Ultrafast heating of FeRh by a femtosecond laser pulse launches a magneto-structural phase transition from an antiferromagnetic to a ferromagnetic state. Aiming to reveal the ultrafast kinetics of this transition, we studied magnetization dynamics with the help of the magneto-optical Kerr effect in a broad range of temperatures (from 4 K to 400 K) and magnetic fields (up to 25 T). Three different types of ultrafast magnetization dynamics were observed and, using a numerically calculated H-T phase diagram, the differences were explained by different initial states of FeRh corresponding to a (i) collinear antiferromagnetic, (ii) canted antiferromagnetic and (iii) ferromagnetic alignment of spins. We argue that ultrafast heating of FeRh in the canted antiferromagnetic phase launches practically the fastest possible emergence of magnetization in this material. The magnetization emerges on a time scale of 2 ps, which corresponds to the earlier reported time-scale of the structural changes during the phase transition.

The integration of micromagnets in microsystems is still at its infancy. There is no well-established technology for patterning permanent magnets with in-plane resolution and thickness on the order of 1 mu m, which is highly desirable for obtaining sizable stray fields in integrated devices. Here we report on the magnetic and structural properties of the W(100)/SmCo(500)/W(100) heterostructure (thickness in nm) after patterning by ion beam etching. Continuous trilayers W/SmCo/W are deposited by sputtering on a Si/SiO 2 substrate and then annealed at 650 degrees C to achieve a remanence magnetization mu 0 Mr = 0.45 T. Rectangular micromagnets (150 x 20 x 0.5 mu m 3 ) are then patterned by ion beam etching. The patterning does not significantly affect the magnetic properties of the SmCo film, apart from the appearance of a vertical shift in the hysteresis loops measured in the +/- 2 T range, which is ascribed to the hardening of magnetic phases upon ion bombardment. As a side effect of etching, tapered edges of the magnets are obtained. However, the stray field generated above 1 mu m height from the magnets is not significantly perturbed by a tapering with an angle up to 45 degrees. This work demonstrates the suitability of our fabrication process for the integration of SmCo permanent micro-magnets in micromechanical system (MEMS).

One of the most appealing features of magnonics is the easy tunability of spin-wave propagation via external magnetic fields. Typically, this requires bulky and power-hungry electromagnets, which are not compatible with device miniaturization. Here, we propose a different approach, exploiting the stray field from permanent micromagnets integrated on the same chip of a magnonic waveguide. In our monolithic device, we employ two SmCo square micromagnets (10 x 10 mu m2) flanking a CoFeB conduit at different distances from its axis, which produces a tunable transverse bias field between 7.5 and 3.0 mT in the conduit region between the magnets. This field is large enough to significantly affect the spin-wave propagation, when an external transverse bias field of 60 mT is applied to stabilize the Damon-Eshbach configuration. Spin waves excited by an antenna just outside the region between the magnets, indeed, enter a region with a variable higher (or lower) effective field depending on the parallel (or antiparallel) alignment between the external and micromagnets fields. Consequently, the attenuation length and phase shift of Damon-Eshbach spin waves can be tuned in a wide range by changing the parallel-antiparallel configuration of the external bias and the distance between SmCo micromagnets and the CoFeB conduit. This work demonstrates the potential of permanent micromagnets for the realization of low-power, integrated magnonic devices with tunable functionalities.

In W/Gd/W/MgO heterostructures, the dependence of mechanical stresses in the Gd film on the crystallographic orientation of the MgO substrate was revealed. Variations in the interplanar spacings in MgO in different orientations create tensile elastic stresses up to 0.22 GPa in the Gd film, which are transferred through the damping layer W. It is found that these stresses affect the isothermal magnetization curves, the corresponding change in the magnetic part of the entropy at the Curie point T_c=293 K, and the relative cooling capacity (RCP). This allows us to consider mechanical stresses as a factor in controlling the magnetocaloric cycle, which increases the efficiency of the refrigeration machine, when mechanical loading is synchronized with the heating-cooling cycles of the ferromagnet. Keywords: magnetocaloric effect, entropy, microdeformations, microstresses.

Integration of inductors on silicon chips is becoming more and more relevant for monolithic electronic applications. In this Letter, we investigate the impact of minor hysteresis loops in an integrated inductor with spiral geometry sandwiched between two soft magnetic layers made of MoNiFe/Cr multilayers. Despite high magnetic susceptibility and low coercivity of optimized multilayers, we find that the inductance is strongly dependent on the AC voltage applied to the device, showing a bell-shaped behavior. Comparing the measured inductance with magneto-optical Kerr effect and vibrating sample magnetometry measurements, we show that the low-signal behavior is limited by domain wall pinning/depinning, which determine the effective susceptibility associated with minor hysteresis loops driven by the applied AC voltage.

Anomalous Hall effect (AHE) in GdFeCo/Ir/GdFeCo multilayered structures attracts great interest because all optical switching, spin-torque, and other effects promise effective application for ultrafast memory element creation. Since AHE is controlled by GdFeCo magnetization, domain dynamics has importance for practical applications. In our work, magnetization reversal in perpendicular GdFeCo/Ir/GdFeCo synthetic ferrimagnets is characterized by AHE measurements. The AHE hysteresis loop obtained with the field applied perpendicular to the sample plane is composed of three sub-loops, and two of them are symmetrically biased with respect to the third one. Switching magnetic fields for two of the three transitions are found to be dependent on magnetic history. In particular, exposure of the sample in the in-plane field leads to reduction of the out-of-plane switching fields in side sub-loops. A multiple series of perpendicular hysteresis loops recorded after exposure under high in-plane field reveals gradual (within 30 min) relaxation of the out-of-plane switching fields to their initial values observed in a non-magnetized sample. Domain wall mobility, limiting switching of the bilayer devices, is complicated due to the coupling between partial domains in each single layer. Unusual dynamics of double domain walls results in unexpected new phenomena affecting electrical processes in bilayer structures.

In synthetic ferrimagnets with perpendicular anisotropy GdFeCo/Ir/GdFeCo, the dependence of the hysteresis loops of the anomalous Hall resistance and the characteristics of the loops on the angle between the magnetic field and the plane of the sample are analyzed. The part of the anomalous Hall resistance corresponding to the spin-orbit torque has been identified. The field dependences of the resistance are sensitive to switching between the magnetic states of the two-layer ferrimagnet and they reproduce the shapes of the magnetization hysteresis loops, calculated with the interlayer exchange interaction, crystal anisotropy, and Zeeman energies at different angles between the field and sample. A slow (~30 min) magnetic relaxation of resistivity hysteresis after reorientation of the sample in magnetic field was found. Specific domain dynamics inherent in two-layer samples was revealed by Kerr microscopy. It was found that slow restoration of the resistivity hysteresis loop is due to domain propagation.

In synthetic ferrimagnets with perpendicular anisotropy GdFeCo/Ir/GdFeCo, the dependence of the hysteresis loops of the anomalous Hall resistance and the characteristics of the loops on the angle between the magnetic field and the plane of the sample are analyzed. The part of the anomalous Hall resistance corresponding to the spin-orbit torque has been identified. The field dependences of the resistance are sensitive to switching between the magnetic states of the two-layer ferrimagnet and they reproduce the shapes of the magnetization hysteresis loops, calculated with the interlayer exchange interaction, crystal anisotropy, and Zeeman energies at different angles between the field and sample. A slow (~30 min) magnetic relaxation of resistivity hysteresis after reorientation of the sample in magnetic field was found. Specific domain dynamics inherent in two-layer samples was revealed by Kerr microscopy. It was found that slow restoration of the resistivity hysteresis loop is due to domain propagation. Keywords: spin Hall effect, spin-orbit torque, magnetic relaxation, domain wall dynamics.

Tb7Fe90B3 microwires with a diameter of 50-100 μm were obtained by the method of ultrafast cooling of the melt. It has been established that the microwires contain the tetragonal Tb2Fe14B1 phase and the cubic TbFe3 phase. These two phases differ in saturation fields of 100 Oe and 10 kOe, respectively. The anisotropy of the coercive force of TbFeB microwires also indicates the coexistence of two magnetic phases. The data obtained are compared with the magnetic properties of PrDyFeCoB microwires, in which the coercive force is isotropic, and magnetization saturation is achieved in lower fields.

In single-ion complexes based on Co 2+ ions, the second and third harmonics of the magnetic susceptibility were found at temperatures of 2-4 K, which exceed the Neel temperature. The maxima of the second and third harmonics of the magnetic susceptibility are observed at a frequency of ~1 Hz, at which the maximum of the first harmonic is observed in a field of 3.2 kOe. An analysis of the dependences of the second and third harmonics of the magnetic susceptibility on the field and temperature showed that the nonlinearity arises as a result of the formation of a spin glass state at temperatures slightly higher than the Neel temperature. In this state, there is no long-range spin order, but there are spin clusters in the spin glass state. The spin-glass state in a compound with a Co 2+ ion with a high magnetic anisotropy is unusual in that the exchange interaction is much smaller than the single-ion anisotropy energy. Keywords: molecular magnets, nonlinear magnetic susceptibility, nanostructures, spin dynamics, spin glass.

Ferromagnetic microparticles significantly affect spin relaxation in the Er3+ single-ion magnet sandwiched in a composite material. The balance of thermal spin noise corresponding to Orbach, Raman, quantum tunneling, and direct relaxation channels is shifted in Er3+ complexes under the influence of surrounding ferromagnetic matrix. There are two competing sources of the electron spin noise controlled by ferromagnetic media. First, internal residual magnetic field delays spin relaxation in the Er3+ complexes due to the Zeeman interaction of the Er3+ spin even in the absence of external magnetic field. Second, chemical bonding between the Er 4d shell and the O 1s shell accelerates magnetic relaxation in the Er3+ ions on the surface of microparticles. Significance of these results is that composite media can be sliced into small elements with a variable frequency of spin noise depending on individual magnetization programmed within each element.

An analysis of the temperature and field dependences of the magnetization of Gd films and microwires was carried out, as well as an isothermal measurement of the magnetic part of the entropy at the Curie temperature. The magnetocaloric effect (MCE), measured as an isothermal increase in the magnetic part of the entropy, in microwires shows two peaks on the temperature dependence of the magnetic part of the entropy, in contrast to one peak in films. In films and microwires, the entropy maximum at 286–293 K, which corresponds to the Curie temperature, depends on the magnetic field, shifting in the same way in films and microwires with an increase in the field at the orientation of the MgO (111) substrate, which provides the maximum mechanical stresses in Gd. In microwires, the second maximum does not change the temperature of 320 K as the field increases to 9 T, but its amplitude increases linearly with the field. This maximum can be caused by a spin-reorientation transition.

In W/Gd/W/MgO heterostructures, the dependence of mechanical stresses in the Gd film on the crystallographic orientation of the MgO substrate was revealed. Variations in the interplanar spacings in MgO in different orientations create tensile elastic stresses up to 0.22 GPa in the Gd film, which are transferred through the damping layer W. It is found that these stresses affect the isothermal magnetization curves, the corresponding change in the magnetic part of the entropy at the Curie point Tc = 293 K, and the relative cooling capacity (RCP). This allows us to consider mechanical stresses as a factor in controlling the magnetocaloric cycle, which increases the efficiency of the refrigeration machine, when mechanical loading is synchronized with the heating-cooling cycles of the ferromagnet.

An analysis of the temperature and field dependences of the magnetization of Gd films and microwires was carried out, as well as an isothermal measurement of the magnetic part of the entropy at the Curie temperature. The magnetocaloric effect (MCE), measured as an isothermal increase in the magnetic part of the entropy, in microwires shows two peaks on the temperature dependence of the magnetic part of the entropy, in contrast to one peak in films. In films and microwires, the entropy maximum at 286-293 K, which corresponds to the Curie temperature, depends on the magnetic field, shifting in the same way in films and microwires with an increase in the field at the orientation of the MgO (111) substrate, which provides the maximum mechanical stresses in Gd. In microwires, the second maximum does not change the temperature of 320 K as the field increases to 9 T, but its amplitude increases linearly with the field. This maximum can be caused by a spin-reorientation transition. Keywords: microwires, thin films, magnetic entropy, magnetic anisotropy, spin-reorientation transition, Curie temperature.

Thick SmCo films of 500 nm thickness were deposited by RF sputtering in W/SmCo/W structures on Si substrate. After annealing at 650 - 750ºC, the as grown soft amorphous structure transforms into a mixture of crystalline Sm2Co17 and SmCo5 hard magnetic phases. Annealing at 650°C leads to film crystallization with an average grain size of 64 nm, coercivity of 0.5 T and remanence magnetization of about 0.5 T for a maximum applied field of 2 T. The remanence magnetization decreases by 20% upon annealing at 750°C, while the average crystalline size and coercivity increase up to 73 nm and 1.1 T, respectively. Series of the First Order Reversal Curves (FORC) recorded in the samples annealed at 650 ºC and at 750 ºC demonstrate redistribution of the switching fields between the softer (Sm2Co17) and harder (SmCo5) phases depending on the strength of interphase interaction. Overall, the higher remanence and sizable coercivity of films annealed at 650°C make them good candidates for the fabrication of micromagnets to be integrated in MEMS systems.

A new method is developed to control the spin relaxation in single-molecular magnets (SMMs) in order to eliminate spin decoherence to the level acceptable for quantum computing at a relaxation frequency of about 10(2) Hz and a temperature of 2 K. A significant part of the SMMs has rapid magnetic relaxation proceeding through several parallel channels sensitive to the presence of an external magnetic field. Some of the relaxation channels in such materials (also called single-ion magnets (SIMs)) are suppressed using an electromagnet in macroscopic volumes of complexes. This is unacceptable when individual SIM complexes are used as qubits and forces us to look for ways to use a local magnetic field and other types of complex interactions in a specially selected environment, which provides the Zeeman interaction in the absence of an external field. We demonstrate that a composite material made of SIM complexes with Er3+ ions and ferromagnetic microparticles exhibits a remanent magnetization, which is sufficient to decrease the spin relaxation frequency in the volume. In magnitude, this effect competes with the well-known effect of hybridization of the orbitals of a complex during its interaction with a metallic surface. Therefore, the microstructuring of an array of complexes in a ferromagnetic matrix can be used to create local regions with a controlled magnetic relaxation frequency.

The integration of magnetic materials in conventional micro electromechanical systems (MEMS) is becoming a hot topic for various applications. Integrated micromagnets are used for magnetic actuation or energy harvesting, due to the sizable forces they allow to produce on movable parts without friction. On the other hand, movable micromagnets can be exploited for reconfiguring the functionalities of integrated magnetic devices exposed to their stray field. Soft magnetic layers are instead used to confine or create magnetic field gradients to be used for sensing applications. In this talk I'll present three specific applications (i) the realization of MEMS magnetometers based on magnetic microresonators interacting with the gradient of a magnetic field produced by a magnetic flux concentrator or a superconductive coil, (ii) the implementation of a magnetic plucking technology for frequency up-conversion in energy harvester and (iii) the development of hybrid magnonic-MEMS devices where the bias field for the propagation of spin waves in spin-textures is provided by some movable micromagnets integrated in a MEMS.

In monoionic complexes based on Co2+ ions, the second and third harmonics of the magnetic susceptibility were found at temperatures of 2–4 K, which exceed the Néel temperature. The maxima of the second and third harmonics of the magnetic susceptibility are observed at a frequency of ~1 Hz, at which the maximum of the first harmonic is observed in a field of 3.2 kOe. An analysis of the dependences of the second and third harmonics of the magnetic susceptibility on the field and temperature showed that the nonlinearity arises as a result of the formation of a spin glass state at temperatures slightly higher than the Néel temperature. In this state, there is no long-range spin order, but there are spin clusters in the spin glass state. The spin-glass state in a compound with a Co2+ ion with a high magnetic anisotropy is unusual in that the exchange interaction is much smaller than the single-ion anisotropy energy.

An analysis of the field and time dependencies of the magnetization of PrDyFeCoB amorphous crystalline microwires is presented. It is found that the magnetization curve near the saturation field has a smoother approximation to saturation than in a ferromagnet, according to theoretical predictions for the spin-glass state of the alloy in the amorphous state within the framework of the random magnetic anisotropy model. Deviations of the dynamics of relaxation of the magnetization of microwires from the logarithmic time dependence and the disappearance of this difference when observing magnetic relaxation in a magnetic field are found. This indicates the typical dynamics of the magnetization reversal of a spin glass in a zero field and the ferromagnetic character of demagnetization in a nonzero field. The results indicate the presence in the PrDyFeCoB microwires of an exotic magnetic state of the material with stochastic local magnetic anisotropy. Keywords: spin glass, ferrimagnet, rare earth alloys, random magnetic anisotropy.

Ta|Pt|GdFeCo|IrMn|Pt multilayer structure with perpendicular magnetic anisotropy in vicinity of the ferrimagnet compensation temperature TK has been carried out. It has been established that the distribution of the magnetic moment in the GdFeCo ferrimagnetic film exchange-coupled to the IrMn antiferromagnet is largely determined by the magnitude and orientation of both the field applied during cooling from room temperature to T = 2 K and the testing field over the entire temperature range. It is shown that the direction of the domain wall moving changes to the opposite one at a fixed value of the amplitude of the magnetic field pulse when the temperature passes through the TK.