Nan Mo

Electric field tunable 60 GHz ferromagnetic resonance response in barium ferrite-barium strontium titanate multiferroic heterostructures

A magnetic-ferroelectric film heterostructure with a large electric field tuning of the ferromagnetic resonance (FMR) mode was fabricated. Pulse laser deposited 30 nm thick Pt electrodes and 3 μm thick barium strontium titanate films on Nb-doped strontium titanate substrates were capped with an unbonded 200 μm thick single crystal in-plane c-axis barium hexaferrite slab. The structure gives a 60 GHz FMR frequency shift of 16 MHz at a bias of 29 V, for an average response of 0.55 MHz/V. The maximum incremental tuning response at 29 V was 1.3 MHz/V. This is a hundredfold improvement over previous results.

Hamiltonian formalism for two magnon scattering microwave relaxation: Theory and applications

A two magnon scattering theory for microwave relaxation in magnetic systems is formulated in the framework of the Hamiltonian formalism. The paper provides general expressions for inhomogeneity coupling coefficients in the case of localized inhomogeneities. An approximate solution for the relaxation rate of the ferromagnetic resonance uniform mode relaxation rate is presented. Two examples of the application of the theory are presented, one for bulk polycrystalline ferrites and one for polycrystalline metallic thin films.

Origins of the damping in perpendicular media : Three component ferromagnetic resonance linewidth in Co-Cr-Pt alloy films

The 9.41GHz ferromagnetic resonance field and linewidth have been measured as a function of the angle (θH) between the external magnetic field and film normal for a series of 17.5nm thick Co–Cr–Pt alloy films. The linewidths ranged from hundreds of Oersted to kiloersted, with different values at θH=0 and θH=90° and additional minima and maxima for θH-values from 16° to 64°. The profiles can be fitted with a combination of inhomogeneity line broadening, grain boundary two magnon scattering, and magnon-electron (m-e) scattering processes, with a notably small Gilbert damping α-value of 0.004 for the m-e term.

The low field microwave effective linewidth in polycrystalline ferrites

High precision measurements on the low and high field effective linewidth ΔHeff at 10GHz have been made on ultradense (UD) and conventionally sintered (CS) polycrystalline yttrium iron garnet (YIG) materials. The high field data confirm previous results on the role of two magnon scattering to low wave number (k) electromagnetic Larmor branch spin waves that lie below the light line. The low field data reveal two important contributions to the effective linewidth. For a field regime from the low k edge of the usual dipole exchange spin wave band down to the point in field (H=HX) where above-the-light-line electromagnetic branch Larmor (EML-HI) spin waves appear, ΔHeff is connected with scattering to relatively high k dipole exchange Larmor (DEL) spin waves. The coupling to these modes comes from grain boundaries in the YIG materials. A grain boundary scattering theory gives reasonable agreement with the data. While the high field effective linewidth due to pseudo in-manifold scattering is larger for the CS...

High-field microwave effective linewidth in polycrystalline ferrites: Physical origins and intrinsic limits

High-precision high-field effective (HFE) linewidth measurements have been made at 10GHz on ultradense hot isostatic pressed and conventionally sintered (CS) yttrium iron garnet ferrite materials. The accuracy was increased ten-fold relative to previous data through the use of a high-quality cavity, a modified measurement technique, and a fine adjustment of the gyromagnetic ratio based on the field-frequency response of the loaded cavity. The HFE linewidths for fields well above the region of degenerate bulk spin-wave band decreases with increasing field and extrapolates to known intrinsic single-crystal linewidths in the extreme high-field limit. From a field point shifted up from the high-field band edge dependent on degenerate dipole-exchange spin waves by 2∕3 the saturation induction, the data track closely the computed density of states for electromagnetic spin waves. In the case of the CS material, for fields below this point, one sees a microstructure-related increase related to conventional modera...

Ferromagnetic resonance and damping in granular Co–Cr films with perpendicular anisotropy

The 17.3 GHz ferromagnetic resonance field (HFMR) and linewidth (ΔH) have been measured as a function of the angle (θH) between the external magnetic field and film normal for a 16 nm thick Co–Cr granular film with uniaxial perpendicular anisotropy. The HFMR(θH) response is significantly different from the uniform rotation prediction. The ΔH(θH) dependence shows major deviations from the Gilbert phenomenological damping model. Both dependences can be modeled simultaneously through a combination of two-magnon scattering processes, inhomogeneity line broadening, and an intrinsic damping from magnon-electron scattering processes, with a Gilbert damping α-value of 0.004.

Tuning of magnetization relaxation in ferromagnetic thin films through seed layers

Tuning of the magnetization relaxation in Fe65Co35 thin films via seed layers was demonstrated. Through the use of different types of seed layers, one can tune substantially both the magnitude and frequency dependence of the relaxation rate η of the film. This tuning relies on the change of the film grain properties with the seed layer and the correlation between grain properties and two-magnon scattering processes. In spite of a significant change of η with the seed layer, the film static magnetic properties remain relatively constant.

High precision metrology based microwave effective linewidth measurement technique.

A precision microwave effective linewidth measurement technique for magnetic samples has been developed. The measurement utilizes a high-Q cylindrical cavity that contains the sample of interest, a highly stable and programable static magnetic field source, a computer controlled network analyzer for cavity center frequency omega c and quality factor Qc determinations, and the standard metrological substitution ABA method for accurate relative omega c and Qc measurements. Sequential long term ABA measurements show that the time and temperature drifts and random errors are the dominant sources of error, with uncertainties in omega c/2pi and Qc in the range of 50 kHz and 25, respectively. The ABA method is applied to eliminate these drifts and minimize the random errors. For measurements over 25 ABA cycles, accuracy is improved to 0.14 kHz for omega c/2pi and 3 for Qc. The temperature variation over a single ABA cycle is generally on the order of 10(-3)-10(-5) degrees C and there is no need for any further temperature stabilization or correction measures. The overall uncertainty in the 10 GHz effective linewidth determinations for a 3 mm diam, 0.5 mm thick polycrystalline yttrium iron garnet (YIG) disk is 0.15 Oe or less, well below the intrinsic single crystal YIG linewidth. This represents a factor of 10 improvement in measurement accuracy over previous work.

Perspective: Local ferromagnetic resonance measurement techniques: "Invited Review Article: Microwave spectroscopy based on scanning thermal microscopy: Resolution in the nanometer range" †Rev. Sci. Instrum. 79, 041101 "2008…‡

According to Vonsovskii, ferromagnetic resonance FMR was unknowingly discovered by Arkad’yev in 1911. The standard citation for the experimental discovery of FMR is to Griffiths for his observation of the rather broad absorption profile and an “anomalous” electron paramagnetic resonance EPR field for in-plane magnetized electroplated ferromagnetic films. One year later, Kittel explained the anomalous FMR fields by taking the dynamic demagnetizing fields into account. FMR is distinct from EPR because the FMR field or frequency is shifted from the usual range of pure electron spin resonance values by substantial amounts because of both static and dynamic demagnetizing field effects, among others. The traditional approaches to the measurement of the FMR response are through shorted waveguide, microwave cavity, or stripline techniques. One usually excites the FMR with a relatively uniform microwave field and obtains a uniform-mode or quasi-uniform-mode response in which all of the precessing spins are excited at the same nominal amplitude and in phase. In spite of the quasi-uniform-mode nature of the response, the actual power absorption profile for a large sample often depends on the local properties that may include a nonuniform microstructure as well as spatial variations in the local magnetic moment, gyromagnetic ratio, magnetocrystalline anisotropy, damping and relaxation processes, and so on. From the late 1950s, various workers have also developed a variety of “local” FMR techniques. A local FMR experiment may be done in two ways. In the first, one excites the sample over a wide area and a local FMR probe is used for detection over a small region of the sample only. The second utilizes both local excitation and local detection. The first practical setup for local FMR measurements used the second approach, in which a thin film sample was excited and detected locally through a small iris of a microwave cavity. This basic and simple near field technique was developed independently by Frait and Soohoo. From the initial work by Frait and Soohoo, there has been evolving work on local FMR approaches. Until recently, these have focused mainly on methods related to near field optics and based on the use of small probes. See Ref. 11 for a non-FMR related review of these general approaches. FMR related citations include work with small coaxial loop FMR probes and dielectric resonator slit probes. These methods represent fairly straightforward variations of the original Frait/Soohoo schemes at different levels of sophistication. These approaches have never been able to achieve spatial resolutions below about 10 m or so. In 1988, a new approach to local FMR based on thermal effects was introduced by Pelzl at Ruhr University, Bochum, Germany. This method has led to a highly sensitive and very fine scale FMR measurement capability down to 10 nm or so. This technique may be termed “scanning thermal FMR microscopy” SThM . For the conventional near field local FMR methods introduced above, a single probe plays a dual role in both the excitation and detection of the magnetic response. In the SThM technique, a microwave cavity is often used to excite the magnetic response in a wall mounted sample. A thermal sensor is then used to probe the sample through a small hole in the wall. The thermal probe simply measures the small changes in the local temperature that go along with the sample heating due to the FMR driven absorbed power. The initial SThM system described in Ref. 16 was based on a photothermal modulation approach. The sample was locally illuminated by a 40 Hz power-modulated He–Ne laser with a 180 m diameter spot size. The modulation induces a localized change in the magnetization of the sample, also at the 40 Hz modulation frequency. This, in turn, gives a modulation in the FMR power absorption and the corresponding local temperature that is then detected by lock-in detection methods. In recent years, Meckenstock et al. have made considerable advances in this basic local FMR thermal detection approach. Reference 17 describes extensions of the method to direct detection without thermal modulation. The nominal resolution in this case was about 100 nm. These authors also developed a variation of the method based on scanning thermal-elastic microscopy SThEM . In this approach, the local thermal-elastic expansion is measured directly by atomic force microscopy AFM techniques. This marriage of methods allows one to bring all of the power and advances in the field of AFM to the table for local FMR measurements. Compared to the previous near field FMR microscopes, the new thermal techniques give a significantly improved spatial resolution. The reported SThEM spatial resolution by Meckenstock et al. was in the range of 10 nm. The use of microwave power modulation and lock-in detection gave good sensitivity and a high signal to noise ratio. REVIEW OF SCIENTIFIC INSTRUMENTS 79, 040901 2008