United States

In the present work, we show that the in situ growth of oriented nanometric aggregates of cobalt oxide can be explored to alter and tune magnetocrystaline anisotropy. As such, the impact of interparticle interaction on magnetic properties of the aggregates is studied. Mono-dispersed cobalt oxide (CoO) nanostructures were prepared through decomposition of metal-organic compound of cobalt (II) in an alcoholic solution under controlled microwave irradiation, below 150 ⁰C. CoO nanostructures were effectively synthesized without the use of any surfactant. The structures were found to be inert to oxidation in ambient air for several months. We attribute the stability of CoO nanospheres to their dense packing, the driving force being, minimization of surface area and surface energy [1]. X-ray diffractometry (XRD) measurements indicate the formation of well crystallized CoO nanoparticles, without the need for post-synthesis annealing. Deconvolution of the active modes in Raman spectra obtained at room temperature reveals the Oh symmetry in rock-salt CoO, induced by microwave-assisted synthesis route [2]. Its effect on magnetic characteristics of CoO is analyzed here. Field-dependent magnetization (MH) and temperature-dependent magnetization (MT) measurements show a phase transition from paramagnetic to superparamagnetic interactions in CoO nanostructures at around 10 K, at which the average particle size was estimated to be 9 nm. The nanostructures however do not exhibit a distinct antiferromagnetic transition around 300 K as the bulk sample [3], but instead show a hysteresis below a blocking temperature of ~10 K (Figure 1). This finding reveals the magnetic behavior of CoO nanostructures that opens new opportunities for future applications as anisotropy source for magnetic recording.References: [1] M. Ibrahim Dar, S. Sampath and S. A. Shivashankar, Journal of Materials Chemistry., Vol. 22, p.22418 (2012) [2] A. V. Ravindra, B. C. Behera and W. Prellier, Journal of Applied Physics, Vol. 116, p.33912 (2014) [3] C.N.R. Rao and B. Raveau, Transition metal oxides, Wiley., (1995)

MMM 2020

Tuning the magnetic anisotropy and phase transition in superparamagnetic cobalt oxide nanostructures synthesized through novel solution-growth technique

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We report magnetic -field -induced rotation of the antiferromagnetic Néel vector in epitaxial CuMnAs thin films. Firstly, using soft x-ray magnetic linear dichroism spectroscopy and magnetometry, we demonstrate spin-flop switching and continuous spin reorientation in films with uniaxial and biaxial magnetic anisotropies, respectively, for applied magnetic fields of the order of 2 T. The remnant antiferromagnetic domain configurations are determined using x-ray photoemission electron microscopy. Next, we show that the Néel vector reorientations are manifested in the longitudinal and transverse anisotropic magnetoresistance. Dependency of the electrical resistance on the orientation of the Néel vector with respect to both the electrical current direction and the crystal symmetry are identified, including a weak 4th fourth-order term which is evident at high magnetic fields. The results provide characterization of key parameters including the anisotropic magnetoresistance coefficients, magnetocrystalline anisotropy, and spin-flop field in epitaxial films of tetragonal CuMnAs, a candidate material for antiferromagnetic spintronics.References: 1. M. Wang, C. Andrews, S. Reimers, et.al. Phys. Rev. B 101, 094429 (2020). 2. P. Wadley, B. Howells, J. Zelezny, et.al. Science 351, 587 (2016).

Spin flop and crystalline anisotropic magnetoresistance in CuMnAs

Antiferromagnets have considerable potential as spintronic materials [1]. Notably, they manifest resonant modes at frequencies higher than can be observed in conventional ferromagnetic materials. Synthetic Antiferromagnets (SAFs), engineered structures of antiferromagnetic (AF) RKKY exchange coupled ferromagnetic(FM)/non-magnetic(NM)/ferromagnetic trilayers, mimic the properties of single phase antiferromagnets whilst possessing greater compatibility with current device technologies such as those used for STT-MRAM production [2,3]. In addition to the conventional acoustic (Kittel) mode, SAFs also possess a higher order resonant (optic) mode [3]. The structure of a SAF is shown in Fig.1a. We report the dynamic properties of SAFs with structure Ta(2 nm)/CoFeB(5 nm)/Ru(tRu)/CoFeB(5 nm)/Pt(4 nm) where tRu is the thickness of the Ru layer, acting as the NM spacer layer. The samples were fabricated using magnetron sputtering and characterized by X-ray Reflectivity (XRR) to measure layer thickness, vector Vibrating Sample Magnetometry (VSM) to verify AF interlayer coupling and a Vector Network Analyser – Ferromagnetic Resonance (VNA-FMR) setup to explore dynamic properties. Fig.1b shows the resonant dynamics of a single CoFeB layer and a CoFeB SAF. The data demonstrate the highest zero field frequencies observed in SAFs to date [4] with resonances of 18 GHz and 21 GHz at the 1st and 2nd instances of AF RKKY coupling respectively. A three part analytical model can reproduce the major features of the measured spectra.References: [1] T. Jungwirth, X. Marti, P. Wadley et al., Nat. Nanotechnol., Vol. 11, p. 231–241 (2016) [2] R. Hao, L. Wang and T. Min, IEEE Trans. Magn., Vol. 55, p. 1-6 (2019) [3] S. Li, C. Wang, X. Chu et al., Adv. Funct. Mater., Vol. 26, p.3738-3744 (2016) [4] H. Waring, N.A.B. Johansson, I. Vera-Marun et al., Phys. Rev. Appl., Vol. 13, p. 034035 (2020)

Zero Field Optic Mode Beyond 20 GHz in a Synthetic Antiferromagnet

Magnetism in strained LaCoO3 (LCO) thin films has been an intriguing scientific subject for the last decade. While the bulk LCO is non-magnetic, strained LCO thin films exhibit robust ferromagnetism with a phase transition temperature (Tc) of 80 K.[1] Since this system is a representative example where epitaxial strain induces non-bulk magnetic properties, it has drawn lots of attentions in the field of magnetic oxide thin films. Despite extensive research over the past decade[2-6], the consensus on the origin of ferromagnetism has not yet to be achieved. A main challenge of this system is a complex microstructure inside the film; the existence of planar defects (seen as dark stripes in high-angle annular dark field scanning transmission electron microscopy (HAADF STEM) images) and their association with oxygen vacancies are important issues. Through the systematic STEM-based investigations, we verified that the planar defects always exist in epitaxial LCOs which undergo a ferroelastic response and the pristine planar defects do not involve oxygen vacancies. Furthermore, we have recently found that the compressed units sandwiched by the dark stripes are responsible for ferromagnetism in epitaxial LCOs. In this presentation, we will show that the ferroelastic nature of ferromagnetic structural units can account for many unusual magnetic properties of this system.References: [1] D. Fuchs et al. Phys. Rev. B 75, 144402 (2007) [2] W. Choi et al. Nano Lett. 12, 4966 (2012) [3] N. Biskup et al. Phys. Rev. Lett. 112, 087202 (2014) [4] L. Qiao et al. Nano Lett. 15, 4677 (2015) [5] J. Fujioka et al. Phys. Rev. Lett. 111, 027206 (2013) [6] G. Sterbinsky et al. Phys. Rev. Lett. 120, 197201 (2018)

Ferroelastic Nature and Ferromagnetism in Epitaxial LaCoO3

The manganites [1,2] and cobaltites [3-5] have been found to be depicting the spin-glass (SG) behaviour; arising out of randomly frozen state of the spins, below a critical temperature. A typical SG is characterized by the absence of long range ordering and magnetic frustration [6]. In LaMn1-y CoyO3 (y = 0.1 - 0.4) competing ferromagnetic (FM) and antiferromagnetic (AFM) interactions result in frustration consequently leading to SG state. In order to investigate the existence of SG behaviour in the present system susceptibility measurement in an ac field of 4 Oe and 0 Oe dc field has been performed at different frequencies. Temperature dependence of in-phase ( χ’ac ) component of the ac susceptibility, show that the peak position which correspond to freezing temperature (Tf ) moves toward higher temperatures and the magnitude of χ’ac decreases as the frequency increases. This is a strong signature of the existence of SG behavior. The parameter called relative shift in freezing temperature (δTf) per decade of frequency, suitable in characterizing a particular SG state [6, 7] is found to be δTf = 0.008 for x = 0.1 and δTf = 0.0053 for x = 0.4; in accordance with the one reported for canonical SG systems [3,6,10]. Temperature variation for out of phase component of ac susceptibility (χac``) show the existence of canonical SG state in both the samples, the prominent feature for which is the zero value of χac`` at temperatures larger than Tf and its finite value below Tf in both the samples. The frequency dependence of Tf is analyzed by using the Vogel-Fulcher (VF)- and Arrhenius –law [6], both of which prove the existence of canonical spin glass state in LaMn1-xCoxO3 ( x = 0.1 - 0.4).References: 1. I. O. Troyanchuk , L. S. Lobanovsky et al J. Magn. Magn.Mater. 210, 63 (2000). 2.X.L. Wang, M. James et al. Supercond. Sci. Technol. 15, 427- 430 (2002) 3. N. Khan, A. Midya, P. Mandal and D. Prabhakaran J. App. Phys. 113, 183909 (2013). 4. V. Kumar, Y. Kumar, R. Kumar et al. J.of App. Phys. 113, 043918 (2013). 5. J. Wu and C. Leighton Phys. Rev. B 67, 174408 (2003). 6. J. A. Mydosh Spin glasses: an experimental introduction (1993 Taylor and Francis); 7. P. Bag, P. R. Baral, and R. Nath Phys. Rev. B 98, 144436 (2018). 8.C. A. M. Mulder, A. J.V. Duyneveldt, and J. A. Mydosh, Phys. Rev. B 25, 515 (1982).

Study of Canonical Spin Glass Behavior in Co doped LaMnO3

The perovskite compound La1-xSrxMnO3 (LSMO) is known for the high spin-polarized currents, which makes it a material of great interest in spintronics [1]. The magnetoresistance (MR) observed was found to depend strongly on the microstructure, i.e., the grain boundaries (GBs) and the interfaces between the grains. By means of electrospinning, one can prepare long (up to 100 µm), free-standing LSMO nanowires. The diameter of the nanowires is in the 200 nm-range, and the individual nanowires are polycrystalline with a grain size of about 15-20 nm, which corresponds to the dimensions obtained via transmission electron microscopy (TEM). The microstructural properties of electrospun La0.8Sr0.2MnO3 (LSMO) nanowires were investigated using SEM, TEM and electron backscatter diffraction (EBSD). By means of EBSD, it is possible to measure the crystallographic orientation of the LSMO grains within an individual nanowire. As the LSMO grains within the nanowires are in the 10-nm range, we employ here parts of the recently developed transmission-EBSD or transmission Kikuchi diffraction technique [2,3] in order to enhance the Kikuchi pattern quality to enable an automated mapping of the crystallographic data. The diffraction results demonstrate that the grain orientation is not random, but there is a texture induced by the shape of the polymer nanowire formed after the electrospinning step. Within an individual nanowire section, the dominating GBs are high-angle grain boundaries, which play an important role in the current flow through the sample (low- and high field magnetoresistance). The data obtained allow further an analysis of the grain shape aspect ratio, and elucidate the grain and grain boundary arrangement within electrospun LSMO nanowires. This work is supported by DFG-project Ko2323-8.References: [1] J. Huang et al., ACS Appl. Mater. Interfaces 10, 42698 (2018) [2] P. W. Trimby, Ultramicroscopy 120, 16 (2012). [3] A. Koblischka-Veneva, M. R. Koblischka, J. Schmauch, et al., J. Microscopy 274, 123 (2019).

Microstructure analysis of electrospun La0.8Sr0.2MnO3 nanowires using electron microscopy and electron backscatter diffraction (EBSD)

Half-doped praseodymium manganites, Pr0.5Ca0.5MnO3, nanoparticles synthesized by sol-gel method are annealed at different temperatures in order to obtain nanoparticles sizes ranging from 15 to 100 nm. HRTEM shows that the nanoparticles are very crystalline with no structural disorder at the surface. Rietveld powder diffraction profile determines that the samples have orthorhombic structure with space group Pnma and cell volume decreases with increasing particle size. The thermodiffraction patterns from 50 to 300 K indicate that the charge ordering (CO) is suppressed for the smallest particles, whereas the largest one still resembles the bulk. Hysteresis loops and ZFC-FC curves evince that, as the most of half-doped (Mn3+:Mn4+ = 1) antiferromagnetic (AFM) charge-ordered manganites, ferromagnetism (FM) appears as particle size decreases. Hysteresis loops at FC procedure show a coercivity increase and a shift to negative field, confirming that both AFM and FM phases not only coexist but are also coupled. In addition, a spin glass behaviour is observed with glass temperature around 50 K. A possible origin of FM phase development is the lacking oxygen ligands at the surface that increases Mn3+ at expenses of Mn4+ in order to maintain the electronic neutrality. In this work, we show that Mn3+:Mn4+ = 1 ratio is preserved in the whole crystalline particle with the exception of the surface, where this ratio changes to Mn3+:Mn4+ >1. This Mn3+ excess makes double exchange Mn3+→ O2-→Mn4+ prevails over the super-exchange giving place to the FM interactions. We show that a “shell” thickness of only one-unit cell, with acell = 0.38 nm, is enough to explain the onset of FM at the surface, whereas the volume remains AFM. Furthermore, the FM to AFM ratio fits to the increase of surface to volume ratio with decreasing particle size.[1].References: [1] H. D. Aliyu, J. M. Alonso, P de la Presa et al., Chem Mater 30, 7138-7145 (2018).

Surface ferromagnetism in Pr0.5Ca0.5MnO3 nanoparticles as a consequence of local imbalance Mn3+:Mn4+ ratio

We report a systematic tracking of the consequence of size reduction to nanoscale on charge order (CO) and magnetic properties of electron doped Sm0.35Ca0.65MnO3 by magnetization and electron paramagnetic resonance (EPR) measurements. The bulk form of this system is charge ordered below 275 K and antiferromagnetic (AFM) below 130 K. The bulk sample and nanoparticles of various sizes (mean diameter ~ 15, 30, 60, 90 nm) were prepared by sol-gel method. Magnetic and X-band EPR measurements were carried out at temperatures from 10-300 K. Our studies show that the highly robust CO in the bulk gets weakened by size reduction and the nanoparticles exhibit ferromagnetic (FM) ordering[1]. Magnetization in the paramagnetic region arising due to FM fluctuations caused by double exchange interaction is found to decrease as particle size decreases. However, at low temperature the trend of FM magnetization as a function of the size is found to be reversed as shown in Fig 1. Analysis of magnetic susceptibilities shows that magnetization in the FM phase is not due to the strengthening of double-exchange FM interactions but it is attributed to the dominance of surface effects. A detailed study on the EPR intensity, line width and g-factor reveals that in 15 nm particles though long range CO disappears, the signature of short range CO persists[2]. Line width variation indicating weakening of the AFM interactions and the development of the short-range FM phase are discussed based on Fig 2. As the particle size decreases the relative contribution of the surface ions increases. The prevalence of translational symmetry breaking, defects and surface imperfections lead to short range FM ordering in the system. We discuss the origin of these effects in terms of the presently existing models.References: [1] S. S. Rao, S. Tripathi, D. Pandey, and S. V. Bhat, Phys. Rev. B 74, 144416 (2006) [2] S. Zhou, Y. Guo, J. Zhao, L. He, C. Wang, and L. Shi, J. Phys. Chem. C 115, 11500 (2011).

Size Dependence of Charge Order and Magnetism in Sm0.35Ca0.65MnO3 – Magnetization and Electron Paramagnetic Resonance Studies

Recently, there have been reports on observation of electrical polarization and/or giant magnetoelectric effect in epitaxial rare-earth iron garnet (RIG) films [1–5] but the mechanism behind it is still in dispute. First-principles methods are employed to understand the existence of such polarization in RIGs. In contrast with previous beliefs, it is found that its occurrence can be explained if the system adopts magnetic multidomains and does neither require local magnetic moments of the rare-earth ions nor non-collinear magnetism. Table I reports the predicted polarization of a studied RIG multidomains (with R=Gd) with DW in the middle of supercells when we consider or not f-electrons of the Gadolinium (G) ions as valence electrons, which we denote as the “with-f” versus the “without-f” cases, respectively. Within each supercell, the two magnetic domains have a magnetic configuration that is reversed with respect to each other. As one can see, domain walls (DW) in GIG gives rise to a polarization even without f-electrons. This is further confirmed by the prediction of polarization in Yttrium (IG) and Lutetium (IG) iron garnet systems having multidomains as well. Furthermore, we found that the microscopic mechanism responsible for this DW-induced-polarization originates from a magnetoelectric effect arising from ferromagnetic interactions between octahedral and tetrahedral Fe ions at the domain walls (note that this interaction is antiferromagnetic in bulk RIG). More precisely, this mechanism is a symmetric exchange-striction mechanism, which, for the electrical polarization, takes the analytical form of Ρ=∑ Pesij Si●Sj, where the summation is over all the spin pairs and Pesij is the polarization coefficient vector associated with the spin pair. Acknowledgments This work is supported by the Department of Energy, Office of Basic Energy Sciences, under Award No. DE-SC0002220References: [1] A. S. Logginov, G. A. Meshkov, A. V. Nikolaev, and A. P. Pyatakov, JETP Lett. 86, 115 (2007). [2] G. V. Arzamastseva, A. M. Balbashov, F. V. Lisovskii, E. G. Mansvetova, A. G. Temiryazev, and M. P. Temiryazeva, J. Exp. Theor. Phys. 120, 687 (2015). [3] A. P. Pyatakov, A. K. Zvezdin, A. M. Vlasov, A. S. Sergeev, D. A. Sechin, E. P. Nikolaeva, A. V. Nikolaev, H. Chou, S. J. Sun, and L. E. Calvet, in Ferroelectrics ( Taylor & Francis Group , 2012), pp. 79–88. [4] V. Koronovskyy and Y. Vakyla, Electron. Mater. Lett. 11, 1028 (2015). [5] I. S. Veshchunov, S. V. Mironov, W. Magrini, V. S. Stolyarov, A. N. Rossolenko, V. A. Skidanov, J. B. Trebbia, A. I. Buzdin, P. Tamarat, and B. Lounis, Phys. Rev. Lett. 115, (2015).

Magnetic-domain-wall-induced electrical polarization in rare-earth iron garnet systems: A first-principle study

Since the first experimental observation of electrical switching of antiferromagnet[1], the experiments on manipulation of antiferromagnetic order are exploded. However, the theoretical models are still very simplified (sigma model, macrospin approximation). This talk will discuss the recent developments of modeling antiferromagnets and ferromagnets considering a full numerical micromagnetic model. We benchmarked the approach with analytical theories for THz oscillator[2], domain wall and skyrmion dynamics[3], while predicting and describing some features for THz detectors, memories[4] and high speed DW motions [5]. Within this approach, it is possible to take into account three contributions for the exchange interactions (one homogeneous and two inhomogeneous) that are important to identify the two time scales of the antiferromagnetic dynamics. The first, at the THz range, that is related to relaxation processes from a misalignment of the antiferromagnetic state. The second one is related to the relaxation of domains and it is of the same order of the one of the ferromagnetic order. In the last part of the talk, It will discussed the origin of memristive behaviour [4] and the effect of grain sizes in the antiferomagnetic switching experiments.References: 1 P. Wadley, et al, Science, 351, 6273 (2016). 2 V. Puliafito, at al, Phys. Rev. B 99, 024405 (2019). 3 L. Sanchez-Tejerina, et al, Phys. Rev. B, 101, 014433, 2020. A. Salimath, et al Phys. Rev. B, 101, 024429, 2020. 4 S. Chen, G. Finocchio, et al, "Antiferromagnetic PtMn memory devices controlled by electric current" Nature Elettronics, 3, 92-98, 2020. 5 Heng-An Zhou, et al, " Compensated magnetic insulators for extremely fast spin-orbitronics", https://arxiv.org/abs/1912.01775.

Micromagnetic modeling of antiferromagnets INVITED

Electron-correlated oxide materials display a plethora of physics phenomena due to their multiples degrees of freedom. Transition-metal oxides, in particular, exhibit coincidental structural, magnetic, and metal-insulator transitions among others. When combined in proximity with single-metallic-ferromagnets, it is possible to enhance the magnetic control at the nanoscale [1]. Understanding the magnetic dynamics in such systems provides the means to untangle the underlying physical phenomena which govern these strongly correlated materials. Here we will present two examples of such systems studied by angular-dependent Ferromagnetic resonance as a function of temperature. In both cases, the magnetic dynamics is governed by the proximity of an oxide undergoing a structural transition with a ferromagnet, as in the case of VOx/Ni heterostructures [2], or in the case of La-Pr-Ca-Mn-O thin films, by their complex magnetic-electronic phase transitions [3]. FMR and micromagnetic simulations allowed us to trace back the details of the phase transitions and to map the changes in anisotropy and the magnetic domain reorganization. Work done in collaboration with Christian Wolowiec, Diego Carranza, Ali Basaran, Amlan Biswas, Michael Fitzsimmons, Arantxa Fraile Rodríguez, Javier Rodriguez-Alvarez, Amílcar Labarta, Xavier Batlle, and Ivan K. Schuller. Funded by the US DoE, Office of Basic Energy Sciences, under Award FG02-87ER-45332 and Colciencias under contract 137-2016. Spanish MINECO projects MAT2015-68772-P and PGC2018-097789-B-I00 and European Union FEDER funds.References: [1] I. Valmianski, et al., in preparation. [2] J. G. Ramirez, et al., Phys. Rev. B 93, 1 (2016). [3] D. Carranza-Celis, et al., in preparation.

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Spin flop and crystalline anisotropic magnetoresistance in CuMnAs

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