R. Shima Edelstein

Lawrence Berkeley National Laboratory, Berkeley, CA, United States

Are you R. Shima Edelstein?

Claim your profile

Publications (15)49.87 Total impact

  • J B Kortright, D M Lincoln, R Shima Edelstein, A J Epstein
    [Show abstract] [Hide abstract]
    ABSTRACT: X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (MCD) at the V L{2,3} and C and N K edges reveal bonding and backbonding interactions in films of the 400 K magnetic semiconductor V[TCNE]x approximately 2. In V spectra, d{xy}-like orbitals are modeled assuming V2+ in an octahedral ligand field, while d{z{2}} and d{x{2}-y{2}} orbitals involved in strong covalent sigma bonding cannot be modeled by atomic calculations. C and N MCD, and differences in XAS from neutral TCNE molecules, reveal spin-polarized molecular orbitals in V[TCNE]x approximately 2 associated with weaker pi bonding interactions that yield its novel properties.
    Physical Review Letters 06/2008; 100(25):257204. · 7.94 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The organic-based magnet VTCNE x , x 2 T c 400 K, is a room temperature magnetic semiconductor with spin polarized valence and conduction bands. It was reported that this material exhibits persistent photoinduced change in both magnetization and conductivity. The simultaneous change in IR spectra by illumination shows photoinduced activation to the metastable state with a small structural change. Here, we employed photoinduced ferrimagnetic resonance FMR study to investigate photoinduced magnetization in VTCNE x film. The FMR spectra display substantial changes in their linewidth and line shift by the illumination indicating substantial increase in random magnetic anisotropy. The results demonstrate optical control of magnetism by changing magnetic anisotropy of the system. © 2008 American Institute of Physics.
    Journal of Applied Physics 02/2008; · 2.21 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (MCD) at the V L2,3 and C and N K edges reveal bonding and backbonding interactions in films of the 400 K magnetic semiconductor V[TCNE] xtilde 2. In V spectra, dxy-like orbitals are modeled assuming V2+ in an octahedral ligand field, while dz2 and dx2-y2 orbitals involved in strong covalent sigma bonding cannot be modeled by atomic calculations. C and N MCD, and differences in XAS from neutral TCNE molecules, reveal spin-polarized molecular orbitals in V[TCNE] xtilde 2 associated with weaker pi bonding interactions that yield its novel properties.
    Physical Review Letters 01/2008; 100. · 7.94 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The V(TCNE){x}, x approximately 2 is an organic-based amorphous ferrimagnet, whose magnetic behavior is significantly affected in the low field regime by the random magnetic anisotropy. It was determined that this material has thermally reversible persistent change in both magnetization and conductivity driven by the optical excitation. Here, we report results of a ferrimagnetic resonance study of the photoinduced magnetism in V(TCNE){x} film. Upon optical excitation (lambda approximately 457.9 nm), the ferrimagnetic resonance spectra display substantial changes in their linewidths and line shifts, which reflect a substantial increase in the random magnetic anistropy. The results reflect the role of magnetic anisotropy in disordered magnets and suggest a novel mechanism of photoinduced magnetism in V(TCNE){x} induced by the increased structural disorder in the system.
    Physical Review Letters 11/2007; 99(15):157205. · 7.94 Impact Factor
  • Jung-Woo Yoo, R. Shima Edelstein, D. M. Lincoln, A. J. Epstein
    [Show abstract] [Hide abstract]
    ABSTRACT: The V(TCNE)x, x˜2 is a fully spin-polarized magnetic semiconductor, whose magnetic order exceeds room temperature (Tc > 350 K), and electronic transport follows hopping mechanism through the Coulomb energy split &*circ; subband. In addition, it was determined that this material has thermally reversible persistent change in both magnetism and conductivity driven by the optical excitation [1]. Here, we report detailed investigation on photo-induced magnetism in V(TCNE)x by employing ferrimagnetic resonance (PIFMR) study with an in-situ light illumination. Upon optical excitation (lambda˜ 457.9 nm), the FMR spectra display substantial change in their linewidth and resonance field. Angular dependence analyses of line shift indicate the increase of unixial anisotropy field in the film caused by the light irradiation. The results demonstrated that the change in overall magnetic anisotropy by the illumination plays an important role in inducing photo- induced magnetism in (TCNE) class magnet. [1] J.-W. Yoo, et al. to be published in Phys. Rev. Lett.
    03/2007;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Concomitant photoinduced magnetic and electrical phenomena are reported for the organic-based magnetic semiconductor V(TCNE)x (x approximately 2; TCNE=tetracyanoethylene; magnetic ordering temperature Tc approximately 400 K). Upon optical excitation (457.9 nm), the system can be trapped in a thermally reversible photoexcited state, which exhibits reduced magnetic susceptibility and increased conductivity with a simultaneous change in IR absorption spectrum. The multiple photonic effects in V(TCNE)x are proposed to originate from structural changes induced by internal excitation in (TCNE)- anions, which lead to relaxation to a long-lived metastable state.
    Physical Review Letters 01/2007; 97(24):247205. · 7.94 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The V(TCNE)x, x∼2 is an organic-based amorphous ferrimagnet, whose magnetic behavior is significantly affected in the low field regime by the random magnetic anisotropy. It was determined that this material has thermally reversible persistent change in both magnetization and conductivity driven by the optical excitation. Here, we report results of a ferrimagnetic resonance study of the photoinduced magnetism in V(TCNE)x film. Upon optical excitation (λ∼457.9 nm), the ferrimagnetic resonance spectra display substantial changes in their linewidths and line shifts, which reflect a substantial increase in the random magnetic anistropy. The results reflect the role of magnetic anisotropy in disordered magnets and suggest a novel mechanism of photoinduced magnetism in V(TCNE)x induced by the increased structural disorder in the system.
    Physical Review Letters 01/2007; 99(15). · 7.94 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: CVD films of V[TCNE]x˜2 are magnetic room temperature and of interest as prototypes in functional organic magnetic systems. In addition to potential technological interest, fundamental questions regarding the electronic structure and spin distribution in V[TCNE]x˜2 films remain, and motivate these measurements of x-ray absorption (XAS) and magnetic circular dichroism (MCD) spectra at the vanadium L, carbon K, and nitrogen K edges. XAS spectra reveal strong multiplet splitting at the V edge and strong pi* features at the C and N edges. The registry of these features at different edges indicates a distinct molecular orbital structure involving all constituents. That this molecular orbital structure supports magnetism is indicated by vanadium MCD spectrum and by the loss of specific XAS and MCD features for oxidized samples. Results are interpreted in the context of prior neutron scattering [1] and EXAFS [2] studies, multiplet calculations of V XAS and MCD, and established C and N XAS features. [1] A. Zheludev, et al., J. Am. Chem. Soc. 116, 7243 (1994). [2] D. Haskel, et al., Phys. Rev. B 70, 054422 (2004).
    03/2006;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Different aspects of the room-temperature (Tc>350K) molecular magnet vanadium tetracyanoethylene (V[TCNE]x˜2) remain only partially understood, including its intra- and inter-molecular bonding and spin distribution, degradation mechanisms, and mesoscopic magnetic and chemical order. Previous studies have shown that control of the local order, structure, spin, and chemical composition determines the magnetic state achieved and its charge transport and dynamic properties^1. In this study, we focus on thin films (˜0.05-0.5 μm) prepared by CVD^2,3 (room temperature conductivity 10-4-10-3 S/cm). Soft x-ray absorption and Magnetic Circular Dichroism (MCD) spectra of the atomic edges are presented, together with XPS, SQUID magnetometry, and EPR spectroscopy. The knowledge gained enables successful demonstration of spin valve devices and photoinduced magnetism in this molecule-based magnetic semiconductor. 1. Pokhodnya, et al, PRB 63, 174408 (2001) 2. Pokhodnya, et al, Adv. Mater. 12, 410 (2000) 3. Shima, et al, MRS Proc. 871E, I7.3 (2005) Supported by DOE grants #DE-FG02-86ER45271, DE-FG02- 01ER45931, DE-AC03-76SF00098, and AFOSR grant #F49620-03-1-0175. NSF-DMR grant #0114098 and Dr. Lisa Hommel are acknowledged for XPS studies.
    03/2006;
  • [Show abstract] [Hide abstract]
    ABSTRACT: We report magnetoresistance (MR) phenomena with temperature and bias dependence in organic semiconductor thin films with either nonmagnetic or magnetic contacts through high field reaching 9T. For nonmagnetic organic thin films such as Alq3 we find a low field MR up to 15%. A similar magnetic field effect has been reported earlier^1 but, as noted, the mechanism remains unclear. We propose a model of the anomalous MR where charge transport is space-charge limited. The current is determined by the e-h recombination rate. The recombination rate is field dependent, analogous to the chemical yield for radical pairs^2. Using an organic- based magnetic semiconductor^3, V[TCNE]x, and Co as magnetic contacts, with a nonmagnetic organic semiconductor (alpha-6T) leads to an order-of-magnitude broader zero-centered MR peak superimposed on a spin-valve effect. Possible origins of this broader MR will be discussed. 1. Francis, et al., New J. Phys. 6 185 (2004); Frankevich, et al., Phys. Rev. B 53 4498 (1996) 2. Steiner and Ulrich, Chem. Rev. 89 51 (1989) 3. Pokhodnya, et al., Adv. Mater. 12 410 (2000); Prigodin, et al., Adv. Mater. 14 1230 (2002); Shima Edelstein, et al., Mater. Res. Soc. Symp. Proc. 871E I7.3 (2005)
    03/2006;
  • [Show abstract] [Hide abstract]
    ABSTRACT: V(TCNE)x˜2 is a fully spin-polarized half- semiconductor, whose magnetic order exceeds room temperature (Tc>350 K), and electronic transport follows hopping mechanism in the Coulomb energy split in &*circ; band. Substantial decrease of magnetization by illuminating with light (lambda=457.9nm) has been found at low temperature (T10^6s at low temperatures and completely relaxes to the state before illumination after annealing upto 250K. Photoinduced ESR analysis indicates strong increase of magnetic anisotropy by light irradiation. We also report substantial increase of conductivity induced by illumination with light (lambda=457.9nm). The temperature dependence of resistivity clearly indicates substantial decrease in activation energy for electronic hopping. The photoinduced effect is proposed to originate from structural changes triggered by pi -> &*circ; excitation in (TCNE) molecules, which leads modification of the orbital wavefunction resulting in changes of magnetic exchange energy J and the activation energy deltaE. ^*Supported in part by AFOSR Grant No. F49620-03-1-0175 and DOE Grant No. DE-FG02-01ER45931 and DE-FG02-86ER45271
    03/2006;
  • N. P. Raju, R. Shima Edelstein, A. J. Epstein
    [Show abstract] [Hide abstract]
    ABSTRACT: We present magnetoresistance and ferrimagnetic resonance (FMR) results on a CVD-prepared [1] thin films (about 0.5 micron thick) [2] of ferrimagnetic semiconductor V(TCNE)x˜2. The temperature dependence of resistance, and the magnetoresistance variation with magnetic field and temperature show similar to the trends reported earlier. [3] FMR spectra reported here show marked differences from the earlier studies on thicker samples (up to 2 microns). [4] Earlier reported FMR shows several sharp peaks compared to only two in the present study. We report temperature dependence of linewidth and integrated intensity for each of these two peaks. The earlier study suggested that the porosity of the sample as one of the possible origins for many peaks. It appears that absence of many sharp FMR peaks in the present sample may reflect less porosity of the thin film which is important for the development of spin-valve devices. 1. K.I. Pokhodnya et. al., Adv. Mater 12, 410 (2000). 2. Shima et. al., MRS Proc. 871E, I7.3 (2005) 3. N.P. Raju et. al., J. Applied Physics 93, 6799 (2003). 4. R. Plachy et. al., Phys. Rev. B 70, 064411 (2004).
    03/2006;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Concomitant photoinduced magnetic and electrical phenomena are reported for the organic-based magnetic semiconductor V(TCNE)x (x˜2; TCNE=tetracyanoethylene; magnetic ordering temperature Tc˜400K). Upon optical excitation (457.9 nm), the system can be trapped in a thermally reversible photoexcited state, which exhibits reduced magnetic susceptibility and increased conductivity with a simultaneous change in IR absorption spectrum. The multiple photonic effects in V(TCNE)x are proposed to originate from structural changes induced by internal excitation in (TCNE)- anions, which lead to relaxation to a long-lived metastable state.
    Physical Review Letters 01/2006; 97(24). · 7.94 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: A multiphotonic response in organic magnetic semiconductor V(TCNE)x (x ~ 2) with a substantial increase of conductivity induced by light (lambda=457.9 nm) is reported. Substantial decrease in activation energy for electronic hopping is due to temperature dependence of resistivity. Photoinduced effects that originated from structural changes triggered by pirarrpi* excitation in (TCNE)ldr- molecules, which leads to modulation of the magnetic exchange energy J and the activation energy DeltaE, is proposed. This mechanism for multiphotonic responses is supported by photoinduced IR studies.
    01/2006;
  • [Show abstract] [Hide abstract]
    ABSTRACT: We describe how the composition of an organic - based magnet can be controlled by varying the Chemical Vapor Deposition (CVD) conditions. A study was conducted for the Co2(CO)8/ TCNE system to form cobalt tetracyanoethylene [Co(TCNE)x, x∼2, a paramagnetic material], and for the V(CO)6/ TCNEx system to form vanadium tetracyanoethylene [V(TCNE)x, x∼2, a ferrimagnetic material]. Thin V(TCNE)x, x∼2 films (∼0.05-0.5 μm) with room temperature conductivity of 10-4<σRT<10-3S/cm and magnetic ordering temperature Tc of up to ∼400K were deposited. The V(TCNE)x, x∼2 thin films have the potential for incorporation in a spin-valve device as one of the magnetic contacts, and are promising candidates to form optically controlled magnetic-based structures.
    MRS Proceedings. 12/2004; 871.

Publication Stats

51 Citations
49.87 Total Impact Points

Institutions

  • 2008
    • Lawrence Berkeley National Laboratory
      • Materials Sciences Division
      Berkeley, CA, United States
  • 2004–2008
    • The Ohio State University
      • Department of Physics
      Columbus, OH, United States