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The magnetic and magnetotransport properties of Ni80Fe20/La0.7Sr0.3MnO3 (NiFe/LSMO) bilayers were investigated after bombarding the LSMO surface with low-energy Ar+ or O2+/Ar+ ion beams before the growth of the top NiFe layer. A variety of magnetic properties are revealed, including an asymmetric two-stepped hysteresis loop with an exchange bias loop shift, and alternatively, a symmetric two-stepped hysteresis loop with an enhanced coercivity. Polarized neutron reflectometry measurements provide details of the magnetic depth profile and interface layer magnetism at different temperatures. The LSMO surface modifications determine a complex magnetic and electric NiFe/LSMO interface having a strong effect on the magnetoresistance of the bilayer. Surface engineering based on ion beam bombardment is presented as a promising technique for optimizing the electronic and magnetic properties of NiFe/LSMO junctions for future device applications. [ABSTRACT FROM AUTHOR]

Significant efforts towards understanding bi-magnetic core-shell nanoparticles are underway currently as they provide a pathway towards properties unavailable with single-phased systems. Recently, we have demonstrated that the magnetism of c-Fe2O3/CoO core-shell nanoparticles, in particular, at high temperatures, originates essentially from an interfacial doped iron-oxide layer that is formed by the migration of Co2+ from the CoO shell into the surface layers of the γ-Fe2O3 core [Skoropata et al., Phys. Rev. B 89, 024410 (2014)]. To examine directly the nature of the intermixed layer, we have used high-resolution transmission electron microscopy (HRTEM) and first-principles calculations to examine the impact of the core-shell intermixing at the atomic level. By analyzing the HRTEM images and energy dispersive spectra, the level and nature of intermixing was confirmed, mainly as doping of Co into the octahedral site vacancies of γ-Fe2O3. The average Co doping depths for different processing temperatures (150 °C and 235 °C) were 0.56 nm and 0.78 nm (determined to within 5% through simulation), respectively, establishing that the amount of core-shell intermixing can be altered purposefully with an appropriate change in synthesis conditions. Through first-principles calculations, we find that the intermixing phase of γ-Fe2O3 with Co doping is ferromagnetic, with even higher magnetization as compared to that of pure γ-Fe2O3. In addition, we show that Co doping into different octahedral sites can cause different magnetizations. This was reflected in a change in overall nanoparticle magnetization, where we observed a 25% reduction in magnetization for the 235 °C versus the 150 °C sample, despite a thicker intermixed layer. [ABSTRACT FROM AUTHOR]

We use a novel solution-based disassociation synthesis scheme of the ionic complex Fe(LiBH4)2 to form Fe nanoparticles. The complex was formed initially using a gentle mechanochemical process, and the Fe nanoparticles emerged after 4 h of ball milling in an air-free environment. Rietveld refinement of x-ray diffraction measurements in an air-free sample holder identified a Im3m α-Fe phase. A room temperature Mössbauer spectrum of the sample presented a six-line spectrum unique to Fe0 metal, and the Fe nanoparticles were extremely well crystallized. Magnetometry results presented a reduced saturation magnetization (e.g., Ms ∼ 85 emu/g at 50 K) that had a Bloch-like T2 temperature dependence, consistent with a gap in the magnon fluctuation spectrum due to finitesize effects. The Fe nanoparticles were magnetically soft, with a coercivity ranging from ∼10 to 20mT with decreasing temperature from 350 K. [ABSTRACT FROM AUTHOR]

A series of core/shell Fe-oxide/SiO2 nanoparticles were produced to form Fe-oxide (1:1 Fe3O4 and c-Fe2O3) cores with diameters of approximately 4.6 nm and shell thicknesses ranging from 3.6 to 5.4 nm. Hot press sintering of core/shell nanoparticles created a nanocomposite of Fe-oxide nanoparticles in a SiO2 matrix. The presence of an iron-orthosilicate at the core-shell interface defines the intrinsic magnetic properties of the nanoparticle systems, resulting in an increase in magnetic anisotropy with thicker SiO2 shell. We find that dipole-dipole interactions are mediated by the overall SiO2 shell, and that these interactions are coupling neighbouring particles' magnetization with increasing correlation lengths. When the nanoparticles from a composite material, packing significantly increases the interaction strengths, altering the overall magnetization of the system so that the iron-oxide cores present an approximate 25% increase in (saturation) magnetization to a bulk-like (∼80 emu/g) value. [ABSTRACT FROM AUTHOR]

We have investigated the role of spontaneously formed interfacial metal silicates on the magnetism of FeCo/SiO2 and Fe49%Co49%V2%/SiO2 core/shell nanoparticles. Element specific x-ray absorption and photoelectron spectroscopy experiments have identified the characteristic spectral features of metallic iron and cobalt from within the nanoparticle core. In addition, metal silicates of iron, cobalt, and vanadium were found to have formed spontaneously at the interface between the nanoparticle core and silica shell. X-ray magnetic circular dichroism experiments indicated that the elemental magnetism was a result of metallic iron and cobalt with small components from the iron, cobalt, and vanadium silicates. Magnetometry experiments have shown that there was no exchange bias loop shift in the FeCo nanoparticles; however, exchange bias from antiferromagnetic vanadium oxide was measured in the V-doped nanoparticles. These results showed clearly that the interfacial metal silicates played a significant role in the magnetism of these core/shell nanoparticles, and that the vanadium percolated from the FeCo-cores into the SiO2-based interfacial shell. [ABSTRACT FROM AUTHOR]

We have examined the effects of core-shell intermixing on the dynamical magnetism of γ-Fe2O3/ MnO nanoparticles. The core and shell phases were identified using x-ray diffraction, and x-ray absorption spectroscopy identified Mn ions in both octahedral and tetrahedral sites, consistent with a significant amount of substitution at the core-shell interface to form an Fe/Mn-ferrite. The dynamical response was probed by Mössbauer spectroscopy, which decouples surface and core spins, and suggested a change in the relaxation behaviour among the spin populations within γ-Fe2O3/MnO relative to the γ-Fe2O3 seed particles. Interestingly, the magnetic relaxation effects at the atomic scale, measured via Möossbauer spectroscopy, were enhanced, indicating that the addition of an MnO shell and intermixing affected the dynamical freezing process which altered the surface magnetism of the γ-Fe2O3 core. Our results show that both the MnO shell and the interfacial intermixed layer are important in determining the core-shell nanoparticle magnetism. [ABSTRACT FROM AUTHOR]

We have fabricated thin films of iron nanocrystallites embedded in a copper matrix using a dual ion beam assisted deposition technique. A secondary End-Hall ion beam bombarded the iron atoms during deposition altering significantly the morphology of the films and allowing for control of the intermixing between iron and copper components. Cross-sectional transmission electron microscopy and x-ray reflectometry experiments indicated that the morphology of the films was that of iron nanocrystallites embedded in a copper matrix. Rietveld refinements of the diffraction pattern identified fcc-copper and amorphous iron. An increased amount of disorder was observed with a reduction in the amount of deposited iron from a 1:1 Fe:Cu ratio to 0.25:0.75 Fe:Cu ratio. Interfacial copper-iron alloys were identified by DC susceptibility experiments through their reduced TC,Alloy (370, 310, and 280 K) compared with that of bulk iron (~1000 K). Element specific x-ray absorption and x-ray magnetic circular dichroism experiments were performed to identify the contributions to the magnetism from the iron and the copper-iron alloy. [ABSTRACT FROM AUTHOR]

Ni3Fe/(Ni, Fe)O thin films with bilayer and nanocrystallite dispersion morphologies are prepared with a dual ion beam deposition technique permitting precise control of nanocrystallite growth, composition, and admixtures. A bilayer morphology provides a Ni3Fe-to-NiO interface, while the dispersion films have different mixtures of Ni3Fe, NiO, and FeO nanocrystallites. Using detailed analyses of high resolution transmission electron microscopy images with Multislice simulations, the nanocrystallites' structures and phases are determined, and the intermixing between the Ni3Fe, NiO, and FeO interfaces is quantified. From field-cooled hysteresis loops, the exchange bias loop shift from spin interactions at the interfaces are determined. With similar interfacial molar ratios of FM-to-AF, we find the exchange bias field essentially unchanged. However, when the interfacial ratio of FM to AF was FM rich, the exchange bias field increases. Since the FM/AF interface 'contact' areas in the nanocrystallite dispersion films are larger than that of the bilayer film, and the nanocrystallite dispersions exhibit larger FM-to-AF interfacial contributions to the magnetism, we attribute the changes in the exchange bias to be from increases in the interfacial segments that suffer defects (such as vacancies and bond distortions), that also affects the coercive fields. [ABSTRACT FROM AUTHOR]

The strength, polarity, and temperature dependence of the exchange bias field in ferromagnet/antiferromagnet thin film systems depend critically on the nature of the interfacial microstructure. Using ion bombardment during film deposition, we have found a way to modulate the interface microstructure, providing unique control over the exchange bias magnetism. We demonstrate interfacial microstructure modulation in Ni80Fe20/(Ni,Fe)O thin films, where different ion bombardment energies alter the exchange coupling between the (Ni,Fe)O and Ni80Fe20 interfaces. One interface configuration provides positive or negative exchange coupling depending on whether the film was cooled with or without an external magnetic field, while another interface configuration results in the more typical negative exchange coupling independent of the field cooling. [ABSTRACT FROM AUTHOR]

The exchange bias field dependence on the Mn-oxide and its microstructure in NiFe/Mn-oxide bilayers was investigated. Transmission electron microscopy results have shown that the bilayer bottom consisted of either α-Mn, rocksalt MnO, or a composite of tetragonal Mn3O4+MnO, depending on the ratio of O2/Ar used during dual ion-beam deposition. Magnetometry results at 5 K indicate that the exchange bias field (Hex∼-300 Oe) is largest in a NiFe/Mn (0%O2/Ar) bilayer. The MnO formation by in situ Mn oxidation results in a decrease in Hex in a NiFe/Mn-oxide (21%O2/Ar) bilayer. In contrast, a further increase in the O2/Ar ratio during deposition results in larger Hex and Hc. This is attributed to the oxidation of MnO into a harder ferrimagnet, Mn3O4. Our results indicate that the antiferromagnetic Mn enabled stronger coupling with NiFe than MnO. In addition, we find that the MnO–Mn3O4 coupling dominates the exchange bias effects at high oxygen concentrations. [ABSTRACT FROM AUTHOR]

Muon spin relaxation has been used to study transverse spin freezing in a-Fe[sub x]Zr[sub 100-x] and a-Fe[sub 90-x]Ru[sub x]Zr[sub 10]. The derived phase diagrams, while generally consistent with model calculations, exhibit some significant differences. Comparison between zero-field and applied field measurements shows that T[sub xy] is strongly affected by a field and falls as B[sup -1], contrary to mean field predictions. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

MnBi is a promising ferromagnetic material for applications at elevated temperatures due to its high Curie temperature, sizable magnetocrystalline anisotropy, and increasing coercivity with temperature. However, fabrication of MnBi thin films with ferromagnetic low-temperature phase (LTP) is conventionally difficult due to the peritectic Mn-Bi reaction and Mn oxidization. In this work, in situ Ar+-ion beam bombardment was introduced to modify the microstructure and magnetic properties of (Mn/Bi)n superlattice. Structural characterization revealed formation of MnBi LTP in the as-deposited ion-beam-bombarded samples. Vacuum annealing further promoted growth of MnBi LTP grains with c-axis orientation, leading to remarkably improved perpendicular magnetic anisotropy. Ion-beam bombardment resulted in a 200% increase in the saturation magnetization of the annealed MnBi thin film. This advancement is attributed to the enhanced interdiffusion and reaction of Mn and Bi in the ion-beam-bombarded thin films. A stability test after 150 days in air revealed a remarkable reduction in ferromagnetism due to MnBi decomposition. This work provides an effective approach for fabrication of high-quality MnBi thin films. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Ni3Fe/(Ni,Fe)O thin films having a nanocrystallite dispersion morphology were prepared by a reactive ion beam-assisted deposition technique. The crystallite sizes of these dispersion-based films were observed to decrease from 8.4±0.3 nm to 3.4±0.3 nm as the deposition flow-rate increased from 2.78% to 7.89% O2/Ar. Thin film composition was determined using selective area electron diffraction images and Multislice simulations. Through a detailed analysis of high resolution transmission electron microscopy images, the nanocrystallites were determined to be Ni3Fe (a ferromagnet), NiO, and FeO (both antiferromagnets). It was determined that the interfacial molar Ni3Fe ratio in the nanocrystallite dispersions increased slightly at first, then decreased as the oxygen content was increased; at 7.89% O2/Ar, the interfacial molar ratio was essentially zero (only NiO and FeO remained). For nanocrystallite dispersion films grown with O2/Ar flow-rate greater than 7.89%, no interfacial (intermixed) Ni3Fe phase was detected, which resulted in no measurable exchange bias. Comparing the exchange bias field between the nanocrystallite dispersion films at 5 K, we observed a decrease in the magnitude of the exchange bias field as the nanocrystallite size decreased. The exchange bias coupling for all samples measured set in at essentially the same temperature (i.e., the exchange bias blocking temperature). Since the ferromagnetic/anti-ferromagnetic (FM/AFM) contact area in the nanocrystallite dispersion films increased as the nanocrystallite size decreased, the increase in the magnitude of the exchange bias could be attributed to larger regions of defects (vacancies and bond distortions) which occupied a significant portion of the FM/AFM interfaces in the nanocrystallite dispersion films. [ABSTRACT FROM AUTHOR]

We have examined the dependence of the magnetism from perpendicular magnetic anisotropy with Co thickness in [Pt/Co] multilayers coupled to a NiO layer. We measure a perpendicular exchange bias field that changes polarity with Co thickness and find that with 2 nm of Co the perpendicular magnetic anisotropy is no longer present. Further evidence of the change in the Co layers preferred orientation is offered by zero field cooled susceptometry, where the low temperature susceptibility is negative for thin Co layers (0.9-1.2 nm) and becomes positive with increasing Co thickness (1.5-2.0 nm). Thin Co layered films also exhibit a compensation point around 100 K. These results indicate that the Co and NiO interface moments are coupled strongly and that there is competition between Pt/Co and Co/Ni at the film layer interfaces that sets the overall anisotropy from perpendicular and parallel configurations. [ABSTRACT FROM AUTHOR]

We have used a protein crystallization technique to fabricate three-dimensional (3D) ordered arrays of magnetoferritin nanoparticles up to a few hundreds of micrometers in size in the form of free standing perfectly faceted crystals. We compare the ac and dc susceptibilities of the nanoparticles in an uncrystallized system (amorphous assembly) and in a macroscopic 3D fcc crystal of nanoparticles. Completely different magnetism is measured due to the different interparticle interactions in the two systems. For example, at temperatures above 50 K the uncrystallized nanoparticle system is superparamagnetic while the 3D crystal displays no superparamagnetism up to 400 K. [ABSTRACT FROM AUTHOR]

A single precursor technique was used to synthesize γ-Fe2O3 nanoparticles with diameters from 5 to 7 nm. The blocking temperature of the nanoparticles essentially increases with Fe precursor concentration. Transmission Mössbauer spectroscopy of the different γ-Fe2O3 nanoparticle systems reveal that the local Fe environments in the nanoparticle vary considerably with Fe precursor concentration. Not only does the nanoparticle size change, but the magnitude of the hyperfine field (a measure of the nanoparticle moment), the amount of chemical disorder in the nanoparticle, and the number of Fe moments in the octahedral and tetrahedral sites of the γ-Fe2O3 spinel structure vary with Fe concentration as well. [ABSTRACT FROM AUTHOR]

The bulk and local magnetisms of ∼10 nm diameter CoFe2O4 nanoparticles have been characterized from 5 to 425 K. Magnetometry results indicate that the nanoparticles have a blocking temperature around 380 K and they exhibit a temperature dependent cubic magnetocrystalline anisotropy similar to that of bulk CoFe2O4 with K(0)∼1.2×106 ergs/cm3. However, the local, atomic-level spin dynamics examined by Mössbauer spectroscopy indicates strongly that rather than exhibiting a typical static, blocked magnetism to superparamagnetism behavior, the CoFe2O4 nanoparticle moments become paramagnetic, with this transition percolating from the surface to the core of the nanoparticle with increasing temperature. [ABSTRACT FROM AUTHOR]

The correlation between the ion-beam bombardment and the exchange bias magnetism in NiFe (10 nm)/Mn (25 nm) bilayers was studied. While the bottom Mn layers bombarded by different Ar ion-beam energies (VEH from 70 to 150 V) retained the same structure, significant differences in exchange bias were observed when in contact with a top NiFe layer. The dependence of the exchange bias field, Hex, with increasing VEH suggests strongly that the Ar ion-beam bombardment process may create uncompensated Mn spins (Hex enhancement) or increase the spin misalignment in ferromagnet (FM)/antiferromagnet (AF) interfaces (Hex decrease), depending on the energy used. A schematic FM/AF spin structure was proposed to explain this unusual exchange bias behavior. [ABSTRACT FROM AUTHOR]

It has been observed that a perpendicular interface contribution to the magnetic anisotropy can rotate the easy magnetization direction from within the film plane to perpendicular to the film plane when the Co thickness in Co/Pt multilayers is below a critical value that is in the range of (sub)nanometers. In this work, mixed CoPt phases mainly consisting of disordered fcc CoPt were found to be responsible for the perpendicular magnetic anisotropy (PMA) for the film deposition end-Hall voltage equal to 140 V. Estimates of mechanisms that could be responsible for the PMA such as “orange peel” coupling, exchange bias, and magnetostriction indicate that they are too weak. We believe that this PMA is driven by the heat of mixing and relatively higher interfacial energy since the Co layer thickness is about 1 nm. [ABSTRACT FROM AUTHOR]

We have examined the local magnetism of 7 nm γ-Fe2O3 nanoparticles as a function of temperature using transmission Mössbauer spectroscopy. Spectra indicate that the nanoparticles exhibit two different spin populations: one is due to the core Fe moments, while the other is from the surface moments. We find that the uniaxial anisotropy of the core increases with temperature from 5 to 75 K, whereupon it remains constant. Simultaneously, the surface moment fluctuations increase in frequency with warming. [ABSTRACT FROM AUTHOR]

The temperature dependent magnetism of EuS(t nm)/Co(t nm) films, where t=3.5, 5.0, 7.8, and 20 nm, reveals two EuS ordering temperatures TC: one magnetic phase order below TC∼16 K that corresponds to pure EuS and the other phase order at a TC>200 K that has a EuS thickness dependence. This enhanced TC is the result of EuS interface moments exchanged coupled to Co moments in an antiparallel configuration. The interface TC varies with inverse EuS film thickness, a behavior that is consistent with a magnetic proximity effect, and the amount of EuS that orders at the enhanced TC decreases with film thickness, t<10 nm. [ABSTRACT FROM AUTHOR]

The structural and magnetic properties of NiCo/(Ni,Co)O bilayers were investigated. X-ray diffractometry results have shown that the top NiCo layer consisted of a fcc NiCo phase. The bilayer bottom was either a pure (Ni,Co)O or a composite [NiCo+(Ni,Co)O] phase, depending on the percent of O2/Ar ratio used during deposition. A double-shifted hysteresis loop exhibiting components that were from positive or negative coupling was observed in the NiCo/(Ni,Co)O (8%O2/Ar) bilayers. The microstructural changes, which result from a combination of deposition oxygen content and the ion-beam bombardment, will result in the unusual exchange bias behavior. [ABSTRACT FROM AUTHOR]

A Ni80Fe20/α-Fe2O3 thin film has been deposited using a dual ion-beam deposition technique where the α-Fe2O3 is formed in situ by O2 bombardment of pure Fe. AC susceptibility results indicate moment fluctuations at temperatures above 150 K. This behavior combined with the thermal fluctuation driven temperature dependence of the negative field coercivity indicates that the α-Fe2O3 nanocolumnar crystallite magnetism evolves from static single domain moments to superparamagnetic moments with warming. 25 kOe field cooling establishes exchange bias with an onset temperature of 50 K. The positive (Hc2) and negative (Hc1) field coercivities exhibit very different temperature dependencies, with Hc1(T) well described by a thermally activated coherent domain wall reversal model, while Hc2(T) is essentially constant except for the lowest temperatures. These results point towards different domain reversal mechanisms in positive and negative fields. [ABSTRACT FROM AUTHOR]

Typically, exchange coupling can only occur when ferromagnetic spins couple to antiferromagnetic spins below the Néel temperature. We present results on a Ni80Fe20/Co3O4 thin film that is composed of ∼10 nm diameter nanocrystalline columns. Field cooling the film reveals low-temperature exchange-bias hysteresis loop shifts that are the same magnitude as those measured in a similar Ni80Fe20/CoO system, although the bulk magnetocrystalline anisotropy is a factor of 5 smaller. The coercivity and exchange-bias loop shift exhibit the same temperature dependence with individual “blocking” temperatures. Most surprising is that the onset temperature of the exchange-bias is nearly four times higher than the bulk Néel temperature. [ABSTRACT FROM AUTHOR]

We present results on a Ni80Fe20/NixFe1-xO thin-film bilayer that shows a positive exchange bias loop shift of ∼90 Oe at 10 K under zero-field-cooled conditions. Zero-field-cooled and field-cooled hysteresis loops were double shifted at temperatures below 200 K. This behavior is due to the presence of a range of antiferromagnetic crystallite sizes in addition to multiple magnetic phases (e.g., FeO, Fe2O3, and NiO). Furthermore, the positive exchange bias loop shift decreases linearly with increasing temperature, with a compensation temperature Tcomp∼220 K, after which negative exchange bias is measured. This temperature dependence of the exchange bias reflects the competition between the Ni80Fe20 ferromagnet and antiferromagnetic Fe oxide and NiO phases as well as a range of local blocking temperatures. We attribute the appearance of a positive exchange bias loop shift at low temperatures to temperature-dependent changes in the interfacial pinning and exchange coupling due to a complex NixFe1-xO structure from energetic ion-beam bombardment effects during the film deposition. [ABSTRACT FROM AUTHOR]

151Eu and 153Eu Mössbauer spectroscopies have been used to study the magnetism of the Ni2+ moments in Eu2BaNiO5 by way of the exchange-induced Eu3+ moments. These independent and complementary probes indicate that at high temperatures all of the moments in the Haldane quantum spin chains are paramagnetic. On cooling below 25 K, fluctuations of ∼90% of the Ni2+ moments slow down, while the remaining 10% remain paramagnetic (indicating the presence of chain defects). Both 151Eu and 153Eu Mössbauer measurements yield the same Ni2+ moment. Furthermore, below 5 K, the 90% chain component becomes static and ordered, in excellent agreement with the ordering temperature expected from the rare-earth exchange energy. [ABSTRACT FROM AUTHOR]

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 Néel 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. [ABSTRACT FROM AUTHOR]

Spin dynamics in a-Fe[sub 90]Sc[sub 10] have been examined with zero-field muon spin relaxation (ZF - µSR) and selective excitation double Mössbauer (SEDM) spectroscopy. Analysis of the µSR data yields T[sup sg] ∼ 105 K in agreement with both Χ[sub ac] and conventional Mössbauer data. SEDM shows fluctuation rates in good agreement with µSR, but reveals an abrupt change in relaxation behavior from spin-flip to diffusive above 90 K (0.9Tsg). [ABSTRACT FROM AUTHOR]

Polysaccharide iron complex, a ferritin analog, has been examined with zero-field muon spin-relaxation at temperatures from 2 to 40 K. Spectra exhibit a clear separation of static moments and collective excitations at low temperatures. At intermediate temperatures, superparamagnetic relaxation is observed and a blocking temperature of T[sub B]=10±2.5 K is measured, in agreement with transmission Mo¨ssbauer spectra and frequency dependent χ[sub ac] data. Superparamagnetic relaxation rates of 20-150 MHz are in agreement with those extrapolated from Mo¨ssbauer spectra using a multilevel magnetic relaxation model. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Using a many-state relaxation formalism and combining it with an accurate expression for the relaxation time of single-domain ferromagnetic particles and a log-normal size distribution we are able to fit Mo¨ssbauer spectra which span temperatures from where a pure hyperfine field distribution is present to where a single broad line occurs due to high relaxation rates. With this model, we have fitted the Mo¨ssbauer spectra of two Fe[sub 3]O[sub 4] ferrofluids, from 12 to 180 K. Fits to the ferrofluid with a mean particle size of 4.5 nm yield an anisotropy energy (K) of 3.0±0.1×10[sup 4] J/m[sup 3] with the onset of magnetic relaxation at 30±5 K while data for a mean particle size of 6.0 nm give K=2.4±0.2×10[sup 4] J/m[sup 3] with magnetic relaxation starting at 55±5 K. These blocking temperatures agree with those extrapolated from frequency dependent χ[sub ac] data. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Analysis of the muon spin relaxation (μSR) signal from a range of a-Fe[sub x]Zr[sub 100-x] alloys shows that the entire sample orders at T[sub c] and that the volume fraction that could be associated with isolated nonmagnetic clusters is less than 3%. Furthermore, the coincidence between the static and dynamic μSR signatures of transverse spin freezing provides a clear confirmation of predictions from numerical models. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Presents information on a study which focused on the use of selective excitation double Mossbauer spectroscopy in separating the influence of chemical disorder from time-dependent effects in alloys. Experimental methods; Results and discussion; Conclusions.

Dynamical freezing of Fe nanocrystallites in a Cu matrix with magnetic ordering of an FeCu interfacial phase provides a unique window into the magnetocaloric effect. The FeCu alloy altered the Fe nanocrystallite surface atoms, and with a magnetic ordering temperature comparable to the dynamical freezing temperature of the nanocrystallites enabled Fe surface atoms to contribute to the overall magnetization. Tuning the amount of interfacial alloy resulted in the control of the magnetic ordering temperature and the magnetocaloric properties. [ABSTRACT FROM AUTHOR]

We explore an emerging device concept based on exchange bias used in conjunction with an antidot geometry to fine tune ferromagnetic resonances. Planar cavity ferromagnetic resonance is used to study the microwave response of NiO/NiFe bilayers with antidot structuring. A large frequency asymmetry with respect to an applied magnetic field is found across a broad field range whose underlying cause is linked to the distribution of magnetic poles at the antidot surfaces. This distribution is found to be particularly sensitive to the effects of exchange bias, and robust in regards to the quality of the antidot geometry. The template based antidot geometry we study offers advantages for practical device construction, and we show that it is suitable for broadband absorption and filtering applications, allowing tunable anisotropies via interface engineering. [ABSTRACT FROM AUTHOR]

The complex permittivity and permeability of superparamagnetic iron-oxide nanoparticles has been quantified using a circular waveguide assembly with a static magnetic field to align the nanoparticle's magnetization. The high sensitivity of the measurement provides the precise resonant feature of nanoparticles. The complex permeability in the vicinity of ferromagnetic resonance is in agreement with the nanoparticle's measured magnetization via conventional magnetometry. A rigorous and self-consistent measure of complex permittivities and permeabilities of nanoparticles is crucial to ascertain accurately the dielectric behaviour as well as the frequency response of nanoparticle magnetization, necessary ingredients when designing and optimizing magnetic nanoparticles for biomedical applications. [ABSTRACT FROM AUTHOR]

Methods to modify the magnetic coercivity and exchange bias field of nanocrystalline antiferromagnetic/ferromagnetic \NiO/\Ni80{\Fe}20 thin films were investigated for bilayers grown using ion-assisted deposition onto different single crystalline substrates. An enhanced coercivity was found at 298 K for the films deposited on single crystalline MgO (100) and \Al_2 \O_3 (11-20) substrates. After field cooling the films to 50 K, the NiO/NiFe bilayer grown on \Al_2 \O_3 (11-20) exhibited the largest exchange bias (\- 25~\Oe). The second part of the study investigated ion-beam modification of the ferromagnetic surface prior to the deposition of the NiO layer. A range of ion-beam bombardment energies (VEH) were used to modify in situ the NiFe surface during the deposition of \NiO/\NiFe/\SiO_2 films. Cross-sectional transmission electron microscopy showed a systematic reduction in the thickness of the NiFe layers with increasing \Ar^ + bombardment energies attributed to etching of the surface. In addition, the bombardment procedure modified the magnetic exchange bias of the composite structure in both the as-prepared and field-cooled state. [ABSTRACT FROM PUBLISHER]

We report the effects of ion-beam bombardment on the room temperature and low temperature magnetic properties of ferromagnetic CoFe/antiferromagnetic (Co,Fe)O thin film bilayers. The films were deposited onto amorphous SiO2 and single crystalline MgO (110)/(100) substrates. Magnetometry showed that ion-beam bombardment was capable of modifying the coercivity and loop shape for the thin film system at room temperature, corresponding to alteration of the effective magneto-crystalline anisotropy field. After field cooling to 50 K, a shifted hysteresis loop was seen for those films containing a proportion of the antiferromagnetic rock-salt (Co,Fe)O phase, with an exchange bias magnitude that depended on the ion-beam bombardment conditions. Our results indicate that matching the substrate with appropriate ion-bombardment conditions provides a promising way to engineer selectively two important types of magnetic anisotropy in ferromagnetic/antiferromagnetic bilayers: magneto-crystalline and exchange bias. [ABSTRACT FROM PUBLISHER]

An interfacial monolayer of CuO in Cu-coated γ-Fe2O3 nanoparticles enables significantly decreased intrinsic surface spin disorder compared to bare γ-Fe2O3 nanoparticles. Element specific x-ray absorption spectroscopy at the L-edges for Cu and Fe indicates that the magnetic moment of the CuO in the shell interacts with the γ-Fe2O3 nanoparticle's surface magnetic moments. This exchange interaction cants the moments of the CuO resulting in a non-zero Cu moment, altering the γ-Fe2O3 nanomagnetism. [ABSTRACT FROM AUTHOR]

Substrate and ion-bombardment effects on exchange bias were explored for CoFe/(Co,Fe)O bilayer films. The (Co,Fe)O component was sputtered onto different substrates and then ion-bombarded before a ferromagnetic CoFe layer was grown. Ferromagnetic resonance reveals that the CoFe is magnetically pinned by the (Co,Fe)O film. We find that substrate type heavily influences the magnitude of exchange bias and also the degree to which the exchange biased system is affected under ion-bombardment. In all cases there is a general decrease in the magnitude of exchange bias and coercivity, and for cases with high energy ion-bombardment unusual changes to the hysteresis loop are observed which may indicate the formation of an additional magnetic phase. [ABSTRACT FROM AUTHOR]

An improved selective excitation double Mössbauer spectrometer has been used to study both static and dynamic disorder in magnetic materials. Static disorder has been identified in a-Fe80B20 and Fe65Ni35, while magnetic relaxation has been measured in a Fe3O4 ferrofluid. Simultaneous static and dynamic disorder have been established in a-Fe92Zr8. [ABSTRACT FROM AUTHOR]

We have investigated the exchange coupling between a bottom FePt thin film layer capped with different Mn-oxides. Results have shown that the magnetization reversal of the soft FePt layer is influenced strongly by the capped Mn-oxide layer (Mn, MnO, and Mn3O4), as revealed by the enhanced coercivities. Typical temperature dependent magnetization between zero-field cooled (ZFC) and field cooled (FC) scans was observed in the Mn-oxide (8%O2/Ar)/FePt bilayer that exhibited a blocking temperature (TB ∼ 120 K) close to the Nèel temperature, TN, of MnO. However, the Mn/FePt bilayer exhibited unusual temperature dependent of M vs. T, implying that intermixing between Mn and FePt interfaces formed an AF FeMn that may have enabled a high irreversibility temperature (Tirr. ∼ 400 K) compared to almost identical ZFC and FC curves from weaker exchange coupling between FePt and the Mn3O4 created with 21 and 30%O2/Ar deposition conditions. [ABSTRACT FROM AUTHOR]

In this study, we investigated the exchange bias (coupling) effects in CoFe/(Co,Fe)O bilayers by using different single crystal substrates of MgO(100) and MgO(110) and Ar ion-beam bombardment on the surface of the bottom antiferromagnet (Co,Fe)O layer before capping with ferromagnet CoFe. In the CoFe/(Co,Fe)O/MgO(110) bilayer, above the irreversibility temperature (Tirr. ∼ 170 K), there was a rapid decrease in M(T) with increasing temperature, unlike the CoFe/(Co,Fe)O/MgO(100) film that showed an increased Tirr. ∼ 300 K and no observable decrease in M(T) above Tirr. The different M vs T zero-field-cooled/field-cooled behavior of the CoFe/(Co,Fe)O bilayers on MgO(100) and MgO(110) indicated that the FM CoFe spin orientations were affected by the different substrates used via exchange coupling to the AF (Co,Fe)O layer altered by MgO. [ABSTRACT FROM AUTHOR]

We have examined the effects of Co doping (0-12% relative to total metal) on the magnetic properties of iron-oxide nanoparticles incorporated in a synthetic ferritin shell. Mössbauer spectroscopy revealed that the nanoparticles consisted of a mixture of Fe3 O4 and γ-Fe2 O3, and that Co was integrated exclusively into the B-sites of the Fe3 O4 phase. Cobalt doping resulted in the formation of Cox Fe3-x O4/γ-Fe2 O3 nanoparticles with x = 0, 0.25, 0.51, 0.56, and 0.63. Magnetometry and susceptometry experiments showed that substantial enhancements in the coercivities, blocking temperatures and anisotropy constants of the magnetoferritin nanoparticles occurred for all Co doping levels. Our results show that the Co altered significantly the local atomic Fe magnetism and enhanced the magnetic nanoparticle anisotropy considerably. [ABSTRACT FROM AUTHOR]

The field dependence of the transverse spin glass phase transition temperature, Txy(B), has been determined both for a - FexZr100-x (x = 90, 91, 92, and 93) using muon spin relaxation and for the bond-frustrated nearest neighbor ±J Heisenberg spin glass using Monte Carlo simulations. In both cases, we find Txy(B) ∝ 1/B, providing direct, quantitative agreement between experiments and simulations. [ABSTRACT FROM AUTHOR]

Evidence of elemental copper magnetism in Cu-coated γ-Fe2O3 nanoparticles was revealed by element specific x-ray absorption spectroscopy and magnetic circular dichroism (XMCD). An interfacial layer of magnetic CuO was discovered that interacted with the Fe3+ surface magnetic moments. This unexpected exchange interaction canted the moments of the CuO, so that a non-zero net magnetic moment was measured. XMCD measurements as a function of field on the copper coated γ-Fe2O3 nanoparticles indicated that the CuO magnetic moment was aligned with the octahedral Fe3+ and its magnetization was modified by the changing applied field. With increasing copper thickness, a stronger field dependence of the Fe3+ and Cu2+ magnetization was observed. [ABSTRACT FROM AUTHOR]

Abstracts are not published in this journal [ABSTRACT FROM AUTHOR]