United States

Magnetofluidic cooling (MFC, Figure 1a) is a passive heat transfer technique governed by the principle of thermomagnetic convection (TMC)1, 2. Under the effect of a thermal and magnetic field gradient, the magnetic fluid’s temperature-dependent response leads to TMC3. However, practical applications require specific device design and materials, mainly determined by the flow behavior of the magnetic fluid. However, the opaque nature of the magnetic fluid limits the experimental flow investigations. We addressed this challenge by a range of experimental setups to measure velocity profiles, vorticity, and temperature profiles. We developed a hybrid PIV method to measure velocity profiles by fabricating a bonding-free 3D PDMS device and analyzed by a MATLAB based image analysis code. The results of the PIV analysis are summarized in Figure 1. We utilized the experimental findings to develop a simulation model and studied the effect of magnetic and hydrodynamic properties of the fluid on the MFC process (Figure 2). Simulation-driven system design was performed4. A ferrofluid was developed and integrated with our MFC system, significantly enhancing device performance. We found that the MFC flow velocity, vorticity increases with increasing HL power, resulting in higher cooling at higher HL powers (Figure 2). Experiments and simulations demonstrated the role of magnetoconvection on device performance. The new device showed enhanced performance viz. cooling up to 200°C at a heat load of 10 W. Our superior device performance is attributed to increased fluid mixing and mass transfer during MFC. Acknowledgments This research is supported by grants from the National Research Foundation, Prime Minister’s Office, Singapore under its Campus of Research Excellence and Technological Enterprise (CREATE) programme.References: 1. V. B. Varma, Energy Efficient Green Magnetic Cooling, 2019. https://youtu.be/caInO_IxnHQ. CREATE Symp.- Climate Change, Singapore 2. R. V. Ramanujan, Pushing Frontiers, Issue 14, 32 (2018). 3. M. S. Pattanaik, V. B. Varma, S. K. Cheekati, G. Prasanna, N. M. Sudharsan and R. V. Ramanujan, Energy Conversion and Management, vol. 198, p. 111819 (2019). 4. V. B. Varma, M. S. Pattanaik, S. K. Cheekati and R. V. Ramanujan, Applied Energy, (2020- in Communication).

MMM 2020

Study of Thermo-Magnetoconvection of a Magnetofluidic Cooling Device

The 65th Annual Conference on Magnetism and Magnetic Materials (MMM 2020) was held in a virtual form. The COVID-19 pandemic, with its associated uncertainty about international travel and the ability to gather in large groups, meant that the Steering Committee decided to cancel the face-to-face meeting that had been planned in Palm Beach, Florida November 2-6, 2020.

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It is over twenty years since the discovery of a giant magnetocaloric effect (GMCE) in Gd-Si-Ge [1], and in that time two low-cost GMCE materials systems have been pursued towards commercialization: Mn-Fe-P-Si [2] and La-Fe-Si [3]. This relatively small number of target room temperature magnetic refrigerants underscores the need for time- and cost-effective magnetocaloric materials discovery projects. The ideal material candidates for magnetocaloric applications must be non-toxic, inexpensive, and readily manufacturable [4]. High- throughput workflows, guided by machine learning and other statistical methods in material informatics, are essential to streamlining the process of screening new possible materials, making the experimental process as targeted and efficient as possible. This presentation describes two approaches to materials screening based on simulated data. The first approach is a modification of the screening method of Bocarsly et al. [5], wherein lattice deformation across the magnetic phase transition (magnetic deformation) was computed based on the difference between unit cell dimension lengths with and without magnetic interactions in models based on density function theory (DFT). We add a phenomenological parameter calculated from the electronic density of states available from the Materials Project [6]. The addition of this new parameter can enhance the prediction of magnetic field-induced entropy change, ΔSM, with nearly 60% better correlation than using magnetic deformation alone. The second approach is motivated by the relative scarcity of consistent magnetic entropy data for stoichiometric compounds, which limits the ability of researchers to cross-check predicted ΔSM with experiment. We describe a method of scraping the Materials Project database for DFT results on magnetically ordered compounds [7] using the Pymatgen open-source Python library [8]. By analyzing the electronic density of states, computed unit cell volume and other parameters obtained through the Materials Project database for over 20,000 materials, we compile a data subset which we use to propose directions for magnetocaloric material research.References: [1] V. Pecharsky et al. 1997 Phys. Rev. Let. 78 4494 [2] N. H. Dung et al. 2011 Adv. Energy Mater 1 1215-1219. [3] M. Katter et al. US Patent Application No. 2012/0043497 A1 (23 Feb. 2012) [4] N. A. Zarkevich et al. 2018 J. Phys. D: Appl. Phys. 51 024002 [5] J. Bocarsly, et al. 2017 Chem. Mat. 29 (4), 1613-1622 [6] A. Jain, S.P. Ong et al. 2013 APL Materials, 2013, 1(1), 011002 [7] S. P. Ong et al. 2015 Comp. Mat. Sci. 97 209–215 [8] S. P. Ong et al. 2013 Comp. Mat. Sci. 68 314–319

Applying Materials Informatics Insights to Magnetocaloric Materials Screening and Discovery

Near-equiatomic iron-rhodium (FeRh) is a fundamentally and technologically interesting material exhibiting giant inverse magnetocaloric effect (MCE), potentially suitable for applications at the diminutive scale such as micro-cooling and medical diagnostic imaging, which require sharp first-order magnetic phase transitions (FOMPTs). Reports on FeRh having a sharp FOMPT in its bulk form are common,1 however the transition broadens considerably in some non-bulk forms, particularly thin-films (thickness < 50nm) and fine particles.2 In this work, an abrupt, bulk-like FOMPT has been demonstrated for the first time in FeRh particles produced using a solvent-free, reactive mechanochemical co-reduction of FeCl2 and RhCl3 precursors. Characterization using structural and magnetic probes indicates that annealing at temperatures ranging from 600°C to 800°C and subsequent quenching enables tuning of composition, microstructure, and physical properties of the particles. Depending upon the heat treatment condition, the fraction of fcc paramagnetic FeRh and B2 FeRh (both ferromagnetic (FM) α’ and antiferromagnetic (AFM) α”) phases can be controlled. The M-T analysis indicates that the first-order magnetostructural transition temperature, Tt, can be tailored between 380K and 325K, and both the △M between the AFM and FM states and the thermal hysteresis can be manipulated as well (Fig. 1). A maximum magnetic entropy change of △Smag ~10 J/kgK at μH=2T was noted in FeRh particles annealed at 800°C. These results will be discussed in the context of the compositional phase diagram and strain accommodation mechanism. Implications of this study on emerging biomedical applications, mainly thermal magnetic resonance imaging and magnetic hyperthermia therapy, will be presented. Acknowledgement: Research at VCU supported by VCU College of Engineering start-up funds and NSF-MRI Grant (1726617). Research in Ames (AL) supported by the M.S.E.D., Office of Basic Energy Sciences of the US DOE. AL is operated for the US DOE by Iowa State University under Contract No. DE-AC02-07CH11358.References: °1L. H. Lewis et. al., J. Phys. D: Appl. Phys. vol. 49. 323002 (2016) 2R. Barua, "Pathways for tailoring the magnetostructural response of FeRh-based systems." Ph.D. Dissertation (2014).

Bulk-like First-order Phase Transition in Mechanochemically Fabricated FeRh Particles

The complex interplay between structural, electronic and magnetic degrees of freedom in Mn-based magnetic Heusler compounds gives rise to a great variety of functional properties suitable for sustainable technological applications (energy conversion devices, sensors, actuators, spintronic components…) [1]. The adaptive lattice allows, by changing composition, the development of different crystallographic and magnetic structures, thus offering the possibility to tune properties and effects. Great efforts have been devoted in the last decades to investigate these materials by combining various experimental techniques and first-principles calculations, nonetheless a comprehensive and coherent description, able to drive the full control of Heusler properties, it is far from being achieved. In this work, we combine bulk-sensitive magnetometry, neutron diffraction and 55Mn nuclear magnetic resonance experiments with first-principles calculations to decouple and clarify the different contributions that define the ferromagnetic structure of off-stoichiometric NiMn-based Heusler compounds. We selected, as test material, the Ni48Mn36In16-xSnx series [2], showing a stable cubic austenitic phase down to 5 K, with a Curie transition slightly above room temperature. The replacement of In with Sn leads to a decrease of the saturation magnetization whereas the Curie temperature is featured by a non-monotonic variation (Fig. 1). First-principles calculations suggested the intrinsic nature of the behaviours of the critical temperature, which turns out to be related to the electronic properties. On the contrary, neutron diffraction and NMR experiments revealed that the decrease of magnetization by increasing the Sn content is caused by the disordering of the Mn magnetic moments sited in the mixed sublattice. The progressive replacement of In by Sn leads to a decrease of ordering temperature of Mn in the mixed site (Fig. 2), due to the destabilization of the complex magnetic interactions.References: [1] T. Graf, C. Felser and S. S. Parkin, Prog. Solid. State Ch., 39, 1-50 (2011). [2] G. Cavazzini, F. Cugini, M. E. Gruner, et al., Scripta Mater., 170, 48–51 (2019).

Decoupling of Magnetic Sublattices in NiMn-based Heusler Compounds

In a recent paper [1], the authors have evidenced how the thermally activated demagnetization processes taking place in magnetite nanoparticles (NPs) play a major role on deteriorating the dc coercivity values measured in these samples at temperatures above ca. 150 K. The mechanisms for that deterioration are the increase with the temperature of both the fluctuation field and the activation volume, the latter reaching at room temperature values of up more than 40 times the average particle volume value. We aim here at the quantitative evaluation of the inter-particles interactions underlying that phenomenology and, particularly, at the identification of the role played by the nanoparticles morphology. For that purpose, we have measured magnetite NPs prepared via oxidative precipitation by controlling their size through the nature of the base and nitrate salt and the ethanol content in the media, and their morphology (either octahedral or spherical) by changing the final base concentration [2]. Particle size, shape and crystal structure were determined by transmission electron microscopy (TEM), high-resolution electron microscopy (HRTEM), and X-rays diffraction (XRD), respectively. The magnetic properties of were measured by means of a vibrating sample magnetometer, working in the temperature range from 5 K up to 290 K by applying a maximum field of 9 T. Figure 1 displays the temperature dependence of the reduced saturation remanence (Mr/MS) clearly showing that in both morphologies, but to a larger extent in the spherical NPs, the measured isotropic powder exhibits reduced remanences below 0.5, which suggests the occurrence of demagnetizing interactions. The actual occurrence of that type of interactions was confirmed from the measurement (at 5 K, 125 K and 200 K) of the field dependencies of the isothermal and demagnetization remanences of the samples and the evaluation of the associated Henkel plots. The differences identified in those plots between the spherical and octahedral NPs are discussed in terms of the particle aggregation into close-flux structures.References: [1] E. Navarro, Y. Luengo, S. Veintemillas, M. P. Morales, F. Javier Palomares, U. Urdiroz, F. Cebollada, and J. M. González; AIP Advances 9, 125143 (2019) [2] Y. Luengo, M. P. Morales, L. Gutiérrez, S. Veintemillas-Verdaguer, J. Mater. Chem. C, 4, 9482-9488 (2016)

Temperature Dependence of the Magnetic Interactions Taking Place in Monodisperse Magnetite Nanoparticles Having Different Morphologies

Magnetic tunnel junction (MTJ) is a promising candidate for the development of ultrasensitive magnetic field sensor (TMR sensor) capable to detect extremely weak magnetic fields (0.5) opening the possibilities for biomedical applications1. The performance of these type of sensors is limited by the high low-frequency noise (1/f) which appears due to the presence of magnetic domain fluctuations or by defects inside the insulator barrier2. In this work, we study the normalization and voltage bias effect on the detection limit in TMR sensors based in MTJs. The multilayer stack is Ta(5)/SyF1/MgO/SyF2/Ta(10), where the synthetic ferrimagnet structures are SyF1: PtMn(25)/CoFe(2.3)/Ru(0.83)/CoFe(1)/Ta(0.1)/CoFeB(1.5) and SyF2: CoFeB(1.5)/Ta(0.1)/CoFe(1)/Ru(2.6)/CoFe(2)/PtMn(16) (nanometers). Samples were processes using optical UV lithography to define the diameter (D) and number of MTJs circular pillars in serie (N) inside the sensor. The RA product was typically around 13-20kΩμm2 at 0 mT and the TMR 200 % at room temperature. In order to provide a good interpretation of the results we propose a theoretical model to extract the real output voltage noise from standard electronic circuits. This model was used on a ¼ Wheatstone bridge setup, where the resistive elements were modelled as fluctuating resistances. Our TMR sensors show a sensitivity of 4 %/mT (<0.1%/mT) at 0mT (P state at -100mT)3. The 1/f noise was quantified by the Hooge-like parameter, which allow to compare the 1/f noise of different types of TMR sensors. We observed that well known equation works only below a threshold voltage, due to the non-linearity of the I-V curve related maybe to different symmetry conduction bands. At low bias voltage, devices show higher values at 0 mT (3.10-8µm2) than in the P state (2–4.10-9µm2). An interesting feature for applications is the almost constant detectivity (100nT/Hz0.5 at 10Hz) as a function of the bias voltage. This is a result of the compromise between the output noise voltage and the sensitivity. The subsequent output power and detection limit normalization allow us to design new TMR sensors with optimal conditions.References: [1] S. Cardoso, D. C. Leitao, T. M. Dias, J. Valadeiro, M. D. Silva, A. Chicharo, V. Silverio, J. Gaspar, and P. P. Freitas, J. Phys. D: Appl. Phys. 50, 213001 (2017). [2] J. Scola, H. Polovy, C. Fermon, M. P. Lecoeur, G. Feng, K. Fahy, and J. M. D. Coey. Appl., Phys. Lett., vol. 90, p. 252501, 2007 [3] J. M. Almeida, P. Wisniowski, and P. P. Freitas IEEE Trans Mag, VOL. 44, NO. 11, (2008)

Detection limit normalization and study of TMR magnetic field sensors

Soft magnetic materials showing a high tunnel magnetoresistance (TMR) effect are needed for improving the sensitivity of magnetic tunnel junction (MTJ) based sensors. We focused the Sendust alloy (Fe85Al5.4Si9.6 wt.%, hereinafter, this is called “FeAlSi”), which has soft magnetic property in D03-ordered structure. FeAlSi electrode is also expected to show a high TMR ratio by Δ1 electron coherent tunneling through MgO barriers as well as Fe electrodes with similar crystal structure to FeAlSi [1]. In previous work [2], we firstly succeeded to fabricate epitaxial D03-FeAlSi films with thickness of 30 nm on MgO substrates. Additionally, we observed good soft magnetic properties comparable to other free layer materials such as NiFe [3]. The purpose of this study is fabrication of MTJs using FeAlSi free layer and investigation of their TMR properties. All films were deposited on Si/SiO2 substrates using magnetron sputtering. The stacking structure for the characterization of magnetic and structural properties was Si/SiO2/Ta(5)/CoFeB(5)/MgO(20)/FeAlSi(30)/Ta(5)(in nm). TMR properties were investigated in the MTJs with stacking structure of Si/SiO2/Ta(5)/CoFeB(5)/MgO(20)/FeAlSi(30)/MgO(2)/CoFeB(3)/Ru(0.85)/CoFe(5)/IrMn(10)/Ta(5)/Ru(10). Crystal structure, magnetic properties, and TMR properties were measured by X-ray diffraction (XRD), vibrating sample magnetometer (VSM), and 4-probe DC measurement. Fig. 1 shows the XRD patterns for FeAlSi films prepared by optimal condition. A fundamental peak of FeAlSi (004) and superlattice peak of FeAlSi (002) from B2-ordered structure were clearly observed. The XRD result indicates that the FeAlSi film grown on SiO2 substrates with an MgO underlayer has (001)-orientation and contains B2-ordered structure. Fig. 2 shows magnetoresistance curve for MTJs with FeAlSi bottom electrode. We firstly observed TMR effect of 18.1% at RT using FeAlSi electrode with relatively small switching field. Although the further improvement of TMR effect is needed, the prepared FeAlSi thin film with soft magnetic property is a promising candidate for the free layer of MTJ based sensors with high sensitivity.References: [1] W. H. Butler et al., Phys. Rev. B, Vol. 63, p.054416 (2001). [2] S. Akamatsu et al., AIP Advances 10, 0 15302 (2020). [3] K. Fujiwara et al., Jpn. J. Appl. Phys., Vol. 52, p.04CM07 (2013).

FeAlSi thin films with low magnetic anisotropy for the free layer of magnetic tunnel junction based sensors

Spin-torque nano-oscillators (STNOs) are spintronic devices operating in frequencies ranging from less than 100 MHz up to a few GHz [1]. Vortex-based STNOs have a distinct configuration exhibiting a vortex magnetization distribution of the free layer in the equilibrium state. STNOs are promising candidates for future-generation implementations, such as rf generation [2], detection [3] or bio-inspired computing [4]. However, their use as magnetic field sensors remains understudied. Vortex-based devices provide a large linear range for magnetic field detection [5] and their nanometric size allows for improved spatial resolution. Such advantages motivate the study of vortex STNOs for sensing applications. One of the main limiting factors for magnetic sensors, notably vortex-based ones, is their intrinsic 1/f noise, which inhibits the device’s detection capabilities at low-frequencies. Here, we study the low-frequency noise of vortex-based STNOs. We determine the Hooge parameter α [6] - a figure of merit used to characterize sensor’s noise properties. In the uniform states, α is in the same order of magnitude as those measured in state-of-the-art TMR sensors with similar RA product, α = 10-11 - 10-10 µm2. In the vortex state, the measured noise level is around one order of magnitude greater than in the parallel state (Fig. 1), due to the increased probability of pinning of the vortex core due to material defects and/or inhomogeneities at the free layer. Furthermore, we examine the impact of the vortex dynamics in the low-frequency noise. Above a certain critical current, the spin transfer torques surpass the intrinsic damping of the free layer resulting in self-sustained oscillations of the vortex core [7]. We find that these dynamics strongly modify the characteristics of the STNOs and, in consequence, the achievable performances of magnetic field sensors relying on magnetic vortex dynamics. We present a detailed analysis of these characteristics and how they relate to the device’s noise properties. The work is supported by the French ANR project “SPINNET” ANR-18-CE24-0012.References: [1] S. Kiselev et al., Nature Vol. 425, 380 (2003) [2] S. Wittrock et al., Phys. Rev. B Vol. 99, 235135 (2019) [3] A. S. Jenkins et al., Nat. Nanotech., Vol. 11, 360 (2016) [4] M. Romera et al., Nature Vol. 563, 230–234 (2018); J. Torrejon et al., Nature, Vol. 547, 428-431 (2017) [5] D. Suess et al., Nature Electronics volume 1, pages 362–370 (2018) [6] F. Hooge and A. Hoppenbrouwers, Physica 45, 386-392 (1969) [7] A. Dussaux et al., Nat. Comm., Vol. 1, 8 (2010)

Analysis of 1/f Low-frequency Noise in Vortex-based Spin-torque Nano-oscillators for Magnetic Sensing Applications

We present nanoscale, resonator-based piezoelectric/magnetostrictive magnetometers that operate on the delta-E effect and are capable of detecting weak magnetic fields at low frequency or DC. As these devices are just hundreds of nanometers thick, and on the order of a hundred microns laterally, they can be produced by lithographic microfabrication techniques. This results in sensors that offer high portability, low cost, low power consumption, and the potential for high spatial resolution in magnetic sensor arrays. However, no prior study exists of a systematic examination of the properties that affect sensing performance in such magnetic sensors. Here, we report on how the deformation of the resonator plate, quantified as curvature, affects the total frequency response of these delta-E effect-based magnetometers. It was found that frequency response changed by as much as two orders of magnitude based on this parameter in devices that are otherwise identical and fabricated on the same Si wafer. Ultimately, a sensor with low curvature was fabricated and a record-high sensitivity to DC magnetic fields, of 4.98 Hz/nT, was achieved. To study this phenomenon in more detail, we measured the Q factor, imaged the magnetic domains using the magneto-optical Kerr effect (MOKE), and performed simulations to shed light on the internal stress distribution. From the Q factor data, we observed a loss mechanism dominated by magnetic loss. Informed by the MOKE images and simulation results, we then tested a simple magnetic model with fitting parameters of active volume fraction, mean easy axis, and effective anisotropy energy density. The model was found to fit the data quite well with low volume fraction but a large easy axis tilt and anisotropy energy.

Curvature Effects on Magnetic Field Sensitivity of Magnetoelectric Nano-Plate Resonators

The CoxFe3-xO4 (0.8≤x≤0) ferrites based magnetic nanoparticle with single domain nature have potential applications in the development ofthe high-density memory devices, magnetic fluid hyperthermia and biomedicine. The present work deals with the experimental investigation of oleic acid coated CoxFe3-xO4 (0.8≤x≤0) nano ferrite based ferrofluids synthesized by the wet chemical co-precipitation method and oleic acid used as surfactant to prevent the agglomeration of magnetic nanoparticles.Structural and magnetic properties of these ferrofluids have been investigated using different characterization techniques. The structural, cations distribution and Rietveld refinement was carried out by the XRD data and the result confirms that single cubic spinel phase belongs to Fd3m space group and the average crystallite size calculated from the XRD data varies from 11 to 14 nm.The Transmission Electron Microscopy (TEM) studies revealed that the particles are spherical in nature and mean particle size varies from 10 to 15 nm and all samples are polycrystalline in nature.The SEM result were also demonstrate the surface morphology of magnetic particle are spherical in shape nanoparticle with least agglomeration. FTIR spectroscopy showed the molecular dynamics and the formation of cubic spinel phase of frequency bands at 545 cm-1and 404 cm-1respectively.The UV-Visible measurement we have estimate the band gap of the materials lies between(2.2 to 2.8& 3.1 to 3.8 e V) of the all the sample form optical absorption spectra fitting by Tauc plot method. The room temperature Vibrating Sample Magnetometer(VSM) analysis revealed that the saturation magnetization and coercivity value varies from 26.18 to 61.78 emu/g and 31.45 to 128.36 Oe, respectively by the increasing the Co concentration. These magnetic properties can be enhanced by the distribution of cations on both A-site and B-site.References: 1.Clara Pereira et al., J. Phys. Chem. C 2016, 120, 2, 1328-1341. 2. R. Tackett et al., JMMM (2008), 320 2755– 2759.

Optimization of cobalt concentration & cation distribution for improved magnetic characteristics and stability of CoxFe3-xO4 mixed ferrite magnetic fluids.

An undulator is a device including two parallel arrays of magnets (separated by a gap) with a polarity in an alternating manner (↑↓↑↓↑↓…). The undulator is an essential part of a Free Electron Laser (FEL) upon which a large body of literature has expressed interest for the applications on various areas such as defence industry, material science and medicine. Also, sub-millimeter period undulators are another interesting approach. The current state of the art of X-FEL uses undulator period down to 3 cm which creates an output radiation wavelength down to 1.5 A using ~14GeV e- beam, ergo big and costly e-beam source is needed. By developing a sub-mm period undulator (i.e. a period of 300µm), it is possible to obtain the same output radiation wavelength with a much less e-beam energy of ~1.4 GeV which could be revolutionary since much smaller FELs could be constructed. Undulators are constructed using either electromagnets or permanent magnets. The latter is the choice due to the fact that no additional power supply is needed. On the other hand, there is a growing interest on L10 phase FePt thin films on the magnetic applications due to their high magnetic anisotropy. In this work, FePt thin films and micro magnets were synthesized by sputtering technique and their magnetic properties were extensively studied. FePt micromagnets could be utilized in next generation sub-mm period undulators. This work was supported by TUBITAK project: 118F319

Study of Thermo-Magnetoconvection of a Magnetofluidic Cooling Device

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