Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session K19: Optical, Thermal and Mechanical Coupling to Spin CurrentsFocus
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Sponsoring Units: GMAG DMP FIAP Chair: Chunhui Du, Harvard Univ Room: LACC 308A |
Wednesday, March 7, 2018 8:00AM - 8:12AM |
K19.00001: Coupling of Heat Carriers in Laser-Induced Thermal Transport in Pt/Co and Pt/CoFeB Bilayers Hyejin Jang, David Cahill Coupling of electrons with phonons or with magnons plays a key role in various transport processes, including ultrafast demagnetization and spin diffusion in magnetic materials. However, experimental determination of coupling strength is challenging due to the difficulty in separately observing an effective temperature of each type of excitation. In this work, we design simple bilayers, consisting of an optically-thick Pt layer and sub-nm-thick ferromagnetic (FM) layer, i.e., Co and CoFeB. The Pt surface is irradiated with a short laser pulse, and the temperature changes of phonons and magnons are observed separately, via the change in reflectance of the Pt surface (i.e., time-domain thermoreflectance), and via the change in magnetization of the FM layer (i.e., time-resolved magneto-optic Kerr effect). We describe the time-evolution of the effective temperatures of electrons, phonons, and magnons by a three-temperature model. We conclude that magnons in Co thermalize rapidly with electrons in Co and that we can derive an improved value of the electron-phonon coupling of Pt from measurements of Pt/Co bilayers. We observe that magnons in CoFeB equilibrate more slowly than magnons in Co and we extract the electron-magnon coupling of CoFeB from measurements of Pt/CoFeB bilayers. |
Wednesday, March 7, 2018 8:12AM - 8:24AM |
K19.00002: Capturing the coupled dynamics of the charge, spin and lattice degrees of freedom through the ferromagnetic phase transition using time-resolved spectroscopies Xun Shi, Phoebe Tengdin, Wenjing You, Cong Chen, Dmitriy Zusin, Yingchao Zhang, Christian Gentry, Adam Blonsky, Mark Keller, Peter Oppeneer, Henry Kapteyn, Zhensheng Tao, Margaret Murnane The coupled interactions of the charge, spin and lattice give rise to intriguing phenomena in quantum materials. Because electrons respond faster than the lattice, ultrafast techniques can uncover which coupled interactions are dominant in a particular material. Here we investigate the ferromagnetic-to-paramagnetic phase transition in nickel using both time-resolved photoemission spectroscopy and magneto-optical Kerr spectroscopy and make several surprising findings. First, the spin system can absorb the laser energy within 20 fs after laser excitation. This is evidenced by a divergence in the transient heat capacity of the spin system as the electron temperature approaches the Curie temperature. Second, we observe a longer and universal timescale of 180 fs in the collapse of the exchange splitting or demagnetization. Moreover, there is a strong fluence dependence of both the phase transition itself and the recovery, which can proceed via two distinct pathways, with timescales of 540 fs and 70 ps, that are attributed to ultrafast spin-spin relaxation and slower timescale phonon coupling, respectively. Our results uncover the clear time sequence of the multiple dynamics and suggest that critical behavior dominates the ultrafast phase transition in nickel. |
Wednesday, March 7, 2018 8:24AM - 8:36AM |
K19.00003: Temperature dependent Mueller matrix of magnetized Ni near the Curie temperature Farzin Abadizaman, Pablo Paradis, Stefan Zollner Optical properties of magnetized Ni are studied in polar configuration. The temperature dependence of the optical constants of magnetized Ni 1000 Å demonstrates an anomaly near the Curie temperature Tc. |
Wednesday, March 7, 2018 8:36AM - 9:12AM |
K19.00004: Optical-helicity-driven optomagnetic field and photo-spin current in metallic systems Invited Speaker: Gyung-Min Choi Interaction among photons, electrons, and spins in condensed matters has opened a research area of opto-spintronics. Circularly polarized photons induce spin density on conduction bands of semiconductors by optical orientation [1, 2], and generate an opto-magnetic field on magnetic moments of insulators by inverse Faraday effect [3, 4]. The observation of optical-helicity-dependent switching of magnetization of metallic ferromagnets has suggested relevant physics in metallic systems [5]. Recently, the opto-magnetic field has been observed in metallic ferromagnets, and its magnitude and direction were explained by interaction energy between angular momentum of photons and magnetization of ferromagnets [6]. For non-magnetic metals, a photo-spin current can occur at interfaces of heavy metals and normal metals due to spin-orbit coupling of heavy metals and inversion symmetry breaking at interfaces [7]. These novel physics enable opto-spintronics in metallic systems with an ultrafast timescale. |
Wednesday, March 7, 2018 9:12AM - 9:24AM |
K19.00005: Spin-flip diffusion in transition metals as a function of temperature Paul Kelly, Kriti Gupta, Rohit S Nair, Ehsan Barati Because of spin-orbit coupling, a spin current injected into a non-magnetic material is not conserved. Valet and Fert characterized the spatial decay of a spin current in terms of a material dependent spin-flip diffusion (sfd) length that has become an extremely important parameter in the field of spin transport. In spite of their importance, almost everything that is known about them is from low-temperature transport measurements. In this work we perform a systematic study of the temperature dependence of sfd lengths for the 4d and 5d transition metal elements using a TB-LMTO based scattering code to calculate the conductance as well as local charge and spin currents. From the decay of a spin current injected into the material, we extract the sfd length. We mimic temperature by displacing atoms randomly in a Gaussian distribution to create a snapshot of frozen thermal lattice disorder. The distribution is characterized by a root mean square displacement and varying this yields different resistivities and sfd lengths. We use the relationship between the resulting resistivities and sfd lengths to determine how generally the Elliott-Yafet relationship is obeyed and whether there are any trends across the d series. |
Wednesday, March 7, 2018 9:24AM - 9:36AM |
K19.00006: Inverse Magneto-Caloric Effect at the Spin Reorientation of Fe2B Alloys Doped with Co Kelly Neubauer, Peter Klavins, Jackson Badger, Valentin Taufour The magneto-caloric effect is usually quantified by a negative variation of entropy when applying field at constant temperature. An inverse effect can also be observed near spin reorientations. We describe the solution growth method used for the synthesis of single crystals of Fe2B alloys with Co substitutions. We can control the spin reorientation temperature using Co doping. The transition occurs in the range 300-0 K for substitutions in the range 11-13%. We report on an inverse magneto-caloric effect at this transition. |
Wednesday, March 7, 2018 9:36AM - 9:48AM |
K19.00007: Spin accumulation on Cu driven by ultrafast demagnetization of Fe, Co, and Ni Im-Hyuk Shin, Byoung-Chul Min, Byeong-Kwon Ju, Gyung-Min Choi Generation of spin currents out of ferromagnets is an essential requirement for spintronics. Recently, measurement of dynamics of magnetization of ferromagnets revealed the existence of ultrafast spin currents during ultrafast demagnetization of metallic ferromagnets. Here, we report direct measurements of transient spin accumulation on a non-magnetic Cu layer driven by ultrafast demagnetization of Fe, Co, and Ni layers. Dynamic behaviors of spin accumulation explicitly shows a close correlation between demagnetization of a ferromagnet and spin accumulation on a non-magnet. Magnitudes of spin accumulation have increased by an order magnitude compared to previous results with [Co/Pt] and [Co/Ni] multilayers. |
Wednesday, March 7, 2018 9:48AM - 10:24AM |
K19.00008: Spin Current Generation by a Surface Acoustic Wave Injection Invited Speaker: Yukio Nozaki Spin angular momentum which is one of the degrees of freedom in electron can be mutually converted with a mechanical rotation according to the conservation low of angular momentum. The conversion from the spin angular momentum to the macroscopic rotation was experimentally demonstrated in ferromagnetic bodies by Einstein and De Haas while Barnett succeeded its inverse conversion. Very recently, from the analytical solution of the Dirac equation with a general covariance, Matsuo et al. theoretically predicted that the same kind of mutual conversion can be realized for free electrons in non-magnetic metals with a weak spin orbital coupling. We demonstrated a conversion of alternating spin current (SC) from a macroscopic rotation generated by surface acoustic wave (SAW) which propagates in a NiFe / Cu bilayer deposited on a LiNbO3 substrate. A ferromagnetic resonance (FMR) excited in the NiFe layer was successfully observed when the fundamental frequency of SAW matched with the FMR frequency. The strength of FMR excitation was strongly suppressed when the Cu layer was removed from the bilayer or an insulating SiO2 layer was inserted in the interface of the bilayer. Furthermore, a decrease in the saturation magentization leads to an increase in the microwave absorption owing to the FMR excitation. These are the clear evidence that the alternating SC generated in Cu layer via spin-rotation coupling (SRC) plays an important role for the FMR excitation. The angular dependence of the strength of FMR excitation quantitatively supports the successful generation of alternating SC using SAW via SRC. Our experimental result will open the way to generate an alternating SC in variety of SAW devices without using ferromagnets and/or nonmagnetic materials with large spin-orbital coupling. |
Wednesday, March 7, 2018 10:24AM - 10:36AM |
K19.00009: An AC technique to directly measure magnetic field induced adiabatic temperature change Prakash Giri, Christian Binek Because magnetic refrigeration might become an attractive alternative to conventional compression refrigeration, there is growing interest in studying equilibrium and AC aspects of the Magneto-Caloric Effect (MCE). We present a dynamic and direct measurement of adiabatic temperature change and compare it with the conventional but indirect DC magnetometric approach utilizing Maxwell’s relation. We use far infra-red detection technique to measure the periodic change in temperature in the archetypical magnetocaloric Gadolinium caused by the application of a low frequency AC magnetic field. For comparison we use SQUID magnetometry to determine isothermal entropy and adiabatic temperature change. The AC method provides additional information about the magnetic properties of the magnetocaloric material such as the temperature dependence of the low frequency magnetic susceptibility. In addition it reveals non-equilibrium effects which are relevant when applying the MCE in a cycling cooling device. |
Wednesday, March 7, 2018 10:36AM - 10:48AM |
K19.00010: Phonon Thermal Edelstein Effect Masato Hamada, Emi Minamitani, Motoaki Hirayama, Shuichi Murakami Spin-rotation coupling in crystals enables us to convert between spin current and mechanical rotations. In this presentation, we focus on the angular momentum of phonons. When the temperature gradient is applied to nonmagnetic crystals without inversion symmetry, the phonon angular momentum is proportional to temperature gradient. This mechanism is analogous to the Edelstein effect in electronic systems. This effect requires crystals with sufficiently low crystallographic symmetries. We calculate the phonon angular momentum generated by temperature gradient in wurtzite-GaN and tellurium by first-principle calculation. Moreover we propose that the phonon angular momentum is converted to a rigid-body rotation of crystals and the magnetization. On the other hand, in magnetic crystals, we discuss the dependence of the phonon angular momentum on magnetic symmetries. |
Wednesday, March 7, 2018 10:48AM - 11:00AM |
K19.00011: Orbital Edelstein effect as a condensed-matter analog of solenoid Shuichi Murakami, Taiki Yoda, Takehito Yokoyama We theoretically study current-induced orbital magnetization in a chiral crystal. This phenomenon is an orbital version of the Edelstein effect. We propose an analogy between the current-induced orbital magnetization and an Ampere field in a solenoid in classical electrodynamics. In order to quantify this effect, we define a dimensionless parameter from the response coefficient relating a current density with an orbital magnetization. This dimensionless parameter can be regarded as a number of turns within a unit cell when the crystal is regarded as a solenoid, and it represents how ``chiral'' the crystal is. By focusing on the dimensionless parameter, one can design band structure which realizes induction of large orbital magnetization. In particular, a Weyl semimetal with all the Weyl nodes close to the Fermi energy can have a large value of this dimensionless parameter, which can exceed that of a classical solenoid. [1] T. Yoda, T. Yokoyama, and S. Murakami, Sci. Rep. 5, 12024 (2015). [2] T. Yoda, T. Yokoyama, and S. Murakami, arXiv:1706.07702. |
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