Electron Paramagnetic Resonance in a Holmium based Single Molecule Magnet

1. End plate 60 +m J 40 20 Frequency (GHz) 0 -20 9 -40 -m 1 J -60 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 8 2 Magnetic Field (Tesla) 7 3 6 4 5 Electron Paramagnetic Resonance in a Holmium based Single Molecule Magnet Lisa Friend1, Sanhita Ghosh2,3, Christopher Beedle3 and Stephen Hill2,3 1Manatee Academy Middle School, Port St. Lucie, Florida 34986, USA 2Department of Physics, Florida State University, Tallahassee, Florida 32306, USA 3National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32310, USA Manatee Academy K-8 Electron Paramagnetic Resonance (EPR) Spectroscopy Ho POM Single Molecule Magnet Analysis and Results Magnified view of EPR is a spectroscopic technique that detects the transitions of unpaired electrons in an applied magnetic field. Absorption peaks occur in a spectrum when the magnetic field tunes the two electron spin states so that their energy difference matches the radiation energy. [Ho0.25Y0.75(W5O18)2]9- • Sample: Holmium based polyoxometalate (POM) • Anisotropic inorganic crystal consisting of a Lanthanide ion (Ho3+) encased in a non-magnetic ligand. • Exhibits Single Molecule Magnet (SMM) properties • Potential candidate for application in spintronics Figure 2.Angle dependent EPR spectra collected at 2K and 50.4GHz. The data represent one of 9 end-plate positions (other positions not shown.) The spectra show the expected 8 transitions as a result of hyperfine coupling. In an applied magnetic field a free electron will precess about the field at a frequency that is contingent on the magnitude of the applied field. When the frequency of the source radiation matches the precession frequency, the electron will absorb energy and move to higher energy states, shown as absorption peaks. B Experiment . Purpose: To observe the EPR signal from a Ho POM single crystal and to identify the orientation of the “easy-axis” of magnetization for the crystal Zeeman Effect: Splitting of a spectral line into components in the presence of a magnetic field due to the lifting of degeneracy of electronic energy levels Hyperfine Coupling: Interaction of electron and nuclear spins giving rise to additional allowed energy levels and thereby leading to splitting of EPR peaks • A rotating cavity endplate enables angle dependent measurements by rotation along 2 orthogonal axes. • A single crystal sample was mounted on the quartz pillar for the experiment. Figure 3. Plot of absorption peak positions from figure 2, as a function of angle orientation. E = energy h = Planck’s constant f = microwave frequency B = magnetic field µB = Boltzmann constant g = g-factor Zeeman Effect Instrumentation • Unpaired 4f outer shell electrons exhibit the Zeeman effect in an applied magnetic field. • Hyperfine coupling due to strong interactions between electron and nuclear spins is expected to cause EPR signal to split into (2 I +1)=8 peaks. Electronic configuration: Ho3+ = [Xe] 4f10 Nuclear spin: I = 7/2 E = hf = BµBg Source Waveguides Cavity Waveguides Detector Figure 4. This plot was obtained by collapsing the corresponding +mj and -mj transitions from the above plot to evaluate the g-value of the crystal. B resonance = hf / µBg • Resonant cavity and waveguide assembly • Frequency range: • 50 – 400 GHz • Magnet: • PPMS superconducting magnet (7T) • Temperature: down to 2 K • Source + Detector: • Schottky diodes and Millimeter Vector Network Analyser (MVNA) B resonance Figure 1.Schematic energy level splitting diagram showing expected transitions (blue arrows) for an electronic transition experiencing nuclear hyperfine coupling, with a nuclear spin of I =7/2. g-value = 1.27 Conclusion Figure 5. Plot showing the peak positions at the minima for 9 end plate orientations The angle dependent data shown above correspond to position ‘5’ for the end-plate and shows the “easy-axis” orientation. References and Acknowledgments M. A. Aldamen et al, Inorganic Chemistry, 48, 3467 (2009). S. Takahashi and S. Hill, Review of Scientific Instruments, 76, 023114 (2005). M. Mola et al, Review of Scientific Instruments, 71, No. 1 (2000). The sample for this experiment was synthesized by E. Coronado’s group in University of Valencia, Spain. I would like to thank the Hill Group for allowing me to participate in this project and for helping me to understand the concepts involved in EPR research. I would also like to thank Pat Dixon and Jose Sanchez for bringing me into the RET program, which provides invaluable experiences for teachers that cannot be achieved anywhere else. Cylindrical cavity chamber with 50.4 GHz as the fundamental mode. • Easy axis orientation was identified through the 2 axis rotation method • g-value for the Ho POM crystal was calculated to be 1.27 Resonant cavity: A cavity in which standing electromagnetic waves can be supported giving rise to resonance modes at specific frequencies. Cavities serve to amplify the signal at the resonance modes which makes this a highly sensitive technique for studying extremely small single crystals. Cavity modes depend on the geometry and dimension of the cavity.