Iron-deficient Ni-Zn spinel ferrite ceramic with composition (Ni_0.66Zn_0.34)_2.3Fe_1.6O₄ was synthesized via hydroxide co-precipitation followed by sintering at 1380 °C. Rietveld refinement of XRD data identified a two-phase structure comprising ferrite (Ni_0.6Zn_0.4Fe₂O₄, 65.6 wt%) and a halite phase (Ni_0.7Zn_0.3O, 34.4 wt%). SEM and EDX mapping confirmed a strong compositional inhomogeneity with spatially separated micrometer-sized Fe-rich and Fe-deficient regions. Magnetization measurements showed reduced saturation magnetization of the ferrite phase, deviation from Bloch's law at low temperatures, and a blocking (freezing) temperature of ∼145 K. These features are attributed to magnetic inhomogeneity within the ferrite phase. It arises from strain-induced spin canting, magnetic clustering, and partial magnetic decoupling caused by the halite phase. The results demonstrate that superparamagnetic-like behavior originates from intragranular magnetic heterogeneity rather than particle size effects.

It is demonstrated that two types of bilinear interlayer exchange coupling (IEC), antiferromagnetic and ferromagnetic, coexist in nanofilms with a paramagnetic spacer. Direct antiferromagnetic IEC is temperature-independent, while indirect ferromagnetic-type IEC arises from spacer ion magnetization, making it proportional to the spacer's magnetic susceptibility. This IEC competition was investigated in an Fe/Cr/FeCr/Cr/Fe structure in the temperature range of 200-400 K. The interplay of these couplings causes a transition from antiferromagnetic behavior at high temperatures to ferromagnetic-type magnetization at low temperatures. The magnetic layers lack growth-induced anisotropy, resulting in equivalent in-plane directions. Experimental hysteresis loops are identical for all field orientations, indicating rotatable-type magnetic anisotropy. A theoretical model incorporating both IEC types and rotatable anisotropy accurately describes the observed magnetic reordering and predicts the corresponding switching fields. This work provides a comprehensive framework for understanding thermally controlled exchange interactions and anisotropy in synthetic antiferromagnets.

Temperature dependent magnetic properties of superconductor-ferromagnet-superconductor (SC/FM/SC) trilayers are studied both experimentally and theoretically, with a focus on ferromagnetic resonance (FMR). The influence of the SC and FM layer thicknesses on the FMR field is examined. To differentiate the mechanisms involved, we additionally investigate structures containing nonmagnetic metallic (M) or insulating (I) spacers (SC/FM/M/SC or SC/FM/I/SC). All the studied multilayers show large reductions in the FMR field below the critical temperature of the SC, except the system containing an insulating spacer (SC/FM/I/SC). This SC-induced FMR-shift (resonance field/frequency) is larger for thicker SC as well as FM layers, reaching a saturation value for very large thicknesses. To explain the measured results, an analytical model is developed, in which the FM-magnetization precession modulates the magnetic flux in the system, thereby inducing an alternating supercurrent in the SC, which in turn produces a dynamic back-action magnetic field on the FM that shifts its resonance frequency. The model considers closed current loops, where the FM layer conductively links the supercurrents flowing in the opposite directions in the two outer SC layers. Our results provide a practical route for increasing the operating frequency of magnonic devices.

Two Ni–Zn spinel ferrites samples with different iron contents were investigated by quantitative X-ray diffractometry and magnetization measurements. The Fe-deficient sample was found to hold 34.4 wt% of halite phase Ni_0.7Zn_0.3O in addition to primary spinel ferrite phase with Ni_0.6Zn_0.4Fe₂O₄ composition, while the near-stoichiometric Ni_0.7Zn_0.3Fe₂O₄ sample contained only 5 wt% of Ni_0.7Zn_0.3O halite. Magnetization study indicated preferentially ferrimagnetic ordering in both samples. The coefficient of magnetocrystalline anisotropy K₁ was calculated from the magnetization data using the law of approach to saturation. The results revealed significant increase of K₁ in Fe-deficient sample that can be employed in microwave absorption applications.

Два зразки нікель-цинкової ферит-шпінелі з різним вмістом заліза досліджувались за допомогою кількісного рентгеноструктурного аналізу та вимірювань намагніченості. Зразок зі зниженим вмістом заліза містив 34,4 мас. % фази галіту Ni0,7Zn0,3O на додаток до первинної фази ферит-шпінелі Ni0,6Zn0,4Fe2O4 , тоді як зразок зі складом близьким до стехіометрії, окрім Ni0,7Zn0,3Fe2O4 , містив лише 5 мас. % галіту Ni0,7Zn0,3O. Дослідження намагніченості продемонстрували переважання феримагнітного впорядкування в обох зразках. Коефіцієнт магнітокристалічної анізотропії K1 розраховано на основі даних намагніченості за методом закону набли ження до насичення. Результати показали значне зростання K1 у зразку зі зниженим вмістом заліза, що може бути засто совано для поглинання випромінювання у мікрохвильовому діапазоні.

We analytically solve the Landau-Lifshitz equations for the collective magnetization dynamics in a synthetic antiferromagnet (SAF) nanoparticle and uncover a regime of barrier-free switching under a short small-amplitude magnetic field pulse applied perpendicular to the SAF plane. We give examples of specific implementations for forming such low-power and ultra-fast switching pulses. For fully optical, resonant, barrier-free SAF switching we estimate the power per write operation to be ∼100 pJ, 10–100 times smaller than for conventional quasi-static rotation, which should be attractive for memory applications.

Spin dynamics in Al-substituted yttrium iron garnets is investigated using broadband ferromagnetic resonance measurements in the temperature range T=200-360 K. Using the measured data, the resonance field and linewidth as well as their temperature dependence are determined, with implications for the uniformity and overall quality of the samples prepared via different chemical fabrication routes. These key parameters governing the spin dynamics in the material are important for its applications in high-speed spintronic and magnonic devices.

Spin dynamics in Al-substituted yttrium iron garnets is investigated using broadband ferromagnetic resonance measurements in the temperature range T = 200–360 K. Using the measured data, the resonance field and linewidth, as well as their temperature dependence, are determined, with implications for the uniformity and overall quality of the samples prepared via different chemical fabrication routes. These key parameters governing the spin dynamics in the material are important for its applications in high-speed spintronic and magnonic devices.

We observe strongly nonlinear spin dynamics in ferromagnetic/antiferromagnetic multilayers, controlled by the number of bilayers in the system, layer thicknesses, and temperature, peaking in magnitude near the Néel point of the antiferromagnetic layers just above room temperature. Well above the Néel transition, the individual ferromagnetic layers are exchange decoupled and resonate independently. As the temperature is lowered toward the Néel point, the ferromagnetic proximity effect through the thin antiferromagnetic spacers transforms the system into a weakly coupled macrospin chain along the film normal, which exhibits pronounced standing spin-wave resonance modes, comparable in intensity to the uniform resonance in the ferromagnetic layers. These findings are supported by our micromagnetic simulations showing clear spin-wave profiles with precessional phase lag along the macrospin chain. Well below the Néel transition, the FeMn layers order strongly antiferromagnetically and exchange-pin the ferromagnetic layers to effectively make the multilayer one macrospin. The appearance and intensity of the high-frequency spin-wave modes can thus be conveniently controlled by thermal gating the multilayer. The nonlinearity in the microwave response of the demonstrated material can approach 100%, which is large compared to nonlinear materials used in, e.g., optics, with second-harmonic generation often at the single percentage level.

Magnetic properties of Y3AlFe4O12 garnet ceramics have been studied over a wide range of temperature (3 – 370 K) and magnetic fields (up to 2 kOe). Effects of varying the temperature on some of the application-specific magnetic characteristics have been analyzed in detail. With the decrease in temperature, the effective anisotropy constant, KEff, is found to sharply rise below ∼150 K, while the saturation magnetization, Ms, changes only slightly (<20 % within 3 – 250 K). The exchange stiffness, Aex, and spin wave stiffness, D, parameters mirror the relatively smooth behavior of the magnetization at low temperatures but display a rapid drop when T approaches TC ≈ 436 K. The temperature dependence of the domain wall thickness and the critical single-domain size correlate with the corresponding values for pure Y3Fe5O12 (YIG) and are in agreement with the theoretical estimates. We conclude by discussing the ways of how to use the obtained results for tailoring the magnetic properties of YIG-based materials for technological applications.