M. Rivas

Influence of the dipolar interactions in the magnetization reversal asymmetry of hard–soft magnetic ribbons

Partial crystallization of the metallic glass Co66Si16B12Fe4Mo2 was performed by annealing at temperatures between 500 and 540 °C for 10–20 min, resulting in crystallite volume fractions of (0.7–5)×10−3 and sizes of 50–100 nm. This two-phase alloy presents a remarkable feature: a hysteresis loop shift that can be tailored by simply premagnetizing the sample in the adequate magnetic field. Shifts as large as five times the coercive field have been obtained which make them interesting for application as magnetic cores in dc pulse transformers. The asymetrical magnetic reversal is explained in terms of the magnetic dipolar field interaction and the observed hysteresis loops have been satisfactorily simulated by a modification of Stoner-Wohlfarth’s model of coherent rotations.

TBIS measurements performed by MOKE for the study of soft ferromagnetic thin films and sandwiches: In-plane and biaxial anisotropies, local anisotropy and dispersion

Abstract Magneto-optic effects have been proved to be a premier tool for the study of magnetic thin films. In the case of films thinner than 1000 A and if there is no appreciable variation of the magnetic structure through the thickness, the magneto-optic effects provide complete information. It is noteworthy that transverse biased initial susceptibility (TBIS) measurement is very sensitive, for it allows a precise determination of the anisotropy field and, moreover, to quantify the anisotropy dispersion distinguishing between short- and long-range magnetization fluctuations in the plane of the film the origin of which is different. If the samples are deposited on glass substrates, by performing TBIS measurements with a magneto-optic transverse Kerr effect (MOKE) we can obtain information of the processes occurring at both film/air and glass/film interfaces, providing information of the variation of the magnetic properties through the thickness. We have successfully applied this technique for the study of thin films and sandwiches, focusing our attention on the following items: 1. (1) Uniaxial in-plane anisotropy, local anisotropy and anisotropy dispersion in transition metal-rare-earth thin films. We have investigated the dependence of the uniaxial anisotropy and its dispersion on the rare-earth substituted and on the composition. Using the ripple theory we have evaluated the local anisotropy as a function of the rare-earth content, and we have found a very nice correlation between macroscopic and local anisotropies, allowing us to establish the origin of the macroscopic anisotropy in these alloys. 2. (2) TBIS measurements can also be used as an alternative technique to measure perpendicular anisotropy. We have developed a theoretical model to explain the particular behaviour of TBIS curves in the most general case when both in-plane and perpendicular anisotropies are present (biaxial anisotropy). From the fit of the experimental curves to the theoretical model we can obtain the in-plane and the perpendicular anisotropy fields. 3. (3) When the samples are not magnetically homogeneous, they may present different magnetic phases which may be made evident by our technique. 4. (4) We recently extended this technique to the study of ferromagnetic double-layered and sandwiched films. In these systems the magnetic and magneto-optical properties are strongly influenced by the coupling at the interfaces. The high sensitivity of our procedure allowed us to distinguish between two contributions to the anisotropy dispersion in sandwiched films: a well-known term due to the in-plane anisotropy skew (long-range fluctuations of the in-plane easy axis) and a contribution due to the existence of a weak perpendicular anisotropy.

Transverse Kerr magnetometry for the study of thin films presenting perpendicular and in-plane anisotropy

Abstract A model for the behaviour of transverse initial susceptibility as a function of dc applied field is developed for the case of thin films exhibiting simultaneous in-plane and perpendicular anisotropy. It is used to deduce in-plane and perpendicular anisotropy fields in a very simple way from transverse susceptibility measurements made by the transverse Kerr effect. The results obtained are cross-checked by those obtained from magnetization curves.

Induced and local anisotropies in amorphous CoZr–rare earth thin films containing Pr, Nd, and Tb

The induced in‐plane and local magnetic anisotropies of rf sputtered (Co93Zr7)100−xREx thin films were investigated as a function of RE content for RE=Pr, Nd, and Tb by transverse biased initial susceptibility measurements at both film/air and glass/film interfaces. The deposition was performed in a magnetic field. The films exhibit a very well defined in‐plane anisotropy with negligible long‐range fluctuations and a small coercive field Hc, in accordance with the experimental data. The overall variations of the transverse susceptibility are related to the ripple. The variations of the local anisotropy with composition were measured. Both the induced anisotropy Ku and the local anisotropy Kloc increase with the increasing amount of RE in the layers. A clear relationship between Ku and Kloc could be established. The results are discussed in terms of the single‐ion anisotropy of the rare earth. While the value of Ku is the same at both interfaces, the ripple constants were found to be slightly different. Th...

Coupling effects in Fe/CoNbZr and Fe/Ag/CoNbZr sandwiches studied by magneto‐optical techniques

Experimental characterization of magnetic anisotropy, coercive field, and anisotropy dispersion of sputtered Fe/CoNbZr and Fe/Ag/CoNbZr sandwiches were carried out by transverse biased initial susceptibility (TBIS) measurements with a magneto‐optic Kerr effect at both film/air and glass/film interfaces. Three different behaviors have been observed depending on the range of magnetization fluctuation. The results indicate that Fe grains play a fundamental role in the type of the dispersion (long range or short range). The results obtained from the coercive field and from x‐ray diffractograms agree with those obtained by TBIS.

The role of local anisotropy in the origin of in-plane uniaxial anisotropy in amorphous CoZrNd thin films

The magnetic properties of rf-sputtered amorphous (Co93Zr7)100−xNdx thin films with 0 < x < 3.5 were investigated by transverse biased initial susceptibility (TBIS) measurements at both film/air and glass/film interfaces. The films exhibit a very well defined in-plane uniaxial anisotropy with negligible long-range fluctuations. A clear relationship could be established between Ku and Kloc, data which is discussed in terms of the single-ion anisotropy of the Nd ions. Although the value of Hk is the same at both interfaces, the as-determined ripple constants were found to be slightly different. This last result is believed to be related to some local defect, the origin of which is also advanced.

Scanning Magneto-Inductive Sensor for Quantitative Assay of Prostate-Specific Antigen

Among analytical tests available for point-of-care diagnostics, the lateral flow immunoassay (LFIA) stands out for its low-cost, speed, portability, and ease of use. By its nature, this paper-based instrument is qualitative, intended to provide a positive/negative reading. LFIA, coupled to a quantitative readout device that did not compromise its advantages, would be a powerful tool for many clinical and biological applications. A promising enabling strategy is the use of superparamagnetic nanoparticles as labels. Their reduction of visual signal compared to gold or latex reporters is compensated by their magnetic induction, which enables absolute quantification with magnetic sensors. In this letter, a magnetic LFIA reader is presented that exploits spontaneous magnetic switching, a characteristic of superparamagnetism, to produce a quantifiable electromagnetic induction in an alternating current carrier. In contrast to other magnetic sensors, this approach does not require the application of external magnetic fields, which greatly reduces its complexity. The capability of the system for bioanalyte quantification has been proved by successfully measuring prostate-specific antigen levels in the interval of clinical interest.

Highly sensitive magneto‐optic transverse Kerr effect measurement system for the detection of perpendicular anisotropy and magnetic phases in thin films

Transverse biased initial susceptibility (TBIS) measurements were made using a magneto‐optical Kerr effect system on samples that exhibited an in‐plane and a perpendicular anisotropy. A change in the slope of both the inverse transverse susceptibility and the hysteresis loop is observed at a certain value of the applied in‐plane field Hs. At values of the applied field higher than Hs the magnetization lies in the plane of the film; at values lower than Hs an out‐of‐plane component appears. By performing TBIS measurements we can also detect different micromagnetic phases magnetically uncoupled with different values of the anisotropy field. The occurrence of a downwards curvature in the inverse susceptibility when the dc field is applied along the hard axis together with the multiple minima when the dc field is applied along the easy axis can be explained by the existence of weakly coupled micromagnetic phases.

Induced, local anisotropy and anisotropy dispersion in amorphous Co-Zr-rare earth thin films

The authors point out that when prepared under adequate conditions, RF sputtered amorphous (Co/sub 0.93/Zr/sub 0.07/)/sub 100-x/ Tb/sub x/ thin films are obtained with fairly uniform magnetic properties and K/sub u/. These layers are suitable for an investigation of the overall effect of the random local anisotropy K/sub loc/ upon the micromagnetic properties of the films. The experimental data were obtained by transverse biased initial susceptibility and ferromagnetic resonance measurements. The value of K/sub loc/ was determined using the ripple theory. K/sub loc/ increases with increasing amount of Tb in the layers, which can be explained by the single-ion anisotropy mechanism. >

Quasi-Static AC FORC Measurements for Soft Magnetic Materials and Their Differential Interpretation

A detailed analysis of the first-order reversal curve (FORC) measurements offers the possibility to separate the different magnetic interactions in multiphase or composite magnetic materials, and then to attain a precise interpretation of the role played by those interactions. In this paper, we present a modified setup based on a classical inductive magnetometer optimized for FORC measurements in soft magnetic materials, together with an improved differential method for the data analysis in order to get an accurate interpretation of the experimental results.