The magnetic properties of hausmannite thin films are investigated in this chapter. The Verdet constant and angle of Faraday rotation are determined. The magnetic anisotropy of Mn3O4 is explained by the measurement of the zero-field cooled (ZFC) and field cooled (FC) curves. This experiment is connected with the presentation of the ferromagnetic to superparamagnetic transition of the hausmannite.
- hausmannite Mn3O4
- Faraday effect
- verdet constant
The hausmannite Mn3O4 can be fabricated by many methods, but the spray pyrolysis method can give it the highest quality. This material is very interesting because it is a transition metal oxide and has application in semiconductor devices . This oxide has two valance states on manganese—Mn2+ and Mn3+. Thus, spinel Mn3O4 occurs in nature as the mineral hausmannite [Mn2+Mn23+O4]. The Mn2+ cations occupy the tetrahedral sites and Mn3+cations occupy the octahedral sites . The nanoparticles of Mn3O4 thin film behave as single-domain ferromagnets. However, above the blocking temperature, the particles behave as paramagnets due to the dominance of thermal fluctuations over the magnetocrystalline anisotropy energy. These nanoparticles have much higher magnetic moments than other paramagnets and are called superparamagnet.
The detailed investigation of magnetic properties of hausmannite thin film is presented in this chapter.
2. Method of preparation and characterization techniques
Several techniques have been used to prepare thin films of these types of transparent and conductive materials to meet the requirements of search and industries such as MOCVD (organometallic chemical vapor deposition) , chemical vapor transport (CVT) , sputtering  and laser ablation [6, 7], which are generally either sophisticated or expensive and hence the need for a simple, easy to meter out and less expensive technique. In addition to these techniques, spray pyrolysis [8–11] has received a little bit of extra attention because of its simplicity and cost-effectiveness as it does not require sophisticated vacuum apparatus. Furthermore, this method can be selected for film production of large area with size grain controllable by controlling the doping concentration. Also, this technique leads to a large production area and it permits also the formation of thin films with possible control of oxygen vacancy by means of the use of both appropriate precursors and postannealing treatments in air [12–15].
Thin ﬁlms of Mn3O4 were grown at 350°C on 1 × 2 cm2 glass substrate using the spray pyrolysis technique. The substrate temperature was fixed using a digital temperature controller with a k-type thermocouple. The aqueous solution with a flow of about 4 ml/min contains magnesium chloride (MnCl2.6H2O) 0.1 M as precursor. The distance between the nozzle and the substrate was about 27 cm. Spray solutions quantity (75 ml) was kept fixed during the growth. The filtered compressed nitrogen air was used as gas carrier at a flow of 4 l/min. The total deposition time was maintained at 20 min. After deposition, the coated substrates were allowed to cool down naturally to room temperature (Figure 1).
The crystalline structure was analyzed by X-ray diffraction, using a Siemens D500 diffractometer with monochromatic CuKα radiation (
3. Magnetic study
The magneto-optic Faraday effect presents the connection between optics, magnetism and atomic physics. Faraday rotation manifests itself as a rotation of the polarization plane of the light passing through the sample in the presence of a magnetic field and is characterized by the Verdet constant (
In the paramagnetic materials, the anomaly factors
The exchange interaction energy leads to the alignment of neighboring atomic moments and this forms magnetic domains. The magnetostatic interaction energy tries to break them into smaller domains oriented antiparallel to each other. The domain size depends on the relative counterbalance between both energies. The system is composed of a single domain, when the magnetostatic energy does not allow the breaking of domains into smaller parts. This condition is connected with the critical value
The spin-orbit coupling and dipolar interaction dictate preferential orientation directions of the magnetic moments because of the finite size of the particles. The magnetic anisotropy energy
If the particles are magneto anisotropic, the calculation of equilibrium magnetization is complicated. The special role for nanoparticles having superficial anisotropy is the violation of local symmetry surroundings and crystal field change that acts on the magnetic ions from the surface. The simplest type of magnetic anisotropy is the easy anisotropy axis.
When external magnetic field is applied over the nanoparticles, it tries to orientate their magnetic moments in the direction of its action. Therefore, if the magnetic field is applied perpendicular of anisotropy axis and the orientation of magnetic moment of the particle is labeled with
The influence of external magnetic field in the orientation of magnetic supermoments is known as Stoner-Wohlfart model . They assume that the coherent rotation of atomic magnetic moments exists and the magnetic field is applied at a certain angle
when , Eq. (8) gives two local minima (the directions of easy magnetization) at
Formula (10) presents the temperature
The magnetization curve increasing to reaching saturation magnetization is measured in the study of the magnetic properties of the hausmannite Mn3O4 which containing nano-objects. To determine the temperature dependence of the magnetic moment Mare carried out two types of measurements—cooling in zero magnetic field (zero-field-cooling, ZFC) and cooling in a nonzero field (field-cooling, FC). The sample is cooled (to liquid helium temperature) during the method of ZFC in the absence of a magnetic field and then a small field (2–5 kOe) is included. The temperature values begin slowly to increase and the magnetic moment (
When and random orientation of the easy magnetization axes of particles, formula (12) is also valid for
Note that the difference between the curves
The orientation of electron spin in the manganese ions is very interesting for study. One of the electrons of the inner shell is responsible for the magnetism and its spin is oriented upwards. If the conductivity electrons move in the same region, where there is the motion of “magnetic” electrons than their spins rotate in the opposite direction. Thus, the conductivity electrons can rotate the electron spins of the other ions. This double interaction is equivalent of the interaction between two “magnetic” electrons which are oriented in one direction. This means that the neighboring spins have to be parallel, which is a result from the action of intermediate environment. This mechanism does not require all electrons to be oriented upwards. It is sufficient that conductivity electrons can be slightly oriented downwards. Thus, the possibility for the rotation of “magnetic” electrons upwards increases.
The energy of electron spin can be presented as (Figure 13):
On the other hand, we can write that
The magnetic moment of the electron is
The energy of interaction between two electrons is expressed by the next equation (Figure 14):
The magneto-optic anomaly factor
The author would like to express here gratefulness to Pr. Dr. Ing. Karem Boubaker, Unité de physique des dispositifs à semi-conducteurs, Tunis EL MANAR University, Tunisia,
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