Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session S30: Magnetic Anisotropy and Permanent Magnets |
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Sponsoring Units: GMAG Chair: David Jiles, Iowa State University Room: 206B |
Thursday, March 5, 2015 8:00AM - 8:12AM |
S30.00001: Magnetic properties of Ce(Fe$_{1-x}$Co$_{x}$)$_{11}$Ti and effects of H and N doping Liqin Ke, Vladimir Antropov The intrinsic magnetic properties of Ce(Fe$_{1-x}$Co$_{x}$)$_{11}$Ti related systems have been studied using electronic structure calculations. Both CeFe$_{11}$Ti and CeCo$_{11}$Ti have uniaxial magnetic anisotropy. Calculations of the magnetic anisotropy of Ce(Fe$_{1-x}$Co$_{x}$)$_{11}$Ti show that the easy magnetization direction changes from uniaxial to in-plane and then back to uniaxial with increasing Co content. This provides an explanation for the interesting non-monotonic behavior of coercivity observed in experiments. The effects of H and N doping on intrinsic magnetic properties are also studied. Both H and N doping increase the magnetization and Curie temperature while N doping has a stronger effect than H doping. Calculated $T_C$ enhancements agree well with experiments and it is found that volume and chemical effects contribute nearly equally to the $T_C$ enhancement with N doping. The uniaxial magnetic anisotropy decreases with N doping, which is consistent with experiments. However, we found that the effect is not only due to the changing of the anisotropy of Ce atom as generally believed. Instead, the decrease of MAE is a collective effect that anisotropy contributions from transition metal sites are also changed with N doping. [Preview Abstract] |
Thursday, March 5, 2015 8:12AM - 8:24AM |
S30.00002: First principles study of doping effects in NdFe$_{11}$Ti permanent magnet compound Yosuke Harashima, Kiyoyuki Terakura, Hiori Kino, Shoji Ishibashi, Takashi Miyake Permanent magnet compounds are required to have large magnetization and strong uniaxial magnetocrystalline anisotropy. NdFe$_{11}$Ti which is one of the ThMn$_{12}$-type rare-earth 3d-transition metal compounds has large magnetic moment due to its high content of Fe. The magnetization and magnetocrystalline anisotropy of the compound can be tuned by nitrogen doping at interstitial sites. NdFe$_{11}$TiN is a good candidate for a permanent magnet compound. Then, it is interesting whether there is another dopant that enhances the magnetic properties. We have investigated doping effects of NdFe$_{11}$TiX where X$=$B, C, N, O, and F by using first principles calculation. These dopants increase the magnetization, and the increase is especially large for N, O, and F doping. The magnetocrystalline anisotropy is estimated from the crystalline electric field parameter \textless r$^{2}$\textgreater A$_{2}^{0}$. NdFe$_{\mathrm{11}}$TiB has negative value of \textless r$^{2}$\textgreater A$_{2}^{0}$ that implies the compound has in-plane anisotropy. As the atomic number of the dopant increases from B to N, \textless r$^{2}$\textgreater A$_{2}^{0}$ is increased, and NdFe$_{11}$TiN has a large positive value, suggesting strong uniaxial anisotropy. Then, \textless r$^{2}$\textgreater A$_{2}^{0}$ turns to decrease as the dopant is changed from N to O to F, and NdFe$_{11}$TiF has a large negative value. In conclusion, we found that N is the most appropriate dopant among B, C, N, O, and F. [Preview Abstract] |
Thursday, March 5, 2015 8:24AM - 8:36AM |
S30.00003: Hyperfine fields of Fe in Nd$_2$Fe$_{14}$B and Sm$_2$Fe$_{17}$N$_3$ Hisazumi Akai, Masako Ogura High saturation magnetization of rare-earth magnets originates from Fe and the strong magnetic anisotropy stems from f-states of rare-earth elements such as Nd and Sm. Therefore the hyperfine fields of both Fe and rare-earth provide us with important pieces of information: Fe NMR enable us to detect site dependence of the local magnetic moment and magnetic anisotropy (Fe sites also contribute to the magnetic anisotropy) while rare-earth NQR directly give the information of electric field gradients (EFG) that are related to the shape of the f-electron cloud as well as the EFG produced by ligands. In this study we focus on the hyperfine fields of materials used as permanent magnets, Nd$_2$Fe$_{14}$B and Sm$_2$Fe$_{17}$N$_3$ from theoretical points of view. The detailed electronic structure together with the hyperfine interactions are discussed on the basis of the first-principles calculation. In particular, the relations between the observed hyperfine fields and the magnetic properties are studies in detail. The effects of doping of those materials by other elements such as Dy and the effects of N adding in Sm$_2$Fe$_{17}$N$_3$ will be discussed. [Preview Abstract] |
Thursday, March 5, 2015 8:36AM - 8:48AM |
S30.00004: Correlations, spin-charge separation, and magnetic anisotropy Ralph Skomski, Priyanka Manchanda Much of the physics of condensed matter reflects electron-electron correlations. On an independent-electron level, correlations are described by a single Slater determinant with broken spin symmetry. This approach includes Hund's rule correlations as well the LSDA and LSDA+U approximations to density-functional theory (DFT). However, from Kondo and heavy-fermion systems it is known that the independent-electron approach fails to describe spin-charge separation in strongly correlated systems, necessitating the use of two or more Slater determinants. Using first-principle and model calculations, we show that spin-charge separation strongly affects the leading rare-earth anisotropy contribution in top-end permanent magnet materials such as Nd$_{2}$Fe$_{14}$B and SmCo$_{5}$. Explicit correlation results are obtained for two limiting cases. First, we derive the density functional for tripositive rare-earth ions in a Bethe-type crystal field. The potential looks very different from the LSDA(+U) potentials, including gradient corrections. Second, we use a simple model to show that Kondo-type spin-charge separation yield a rare-earth anisotropy contribution absent in the independent-electron approach. This research is supported by DOE (DE-FG02-04ER46152). [Preview Abstract] |
Thursday, March 5, 2015 8:48AM - 9:00AM |
S30.00005: Dy-Free Nd-Fe-B Based Permanent Magnets Arjun Pathak, Mahmud Khan, Karl Gschneidner, Jr., Ralph McCallum, Vitalij Pecharsky Nd$_{2}$Fe$_{14}$B based permanent magnets are the current state of the art for high performance magnets. The prototype crystallize in the $P$4$_{2}$ \textit{/ mnm} tetragonal crystal structure, where the Nd atoms occupy the \textit{4f} and \textit{4g }sites, Fe atoms occupy six different atomic sites (\textit{16k}$_{1}$, \textit{16k}$_{2}$, \textit{8j}$_{1}$, \textit{8j}$_{2}$, \textit{4e}, \textit{4c}), and B occupies only the \textit{4g} site. The leading contribution to the magnetocrystalline anisotropy in Nd$_{2}$Fe$_{14}$B energy comes from the Nd ions, which strongly prefer a $c$-axis alignment at ambient temperature. Nd$_{2}$Fe$_{14}$B permanent magnet has excellent magnetic properties at room temperature but has poor high temperature properties (T\textgreater 400 K). A small amount of Dy (up to 10{\%}) is substituted for Nd in Nd$_{2}$Fe$_{14}$B to increase the high temperature performance. Although Dy containing Nd$_{2}$Fe$_{14}$B magnets are desired for high temperature applications, the high price and limited supply of Dy urges the development of Dy-free permanent magnets. Here, we discuss the magnetic properties of several Dy-free Nd-Fe-B based nanostructured magnets and propose alternatives for Dy-based Nd$_{2}$Fe$_{14}$B permanent magnets for high temperature applications such as electric drive motors and wind turbines. This work was supported by the U.S.DOE, ARPA-E, Rare Earth Alternatives in Critical Technologies for Energy (REACT). The research was performed at the Ames Laboratory which is operated for the U.S. DOE by Iowa State University under contract {\#}DE-AC02-07CH11358. [Preview Abstract] |
Thursday, March 5, 2015 9:00AM - 9:12AM |
S30.00006: Axial Magnetic Anisotropy from Two Systems Fe$_2$B and Co$_2$B with Planar Anisotropy Valentin Taufour, Tej Lamichhane, Sergey L. Bud'ko, Anton Jesche, Alan I. Goldman, Kevin W. Dennis, R. William McCallum, Vladimir Antropov, Paul C. Canfield Growth of single crystals of (Fe$_{1-x}$Co$_x$)$_2$B ($0\leq x\leq1$) and detailed characterization of their magnetic properties will be presented. Despite the fact that both Fe$_2$B and Co$_2$B show a planar anisotropy at room temperature, we observe a uniaxial anisotropy at intermediate doping which makes (Fe,Co)$_2$B a promising system for permanent magnet applications in a system without rare-earth element. Comparison with recent band structure calculations will be presented. The temperature dependence of the anisotropy measured on single crystals from $2$~K to $1000$~K shows some unusual variations with an increase of the magnetic anisotropy with increasing temperature at some specific substitution. \\[4pt] This work is supported by the Critical Materials Institute, an Energy Innovation Hub funded by the US DOE and by the Office of Basic Energy Science, Division of Materials Science and Engineering. Ames Laboratory is operated for the US DOE by Iowa State University under Contract No. DE-AC02-07CH11358. [Preview Abstract] |
Thursday, March 5, 2015 9:12AM - 9:24AM |
S30.00007: Electronic structure calculations of the temperature dependence of magnetocrystalline anisotropy in (Fe$_{1-x}$Co$_x$)$_2$B alloys Ivan Zhuravlev, Liqin Ke, Vladimir Antropov, Kirill Belashchenko A number of important magnetic systems exhibit anomalous temperature dependence of the magnetocrystalline anisotropy (MCA), such as a spin-reorientation transition or an MCA increasing with temperature. The mechanisms of such anomalies vary. In Nd-Fe-B magnets the spin-reorientation transition is likely due to the disordering of Nd spins, while in MnBi the MCA quickly increases with temperature due to thermal expansion. (Fe$_{1-x}$Co$_x$)$_2$B alloys present another example of highly anomalous temperature-dependent MCA. Our calculations show that these anomalies are not due to thermal expansion. We therefore study the effects of spin disorder on MCA and on the electronic structure of this system using our implementation of the vector disordered-local moment (DLM) method with spin-orbit coupling within the Green's function-based linear muffin-tin orbital (LMTO) method. We also consider the influence on MCA of the magnitude of the spin moments of Fe and Co. The results show that the observed anomalies are associated with the effects of thermal spin fluctuations on the electronic structure. [Preview Abstract] |
Thursday, March 5, 2015 9:24AM - 9:36AM |
S30.00008: Superparamagnetism in the martensitic phase of the magnetic shape-memory alloys Ni$_{50-x}$Co$_{x}$Mn$_{40}$Sn$_{10}$ Shaojie Yuan, Philip Kuhns, Michael Hoch, James Brooks, Arneil Reyes, Vijay Srivastava, Daniel Phelan, Richard James, Chris Leighton The Ni-Mn based shape magnetic memory alloys have attracted considerable attention because of their interesting magnetic properties, including intrinsic superparamagnetism and intrinsic exchange-bias effects, which are found in the martensitic phase of these materials. Here, we report on the results of zero-field $^{55}$Mn NMR measurements made on the alloys Ni$_{50-x}$Co$_{x}$Mn$_{40}$Sn$_{10}$ with x=7 and x=14. The results show that Co substitution not only changes the electronic configuration of a fraction of the Mn ions but also alters the magnetic interaction amongst these ions, leading to marked changes in the low temperature antiferromagnetic and ferromagnetic components compared to x=0. For both x=7 and x=14 our analysis shows that the Mn ions form a new ferromagnetic nanoclusters in small Co rich regions of the sample. Based on the temperature dependence of the NMR spectral features, we propose a method to estimate the superparamagnetic cluster size distribution. [Preview Abstract] |
Thursday, March 5, 2015 9:36AM - 9:48AM |
S30.00009: Huge Magnetocrystalline Anisotropy in UMn$_2$Ge$_2$ David Parker, Nirmal Ghimire, Ryan Baumbach, Eric Bauer, Ling Li, David Mandrus, John Singleton, David Singh We present an experimental finding, as predicted theoretically by one of the authors, of an extremely high uniaxial magnetic anisotropy energy - approaching 15 MJ/m$^{3}$ - in the 122 actinide ferromagnet UMn$_{2}$Ge$_{2}$. This large MAE appears to originate in the extremely strong Uranium spin-orbit coupling and the sizable orbital moment (approaching 2 $\mu_B$) on this atom. Implications for other 122 compounds are discussed. [Preview Abstract] |
Thursday, March 5, 2015 9:48AM - 10:00AM |
S30.00010: Magnetic Anisotropy in UMn$_{2}$Ge$_{2}$ Morgann Berg, Alex de Lozanne, Ryan Baumbach, Jeehoon Kim, Eric Bauer, Joe Thompson, Filip Ronning UMn$_{2}$Ge$_{2}$, a permanent magnet, is a ternary intermetallic compound with a tetragonal crystal structure of type ThCr$_{2}$Si$_{2}$ and with space group I4/mmm. Local U and Mn moments in UMn$_{2}$Ge$_{2}$ order on their respective sublattices at temperatures near 100 and 380 K, respectively. Previous x-ray diffraction, Kerr rotation angle, and SQUID magnetometry data support the commonly accepted notion that U moments order at low temperature and align Mn moments along the c-axis, introducing anisotropy. Previous results obtained using a multi-mode atomic force microscope in magnetic force microscopy (MFM) mode indeed confirmed that UMn$_{2}$Ge$_{2}$ displays uniaxial anisotropy with an easy axis coinciding with the c-axis of the material. However, the branching domains in UMn$_{2}$Ge$_{2}$ consistent with uniaxial anisotropy were observed all the way up to room temperature by MFM. This indicates that the effect of uranium moments on the magnetic microstructure of UMn$_{2}$Ge$_{2}$ is not limited to low temperatures near the ordering temperature of the uranium sublattice. We further investigate closure domains in the surface of UMn$_{2}$Ge$_{2}$ and report on characteristics and signatures of anisotropy revealed by the orientation and periodic structures of closure domains. [Preview Abstract] |
Thursday, March 5, 2015 10:00AM - 10:12AM |
S30.00011: Lattice Monte Carlo Simulation Study Atomic Structure of Alnico 5-7 Permanent Magnets Manh Cuong Nguyen, Xin Zhao, Cai-Zhuang Wang, Kai-Ming Ho The fluctuations and increases in price and the issues in supply recently of rare earth metals re-heated the sought for non-rare earth permanent magnets. Alnico permanent magnets have been considered as promising replacements for rare earth-based permanent magnets due to the superiors in the magnetic performance at high temperature and the abundances of the constituent elements. Using lattice Monte Carlo simulation in combination with cluster expansion method we study the atomic structure of alnico 5-7 permanent magnets. We observed the phase separation into FeCo-rich and NiAl-rich phases in alnico 5-7 at low temperature, which is consistent with experiment. The phase boundary between these two phases is quite sharp. Both FeCo-rich and NiAl-rich phases are in B2 ordering with Fe and Al sitting on ?-site and Ni and Co sitting on ?-site. The degree of order of NiAl-rich phase is quite higher than that of FeCo-rich phase and it decreases with temperature slower than that of FeCo-rich phase. We also observed a small and increasing with annealing temperature magnetic moment in NiAl-rich phase, implying that the magnetic properties of alnico 5-7 could be improved by lowering annealing temperature to diminish the magnetism in NiAl-rich phase. [Preview Abstract] |
Thursday, March 5, 2015 10:12AM - 10:24AM |
S30.00012: Strong magneto-crystalline anisotropy in transition metal intercalated dichalcogenides Vaideesh Loganathan, Andriy Nevidomskyy, Jian-Xin Zhu A figure of merit for hard ferromagnets is proportional to the magneto-crystalline anisotropy energy (MAE), which measures the energy cost of deviations from easy-axis magnetization. Materials containing elements with strong spin-orbit coupling and large ordered moment have been found to be strongly anisotropic. Due to the scarce availability of rare earths, hard magnets without rare earths are desirable, and intercalated metal dichalcogenides, $A_{0.25}MS_2$ (A=Fe,Mn; M=Ta,Nb), are one such candidate. We have performed first-principles LDA+U calculations on these materials. The MAE was calculated using two approaches, from the total energy difference between easy and hard magnetic directions using the non-collinear method; and using the torque method. Along with a saturated moment of $4 \mu B$, we find MAE of about 10meV. This corresponds to an anisotropy field of about 60 T, comparable to those of rare-earth magnets. Substitution of Ta with Nb yields minor change in MAE, suggesting that the spin-orbit coupling effect contributes less to the anisotropy than the crystal structure. We find that the MAE in Fe intercalated compounds strongly depends on the value of the Hubbard U. We also study the effect on MAE of lattice strain, which can be used to tune the anisotropy. [Preview Abstract] |
Thursday, March 5, 2015 10:24AM - 10:36AM |
S30.00013: Theoretical study of local magnetocrystalline anisotropy of $\varepsilon $-Fe$_{2}$O$_{3}$ Daisuke Hirai, Shinji Tsuneyuki, Yoshihiro Gohda Magnetocrystalline anisotropy (MCA) is positively correlated with corercivity that is one of important magnetic figures of merit for applications such as high-density magnetic recording media, high-frequency electromagnetic wave absorbers, and permanent magnets. In general, MCA is given for an entire phase of a material. In light of materials engineering, however, MCA information at respective atoms in crystals is useful to identify weak parts for possible nucleation cores of magnetic reversal and design the local MCA. Considering these facts, we examined the local MCA of hard magnetic materials on the basis of density functional theory and the second perturbation theory on spin-orbit interaction [1]. We studied the magnetic properties of $\varepsilon $-Fe$_{2}$O$_{3}$, which shows the largest coercivity among all the metal oxides [2]. Particularly, we tried to elucidate the effect of an oxygen vacancy on the magnetic properties. As a result, we clarified that the vacancy enhances both the magnetic moment and MAE.\\[4pt] [1] Z. Torbatian et al., Appl. Phys. Lett. 104, 242403 (2014);\\[0pt] [2] S. Ohkoshi \textit{et al.}, Bull. Chem. Soc. Jpn. \textbf{86}, 897 (2013). [Preview Abstract] |
Thursday, March 5, 2015 10:36AM - 10:48AM |
S30.00014: First principles study of Al substituted strontium hexaferrite Vivek Dikshit, Sultana Ababtin, Chandani Nandadasa, Sungho Kim, Seong-Gon Kim, Jihoon Park, Yang-Ki Hong We have studied the site occupancy and magnetic properties of Al substituted M-type Strontium hexaferrite, SrFe$_{12-x}$Al$_{x}$O$_{19}$ with x $=$ 0.5 and x $=$ 1.0 using density functional theory (DFT). For x $=$ 0.5 case, an Al atom preferentially occupy the 2a site at T $=$ 0 K, this trend endures up to 220 K beyond this temperature Al atom is more likely to occupy the 12k site. For the x $=$ 1.0 case, the site preference probability is maximum when two Al atoms occupy the 2a and the 12k sites. We found that magnetic anisotropy of SrFe12-xAlxO19 increases as the concentration of Al atoms increases, while there is a reduction in the magnetic moment per unit cell by 5$\mu $B and 10$\mu $B in the case of x $=$ 0.5 and x $=$ 1.0, respectively. Our results agree with the available experimental results on Al substituted strontium hexaferrite. [Preview Abstract] |
Thursday, March 5, 2015 10:48AM - 11:00AM |
S30.00015: Size Reduction Techniques for Large Scale Permanent Magnet Generators in Wind Turbines Helena Khazdozian, Ravi Hadimani, David Jiles Increased wind penetration is necessary to reduce U.S. dependence on fossil fuels, combat climate change and increase national energy security. The U.S Department of Energy has recommended large scale and offshore wind turbines to achieve 20{\%} wind electricity generation by 2030. Currently, geared doubly-fed induction generators (DFIGs) are typically employed in the drivetrain for conversion of mechanical to electrical energy. Yet, gearboxes account for the greatest downtime of wind turbines, decreasing reliability and contributing to loss of profit. Direct drive permanent magnet generators (PMGs) offer a reliable alternative to DFIGs by eliminating the gearbox. However, PMGs scale up in size and weight much more rapidly than DFIGs as rated power is increased, presenting significant challenges for large scale wind turbine application. Thus, size reduction techniques are needed for viability of PMGs in large scale wind turbines. Two size reduction techniques are presented. It is demonstrated that 25{\%} size reduction of a 10MW PMG is possible with a high remanence theoretical permanent magnet. Additionally, the use of a Halbach cylinder in an outer rotor PMG is investigated to focus magnetic flux over the rotor surface in order to increase torque. [Preview Abstract] |
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