Traditional lock-in amplifiers employ a simple IIR filter which can have long measurement times to produce acceptable noise reduction. Using AC field Hall Effect measurements, we demonstrate that a combination of digital lock-in output stage filters, FIR and IIR low pass filters, can produce improved noise reduction and measurement time over the traditional approach of independent IIR filters or FIR filters. We will present step response, equivalent noise bandwidth and autocorrelation times for a n pole IIR filter (n <=4) followed by a filter response of any finite length. We will demonstrate that the performance of the hybrid output filter, with time measured in units of FIR time, depend only on the ratio of the IIR time constant to the FIR time. Finally, we will present criteria for statistically independent samples for the various filter configurations. AC field Hall Effect characterization is carried out with very low AC field frequency, typically 100 mHz, to avoid inductive pick-up in the measurement. Measurement times for a single voltage reading are as long as 60 seconds. The standard protocol for the Hall effect measurement includes current reversal and two different geometries which increases the number of voltage readings to 6 and results in a 6-minute total observation time to measure a single Hall voltage. Multiple Hall voltage readings are needed to calculate the of standard error of the Hall voltage and increases the measurement time in excess of 1 hour. An effective hybrid filter approach for AC field Hall effect measurement promises shorter measurement times or lower noise in the same observation window. Because it is a well understood material, an InAs sample was chosen as a model system for performance testing of the hybrid output filter in an AC field Hall effect measurement. Using currents of 10 microamps, the Hall voltage will be on the order of a few microvolts for this InAs sample depending on the field magnitude. In this work, an InAs sample with 6.52e16 cm-3 carrier density was first measured in a Lake Shore Cryotronics 8407 using the AC field option of the 8400 system. Using a 10 microamp excitation current and 0.644 T AC field amplitude, the magnitude of the measured Hall voltage was 2.44 microvolts with a 119 nanovolt standard deviation. The measurement electronics were subsequently replaced with the Lake Shore Cryotronics Model M81 Synchronous Source Measure platform equipped with a balanced current source (BCS-10) and voltmeter (VM-10) and the Hall effect measurement repeated with the same magnet and power supply. The M81 lock-in was configured with a hybrid output filter consisting of an IIR filter with a 4 second time constant and 12 dB roll-off combined with a FIR filter with 40 second averaging time which results in a total settle time of the filter of 60 seconds. The measured Hall voltage was 2.47 microvolts with a standard deviation of the Hall voltage of 37 nanovolt – a factor 3.2 improvement in measurement uncertainty over the 8400 system.

Improved AC Field Hall Measurements Using Hybrid Filters

InGaAs quantum wells (QW) grown by MOVPE are used to extend the absorption edge of GaAs photovoltaics into the infrared, increasing the current density with minimal loss in voltage as compared to a bulk GaAs device. GaAsP strain compensation is used to provide a tensile offset to the compressive strain in the wells, allowing growth of 50 pairs of strain balanced wells which did not exhibit detrimental effects of lattice relaxation/defect formation on photovoltaic figures of merit. Significant work was performed to optimize the growth and interfaces of these films to prevent step bunching and poor surface morphology and device performance when grown on offcut substrates, which are necessary for e.g. growth on non-polar germanium for a bottom junction or atomic ordering of InGaP for a top junction. A distributed Bragg reflector (DBR) is incorporated below the device, effectively increasing the optical path through the QWs and nearly doubling their sub-GaAs-bandgap collection per well. Dual-junction InGaP/GaAs devices were designed using a drift-diffusion model incorporating the optical effects of each subcell, connecting tunnel junction, and anti-reflective coating (ARC) along with the expected contribution of a varying number of QWs. These parameters were co-optimized to design control and QW-enhanced designs which were each nominally current-matched under AM0. Dual-junction devices were grown with an InGaP top junction coupled to either a control GaAs bottom cell without quantum wells or to the previously developed 50-QW enhanced GaAs bottom cell. Once fabricated, the QW-enhanced device provided an extra 2.6 mA/cm2 from sub-gap absorption with a minimal loss in voltage (11 mV) compared to a control device without QWs or DBR, providing an absolute efficiency increase of 3.6% under AM0.

InGaAs/GaAsP Multiple Quantum Well Enhanced Multijunction Photovoltaics

Fine-tuning pre-trained models has been ubiquitously proven to be effective in a wide range of NLP tasks. However, fine-tuning the whole model is parameter inefficient as it always yields an entirely new model for each task. Currently, many research works propose to only fine-tune a small portion of the parameters while keeping most of the parameters shared across different tasks. These methods achieve surprisingly good performance and are shown to be more stable than their corresponding fully fine-tuned counterparts. However, such kind of methods is still not well understood. Some natural questions arise: How does the parameter sparsity lead to promising performance? Why is the model more stable than the fully fine-tuned models? How to choose the tunable parameters? In this paper, we first categorize the existing methods into random approaches, rule-based approaches, and projection-based approaches based on how they choose which parameters to tune. Then, we show that all of the methods are actually sparse fine-tuned models and conduct a novel theoretical analysis of them. We indicate that the sparsity is actually imposing a regularization on the original model by controlling the upper bound of the stability. Such stability leads to better generalization capability which has been empirically observed in a lot of recent research works. On the other hand, the random and rule-based methods do not utilize task-specific data information while the projection-based approaches suffer from the projection discontinuity problem. To solve these problems, we propose a novel Second-order Approximation Method (SAM) which approximates the original problem with an analytically solvable optimization function. The tunable parameters are determined by directly optimizing the approximation function. We conduct extensive experiments on several tasks. The experimental results show that our proposed SAM model outperforms many strong baseline models and it also verifies our theoretical analysis.

On the Effectiveness of Parameter-Efficient Fine-Tuning

Magnetic Tunnel junctions (MTJs) with Co2FeAl Heusler alloy electrode are expected to show giant TMR ratio at room temperature (RT) because of its half-metallicity. In previous work, although Co2FeAl/MgO/CoFe-MTJ showed a TMR ratio of 330% at RT [1], it is still smaller than those for MTJs with CoFeB electrodes. In order to realize a large TMR ratio at RT, it is necessary to fabricate high-quality Heusler alloy electrodes and to control the interface at the atomic level. Therefore, I focused on molecular beam epitaxy (MBE) technique, which enables the fabrication of highly ordered Heusler alloy thin films [2] and atomic level interfacial modification [3]. In this work, we prepared epitaxial Co2FeAl thin films by MBE technique and characterized their magnetic properties to identify the half-metallicity of the Co2FeAl films. In addition, TMR effect in Co2FeAl/MgO/CoFe-MTJs fabricated by MBE were investigated.
The films were deposited on MgO(001) substrate using MBE and sputtering methods. The stacking structure of the Co2FeAl films were Cr(20)/Co2FeAl(50)/MgO(5) (unit of nm). After the fabrication of Co2FeAl layers, the films were annealed at Ta=200~700°C. Structural and magnetic properties were measured by XRD, VSM and FMR. The stacking structure of MTJs was Cr(20)/Co2FeAl(30)/MgO(2.0)/CoFe(5)/IrMn(10)/Cr(5) (unit of nm). After the microfabrication, the MTJ devices were annealed at TMTJ=275~425°C under 1T magnetic fields. TMR effects were measured by DC 4-probe method.
The Co2FeAl film with Ta=600°C showed high B2 order parameter and high magnetization. In addition, as shown in Fig.1, it showed very small damping constant α of 1.8×10-3, indicating that the film has good half-metallicity. Fig.2 shows the magnetoresistance curves of the MTJs measured at RT. A relatively large TMR ratio of 125% at TMTJ=400°C was observed and the TMR ratio can be improved by insertion of ultrathin Mg or Al into the Co2FeAl/MgO interface due to suppression of the interfacial state and oxidation.
This work was supported by NEDO, JSPS, GP-Spin Program and JST SPRING, Grant Number JPMJS2114. 
**References** [1] W. Wang, H. Sukegawa and K. Inomata, Appl. Phys. Lett. 95, 182502 (2009)
[2] M. Oogane, A. P. McFadden and C. J. Palmstrøm, Appl. Phys. Lett. 112, 262407 (2018)
[3] K. Himi, K. Takahashi and S. Mitani, Appl. Phys. Lett. 78, 1436 (2001)
 
![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/75375eae34e12d781b9ef3a55bce6e29.png)

Damping constant of Co2FeAl thin film deposited by molecular beam epitaxy and tunnel magnetoresistance effect using them

We have investigated the effect of Si surface atomically flattening on the memory characteristics of MFSFETs utilizing ferroelectric HfN (Fe-HfN) gate insulator for analog memory application. The Fe-HfN does not form the interfacial layer so that the Si surface flatness directly affects to the MFSFET characteristics. Not only the static and memory characteristics but the low-frequency noise characteristics of MFSFET were significantly improved by the Si surface atomically flattening which is important for the analog memory application.

MFSFET with Ferroelectric HfN for Analog Memory Application

In Advanced Lab classes, students often perform labs on radioactivity and nuclear spectroscopy as well as labs measuring several fundamental constants. Advanced labs classes can pose a problem for smaller public institutions as these&nbsp;classes usually have low enrollments and having multiple traditional lab setups is costly. However, advances in digital electronics have enabled the development of low cost (&lt;$2000), easy to use nuclear spectroscopy systems that can provide multiple lab activities. In addition to the traditional radioactivity and shielding labs, these systems can also be used with exempt radioactive sources to provide other laboratory activities including measuring to around 5%: 1) the charge of an electron using x-ray fluorescence and 2) the mass of an electron using Compton scattering. This&nbsp;eliminates&nbsp;the need for additional laboratory equipment making such labs more accessible to both undergraduate institutions and high schools.&nbsp;

A Simple Nuclear Lab To Measure The Electron's Rest Mass

There is a large gap in energy product (BHmax) between permanent magnets based on rare-earth elements (REE) and those which are REE-free. Although the need for REE-magnets in the highest performance devices is indisputable, there are many other applications with lower magnetic demand for which REE-magnets are still employed, in the absence of REE-free alternatives which can meet the requirements. Thus, providing new REE-free options with BHmax values within this gap would allow reducing the use of these critical resources. A common approach to address the mentioned issue is trying to enhance the properties of REE-free hard phases (e.g., ferrites) by combining them with soft magnetic phases in a composite material.
In the present work, the nature of the magnetic coupling of two magnetic phases (hard-soft) in atomic contact has been studied. First, the magnetic domain configuration of an isolated SrFe12O19 platelet shaped nanoparticle (hard phase) was studied by means of photoemission electron microscopy (PEEM) combined with x-ray absorption (XAS) and magnetic circular dichroism (XMCD) at the CIRCE beamline at the ALBA Synchrotron Light Facility,[1] concluding the particle was in a single-domain state with the magnetization direction perpendicular to the platelet plane. Afterwards, a 1.2 nm layer of metallic Co (soft phase) was deposited on top of the platelet by molecular beam epitaxy (MBE) and thorough characterization of the sample at the same synchrotron beamline revealed the magnetization of the soft metallic overlayer remained in-plane despite the out of plane magnetization displayed by the hard platelet. Thus, the two phases presented completely uncorrelated magnetic domain patterns.[2]
Our experiments therefore suggest that the two magnetic phases must not be exchange coupled to one another, which is somewhat unexpected for a system in which: (i) intimate contact between the phases is presumed, as the Co overlayer is directly grown onto the SrFe12O19 surface, and (ii) the soft phase dimension is clearly below the threshold size for rigid coupling. However, the dominant energy component here appears to be the shape anisotropy term, which for this specific case is calculated to overcome the Zeeman energy of the perpendicular magnetic field generated by the hard ferrite.
[1] L. Aballe, M. Foerster, E. Pellegrin, J. Nicolas, S. Ferrer, J. Synchrotron Radiat. 2015, 22, 745.
[2] G. D. Soria, C. Granados-Miralles, A. Mandziak, P. Jenuš, M. Saura-Múzquiz, M. Christensen, M. Foerster, L. Aballe, J. F. Fernández, J. de la Figuera, A. Quesada, J. Phys. D. Appl. Phys. 2021, 54, 054003.

Investigating the magnetic coupling of hard (SrFe12O19) and soft (Co) phases in atomic contact

Molecular Dynamics for the Computational Discoveries in Science
 By
 Dr. Nish Sonwalkar
 contact: Dr. Nish Sonwalkar nish@alum.mit.edu

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Molecular Dynamics for Computational Discoveries in Science
 by
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 Contact: nish@alum.mit.edu

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Molecular Dynamics for Computational Discoveries in Science
 by 
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Molecular Dynamics for Computational Discoveries in Science
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Improved AC Field Hall Measurements Using Hybrid Filters

InGaAs/GaAsP Multiple Quantum Well Enhanced Multijunction Photovoltaics

On the Effectiveness of Parameter-Efficient Fine-Tuning

Damping constant of Co2FeAl thin film deposited by molecular beam epitaxy and tunnel magnetoresistance effect using them

MFSFET with Ferroelectric HfN for Analog Memory Application

A Simple Nuclear Lab To Measure The Electron's Rest Mass

Investigating the magnetic coupling of hard (SrFe12O19) and soft (Co) phases in atomic contact

Molecular Dynamics for Computational Discoveries in Science Segment 6

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Molecular Dynamics MOOC: 1D Motion

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