Terahertz emission spectroscopy of laser-induced spin dynamics in TmFeO3 and ErFeO3 orthoferrites

Copyright © 2014 American Physical SocietyUsing the examples of laser-induced spin-reorientation phase transitions in TmFeO3 and ErFeO3 orthoferrites, we demonstrate that terahertz emission spectroscopy can obtain novel information about ultrafast laser-induced spin dynamics, which is not accessible by more common all-optical methods. The power of the method is evidenced by the fact that, in addition to the expected quasi-ferromagnetic and quasi-antiferromagnetic modes of the iron sublattices, terahertz emission spectroscopy enables detection of a resonance optically excited at an unexpected frequency of ∼0.3–0.35 THz. By recording how the amplitude and phase of the excited oscillations depend on temperature and applied magnetic field, we show that the unexpected mode has all the features of a spin resonance of the Fe3+ ions. We suggest that it can be assigned to transitions between the multiplet sublevels of the 6A1 ground state of the Fe+3 ions occupying rare-earth positions.European Commission's 7th Framework Program (FP7/2007–2013)Engineering and Physical Sciences Research Council (EPSRC)Netherlands Organization for Scientific Research (NWO)Foundation for Fundamental Research on Matter (FOM)ERCRussian GovernmentRFB

THz emission from Co/Pt bilayers with varied roughness, crystal structure, and interface intermixing

Ultrafast demagnetization of Co/Pt heterostructures induced by a femtosecond 800-nm laser pulse launches a spin current from Co to Pt and subsequent conversion of the spin current to a charge current in the Pt layer due to the inverse spin-Hall effect. At the same time, due to the spin-dependent photogalvanic effect, a circularly polarized femtosecond laser pulse also generates a photocurrent at the Co/Pt interface. Both ultrashort photocurrent pulses are effectively detected in a contactless way by measuring the THz radiation they emit. Here we aim to understand how the properties of the Co/Pt interface affect the photocurrents in the bilayers. By varying the interfacial roughness, crystal structure, and interfacial intermixing, as well as having an explicit focus on the cases when THz emissions from these two photocurrents reveal opposite trends, we identify which interface properties play a crucial role for the photocurrents. In particular, we show that by reducing the roughness, the THz emission due to the spin-dependent photogalvanic effect reduces to zero while the strength of the THz emission from the photocurrent associated with the inverse spin-Hall effect increases by a factor of 2. On the other hand, while intermixing strongly enhances the THz emission from the inverse spin-Hall effect by a factor of 4.2, THz emission related to the spin-dependent photogalvanic effect reveals the opposite trend. These findings indicate that microstructural properties of the Co-Pt interface play a decisive role in the generation of photocurrents

A compartive study of the retentive capability of the Sydney mini-screw with 6mm orthodontic anchorage miniscrews in the tibia and femur of New Zealand rabbits by removal torque test

Aim: To investigate the retentive capability of the Sydney Mini-screw with injectable bone cement by removal torque. Method: 16 New Zealand White rabbits were divided evenly into 2 groups, T1 0 week to assess primary stability and T2 8 weeks to test secondary stability. Three groups of miniscrews Sydney Mini-screw with Cement (SMSC) N=12, Sydney Miniscrew without cement (SMS) N=10 and control Aarhus (CA) 6mm screw N=10 were placed randomly and evenly between the right and left tibial and femoral sites. The SMSC and SMS required predrilling of a pilot hole and the SMSC had injectable bone cement PRODENSE. Removal torque was measured and Friedman's Test and two-sample t-test were used for statistical analysis, where appropriate. Results: Removal torque values at T1 for CA, SMS, SMSC were not significantly different (p=0.072) but were significantly different at T2 (p=0.012). Only SMS (p=0.006) showed statistically significant difference between T1 and T2. The different surgical locations at T2 did not statistically differ from each other either (p=0.948). Conclusion: Sydney Miniscrew with and without cement had significantly higher secondary stability and had a trend towards increased primary compared to a normal control miniscrew. More research is required with an increased sample size

Selective Excitation of Terahertz Magnetic and Electric Dipoles in Er3+ Ions by Femtosecond Laser Pulses in ErFeO3

We show that femtosecond laser pulse excitation of the orthoferrite ErFeO3 triggers pico- and subpicosecond dynamics of magnetic and electric dipoles associated with the low energy electronic states of the Er3+ ions. These dynamics are readily revealed by using polarization sensitive terahertz emission spectroscopy. It is shown that by changing the polarization of the femtosecond laser pulse one can excite either electric dipole-active or magnetic dipole-active transitions between the Kramers doublets of the 4I15/2 ground state of the Er3+(4f11) ions. These observations serve as a proof of principle of polarization-selective control of both electric and magnetic degrees of freedom at terahertz frequencies, opening up new vistas for optical manipulation of magnetoelectric materials

Магнитооптическое исследование гольмиевого феррита-граната Ho₃Fe₅O₁₂

Объемный гольмиевый феррит-гранат Ho₃Fe₅O₁₂ является кубическим ферримагнетиком с температурой Кюри TC = 567 К и точкой компенсации намагниченности Tcomp в области 130–140 К. В работе представлены результаты магнитооптического исследования эпитаксиальной пленки гольмиевого ферритаграната Ho₃Fe₅O₁₂ толщиной ~10 мкм, выращенной на подложке типа (111) гадолиний-галлиевого граната Gd₃Ga₅O₁₂. Особенностью данной структуры является близость параметров объемного материала, из которого выращена пленка, и подложки. Изучены температурные и полевые зависимости эффекта Фарадея, а также температурная зависимость доменной структуры в нулевом поле. Установлено, что точка компенсации для данной структуры составляет 127 К. Показано, что температурная зависимость характерного размера доменной структуры имеет расходимость в этой точке. На основании полученных результатов установлено, что магнитная анизотропия этого материала определяется одноосным и кубическим вкладами, причем каждый из них характеризуется различной температурной зависимостью. Сложная форма петель гистерезиса и резкая перестройка доменной структуры при изменении температуры указывают на наличие переходов из коллинеарной в неколлинеарную фазу. Была также исследована генерация второй оптической гармоники под действием лазерных импульсов длительностью 100 фс и центральной энергией фотона E = 1,55 эВ. Электродипольный вклад в генерацию второй гармоники, как кристаллографический, так и магнитный, был надежно зарегистрирован, несмотря на малое рассогласование параметров пленки феррита-граната и подложки.Об’ємний гольмієвий ферит-гранат Ho₃Fe₅O₁₂ є кубічним феримагнетиком з температурою Кюрі TC = 567 К і точкою компенсації намагніченості Tcomp в області 130–140 К. В роботі представлено результати магнітооптичного дослідження епітаксіальної плівки гольмієвого ферит-гранату Ho₃Fe₅O₁₂ з товщиною ∼ 10 мкм, яку вирощено на підкладці типу (111) гадоліній-галієвого гранату Gd₃Ga₅O₁₂. Особливістю цієї структури є близькість параметрів об'ємного матеріалу, з якого вирощено плівку, і підкладки. Вивчено температурні та польові залежності ефекту Фарадея, а також температурна залежність доменної структури в нульовому полі. Встановлено, що точка компенсації для цієї структури складає 127 К. Показано, що температурна залежність характерного розміру доменної структури має розбіжність в цій точці. На підставі отриманих результатів встановлено, що магнітна анізотропія цього матеріалу визначається одновісним і кубічним вкладами, причому кожен з них характеризується різною температурною залежністю. Складна форма петель гістерезису і різка перебудова доменної структури при зміні температури вказують на наявність переходів з колінеарної в неколінеарну фазу. Було також досліджено генерацію другої оптичної гармоніки під дією лазерних імпульсів тривалістю 100 фс і центральною енергією фотона E = 1,55 еВ. Електродипольний вклад в генерацію другої гармоніки, як кристалографічний, так і магнітний, був надійно зареєстрований, незважаючи на мале розузгодження параметрів плівки ферит-гранату та підкладки.Bulk holmium iron garnet Ho₃Fe₅O₁₂ is a cubic ferrimagnet with Curie temperature TC = 567 K and magnetization compensation point between 130–140 K. The magneto-optical data are presented for a holmium iron garnet Ho₃Fe₅O₁₂ epitaxial ~10 μm thick film grown on a (111)-type gadolinium-gallium garnet Gd₃Ga₅O₁₂ substrate. A specific feature of this structure is the closeness of the parameters of the bulk material, from which the film has been grown, to the substrate ones. Temperature and field dependences of the Faraday effect and temperature dependence of the domain structure in zero field are investigated. The compensation point of the structure is found to be Tcomp = = 127 K. It is shown that the temperature dependence of characteristic size of the domain structure diverges at this point. It is established that the magnetic anisotropy of the material is determined by both uniaxial and cubic contributions, each being characterized by different temperature dependence. A complex form of hysteresis loops and a sharp changes of the domain pattern with varying temperature suggest that there occur collinear–noncollinear phase transitions. Study of the optical second harmonic generation was carried out using 100 fs laser pulses with central photon energy E = 1,55 eV. The electric dipole contribution (both crystallographic and magnetic) to the second harmonic generation was observed with high reliability despite a small mismatch of the film and substrate parameters

Ultrafast optical modification of exchange interactions in iron oxides

Ultrafast non-thermal manipulation of magnetization by light relies on either indirect coupling of the electric field component of the light with spins via spin-orbit interaction or direct coupling between the magnetic field component and spins. Here we propose a scenario for coupling between the electric field of light and spins via optical modification of the exchange interaction, one of the strongest quantum effects with strength of 10(3) Tesla. We demonstrate that this isotropic opto-magnetic effect, which can be called inverse magneto-refraction, is allowed in a material of any symmetry. Its existence is corroborated by the experimental observation of terahertz emission by spin resonances optically excited in a broad class of iron oxides with a canted spin configuration. From its strength we estimate that a sub-picosecond modification of the exchange interaction by laser pulses with fluence of about 1 mJ cm(-2) acts as a pulsed effective magnetic field of 0.01 Tesla.European Commission’s 7th Framework Program (FP7/2007–2013)EPSRCNetherlands Organization for Scientific Research (NWO)Foundation for Fundamental Research on Matter (FOM)European Research CouncilRussian Ministry of Education and ScienceRFBR-NSFC projectBureau of International CooperationNSFC projectNSFC-NWOEU Seventh Framework ProgramNWO by a Rubicon grantEuropean Commission (FP7-ICT-2013-613024–GRASP

Laser stimulated THz emission from Pt/CoO/FeCoB

The antiferromagnetic order can mediate a transmission of the spin angular momentum flow, or the spin current, in the form of propagating magnons. In this work, we perform laser stimulated THz emission measurements on Pt/CoO/FeCoB multilayers to investigate the spin current transmission through CoO, an antiferromagnetic insulator, on a picosecond timescale. The results reveal a spin current transmission through CoO with the diffusion length of 3.0 nm. In addition, rotation of the polarization of the emitted THz radiation was observed, suggesting an interaction between the propagating THz magnons and the Néel vector in CoO. Our results not only demonstrate the picosecond magnon spin current transmission but also the picosecond interaction of the THz magnons with the Néel vector in the antiferromagnet

Ultrafast control of magnetic interactions via light-driven phonons

Resonant ultrafast excitation of infrared-active phonons is a powerful technique with which to control the electronic properties of materials that leads to remarkable phenomena such as the light-induced enhancement of superconductivity1,2, switching of ferroelectric polarization3,4 and ultrafast insulator-to-metal transitions5. Here, we show that light-driven phonons can be utilized to coherently manipulate macroscopic magnetic states. Intense mid-infrared electric field pulses tuned to resonance with a phonon mode of the archetypical antiferromagnet DyFeO3 induce ultrafast and long-living changes of the fundamental exchange interaction between rare-earth orbitals and transition metal spins. Non-thermal lattice control of the magnetic exchange, which defines the stability of the macroscopic magnetic state, allows us to perform picosecond coherent switching between competing antiferromagnetic and weakly ferromagnetic spin orders. Our discovery emphasizes the potential of resonant phonon excitation for the manipulation of ferroic order on ultrafast timescales6

An effective magnetic field from optically driven phonons

Light fields at terahertz and mid-infrared frequencies allow for the direct excitation of collective modes in condensed matter, which can be driven to large amplitudes. For example, excitation of the crystal lattice has been shown to stimulate insulator–metal transitions, melt magnetic order or enhance superconductivity. Here, we generalize these ideas and explore the simultaneous excitation of more than one lattice mode, which are driven with controlled relative phases. This nonlinear mode mixing drives rotations as well as displacements of the crystal-field atoms, mimicking the application of a magnetic field and resulting in the excitation of spin precession in the rare-earth orthoferrite ErFeO3. Coherent control of lattice rotations may become applicable to other interesting problems in materials research—for example, as a way to affect the topology of electronic phases

Peculiarities of the stochastic motion in antiferromagnetic nanoparticles

Antiferromagnetic (AFM) materials are widely used in spintronic devices as passive elements (for stabilization of ferromangetic layers) and as active elements (for information coding). In both cases switching between the different AFM states depends in a great extent from the environmental noise. In the present paper we derive the stochastic Langevin equations for an AFM vector and corresponding Fokker-Planck equation for distribution function in the phase space of generalised coordinate and momentum. Thermal noise is modeled by a random delta-correlated magnetic field that interacts with the dynamic magnetisation of AFM particle. We analyse in details a particular case of the collinear compensated AFM in the presence of spin-polarised current. The energy distribution function for normal modes in the vicinity of two equilibrium states (static and stationary) in sub- and super-critical regimes is found. It is shown that the noise-induced dynamics of AFM vector has pecuilarities compared to that of magnetisation vector in ferromagnets.Comment: Submitted to EPJ ST, presented at the 4-th Conference on Statistical Physics, Lviv, Ukraine, 201