Dynamic magnetic susceptibility of systems with long-range magnetic order

The utility of the tunnel diode resonator (TDR) as an instrument for the study of magnetically ordered materials has been expanded beyond simple demonstration purposes. Results of static applied magnetic field dependent measurements of the dynamic magnetic susceptibility, χ, of various ferromagnetic (FM) and antiferromagnetic (AFM) materials showing a range of transition temperatures (1-800 K) are presented. Data were collected primarily with a tunnel diode resonator (TDR) at different radio-frequencies (∼10-30 MHz). In the vicinity of TC local moment ferromagnets show a very sharp, narrow peak in χ which is suppressed in amplitude and shifted to higher temperatures as the static bias field is increased. Unexpectedly, critical scaling analysis fails for these data. It is seen that these data are frequency dependent, however there is no simple method whereby measurement frequency can be changed in a controllable fashion. In contrast, itinerant ferromagnets show a broad maximum in χ well below TC which is suppressed and shifts to lower temperatures as the static bias field is increased. The data on itinerant ferromagnets are fit to a semi-phenomenological model that suggests the sample response is dominated by the uncompensated minority spins in the conduction band. Concluding remarks suggest possible scenarios to achieve frequency resolved data using the TDR as well as other fields to which the apparatus may be applied

Distinguishing local moment versus itinerant ferromagnets: Dynamic magnetic susceptibility

Radio-frequency measurements of dynamic magnetic susceptibility of various ferromagnets show striking differences between local-moment ferromagnetism (LFM) and weak itinerant ferromagnetism (IFM) ferromagnetic systems. LFMs show a very sharp peak in susceptibility in the vicinity of the Curie temperatureTC that rapidly decreases in amplitude and shifts to higher temperature with the application of a weak dc bias field. In stark contrast, the generally accepted IFM systems show no peak, but rather a broad maximum well below TC. The temperature of this maximum shifts to lower values and the amplitude is suppressed with an applied dc field

Contact-less measurements of Shubnikov-de Haas oscillations in the magnetically ordered state of CeAgSb2_2 and SmAgSb2_2 single crystals

Shubnikov - de Haas oscillations were measured in single crystals of highly metallic antiferromagnetic SmAgSb$_{2}$ and ferromagnetic CeAgSb$_{2}$ using a tunnel diode resonator. Resistivity oscillations as a function of applied magnetic field were observed via measurements of skin depth variation. The effective resolution of $\Delta\rho\simeq20$ p$\Omega$ allows a detailed study of the SdH spectra as a function of temperature. The effects of the Sm long - range magnetic ordering as well as its electronic structure ($4f$-electrons) on the Fermi surface topology is discussed

Multiple nearest-neighbor exchange model for the frustrated magnetic molecules Mo72Fe30 and Mo72Cr30

Our measurements of the differential susceptibility dM/dH of the frustrated magnetic molecules Mo72Fe30 and Mo72Cr30 reveal a pronounced dependence on magnetic field (H) and temperature (T) in the low H - low T regime, contrary to the predictions of existing models. Excellent agreement with experiment is achieved upon formulating a nearest-neighbor classical Heisenberg model where the 60 nearest-neighbor exchange interactions in each molecule, rather than being identical as has been assumed heretofore, are described by a two-parameter probability distribution of values of the exchange constant. We suggest that the probability distribution provides a convenient phenomenological platform for summarizing the combined effects of multiple microscopic mechanisms that disrupt the idealized picture of a Heisenberg model based on a single value of the nearest-neighbor exchange constant.Comment: 8 pages, 5 figure

Magnetic structure of Dy3+ in hexagonal multiferroic DyMnO3

Element specific x-ray resonant magnetic scattering (XRMS) investigations were undertaken to determine the magnetic structure of the multiferroic compound, hexagonal DyMnO3. In the temperature range from 68 K down to 8 K the Dy3+ moments are aligned and antiferromagnetically correlated in the c direction according to the magnetic representation Γ3. The temperature dependence of the observed intensity can be modeled assuming the splitting of ground-state doublet crystal-field levels of Dy3+ by the exchange field of Mn3+. XRMS together with magnetization measurements indicate that the magnetic representation is Γ2 below 8 K

Dynamic magnetic susceptibility of systems with long-range magnetic order

The utility of the tunnel diode resonator (TDR) as an instrument for the study of magnetically ordered materials has been expanded beyond simple demonstration purposes. Results of static applied magnetic field dependent measurements of the dynamic magnetic susceptibility, χ, of various ferromagnetic (FM) and antiferromagnetic (AFM) materials showing a range of transition temperatures (1-800 K) are presented. Data were collected primarily with a tunnel diode resonator (TDR) at different radio-frequencies (∼10-30 MHz). In the vicinity of TC local moment ferromagnets show a very sharp, narrow peak in χ which is suppressed in amplitude and shifted to higher temperatures as the static bias field is increased. Unexpectedly, critical scaling analysis fails for these data. It is seen that these data are frequency dependent, however there is no simple method whereby measurement frequency can be changed in a controllable fashion. In contrast, itinerant ferromagnets show a broad maximum in χ well below TC which is suppressed and shifts to lower temperatures as the static bias field is increased. The data on itinerant ferromagnets are fit to a semi-phenomenological model that suggests the sample response is dominated by the uncompensated minority spins in the conduction band. Concluding remarks suggest possible scenarios to achieve frequency resolved data using the TDR as well as other fields to which the apparatus may be applied.</p

Distinguishing local moment versus itinerant ferromagnets: Dynamic magnetic susceptibility

Radio-frequency measurements of dynamic magnetic susceptibility of various ferromagnets show striking differences between local-moment ferromagnetism (LFM) and weak itinerant ferromagnetism (IFM) ferromagnetic systems. LFMs show a very sharp peak in susceptibility in the vicinity of the Curie temperatureTC that rapidly decreases in amplitude and shifts to higher temperature with the application of a weak dc bias field. In stark contrast, the generally accepted IFM systems show no peak, but rather a broad maximum well below TC. The temperature of this maximum shifts to lower values and the amplitude is suppressed with an applied dc field.The following article appeared in Journal of Applied Physics (2008): 07D302 and may be found at http://dx.doi.org/10.1063/1.2832349.</p

Contactless measurements of Shubnikov-de Haas oscillations in the magnetically ordered state of CeAgSb2 and SmAgSb2 single crystals

Shubnikov-de Haas oscillations were measured in single crystals of highly metallic antiferromagnetic SmAgSb2 and ferromagnetic CeAgSb2 using a tunnel diode resonator. Resistivity oscillations as a function of applied magnetic field were observed via measurements of skin depth variation. The effective resolution of Delta rho similar or equal to 20 p Omega allows a detailed study of the SdH spectra as a function of temperature. The effects of the Sm long-range magnetic ordering as well as its electronic structure (4f electrons) on the Fermi surface topology is discussed.This article is published as Prozorov, Ruslan, Matthew D. Vannette, German D. Samolyuk, Stephanie A. Law, Sergey L. Bud’ko, and Paul C. Canfield. "Contactless measurements of Shubnikov-de Haas oscillations in the magnetically ordered state of Ce Ag Sb 2 and Sm Ag Sb 2 single crystals." Physical Review B 75, no. 1 (2007): 014413. DOI: 10.1103/PhysRevB.75.014413. Copyright 2007 American Physical Society. Posted with permission

Orchard Management and Landscape Context Mediate the Pear Floral Microbiome

Crop-associated microbiota are a key factor affecting host health and productivity. Most crops are grown within heterogeneous landscapes, and interactions between management practices and landscape context often affect plant and animal biodiversity in agroecosystems. However, whether these same factors typically affect crop-associated microbiota is less clear. Here, we assessed whether orchard management strategies and landscape context affected bacterial and fungal communities in pear (Pyrus communis) flowers. We found that bacteria and fungi responded differently to management schemes. Organically certified orchards had higher fungal diversity in flowers than conventional or bio-based integrated pest management (IPM) orchards, but organic orchards had the lowest bacterial diversity. Orchard management scheme also best predicted the distribution of several important bacterial and fungal genera that either cause or suppress disease; organic and bio-based IPM best explained the distributions of bacterial and fungal genera, respectively. Moreover, patterns of bacterial and fungal diversity were affected by interactions between management, landscape context, and climate. When examining the similarity of bacterial and fungal communities across sites, both abundance- and taxon-related turnovers were mediated primarily by orchard management scheme and landscape context and, specifically, the amount of land in cultivation. Our study reveals local- and landscape-level drivers of floral microbiome structure in a major fruit crop, providing insights that can inform microbiome management to promote host health and high-yielding quality fruit. IMPORTANCE Proper crop management during bloom is essential for producing disease-free tree fruit. Tree fruits are often grown in heterogeneous landscapes; however, few studies have assessed whether landscape context and crop management affect the floral microbiome, which plays a critical role in shaping plant health and disease tolerance. Such work is key for identification of tactics and/or contexts where beneficial microbes proliferate and pathogenic microbes are limited. Here, we characterize the floral microbiome of pear crops in Washington State, where major production occurs in intermountain valleys and basins with variable elevation and microclimates. Our results show that both local-level (crop management) and landscape-level (habitat types and climate) factors affect floral microbiota but in disparate ways for each kingdom. More broadly, these findings can potentially inform microbiome management in orchards for promotion of host health and high-quality yields