8/14/2019 Multiferroic Crystals poster

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Multiferroic CrystalsIntroduction

This poster presents background information on the BSc project, Multiferroic

Crystals. Multiferroic crystals are, as the name suggests, multiferroic. The term

multiferroicwas first proposed by Schmid to describe materials which display more

than one of the primary ferroic properties such as; [1]

Ferroelectricity

Ferromagnetism

Ferroelasticity

Ferrotoroidicity

The crystalline materials dealt with here only display ferroelectric and ferromagnetic

properties.

This hysteresis curve shows how electric polarisation (P) depends on

applied electric field (E),and also how the magnetisation (M)depends on

the applied magnetic field (B). As the applied field increases, P or M

increases from a to b. Once the applied field is removed, P or M rests at c.

So P and M cannot be zero once their respective field is applied. [3]

E, B

P, M

FerromagnetismThis is very common as it is the mechanism which causes magnetism in the everyday

sense. Permanent magnets and the m aterials attracted to them are so because they

are ferromagnetic. This is caused when a material has a spontaneous magnetisation

which can be altered by application of a magnetic field. Similarly to ferroelectricity,

this is best described by a hysteresis curve as shown above.

Ferroelectricity [2]

A material is said to be ferroelectric if it has a spontaneous electric polarisation

which can be reversed by the application of an electric field. This is best described

by the hysteresis curve below.

Materials combining these properties will display a coupling between them. The

stronger the coupling, the more potential there is for use of a material as it is this

coupling which sets multiferroics aside from other materials.

Many materials do not have very strong coupling but LuFe2O4(lutetium iron oxide)

shows promising signs that it may become very useful.

One of the most effective uses would be in data storage. Coupling in LuFe 2O4means

that data could be written electrically and read magnetically, thus combining the

best parts of current data storage methods. [4]

LuFe2O4Lutetium iron oxide is part of a family of crystals which share the same structure. This

group is known as RFe2O4 (where R is; Y, In, Sc, and Lanthanides from Dy to Lu). The

structure of these crystals is shown below.

This shows the structure of the RFe2O4crystals. They are comprised of two

layers, T and W. The T layer is made of triangular levels of O, R and Fe

which are stacked up the c-axis. The W layer is made entirely of iron atoms

in a triangular lattice. These are an even mix of Fe2+and Fe3+atoms which

leads to an average of Fe

2.5+

. [5]

Ferroelectricity arises as a result of charge ordering within the RFe 2O4 structure [6].

This charge ordering is caused by charge frustration in the W layer which occurs when it

is not possible to satisfy all the interactions in a system to find a ground state [7, 8].

To carry out experiments and research on this material requires a high quality single

crystal so that the entire material is aligned. The image below shows a single crystal as

there are no protrusions or separate cr ystals growing off in different directions.

This image shows a single crystal of LuFe2O4. This was grown using the floating zone

technique described in the next section.

Powder SynthesisTo grow a single crystal like the one above takes a lot of time. To make a LuF2 2O4

crystal, exact proportions of Lu2O3 (lutetium oxide) and Fe2O3 (iron oxide) must be

weighed out and ground together into a fine powder. This powder is then heated

several times in different atmospheres for varying lengths of time. This will eventually

produce a polycrystalline rod for use in the crystal growth. A typical method would be;

1. Heat for 12 hours at 1200C in a CO2/CO = 5 atmosphere

2. Grind powder

3. Heat for 24 hours at 1200C in an Ar atmosphere

4. Grind powder and compress into rod

5. Heat for 24 hours at 1250C in a CO2/CO = 5 atmosphere

(CO2/CO = 5 indicates the ratio of carbon dioxide to carbon monoxide in the furnace atmosphere)

Crystal GrowthOnce the final rod has been baked, it is time to use the floating zone technique. This

uses an optical floating zone furnace (as shown below) to actually grow the crystal.

Feed Rod

Seed Rod

Rotation of Seed Rod

Rotation of Feed Rod

Halogen bulb

Focusing Mirrors

Growth Atmosphere

Crystal Growth

Molten Zone

This diagram shows the components of the optical floating zone furnace. The black

arrow show how the focusing mirrors direct light from the halogen bulbs onto the

molten zone. (Created from an original image found in [9])

The rod produced from the powder synthesis will now become the feed rod from

which the crystal will grow. The seed rod is the same material but should have a crystal

structure.

The two rods are set rotating in opposite directions in order that the temperature and

composition of the molten zone become homogeneous. An atmosphere, commonly

CO2/CO, is introduced before the lamps are turned on . Mirrors focus the heat onto the

tip of the feed rod so it starts to melt. This must then be maneuvered so that it meets

and sticks to the tip of the seed rod. Now, the focal point of the mirrors is moved

slowly up the feed rod (at around 1mm per hour) thus melting it further up. This allows

the molten zone to solidify onto the seed rod with a crystal structure. The feed rod is

sometimes raised very slowly (at 0.3mm per hour) during the growth to ensure the

crystal grows very thin and remains a single crystal.

References[1] - H. Schmid, Multiferroic Magnetoelectrics, Ferroelectrics 162, 317.[2] - M. Lines & A. Glass (1979). Principles and applications of ferroelectrics and related

materials. Clarendon Press, Oxford. ISBN 0-19-851286-4.

[3] - http://www.codecogs.com/reference/physics/magnetism/magnetic_hysteresis.php

(Retrieved on 28/11/2012)

[4] - W. Erenstein, N. D. Mathur and J. F. Scott, Multiferroic and Magnetoelectric Materials,

Nature 442, p. 759-765

[5] - M.A. Subramanian, T. He, J. Chen, N.S. Rogado, T.G. Calvarese and A.W. Sleight, Adv.

Mater. 18, 1737-1739 (2006).

[6] - J. van der Brink and D.I. Khomskii, J.Phys: Condens. Matter 20, 434217 (2008).

[7] - N. Ikeda, J. Phys.: Condens. Matter 20, 434218 (2008).

[8] - S. Blundell, Magnetism in Condensed Matter, Oxford Master Series in Condensed

Matter Physics.

[9] - Christo Guguschev, Jens Gtze, and Matthias Gbbels, American Mineralogist 95,

p. 449-455

Ben Brown, Department of Physics,