and of Al were found to alter the magnetic attributes, such as the initial susceptibility and metamagnetic transition field, of the giant magnetocaloric material Gd5(Ge,Si)4 and thereby increase the entropy change for a given applied magnetic field, relative to uncoated samples. The experimental data strongly suggest that these coatings confer a strain that propagates through the material and enhances the effect of the magnetic field in the metamagnetic phase transition range. A simplified Landau theory of coupled magnetic and structural transitions was developed for Gd5(Si1−xGex)4 and used to analyze the thermodynamics and phase transitions of this material under the influence of magnetic fields and elastic strain fields.

A large positive magnetic entropy change ΔSM is observed in the metallic antiperovskite Mn3GaC near its first-order metamagnetic transition temperature 159 K where the stoichiometric compound transforms from an antiferromagnetic to a canted ferromagnetic state accompanied by a discontinuous volume change of −0.46% without change of symmetry. The unusual field dependence of the ΔSM of Mn3GaC shows a very rapid linear increase from zero to a saturation value within a field interval smaller than 1 T. The broadening of the peak of ΔSM to low temperature with increasing field change creates an ΔSM plateau in the temperature dependence of the MCE, which is of significance for practical application of these materials in the Ericsson-cycle magnetic refrigerator.

Nanocomposite films of Ni80Fe20/NixFe1−xO were prepared by a dual ion-beam deposition technique. The structural and magnetic properties of nanocomposite films fabricated with oxygen content in the deposition assist beam ranging from 0% to 55% were studied. The dependence of the resistivity on oxygen percent shows that the compositions with exchange-enhanced coercivity are close to a percolation threshold. A strong temperature dependence of coercivity Hc and exchange bias field Hex is found in these composite films. Films prepared with 46% O2 in the assist beam exhibit an enhanced Hc relative to Permalloy (Ni80Fe20) and a characteristic shifted hysteresis loop indicative of exchange coupling between the constituent metal and oxide phases. At T=10 K, films prepared with 44% O2 in the assist beam have an exchange shift Hex∼−225 Oe with a blocking temperature TB∼100 K that reflects the low Neel temperatures of FeO-rich NixFe1−xO solid solutions.

Exchange coupling across the interface in polycrystalline exchange-spring thin films of L10 CoPt and hcp Co has been investigated with magnetic measurements and micromagnetic modeling. For thick enough Co films, the reversal in the second quadrant of the hysteresis loop is multiphase, with full reversal of the soft Co phase accomplished before reversal of the hard CoPt phase. Therefore the exchange field acting on the soft phase is determined as the field of maximum susceptibility in the soft-phase reversal portion of the second quadrant. Experimentally, it is found that the exchange field is a linear function of the inverse thickness of the soft phase. This result is in agreement with the exchange field calculated from the associated micromagnetic modeling only under the condition that the magnitude of the exchange coupling constant between spins across the CoPt-Co interface is similar to that characterizing the bulk CoPt. Experimental and modeling-derived results do not agree for decreased interfacial e...

Experimental studies were performed on model bilayers of fully ordered L10 CoPt with thicknesses of 25, 50, or 100 nm, capped by Co of varying thicknesses from 2.6–75 nm. The bilayers were subjected to different annealing conditions; transmission electron microscopy and x-ray diffraction supplied data concerning the crystallographic structure, microstructure, and texture of the constituent phases in the bilayers. The degree of exchange coupling was investigated primarily with magnetic measurement and magnetic force microscopy. Samples subjected to annealing treatments at 300 °C showed evidence of grain growth and increased texture of the hexagonally close-packed (hcp) cobalt layer relative to the as-deposited condition. The degree of exchange coupling in these materials improved only slightly with annealing, as evidenced by both magnetic recoil curve measurement and magnetic force microscopy. Samples subjected to a higher-temperature annealing treatment of 550 °C evidenced dramatic improvement in the magn...

DC demagnetization (DCD), isothermal remanent magnetization (IRM), and reversible magnetization measurements were used to examine the magnetic reversal behavior in melt-quenched samples of composition Nd13.9(Fe0.92Co0.08)80.3B5.3Ga0.5 subjected to different levels of thermomechanical deformation (die upsetting). The reversible magnetization measurements indicate that in the thermally demagnetized condition domain walls are present within the material for all levels of deformation. During demagnetization from a saturated state, domain walls are present in a minority of grains at low levels of deformation but these domain walls disappear at higher levels of deformation. This behavior is explained in terms of changes in the relative strengths of nucleation and pinning fields. Comparison of these results to previous results obtained on similar materials indicate that changes in the magnetization behavior with degree of deformation are more marked in samples which are free of Ga, indicating that Ga additions a...

To understand the effects of nucleation fields and intergranular dipolar interactions on the magnetization reversal mechanism, recoil curves from the major hysteresis loop have been measured on a sample of sintered PrFeB as a function of temperature from 150 to 300 K. At room temperature the reversible magnetization behavior indicates a reversal mechanism of nucleation of domain walls whose motion after nucleation is resisted by dipolar fields. As the temperature is reduced, the coercivity, and hence the nucleation field, is observed to increase while the dipolar fields, dependent on microstructure and saturation magnetization, remain approximately constant. These temperature-dependent changes in the relative magnitudes of the dipolar field and nucleation field cause the reversible magnetization behavior to change from domain wall motion to rotation. This change in behavior is attributed to the supposition that at temperatures where the nucleation field exceeds the dipolar field, once nucleated, domain walls are swept out of the material.

A model has been developed to examine the effect that different distributions of pinning and nucleation fields have upon the reversible magnetization behavior. The model allows each grain to be in one of three states, saturated in the positive and negative directions or containing a domain wall. The nucleation and pinning field distributions determine the rate of change of the number of grains in the three states with field. The results indicate that reversible magnetization curves display a well-defined minimum where the domain wall pinning fields exceed the reverse domain nucleation fields. Where the domain wall nucleation fields exceed the domain wall pinning fields no minimum is seen. The exact shape of the reversible magnetization curve and the position of the minimum are indicative of the dynamics of the reversal process. This model is able to reproduce the form of reversible magnetization curves measured previously in a sintered isotropic NdFeB based magnet and a Sm2(Co, Fe, Cu, Zr)17 type magnet.

Elevated-temperature superconducting quantum interference device magnetometry was employed to investigate the detailed phase constitution of nominally glassy stoichiometric Nd2Fe14B melt-spun ribbons modified with 6 wt % Ti/C and subjected to various degrees of quenching by circumferential wheel speed variations of 40, 35, and 20 m/s. While previous microstructural characterization indicated that these materials were completely amorphous, ac susceptibility and magnetization measurements analyzed within the framework of superparamagnetism indicates that the materials actually contain a concentration of crystalline phase clusters of Nd2Fe14B and α-Fe that remained undetected by previous microstructural characterization due to their small size (diameter

Thin film CoPt/Co bilayers have been prepared as a model system to investigate the relationship between microstructure and exchange coupling in two-phase nanocomposite permanent magnets. The bilayers were prepared by magnetron sputter deposition of near-equiatomic CoPt with a thickness of 25 nm onto oxidized Si wafers. In the as-deposited state, CoPt had the A1 (fcc) structure and was magnetically soft. Before reinsertion into the sputtering chamber for the deposition of 2.8–16.7 nm thick Co layers, the CoPt films were annealed at 700 °C for 120 min to produce the magnetically hard, fully ordered L10 phase. The presence of exchange coupling in the bilayers was verified by magnetic hysteresis and recoil measurements and showed that only for Co thicknesses below 6.3 nm was this layer (in its as-deposited state) coupled through its full thickness to the CoPt layer. Annealing the bilayer samples at 300 and 550 °C for 20 min resulted in improvement of the interlayer magnetic coupling and produced clear differe...

Magnetic nanocomposite alloys developed from amorphous precursors by annealing often exhibit metastable transition phases that ultimately control the kinetics of the nucleation and growth of the desired RE2Fe14B-type phase. It is thus important to understand the effect of starting composition on the formation of the metastable transition phases found in the nanocomposite system. The relationships between composition and the nature of metastable phase formation in the low-boron content melt-spun nanocomposite alloy Nd2[Co0.06(Fe1−xCrx)0.94]23.2B1.48 (0⩽x

Positron annihilation spectroscopy utilizing the Doppler broadening of the annihilation radiation (DBAR) method was performed on stoichiometric Nd2Fe14B ribbons containing 0, 2, and 6 wt % Ti–C that were melt spun to the x-ray amorphous state at a wheel speed of 40 m/s. DBAR data indicate that the as-spun samples are very homogeneous. It is found that an increase in the concentration of alloying additions promotes an increase in the shape parameter (“S parameter”) of the symmetric Doppler-broadened positron–electron annihilation peak. This increase in the S parameter may be interpreted as an increase in the amount of open volume in the quenched glass. The presence of increased open volume is concurrent with an increased stability of the glass against crystallization. However, at the present time it is not clear if the increased thermal stability is a direct product of the increased open volume, or if stereochemical considerations determine the stability of the glass.

FeNi having the tetragonal L10 crystal structure is a promising new rare-earth-free permanent magnet material. Laboratory synthesis is challenging, however, tetragonal L10 FeNi—the mineral “tetrataenite”—has been characterized using specimens found in nickel-iron meteorites. Most notably, the meteorite NWA 6259 recovered from Northwest Africa is 95 vol. % tetrataenite with a composition of 43 at. % Ni. Hysteresis loops were measured as a function of sample orientation on a specimen cut from NWA 6259 in order to rigorously deduce the intrinsic hard magnetic properties of its L10 phase. Electron backscatter diffraction showed that NWA 6259 is strongly textured, containing L10 grains oriented along any one of the three equivalent cubic directions of the parent fcc structure. The magnetic structure was modeled as a superposition of the three orthonormal uniaxial variants. By simultaneously fitting first-quadrant magnetization data for 13 different orientations of the sample with respect to the applied field d...

Rapidly solidified samples of Nd9.5Fe84.5B6 with and without 3 at. % TiC were prepared by melt spinning and melt extraction and then annealed in vacuum (3×10−6 Torr) at temperatures from 600 to 750 °C. For alloys melt spun under similar conditions, the overquenched state was achieved at wheel speeds >10 m/s for the TiC added alloy while >20 m/s was necessary without TiC. The overquenched samples contained a smaller fraction of α-Fe in smaller grains than the undercooled samples where Fe dendrites formed near the free surface during solidification. These Fe dendrites were not removed by annealing. In addition, large orientated 2-14-1 grains nucleated on the Fe dendrites. This combination is detrimental to the magnetic properties. The addition of TiC results in improved control of the microstructure over a larger fraction of the ribbon volume enhancing the magnetic properties.

To elucidate the interphase interactions inherent to nanocomposite magnetic alloys, measurements of remanence Br, and coercivity Hci were made on a series of four meltspun, remanence-enhanced nanocomposite alloys consisting solely of various amounts of Nd2Fe14B and α-Fe. The phase constitution and microstructural scale of the alloys were characterized with synchrotron x-ray diffraction. Magnetic measurements were made using superconducting quantum interference device (SQUID) magnetometry on evacuated and encapsulated samples in the temperature range of 300 K⩽T⩽750 K, in order to characterize the α-Fe component independently of the Nd2Fe14B component. The high-temperature coercivities of the samples increase with the amount of α-Fe present in the samples, ranging from an average value of approximately 75 Oe for the sample with 14 wt % excess Fe to over 400 Oe at 700 K for the sample with 27 wt % excess Fe. The relatively high coercivities of the samples found at elevated temperatures imply that a tabular m...

The stray flux manifestations of surface magnetic domains found in as-grown Nd2Fe14B single crystals were observed, with a resolution in the range of 1 μm, using conventional scanning electron microscopy (SEM) without instrumental modifications. A modified image-distortion mode was applied to image the three-dimensional stray flux emanating from the sample. The simplicity of the technique and the ready adaptability of the SEM to such modifications as in situ current and magnetic field application suggest that the results of this study may be extended to investigations of other materials of technological interest, such as perpendicular media disks.

Determination of the quantitative relationship between crystallographic texture and magnetic properties in advanced permanent magnets may be hampered by complex microstructures, which complicate methods that rely on diffraction, or by interparticulate interactions, which adversely affect methods based on magnetic remanence measurements. To this end, new techniques in the determination of texture of bulk permanent magnets are being explored to overcome these inherent experimental difficulties. The analysis of inverse paramagnetic susceptibility measurements constitutes a new method to investigate crystallographic texture. Such measurements also provide Curie temperature data, which is sensitive to chemical changes that may have occurred in the magnetic phase during processing. The mathematical formalism underlying the analysis of inverse susceptibility measurements is outlined, and is used to evaluate magnetic measurements taken from a series of Nd{sub 2}Fe{sub 14}B magnets that have been processed by different means, and thus contain different degrees of texture. While this method does provide qualitative information concerning the relative crystallographic alignment of magnet samples, it needs calibration to obtain an explicit value for a texture order parameter.

In order to investigate factors affecting coercivity a series of two-phase Nd2Fe14B-based nanocomposite alloys with different excess iron concentrations were produced by melt-spinning methods. The constituent grain size was estimated by diffraction methods to be in the range of 150–500 A, and room-temperature demagnetization curves verify that the alloys studied exhibit a modest remanence enhancement. Isothermal remanence magnetization (IRM) and dc demagnetization (DCD) measurements performed at temperatures in the range 275 K⩽T⩽350 K illustrate that the coercivity and irreversible magnetization develop in a bimodal, incoherent manner from a demagnetized state but upon demagnetization from a saturated state the system evinces collective, exchange-coupled behavior as illustrated by the reversible magnetization Mrev. The temperature dependencies and values of the irreversible susceptibility χirr (DCD) suggest that a moderating phase with a magnetic anisotropy intermediate to the two constituent main phases ...

It is known that the addition of alloying elements to the Nd2Fe14B-based alloys significantly inhibits the onset of the crystallization in particles produced by the inert gas atomization (IGA) process. Transmission electron microscopy (TEM) was performed on a selection of as-atomized particles synthesized by IGA methods with the addition of 3 wt % TiC. TEM samples were successfully prepared using the wedge technique. Large particles (>10 μm) tend to consist of well-defined grains of Nd2Fe14B while the submicron size particles are generally amorphous and particles with diameter d in the size range, 1 μm

An intriguing pathway for the realization of metallic, rare-earth-free magnets is described. Creation of a so-called “exchange-bias” permanent magnet is contemplated that mimics the microstructure of Alnico magnets, comprised of FeCo-based precipitates that are separated by a metallic antiferromagnetic phase that replaces non-magnetic NiAl-rich phase of alnico. In this manner, the existing shape anisotropy is augmented by exchange-bias anisotropy through interphase coupling, providing enhanced coercivities. As a proof of concept, nanocomposite alloys have been fabricated from rapidly solidified Fe34Co33Mn33 ribbons that phase separate into an antiferromagnetic Mn-rich γ-phase and ferromagnetic α-FeCo nanoprecipitates upon post-solidification processing. A progressive enhancement of coercivity, remanence, and exchange bias at T

The effect of atomic substitutions on the magnetization, exchange, and magnetocrystalline anisotropy energy of L10-ordered FeNi (tetrataenite) is computationally investigated. The compound naturally occurs in meteorites but has attracted renewed attention as a potential material for permanent magnets, and elemental additives will likely be necessary to facilitate the phase formation. Our density functional theory calculations use the Vienna ab-initio simulation package, applied to 4-atom unit cells of Fe2XNi and 32-atom supercells (X = Al, P, S, Ti, V, Cr, Mn, Fe, Co). While it is found that most additives deteriorate the magnetic properties, there are exceptions: excess substitutional Fe and Co additions improve the magnetization, whereas Cr, S, and interstitial B additions improve the magnetocrystalline anisotropy.

In this work, we demonstrate that the magnetocaloric response of FeRh-based compounds may be tailored for potential magnetic refrigeration applications by chemical modification of the FeRh lattice. Alloys of composition Fe(Rh1−xAx) or (Fe1−xBx)Rh (A = Cu, Pd; B = Ni; 0

A granular system of FeRh-based nanoprecipitates (∼10–15 nm diameter) embedded in a rapidly solidified copper ribbon matrix was found to transit from a metastable tetragonal L10 (AuCu-1-type) structure to a stable B2 (CsCl-type) structure upon annealing-induced coarsening to ∼94 nm. The hysteretic magnetic transition observed at ∼100 K develops a gradual broadening that accompanies the L10 → B2 crystal structure transition. It is proposed that the Cu matrix influences the structural and magnetic properties of the FeRh-based nanoparticles through interfacial strain and chemical effects. These results emphasize the sensitivity of the magnetostructural response of FeRh to changes in the nanostructural scale, and provide pathways for tailoring the transition.

Effects of nanostructuring on the magnetostructural response of the near-equiatomic FeRh phase were investigated in nanocomposite materials synthesized by rapid solidification and subsequent annealing of an alloy of nominal atomic composition (FeRh)5Cu95. Transmission electron microscopy studies confirm attainment of a phase-separated system of nanoscaled (∼10–15 nm diameter) precipitates, consistent with FeRh embedded in a Cu matrix. These nanoprecipitates are crystallographically aligned with the coarse-grained Cu matrix and possess an L10-type (CuAu 1) structure, in contrast to the B2 (CsCl)-type structure of bulk FeRh. It is proposed that the face-centered cubic crystal structure of the Cu matrix serves as a template for the formation and stabilization of the L10 structure in the FeRh nanoprecipitates. Magnetic measurements highlight the existence of multiple magnetic phases in the material exhibiting spin-glass (T ≤ 15 K), ferromagnetic and paramagnetic (T > 20 K) behavior. A thermally hysteretic mag...

Factors which contribute to magnetostructural transition control have been demonstrated by study of the effects of Au incorporation on the magnetic and structural character of CsCl-structured equiatomic FeRh thin films. Sputtered films were capped with 2 nm of Au deposited at 873 K and at 323 K and subsequently characterized with magnetometry and synchrotron-based structural probes. Diffusion of Au into the FeRh film layer at 873 K is confirmed by a reduction in the Au capping layer thickness relative to the film capped at 323 K. The impact of Au diffusion on the FeRh magnetostructural character is noted by a decrease in the onset of the transition temperature, a thermally broadened first-order transition and an increased sensitivity of the transition to applied magnetic field. Additionally, magnetization data indicate that Au diffusion causes retention of the ferromagnetic phase well below the normal magnetostructural transition temperature. These results are attributed to a multiphase FeRh film layer created by thermally driven Au diffusion.

Binary alloys of Fe and Co have among the highest magnetizations of any transition metal alloy systems, but their affinity to form oxides act to reduce the magnetization of nanoparticles as their size is reduced below ∼30 nm. Here, we demonstrate the synthesis of single phase, size-controlled FeCo nanoparticles having magnetization greater than 200 emu/g via a non-aqueous method in which ethylene glycol served as solvent and reducing agent as well as surfactant. Experiments indicated pure-phase FeCo nanoparticles, having saturation magnetization up to 221 emu/g for sizes of 20–30 nm, in single batch processes resulting in > 2 g/batch. Post-synthesis oxidation of nanoparticles was investigated until very stable nanoparticles were realized with constant magnetization over time.

Silver manganese alloys of composition Ag100−xMnx (25 ≤ x ≤ 40) were prepared by rapid solidification to investigate their reported antiferromagnetic character. Thorough characterization of their structural, compositional, and magnetic features indicates the formation of a phase-separated fcc-type structure in all studied alloys with lattice parameters and coherently diffracting region sizes on the order of 40–100 nm that are strongly correlated with the Mn content. Magnetic measurement reveals a blocking temperature TB ∼ 105 K that is accompanied by unprecedented exchange bias values up to 1.7 T (17 kOe) at low temperatures. The magnitudes of the exchange bias field and the blocking temperature increase with increasing Mn concentration, suggesting a very sensitive balance between Mn content and magnetic character in these alloys that fosters tuning of the resultant exchange bias.

Fe3O4 were synthesized nanoparticles by thermal decomposition method with oleic acid as the surfactant, and to make them suitable for aqueous environments, dopamine ligand exchange was carried out on the particles. The nanoparticle size and phase was quantified by transmission electron microscopy (TEM) and x-ray diffraction (XRD), respectively. Superconducting quantum interference device magnetometry confirmed superparamagnetic behavior in both nanoparticles. A surprising and significant increase in the remanence MR, saturation magnetization MS, and blocking temperature TB of the particles was found after dopamine functionalization, even though TEM and XRD studies revealed no change in the particles’ size and/or structure. The results are consistent with an increase in the magnetic size of the nanoparticle core induced by the dopamine ligand exchange process. These effects are tentatively attributed to surface bonding effects that alter the canted magnetic state of the Fe3O4 nanoparticles.

Epitaxial Fe48Rh52 thin films of nominal thickness 500A were grown on MgO (001) substrates via molecular beam epitaxy and capped with 20A of either Au or MgO. The presence of the anticipated magnetostructural phase transition from antiferromagnetism to ferromagnetism at ∼350K was confirmed by superconducting quantum interference device and magneto-optic Kerr effect magnetometry. X-ray magnetic circular dichroism (XMCD) experiments were carried out at the L2,3 edges; measurements were done in both total electron yield (TEY) mode and indirect transmission mode. At room temperature, the indirect transmission XMCD data show no detectable dichroism, consistent with bulk antiferromagnetic behavior. However, room-temperature TEY data originating from the films’ surface reveal an appreciable dichroic signal indicating the presence of ferromagnetism. This near-surface/interfacial magnetism may be modified by choice of capping layer; the MgO-capped sample exhibited a considerably smaller ferromagnetic XMCD signal. The results have potential impact on the application of FeRh thin films as temperature-variable pinning layers in exchanged-biased systems.

Simultaneous high-Curie temperature (TC) ferromagnetism and enhanced temperature-independent paramagnetism (χTIP) is quantified in TiO2 anatase nanorods doped with nominal 0.5 at % Fe synthesized by a hydrothermal route. The nanorod dimensions are 8–35 nm in width and several hundred nanometers in length and possess an Fe concentration of 0.3–1.0 at %; no evidence of pure iron nanoparticles is detected. Magnetometry shows ferromagnetism at low fields that transitions to paramagnetism at higher fields with an enhanced temperature-independent susceptibility >100 times that of pure bulk anatase TiO2. The enhanced magnetism is tentatively attributed to the defect structure of the nanorods.

The magnetic character of Pd40FexNi40−xP20, a metallic glass where x=17.5, is experimentally studied over a temperature range of 10–375K. Hysteresis and temperature-dependent magnetization measurements derived from superconducting quantum interference device magnetometry on the amorphous alloy provide insight into the role of applied magnetic field on magnetic phase formation during the nanocrystallization process. Specifically, the microstructure of the melt-spun form of Pd40(Fe,Ni)40P20 is discussed in correlation to the observed superparamagnetic and spin-glass behavior. Despite the extremely high quenching rate used to synthesize the alloy, this composition shows evidence of chemical segregation on the nanoscale in the as-solidified state.

An increase in the magnitude of resistivity and magnetic susceptibility (10×) with increase of oxygen in the assist beam during reactive ion-beam assisted deposition of Pd94Mn6 nanocomposite films is reported. These results imply that a portion of the oxygen is implanted in the as-deposited films within the Pd matrix, forming a metastable (PdMnO) solid solution that creates an increase in the effective electronic density of states at the Pd Fermi level and fosters an electronic state that approaches the cooperative magnetism. Vacuum annealing of films is hypothesized to foster the formation of Mn oxides that exhibit antiferromagnetism in the temperature range 50

The exchange-spring behavior found in Sm–Co (20nm)∕Fe epitaxial bilayer films was investigated by analyzing major hysteresis and recoil curves as a function of anneal conditions. The hard layer consists of nanocrystalline intermetallic Sm–Co hexagonal phases (majority phase Sm2Co7 with SmCo3 and SmCo5). Recoil curves, obtained from the successive removal to remanence and reapplication of an increasingly negative field from the major demagnetization curve, reveal the reversible and irreversible components of the magnetization. The Sm–Co thickness was fixed at 20nm while the Fe thicknesses of 10 and 20nm were studied, with ex situ annealing carried out in evacuated, sealed silica tubes at different temperatures. The peak in the recoil curve area is associated with the coercivity of the hard phase. The development of the soft component magnetization is revealed by the departure of the recoil area from zero with application of a reverse field. These two features together confirm that annealing stabilizes the ...

Two series of epitaxial CoPt and FePt films, with nominal thicknesses of 42 or 50 nm, were prepared by sputtering onto single-crystal MgO(001) substrates in order to investigate the chemical ordering and the resultant magnetic properties as a function of alloy composition. In the first series, the film composition was kept constant, while the substrate temperature was increased from 144 to 704 °C. In the second series the substrate temperature was kept constant at 704 °C for CoPt and 620 °C for FePt, while the alloy stoichiometry was varied in the nominal range of 40–60-at. % Co(Fe). Film compositions and thicknesses were measured via Rutherford backscattering spectrometry. The lattice and long-range order parameter for the L10 phase were obtained for both sets of films using x-ray diffraction. The room-temperature magnetocrystalline anisotropy constants were determined for a subset of the films using torque magnetometry. The order parameter was found to increase with increasing temperature, with ordering...

Isolated NiAs-type MnBi nanoparticles of 10 nm diameter, embedded in a Bi matrix, were fabricated by melt-spinning and then annealing at 528 K for 1 h. A large coercivity, over 16 kOe at 300 K, with a positive temperature coefficient is observed. In agreement with bulk results, ac susceptibility measurements show a spin reorientation around 90 K coincident with an abrupt decrease in the coercivity with decreasing temperature. The high coercivity at 300 K is attributed to a mixture of nucleation and pinning mechanisms for magnetic reversal.

In order to understand the effect of solute additions on the crystallization behavior and to directly measure the crystallization pathways, a series of high temperaturex-ray diffraction (HTXRD) experiments were performed on rapidly-solidified Nd 2 Fe 14 B alloys from room temperature to 1073 K using high-energy synchrotron radiation and a Debye–Scherrer geometry. The relative phase fractions of crystalline and amorphous material and changes in lattice parameters were determined as a function of temperature using Rietveld analysis. The effect of solute additions on the lattice parameters of the Nd 2 Fe 14 B unit cell was clearly evident. As-quenched samples measured at room temperature consist of predominantly glassy Nd–Fe–B (up to 90 wt. %), 8–9 wt. % crystalline Nd 2 Fe 14 B and the remainder α-Fe. From room temperature up to the Curie temperature (T c ) for Nd 2 Fe 14 B , the unit cell volume contracts primarily due to contraction in the basal plane. From 600 K up to the crystallizationtemperature the thermal expansion is linear with a positive slope. There is a second inflection in the thermal expansion at the crystallizationtemperature. This inflection is indicative of solute ejection from the devitrified structure while the negative thermal expansion found for T K is a result of spontaneous magnetostriction which reflects the local anisotropies in the Nd 2 Fe 14 B structure.

The unique sample preparation technique of reactive ion beam assisted deposition has been used to make a thin film sample of interdispersed Ni and NiO nanocrystallites. The ferromagnetic and antiferromagnetic crystallites in the film provide a unique geometry that lies between that of bulk exchange biased systems and layered thin films, and provides clear evidence of exchange effects. Interphase magnetic exchange is demonstrated by suppression of the Neel and Curie temperatures of the components, as well as by the typical hysteresis loop shift that is a measure of the exchange field strength. Furthermore, a strong temperature and maximum applied field dependence for both the coercivity and exchange field strength is present, indicating competition between the exchange interaction of the Ni and NiO nanocrystallites and the external field and NiO surface magnetic coupling interactions.

A detailed study of the microstructure and composition in hot‐pressed (MQ‐2) and die‐upset (MQ‐3) magnet alloys based on the Nd2Fe14B composition, utilizing high resolution and analytical transmission electron microscopy, is reported. The initial magnetic properties of the two samples show different behaviors, which are attributed to the difference in the anisotropy of the grain structure and the grain boundaries. The hot‐pressed sample shows faceted grains of the 2‐14‐1 phase, while die‐upset sample shows plate‐like grains, together with larger equiaxed grains that contain a speckling of precipitates in the grain interior. The grain structure and composition remain rather similar in the two samples. The grain boundary phase averages ∼1–≳10 nm in width. The thicker grain boundaries are Nd‐rich, while the thinner grain boundaries in the hot‐pressed sample exhibit an Fe‐rich composition near that of the NdFe3 phase. Nd‐rich phases are found at the grain boundary junctions of both samples, with the Nd:Fe ratio near 7:3 in the die‐upset sample, and up to 3:2 in the hot‐pressed sample. The significance of the microstructure and the grain boundary phases on the magnetic behavior in the two samples is discussed.

Quantitative evidence of a ferromagnetic Fe‐rich minor phase present in both melt‐quenched/die‐upset and sinteredmagnets based on the Nd2Fe14B composition is presented. Full hysteresis loops were obtained at elevated temperatures (650 K≤T≤800 K) and were subsequently decomposed to obtain the saturationmagnetizationM s of the minor phase as well as the paramagnetic slope of the 2‐14‐1 major phase as a function of temperature. Assuming a composition of pure iron, the calculated volume % of the impurity phase in the die‐upset magnets is consistent with that obtained from previous observations of the widths and geometry of the intergranular phase found in the same magnets. The magnetization of the ferromagnetic impurity phase in the melt‐quenched magnets decreases more rapidly than that of pure iron; extrapolation indicates a Curie temperature in the range 925 K≤T≤975 K. The paramagneticsusceptibility of the Nd2Fe14B main phases exhibits Curie–Weiss behavior with the same paramagneticCurie temperature for both sintered and die‐upset samples. The Curie constants differ, however, probably due to the different degrees of crystallographic alignment.

The Jiles–Atherton theory is based on considerations of the dependence of energy dissipation within a magnetic material resulting from changes in its magnetization. The algorithm based on the theory yields five computed model parameters, MS, a, α, k, and c, which represent the saturation magnetization, the effective domain density, the mean exchange coupling between the effective domains, the flexibility of domain walls and energy‐dissipative features in the microstructure, respectively. Model parameters were calculated from the algorithm and linked with the physical attributes of a set of three related melt‐quenched permanent magnets based on the Nd2Fe14B composition. Measured magnetic parameters were used as inputs into the model to reproduce the experimental hysteresis curves. The results show that two of the calculated parameters, the saturation magnetization MS and the effective coercivity k, agree well with their directly determined analogs. The calculated a and α parameters provide support for the ...