Physics & Astronomy, Dept. of

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Synthesis of Mn-Zn ferrite from Na2O-B2O3 flux under CO-CO2 ambients
(American Institute of Physics, 1982-03) Plaskett, T. S.; Shivashankar, S. A.; Gilbert, B. L.; Olson, B. L.; Chien, C.-L.; Herman, D.A. Jr.
Synthesis was carried out by dissolving MnO, ZnO, and Fe2O3 in the molar ratio 0.42:0.44:1.07 in fluxes of Na2O:xB2O3 at 900 °C and slow cooling under ambients of CO-CO2 mixtures; x varied from 1.34 to 2.0 and Co-Co2 from 0.06 to 1.0% CO. The composition and oxidation state of the precipitated ferrite were dependent on both CO content and x. The composition of the ferrite ranged between Mn0.44Zn0.40Fe2.16O4 and Mn0.09Zn0.06Fe2.85O4 and the Fe2+ between 0.16 and 0.85 moles/formula unit. By using reactive gas mixtures, the cation oxidation states were maintained constant during cooling. Mössbauer spectroscopy and coulometric analysis were used to establish that Mn3+ was unstable in the flux in the presence of Fe2+. The results obtained by these experiments establish the optimum conditions necessary to grow Mn-Zn ferrite films by liquid phase epitaxy (LPE). The saturation magnetization of the synthesized material was between 4700 and 5200 G; the Curie temperature increased with Fe2+ from 180 to 490 °C. These properties agree with measurements previously made on LPE films of this material.
Magnetic dynamics of La2CuO4 studied by Mossbauer spectroscopy
(American Institute of Physics, 1990-05-01) Tang, H.; Gang, Xiao; Singh, Avinash; Tesanovic, Zlatko; Chien, C.-L.; Walker, J. C.
By doping a very small amount of 57Fe into La2CuO4, Mossbauer spectroscopy has been applied to study the magnetic property of the parent compound. From the measurement of the magnetic hyperfine field at the Fe nuclei for various temperatures between 4.2 K and TN, the temperature dependence of the sublattice magnetization for La2CuO4 has been discussed. A theoretical calculation shows that, with temperature increasing a 3D-2D dimensional crossover occurs in the magnetic dynamics of an anisotropic antiferromagnetic, which is indeed confirmed by the data. The best fit to the data using this theory yields J=1600 K and r=0.011.
Effect of spacer material on the magnetic surface anisotropy in ultrathin Fe70B30 multilayer films
(American Institute of Physics, 1991-04-15) Hicken, R. J.; Rado, G. T.; Chien, C.-L.
It has been found recently that the magnetic surface anisotropy Ks in Fe70B30/Ag multilayer films decreases monotonically with magnetic layer thickness (2L) for 2L<16.5 Å. In order to determine possible effects of the spacer material on the surface anisotropy in the aforementioned system, Ag has been replaced with Al2O3 and ferromagnetic resonance (FMR) measurements have been made on these films. These Fe70B30/Al2O3 films were fabricated by magnetron sputtering and were characterized by X-ray-diffraction and vibrating sample magnetometer (VSM) measurements in addition to FMR. In the region where Ks depends upon 2L, the data is insufficient to confirm the thickness dependence of Ks that was observed in Fe70B30/Ag, while in the region where Ks is independent of 2L, the values of Ks deduced for Fe70B30/Ag and Fe70B30/Al2O3 are in good agreement. The latter is particularly interesting in light of the enormous difference in conductivity between Ag and Al2O3.
A strongly fluctuating quasi-two-dimensional insulator
(American Institute of Physics, 1991-04-15) Cooper, A.S.; Espinosa, G.P.; Broholm, C.; Aeppli, G.
The authors describe neutron-scattering data for SrCr8-xGa4+xO19, a layered compound containing planes of S=3/2 Cr3+ ions which form Kagome lattices. Despite strong antiferromagnetic interactions (θCW≈-500 K), fluctuations account for more than 75% of the free ion moment at 1.5 K. The spectrum (averaged over reciprocal space) is gapless and resembles that of a 2D long-range-ordered antiferromagnet, as does the low-temperature specific heat of the compound. Even so, the static correlation length does not exceed 7±2 Å. Monte-Carlo simulations of the antiferromagnetic three-state Potts model on the Kagome lattice show that this model does not have a finite temperature phase-transition. Even at T=0, there does not seem to be true long-range antiferromagnetic-order. However, the magnetic correlations decay algebraically rather than exponentially. The implications of this result for the ground-state of SrCr8-xGa4+xO19 is discussed.
Comparison of the electron-spin-resonance linewidth in multilayered CuMn spin glasses with insulating versus conducting interlayers
(American Institute of Physics, 1994-05-15) Leslie-Pelecky, D. L.; VanWijland, F.; Hoff, C. N.; Cowen, J.A.; Gavrina, A.; Chien, C.-L.
The temperature-dependent electron-spin-resonance linewidth ΔH(T) may be used to investigate the effect of the geometry and interlayer material on the magnetic properties of multilayered systems. We compare ΔH(T) in CuMn/Al2O3 multilayers with previous measurements of CuMn/Cu samples. CuMn/Al2O3 samples with CuMn thicknesses, WSG, from 40 Å to 20000 Å obey the same form as the CuMn/Cu system, but show quantitative differences in the fitting parameters. The linewidths of the CuMn/Al2O3 samples, even in the bulk, are systematically larger than the linewidths for the CuMn/Cu samples, suggesting that the ESR linewidth is sensitive to differences in sample growth and structure. The value of the minimum linewidth decreases with decreasing WSG in the CuMn/Al2O3 series, but remains constant in the CuMn/Cu series. Although susceptibility measurements of the freezing temperature Tf do not differentiate between samples with WSG≥5000 Å, the ESR linewidth is sensitive to changes at larger length scales. This experiment emphasizes the importance of considering both the total sample thickness, as defined by the range of the conduction electrons, and the spin-glass layer thickness in analyzing the ESR linewidth in multilayers.
Strong magnetic fluctuations in transition metal oxides
(American Institute of Physics, 1996-04-15) Aeppli, G.; Lee, S.-H.; Broholm, C.; DiTusa, J. F.; Bao, W.
Most magnets have long-range magnetic order when the thermal energy is less than the local magnetic exchange energy (T<|ΘCW|) effects such as reduced dimensionality and frustration, 1 however, can suppress the ordering transition and lead to unusual cooperative paramagnetic phases at low temperatures. We review neutron scattering experiments exploring such short-range-ordered phases in insulating transition metal oxides. We discuss (V1-xCrx)2O3, in which orbital fluctuations appear to limit spin correlations to within small “molecular” clusters, SrCr9pGa12-9pO19, in which geometrical frustration allows local antiferromagnetic constraints to be fulfilled without long-ranged order, and Y2BaNiO5, in which magnetic interactions occur only within chains of spins which are unable to order because of the Haldane effect. Emphasis is placed on the common features of exchange interactions in these oxides and the important role which magnetic neutron scattering has played in understanding the unusual magnetic phenomena.
Long-range exchange coupling between a ferromagnet and an antiferromagnet across a nonmagnetic spacer layer
(American Institute of Physics, 1997-04-15) Gokemeijer, N.J.; Ambrose, T.; Chien, C.-L.; Wang, N.; Fung, K.K.
The antiferromagnet/ferromagnet exchange coupling giving rise to a shifted hysteresis loop has usually been considered an interfacial effect. We show evidence that this exchange coupling between an antiferromagnet (CoO) and a ferromagnet (Ni81Fe19) is long range in nature. Exchange coupling has been observed in tri-layer films consisting of a nonmagnetic noble metal (Ag, Au, and Cu) spacer layer sandwiched between 300 Å CoO and 300 Å NiFe. The strength of the coupling decreases with increasing spacer layer thickness and vanishes at about 55 Å. This suggests that the antiferromagnetic/ferromagnetic exchange coupling is beyond an interfacial effect, and that conduction electrons may be involved in the mediation of the coupling
Studies of deposition order anomaly of exchange coupling in bilayers and trilayers of NiFe and CoO
(American Institute of Physics, 1997-04-15) Ambrose, T.; Leifer, K.; Hemker, K. J.; Chien, C. L.
The temperature dependence of the exchange field and coercivity and the effect of deposition order have been studied in a series of bilayer (NiFe/CoO and CoO/NiFe) and trilayer (NiFe/CoO/NiFe) films. A profound difference in the exchange field temperature behavior was observed in samples with NiFe deposited on top of CoO compared to samples with CoO deposited on top of NiFe. When CoO is on top of NiFe, the exchange field follows a linear temperature dependence, while for samples with NiFe on top of CoO, the exchange field has a plateau followed by a rapid decrease. These distinct temperature dependences are reproduced in the NiFe/CoO/NiFe trilayer which contains both geometries. Transmission electron microscopy has been employed to study the interface between the NiFe and CoO layers.
Dependence of exchange coupling on antiferromagnetic layer thickness in NiFe/CoO bilayers
(American Institute of Physics, 1998-06-01) Ambrose, T.; Chien, C.-L.
A systematic study of the dependence of exchange coupling in NiFe/CoO bilayers on CoO layer thickness tAF from 5 to 500 Å has been made. For large CoO thicknesses (tAF>100 Å), the exchange field varies as 1/tAF, whereas for small CoO thicknesses (tAF<100 Å), finite-size scaling of the Néel temperature TN and also the blocking temperature TB dominate.
Doubly exchange-biased NiCoO/NiFe/Cu/NiFe/NiCoO spin valves
(American Institute of Physics, 1999-04-15) Ambrose, T.; Liu, K.; Chien, C.-L.
A new type of doubly exchange-biased Ni50Co50O/NiFe/Cu/NiFe/Ni30Co70O spin-valve structure, with two ferromagnetic layers exchange biased in opposite directions by two different antiferromagnetic layers is demonstrated. By field cooling in magnetic fields of opposite directions, the resultant hysteresis loop consists of two loops shifted in opposite directions from the zero magnetic field. The NiFe layers are in the antiparallel state in zero magnetic field, and the switching to the parallel state can be tuned by varying the exchange bias. The modified spin valves also show potential for suppressed Barkhausen noise.
Giant positive magnetoresistance of Bi nanowire arrays in high magnetic fields
(American Institute of Physics, 1999-04-15) Hong, Kimin; Yang, F.Y.; Liu, Kai; Reich, D.H.; Searson, P.C.; Chien, C.-L.; Balakirev, F. F.; Boebinger, G. S.
We have studied the magnetoresistance of electrodeposited Bi wires with diameters between 200 nm and 2 μm in magnetic fields up to B=55 T. In zero field, the resistance increases with decreasing temperature, indicating that the mean free path is strongly influenced by the nanowire geometry. The high-field magnetoresistance shows strong dependence on field orientation; typically 200% for B parallel to the wires, and 600%-800% for B perpendicular to the wires. The perpendicular magnetoresistance is well described by a modified two-current model which suggests that the high-field response of the arrays is fairly insensitive to the wire diameter, and is dominated by bulk properties of Bi.
Memory effects of exchange coupling in CoO/Ni81Fe19 bilayers
(American Institute of Physics, 1999-04-15) Gokemeijer, N.J.; Chien, C.-L.
By changing the field-cooling procedure, the value and even the sign of the exchange field HE in CoO/Ni81Fe19 bilayers can be greatly altered. The value of HE can be made to vanish at any temperature below TN. We also show that the magnetization of the Ni81Fe19 is the crucial parameter in the field-cooling procedure. Changing the magnetization during field cooling shows that the exchange bias is an accumulative memory effect, dependent on the thermal and field history of the bilayer. The enhanced coercivity which accompanies the exchange bias, depends only on temperature and not on the field-cooling history.
Comparison of surface spin wave modes at Fe/MnF2 and Fe/Mn interfaces
(American Institute of Physics, 2000-11-01) Siu, I. L.; Islam, M. N.; Walker, J. C.
We have studied the thermal demagnetization in semi-infinite ferromagnets in Fe/MnF2 and Fe/Mn bilayers using Mössbauer spectroscopy. We find that the hyperfine field at the Fe/MnF2 interface follows a quasilinear temperature dependence, which reverts to a T3/2 dependence further into the bulk. The region in which linear temperature dependence was observed also showed significantly higher spin canting than in the film's bulk layers. The interface in the Fe/Mn system immediately showed a T3/2 dependence which persisted deeper into the bulk. We attribute the linear temperature behavior to surface spin wave modes created by a perpendicular surface anisotropy at the interface. This behavior diminishes farther away from the interface, until the hyperfine field goes like T3/2 as expected for bulk, 3D spin waves. We conclude that the perpendicular surface anisotropy is much stronger at the Fe/MnF2 than the Fe/Mn interface.
Magnetic core loss of ultrahigh strength FeCo alloys
(American Institute of Physics, 2003-05-15) Cheng, X.M.; Zhang, X.K.; Zhang, D.Z.; Lee, S.H.; Duckham, A.; Weihs, T. P.; Cammarata, R.C.; Chien, C.-L.
Hiperco® 50 alloy heat treated between 450 and 650°C exhibits superior mechanical properties. We report the measurements of the ac core loss at various frequencies up to 4500 Hz of the Hiperco® 50 alloy samples annealed at 450 and 650°C. The 650°C annealed specimens have lower ac core loss than that of the 450°C annealed ones. The total core loss, consisting of contributions from hysteresis core loss and eddy-current core loss, depends on frequency f as af+bf2. The eddy-current loss of a single laminate is minor compared to the hysteresis loss.
Cell manipulation using magnetic nanowires
(American Institute of Physics, 2003-05-15) Hultgren, A.; Tanase, M.; Chen, C. S.; Meyer, G. J.; Reich, D.H.
The use of magnetic nanowires is demonstrated as a method for the application of force to mammalian cells. Magnetic separations were carried out on populations of NIH-3T3 mouse fibroblast cells using ferromagnetic Ni wires 350 nm in diameter and 35 μm long. Separation purities in excess of 90% and yields of 49% are obtained. The nanowires are shown to outperform magnetic beads of comparable volume.
Magnetic properties of epitaxial Mn-doped ZnO thin films
(American Institute of Physics, 2003-05-15) Cheng, X.M.; Chien, C.-L.
Epitaxial ZnO thin films doped with 7% Mn have been made by reactive rf magnetron sputtering onto (112_0) sapphire substrates at 400°C. X-ray diffraction measurements reveal that the Zn0.93Mn0.07O film has a (0001) wurtzite single-crystal structure with a rocking curve width of 0.98°. UV-VIS absorption spectra show a band gap of 3.25 eV for pure ZnO films and 3.31 eV for the Zn0.93Mn0.07O film with states extending into the gap. The Auger electron spectroscopy shows homogeneous distribution of Mn in the film. The magnetic properties of the Zn0.93Mn0.07O film have been measured by a superconducting quantum interference device magnetometer at various temperatures with fields up to 5 T. No ferromagnetic ordering has been observed at temperature at 5 K. Instead, paramagnetic characteristics with a Curie-Weiss behavior have been observed.
Biological applications of multifunctional magnetic nanowires
(American Institute of Physics, 2003-05-15) Reich, D.H.; Tanase, M.; Hultgren, A.; Bauer, L. A.; Chen, C. S.; Meyer, G. J.
Magnetic particles that can be bound to cells and biomolecules have become an important tool for the application of force in biology and biotechnology. Multifunctional magnetic nanowires fabricated by electrochemical deposition in nanoporous templates are a type of magnetic carrier that offers significant potential advantages over commercially available magnetic particles. Recent experimental work aimed at developing these wires for this purpose is reviewed. Results on chemical functionalization of Au and Au/Ni wires and magnetic manipulation of wires in suspension are described. Fluorescence microscopy was used to demonstrate the covalent binding of thiol-terminated porphyrins to Au nanowires, and to optimize functionalization of two-segment gold-nickel nanowires for selectivity and stability of the nanowire-molecule linkages. Magnetic trapping is a technique where single nanowires are captured from fluid suspension using lithographically patterned micromagnets. The influence of an external magnetic field on this process is described. The dynamics of magnetic trapping is shown to be well described by a model based on the interplay of dipolar forces and viscous drag.
Exchange coupling between Cr and ferromagnetic thin films
(American Institute of Physics, 2003-05-15) Yang, F.Y.; Chien, C.-L.
Exchange bias has been observed in Ni81Fe19/Cr bilayers. The exchange bias field for Cr thickness tCr larger than 62 Å increases with tCr and levels off at tCr=150 Å, whereas coercivity increases without saturation. The blocking temperature increases with the Cr thickness. Due to the presence of commensurate spin density waves in the thicker but strained Cr layers, blocking temperature as much as 425 K has been observed. Exchange bias has also been observed in Co/Cr bilayers.
Longitudinal and perpendicular exchange bias in FeMn/(FeNi/FeMn)n multilayers
(American Institute of Physics, 2003-05-15) Sun, L.; Zhou, S.M.; Searson, P.C.; Chien, C.-L.
Exchange bias in ferromagnetic (FM)/antiferromagnetic (AF) bilayers is usually investigated in the longitudinal configuration with the exchange coupling established in the film plane. In this work, we report on the perpendicular exchange bias in FeMn(8 nm)/[FeNi(2 nm)/FeMn(8 nm)]n multilayers induced by perpendicular field cooling. The thin FeNi layers give rise to large values of the exchange field and coercivity, and n=15 allows a sufficiently large magnetization for the measurements. Even though the soft FeNi layers have an intrinsic in-plane anisotropy, perpendicular exchange bias has been observed after cooling in a perpendicular external field. The exchange field in the perpendicular configuration is about 0.85 that of the longitudinal case. In both the longitudinal and perpendicular configurations, the exchange field decreases quasilinearly with temperature. The squareness of perpendicular hysteresis loops decreases with increasing temperature.
Quasi-Langmuir-Blodgett thin film deposition of carbon nanotubes
(American Institute of Physics, 2004-03-15) Gruner, G.; Gabriel, J.-C. P.; Armitage, N. P.
The handling and manipulation of carbon nanotubes continues to be a challenge to those interested in the application potential of these promising materials. To this end, we have developed a method to deposit pure nonoriented nanotube films over large flat areas on substrates of arbitrary composition. The method bears some resemblance to the Langmuir-Blodgett deposition method used to lay down thin organic layers. We show that this redeposition technique causes no major changes in the films' microstructure and that they retain the electronic properties of as-deposited films laid down on an alumina membrane.

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