arXiv subjects

Casey W. Miller

Publications and source records attributed to Casey W. Miller.

At least 19 recordsLinked to original sources

arXiv2024

Modeling when and how physics PhD students search for a research group: the role of interests and prior research experiences in timely group integration

Studying the factors that influence the quality of physics PhD students' doctoral experiences, especially those that motivate them to stay or leave their programs, is critical for providing them with more holistic and equitable support. Prior literature on doctoral attrition has found that students with clear research interests who establish an advisor-advisee relationship early in their graduate careers are most likely to persist. However, these trends have not been investigated in the context of physics, and the underlying reasons for why these characteristics are associated with leaving remain unstudied. Using semi-structured interviews with 40 first and second year physics PhD students, we construct a model describing the characteristic pathways that physics PhD students take while evaluating interest congruence of prospective research groups. We show how access to undergraduate research and other formative experiences helped some students narrow their interests and look for research groups before arriving to graduate school. In turn, these students reported fewer difficulties finding a group than students whose search for an advisor took place during the first year of their PhD. Lastly, we identify two characteristic types of students at a higher risk of leaving their programs: students who enter graduate school with broad interests and struggle to find a group, and students who join a research group early based on research interest alone and subsequently encounter issues with a negative mentoring relationship. This work serves as a major step toward creating a comprehensive model of how PhD students find a research group, and opens the door for future work to investigate how factors such as group culture and working environment impact the search process.

physics.ed-ph↗

arXiv2024

Physics PhD student perspectives on the importance and difficulty of finding a research group

Joining a research group is one of the most important events on a graduate student's path to becoming an independent physics researcher and earning a PhD. However, graduate students' perspectives on the experience of finding a research group are not well-documented in the literature. Understanding these perspectives is crucial for evaluating whether departments are providing students with adequate support while they search for a research group, and how difficulties during this process contribute to attrition. Semi-structured interviews with N=20 first and second year physics PhD students reveal that incoming graduate students see joining a research group as a significant decision, and recognize that it may impact whether they will be able to complete the program. We found that students who struggled to find a group felt isolated and worried about falling behind their peers, whereas students who were able to immerse themselves in a positive group environment reported increased sense of belonging in their programs. The process of finding a research group often held differential importance for students identifying as women and non-binary, who at times reported having to deprioritize their preferred research topic in order to be part of a more inclusive working environment. Although incoming graduate students characterized joining a research group as a significant decision, they often felt unprepared to make it. Moreover, they perceived an overall lack of guidance and structure from their departments, and characterized coursework as a barrier to searching for a group. Our findings suggest that providing students with better support during their group search process could help improve retention, particularly for traditionally underrepresented students, and improve students' overall satisfaction in their graduate programs.

physics.ed-ph↗

arXiv2023

Inequities and misaligned expectations in PhD students' search for a research group

Joining a research group is one of the most important events on a graduate student's path to earning a PhD, but the ways students go about searching for a group remain largely unstudied. It is therefore crucial to investigate whether departments are equitably supporting students as they look for an advisor, especially as students today enter graduate school with more diverse backgrounds than ever before. To better understand the phenomenon of finding a research group, we use a comparative case study approach to contrast important aspects of two physics PhD students' experiences. Semi-structured interviews with the students chronicled their interactions with departments, faculty, and the graduate student community, and described the resources they found most and least helpful. Our results reveal significant disparities in students' perceptions of how to find an advisor, as well as inequities in resources that negatively influenced one student's search. We also uncover substantial variation regarding when in their academic careers the students began searching for a graduate advisor, indicating the importance of providing students with consistent advising throughout their undergraduate and graduate experiences.

physics.ed-ph↗

arXiv2019

Typical Physics PhD Admissions Criteria Limit Access to Underrepresented Groups but Fail to Predict Doctoral Completion, including some additional information

This work aims to understand how effective the typical admissions criteria used in physics are at identifying students who will complete the PhD. Through a multivariate statistical analysis of a sample that includes roughly one in eight students who entered physics PhD programs from 2000-2010, we find that the traditional admissions metrics of undergraduate GPA and the Graduate Records Examination (GRE) Quantitative, Verbal, and Physics Subject Tests do not predict completion in US physics graduate programs with the efficacy often assumed by admissions committees. We find only undergraduate GPA to have a statistically significant association with physics PhD completion across all models studied. In no model did GRE Physics or GRE Verbal predict PhD completion. GRE Quantitative scores had statistically significant relationships with PhD completion in two of four models studied. However, in practice, probability of completing the PhD changed by less than 10 percentage points for students scoring in the 10 ^th vs 90 ^th percentile of US test takers that were physics majors. Noting the significant race, gender, and citizenship gaps in GRE scores, these findings indicate that the heavy reliance on these test scores within typical PhD admissions process is a deterrent to increasing access, diversity, and equity in physics. Misuse of GRE scores selects against already-underrepresented groups and US citizens with tools that fail to meaningfully predict PhD completion. This is a draft; see the journal for the published version. Additionally included in blue text are several responses to queries about this work.

physics.soc-ph↗

arXiv2018

Nanoscale Magnetic Behavior Localization in Exchange Strength Modulated Ferromagnets

Although ferromagnetism is in general a long-range collective phenomenon, it is possible to induce local spatial variations of magnetic properties in ferromagnetic materials. For example, systematic variation of the exchange coupling strength can be used to create systems that behave as if they are comprised of virtually independent segments that exhibit "local" Curie temperatures. Such localization of thermodynamic behavior leads to boundaries between strongly and weakly magnetized regions that can be controllably moved within the material with temperature. The utility of this interesting functionality is largely dependent on the inherent spatial resolution of magnetic properties - specifically the distance over which the exchange strength and corresponding properties behave locally. To test the degree to which this type of localization can be realized in materials, we have fabricated epitaxial films of Co[1-x]Ru[x] alloy featuring a nanometer scale triangular wave-like concentration depth profile. Continuous nanoscale modulation of the local Curie temperature was observed using polarized neutron reflectometry. These results are consistent with mean-field simulations of spin systems that encompass the possibility of delocalized exchange coupling, and show that composition grading can be used to localize magnetic properties in films down to the nanometer level. Since this is demonstrated here for an itinerant metal, we assert that for virtually any modulated magnetic material system, collective effects can be suppressed to length scales smaller than about 3 nm, so that magnetic behavior overall can be well described in terms of local material properties.

cond-mat.mtrl-sci↗

arXiv2015

Moderate Positive Spin Hall Angle in Uranium

We report measurements of spin pumping and the inverse spin Hall effect in Ni80Fe20/Uranium bilayers designed to study the efficiency of spin-charge interconversion in a super-heavy element. We employ broad-band ferromagnetic resonance on extended films to inject a spin current from the Ni80Fe20 (permalloy) into the uranium layer, which is then converted into an electric field by the inverse spin Hall effect. Surprisingly, our results suggest a spin mixing conductance of order 2x10 e19 m-2 and a positive spin Hall angle of 0.004, which are both merely comparable to those of several transition metals. These results thus support the idea that the electronic configuration may be at least as important as the atomic number in governing spin pumping across interfaces and subsequent spin Hall effects. In fact, given that both the magnitude and the sign are unexpected based on trends in d-electron systems, materials with unfilled f-electron orbitals may hold additional exploration avenues for spin physics.

cond-mat.mes-hall↗

arXiv2015

Spatial evolution of the ferromagnetic phase transition in an exchange graded film

A combination of experiments and numerical modeling was used to study the spatial evolution of the ferromagnetic phase transition in a thin film engineered to have a smooth gradient in exchange strength. Mean-field simulations predict, and experiments confirm that a 100 nm Ni[x]Cu[1-x] alloy film with Ni concentration that varies by 9 % as a function of depth behaves predominantly as if comprised of a continuum of uncoupled ferromagnetic layers with continuously varying Curie temperatures. A mobile boundary separating ordered and disordered regions emerges as temperature is increased. We demonstrate continuous control of the boundary position with temperature, and reversible control of the magnetically ordered sample volume with magnetic field.

cond-mat.mtrl-sci↗

arXiv2013

Depth dependent magnetization profiles of hybrid exchange springs

We report on the magnetization depth profile of a hybrid exchange spring system in which a Co/Pd multilayer with perpendicular anisotropy is coupled to a CoFeB thin film with in-plane anisotropy. The competition between these two orthogonal anisotropies promotes a strong depth dependence of the magnetization orientation. The angle of the magnetization vector is sensitive both to the strength of the individual anisotropies and to the local exchange constant, and is thus tunable by changing the thickness of the CoFeB layer and by substituting Ni for Pd in one layer of the Co/Pd stack. The resulting magnetic depth profiles are directly probed by element specific x-ray magnetic circular dichroism (XMCD) of the Co, Fe, and Ni layers located at different average depths. The experimental results are corroborated by micromagnetic simulations.

cond-mat.mtrl-sci↗

arXiv2013

Admissions Criteria and Diversity in Graduate School

In this work, I point out the negative implications for diversity in graduate school resulting from the use of cutoff scores on the GRE in the admissions process. In light of the data presented, as well as a swelling body of evidence suggesting no long term correlation with research success, I pose several challenges to the community related to the continued use of the GRE.

physics.ed-ph↗

arXiv2011

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 $\textrmÅ$)/[Gd(400 $\textrmÅ$)/W(50 $\textrmÅ$)]$_8$ 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 2-D Ising model may suggest that finite size effects play an important role in this system. Together, these observations suggest that nanostructuring may be a promising route to tailoring the magnetocaloric response of materials.

cond-mat.mes-hall↗

arXiv2011

Exchange bias of mu-metal thin films

The exchange bias of the soft ferromagnet mu-metal, Ni77Fe14Cu5Mo4, with the metallic antiferromagnet Fe50Mn50 has been studied as a function of ferromagnet thickness and buffer layer material. Mu-metal exhibits classic exchange bias behavior: the exchange bias (HEB) and coercive fields scale inversely with the ferromagnet's thickness, with HEB varying as the cosine of the in-plane applied field angle. While the exchange bias, coercivity, and exchange energy are greatest when the buffer layer material is (111) oriented Cu, amorphous Ta buffers allow the mu-metal to retain more of its soft magnetic character. The ability to preserve soft ferromagnetic behavior in an exchange biased heterostructure may be useful for low field sensing and other device applications.

cond-mat.mes-hall↗

arXiv2010

A student's guide to searching the literature using online databases

A method is described to empower students to efficiently perform general and literature searches using online resources. The method was tested on undergraduate and graduate students with varying backgrounds with scientific literature. Students involved in this study showed marked improvement in their awareness of how and where to find accurate scientific information.

physics.ed-ph↗

arXiv2007

Dynamic Spin-Polarized Resonant Tunneling in Magnetic Tunnel Junctions

Precisely engineered tunnel junctions exhibit a long sought effect that occurs when the energy of the electron is comparable to the potential energy of the tunneling barrier. The resistance of metal-insulator-metal tunnel junctions oscillates with an applied voltage when electrons that tunnel directly into the barrier's conduction band interfere upon reflection at the classical turning points: the insulator-metal interface, and the dynamic point where the incident electron energy equals the potential barrier inside the insulator. A model of tunneling between free electron bands using the exact solution of the Schroedinger equation for a trapezoidal tunnel barrier qualitatively agrees with experiment.

cond-mat.mtrl-sci↗

arXiv2007

Steric quenching of the switchable mirror effect Phys. Rev. B 75, 104109 (2007)

Scandium was substituted for yttrium to observe the effect of unit cell size on the optical metal-to-insulator (MIT) transition in the Y_(1-z)Sc_(z)H_(x) alloy system. The optical transmittance decreases significantly for z>0.10. Simultaneous electrical resistivity measurements confirm the transition from trihydride to dihydride behavior with increasing z. These observations imply a quenching of the MIT when the unit cell volume falls below a critical level that is consistent with the boundary between trihydride and non-trihydride forming rare-earth elements. A combinatoric model reveals this formation boundary corresponds to two or more Sc per unit cell.

cond-mat.mtrl-sci↗

arXiv2006

Dynamics of Spontaneous Magnetization Reversal in Exchange Biased Heterostructures

The dependence of thermally induced spontaneous magnetization reversal on time-dependent cooling protocols was studied. Slower cooling and longer waiting close to the Nèel temperature of the antiferromagnet ($T_N$) enhances the magnetization reversal. Cycling the temperature around $T_N$ leads to a thermal training effect under which the reversal magnitude increases with each cycle. These results suggest that spontaneous magnetization reversal is energetically favored, contrary to our present understanding of positive exchange bias.

cond-mat.mtrl-sci↗

arXiv2006

Quantitative Determination of the Adiabatic Condition Using Force-Detected Nuclear Magnetic Resonance

The adiabatic condition governing cyclic adiabatic inversion of proton spins in a micron-sized ammonium chloride crystal was studied using room temperature nuclear magnetic resonance force microscopy. A systematic degradation of signal-to-noise was observed as the adiabatic condition became violated. A theory of adiabatic following applicable to cyclic adiabatic inversion is reviewed and implemented to quantitatively determine an adiabaticity threshold $(γH_1)^2/(ω_{osc}Ω) = 6.0$ from our experimental results.

cond-mat.mtrl-sci↗

arXiv2006

Novel Fabrication of Micromechanical Oscillators with Nanoscale Sensitivity at Room Temperature

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^{-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 ~10^{-16} N in a 1 Hz bandwidth, 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 ~85% and alignment better than 1000 nm. Post-fabrication focused-ion-beam milling was used to further pattern the integrated magnetic structures when nanometer scale dimensions were required.

cond-mat.mes-hall↗

arXiv2006

Superiority of the $h$-index over the Impact Factor for Physics

Focusing specifically on physics periodicals, I show that the journal Impact Factor is not correlated with Hirsch's $h$-index. This implies that the Impact Factor is not a good measure of research quality or influence because the $h$-index is a reflection of peer review, and thus a strong indicator of research quality. The impact gap between multidisciplinary journals and physics-only journals is significantly reduced when $h$ is used instead of the Impact Factor. Additionally, the impact of journals specializing in review articles is inherently deflated using $h$ because of the limited number of annual publications in such periodicals. Finally, a reordering of the top ranking journals occurs with $h$ when only the physics articles of multidisciplinary journals are considered, falling more in line with the average physicist's interpretation of a journal's prestige.

physics.soc-ph↗