Control and control again: change in magnetic direction due to voltage in Fe / BaTiO3



    Physical phenomena and processes are present in everything that surrounds us (chemical ones, too, but not about them today). Sit at the computer - physics, look out the window at the birdies - physics, overexposed the meat on fire, and it turned into a coal, this is also physics. From the gigantic to the smallest objects in the Universe, everywhere there are various manifestations of physics — properties, characteristics, phenomena and processes. And what do many scientists so want to get, knowing almost everything about a process? Of course, control. The control of physical processes can give a lot of useful pluses, but the achievement of this control is a very complex process, often associated with something that is not completely clear. Today we will consider a study in which a group of scientists decided to demonstrate the possibilities of the magnetoelectric (ME) effect, more precisely, how can one gain control over the magnetic directivity and ordering by means of an electric field at room temperatures. How exactly is this realized, what is obtained from this and what are the prospects? Answers, as always, await us in the report of the research group. Go.

    Study basis


    As already mentioned in the prologue, everything and everything in this study is based on the magnetoelectric (ME) effect. So what is it? The ME effect is the relationship between magnetism and the electric field — electric polarization in an external magnetic field or magnetization in an external magnetic field. One reinforces the other. Interesting thing, but very picky in terms of temperature. In most single-crystal materials with an ME effect, the Curie temperature is quite low, that is, this effect is manifested only at temperatures significantly lower than room temperatures. And this greatly limits the practical application of the ME effect, despite all its usefulness.

    This annoying drawback can be corrected using not monostructured materials, that is, consisting of one substance, but composite ones from several, more precisely from ferrites and piezoelectrics. For ferrites are very sensitive from a magnetic point of view to an external electric field.

    Researchers themselves know this firsthand and give an example of a composite of ferromagnetic Fe (iron) and BaTiO 3 (barium titanate, BTO), which is both ferroelectric and ferroelastic (SC).
    Ferroelectricity (or ferroelectricity) * is the occurrence of spontaneous polarization in a crystal at a certain temperature even without an external electric field.

    And ferroelastic is called single-crystal substances, whose crystal lattice can spontaneously deform with decreasing temperature and phase transition.
    In addition, researchers want to gain control not only over the entire heterostructure as a whole, so to speak, but over individual nanostructures and nanoparticles. And that may be useful in a recent study of the control over the electric field of super-paramagnetic Ni nanoparticles, since this allows manipulating magnetism on a nanometer scale by affecting the magnetoelastic anisotropy of a variable voltage.

    In this work, scientists decided to study the ME effect in more detail by analyzing the magnetic domain structures of a Fe nanocrystalline film grown on a BaTiO 3 substrate(5x5 mm, 0.5 mm thick). According to scientists, they were able to prove that superparamagnetic particles can exhibit the properties of a superferromagnetic state at room temperature, and this is due to the effect of an electric field on magnetoelastic anisotropy.

    At room temperature, the BTO crystal lattice is tetragonal (c = 4. 036 Å, a = b = 3.992 Å). The polarization of an FE crystal is always directed along the c axis . In addition, the FE regions a1 – a2 with domains in which the directions of polarization and the tetragonal lattice alternate between two orthogonal directions relative to the substrate and the domain walls along [110] pc coexist with regions a 1 –c and a 2–C with in-plane / out-of-plane polarization and domain walls along [100] pc and [010] pc .
    Å is a unit of measurement of length (in our case, thickness), 1 Å = 10 −10 m or 0.1 nm.
    Domain zones a 1 and a 2 lead to 1.1% uniaxial deformation of the lattice (c - a) / a in the substrate itself, and domain zones with isotropically deformed (a = b).

    It is worth noting that the Fe film has a region of thickness gradient (“wedge”, if from the point of view of geometry) 30 μm wide, dividing the sample in half. In this section, the thickness of Fe (t Fe ) varies along the [¯100] pc BTO direction from 0.5 to 3 nm (nanometers). In all other regions, the thickness of Fe is unchanged: either 0.5 nm or 3 nm. Scientists have confirmed the state of the wedge using X-ray absorption spectroscopy (RAS) and X-ray magnetic circular dichroism (RMCD).
    X-ray magnetic circular dichroism * is the difference between the two PAC spectra obtained in a magnetic field with left and right circularly polarized light.

    Further, the sample was coated with an Al protective film 3 nm thick. After measurements using X-ray photoemission electron microscopy, the atomic structure of the sample was verified by a transmission scanning electron microscope.

    Research results



    Image No. 1

    To begin with briefly about what we see above. Images 1a and 1b are X-ray absorption spectroscopy images from the edges of Fe L 3 and Ti L 2 , respectively. These images confirm the gradient of the Fe film thickness, attenuating the Ti signal from BTO ( 1c ).

    Scanning the energy of horizontally polarized incident x-ray radiation made it possible to obtain the spatial spectrum of Ti L 2,3 and Fe L 2,3 ( 1d ). No changes were observed regarding the shape of the spectrum of Ti L2.3 over the region of the Fe wedge. Unlike Fe LL 2,3, a shape of the spectrum that changes with the thickness of the iron film. These changes are best seen in the spectral region L 2 of the edge ( 1e ), where changes in the degree of oxidation of iron affect the shape of the spectrum.

    Thus, the spectrum of the portion of the Fe film with the maximum thickness (3 μm) is similar to the spectrum of bulk Fe, but when the film is thinned to an experimental minimum of 0.5 μm, the spectrum acquires FeO x features (marked by black arrows on ).

    Such an observation is practical evidence of the presence of an intermediate FeOx layer between the main Fe and BaTiO 3 layers , the thickness of which should be approximately 2–3 Å.

    Magnetically contrasting images of the RCDM of the domain zones of the Fe ( 1f ) wedge did not show the imprints (effects) of the FE / BTO domain zones. Scientists, on the contrary, expected such prints, based on the principles of magnetostriction.
    Magnetostriction * - changes in the volume and size of the body due to changes in its magnetization.
    At the same time, scientists note that the absence of such prints does not exclude the presence of a slight transfer of deformation between the substrate and the iron film, i.e., the lattice mismatch in less than 10%.

    Also in image 1f, we see a clear and sharp transition between the paramagnetic (white color) and ferromagnetic states (blue color) with a change in the thickness of the iron film. A comparison of the RCDM of the wedge profile with the thickness profile of Fe ( 1g ) along one line showed a critical film thickness (t FM ) of 13 Å, at which such a sharp transition from one magnetic state to another occurs.

    And here it is important to note that at a temperature of 320 K, the value of the critical thickness of a ferromagnet in highly ordered coherent epitaxial films is approximately 1 monoatomic layer. And this is much less than in the case of the studied composite. Accordingly, this indicates the presence of a state of superparamagnetism at t FeFM, that is, when the thickness of Fe (in the experiment) is greater than the thickness of the ferromagnet (in theory). And this may be due to the nanocrystalline structure of the film.


    Image No. 2

    The image above is the result of dark-field microscopy of the region with the largest thickness of the iron film (3 μm). Here we see a uniform Fe layer of a nanocrystalline structure with grains (crystallites) of 2-3 nm in size. In this case, the planar distance of one of these grains is 2.86 Å, which correlates with the body-centered symmetry (syngony) of the iron crystal lattice.


    Image No. 3

    And now the most important thing is the magnetoelectric effect and its dependence on voltage.

    Before starting the voltage tests, the sample was first cooled to 60 K and then heated again to 320 K. This procedure changed the initial structure of the magnetic domains of the iron layer.

    Image 3a shows a RCDM image at V = 0 V, that is, in the absence of a voltage effect on a given portion of the sample. In the thickened region of the ferromagnetic wedge, magnetic domains (blue and white stripes) whose walls are oriented along [¯110] pc are clearly visible . The direction of magnetization inside these domain zones “goes” either along [010] pc / [0¯10] pc (new white bars) or along [100] pc(original blue stripes). A similar structure of magnetic domains with the formation of sites rotated 90 ° relative to the initial position can be associated with the aforementioned thermal cycle or with the temperature difference in this cycle due to ferroelasticity.

    Further, the sample was exposed to a voltage of V = +74 V, which made the magnetic domains more distinct ( 3b ). After an hour of this voltage, new magnetic domains with a direction along [100] pc (blue) or [010] pc / [0¯10] pc (white) became visible . This is shown in picture 3c . According to scientists, the new domain zones along the [100] pc direction resemble ferroelastic walls a 1−c. This means that the BTO substrate was transformed into a V 1 −c V ferroelastic (SC).

    The 3d image shows all magnetic domain zones already at a voltage of 170 V. Each zone was marked with a Latin letter depending on the magnetic direction:

    α - [010] pc / [0¯10] pc (white zones);
    β - [100] pc (blue areas);
    γ is the region where the imprints of the former ferroelastic domains a T 1 −a T 2 are preserved .

    The increase in voltage led to the displacement and exchange of the position of the domain zones. Theoretically, this coexistence of several different magnetic domain zones is understandable, but in practice it was demonstrated for the first time.

    Scientists did not see any particular dependence of the magnetic domains along the [100] pc axis on the gradient of the thickness of the iron film. But at the same time, they note that the effect of voltage led to an increase in ferromagnetic properties closer to the minimum thickness of Fe (in the regions β).

    Further, an exposure was carried out with voltage from +170 V to -170 V, which confirmed the above statement of scientists regarding β regions.


    Image No. 4

    If we compare the pictures 3d and 4a, then you can see changes in these areas (expansion, and then narrowing). This is associated with relaxation of deformation in polycrystalline materials that occurs over time. Only those regions of the Fe layer remained unchanged where the BTO domains were transformed quite recently. Pictures 4b and 4c show dashed lines along the [100] pc , which indicate the areas where the transition from α to β occurred.

    Graph 4d shows the results of an analysis of the extent of the ferromagnetic regions. We see that the growth of the long-range magnetic order extends to 1.3 μm along the [100] pc .
    Long- range magnetic order * - the order of orientation of the magnetic moments of atoms, extending over distances much more than interatomic.
    For a more detailed acquaintance with the nuances and details of the study, I strongly recommend that you look into the report of the research group .

    Epilogue


    The most basic conclusion that can be drawn from this experiment is that the local expansion of ferromagnetism towards narrower regions of the Fe layer is feasible by means of a controlled action of a certain voltage on the sample. Scientists attribute this process to the magnetoelastic modification of magnetic anisotropy associated with iron crystals, which in turn leads to the appearance of a superparamagnetic / superferromagnetic transition at room temperature.

    This study took us one step closer to understanding how magnetic ordering in an electric field can be controlled, while controlling the size of ferromagnetic domains, reducing it to the size of FE domains, using structures from thin films instead of single crystals.

    Control and control again. It is not enough for us to understand how certain physical or chemical processes occur around us, we want to control them. And scientists are doing everything possible to curb even those processes that until now were considered to be controlled only by nature itself. We can only hope that such studies, like the one we have examined today, will be aimed at creation, not destruction.

    Thank you for your attention, remain curious and have a great working week, guys.

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