H-T Phase Diagram of Rare-Earth -- Transition Metal Alloy in the Vicinity of the Compensation Point

Anomalous hysteresis loops of ferrimagnetic amorphous alloys in high magnetic field and in the vicinity of the compensation temperature have so far been explained by sample inhomogeneities. We obtain H-T magnetic phase diagram for ferrimagnetic GdFeCo alloy using a two-sublattice model in the paramagnetic rare-earth ion approximation and taking into account rare-earth (Gd) magnetic anisotropy. It is shown that if the magnetic anisotropy of the $f$-sublattice is larger than that of the $d$-sublattice, the tricritical point can be at higher temperature than the compensation point. The obtained phase diagram explains the observed anomalous hysteresis loops as a result of high-field magnetic phase transition, the order of which changes with temperature. It also implies that in the vicinity of the magnetic compensation point the shape of magnetic hysteresis loop is strongly temperature dependent.Comment: 8 pages, 3 figure

Laser induced THz emission from femtosecond photocurrents in Co/ZnO/Pt and Co/Cu/Pt multilayers

The ultrashort laser excitation of Co/Pt magnetic heterostructures can effectively generate spin and charge currents at the interfaces between magnetic and nonmagnetic layers. The direction of these photocurrents can be controlled by the helicity of the circularly polarized laser light and an external magnetic field. Here, we employ THz time-domain spectroscopy to investigate further the role of interfaces in these photo-galvanic phenomena. In particular, the effects of either Cu or ZnO interlayers on the photocurrents in Co/X/Pt (X = Cu, ZnO) have been studied by varying the thickness of the interlayers up to 5 nm. The results are discussed in terms of spin-diffusion phenomena and interfacial spin-orbit torque.Comment: 15 pages, 6 figures, 2 table

Selection Rules for All-Optical Magnetic Recording in Iron Garnet

Finding an electronic transition a subtle excitation of which can launch dramatic changes of electric, optical or magnetic properties of media is one of the long-standing dreams in the field of photo-induced phase transitions [1-5]. Therefore the discovery of the magnetization switching only by a femtosecond laser pulse [6-10] triggered intense discussions about mechanisms responsible for these laser-induced changes. Here we report the experimentally revealed selection rules on polarization and wavelengths of ultrafast photo-magnetic recording in Co-doped garnet film and identify the workspace of the parameters (magnetic damping, wavelength and polarization of light) allowing this effect. The all-optical magnetic switching under both single pulse and multiple-pulse sequences can be achieved at room temperature, in narrow spectral ranges with light polarized either along or crystallographic axes of the garnet. The revealed selection rules indicate that the excitations responsible for the coupling of light to spins are d-electron transitions in octahedral and tetrahedral Co-sublattices, respectively

Laser-driven quantum magnonics and THz dynamics of the order parameter in antiferromagnets

The impulsive generation of two-magnon modes in antiferromagnets by femtosecond optical pulses, so-called femto-nanomagnons, leads to coherent longitudinal oscillations of the antiferromagnetic order parameter that cannot be described by a thermodynamic Landau-Lifshitz approach. We argue that this dynamics is triggered as a result of a laser-induced modification of the exchange interaction. In order to describe the oscillations we have formulated a quantum mechanical description in terms of magnon pair operators and coherent states. Such an approach allowed us to} derive an effective macroscopic equation of motion for the temporal evolution of the antiferromagnetic order parameter. An implication of the latter is that the photo-induced spin dynamics represents a macroscopic entanglement of pairs of magnons with femtosecond period and nanometer wavelength. By performing magneto-optical pump-probe experiments with 10 femtosecond resolution in the cubic KNiF$_3$ and the uniaxial K$_2$NiF$_4$ collinear Heisenberg antiferromagnets, we observed coherent oscillations at the frequency of 22 THz and 16 THz, respectively. The detected frequencies as a function of the temperature ideally fit the two-magnon excitation up to the N\'eel point. The experimental signals are described as dynamics of magnetic linear dichroism due to longitudinal oscillations of the antiferromagnetic vector.Comment: 25 pages, 10 figure

Meson model for f_0(980) production in peripheral pion-nucleon reactions

The Juelich model for pion-pion-scattering, based on an effective meson-meson Lagrangian is applied to the analysis of the S-wave production amplitudes derived from the BNL E852 experiment pi^- p -> pi^0 pi^0 n for a pion momentum of 18.3 GeV. The unexpected strong dependence of the S-wave partial wave amplitude on the momentum transfer between the proton and neutron in the vicinity of the f_0(980) resonance is explained in our analysis as interference effect between the correlated and uncorrelated pi^0 pi^0 pairs.Comment: 6 pages, 7 figures, formulas added, typos removed, new figure

Macrospin dynamics in antiferromagnets triggered by sub-20 femtosecond injection of nanomagnons

The understanding of how the sub-nanoscale exchange interaction evolves in macroscale correlations and ordered phases of matter, such as magnetism and superconductivity, requires to bridging the quantum and classical worlds. This monumental challenge has so far only been achieved for systems close to their thermodynamical equilibrium. Here we follow in real time the ultrafast dynamics of the macroscale magnetic order parameter in the Heisenberg antiferromagnet KNiF 3 triggered by the impulsive optical generation of spin excitations with the shortest possible nanometre wavelength and femtosecond period. Our magneto-optical pump-probe experiments also demonstrate the coherent manipulation of the phase and amplitude of these femtosecond nanomagnons, whose frequencies are defined by the exchange energy. These findings open up opportunities for fundamental research on the role of short-wavelength spin excitations in magnetism and strongly correlated materials; they also suggest that nanospintronics and nanomagnonics can employ coherently controllable spin waves with frequencies in the 20 THz domain

Sub-picosecond exchange-relaxation in the compensated ferrimagnet Mn2_2Rux_xGa

We study the demagnetization dynamics of the fully compensated half-metallic ferrimagnet Mn$_2$Ru$_x$Ga. While the two antiferromagnetically coupled sublattices are both composed of manganese, they exhibit different temperature dependencies due to their differing local environments. The sublattice magnetization dynamics triggered by femtosecond laser pulses are studied to reveal the roles played by the spin and intersublattice exchange. We find a two-step demagnetization process, similar to the well-established case of Gd(FeCo)$_3$, where the two Mn-sublattices have different demagnetization rates. The behaviour is analysed using a four-temperature model, assigning different temperatures to the two manganese spin baths. Even in this strongly exchange-coupled system, the two spin reservoirs have considerably different behaviour. The half-metallic nature and strong exchange coupling of Mn$_2$Ru$_x$Ga lead to spin angular momentum conservation at much shorter time scales than found for Gd(FeCo)$_3$ which suggests that low-power, sub-picosecond switching of the net moment of Mn$_2$Ru$_x$Ga is possible.Comment: 5 pages, 3 figures, J. Phys.: Condens. Matter (2021

Femtosecond control of electric currents at the interfaces of metallic ferromagnetic heterostructures

The idea to utilize not only the charge but also the spin of electrons in the operation of electronic devices has led to the development of spintronics, causing a revolution in how information is stored and processed. A novel advancement would be to develop ultrafast spintronics using femtosecond laser pulses. Employing terahertz (10$^{12}$ Hz) emission spectroscopy, we demonstrate optical generation of spin-polarized electric currents at the interfaces of metallic ferromagnetic heterostructures at the femtosecond timescale. The direction of the photocurrent is controlled by the helicity of the circularly polarized light. These results open up new opportunities for realizing spintronics in the unprecedented terahertz regime and provide new insights in all-optical control of magnetism.Comment: 3 figures and 2 tables in the main tex

Optical determination of the Néel vector in a CuMnAs thin-film antiferromagnet

Recent breakthroughs in electrical detection and manipulation of antiferromagnets have opened a new avenue in the research of non-volatile spintronic devices.1-10 Antiparallel spin sublattices in antiferromagnets, producing zero dipolar fields, lead to the insensitivity to magnetic field perturbations, multi-level stability, ultrafast spin dynamics and other favorable characteristics which may find utility in fields ranging from magnetic memories to optical signal processing. However, the absence of a net magnetic moment and the ultra-short magnetization dynamics timescales make antiferromagnets notoriously difficult to study by common magnetometers or magnetic resonance techniques. In this paper we demonstrate the experimental determination of the Néel vector in a thin film of antiferromagnetic CuMnAs9,10 which is the prominent material used in the first realization of antiferromagnetic memory chips.10 We employ a femtosecond pump-probe magneto-optical experiment based on magnetic linear dichroism. This table-top optical method is considerably more accessible than the traditionally employed large scale facility techniques like neutron diffraction11 and Xray magnetic dichroism measurements.12-14 This optical technique allows an unambiguous direct determination of the Néel vector orientation in thin antiferromagnetic films utilized in devices directly from measured data without fitting to a theoretical model