In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

The key methods for the preparation of magnetic nanoparticles are described systematically. The experimental data on their properties are analysed and generalised. The main theoretical views on the magnetism of nanoparticles are considered.

https://iopscience.iop.org/article/10.1070/RC2005v074n06ABEH000897/pdf

Magnetic nanoparticles: preparation, structure and properties

Preface. Contents of volumes 1-6. 1. Magnetism in ultrathin transition metal films (U. Gradmann). 2. Energy band theory of metallic magnetism in the elements (V.L. Moruzzi, P.M. Marcus). 3. Density functional theory of the ground state magnetic properties of rare earths and actinides (M.S.S. Brooks, B. Johansson). 4. Diluted magnetic semiconductors (J. Kossut, W. Dobrowolski). 5. Magnetic properties of binary rare-earth 3d-transition-metal intermetallic compounds (J.J.M. Franse, R.J. Radwanski). 6. Neutron scattering on heavy fermion and valence fluctuation 4f-systems (M. Loewenhaupt, K.H. Fischer). Author index. Subject index. Materials index.

Handbook of Magnetic Materials

Ferromagnetism usually only occurs in materials containing elements that form covalent 3d and 4f bonds. Its occurrence in pure carbon is therefore surprising, even controversial. A systematic magnetic force microscope study indicates that ferromagnetism in graphite is the result of localized spins that arise at grain boundaries.

/pdf/room-temperature-ferromagnetism-in-graphite-driven-by-two-36ae7e3kaz.pdf

Room-temperature ferromagnetism in graphite driven by two-dimensional networks of point defects

Debjani Karmakar,1 S. K. Mandal,2 R. M. Kadam,3 P. L. Paulose,4 A. K. Rajarajan,5 T. K. Nath,2 A. K. Das,2 I. Dasgupta,6 and G. P. Das7 1Technical Physics & Prototype Engineering Division, Bhabha Atomic Research Center, Mumbai 400085, India 2Department of Physics & Meteorology, Indian Institute of Technology, Kharagpur 721302, India 3Radiochemistry Division, Bhabha Atomic Research Center, Mumbai 400085, India 4Tata Institute of Fundamental Research, Mumbai 400005, India 5Solid State Physics Division, Bhabha Atomic Research Center, Mumbai 400085, India 6Department of Physics, Indian Institute of Technology Bombay, Mumbai 400076, India 7Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata 700032, India Received 18 August 2006; revised manuscript received 7 December 2006; published 2 April 2007

Ferromagnetism in Fe-doped ZnO nanocrystals: Experiment and theory

Mechanical alloyed Ho3+ doping in CoFe2O4 spinel ferrite and understanding of magnetic nanodomains

The grain size of α-Fe2O3 decreases to ∼20 nm by 64 h mechanical milling of the bulk sample. X-ray diffraction pattern suggested identical crystal structure in bulk and mechanical milled samples. Magnetic study (at temperatures of 100–900 K and fields of 0–±15 kOe) showed many interesting features during the decrease in grain size in antiferromagnetic α-Fe2O3, e.g., suppression of Morin transition, enhancement in low temperature magnetization, magnetic blocking at high temperature, exchange bias effect, and unusual relaxation of magnetic spin moment. We understand the results in terms of core-shell spin structure of nanograins, where the core part essentially retained the magnetic structure of the bulk sample and the magnetic structure of the shell part is modified due to grain size reduction and surface modification during mechanical milling. Core-shell structure also plays an important role in exhibiting the increasing soft ferromagnetic character in the present hematite samples. The in field magnetic relaxation at room temperature revealed some interesting properties of the magnetic spin ordering in hematite system.

Surface magnetism, Morin transition, and magnetic dynamics in antiferromagnetic α-Fe2O3 (hematite) nanograins

Our magnetic investigation of ${\mathrm{CoRh}}_{2}{\mathrm{O}}_{4}$ nanoparticles show the signature of antiferromagnetic ordering at ${T}_{N}\ensuremath{\approx}27\mathrm{K},$ same as that of the bulk sample, for all particle sizes down to $\ensuremath{\approx}16\mathrm{nm}.$ However, we observe a systematic magnetic enhancement below ${T}_{N},$ being larger for a smaller particle size. We propose a core-shell model for the magnetization of nanoparticles, with the core retaining its antiferromagnetic order and the enhancement of magnetization below ${T}_{N}$ attributed to the increasing number of frustrated shell (surface) spins. The scaling analysis of low temperature magnetization shows that the system approaches the limit of superparamagnetism for the particle size $\ensuremath{\sim}8\mathrm{nm}.$ Our model also predicts an alternation of exchange interactions along the inter-particle distance, very similar to RKKY type interactions in the metallic spin glass systems.

Magnetic enhancement in antiferromagnetic nanoparticle of CoRh 2 O 4

Anomaly in cluster glass behaviour of Co0.2Zn0.8Fe2O4 spinel oxide

We report the magnetic behavior of spinel chromite $\mathrm{Mn}{\mathrm{Cr}}_{2}{\mathrm{O}}_{4}$. Bulk $\mathrm{Mn}{\mathrm{Cr}}_{2}{\mathrm{O}}_{4}$ shows a sequence of magnetic states, i.e., paramagnetic (PM) to collinear ferrimagnetic (FM) state below ${T}_{C}\ensuremath{\sim}45\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and collinear FM state to noncollinear FM state below ${T}_{S}\ensuremath{\sim}18\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Decrease of particle size reduces the noncollinear spin structure and consequently, magnetic transition at ${T}_{S}$ decreases in nanoparticle samples. However, ferrimagnetic order is still dominating in nanoparticles, except the observation of superparamagnetic-like blocking and decrease of spontaneous magnetization. This, according to the core-shell model of ferrimagnetic nanoparticles, may be due to surface disorder effects of nanoparticles. The system also shows the increase of ${T}_{C}$ in nanoparticle samples, which is not consistent with the core-shell model. The analysis of the $M(T)$ data, applying spin wave theory, has shown an unusual Bloch exponent value 3.35 for bulk $\mathrm{Mn}{\mathrm{Cr}}_{2}{\mathrm{O}}_{4}$, which decreases and approaches 1.5, a typical value for any standard ferromagnet, with decreasing particle size. We have also observed the lattice expansion in $\mathrm{Mn}{\mathrm{Cr}}_{2}{\mathrm{O}}_{4}$ nanoparticles. The present work shows the correlation between a systematic increase of lattice parameter and the gradual decrease of $B$ site noncollinear spin structure in $\mathrm{Mn}{\mathrm{Cr}}_{2}{\mathrm{O}}_{4}$.

R.N. Bhowmik

Papers

Mechanical alloyed Ho3+ doping in CoFe2O4 spinel ferrite and understanding of magnetic nanodomains

Surface magnetism, Morin transition, and magnetic dynamics in antiferromagnetic α-Fe2O3 (hematite) nanograins

Magnetic enhancement in antiferromagnetic nanoparticle of CoRh 2 O 4

Anomaly in cluster glass behaviour of Co0.2Zn0.8Fe2O4 spinel oxide

Lattice expansion and noncollinear to collinear ferrimagnetic order in a MnCr 2 O 4 nanoparticle