United States

The data storage industry is currently migrating to Heat Assisted Magnetic Recording (HAMR) technology for the next generations of hard drives. HAMR allows the use of high anisotropy media by introducing laser pulses to lower the coercivity to writable levels at elevated temperatures. However, the long-term stability of written data remains a problem, especially when it may be subjected to high temperatures during the writing or rewriting of neighbouring tracks. A simple model is desirable for the evaluation of energy barriers for complex media. Concerning the long term and controlled stability of the recorded information, we developed a numerical model which allows us to perform energy barrier calculations for multispin systems. Our work is based on the Lagrange multiplier method of finding points of extremum for functions subject to equality constraints. While this approach has been previously used in conjunction with the Landau-Lifshitz-Gilbert equation [1-2], we were able to integrate the Lagrange optimisation strategy in a direct minimisation tool. Making use of our model, we investigate several spin chain systems, mapping the minimum energy paths (MEPs) and extracting the corresponding energy barriers, while characterising the specific reversal mechanisms, ranging from the extremes of coherent rotation and domain wall formation (see Fig.1). Furthermore we use our description to explore the exchange spring behaviour and the reduction of the switching field and energy barrier via the hard/soft coupling [3].References: [1] D.A. Garanin and H. Kachkachi, Phys. Rev. Lett. 90, 65504,(2003). [2] R. Yanes, O. Chubykalo-Fesenko, H. Kachkachi, Phys. Rev. B 76, 064416 (2007) [3] K. Yu. Guslienko, O. Chubykalo-Fesenko, O. Mryasov, Phys. Rev. B 70, 104405 (2004)

MMM 2020

Energy barrier calculations in multispin systems for magnetic recording purposes

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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technical paper

Magnetic nanoparticles have recently garnered interest for applications as MRI temperature contrast agents for Magnetic Resonance Imaging Thermometry (tMRI)[1]. Magnetic materials which can be used to noninvasively monitor body temperature would be greatly beneficial both for diagnostic purposes as well as MRI guided thermal procedures such as laser ablation of tumors. However, materials studied previously [1,2] are toxic. In this work we present an MRI thermometry study of MgZn ferrite microparticles. Such materials where shown in an earlier study to be less toxic than previously studied ferrites [3]. MgZn ferrites were prepared using a standard ceramic method and size distribution analysis was carried out using Scanning Electron Microscopy (SEM). Proton NMR linewidth, T1, and T2 relaxation times where measured of solutions containing MgZn microparticles ata 3T. T2* weighted images were acquired in a high magnetic field of 3T and low magnetic fieeld of 0.2T scanners (Figure 1) to investigate temperature dependent contrast in images. In addition, tests of particle stability in solution where carried out and Inductively Coupled Plasma Mass Spectroscopy (ICP-MS) was used to determine dissolution of Mg and Fe in aqueous solutions of MgZn particles. results show that MgZn particles are promising compounds for tMRI as images show show strong dependence of image brightness vs temperature. (Figure 2). MgZn compounds have shown a lesserdegree of toxicity putting us closer to the ability to use ferrite particles as tMRI contrast agents in a clinical setting.References: [1] J. Hankiewicz, Z. Celinski, and K. Stupic, Nat. Commun, 7 (2016) [2] J. Hankiewicz, J. Stoll, and J. Stroud, J. Magnet. Magnet. Mater., 469 , pp. 550-557, (2019) [3] N. Alghamdi, J. Stroud, and M. Przybylski, Structural, Magnetic and Toxicity Studies of Ferrite particles for Magnetic Resonance Imaging Thermometry, J. Magnetism and Magnetic Materials. 497 , (2019)

Development of Biocompatible Ferrite Particles for Magnetic Resonance Imaging

Traditional inductive methods used for Magnetic Resonance Imaging (MRI) suffer from significant challenges due to the low signal-to-noise ratio (SNR) arising from the weak nuclear magnetic moment of protons when they precess about a large magnetic field at room temperature [1]. In an attempt to amplify the magnetic flux, it has been proposed to use magnetic nanoparticles of various geometrical shapes that are inserted into an NMR inductive coil [1-3]. Thus, the magnetic flux due to the response of the ensemble of proton spins to the RF field can be significantly enhanced as these nanoparticles have high transverse magnetic susceptibility under appropriate DC bias conditions. Prior work did not consider random thermal magnetization fluctuations in such nanomagnets that affect the SNR. In this work, we simulated the magnetization dynamics of a single-domain ellipsoidal nanomagnet driven by an AC field (HAC) along the easy axis for different bias fields (HDC) along the hard axis of the nanomagnet (Fig 1a). Importantly, our simulation also included the effect of room temperature thermal noise as shown in Fig 2. For HDC =hc, the energy profile is nearly flat (Fig 1b) that leads to the maximum magnetization amplification (Fig 1c) in response to the driving AC field that mimics the field due to proton spins precessing at nuclear magnetic resonance (NMR) frequency. However, the flat energy profile also increases the random magnetization fluctuations due to the thermal noise (Fig 2). Nevertheless, the SNRs for h=0, 0.5hc, 0.75hc, 1.0hc and 1.5hc are 0.003, 0.29, 0.63, 0.7 and 0.23 respectively showing h= hc provides the highest SNR. However, these are considering a single nanomagnet only. We will further investigate improvement of NMR detection SNR as a function of the coil-filling factor (millions of magnetic nanoparticles), nanoparticle shape, applied NMR polarizing field, and narrow band filtering to increase the SNR > 1000.References: 1. M. Barbic, A. Scherer, Solid State Nuclear Magnetic Resonance 28, 91 (2005). 2. M. Barbic, H. Elbidweihy, Journal of applied physics 120, 104506 (2016). 3. H.Elbidweihy, R. Smith and M. Barbic, AIP Advances 9, 045031 (2019).

Signal to Noise Ratio of Single Domain Nanomagnet Amplification of Nuclear Magnetic Resonance Signals at Ambient Temperature

Common symptoms of Corona virus 2019 (COVID-19) include: (i) shortness of breath or difficulty breathing, (ii) cough and (iii) fever. Current detectors provide limited information and possess lengthy processing time, or are physically contacted to patients. Therefore, there is an urgent need for developing contactless devices that enable early and fast detection of COVID-19 and track their growth rates in real time. In this study, we have integrated an ultra-sensitive magnetic sensor technology [1] with machine learning to develop an innovative, contactless diagnostic device that can sense the breathing symptoms of COVID-19 at multiple stages of disease progress and provide crucial information on the growth rate by patient of this deadly virus. We add predictive analytics, with the help of machine learning, to provide a good understanding of what can and cannot be said about a patient’s health condition status and a possible treatment plan, based on the available data. Figure 1a shows a tested person’s characteristic breathing pattern and how his breathing amplitude varies within 60 seconds. This “shortness of breath” feature (at 35th s and 50th s) is not detectable using a commercially available respiratory monitor. The sensor also provides an accurate measure on strength of his breath at multiple stages, as shown in Figure 1b for the normal breath – Stage I, the deepest breath – Stage II, and the intermediate/random breath – Stage III.References: [1] O. Thiabgoh, T. Eggers, and M. H. Phan, A new contactless magneto-LC resonance technology for real time respiratory motion monitoring, Sensors and Actuators A 265, 120 (2017).

Real-Time Tracking of Coronavirus Progress Using Magnetic Sensing and Machine Learning

Granular L10-FePt thin film deposited on (002) textured MgO underlayers have come long way in the support of the present HAMR media areal density capability. However, further reduction of the center-to-center pitch distance between adjacent grains below 6nm is meeting the nucleation density limit of this media/underlayer material combination [1]. Here, we present a novel fabrication method utilizing metal-oxide/nitride materials systems with two-phase separation to develop a pre-defined and robust oxide/nitride template to increase the nucleation density and the columnar growth of L10-ordered FePt grains. The template is directly formed on top of MgO underlayer by co-sputtering metal and oxide/nitride materials, resulting in metal particles embedded in the oxide/nitride matrix. This is followed by the selective etching of the metal nanoparticles from the matrix [2]. Experimental demonstration shows that FePt can be backfilled into the “craters” left by the metal nanoparticles with the correct texture on the MgO underlayer. By comparing the mean pitch distance of FePt grains without and with template layer in Fig. 1, it is shown that the nucleation density of FePt grains is determined by the areal density of the “craters” formed in Au-SiO2 template layer. With the help of such a fabrication method, the material choices for further reducing the pitch distance of the FePt grains in HAMR media are expanded. The several combinations of various matrix materials forming the template and their results on nucleation and growth of FePt will be presented and discussed.References: [1] I. Suzuki, J. Wang, Y. Takahashi, K. Hono, Journal of Magnetism and Magnetic Materials, Vol. 500, 166418 (2020) [2] B. Zhou, C.c. Xu, B. S. D. C. S. Varaprasad, D. Laughlin, J. Zhu, 2019 MMM conference (2019)

Templated Growth of L10-FePt for HAMR Media

Writing tracks narrowly spaced in the crosstrack direction appears to increase the User Areal Density (UAD), but can lead to erasure of the adjacent tracks. This phenomena is called Adjacent Track Erasure (ATE). Our research aims to establish the numerical extent of ATE and introduce efforts to reduce this ATE for different HAMR media. This research can help increase the HAMR storage density by making HAMR media resistant to ATE. The ATE in HAMR is limited to tracks immediately adjacent to the track being overwritten [1]. For a single write, we have previously found that the optimum track spacing between these tracks to maximize the UAD for a noise free reader is 70% of the heat spot FWHM [2]. By implementing micromagnetic simulations, we find the single layer FePt media is more susceptible to ATE as its thickness decreases. The 3nm-6nm high temperature Thermal ECC media [3] has a slightly higher ATE susceptibility compared to the 9nm single layer FePt media. In contrast the 3nm-6nm low temperature Thermal ECC media [4] has a significantly higher ATE susceptibility (Table 1). To reduce the ATE, we change the applied field angle and introduce exchange coupling between grains of the media. The single layer FePt media ATE is impervious to the applied field angle although the ATE reduces as the field angle reduces for the high temperature Thermal ECC media. The ATE reduces for the single layer FePt media and the high temperature Thermal media with optimized exchange coupling (IGC) values. The ATE for the single layer FePt and high temperature Thermal ECC media using these optimization techniques is shown in Fig. 2. Both these optimization techniques however do not sufficiently improve the ATE susceptibility for the low temperature Thermal ECC media. We believe the ATE is proportional to the thermal stability factor at a temperature just below the write temperature. This hypothesis correctly predicts the ATE in the single layer FePt media assuming coherent rotation. For Thermal ECC media, other mathematical techniques that capture incoherent rotation can be used to verify this hypothesis.References: [1] S. Kalarickal, A. Tsoukatos, S. Hernandez, C. Hardie and E. Gage, "Adjacent Track Interference in Heat-Assisted Magnetic Recording: Impact and Implications," in IEEE Transactions on Magnetics, vol. 55, no. 7, pp. 1-4, July 2019 [2] Z.Liu, Y.Jiao and R.H. Victora, “Composite media for high density heat assisted magnetic recording”, Appl. Phys. Lett. 108, 232402(2016) [3] Z.Liu & R.H.Victora (2016). “Composite structure with superparamagnetic writing layer for heat-assisted magnetic recording”, IEEE Transactions on Magnetics, vol. 52, no. 7, pp. 1-4, July 2016 [4] N.A. Natekar, W.Tipcharoen & R.H.Victora, “Composite media with reduced write temperature for heat assisted magnetic recording”, Journal of Magnetism and Magnetic Materials 486 (2019) 165253

Analysis of Adjacent Track Erasure for the HAMR Media

Heat-assisted magnetic recording (HAMR) is the most promising candidate for next generation data storage. Previous study suggests that user areal density in HAMR with composite media can reach up to 4.7 Tb/in2 [1]. On the other hand, shingled magnetic recording has been adopted to keep the growth in user areal density for current state-of-art perpendicular magnetic recording. In this work, Heat-assisted Shingled Magnetic Recording (HSMR) is studied. It is known that adjacent track erasure can degrade signal-to-noise ratio (SNR) in conventional HAMR especially when the number of adjacent track write increases [2]. In HSMR, since there will be only one adjacent track write due to the shingled-style writing, HSMR should have better performance than conventional HAMR under the same cross-track density. As a consequence of transition curvature in HAMR [3], the shape of the remaining track after shingled writing is a half arc (Fig. 1). Thus, the read head should also be rotated by a certain angle (θ) to capture the effect from the arc shape of the transition. Table 1 shows the SNR for different track pitch and rotation angle of the read head for a heat spot with peak temperature of 850 K and full width at half maximum (FWHM) of 30 nm. It can be seen that SNR is enhanced by around 1 dB if the read head is rotated an optimal amount. Bit error rate is then calculated with a 1-D equalizer and an improved Viterbi detector [4]. It is shown that for a three-read head arrays, the user areal density can approach up to 5 Tb/in2.References: 1 Z. Liu, Y. Jiao, and R.H. Victora, Applied Physics Letters 108, 232402 (2016) 2 S. Kalarickal, A. Tsoukatos, S. Hernandez, C. Hardie, and E. Gage, IEEE Transactions on Magnetics 1 (2019) 3 J.-G.J. Zhu and H. Li, IEEE Transactions on Magnetics 53, 1 (2017) 4 Y. Wang, M.F. Erden, and R.H. Victora, IEEE Magnetics Letters 3, (2012)

Rotated Read Head for High Density Heat-Assisted Shingled Magnetic Recording

Micromagnetic modelling enables the accurate simulation of large magnetic systems with a significantly reduced computational cost compared to atomistic simulations. Through micromagnetic modelling it is possible to simulate systems consisting of thousands of grains over a range of timescales from nanoseconds to years. Here we present an open-source multi-timescale micromagnetic code, called MARS, consisting of the key solvers Landau-Lifshitz-Gilbert (LLG), Landau-Lifshitz-Bloch (LLB) and Kinetic Monte Carlo (kMC). The included solvers have been rigorously tested and are bundled into an overarching solver class to enable the creation of simulations which can utilise any of the available solvers at run-time. This combination of solvers also enables multi-timescale simulations with dynamic switching between solvers for specified timescales. The granular systems used are generated via a Voronoi construction and can consist of multiple or single layers. Each layer is assigned its own material with separate material configuration files. MARS is designed to use atomistic parameterisation for the material parameter inputs, obtained from software such as VAMPIRE [1]. MARS includes numerous bundled simulation types including but not limited to: FORC, FMR, HAMR writing. Fig.1 shows the result of a HAMR writing simulation of a pseudorandom bit sequence via the LLB solver followed by the read back of the system after a week via the kMC solver. Such simulations provide an easy method to test a system’s suitability for data writing and long term storage. A detailed model of the HAMR write head is provided including custom field and temperature profiles, skew angle and near-field transducer to write head spacing.References: [1] R. Evans, W. Fan and P. Chureemart, et al. Journal of Physics: Condensed Matter, Vol. 26, p103202 (2014)

Models of Advanced Recording Systems (MARS) - A Multi-Timescale Micromagnetic Code for Granular Film Simulations

Proton magnetic resonance imaging (1H-MRI) is one of the most powerful noninvasive biomedical imaging techniques to date. Conventional 1H-MRI can deliver pictures of exquisite anatomical detail, which is revolutionary in terms of its window into the body. A clear next step would be the delivery of novel MRI methods and molecular imaging probes to enable a window into the physiological chemistry of the body. The chemistry of the body governs nearly all function and dysfunction. Hence, the ability to noninvasively detect said chemistry would revolutionize diagnostic possibilities. We will present two efforts in our lab to develop novel molecular probes for chemical sensitivity. Notably, our efforts focus on magnetic species that are relatively exotic to the commonly used Gd3+ contrast agents: Cobalt-59 nuclear spins and high-spin metal ions (Fig. 1). Our first effort is directed at understanding the temperature-sensitivity of the Cobalt-59 nuclear magnetic resonance (NMR) signal in highly stable Co(III) complexes. Molecules containing the Cobalt-59 nucleus were recognized in 1980 as effective NMR thermometers owing to a highly sensitive chemical shift.1 Indeed, the chemical shifts for Cobalt-59-containing molecules exhibit two orders of magnitude higher sensitivities (on the order of 1-3 ppm/°C) than traditional 1H signals (0.01 ppm/°C).2 If translated into an applicable chemical probe, Cobalt-59 NMR probes could thus facilitate novel imaging techniques for, e.g., monitoring thermal ablation of tumors3 or thermal drug delivery.4 Yet, the exact design parameters to deliver molecular 59Co thermometers with even higher sensitivity (for higher-resolution imaging) are lacking. We will present novel results from our efforts to understand and control temperature sensitivity by molecular design.5,6 A second effort in our group to be presented is developing metal complexes for an imaging scheme that uses the electron analog of nuclear magnetic resonance: electron paramagnetic resonance imaging (EPRI). EPRI is capable of delivering local chemical information, including pH and oxygen content.7 Given the inherent tunability of the chemical properties of unpaired electrons, the potential exists for far more areas of chemical sensitivity relative to nuclei. Hence, merging EPRI with the anatomical detail of 1H-MRI could provide revolutionary new imaging techniques. A key challenge to doing so, however, is the fact that conventional EPRI molecular imaging probes (organic radicals) would use high-frequency microwaves for function in a typical 1.5 T MRI scanner. High-frequency microwaves are absorbed by water, which would heat and injure tissue. Molecules containing high-spin metal ions, owing to a unique electronic feature called zero-field splitting, may surmount this challenge. However, to fully realize this potential, significant insight is necessary in controlling the properties of metal ions under the action of low-frequency microwaves. In the second part of the presentation, we will touch on our efforts in this area.References: [1] G. C. Levy, T. J. Bailey and D. A. Wright, J. Magn. Reson. Vol. 37, p. 353–356 (1980). [2] J. C. Hindman, J. Chem. Phys. Vol. 44, p. 4582–4592 (1966). [3] M. Ahmed, L. Solbiati and C. L. Brace et al., J. Vasc. Interv. Radiol. Vol. 25 , p. 1691-1705.e4 (2014). [4] N. Hijnen, N.; S. Langereis and H. Grüll, Adv. Drug Deliv. Rev. Vol. 72, p. 65–81 (2014). [5] T. M. Ozvat, M. E. Peña and J. M. Zadrozny, Chem. Sci. Vol. 10, p. 6727–6734 (2019). [6] T. M. Ozvat, G. E. Sterbinsky and A. J. Campanella et al., Dalton Trans. Advance Article [DOI: 10.1039/D0DT01391A] (2020). [7] In Vivo EPR (ESR); L. J. Berliner, Ed.; Springer US: Boston, MA (2003).

Controlling Exotic Molecular Spins Toward Next-Generation MRI Probes INVITED

The ultrafast optical control of the magnetic order rapidly emerged as a promising approach in ultrafast magnetism. Yet, more than a decade after the discovery of complete and deterministic magnetization switching induced by femtosecond circularly polarized laser pulses1, many fundamental questions remain unanswered. In particular, the issue of the relative contribution of pure thermal or nonthermal effects induced by the laser pulses is crucial for the understanding of the magnetization reversal mechanism. Several theoretical models have been developed for multipulse all-optical helicity-dependent switching (AO-HDS) without reaching a consensus. Using circular polarization, transfer of light angular momentum and the emergence of an effective magnetic field via the inverse Faraday effect (IFE), a nonthermal optomagnetic effect, may occur2. In contrast, it has been argued that AO-HDS could originate from a purely heating mechanism3. In this case, the asymmetry would arise from the difference in light absorption between left- and right-circular polarization due to the magnetic circular dichroism (MCD). By conducting helicity-dependent ultrafast magnetization dynamics in a CoTb ferrimagnetic alloy that exhibits AO-HDS, we are able to quantitatively determine the MCD and resolve its role in the switching mechanism4. We find a MCD of ~1% from time-resolved Kerr effect measurements (Fig. 1a). Unequivocal interpretation of the sign of the dichroism is provided by performing AOS and, for the first time in a ferrimagnet, fs laser-induced domain wall motion experiments (Figs 2a-2b). We demonstrate that AOS occurs when the magnetization is initially in the most absorbent state, according to the light helicity. Moreover, we evidence that the MCD creates a thermal gradient that drives a domain wall toward hotter regions. Our experimental results are in agreement with the purely thermal models of AOS reported in the literature.References: 1C. D. Stanciu et al., PRL 99, 047601 (2007) 2T. D. Cornelissen et al., APL 108, 142405 (2016) 3J. Gorchon et al., PRB 94, 020409(R) (2016) 4Y. Quessab et al., PRB 100, 024425 (2019)

Resolving the role of magnetic circular dichroism in multishot helicity-dependent all-optical switching INVITED

New fascinating phenomena involving coupled spin waves and phonons are observed at the magneto-elastic resonance: magnetic field dependent spin Seebeck effect [1], acoustically-assisted spin pumping [2, 3] and magnetization swiching [4, 5]. In this context, it has been shown that an efficient mean to excite phonons is by surface acoustic waves (SAW). The integration of SAW-FMR (ferromagnetic resonance) mechanism in spintronic and magnonic devices are claimed to operate with very low power. However, most ferromagnetic materials have precession frequencies well above 5 GHz, i.e. above the highest frequency reached by today's SAW technologies. This restricts the research on SAW-FMR to low magnetization and/or low magnetic anisotropy materials, such as (Ga,Mn)As or (GaMn)(As,P), Ni, and YIG. Here, we show that thin films of Fe epitaxially grown on GaAs(001), a spintronic and magnonic compatible magnetoelastic and piezoelectric heterostucture, give the opportunity to obtain SAW-FMR, at room temperature, below 1 GHz despite the high Fe magnetization. This is obtained by exploiting the softening of the magnetic stiffness when the field is applied along a hard axis, at moderate intensity (60 mT). Slight variations of the magnetic field direction, less than 0.1 deg off, increase dramatically magnetization precession frequency leading to off-resonance interaction and suppressing SAW attenuation. The observed phenomena are described by adopting a very simple magnetization dynamics model that catches the physics of the interaction. The observation of resonant, low frequency and highly directional magnetoelastic coupling in Fe suggests that SAW mature technology offers the opportunity to magnonics and spintronics to handle magnetization precession in a remote way, without using any dissipative electron current. [6]References: [1] Takashi Kikkawa, et al., Phys. Rev. Lett. 117, 207203, (2016) [2] M. Weiler, et al. Phys. Rev. Lett. 108, 176601, (2012) [3] Ken ichi Uchida, et al., Solid State Communications 198, 26, (2014) [4] L. Thevenard,et al. Phys. Rev. B 93, 134430, (2016) [5] P. Kuszewski, et al. Journal of Physics: Condensed Matter 30, 244003 (2018) [6] J.-Y. Duquesne, P. Rovillain, et al. Phys. Rev. Appl. 12, 024042 (2019)

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