United States

Magnetization reversal in NiFe/IrMn exchange-biased thin films was investigated under thermal cycling in an external magnetic field, applied opposite to the direction of
the exchange bias field. Thermal hysteresis of magnetization accompanied by changes in magnetization polarity was observed in the applied field close to the exchange bias value. This
effect appears when thermally induced variations of the exchange bias exceed the corresponding variations in coercivity. The amplitude of magnetization reversal in NiFe/IrMn structures
exceeds ∼100 times the corresponding amplitude in spin-crossover molecular compounds. The observed bistability of the magnetic state, revealed by thermal hysteresis, gradually
disappears with an increase in the number of cooling–heating thermal cycles, that indicates an irreversible quenching of the interfacial magnetization configuration. This effect paves the
way for the creation of a new class of switching devices with thermally assisted bistability in the ferromagnetic state.
The work was supported by AAAA-A19-119092390079-8 government task of Institute of Problems of Chemical Physics.

MMM 2022

Effect of thermal cycling on exchange bias in NiFe/IrMn bilayers

poster

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Cubic spinel ferrites are a class of oxides represented by the chemical formula AB2O4 with A and B being divalent and trivalent cations occupying octahedral and
tetrahedral voids. Magnetization arises from superexchange interactions between cations via the oxygen anion. Among these, cobalt ferrites have been shown to undergo spinodal
decomposition to form periodic microstructures at the nm length scale. The resultant changes in magnetic properties have been shown to be dependent on the periodicity and degree of
decomposition. Complex nanostructured microstructures with concomitant spatial variations of intrinsic magnetic properties and magnetic interactions can yield wasp-waisted major
hysteresis loops indicative of multiple magnetization reversal events1. Detailed analysis requires advanced magnetometry techniques, in order to disentangle reversible, irreversible, and
viscous magnetization processes and their dependence on previous magnetic history.
In this work, cobalt ferrite samples have been synthesized using standard powder processing techniques. Different degrees of decomposition have been achieved by varying the annealing
time within the coherent spinodal range. TEM has been used to determine periodicity, degree of decomposition, lattice mismatch and the associated coherency strains between the
decomposed phases. Magnetization processes occurring in these materials have been investigated through vibrating sample magnetometry using major M-H loops and temperaturedependent
magnetization. These measurements have been complemented with non-saturating high-resolution first order reversal curve (FORC) diagrams obtained with different preconditioning
fields, in order to explore magnetic states inaccessible to the major loops. Various switching events corresponding to the features observed in the FORC diagrams have been
identified and correlated with microstructural observations. Finally, a better insight into the nature of the switching events has been obtained with Rayleigh loop measurements along
selected FORCs. 

**References:** 

1. M. V. Suraj, A. Talaat, B. C. Dodrill, Y. Wang, J. K. Lee, and P. R. Ohodnicki Jr., AIP Advances 12, 035031 (2022) 

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Understanding magnetization reversal mechanisms and exchange interactions in nanostructured cobalt ferrites using first order reversal curves (FORC)

When two materials are brought into close proximity, the magnetic proximity effects, including the exchange anisotropy, exchange bias (EB), and coercivity, are among the
most fascinating and active materials research areas today. Among them, the interlayer exchange coupling (IEC) plays a crucial role to manipulate the magnetic hysteresis loop in a variety
of solid-state magnetic heterostructures. Such IEC can be understood by the equilibrium field-like spin torque (FLST) of magnetic tunnel junctions (MTJs) in noncollinear magnetic
configuration. Recently, electrical and spin switches across a single organic molecule connecting ferromagnetic electrodes are also burgeoning fields for possible applications in the socalled
molecular spintronics. The ability to calculate the noncollinear spin torque effect and spin dynamics of magnetic heterostructures is difficult but important to include the complex
structural, electronic, and magnetic properties at spinterfaces for nanoscale spintronics devices.
In this study, the molecular scale magnetic proximity effect is proposed in single-molecule magnetic junctions (SMMJs) consisting of an amine-ended 1,4-benzenediamine (BDA) molecule
coupled to two ferromagnetic Co electrodes. The magnetotransport calculation is carried out using the generalized Landau-Lifshitz-Gilbert (LLG) equation combined with density
functional theory (DFT) and our self-developed JunPy calculated spin-torque effect. We first demonstrate that the equilibrium FLST plays a crucial role in the strain-controlled exchange
bias with current-controlled magnetic coercivity in single-molecule magnetic junctions [1]. In consideration of the resonant tunnelling and the multi-reflection processes at interfaces, our
DFT+JunPy+LLG calculation results [2,3] illustrate the underlying mechanism in an important aspect, namely, molecular scale exchange bias effect, via the modulation of angular
dependence of equilibrium FLST in amine-ended SMMJs with complex contact deformation and interfacial bonding during breaking junction techniques. This work is supported by the
Ministry of Science and Technology (108-2628-M-008-004-MY3). 

**References:** 

[1] Y. -H. Tang and B. -H. Huang, Phys. Rev. Research 3, 033264 (2021) 
[2] K. -R. Chiang and Y. -H. Tang, ACS Omega 6, 19386 (2021) 
[3] Y. -H. Tang, Y. -C. Chuang, and B. -H. Huang, submitted to Front. Phys. : Charge, Heat and Spin Transport Through Molecular Nanostructures

Underlying mechanism for exchange bias in single molecule magnetic junctions

Iron oxide-based magnetic nanoparticles offer ample possibilities for a wide range of biomedical applications such as magnetic resonance imaging, hyperthermia treatment,
targeted drug delivery, etc. [1] Understanding the complex magnetic textures in the microscopic dimensions by tailoring the size and morphology of these multifunctional structures is of
utmost importance for controlled and optimized applications. The coexistence of multiple phases in these nanostructures provides an additional degree of freedom to tune their
functionalities. Despite extensive investigations on the magnetic properties of core-shell iron oxide nanostructures with different shapes, sizes and morphologies, limited studies are
available on the influence of phase coexistence on iron oxide nanocubes. Here we report on a comprehensive investigation of magnetic properties of multiphase iron oxide nanocubes with
different sizes ranging between 10-43 nm synthesized using a chemical decomposition technique.[2] The exposure to air resulted in an α-Fe2O3 shell layer on the surface of the nanocubes,
with the core formed of a possible composite mixture of Fe3O4 and FeO phases. Magnetometry indicative of the Morin (α-Fe2O3) and Verwey (Fe3O4) transitions at ~250 K and ~120 K,
respectively identify the two phases, while FeO, the third phase is identified through XRD and Raman spectra. The smaller nanocubes up to average size 24 nm showed the exchange bias
(EB) effect below 250 K with 15 nm showing the strongest. The EB effect is attributed to the strong interfacial coupling between the ferrimagnetic (FiM) Fe3O4 phase and
antiferromagnetic (AFM) FeO phase. The Fe3O4/α-Fe2O3 (FiM/AFM) interfaces are found not to contribute to the EB effect. [3] Our results have enabled us to view the basis of the EB
effect in multi-phase iron oxide nanosystems and could open up possibilities to design tunable exchange-coupled nanomaterials with desirable magnetic properties for biomedical and
spintronic applications. 


**References:** 

[1] C. S. S. R. Kumar and F. Mohammad, Adv. Drug Deliv. Rev. 63, 789 (2011). 
[2] H. Khurshid, S. Chandra, W. Li, M. H. Phan, G. C. Hadjipanayis, P. Mukherjee, and H. Srikanth, Journal of Applied Physics, Vol. 113 (2013). 
[3] S. B. Attanayake, A. Chanda, R. Das, M. H. Phan, and H. Srikanth, AIP Adv. 12, 035136 (2022).

Tunable Magnetism and Exchange Bias effect in Multiphase Iron oxide Nanocubes

Co3O4 is a transition metal oxide with atypical exchange interactions that propagate through nonmagnetic ions giving rise to weak antiferromagnetism[1]. Thus, small
changes to the exchange pathways can lead to large changes in the observed magnetism. Cu, Ni, and Fe-doped Co3O4 nanoplates were synthesized (Fig. 1) to investigate the differences in
magnetic response as a result of the new exchange interactions, including contributions from the surface and core[2]. Through the addition of other transition metal ions we disrupt the
intrinsic exchange interactions and investigate the effect on the electronic and magnetic structures. The electronic structure of doped materials can be altered to form ligand holes in the
oxygen band. Instead of the holes residing on the metal ion – creating a mixed valence structure of 2+ and 3+, the holes occupy the oxygen band creating a ligand hole[3,4]. This manifests
in interesting ways such as modifying the exchange interactions through delocalization of the electrons.
The addition of dopants into the A (tetrahedral) site leads to a change in the number of unpaired spins, decreasing the strength of the extended exchange interactions responsible for the
antiferromagnetic order. Occupation of the B (octahedral) site leads to stronger ferromagnetic interactions. By replacing the Co3+ ions which have no magnetic moment with Cu, Ni, or Fe
ions, we introduce unpaired spins into the B site. Fe-doped Co3O4 shows the strongest exchange interactions. For example, 25% atm. Fe shows the largest magnetic saturation at roomtemperature
compared to similar dopings of Ni, and Cu (Fig. 2). The strong magnetic response is a result of Fe3+ occupying the B site interacting with Co2+ on the A site. Interestingly,
both Cu and Ni occupy the B site with a 2+ oxidation state (replacing Co3+) which disrupts the charge balance of the system. This forces Cu and Ni to appropriate electrons from the
surrounding oxygen ions. This behaviour can modify the existing exchange interactions, similar to what happens in the high temperature superconducting cuprates. 

References 

[1] - W.L. Roth, J Phys. Chem. Solids, Vol. 25, 1, p.1-10 (1964). 
[2] - M. Shepit, V.K. Paidi, C.A. Roberts, et al., Sci Rep., Vol. 10, 20990 (2020). 
[3] - M. Shepit, V.K. Paidi, C.A. Roberts, et al., Phys. Rev. B, Vol. 103, 024448 (2021). 
[4] - G. Sawatsky, R. Green, Quantum Materials: Experiment and Theory, Autumn School on Correlated Electrons (Jülich), (2016). 

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Modifying the Exchange Interactions of (Cu, Ni, and Fe) Doped Co3O4 Nanoplates via Ligand Holes

Although materials exhibiting exchange bias (EB) phenomena have found use in many modern technologies[1], [2], much of their underlying physics remain highly debated.
For bulk alloys, EB cannot be understood within the context of interfacial antiferromagnetic (AFM) / ferromagnetic (FM) interactions[3]. Rather, for certain Mn-rich Heusler alloys, EB is
better understood within the context of many different exotic magnetic interactions[4], [5]. For the Mn-Ni-Ga system, excess Mn can fill free Ni / Ga lattice sites leading to competing FM /
AFM type interactions[6]. Competition is believed to govern EB effect in these systems. Considering the common valence structure and smaller radius of Cr atoms, it is interesting to study
the effect of Mn-Cr swapping. The smaller Cr atoms should reduce nearest neighbor bond radii and promote AFM behavior. Here we report on a set of Ni2Mn1.4-xCrxGa0.6 (0.00 ≤ x ≤
0.40) compounds fabricated via standard arc-casting and annealing procedures. At dilute Cr content, x ≤ 0.10, AFM interactions in the system were enhanced. This behavior resulted in
enhanced EB properties of the system. The x = 0.10 sample exhibited maximum EB fields of HEB ~ 723 Oe and HEB ~ 300 Oe for ZFC / FCC conditions respectively. For higher Cr
content, the FM response of the system was enhanced, and a secondary precipitate was observed as the bulk of the samples began to reject new Cr atoms. The observed EB phenomena of
samples containing Cr content in excess of x = 0.25 was suppressed significantly. Relevant structural, compositional, and magnetic data will be presented for the Ni2Mn1.4-xCrxGa0.6
system. 

**References:** 

[1] M. Bibes, J. Villegas and A. Barthelemy, Adv. Phys., Vol. 60, p.5-84 (2011) 
[2] C. Tsang, R. Fontana and T. Lin, IEEE Trans. Magn., Vol. 30, p.3801-3806 (1994) 
[3] J. Nogués and I. Schuller, J. Magn. Magn. Mater., Vol. 192, p.203-232 (1999) 
[4] M. Khan and A. Albagami, J. All. Comp., Vol. 727, p.100-106 (2017) 
[5] Y. Zhang, J. Li, and F. Tian, Intermetallics, Vol. 107, p.10-14 (2019) 
[6] P. Lazpita, J. M. Barandiaran and J. Feuchtwanger, J. Phys.: Conf. Ser., Vol. 325, p.012016 (2011)

Exchange Bias Properties of Cr Doped Ni2Mn1.4 xCrxGa0.6 Heusler Alloys

Synthetic antiferromagnetic nanoplatelets (NPs) with perpendicular magnetic anisotropy (PMA) are promising candidates for nano-torque-related applications [1]. To
manipulate the NPs, a good understanding of the homogeneity in magnetic and structural properties is required. Most techniques measure these properties based on ensemble techniques
[2], however, an efficient and easy magnetic characterization technique on single particle level, down to the nanometer range, is currently lacking.
Here, we present a characterization method based on photothermal magnetic circular dichroism (PT MCD), which measures the differential absorption of left and right circularly polarized
light of an individual magnetic nanoparticle due to the polar Kerr effect [3]. In this way the hysteresis loop on a single 120 nm diameter SAF NP is obtained (see Fig 1b). When compared
to SQUID (of ~106 particles) as shown in Fig. 1c, the loop exhibits comparable behavior i.e., the antiferromagnetic state at zero applied field and a sharp magnetization switch at a large
field. We notice that the values of the switching fields (BL and BH as schematically represented in Fig. 1b) are different from the SQUID measurement, which is due to the increased
temperature of the NP during the PT MCD measurement. The statistics of the switching fields by PT MCD are shown in Fig 1d where a difference in the distribution is observed for the BH
and BL switch, which we attribute to a change in the dominant magnetic reversal mechanism of the NPs [4].
Our results will pave the way to a deeper understanding of torque based applications using PMA-SAF NPs and introduce the PT MCD method to the magnetic-nanoparticle community [5]. 

**References:** 

[1] Mansell, R., et al. Scientific reports 7.1 (2017): 1-7. 
[2] Wasielewski, M. R., et al. Nature Reviews Chemistry 4.9 (2020): 490-504. 
[3] Spaeth, Patrick, et al. Nano letters 22.9 (2022): 3645-3650. 
[4] Thomson, T., et al. Physical review letters 96.25 (2006): 257204. 
[5] Adhikari, S, Li, J., et al. In-Preparation (2022) 

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Magnetization Switching of Single 120 nm Diameter Synthetic Antiferromagnetic Nanoplatelets with Perpendicular Magnetic Anisotropy

Nanomotors —such as drug delivery vehicles or self-assembled structures requires efficient manipulation of nanoparticle motion. Magnetic field manipulation provides a
contactless and sensitive solution to the issue.
Here, magnetic nanowires (MNWs) of various lengths and radii were fabricated via electrochemical deposition into nanoporous templates. As a working electrode one side of templates
was sputtered with a Cu film. Suspensions were created by removing the MNWs from the template firstly by etching away the Cu film, and then dissolving the template in NaOH. The
MNWs were then rinsed several times with deionized water, then redispersed using ultrasonic agitation and placed into the liquid to be studied. A rotating external magnetic field can apply
a magnetic torque to balance the drag torque, caused by liquid, and rotate the MNWs in sync with the field. Magnetic torque arises when the magnetic field is not parallel to the nanowires.
In addition, nanowire size has large effect on their rotation speed - a longer MNW will experience slower acceleration and larger drag, while a MNW with larger diameter rotates faster. At
low frequency rotations of the magnetic field, the nanowires rotate synchronously with the applied field. When the frictional torque exceeds the magnetic torque, the MNW no longer
synchronizes with the rotating external field. When the driving field frequency is very high the nanowire will keeps oscillating around its position. By exploring MNW diameters from 20-
200nm and lengths from 1-5μm and rotation frequencies from 60-200rpm in liquids from aqueous to viscous (including cryopreservation agents for the new field of nanowarming organs
and tissues), a phase diagram of MNW control has been created.

Magnetic nanowires rotation in suspension under the influence of magnetic field

Magnetic iron oxide nanoparticles have been extensively studied in the biomedical field due to their diverse application such as magnetic hyperthermia, targeted drug
delivery. Recently, larger magnetic particles with a vortex ground state has been recognized as useful biomedical materials, because of high saturation magnetization and wider tunability of
a particle size from sub-100 nm to submicrometer scale. The vortex particles showed a high heating efficiency, which is comparable to that of superparamagnetic nanoparticles and is
enhanced with the number of vortices [1,2]. However, spin reorientation process under alternating fields, for submicron particles in particular, has not been fully understood, though the
understanding is of crucial importance for the improvement of the biomedical performance. Here, we report results of magnetic hysteresis scaling for hollow Fe3O4 spherical submicron
particles with a vortex structure [3,4]. A scaling coefficient is deduced from the power law scaling and examined to elucidate the relation with particle morphology.
We examined hollow Fe3O4 spherical particles with varying 400-700 nm diameter [4]. A set of symmetrical minor loops were measured with increasing field amplitude step-by-step up to
2 kOe. Fig. 1 shows a typical example of a hysteresis scaling at T = 300 K for various particle sizes. The relation between hysteresis loss and remanence of minor loops follows a power law
with an exponent of ~1.5 in a limited field range (< ~ 500 Oe), where a vortex state is stabilized. Such exponent was universally obtained for other particle size and in a wide temperature
range of 10-300 K. As shown in the inset, the scaling coefficient monotonically decreases with increasing inner/outer diameter ratio and temperature, indicating a decrease of domain wall
pinning strength [5] and an increasing vortex mobility under alternating fields. The results demonstrate that the scaling coefficient is an useful physical parameter for evaluating a vortex
mobility in a ferromagnetic particle. 

**References:** 

[1] N. A. Usov, et al., Sci. Rep. 8, 1224 (2018). 
[2] D. W. Wong, et al., Nanoscale Res. Lett. 14, 376 (2019). 
[3] N. Hirano, et al., Appl. Phys. Lett. 119, 132401 (2021). 
[4] M. Chiba et al., J. Magn. Magn. Mater. 512, 167012 (2020). 
[5] S. Kobayashi et al., J. Appl. Phys. 107, 023908 (2010). 

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Magnetic vortex mobility for hollow Fe3O4 submicron particles studied by a hysteresis scaling technique

Body Magneto-acoustic devices, integrating ferri- and ferro-magnetic materials with surface and bulk acoustic wave transducers, are of interest for nonlinear signal processing
applications such as frequency selective and tunable filtering, signal correlation and isolation [1-3]. Typically, the devices are fabricated by sputter depositing the piezoelectric material
needed for acoustic transduction onto the magnetic substrate. Lack of epitaxy as well as thermal budget constraints in the sputtering processes, however, restrict the combinations of
piezoelectric and magnetic materials possible for these devices. In this work, we present an alternate fabrication approach based on heterogeneous integration whereby the acoustic
transducer is grown on an epitaxially suitable substrate and subsequently transferred to the desired magnetic material [4], allowing for device design and performance to be optimized
unhindered by process limitations. A magnetoelastic high overtone bulk acoustic resonator (ME-HBAR) is implemented as an example device, as shown in Fig. 1. A GaN acoustic
transducer with a top Al electrode is grown on epitaxially compatible 4H-SiC and released for transfer onto a Cu/YIG substrate by etching a sacrificial NbN layer. Cu serves as the bottom
electrode of the acoustic transducer and an acoustic matching layer.
The frequency response of the ME-HBAR is measured as a function of magnetic field vector. Two representative responses corresponding to zero field and Bx = 56 mT (in-plane
component) and Bz = 180 mT (normal component) are shown in Fig. 2. Around 2.75 GHz, the resonant acoustic modes are strongly attenuated and shifted in frequency due to hybridizing
with spin wave modes of similar frequency and wavelength to form coupled magneto-elastic waves [5]. Sufficiently far from the crossover region (not shown), the frequency shift is
attributed to the ΔE effect, which together with the lossy hybridization region may be used to realize tunable bandpass and notch filters respectively. 

**References:** 

1. X. Liang, C. Dong, H. Chen, et al., Sensors, Vol. 20 p. 1532 (2020) 
2. Y Li, C. Zhao, W. Zhang, et al., APL Materials, Vol. 9, art. 060902 (2021) 
3. M. Frommberger, Ch. Zanke, A. Ludwig, et al., Microelectronic Engineering, Vol. 67, p. 588 (2003) 
4. B. Downey, A. Xie, S. Mack, et al., IEEE 2020 Device Research Conference, p. 1 (2020) 
5. C. Kittel, Phys. Rev., Vol. 110, p. 836 (1958) 

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Heterogeneously Integrated Magneto Acoustic Resonators

High-entropy alloys (HEAs), instead of employing one or two main constituents like in traditional alloy development, adopt a different design concept where it combines a blend of multi-principal elements in large concentrations. This design idea enables them with large values of configurational entropy of mixing and a vast compositional space which offers a huge window of exploration opportunities. Magnetocaloric HEAs have been limited to low-temperature range or showing sub-par performance till recent extensions to the first-generation equiatomic HEA design approach have been devised [1, 2]. The magnetocaloric performance of equiatomic magnetocaloric HEAs heavily depends on the choice of element selection and thus the likelihood of relying on the rare-earth (RE) elements due to their intrinsic large magnetic moments. Extending beyond the equiatomic point towards non-equiatomic in the HEA region combined with directed search strategies, overcomes the above-mentioned limitations of magnetocaloric HEAs: the low temperature limit is improved (48 % increase as displayed in Fig 1) and â‰¥ 5-fold performance enhancement (Fig 2) [3-5]. In this talk, we will show the various design strategies of magnetocaloric HEAs and how to tackle the vast HEA space to tune for the desired magnetocaloric properties. Work supported by Grant PID2019-105720RB-I00 funded by MCIN/AEI/ 10.13039/501100011033. Additional support ConsejerÃ­a de EconomÃ­a, Conocimiento, Empresas y Universidad de la Junta de AndalucÃ­a (grant P18-RT-746), US Air Force Office of Scientific Research (FA8655-21-1-7044) and Sevilla University under VI PPIT-US program.

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Effect of thermal cycling on exchange bias in NiFe/IrMn bilayers

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Understanding magnetization reversal mechanisms and exchange interactions in nanostructured cobalt ferrites using first order reversal curves (FORC)

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