Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session S40: Nanoscale Designed Magnetic FilmsFocus Live
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Sponsoring Units: GMAG DMP Chair: Patrick Quarterman, National Institute of Standards and Technology |
Thursday, March 18, 2021 11:30AM - 12:06PM Live |
S40.00001: Materials for nanoscale spatial control of ferromagnetic phase transitions Invited Speaker: Lorenzo Fallarino Multilayer structures have revealed an impressive variety of collective properties, and they represent an established approach to materials design with widespread applications [1]. One example are exchange spring systems, in which low-moment magnetically hard and high-moment magnetically soft layers are strongly coupled [2], a concept that was further advanced by utilizing a magnetic anisotropy gradient along the thickness [3]. Subsequent experimental efforts have led to the discovery that compositional grading can also be used to fabricate all-ferromagnetic films that exhibit a magnetic behavior dominated by local Curie temperatures representing the local exchange coupling strength only [4]. Such materials were demonstrated to exhibit a quasi-phase-boundary between strongly and weakly magnetized regions that can be continuously and reversibly moved along the depth of the films, which in turn allows a temperature-dependent (de-)correlation of the magnetic state and reversal behavior [5]. Correspondingly, phase transitions in such graded compositional materials may be more complex than in usual ferromagnets. Due to the exchange strength profile, the resulting magnetic state along the thickness may exhibit a temperature dependent confinement in contrast to bulk-like conventional magnetic systems, and thus lead to significantly altered critical behavior if compared to conventional scaling and universality [6]. |
Thursday, March 18, 2021 12:06PM - 12:18PM Live |
S40.00002: Controlling magnetic configuration in the soft/hard bilayers probed by PNR Nan Tang, Dustin Gilbert, Brian James Kirby Hard-soft magnetic composites have played an important role in the data storage and permanent magnetic technology [1]. Understanding the full range of interactions within hard-soft coupled systems is critical to maximizing their benefit in composite applications [2]. Here, we present polarized neutron reflectometry (PNR) to experimentally study magnetic configurations in hard/soft bilayers. By designing saturation magnetization of hard () and soft () magnets and the soft layer thickness (), the magnetic interactions, and resulting configuration, is systematically varied. When less than the critical exchange length (, the magnetic moments of soft layer are rigidly coupled to magnetic hard layer. While for , magnetic moments in the soft layer becomes relaxed in plane. This relaxation can be slowly released in thicker magnetic soft layers with lower and higher. These results are then supported with micromagnetic simulation. |
Thursday, March 18, 2021 12:18PM - 12:30PM Live |
S40.00003: Magnetic Modulation In Ferrimagnetic Fe1-XGdx Thin Film Multilayer Structures By Variation of Compensation Temperature Jenae Shoup, Brian Kirby, Dario Arena Metallic ferrimagnets consisting of rare earth (RE) / transition metal (TM) alloys exhibit an intriguing magnetic compensation of the RE/TM magnetic sublattices as a function of composition and temperature. We have been examining Fe1-xGdx thin films where the Gd content has been modified systematically. Static magneto-optic Kerr effect measurements of monolithic films are used to determine the composition that has magnetic compensation at room temperature. The compensation temperatures of the samples are then verified with temperature dependent magnetometry. Magnetic bi-layers and multilayers with dissimilar Fe1-xGdx ratios are also explored; for example, ~26% Gd (compensation at 140K) and ~25% Gd (compensation at 280K) bi-layers exhibit non-trivial magnetization vs. temperature behavior. These films are structurally nearly homogeneous while the magnetic profile differs dramatically with temperature variations. The magnetic interface of such structures is undetermined and we will use polarized neutron reflectometry (PNR) to examine spin ordering in the interfacial region as a function of field and temperature. |
Thursday, March 18, 2021 12:30PM - 12:42PM Live |
S40.00004: In-Depth Structural and Magnetic Characterizations of Fe and CrN Bilayer System Khan Alam, Arthur Smith, Kai Sun, Andrew Foley Iron and chromium nitride bilayer is an ideal system for exchange biasing and sensing applications as the Neel temperature of CrN is 280 K and Curie temperature of Fe 1043 K. For better growth and tunability of the bilayer, it is crucial to understand its crystal and magnetic structures in detail. In this project, we grow high quality epitaxial Fe/CrN bilayer thin films on MgO(001) by molecular beam epitaxy. All bilayers grow in 001 orientation on MgO(001) substrate with uniform layer thicknesses and sharp interfaces. Our data reveals the epitaxial relationship between Fe and CrN crystals and the magnetic structures of Fe and CrN. We found anisotropies of Fe and CrN are parallel to [110]MgO. We present a model that combines the crystal and magnetic structures of the Fe/CrN bilayer and fully explains all results. |
Thursday, March 18, 2021 12:42PM - 12:54PM Live |
S40.00005: Magneto-ionic Enhancement of Exchange Bias in MnN/CoFe Christopher Jensen, Alberto Quintana, Patrick Quarterman, Alexander Grutter, Kai Liu The magneto-ionic effect has shown promise as an effective approach to control magnetic properties through controlled motion of ionic species, which can be tailored under an applied electric field.1 For example, magneto-ionic control of exchange bias may lead to reduction in energy costs and the development of novel devices.2 We have investigated nitrogen-ion induced exchange bias effects in MnN/CoFe films grown on a Ta seed layer.3 Upon field cooling across the Néel temperature of MnN, a significant exchange bias of ~ 620 Oe is induced. Polarized neutron reflectometry (PNR) shows that nitrogen migrates from MnN into the Ta, the latter acts as a nitrogen getter. Subsequently nitrogen is driven from the Ta back into MnN by applying an electric field, confirmed by PNR. The increase in nitrogen content in MnN leads to an increase in exchange bias of ~ 30 Oe (5%). Preliminary studies also show that this electric field effect is reversible. These results indicate a sensitive nitrogen-ion based magneto-ionic approach to tuning exchange bias. |
Thursday, March 18, 2021 12:54PM - 1:06PM Live |
S40.00006: Nanoscale manipulation of magnetic domains by strain-induced proximity Xavier Batlle, Ilya Valmianski, Arantxa Fraile Rodríguez, Javier Rodriguez Alvarez, Montserrat Garcia del Muro, Christian T Wolowiec, Florian Kronast, Juan G. Ramirez, Ivan Schuller, Amílcar Labarta Coupling between lattice and spin degrees of freedom without the use of magnetic fields allows for efficient spintronic devices. Hybrid nanostructures composed of a vanadium oxide undergoing a first-order, structural phase transition (SPT) and a ferromagnet (FM) offer a promising route to control of magnetism by strain-induced proximity. Up to 500% coercivity increase was found in Ni/V2O3 bilayers in a very narrow T range. By synchrotron-based X-ray microscopy we show a reconfiguration of the FM domain pattern across the V2O3 SPT. The lateral correlation length of the Ni domains shows a significant increase at the SPT and a broad distribution of the local transition temperatures is found. These findings are supported by static and dynamic magnetometry measurements and micromagnetic simulations. All the above point to nanoscale phase coexistence of two Ni magnetic anisotropies induced by interfacial stress transfer across the SPT of V2O3. This allows to manipulate magnetic domains at the nanoscale and pursues to engineer coercive fields for novel data storage architectures based on straintronics. |
Thursday, March 18, 2021 1:06PM - 1:18PM Live |
S40.00007: Observations of Exchange Bias in Patterned Bilayers using Three-Axis Torque Magnetometry. John Thibault, Katryna Fast, Vince Sauer, Michael Dunsmore, Joe Losby, Zhu Diao, Eric J Luber, Miro Belov, Mark R Freeman We report studies of exchange bias in circular disk bilayers of 20nm Permalloy/20nm cobalt oxide, lithographically patterned on nanomechanical resonators. Torque magnetometry is well-suited to thin film studies of exchange bias since the measurements are sensitive to magnetic behavior at the bilayer interface1. The nanomechanical resonators employed here are of unique design whereby three orthogonal components of field induced torque, and thus three orthogonal components of anisotropy, can be measured simultaneously2. The three axis technique provides a platform for obtaining standard hysteresis curves while permitting measurement of rotational hysteresis with no requirement to reorient the sample or the field generators. Shifted hysteresis loops and rotational hysteresis are observed at cryogenic temperatures thus confirming exchange bias at the Permalloy/cobalt oxide interface. A strongly temperature-dependent and magnetic history-independent contribution to out-of-plane torque that may be an indicator of disordered exchange bias will be discussed. Micromagnetic simulations of mechanical torque lend further insight through qualitative and quantitative comparisons with the measured magnetic behavior. |
Thursday, March 18, 2021 1:18PM - 1:30PM Live |
S40.00008: First-principles calculations of spin-orbit torques in a Mn2Au/HM bilayer Wuzhang Fang, Kirill Belashchenko Mn2Au is a collinear AFM and has a high Néel temperature above 1000 K. It has a tetragonal structure with a combination of inversion symmetry and time reversal symmetry which is also the symmetry of CuMnAs. This particular symmetry gives rise to opposite current-induced spin polarization on each sublattice when applying an electric field. The staggered spin polarization results in a non-staggered field-like (FL) spin-orbit torque (SOT), which is suitable for the switching of the AFM order. |
Thursday, March 18, 2021 1:30PM - 1:42PM Live |
S40.00009: Spin-reorientation transition in an Fe double-layer on W(110) induced by Dzyaloshinsky-Moriya interaction Balázs Nagyfalusi, László Udvardi, László Szunyogh, Levente Rozsa Controlling the preferred direction of the magnetic moments is essential for the design of spintronic devices based on ultrathin films and |
Thursday, March 18, 2021 1:42PM - 1:54PM Live |
S40.00010: Structural and Magnetotransport Properties of Epitaxial Ferrimagnetic Mn4N Thin Films on GaN by Plasma-assisted Molecular Beam Epitaxy Zexuan Zhang, Yongjin Cho, Celesta S Chang, Mingli Gong, Shaoting Ho, Jashan Singhal, Huili Grace Xing, Debdeep Jena The III-nitride family of wide bandgap semiconductors are of great importance for diverse applications ranging from solid-state lighting to RF and power electronics (D. Jena, Jpn. J. Appl. Phys. 58, SC0801 (2019)). Integration of ferromagnets on GaN will be important building block not only for device applications aiming to merge logic, memory and communication components but also for studying spin-related phenomena in non-centrosymmetric semiconductors. |
Thursday, March 18, 2021 1:54PM - 2:06PM Live |
S40.00011: Temperature-dependent magnetic transitions in CoCrPt-Ru-CoCrPt synthetic ferrimagnets (SFM) Bradlee Beauchamp, Ernesto Marinero The magnetic orientations and switching fields of a CoCrPt-Ru-CoCrPt SFM have been studied in the temperature range from 2K to 300K. Two sets of magnetic transitions are observed. The first set exhibits three magnetic transitions (50K – 370K ), whereas in the second, only two transitions (2K - 50K). The observed magnetic hysteresis curves of the synthetic ferrimagnet are explained using the energy diagram technique framework pioneered by Koplak et al. [1] which accurately describes the competition between interlayer exchange coupling energy, Zeeman energy, and anisotropy energy in the system. In this work we expand the framework to include synthetic ferrimagnets (SFMs) comprising higher perpendicular magnetic anisotropy materials and large (4X) interlayer exchange coupling energies which are promising for the development of ultrafast (ps) magnetic switching free layers in MTJ structures. Furthermore, we apply the analysis to predict SFM magnetic hysteresis curves in a temperature regime that includes temperature extrema that a synthetic ferrimagnet would be expected to reliably operate at, were it to be utilized as a free layer in a memory or sensor spintronic device. |
Thursday, March 18, 2021 2:06PM - 2:18PM Live |
S40.00012: Electric-field control of ferromagnetic regions to form intrinsic tunnel junctions in dynamically phase separated manganites Ambika Shakya, Amlan Biswas Thin films of (La1-yPry)1-xCaxMnO3 (LPCMO) grown on (110) NdGaO3 (NGO) substrates show micrometer scale electronic phase separation into ferromagnetic metallic (FMM) and insulating regions. Previous studies have shown that the FMM regions can be tuned in the dynamic (fluid) phase separated state using an external electric field. We have fabricated micrometer-scale gold contact patterns on LPCMO thin films to apply a non-uniform electric field and probe the magnetotransport properties of such phase separated regions, simultaneously. We have observed tunneling magnetoresistance (TMR) behavior in the magnetic easy as well as hard axis directions of LPCMO. The in-plane magnetic field sweeps near the insulator-to-metal transition temperature revealed hysteretic, resistance jumps; a clear signature of TMR. The switching field from high to low resistance reduces with the decreasing temperature. In addition, the application of high voltage along the easy axis, increases the height of the resistance peaks during field sweeps along the hard axis direction and vice versa. Hence, our data show the formation of micrometer-scale intrinsic magnetic tunnel junctions in phase separated manganites, which can be further tuned using a non-uniform electric field. |
Thursday, March 18, 2021 2:18PM - 2:30PM On Demand |
S40.00013: Site preference and magnetic properties of Mn substituted M-type strontium hexaferrite Bipin Lamichhane, Dinesh Thapa, Chandani Nandadasa, Minyeong Choi, Yang-Ki Hong M-type strontium hexaferrite (SFO) has various technological applications. In order to tune the magnetic properties of SFO, different elements are used to substitute Fe+3. In this work, we use Mn to substitute Fe for possible technical applications. The first principle total-energy calculations based on density functional theory is used to study the site preference and magnetic properties of Mn substituted M-type strontium hexaferrite (MSFO) with x = 0 and x = 1.0. The exchange-correlation energy is described by the GGA of Perdew–Burke–Ernzerhof (PBE). The calculation of the substitution energy of Mn in (SFO) shows that in the ground state configuration Mn+3 ions preferentially occupy two of the majority sites, 12k and 2a, which reduces saturation magnetization since Mn has lesser magnetic moment than Fe. Furthermore, formation probability analysis is done to find occupancy at annealing temperature (>1000K), Our results show 12k site is most likely to occupy at high temperature. This causes a decrease in saturation magnetization, coercivity which agrees well with a previous experimental study. |
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