MAGNETISM IN NANOCHEMISTRY Flashcards
For which type of compounds is Curie-Weiss paramagnetism observed at high temperatures? Describe the Curie-Weiss Law.
Curie-Weiss paramagnetism is observed at high temperatures for compounds with unpaired electrons. The Curie-Weiss law is: X = C/(T-Ø), X = susceptibility, C = Curie constant, T = abs. temp., Ø = Weiss constant.
Describe M(H) behavior for a typical ferromagnetic bulk material. Define saturation magnetization, remanence, coercivity, hysteresis loop. What is the difference between a hard and a soft magnetic material?
Saturation magnetization is the state reached when an increase in applied external magnetic field H cannot increase the magnetization of the material further. Remanence is the magnetization left behind in a ferromagnetic material (such as iron) after an external magnetic field is removed.
Coercivity is the resistance of a magnetic material to changes in magnetization, equivalent to the field intensity necessary to demagnetize the fully magnetized material.
Hysteresis is the dependence of the state of a system on its history. For example, a magnet may have more than one possible magnetic moment in a given magnetic field, depending on how the field changed in the past.
Hard: hard to magnetize and demagnetize. Very large remanence, high coercivity.
Soft: easy to magnetize and demagnetize. Minimal remanence, low coercivity. Usually ferrites.
- Explain magnetic domains in a bulk material. What occur on subjecting the material to a high magnetic field? What is the typical size of a magnetic domain?
Magnetic domains (Weiss domains) are the microscopic areas inside which the spins are ordered before being subjected to a magnetic field, size 1-100 microns (in bulk). Upon applying a high magnetic field the domains parallel to the field will grow while the others will diminish until the whole material is oriented parallel to the magnetic field. In high field the magnetization of the various domains will orient more and more and finally become parallel to the applied magnetic field, and we achieve saturation magnetization
Describe the difference between a ferromagnet and a ferrimagnet. Give examples of relevant compounds. What is the chemical difference between magnetite and maghemite?
Ferro- and ferrimagnets both have net magnetic moment. For ferromagnets the spins are ordered parallel to each other, while ferrimagnets can have antiparallel ordering of spins with two different magnetic moments (two different magnetic cations).
Fe3O4 Magnetite Fe(III) and Fe(II)
Fe2,67O4 Maghemite Fe-vacanciesFe(III)
What is a nanomagnet? Give examples from different categories of materials.
A nanomagnet is a single domain particle that shows remanence (keeps the magnetization).
Can be a metal NP; Fe, Co, Ni and their alloys (rich on these element).
Oxides like magnetite and maghemite; other spinels MFe2O4 etc.
Molecular magnets(organometallic compounds).
What is a single molecule magnet? Give an example.
A molecule that can be magnetized in a magnetic field, that keeps the magnetization after the field has been switched off, and the magnetization is molecular in origin (no interaction with other molecules necessary).
Usually magnetic cations with organic linkers, Fe8.
What is the typical size of a single domain magnetic nanoparticle? Explain what is understood with the term blocking temperature.
Single domain nanoparticles will have a larger size than those required in order to observed superparamagnetic behavior. It depends on the chemical system. The upper range is in the 75-150 nm range.
Blocking temperature is the transition temperature where you go from a superparamagnetic case to a blocked case.
Explain the key properties of a superparamagnetic particle. What determines the maximum magnetization of such a superparamagnetic particle? Compare the M(H) curve of a superparamagnetic particle with those you draw in task 2.
Key parameters: size and saturation magnetization and magnetic susceptibility in low fields; maximum magnetization when all particles are aligned with the magnetic field –is determined by the number of magnetic atoms and their spins –in the sample. Superparamagnetic particles have a high magnetic susceptibility, and do not show remanence above the blocking temperature.
Why is the Neel relaxation time for the nanoparticle an important parameter when it comes to measurement of the magnetic properties? Which role has temperature or the frequency of an alternating (AC) magnetic field on the measured data/properties? Make relevant sketches.
The Neél relaxation time is the mean time for spin to flip due to thermal fluctation. When doing a DC experiment the collection time is constant and the temperature varied. This means that at a certain temperature the material will suddenly show magnetization with an open M(H) loop, and we are then in the blocked case. If you measure with AC where the Neél relaxation time and the measurement time are close to each other in size, you will observe a frequency dependence for the susceptibility.
Consider magnetite. For which particle size do you expect to observe the highest coercivity? Explain.
Max coercivity for the largest possible single domain particle – is the most magnetic one without splitting into more domains.
What is understood with exchange bias? What is the origin for the particular magnetic behavior? Give a chemical example. Draw a relevant M(H) curve.
This is an interface phenomenon between two different magnetic orderings. The conflicting magnetic orderings push the center of the hysteresis loop away from origin.
How can nanoparticles be used in medicine with respect to hyperthermia treatment?
Nanoparticles are transferred to the cancer tumor where they attach. When you apply an AC magnetic field it heats up due to flipping of spins. This kills the cancer cells.
Explain how the amount of metallic nickel in a nanocatalyst (Ni on an LDH support) can be determined by means of magnetic measurements.
You measure M(H) and look at the saturation magnetization. Higher saturation -> more metallic nickel.
What are ferrofluids? How are they made, and give examples of applications.
Ferrofluids are iron oxide nanoparticles in a suspension. Surfactants are needed to avoid agglomeration. Particle size is small – they have to be in the regime of superparamagnetism. Ferrofluids are as a magnetic seal.
