Bulk Magnetometry Research Articles

Onset of antiferromagnetism in heavy-fermion metals

There are two main theoretical descriptions of antiferromagnets. The first arises from atomic physics, which predicts that atoms with unpaired electrons develop magnetic moments. In a solid, the coupling between moments on nearby ions then yields antiferromagnetic order at low temperatures. The second description, based on the physics of electron fluids or 'Fermi liquids' states that Coulomb interactions can drive the fluid to adopt a more stable configuration by developing a spin density wave. It is at present unknown which view is appropriate at a 'quantum critical point' where the antiferromagnetic transition temperature vanishes. Here we report neutron scattering and bulk magnetometry measurements of the metal CeCu(6-x)Au(x), which allow us to discriminate between the two models. We find evidence for an atomically local contribution to the magnetic correlations which develops at the critical gold concentration (x(c) = 0.1), corresponding to a magnetic ordering temperature of zero. This contribution implies that a Fermi-liquid-destroying spin-localizing transition, unanticipated from the spin density wave description, coincides with the antiferromagnetic quantum critical point.

Polarized neutron reflectivity studies of biquadratic coupling in [Fe/Cr] (100) and [Fe/Al] (100) superlattices and films (invited) (abstract)

Sensitivity to magnetic atoms and low intrinsic absorption characterize the interaction of neutrons with matter. Consequently, polarized neutron reflectivity provides a unique means of performing depth-resolved vector magnetometry. We have used this technique to determine the magnetization depth profiles of Fe/Cr superlattices. Superlattices of bilayer composition [55 Å Fe/17 Å Cr], grown at 523 K, exhibit biquadratic coupling with large saturation fields (∼3 kOe), while those grown at 293 K are ferromagnetically ordered. We have directly measured the evolution of the coupling angle between adjacent Fe layers as a function of applied field and will discuss how bilinear, biquadratic, and external field terms produce the observed order. The weaker coupling found in the Fe/Al system makes possible the investigation of a range of spin configurations at temperatures that do not endanger the sample. We have mapped the phase diagram of a [42 Å Fe/12 Å Al/39 Å Fe] (100) trilayer and find evidence of biquadratic coupling at low temperatures and fields (e.g., when H=180 Oe, the Fe layer spins relax away from ferromagnetic alignment below T≊170 K). Our measurements agree qualitatively with energy minimization calculations and the results of bulk magnetometry.

Structural and magnetic characteristics of the Cfmm2Fe14-xCoxB system (Cfmm = Ce-free mischmetal)

Intermetallics of Cfmm2Fe14-xCoxB systems, where Cfmm stands for Ce-free mischmetal, were synthesized. The structural and magnetic properties were studied by means of bulk magnetometry for the entire composition range x = 0–14. All materials have a tetragonal crystal structure with decreasing lattice parameters upon Co substitution. The Curie temperature, Tc, rises with x. At room temperature, saturation magnetization, Ms, shows a maximum at x ≈ 2.0, while the anisotropy field, HA, and the theoretical energy product, (BH)max, exhibit weak maxima at around x = 4 and then gradually decrease for higher Co concentrations. X-ray diffraction patterns for the sample studied with (a) zero and (b) 20 kOe applied field show that magnetization lies along the c-axis at room temperature.

Observation of a low temperature magnetic phase transition in nonsuperconducting La 2− xSr xCuO 4+ y by macroscopic magnetization and electron spin resonance

We show that the transition to the low temperature ( T ≲ 12 K) magnetic phase (LTMP) of La 2 − x Sr x CuO 4 + y (LSCO) for 0.02 ≲ x ≲ 0.03 is clearly observed via bulk magnetometry in vacuum-annealed samples. The transition temperature T f depends on the strontium concentration and is insensitive to the vacuum treatment. Measurements of an electron spin resonance (ESR) signal are also described. The temperature dependent amplitude of this signal displays a clear anomaly at T f, suggesting that the observed spins participate in the LTMP. Arguments are presented for the intrinsic nature of the observed ESR signal and the LTMP. We compare temperature dependent effects in both probes to expectations for spin glasses, and relate our results to consideration of the nature of the LTMP, and of the origin of the spin system from which the LTMP is formed.

Surface magnetization of Co70V10B20 and Fe77Cr6B17 amorphous alloys

Magnetization has been measured as a function of in-plane applied field and temperature by measuring the spin polarization of low-energy electrons (LEESPA) excited from the surface by an unpolarized electron beam (Ep=2 keV). These extremely surface sensitive data (probing <25 Å), taken on surfaces characterized by Auger electron spectroscopy, are compared with results obtained by magneto-optic Kerr effect (MOKE probing <250 Å) and bulk magnetometry. The LEESPA M-H loops show considerably lower remanence than MOKE indicating appreciable surface anisotropy. The LEESPA polarization approaches zero at approximately TC =170 °C for both the Co and Fe base alloys with less curvature than bulk magnetization does. The bulk TC’s are 20 °C higher than those measured by LEESPA.

Magnetic behavior of R 2Co 14B hydrides (R = La, Pr, Sm, Gd, Tb and Y)

The structure and magnetic properties of R 2Co 14B sysstems(R = La, Pr, Nd, Sm, Gd, Tb and Y) and their hydrides were studied by means of bulk magnetometry. All R 2Co 14B hydrides presently studied occur in the tetragonal Nd 2Fe 14B-type crystal structure. The composition-temperature isotherms measured fro selected Gd- and Nd- containing systems exhibit some indication of a platuau pressure at higher hydrogen concentrations. Hydrogenation expands the unit volume, V c, by 1.5 to 3.0%, depending on the nature of R and the content of hydrogen. It was found that introduction of hydrogen into the lattice decreases M s of the Co sublittice. This is attributed to the effect of electron charge transfer from Hto Co-3d sublittice. Hydrogennation significantly decreases the anistropy fields, H A, and the spin-reorientation transition temperatur, T SR, for Prand Tb-based intermetallics. The results indicates that the hydrogen makes the compounds magnetically softer, which is attributed to the influence of hydrogen on both the 3d and R sublittices. Two types of spin-reorientation transition for the Nd 2Co 14B system were observed. Hydrogenaration reduces both the low transition temperature, T SR 1 , and the high transition temperature, T SR 2 , which is explained using the Boltich-Wallace mechanism.

Spin reorientations in R 2Fe 14- xCo xB systems (R = Pr, Nd and Er)

The effect of cobalt substitution on spin reorientation phenomena in R 2Fe 14- x Co x B systems (R = Pr, Nd, Er) was studied by means of bulk magnetometry in the temperature range 4.2–1100 K. It was established that in the Nd-based system the introduction of cobalt resulted not only in a shift of the low temperature spin reorientation (cone to axis) but also triggered, for x ⩾ 10, an appearance of a second spin reorientation (axis to plane) at high temperature. In the Pr-based system, for x ⩾ 9.5, a high temperature spin reorientation (axis to plane) was also observed. Its dependence on Co content was determined. For the Er-based system, an increase of the spin reorientation temperature (plane to axis) was observed as more cobalt was introduced into that system. It was also established that the tetragonal single-phase materials in this system exist only up to x = 5. Temperature-composition diagrams are presented, indicating types of spin arrangements observed in the investigated systems.