PubMed Health⌕ Search

Biomedical subjects

C Leighton

Publications and source records attributed to C Leighton.

At least 19 recordsLinked to original sources
PubMed2006

Exchange bias as a probe of the incommensurate spin-density wave in epitaxial Fe/Cr(001).

We report clear multiple period oscillations in the temperature dependence of exchange bias in an Fe thin film exchange coupled to a neighboring Cr film. The oscillations arise due to an incommensurate spin-density wave in the Cr, with wave vector perpendicular to the Fe/Cr(001) interface. The exchange bias and coercivity allow for a determination of the extent of the thermally driven wavelength expansion, the (strain-suppressed) spin-flip transition temperature, and the Cr Néel temperature, which show a crossover from bulklike to finite-size behavior at a Cr thickness of approximately 1100 A. The data are consistent with a transition from a transverse to longitudinal wave on cooling.

Journal Article↗
PubMed2006

Evolution with composition of the d-band density of states at the Fermi level in highly spin polarized Co1-xFexS2.

Highly spin polarized (SP) and half-metallic ferromagnetic systems are of considerable current interest and of potential importance for spintronic applications. Recent work has demonstrated that Co1-xFexS2 is a highly polarized ferromagnet (FM) where the spin polarization can be tuned by alloy composition. Using 59Co FM-NMR as a probe, we have measured the low-temperature spin relaxation in this system in magnetic fields from 0 to 1.0 T for 0<or=x<or=0.3. The 59Co spin-lattice relaxation rates follow a linear T dependence. Analysis of the data, using expressions for a FM system, permits information to be obtained on the d-band density of states at the Fermi level. The results are compared with independent density of states values inferred from electronic specific heat measurements and band structure calculations. It is shown that FM-NMR can be an important method for investigating highly SP systems.

Journal Article↗
PubMed2006

Artificial 'spin ice' in a geometrically frustrated lattice of nanoscale ferromagnetic islands.

Frustration, defined as a competition between interactions such that not all of them can be satisfied, is important in systems ranging from neural networks to structural glasses. Geometrical frustration, which arises from the topology of a well-ordered structure rather than from disorder, has recently become a topic of considerable interest. In particular, geometrical frustration among spins in magnetic materials can lead to exotic low-temperature states, including 'spin ice', in which the local moments mimic the frustration of hydrogen ion positions in frozen water. Here we report an artificial geometrically frustrated magnet based on an array of lithographically fabricated single-domain ferromagnetic islands. The islands are arranged such that the dipole interactions create a two-dimensional analogue to spin ice. Images of the magnetic moments of individual elements in this correlated system allow us to study the local accommodation of frustration. We see both ice-like short-range correlations and an absence of long-range correlations, behaviour which is strikingly similar to the low-temperature state of spin ice. These results demonstrate that artificial frustrated magnets can provide an uncharted arena in which the physics of frustration can be directly visualized.

Journal Article↗
PubMed2005

Co1-xFexS2: a tunable source of highly spin-polarized electrons.

In the emerging field of spin-electronics ideal ferromagnetic electron sources would not only possess a high degree of spin polarization, but would also offer control over the magnitude of this polarization. We demonstrate here that a simple scheme can be utilized to control both the magnitude and the sign of the spin polarization of ferromagnetic CoS2, which we probe with a variety of techniques. The position of the Fermi level is fine-tuned by solid solution alloying with the isostructural diamagnetic semiconductor FeS2, leading to tunable spin polarization of up to 85%.

Journal Article↗
PubMed2005

Intergranular giant magnetoresistance in a spontaneously phase separated perovskite oxide.

We present small-angle neutron scattering data proving that, on the insulating side of the metal-insulator transition, the doped perovskite cobaltite La(1-x)Sr(x)CoO(3) phase separates into ferromagnetic metallic clusters embedded in a nonferromagnetic matrix. This induces a hysteretic magnetoresistance, with temperature and field dependence characteristic of intergranular giant magnetoresistance (GMR). We argue that this system is a natural analog to the artificial structures fabricated by depositing nanoscale ferromagnetic particles in a metallic or insulating matrix; i.e., this material displays a GMR effect without the deliberate introduction of chemical interfaces.

Journal Article↗
PubMed2003

Magnetic phase separation in La1-xSrxCoO3 by 59Co nuclear magnetic resonance.

59Co NMR measurements on La1-xSrxCoO3 reported here establish unequivocally, for the first time, the coexistence of ferromagnetic regions, spin-glass regions, and hole-poor low spin regions at all x values from 0.1 to 0.5. A zero external field NMR spectrum, which is assigned to the ferromagnetic regions, has a spectral shape that is nearly x independent at 1.9 K, as are the relaxation times, T1 and T2. The integrated spectral area increases rapidly with x up to x = 0.2 and then decreases slightly for larger x. In a field of 9.97 T, a narrow NMR line is observed at 102 MHz, identical to that found in x = 0 samples in previous work. The integrated intensity of this spectrum decreases rapidly with increasing x, and is ascribed to hole-poor low spin regions. Beneath this spectrum, a third broad line, with a peak at 100 MHz, is assigned to a spin- or cluster-glass-like phase.

Journal Article↗
PubMed2001

Two-stage magnetization reversal in exchange biased bilayers.

MnF(2)/Fe bilayers exhibit asymmetric magnetization reversal that occurs by coherent rotation on one side of the loop and by nucleation and propagation of domain walls on the other side of the loop. Here, we show by polarized neutron reflectometry, magnetization, and magnetotransport measurements that for samples with good crystalline "quality" the rotation is a two-stage process, due to coherent rotation to a stable state perpendicular to the cooling field direction. The result is remarkably asymmetrically shaped hysteresis loops.

Journal Article↗
PubMed2001

Drug treatment of scleroderma.

Scleroderma or systemic sclerosis is a rare condition with many clinical manifestations including Raynaud's phenomenon. As with many other rarely encountered diseases, drug therapy for scleroderma is often empirical with little evidence in the form of randomised controlled trials to aid drug choice. Raynaud's phenomenon has been recognised for well over 100 years. A considerable number of clinical trials in this area have demonstrated unequivocally the use of nifedipine as a gold standard. Large studies have also demonstrated the efficacy of iloprost. However, this drug is not as yet licensed for scleroderma in the UK or elsewhere. This presents an additional problem as information regarding the use and administration of unlicensed drugs is often sparse and post-marketing surveillance to assess safety is not routinely performed. When looking at the other distinct conditions encountered by a patient with scleroderma it becomes evident that trials are often retrospective or limited in patient numbers. Studies investigating the use of methotrexate, antithymocyte globulin and cyclophosphamide in patients with scleroderma have been very small and in some cases not well designed. The major work on penicillamine was a retrospective trial. Again these drugs are not licensed for use in scleroderma. Drug therapy for pulmonary hypertension secondary to scleroderma closely follows that outlined for primary pulmonary hypertension. In the US there is a patient registry for primary pulmonary hypertension that has enabled well designed, large-scale studies to demonstrate the benefits of epoprostenol in severe primary pulmonary hypertension. Hence, research in this area has progressed considerably over the last decade. Clearly, a considerable amount of work is being carried out to elucidate new treatment regimens for scleroderma, however, evaluation of these studies is proving to be a difficult process. Designated hospital centres for scleroderma (there are currently 2 in the UK), better markers of disease activity and methods to measure improvement or deterioration in affected organs, should enable research into aetiology, disease progression and treatment to be carried out on a larger scale resulting, hopefully, in more conclusive answers.

Gastrointestinal Diseases↗
PubMed2000

Asymmetric magnetization reversal in exchange-biased hysteresis loops

Polarized neutron reflectometry is used to probe the in-plane projection of the net-magnetization vector M--> of polycrystalline Fe films exchange coupled to twinned (110) MnF (2) or FeF (2) antiferromagnetic (AF) layers. The magnetization reversal mechanism depends upon the orientation of the cooling field with respect to the twinned microstructure of the AF, and whether the applied field is increased to (or decreased from) a positive saturating field; i.e. , the magnetization reversal is asymmetric. The reversal of the sample magnetization from one saturated state to the other occurs via either domain wall motion or magnetization rotation on opposite sides of the same hysteresis loop.

Journal Article↗
PubMed2000

Coercivity enhancement in exchange biased systems driven by interfacial magnetic frustration

We report the temperature and cooling field dependence of the coercivity of exchange biased MnF(2)/Fe bilayers. When the antiferromagnetic surface is in a state of maximum magnetic frustration and the net exchange bias is zero, we observe a strong enhancement of the coercivity, which is proportional to the exchange coupling between the layers. Hence, the coercivity can be tuned in a reproducible and repeatable fashion in the same sample. We propose that a frustrated interface provides local energy minima which effectively pin the propagating domain walls in the ferromagnet, leading to an enhanced coercivity.

Journal Article↗
PubMed2000

Efficacy of the echinocandin caspofungin against disseminated aspergillosis and candidiasis in cyclophosphamide-induced immunosuppressed mice.

The in vivo efficacy of the echinocandin antifungal caspofungin acetate (caspofungin; MK-0991) was evaluated in models of disseminated aspergillosis and candidiasis in mice with cyclophosphamide (CY)-induced immunosuppression. Caspofungin is a 1, 3-beta-D-glucan synthesis inhibitor efficacious against a number of clinically relevant fungi including Aspergillus and Candida species. Models of CY-induced transient or chronic leukopenia were used with once daily administration of therapy initiated 24 h after microbial challenge. Caspofungin was effective in treating disseminated aspergillosis in mice that were transiently leukopenic (significant prolongation of survival at doses of > or =0.125 mg/kg of body weight and a 50% protective dose [PD(50)] of 0.245 mg/kg/day at 28 days after challenge) or chronically leukopenic (50 to 100% survival at doses of > or =0.5 mg/kg and PD(50)s ranging from 0.173 to 0.400 mg/kg/day). Caspofungin was effective in the treatment and sterilization of Candida infections in mice with transient leukopenia with a 99% effective dose based on reduction in log(10) CFU of Candida albicans/gram of kidneys of 0.119 mg/kg and 80 to 100% of the caspofungin-treated mice having sterile kidneys at caspofungin doses from 0.25 to 2.0 mg/kg. In Candida-infected mice with chronic leukopenia, caspofungin was effective at all dose levels tested (0.25 to 1.0 mg/kg), with the log(10) CFU of C. albicans/gram of kidneys of caspofungin-treated mice being significantly lower (>99% reduction) than that of sham-treated mice from day 4 to day 28 after challenge. Also, 70 to 100% of the caspofungin-treated, chronic leukopenic mice had sterile kidneys at caspofungin doses of 0.5 to 1.0 mg/kg from day 8 to 28 after challenge. Sterilization of Candida infections by caspofungin in the absence of host leukocytes provides compelling in vivo evidence for fungicidal activity against C. albicans. Further human clinical trials with caspofungin against serious fungal infections are in progress.

Animals↗
PubMed2000

Drug-induced aseptic meningitis: diagnosis and management.

Drug-induced aseptic meningitis (DIAM) has been reported as an uncommon adverse reaction with numerous agents. It is a diagnosis of exclusion, and clinical signs and CSF findings vary greatly. The body of evidence regarding DIAM is largely in the form of anecdotal case reports and must be interpreted carefully bearing this in mind. The major categories of causative agents are nonsteroidal anti-inflammatory drugs, antimicrobials, intravenous immunoglobulin, intrathecal agents, vaccines and a number of other less frequently reported agents. There appears to be an association between DIAM and connective tissue disease, particularly systemic lupus erythematosus, and ibuprofen. There are 2 major proposed mechanisms for DIAM. The first involves direct irritation of the meninges by intrathecal administration of the drug, and the second involves immunological hypersensitivity to the drug, most likely type III and type IV hypersensitivity. Recognition and diagnosis of DIAM is important, as it is treatable by withdrawal of the drug and recurrence is prevented. The outcome of DIAM is generally good, usually without long term sequelae. This article describes the case reports of DIAM in the current literature and discusses the diagnosis and management of this rare complication.

Anti-Infective Agents↗
PubMed1999

Pretreatment factors predict overall survival for patients with low-grade glioma: a recursive partitioning analysis.

PURPOSE: Three databases were pooled and analyzed to determine which groupings of prognostic factors best predicted overall survival for patients with low-grade gliomas treated with surgery and immediate or delayed radiotherapy. METHODS AND MATERIALS: Databases of patients with low-grade gliomas compiled at the London Regional Cancer Centre (LRCC), the Norwegian Radium Hospital (NRH), and the University of California, San Francisco (UCSF) were merged. Inclusion criteria for the pooled analysis included: age > or =18 years and histologically confirmed low-grade (World Health Organization Grade II) supratentorial fibrillary astrocytoma, oligodendroglioma or mixed oligoastrocytoma. Factors analyzed for prognostic significance included: age at diagnosis, gender, seizures at presentation, presence of enhancement on computed tomography (CT) or magnetic resonance imaging (MRI), Karnofsky Performance Status (KPS) at diagnosis, histology, extent of surgical resection, timing of radiotherapy, and treating institution. Univariate and multivariate analysis of overall survival for these factors was performed. Recursive partitioning was performed to generate prognostic groups using these factors. RESULTS: From the combined databases, 401 patients were eligible for analysis. Median survival for the entire group was 95 months/7.9 years. On univariate analysis age 18-40, presence of seizures at presentation, KPS > or =70, treating institution, and absence of contrast enhancement were associated with improved overall survival. On multivariate analysis, these factors remained independent predictors of improved overall survival. Recursive partitioning analysis yielded four prognostic groups with statistically different median survivals (MS): Group I (n = 41: KPS <70, age >40) MS 12 months; Group II (n = 34: KPS > or =70, age >40, enhancement present) MS 46 months; Group III (n = 138: KPS <70, age 18-40 or KPS > or =70 age >40, no enhancement) MS 87 months; Group IV (n = 188: KPS > or =70, age 18-40) MS 128 months. CONCLUSION: Clusters of pretreatment prognostic factors described subgroups of low-grade glioma patients with divergent overall survivals. Consideration of these prognostic subgroups may be important when considering timing of interventions for these patients and in the stratification of patients for clinical trials.

Adult↗
PubMed1999

Low grade glioma: a measuring radiographic response to radiotherapy.

PURPOSE: We set out to determine the rate of response of low-grade (WHO Grade II) gliomas to radiotherapy and analyze the relationship between radiographic response, symptom control and patient survival. METHODS: Patients were eligible for this study if they had received radiotherapy for pathologically confirmed, residual, supratentorial low-grade astrocytoma, oligodendroglioma, or mixed glioma, and imaging studies (baseline and follow-up) were available for review. Percent change in tumor size and rate and timing of response were determined by maximum linear measurement, area measurement, volume measurement using an ellipsoid model, and volume measurement by image segmentation. For each method, response to radiotherapy was defined firstly as a > or = 50% decrease in tumor size (partial response), and secondly as a decrease equivalent to a 50% area decrease (normalized partial response). Relationships between radiographic response, clinical improvement and progression-free survival were analyzed using a Cox Proportional Hazard's model. RESULTS: Twenty-one patients in a database (13 male, 8 female; ages 22-66 years) met the eligibility criteria. Twenty were imaged by computed tomography, 18 had an astrocytoma and 15 were irradiated soon after surgery. Responses were common and not felt to be due to a steroid effect. Use of normalized response criteria improved agreement between assessment of response as determined by the 4 methods. Median time to maximum radiographic improvement was 2.8 months (range, 1.5-11). Sixteen patients (76%) were improved neurologically, the median time to progression was 4.8 years and the 5-year progression-free survival rate was 43%. We did not detect a statistically significant association between response (as measured by any method), symptomatology and progression-free survival. CONCLUSIONS: Low-grade gliomas are moderately radioresponsive. Use of volume measurement may over-estimate the number of partial responses unless a volume reduction equivalent to a 50% area decrease is used to define response. The best way to measure response remains uncertain because neither visual, area, nor volume changes confidently predicted clinical outcomes.

Adult↗
PubMed1998

A change of heart.

Explore the source record for details and available documents.

Coronary Disease↗
PubMed1997

Supratentorial low-grade glioma in adults: an analysis of prognostic factors and timing of radiation.

PURPOSE: To review the outcomes of patients with low-grade glioma diagnosed by modern imaging and treated at a center where postponing radiotherapy was common practice. METHODS: We reviewed the records of patients (age > or = 18 years) with pathologically confirmed supratentorial low-grade fibrillary astrocytoma, oligodendroglioma, and mixed glioma treated at a regional cancer center in Canada between 1979 and 1995. RESULTS: Median survival for the entire group (N = 167; mean age 40.6 years) was 10.5 years with 5- and 10-year survival rates of 72% and 50%, respectively. Median progression-free survival was 4.9 years with 5- and 10-year progression-free rates of 50% and 12%, respectively. Overall and progression-free survivals were longer for patients with an oligodendroglioma or mixed glioma than with astrocytoma (median 13 v 7.5 years, P = .003; progression-free 5.6 v 4.4 years, p = .054). Age at diagnosis < or = 40 years, seizures at presentation, minimal residual tumor after surgery, Karnofsky performance status > or = 70, and oligodendroglioma or mixed glioma pathology were associated with significantly longer median survival on univariate and multivariate analyses. Radiotherapy deferred until tumor progression (v immediate radiotherapy) was associated with longer survival on univariate analysis, but an imbalance in other variables accounted for this advantage such that timing of radiotherapy was not an independent (favorable or adverse) prognostic factor on multivariate analysis. CONCLUSION: Patients with low-grade glioma diagnosed by modern imaging can be expected to live a long time; timing of radiotherapy may be a less important determinant of survival than nontreatment variables and residual tumor bulk.

Actuarial Analysis↗