Optimization Of Soft Magnetic Properties For Co-based Microwires

The phenomenon that the impedance of magnetic materials changes significantly under the external magnetic field is called Giant Magnetoimpedance(GMI) effect. Because of its high sensitivity and fast response, so that it can be potentially used in magnetic recording and sensors. Since the sensors are going towards miniaturization and high sensitivity, glass-covered micro wires with the excellent GMI properties will be employed extensively. This thesis first introduces the GMI effect, soft magnetic properties, magnetostriction coefficient and the recent research of glass-covered microwires. Then, the Taylor-Ulitovsky method is presented which is used to prepare glass-covered microwires. The results focus on GMI and soft magnetic properties of microwires with different dimension and annealing process. The main contents are as follows:1. Dimension (length:0.1～8 cm, diameter:20～50μm)(1) GMI decreases with length of wires decreasing.(2) It is proved that the soft magnetic properties get worse when the wires get shorter. Specifically, the coercivity turns higher if the permeability turns lower. GMI and field sensitivity first increased and then decreased with the increasing of the microwires diameter. When the diameter is 29μm, the MI ratio reaches to the maximum value of 565%, and the field sensitivity reaches at 252%/Oe.2. Annealing (Current density:0～2.0×108 A/m2, Annealing period:0～20 min, External stress:0～500 MPa)(1) Annealing with different current density will change the ratio of the internal circumferential magnetic domain structures correspondingly. Crystallizing extent of microwires varies under different annealing time. Stronger external stress will get both more circumferential structures and higher circumferential anisotropy.(2) Stronger GMI of the wires does not make higher field sensitivity. The field sensitivityζwill not get to be maximum value unless GMI matches the circumferential anisotropy best.(3) Annealing with over heavy current or too long time will cause wires crystallized, which makes coercivity high. Over strong external stress leads to weak soft magnetic properties as it causes internal flaky particles in wires. In this thesis, we can draw the following conclusions. Annealing the microwire 25μm/29μm in diameter 2 cm in length with current density of 1.2×108 A/m2 without external stress for 10 minutes, we can obtain the best soft magnetic properties and highest field sensitivity of 602%/Oe. In this case, the small and uniform particles, combined with circumferential magnetic domain structures makes coercivity weaker.