Volume 37, Issue 3, 01 March 1966
Index of content:
37(1966); http://dx.doi.org/10.1063/1.1708482View Description Hide Description
The octahedral cubic symmetry at a Tm site in TmN gives a singlet (nonmagnetic) level as the ground state of the Tm3+ ion in the crystal field. We have used pulsed magnetic fields up to 265 kOe to counteract the crystal‐field effects and induce large ionic moments. Magnetizations measured at 4.2°, 20.4°, and 77°K show an approach to saturation at the highest fields used. For 4.2°K, the moment at 265 kOe is 4.5 μB per molecule as compared to the saturation value of 7 μB. The experimental results for the magnetization and susceptibility are compared to the theoretical results for crystal‐field calculations. This comparison enables us to discuss the way in which exchange forces enter the magnetization process at high magnetic fields as the ground state becomes polarized. The fourth‐ and sixth‐order crystal‐field parameters have been evaluated by observing paramagnetic resonance at 9.35 Gc/sec for the Γ5 (2) excited triplet.
37(1966); http://dx.doi.org/10.1063/1.1708483View Description Hide Description
The antiferromagnetictransition in UO2 at 30.8°K has been examined in a neutron diffraction study. Magnetic intensity changes measured through the Néel temperature revealed that the transition is an extremely sharp one: 50% magnetization is established within ΔT/TN =0.001. As a further test of the first‐order nature of the transition, the magnetic scattering above the Néel temperature was studied to see if critical scattering could be observed. While, as would be expected in any case, some short‐range correlation in magnetization density was indicated by the observation of a diffuse peak, the absence of the sharp critical scattering characteristic of a second‐order transition confirmed the first‐order nature of the phase change. In addition to the above measurements, an extensive study of the magnetic structure was carried out at 5°K to determine the U4+form factor.Magnetic ordering of the first kind on the face‐centered cubic sites occupied by the U4+ ions in the fluorite‐type structure was reported earlier by Henshaw and Brockhouse,1 and this was confirmed in the present study. The form factor, determined from 23 independent (hhl) single‐crystal intensity measurements over a sin θ/λ range extending to about 0.8, indicate a 5f 2 electronic configuration with an effective U4+ moment of 1.75±0.05 μB and some degree of asphericity in magnetization density. Details are to be published in The Physical Review.
37(1966); http://dx.doi.org/10.1063/1.1708484View Description Hide Description
A study has been made of the magnetic properties of the spinels ACr2X4, where A is diamagnetic zinc or cadmium, and X is oxygen, sulfur, or selenium. CdCr2Se4 and CdCr2S4 are apparently ferromagnetic with moments somewhat below the predicted value of 6.0 μB/molecule. By contrast, ZnCr2Se4 and ZnCr2S4 are antiferromagnetic at low temperature despite having positive asymptotic Curie points (TA ). We found very little thermal variation of cell size in these materials, which further corroborates Lotgering's conclusion that the antiferromagnetism is due to next‐nearest‐neighbor interactions.
Disagreement between experimental values of TA/TC and the theoretical value calculated assuming nearest‐neighbor interactions indicate that next‐nearest‐neighbor interactions are playing a role in the ferromagnetic materials as well.
Comparisons of TA and TN in these materials indicate that the magnetic interactions are strongly dependent upon the diamagnetic A‐site ion. However, the data cannot uniquely attribute this effect to an exchange coupling directly involving the A‐site cation.
37(1966); http://dx.doi.org/10.1063/1.1708485View Description Hide Description
Electrical transport properties were measured on the recently discovered ferromagnetic spinels CdCr2S4 (Tc =85°K) and CdCr2Se4 (Tc =130°K). The large Cr−Cr separation (≥3.63 Å) excludes metallic conductivity due to Cr−Cr overlap. Electrical conductivity,Seebeck effect, Hall effect, and magnetoresistance measurements were made on high‐density, polycrystalline samples (CdCr2S4: 99.6%; CdCr2Se4: 99.9% of the theoretical density). Both materials show a negative temperature coefficient of the resistivity in the para‐ and ferromagnetic region without any discontinuity at the Curie temperature. The room‐temperature conductivities of CdCr2S4 and of CdCr2Se4 are 5×10−4 (Ω·cm)−1. At 77°K the conductivity of CdCr2S4 is 3.5×10−10 (Ω·cm)−1 and of CdCr2Se4 6.7×10−5 (Ω·cm). The Seebeck coefficient for both materials is rather small: (CdCr2S4: −60 μV/°K; CdCr2Se4: +60 μV/°K), which indicates that the materials are almost compensated. The room‐temperature mobility of CdCr2Se4 is 7 cm2/Vsec. In the ferromagnetic region CdCr2Se4 exhibits a negative magnetoresistanceeffect. An applied field of 7 kG results in a 4% increase of the conductivity. Band formation in CdCr2Se4 due to overlap of the 4p orbitals of the Se ions is proposed.
37(1966); http://dx.doi.org/10.1063/1.1708486View Description Hide Description
A variety of relatively transparent materials containing Eu+ + have become available, thus enabling a study to be made of the magneto‐optical properties of the Eu+ + ion in widely different environments. The following materials are reviewed: Eu2SiO4 (orthorhombic structure with a ferromagneticCurie temperatureTc of 7°K), dilute Eu+ + in CaF2(fluorite structure), EuF2(fluorite structure, Tc ≈0), EuO−Al2O3−B2O3glasses (Tc =0), EuSe (NaCl structure, Tc =7°K), and EuO (NaCl structure, Tc =69°K).
We discuss the magneto‐optical properties of the ``nonmagnetic'' materials (CaF2: Eu+ +, EuF2, Eu glasses) in terms of an intra‐atomic 4f 7→4f 65dtransition. Using this transition, calculation of the Faraday and Cotton‐Mouton birefringence in EuF2 gives reasonable agreement with experiment. The magneto‐optical properties of the ``magnetic materials'' (Eu chalcogenides) are discussed in terms of an interatomic (4f 7) (4f 7) → (4f 6) (4f 75d) transition. This transition gives reasonable agreement with the observed magnitudes of the magnetic absorption edge shifts in EuSe and EuO.
37(1966); http://dx.doi.org/10.1063/1.1708487View Description Hide Description
Magnetization studies on an aggregate of single crystals of ferrimagnetic CoMnO3 oriented with their trigonal axes parallel gave a value of K 1+2K 2= −1.4/×107 erg/cm3 at 300°K for the magnetocrystalline anisotropy expressed in the usual form: K 0+K 1 sin2θ+K 2 sin4θ. The very large anisotropy is believed to arise from partial removal of the orbital degeneracy of the 4 T 1ground state by a trigonal component of the crystal field. The anisotropy energy was calculated following the method of Slonczewski except that the entire ground‐state manifold was treated rather than using a perturbation approach. The fact that the moments lie perpendicular to the trigonal axis makes the more complete calculation necessary. The observed anisotropy energy was obtained for a trigonal splitting of 1250 cm−1 and a spin orbital parameter (αλ) of 204 cm−1. The sign of the trigonal splitting is such that an orbital singlet lies lowest.
37(1966); http://dx.doi.org/10.1063/1.1708488View Description Hide Description
A simple model accounts semiquantitatively for the first‐order magnetic phase change observed recently in UO2 by Frazer, Shirane, Cox, and Olsen. The model assumes that the electronic structure of the paramagnetic U4+ ion consists of a nonmagnetic singlet ground state and a low‐lying magnetic triplet, and that only bilinear isotropic exchange interactions are present. In a molecular‐field theory the triplet is split by an internal field proportional to the magnetization. If the molecular field is sufficiently strong one of the components of the triplet will lie, in the magnetic state, below the singlet, and a self‐consistent magnetic solution is obtained at T=0. Increasing the temperature causes the magnetization to be reduced, and the low lying component of the triplet is raised in energy. It is shown that a ``catastrophe'' may occur at some critical temperature so that the magnetization is reduced discontinuously to zero. It is also found that, depending on the ratio of the singlet‐triplet energy difference to the molecular field splitting of the triplet, one obtains either no magnetic ordering, a first‐order phase change, or a second‐order transition.
37(1966); http://dx.doi.org/10.1063/1.1708489View Description Hide Description
The room‐temperature internal magnetic fieldHi , electric quadrupole coupling ε, and isomer shift IS of divalent 57Fe in CoO have been measured as a function of pressure up to about 250 kbar. Radioactive samples were prepared by heating 57Co‐enriched Co3O4 in vacuum above 1000°C. The main features of the velocity spectra are qualitatively similar to those of the temperature study of this system as reported by Wertheim,1 except that the fraction of trivalent 57Fe observed is substantially reduced.2,3 As the pressure is raised the velocity spectrum unfolds into a magnetic hyperfine pattern in which Hi increases from zero to about 210 kOe at 250 kbar. This value is somewhat higher than the low‐temperature saturation value of approximately 180 kOe. The rate of increase of Hi with pressure, coupled with low‐temperature and x‐ray pressure‐volume measurements,4 shows that the Néel temperatureTN (291°K at atmospheric pressure) increases with decreasing volume in a manner consistent with the results of Bloch et al. 5 [d(lnTN )/d(lnV)≅ −3 at atmospheric pressure]. The velocity spectra also indicate small changes in both IS and ε with pressure, the former varying from −1.16 mm/sec at atmospheric pressure to −1.08 mm/sec at 250 kbar, relative to a stainless steel absorber. Both the change in IS and saturation value of Hi are consistent with an expansion of the 3dcharge density and an accompanying increase in the 3s charge and spin density at the nucleus, because of less screening. The parameter, ε=¼e 2 qQ, in the antiferromagnetic phase is observed to approximately double from −0.03 mm/sec at 100 kbar to −0.06 mm/sec at 250 kbar. (These values are based on the unproven assumption that Hi is parallel to the axis of an axially symmetric electric field gradient tensor.)
Effects of Hydrostatic Pressure on the Magnetic Ordering Temperatures and the Magnetization of Some Ionic Compounds37(1966); http://dx.doi.org/10.1063/1.1708490View Description Hide Description
We have measured the variation of the magnetic‐ordering temperatures θ of CoO and of several ferritegarnets as a function of hydrostatic pressure up to 6000 bar. One observes that ∂(log θ)/∂(log V), where V is the volume, is of the order of −3.2. An analysis of the thermal variation of the paramagneticsusceptibility gave the same result. We have measured at room temperature the variation with pressure and field of the magnetization of some ferritegarnets; the observed results are related to the variation with pressure of the compensation and Curie points.
Linear Magnetoelectric Effects in Gallium Iron Oxide at Low Temperatures and in High Magnetic Fields37(1966); http://dx.doi.org/10.1063/1.1708491View Description Hide Description
The magnetoelectric (ME) experiments1 on monocrystals of the piezoelectric ferromagnet Ga2−x Fe x O3 (x≈1) are extended to low temperatures (T≳4.2°K) and high magnetic fields (H≲150 kOe). It is found that: (1) The linear ME susceptibility α, which has a maximum1 at some T=T max, exhibits a minimum at some T=T min satisfying 4.2°K<T min<T max; moreover, the value of α is almost as large at 4.2°K as at T max. (2) If H is sufficiently large so that the direction u=M s /Ms of the spontaneous magnetization M s is parallel to H, then the electric polarizationPb induced at constant T along the polar crystal axis (b axis) is given by Pb =ηu+αu H+βu H 2, where βu H≪α for H≲150 kOe. The ``spontaneous ME polarization'' ηu is strongly u‐dependent. However, αu is approximately u‐independent, its u‐dependent part being only about 2% at 145.2°K and 77.4°K and about 10% at 4.2°K. Since the (approximate) u‐independence of αu indicates that the details of the spin canting assumed in the previous theoretical model1 are inapplicable to Ga2−x Fe x O3, a new thermodynamic description of ME effects is presented which does apply to ferromagnets having u ∥ H and satisfactorily describes (2). Finally, a thermodynamic calculation is given of a new mechanism of linear ME effects, namely the combined action of forced magnetostriction (i.e., a kind of piezomagnetism) and piezoelectricity. This mechanism, which ultimately arises from the strain dependence of exchange (and other) interactions, must exist in all unclamped piezoelectric ferromagnets having u ∥ H, and is probably significant in Ga2−x Fe x O3 near the Curie point.
37(1966); http://dx.doi.org/10.1063/1.1708492View Description Hide Description
High‐magnetic‐field‐Mössbauer and magnetic‐moment experiments have been performed with Ga2−x Fe x O3. This magnetic system is of considerable interest because it is piezoelectric, weakly magnetic,1 and magnetoelectric.2 The crystal structure has been determined3 and a magnetic ordering inferred.3 The magnetic‐moment measurements were made using a vibrating sample magnetometer in fields up to 75 kOe. The samples used for the Mössbauer absorption experiments consisted of 57Fe enriched powders, grown from a flux, embedded in lucite, and having x=0.8 and 1.2. A mosaic absorber made of small x‐ray oriented single crystals was also studied. Measurements were made over the temperature range 4.2° to 320°K and in external magnetic fields up to 130 kOe. At low temperatures the zero‐external field‐absorption spectra indicate that there are at least two magnetically nonequivalent sites. In a large external field the hyperfine spectrum lines corresponding to Δm=0 vanish, while each of the outer lines (corresponding to Δm=±1) splits into two well‐resolved components of unequal intensity. Detailed analysis of the experimental results for both the single crystal and polycrystalline absorbers indicates that at zero external field all spins lie in the a−c plane (c<a<b), that the observed moment is due to ferrimagnetism4 rather than to a canted spin structure, and that the spins are unequally divided between the two sublattices.
A detailed account of these measurements will be published elsewhere.
37(1966); http://dx.doi.org/10.1063/1.1708493View Description Hide Description
Ferroelectricity and weak ferromagnetism have been found to set on simultaneously in Ni3B7O13I at about 64°K. This is evidenced by dielectrichysteresis, spontaneous Faraday effect, quadratic magnetoelectric hysteresis, etc. The strong coupling between the mutually perpendicular spontaneous polarization——and spontaneous magnetization——is such that, when the former is reversed, the latter turns by 90°. The magnetic point group is most probably m′m2′. Dielectric constant, magnetic, and magnetoelectric susceptibilities and magnetic coercive field are shown as a function of temperature.