Non-magnetic Contributions Research Articles

Current switching of the antiferromagnetic Néel vector in Pd/CoO/MgO(001)

Recently, the electrical switching of antiferromagnetic (AFM) order has been intensively investigated because of its application potential in data storage technology. Herein, we report the current switching of the AFM N\'eel vector in epitaxial Pd/CoO films as a function of temperature. Using combined measurements of Hall resistance (HR) and x-ray magnetic linear dichroism (XMLD) below and above the AFM N\'eel temperature, we unambiguously identified both magnetic and nonmagnetic contributions to the current-induced HR change. Through magnetic field-induced HR measurements, we quantitatively determined the percentage of current-induced CoO spin switching. Further, we showed that the thermal effect dominated the CoO magnetic switching more in samples with a thinner Pd layer and that samples with a thicker CoO layer required higher thermal activation for current-induced magnetic switching. These results provide a clear and comprehensive picture of current-induced AFM spin switching across the AFM N\'eel temperature.

Study on high damping L12/D03 biphase Fe-27Ga alloys with near-zero magnetostriction and its damping mechanism

A novel L12/D03 biphase Fe-27Ga alloy with both high damping (0.007, equivalent to a specific damping capacity (SDC) of 0.04) and the near-zero (∼−4 ppm) magnetostriction has been prepared by using an appropriate heat treatment process: annealing at 480 °C for 1 h. Here the L12 phase proportion is about 44.8%. More importantly, the damping mechanism of L12/D03 biphase Fe-27Ga alloy has been deeply studied based on the experiments of amplitude-dependent IF measurements with and without magnetic field, combined with the magnetostriction measurements. All the results indicate that for the L12/D03 biphase Fe-27Ga alloy, D03 phase with a magnetic damping plays a dominant role under a lower strain amplitude while L12 phase with a non-magnetic damping plays a main role under a higher strain amplitude. At the same time, the L12/D03 biphase Fe-27Ga alloy with more L12 phase has a higher damping capacity under a higher strain amplitude, which is mainly the non-magnetic damping contribution originated from the twin boundaries movement. This can provide a new way and design concept for high damping Fe-Ga alloys to satisfy more application requirements, such as micro-vibration field, strong vibration field, magnetic or non-magnetic field.

Analytic description and optimization of magneto-optical Kerr setups with photoelastic modulation.

Instruments based on the magneto-optical Kerr effect are routinely used to probe surface magnetic properties. These tools rely on the characterization of the polarization state of reflected light from the sample to collect information on its magnetization. Here, we present a theoretical optimization of common setups based on the magneto-optical Kerr effect. A detection scheme based on a simple analyzer and photodetector and one made from a polarizing beam splitter and balanced photodetectors are considered. The effect of including a photoelastic modulator (PEM) and a lock-in amplifier to detect the signal at harmonics of the modulating frequency is studied. Jones formalism is used to derive general expressions that link the intensity of the measured signal to the magneto-optical Fresnel reflection coefficients for any orientation of the polarizing optical components. Optimal configurations are then defined as those that allow measuring the Kerr rotation and ellipticity while minimizing nonmagnetic contributions from the diagonal Fresnel coefficients in order to improve the signal-to-noise ratio (SNR). The expressions show that with the PEM, setups based on polarizing beam splitters inherently offer a twofold higher signal than commonly used analyzers, and the experimental results confirm that the SNR is improved by more than 150%. Furthermore, we find that while all proposed detection schemes measure Kerr effects, only those with polarizing beam splitters allow measuring the Kerr rotation directly when no modulator is included. This accommodates, for instance, time-resolved measurements at relatively low laser pulse repetition rates. Ultrafast demagnetization measurements are presented as an example of such applications.

Magnetic Sensitivity Distribution of Hall Devices in Antiferromagnetic Switching Experiments

We analyze the complex impact of the local magnetic spin texture on the transverse Hall-type voltage in device structures utilized to measure magnetoresistance effects. We find a highly localized and asymmetric magnetic sensitivity in the eight-terminal geometries that are frequently used in current-induced switching experiments, for instance, to probe antiferromagnetic materials. Using current-induced switching of antiferromagnetic $\mathrm{Ni}\mathrm{O}/\mathrm{Pt}$ as an example, we estimate the change in the spin Hall magnetoresistance signal associated with switching events based on the domain-switching patterns observed via direct imaging. This estimate correlates with the actual electrical data after subtraction of a nonmagnetic contribution. Here, the consistency of the correlation across three measurement geometries with fundamentally different switching patterns strongly indicates a magnetic origin of the measured and analyzed electrical signals.

Observation of current-induced switching in non-collinear antiferromagnetic IrMn3 by differential voltage measurements

There is accelerating interest in developing memory devices using antiferromagnetic (AFM) materials, motivated by the possibility for electrically controlling AFM order via spin-orbit torques, and its read-out via magnetoresistive effects. Recent studies have shown, however, that high current densities create non-magnetic contributions to resistive switching signals in AFM/heavy metal (AFM/HM) bilayers, complicating their interpretation. Here we introduce an experimental protocol to unambiguously distinguish current-induced magnetic and nonmagnetic switching signals in AFM/HM structures, and demonstrate it in IrMn3/Pt devices. A six-terminal double-cross device is constructed, with an IrMn3 pillar placed on one cross. The differential voltage is measured between the two crosses with and without IrMn3 after each switching attempt. For a wide range of current densities, reversible switching is observed only when write currents pass through the cross with the IrMn3 pillar, eliminating any possibility of non-magnetic switching artifacts. Micromagnetic simulations support our findings, indicating a complex domain-mediated switching process.

Universality in the temperature dependence of the heat capacity of magnetic solids

It is shown that the temperature dependence of the heat capacity of the magnetic solids can completely be described by a sequence of universal power functions of temperature. Characteristic for universality is that each power function holds over a finite temperature range and has a rational exponent. The analytical change from one to the adjacent power function is a typical crossover event. Universality reveals that the temperature dependence of the heat capacity is determined by a boson field whereas the absolute values are given by all magnetic and non-magnetic inter-atomic interactions. Universality for temperatures outside the critical range at Tc, i.e. for temperatures for which the phonons dominate the heat capacity has to be characterized as non-intrinsic, arising from interactions of the phonons with the bosons of the continuous magnetic medium. As we have shown earlier, the bosons of the continuous magnetic solid are essentially magnetic dipole radiation generated via stimulated emission by the precessing spins. We have called them Goldstone bosons. The interactions of the Goldstone bosons with the magnons modify the wave-vector dependence of the magnons. For cubic crystals the dispersions along [ζ, 0, 0] direction are essentially as for the linear spin chain, i.e. one-dimensional. As the different rational exponent values in the temperature power function of the heat capacity show, there exists a number of distinct modes of interaction between the Goldstone boson field and the phonons. The actual exponent depends additionally on the proportion between the magnetic and the non-magnetic energy contributions and therefore changes with temperature. The observed exponents are, however, difficult to interpret.

Magnetic and magnetothermal properties of Gd75Cu25 prepared by melt-spinning

Magnetic and magnetothermal properties of melt-spun Gd75Cu25 alloy were studied. A second-order magnetic phase transition is observed at the Curie temperature of 144 K. The peak value of the magnetic entropy change −ΔSmpk = 7.0 J kg−1 K−1 is observed near the Curie point under the magnetic field change 0–5 T, while the refrigerant capacity is found to be 594 J kg−1. The specific heat exhibits a wide transition near the Curie temperature, and the non-magnetic contributions to the total specific heat are described using the Debye temperature ΘD = 155 K and the electronic specific heat coefficient γ = 65 mJ mol−1 K−2.

Concurrent magneto-optical imaging and magneto-transport readout of electrical switching of insulating antiferromagnetic thin films

We demonstrate stable and reversible current induced switching of large-area (>100 μm2) antiferromagnetic domains in NiO/Pt by performing concurrent transport and magneto-optical imaging measurements in an adapted Kerr microscope. By correlating the magnetic images of the antiferromagnetic domain changes and magneto-transport signal response in these current-induced switching experiments, we disentangle magnetic and non-magnetic contributions to the transport signal. Our table-top approach establishes a robust procedure to subtract the non-magnetic contributions in the transport signal and extract the spin-Hall magnetoresistance response associated with the switching of the antiferromagnetic domains, enabling one to deduce details of the antiferromagnetic switching from simple transport measurements.

Parameterization of dilute Ising model for iron-containing lanthanum gallate and aluminate solid solutions based on first-principles calculations

Calculations of the cubic models of two solid solutions LaGa0.5Fe0.5O3 and LaAl0.5Fe0.5O3 have been performed within the hybrid density functional theory. Multiple configurations have been considered for the solid solutions resulting from a different distribution of Fe atoms over the p-metal (Ga or Al) positions accounting for different spin orientation for Fe atoms themselves. It was demonstrated that 27 structures for both LaGa0.5Fe0.5O3 and LaAl0.5Fe0.5O3 should be treated to account for all the possible configurations in case of cubic 2 × 2 × 2 supercell. Optimized geometry, energy, and electron properties were calculated for all the obtained configurations. Statistical weights and probabilities were estimated for each symmetry non-equivalent configuration of the solid solutions within the canonical ensemble. A new parameterization for the dilute Ising model has been proposed. In this model, we account for non-magnetic contributions, which are absent in the simple Ising model, using the lattice approach based on the concept of interchange energy. Two model parameters (the magnetic coupling constant and interchange energy) were fitted to the calculated total energies of all considered configurations of both solid solutions. The dilute Ising model confirmed the benefit of Fe-clustering in doped lanthanum gallate against aluminate. Different signs of the estimated interchange energy enable us to explain the reasons for such differences.

The low temperature specific heat of single crystal Er-Al2 compound

The low temperature heat capacity of the ErAl2 single crystal compound have been measured in the temperature 2– 40 K using a modified adiabatic continues heating technique. The magnetic contributions to the total specific heat were extracted using similar but nonmagnetic “lattice blank”, which gives a good description of the non-magnetic contributions to the heat capacities. The magnetic specific heat exhibits large anomaly at the highly cooperative magnetic phase transition, which lies at about 12 K. A broad Schottky anomaly was shown centered at about 30 K. The magnetic contributions to the heat capacity of the ErAl2 compound, Cm(T) have been calculated using mean field approximation. The Hamiltonian has been taken to be the sum of crystal field and isotropic exchange terms. A computer program based on the molecular field approximation has been written to calculate the crystal field levels and the magnetic contributions to the heat capacity as a function of temperature in the ErAl2 compound. The critical exponent analysis showed that α′ ≈ α and A′/A ≈ 1.9. The low temperature magnetic specific heats behave as T3/2 in support of the spin wave excitations predictions.

Magnetic and nonmagnetic contributions to the heat capacity of metamagnetic shape memory alloy

A quantitative theoretical analysis of the heat capacity of metamagnetic shape memory alloy (MMSMA) is reported. It describes the peculiarities of the heat capacity of Ni-Mn-Sn alloy, which undergo a paramagnetic-ferromagnetic phase transition and a magnetostructural phase transformation (MPT) from ferromagnetic austenite to a weakly magnetic martensite. Good agreement between the experimental and theoretical temperature dependences of the heat capacity is observed. It is shown that the heat capacity of MMSMA can be presented as the sum of three terms: (i) a nonmagnetic term describing the heat capacity of crystal lattice, when the alloy temperature is well below the MPT temperature or above Curie temperature; (ii) a nonmagnetic term caused by the martensitic lattice distortion, which arises in the course of magnetostructural transformation; and (iii) a magnetic term related to the magnetic ordering of austenite and martensite. For Ni-Mn-Sn alloy, all terms appeared to be comparable in magnitude and, therefore, none of them can be ignored a priori. A similar behavior is expected for other MMSMAs.

Dynamics of exciton magnetic polarons in CdMnSe/CdMgSe quantum wells: Effect of self-localization

We study the exciton magnetic polaron (EMP) formation in (Cd,Mn)Se/(Cd,Mg)Se diluted-magnetic-semiconductor quantum wells using time-resolved photoluminescence (PL). The magnetic field and temperature dependencies of this dynamics allow us to separate the non-magnetic and magnetic contributions to the exciton localization. We deduce the EMP energy of 14 meV, which is in agreement with time-integrated measurements based on selective excitation and the magnetic field dependence of the PL circular polarization degree. The polaron formation time of 500 ps is significantly longer than the corresponding values reported earlier. We propose that this behavior is related to strong self-localization of the EMP, accompanied with a squeezing of the heavy-hole envelope wavefunction. This conclusion is also supported by the decrease of the exciton lifetime from 600 ps to 200 - 400 ps with increasing magnetic field and temperature.

Lattice specific heat for the RMIn5 (R=Gd, La, Y; M=Co, Rh) compounds: Non-magnetic contribution subtraction

We analyze theoretically a common experimental process used to obtain the magnetic contribution to the specific heat of a given magnetic material. In the procedure, the specific heat of a non-magnetic analog is measured and used to subtract the non-magnetic contributions, which are generally dominated by the lattice degrees of freedom in a wide range of temperatures.We calculate the lattice contribution to the specific heat for the magnetic compounds GdMIn5 (M=Co, Rh) and for the non-magnetic YMIn5 and LaMIn5 (M=Co, Rh), using density functional theory based methods. We find that the best non-magnetic analog for the subtraction depends on the magnetic material and on the range of temperatures. While the phonon specific heat contribution of YRhIn5 is an excellent approximation to the one of GdCoIn5 in the full temperature range, for GdRhIn5 we find a better agreement with LaCoIn5, in both cases, as a result of an optimum compensation effect between masses and volumes. We present measurements of the specific heat of the compounds GdMIn5 (M=Co, Rh) up to room temperature where it surpasses the value expected from the Dulong–Petit law. We obtain a good agreement between theory and experiment when we include anharmonic effects in the calculations.

Electric contributions to magnetic force microscopy response from graphene and MoS2 nanosheets

Magnetic force microscopy (MFM) signals have recently been detected from whole pieces of mechanically exfoliated graphene and molybdenum disulfide (MoS2) nanosheets, and magnetism of the two nanomaterials was claimed based on these observations. However, non-magnetic interactions or artefacts are commonly associated with MFM signals, which make the interpretation of MFM signals not straightforward. A systematic investigation has been done to examine possible sources of the MFM signals from graphene and MoS2 nanosheets and whether the MFM signals can be correlated with magnetism. It is found that the MFM signals have significant non-magnetic contributions due to capacitive and electrostatic interactions between the nanosheets and conductive cantilever tip, as demonstrated by electric force microscopy and scanning Kevin probe microscopy analyses. In addition, the MFM signals of graphene and MoS2 nanosheets are not responsive to reversed magnetic field of the magnetic cantilever tip. Therefore, the observed MFM response is mainly from electric artefacts and not compelling enough to correlate with magnetism of graphene and MoS2 nanosheets.

Assessments of coefficients of linear thermal expansions for magnetic elements Fe, Co and Ni

The coefficients of linear thermal expansions (CLEs) of magnetic elements Fe, Co and Ni were assessed from experimental information using theoretical models combined with MATLAB calculations. Model parameters can be determined accurately, and the assessed data are in good agreement with the experimental results. To facilitate the assessments, theories of thermal expansion were applied to separate CLEs into its nonmagnetic and magnetic components. The calculations of nonmagnetic contribution to CLEs were based on the modified Gruneisen–Debye model, in which the Debye temperature was regarded as an undetermined constant. In order to put the prediction of CLEs at the magnetic transition region on a sound physical basis, two kinds of theoretical models were innovatively used to calculate the magnetic contribution to CLEs, i.e., the Bragg–Williams model and the Fermi–Dirac distribution function. Model parameters were evaluated from experimental data using least square method. Detailed comparisons were made with the published experimental data and the calculated total CLEs. A satisfactory agreement is reached.

Paramagnetic dust particles in rf-plasmas with weak external magnetic fields

Experimental studies on dusty plasmas containing systems of (super-)paramagnetic dust particles are presented. In our experiments, external (homogeneous as well as inhomogeneous) magnetic fields in the mT range are applied to study the effect on single particles or few-particle systems that are trapped inside the sheath region. The behavior of the paramagnetic dust particles is considerably different than that of dielectric plastic particles, which are widely used in dusty plasmas. It is revealed that especially non-magnetic contributions play an important role in the interaction between superparamagnetic particles.

Nonlocal nonlinear magneto-optical response of a magnetoplasmonic crystal

Surface plasma resonance (SPR) excitation in a Co-based two-dimensional magnetoplasmonic crystal was found to strongly enhance the second-harmonic generation (SHG) efficiency. Large changes in the phase shift between magnetic and nonmagnetic SHG contributions were observed in the transversal Kerr effect as a function of the incidence angle. The activation of a nonlocal quadrupole mechanism of the magnetic SHG due to the SPR excitation was found to lead to an unusual phase behavior in the vicinity of the resonance.

Ultrafast element-specific magnetization dynamics of complex magnetic materials on a table-top

We review recent progress in femtosecond magnetization dynamics probed by extreme ultraviolet pulses from high-harmonic generation. In a transverse magneto-optical Kerr geometry, we established an ultrafast, element-specific experimental capability – on a table-top – for the measurement of magnetization dynamics in complex multi-sublattice magnets and multilayer magnetic structures. We show that this newly introduced technique is an artifact-free magnetic sensor, with only negligible non-magnetic (optical) contributions from the transient variation of the refractive index due to the presence of a non equilibrium hot-electron distribution. We then use these new experimental capabilities of ultrahigh time-resolution, combined with element-specific simultaneous probing, to disentangle important microscopic processes that drive magnetization dynamics on femtosecond timescales. We elucidate the role of exchange interaction on magnetization dynamics in strongly exchange-coupled alloys, and the role of photo-induced superdiffusive spin currents in magnetic multilayer stacks.

Electron- and phonon-mediated ultrafast magnetization dynamics of Gd(0001)

Here we report on the ultrafast magnetization dynamics of Gd(0001), which we investigated as a function of equilibrium temperature by employing the femtosecond time-resolved magneto-optical Kerr effect (MOKE) and modeling by the Landau-Lifshitz-Bloch equation in combination with the two-temperature model. Based on the observed temperature-dependent transient MOKE signals, we separate the magnetization dynamics into two regimes at delays of (i) a few picoseconds and (ii) several 100 femtoseconds. In the picosecond regime, the demagnetization time determined from the experiment increases with temperature from 0.8 ps at 50 K to 1.5 ps at 280 K. A successful description of this observation was achieved by considering the dynamics of the $4f$ spin system coupled to $5d$ conduction electrons within two coupling mechanisms: (a) through electronic scattering and (b) spin-flip scattering mediated by phonons. We conclude that at temperatures below the Debye temperature, a hot electron-mediated process describes the experimentally found demagnetization times of $\ensuremath{\approx}$$0.8$ ps well. At higher temperatures phonon-mediated processes have to be included to explain the 2 times longer demagnetization time. In the second regime at time delays of few 100 fs we find an increase in the MOKE rotation and ellipticity at 50 K at delays before demagnetization sets in. Above 50 K the transient changes in rotation and ellipticity are of opposite sign. We explain this behavior by competing magnetic and nonmagnetic contributions in the transient MOKE signals at these delays directly after optical excitation when excited phonons do not yet facilitate angular momentum transfer to the lattice.

Magnetic properties of cobalt nanowires: Study by polarized SANS

Structural and magnetic properties of two-dimensional spatially ordered system of ferromagnetic cobalt nanowires embedded into Al2O3 matrix have been studied using polarized small-angle neutron scattering. A comprehensive analysis of contributions to the scattering intensity was carried out, including nonmagnetic (nuclear) contribution, magnetic contribution depending on the magnetic field, and nuclear–magnetic interference indicating the correlation between the magnetic and nuclear structures. Experiments have revealed an anomalously low value of the magnetic contribution as compared to the nuclear one. This behavior is interpreted in terms of low coherence of the magnetic structure caused by the anisotropy of Co crystallites as compared with the large coherency of nuclear structure of nanowires.