Spin-wave coupling to electromagnetic cavity fields in dysposium ferrite

Coupling of spin-waves with electromagnetic cavity field is demonstrated in an antiferromagnet, dysprosium ferrite (DyFeO3). By measuring transmission at 0.2-0.35 THz and sweeping sample temperature, magnon-photon coupling signatures were found at crossings of spin-wave resonances with Fabry-Perot cavity modes formed in samples. The obtained spectra are explained in terms of classical electrodynamics and a microscopic model.Comment: 3 pages, 2 figure

Low temperature hopping magnetotransport in paramagnetic single crystals of cobalt doped ZnO

Long needle-shaped single crystals of Zn1-xCoxO were grown at low temperatures using a molten salt solvent technique, up to x=0.10. The conduction process at low temperatures is determined to be by Mott variable range hopping. Both pristine and cobalt doped crystals clearly exhibit a crossover from negative to positive magnetoresistance as the temperature is decreased. The positive magnetoresistance of the Zn1-xCoxO single crystals increases with increased Co concentration and reaches up to 20% at low temperatures (2.5 K) and high fields (>1 T). SQUID magnetometry confirms that the Zn1-xCoxO crystals are predominantly paramagnetic in nature and the magnetic response is independent of Co concentration. The results indicate that cobalt doping of single crystalline ZnO introduces localized electronic states and isolated Co2+ ions into the host matrix, but that the magnetotransport and magnetic properties are decoupled.Comment: 7 pages, 9 figures, submitted to Physical Review

Thermodynamics of continuous media with electromagnetic fields

The thermodynamics of an electrically charged, multicomponent continuous medium with electromagnetic fields is analysed in the non-relativistic limit. Applying locally the first and second law of thermodynamics and Maxwell's equations for a linear theory of electromagnetism, three equations characterising the continuous medium are derived: a thermostatic equilibrium equation, a reversible and an irreversible thermodynamic evolution equation. For a local thermodynamic equilibrium, explicit expressions for the temperature and the chemical potentials in terms of the electromagnetic fields are obtained. The linear phenomenological relations describe novel effects of non-uniform electromagnetic fields on the transport equations and account also for magnetoresistance and optical tweezer

The phase transition in the localized ferromagnet EuO probed by muSR

We report results of muon spin rotation measurements performed on the ferromagnetic semiconductor EuO, which is one of the best approximations to a localized ferromagnet. We argue that implanted muons are sensitive to the internal field primarily through a combination of hyperfine and Lorentz fields. The temperature dependences of the internal field and the relaxation rate have been measured and are compared with previous theoretical predictions.Comment: 4 pages, 4 figure

Effect of oxidation of cobalt-based nanowires on NMR spin-lattice relaxation

Nuclear spin-lattice relaxation measurements were performed on Co-based magnetic nanowires, in zero field and in 1 T applied field. A measurement method is developped that allows a confident interpretation of the experimental data. An enhanced relaxation due to a thermally activated phenomenon associated to oxidation is reported for the first time in the nuclear magnetic resonance of ferromagnetic system

Antiferromagnetic resonance in α\alpha-Fe2_2O3_3 up to its N\'eel temperature

Hematite ($\alpha$-Fe$_2$O$_3$) is an antiferromagnetic material with a very low spin damping and high N\'eel temperature. The temperature dependence of the antiferromagnetic resonance in a bulk single crystal of hematite was characterized from room temperature up to the N\'eel temperature in the frequency range of 0.19-0.5 THz. From these data, the N\'eel temperature was estimated as 966 K

Cavity-mediated coupling of antiferromagnetic spin waves

Coupling of space-separated oscillators is interesting for quantum and communication technologies. In this work, it is shown that two antiferromagnetic oscillators placed inside an electromagnetic cavity couple cooperatively to its terahertz modes and, in effect, hybridized magnon-polariton modes are formed. This is supported by a systematic study of reflection spectra from two parallel-plane slabs of hematite ($\alpha$-Fe$_2$O$_3$), measured as a function of their temperatures and separation distance, and modeled theoretically. The mediating cavity was formed by the crystals themselves and the experiment was performed in a practical distance range of a few millimetres and above room temperature. Cavity-mediated coupling allows for engineering of complex resonators controlled by their geometry and by sharing properties of their components

Establishing the fundamental magnetic interactions in the chiral skyrmionic Mott insulator Cu2OSeO3 by terahertz electron spin resonance

The recent discovery of skyrmions in Cu$_2$OSeO$_3$ has established a new platform to create and manipulate skyrmionic spin textures. We use high-field electron spin resonance (ESR) spectroscopy combining a terahertz free electron laser and pulsed magnetic fields up to 64 T to probe and quantify its microscopic spin-spin interactions. Besides providing direct access to the long-wavelength Goldstone mode, this technique probes also the high-energy part of the excitation spectrum which is inaccessible by standard low-frequency ESR. Fitting the behavior of the observed modes in magnetic field to a theoretical framework establishes experimentally that the fundamental magnetic building blocks of this skyrmionic magnet are rigid, highly entangled and weakly coupled tetrahedra.Comment: 5 pages, 3 Figure

Current-induced two-level fluctuations in pseudo spin-valves (Co/Cu/Co) nanostructures

Two-level fluctuations of the magnetization state of pseudo spin-valve pillars Co(10 nm)/Cu(10 nm)/Co(30 nm) embedded in electrodeposited nanowires (~40 nm in diameter, 6000 nm in length) are triggered by spin-polarized currents of 10^7 A/cm^2 at room temperature. The statistical properties of the residence times in the parallel and antiparallel magnetization states reveal two effects with qualitatively different dependences on current intensity. The current appears to have the effect of a field determined as the bias field required to equalize these times. The bias field changes sign when the current polarity is reversed. At this field, the effect of a current density of 10^7 A/cm^2 is to lower the mean time for switching down to the microsecond range. This effect is independent of the sign of the current and is interpreted in terms of an effective temperature for the magnetization.Comment: 4 pages, 5 figures, revised version, to be published in Phys. Rev. Let

Orbital domain state and finite size scaling in ferromagnetic insulating manganites

55Mn and 139La NMR measurements on a high quality single crystal of ferromagnetic (FM) La0.80Ca20MnO3 demonstrate the formation of localized Mn(3+,4+) states below 70 K, accompanied with strong anomalous increase of certain FM neutron Bragg peaks. (55,139)(1/T1) spin-lattice relaxation rates diverge on approaching this temperature from below, signalling a genuine phase transition at T(tr) approx. 70 K. The increased local magnetic anisotropy of the low temperature phase, the cooling-rate dependence of the Bragg peaks, and the observed finite size scaling of T(tr) with Ca (hole) doping, are suggestive of freezing into an orbital domain state, precursor to a phase transition into an inhomogeneous orbitally ordered state embodying hole-rich walls.Comment: 4 pages, 4 figure