United States

Heat capacity is a basic physical quantity most important for studying magnetocaloric materials. Not only it reflects how much heat can be stored in the material, but it also allows to quantify the two main performance parameters: the isothermal entropy change Î”&lt;i&gt;S&lt;/i&gt; and the adiabatic temperature change Î”&lt;i&gt;T&lt;/i&gt;&lt;sub&gt;ad&lt;/sub&gt;. On top of that, being sensitive to structural, electronic and magnetic degrees of freedom, heat capacity also provides fundamental insights. In Fe&lt;sub&gt;2&lt;/sub&gt;P materials, at the exception of the binary parent, heat capacity studies were so far limited to indirect determinations of the magnetocaloric effect. Here, we will present an extensive heat capacity study of a prototypical MnFe(P,Si,B) compound. Semi-adiabatic relaxation calorimetry, commercial heat flow and home-made Peltier cells differential scanning calorimeters were used to record the heat capacity both around first-order ferromagnetic transition and at low temperatures. This systematic investigation fills several knowledge gaps on the promising MnFe(P,Si,B) magnetocaloric materials.&lt;br/&gt;First, we will describe the development of a Peltier cell differential scanning calorimeter designed â€œas an optionâ€ for commonplace commercial cryostat equipped with high magnetic field. This system can be used to measure &lt;i&gt;directly&lt;/i&gt; the isothermal entropy change Î”&lt;i&gt;S&lt;/i&gt; induced by a magnetic field and it even allows targeting the cyclic (reversible) effect due to successive magnetization/demagnetization. In MnFe&lt;sub&gt;0.95&lt;/sub&gt;P&lt;sub&gt;0.585&lt;/sub&gt;Si&lt;sub&gt;0.34&lt;/sub&gt;B&lt;sub&gt;0.075&lt;/sub&gt;, an exceptionally large cyclic entropy change at intermediate field (Î”&lt;i&gt;S&lt;/i&gt;&lt;sub&gt;cyclic&lt;/sub&gt; = 13.2 J kg&lt;sup&gt;-1&lt;/sup&gt; K&lt;sup&gt;-1&lt;/sup&gt; for Âµ&lt;sub&gt;0&lt;/sub&gt;Î”&lt;i&gt;H&lt;/i&gt; = 1 T) is observed.&lt;br/&gt;Then, the heat capacity is analyzed over a broad temperature range to provide an estimate of several important parameters, such as the electronic density of states (&lt;i&gt;N&lt;/i&gt;(E&lt;sub&gt;F&lt;/sub&gt;)â‰ˆ 7.2 states eV&lt;sup&gt;-1&lt;/sup&gt;f.u.&lt;sup&gt;-1&lt;/sup&gt;) or an estimate of the Debye temperature &lt;i&gt;Î¸&lt;/i&gt;&lt;sub&gt;D&lt;/sub&gt; â‰ˆ 455 K. Different methods (non-magnetic reference, Debye function, phonons calculations) were used to model the lattice contribution to the heat capacity. Our study highlights the difficulty to disentangle lattice, electronic and magnetic contributions in Fe&lt;sub&gt;2&lt;/sub&gt;P-type materials. It nonetheless reveals that the magnetic entropy plus latent heat is considerably larger in MnFe(P,Si,B) than in the parent Fe&lt;sub&gt;2&lt;/sub&gt;P.

MMM 2022

Heat Capacity and Giant Magnetocaloric Effect of a MnFe(P,Si,B) Compound investigated by semi adiabatic and Peltier Cell Calorimetry Techniques

Heat capacity is a basic physical quantity most important for studying magnetocaloric materials. Not only it reflects how much heat can be stored in the material, but it also allows to quantify the two main performance parameters: the isothermal entropy change Î”S and the adiabatic temperature change Î”Tad. On top of that, being sensitive to structural, electronic and magnetic degrees of freedom, heat capacity also provides fundamental insights. In Fe2P materials, at the exception of the binary parent, heat capacity studies were so far limited to indirect determinations of the magnetocaloric effect. Here, we will present an extensive heat capacity study of a prototypical MnFe(P,Si,B) compound. Semi-adiabatic relaxation calorimetry, commercial heat flow and home-made Peltier cells differential scanning calorimeters were used to record the heat capacity both around first-order ferromagnetic transition and at low temperatures. This systematic investigation fills several knowledge gaps on the promising MnFe(P,Si,B) magnetocaloric materials. First, we will describe the development of a Peltier cell differential scanning calorimeter designed â€œas an optionâ€ for commonplace commercial cryostat equipped with high magnetic field. This system can be used to measure directly the isothermal entropy change Î”S induced by a magnetic field and it even allows targeting the cyclic (reversible) effect due to successive magnetization/demagnetization. In MnFe0.95P0.585Si0.34B0.075, an exceptionally large cyclic entropy change at intermediate field (Î”Scyclic = 13.2 J kg-1 K-1 for Âµ0Î”H = 1 T) is observed. Then, the heat capacity is analyzed over a broad temperature range to provide an estimate of several important parameters, such as the electronic density of states (N(EF)â‰ˆ 7.2 states eV-1f.u.-1) or an estimate of the Debye temperature Î¸D â‰ˆ 455 K. Different methods (non-magnetic reference, Debye function, phonons calculations) were used to model the lattice contribution to the heat capacity. Our study highlights the difficulty to disentangle lattice, electronic and magnetic contributions in Fe2P-type materials. It nonetheless reveals that the magnetic entropy plus latent heat is considerably larger in MnFe(P,Si,B) than in the parent Fe2P.

technical paper

#### Welcome to the 67th Annual Conference on Magnetism and Magnetic Materials, MMM 2022!

The Conference was held from October 31 to November 4 and offers both in-person and on-demand content.

MMM 2022 is sponsored jointly by AIP Publishing and the IEEE Magnetics Society. Members of the international scientific and engineering communities interested in recent developments in fundamental and applied magnetism are invited to attend and contribute to the technical sessions. The technical program will include invited and contributed papers in oral and poster sessions, invited symposia, a tutorial, and several special sessions. This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

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Antiferromagnets have unique attributes such as high-speed dynamics and small stray fields, forming a new paradigm of spintronics (1,2). In particular, non-collinear antiferromagnets with chiral spin structures have attracted increasing attention as they exhibit various intriguing topological phenomena (3,4). Recent studies also demonstrated current-induced reversal (5) and rotation (6) of the chiral-spin structure in thin-film systems. An important issue that has remained to be explored is the stability of antiferromagnetic spin structure against thermal fluctuation, which determines the retention time of the spin structure as well as the critical current for the electrical manipulation. Here, we fabricate non-collinear antiferromagnetic D019-Mn3Sn nanodots with various diameters and measure their thermal stability factor Δ.
We deposit W(2)/Ta(3)/Mn3Sn(20)/MgO(1.3)/Ru(1) (in nm) on MgO(110) substrate by magnetron sputtering (7), followed by annealing at 500 oC for an hour. We confirm the M-plane-oriented epitaxial growth of Mn3Sn layer by X-ray diffraction. The stack was processed into circular dot devices with a diameter D of 175-1000 nm using electron beam lithography and Ar ion milling. Figure 1 shows Hall resistance RH as a function of the magnetic field H for different D together with the scanning electron microscopy (SEM) image. We then evaluate Δ of Mn3Sn dot through the measurement of switching probability versus amplitude of pulse magnetic field. For the analysis, we assume a six-fold in-Kagome-plane anisotropy to model the magnetostatic energy of Mn3Sn. Figure 2 shows D dependence of Δ. The reduction of Δ at D of less than about 300 nm is observed, indicating a single domain reversal of spin structure in this scale, consistent with a previous report on Mn3Sn nanowire (8). Our result offers an important insight to understand the thermal activation of antiferromagnets, allowing one to design reliable and efficient antiferromagnetic devices. 

This work was partly supported by JSPS Kakenhi (Nos. 19H05622 and 22K14558), Casio foundation (No. 39-11), and RIEC Cooperative Research Projects. 

**References:** 
(1) T. Jungwirth et al., Nat. Nanotechnol. 11, 231 (2016). 
(2) V. Baltz et al., Rev. Mod. Phys. 90, 015005 (2018). 
(3) S. Nakatsuji et al., Nature 527, 212 (2015). 
(4) A. Nayak et al., Sci. Adv. 2, e1501870 (2016). 
(5) H. Tsai et al., Nature 580, 608 (2020). 
(6) Y. Takeuchi et al., Nat. Mater. 20, 1364 (2021). 
(7) J.-Y. Yoon et al., Appl. Phys. Express 13, 013001 (2020). 
(8) H. Bai et al., Appl. Phys. Lett. 117, 052404 (2020). 

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Thermal stability of non collinear antiferromagnetic Mn3Sn nanodot

The discovery of two-dimensional van der Waals ferromagnets [1,2] has been long anticipated since the effects of spatial dimensionality on criticality were recorded [3,4]. The potential applications of this new class of materials seem vast within the field of spintronics. Fe3GeTe2 has already proved highly efficient for spin-orbit torque switching [5], yet rich skyrmionic textures [6,7] and semi-metallicity [8] are also found within its roster of ever-growing phenomena. Fe5GeTe2, a related compound with a higher Tc [9], presents its own unique set of magnetic order that can be tuned with doping [10] and gate voltage [11]. In a recent collaboration [12] we found spin-orbit torques within bulk Fe3GeTe2 due to a site preference that breaks the crystal’s centrosymmetry [13]. Meanwhile, in Fe5GeTe2, we found skyrmionic textures present even up to room temperature [14]. To understand the origins of these effects one must delve into the underlying electronic structure to provide a comprehensive understanding of the associated spin dynamics.

Using a variety of first-principles methods [12,14,15] we evaluate the exotic phenomena observed in both Fe3GeTe2 and Fe5GeTe2. In Fe3GeTe2 strong ‘hidden’ current-induced torques are harvested by each of the two-dimensional Fe3GeTe2 layers separately. Analysis of the electronic structure elucidates an interplay of spin and orbital degrees of freedom which have a profound impact on spin-orbit torques. A drastic difference in the behavior of the individual components of the torque results in non-trivial switching. In Fe5GeTe2 we use first principles methods in combination with atomistic spin-dynamics simulations to characterize the domain wall and subsequent spin textures. These distinct features from both compounds present an opportunity to tailor the spin and orbital dynamics with Fe concentration, opening a new era of van der Waals spin orbitronics. 
**References** [1] C. Gong et al., Nature 546, 265 (2017).
[2] B. Huang et al., Nature 546, 270 (2017).
[3] R. B. Griffiths, Physical Review 136, 437 (1964).
[4] N. D. Mermin et al., Physical Review Letters 17, 1133 (1966).
[5] M. Alghamdi et al., Nano Letters 19, 4400 (2019).
[6] B. Ding et al., Nano Letters 20, 868 (2020).
[7] H.-H. Yang et al., 2D Materials 9, 025022 (2022).
[8] K. Kim et al., Nature Materials 17, 794 (2018).
[9] A. F. May et al., ACS Nano 13, 4436 (2019).
[10] A. F. May et al., Physical Review Materials 4, 074008 (2020).
[11] C. Tan et al., Nano Letters 21, 5599 (2021).
[12] F. Martin et al., arXiv:2107.09420.
[13] A. Chakraborty et al., Advanced Materials 34, 2108637 (2022).
[14] M. Schmitt et al., arXiv:2204.11348.
[15] T. G. Saunderson at al., arXiv:2204.13052.

Harnessing the spin and orbital dynamics of iron based 2D van der Waals ferromagnets

Current-induced spin-orbit torque (SOT) originating from the spin-Hall effect (SHE) has attracted attention due to their potential applications for SOT-MRAM, skyrmion and domain wall devices. For their applications, heavy metal with high spin Hall angle (|θSH|) and low resistivity (ρxx) is necessary for an efficient SOT operation. We have proposed Co/Pt/(Ir or Ru)/Pt/Co synthetic antiferromagnetic (AF) systems for SOT-MRAM (Fig. 1), and found that the Pt/Ir/Pt spacer layer exhibits AF interlayer exchange coupling (Jex) as well as large || and low ρxx [1, 2].
In this study, we study the current-induced SOT in perpendicularly magnetized Co/Pt/Ir/Pt/Co synthetic AF system which exhibits nearly compensated magnetization. The synthetic AF structures with various Pt, Ir and Co layer thicknesses were prepared by using UHV sputtering system. The stacks were patterned into Hall bar devices and magnitude of the was evaluated from the shift of the reversal magnetic field due to the SOT current. The results were compared with those of the ferromagnetic stack systems with Co/Pt and Co/[Pt/Ir]-multilayer structures.
Figures 2(a) and 2(b) show the typical current-induced SOT switching under various fixed external magnetic fields (Hy) in Sample A (Co/Pt structure) and Sample B (synthetic AF structure), respectively. The magnetizations of the two Co layers in the synthetic AF can be switched between two anti-parallel states simultaneously by SOT. The estimated values of current density () at Hy=0 mT for Samples A and B are 7.9×107 and 4.2×107 A/cm2, respectively [3]. The magnitude of in Sample B is about half that in Sample A. The magnitudes of estimated |θSH| for Samples A and B are 7.0% and 15.6%, respectively [3], which is consistent with SOT switching behavior (Fig. 2). In this presentation, we will also show the dependence of the magnitude of SHE on the strength of Jex in the synthetic AF system. This work was supported by the CIES Consortium, Spin-RNJ, RIEC, JST OPERA (JPMJOP1611), MEXT Next generation X-nics and JSPS KAKENHI (JP19H00844, JP21K18189).
 
**References** [1] Y. Saito, N. Tezuka, S. Ikeda and T. Endoh, Phys. Rev. B 104,064439 (2021). [2] Y. Saito, S. Ikeda and T. Endoh, Appl. Phys. Lett. 119, 142401 (2021). [3] Y. Saito, S. Ikeda and T. Endoh, Phys. Rev. B 105, 054421 (2022). 

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Enhancement of current to spin current conversion efficiency in synthetic antiferromagnetic layer system

Substantial gaps remain in the understanding of spin transport in low-Z nonmagnetic (N) metals, despite their use in spintronic technology. While Elliot-Yafet spin relaxation is known to prevail in such metals, the βi constants that relate spin relaxation times to momentum relaxation times are only known for certain defect types (i), primarily in simple metals like Cu [1] and Al [2]. Other low-Z metals with potentially long spin diffusion lengths have been barely explored [3], and their βi remain unknown. In this work we thus explore spin transport in Mg, using nonlocal spin valves (NLSVs). Mg-based NLSVs were deposited by UHV evaporation on Si/SiNx using Al and Ti seed layers. As shown by specular wide-angle X-ray diffraction (Fig. 1(a)) and (0002) pole figures (Fig. 1(b)), strong out-of-plane (0001) texture arises, while in-plane texture (Fig. 1(c)) is negligible. Significantly, the (0001) texture of HCP Mg is significantly stronger with HCP Ti vs. FCC Al seeds (right and left of Figs. 1(b,c)). These Mg films exhibit the lowest residual resistivities (0.99 μΩ cm) and highest residual resistivity ratios (>6) reported for polycrystalline Mg films and nanowires. In NLSVs, the low-temperature spin diffusion length, βphonon, and βdefect reach 550 nm, 28000, and 5300 in Ni80Fe20/Al(5 nm)/Mg(100 nm) NLSVs, compared to 370 nm, 16000, and 1200 in Ni80Fe20/Ti(4 nm)/Mg(100 nm) NLSVs (Fig. 2). We hypothesize that these improvements in spin transport parameters in the Al-seed case arise due to weaker (0001) texture. Specifically, recent spin-hot-spot-based [4] theoretical predictions for HCP Mg predict 2200% higher β for spins injected along the Mg c-axis vs. in the basal plane [5]; our results may constitute the first experimental evidence for this highly anisotropic spin relaxation. This work was supported by the NSF. 
**References** [1] J. D. Watts, L. O’Brien, J. S. Jeong et al., Phys. Rev. Mater., Vol. 3, p.124409 (2019). [2] J. D. Watts, J. T. Batley, N. A. Rabideau et al., Phys. Rev. Lett., Vol. 128, p.207201 (2022). [3] H. Idzuchi, Y. Fukuma, L. Wang et al., Appl. Phys. Exp., Vol. 3, p.049202 (2010). [4] J. Fabian and S. Das Sarma, Phys. Rev. Lett., Vol. 81, p.5624 (1998). [5] B. Zimmermann, P. Mavropoulos, N. Long et al., Phys. Rev. B, Vol. 93, p.144403 (2016) 
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Spin Transport in Mg based Nonlocal Spin Valves

The high cost of CoFe alloys has prevented their widespread use regardless of their promising properties: high magnetisation, high Curie temperature, as soft magnetic materials for high temperature applications. Our research has shown that the additional of AlMn can be used to create CoFeMnAl alloys, which have reduced Co content but similar saturation magnetisation as CoFe alloys, therefore providing a more economical alternative to pure CoFe soft magnets. The research studied how the addition of Al to CoFeMn0.5 changed the alloy phases and hence the magnetic properties. The CoFeMn0.5Alx alloys (0 < x <0.75) were fabricated using arc-melting, and characterised using X-ray diffraction (phases present), scanning electron microscope (composition) and SQUID magnetometry (magnetic properties as a function of magnetic field and temperature). It was found that for Al content of x < 0.2, there existed mixed FCC and BCC phases, while with further addition of Al, the BCC phase was stabilised (Figure 1a.). This lead to an increase in the saturation magnetisation and a decrease in the coercive field (Figure 1b). The alloy CoFeMn0.5Al0.25 was determined to have a competitive saturation magnetisation compared to CoFe, but with the reduction of Co concentration in the alloy should also be cheaper to produce. 
**References:** 

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Figure 1a. XRD of the CoFeMn0.5Alx alloys. 1b Saturation magnetisation and coercive field as a function of Al concentration.

CoFeMn0.5Alx: High magnetization CoFe based soft alloys with reduced Co content

Magnetic Tunnel Junctions (MTJs) are multi-layered devices that use tunnel magnetoresistance (TMR) to detect changes in magnetic field. [1]. MTJs with specific characteristics are relatively complex to fabricate due to the high dependence of their magnetic properties, especially perpendicular magnetic anisotropy (PMA), on individual film thicknesses and interfaces. Post-growth modification tools, such as ion irradiation [2], are perfect when the properties of such structures are to be modified. Ion irradiation is a versatile tool that causes structural changes in the material’s lattice by forming point defects, that in turn cause changes in the magnetic properties, which can be controlled by tuning the fluence, energy, and ion current density of the ion beam [3]. 
In this study, we use 30 keV argon and neon ions to modify the magnetic properties of a MTJ half stack [Sub-SiO2/ Ta (5)/ MgO (1.5)/ Co40Fe40B20 (1.6)/ W (0.5)/ Ta (2)] by irradiating with fluences between 1×1013 ions/cm2 and 1x1015 ions/cm2. The low fluence ion irradiation (<1014 ions/cm2) changed the anisotropy of magnetic layers from out-of-plane to in-plane, and even achieve a canted anisotropy. The calculated displacement per atom (DPA) in CoFeB using the TRIM [4] simulation tool ranges between 0.13 and 32.75 for fluences 1×1013 ats/cm2 and 1x1015 ions/cm2. We found that an irradiation of even 1013 ions/cm2 fluence may cause a reduction in effective anisotropy by around 53%. We envisage that low fluence irradiation (<1014 ions/cm2) limits significant intermixing of layers which avoids the formation of a dead layer in the stack and degradation of TMR performance. These findings are important for fabricating better magnetic sensors with lower signal-to-noise ratios, MRAMs with voltage tunability, tuning properties of spin-orbit-torque based devices, etc. Detailed experimental results obtained from Magneto Optical Kerr Effect Spectroscopy (MOKE) and superconducting quantum interference device (SQUID) supported by DYN-TRIM simulations will be presented in the conference. 
**References:** [1] C. Zheng, K. Zhu, S.C. de Freitas, J.Y. Chang, J.E. Davies, P. Eames, P.P. Freitas, O. Kazakova, C.G. Kim, C.W. Leung, S.H. Liou, A. Ognev, S.N. Piramanayagam, P. Ripka, A. Samardak, K.H. Shin, S.Y. Tong, M.J. Tung, S.X. Wang, S. Xue, X. Yin, P.W.T. Pong, Magnetoresistive Sensor Development Roadmap (Non-Recording Applications), IEEE Transactions on Magnetics. 55 (2019) 1–30. 
[2] J. V. Kennedy, W.J. Trompetter, P.P. Murmu, J. Leveneur, P. Gupta, H. Fiedler, F. Fang, J. Futter, C. Purcell, Evolution of Rutherford’s ion beam science to applied research activities at GNS Science, J R Soc N Z. (2021) 1–18. 
[3] A. Mahendra, P. Gupta, S. Granville, J. Kennedy, Tailoring of magnetic anisotropy by ion irradiation for magnetic tunnel junction sensors, Journal of Alloys and Compounds. 910 (2022) 164902. 
[4] J.F. Ziegler, M.D. Ziegler, J.P. Biersack, SRIM - The stopping and range of ions in matter (2010), Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms. 268 (2010) 1818–1823.

Tailoring Perpendicular Magnetic Anisotropy of Cobalt Iron alloy (CoFeB) Using Ion Irradiation

Investigations on rare-earth intermetallics are at the forefront as they exhibit novel physical phenomena and can be used for spintronics, magnetic refrigeration, and memory applications. R2T3X5 series (R-rare-earth element, T-Transition element, and X-p-block element) is one of these well-studied systems with novel physical phenomena[1,2], whose application perspectives must be further explored. Hence, in this work, magnetic, magnetoresistance(MR), and magnetocaloric(MCE) properties of Tb2Co3Ge5 from the same series have been investigated to gain a better understanding of its physical features and potential applications[3]. The compound has been prepared by arc-melting technique followed by 15 days of annealing at 1023K. The compound crystallizes in Lu2Co3Si5 structure (SG - I12/c1) and exhibits successive antiferromagnetic transitions at T=31.5 K, 19.4 K, and 11 K, respectively. The antiferromagnetic character is confirmed by the modified Curie-Wiess behavior in the inverse magnetic susceptibility (B=10 kOe) with Curie-Weiss temperature (Î¸P)= -8.2 K. Further studies of heat capacity and electrical resistivity show that the crystal electric field effect has an effect and that, at low temperatures, there is a magnon gap of 17.6 K, which is a sign of the complex antiferromagnetic ground state. As the magnetic field(B) increases, the antiferromagnetic ground state shows a metamagnetic transition around the critical magnetic field, Bc=40 kOe, which has an impact on both MR and MCE. As shown in Fig.1, the MR properties reveal a crossover from positive to negative around Bc, due to the presence of metamagnetic transition. At higher fields, B=90 kOe, the negative MR reaches a maximum of 51% at 12 K. Similarly, variation of B above Bc alters the inverse MCE to conventional type with enhancement in MCE properties (See Fig.2). The crossover in the MR and MCE is more suitable for the magnetic refrigeration application[4] as it can switch between cooling and heating cycles as B varies. Acknowledgments: ITMS: 313011D232 ERDF; and also by VEGA1/0705/20, 1/0404/21.

Influence of Metamagnetic transition on Magnetocaloric and Magnetoresistance properties of Tb2Co3Ge5

Magnetometer with diamond nitrogen-vacancy (NV) centers [1] is a highly-sensitive magnetic sensor under the application of a highly uniform (99.9%) bias magnetic field
[2]. The integration of magnetometer with diamond NV magnetic sensors is required for various applications. However, there is a problem about the design of small magnet generating
uniform magnetic field in the conventional methods. Therefore, in the study, we propose innovative method based on inverse problem approach for optimizing the shape of magnet.
To optimize the shape of magnet, we estimate the shape of the magnet from the perfectly uniform magnetic field we set up. The magnet is groups of magnetic moment m as shown in Fig.
1(a). Figure 1(b) shows the optimized shape of magnet from the model of vertical rectangle. In Figure 1(c), we depicted cross-section of each z-stage of optimized the magnet vertical
rectangle. Figure 1(d) shows the magnetic field Bz on the diamond surface. The uniformity was 98.5% before optimization and improved to 99.9% after optimization on the diamond
surface. However, the uniformity on the entire diamond was low at 89.0%. To achieve the target magnetic field uniformity, we changed the thickness of the diamond NV center from 2mm
to 0.1 mm and model from vertical rectangle to horizontal rectangle. Figure 2(a)(b) show the model of horizontal rectangle and the optimized shape of magnet. In Fig. 2(c), we depicted
cross-section of each z-stage of optimized the magnet of horizontal rectangle. The magnetic field on the diamond surface was 99.6% and the uniformity of the entire diamond improved to
97.5%. We developed a method to determine the shape of a magnet from a highly uniform magnetic field. As the next step, we fabricate the optimized small magnet for diamond NV center
and evaluate the improvement in magnetic sensitivity. 

**Acknowledgement:** 

This work was supported by the Uehara Memorial Foundation and Hitachi Global Foundation. 

**References:** 

[1] A. Kuwahata et al., Scientific Reports 10 (1), 1-9,(2020). 
[2] Alexander P. Nizovtsev et al., nanomaterials 11 (5), 1-13 (2021). 

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Development of a compact magnet for applying a highly uniform magnetic field to a diamond magnetic sensor by inverse problem analysis

Magnetic particle imaging (MPI) utilizes the nonlinear magnetic response of superparamagnetic iron oxide nanoparticles (SPIONs) for applications such as cell tracking,
neuroimaging, and targeted hyperthermia. As SPIONs are central to MPI, understanding their properties is critical. A magnetic particle spectrometer (MPS) is often used to asses the
SPIONs’ magnetic properties. Despite MPS’s utility in facilitating MPI research and nanoparticle chemistry, among other uses, there are limited commercial MPS devices and few
comprehensive open-source resources on their construction and use. Here, we report the detection limit of an open-source MPS device [1]. 

**Methods:** 

Sensitivity limits were determined from an 11 phantom dilution series, each 4 μL of Synomag-D 70nm (Lot:16321104-02, Micromod Partikeltechnologie GmbH, Germany).
Samples were placed in the apex of a microcentrifuge tube, ranging from undiluted (6 mg Fe/mL) to 0.98 μg Fe/mL. For each, we acquired three data-sets with 40 acquisitions (sinusoidal
drive field, 10 mT, 23.8 kHz, 29 ms) with the sample in and 40 with the sample out. The mean of sample-in minus sample-out signal was plotted against SPION mass and fit using a linear
regression. The detection limit was defined as the Fe mass at which the regression intercepted the standard deviation of the sample out acquisitions. We repeated this analysis for the 3rd,
5th, and 7th harmonic component of the signal.
Results: The measured detection limit is 1.3 ng (fit R2=0.999) for the 3rd harmonic component of the signal, 4 ng for the 5th harmonic, and 7 ng for the 7th. 

**Discussion:** 

This open-source MPS device is relatively cheap and simple and has a sensitivity comprable or higher than other similar systems [2, 3, 4]. Iron laden cells carry roughly 50 pg
per cell [4], thus we anticipate this detection limit to correspond to about 26 cells for those studies. Further, as the acquisition is only 29 ms per sample, and minimal sample preparation, it
is well-suited for synthesis process validation. 

**References** 

[1] E. Mattingly, E. E. Mason, K. Herb, M. Sliwiak, K. Brandt, C. Z. Cooley, and L. L. Wald. OS-MPI: An open-source magnetic particle imaging project. International Journal
on Magnetic Particle Imaging, 6(2):1–3, 2020. 
[2] M. Graeser, A. Von Gladiss, M. Weber, and T. M. Buzug. Two dimensional magnetic particle spectrometry. Physics in Medicine and Biology,
62(9):3378–3391, 2017. 
[3] N. Garraud, R. Dhavalikar, M. Unni, S. Savliwala, C. Rinaldi, and D. P. Arnold. Benchtop magnetic particle relaxometer for detection, characterization and analysis of magnetic
nanoparticles. Physics in Medicine and Biology, 63(17), 2018. 
[4] N. Loewa, F. Wiekhorst, I. Gemeinhardt, M. Ebert, J. Schnorr, S. Wagner, M. Taupitz, and L. Trahms. Cellular uptake of magnetic nanoparticles quantified by magnetic particle
spectroscopy. IEEE Transactions on Magnetics, 49(1):275–278, 2013. 

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The Detection Limit of an Open Source Magnetic Particle Spectrometer

Interest in superconducting computing is being driven by two concerns. On the one hand, large-scale computing facilities and data centers are using electrical power at an ever-increasing rate. Projections suggest that a fully superconducting computer would consume considerably less power than conventional semiconductor-based computers [1]. On the other hand, a quantum computer based on superconducting qubits would benefit from having a classical superconducting processor nearby in the cold space, to avoid a plethora of control and readout lines between room temperature and the cold space [2]. Building a large-scale memory for a superconducting computer is a challenge. One approach is to use Josephson junctions containing ferromagnetic materials as the basic memory element for such a memory [3]. In that approach, the information storage device is a Josephson junction containing a pseudo spin valve. Depending on the relative orientation of the two magnetizations in the spin valve, and with suitably chosen thicknesses of both magnetic layers, the ground-state phase difference across the junction can be either zero or π. We have demonstrated two different types of junctions that can be controllably switched between the 0-phase state and the π-phase state [4,5]. While such junctions are promising components for a scalable superconducting memory [6], much more work needs to be done to enable scaling of this approach to a large-scale memory array.
Work supported by the US DOE under grant DE-FG02-06ER46341, by Northrop Grumman Corporation, and by IARPA via U.S. Army Research Office contract W911NF-14-C-0115. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the ODNI, IARPA, or the U.S. Government. 
**References** [1] D.S. Holmes, A.L. Ripple, & M.A. Manheimer, IEEE Trans. Appl. Supercond. 23, 1701610 (2013). 
[2] E. Leonard et al., Phys. Rev. Applied 11, 014009 (2019). 
[3] A.Y. Herr & Q.P. Herr, US Patent 8,270,209 (2012). 
[4] E. C. Gingrich, B. M. Niedzielski, J. A. Glick, Y. Wang, D. L. Miller, R. Loloee, W. P. Pratt Jr., and N. O. Birge, Nature Phys. 12, 564 (2016). 
[5] J.A. Glick, V. Aguilar, A. Gougam, B.M. Niedzielski, E.C. Gingrich, R. Loloee, W.P. Pratt, Jr., and N.O. Birge, Science Advances 4, eaat9457 (2018). 
[6] I. M. Dayton et al., IEEE Magnetics Lett. 9, 3301905 (2018).

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