Rare Earth-transition Metal Alloys Research Articles

Magneto-optic Kerr effect in a spin-polarized zero-moment ferrimagnet

The magneto-optical Kerr effect (MOKE) is often assumed to be proportional to the magnetisation of a magnetically ordered metallic sample; in metallic ferrimagnets with chemically distinct sublattices, such as rare-earth transition-metal alloys, it depends on the difference between the sublattice contributions. Here we show that in a highly spin polarized, fully compensated ferrimagnet, where the sublattices are chemically similar, MOKE is observed even when the net moment is strictly zero. We analyse the spectral ellipsometry and MOKE of Mn 2 Ru x Ga, and show that this behaviour is due to a highly spin-polarized conduction band dominated by one of the two manganese sublattices which creates helicity-dependent reflectivity determined by a broad Drude tail. Our findings open new prospects for studying spin dynamics in the infra-red.

Size induced large upward shift of magnetic transition temperatures in nanowires of rare-earth transition metal alloy GdxCo1−x(x = 0.4)

It has been observed that in nanowires (diameter ∼ 100 nm) of a rare-earth (RE) transition metal (TM) alloy GdxCo1−x(x = 0.4) the magnetic transition temperatures like the magnetization compensation temperatures (Tcm) can be shifted upward by nearly 350 K compared to that of the bulk and be made as high as 850 K. The saturation magnetization (MS) remains high over the whole temperature range compared to that the bulk and at room temperature the enhancement in MS is 33%. This enhances the applicability of the RE-TM alloy to well above the room temperature. The magnetic experiments were carried out in the temperature range 80 K–1100 K on the alloy nanowires grown by electrochemical synthesis in the nanopores of Anodic Aluminium Oxide (AAO) templates. It has been established that the large shift in the Tcm is linked to qualitative change in the nature of the anisotropy energy EA on size reduction, in particular in the temperature derivative dEA/dT that reaches more or less temperature independent value in the nanowires.

Spatially resolved investigation of all optical magnetization switching in TbFe alloys

Optical control of magnetization using femtosecond laser without applying any external magnetic field offers the advantage of switching magnetic states at ultrashort time scales. Recently, all-optical helicity-dependent switching (AO-HDS) has drawn a significant attention for potential information and data storage device applications. In this work, we employ element and magnetization sensitive photoemission electron microscopy (PEEM) to investigate the role of heating in AO-HDS for thin films of the rare-earth transition-metal alloy TbFe. Spatially resolved measurements in a 3–5 μm sized stationary laser spot demonstrate that AO-HDS is a local phenomenon in the vicinity of thermal demagnetization in a ‘ring’ shaped region. The efficiency of AO-HDS further depends on a local temperature profile around the demagnetized region and thermally activated domain wall motion. We also demonstrate that the thickness of the film determines the preferential switching direction for a particular helicity.

Ultrafast dynamics in ferrimagnetic materials with a quantum Monte Carlo atomistic model

We study of the ultrafast dynamics of the atomic angular momentum in ferrimagnets irradiated by laser pulses. My apply a quantum atomistic spin approach based on the Monte Carlo technique. Our model describes the coherent transfer of angular momentum between the spin and the orbital momentum as well as the quenching of the orbital momentum induced by the lattice field. The Elliott-Yafet collision mechanism is also included. We focus on elementary mechanisms that lead to the dissipation of the total angular momentum in a rare earth-transition metal (RE-TM) alloy in which the two sublattices have opposite spin orientation. Our model shows that the observed ultrafast quenching of the magnetization can be explained microscopically by the transfer of spin between the sublattices and by the quenching of the localized orbital angular momentum.

Increasing de-/hydriding capacity and equilibrium pressure by designing non-stoichiometry in Al-substituted YFe2 compounds

Abstract Non-stoichiometric YFe 1.5 Al 0.5-x , YFe 1.6 Al 0.4-x and YFe 1.7+x Al 0.3 ( x = 0.1 and 0.2) compounds with single C15 Laves phase were successfully prepared. Compared with stoichiometric compounds, enhanced hydrogenation capacity and hydrogen absorption rate were obtained in hypo-stoichiometric compounds, and increased dehydrogenation equilibrium pressure was achieved for hyper-stoichiometric compounds. The YFe 1.6 Al 0.2 compound shows the maximum reversible absorption capacity of 3.04 H/f.u. (1.63 wt.%), which is 18% higher than that of YFe 1.5 Al 0.5 . Faster hydrogenation kinetics could be obtained in all of the hypo-stoichiometric samples, completing hydrogenation within 300 s at 100 °C. The YFe 1.7+x Al 0.3 compounds showed elevated desorption equilibrium pressure, indicating higher dehydrogenation capacity than that of stoichiometric compounds. The present work demonstrates the positive effect of non-stoichiometry on the Y-Fe-Al ternary compounds and plays a guiding role for improving the hydrogen storage properties of AB 2 -type rare earth-transition metal alloys.

Laser-initiated magnetization reversal and correlated morphological effects visualized within situFresnel transmission electron microscopy

Laser-initiated switching of magnetization direction in ferrimagnetic rare-earth--transition-metal (RE-TM) alloys---whether laser induced or photothermal via compensation point---is being vigorously pursued owing to the promise of extending operating frequencies of magnetic devices into the terahertz regime. Despite intense interest, however, the effects of repeated laser exposure on the film structure and subsequent switching behavior have yet to be investigated. In order to better understand the correlated effects of femtosecond-laser irradiation on both the magnetic response and photoinduced morphological variations of RE-TM alloys, we performed in situ Fresnel transmission electron microscopy (TEM) on $\mathrm{T}{\mathrm{b}}_{23}\mathrm{C}{\mathrm{o}}_{77}$ thin films with Ta protecting layers. Via optical access to the specimen in a modified TEM, we irradiated the thin films in situ with both individual and series of femtosecond optical pulses, and correlated laser-induced changes in magnetic domain-wall formation and growth with photothermal crystal formation and accompanying pinned magnetic sites. We find that, for a range of applied laser fluences and numbers of individual pulses, several distinct regions are formed displaying varied magnetic behavior (switchable, nonswitchable, demagnetized) and morphological features (small-to-large crystal-grain variations). Through a series of systematic studies, we quantified these linked magnetic and morphological properties as a function of laser fluence, number of pulse-train cycles, and number of individual femtosecond-laser pulses and the duration between each. Our results show how the sensitive connection between magnetic behavior and morphological structure can emerge in magneto-optic experiments across several parameters, thus illustrating the need for rigorous characterization so that potential operating regimes may be universally identified.

Reversible hydriding in YFe2-xAlx (x = 0.3, 0.5, 0.7) intermetallic compounds

To restrain the irreversible disproportionating reaction of YFe2 phase in the hydrogenation process, the structure of YFe2 was modified by partly substituting Fe with Al. And then, the hydrogen storage properties of YFe2-xAlx (x = 0.3, 0.5, 0.7) alloys were systematically investigated. It was found that the C15 Laves phase structure of those three Y-Fe-Al alloys kept stable at different states of as-annealed, hydrogenated and dehydrogenated, but their lattice constants vary with the Al and hydrogen contents. The YFe1.7Al0.3 alloy shows maximum reversible hydrogen storage capacity of 1.38 wt%, which is lower than its initial hydrogen absorption capacity of 1.56 wt%. All the three Y-Fe-Al alloys show fast hydrogenation rate at relative low temperature of 100 °C, however, their dehydrogenation properties are still limited due to their rather low desorption equilibrium pressure. The present finding provides a possibility of developing new types of rare earth-transition metal alloys for hydrogen storage.

Magnetic spin moment reduction in photoexcited ferromagnets through exchange interaction quenching: beyond the rigid band approximation

The exchange interaction among electrons is one of the most fundamental quantum mechanical interactions in nature and underlies any magnetic phenomena from ferromagnetic ordering to magnetic storage. The current technology is built upon a thermal or magnetic field, but a frontier is emerging to directly control magnetism using ultrashort laser pulses. However, little is known about the fate of the exchange interaction. Here we report unambiguously that photoexcitation is capable of quenching the exchange interaction in all three 3d ferromagnetic metals. The entire process starts with a small number of photoexcited electrons which build up a new and self-destructive potential that collapses the system into a new state with a reduced exchange splitting. The spin moment reduction follows a Bloch-like law as , where ΔE is the absorbed photon energy and β is a scaling exponent. A good agreement is found between the experimental and our theoretical results. Our findings may have a broader implication for dynamic electron correlation effects in laser-excited iron-based superconductors, iron borate, rare-earth orthoferrites, hematites and rare-earth transition metal alloys.

Low-remanence criterion for helicity-dependent all-optical magnetic switching in ferrimagnets

We demonstrate that a low-remanent sample magnetization ${M}_{\mathrm{R}}$ is crucial for all-optical magnetic switching (AOS) in ferrimagnets and ferrimagnet heterostructures. ${M}_{\mathrm{R}}$ may be devised by the composition of the material. However, it can also be controlled in situ by changing the sample temperature because it affects ${M}_{\mathrm{R}}$. We show that increasing the lattice temperature by laser pulses or a simple heating resistor enables AOS in a ferrimagnetic Tb-Fe film, which does not exhibit AOS at room temperature. We reconcile earlier contradicting results for AOS in heated and cooled magnetic films by applying the low-remanence criterion. It also applies to other existing rare-earth transition-metal (RE-TM) alloys, such as GdFeCo or Tb-Co, to ferrimagnet heterostructures, and to RE-free synthetic ferrimagnets.

Determination of Crystallographic Phase and Estimation of Order Degree for Rare Earth-Transition Metal Alloy Films with Hexagonal Structures

Determination of Crystallographic Phase and Estimation of Order Degree for Rare Earth-Transition Metal Alloy Films with Hexagonal Structures

Dependence of all-optical magnetic switching on the sublattice magnetization orientation in Tb-Fe thin films

We demonstrate that the direction of all-optical switching (AOS) in rare-earth transition-metal (RE-TM) alloy Tb-Fe thin films depends on the orientation of the sublattice magnetization and not on the direction of the resulting net magnetization. For this purpose, we investigated the AOS ability for a sample dominated by the Fe sublattice magnetization (Tb24Fe76) and another dominated by the Tb sublattice (Tb30Fe70). This finding of the sublattice dependence of AOS contributes to the understanding of switching in RE-TM multilayered thin films and heterostructures.

Controlling the polarity of the transient ferromagneticlike state in ferrimagnets

After the application of an ultrashort laser pulse, the antiferromagnetic alignment in rare earth-transition metal alloys can temporarily become ferromagnetic with the rare-earth polarity. Proposed models merely describe this effect, without showing the route for its manipulation. Here we use extensive atomistic spin model simulations and micromagnetic theory for ferrimagnets at elevated temperatures to predict that the polarity of this transient ferromagnetic-like state can be controlled by initial temperature. We show that this arises because the magnetic response of each lattice has a different temperature dependence, at low temperatures the transition metal responds faster than the rare earth, while at high temperatures this role is interchanged. Our findings contribute to the physical understanding and control of this state and thus open new perspectives for its use in ultrafast magnetic devices.

Effect of damping on the laser induced ultrafast switching in rare earth-transition metal alloys

In this paper, we present simulations of thermally induced magnetic switching in ferrimagnetic systems performed with a Landau-Lifshitz-Bloch (LLB) equation for damping constant in a wide range of values. We have systematically studied the GdFeCo ferrimagnet with various concentrations of Gd and compared for some values of parameters the LLB results with atomistic simulations. The agreement is remarkably good, which shows that the dynamics described by the ferrimagnetic LLB is a reasonable approximation of this complex physical phenomenon. As an important element, we show that the LLB is able to also describe the intermediate formation of a ferromagnetic state which seems to be essential to understand laser induced ultrafast switching. The study reveals the fundamental role of damping during the switching process.

Subpicosecond magnetization dynamics in TbCo alloys

Since the discovery of all-optical magnetization switching in rare-earth transition-metal alloys the underlying magnetization dynamics of multisublattice magnets has become a hot topic of modern magnetism. We studied the ultrafast magnetization dynamics in TbCo alloys as a function of the alloy composition and the laser fluence using either 800 nm or 400 nm probe pulses. Direct comparison between TbCo samples with different compositions for equal excitation conditions demonstrates that the magnetization dynamics of the Co sublattice strongly depends on the Tb concentration. For ${\mathrm{Tb}}_{32}{\mathrm{Co}}_{68}$ the magnetization of the sublattices can even transiently be reversed on a subpicosecond time scale.

Engineered materials for all-optical helicity-dependent magnetic switching

The possibility of manipulating magnetic systems without applied magnetic fields have attracted growing attention over the past fifteen years. The low-power manipulation of the magnetization, preferably at ultrashort timescales, has become a fundamental challenge with implications for future magnetic information memory and storage technologies. Here we explore the optical manipulation of the magnetization in engineered magnetic materials. We demonstrate that all-optical helicity-dependent switching (AO-HDS) can be observed not only in selected rare earth-transition metal (RE-TM) alloy films but also in a much broader variety of materials, including RE-TM alloys, multilayers and heterostructures. We further show that RE-free Co-Ir-based synthetic ferrimagnetic heterostructures designed to mimic the magnetic properties of RE-TM alloys also exhibit AO-HDS. These results challenge present theories of AO-HDS and provide a pathway to engineering materials for future applications based on all-optical control of magnetic order.

Temperature Dependence of All-Optical Ultrafast Magnetization Switching in TbFeCo

Using a time-resolved pump-probe system, we experimentally demonstrated that all-optical magnetization switching in TbFeCo can be observed at both room temperature and a low temperature of 110 K. This switching can only be triggered by a circularly polarized pump beam within a very narrow range of pulse energies. We believe these findings will lead to a better understanding of optically induced magnetization reversal on rare-earth transition metal alloys and provide an ultrafast and efficient way for future magnetic recording technique.

Highly efficient all-optical switching of magnetization in GdFeCo microstructures by interference-enhanced absorption of light

Employing magnetization-sensitive microscopy techniques, we address the light-induced magnetization dynamics of patterned samples of rare-earth transition metal alloys. We experimentally find that smaller structures require a lower energy density to undergo all-optical switching of the magnetization. With the aid of simulations we explain this reduction in terms of enhanced light absorption by interference of the light within the structure. This results in a decrease of about 60% in the energy densities required for all-optical switching compared with those in continuous thin films of the same alloy. Moreover, we envisage that an energy lower than 10 fJ should be sufficient to switch a 20 × 20 nm 2 structure.

Influence of Temperature on Structure and Magnetic Properties of Exchange Coupled TbCo/FeNi Bilayers

Among amorphous films of rare earth-transition metal (RE-TM) alloys as exchange-biasing layers in magnetoresistive heads and spin-valve sensors, the amorphous Tb-Co films have most high practical potential. In the present work the influence of the temperature and the heat treatment parameters on the structure and magnetic properties was studied for exchange bias FeNi/Tb35Co65 bilayers annealed in vacuum or a nitrogen flow. A simple explanation of the dependence of the magnetic properties on the temperature and the heat treatment parameters connected with structural changes in each one of the layers was proposed.

Contributions of electron and phonon transport to the thermal conductivity of GdFeCo and TbFeCo amorphous rare-earth transition-metal alloys

We experimentally investigate the electron and phonon contributions to the thermal conductivity of amorphous GdFeCo and TbFeCo thin films. These amorphous rare-earth transition-metal (RE-TM) alloys exhibit thermal conductivities that increase nearly linearly with temperature from 90 to 375 K. Electrical resistivity measurements show that this trend is due to an increase in the electron thermal conductivity over this temperature range and a relatively constant phonon contribution to thermal conductivity. We find that at low temperatures (∼90 K), the phonon systems in these amorphous RE-TM alloys contribute ∼70% to thermal conduction with a decreasing contribution as temperature is increased.

Transport spin polarization of the rare-earth transition-metal alloy Co1−xGdx

We present point-contact Andreev reflection (PCAR) measurements of the transport spin polarization of ferrimagnetic Co${}_{1\ensuremath{-}x}$Gd${}_{x}$ alloys. At the compensation point of the alloy, where the magnetization of the independent Co and Gd subnetworks are equal and the net magnetization of the alloy is close to zero, there remains a finite transport spin polarization. The composition dependence of the transport spin polarization measured by PCAR is quite different from that of the tunneling spin polarization.