T. R. McGuire

Anisotropic magnetoresistance in ferromagnetic 3d alloys

The anisotropic magnetoresistance effect in 3d transition metals and alloys is reviewed. This effect, found in ferromagnets, depends on the orientation of the magnetization with respect to the electric current direction in the material. At room temperature, the anisotropic resistance in alloys of Ni-Fe and Ni-Co can be greater than 5%. The theoretical basis takes into account spin orbit coupling and d band splitting. Other properties such as permeability, magnetostriction, and Hall voltage have no simple relationship to magnetoresistance. Anisotropic magnetoresistance has an important use as a magnetic field detector for digital recording and magnetic bubbles. Such detectors because of their small size are fabricated using thin film technology. Film studies show that thickness, grain size, and deposition parameters play a significant role in determining the percentage change in magnetoresistance. In general, the change is smaller in films than bulk materials. Several tables and graphs that list bulk and film data are presented.

Ferromagnetic Europium Compounds

Ferromagnetism has been found in several divalent europium compounds. Some of these materials are particularly simple in both crystal and magnetic structure and are ideal for experimental and theoretical study. The materials we have investigated can be divided into three groups, europium chalcogenides, europium halogens, and europium silicates. Magnetic information on these compounds are summarized and theoretical results reviewed.The europium chalcogenide series all have the rocksalt structure, the oxide, sulfide, and selenide being ferromagnetic, and it is on these materials that the most work has been done. This is especially true of EuS where detailed moment, specific heat, and nuclear magnetic resonance measurements are available. The oretical analysis indicates that there is positive exchange between nearest‐neighbor europium atoms which decreases with increasing atom spacings. There is also a weaker negative superexchange via the anions in these compounds which is responsible for antiferromagnetism...

Growth of YBa2Cu3Ox single crystals

We report a set of conditions for crystal growth of the high‐temperature superconductor YBa2Cu3Ox. The as‐grown single crystals have critical temperatures up to 85 K. Preliminary studies have shown that the transition temperatures can be increased by thermal annealing in oxygen, as in ceramic samples. The crystals are in suitable dimensions for definitive magnetic, optical, and transport measurements.We report a set of conditions for crystal growth of the high-temperature superconductor YBa/sub 2/Cu/sub 3/O/sub x/. The as-grown single crystals have critical temperatures up to 85 K. Preliminary studies have shown that the transition temperatures can be increased by thermal annealing in oxygen, as in ceramic samples. The crystals are in suitable dimensions for definitive magnetic, optical, and transport measurements.

New ferromagnetic 5:4 compounds in the rare earth silicon and germanium systems

Abstract The rare earth elements Gd, Tb, Dy, Ho, and Er form compounds with Ge and Si at the 5 : 4 stoichiometry which have orthorhombic crystal structures. The suicides and germanides are in the same space group and have very similar lattice constants, but incomplete solid solubility in the system Gd 5 Si 4 Gd 5 Ge 4 indicates that they are not isostructural. Magnetic measurements show that the suicides are ferromagnetic with relatively high ordering temperatures. The germanides are antiferromagnetic, but replacement of a small amount of Ge by Si in Gd 5 Ge 4 produces solid solutions which are ferromagnetic at low temperatures and have intermediate transitions to antiferromagnetism before becoming paramagnetic.

Growth and giant magnetoresistance properties of La‐deficient LaxMnO3−δ (0.67≤x≤1) films

Epitaxial thin films of lanthanum‐deficient LaxMnO3−δ (0.67≤x≤1) have been grown on (100) SrTiO3 substrates by pulsed laser deposition. The as‐deposited films exhibit a ferromagnetic transition at temperatures ranging from 115 to 240 K, with the transition temperature (Tc) increasing with higher La deficiency. A sharp drop in resistivity and negative magnetoresistance is observed close to Tc, a behavior similar to that observed in divalent substituted La1−xMxMnO3−δ (M=Ba, Sr, Ca, Pb) films. Postannealing the films in O2 reduces the resistivity and raises the Tc to values close to room temperature. A magnetoresistance value of 130% (Δρ/ρH) has been obtained at 300 K at 4 T for a post‐annealed film with x=0.75.

Growth of YBa/sub 2/Cu/sub 3/O/sub x/ single crystals

We report a set of conditions for crystal growth of the high‐temperature superconductor YBa2Cu3Ox. The as‐grown single crystals have critical temperatures up to 85 K. Preliminary studies have shown that the transition temperatures can be increased by thermal annealing in oxygen, as in ceramic samples. The crystals are in suitable dimensions for definitive magnetic, optical, and transport measurements.We report a set of conditions for crystal growth of the high-temperature superconductor YBa/sub 2/Cu/sub 3/O/sub x/. The as-grown single crystals have critical temperatures up to 85 K. Preliminary studies have shown that the transition temperatures can be increased by thermal annealing in oxygen, as in ceramic samples. The crystals are in suitable dimensions for definitive magnetic, optical, and transport measurements.

Magnetic Structure of EuTiO3

Europium titanate has the cubic perovskite structure containing divalent Eu (7 μB) and tetravalent Ti. From magnetic measurements we find that EuTiO3 is one of the few antiferromagnetic materials with a positive θ (TN=5.3°K, θ=3.8°K). At 1.3°K the magnetic moment (σ) increases linearly with field to 10 kOe; above 14 kOe the moment saturates and σ=156 emu/gm (6.93 μB) at 20 kOe. Powder neutron‐diffraction work indicates that EuTiO3 has the Type G magnetic structure in which a given Eu++ has six nearest‐neighbor europium ions antiparallel and 12 next‐nearest‐neighbor europium ions parallel. In a perovskite structure where only the 12‐coordinated ion is magnetic, i.e., Eu++, the molecular field relations for a two sublattice model yield J1/k=−0.021°K, where J1 is the effective intersublattice exchange interaction, and J2/k=0.040°K, where J2 is the effective intrasublattice exchange interaction. The signs of J1 and J2 are opposite to those found in the europium chalcogenide series. The chalcogenides, however,...

Magnetic Structure and Exchange Interactions in Cubic Gadolinium Compounds

The two groups of metallic compounds Gd3+X3− (X=N, P, As, Sb, Bi) and Gd3+Z2− (Z=S, Se, Te) have the rocksalt structure and except for ferromagnetic GdN they are antiferromagnetic. Powder neutron diffraction measurements on GdS, GdSe, GdSb, and GdBi show them to have order of the second kind. The exchange interactions are discussed on the basis of the magnetic structure and susceptibility measurements.

Magnetoresistance and Hall effect of chromium dioxide epitaxial thin films

Epitaxial CrO2 thin films have been grown on TiO2(100) and Al2O3(0001) substrates by atmospheric pressure chemical vapor deposition. The films have a Curie temperature (Tc) of around 393 K with the ones grown on TiO2 exhibiting in-plane uniaxial magnetic anisotropy. They also display metallic characteristics, with room temperature resistivity of about 285 μΩ cm, dropping by about two orders of magnitude upon cooling down to 5 K. Magnetoresistance (MR) properties of the films have been measured with the magnetic field in the plane. For a field of 40 kOe, a positive transverse MR of about 25% at 5 K and a negative MR of about 7% at near Tc have been observed. In addition, Hall resistivity has been measured with magnetic field up to 40 kOe. A positive ordinary Hall effect is found at low temperatures, indicating the conduction carriers are holes.

Ferromagnetism in Rare‐Earth Group VA and VIA Compounds with Th3P4 Structure

The bcc structure I43d−Td6(Th3P4) occurs as a defect structure in 2:3 compounds of rare‐earth elements with S, Se, and Te, or as an inverted structure in 4:3 compounds with group VA elements. The structural, electric, and magnetic properties of the metallic 4:3 compounds and the 2:3 rare‐earth semiconductors have been investigated with specific reference to gadolinium, for which the S ground state gives the minimum crystal field effect.Gd4Bi3 and Gd4Sb3 are both ferromagnetic with a saturation magnetization obeying the spin‐wave T32 law up to 0.8 of the Curie temperature. The Curie temperature Tc varies in the solid solution system Gd4Sb3– Gd4Bi3 with composition from 260° to 340°K. The semiconducting compound Gd2Se3(ρRT=3 Ω cm) has been found to be antiferromagnetic below TN=6°K. Solution of Gd in the holes of the defect Th3P4 structure decreases the electrical resistivity without a measurable variation of lattice constant (a0=8.718 A). With increasing conductivity the material changes from antiferromag...