Observation of isotropic critical spin fluctuations in Gd

Collins, Gary S.; Chowdhury, A. R.; Hohenemser, C.

doi:10.1103/physrevb.33.4747

Cited by 29 publications

(15 citation statements)

References 10 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…We also report on measurements of the muon spin relaxation (µSR) rate in high purity single crystal samples of Gd. A comparison of the theoretical predictions with these and earlier [4,6] µSR measurements as well as perturbed angular correlation (PAC) and Moessbauer data [1][2][3] gives a coherent picture of the dynamic and static critical behavior of Gd and resolves the puzzling situation described above.We consider a system with N identical spins fixed on the sites of a three dimensional lattice. Taking into account magnetocrystalline anisotropy as well as dipolar interaction it is described by the Hamiltonian…”

supporting

confidence: 56%

“…This may not be easily interpreted in terms of one or the other universality class. This is even more so, as the static critical exponents for the various universality classes are of comparable magnitude.A surprising and yet unexplained observation was made by a measurement of the critical dynamics using hyperfine methods [1][2][3]. The critical exponent w for the autocorrelation time τ c , which should scale as τ c ∝ (T − T c ) −w in the asymptotic regime, was found to be w ≈ 0.5.…”

mentioning

confidence: 99%

See 1 more Smart Citation

Determination of the Universality Class of Gadolinium

et al. 1997

View full text Add to dashboard Cite

We resolve a longstanding puzzle for the static and dynamic critical behavior of Gadolinium by a combined theoretical and experimental investigation. It is shown that the spin dynamics of a three dimensional ferromagnet with hcp lattice structure and a spin-spin interaction given by both exchange and dipole-dipole interaction belongs to a new dynamic universality class, model J * . Comparing results from mode coupling theory with results from three different hyperfine interaction probes we find quantitative agreement. The crossover scenario for the wavevector dependence of the hyperfine relaxation rate is determined by a subtle interplay between three length scales: the correlation length, the dipolar and the uniaxial wave vector.PACS numbers: 75.40. Gb, 75.40.Cx, 76.75.+i, 76.80.+y The spin dynamics of simple ferromagnets in the vicinity of their Curie point T c are archetypical examples of dynamic critical phenomena near second-order phase transitions. Much experimental and theoretical effort has been put into identifying the dynamic universality classes and assigning them to magnetic substances. Nevertheless, experimental observations on gadolinium [1][2][3][4][5][6][7] remained a puzzle up to now. Because of its large localized magnetic moment, and the fact that it is an Sstate ion, Gd should have a very small magnetocrystalline anisotropy and therefore be much better a model system for an isotropic Heisenberg magnet than either Fe, Ni or EuO. As a consequence it should belong to the model J dynamic universality class in the classification scheme of Ref. [8]. The measured static and especially dynamic critical exponents are, however, not at all compatible with model J. The objective of this paper is to resolve this longstanding seemingly contradictory situation by a combined theoretical and experimental study.Early experimental observations [9] clearly demonstrate that Gd has an easy axis which coincides with the hexagonal axis of its hcp lattice. The origin of such an easy axis cannot be understood from the magnetocrystalline anisotropy. But, based on a mean field theory [10] it has been argued that a combined effect of the lattice structure and dipolar interaction favors the c-axis as the easy direction. This view is supported by measurements [7] of the c-axis and basal-plane susceptibility on a single crystal of Gd. It is found that the basal-plane susceptibility crosses over from a singular behavior to a constant at a characteristic temperature scale which can be accounted for by dipolar effects. The analysis of the static critical behavior [7] is, however, complicated by the fact that all experiments are done in the non-asymptotic regime where superposed crossover lead to complex temperature dependences. This may not be easily interpreted in terms of one or the other universality class. This is even more so, as the static critical exponents for the various universality classes are of comparable magnitude.A surprising and yet unexplained observation was made by a measurement of the critical dynamics u...

show abstract

supporting

confidence: 56%

mentioning

confidence: 99%

Determination of the Universality Class of Gadolinium

et al. 1997

View full text Add to dashboard Cite

show abstract

“…In contrast, it lies significantly lower than the Heisenberg prediction (z = 5/2). 48 It is worth noting here that the value of the z exponent reflects that it is dipolar interactions and not electron delocalization that produces the observed deviation from Heisenberg critical behavior. A priori, both could act as the perturbation to short-range exchange interactions that causes the crossover.…”

Section: B Heisenberg To Mean-field Drift Due To Long-range Dipolar mentioning

confidence: 89%

“…η = 0.034 for a three-dimensional ferromagnet. 48 Without direct measurements of ξ (T ), ν can be eliminated from the equation via the scaling relation z = 2 + α/ν and α can been obtained from the other static exponents (α = 2 − 2β − γ = −0.14). This yields a value of z ≈ 1.91, which compares well with the prediction for a dipolar ferromagnet (1.98), 42 45 with similar crossover behaviors.…”

Section: B Heisenberg To Mean-field Drift Due To Long-range Dipolar mentioning

confidence: 99%

Critical behavior in the inhomogeneous ferromagnetSrFe0.80Co0.20O3.
Lago
¹
,
Rosseinsky
²
,
Blundell
³

et al. 2011
Phys. Rev. B
11
0
6
0
View full text Add to dashboard Cite

A detailed muon spin relaxation (μSR) and magnetization study is presented of the paramagnetic-toferromagnetic transition in the compound SrFe 0.80 Co 0.20 O 3 . The critical exponents derived from the static critical analysis are close to the theoretical predictions for the Heisenberg model in three dimensions. However, a small drift toward mean-field values is interpreted as arising from the presence of long-range dipolar interactions between the Fe(IV) centers. The evolution of spin dynamics across the transition determined from the μSR study is consistent with this interpretation. μSR and magnetization data also provide evidence of an inhomogeneous magnetic state both above and below T c , placing this system in line with other double-exchange materials such as manganites and cobaltites where spontaneous electronic and magnetic phase separation appears conspicuous and explains much of the encountered experimental phenomenology.

show abstract

“…As the temperature is reduced further, there is a second crossover temperature to Ising behavior approximately 0.5 -1 K above Tr. [3].The dynamics properties of gadolinium have been studied mainly by two hyperfine techniques: the perturbed angular correlation method [4] and the positive muon spin relaxation and rotation spectroscopy (ttSR) [5,6]. There is little information from neutron experiments [7].…”

mentioning

confidence: 99%

Possibility of observation of the critical paramagnetic longitudinal spin fluctuations in gadolinium by muon spin rotation spectroscopy

Réotier

Yaouanc

1994

Phys. Rev. Lett.

View full text Add to dashboard Cite

The theory described here showers that the muon spin depolarization rates measured on gadolinium in the critical paramagnetic region give direct information on the longitudinal (along the wave vector q) spin fluctuations. In the Ising regime it is possible to distinguish the parallel (along the c axis) from the perpendicular fluctuations.PACS numbers: 76.75.+i, 75.40.Gb, 75.30.Gw Gadolinium is a rare earth metal which crystallizes in the hexagonal close packed structure (hcp) and exhibits simple ferromagnetic ordering along the c axis at temperatures near the Curie temperature, Tr: = 293 K. Its ferromagnetism is due to the Ruderman-Kittel-Kasuya-Yosida interaction between the Gds+ ions which are in the Sq/q state. This ionic electronic structure suggests that the magnetocrystalline anisotropy is very small. Therefore a simple Heisenberg behavior was expected for the magnetic properties of gadolinium near Tr. . Contrary to these expectations, measurements have shown that these properties are quite complicated. It is only recently that a consistent experimental picture of the static properties has emerged [1,2]. It seems that all the complications are due to the magnetic dipole-dipole interactions. These long range interactions cause a crossover as the temperature is reduced in the paramagnetic region from an isotropic Heisenberg regime to a dipolar Heisenberg regime with a crossover temperature of approximately 4 K above Tr. . As the temperature is reduced further, there is a second crossover temperature to Ising behavior approximately 0.5 -1 K above Tr. [3].The dynamics properties of gadolinium have been studied mainly by two hyperfine techniques: the perturbed angular correlation method [4] and the positive muon spin relaxation and rotation spectroscopy (ttSR) [5,6]. There is little information from neutron experiments [7]. lsSR measurements performed on a single crystal seem to show that the magnetic fluctuations have a pronounced anisotropy when the temperature is approached to within -0.5 K from Tr. [5]. The lsSR data recorded on a polycrystalline sample indicate that the magnetic fluctuations are almost temperature independent for 1 K + T -Tr: + 10 K [6]. Because pSR measurements can be carried out in a wide temperature range which can extend from Tc, up to very high temperature, they should allow us to study in detail the different crossovers. Up to now no quantitative information has been extracted from these measurements mainly because the relation between the data and the correlation functions which characterize the fluctuations of the Gd + total moments has not been available. Recently it has been shown that it is possible to understand quantitatively pSR data recorded in cubic paramagnets [8]. The purpose of this Letter is to provide a theoretical framework for the interpretation of the available ltSR data on gadolinium and to initiate further and more accurate experiments. For an anisotropic crystal structure, the dipolar part of the Hamiltonian is the sum, near the Brillouin zone center, of two te...

show abstract

Observation of isotropic critical spin fluctuations in Gd

Cited by 29 publications

References 10 publications

Determination of the Universality Class of Gadolinium

Determination of the Universality Class of Gadolinium

Critical behavior in the inhomogeneous ferromagnetSrFe0.80Co0.20O3.
Lago
¹
,
Rosseinsky
²
,
Blundell
³

et al. 2011
Phys. Rev. B
11
0
6
0
View full text Add to dashboard Cite

Possibility of observation of the critical paramagnetic longitudinal spin fluctuations in gadolinium by muon spin rotation spectroscopy

Contact Info

Product

Resources

About

Observation of isotropic critical spin fluctuations in Gd

Cited by 29 publications

References 10 publications

Determination of the Universality Class of Gadolinium

Determination of the Universality Class of Gadolinium

Critical behavior in the inhomogeneous ferromagnetSrFe0.80Co0.20O3.Lago1, Rosseinsky2, Blundell3 et al. 2011Phys. Rev. B11060View full textAdd to dashboardCite

Possibility of observation of the critical paramagnetic longitudinal spin fluctuations in gadolinium by muon spin rotation spectroscopy

Contact Info

Product

Resources

About

Critical behavior in the inhomogeneous ferromagnetSrFe0.80Co0.20O3.
Lago
¹
,
Rosseinsky
²
,
Blundell
³

et al. 2011
Phys. Rev. B
11
0
6
0
View full text Add to dashboard Cite