Multiferroics as Data Storage Elements E. Raajeshwar & T.G. Premkumar Bannari Amman Institute of Technology

Ferroelectricity Property of certain non-conducting or dielectrics, that exhibit spontaneous electric polarization. Electric polarization-separation and alignment of electric dipoles in the direction of electric field. Can be reversed by appropriate electric field. It exhibits only in some range of temperatures

Ferroelectricity A good example for ferroelectric material is iron. Iron atoms being tiny magnets, spontaneously align themselves in clusters called ferromagnetic domains, which in turn can be oriented predominantly in a given direction by application of an external magnetic field.

Ferromagnetism Mechanism by which certain materials are made into magnets or are attracted towards them. Strongest of all forces in magnetism. Para magnetism, Diamagnetism and Antiferromagnetism are other types. These are detected only by sensitive instruments.

Ferromagnetism Electron behaves as a tiny magnet due to magnetic moment. Quantum mechanical spin. Due to this property, the magnetic field can either be pointing “up” or “down”. Main source is from quantum mechanism spin.

Ferromagnetism Arises when the magnetic dipoles in same direction Tiny magnets arranged in a single direction creates a macroscopic field. Fully filled orbital does not contribute to ferromagnetism. Only partially filled contribute to ferromagnetism.

Ferromagnetism Only a few materials are ferromagnetic. E.g. Iron, cobalt, nickel, lodestone.

Data storage in FeRAM and MRAMs Data is burnt by switching magnetic states. Enables efficient and fast write options. Reading is comparatively slower. High current is required to write. Data is burnt by switching electrical states. Enables efficient and fast read options. Writing is comparatively slower. High cost and low density.

What happens when both properties combine? Ultimate memory devices arises! Multiferroics are elements capable of combining more than one ferroic property. They exhibits both ferro electric and ferro magnetic properties.

Data storage in multiferroics The multiferroic material consists of a magnetostrictive thin film and a piezoelectric substrate. This oscillating field produces alternating strain-induced magnetic anisotropy in the magnetostrictive layer. Oscillating electric field Piezoelectric crystal AC Voltage

The four state memory Voltage driven spin wave excitation could be used for low-dissipation spin wave based logic and memory elements. In multiferroics strong coupling between electric and magnetic states exists So available switchable states (+P,+M), (+P,-M), (-P,+M) and (-P,-M) are not independent. They are available in either (+P,+M) and (-P,-M) or (+P,-M) and (-P,+M). Thus the device is restricted to two states similar to conventional memory elements.

The four state memory This problem is overcame by forming a ferromagnetic-magnetoelectric tunnel junction. This results in four state memory effect.

Advantages over conventional data storage Conventional storage Multiferroic storage Uses movement of electrons. Leakage of electrons are possible which enables energy loss in conventional semiconductor technology. Uses spin based electronics. By using spin waves, leakage energy can be reduced which could help making computer processors 1000 times efficient and faster

Limitations Most of the known multiferroic elements retains their property only at very low temperatures. Examples: BiFeO3, EuTiO3, BiMnO3, etc., This restricts the usage of such efficient elements in electronic devices which works at room temperature.

Room temperature multiferroics BaTiO3 exhibits multiferroic properties at room temperature. It is achieved by depositing ten atom thin film of cobalt and iron on the top of a four atom thin BaTiO3.