The magnetization of amorphous sputtered films of a-RxCo1-, with x approximate to 0.25 and R = Dy, Tb, or Tm, is investigated by magnetometry, anomalous Hall effect, and magneto-optic Kerr effect to understand how the magnetic structure of the films is influenced by temperature and the quadrupole and higher multipole moments of the rare-earth charge distribution. Square magnetic hysteresis loops with perpendicular magnetic anisotropy and divergent coercivity that reaches 3.5 T in the vicinity of the compensation temperature T-comp are observed at 175 and 200 K for Dy and Tb films, respectively, but the coercivity in Tm films never exceeds 0.5 T and shows no divergence near the compensation at 50 K. The temperature dependence of the net rare-earth moment is inferred from the cobalt moment of soft ferromagnetic a-YxCo1-x. The magnitude of the second-order random anisotropy energy exceeds the antiparallel R-Co exchange coupling for all three rare earths. The negative quadrupole moments of Dy and Tb lead to random easy-axis anisotropy with large coercivity. The positive quadrupole moment of Tm favors random hard-axis anisotropy where each Tm has an easy plane. The resulting sperimagnetic ground states are modeled by a distribution of rare-earth moments within a cone of half-angle theta(0) whose axis is antiparallel to the ferromagnetic axis of cobalt. The reduced moment < J(z)>/J at T = 0 is calculated from a one-atom Hamiltonian as a function of alpha, the ratio of uniaxial anisotropy to exchange energy per rare-earth atom for different angles theta between the local anisotropy axis, and the ferromagnetic Co axis. Extrapolated values of < J(z)>/J are similar to 0.75 at low temperature for both D-y and Tb, with a sharp increase <10 K attributed to higher-order multipole moments. The hard-axis random anisotropy resulting from the positive quadrupole moment of Tm leads to a larger low-temperature value of < J(z)>/J = 0.84. On increasing temperature, the magnitude of the rare-earth moment and the local random anisotropy that creates the sperimagnetism are reduced; the cone angle narrows, but the noncollinear structure persists well above room temperature for Dy but not for Tb, a difference related to the opposite signs of their hexadecapole moments. A temperature-dependent spin-flop field observed near compensation in a-Dy25Co75 extrapolates to 2.0 T at Tcomp, a remarkably low value that is associated with the nonrigid character and high transverse susceptibility of the frustrated Dy subnetwork. An x-ray photoemission electron microscopy investigation of partial single-pulse all-optical switching in a 10 nm a-Dy25Co75 film as a function of temperature establishes that the process is stochastic, and unrelated to inhomogeneities in the films. The size of the sperimagnetic domains in the unmagnetized state is similar to 200 nm, <300 K.

Exploring anisotropy and diverse magnetization dynamics in specimens with vanishing magnetic moments presents a significant challenge using traditional magnetometry, as the low resolution of existing techniques hinders the ability to obtain accurate results. In this study, we delve deeper into the examination of magnetic anisotropy and quasistatic magnetization evolution in Mn2RuxGa (MRG) thin films, as an example of a compensated ferrimagnetic half-metal, by employing anomalous Hall effect measurements within a tetragonal crystal lattice system. Our research proposes an innovative approach to accurately determine the complete set of anisotropy constants of these MRG thin films. To achieve this, we perform anomalous Hall voltage curve fitting, using torque models under the macrospin approximation, which allow us to obtain room-temperature outof-plane anisotropy constants K1 = 4.0 x 104 J m-3 (K1/M = 0.655 T) and K2 = 2.54 x 104 J m-3 (K2/M = 0.416 T), along with a weaker in-plane anisotropy constant K3 = 3.48 x 103 J m-3 (K3/M = 0.057 T). By additionally employing first-order reversal curves and classical Preisach hysteresis (hysterons) models, we are able to validate the efficacy of the macrospin model in capturing the magnetic behavior of MRG thin films. Furthermore, our investigation substantiates that the complex steady-state magnetization behavior of MRG thin films can be effectively modeled using a combination of hysteronic and torque models. This approach facilitates the exploration of both linear and nonlinear steady-state magnetization evolution, in the presence of an external magnetic field and/or current-induced effective fields, generated by the spin-orbit torque and spin transfer torque mechanisms. The detailed understanding of the quasiequilibrium magnetization behavior is a key prerequisite for the exploitation of in-phase and out-of-phase resonance modes in this material class, for high-bandwidth modulators/demodulators, filters, and oscillators for the high-GHz and low-THz frequency bands.

Ferrimagnetic compensation in Mn 4−x Ga x N (00.15 and it occurs at 235 K when x = 0.18. A greater amount of Ga-doping is required to achieve compensation in the bulk, where room-temperature compensation is at x = 0.26. The differences are discussed in terms of Ga site occupancy in the Mn 4 N structure, and an effect of the change of easy axis of the triangular ferrimagnetic spin structure from [111] in the bulk to [001] in the tetragonally-distorted films.

Thin films of Mn 2 Ru x Ga grown by sputtering at low temperature, T sub = 225 ◦ C, are nearly defect free and have in-plane grain size exceeding 180 nm. A higher T sub of 425 ◦ C leads to an increased density of defects and reduced in-plane coherence. The near-perfect films grown at low temperature exhibit a pinched wasp-waist magnetic hysteresis that is a consequence of the frustration induced by exchange interaction and site-specific anisotropy. At higher temperatures, the long-range crystalline order is broken by the presence of defects and simpler square hysteresis loops are recovered.

The magnetization of amorphous DyCo3 and TbCo3 is studied by magnetometry, anomalous Hall effect and magneto-optic Kerr effect to understand the temperature-dependent magnetic structure. A square magnetic hysteresis loop with perpendicular magnetic anisotropy and coercivity that reaches 3.5 T in the vicinity of the compensation temperature is seen in thin films. An anhysteretic soft component, seen in the magnetization of some films but not in their Hall or Kerr loops is an artefact due to sputter-deposition on the sides of the substrate. The temperature-dependence of the net rare earth moment from 4-300K is deduced, using the cobalt moment in amorphous YxCo1-x. The single-ion anisotropy of the quadrupole moments of the 4f atoms in the randomly-oriented local electrostatic field gradient overcomes their exchange coupling to the cobalt subnetwork, resulting in a sperimagnetic ground state where spins of the noncollinear rare-earth subnetwork are modelled by a distribution of rare earth moments within a cone whose axis is antiparallel to the ferromagnetic axis z of the cobalt subnetwork. The reduced magnetization (Jz)/J at T=0 is calculated from an atomic Hamiltonian as a function of the ratio of anisotropy to exchange energy per rare-earth atom for a range of angles between the local anisotropy axis and -z and then averaged over all directions in a hemisphere. The experimental and calculated values of (J-z)/J are close to 0.7 at low temperature for both Dy and Tb. On increasing temperature, the magnitude of the rare earth moment and the local random anisotropy that creates the cone are reduced; the cone closes and the structure approaches collinear ferrimagnetism well above ambient temperature. An asymmetric spin flop of the exchange-coupled subnetworks appears in the vicinity of the magnetization compensation temperatures of 175K for amorphous Dy0.25Co0.75 and 200 K for amorphous TbCo3.

The full magneto-galvanic tensor of the non-centrosymmetric Heusler alloy Mn 2 Ru 09 Ga in thin film form was determined. The samples are expected to exhibit single layer spin-orbit torques due to the lack of a crystal centre of inversion and due to the broken symmetry at the thin film interfaces. By analysing the first and second harmonic response of the longitudinal and transverse voltages we extract the magnitude and symmetry of the current-induced spin-orbit torque as well as the contributions originating from thermal gradients induced by the bias current. The dominating torque term is damping-like (τ DL ), due to the thin-film geometry, and reaches a magnitude of τ DL = 4.38 × 10 −14 TA −1 m 2 .

Exploring anisotropy and diverse magnetization dynamics in specimens with vanishing magnetic moments presents a significant challenge using traditional magnetometry, as the low resolution of existing techniques hinders the ability to obtain accurate results. In this study, we delve deeper into the examination of magnetic anisotropy and quasi-static magnetization dynamics in \mrg\,(MRG) thin films, as an example of a compensated ferrimagnetic half-metal, by employing anomalous Hall effect measurements within a tetragonal crystal lattice system. Our research proposes an innovative approach to accurately determine the complete set of anisotropy constants of these MRG thin films. To achieve this, we perform anomalous Hall voltage curve fitting, using torque models under the macrospin approximation, which allow us to obtain out-of-plane anisotropy constants $K_1=4.0\times10^4$ J m$^{-3}$ ($K_1/M=0.655$\,T) and $K_2=2.54\times10^4$ J m$^{-3}$ ($K_2/M=0.416$\,T), along with a weaker in-plane anisotropy constant $K_3=3.48\times10^3$ J m$^{-3}$ ($K_3/M=0.057$\,T). By additionally employing first-order reversal curves (FORC) and classical Preisach hysteresis (hysterons) models, we are able to validate the efficacy of the macrospin model in capturing the magnetic behavior of MRG thin films. Furthermore, our investigation substantiates that the complex quasi-static magnetization dynamics of MRG thin films can be effectively modelled using a combination of hysteronic and torque models. This approach facilitates the exploration of both linear and non-linear quasi-static magnetization dynamics, in the presence of external magnetic field and/or current-induced effective fields, generated by the spin-orbit torque and spin transfer torque mechanisms.

Thin films of Mn4N and related ternary metallic perovskites with perpendicular anisotropy are interesting for spintronics, but their magnetic structures differ from the triangular ferrimagnetism of the bulk. A temperature-independent anomalous Hall conductivity of -90 52-1 cm-1 was found in Mn4N films, in addition to the normal temperature-dependent contribution. Based on known spin structures of bulk Mn3ZN compounds and the distance dependence of the Mn-Mn exchange for first- and second-neighbor Mn-Mn pairs, we propose a topological noncollinear spin structure for the Mn4N films with perpendicular anisotropy. The Letter shows how a small change in symmetry of the spin structure can influence the magnetotransport properties of frustrated ferrimagnetic films.

Repeated uniform switching of the magnetization of thin films of ferrimagnetic amorphous Gdx(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 DyxCo1−x and TbxCo1−x with x ≈ 0.25 irradiated with single 200 fs pulses of 800 nm laser light. The samples have strong local random anisotropy due to the non-S state rare earth. The compensation temperature of the films is ≤180 K, and their Curie temperature is ≈500 K. They are largely switched by the first pulse, and subsequent pulses lead to partial re-switching of a decreasing amount of the irradiated area, with a granular structure of submicrometer regions of switched and unswitched material. Individual switched regions about 700 nm in size are observed around the edge of the irradiated spots where the fluence is at the threshold for switching. Results are discussed in terms of a random anisotropy model where the ratio of local anisotropy to exchange is temperature dependent.

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?. Loss of perpendicular anisotropy in MRG is strongly correlated with a reduction in the substrate-induced tetragonality due to relaxation of the crystal structure. In the absence of diffusion, strain and chemical ordering within MRG are correlated. The limited solubility of both Hf and Mo in MRG is a source of additional valence electrons, which results in an increase in compensation temperature T-comp. This also stabilises perpendicular anisotropy, compensating for changes in strain and defect density. The reduction in squareness of the MRG hysteresis loop measured by anomalous Hall effect is < 10%, making it useful in active devices. Furthermore, a CoPt3 phase with (220) texture in the perpendicular Co/Pt free layer promoted by a Mo spacer layer is the only one that retains its perpendicular anisotropy on annealing.

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_xCo_1-x and Tb_xCo_1-x with x≃ 0.25 irradiated with single 200 fs pulses of 800 nm laser light. The samples have strong local random anisotropy due to the non-S state rare earth. The compensation temperature of the films is ≤ 180 K and their Curie temperature is ≃ 500 K. They are mostly switched by the first pulse, and subsequent pulses lead to partial re-switching of a decreasing amount of the irradiated area, with a granular structure of submicron regions of switched and unswitched material. Individual switched domains about 700 nm in size are observed around the edge of the irradiated spots where the fluence is at the threshold for switching. Results are discussed in terms of a random anisotropy model where the ratio of local anisotropy to exchange is temperature dependent and close to the threshold for strong pinning.

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 pulses of 800 nm laser light. The samples have strong local random anisotropy due to the non-S state rare earth. The compensation temperature of the films is $\le$ 180 K and their Curie temperature is $\simeq$ 500 K. They are mostly switched by the first pulse, and subsequent pulses lead to partial re-switching of a decreasing amount of the irradiated area, with a granular structure of submicron regions of switched and unswitched material. Individual switched domains about 700 nm in size are observed around the edge of the irradiated spots where the fluence is at the threshold for switching. Results are discussed in terms of a random anisotropy model where the ratio of local anisotropy to exchange is temperature dependent and close to the threshold for strong pinning.

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 predicted at exceptionally high temperature. Any of this would be of interest for spin electronics. Here, we investigate the magnetic properties of bulk, single-phase V3Al, CrVTiAl and the corresponding Ga compounds, with and without Fe-57 doping. Results are compared with data on the constituent elements. We conclude that all the as-cast alloys show some degree of B2-type ordering, but all of them are Pauli paramagnets with dimensionless susceptibilities close to the average of the atomic constituents. Prolonged annealing of the single-phase as-cast alloys leads to phase segregation. Density functional theory calculations on V3X and CrVTiX with X = B, Al, Ga, and In confirm that different atomic arrangements on the four interpenetrating face-centered cubic sublattices of the Heusler structure could indeed lead to unusual magnetic properties, but both magnetism and semiconductivity are destroyed by disorder. The energy and entropy differences between different ordered magnetic phases preclude the stabilization of any single one of them. All are metastable and inaccessible in alloys prepared from the melt.

Due to its negligible spontaneous magnetization, high spin polarization and giant perpendicular magnetic anisotropy, Mn 2 Ru x Ga (MRG) is an ideal candidate as an oscillating layer in THz spin-transfer-torque nano-oscillators. Here, the effect of ultrathin Al and Ta diffusion barriers between MRG and MgO in perpendicular magnetic tunnel junctions is investigated and compared to devices with a bare MRG/MgO interface. Both the compensation temperature, T comp , of the electrode and the tunneling magnetoresistance (TMR) of the device are highly sensitive to the choice and thickness of the insertion layer used. High-resolution transmission electron microscopy, as well as analysis of the TMR, its bias dependence, and the resistance-area product allow us to compare the devices from a structural and electrical point of view. Al insertion leads to the formation of thicker effective barriers and gives the highest TMR, at the cost of a reduced T comp . Ta is the superior diffusion barrier which retains T comp , however, it also leads to a much lower TMR on account of the short spin diffusion length which reduces the tunneling spin polarization. The study shows that fine engineering of the Mn 2 Ru x Ga/barrier interface to improve the TMR amplitude is feasible.

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 high-frequency spin dynamics, especially in thin films. Here we show that spin-obit torque in a single layer of the highly spin-polarized compensated ferrimagnet Mn2RuxGa is remarkably efficient at generating spin-orbit fields μ_0H_eff, which approach 0.1x10-10 T m2/A in the low-current density limit – almost a thousand times the Oersted field, and one to two orders of magnitude greater than the effective fields in heavy metal/ferromagnet bilayers. From an analysis of the harmonic Hall effect which takes account of the thermal contributions from the anomalous Nernst effect, we show that the antidamping component of the spin-orbit torque is sufficient to sustain self-oscillation. Our study demonstrates that spin electronics has the potential to underpin energy-frugal, chip-based solutions to the problem of ultra high-speed information transfer.

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 non-centrosymmetric materials that exhibit finite dissipative spin-orbit torque. We compute the steady-state phase diagram as a function of the current and spin-orbit torque magnitude. The anisotropic magnetoresistance causes the conversion of the resulting AF oscillations to a terahertz AC output voltage. These findings provide an attractive and novel route to design terahertz antiferromagnetic spin-orbit torque oscillators in simple single-layer structures.

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 polarized, fully compensated ferrimagnet, where the sublattices are chemically similar, a signal is observed even when the net moment is negligible. We analyze the spectral ellipsometry and MOKE of Mn2Rux Ga and show that this behavior is due to a highly spin-polarized conduction band dominated by one of the two manganese sublattices (4c) which creates helicity-dependent reflectivity dominated by a broad Drude tail. Our findings open prospects for studying spin dynamics in the infrared.