J M D Coey

Trinity College Dublin | TCD · School of Physics

Energy-level diagrams for cubic metallic Fe 4 N and Mn 4 N were proposed by Goodenough in the late 1960s. Fe 4 N is ferromagnetic, but Mn 4 N is ferrimagnetic with a large moment on Mn c at the cube corner site and a much smaller antiparallel contribution from Mn f at the three face-centre sites. Neutron diffraction revealed noncollinear ferrimagne...

Compensated ferrimagnets show no net magnetization like antiferromagnets, but their transport and magneto-optic properties resemble those of ferromagnets, thereby creating opportunities for applications in high-frequency spintronics and low energy loss communications. Here we study the modification of the noncollinear triangular ferrimagnetic spin...

Compensated ferrimagnets, like antiferromagnets, show no net magnetization but their transport and magneto-optic properties resemble those of ferromagnets, thereby creating opportunities for applications in high-frequency spintronics and low-energy loss communications. Here we study the modification the noncollinear ferrimagnetic spin structure of...

Perpendicular heterostructures based on a ferrimagnetic Mn2RuxGa (MRG) layer and a ferromagnetic Co/Pt multilayer were examined to understand the effects of different spacer layers (V, Mo, Hf, HfOx and TiN) on the interfaces with the magnetic electrodes, after annealing at 350∘C. Loss of perpendicular anisotropy in MRG is strongly correlated with a...

Repeated uniform switching of the magnetization of thin films of ferrimagnetic amorphous Gd$_{x}$(FeCo)$_{1-x}$ in response to single fast laser pulses is well established. Here we report unusual toggle switching in thin films of sperimagnetic amorphous Dy$_x$Co$_{1-x}$ and Tb$_x$Co$_{1-x}$ with $\it{x} \simeq$ 0.25 irradiated with single 200 fs pu...

Three natural specimens of hematite are characterized using X-ray diffraction, magnetometry, IR/Raman and Mössbauer spectroscopy, with focus on the Morin transition. All are exfoliated by grinding and prolonged sonication in dimethylformamide, followed by centrifuging to extract exfoliated debris from the supernatant. This material is characterized...

The ruthenium content of half-metallic Mn2RuxGa thin films, with a biaxially strained inverse Heusler structure, controls the ferrimagnetism that determines their magnetic and electronic properties. An extensive study of MnyRuxGa films on MgO (100) substrates with 1.8≤y≤2.6 and x=0.5, 0.7 or 0.9, including crystallographic, magnetic order, magneto-...

Controlling supramolecular polymerization is of fundamental importance to create advanced materials and devices. Here we show that the thermodynamic equilibrium of Gd3+-bearing supramolecular rod networks is shifted reversibly at room temperature in a static magnetic field of up to 2 T. Our approach opens opportunities to control the structure form...

The electrosorption of Gd3+ ions from aqueous 70 mM Gd(NO3)3 solution in monolithic carbon aerogel electrodes was recorded by dynamic neutron imaging. The aerogels have a bimodal pore size distribution of macropores and mesopores centred at 115 nm and 15 nm, respectively. After the uptake of Gd3+ ions by the negatively charged surface of the porous...

Calculations suggest that ordered Heusler alloys with 18 valance electrons could exhibit a variety of unusual electronic and magnetic states that are absent in the constituent elements. They include magnetic semiconductors, spin gapless semiconductors, compensated ferrimagnetic half-metals, and metallic antiferromagnets. Magnetic order has been pre...

Perpendicular heterostructures based on a ferrimagnetic Mn2RuxGa (MRG) layer and a ferromagnetic CoPt multilayer were examined to understand the effects of different spacer layers (V, Mo, Hf, HfOx and TiN) on the interfaces with the magnetic electrodes, especially after annealing at 350 C. Loss of perpendicular anisotropy in MRG is strongly correla...

All-optical reswitching has been investigated in the half-metallic Heusler ferrimagnet Mn2Ru0.9Ga, where Mn atoms occupy two inequivalent sites in the XA-type structure. The effect of a second 200 fs, 800 nm laser pulse that follows the first pump pulse, when both are above the threshold for switching, is studied as a function of t12, the time betw...

The connection between chirality and electromagnetism has attracted much attention through the recent history of science, allowing the discovery of crucial nonreciprocal optical phenomena within the context of fundamental interactions between matter and light. A major phenomenon within this family is the so-called Faraday chiral anisotropy, the lon...

Antiferromagnetic insulators are a ubiquitous class of magnetic materials, holding the promise of low-dissipation spin-based computing devices that can display ultra-fast switching and are robust against stray fields. However, their imperviousness to magnetic fields also makes them difficult to control in a reversible and scalable manner. Here we d...

Compensated ferrimagnets are promising materials for fast spintronic applications based on domain-wall motion as they combine the favorable properties of ferromagnets and antiferromagnets. They inherit from antiferromagnets immunity to external fields, fast spin dynamics, and rapid domain-wall motion. From ferromagnets they inherit straightforward...

We study the demagnetization dynamics of the fully compensated half-metallic ferrimagnet Mn2RuxGa. 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 pul...

Compensated ferrimagnets are promising materials for fast spintronic applications based on domain wall motion as they combine the favourable properties of ferromagnets and antiferromagnets. They inherit from antiferromagnets immunity to external fields, fast spin dynamics and rapid domain wall motion. From ferromagnets they inherit straightforward...

All-optical re-switching has been investigated in the half-metallic Heusler ferrimagnet Mn2Ru0.9Ga, where Mn atoms occupy two inequivalent sites in the XA-type structure. The effect of a second 200 fs 800 nm pump pulse that follows a first pulse, when both are above the threshold for switching, is studied as a function of t12, the time between them...

Variation of Mn content in Mn$_2$Ru$_x$Ga thin films influences chemical disorder and crystal structure, which strongly influence material properties. Mn$_y$Ru$_x$Ga thin films with $1.2 < y < 2.6$ for $x$ = 0.5, 0.7 and 0.9 were deposited by DC magnetron sputtering onto MgO (100) to investigate the influence of composition on magnetic order, magne...

The development of high‐density magnetic recording media is limited by superparamagnetism in very small ferromagnetic crystals. Hard magnetic materials with strong perpendicular anisotropy offer stability and high recording density. To overcome the difficulty of writing media with a large coercivity, heat‐assisted magnetic recording was developed,...

Spin orbit torque driven switching is a favourable way to manipulate nanoscale magnetic objects for both memory and wireless communication devices. The critical current required to switch from one magnetic state to another depends on the geometry and the intrinsic properties of the materials used, which are difficult to control locally. Here we dem...

Energy-efficient control of magnetization without the help of a magnetic field is a key goal of spintronics. Purely heat-induced single-pulse all-optical toggle switching has been demonstrated, but so far only in Gd-based amorphous ferrimagnet films. In this work, we demonstrate toggle switching in films of the half-metallic ferrimagnetic Heusler a...

The development of high-density magnetic recording media is limited by the superparamagnetism in very small ferromagnetic crystals. Hard magnetic materials with strong perpendicular anisotropy offer stability and high recording density. To overcome the difficulty of writing media with a large coercivity, heat assisted magnetic recording (HAMR) has...

A model is presented to explain the temperature-independent saturating paramagnetic response of some transparent oxides with no unpaired d-electrons. It is based on an array of N surface defect-related electrons with bound states in the gap that can form a coherent mesoscopic many-electron state in response to fluctuations of the zero-point electro...

Understanding how a single laser pulse can toggle magnetization in a compensated 3d ferrimagnet is a critical problem in ultrafast magnetism. To resolve it, we test single-shot all-optical switching of magnetization in Mn2RuxGa at different temperatures using femto- to picosecond pulses in the visible to far-infrared spectral ranges. The switching...

Understanding and controlling supramolecular polymerization are of fundamental importance to create advanced materials and devices. Many stimuli have been explored in the past decades, but magnetic fields and field gradients have received little attention. This is because magnets do not provide enough magnetic energy to overcome thermal noise at th...

Spin orbit torque driven switching is a favourable way to manipulate nanoscale magnetic objects for both memory and wireless communication devices. The critical current required to switch from one stored magnetic state to another depends on the multilayer structure of the device and the intrinsic properties of the materials used, which are difficul...

As-cast ingots of Mn100−xZx with Z = Al, Ge, Mn, Fe and Co all have the A13 βMn structure up to some maximum value of x. The magnetic susceptibility of pure βMn is ≈ 9 × 10⁻⁴ from 4 to 400 K, but the field-cooled and zero-field-cooled susceptibilities of all doped samples increase at low temperature and they bifurcate at a spin-freezing temperature...

Sm(CoFeCuZr)z permanent magnets are of great technological interest due to their good magnetic performance and excellent thermal stability. The Fe content plays a key role for magnetic properties, determining the maximum energy product and the highest working temperature. Here we investigated the Fe site preferences in Sm(CoFeCuZr)z magnets with Fe...

A flux source with four rotatable permanent magnet rods having access from two perpendicular directions through gaps exceeding the rod diameter can produce a continuously-variable flux density of up to ±300 mT at the centre when high-grade Nd-Fe-B magnets are used. The trade-off between field, access and torque in this mangle design is discussed. T...

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 las...

An electric current passing through a magnetic conductor can generate a dissipationless transversal current of topological Berry curvature origin. This anomalous Hall effect requires the breaking of spin-degeneracy of electronic bands, conventionally arising from a macroscopic moment in ferromagnets, or a non-collinear magnetic order in complex mag...

The method of neutron imaging was adopted to map the concentration evolution of aqueous paramagnetic Gd(NO3)3 solutions. Magnetic manipulation of the paramagnetic liquid within a miscible nonmagnetic liquid is possible by countering density-difference driven convection. The formation of salt fingers caused by double-diffusive convection in a liquid...

The method of neutron imaging was adopted to map the concentration evolution of aqueous paramagnetic Gd(NO3)3 solutions. Magnetic manipulation of the paramagnetic liquid within a miscible nonmagnetic liquid is possible by countering density-difference driven convection. The formation of salt fingers caused by double-diffusive convection in a liquid...

Here we study both static and time-resolved dynamic magnetic properties of the compensated ferrimagnet Mn2RuxGa from room temperature down to 10 K, thus crossing the magnetic compensation temperature TM. The behavior is analyzed with a model of a simple collinear ferrimagnet with uniaxial anisotropy and site-specific gyromagnetic ratios. We find a...

Here we study both static and time-resolved dynamic magnetic properties of the compensated ferrimagnet from room temperature down to 10K, thus crossing the magnetic compensation temperature $T_{M}$. The behaviour is analysed with a model of a simple collinear ferrimagnet with uniaxial anisotropy and site-specific gyromagnetic ratios. We find a maxi...

Viable and energy-efficient control of magnetization without the help of a magnetic field is a key goal of spintronics. Purely thermal single-pulse all-optical toggle switching has been demonstrated, but so far only in Gd based amorphous ferrimagnet films. In this work, we demonstrate thermal single-pulse toggle switching in the half-metallic compe...

It is difficult to obtain dispersed particles of SmCo5 by calciothermic reduction because of sintering during the high-temperature reaction. This study presents a new strategy to synthesize dispersible SmCo5 particles by co-precipitating a precursor containing amorphous Sm(OH)3 and coherent nanoscale Co(OH)2 and Ca(OH)2 crystallites. The Ca(OH)2 de...

div>Solid walls become increasingly important when miniaturizing fluidic circuitry. They limit flow-rates achievable for a given pressure drop, and are plagued by fouling. Approaches to reduce the wall interactions include hydrophobic coatings, liquid-infused porous surfaces, nanoparticle surfactant jamming, changing the surface electronic structur...

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

There is an increasing demand for permanent magnets with high‐temperature stability. Normally, the magnets are heat treated above the operating temperature range to saturate the irreversible losses, thereby producing magnets that are stable, but with significantly degraded performance. In addition, uncertainty about the losses in differently shaped...

Rare earth permanent magnets constitute a mature technology, but the shock of the 2011 rare earth crisis led to the re-evaluation of many ideas from the 1980s and 1990s about possible new hard magnets containing little or no rare earth (or heavy rare earth). Nd–Fe–B magnets have been painstakingly and skillfully optimized for a wide range of applic...

Oxides of non-magnetic cations exhibit elusive signs of weak temperature-independent ferromagnetism. The effect is associated with surface defects, but it defies conventional explanation. Possible hypotheses are a spin-split defect impurity band, or giant orbital paramagnetism related to zero-point vacuum fluctuations.

New-generation magnetostrictive applications in micromanipulation instruments, torque sensing and transducers require materials that offer a combination of large magnetostriction and good structural properties. Fe100-xGax-based (x = 17-19) alloys are potential candidates. In this work, the solidification behavior of Tb-doped FeGa alloys is investig...

Antiferromagnets and compensated ferrimagnets offer opportunities to investigate spin dynamics in the 'terahertz gap' because their resonance modes lie in the 0.3 THz to 3 THz range. Despite some inherent advantages when compared to ferromagnets, these materials have not been extensively studied due to difficulties in exciting and detecting the hig...

We show how a charge current through a single antiferromagnetic layer can excite and control self-oscillations. Sustained oscillations with tunable amplitudes and frequencies are possible in a variety of geometries using certain classes of noncentrosymmetric materials that exhibit finite dissipative spin-orbit torque. We compute the steady-state ph...

Spintronic devices based on antiferromagnetic (AFM) materials hold the promise of fast switching speeds and robustness against magnetic fields. Different device concepts have been predicted and experimentally demonstrated, such as low-temperature AFM tunnel junctions that operate as spin-valves, or room-temperature AFM memory, for which either ther...

Following its discovery in the aftermath of Nd 2 Fe 14 B, Sm 2 Fe 17 N 3 seemed to offer intrinsic magnetic properties that were superior or comparable to those of its famous predecessor. But the promise of the new material to challenge Nd 2 Fe 14 B was not realized, mainly because the 2:17 nitride powder, prepared by a low-temperature gas-phase in...

Magnetotransport is investigated in thin films of the half-metallic ferrimagnet Mn2RuxGa in pulsed magnetic fields of up to 58 T. A nonvanishing Hall signal is observed over a broad temperature range, spanning the compensation temperature (155 K), where the net magnetic moment is strictly zero, the anomalous Hall conductivity is 6673Ω−1m−1, and the...

Pulsed electrochemical and electroless techniques for removing antimony and lead from water using sacrificial iron sheets are analyzed in terms of number of pulses and electrolysis time (up to 20 minutes), or pH and exposure time (up to 12 days) respectively. The amount of Sb or Pb in a test aqueous solution containing 10 ppm could be reduced to <...

The magneto-optical Kerr effect (MOKE) is often assumed to be proportional to the magnetization 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 po...

Electrolyte gating is a powerful means for tuning the carrier density and exploring the resultant modulation of novel properties on solid surfaces. However, the mechanism, especially its effect on the oxygen migration and electrostatic charging at the oxide heterostructures, is still unclear. Here we explore the electrolyte gating on oxygen-deficie...

We have a choice of methods for examining domains at the surface of a ferromagnet that depend on probing the stray field distribution, but these methods do not work in antiferromagnets or compensated ferrimagnets, which produce no stray field. The discovery of compensated ferrimagnetic half metals allows for the local magnetization state to be obse...

Magnetotransport is investigated in thin films of the half-metallic ferrimagnet Mn$_2$Ru$_x$Ga in pulsed magnetic fields of up to 58 T. A non-vanishing Hall signal is observed over a broad temperature range, spanning the compensation temperature 155 K, where the net magnetic moment is strictly zero, the anomalous Hall conductivity is 6673 $\Omega^{...

Spin-orbit torques can control antiferromagnetic dynamics. We determine the form of the reactive and dissipative spin-orbit torques (SOT) in the non-centrosymmetric material CuMnAs, and show how a charge current in a single antiferromagnetic layer can excite and control self-oscillations. We predict that sustained oscillations with tunable amplitud...

The volume change on magnetic saturation of a large single crystal of Fe83Ga17 is measured by Joule's liquid displacement method. The crystal was tested in the as-grown state, and after annealing at 760 °C followed by slow cooling at 10 °C/min or quenching in water. In each case an upper limit to any volume change is < 5 ppm, establishing that the...

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 po...

The discovery of a tenfold increase in magnetostriction of Fe by alloying nonmagnetic Ga was a breakthrough in magnetostrictive materials. The large magnetostriction is attributed to tetragonal nanoheterogeneities dispersed in the bcc matrix. A further remarkable fivefold increase is achieved by trace rare earth doping (<1 at%) up to a value of ≈15...

Complex-oxide materials exhibit physical properties that involve the interplay of charge and spin degrees of freedom. However, an ambipolar oxide that is able to exhibit both electron-doped and hole-doped ferromagnetism in the same material has proved elusive. Here we report ambipolar ferromagnetism in LaMnO3, with electron-hole asymmetry of the fe...