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The recent discovery of 2D magnetic order in van der Waals materials has stimulated a renaissance in the field of atomically thin magnets. This has led to promising demonstrations of spintronic functionality such as tunneling magnetoresistance. The frantic pace of this emerging research, however, has also led to some confusion surrounding the underlying phenomena of phase transitions in 2D magnets. In fact, there is a rich history of experimental precedents beginning in the 1960s with quasi‐2D bulk magnets and progressing to the 1980s using atomically thin sheets of elemental metals. This review provides a holistic discussion of the current state of knowledge on the three distinct families of low‐dimensional magnets: quasi‐2D, ultrathin films, and van der Waals crystals. It highlights the unique opportunities presented by the latest implementation in van der Waals materials. By revisiting the fundamental insights from the field of low‐dimensional magnetism, this review highlights factors that can be used to enhance material performance. For example, the limits imposed on the critical temperature by the Mermin–Wagner theorem can be escaped in three separate ways: magnetocrystalline anisotropy, long‐range interactions, and shape anisotropy. Several recent experimental reports of atomically thin magnets with Curie temperatures above room temperature are highlighted. [ABSTRACT FROM AUTHOR]

Neutron reflectometry was used to determine the distribution of salt ions and water in thin poly(acrylic acid) and poly(allylamine hydrochloride) polyelectrolyte multilayers assembled with and without salt. Increasing salt concentration reverses the exclusion of water from the substrate region, eventually leading to an asymmetric segregation of water near the substrate at high salt concentration. The counterions were found to localize near the substrate in films that were either assembled with salt or were exposed to salt solutions. In addition, the capping layer of the film was found to greatly influence the counterion distribution in the multilayer. [ABSTRACT FROM AUTHOR]

The magnetic anisotropy and the in-plane magnetization reversal mechanism of Ga0.945Mn0.055As thin films, grown on GaAs(001), were investigated. Samples A and B, with sample A grown 45 °C lower than sample B, were analyzed by magnetization hysteresis measurements and polarized neutron reflectivity (PNR) magnetization reversal experiments. Magnetization hysteresis loops and temperature-dependent magnetization curves accounted for an in-plane uniaxial magnetic anisotropy, with a significant out-of-plane magnetization, for sample A, while an in-plane biaxial magnetic anisotropy with in-plane <100> as easy axes was evidenced for sample B. PNR magnetization reversal experiments showed the occurrence of a spin-flip maximum upon magnetization reversal only for sample B. A mechanism of magnetization reversal proceeding by 180° domain-wall nucleation and propagation is proposed for sample A, while an incoherent rotation mechanism by 90° domains is proposed for sample B. The reversal mechanism is shown to be correlated to the anisotropy of the samples which depends on the T/Tc ratio. [ABSTRACT FROM AUTHOR]

The magnetic anisotropy and the in-plane magnetization reversal mechanism of Ga(sub 0.945)Mn(sub 0.055)As thin films, grown on GaAs(001), are investigated. Samples A and B, with sample A grown 45 degree Celsius lower than sample B are analyzed by magnetization hysteresis measurements and polarized neutron reflectivity (PNR) magnetization reversal experiments.

Electrochemical behavior of Ni in an alkaline heavy water electrolyte was studied using cyclic voltammetry and polarized neutron reflectometry. Delays in the hydrogen and oxygen evolution reactions on Ni electrodes and slower kinetics of the reactions in heavy water compared to light water were observed. However, in both 0.01 mol/L NaOH light and heavy water solutions, Ni oxidation and passive film formation occurred at the same potential with similar passive current densities indicating a minor isotope effect of deuterium on passive film formation and growth. Polarized neutron reflectometry detected a passive film growth coefficient of 11.5 ± 1.1 Å/V. The Ni consumption coefficient was determined to be 7.3 ± 1.5 Å/V. Ni oxidation and passive film growth occurred mostly during the transient period after the applied potential increase but was negligible at the steady state. The electric field strength across the passive film was 8 x 106 V/cm. The Pilling-Bedworth ratio of the passive film during potentiostatic growth was close to the value of NiO grown on bulk Ni. This work demonstrated the capability of neutron reflectometry in the study of passive films on metals and alloys. [ABSTRACT FROM AUTHOR]

The dissolution of a Ni (111) thin film exposed to a 0.1 M NaCl (pH4) solution was monitored for a period of 816 hours using X-ray reflectometry. The dissolution rate of the Ni (111) film decreased from approximately 9 Å/h at the beginning of the immersion experiment to 0.05 Å/h after 816-hour exposure. After reaching a steady state between formation and dissolution, the thickness of the NiO passive film remained constant at approximately 18 Å during the immersion experiment. [ABSTRACT FROM AUTHOR]