J. Torrejón

Fabrication and magnetic properties of hard/soft magnetostatically coupled FePt∕FeNi multilayer microwires

A family of multilayer microwires with hard/soft biphase magnetic behavior is here introduced. The microwires consist of a Fe63Pt27Si10 hard magnetic nucleus and a Fe20Ni80 soft outer shell separated by an intermediate insulating Pyrex glass microtube. The precursor FePtSi glass-coated microwire is fabricated by quenching and drawing technique, and its L10 hard magnetic phase is grown by postannealing treatment technique. The polycrystalline FeNi soft magnetic outer shell has been deposited by electroplating. The analysis of the low-field hysteresis loops of the FeNi soft phase after premagnetizing until near magnetic saturation provides information about the magnetostatic coupling between phases. The FeNi magnetization curve is shifted toward positive field when the FePt remanent magnetization is positive and vice versa. A systematic analysis of the magnetostatic coupling and the corresponding bias field arising from uncompensated poles of the premagnetized FePt hard phase has been performed. The strengt...

Asymmetric magnetoimpedance in self-biased layered CoFe/CoNi microwires

The magnetoimpedance (MI) effect has been investigated in a family of multilayer microwires with biphase magnetic behavior consisting of a soft nucleus (CoFeSiB), an intermediate nonmagnetic insulating layer, and a hard outer shell (CoNi). The MI response of the soft phase can be tailored by its magnetostatic coupling with the hard phase. The hard outer shell, in its remanence state, creates a bias field in the soft nucleus that shifts the magnetization process and results in an asymmetric MI response. The amplitude of that bias field is determined by the geometric characteristics and the magnetic state of the hard phase. Furthermore, a near linear MI behavior with high sensitivity was realized around zero operation field point, with the advantage of not employing external biasing fields and additional coils. This makes biphase microwires exhibiting self-bias and asymmetric MI very attractive as sensing elements in magnetic-field sensor devices and materials.

Nonlinear magnetoimpedance and parametric excitation of standing spin waves in a glass-covered microwire

The giant magnetoimpedance of an 8.5 μm glass-covered amorphous microwire was investigated in the frequency range of 10 MHz–3.5 GHz. It was found that when the exciting microwave current exceeds some threshold value, a periodic fine structure appears in the frequency dependence of the complex impedance. The appearance of this nonlinear phenomenon is interpreted to be a consequence of the parametric excitation of standing spin waves.

Magnetostatic coupling in soft/hard biphase magnetic systems based on amorphous alloys

We report on magnetic bias in biphase systems arising from by dipolarlike interaction. Composites consist of amorphous ribbons and wires prepared by rapid solidification techniques, surrounded by an electroplated layer. Amorphous metallic core is (Co0.94Fe0.06)72.5Si12.5B15 with ultrasoft magnetic behavior (coercivity of 10–10−1A∕m), and the electroplated coating is crystalline Co85Ni15 alloy with harder magnetic character (coercivity of 104A∕m. After premagnetizing in a dc saturating field (106A∕m), the low-field (±103A∕m) hysteresis loops are ascribed to the magnetization process of the soft core and exhibit a typical shift towards the direction of the premagnetizing field. This shift is ascribed to the magnetostatic field generated by uncompensated poles at the edges of the hard layer, which remains close to its remanence state. The influence of the geometry of both phases on magnetostatic energy term allows us to tailor the bias field and susceptibility of the core. These results open other possibilit...

Temperature-Dependent Magnetic Properties of Magnetically Biphase Microwires

The magnetic behaviour of soft/hard biphase magnetic microwires has been studied as a function of temperature in the range 25°C to 900°C. 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.

Double-absorption ferromagnetic resonance in biphase magnetic microwires

We report a ferromagnetic resonance study of biphase magnetic microwires consisting of soft amorphous nucleus, intermediate nonmagnetic layers, and harder outer crystalline shell. Real and imaginary impedance components are investigated under increasing static axial magnetic field with a network analyzer in the microwave frequency range for selected microwires with different soft nuclei. Natural ferromagnetic resonance is even observed for particular microwires with strong axial anisotropy. The presence of a hard phase induces a second absorption peak at frequencies lower than those of the soft phase. Moreover, magnetic anisotropy of different soft phases is deduced from the evolution of resonance frequency with applied field.

Magnetostatic bias in a composite hard/soft/hard microlayer

A composite magnetically hard/soft/hard trilayer ribbon with micrometric thickness has been prepared by combined melt-spinning and electroplating techniques. Its main peculiarities relate to the existence of a magnetic coupling between hard and soft magnetic phases resulting in bias magnetic field acting on the soft layer. A systematic study of the low-field hysteresis loops has been performed after premagnetizing under a direct current saturating field as a function of the geometrical dimensions of both types of layers. The analysis of the observed bias effect has allowed us to conclude the magnetostatic origin of the tailorable coupling. The present investigation in microscale system points out to the potential technological applications that in a further step can be developed in nanomagnetic systems.

Circular Magnetoelastic Anisotropy Induced in the Nucleus of an FeSiB-CoNi Soft-Hard Bi-Phase Microwire

Multilayer amorphous microwires have been prepared coating a precursor FeSiB amorphous nucleus with a crystalline CoNi outer layer. The FeSiB as-cast amorphous wire is characterised by an unique bistable behavior where magnetization reverses from one remanent state to another through a single large Barkhausen event at a field value known as switching field, Hsw. In multilayer microwires, the presence of a second magnetic phase, a magnetically harder CoNi layer, substantially modifies the values of Hsw via two coupling mechanisms: magnetoelastic and magnetostatic interactions between the two phases. In this paper, a systematic study on the effects of the geometrical characteristic of the outer phase on the Hsw behavior is presented. M-H analysis and fluctuation studies by induction technique were performed on a family of samples having different CoNi thicknesses.