Acoustomagnetic pulse experiments in LiNbO3∕Mn12 hybrids (original) (raw)
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Acoustomagnetic pulse experiments in LiNbO[sub 3]∕Mn[sub 12] hybrids
Applied Physics Letters, 2006
We report here on the influence of surface acoustic waves ͑SAWs͒ on the magnetization of a Mn 12-acetate single crystal. The crystal was mounted on the surface of a piezoelectric LiNbO 3 substrate containing an interdigital transducer for the excitation of SAWs. The magnetization of the crystal was measured using a rf superconducting quantum interference device with a time resolution of 1 s. The piezoelectric material was excited by SAW pulses of different frequencies produced by applying microwave pulses to the transducer. Our data show that molecular magnets onto the LiNbO 3 surface can be used as very sensitive detectors of the SAW frequency and intensity.
Macroscopic quantum effects generated by acoustic waves in a molecular magnet
We have shown that the size of the magnetization step due to resonant spin tunneling in a molecular magnet can be strongly affected by sound. The transverse-acoustic wave can also generate macroscopic quantum beats of the magnetization during the field sweep. Single-molecule magnets SMMs have attracted much interest because they provide possibility to observe quantum effects at the macroscopic scale. Among these effects are stepwise magnetization curve caused by resonant spin tunneling, 1,2 topological interference of tunneling trajectories, 3 and crossover between classical and quantum superparamagnetisms. 4,5 The Landau-Zener theory has been used to describe spin transitions that occur during the field sweep. 6 It has been recognized that spin-phonon interactions play an important role in the dynamics of spins in molecular magnets. 7–13 Possibility of Rabi oscillations of spins caused by the acoustic wave has been studied. 14 In recent years the effect of sound on molecular magnets has been explored in experiment. 15 In this paper we show that sound can significantly affect the size of the magnetization step due to resonant spin tunneling. In the presence of the field sweep an acoustic wave can also generate quantum beats of the mag-netization of a macroscopic sample. We compute the parameters of the sound that are necessary to observe these effects. The effect we are after is illustrated in Fig. 1. The lower curve shows how the sound modulates the distance between spin energy levels in the absence of the field sweep. In this case the phase of Rabi oscillations caused by a propagating sound wave depends on coordinates such that the oscillations average out over the volume of the sample if the latter is large compared to the wavelength of sound. In the presence of the field sweep the upper curve the phase of the Rabi oscillations is still a function of coordinates. However, the Landau-Zener probability of spin transitions that contribute to the oscillations depends on the rate of the field sweep. That rate becomes modulated by the sound. Consequently, the regions of the sample that contribute most to the dynamics of the magnetization add their contributions constructively. The resulting oscillations of the magnetic moment of the sample can be observed in a macroscopic experiment. Note that the quantum oscillations of the magnetization driven by the sound wave should be more robust with respect to decoherence and, thus, easier to observe than the free oscillations. We consider a crystal of single-molecule magnets with the Hamiltonian H SMM = − DS z 2 − g B H z S z + H trans , 1 where S i are Cartesian components of the spin operator and D is the second-order anisotropy constant. The second term is the Zeeman energy due to the longitudinal field H z , with g being the gyromagnetic factor and B being the Bohr mag-neton. The last term includes the transverse magnetic field and the transverse anisotropy, which produce level splitting. Local rotation produced by a transverse-acoustic wave of frequency = c t k, wave vector k, and amplitude u 0 , polarized along the y axis and running along the x axis is given by 16 r = 1 2 ku 0 coskx − tzˆ. 2 Due to the rotation of the local anisotropy axis by sound, the spin Hamiltonian becomes 9 H = e −i·SˆH SMM e i·Sˆ. 3 The simplest solution of the problem for an individual spin can be obtained in the coordinate frame that is rigidly coupled to the local crystallographic axes. The wave functions in the laboratory and lattice frames, and lat , are related through lat = e i·Sˆ, 4 while the spin Hamiltonian in the lattice-frame is given by 9,17,18 H lat = H SMM − S ˆ · 5 with FIG. 1. Schematic of the time dependence of the distance between spin energy levels. Thin solid line: the effect of the acoustic wave without the field sweep. Thick solid line: field sweep is modulated by the acoustic wave.
First Evidence of Surface SH-Wave Propagation in Cubic Piezomagnetics
Journal of Electromagnetic Analysis and Applications, 2010
This theoretical work provides with results of characteristics calculation of the ultrasonic surface Zakharenko waves (USZWs) existing in piezomagnetic cubic monocrystals of class m3m that can be readily used for non-destructive testing. The piezomagnetic waves propagate in direction [101] corresponding to relatively easy magnetization for the following piezomagnetics: Galfenol, Terfenol-D, and CoFe 2 O 4 with cubic structures. The phase velocities of the USZW-waves and the coefficient of magnetomechanical coupling (CMMC) K 2 were calculated for the crystals. It was found that the coefficient K 2 for piezomagnetics with K m 2 > 1/3 and K m
PACS 75.50.Xx – Molecular magnets
- For the first time, the morphology and dynamics of spin avalanches in Mn12-Acetate crystals using magneto-optical imaging has been explored. We observe an inhomogeneous relaxation of the magnetization, the spins reversing first at one edge of the crystal and a few milliseconds later at the other end. Our data fit well with the theory of magnetic deflagration, demonstrating that very slow deflagration rates can be obtained, which makes new types of experiments possible. Synthesized in 1980, Mn12-Acetate is a crystal composed of a large number (typically of the order of 1017) of [Mn12O12(CH3COO)16·(H2O)4]·2CH3COOH·4H2O molecules, each with a large spin S = 10 µB [1]. Its mono-disperse structure, strong spin anisotropy as well as the hysteretic behavior at low temperatures [2] have attracted a lot of interest. From a fundamental point of view, Mn12-Acetate offers an unique playground to study phenomena at the frontier between classical and quantum mechanics, whereas from the point of...
Acoustic ferromagnetic resonance and spin pumping induced by surface acoustic waves
Journal of Physics D: Applied Physics, 2020
Voltage induced magnetization dynamics of magnetic thin films is a valuable tool to study anisotropic fields, exchange couplings, magnetization damping and spin pumping mechanism. A particularly well established technique is the ferromagnetic resonance (FMR) generated by the coupling of microwave photons and magnetization eigenmodes in the GHz range. Here we review the basic concepts of the so-called acoustic ferromagnetic resonance technique (a-FMR) induced by the coupling of surface acoustic waves (SAW) and magnetization of thin films. Interestingly, additional to the benefits of the microwave excited FMR technique, the coupling between SAW and magnetization also offers fertile ground to study magnon-phonon and spin rotation couplings. We describe the in-plane magnetic field angle dependence of the a-FMR by measuring the absorption / transmission of SAW and the attenuation of SAW in the presence of rotational motion of the lattice, and show the consequent generation of spin current by acoustic spin pumping.
Open Journal of Acoustics, 2015
This comparative study acquaints the reader with some properties of the eighth and tenth new shear-horizontal surface acoustic waves (SH-SAWs) propagating along the free surface of the magnetoelectroelastic (6 mm) medium. These new nondispersive SH-SAWs cannot exist when the electromagnetic constant α is equal to zero. The piezoelectromagnetic SH bulk acoustic wave and the surface Bleustein-Gulyaev-Melkumyan (BGM) wave are also chosen for comparison. The main problem of this report is the demonstration of the fact that the new waves can propagate slower than the BGM wave. This problem can be very important due to the fact that among the other known SH-SAWs the BGM wave can propagate significantly slower than the corresponding SH bulk acoustic wave. Two new SH-SAWs are analytically and graphically studied in dependence on the electromagnetic constant α. For the graphical study, two (6 mm) composites are used: BaTiO 3 -CoFe 2 O 4 and PZT-5H-Terfenol-D. For the second composite it is solidly demonstrated that for small values of α, the eighth new SH-SAW cannot exist and its velocity starts with zero at some small threshold value of α rapidly reaching the BGM-wave velocity. This means that a weak magnetoelectric effect can dramatically slow down the speed of either new SH-SAW. As a result, the studied new SH-SAWs can be suitable for creation of new technical devices to sense the magnetoelectric effect. For the analytical study, extreme and inflexion points were evaluated in the velocities' dependencies on the value of α.
Journal of King Saud University - Science, 2017
It is expected that this theoretical report finalizes the research regarding to the shear-horizontal surface acoustic wave (SH-SAW) propagation along the suitable surface of the transversely isotropic (6 mm) piezoelectromagnetics. This report examines extra two new SH-SAWs, the existence of which dramatically depends on the small electromagnetic constant that is responsible for the magnetoelectric effect. This study also provides some comparison with the previously obtained theoretical results and the phenomenon called the Goldstone excitation. The obtained results can be useful for educational purposes, creation of novel technical devices based on the magnetoelectric effect that can find applications in spintronics, further theoretical treatments of the SH-wave propagation in plates, nondestructive testing and evaluation of apt surfaces and plates, etc.
Single-Molecule Magnets: A New Class of Tetranuclear Manganese Magnets
Inorganic Chemistry, 2000
The preparation, X-ray structure, and detailed physical characterization are presented for a new type of singlemolecule magnet [Mn 4 (O 2 CMe) 2 (pdmH) 6 ](ClO 4 ) 2 (1). Complex 1‚2MeCN‚Et 2 O crystallizes in the triclinic space group P1 h, with cell dimensions at 130 K of a ) 11.914(3) Å, b ) 15.347(4) Å, c ) 9.660(3) Å, R ) 104.58(1)°, ) 93.42(1)°, γ ) 106.06(1)°, and Z ) 1. The cation lies on an inversion center and consists of a planar Mn 4 rhombus that is mixed-valent, Mn III 2 Mn II 2 . The pdmHligands (pdmH 2 is pyridine-2,6-dimethanol) function as either bidentate or tridentate ligands. The bridging between Mn atoms is established by either a deprotonated oxygen atom of a pdmHligand or an acetate ligand. The solvated complex readily loses all acetonitrile and ether solvate molecules to give complex 1, which with time becomes hydrated to give 1‚2.5H 2 O. Direct current and alternating current magnetic susceptibility data are given for 1 and 1‚2.5H 2 O and indicate that the desolvated complex has a S ) 8 ground state, whereas the hydrated 1‚2.5H 2 O has a S ) 9 ground state. Ferromagnetic interactions between Mn III -Mn II and Mn III -Mn III pairs result in parallel spin alignments of the S ) 5 / 2 Mn II and S ) 2 Mn III ions. High-frequency EPR spectra were run for complex 1‚2.5H 2 O at frequencies of 218, 328, and 436 GHz in the 4.5-30 K range. A magnetic-field-oriented polycrystallite sample was employed. Fine structure is clearly seen in this parallel-field EPR spectrum. The transition fields were least-squares-fit to give g ) 1.99, D ) -0.451 K, and B 4°) 2.94 × 10 -5 K for the S ) 9 ground state of 1‚2.5H 2 O. A molecule with a large-spin ground state with D < 0 can function as a single-molecule magnet, as detected by techniques such as ac magnetic susceptibility. Out-of-phase ac signals ( ′′ M ) were seen for complexes 1 and 1‚2.5H 2 O to show that these complexes are single-molecule magnets. A sample of 1 was studied by ac susceptibility in the 0.4-6.4 K range with the ac field oscillating at frequencies in the 1.1-1000 Hz range. A single peak in ′′ M vs temperature plots was seen for each frequency; the temperature of the ′′ M peak varies from 2.03 K at 995 Hz to 1.16 K at 1.1 Hz. Magnetization relaxation rates were evaluated in this way. An Arrhenius plot gave an activation energy of 17.3 K, which, as expected, is less than the 22.4 K value calculated for the thermodynamic barrier for magnetization direction reversal for an S ) 8 complex with D ) -0.35 K. The 1‚2.5H 2 O complex with an S ) 9 ground state has its ′′ M peaks at higher temperatures.