Longitudinal Hysteresis Loops Research Articles

Asymmetric Hysteresis Loops in Co Thin Films

Asymmetric magnetic hysteresis loops are usually found in exchange bias (EB) systems, typically after field cooling a system below the Néel temperature of an antiferromagnet exchange coupled to a ferromagnet. Alternatively, asymmetric hysteresis loops may occur due to undetected minor loops or in systems with a rotational anisotropy. Here, we report on an exchange bias thin film system MgO(100)/Co/CoO, examined at room temperature, which is far above the blocking temperature, by the magneto-optical Kerr effect (MOKE). While the longitudinal hysteresis loops partly show steps which are well-known from diverse purely ferromagnetic systems, the transverse hysteresis loops exhibit clear asymmetries, similar to exchange biased systems at low temperatures, and unusual transverse magnetization values at saturation. Since minor loops and a rotational anisotropy can be excluded in this case, this asymmetry can possibly be a residue of the exchange bias coupling at lower temperatures.

Stress-Annealing Induced Anisotropy in FeCrCuNbSiB Nanocrystalline Wires

A few of amorphous wires of compositions Fe73.5-xCrxCu1Nb3Si13.5B9 (x = 0, 3 and 7) were submitted to current-annealing under the application of tensile stresses (s = 0 – 228 MPa). The measurement of longitudinal hysteresis loops, M(H) , was performed on the samples, from which the stress-annealing induced transverse anisotropy constant, K, were determined. We found that the magnitude of K and the s - sensitivity of K, dK/ds, decrease with increasing the amount of Cr. These results have been explained by the Herzer’s back-stress theory by considering the reduced magnetostriction with increasing the amount of Cr. Additionally, an abnormal disappearing K for large-stress-annealed samples of x = 0 was observed and it was discussed by a few of possible mechanisms.

Study of magnetization reversal and anisotropy of single crystalline ultrathin Fe/MgO (001) film by magneto-optic Kerr effect**Project supported by the National Natural Science Foundation of China (Grant Nos. 11274033, 11474015, and 61227902), the Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20131102130005), and the Beijing Key Discipline Foundation of Condensed Matter Physics.

The magnetization reversal process of Fe/MgO (001) thin film is investigated by combining transverse and longitudinal hysteresis loops. Owing to the competition between domain wall pinning energy and weak uniaxial magnetic anisotropy, the typical magnetization reversal process of Fe ultrathin film can take place via either an “l-jump” process near the easy axis, or a “2-jump” process near the hard axis, depending on the applied field orientation. Besides, the hysteresis loop presents strong asymmetry resulting from the variation of the detected light intensity due to the quadratic magneto-optic effect. Furthermore, we modify the detectable light intensity formula and simulate the hysteresis loops of the Kerr signal. The results show that they are in good agreement with the experimental data.

Hysteresis characteristics of an analytical vector hysteron

The analytical way to describe uniaxial single-domain particles, using new symmetry considerations on the magnetic anisotropy, as a vector hysteresis unit (vector hysteron), is presented. The main characteristics of the vector hysteron such as the longitudinal hysteresis loops, transversal hysteresis loops and rotational hysteresis loops are presented. An extension of the vector hysteron and a vector hysteron that can be applied to the ferroelectric hysteresis are also presented.

Magnetic Dynamic Interaction in Amorphous Microwire Array

The magnetic dynamic interaction in ferromagnetic microwire arrays has been studied based on thin Co-based glass-coated amorphous microwires. The longitudinal hysteresis loops of the microwire arrays consisting of different number of wires have been measured statically with no current flowing through the arrays and dynamically with ac current flowing through the arrays. The results show that with the number of wires increases the transverse anisotropy of the arrays has been enhanced, especially when an ac current was applied to the arrays, an additional circumferential anisotropy was induced. The results are explained by a domain unification effect in the form of domain wall displacement. Moreover, the changes in the magneto-impedance effect with the number of wires in the arrays indicate that the domain unification effect is frequency dependant and related to the dimension of the arrays. The domain wall motion contributes to the magneto-impedance up to a critical frequency beyond which only domain rotation occurs. Below the critical frequency, the initial permeability of the arrays increases with the increase of the number of wires while above the critical frequency, the trend inverses. The magneto-impedance response of the microwire arrays is a consequence of the dynamic interaction of domain unification and eddy current.

Study of the Noise in Multicore Orthogonal Fluxgate Sensors Based on Ni-Fe/Cu Composite Microwire Arrays

In this paper, a set of microwire arrays based on Ni-Fe/Cu composite wires were fabricated as the multicore sensing elements with different number of wires and different structures. The longitudinal hysteresis loops showed that the anisotropy of the arrays changed in different structures. The sensitivity of the multicore orthogonal fluxgate sensor increased exponentially with the increase of number of wires in the arrays, whereas the noise level depended on the working mode and structure of the microwire arrays. In the fundamental mode, a minimum noise level of 42 pT/rtHz@1 Hz was obtained with a 5-wire array, and in the second harmonic mode a minimum noise level of 177 pT/rtHz@1 Hz was obtained with the same array. Investigation of the relationship between the magnetic properties and the microwire array structures revealed a collective compactedness value which is able to describe the interaction effect in wires and to predict the favorable array structure based on the ratio of effective contact volume to the total material volume.

Circular Magnetization Processes in CoFeNi Electroplated Wires

Circular magnetization processes in electroplated wires is an important topic having straight connection with sensor applications of these soft magnetic materials. In present work the longitudinal and circular hysteresis loops were measured and corresponding magnetization processes were studied in Cu98Be2/Co16Fe20Ni64 wires. The longitudinal hysteresis loops, Mz-Hz, were measured by inductive technique in a frequency range of 10 to 70 Hz. The circular magnetization curves (Mφ- Hφ) were measured for frequencies of 50 and 100 kHz in the Hφ field up to 1500A/m for different values of the axial external field of 0 to 500 A/m. The longitudinal and circular magnetization curves are comparatively analyzed.

Magnetization reversal and soft modes in nanorings: Transitions between onion and vortex states studied by Brillouin light scattering

We present a joint experimental and theoretical investigation of the magnetization reversal process in an array of circular permalloy nanorings. The longitudinal hysteresis loop measured by magneto-optic Kerr effect exhibits the typical two-step switching behavior corresponding to transition from the onion to the vortex state and vice versa with plateaux in each loop branch. The frequencies of the spin excitations in these configurations calculated by means of the dynamical matrix approach are in good agreement with those measured by Brillouin light scattering. Several types of spin modes are found localized in different regions of the rings. It is found that the onion-to-vortex and the vortex-to-onion transitions are triggered by soft spin modes. The shape and symmetry of the dynamic magnetization profile of these soft modes provide the initial modifications of the magnetization at the onset of the transitions.

Low field microwave absorption and magnetization process in CoFeNi electroplated wires

Ferromagnetic resonance (FMR), Ferromagnetic antirresonance (FMAR) and low field magnetoimpedance (MI) are the characteristic features of high frequency losses in applied fields. While some results on FMR and FMAR in CoFeNi electroplated wires were reported earlier, here we present microwave absorption in CuBe wires electroplated by 1μm FeCoNi magnetic layer at very low fields. These data are comparatively analysed together with longitudinal hysteresis loops in order to reveal the correlation between power absorption and magnetization processes. Microwave studies are made by using the cavity perturbation method at 9.65 GHz for a DC field parallel to the sample axis, and with microwave magnetic field hrf parallel or perpendicular to the wire axis. Two peaks have been observed in all samples, one is due to FMR, and the other is, at very low fields, related to MI. The MI peaks represent minima in power absorption. By comparing with the hysteresis loop we remark the close correspondence between the MI phenomena in the axial mode and the concomitant magnetization process.

Rotational hysteresis of the exchange anisotropy direction in Co∕FeMn thin films

The effects of rotating an applied field on the exchange anisotropy in Co∕FeMn thin films have been investigated. When the applied field is initially along the cooling field direction, the longitudinal hysteresis loop has a maximum coercivity and the transverse hysteresis loop is flat, indicating that the exchange field is along the cooling field direction. When the applied field angle is rotated away and then restored to the original field cooling direction, the exchange anisotropy direction has changed. The rotation of the exchange field direction trails the applied field and is hysteretic. The rotational hysteresis of the exchange field direction is due to the weak anisotropy in thin FeMn layers and decreases with increasing FeMn thickness.

Exchange coupling in symmetric Co/Cr/Co trilayer: a Brillouin light scattering study

A detailed Brillouin light scattering (BLS) investigation of spin waves in Co(20 nm)/Cr(dCr)/Co(20 nm) symmetric trilayer with wedge-shaped Cr interlayer has been carried out. Results showed that the interlayer exchange coupling oscillates, with antiferromagnetic peaks near the Cr layer thickness of 0.7 and 2.5 nm. These oscillations can be satisfactorily reproduced by a calculation based on the RKKY theory. An independent confirmation of the results obtained by BLS has been derived by measuring the saturation field of the longitudinal hysteresis loop as a function of the Cr thickness by magneto-optic Kerr effect magnetometry. Using the magnetic parameter determined by the above analysis, we have calculated the Brillouin cross section for the different Cr spacer thickness obtaining a very good agreement with the measured spectra.

An analysis of interacting bistable magnetic microwires: from ordered to chaotic behaviours

Fe-based magnetostrictive amorphous wires are bistable exhibiting square-shaped longitudinal hysteresis loops. This is a consequence of their particular domain structure that allows them to be approached as magnetic dipoles. Arrays of such bistables wires are accordingly shown to be coupled by magnetostatic interactions. Furthermore, an analysis of these interactions allows us to conclude that depending on the relative strength of the magnetostatic and Zeeman interactions, ordered, complex or chaotic behaviours are found in the temporal variation of the array magnetisation.

Peculiar behavior of the magneto-impedance in Fe-based amorphous wires under torsion

The inductive ( ωL) and resistive ( R) components of the impedance of an amorphous wire (Fe 77.5Si 7.5B 15) subjected to torsional stresses are measured. For a critical value of the AC driving current L abruptly increases due to irreversible magnetization processes in the circumferential configuration. Suitable combinations of the AC driving current amplitude and frequency give rise to a peak in the resistive variations and a minimum in the inductance. The inductance shows a bistable asymmetric hysteresis loop ( L vs. H). The torsional stress increases the coercivity of the longitudinal hysteresis loop of the wire but favors the reaching of the saturation magnetization at lower fields.