Wolfgang Kleemann

Magnetoelectric exchange bias systems in spintronics

Magnetoelectric switching of perpendicular exchange bias is observed in a Co∕Pt multilayer attached to single crystalline magnetoelectric antiferromagnetic Cr2O3(111). The exchange bias field HEB can be set to positive or negative values by applying an electric field Efr either parallel or antiparallel to the magnetic freezing field Hfr while cooling to below the Neel temperature. Based on this result, the antiferromagnetic spin state can be used as a medium for data storage. The authors propose magnetic random access memory cells and magnetic logic devices, which are purely voltage controlled.

Two-stage processes of electrically induced-ferroelectric to relaxor transition in 0.94(Bi1/2Na1/2)TiO3-0.06BaTiO3

The stability of electrically induced long-range ferroelectric order in a relaxor 0.94(Bi1/2Na1/2)TiO3-0.06BaTiO3 ceramic material has been investigated by temperature-dependent X-ray diffraction and electrical property measurements. The depolarization and ferroelectric-to-relaxor transition are identified as separate and discrete processes. It is observed that the induced ferroelectric domains first lose their ferroelectric/ferroelastic texture coincident with a peak signal in the thermally induced depolarization current. With further increase in temperature, the detextured ferroelectric domains are dissociated into nanoscale entities. This fragmentation marks the ferroelectric-to-relaxor transition. It is suggested that the ferroelectric-to-relaxor transition has features of a second order phase transition.

Electric in-plane polarization in multiferroic CoFe2O4/BaTiO3 nanocomposite tuned by magnetic fields

Ferrimagnetic CoFe2O4 nanopillars embedded in a ferroelectric BaTiO3 matrix are an example for a two-phase magnetoelectrically coupled system. They operate at room temperature and are free of any resource-critical rare-earth element, which makes them interesting for potential applications. Prior studies succeeded in showing strain-mediated coupling between the two subsystems. In particular, the electric properties can be tuned by magnetic fields and the magnetic properties by electric fields. Here we take the analysis of the coupling to a new level utilizing soft X-ray absorption spectroscopy and its associated linear dichroism. We demonstrate that an in-plane magnetic field breaks the tetragonal symmetry of the (1,3)-type CoFe2O4/BaTiO3 structures and discuss it in terms of off-diagonal magnetostrictive-piezoelectric coupling. This coupling creates staggered in-plane components of the electric polarization, which are stable even at magnetic remanence due to hysteretic behaviour of structural changes in the BaTiO3 matrix. The competing mechanisms of clamping and relaxation effects are discussed in detail.

RANDOM FIELDS IN RELAXOR FERROELECTRICS — A JUBILEE REVIEW

Substitutional charge disorder as in PbMg1/3Nb2/3O3, structural cation vacancies as in SrxBa1-xNb2O6 and isovalent substitution of off-centered cations as in BaTi1-xSnxO3 and BaTi1-xZrxO3 give rise to quenched electric random-fields (RFs), which we proposed to be at the origin of the peculiar behavior of relaxor ferroelectrics 20 years ago. These are, e.g. a strong frequency dispersion of the dielectric response and an apparent lack of macroscopic symmetry breaking in the low temperature phase. Both are related to mesoscopic RF-driven phase transitions, which give rise to irregularly shaped quasi-stable polar nanoregions below the characteristic temperature T*, but above the global transition temperature Tc. Their co-existence with the paraelectric parent phase can be modeled by time-dependent field equations under the control of quenched RFs and stress-free strain (in the case of order parameter dimension n ≥ 2). Transitions into global polar order at Tc may occur in uniaxial relaxors as observed on the uniaxial relaxor ferroelectric Sr0.8Ba0.2Nb2O6 and come close to RF Ising model criticality. Re-entrant relaxor transitions as observed in solid solutions of Ba2Pr0.6Nd0.4(FeNb4)O15 are proposed to evidence the coexistence of distinct normal and relaxor ferroelectric phases within the framework of percolation theory.

Dynamic phase transitions in ferroic systems with pinned domain walls

The creep and relaxation of domain walls under an ac electric field that are observed in an ideal model system, periodically poled superionic KTiOPO4 (KTP), appear to occur in different regimes that are separated by a dynamic phase transition at a well-defined frequency, f m = 0.003 Hz, at temperature T = 233 K. The power-law dispersion of the creep susceptibility, χ ∝ 1 + (iωτ)− β (with β ≈ 0.4), and the large nonlinearity encountered for f<f m is contrasted with the Cole–Cole-type relaxational dispersion, χ ∝ [1 + (iωτ)1− α −1 (with α ≈ 0.3), for f > f m. Similar creep-to-relaxation transitions are observed at low frequencies in other ferroic systems with weak disorder: the multidomained uniaxial relaxor Sr0.69Ba0.31Nb2O6 (SBN), the quantum ferroelectric domain state of SrTi18O3 (STO18) and the superferromagnetic nanoparticle system [(Co80Fe20(1.4 nm)/Al2O3(3 nm)]10, which appear to belong to the same dynamical universality class.

Ferroelectric Domains in SrxBa1 − xNb2O6 Single Crystals (0.4 ≤ × ≤ 0.75)

The crossover of single crystalline Sr x Ba 1 − x Nb 2 O 6 (SBN) from ferroelectric to relaxor behavior when increasing the Sr 2+ content from x = 0.4 to x = 0.75 is investigated by dielectric low-frequency susceptometry and scanning piezo-force microscopy. Increasing quenched random electric fields are conjectured to be responsible for the observed increasing dielectric polydispersivity and the growing fractality of the polar 180° domains. A simple statistical model taking into account the varying occupation of the different A-sites with vacancies, Sr 2+ and Ba 2+ ions, and their influence on the average RF strength is proposed to describe the transformation to the relaxor behavior with increasing x.

Scaling behaviour of strontium titanate

The dielectric susceptibility, = ´ - i´´, of nominally pure strontium titanate is measured between 4 and 70 K as a function of an external bias field. Static scaling analysis of the susceptibility reveals a very good scaling behaviour with a quasi-critical exponent a = /( - 1) = 2.00 ± 0.02 extracted consistently from two different experiments.

Domainlike precursor clusters in the paraelectric phase of the uniaxial relaxor Sr0.61Ba0.39Nb2O6

Dielectric spectra measured within 10−3Hz⩽f⩽104Hz in the paraelectric state of Sr0.61Ba0.39Nb2O6 (SBN61) at T>320K reveal a broadened Debye-type relaxation at f≈100Hz and a power-law low-f increase at f<10−1Hz. These anomalies are attributed to the dynamic modes wall segment relaxation and creep of the interfaces between polar nanoregions (PNRs) and paraelectric bulk. This structure is self-organized by the statistical distribution of quenched random fields due to the charge disorder of SBN61. The PNRs and their thermal evolution are visualized by piezoresponse microscopy.

Second-harmonic study of polar symmetry and domain structure in SrTi18O3

The ferroelectric domain structure of SrTi18O3 is studied via the azimuthal dependence of the second-harmonic (SH) light intensity below Tc≈25 K. Tensor analysis of the SH response reveals locally variant mixtures of eight triclinic polar domains, which transform into an orthorhombic single domain under an electric field E applied perpendicularly to the tetragonal axis, with d22 being the dominant coupling constant.

Multiferroically composed exchange bias systems

Magnetoelectric (ME) antiferromagnetic Cr2O3, being exchange coupled to a ferromagnetic multilayer (Pt/Co/Pt)n, n ≥ 1, is treated as a multiphase multiferroic material with sophisticated multifunctional properties. It is shown that the exchange bias (EB) of the ferromagnetic hysteresis loop cannot only be controlled by the magnetic freezing field, H fr, but additionally also by an external electric freezing field, E fr, via the ME effect of Cr2O3. Either a gradual shift or complete switching from −H EB to +H EB on the magnetic field axis are possible. Here the influence of E fr at constant H fr on both the coercive field, H c, and the antiferromagnetic domain wall susceptibility, Δχ, and on the temperature dependence of the ME controlled quantities H c(T) and H EB(T) is reported.