Perpendicular exchange bias and its control by magnetic, stress and electric fields

erpendicular exchange bias (PEB) involving perpendicular magnetic anisotropy (PMA) in both the antiferromagnetic (AF) pinning and the ferromagnetic (FM) sensor layer is expected to become important in future perpendicular recording and sensing devices. Further, because of the reduced spin dimensionality, PEB promises to be easier understandable than the conventional planar exchange bias (EB). In addition to its first realization using the Ising-type AF compounds FeF2 and FeCl2 we have tested control strategies of EB being alternative to the conventional magnetic and thermal ones. Indeed, specific symmetry properties of the pinning layer have been shown to enable mechanical (viz. piezomagnetic via FeF2) and electric control (viz. magneto-electric via Cr2O3) of EB, respectively. Electric control promises to become relevant for TMR devices in MRAM technology

Perpendicular exchange bias and its control by magnetic, stress and electric fields

erpendicular exchange bias (PEB) involving perpendicular magnetic anisotropy (PMA) in both the antiferromagnetic (AF) pinning and the ferromagnetic (FM) sensor layer is expected to become important in future perpendicular recording and sensing devices. Further, because of the reduced spin dimensionality, PEB promises to be easier understandable than the conventional planar exchange bias (EB). In addition to its first realization using the Ising-type AF compounds FeF2 and FeCl2 we have tested control strategies of EB being alternative to the conventional magnetic and thermal ones. Indeed, specific symmetry properties of the pinning layer have been shown to enable mechanical (viz. piezomagnetic via FeF2) and electric control (viz. magneto-electric via Cr2O3) of EB, respectively. Electric control promises to become relevant for TMR devices in MRAM technology

Robust isothermal electric switching of interface magnetization: A route to voltage-controlled spintronics

Roughness-insensitive and electrically controllable magnetization at the (0001) surface of antiferromagnetic chromia is observed using magnetometry and spin-resolved photoemission measurements and explained by the interplay of surface termination and magnetic ordering. Further, this surface in placed in proximity with a ferromagnetic Co/Pd multilayer film. Exchange coupling across the interface between chromia and Co/Pd induces an electrically controllable exchange bias in the Co/Pd film, which enables a reversible isothermal (at room temperature) shift of the global magnetic hysteresis loop of the Co/Pd film along the magnetic field axis between negative and positive values. These results reveal the potential of magnetoelectric chromia for spintronic applications requiring non-volatile electric control of magnetization.Comment: Single PDF file: 27 pages, 6 figures; version of 12/30/09; submitted to Nature Material

Large Orbital Magnetic Moment and Coulomb Correlation effects in FeBr2

We have performed an all-electron fully relativistic density functional calculation to study the magnetic properties of FeBr2. We show for the first time that the correlation effect enhances the contribution from orbital degrees of freedom of $d$ electrons to the total magnetic moment on Fe$^{2+}$ as opposed to common notion of nearly total quenching of the orbital moment on Fe$^{2+}$ site. The insulating nature of the system is correctly predicted when the Hubbard parameter U is included. Energy bands around the gap are very narrow in width and originate from the localized Fe-3$d$ orbitals, which indicates that FeBr2 is a typical example of the Mott insulator.Comment: 4 pages, 3 figures, revtex4, PRB accepte

Field dependent competing magnetic ordering in multiferroic Ni3V2O8

The geometrically frustrated magnet Ni3V2O8 undergoes a series of competing magnetic ordering at low temperatures. Most importantly, one of the incommensurate phases has been reported to develop a ferroelectric correlation caused by spin frustration. Here we report an extensive thermodynamic, dielectric and magnetic study on clean polycrystalline samples of this novel multiferroic compound. Our low temperature specific heat data at high fields up to 14 Tesla clearly identify the development of a new magnetic field induced phase transition below 2 K that shows signatures of simultaneous electric ordering. We also report temperature and field dependent dielectric constant that enables us to quantitatively estimate the strength of magneto-electric coupling in this improper ferroelectric material.Comment: 18 pages, 4 figures. Accepted for publication in Euro. Phys. Let

Neutron scattering study of transverse magnetism

In order to clarify the nature of the additional phase transition at H1 (T) \u3c Hc (T) of the layered antiferromagnetic (AF) insulator FeBr2 as found by Aruga Katori et al. (1996) we measured the intensity of different Bragg-peaks in different scattering geometries. Transverse AF ordering is observed in both AF phases, AFI and AFII. Its order parameter exhibits a peak at T1 = T (H1) in temperature scans and does not vanish in zero field. Possible origins of the step-like increase of the transverse ferromagnetic ordering induced by a weak in-plane field component when entering AFI below T1 are discussed

Quantum Spin Glasses

Ising spin glasses in a transverse field exhibit a zero temperature quantum phase transition, which is driven by quantum rather than thermal fluctuations. They constitute a universality class that is significantly different from the classical, thermal phase transitions. Most interestingly close to the transition in finite dimensions a quantum Griffiths phase leads to drastic consequences for various physical quantities: for instance diverging magnetic susceptibilities are observable over a whole range of transverse field values in the disordered phase.Comment: 10 pages LaTeX (Springer Lecture Notes style file included), 1 eps-figure; Review article for XIV Sitges Conference: Complex Behavior of Glassy System