H. Zähres

Synthesis of tailored composite nanoparticles in the gas phase

We report on a method to obtain tailored nanoparticle aggregates of two components in the gas phase. The method is based on the modification of the Brownian collision rate by charging the nanoparticles. Particles of different components are charged oppositely in order to obtain composite nanoparticle aggregates via preferential coagulation. The resulting composite aggregates are uncharged, which allows for their separation from both, charged unaggregated particles and charged aggregates of only one component. The mean size and standard deviation of each particle component can be adjusted by means of differential mobility analysis. Experimental results are presented for composites of PbS and Ag.

The magnetic states and magnetoelasticity in bcc invar-type Cr75Fe25−xMnx alloys

Abstract The substitution of Fe by Mn in the bcc ferromagnetic system Cr75Fe25 decreases the Curie temperature and eventually destroys long range FM order at 3 at% Mn. At higher Mn concentrations antiferromagnetic order sets in. Resistivity measurements, together with recent magnetization measurements show that for Fe concentrations 10

Coexisting antiferromagnetism and ferromagnetism in Fe-Ni Invar

Abstract We have measured the temperature dependence of the thermal expansion in fields of 0, 2, 4 and 6 T and the forced magnetostriction of Fe65.5Ni34.5 Invar. Both the magnetostriction and the thermal expansion coefficient show anomalies at about 20 K which we associate with antiferromagnetic γ-Fe clustering in the ferromagnetic matrix.

Anti‐Invar in Fe‐Ni

Anti‐Invar behavior in a material can be characterized by an anomalously large and a weak temperature‐dependent thermal‐expansion coefficient, when compared to the respective Gruneisen lattice expansion. It is just the opposite of the Invar effect, which is characterized by an anomalously small thermal‐expansion coefficient. Common to Invar and anti‐Invar is the fact that both posess moment‐volume instabilities. Anti‐Invar is observed in the paramagnetic state. It occurs in γ‐Fe and in a number of 3d fcc binary and ternary alloys. In FexNi100−x alloys it is observed in the concentration range 70≤x≤100 at. % within the fcc stability range. The effect vanishes as the Invar concentration, x=65 at. %, is approached. To examine the valence electron concentration dependence of the anti‐Invar effect the thermal expansion has been measured in the fcc state of FexNi100−x for 63≤x≤100 at. %. Using a model based on moment‐volume instabilities in conjunction with a thermal activation process the size of the anti‐Inva...

Magnetovolume effects in Fe80-xNixCr20 alloys

Abstract Temperature and magnetic field dependences of the thermal expansion between 4 and 300 K and in fields up to 6 T were made on Fe80-xNixCr20 for 14⩽x⩽49 at%. This concentration range covered the regions in which the samples were antiferromagnetic, paramagnetic and ferromagnetic as well as spin glass and reentrant spin glass at low temperatures. We develop a method of determining the lattice contribution to the thermal expansion for such systems showing mixed magnetic behavior and analyze the present data accordingly. We find in ferromagnetic samples large magnetic contributions to the thermal expansion even at temperatures much higher than the Curie temperature. The field dependence of the lenght change shows behavior which is characteristic of the magnetic state of the system.

The study of the thermal properties of γ-Mn from Mn-Cu alloys

Abstract The addition of small amounts of Cu to Mn stabilizes the fcc phase in Mn (γ-Mn) in a broad temperature range which is otherwise stable in a narrow limit around 1400 K. These alloys undergo an fcc-fct transition on cooling which is accompanied by an abrupt appearance of antiferromagnetism. In this paper we present specific heat and thermal expansion data of three Mn-Cu alloys containing 4.42, 6.75 and 8.55 at% Cu throughout their stability range. The specific heat data reflects the features of the structural magnetic transition and suggests a high temperature anharmonic part which is supported by the thermal expansion data.

Elastic and nonlinear acoustic properties and thermal expansion of rare-earth metaphosphate glasses

Abstract The thermal expansions and the ultrasonic wave velocities and attenuations of (Sm2O3)0.234(P2O5)0.776 and (La2O3)0.222 (P2O5)0.778 and mixed (La2O3)x (Sm2O3)y(P2O5)0.75 (where x + y = 0.25) metaphosphate glasses have been measured as functions of temperature. The change in the ultrasonic wave velocity induced by application of hydrostatic pressure up to 0.16 GPa has also been measured at selected temperatures between room temperature and 375 K. The experimental results provide the temperature dependences of the adiabatic elastic stiffnesses C11 and C44 and related elastic properties and provide the hydrostatic-pressure derivatives (∂C 11/∂P)P=0 and (∂C 44/∂P)p=0 of the elastic stiffnesses and (∂B S/∂P)P=0 of the bulk modulus. The results obtained for C IJ and (∂C IJ/∂P)P=0 are used to determine the long-wavelength acoustic-mode Gruneisen parameters; these quantify the vibrational anharmonicity, which is essential information for developing the acoustic mode contribution to the thermal expansion o...

In situ multifrequency ferromagnetic resonance and x-ray magnetic circular dichroism investigations on Fe/GaAs(110): Enhanced g-factor

We determined the magnetic anisotropy energy and g-factor of an uncapped 10 nm thick Fe/GaAs(110) film using a setup that allows frequency (1.5–26.5 GHz) as well as angular dependent ferromagnetic resonance measurements under ultrahigh vacuum conditions. The g-factor g=2.61±0.1 is unusually high at room temperature and can be interpreted as the result of an increased orbital moment due to strain. This interpretation is supported by more surface sensitive x-ray magnetic circular dichroism measurements which yield g=2.21±0.02 measured at remanence. The difference in g may be the result of magnetic field dependent magnetostriction which influences the orbital moment.

Magnetovolume in fcc ferro- and antiferromagnetic 3d-metal alloys

Thermal expansion measurements as a function of temperature and magnetic field were carried out on Fe‐Ni‐Cr and Fe‐Ni‐Mn alloys. The magnetic contribution to the thermal expansion was determined with respect to the paramagnetic reference samples in the Fe‐Ni‐Cr and Fe‐Ni‐Mn series. We observe a general behavior that alloys which order ferromagnetically show a positive magnetovolume effect which persists to temperatures much higher than the Curie temperature, whereas in the case of antiferromagnetic ordering the magnetovolume effect is positive below temperatures slightly higher than the Neel temperature, and negative above it.

Bolometer detection of magnetic resonances in nanoscaled objects

We report on a nanoscaled thermocouple (ThC) as a temperature sensor of a highly sensitive bolometer for probing the dissipative damping of spin dynamics in nanosized Permalloy (Py) stripes. The Au-Pd ThC based device is fabricated by standard electron beam lithography on a 200 nm silicon nitride membrane to minimize heat dissipation through the substrate. We show that this thermal sensor allows not only measurements of the temperature change on the order of a few mK due to the uniform resonant microwave (MW) absorption by the Py stripe but also detection of standing spin waves of different mode numbers. Using a 3D finite element method, we estimate the absorbed MW power by the stripe in resonance and prove the necessity of using substrates with an extremely low heat dissipation like a silicon nitride membrane for successful thermal detection. The voltage responsivity and the noise equivalent power for the ThC-based bolometer are equal to 15 V W(-1) and 3 nW Hz(-1/2), respectively. The ThC device offers a magnetic resonance response of 1 nV/(μ(B) W) corresponding to a sensitivity of 10(9) spins and a temperature resolution of 300 μK under vacuum conditions.