Session S54: Tuning Magnetism and Transport in Nano-Composites and Heterostructures

3D Metallic Nanowire Networks

Networks of interconnected magnetic nanowires offer an exciting platform to explore 3-dimensional (3D) nanomagnetism, where their structure, topology and frustration may be used as additional degrees of freedom to tailor the materials properties. We have achieved quasi-ordered metallic nanowire networks over cm-scale areas, using multiple angle ion-tracking and electrochemical deposition, with density as low as 40 mg/cm³ [1]. New magnetization reversal mechanisms in cobalt networks are captured by the first-order reversal curve (FORC) method, which demonstrate the evolution from strong demagnetizing dipolar interactions to intersections-mediated domain wall pinning and propagation, and eventually to shape-anisotropy dominated magnetization reversal. Such networks offer interesting potentials for 3D magnetic memory and logic applications, where the spin textures may be manipulated in a contactless fashion by chemisorption [2,3]. They also may be used to implement repeatable multi-state memristors in neuromorphic circuits, due to the expected discrete nature of domain wall motion through the intersecting networks. Interestingly, another random configuration of metallic nanowire networks has found applications in deep-submicron particulate filtration, relevant to combatting COVID-19 and air pollution, due to the extremely large surface areas and the excellent mechanical strength [4,5].

Work done in collaboration with Edward C. Burks, Dustin A. Gilbert, Gong Chen, Dhritiman Bhattacharya, James Malloy, Alberto Quintana, Christopher J. Jensen, Peyton D. Murray, Chad Flores, Thomas E. Felter, Supakit Charnvanichborikarn, Sergei O. Kucheyev, Jeffrey D. Colvin, Andreas Schmid, and Gen Yin.

[1] E. C. Burks, D. A. Gilbert, P. D. Murray, C. Flores, T. E. Felter, S. Charnvanichborikarn, S. O. Kucheyev, J. Colvin, G. Yin and Kai Liu, Nano Lett., 21, 716 (2021).

[2] G. Chen, A. Mascaraque, H.Y. Jia, B. Zimmermann, M. Robertson, R. Lo Conte, M. Hoffmann, M.A.G. Barrio, H.F. Ding, R. Wiesendanger, E. Michel, S. Blügel, A. Schmid, and Kai Liu, Sci. Adv. 6, eaba4924 (2020).

[3] G. Chen, M. Robertson, M. Hoffman, C. Ophus, A. L. F. Cauduro, R. Lo Conte, H. F. Ding, R. Wiesendanger, S. Blügel, A. K. Schmid, and Kai Liu, Phys. Rev. X, 11, 021015 (2021).

[4] J. Malloy, A. Quintana, C. J. Jensen, and Kai Liu, Nano Lett., 21, 2968 (2021).

[5] Phase 1 Winner of BRADA-NIOSH Mask Innovation Challenge: https://drive.hhs.gov/mask_challenge.html   

Large refrigerant capacity in rare earth free nanostructures of matrix embedded Fe nanoparticles

A magnetocaloric effect with sizable isothermal entropy change (△S) maintained over a broad range of temperature (10K ˂T˂ 300K) is reported for a rare earth free nanoparticle heterostructure. The nanostructure is composed of environment friendly and non-toxic iron (Fe) nanoparticles confined in layers of a titanium nitride (TiN) thin-film matrix. The magnetocaloric Fe particles of uniform shape and narrow size distribution are embedded in a TiN/Fe/TiN multilayered pattern using a pulsed laser deposition (PLD) method. Crystallographic studies carried out using x-ray diffraction have indicated the epitaxial nature of TiN film. M  vs.T data at various fields, allows application of the Maxwell relation which provides quantitative information about the isothermal entropy change in the Fe nanoparticulate system. Our data show a magnetocaloric effect (MCE) with a |△S^max| of 4.18×10³  J/K m³ in the magnetic field range of 0.1 T ˂ μ₀H ˂ 3 T for the TiN/Fe/TiN sample having Fe particles size of ~15 nm. At lower applied fields (0.0025T ˂ μ₀H ˂ 0.075T) the change in △S vs T peaks in the vicinity of the blocking temperature (T_B∼60K). For T>T_B, dynamic hysteresis is absent and the magnetocaloric effect becomes potentially useful for near room temperature cooling applications. The weak temperature dependence of △S over a wide range of temperature brings about a maximum refrigerant capacity value of 7.40×10⁵ J/m³ (94 J/kg) at 3 T that is comparable with other attractive rare-earth-free nanostructured transition metal magnetocaloric materials.   

Elucidation of relaxation dynamics of ferrite nanoparticles at nanosecond timescales

Magnetic nanoparticles (MNPs) are becoming increasingly important as tracers for non-invasive, in vivo magnetic imaging (e.g. MPI¹), magnetic hyperthermia, and thermometry². For these applications, MNP tracers are driven by AC magnetic fields and detected by their modulated magnetic moments. The observed magnetization dynamics, which span broad timescales from seconds to nanoseconds³, are governed by a complex set of parameters, including size, morphology, magnetic anisotropy, saturation magnetization, and interactions.⁴ Here, we present time and frequency domain measurements for our synthesized ferrite and cobalt-doped ferrite nanoparticles suspended in solution over a range of temperatures (240-330 K), magnetic field amplitudes (0-20 mT_RMS), and frequencies (DC-50 MHz) to elucidate relaxation mechanisms and quantify magnetic monodispersity.

[1] Gleich et al. Nature, 435, p. 1214-7 (2005)

[2] Bui et al. J. Appl. Phys., 128, 224901, p. 1-9 (2020)

[3] Deissler et al. Med. Phys. 41, 012301 (2014)

[4] Anderson et al. Phys. Rev. B, 94, p. 1-8 (2016)   

Magnetic field dependence of the recombination times in CdSe/CdMnS core/shell nanaoplatelets

We have performed time resolved photoluminescence (PL) studies of colloidal nanoplatelets (NPL) at T = 5 K in the presence of a magnetic field B.  The NPLs consist of a CdSe core surrounded by a CdMnS shell.  The PL has one component  associated with exciton recombination (channel 1) and another with the recombination of electrons with holes localized at the CdSe/CdMnS interfaces (channel 2) [1].   The band edge emission time evolution is described by two time constants.  A shorter time τ₁ associated with channel 1 which does not depend on B and  a longer time τ₂  associated with channel 2.  The time τ₂ for the  σ+  (σ- ) component increases (decreases) with magnetic field.  Furthermore, we observe the B dependence of the two slow times gets smaller at T = 10 K compared to T = 5 K. We have developed a model to interpret these results. [1] J.R. Murphy et al., Appl. Phys. Lett. 2016, 108, 242406   

Distinguishing two-component anomalous Hall effect from topological Hall effect in magnetic topological insulator MnBi2Te4

In transport, the topological Hall effect (THE) is widely interpreted as a sign of chiral spin textures, like magnetic skyrmions. However, the co-existence of two anomalous Hall effects (AHE) could give rise to similar non-monotonic features or "humps", making it difficult to distinguish between the two. Here we demonstrate that the "artifact" two-component anomalous Hall effect can be clearly distinguished from the genuine topological Hall effect by three methods: 1. Minor loops 2. Temperature dependence 3. Gate dependence. One of the minor loops is a single loop that cannot fit into the full AHE loop under the assumption of AHE+THE. In addition, by increasing the temperature or tuning the gate bias, the emergence of humps is accompanied by a polarity change of the AHE. Using these three methods, one can find the humps are from another AHE loop with a different polarity. Our material is a magnetic topological insulator MnBi₂Te₄ grown by molecular beam epitaxy, where the presence of the secondary phase MnTe₂ on the surface contributes to the extra positive AHE component. Our work may help future researchers to exercise cautions and use these three methods to examine carefully in order to ascertain genuine topological Hall effect.   

Py-Cu Graded-Index Magnonic Waveguides

Spin waves (magnons) have many optical analogies, including the law of reflection, which allows total internal reflection (TIR, akin to a fiber optic cable). For this reason, we’re working to create a magnonic waveguide made from Py-Cu multilayered films.

 

We grow films via sputter deposition with a 10 nm core of Py_0.8Cu_0.2 that is symmetrically bound by 10 nm of Cu and 10 nm of Py_0.6Cu_0.4. Py-Cu alloys are attractive for this study because we can easily tune magnetic properties by changing the Cu content (T_C decreases as Cu content increases).

 

We measure samples using ferromagnetic resonance (FMR) with frequencies from 3-20 GHz and fit results to the Kittel equation. We extract values for the Gilbert damping parameter and inhomogeneous broadening, which informs us of how magnons propagate through each film.

 

Ideally, if we reverse the magnetic layers, instead of seeing total internal reflection, we should observe a complete spin absorber.   

Identification of skyrmion transition mechanisms by sub-10 nm maps of the transition rate

In addition to the conventional radial symmetric collapse of magnetic skyrmions, recent studies predicted the occurrence of skyrmion annihilation processes via the chimera skyrmion state [1-3]. Here, we demonstrate the realization of both the radial symmetric and the chimera transition mechanism in the ultra-thin film system fcc-Pd/Fe/Ir(111) [4]. Scanning tunneling microscopy is used to create transition rate maps of magnetic switching events induced by single electron events. In combination with energy density maps of the transition states obtained by atomistic spin simulations parametrized from first principles, they allow for the identification of both annihilation mechanisms. It is further shown, that a transition between both mechanisms can be achieved by the application of external in- and out-of-plane magnetic fields, yielding a sound agreement between experiment and theory.

[1] Meyer et al., Nat. Commun. 10, 3823 (2019)

[2] Heil et al., Phys. Rev. B 100, 134424 (2019)

[3] Desplat et al., Phys. Rev. B 99, 174409 (2019)

[4] Muckel et al., Nat. Phys. 17, 395-402 (2021)   

Spin transport and proximity induced magnetism in thin film structures

Critical physical mechanisms occur across the interfaces between magnetic (FM) and non-magnetic heavy metal (HM) thin-film layers. Key examples are the enhancement of magnetic damping, which occurs via interfacial effects and the pumping of spin-current into NM layers, and spin-orbit torque (SOT) switching from the propagation of spin-current from a HM into a FM layer. The linkage between different interfacial phenomena has been the subject of debate, including the role of proximity induced magnetisation (PIM) in spin transport across FM/HM interfaces [1]. Debate has also surrounded the spin-diffusion length from spin-pumping analysis [2] and spin-pumping through insulating layers [3]. The focus here is on spin-transport across the interface. PIM in Pt is discussed first in heavy metals layered with ferro and ferri-magnetic materials [4]. Spin transport across FM/NM interfaces is then introduced and the effects of interface structure and NM thickness and a new fuller physical description for the analysis of spin-transport from spin-pumping in FM/NM is presented [2]. This shows that both the NM and FM layer thicknesses should be systematically studied and also shows the need to incorporate a thickness dependence for the spin-diffusion length in the NM layer [2]. Finally, very recent work demonstrating the relationship between PIM and enhanced damping is described. Element specific x-ray magnetic circular dichroism and ferromagnetic resonance measurements in both CoFe/Au/Pt and NiFe/Au/Pt thin film samples with varying Au thickness show an approximately linear relationship between the magnitude of Pt PIM and the damping enhancement [1].   

Scanning NV Magnetometry for Magnetic Memory Devices

Scanning NV magnetometry (SNVM) is an emerging quantum sensing technique which allows to measure minute magnetic fields with nanoscale resolution. We present a specific use-case of SNVM: the characterization of magnetic nanowires. Magnetic nanowires are among the essential building-blocks of contemporary spintronic devices [1] since their magnetic properties can be tuned by their geometry, and their fabrication is compatible with standard semiconductor fabrication schemes. While their topography and homogeneity can be well characterized with established techniques, it remains difficult to access their microscopic magnetic properties which are key to improve device performance.

Here, we demonstrate magnetic imaging of ultra-scaled magnetic nanowires by SNVM [2]. The imaging reveals the presence of weak magnetic inhomogeneities inside in-plane magnetized nanowires that are largely undetectable with standard metrology. In this context, we will discuss the potential of SNVM for semiconductor device analysis.

[1] Parkin, S., & Yang, S. H. (2015). Memory on the racetrack. Nature Nanotechnology, 10(3), 195–198.

[2] Celano U., et al., submitted   

Tuning of exchange bias and magnetoresistance of self-assembled vertically aligned La0.7Sr0.3MnO3:NiO nanocomposite thin films via microstructure induced strain

The microstructures and interfaces of two-phase vertically aligned nanocomposite (VAN) thin films play a key role in the design of spintronic device architectures and their multifunctional properties.[1,2] Here, we show how the microstructures in self-assembled VAN thin films of La0.7Sr0.3MnO3:NiO (LSMO:NiO) can be effectively tuned from nanogranular to nanocolumnar and to nanomaze by controlling the number of laser shots from the two constituent phase targets in the PLD film growth. The observed microstructural induced strain is found to significantly enhance the magnetoresistance in a very broad temperature range between 10 K and 240 K and to modulate the in-plane disorder-induced exchange bias, whose origin was investigated in detail by magnetotransport and X-ray magnetic circular dichroism measurements. As a consequence of out-of-plane tensile strain, a systematic variation in the Mn3+/Mn4+ content across the vertical interface is also observed by means of X-ray absorption spectroscopy. Our results [3] show that fine-tuning of the microstructure-induced out-of plane tensile strain, interfacial disorder and grain boundaries can be used to effectively modifying the exchange bias, magnetotransport and the electronic structural properties of these LSMO:NiO VAN thin films.   

Magneto-Optical study of Bound Excitons in Semimagnetic Quantum Ring

Embedding magnetic impurity ions, like Mn2+, within semiconductor nanostructures has been a promising approach for magnetic tuning of the electronic states which are well illustrated in the present magneto-optical study of bound excitons in a

CdTe/Cd1-xMnxTe Quantum Ring (QR). The Giant Zeeman Splitting (GZS)[1,2] between the s + and s- Excitonic transitions in the Faraday configuration, due to the magnetic exchange interaction (sp-d exchange) between the localized substitutional magnetic impurity ions and the delocalized charge carriers, has been investigated for various concentration (x) of the magnetic ions, using the variational technique in the effective mass approximation invoking mean-field theory with the inclusion of modified Brillouin function. The enhancement of GZS, and in turn, the effective Landé g-factor with the application of magnetic field (g) is strikingly manifested in Type I –Type II transition in the band structure, as ‘g tremendously reduces the potential barrier height in the valence band as compared to the conduction band. This highlights the extraordinary magneto-optical properties including Giant Faraday Rotation (GFR)[3]. The results are discussed and compared in all the three magnetic regimes: (i) low ‘x’ at all T, (ii) arbitrary ‘x’ at high T, and (iii) large ‘x’ at low T where the magnetic saturation occurs[4]. The obtained results show that the excitonic Zeeman splitting is large for low ‘x and T’ as compared to high ‘x and T’ where the possibilities for the manganese ions to form antiferromagnetic pairs is maximized.

[1] Charles, J Barrows et. al, J. Phys. Chem. Lett. 2015, 6, 3076-3081.

[2] Chi, Li et. Al., J. Am. Chem. Soc. 2020, 142, 20616-20623.

[3] Seongmin, Ju et. al, Phys. Status Solidi A 2019, 1800549 (1-7).

[4] Bartholomew, D U et. al., Phys. Rev. B 1986, 34, 6943-6950.