Casey W. Miller

Enhanced giant magnetoimpedance effect and field sensitivity in Co-coated soft ferromagnetic amorphous ribbons

A 50 nm-thick Co film has been grown either on the free surface (surface roughness, ∼6 nm) or on the wheel-side surface (surface roughness, ∼147 nm) of Co84.55Fe4.45Zr7B4 amorphous ribbons. A comparative study of the giant magnetoimpedance (GMI) effect and its field sensitivity (η) in the uncoated and Co-coated ribbons is presented. We show that the presence of the Co coating layer enhances both the GMI ratio and η in the Co-coated ribbons. Larger values for GMI ratio and η are achieved in the sample with Co coated on the free ribbon surface. The enhancement of the GMI effect in the Co-coated ribbons originates mainly from the reduction in stray fields due to surface irregularities and the enhanced magnetic flux paths closure. These findings provide good guidance for tailoring GMI in surface-modified soft ferromagnetic ribbons for use in highly sensitive magnetic sensors.

Thermomagnetic analysis of FeCoCrxNi alloys: Magnetic entropy of high-entropy alloys

The equimolar alloy FeCoCrNi, a high-entropy alloy, forms in the face-centered-cubic crystal structure and has a ferromagnetic Curie temperature of 130 K. In this study, we explore the effects of Cr concentration, cold-rolling, and subsequent heat treatments on the magnetic properties of FeCoCrxNi alloys. Cr reductions result in an increase of the Curie temperature, and may be used to tune the TC over a very large temperature range. The magnetic entropy change for a change in applied field of 2T is ΔSm = −0.35 J/(kg K) for cold-rolled FeCoCrNi. Cold-rolling results in a broadening of ΔSm, where subsequent heat treatment at 1073 K sharpens the magnetic entropy curve. In all of the alloys, we find that upon heating (after cold-rolling) there is a re-entrant magnetic moment near 730 K. This feature is much less pronounced in the as-cast samples (without cold-rolling) and in the Cr-rich samples, and is no longer observed after annealing at 1073 K. Possible origins of this behavior are discussed.

Magnetocaloric effect in Gd/W thin film heterostructures

In an effort to understand the impact of nanostructuring on the magnetocaloric effect, we have grown and studied gadolinium in MgO/W(50 A)/[Gd(400 A)/W(50 A)]8 heterostructures. The entropy change associated with the second-order magnetic phase transition was determined from the isothermal magnetization for numerous temperatures and the appropriate Maxwell relation. The entropy change peaks at a temperature of 284 K with a value of approximately 3.4 J/kg K for a 0–30 kOe field change; the full width at half max of the entropy change peak is about 70 K, which is significantly wider than that of bulk Gd under similar conditions. The relative cooling power of this nanoscale system is about 240 J/kg, somewhat lower than that of bulk Gd (410 J/kg). An iterative Kovel–Fisher method was used to determine the critical exponents governing the phase transition to be β=0.51, and γ=1.75. Along with a suppressed Curie temperature relative to the bulk, the fact that the convergent value of γ is that predicted by the tw...

Impact of interfacial roughness on tunneling conductance and extracted barrier parameters

The net tunneling conductance of metal-insulator-metal tunnel junctions is studied using a distribution of barrier thicknesses consistent with interfacial roughness typical of state-of-the-art tunnel junctions. Moderate amounts of roughness cause the conductance to resemble that of much thinner and taller barriers. Fitting numerically generated conductance data that include roughness with models that assume a single-thickness barrier leads to erroneous results for both the barrier height and width. Rules of thumb are given that connect the roughness to the real space mean thickness and the thickness inferred from fitting the net conductance with traditional tunneling models.

Novel fabrication of micromechanical oscillators with nanoscale sensitivity at room temperature

In this paper, we report on the design, fabrication, and implementation of ultrasensitive micromechanical oscillators. Our ultrathin single-crystal silicon cantilevers with integrated magnetic structures are the first of their kind: They are fabricated using a novel high-yield process in which magnetic film patterning and deposition are combined with cantilever fabrication. These novel devices have been developed for use as cantilever magnetometers and as force sensors in nuclear magnetic resonance force microscopy (MRFM). These two applications have achieved nanometer-scale resolution using the cantilevers described in this work. Current magnetic moment sensitivity achieved for the devices, when used as magnetometers, is 10/sup -15/ J/T at room temperature, which is more than a 1000-fold improvement in sensitivity, compared to conventional magnetometers. Current room temperature force sensitivity of MRFM cantilevers is /spl sim/10/sup -16/ N//spl radic/Hz, which is comparable to the room temperature sensitivities of similar devices of its type. Finite element modeling was used to improve design parameters, ensure that the devices meet experimental demands, and correlate mode shape with observed results. The photolithographic fabrication process was optimized, yielding an average of /spl sim/85% and alignment better than 1 /spl mu/m. Postfabrication-focused ion-beam milling was used to further pattern the integrated magnetic structures when nanometer scale dimensions were required.

Tunable magnetocaloric effect in transition metal alloys.

The unpredictability of geopolitical tensions and resulting supply chain and pricing instabilities make it imperative to explore rare earth free magnetic materials. As such, we have investigated fully transition metal based “high entropy alloys” in the context of the magnetocaloric effect. We find the NiFeCoCrPdx family exhibits a second order magnetic phase transition whose critical temperature is tunable from 100 K to well above room temperature. The system notably displays changes in the functionality of the magnetic entropy change depending on x, which leads to nearly 40% enhancement of the refrigerant capacity. A detailed statistical analysis of the universal scaling behavior provides direct evidence that heat treatment and Pd additions reduce the distribution of exchange energies in the system, leading to a more magnetically homogeneous alloy. The general implications of this work are that the parent NiFeCoCr compound can be tuned dramatically with FCC metal additives. Together with their relatively lower cost, their superior mechanical properties that aid manufacturability and their relative chemical inertness that aids product longevity, NiFeCoCr-based materials could ultimately lead to commercially viable magnetic refrigerants.

Impact of interfacial magnetism on magnetocaloric properties of thin film heterostructures

Polarized neutron reflectometry was used to determine the depth profile of the magnetic moment per Gd atom, mGd, in a Gd(30 nm)/W(5 nm) multilayer. Despite sharp interfaces observed by transmission electron microscopy, mGd is systematically suppressed near the Gd-W interfaces. Because the peak magnetic entropy change is proportional to mGd2/3, this results in a reduction of the maximum achievable magnetocaloric effect in Gd-W heterostructures. By extension, our results suggest that creating materials with Gd-ferromagnet interfaces may increase the mGd relative to the bulk, leading to enhanced magnetocaloric properties.

The impact of barrier height distributions in tunnel junctions

We demonstrate that including continuous and discrete tunnel barrier height distributions in otherwise traditional tunneling formalisms enables straightforward modeling of several phenomena important to tunneling. Random barrier height inhomogeneities significantly impact the tunneling conductance, as evidenced by ideal tunneling models extracting faulty barrier parameters, with the incurred errors strongly dependent on the variance. Thermal smearing is addressed by transferring the energy distribution from the electrons to the barrier potential energy, thereby enabling zero-temperature tunneling models to model temperature dependent tunneling. For discrete tunneling channels, a secondary, impuritylike channel is shown to dominate the net conductance at surprisingly low impurity levels, implying that the observation of intrinsically large barrier heights is highly unlikely with transport measurements. Finally, spin-filter tunneling is modeled with independent tunneling channels whose barrier heights are l...

Magnetocaloric effect in nanoscale thin films and heterostructures

This review focuses on the magnetocaloric effect with special attention to nanoscale thin films and heterostructures. The authors outline the general phenomenon of the magnetocaloric effect and discuss how using materials in reduced dimensions can impact this emerging area. The authors note works of significance to date and highlight general features emanating from the community. They provide important details related to sample fabrication, relevant metrology, and discuss advanced data analyses, all of which are done in a tutorial fashion. Finally, the authors provide an outlook for the application of nanoscience to magnetocalorics.

Effects of preparation conditions on the magnetocaloric properties of Gd thin films

The impact of the deposition temperature and chamber gettering on Ta(5 nm)/Gd(30 nm)/Ta(5 nm) thin films magnetocaloric effect properties was investigated. Increasing the deposition temperature generally improves the entropy peak (magnitude, full width at half max, and temperature of the peak) but also leads to significant oxidation. Gettering the chamber prior to deposition not only reduced this oxidation issue but also increased the relative cooling power of films grown at elevated temperatures by as much as 33% over ungettered samples.