Low frequency magnetoresistive noise in spin-valve structures

Diao

Applied Physics Letters

et al. 2010

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Magnetization switching is investigated in ring-shaped MgO-based magnetic tunnel junctions with 168% tunneling magnetoresistance. Besides the forward and reverse onion states, two vortex states and several metastable states are observed for the ferromagnetic free layer. Electrical noise is used to characterize the low frequency magnetization dynamics; a stationary 1 / f noise spectrum is observed within each magnetic state but they are separated by noise peaks which show a 1 / f 2 spectrum that is associated with slow random telegraph fluctuations. In the 1 / f region, the normalized magnetic noise parameter, ␣ mag , is shown to be consistent with the fluctuation-dissipation theorem.

supporting

confidence: 56%

supporting

confidence: 53%

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Magnetic noise in MgO-based magnetic tunnel junction rings

Diao

Applied Physics Letters

et al. 2010

Self Cite

“…The fluctuation-dissipation theorem (FDT) relates the magnetization fluctuations and the susceptibility [19,20]. For a sample in thermal equilibrium which exhibits a linear response, the power spectrum of the magnetic moment S m ðfÞ is given by [21,22] …”

Section: -2mentioning

confidence: 99%

Magnetic Noise in Structured Hard Magnets

et al. 2010

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The anomalous Hall effect of wires patterned from ðCo 90 Fe 10 =PtÞ n multilayers, with 10 n 50, is used to determine the magnetization process in a small volume of maze domains. Time-independent 1=f noise appears in samples with a quality factor Q < 1 at points on the hysteresis loop where the magnetization reverses continuously. The magnetic noise is associated with reversible excursions of segments of a domain wall $100 nm long. Barkhausen jumps are observed close to either the switching field or the saturation field where the noise power spectrum varies as 1=f 1:7 , and its magnitude decays with time. DOI: 10.1103/PhysRevLett.104.047202 PACS numbers: 75.60.Ej, 75.75.Àc, 75.50.Ww, 85.75.Nn Hysteresis is the defining property of a ferromagnet. The irreversible, nonlinear, and time-dependent response of the magnetization to an external magnetic field is traced as a time-or frequency-dependent open loop, which represents the sequence of metastable states occupied by the magnet as it adapts its domain configuration to the external field. Magnetic noise associated with the loop was first heard by Barkhausen in 1919, when he detected irreversible domain wall movement in nickel wires via the emf generated in a pickup coil by the flux jumps [1,2]. Barkhausen noise provides the basis for a method of nondestructive testing of steel parts under ac excitation [3]. It has been used to study correlations in magnetization reversal cascades by recording the jumps with a magneto-optical microscope [4].Single-domain particles of hard magnets may exhibit coherent magnetization reversal-the classical StonerWohlfarth behavior [5]-when they are about 10 nm in size, or less [6]. Uniformly magnetized films of similar thickness having spontaneous magnetization M and perpendicular anisotropy K u exhibit perpendicular magnetization when the quality factor Q ¼ 2K u = 0 M 2 is greater than 1. The hysteresis may then be a simple square loop like that of a Stoner-Wohlfarth particle, with an abrupt switch from magnetization þM to ÀM at the coercive field H c , when the applied field is along the easy axis. Perpendicular magnetization remains possible when Q < 1, but in a multidomain state, which is stabilized by the stray field created by a maze of stripe domains. The magnetization switches from a uniformly magnetized state in a positive applied field to a maze domain state, which is then progressively demagnetized by domain wall motion. The reverse stripes broaden and the normal stripes narrow before breaking up into segments and bubbles, and eventually disappear as the sample reaches saturation in the reverse direction [7,8]. Thin film multilayers of Fe or Co and Pt or Pd with perpendicular magnetization are of great interest for magnetoelectronics, magnetic recording and studies of the physics of magnetic interactions at the nanoscale level. In this Letter, we show how the magnetic noise due to fluctuations in the magnetization of a small sample volume changes as we go around the hysteresis loop. The fluctuations are measured using t...

Journal of Applied Physics

“…2 (c)]. As discussed in the previous section, when the field opposing the reference layer becomes comparable to B AF , the free layer is no longer the dominant source of noise [15]. Although the fluctuations of the reference layer do not cause a clearly observable change in the resistance, they can dominate the spectrum at θ = π: whereas the free layer is stabilized by the applied field, larger fields will continue to destabilize the SAF layers, providing additional mechanisms for fluctuations to occur.…”

Section: B Noise As a Function Of Magnetic Field Orientationmentioning

confidence: 72%

Low frequency noise characteristics of submicron magnetic tunnel junctions

Zhong

Chen

Garzon

et al. 2011

We report that low frequency (up to 200 kHz) noise spectra of magnetic tunnel junctions with areas 10 −10 cm 2 at 10 Kelvin deviate significantly from the typical 1/f behavior found in large area junctions at room temperature. In most cases, a Lorentzian-like shape with characteristic time between 0.1 and 10 ms is observed, which indicates only a small number of fluctuators contribute to the measured noise. By investigating the dependence of noise on both the magnitude and orientation of an applied magnetic field, we find that magnetization fluctuations in both free and reference layers are the main sources of noise in these devices. At small fields, where the noise from the free layer is dominant, a linear relation between the measured noise and angular magnetoresistance susceptibility can be established.