Search

Thin film ferromagnet/antiferromagnet (F/AF) exchange biased structures that are widely used in GMR spin valves are considered nowadays as promising systems for antiferromagnetic spintronic and spin-orbitronic devices. Here, the temperature dependences of magnetization dynamics in Co/IrMn and Co/FeMn F/AF structures are investigated using ferromagnetic resonance (FMR) in comparison to a free Co layer. A strong additional decrease in the resonance field was observed in Co/IrMn with a temperature decrease attributed to the rotatable anisotropy increase, which almost vanished at room temperature. In contrast to Co/IrMn, the contribution of the rotatable anisotropy in Co/FeMn is much weaker, even though it exists at RT, it is negative, and slightly varies with the temperature and resonance field shift in Co/FeMn. This is mainly due to unidirectional exchange anisotropy. FMR linewidth for the free Co layer increases with decreasing temperature and is accompanied with a slow relaxation process, while the additional contribution to FMR line broadening in Co/IrMn and Co/FeMn structures is correlated with variation in the exchange anisotropy. The observed results are discussed based on structural and surface morphology and magnetization reversal characterization using X-ray diffraction, atomic force microscopy, and vibrating sample magnetometry data. [ABSTRACT FROM AUTHOR]

In this letter, we address a technical peculiarity of wide-field magneto-optical Kerr microscopy, which may easily lead to a misinterpretation of the domain contrast on curved surfaces. On the example of circumferentially magnetized domains in ferromagnetic microwires, we show that the checkerboard domain pattern, which typically shows up in polar Kerr sensitivity, is caused by the superposition of longitudinal Kerr contrasts that arise from in-plane magnetization components and inclined light incidence rather than from the magnetization components perpendicular to the focal plane as claimed in the literature. Experimental ways to avoid such peculiar contrasts are demonstrated. [ABSTRACT FROM AUTHOR]

Micromagnets in the form of microwires have become a subject of interest in various applications including magnetic field bias elements in miniature actuators, sensors, and manipulators. While the preparation of hard magnetic microwires with sufficiently high magnetization is a difficult task, it can be achieved by using specific crystallization techniques. We present a technique for the fabrication of micromagnets via controlled heat treatment of amorphous microwires using an electrical current. This crystallization technique converts a Co-rich microwire from a soft to a hard magnetic material. A large increase in coercivity (up to 580 Oe) is obtained after optimal current annealing (100 mA during 30–60 min). The crystallization consists of fine grains of mainly hcp-Co with an average size of 74 nm. [ABSTRACT FROM AUTHOR]

The magnetic behaviour of soft/hard biphase magnetic microwires has been studied as a function of temperature in the range 25 \ ^\circC to 900 ^\circC. The microwires consist of an ultrasoft CoFe-based vanishing magnetostriction amorphous core covered by insulating Pyrex coating prepared by quenching and drawing, plus an electroplated CoNi magnetically harder external shell. The magnetization process has been analyzed through the study of the hysteresis loops and their parameters like saturation magnetization and coercivity of the different phases, measured in a vibrating sample magnetometer. The magnetically biphase character has been first confirmed by room-temperature measurements for wires with different thickness. The observed dependence of magnetization and coercivity on temperature is correlated with the overcoming of the Curie temperature and crystallization of the amorphous soft core. [ABSTRACT FROM AUTHOR]

The effect of the annealing temperature Tann on the magnetic properties of cobalt ferrite nanoparticles embedded in an amorphous silica matrix (CoFe2O4/SiO2), synthesized by a sol-gel auto-combustion method, was investigated by magnetization and AC susceptibility measurements. For samples with 15% w/w nanoparticle concentration, the particle size increases from ~2.5 to ~7 nm, increasing Tann from 700 to 900 °C. The effective magnetic anisotropy constant (Keff) increases with decreasing Tann, due to the increase in the surface contribution. For a 5% w/w sample annealed at 900 °C, Keff is much larger (1.7 × 106 J/m3) than that of the 15% w/w sample (7.5 × 105 J/m3) annealed at 700 °C and showing comparable particle size. This indicates that the effect of the annealing temperature on the anisotropy is not only the control of the particle size but also on the core structure (i.e., cation distribution between the two spinel sublattices and degree of spin canting), strongly affecting the magnetocrystalline anisotropy. The results provide evidence that the magnetic anisotropy comes from a complex balance between core and surface contributions that can be controlled by thermal treatments. [ABSTRACT FROM AUTHOR]

Highlights • Trilayer Ni x Fe (1−x) (10нм)/IrMn(t AF)/Ni x Fe (1−x (10нм)/thin film structure. • Low-nickel, high-nickel permalloy. • Exchange bias, noncollinearity and magnetization of bottom and top ferromagnetic layers. • Ferromagnetic coupling through the antiferromagnetic spacer. • Rotatable anisotropy. Abstract Magnetic properties of trilayer double-exchange biased Si/Ta(30 nm)/Ni x Fe (1−x) (10 nm)/IrMn(t AF)/Ni x Fe (1−x (10 нм)/Ta(30 нм) thin film structures with x = 0.4 and x = 0.75 were investigated. The exchange bias, noncollinearity of unidirectional and uniaxial anisotropy and magnetization in both bottom and top ferromagnetic layers as a function of the antiferromagnetic (AF) layer thickness in the range from 2 to 50 nm were investigated. The obtained effects support the model of ferromagnetic coupling through the antiferromagnetic spacer and the model of rotatable anisotropy contribution at small AF thickness. It was found that usage of low-nickel permalloy in exchange biased systems has advantages of higher magnetization, a higher magnetic ordering at the interface and smaller critical thickness of antiferromagnetic layer to set the exchange bias, though provides a lower exchange bias compared with normal permalloy composition. [ABSTRACT FROM AUTHOR]

A method for calculating micromagnetic state of ferro- or ferrimagnetic single-crystals based on the Néel's method of phases is proposed. The standard Néel technique requires different approaches to calculation of micromagnetic state of samples with different anisotropy types. Furthermore, this technique cannot be used to calculate magnetization curves of materials with a complex anisotropy type, in which the first-order magnetization process (FOMP) occurs. On the contrary, the technique proposed in the present work makes it possible to calculate micromagnetic state of a sample within one unified approach. This technique has no limitations in terms of the anisotropy type as well. In case of the FOMP, the simulation methods that we used show results different from conventional calculation methods. The reason is that the conventional methods imply coherent rotation of magnetization in single domain particle (so-called Stoner–Wohlfarth model). We explain this discrepancy by the fact that a magnetic domain structure appears in the region of the FOMP. In the present work we show that magnetization processes do not occur in a jump under the FOMP but gradually pass though nucleation and new high-field phase growing, which substitutes for the low-field phase. [ABSTRACT FROM AUTHOR]