LABORATORY FOR MAGNETIC MATERIALS
Learning outcomes of the course unit
Objective of the course is to provide a panoramic view of the modern magnetometric techniques aimed at the characterization of magnetic materials, in particular of those having technical applications. With a propaedeutic purpose, the techniques for magnetic field generation will be described together with the main methods for the measurement of magnetic quantities, like as magnetization, magnetic susceptibility and magnetic anisotropy. Besides the description and use of conventional instruments, also special ad hoc experimental set ups and measurement techniques will be illustrated, which are part of the research activity. For what concerns the materials utilized in the experiments, one may mention: the rare earth-transition metals intermetallic compounds and the ferrimagnetic hexagonal oxides, which both find applications in the field of permanent magnets; the alloys and compounds with potential appliance in the field of magnetic refrigeration; the magnetic thin films and multilayers, which have promising relevance in the microsystem technology and in the information storage. The measurements performed on such materials will be aimed at the determination of the main parameters, which are important both for applications and theory.
- Elettromagnetism: theory and laboratory
- General Chemistry
- Fundamentals of Solid State Physics
Course contents summary
Short account on the magnetic properties of matter:
- Dia-, para-, ferro- and ferrimagnetism. Curie and Curie-Weiss laws. Antiferromagnetism: Néel’s model;
- Intrinsic properties: magnetization; Curie temperature; thermodynamic theory of phase transitions; magnetic phase analysis. The magnetic anisotropy: shape anisotropy and demagnetizing field; magnetocrystalline anisotropy: phenomenological description. Spin reorientation transitions (SRT) and First-Order Magnetization processes (FOMP);
- Extrinsic properties: magnetic domains and domain walls, fine particles. The magnetization reversal mechanisms; single domain particles, coherent rotation, Stoner-Wohlfarth model; non-uniform processes, domain wall motion, Brown’s paradox;
- Application of magnetic materials: permanent magnets and magnetic recording.
- Magnetic field generation and measurement:
- Magnetization measurement: induction techniques, open and closed circuits; extraction magnetometer, vibrating sample magnetometer. The SQUID magnetometer;
- Magnetization measurement: force methods; Stationary pendulum magnetometer, Alternating Force Gradient Magnetometer;
- Magnetic susceptibility measurement: the ac susceptometer;
- Magnetic anisotropy measurements; the Singular Point Detection (SPD) technique.
- magnetic characterization of dia- and paramagnets: susceptibility, Curie constant;
- magnetic characterization of antiferromagnets: Néel temperature, Curie—Weiss law;
- magnetic characterization of ferromagnets: Curie temperature, hysteresis loop;
- the effect of demagnetizing field: soft ferromagnets;
- Belov-Goryaga method to individuate the Curie transition;
- Skin effect: frequency dependence of complex magnetic susceptibility in metal conductors.
Specialistic experiments (examples):
- Study of spin reorientations in Ba or Sr hexaferrite crystals;
- Analysis of the estrinsic properties of ferromagnetic thin films: butterfly curves, magnetic viscosity and recoil loops;
- Analysis of the magnetocaloric effect around the Curie point of a ferromagnet;
- Determination of critical fields in ferromagnetic polycrystals: SPD technique and complex magnetic susceptibility;
- Magnetic characterization of magnetite thin films: Verwey transition, exchange-bias effect.
Other useful textbooks:
Introduction to Magnetism and Magnetic Materials
Chapman & Hall, London 1991
Solid State Magnetism
Edward Arnold, London 1991 ISBN 0340545526
The student must participate to every laboratory session. At the end of the course, he must submit two individual relations: the first concerning an experiment of the propaedeutic series; the second concerning an experiment of the specialistic series. The relations will be discussed on the basis of a presentation in form of seminar.