arXiv subjects

W. Kleemann

Publications and source records attributed to W. Kleemann.

12 recordsLinked to original sources

arXiv2010

Interface driven magnetoelectric effects in granular CrO2

Antiferromagnetic and magnetoelectric Cr2O3-surfaces strongly affect the electronic properties in half metallic CrO2. We show the presence of a Cr2O3 surface layer on CrO3 grains by high-resolution transmission electron microscopy. The effect of these surface layers is demonstrated by measurements of the temperature variation of the magnetoelectric susceptibility. A major observation is a sign change at about 100 K followed by a monotonic rise as a function of temperature. These electric field induced moments in CrO3 are correlated with the magnetoelectric susceptibility of pure Cr2O3. This study indicates that it is important to take into account the magnetoelectric character of thin surface layers of Cr2O3 in granular CrO2 for better understanding the transport mechanism in this system. The observation of a finite magnetoelectric susceptibility near room temperature may find utility in device applications.

cond-mat.str-el↗

arXiv2009

Epitaxial growth and magnetoelectric relaxor behavior in multiferroic 0.8Pb(Fe1/2Nb1/2)O3-0.2Pb(Mg1/2W1/2)O3 thin films

We present electric and magnetic properties of 0.8Pb(Fe1/2Nb1/2)O3-0.2Pb(Mg1/2W1/2)O3 films epitaxially grown on (001) SrTiO3 substrates using pulsed laser deposition. A narrow deposition window around 710 oC and 0.2 mbar has been identified to achieve epitaxial single-phase thin films. A typical Vogel-Fulcher relaxor-like dielectric and magnetic susceptibility dispersion is observed, suggesting magnetoelectric relaxor behavior in these films similar to the bulk. We determine a magnetic cluster freezing temperature of 36 K, while observing weak ferromagnetism via magnetic hysteresis loops up to 300 K.

cond-mat.mtrl-sci↗

arXiv2009

Spin-lattice coupling in multiferroic Pb(Fe1/2Nb1/2)O3 thin films

We have made magnetization and x-ray diffraction measurements on an epitaxial Pb(Fe1/2Nb1/2)O3 200 nm film. From the temperature dependence of the out-of-plane lattice parameter we can assign a Burns' temperature at Td ~ 640 K, a temperature at T* ~ 510 K, related to the appearance of static polar nanoregions, and an anomaly occurring at 200 K. The latter is precisely the Néel temperature TN determined from magnetization and points to spin-lattice coupling at TN ~ 200 K. We also observe "weak ferromagnetism" up to 300K and propose superantiferromagnetic clusters as a plausible scenario to explain this hysteresis above TN.

cond-mat.mtrl-sci↗

arXiv2006

Magnetic field-dependent dielectric constant in La2/3Ca1/3MnO3

We report a rather high dependence of the dielectric permittivity on the magnetic field in La2/3Ca1/3MnO3. The variation is maximum at around 270 K, little above the Curie temperature, TC, and it reaches a 30% under only 0.5 T. We attribute this phenomenon to the space-charge or interfacial polarization produced between the insulator and the metallic regions segregated intrinsically in the material above TC.

cond-mat.str-el↗

arXiv2005

Collective states of interacting ferromagnetic nanoparticles

Discontinuous magnetic multilayers [CoFe/Al2O3] are studied by use of magnetometry, susceptometry and numeric simulations. Soft ferromagnetic Co80Fe20 nanoparticles are embedded in a diamagnetic insulating a-Al2O3 matrix and can be considered as homogeneously magnetized superspins exhibiting randomness of size (viz. moment), position and anisotropy. Lacking intra-particle core-surface ordering, generic freezing processes into collective states rather than individual particle blocking are encountered. With increasing particle density one observes first superspin glass and then superferromagnetic domain state behavior. The phase diagram resembles that of a dilute disordered ferromagnet. Criteria for the identification of the individual phases are given.

cond-mat.mtrl-sci↗

arXiv2005

Bias field effect on the temperature anomalies of dielectric permittivity in PbMg1/3Nb2/3O3- PbTiO3 single crystals

In contrast to ordinary ferroelectrics where the temperature, Tm, of the permittivity maximum monotonically increases with bias field, E, in (1-x)PbMg1/3Nb2/3O3-(x)PbTiO3 (0<x<0.35) single crystals, Tm was found to remain constant or decrease with E up to a certain threshold field, Et, above which Tm starts increasing. Et decreases with x and almost disappears at about x=0.4. We explain this field dependence by the quenched random fields and coupling between the orientable dipoles with average polarization. For crystals with 0.06<x<0.13, the T-E phase diagrams are constructed, which include the relaxor, ferroelectric, glass and mixed ferroelectric-glass phases. The boundary between the ferroelectric and relaxor phases corresponds to a first-order phase transition and is characterized by thermal hysteresis with a large curvature of the line obtained in the field cooling mode, and by the presence of the end-point above which there is no difference between the ferroelectric and relaxor phases. The glass phase has a nearly field-independent boundary in the vicinity of the Vogel-Fulcher temperature TVF. The coupling between the glass order parameter and polarization changes the ferroelectric boundary at temperatures lower than TV

cond-mat.mtrl-sci↗

arXiv2005

Models for the magnetic ac susceptibility of granular superferromagnetic CoFe/Al$_2$O$_3$

The magnetization and magnetic ac susceptibility, $χ= χ' - i χ''$, of superferromagnetic systems are studied by numerical simulations. The Cole-Cole plot, $χ''$ vs. $χ'$, is used as a tool for classifying magnetic systems by their dynamical behavior. The simulations of the magnetization hysteresis and the ac susceptibility are performed with two approaches for a driven domain wall in random media. The studies are motivated by recent experimental results on the interacting nanoparticle system Co$_{80}$Fe$_{20}$/Al$_{2}$O$_{3}$ showing superferromagnetic behavior. Its Cole-Cole plot indicates domain wall motion dynamics similarly to a disordered ferromagnet, including pinning and sliding motion. With our models we can successfully reproduce the features found in the experimental Cole-Cole plots.

cond-mat.dis-nn↗

arXiv2002

Cole-Cole analysis of the superspin glass system Co80Fe20/Al2O3

Ac susceptibility measurements were performed on discontinuous magnetic multilayers [Co80Fe20(t)/Al2O3(3nm)]x10, t = 0.9 and 1.0nm, by Superconducting Quantum Interference Device (SQUID) magnetometry. The CoFe forms nearly spherical ferromagnetic single-domain nanoparticles in the diamagnetic Al2O3 matrix. Due to dipolar interactions and random distribution of anisotropy axes the system exhibits a spin-glass phase. We measured the ac susceptibility as a function of temperature 20 < T < 100K at different dc fields and as a function of frequency 0.01 < f < 1000Hz. The spectral data were successfully analysed by use of the phenomenological Cole-Cole model, giving a power-law temperature dependence of the characteristic relaxation time tau_c and a high value for the polydispersivity exponent, alpha = 0.8, typical of spin glass systems.

cond-mat.dis-nn↗

arXiv1999

Two-dimensional behavior of the sublattice magnetization in three-dimensional Ising antiferromagnets

A three-dimensional layered Ising-Antiferromagnet with a ferromagnetic intra-layer coupling to z neighbors, zJ > 0, and an antiferromagnetic interlayer coupling to z' neighbors, z'J' < 0, is investigated by Monte Carlo simulations on a hexagonal lattice. The physical nature of the anomalous temperature bahavior of the sublattice magnetizations, which is found for certain values of r=zJ/z'J' and z' in magnetic fields is explained in terms of successive phase transitions. They take place on the ferromagnetic 2-dimensional spin-down sublattice at T = T_c^{2d}, smeared by a finite stabilizing molecular field, and on both antiferromagnetically coupled sublattices at T_c^{3d} > T_c^{2d}.

cond-mat.stat-mech↗

arXiv1998

Magnetic neutron scattering investigation of the field-induced Griffiths phase in FeCl2

Quasielastic neutron scattering has been utilized to investigate the Griffiths phase induced by an axial magnetic field in FeCl2 at temperatures Tc(H)<=T<=TN, where Tc(H) is the transition temperature in an external magnetic field H. We present the temperature dependence of the integrated neutron scattering intensity at fixed scattering vectors for various magnetic fields. On cooling below TN the antiferromagnetic short-range fluctuations decrease due to an increase of the volume fraction which is already antiferromagnetically ordered. A weighted average of the local contributions to the antiferromagnetic susceptibility quantitatively describes the scattering cross section.

cond-mat.stat-mech↗

arXiv1998

Crossover from pure Ising to random exchange dominated behavior of the two-dimensional antiferromagnet Rb2Co1-xMgxF4

The temperature dependence of the uniform susceptibility, chi, of diluted two-dimensional Ising antiferromagnets Rb2Co1-xMgxF4,0<=x<=0.4, is investigated in the limit of vanishing external field. Novel data for x = 0.15 are compared with those obtained for x = 0 and 0.4 by Breed et al. (1969) and Ikeda (1983), respectively. Whereas in the pure case, x = 0, Fishers (1962) "energetic contribution" dominates, Aharony et al.s (1979,1986) "random contribution" becomes increasingly important with increasing x. Taking into account both terms not only with respect to the global ordering temperature, TN(x), but also in relation to the "local" phase transition temperatures TN throughout the Griffiths regime,TN(x)<=TN<=TN(0), a satisfactory quantitative description of chi vs. T is deduced for any x above the percolation threshold.

cond-mat.dis-nn↗

arXiv1998

Field-induced transverse spin ordering in FeBr2

Weak first-order phase transitions from axial to oblique spin ordering in FeBr2 are evidenced by SQUID magnetometry in axial fields H1(T) above the multicritical point, Hm = 2.4 MA/m, Tm = 4.6 K, and below the antiferro-to-paramagnetic phase line, Hc(T), in agreement with recent specific heat data (Aruga Katori et al., 1996). The ordering of the in-plane moments is probably due to non-diagonal coupling to the longitudinal ones, both of which increase dis-continuously at H1(T) only under an additional symmetry-breaking transverse field.

cond-mat.stat-mech↗