Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session H21: Magnetic Phenomena in Bulk Oxides IFocus
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Sponsoring Units: GMAG DMP DCOMP Chair: Onur Erten, Rutgers Univ Room: LACC 309 |
Tuesday, March 6, 2018 2:30PM - 2:42PM |
H21.00001: Trends in the Magnetocaloric Response in the Half-Heusler/Heusler System MnNi1+xSb Emily Levin, Joshua Bocarsly, Kira Wyckoff, Tresa Pollock, Ram Seshadri The magnetocaloric effect (MCE), a change in temperature upon application of a magnetic field, has been widely studied for applications in magnetic refrigeration and thermomagnetic power generation. A density functional theory (DFT) based proxy for the gravimetric isothermal entropy change ΔSM, the usual metric of the MCE, suggests a larger magnetocaloric response in the half-Heusler MnNiSb than in the Heusler MnNi2Sb. Compositions between these endmembers were prepared via assisted microwave reaction. We observe a solid solution via synchrotron X-ray diffraction, and refinements agree with DFT, suggesting excess Ni is added to a symmetrically distinct site. Saturation magnetization is maximized at an intermediate composition, while the Curie temperature (TC) decreases from 750K to 350K with increasing Ni content. The highly tunable TC in this range is ideal for low grade waste heat recovery. ΔSM monotonically decreases with Ni content, while concurrent broadening of the magnetic transition due to longer range magnetic interactions yields a maximum in refrigerant capacity close to the Heusler composition. The increase in |ΔSM| with decreasing Ni may be extended to other MnNi2Z Heusler systems that are currently under investigation for use in magnetocaloric refrigeration. |
Tuesday, March 6, 2018 2:42PM - 2:54PM |
H21.00002: NMR Investigation of Spin Spiral ZnCr2Se4 Sejun Park, Sangil Kwon, Soonchil Lee, Seunghyun Khim, Dilip Bhoi, Kee Kim The electron spins of ZnCr2Se4 order antiferromagnetically while the nearest neighbor exchange interaction is ferromagnetic. Therefore, exchange interaction between ions at least up to 4th nearest neighbor should be considered, which makes the description of the spin system very complex and challenging. In this work, we measure zero field nuclear magnetic resonance (NMR) of 53Cr (I=3/2) showing complex features with six peaks. Below 6 K, asymmetric anomaly in the spectra appears, similar to the previously reported NMR data of BiFeO3. This observation is attributed to the change in T2 relaxation mechanism of 53Cr nuclei indicating unexpected state change. Complex spectra with six peaks and powder-like signal distribution are observed indicating the presence of anisotropic hyperfine interaction and nuclear quadrupole resonance. Our measurement also reconfirms the spin spiral ordering. |
Tuesday, March 6, 2018 2:54PM - 3:06PM |
H21.00003: Spin Relaxation of Impurity centers in Oxides and Semiconductors in Fields up to 14 T Johan Van Tol The Spin Lattice Relaxation (SLR) and spin decoherence times can be a strong function of magnetic field, especially at low temperatures. These parameters are of crucial significance to possible applications like spin-based information storage or spin-based quantum information processing. However, experimental data above fields of 3 Tesla are lacking. Here we report a study of the field dependence of both SLR and decoherence times in various materials at high magnetic fields by a direct time-domain measurement of T1 and T2 relaxation times with pulsed electrons paramagnetic resonance (EPR) at 120, 240, 336, and 395 GHz. Below 80K for Mn2+, V2+, and Cr3+ impurities in MgO the SLR is strongly field dependent, and shows an approximate B2 dependence, while in N-doped SiC the SLR is strongly dependent on magnetic field at temperatures below 12K with an approximate B4 dependence. These results are compared to theoretical and experimental phonon densities in these materials and to calculated values of the direct single-phonon spin lattice relaxation. |
Tuesday, March 6, 2018 3:06PM - 3:18PM |
H21.00004: Novel high-coercivity Fe16-xAlxN2 alloy Jinho Byun, Taewon Min, Hyoung Jeen Jeen, Sungkyun Park, Jaekwang Lee Iron nitrides (Fe16N2) have recently attracted considerable attentions for the future rare-earth (RE) free permanent magnets due to its low cost and high magnetization compared to other RE-free magnetic materials. Despite such a high remanent magnetization, its usefulness is restricted because it is easily de-magnetized. Here, using the first-principles density functional theory calculations, we extensively investigated the structure evolution, stability and magnetic properties of Fe16-xAlxN2 alloys by varying Al contents. We find that substituting Fe by Al in Fe16N2 greatly improves the magnetic anisotropy of the alloys, and their coercivities can increase by about 300%compared to that ofFe16N2. We thus expect that these Fe16-xAlxN2 alloys will be promising candidates for strong rare-earth free permanent magnets. |
Tuesday, March 6, 2018 3:18PM - 3:30PM |
H21.00005: Optimizing the soft magnetic behavior of iron nitride Todd Monson, Tyler Stevens, Stan Atcitty, Baolong Zheng, Yizhang Zhou, Enrique Lavernia The γ’ phase of iron nitride holds tremendous potential as a new soft magnetic material with a saturation magnetic polarization of 1.89 T and a high electrical resistivity (> 20,000 µΩcm) capable of mitigating eddy current losses. However, until recently γ’–Fe4N had only been synthesized and studied as a thin film, making the fabrication of devices such as inductive cores impossible. New low temperature consolidation techniques, such as spark plasma sintering (SPS), have enabled the fabrication of bulk γ’–Fe4N samples and devices while avoiding thermal decomposition. Although the synthesis of bulk γ’–Fe4N was a significant achievement, efforts are continuing to enhance its soft magnetic properties by improving phase purity and controlling grain size, while further increasing electrical resistivity. These efforts, which include advances in the synthesis of raw materials, processing techniques, and composite approaches will be described. |
Tuesday, March 6, 2018 3:30PM - 3:42PM |
H21.00006: Orbital Moment Unquenching and Large Enhancement of Local Magnetic Anisotropy in the M-type Sr-Hexaferrites by La and Co Substitutions Jaeyoung Kim, Yoon Young Koh The hard magnetic property M-type hexaferrites are originated basically from the uniaxial crystal structure of magnetoplumbite and there have been a lot of works to enhance the property by modifying the magentic crystalline anisotropy energy or local spin magnetic moments through substituting varieties of cations for Sr and Fe sites. For example, smaller size of La3+ ions induce a structural transition at low temperature and the excess charges by them can increase the ratio of Fe2+ ions with smaller spin magnetic moment in the antiparallel sites. The substituted Co ions for Fe sites have smaller local magnetic moments than Fe ions and their uneven distribution in differerent local sites is expected to change the magnetization too. In this work, we have measured X-ray absorption spectroscopy and X-ray magnetic circular dichroism on a series of M type Sr hexaferrites which have different substitution ratios of La and Co. They directly show that the Fe2+ are formed by the La and Co substitution and they are localized mostly at Oh sites. Also, we will show that the local symmetry distortion at Oh sites taken by Fe2+ induces a large amount of orbital magnetic moment at the Fe2+ sites. |
Tuesday, March 6, 2018 3:42PM - 3:54PM |
H21.00007: First-Principles Study on the Role of B in Nd2Fe14B Yasutomi Tatetsu, Yosuke Harashima, Takashi Miyake, Yoshihiro Gohda Rare-earth-Fe based magnetic compounds R2Fe17 (R = rare earth) are not capable of becoming strong magnets due to their low Curie temperatures. Sagawa tried to make the ferromagnetism of the compounds stronger through magneto-volume effect. He added boron (B) to Nd2Fe17 in order to stretch the Fe-Fe distances, which led the discovery of Nd2Fe14B, the strongest magnet known[1]. However, according to the first-principles studies on (Y, Gd)2Fe14B with the same structure as Nd2Fe14B[2], the magnetic moment ms of Fe is reduced after adding B, implying that B does not enhance the magnetic properties of Nd2Fe14B. |
Tuesday, March 6, 2018 3:54PM - 4:06PM |
H21.00008: Simultaneous in-situ specific heat and x-ray diffraction measurements on Fe2P Kristin Willa, Zhu Diao, Donato Campanini, Ulrich Welp, Matthias Hudl, Zahirul Islam, Wai-Kwong Kwok, Andreas Rydh The hexagonal transition metal pnictide Fe2P undergoes both a structural and a magnetic phase transition at 217 K. Strong magneto-elastic coupling turns the usually 2nd order paramagnet-to-ferromagnet transition into a 1st order one, making this material a perfect candidate to study the interplay between phase transitions. By integrating a nanocalorimeter platform into a 4.5 T magnet at an x-ray beamline, we have measured the sample’s specific heat and x-ray diffraction simultaneously. This multi-modal approach confirms that the structural and magnetic transitions appear at the same temperature. We observe that applying an external magnetic field broadens the structural transition. Additionally, we investigated the hysteresis of the transition in both specific heat and lattice parameter where the precise temperature control of the nanocalorimeter allows to line up the features observed in both quantities. X-ray scans were taken at different temperatures through the transition which reveal peak splitting in the FM phase that persists into the superheated region indicating the appearance of twinning or a domain structure. |
Tuesday, March 6, 2018 4:06PM - 4:18PM |
H21.00009: Orbital domain dynamics in Magnetite below the Verwey transition Roopali Kukreja, Nelson Hua, Josh Ruby, Andi Barbour, Wen Hu, Claudio Mazzoli, Stuart Wilkins, Eric Fullerton, Oleg Shpyrko Mesoscale phenomena play an important role in the dynamics of phase transitions in correlated systems. In order to fully understand and tailor their functionalities detailed access to the nanoscale regime, correlation length scales and temporal evolution of the order parameter is required. In this study, we utilized X-ray photon correlation spectroscopy (XPCS) to investigate the orbital domain dynamics of the insulating phase undergoing the insulator-to-metal transition in magnetite (Tv=123 K). XPCS provides a unique way to characterize disorder (domains, phase separation etc.) and their correlations by allowing us to capture high resolution coherent speckle patterns. By tuning to the Fe L3-edge, we studied the evolution of the orbitally ordered superlattice peak present in the low temperature insulating phase and forbidden in the metallic phase. We observed two distinct regimes below the Verwey transition. In the first regime at lower temperatures, magnetite follows an Arrhenius behavior and the characteristic timescale for orbital fluctuations decreases as the temperature increases. In the second regime near Tv, magnetite phase separates into metallic and insulating domains, and the kinetics of phase transition is dictated by metallic-insulating interfacial boundary conditions. |
Tuesday, March 6, 2018 4:18PM - 4:30PM |
H21.00010: Giant, pressure-induced magnetic switching in
the ruthenate Sr4Ru3O10 Wenhai Song, Yu Zhang, Hao Zheng, Jasminka Terzic, Hengdi Zhao, Yifei Ni, Gang Cao The triple-layered Sr4Ru3O10 is a magnetic metal with a magnetic transition at 105 K and a low-spin S=1 state. It presents a strong ferromagnetic behavior along the magnetic easy-axis that is uncommonly aligned with the c-axis, but unusual antiferromagnetic and metamagnetic characteristics within the basal plane at ambient pressure. This sharply contrasts its sister compound, the ferromagnetic metal SrRuO3. The central findings of this work are that the application of relatively low pressures (up to 1 GPa) can readily switch the direction of magnetic moments from the c-axis to the basal plane, where the basal-plane magnetization is nearly saturated with an enhanced order moment of 0.8 Bohr magneton/Ru at 1 GPa and 2 Tesla. The magnetic switching also results in drastic changes in transport properties. All results will be presented and discussed along with comparison drawn with other related compounds. |
Tuesday, March 6, 2018 4:30PM - 4:42PM |
H21.00011: Coexistence of Shubnikov–de Haas quantum oscillations and an antiferromagnetic insulating state in Ca3Ru2O7 Hao Zheng, Hengdi Zhao, Jasminka Terzic, David Graf, Wenhai Song, Yifei Ni, Yu Zhang, Gang Cao The bilayered ruthenate Ca3Ru2O7 is known to be an antiferromagnetic (AFM) insulator with a charge gap of 0.1 eV. It undergoes a first-order metal-insulator transition at 48 K and an AFM transition at 56 K. Of a wide range of exotic phenomena of this system, the conflicting hallmarks of Shubnikov–de Haas (SdH)quantum oscillations and the AFM insulating state are the focus of our recent reinvestigation, which includes measurements of magnetic torque and magnetoresisticity at magnetic fields up to 36 Tesla at NHMFL. Our new results confirm strong SdH oscillations with low frequencies ranging from 20 Tesla to 120 Tesla. It is also remarkable that Ca3Ru2O7 features a pronounced resistivity saturation below 15 K, which, along with other phenomena, may signal a novel state existent in this oxide. All results will be presented and discussed along with comparison drawn with other relevant compounds. |
Tuesday, March 6, 2018 4:42PM - 4:54PM |
H21.00012: Nonlinear Pauli Susceptibilities in Sr$_3$Ru$_2$O$_7$ and the Universal Features of Itinerant Metamagnetism Bellave Shivaram, Jing Luo, Gia-Wei Chern, Daniel Phelan, Rosalba Fittipaldi, Antonio Vecchione We present the measurements of the third order, $\chi_3$ and fifth order, $\chi_5$, susceptibilities (for magnetic fields both parallel and perpendicular to the c-axis) in an itinerant oxide metamagnet, Sr$_3$Ru$_2$O$_7$. These susceptibilities exhibit maxima in their temperature dependence such that $T_1 \approx 2T_3 \approx 4T_5$ where the $T_i$ are the position in temperature where a peak in the $i$-th order susceptibility occurs. Similar relations between these maximum temperatures are also observed in several other metamagnetic materials. Here we show that these relations are a universal feature of electron systems whose Fermi level is proximate to a van Hove singularity (vHS). |
Tuesday, March 6, 2018 4:54PM - 5:06PM |
H21.00013: Evolution of Griffiths phase and critical exponents by site dilution of Ga3+ in SrRuO3 Renu Gupta, Ashim Kumar Pramanik Here, we report the evolution of magnetic behavior in doped Ru-based perovskite structure of SrRu1-xGaxO3 series. The parent SrRuO3 has an orthorhombic-Pbnm structure which also sustained by Ga doping and also showing a significant change in a structural parameter at 300 K. Polycrystalline SrRuO3 is known to as an itinerant ferromagnet (FM) with Tc ~ 160 K and having large bifurcation between Zero-field-cooled and field-cooled (FC) magnetization below Tc which is decrease by Ga doping. The inverse susceptibility (χ-1) in paramagnetic (PM) state and high-temperature region fitted by modified Curie-Weiss law (CWL). Effective moment (µeff) and θp decreases by Ga doping. The χ-1 (T) revealed some upward deviation by Ga doping from CWL behavior above Tc due to formation of FM clusters in the PM background which is characterized as Griffiths phase. Critical exponent fitting of FC magnetization of SrRuO3 close to Tc has showing mean field type behavior (β = 0.5) which is increases by Ga doping. Isothermal magnetization at 5 K of SrRuO3 is showing hysteresis which also sustained by Ga doping with decreasing saturation moment (µH). The Arrott's plots also give confirmation of FM nature with decreasing value of spontaneous magnetization (MS). |
Tuesday, March 6, 2018 5:06PM - 5:18PM |
H21.00014: The magnetic field effect on the two competing magnetic phases in Ca$_2$Ru$_{0.92}$Fe$_{0.08}$O$_4$ Songxue Chi, Feng Ye, Jaime A. Fernandez-Baca, Gang Cao Substitution of Fe for Ru in Ca$_2$RuO$_4$ weakens the structural distortion and induces negative thermal expansion. Additionally, it introduces a $B$-centered stacking in the collinear magnetic structure and competes with the original $A$-centered magnetic phase. As the Fe-doping increases, the $B$-center phase becomes more predominant and eventually takes over as the substitution reaches 12 percent. The competition of these two magnetic phases is also susceptible to magnetic field. Our neutron diffraction measurement on Ca$_2$Ru$_{0.92}$Fe$_{0.08}$O$_4$ illustrates the field-effect on the two magnetic sublattices and underlines the importance of orbital physics. |
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