Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session H3: Collective Effects in Molecular Magnets |
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Sponsoring Units: GMAG Chair: Yosi Yeshurun, Ber-Ilan University Room: Ballroom A3 |
Tuesday, March 22, 2011 8:00AM - 8:36AM |
H3.00001: Experimental realization of random-field Ising ferromagnetism in a molecular magnet Invited Speaker: The longitudinal magnetic susceptibility of single crystals of the molecular magnet Mn$_{12}$-acetate obeys a Curie-Weiss law, indicating a transition to a ferromagnetic phase at $\sim $ 0.9 K [1,2]. With increasing magnetic field applied transverse to the easy axis, a marked change is observed in the temperature dependence of the susceptibility, with a considerably more rapid suppression of the Curie-Weiss temperature than predicted by mean-field theory for an ordered single crystal. Our results can instead be fit by a Hamiltonian for a random-field Ising ferromagnet in a transverse magnetic field, where the randomness derives from the intrinsic distribution of locally tilted magnetic easy axes known to exist in Mn$_{12}$-acetate crystals. Mn$_{12}$-ac and other single molecule magnets may thus serve as clean model systems for the study of random field ferromagnetism where the random fields are controllable and considerably larger than typical hyperfine fields. This discovery promises to enable widespread and convenient experimental study of magnetism in a random field in a broad class of new materials.\\[4pt] Work performed by and in collaboration with: Bo Wen, and Lin Bo, Physics Dept. City College of New York, CUNY (funded by NSF-DMR-0451605), P. Subedi and A. D. Kent, Physics Dept., NYU, (funded by NSF-DMR-0506946 and ARO-W911NF-08-1-0364) Y. Yeshurun, Physics Dept., Bar Ilan U, (funded by Deutsche Forschungsgemeinschaft), A. J. Millis, Physics Dept. Columbia U. (funded by DMR DMR-0705847), C. Lampropoulos and G. Christou, Chemistry Dept., U. of Florida (funded by NSF -CHE-0910472). \\[4pt] [1] B. Wen et al., Phys. Rev. B 82, 014406 (2010). \\[0pt] [2] Luis, et al., Phys. Rev. Lett. 95, 227202 (2005). [Preview Abstract] |
Tuesday, March 22, 2011 8:36AM - 9:12AM |
H3.00002: Pure and Random--field quantum criticality in dipolar Ising magnets Invited Speaker: A theoretical model for $Mn_{12}$ acetates and related materials is derived. Isomer effects present in some families of host acetate materials are argued to lead to a random field of a strength which may be tuned by a magnetic field applied in a direction perpendicular to the easy axis of the $Mn_{12}$ unit. A mean field phase diagram is presented and consequences of beyond-mean-field physics are outlined. Measureable consequences in the experimentally accessible high temperature regime are presented and in this regime the importance of a complete treatment of the molecular level structure is emphasized. Open theoretical problems are described. Work reported in Phys. Rev. B 82, 014406 (2010) and Phys. Rev. B 82, 174405 (2010)). and performed in collaboration with: M. Sarachik, Bo Wen, and Lin Bo, Physics Dept. City College of New York, CUNY (funded by NSF-DMR-0451605), P. Subedi and A. D. Kent, Physics Dept., NYU, (funded by NSF-DMR-0506946 and ARO-W911NF-08-1-0364) Y. Yeshurun, Physics Dept., Bar Ilan U, (funded by Deutsche Forschungsgemeinschaft), C. Lampropoulos and G. Christou, Chemistry Dept., U. of Florida (funded by NSF -CHE-0910472). \\[4pt] [1] Phys. Rev. B 82, 014406 (2010).\\[0pt] [2] Phys. Rev. B 82, 174405 (2010)). [Preview Abstract] |
Tuesday, March 22, 2011 9:12AM - 9:48AM |
H3.00003: Deflagration, fronts of tunneling, and dipolar ordering in molecular magnets Invited Speaker: Although there is no exchange interaction in crystals of molecular magnets characterized by a giant effective spin $S$ ($S$ =10 for Mn$_{12}$, and Fe$_{8})$, magnetic field $B^{(D)}$ generated by magnetic moments \textit{g$\mu $}$_{B}S$ of magnetic molecules creates energy bias $W^{(D) }$=2\textit{Sg$\mu $}$_{B} B^{(D)}$ on a molecule that largely exceeds the tunnelling splitting $\Delta $ of matching quantum states on different sides of the anisotropy barrier. Thus the dipolar field has a profound influence on the processes of tunnelling and relaxation in molecular magnets. Both theoretical and experimental works showed a slow non-exponential relaxation of the magnetization in both initially ordered and completely disordered states since most of the spins are off tunneling resonance at any time. Recently a new mode of relaxation via tunneling has been found, the so-called fronts of tunneling, in which (within a 1$d$ theoretical model) dipolar field adjusts so that spins are on resonance within the broad front core. In this ``laminar'' regime fronts of tunnelling are moving fast at speeds that can exceed that of the temperature-driven magnetic deflagration, if a sufficiently strong transverse field is applied. However, a ``non-laminar'' regime has also been found in which instability causes spins to go off resonance and the front speed drops. In a combination with magnetic deflagration, the laminar regime becomes more stable and exists in the whole dipolar window 0$\le W \quad \le W^{(D)}$ on the external bias $W$, where the deflagration speed strongly increases. Another dipolar effect in molecular magnets is dipolar ordering below 1 K that has recently been shown to be non-uniform because of formation of magnetic domains. An object of current research is possible non-uniformity of magnetic deflagration and tunneling fronts via domain instability that could influence their speed. [Preview Abstract] |
Tuesday, March 22, 2011 9:48AM - 10:24AM |
H3.00004: Experiments on Magnetic Deflagration Invited Speaker: Magnetic deflagration was first observed in molecular magnets [1,2] and then in glassy magnetic materials like manganites [3,4] and intermetallic systems like Gd$_{5}$Ge$_{4}$ [5]. The role of the chemical energy is played by the magnetic energy of the material. In the case of a molecular magnet, this is Zeeman energy, while in manganites and Gd$_{5}$Ge$_{4}$ the free energy is a combination of the Zeeman energy and the energy of the metastable magnetic phase. In molecular magnets both the ignition process and the speed of the flame are assisted by quantum spin reversal [2]. There also exists some evidence of the transition from deflagration to detonation [6]. Various experimental techniques have been used to detect the speed of the magnetic flame. They include SQUID magnetometry, Hall bars and coils. Magnetic deflagration has been ignited by local heating, application of external fields, by surface acoustic waves and microwaves. High frequency EPR measurements of the population of spin levels permitted observation of magnetic deflagration in real time. The talk will review these experiments and their interpretation. \\[4pt] [1] Y. Suzuki et al. Phys. Rev. Lett. 95, 147201 (2005). \\[0pt] [2] A. Hernandez-Minguez et al. Phys. Rev. Lett. 95, 217205 (2005). \\[0pt] [3] F. Macia et al. Phys. Rev. B79, 092403 (2009). \\[0pt] [4] F. Macia et al. Phys. Rev. B76, 174424 (2007). \\[0pt] [5] S. Velez et al. Phys. Revc. B81, o64437 (2010). \\[0pt] [6] W. Decelle et al. Phys. Rev. let. 102, 027203 (2009) [Preview Abstract] |
Tuesday, March 22, 2011 10:24AM - 11:00AM |
H3.00005: Electronic Structure and Transport Through Single Molecule Magnets Invited Speaker: Over the past decade, single-molecule magnets have drawn considerable attention due to observed magnetic quantum tunneling and interference and a possibility of using them for information storage or devices. There have been so far significant experimental efforts to build and characterize monolayers of single-molecule magnets on various surfaces or single-molecule magnets connected to electrodes. There is need to understand changes of electronic and magnetic properties of single-molecule magnets in those environments using quantum mechanical simulations. We simulate, within density-functional theory, a nanostructure in which prototype single-molecule magnets Mn12 are adsorbed onto a gold surface. We investigate coupling between the Mn12 and the surface and discuss electronic structure and magnetic anisotropy of the Mn12 on a gold surface in comparison to an isolated Mn12. In addition, we present electron transport properties through a Mn12 bridged between gold electrodes, using the nonequilibrium Green's function method in conjunction with density-functional theory. We discuss a possibility of using a Mn12 molecule as a spin filter and an effect of interface geometry and bonding type on transport across a Mn12. [Preview Abstract] |
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