F. T. Parker

Magnetic cooling near Curie temperatures above 300 K

Selection of materials and expected magnetocaloric effects are discussed for magnetic cooling applications at elevated temperatures (400–800 K). Various considerations result in the selection of rare earth‐transition metal compounds such as Sm2Fe17−xCox for this task. These materials offer a wide range of suitable magnetic ordering temperatures as a function of x. They also show relatively high effective magnetic moments per volume. Molecular field models are developed for analytically predicting entropy changes at and above the ordering temperature. Concomitant adiabatic cooling ΔT is accordingly computed for these compounds near the ordering temperatures. It is found that for a family of compounds ΔT values increase somewhat with increasing ordering temperatures due to the decreasing influence of the lattice heat capacity at higher temperatures. Adiabatic cooling of ΔT=−7.5 K at 70 kOe to ΔT=−9.2 K at 70 kOe is predicted for materials Y2Fe17−xCox near their Curie points of 300 and 600 K, respectively (c...

Magnetic anomalies in NiO nanoparticles

As first noted by Neel, antiferromagnetic nanoparticles could exhibit superparamagnetic relaxation of their spin lattices as well as permanent moments arising from uncompensated surface spins. Several samples of antiferromagnetic NiO nanoparticles with average sizes ranging from 50 to >800 A were investigated in the present study. In addition to the inverse dependence on average particle size of the susceptibility predicted by Neel, and previously reported, some unusual behavior was observed. Above the blocking temperatures (TB) of the particles, the reversible magnetization could not be fit with a Langevin function that was consistent with the physically reasonable moment representing the uncompensated spins. For the 53 A diameter particles, both zero-field-cooled (ZFC) and field-cooled (FC) loops below TB exhibit large coercive forces (several kOe) and the loops showed irreversibility up to 50 kOe. In addition, in the FC state below TB the hysteresis loops were strongly shifted. The latter behavior may ...

Anomalous properties of magnetic nanoparticles

Abstract Nanoparticles of ferrimagnetic NiFe2O4 and antiferromagnetic NiO exhibit a variety of anomalous magnetic properties. The lower coordination of surface spins is responsible in both cases for the observed behavior. This conclusion is supported by calculations of field-dependent spin distributions in these nanoparticles.

Giant magnetoresistance in heterogeneous Cu-Co and Ag-Co alloy films (invited)

Giant magnetoresistance in sputtered single films of Cu-Co and Ag-Co heterogeneous alloys is discussed. The films consist of Co-rich precipitates in a nonferromagnetic matrix. The Ag-Co films have higher DELTArho/rho and DELTArho values than the Cu-Co films, possibly due to less Co dissolved in the Ag matrix. DELTArho scales inversely with precipitate particle size, implying that Co-rich clusters less-than-or-equal-to 20 angstrom diameter may be most effective for spin dependent scattering. This trend of the data and a phenomenological model suggest that interfacial spin dependent scattering is significantly stronger than the scattering within the Co-rich particles.

Non-oscillatory antiferromagnetic coupling in sputtered Fe/Si superlattices

A series of sputtered Fe(30{Angstrom})/Si(x) superlattices were grown for x=10--40{Angstrom}. Magnetization and Kerr hysteresis loops, and neutron-reflectively measurements identify antiferromagnetic (AF) coupling of the Fe layers at room temperature for x=15{Angstrom} nominal thickness, with switching fields of 6kOe. X-ray structural analysis indicate that the spacer medium is crystalline for x<20{Angstrom}, while sputtered Si is amorphous (a). Failure to detect oscillations in the AF coupling for thicker Si layers is due to the formation of a-Si, as opposed to the crystalline silicide responsible for the coupling.

X-ray diffraction and Mössbauer studies of structural changes and L10 ordering kinetics during annealing of polycrystalline Fe51Pt49 thin films

Room-temperature x-ray diffraction and Mossbauer effect techniques have been used to characterize the structural features and local atomic environments of sputtered Fe51Pt49 thin films following various isothermal treatments. Both techniques show that no significant changes occur in the chemically ordered L10 tetragonal phase after it has formed. In contrast, changes in the disordered face-centered-cubic (fcc) phase are observed prior to the transformation into the ordered tetragonal phase. Mossbauer measurements indicate the development of increasing short-range order in the disordered fcc phase with increasing annealing temperature. Asymmetries in the fcc x-ray diffraction profiles also suggest the presence of lattice distortions caused by atomic size differences commonly found in the quenched disordered fcc phase of materials that form ordered structures. Quasi-real-time kinetic measurements of the disorder→order transformation in sputtered Fe51Pt49 thin films within the temperature range 300 °C⩽T⩽400 ...

Magnetic decoupling in sputtered Fe/Si superlattices and multilayers

Sputtered Fe/Si superlattices were grown to study the magnetic coupling between ferromagnetic Fe layers (30 A thick) for Si spacer‐layer thicknesses (tSi) between 10 and 40 A. The material is ferromagnetical for tSi≤13 A and antiferromagnetically coupled for 13 A≤tSi≤17 A. For tSi≥17 A the Fe layers are uncoupled. X‐ray analysis indicates that the system is well layered, but that the crystal structure remains coherent only for tSi≤17 A. These results, along with our Mossbauer investigation, strongly suggest that the Si layer is crystalline for tSi≤17 A, and is silicide in nature. For thicker spacers, Si becomes amorphous. We propose a model of the layering that is consistent with the known properties of Fe silicide.

Magnetic anomalies in single crystal Fe3O4 thin films

Fe3O4 thin films were reactively sputtered onto 〈100〉 and 〈110〉 MgO substrates. X‐ray diffraction data indicate single crystal Fe3O4 films under a tensile stress. Magnetization data show good agreement with bulk Fe3O4 for Ms, but also show the presence of a large anisotropy component manifest in the lack of saturation in fields up to 70 kOe. Conversion electron Mossbauer spectroscopy data taken at zero field also show good agreement with bulk Fe3O4 for the values of the hyperfine fields and isomer shifts of the two Fe3O4 sites, indicate good stoichiometry, but indicate the presence of a large anisotropy component randomizing the moments. In‐plane torque measurements on 〈100〉 oriented Fe3O4 are consistent with bulk Fe3O4 magnetocrystalline anisotropy. However, in‐plane torque measurements on 〈110〉 oriented Fe3O4 show the presence of a uniaxial anisotropy superimposed on the crystalline terms. This anisotropy, modeled as being due to the stress induced by epitaxy, changes the easy axis from the 〈111〉 direct...

Magnetically soft, high-moment, high-resistivity thin films using discontinuous metal/native oxide multilayers

Multilayers consisting of discontinuous metal layers with native oxide surfaces have been fabricated using CoxFe100−x alloys ranging in composition from pure Co to pure Fe. For the Fe-containing compositions, the composites are magnetically soft with resistivities in the range of 100 to 2000 μΩ cm. Mossbauer spectroscopy indicates a magnetically ordered Fe-oxide component, and the magnetic moment of the oxide phase for pure Fe/native oxide multilayers has been determined.

Mössbauer effect determination of chemical segregation in sputtered Co‐Cr films

Mossbauer spectroscopy has been used to determine several of the features of sputtered Co‐Cr films. Isotopically enriched 57Fe was used as a probe in the films and in bulk alloys employed as calibration standards. Film stoichiometries from 14.5 to 32 at. % Cr were measured. Three phases are observed. For films sputtered at 200 °C substrate temperature, one phase (about 40 at. % of the films) has a composition close to the overall film composition. The second and most accurately determined phase is Co rich, showing increasing Cr concentration and decreasing mole fraction with increasing overall Cr content. For example, a 21.3 at. % Cr film contains a Co‐rich phase with 6.0 at. % Cr. The third phase is Cr rich and nonmagnetic. Compositions have been determined indirectly through the film saturation moments. Assuming the Fe:Co ratio is constant in the various phases, the third phase contains about 37 at. % Cr for nearly all overall film compositions, close to the σ‐phase boundary. The relative amount of this...