3. EPITAXIAL THIN FILMS
Table 3.5: Coercive field (Hc) for FePt fim grown on SrTiO3 and MgO substrates as a function of thickness.
Substrate Film thickness (nm) HC(kOe)
SrTiO3 6 29±1
MgO 6 24±0.7
SrTiO3 3.5 25±0.8
MgO 3.5 20.5±0.6
STO.
These results point out as ordered FePt ultra thin films can be obtained on both sub-strate characterized by homogenous and well separated grains morphology.
3.3 Magnetite epitaxial thin films
2.1. Magnetite: an overview 63
In the inverse spinel structure, specified generally as AB2O4, two usually non equivalent metal ions, A and B, are embedded in a cubically face-centered lattice of O2 ions. The structure is cubic, with space group Fd¯3m, and from the ionic point of view, the magnetite general chemical formula can be written as [F e3+]A[F e2+, F e3+]BO4, where octahedral iron ions are indicated as red circles (B sites) and the tetrahedral iron ions as blue circles (A sites) (see figure 2.2). Therefore, Fe2+ and Fe3+ ions coexist at the same crystallographic site in the inverse spinel structure.
Figure 2.2: The crystal structure of spinel Fe3O4. Blue atoms are tetrahedrally coordinated Fe2+; red atoms are octahedrally coordinated, 50/50 Fe2+/ Fe3+; white atoms are oxygen [94].
Below TC= 860 K magnetite orders ferrimagnetically [95]. This magnetic ordering follows Néels two-sublattice model and implies that the A and B site ions are aligned ferromagnetically within each sublattice, and antiferromag-netically between the two sublattices. The saturation magnetic moment has been determined to be 4.1 µB/f.u. at 300 K, which supports the validity of this model, which predicts:
µT OT AL= µ(F e3+B) + µ(F e2+B) µ(F e3+A) = 5µB+ 4µB 5µB
µT OT AL= 4µB
The Verwey transition
Around 120 K magnetite undergoes a phase transition nowadays referred to as the Verwey transition. This transition has been widely studied during the last century. It started in 1929 when Russell W. Millar found a maximum in the specific heat of magnetite at 114.15 K [96], three years later C. H.
Li reported that the magnetic properties of the crystal abruptly changed at the same temperature [97] and the following years, other authors studied the change in magnetite physical properties at this first-order transition [76].
Figure 3.22: - The crystal structure of spinel Fe3O4. Blue atoms are tetrahedrally coordinated Fe3+; red atoms are octahedrally coordinated 50% Fe3+ and 50% Fe2+. The other are oxygen atoms. After [140]
in Arago´n (INA) by Doc. Julia Orna. Here I recall the main results reported in her Ph.D thesis [140]. During these studies, epitaxial Fe3O4 thin films with thickness of 45 nm have been grown on MgO (100) substrates by means of the PLD system, with a base pressure lower than 10 8 mbar, at the growth temperature TG=650 C. The PLD technique preserves the stoichiometry from the target into the deposited film for simple oxides. Therefore, the e↵ect of the deposition rate has been studied at the first place, as it can influence both the crystalline quality of the film and its stoichiometry: too low deposition rates may produce an oxidation of the deposited material, performing an in situ reactive PLD process. After the deposition rate optimization, the influence of the growth temperature on the epitaxial growth of magnetite on MgO (100) has been studied. In order to find the optimal temperature for the epitaxial growth of magnetite on MgO (100), magnetite thin films (with thickness of 45 nm) have been grown at dif-ferent temperatures from 300 C to 800 C at the optimized deposition rate (36 ˚A/min.).
The symmetrical ✓ 2✓ scan around the (200) Bragg peak from the MgO substrate has shown the Fe3O4 (400) reflection and its Laue oscillations up to 10th order Fig. 3.23).
As the Laue oscillations have its origin in the finite number of di↵ractive layers, from their periodicity q, a coherence length ✏ = 2⇡/ q = 47± 0.5 nm was determined. The low-resolution TEM image in figure 3.24(a) has shown a continuous and homogeneous film. Furthermore, HRTEM images evidence the high-quality and epitaxial growth of Fe3O4 on MgO, showing a flat and smooth interface with MgO Fig. 3.24(b) ) and the presence of structural growth defects called antiphase boundaries (APBs), which are defects originated from stress-relaxation mechanism.
From the magnetization measurements as a function of the applied magnetic field at
3. EPITAXIAL THIN FILMS
Figure 3.23: - ✓ 2✓ from a 50 nm thick film grown at the optimal growth parameters.
After [140].
80 Chapter 2. Magnetite epitaxial thin films
A scanof the asymmetrical Fe3O4(226) reflection (figure 2.20) was measured by fixing the position angles of the sample in the Fe3O4(226) reflec-tion and rotating in-plane the sample along 360o. The eight peaks of similar intensity from the (311) planes were found, revealing the epitaxial in-plane orientation of the magnetite thin film: MgO[100]//Fe3O4[100].
Figure 2.17: scanof the asymmetrical Fe3O4(311) reflection.
2.2.5 Morphological and surface characterization
If we want to use the magnetite film as part of a spintronic device such a mag-netic tunnel junction, continuity, homogeneity and low roughness of the layer are key issues. Therefore, an exhaustive morphological and surface character-ization is necessary.
The low-resolution TEM image in figure 2.18 shows that the film is contin-uous and quite homogeneous at lateral sizes of the order of the micron.
100 nm
Au
MgO Fe3O4
Figure 2.18: Low-resolution TEM image from a MgO (001) // (40 nm) Fe3O4/ (45 nm) Au film. Measurement performed by Dr. C. Magen at the Centre d’Elaboration de Matériaux et d’Etudes Structurales, CNRS in Toulouse (France).
(a)
2.2. Growth optimization 81
High-resolution transmission electron microscopy (HRTEM) images evi-dence the high-quality epitaxial growth of Fe3O4on MgO, presenting a flat and smooth interface with MgO (Figure 2.19) and the presence of APBs.
Figure 2.19: Cross section HRTEM image of a MgO (001) // Fe3O4film. The different crystalline planes and distances in both Fe3O4and MgO substrate are indicated.
Measurement performed by Dr. C. Magen at the Centre d’Elaboration de Matériaux et d’Etudes Structurales, CNRS in Toulouse (France).
Electron diffraction measurements confirm the epitaxial growth and good crystallinity of the films. The diffraction image of the substrate indicates the expected cubic structure of MgO, whereas the diffraction image of the film indicates also cubic structure of Fe3O4, with additional diffraction spots at intermediate positions with respect to the MgO image due to the doubling of the lattice parameter in Fe3O4.
(b)
Figure 3.24: ✓ 2✓ from a 40 nm thick film grown at the optimal growth parameters and b) cross section HRTEM image of a MgO (100) // Fe3O4 film. After [140].
3.3 Magnetite epitaxial thin films
room temperature Fig. 2.23), a saturation magnetization value of 440 emu/cm3 was obtained, which is about 10% less than the value reported for bulk Fe3O4. However, this e↵ect has been observed before in high quality magnetite films, and is generally explained due to the presence of APBs with antiferromagnetic interactions or due to the epitaxial strain. The Verwey transition at Tv = 111 K, observed by means of
2.2. Growth optimization 83
2.2.6 Magnetic and electrical transport properties
Once the structural and superficial quality of the magnetite films was con-firmed, the magnetic characterization was performed by means of SQUID mag-netometry. From the magnetization measurements as a function of the applied magnetic field at room temperature (figure 2.23), we obtained a value of 440 emu/cm3for the saturation magnetization, which is about 10% less than the value reported for bulk Fe3O4. However, this effect has been observed before in high quality magnetite films, and is generally explained due to the presence of APBs with antiferromagnetic interactions or due to the epitaxial strain (see section 2.1.2).
The Verwey Transition can be observed both in magnetization (figure 2.24) and transport measurements (figure 2.25) as a function of temperature.
In first place, and by means of SQUID magnetometry, the transition was observed in magnetization measurements (figure 2.24). The Verwey Transition Temperature was found at 110 K, which is a value lower than the one measured in single-crystals (TV ⇠ 120 K). However, a decrease of TV has been observed by other authors in epitaxial thin films for thickness below 100 nm, and related with epitaxial strain [122, 124, 126, 128, 145] and the presence of defects such as APBs [119, 125] (for more details see section 2.1.1).
Indeed, the existence of the transition evidences a high stoichiometry, as slight deviations can lead to the disappearance of the transition (section 2.1.1).
Figure 2.23: Magnetization measure-ment as a function of magnetic field at room temperature (300 K).
Figure 2.24: Magnetization as a function of the temperature in an applied field of 500 Oe. The sample is heated after a ZFC.
Figure 3.25: - Magnetization measurement as a function of magnetic field at room temperature (left) and FC , ZFC measurement with applied field of 500 Oe (right). After [140].
transport measurements as a function of the temperature, has been identified as a sub-stantial increase of the resistivity at the transition temperature. Spanish collaborators98 Chapter 2. Magnetite epitaxial thin films
-10 -5 0 5 10
H(kOe)
-10 -5 0 5 10
-60 -50 -40 -30 -20 -10 0
T=300K 150nm 40nm
15nm 9nm
U(P:cm xy 5nm
H(kOe)
J//[110]
0
Figure 2.37: Transversal resistivity as a function of the applied magnetic field ✓ = 45 for several thin-film thicknesses at room temperature with current direction J//[110]
[75, 151].
As a function of temperature the planar Hall effect increases in magnitude, reaching colossal values below the Verwey transition, 16 m⌦·cm at T=73 K for the 20-nm-thick film. This result demonstrates the potentiality of the planar Hall effect in magnetite thin films for studies of magnetization processes as well as for sensitive low-field detection.
Anomalous Hall Effect
In magnetic materials, when a magnetic field is applied perpendicularly to the film plane and a current is injected in the x direction, in addition to the transversal voltage proportional to the applied magnetic field that corresponds to the Ordinary Hall Effect (OHE), it appears a term derived from the spon-taneous magnetization of the material. This additional contribution is the Anomalous Hall Effect (AHE), accounted in the last term in equation 2.10.
Figure 3.26: - Transversal resistivity as a function of the applied magnetic field ✓=45 for several thin-film thicknesses at room temperature with current direction J//[110]. After [140].
have systematically characterized the magnetoresistance of magnetite films in di↵erent geometries, i.e. Hall e↵ect and planar Hall e↵ect, for several film thicknesses and as a
103
3. EPITAXIAL THIN FILMS
function of temperature, obtaining interesting published results [143, 144, 145]. The measurements have been done with the applied magnetic field forming a fixed angle with the current, ✓=45 , since the signal will be maximum in this geometry when satu-ration in magnetization is reached. A value at room temperature of| ⇢xy |⇡ 60µ⌦ · cm is obtained for the film thickness of 5 nm. The peaks associated with the coercive field shift towards zero field as the sample thickness is decreased, indicating a evolution of the films towards superparamagnetic behavior.