Magnetic properties of materials Response of a material to a magnetic field M M ~ Magnetic susceptibility, H H M = Magnetisation H = Magnetic field Classification based on response to a magnetic field • Paramagnetic attracted by a magnetic field • Diamagnetic repelled by a magnetic field Diamagnetic • Paired electrons • : negative, temperature-independent • H2, H2O, NaCl, C6H6 T 0 Courtsey:http://research.pbsci.ucsc.edu/chemistry/ li/teaching/chem268/Magnetic%20properties.pdf Paramagnetic • Unpaired electrons • : positive, decreases with temperature • O2, CuSO4, DPPH (diphenyl picryl hydrazide) Curie law 0 C T T Cooperative magnetic interactions • Ferromagnetism • Fe, Co, CrO2 Curie-Weiss law C T • Antiferromagnetism • Cr, FeCl2, MnO • Ferrimagnetism • Fe3O4, Y3Fe5O12 1 0 T Paramagnetism 1 1 Curie temp. TC T 0 Ferromagnetism 1 Néel temp. TN T 0 Antiferromagnetism T 0 Ferrimagnetism • permanent magnets • high Hc, Mr (hard magnets), high TC • transformers • low HC, Mr (soft magnets) M • information storage • soft magnets, unique hysterisis loops H • magnetic shape memory alloys (MSMA) • SMA : deformed at low T, shape restored at high T • structural transition • Nitinol (Ni-Ti) • MSMA : large strain under magnetic field (Ni2MnGa) • magnetic anisotropy and structural transition martensite austenite Courtsey: http://en.wikipedia.org/wiki/File:Sma_wire.jpg • spintronics • spin of electrons (or nuclei) control oproperties like resistance • giant magnetoresistance: alternating ferro and nonmagnetic layers • resistance: small (parallel magnetizations); large (antiparallel magnetizations) • FeCr • hard disk drive Problem Set 1. What would be the relative advantage(s) and disadvantage(s) of replacing iron cores by ferrite cores in transformers ? 2. Why is it desirable to prepare acicular ferromagnetic particles for applications in recording materials. 3. A monoatomic ferromagnet has Curie temperature of 630 K and each atom contributes 0.6 BM towards the magnetic moment. Calculate the internal field in this material. 4. Given the following data for the ratio of lattice constant (rab) to orbital radius (ro) for various metals: rab/ro Fe 3.26 Co 3.64 Ni 3.94 Cr 2.60 Mn 2.94 Gd 3.10 explain why Mn (rab = 2.58Å) is not ferromagnetic while MnAs (rab = 2.85Å) and MnSb (rab = 2.89Å) are ferromagnetic (see for example the simple model by Bethe modified by Slater; ref.: A. J. Dekker). 5. Find examples of single molecule magnets reported in recent literature.