Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session P19: Magnetic Nanoparticles: Curved Geometries and AnisotropyFocus Session
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Sponsoring Units: GMAG DMP Chair: Vincent Vlaminck, Univ San Francisco de Quito Room: LACC 308A |
Wednesday, March 7, 2018 2:30PM - 3:06PM |
P19.00001: Tailoring magnetic domain walls in cylindrical nanowires Invited Speaker: Cristina Bran Cylindrical magnetic nanowires, NWs, are considered promising for technological applications as they offer new ways to design their magnetization reversal processes. They present important advantages such as: the possibility to tailor the domain wall (DW) shape by adjusting the nanowires geometry or anisotropy, the stability of DW during its motion and suppression of Walker breakdown [1-3]. Here we show a way of controlling DW propagation in these one-dimensional nanostructures by specific designs in geometry (i.e., modulations of diameter), or in magnetic anisotropy (i.e., modulations in the composition) [4]. In this way, the modulation will act as pinning center for nucleation, pinning and depinning of DWs. These local variations changes the typical single-domain state present in soft NWs, as well as the mono domain wall-type switching of the magnetization based on the DW nucleation and propagation. A detailed description of the magnetic configuration of individual cylindrical Co-based NWs is presented by combining XMCD-PEEM which allows resolving the surface and the internal magnetic structure of NWs, magneto-optical Kerr effect and micromagnetic simulations. |
Wednesday, March 7, 2018 3:06PM - 3:18PM |
P19.00002: Effect of annealing temperature on the structural and magnetic properties of nanocomposites of Cobalt Ferrite and Multi-layer Graphene Himanshu Verma, Dereje Seifu, Shashi Karna, Haiping Hong Graphene has emerged as a favorite research material for various technological applications. Pristine graphene is a diamagnetic material because of the delocalized π-bonding network. Nanocomposites of graphene with other ferromagnetic materials such as cobalt ferrite reflects multi-functional properties useful for storage devices, sensors, and photocatalytic applications. Nanoparticles of anisotropic diamagnetic multi-layer graphene (MLG) and ferrimagnetic CoFe2O4 (CFO) were synthesized through a one-step sonication process in the presence of a surfactant. The particle size, crystal structure, and magnetic properties of as synthesized nanocomposite were investigated by X-Ray Diffractometer (XRD), Atomic Force Microscopy (AFM), Magnetic Force Microscopy (MFM), Scanning Electron Microscopy (SEM), and Vibrating Sample Magnetometer (VSM). The nanocomposite presented a super-paramagnetic behavior. The saturation magnetization increased significantly with increased annealing temperature as a result of improved crystallinity. The saturation magnetization (Ms) at 400 oC annealing temperature observed was 400 emu/g. A trifold increase was observed for Ms (1200 emu/g) at 600 oC. |
Wednesday, March 7, 2018 3:18PM - 3:30PM |
P19.00003: First-principles study of the voltage-controlled magnetic anisotropy at the surface of the MnPt antiferromagnet Po-Hao Chang, Wuzhang Fang, Kirill Belashchenko Using antiferromagnets in microwave spin wave logic devices could increase their speed and reliability compared to ferromagnets. Spin waves can be excited parametrically in an ultrathin magnetic strip using voltage-controlled magnetic anisotropy (VCMA). MnPt is an attractive material for this purpose thanks to its small and easily tunable magnetocrystalline anisotropy. Here we study VCMA at the surface of MnPt using local-orbital-based first-principles calculations. In order to isolate the response of the interfacial magnetic anisotropy to the external field, slabs of several differet thicknesses are examined. Different surface terminations are considered. |
Wednesday, March 7, 2018 3:30PM - 3:42PM |
P19.00004: Abstract Withdrawn
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Wednesday, March 7, 2018 3:42PM - 3:54PM |
P19.00005: Temperature Dependence of Exchange Coupled Multilayers Kristen Repa, Frank Schooner, Luke Stohrer, Casey Miller We present results on an exchange coupled multilayer thin film structure consisting of PyxCu1-x (x = 0.4, 0.8, and 1.0) that are exchanged biased (EB) with IrMn, and capped with Ta. The PyxCu1-x alloy materials are ordered from bottom to top: Py0.8Cu0.2 (20 nm)/ Py0.4Cu0.6 (20 nm)/ Py (20 nm). Below TC of the Py0.4Cu0.6 layer, both ferromagnetic layers exhibit EB. As the temperature increases and the system approaches TC for the Py0.4Cu0.6 layer, the portion of the loop corresponding to the bottom, Py0.8Cu0.2, layer shifts closer to zero. When T > TC for the Py0.4Cu0.6 layer, the coupling between the top and bottom layers is broken and EB is only observed for the layer in direct contact with IrMn. Vibrating sample magnetometry measurements indicate that the TC of Py0.4Cu0.6 is approximately 100 K, however, we observe EB of the bottom film layer at temperatures greater than TC. This could be the result of either a proximity effect altering the ordering temperature of the Py0.4Cu0.6 or orange peel coupling. |
Wednesday, March 7, 2018 3:54PM - 4:06PM |
P19.00006: Room temperature VCMA effect in SrTiO3\Co\Pt trilayers and the impact of the interface Bart Vermeulen, Johan Swerts, Sébastien Couet, Mihaela Popovici, iuliana radu, kristiaan temst, Guido Groeseneken, Koen Martens The standard material stacks with Perpendicular Magnetic Anisotropy (PMA) using MgO as a dielectric have an inherently limited Voltage Control of Magnetic Anisotropy (VCMA) effect1,2. A higher permittivity of the gate dielectric increases the injected charge, likely enhancing the VCMA effect. The VCMA effect has recently been demonstrated on SrTiO3(STO)\Co\Pt trilayers3: one condition is a minimal thickness of the magnetic dead layer (MDL) between STO and Co. |
Wednesday, March 7, 2018 4:06PM - 4:18PM |
P19.00007: Magnetic anisotropy analysis on recent Cr layered compound with large magnetic anisotropy and the role of 5p spin orbit interaction. Donghwan Kim, Kyoo Kim, Kyung-Tae Ko, Jun Sung Kim, Younghak Kim, Jae-Young Kim, Sang-Wook Cheong, Jae-Hoon Park Recently, 2D ferromagnetism on single layer Cr based layered compounds is discovered. These systems circumvent the Mermin-Wagner theorem by having large magnetic anisotropy. However, based on the conventional anisotropy mechanisms, it is very hard to have that much magnetic anisotropy (~ 1 T) in Cr3+ (d3) systems. We have studied their magnetic anisotropy by Cr L-edges XMCD and 4 site Full multiplet cluster calculation and found that the single ion anisotropy is not a major origin of the anisotropy. Instead, the spin-orbit interaction of 5p orbitals (~ 500 meV for Te, ~ 630 meV for I) plays an important role. |
Wednesday, March 7, 2018 4:18PM - 4:30PM |
P19.00008: Coexistence of Hard and Soft Magnetic Phases in Single Layer Amorphous Tb-Co Thin Films. Alejandro Ceballos, Michalis Charilaou, Frances Hellman Amorphous Tb-Co thin films prepared as layers with a thin Ta underlayer and either a Ta or Pt overlayer (Ta(2 nm)/a-TbxCo1-x (t)/ Ta or Pt (7 nm), with t varying from 1-16 nm), show evidence of both a soft and a hard magnetic phase despite no sign of this at room temperature. Low temperature magnetometry measurements reveal the decoupling of the two magnetic phases with decreasing temperature which we attribute to increased anisotropy energy at lower temperatures. The different t allowed us to separate out the effects of Ta or Pt overlayers. By decreasing the film thickness to 2 nm, slightly above the superparamagnetic limit found at 1 nm, a soft phase was isolated and found to be present in all the films which was also seen with x-ray reflectivity measurements, suggesting that magnetic nanoparticles form for the first 2 nm which are then buried by a homogeneous film on top responsible for the hard phase. This model was validated using micromagnetic simulations. The different effects of Ta and Pt capping layers on the magnetic properties lead to significant differences in magnetization, dominated by the enhanced anisotropy. |
Wednesday, March 7, 2018 4:30PM - 4:42PM |
P19.00009: Tuning magnetic anisotropy of ultrathin Fe films by coating with graphene or h-BN Nguyen Cuong Recent advances in the miniaturization progress of spintronic devices requires the ultrathin Fe film with perpendicular magnetic easy axis for many promising applications such as high density nonvolatile memory and high thermal stability logic chips. Therefore, it is important to achieve a large magnetic anisotropy of ultrathin Fe film with high thermal, chemical stability. In this work, we demonstrate the possibility of tuning magnetic anisotropy of ultrathin Fe films by coating with graphene or h-BN layer based on first-principles total-energy calculations. Our calculations show that graphene or h-BN coating layer can dramatically increases the magnetic anisotropy energy of ultrathin Fe film about 1.5-2 times, compared with its value of pristine Fe surfaces. Furthermore, we also show that the effect of graphene/h-BN coating layer is robust with the thickness of Fe film. These findings suggest a possible approach to realize the ultrathin Fe film with large and robust perpendicular magnetic anisotropy by coating with atomic-layer materials such as graphene, h-BN for future spintronic devices. |
Wednesday, March 7, 2018 4:42PM - 4:54PM |
P19.00010: Large perpendicular magnetic anisotropy in the transition metal/nitride thin films Jie-Xiang Yu, Jiadong Zang Enabling large perpendicular magnetic anisotropy (PMA) in transition metal thin films is a pathway towards nanomagnetism and next generation magnetic random-access memory devices. In this talk, we focus on PMA in the transition metal/nitride thin films based on first-principles calculations. The strength of PMA approaches the atomic limit of anisotropy for an isolated atom. It is the combined effect of spin-orbit interaction and electron correlation that plays a key role. As a consequence, PMA is predominated by the first order perturbation of the spin-orbit coupling, in contrast to conventional transition metal/oxide thin films. This work might open a new field of magnetism on transition metal/nitride interfaces. |
Wednesday, March 7, 2018 4:54PM - 5:06PM |
P19.00011: Tuning magnetic anisotropy energy in a Fe/Pt multilayered nanowire* Anish Senapati, Meghnath Jaishi, Ranjit Pati Magnetic anisotropy energy (MAE) arises from the directional dependent magnetic behavior of the materials. Magnetic data storage requires the MAE to be higher than kBT (kB is the Boltzmann constant and T is the room temperature) so that unintentional magnetization switching can be averted. Considerable efforts are made in recent years to find low dimensional materials with tunable MAE for their potential application in ultra-high density data storage device. Though spin-orbit coupling happens to be the prime cause, it is found that the dimensionality and symmetry also plays a key role in the enhancement of MAE with the recent findings that show the low symmetry nanoscale structures possess much larger MAE than the high symmetry bulk materials. Herein, using a periodic density functional approach that includes spin-orbit interaction, we have investigated the MAE in Fe/Pt multilayered nanowire; the role of Pt spacer in tuning MAE is examined. Based on our calculation, we predict an increase of an order of magnitude in MAE with the decrease of the Pt spacer layer thickness. By decreasing the number of Pt spacer layers from eight to seven in the nanowire supercell, the MAE is found to increase from 2.9 meV to 21 meV. |
Wednesday, March 7, 2018 5:06PM - 5:18PM |
P19.00012: Magnetoelasticity of CoFe thin films Daniel Schwienbacher, Matthias Pernpeintner, Mathias Weiler, Eric Edwards, Hans Nembach, Justin Shaw, Rudolf Gross, Hans Huebl Ultra-low magnetic damping materials are critical for spintronic applications. Schoen et al. [1] recently discovered ultra-low magnetic damping in a metallic CoFe thin film. Magnetoelastic properties critically affect spintronic devices, however they are difficult to quantify in thin films. Here, we use SiN nanostring resonators loaded with CoFe thin films for measuring the magnetoelastic properties and find λ = (-27.2 ± 4.1 )× 10-6 for 10 nm thick Co25Fe75 films. This opens exciting perspectives for combining magnetoelastics with ultra-low damping materials. |
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