ABSTRACT
Superparamagnetism is an unwanted property of small magnetic particles where the magnetization of the particle flips randomly in time, due to thermal noise. There has been an increased attention in the properties of superparamagnetic particles recently, because of their potential applications in high density storage and medicine. In electron transport through single nanometer scale magnetic particles, the current can also cause the magnetization to flip randomly in time, even at low temperature. Here we show experimental evidence that when the current is then reduced towards zero in the applied magnetic field, the magnetization can reliably freeze about a higher anisotropy-energy minimum, where it tends to be inverted with respect to the magnetic field direction. Specifically, we use spin-unpolarized tunneling spectroscopy of discrete levels in single Ni particles 2-4 nm in diameter at mK-temperature, and find that the the magnetic excitation energy at the onset of current decreases when the magnetic field increases, reaching near degeneracy at nonzero magnetic field. We discuss the potential for spintronic applications such as current induced magnetization switching without any spin-polarized leads.
ABSTRACT
This work reports the first study of current-driven magnetization noise in a single, nanometerscale, ferromagnetic (Co) particle, attached to normal metal leads by high-resistance tunneling junctions. As the tunnel current increases at low temperature, the magnetic switching field decreases, its probability distribution widens, while the temperature of the environment remains nearly constant. These observations demonstrate nonequilibrium magnetization noise. A classical model of the noise is provided, where the spin-orbit interaction plays a central role in driving magnetic tunneling transitions.
ABSTRACT
It is shown that the dielectric response of Ba(0.77)Sr(0.23)TiO(3) nanoparticles at temperatures below 200 K has a frequency and temperature dependence in agreement with the Debye theory with a single relaxation time, which exhibits the Arrhenius law. By contrast, at temperatures above 210 K the dielectric response exhibits a broad range of relaxation times characteristic of relaxor-ferroelectrics. We suggest that the single relaxation time at low temperature originates from a frustration effect, in analogy with frustrated antiferromagnetism.
ABSTRACT
We report a size-manipulable synthesis of single-crystalline nanorods/nanowires of barium manganite (BaMnO(3)) and barium titanium manganite (BaTi(1/2)Mn(1/2)O(3)) by using the composite-hydroxide-mediated approach. The synthesis cleanly yields nanorods with a hexagonal perovskite structure. Typical nanorods have widths ranging between 50 and 100 nm, and the lengths can be easily controlled by time and temperature or by adding a small amount of water during the synthesis process. Resistance measurement shows that a phase transition happened at 58 K on BaMnO(3). The photoluminescence spectrum of BaTi(1/2)Mn(1/2)O(3) presents two emission peaks at wavelengths of 465 and 593 nm, corresponding to blue and green fluorescence. The ability to synthesize nanorod manganites of a desired length should enable detailed investigations of the size-dependent evolution of magnetism, magnetoresistance, nanoscale phase separation, and realization of a nanodevice of magnetic sensors.
Subject(s)
Barium Compounds/chemical synthesis , Manganese Compounds/chemical synthesis , Nanotubes/chemistry , Nanowires/chemistry , Titanium/chemistry , Barium Compounds/chemistry , Crystallization , Manganese Compounds/chemistry , Particle SizeABSTRACT
We present measurements of mesoscopic resistance fluctuations in cobalt nanoparticles and study how the fluctuations with bias voltage, bias fingerprints, respond to magnetization-reversal processes. Bias fingerprints rearrange when domains are nucleated or annihilated. The domain wall causes an electron wave function-phase shift of approximately equal to 5pi. The phase shift is not caused by the Aharonov-Bohm effect; we explain how it arises from the mistracking effect, where electron spins lag in orientation with respect to the moments inside the domain wall. Dephasing time in Co at 0.03 K is short, tau phi approximately 1 ps, which we attribute to the strong magnetocrystalline anisotropy.
