Probing Magnetic Order with Neutron Scattering

Transcription

1 Probing Magnetic Order with Neutron Scattering G.J. Mankey, V.V. Krishnamurthy, F.D. Mackey and I. Zoto University of Alabama in collaboration with J.L. Robertson and M.L. Crow Oak Ridge National Laboratory S. Maat and E.E. Fullerton IBM Almaden Research Center MINT Spring Review 2002 Sponsored by ARO DAAH , NSF MRSEC DMR , DOE DMR DE-AC05-96OR22464 and DE-FG02-02ER WWW:

2 Antiferromagnetic Thin Films Goal: Understand exchange bias by relating microstructure with measured magnetic data. Experiments: Produce single-crystal layers for study. Determine spin structure of FeMn with neutron scattering. Measure critical behavior of FeMn with temperaturedependent neutron scattering. Correlate with exchange bias measurements of bilayer films.

3 F/AF Exchange Bias When a ferromagnet (F) is deposited on an antiferromagnet (AF) in an applied field, the hysteresis loop of the F film is altered in two ways: There is a bias (or shift) of the hysteresis loop by an amount called H p or the pinning field. There is an enhancement of the coercive field, H c, particularly along the direction of the applied field.

4 Hypothetical Spin Arrangements of AF FeMn 1Q 2Q 3Q Three spin structures for FeMn are proposed. Each should give distinctly different anisotropy behavior when inserted in an AF/F bilayer system.

5 Spin Hamiltonian The anisotropic Heisenberg Hamiltonian is described by two coupling parameters. J z is the out of plane coupling. J is the in-plane coupling. Special cases are Ising (J = 0), XY (J z = 0), isotropic Heisenberg (J z = J ). The ratio of J z / J gives the anisotropy. The spin ordering in itinerant systems like FeMn is not adequately described by this model.* * S. Maat, Ph.D. dissertation, UA (2000).

6 Determining Spin Ordering from Power Laws The critical behavior depends on the universality class of the system. The magnetization power law exponent, β, is for the 2D Ising model, 0.24 for the finite-size 2D XY model and 0.34 for the 3D Heisenberg model. Determination of β provides an insight into the type of magnetic ordering.

7 2D to 3D Dimensional Crossover The power law exponent β increases in the range of film thickness of 5 to 10 monolayers. Ref: F. Huang, et al., Phys. Rev. B 49, 3962 (1994). For in-plane magnetized films (a) and (b) the crossover is from finite-sized 2D XY to 3D Heisenberg model. For perpendicular magnetized films the crossover is from 2D Ising to 3D Heisenberg model. Do AF films exhibit a similar behavior?

8 Antiferromagnetic Spin Ordering in FePt 3 Two types of spin ordering are observed in the bulk material. Films deposited on MgO(110) exhibit only [1/2 1/2 0] order. Films deposited on sapphire substrates exhibit both types of spin ordering. Epitaxial strain plays a significant role in determining the spin structure of AF films. Ref. S. Maat, et al., Phys. Rev. B 63, (2001).

9 Spin Ordering in FePt 3 /MgO(110) The power law fit is good for all temperatures measured. A characteristic exponent for the finite-size 2D XY model of β = 0.23 is extracted from the data. In layered systems, even a small amount of interplanar coupling makes the system behave as Heisenberg. Can we learn something about the spin ordering from the critical behavior? Ref: S. Maat, et al., Phys. Rev. B 63, (2001).

10 RHEED for Si(110) / Cu(20nm) / Ni 80 Fe 20 (10nm) / Fe 60 Mn 40 (t) t = 20 nm t = 60 nm t = 40 nm t = 100 nm FeMn films can be produced in the metastable fcc phase up to a thickness of 40 nm. Above t = 40 nm, the films revert to the more stable bcc phase as confirmed by RHEED measurements. Thicker films are needed for neutron measurements--make multilayer samples. Ref: C. Liu et al., J. Vac. Sci. Technol. A 19, 1213 (2001).

11 Probing Antiferromagnetic Ordering with Unpolarized Neutrons For unpolarized neutrons dσ = b 2 + p 2 q 2 where b is the nuclear scattering length, p is the magnetic scattering amplitude and q 2 =sin 2 α with α = the angle between the scattering plane normal and the magnetic moment. At half-order diffraction locations, only the magnetic scattering contributes, so the diffracted intensity is proportional to the sublattice magnetization in the scattering plane.

12 Neutron Scattering Results Si<110>/Cu(100)/[FeMn(5)/Cu(5)]x50/Cu(20) Chemical Magnetic Radial Rocking Antiferromagnetic ordering in and out of the plane is observed for the FeMn layers. Chemical and magnetic diffraction data gives comparable atomic and magnetic correlation lengths. Comparison of intensities shows all of the FeMn exhibits antiferromagnetic ordering.

13 Temperature-Dependent Neutron Scattering [1-1 0] Magnetic Peak Intensity [10 nm Cu(111) / 10 nm Fe 50 Mn 50 ] 50 First Heating Cycle Second Heating Cycle Third Heating Cycle After 275 ºC Anneal Temperature (ºC) For the first heating cycle, the Néel temperature is close to the bulk value for Fe 50 Mn 50. An irreversible structural change occurs at 200 ºC. Is it diffusion from the FeMn into the Cu interlayer? An enhanced Néel temperature results from the structural change. The raw data must be corrected for the Debye-Waller factor to find the critical exponent.

14 Corrected Data First Cycle Second Third For the first heating cycle, a Néel temperature of 510K and critical exponent of are found, consistent with bulk 3D Heisenberg behavior. Subsequent heating cycles showed that annealing to 480 K irreversibly changes the microstructure of the multilayer, resulting in a reduction in the magnetization, a reduction of the critical exponent, and an increase of the Néel temperature.

15 Conclusions Neutron diffraction cannot distinguish between domains of 1Q or 2Q and a single-domain 3Q structure. Heisenberg-like critical behavior is observed for FeMn/Cu multilayers. Access to temperatures near the Néel temperature is limited by the activation of thin film diffusion processes around 200 ºC. Diffusion is dominated by silicide formation, so investigation of multilayers on single crystal Cu or alternative substrates remains a possibility.

16 Future Work More detailed measurements with 0.9 T N < T < T N to accurately determine the critical exponent β. Choose materials with lower T N and interfaces which are more robust against annealing cycles. Make F/AF superlattices to compare temperature dependent exchange bias behavior with critical behavior. Participate in instrument development at the High Flux Isotope Reactor to improve spectrometer efficiency allowing measurements of thinner films.