United States

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).

MMM 2020

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

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

Antiferromagnets have gained significant interest in recent years due to their potential application in antiferromagnetic spintronic devices and neuromorphic computing where the antiferromagnet is the active element. A current problem in the development of such devices is a lack of understanding of the magnetic properties of antiferromagnets. Atomistic modelling is a powerful tool in understanding these properties as it has the ability to model the materials in atomistic detail but on a scale comparable to realistic device sizes. Here, an atomistic model [1] is used to model the technologically relevant anti-ferromagnetic material Iridium Manganese to calculate the ground state spin structure and magnetic anisotropy. Previously, only the properties of fully ordered Iridium Manganese alloys or disordered IrMn3 have been investigated due to the complex material anisotropy of disordered crystal structures. Here, we have overcome this problem by modelling the anisotropy using the Néel pair anisotropy model which has the capability to accurately model any order or composition of IrMn [2]. Initially, the model was validated by comparing the results for ordered and disordered IrMn3, against previous experimental and theoretical calculations. The properties of never before theoretically studied compositions and orders of IrMn were calculated with particular focus on the technologically relevant compositions close to IrMn4 - IrMn5 used in realistic devices. It was found that all the technologically relevant alloys of disordered IrMn have the same ground state structure. A full composition study of the fully ordered and fully disordered states was undertaken, and it was found that in both cases the Néel temperature increases as the Mn content is increased, stabilizing the spin structure. These findings are an important step towards understanding the fundamental properties of antiferromagnets for applications.References: [1] R. F.L. Evans, et al, J Phys Cond. Matter, 26, (2014) [2] S. Jenkins, et al Physical Review B, 100, (2019)

Atomistic spin dynamics simulations of Iridium Manganese Alloys

Magnetization switching plays a central role in the field of magnetism-based devices. A concrete example is the case of magnetic textures which play the role of carriers of information. A precise control of their mobility becomes crucial for potential applications such as racetrack memories [1], sensors [2], and logic-devices [3]. Both ferromagnets (FM) and antiferromagnets (AFM) can host multiple types of these textures. While FM have been investigated in depth, only recently a mechanism to excite AFM was proposed based on the fact that there are certain materials in which their magnetic sublattices form inversion partners, being possible to generate a current-induced Néel spin-orbit (SO) field [4]. With this objective in mind, we have evaluated the dynamics of a one-dimensional Néel-like DW in an embedded FM layer of the metallic layered Mn2Au excited through the Néel SO field. To do this, we have combined atomistic spin dynamics simulations of the real crystalline structure together with an analytical model that combines the Lagrangian formalism with the collective coordinates approach. When theoretically formalizing the problem, complications arise to define an unique Néel order parameter due to the complexity of the system. To avoid this, we have introduced a pair of staggered AFM-coupled Néel vectors. Taking this into account, we have evaluated three consecutive correlated SO field-based regions: one of acceleration driven by a ramped stimulus, another of steady-steady at constant speed, and a friction-driven regime in the absence of external fields. Traces of the inertial dynamics of the magnetic texture and a great stability of the DW even at speeds very close to the maximum magnon group velocity of the medium have been observed. On the other hand, it has been possible to predict the distance traveled by the magnetic soliton in its friction-driven motion, which is of the order of 5-10 times greater than the DW width at rest for moderate fields of 10-60 mT. This opens up the possibility of exploiting the phenomenon of inertia to reduce the power consumption in those technological applications such as racetrack memories in which the position of the magnetic texture needs to be precisely determined.References: [1] S. S. Parkin, M. Hayashi, and L. Thomas, Science, Vol. 320, p. 190-194 (2008) [2] R. Mattheis, S. Glathe, M. Diegel, et al., Journal of Applied Physics, Vol. 111, p. 113920 (2012) [3] P. Wadley, B. Howells, J. Zelezný, et al., Science, Vol. 351, p. 587-590 (2016) [4] J. Zelezný, H. Gao, K. Výborný, et al., Physical Review Letters, Vol. 113, p. 157201 (2014)

Inertial Domain Wall Motion Driven by Staggered Spin-Orbit Fields in Layered Antiferromagnets

Antiferromagnets are promising materials for spintronic applications due to their unique features such as ultrafast spin dynamics and robustness against external magnetic fields. An efficient way to manipulate the antiferromagnetic (AFM) order is to use spin-orbit torque (SOT), which utilizes the conversion of electric current to spin through the spin-orbit interaction. Current-induced switching of the AFM order has been demonstrated experimentally in several antiferromagnets including Mn2Au. It has a broken sublattice inversion symmetry, which gives rise to opposite spin polarization on each sublattice under an electrical current. The staggered spin polarization results in a non-staggered field-like (FL) SOT, which is suitable for the switching of the AFM order. It is also possible to utilize the damping-like SOT originated from a spin current generated by a heavy metal (HM) layer adjacent to Mn2Au. Here by using first-principles calculations, we calculate the SOTs of a Mn2Au/HM bilayer, where HM is W or Pt. By expanding the SOT using vector spherical harmonics, layer-resolved angular dependence of SOT is obtained. For FL SOT, it has a large contribution from the Mn2Au and a small contribution from the HM. For DL SOT, it has most of the contribution from the HM. FL SOT is non-staggered on each sublattice and it dominates the behavior of spin dynamics. DL SOT is staggered on each sublattice so that its net effect on spin dynamics is negligible.

Spin-oribit torques in Mn2Au/HM bilayers

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

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.

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