1 Element -Specific First Order Reversal Curves Measured by Magnetic Transmission X-ray Microscopy

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Element-Specific First Order Reversal Curves Measured by Magnetic

Transmission X-ray Microscopy

Dustin A. Gilbert,1 Mi-Young Im,2 Kai Liu,3 Peter Fischer2, 4

1Materials Science Department, University of Tennessee, Knoxville, TN 37996, USA

2Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720,

USA

3Physics Department, Georgetown University, Washington DC 20057, USA

4Physics Department, University of California, Santa Cruz, CA 95616, USA

Abstract: The first order reversal curve (FORC) method is a macroscopic measurement technique

which can be used to extract quantitative, microscopic properties of hysteretic systems. Using

magnetic transmission X-ray microscopy (MTXM), local element-specific FORC measurements

are performed on a 20 nm thick film of CoTb. The FORCs measured with microscopy reveal a

step-by-step domain evolution under the magnetic field cycling protocol, and provide a direct

visualization of the mechanistic interpretation of FORC diagrams. They are compared with

magnetometry FORCs and show good quantitative agreement. Furthermore, the high spatial

resolution and element-specific sensitivity of MTXM provide new capabilities to measure FORCs

on small regions or specific phases within multicomponent systems, including buried layers in

heterostructures. The ability to perform FORCs on very small features is demonstrated with the

MTXM-FORC measurement of a rectangular microstructure with vortex-like Landau structures.

This work demonstrates the confluence of two uniquely powerful techniques to achieve

quantitative insight into nanoscale magnetic behavior.

Introduction

Research and design of modern magnetic materials frequently requires a microscopic

understanding of the structure and properties of the system,1-3 which can be challenging to

determine with macroscale measurements.4-6 On the other hand, advanced magnetic imaging, such

as magnetic transmission X-ray microscopy (MTXM)7-12, Lorentz transmission electron

microscopy,13 spin-polarized scanning tunneling microscopy,14,15 spin-polarized low energy

electron microscopy (SPLEEM),16,17 photoemission electron microscopy,18,19 and scanning

electron microscopy with polarization analysis,20-22 can provide critical visualization of real space

magnetic configurations. However, magnetic imaging over local areas face limitations to

effectively and quantitatively resolve the sometimes large variations in materials properties of a

system. An ideal research technique would bridge these two domains: quantitative macroscale

measurements with exemplary spatial resolution or microscopy that can quantitatively survey

magnetic property variations.6

The first order reversal curve (FORC) technique has shown promise as a bridge of the

former type.9,23-28 This technique analyzes the evolution of the magnetization along a series of

partial hysteresis loops – macroscopic measurements – to extract magnetic characteristics such as

microscopic interaction fields and intrinsic coercivity distributions. These properties often are not

associated with any particular regions of the sample, but represent sample-scale ensemble-

averaged distributions. Furthermore, FORC distributions have been used extensively to provide

insights into the magnetization reversal processes,9,29-38 yet direct confirmation with magnetic

imaging, particularly one carried out using the same magnetic field cycling protocol, has been

lacking. In fact, performing FORC measurements with microscopy which is sensitive to magnetic

structure provides a mechanism to extract quantitative insights from microscopic probes,

becoming a bridge of the latter type.

In this work, we combine the FORC technique with MTXM to quantitatively evaluate

magnetization reversal behavior.26-28,39,40 Combining these two techniques allows the magnetic

interaction fields and intrinsic coercivity distributions to be quantitatively extracted from local

areas. MTXM further provides the ability to highlight specific regions or phases3 using its element-

specific, high-resolution11 imaging capability. The MTXM-FORCs measured on a CoTb film with

perpendicular anisotropy are directly compared to FORCs measured on the same sample with

alternating gradient magnetometry (AGM). Since the FORC technique has been proposed as a

macroscopic measurement which can “fingerprint” microscopic reversal behavior,9,29-38,41

comparing FORC measurements captured with macroscopic techniques and microscopy presents

an ideal opportunity to confirm this capability.42,43 The results of this work lay the foundation for

broader applications of FORC and MTXM, including applying the FORC technique to exceedingly

small samples to quantitatively extract microscopic information, as demonstrated on a single

permalloy (Py) microstructure with vortex-like flux-closure structures.

Experiment

In the first sample, a single magnetic film of Pt(5 nm)/CoTb(20 nm)/Pt(5 nm) with

perpendicular magnetic anisotropy (PMA) was grown by sputtering on an X-ray transparent

amorphous Si3N4 (200 nm) membrane in a 5 mTorr Ar atmosphere, in an ultrahigh vacuum

sputtering chamber. The Pt layers were used as an adhesion/seed layer and capping layer,

respectively. MTXM images were obtained in the out-of-plane geometry on the XM-1 microscope

(BL 6.1.2) at the Advanced Light Source using 778 eV X-rays, probing the Co L3 edge.7 Fresnel

zone-plate optics were used for X-ray focusing and imaging, and the full field images were

recorded by an X-ray sensitive CCD camera. Magnetic contrast was obtained using the XMCD

effect at the Co L3 edge. The use of XMCD makes this technique inherently element-specific. For

the present CoTb system, which consist of only a single magnetic phase, the element specific

sensitivity does not provide an additional insight, but it can be used to separate the magnetic signals

in a system with more than one magnetic material or phase. To obtain FORC data, XMCD images

were recorded at each magnetic field step following the FORC sequence described below. From

the images, the area of the black and white contrasted domains, AWhite and ABlack were measured,

corresponding to the positive and negative out-of-plane directions, respectively. A normalized

magnetization is defined as  

. Magnetometry measurements were performed

at room temperature using an alternating gradient magnetometer (AGM). The magnetization from

the AGM is identified by the variable M.

In the second sample, a single 40 nm thick, rectangular (3 m × 2 m) microstructure of

permalloy (Ni80Fe20) with in-plane magnetization was prepared on a Si3N4 windows using

electron-beam lithography and liftoff techniques. For this size and shape of microstructure, the

magnetization curls into a pair of vortex structures, e.g. Landau patterns, with opposite

circularities; circularity is defined as the direction of the in-plane winding of the chiral structure

and can be clockwise or counter-clockwise.22,44 MTXM images were obtained using circularly

polarized 708 eV X-rays, probing the Fe L3 edge.8,34 Imaging was performed in a tilted geometry,

with the X-rays impinging at 30° relative to the sample normal, capturing the in-plane component

of the magnetization. The black and white contrasts indicate regions with magnetization parallel

and antiparallel to the positive magnetic field direction, respectively. The magnetic sensitivity was

enhanced by subtracting images taken with left- and right-circularly polarized X-rays. The

magnetization from a single microstructure is calculated to be 180 pemu, which is beyond the

sensitivity of conventional magnetometers.

First order reversal curve measurements for both MTXM and AGM were performed

following a previously reported magnetic field sequence.9,26,45 From positive saturation the applied

magnetic field is reduced to a scheduled reversal field, HR. At HR the field sweep direction is

reversed and the magnetization, M or MMic., is measured as the applied field, H, is increased back

to positive saturation. This process is repeated for HR between the positive and negative saturated

states, thus measuring a family of FORCs.9,31 A mixed second order derivative is applied to the

dataset to extract the FORC distribution:    







 . Along each FORC

branch (increasing H at a constant HR) the derivative 

 captures domain growth or ‘up-

switching’ events. The derivative 

 distinguishes new up-switching events on adjacent FORC

branches.26 It is thus important to note that the absence of any feature in the FORC diagram only

indicates that the magnetization is changing at the same rate as that on adjacent FORCs (adjacent

in HR).

Results

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摘要：

1Element-SpecificFirstOrderReversalCurvesMeasuredbyMagneticTransmissionX-rayMicroscopyDustinA.Gilbert,1Mi-YoungIm,2KaiLiu,3PeterFischer2,41MaterialsScienceDepartment,UniversityofTennessee,Knoxville,TN37996,USA2MaterialsSciencesDivision,LawrenceBerkeleyNationalLaboratory,Berkeley,CA94720,USA3PhysicsD...

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1 Element -Specific First Order Reversal Curves Measured by Magnetic Transmission X-ray Microscopy

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