Reduced Curie Temperature Research Articles

Element-specific investigation of the magnetization of ultrathin Fe on a V(1 1 0) single crystal

The magnetization of ultrathin Fe deposited in situ onto a V(1 1 0) single crystal has been studied by means of X-ray magnetic circular dichroism measurements at the L 2,3 absorption edges of Fe and V as a function of the Fe coverage. The Fe magnetization sets on at an Fe thickness of 2 ML in accordance with an earlier Polarized Neutron Reflectometry investigation. It increases rapidly with Fe thickness and asymptotically reaches the bulk magnetization at 5 ML. In the critical Fe thickness regime between 2 and 5 ML it significantly increases by cooling from room temperature to 110 K due to the reduced Curie temperature of the ultrathin Fe film. In contrast, the onset of the Fe magnetization at 2 ML is barely influenced by cooling which may be due to the coalescence of the Fe islands observed earlier with low-energy electron diffraction. Residual O contamination on the V surface reduces the Fe magnetization due to the formation of Fe oxide.

Manipulation of the Curie temperature and the magnetic moments of ultrathin Ni and Co films by Cu-capping

Temperature-dependent studies are performed in situ on Ni/Cu(0 0 1) and Co/Cu(0 0 1) ultrathin films by AC-susceptibility and the element-specific X-ray magnetic circular dichroism. It is shown that the growth of a Cu capping layer reduces considerably the Curie temperature by about 30–40 K for Ni and 100 K for Co films. The effect is practically completed after the growth of the first few monolayers of the Cu capping layer. Further growth of Cu does not alter significantly the Curie temperature. We show that the Cu-capping results in a decrease of the magnetic moments and the relation of it to the reduced Curie temperatures is discussed.

Magnetic structure of Fe/Gd multilayers determined by resonant x-ray magnetic scattering

The magnetic structures of a $({\mathrm{F}\mathrm{e}/\mathrm{G}\mathrm{d})}_{15}$ multilayer are determined by resonant x-ray magnetic scattering using circular polarized light of energies tuned close to the Gd L and the Fe K absorption edges. Difference superlattice Bragg peaks observed by flipping the photon helicity show that the magnetic moments of the Gd layers are directed antiparallel to the in-plane applied field at temperatures higher than 180 K, and are twisted below. The local Gd magnetizations in each 5.4-nm-thick layer are highly nonuniform in both magnitude and twist angle in the out-of-plane direction: the interface sublayers nearly fully magnetize at room and low temperatures under the influence of the adjacent Fe magnetizations, whereas the central sublayers show measurable spontaneous magnetizations at 200 K and below. An application of the $1\ensuremath{-}{(T/T}_{c})$ law shows a reduced Curie temperature ${(T}_{c}=214\mathrm{K})$ compared with bulk Gd for the central sublayers, while ${T}_{c}=1023\mathrm{K}$ for the interface sublayers. The interface and central sublayers exhibit distinct twist behaviors as a function of temperature below the compensation temperature, indicating the short-range nature of the Fe-Gd interaction. The element-specific resonant x-ray scattering confirmed the antiferromagnetic arrangement of the Gd and Fe moments at room temperature.

Magnetization of thin Gd films on W(110) near the Curie temperature

In situ temperature dependent magneto-optic Kerr effect measurements in ultrahigh vacuum are presented for 80 and 300 Å thick Gd(0001) films. Films were deposited at 300 K and stepwise annealed to 820 K. It was found that the remanent magnetization Mr(T) depends on the annealing temperature. The as-deposited films have a reduced Curie temperature Tc, e.g., Tc(80 Å)=273 K, which gradually shifts up to the bulk Tc of Gd upon annealing. The changes in Mr(T) are correlated with a sharpening of the Gd low energy electron diffraction pattern. The best magnetic homogeneity and structural coherence is found for films annealed to 620 K. At higher temperatures the films break up into large three-dimensional islands with nearly uncovered W areas in between (W Auger is observed). The magnetic behavior was simulated with a model which assumes a Gaussian distribution of Tc values. From these simulations and the known bulk pressure dependence of Tc we estimate the mean strain and the strain distribution within the films.

Magnetic properties of Ag/Ni multilayered films

The magnetic properties of Ag/Ni multilayered films, prepared by the vacuum evaporation method, as a function of different thicknesses of Ni and Ag layers have been studied. It has been found that the magnetic moment of the Ni layer decreases greatly with increase of the thickness of the Ag layer. The temperature dependence of magnetization exhibits the features of Bloch’s T3/2 law with a large value of the Bloch coefficient, B3/2, which also depends on the thickness of the Ag layer when the thickness of the Ni layer is fixed at 30 Å. These phenomena can be explained by means of the itinerant electron theory. Furthermore, the magnetic size effect of Ag/Ni multilayered films, such as a reduced Curie temperature as a function of the thickness of the Ni layer tNi and a linear dependence of magnetization on temperature for T<175 K when tNi=10 Å, have been discussed.

A Comparison of Growth and Magnetic Properties of FE(ll0)/AG(lll) and FE(100)/AG(100) Multilayers

AbstractTwo series of Fe/Ag multilayers were grown in a Perkin-Elmer 430B MBE system, one of the Fe(110)/Ag(lll) orientation and another of the Fe(100)/Ag(100) orientation. Vastly different techniques were developed by this group and others to achieve epitaxial growth of both of these systems. Using RHEED, it was inferred that the optimal growth of Fe(110) on Ag(lll) occurred at a substrate temperature of 180° C. In contrast, the growth of Fe(lO0)/Ag(100) proceeded with the sharpest RHEED streaks at a reduced substrate temperature. We believe that these fundamentally different growth parameters are the result of physically different growth modes, conjectured to be: edge growth (Fe 110), and a more nucleated growth (Fe 100).Accordingly, dissimilar magnetic interfacial properties are also strongly in evidence, accounted for by the structural differences associated with the different Fe planes. Furthermore, Fe(110) layers as thin as 3 ML were grown on Ag(lll) and showed no superparamagnetism and a genuine 2-dimensional behavior of M(T). However, the Fe(100) on Ag(100) multilayers in a similar thickness range exhibited strong relaxation and a comparatively reduced Curie temperature.

Ultrathin magnetic Fe films on W(001): A spin-polarized inverse photoemission study of Fe electronic structure evolution (abstract)

By using spin-dependent electron spectroscopic techniques, one can explore the relevant problems in 3d metallic ferromagnetism, including temperature effects. Current surface science methods of ultrathin-film preparation and characterization make it possible to study interesting two dimensional magnetic systems. In the present work, ultrathin epitaxially grown Fe films on W(001) have been investigated by spin- and angle-resolved inverse photoemission spectroscopy. The evolution of the electronic structure has been studied for thickness from one monolayer to ‘‘bulklike’’ Fe thin films. In-plane remanent magnetization is observed. In the thick bulklike sample, transitions to majority and minority states near the H point of bcc Fe were identified, in contrast with data from bulk Fe(001).1 The thinner Fe films showed a trend of ‘‘collapsing’’ d bands at H′25. The results suggested a reduced Curie temperature for monolayers thick films. Information obtained for examining the correlation length in the itinerant ferromagnetic systems does not support the local-band picture at finite temperatures.2

Magnetic properties of single crystal rare-earth Gd-Y superlattices

The first coherent single crystal rare earth superlattices of Gd n T m were grown by metal molecular beam epitaxy. The magnetic results based on magnetization measurements are consistent with a model in which the central Gd region in each array behaves like an ideal ferromagnetic thin film even for n as small as 5. The two interfacial Gd layers at the sides appear not to order ferromagnetically at low temperatures and have a susceptibility independent of n. The temperature dependence of the magnetic moment of the isolated Gd arrays (3⩽ n⩽21) is similar to the behavior seen in bulk Gd with reduced Curie temperatures.

Renormalisation group calculations for the dilute Heisenberg magnet

A position-space renormalisation group calculation based on the Migdal approximation is presented for a randomly diluted Heisenberg magnet. A crossover from percolative to thermal critical behaviour was found at low temperatures and the associated crossover exponent has been calculated. To investigate the behaviour at higher temperatures, an approximate recursion formula was used for the spin system. The calculated variation in the reduced Curie temperature with the concentration of impurities is in fairly good agreement with series-expansion results.

On the Curie points and high temperature susceptibilities of Heisenberg model ferromagnetics

The first six coefficients in the expansion of the susceptibility χ, and its inverse, χ −1, in ascending powers of the reciprocal temperature, have been determined for the Heisenberg model of a ferromagnetic, for any spin value, S, and any lattice. The first five coefficients appropriate to the magnetic specific heat, C, have also been found. For the body-centred and face-centred cubic lattices, the χ and C coefficients are tabulated for half-integral S from 1/2 to 3. From these coefficients estimates have been made of the reduced Curie temperatures, θs c= k T c/J. It is found that for the simple, body-centred and face-centred cubic lattices the formula reproduces the estimated Curie temperatures fairly accurately. Here X=S(S+1) and z is the lattice coordination-number. It is found that, suitably scaled, the theoretical curves for inverse susceptibility against temperature above the Curie point are rather insensitive to the spin value and to the precise lattice structure. The ratio of their initial to the...