Robert D. McMichael

Magnetocaloric effect in superparamagnets

The magnetocaloric effect is calculated for superparamagnetic materials as a function of temperature, field and cluster size. Assuming classical behavior, a universal curve is calculated from which an optimum cluster moment may be found for maximum entropy change upon application of a given field H at a given temperature T. Quantum effects are shown to be small for temperatures above 10 K and fields less than a few tesla. A comparison with results for a spin-72 paramagnet such as Gd3Ga5O12 (GGG) is made, which indicates that superparamagnetic materials such as magnetic nanocomposites offer the possibility of extending the upper useful temperature limit of paramagnetic materials for magnetic refrigeration.

Oxygen as a surfactant in the growth of giant magnetoresistance spin valves

We have found a novel method for increasing the giant magnetoresistance (GMR) of Co/Cu spin valves with the use of oxygen. Surprisingly, spin valves with the largest GMR are not produced in the best vacuum. Introducing 5×10−9 Torr (7×10−7 Pa) into our ultrahigh vacuum deposition chamber during spin-valve growth increases the GMR, decreases the ferromagnetic coupling between magnetic layers, and decreases the sheet resistance of the spin valves. It appears that the oxygen may act as a surfactant during film growth to suppress defects and to create a surface which scatters electrons more specularly. Using this technique, bottom spin valves and symmetric spin valves with GMR values of 19.0% and 24.8%, respectively, have been produced. These are the largest values ever reported for such structures.

Enhanced magnetocaloric effect in Gd3Ga5−xFexO12

The working refrigerant material in the majority of magnetic refrigerators has been Gd3Ga5O12 (GGG) which has an upper temperature limit near 15 K. In this paper we report on the field‐induced adiabatic magnetic entropy change, ΔSm(H,T), of a series of iron‐substituted gadolinium garnets (GGIG) Gd3Ga5−xFexO12 which have the potential to increase the working temperature range or to reduce the field requirements of cryogenic magnetic refrigeration. Depending on Fe concentration, x, the entropy change of these materials at applied fields of 0.9 and 5.0 T is much greater than that of GGG, especially at temperatures above 15 K. At low Fe concentrations, the results are consistent with formation of magnetically ordered clusters of spins at low temperatures. Room temperature electron paramagnetic resonance measurements show that Fe3+ ions mediate exchange interactions which are responsible for clustering at low temperatures.

Magnetic and optical properties of γ‐Fe2O3 nanocrystals

γ-Fe2O3 nanocrystals with a mean radius of 4.2 nm have been synthesized in a polymer matrix by an ion exchange and precipitation reaction. Magnetization and susceptibility data from experiment and computer simulations indicate that the system is superparamagnetic. The optical absorption edge is red shifted with respect to that of an epitaxially-grown single-crystal film of γ-Fe2O3. The red shift is attributed to lattice strain in the small particles.

Magnetic normal modes of nanoelements

Micromagnetic calculations are used to determine the eigenfrequencies and precession patterns of some of the lowest-frequency magnetic normal modes of submicron patterned elements. Two examples are presented. For a Permalloy-like ellipse, 350nm×160nm×5nm thick in zero field, the lowest frequency normal mode at 4GHz corresponds to precession in the “ends” of the ellipse. Other resonant frequencies are compared with the frequencies of spinwaves with discrete wave vectors. For a normally magnetized 50nmdiameter×15nm thick cobalt disk, the calculated eigenfrequencies increase linearly with applied field, mimicking the behavior of the experimental critical current for spin transfer instabilities in an experimental realization of this disk.

Magnetoresistance values exceeding 21% in symmetric spin valves

We report values of the giant magnetoresistance (GMR) effect exceeding 21% in symmetric spin valves, the highest values ever reported for such structures. The key elements in this achievement are the use of a Co/Cu/Co/Cu/Co multilayer in which the center Co layer is substantially thicker than the outer Co layers and the use of the antiferromagnetic insulator NiO at the top and bottom to pin the adjacent Co layers magnetically. The relative Co layer thicknesses suggest that some specular scattering of conduction electrons may occur at the metal/insulator interfaces and may enhance the GMR.

Demagnetized‐state dependence of Henkel plots. I. The Preisach model

The interpretation of interaction through Henkel plots and Δm plots has become popular in recent years; however, the demagnetized state is often not specified. In this paper, the demagnetized state dependence of Henkel plots is calculated using the classical Preisach model, the moving model, and the complete‐moving‐hysteresis model. In the calculation of the virgin remanence curve, ac, dc+, and dc− demagnetized states were used. The resulting collection of Henkel plots contains examples of ‘‘up’’ and ‘‘down’’ deviations from the Wohlfarth line by changing the demagnetized state only.

Spin-wave propagation in the presence of interfacial Dzyaloshinskii-Moriya interaction

In ferromagnetic thin films, broken inversion symmetry and spin-orbit coupling give rise to interfacial Dzyaloshinskii-Moriya interactions. Analytic expressions for spin-wave properties show that the interfacial Dzyaloshinskii-Moriya interaction leads to non-reciprocal spin-wave propagation, i.e. different properties for spin waves propagating in opposite directions. In favorable situations, it can increase the spin-wave attenuation length. Comparing measured spin wave properties in ferromagnet

Optimizing the giant magnetoresistance of symmetric and bottom spin valves (invited)

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Ferromagnetic resonance linewidth in thin films coupled to NiO

normal metal bilayers and other artificial layered structures with these calculations can provide a useful characterization of the interfacial Dzyaloshinskii-Moriya interactions.