Magnetism in Nanochemistry

Question 1

For which type of compounds is Curie-Weiss paramagnetism observed at high temperatures? Describe the Curie-Weiss law.

Answer 1

CW-paramagnetism is observed for compounds with unpaired electrons, typically d- or f-elements, but can also occur in molecules (O2) and radicals (NO).

Curie-Weiss law: X(Chi)= C/(T-Theta)
The theta makes the (straight) line cross the T-axis at a negative or positive value

Question 2

Describe M(H) behavious for a typical ferromagnetic bulk material. Define saturation magnetization, remanence, coercivity, hysteresis loop.

Answer 2

• Saturation magnetization:
• Remanence: Remaining magnetism in a (ferromagn.) material after a magnetic field has been removed (opp-og-ned)
• Coercivity: The ability of a ferromagnetic material to withstand an external magnetic field without becoming demagnetized. (Sidelengs)
• Hysteresis loop

Hard vs soft magnet (later question)

Question 3

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?

Answer 3

• Magnetic domains (Weiss domains) are microscopic areas inside a bulk where the spins are ordered (before being subjected to a MF)
• Applying a high MF will make the parallell domains grow, while the other domains will diminish :( In the end, every domain is parallell to the field. The domains will orient to become parallell.
• Typical size of magnetic domain: 1 - 100 mikrons (in bulk)

Question 4

Describe the difference between a ferromagnet and a ferrimagnet. Give examples of relevant compounds. What is the chemical difference between magnetite and maghemite?

Answer 4

Ferro and ferri both have net magnetic moment. Ferri have antiparallell spins, but they are not at the same magnitude, so some magnetization will remain. –>

Question 5

What is a nanomagnet? Give examples from different categories of materials.

Answer 5

• A nanomagnet is a single particle domain that shows remanence
• Can be a metal NP; Fe, Co, Ni (and their alloys)
  Oxides like magnetite and maghemite; other spinels
  Molecular magnets

Question 6

What is a single molecule magnet? Give an example

Answer 6

Molecule that can be magnetized in a magnetic field, keeping the magnetization after the field has been switched off. Magnetization molecular in origin.
Usually magnetic cations with organic linkers, Fe8

Question 7

What is the typical size of a single domain magnetic nanoparticle? Explain what is understood with the term “blocking temperature”.

Answer 7

Typical size: 3 - 50 nm (depends on material)
- Blocking temperature: The transition temperature where the situation goes from a superparamagnetic case to a blocked case –> a superparamagnetic material will change its properties below this.

Keywords: Néel relaxation temperature

Question 8

Explain the key properties of a superparamagnetic particle. What detemines the maximum magnetization of such a superparamagnetic particle? Compare the M(H) curve of a superparamagnetic paricle with those you draw in task 2.

Answer 8

Size and saturation magnetization and magnetic susceptibility in low field; maximum magnetization when all particles are aligned with the magnetic field in the sample - this is determined by number of magnetic atoms and their spins. High magnetic susceptibility, does not show remanence above the blocking temperature.

Question 9

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.

Answer 9

The mean time for spin to flip due to thermal fluctuation. When doing a DC experiment the collection time is constant and the T varied. At a certain T, the material will suddenly show magnetization with an open M(H)-loop –> BLOCKED BITCH

Question 10

Consider magnetite. For which particle size do you expect to observe the highest coercivity? Explain

Answer 10

Magnetite - Fe3O4

Question 11

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.

Answer 11

An interface phenomenon between two different magnetic orderings. The difference will pust the hysteresis loop away from the origin.

Question 12

How can naoparticles be used in medicine with respect to hyperthermia treatment?

Answer 12

doy

Question 13

Explain how the amount of metallic nickel in a nanocatalyst (Ni on an LDH support) can be determined by means of magnetic measurements.

Answer 13

Measure M(H), look at saturation magnetization - higher saturation –> more metallic nickel

Question 14

What are ferrofluids? How are they made, and give examples of applications.

Answer 14

Iron oxide nanoparticles in a suspension, with surfactants helping with avoiding agglomoration.
- Particle sizes: In the regime of superparamagnetism

Question 15

What is the difference between a hard and soft magnetic material?

Answer 15

Hard magnet:

• Hard to magnetize and demagnetize
• Large remanence M_R
• High coercivity H_C
• (High Curie temperature)

Soft magnet:

• Easy to magnetize and demagnetize
• Minimal remanence
• Low coercivity